Gamil Guirgis - Academia.edu (original) (raw)
Papers by Gamil Guirgis
The rotational spectrum of diethylsilane has been assigned using broadband chirped-pulse Fourier-... more The rotational spectrum of diethylsilane has been assigned using broadband chirped-pulse Fourier-transform microwave (CP-FTMW) spectroscopy. Previously, Fourier-transform microwave rotational spectra were observed using a Balle-Flygare type instrument for the ^{28}Si isotopologues of the gauche-gauche, trans-gauche, and trans-trans conformers. In the present study, a broadband microwave spectrum was obtained at the University of Virginia, taking advantage of the ability to perform deep signal averaging to increase the measurement sensitivity. To obtain a full structural determination of the conformers of this molecule, spectra for the ^{29}Si, ^{30}Si, and single ^{13}C substitutions for the gauche-gauche, the trans-gauche, and the trans-trans species were assigned. Substitution (r_s) structures and inertial fit (r_0) structures were determined and a comparison between the experimental and ab initio structures will be presented. For the ^{28}Si isotopologues, the percent differences between the experimental and ab initio rotational constants are less than 1.5% for the trans-trans and trans-gauche and are between 2.0 and 5.0% for the gauche-gauche conformer. The structural parameters will be compared between this molecule, diethylgermane and other silicon containing molecules and the relative abundances of the three conformers will be discussed. S.A. Peebles, M.M. Serafin, R.A. Peebles, G.A. Guirgis, and H.D. Stidham J. Phys. Chem. A, (2009), DOI: 10.1021/jp811049n.
J Mol Struct, 1998
Chloromethyl dimethyl fluorosilane (CH 2Cl(CH 3) 2SiF) was synthesized and investigated by vibra... more Chloromethyl dimethyl fluorosilane (CH 2Cl(CH 3) 2SiF) was synthesized and investigated by vibrational spectroscopy and by ab initio quantum chemical methods. Raman spectra of the liquid were obtained at seven temperatures between 295 and 174 K, and spectra of the amorphous and crystalline solids were recorded. The infrared spectra of the vapour, of the amorphous and crystalline states were obtained between 4000 and 50 cm -1. The compound was mixed with argon and nitrogen and the vapour mixture was deposited on a CsI window at 5 and at 15 K, the infrared spectra were recorded in the range 4000-400 cm -1 before and after annealing. The spectra reveal that the compound exists as anti and gauche conformers in the vapour, liquid, in the unannealed matrices and in the amorphous solid. Six cases were observed when infrared and Raman bands present in the fluid phases vanished after crystallization. Raman temperature studies in the liquid gave ΔconfH = 0.2 ± 0.15 kJ mol -1. The gauche conformer was the low energy conformer and the only one present in the crystal. The IR bands vanishing in the argon and nitrogen matrix spectra after annealing to ca. 28-34 K, suggested that the anti conformer had a lower energy than the gauche in both matrices. The conformational barrier was estimated to be ca. 7 kJ mol -1. Ab initio calculations at the HF/3-21G∗, HF/6-31G∗, HF/6-311G∗ and MP2/6-31G∗ levels of approximation gave optimized geometries, IR and Raman intensities and vibrational frequencies for the anti and gauche conformers. After scaling, a reasonably good agreement between the experimental and calculated wavenumbers for the anti and gauche conformers was obtained.
Journal of Raman Spectroscopy, 1994
Spectrochimica Acta Part a Molecular and Biomolecular Spectroscopy, 2008
Variable temperature (-55 to -100 degrees C) studies of the infrared spectra (4000-400 cm(-1)) of... more Variable temperature (-55 to -100 degrees C) studies of the infrared spectra (4000-400 cm(-1)) of cyclobutanol, c-C4H7OH dissolved in liquid xenon have been carried out. The infrared spectrum (4000-100 cm(-1)) of the gas has also been recorded. From these data two of the four possible stable conformers have been confidently identified and their order of stabilities has been experimentally determined where the first indicator is for the position of attachment of the hydroxyl group on the bent cyclobutyl ring (Eq=equatorial or Ax=axial) and the second one (t=trans, g=gauche) is the relative position of the hydroxyl rotor, i.e. rotation around the ring C-O bond. The enthalpy difference between the most stable Eq-t conformer and the second most stable rotamer, Eq-g, has been determined to be 200+/-50 cm(-1) (2.39+/-0.60 kJ/mol). This experimentally determined order is consistent with the order of stability predicted by ab initio calculations Eq-t>Eq-g>Ax-g>Ax-t. Evidence was obtained for the third conformer Ax-g which is predicted by ab initio calculations to be less stable by more than 650cm(-1) than the Eq-t form. The percentage of each conformer at ambient temperature is estimated to be Eq-t (50%), Eq-g (47%) and Ax-g (3%). The conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios and vibrational frequencies have been obtained for all of the conformers from MP2(full)/6-31G(d) ab initio calculations. The optimized geometries and conformational stabilities have been obtained from ab initio calculations utilizing several different basis sets up to MP2(full)/aug-cc-pVTZ and from density functional theory calculations by the B3LYP method. By utilizing previously reported microwave rotational constants for the Eq-t conformer combined with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r0 parameters have been obtained. The determined heavy atom structural parameters for the Eq-t conformer are: the distances C1-C4=1.547(5) angstroms, C4-C6=1.552(5)angstroms, C-O=1.416(5) angstroms and angles angleC6C4C1=86.6(5) degrees , angleC4C1C5=88.9(5) degrees and angleC6C5C1C4=22.8(5) degrees . The results are discussed and compared to the corresponding properties of some similar molecules.
Journal of Molecular Structure, Mar 1, 1998
The Raman (3100-10 cm -1) and i.r. (3200-400 cm -1) spectra of difluoroacetyl fluoride, CHF 2CFO,... more The Raman (3100-10 cm -1) and i.r. (3200-400 cm -1) spectra of difluoroacetyl fluoride, CHF 2CFO, have been recorded for the gaseous and solid phases. Additionally, the Raman spectrum of the liquid phase has been recorded and qualitative depolarization values have been obtained. Based on these data, it is shown that both the gauche and trans (hydrogen atom trans to the fluorine atom) conformers exist in the fluid phases. For the sample dissolved in liquified xenon, a variable temperature study gave an enthalpy difference of 88 ± 18 cm -1 (251 ± 50 cal mol -1) with the gauche conformer the more stable rotamer. This result is at variance with the ab initio calculations where the trans conformer is predicted to be more stable when electron correlation to second order is utilized. At ambient temperature the gauche conformer is the predominant rotamer in the liquid phase and is the only conformer present in the solid. Complete assignments of observed bands to fundamental vibrations of the gauche conformer are reported and a nearly complete assignment is obtained for the trans conformer. The C-H distance ( r0 has been obtained from the isolated carbon-hydrogen stretching frequencies for the trans and gauche forms with values of 1.090 and 1.093 Å, respectively. Additionally, other r0 parameters have been obtained from the previously reported rotational constants for five isotopomers of difluoroacetyl fluoride. All the experimental data are compared with the corresponding quantities obtained from ab initio RHF/3-21G, RHF/6-31G∗, MP2/6-31G∗ and MP2/6-31++G∗ calculations. From the RHF/3-21G calculation the gauche conformer is predicted to be the more stable conformer by 222 cm -1, but the trans conformer is predicted to be more stable by 105, 19 and 56 cm - from the RHF/6-31G∗, MP2/6-31G∗ and MP2/6-31++G∗ calculations, respectively. The structural parameters, vibrational frequencies and harmonic force field have been calculated for both conformers and the values are compared to the experimental data where appropriate.
Spectrochimica Acta Part a Molecular and Biomolecular Spectroscopy, Oct 1, 1999
The infrared spectra (3500-50 cm -1) of the gas and solid and the Raman spectrum (3500-30 cm -1) ... more The infrared spectra (3500-50 cm -1) of the gas and solid and the Raman spectrum (3500-30 cm -1) of solid 2-pentyne, CH 3CH 2CCCH 3, have been recorded. Additionally, the infrared spectrum (3500-400 cm -1) of a xenon solution has been recorded. A complete vibrational assignment is proposed based on infrared band contours, relative intensities, depolarization values, and group frequencies. The assignment is supported by normal coordinate calculations utilizing ab initio force constants. The internal rotational barrier for the CH 3 rotor of the ethyl group was determined to be 1285 cm -1 from the torsional transitions whereas that for the CH 3 rotor attached to the carbon of the triple bond has nearly free rotation. Complete equilibrium geometries have been determined employing several basis sets at the levels of restricted Hartree-Fock (HF), and/or with full electron correlation by the perturbation method to second order (MP2) as well as with a hybrid density functional theory (B3LYP). The results are discussed and compared to those obtained for some similar molecules.
Journal of Raman Spectroscopy, 1992
ABSTRACT
Struct Chem, 1999
The infrared (3200 to 400 cm-1 ) spectra of gaseous and solid and Raman (3200 to 20 cm-1) spectra... more The infrared (3200 to 400 cm-1 ) spectra of gaseous and solid and Raman (3200 to 20 cm-1) spectra of liquid and solid ethyl chlorosilane-Si-d2, CH3CH2SiD2Cl, have been recorded. Both the gauche and transconformers have been identified in the fluid phases, but only the ...
Journal of Molecular Structure, 2008
The infrared spectra of 3-butyn-1-ol, HC tbnd CCH 2CH 2OH, have been recorded as a vapour in the ... more The infrared spectra of 3-butyn-1-ol, HC tbnd CCH 2CH 2OH, have been recorded as a vapour in the range 3600-50 cm -1 and as a liquid between 3600 and 400 cm -1. Additional spectra of the alcohol isolated in an argon matrix at ca. 5 K were obtained and spectra were recorded after annealing to various temperatures between 10 and 35 K. Raman spectra of the liquid were recorded at room temperature and at various temperatures between 295 and 143 K. Spectra of an amorphous solid were recorded at 78 K. In spite of several attempts and many different annealing temperatures, the sample crystallized neither in the IR nor in the Raman cryostats. In the variable temperature Raman spectra, some bands of the liquid changed in relative intensity and were interpreted in terms of conformational equilibria between two of the five possible conformers. Complete assignments were made for all the bands of the most stable conformer gg, in which the OH group is approaching the triple bond, forming an intramolecular hydrogen bond. From various bands assigned to a second conformer aa, in which OH is oriented anti to the sbnd C tbnd C sbnd bond, or a third conformer ag, the conformational enthalpy difference was found to be Delta confH( ag-gg) = 0.9 kJ mol -1 in the liquid. The two highest energy conformers g'g and ag were not detected. Quantum-chemical calculations have been carried out at the MP2 and B3LYP levels with a variety of basis sets. The calculations revealed that gg was the low energy conformer and CBS-QB3 calculations suggested the gg conformer was more stable by 5.4 and 4.2 kJ mol -1 relative to ag and aa, respectively, in the vapour. Vibrational wavenumbers and infrared and Raman band intensities for the three low energy conformers are reported from B3LYP/cc-pVTZ calculations.
The Journal of Physical Chemistry a, Apr 1, 2008
Infrared spectra (4000 to 400 cm(-1)) of the gas and variable temperature xenon solutions, and th... more Infrared spectra (4000 to 400 cm(-1)) of the gas and variable temperature xenon solutions, and the Raman spectrum of the liquid have been recorded for cyclopropylisocyanate. The enthalpy difference has been determined to be 77 ± 8 cm(-1) (0.92 ± 0.10 kJ/mol) with the trans form more stable than the cis conformer with 59 ± 2% present at ambient temperature. By utilizing three rotational constants for each conformer, combined with structural parameters predicted from MP2(full)/6-311+G(d,p) calculations, the adjusted r(0) parameters have been obtained. Heavy atom structural parameters for the trans [cis] conformers are the following: distances (Å) (C-C(2,3)) = 1.509(3) [1.509(3)], (C(2)-C(3)) = 1.523(3) [1.521(3)], (C-N) = 1.412(3) [1.411(3)], (N═C) =1.214(3) [1.212(3)], (C═O) = 1.163(3) [1.164(3)]; angles (°) ∠CCN = 116.7(5) [120.1(5)], ∠CNC = 136.3(5) [137.6(5)]. The centrifugal distortion constants have been predicted from ab initio and DFT calculations and are compared to the experimentally determined values.
Journal of Molecular Structure, Apr 20, 1998
The infrared (3500 to 30 cm -1) spectra of gaseous and solid and the Raman (3500 to 10 cm -1) spe... more The infrared (3500 to 30 cm -1) spectra of gaseous and solid and the Raman (3500 to 10 cm -1) spectra of the liquid with quantitative depolarization ratios and solid chloromethyl methyl silane, ClCH 2SiH 2CH 3, have been recorded. Similar data have also been recorded for the Si-d 2 isotopomer. These data indicate that two conformers are present in the fluid states but only one conformer is present in the annealed crystalline state. The mid-infrared spectra of the sample dissolved in liquified xenon as a function of temperature (- 100 to - 70°C) have been recorded. Utilizing conformer pairs at 738 ( gauche), 685 ( gauche), and 700 ( trans) cm -1 the enthalpy difference has been determined to be 180 ± 18 cm -1 (515 ± 51 cal mol -1) with the gauche conformer the more stable species. However, in the spectrum of the solid, the trans conformer is the stable rotamer remaining after the sample is well annealed. Utilizing the SiH stretching frequencies from the infrared spectrum of the ClCH 2SiHDCH 3 isotopomer, the two SiH bond distances are calculated to be 1.482 and 1.487 Å for the gauche conformer. The optimized geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies are reported for both conformers from {RHF}/{6-31 G∗ } and/or {MP2 }/{6-31 G∗ } ab initio calculations. The gauche conformer is predicted to be the more stable rotamer from both ab initio calculations in agreement with the experimental results. The other calculated quantities are compared to the experimentally determined values where applicable as well as with some corresponding results for some similar molecules.
Journal of Molecular Structure, 2003
The infrared spectra (3500-400 cm -1) of methylvinyldichlorosilane, CH 2CHSi(CH 3)Cl 2, and dime... more The infrared spectra (3500-400 cm -1) of methylvinyldichlorosilane, CH 2CHSi(CH 3)Cl 2, and dimethylvinylchlorosilane, CH 2CHSi(CH 3) 2Cl, dissolved in liquid krypton have been recorded. From temperature (-105 to -150 °C) dependent FT-IR spectra of these solutions, it is shown that the cis conformer (methyl group eclipsing the double bond) of methylvinyldichlorosilane is the more stable form with an enthalpy difference of 41±10 cm -1 (0.48±0.12 kJ/mol) which is at variance with most of the theoretical predictions. Similar studies for dimethylvinylchlorosilane give an enthalpy difference of 25±10 cm -1 (0.30±0.12 kJ/mol) with the gauche conformer (methyl group eclipsing the double bond) the more stable rotamer which is consistent with the ab initio calculations except with the largest basis set MP2/6-311+G(2d,2p) utilized. Both ab initio calculations with full electron correlation by the perturbation method to the second order (MP2) and density functional theory calculations by the B3LYP method have been carried out with a variety of basis sets up to 6-311G(2d,2p) with and without diffuse functions for all four of the CH 2CHSiCl nY 3- n (Y=H or CH 3, n=1 or 2) molecules. Only for the CH 2CHSiH 2Cl molecule are all of the predictions that the gauche conformer is the more stable rotamer consistent with the experimental results whereas for the other three molecules the conformation stability predictions changed with the MP2/6-311G(2d,2p) calculations compared to those with the smaller basis sets. With such small energy differences between conformers, one cannot rely on ab initio calculations to predict the correct conformer stability for these type of molecules.
Journal of Molecular Structure, 2002
The Raman spectra (3200±30 cm 21 ) of liquid and solid, and infrared spectra of gaseous and solid... more The Raman spectra (3200±30 cm 21 ) of liquid and solid, and infrared spectra of gaseous and solid ethyl chlorogermane, CH 3 CH 2 GeH 2 Cl, have been recorded. Variable temperature (2105 to 2150 8C) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out. From these data, the enthalpy difference has been determined to be 143^30 cm 21 (1.71^0.36 kJ mol 21 ), with the more stable form, the gauche conformer, which is consistent with the prediction from ab initio calculations at both the Hartree±Fock level and with electron correlation by the perturbation method to second order. It is estimated that 80% of the sample is in the gauche form at ambient temperature. A complete vibrational assignment is proposed for both the gauche and trans conformers based on infrared band contours, relative intensities, depolarization values, and group frequencies which is supported by normal coordinate calculations utilizing the force constants from the ab initio MP2/6-31G(d) calculations. The optimized geometries and conformational stabilities have also been obtained from ab initio calculations utilizing several different basis sets with full electron correlation by the perturbation method up to MP2/6-311 1 G(2d,2p). The r 0 adjusted structural parameters for methyl chlorogermane and ethyl germane have been obtained from a combination of the previously reported microwave rotational constants and ab initio predicted parameters. The results are discussed and compared to some corresponding results for several related molecules. q
Journal of Raman Spectroscopy, Mar 1, 2010
The Raman (3200-30 cm−1) and/or infrared spectra (3500 to 400 cm−1) of gaseous, liquid and solid ... more The Raman (3200-30 cm−1) and/or infrared spectra (3500 to 400 cm−1) of gaseous, liquid and solid dimethylsilylisocyanate, (CH3)2 Si(H)NCO, have been recorded. The MP2(full) calculations, employing a variety of basis sets with and without diffusion functions, have been used to predict the structural parameters, conformational stability, vibrational fundamental wavenumbers, Raman activities, depolarization values and infrared intensities to support the vibrational assignment. The low wavenumber Raman spectrum of the gas with a significant number of Q-branches for the SiNC(O) bend is consistent with an essentially linear SiNCO moiety. The ab initio calculations supported this conclusion as all possible orientations of the NCO moiety lead to nearly the same energy. This result is at variance with the conclusion from the electron diffraction study that the heavy atom skeleton was bent with an angle of 152(5)° with one stable cis conformer. It is believed that this reported angle difference from 180° is due to the shrinkage effect. The SiH distance of 1.486 Å has been obtained from the isolated SiH stretching wavenumber. From the adjustment of the ab initio MP2(full)/6-311+G(d,p) predicted structural parameters, a proposed structure is reported, which is expected to give rotational constants within a few megahertz of the actual ones. These experimental and theoretical results are compared with the corresponding quantities of similar molecules. Copyright © 2009 John Wiley & Sons, Ltd.
The Journal of Physical Chemistry a, Jun 1, 2010
H) has been investigated for the first time in the 28-80 GHz spectral interval at a temperature o... more H) has been investigated for the first time in the 28-80 GHz spectral interval at a temperature of -30°C. The spectrum of the ground vibrational state of one conformer characterized by an anticlinal orientation for the CdC-C-Si chain of atoms and a synclinal conformation for the CsCsSisH link has been assigned. This rotamer was found to be at least 2 kJ/mol more stable than further rotameric forms. The spectroscopic investigation has been augmented with quantum chemical calculations employing the MP2 and B3LYP methods using the 6-311++G(3df,3pd) basis set. The theoretical predictions are generally in good agreement with the experimental results.
J Mol Struct, 1997
The infrared (3500 to 40 cm−1) and Raman (3500 to 20 cm−1) spectra of gaseous and solid dimethyl ... more The infrared (3500 to 40 cm−1) and Raman (3500 to 20 cm−1) spectra of gaseous and solid dimethyl vinyl chlorosilane, CH2CHSi(CH3)2Cl, have been recorded. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values obtained. Both the cis and gauche conformers have been identified in the fluid phases but only the gauche conformer remains in the solid. Variable temperature (− 60 to − 100°C) studies of the infrared spectra of dimethyl vinyl chlorosilane dissolved in liquid xenon have been recorded. From these data the enthalpy difference has been determined to be 41 ± 9 cm−1 (117 ± 26 cal mol−1) with the gauche conformer the more stable rotamer. A complete vibrational assignment is proposed for the gauche conformer based on infrared and Raman relative intensities, depolarization values and group frequencies. Several of the fundamentals for the cis conformer have also been identified. The vibrational assignments are supported by normal coordinate calculations. Complete equilibrium geometries have been determined for both rotamers by ab initio calculations employing the 3-21G∗ and 6-31G∗ basis sets at the levels of the restricted Hartree-Fock (RHF) and Møller-Plesset (MP) to second order. The frequencies for the normal modes and the potential energy terms for the conformer interconversion have been calculated at the level. The results are discussed and compared to those obtained for some similar molecules.
Journal of Molecular Structure, Feb 20, 1996
The Raman (3200-10 cm -1) spectra of liquid and solid and mid-infrared (3400-400 cm -1) spectra o... more The Raman (3200-10 cm -1) spectra of liquid and solid and mid-infrared (3400-400 cm -1) spectra of gaseous and solid 3-methyl-2-butenoyl chloride (3,3-dimethylacryloyl chloride), (CH 3) 2CC(H)CClO, have been recorded. Additionally, the far-infrared (350-65 cm -1) spectrum of the gas has been recorded. These spectral data have been interpreted on the basis that the syn conformer (the CC double bond is oriented cis to the CO bond) is the only stable conformer present in all three phases. This conclusion is supported by ab initio calculations where a second conformer is predicted to be nearly 5 kcal mol -1 higher in energy. A complete vibrational assignment of the normal modes is provided. The structural parameters, force constants, and vibrational frequencies for the syn conformer have been determined from ab initio calculations employing the {RHF}/{3-21 G*} and {RHF}/{6-31 G*} basis sets, as well as with electron correlation at the {MP2 }/{6-31 G*} level. The theoretical results are compared to the experimental values, as well as with the corresponding quantities obtained for some similar molecules where appropriate.
Journal of Molecular Structure, Nov 9, 1999
The infrared (3500-30 cm -1) spectra of gaseous and solid and the Raman (3500-10 cm -1) spectra o... more The infrared (3500-30 cm -1) spectra of gaseous and solid and the Raman (3500-10 cm -1) spectra of liquid with quantitative depolarization ratios and solid 2-chloroethyl silane, ClCH 2CH 2SiH 3, have been recorded. Similar data have been recorded for the Si-d 3 isotopomer. These data indicate that two conformers, trans and gauche, are present in the fluid states but only one conformer, trans, is present in the solid. The mid-infrared spectra of the sample dissolved in liquified xenon as a function of temperature (-55 to -100°C) has been recorded. The enthalpy difference between the conformers has been determined to be 181±12 cm -1 (2.17±0.14 kJ/mol) with the trans rotamer the more stable form. From the isolated Si-H frequencies from the Si-d 2 isotopomer the ro Si-H distances of 1.484 and 1.483 Å for the trans and 1.481 for the gauche conformers have been obtained. Ab initio calculations have been carried out with several different basis sets up to MP2/6-311+G** from which structural parameters and conformational stabilities have been determined. With all the basis sets the trans form is predicted to be the more stable conformer which is consistent with the experimental results. These results are compared to the corresponding quantities for the carbon analogue.
Journal of Molecular Structure, 1999
Infrared spectra (4000±50 cm 21 ) of the vapor, amorphous and crystalline solids and Raman spectr... more Infrared spectra (4000±50 cm 21 ) of the vapor, amorphous and crystalline solids and Raman spectra (3600±10 cm 21 ) of the liquid with qualitative depolarization data as well as the amorphous and crystalline solids of methylaminothiophosphoryl di¯uoride, CH 3 N(H)P(S)F 2 , and three deuterated species, CD 3 N(H)P(S)F 2 , CH 3 N(D)P(S)F 2 , and CD 3 N(D)P(S)F 2 , have been recorded. The spectra indicate that in the vapor, liquid and amorphous solid a small amount of a second conformer is present, whereas only one conformer remains in the low temperature crystalline phase. The near-infrared spectra of the vapor con®rms the existence of two conformers in the gas phase. Asymmetric top contour simulation of the vapor shows that the trans conformer is the predominant vapor phase conformer. From a temperature study of the Raman spectrum of the liquid the enthalpy difference between the trans and near-cis conformers was determined to be 368^15 cm 21 (4.41^0.2 kJ/mol), with the trans conformer being thermodynamically preferred. Ab Initio calculations with structure optimization using the 6-31G(d) and 6-3111G(d,p) basis sets at the restricted Hartree±Fock (RHF) and/or with full electron correlation by the perturbation method to second order (MP2) support the occurrence of near-trans (,58 from trans) and near-cis (,208 from cis) conformers. From the RHF/6-31G(d) calculation the near-trans conformer is predicted to be the more stable form by 451 cm 21 (5.35 kJ/mol) and from the MP2/6-3111G(d,p) calculation by 387 cm 21 (4.63 kJ/mol). All of the normal modes of the near-trans rotamer have been assigned based on infrared band contours, depolarization values and group frequencies and the assignment is supported by the normal coordinate calculation utilizing harmonic force constants from the MP2/6-31G(d) ab initio calculations. q
Canadian Journal of Analytical Sciences and Spectroscopy, 2003
The rotational spectrum of diethylsilane has been assigned using broadband chirped-pulse Fourier-... more The rotational spectrum of diethylsilane has been assigned using broadband chirped-pulse Fourier-transform microwave (CP-FTMW) spectroscopy. Previously, Fourier-transform microwave rotational spectra were observed using a Balle-Flygare type instrument for the ^{28}Si isotopologues of the gauche-gauche, trans-gauche, and trans-trans conformers. In the present study, a broadband microwave spectrum was obtained at the University of Virginia, taking advantage of the ability to perform deep signal averaging to increase the measurement sensitivity. To obtain a full structural determination of the conformers of this molecule, spectra for the ^{29}Si, ^{30}Si, and single ^{13}C substitutions for the gauche-gauche, the trans-gauche, and the trans-trans species were assigned. Substitution (r_s) structures and inertial fit (r_0) structures were determined and a comparison between the experimental and ab initio structures will be presented. For the ^{28}Si isotopologues, the percent differences between the experimental and ab initio rotational constants are less than 1.5% for the trans-trans and trans-gauche and are between 2.0 and 5.0% for the gauche-gauche conformer. The structural parameters will be compared between this molecule, diethylgermane and other silicon containing molecules and the relative abundances of the three conformers will be discussed. S.A. Peebles, M.M. Serafin, R.A. Peebles, G.A. Guirgis, and H.D. Stidham J. Phys. Chem. A, (2009), DOI: 10.1021/jp811049n.
J Mol Struct, 1998
Chloromethyl dimethyl fluorosilane (CH 2Cl(CH 3) 2SiF) was synthesized and investigated by vibra... more Chloromethyl dimethyl fluorosilane (CH 2Cl(CH 3) 2SiF) was synthesized and investigated by vibrational spectroscopy and by ab initio quantum chemical methods. Raman spectra of the liquid were obtained at seven temperatures between 295 and 174 K, and spectra of the amorphous and crystalline solids were recorded. The infrared spectra of the vapour, of the amorphous and crystalline states were obtained between 4000 and 50 cm -1. The compound was mixed with argon and nitrogen and the vapour mixture was deposited on a CsI window at 5 and at 15 K, the infrared spectra were recorded in the range 4000-400 cm -1 before and after annealing. The spectra reveal that the compound exists as anti and gauche conformers in the vapour, liquid, in the unannealed matrices and in the amorphous solid. Six cases were observed when infrared and Raman bands present in the fluid phases vanished after crystallization. Raman temperature studies in the liquid gave ΔconfH = 0.2 ± 0.15 kJ mol -1. The gauche conformer was the low energy conformer and the only one present in the crystal. The IR bands vanishing in the argon and nitrogen matrix spectra after annealing to ca. 28-34 K, suggested that the anti conformer had a lower energy than the gauche in both matrices. The conformational barrier was estimated to be ca. 7 kJ mol -1. Ab initio calculations at the HF/3-21G∗, HF/6-31G∗, HF/6-311G∗ and MP2/6-31G∗ levels of approximation gave optimized geometries, IR and Raman intensities and vibrational frequencies for the anti and gauche conformers. After scaling, a reasonably good agreement between the experimental and calculated wavenumbers for the anti and gauche conformers was obtained.
Journal of Raman Spectroscopy, 1994
Spectrochimica Acta Part a Molecular and Biomolecular Spectroscopy, 2008
Variable temperature (-55 to -100 degrees C) studies of the infrared spectra (4000-400 cm(-1)) of... more Variable temperature (-55 to -100 degrees C) studies of the infrared spectra (4000-400 cm(-1)) of cyclobutanol, c-C4H7OH dissolved in liquid xenon have been carried out. The infrared spectrum (4000-100 cm(-1)) of the gas has also been recorded. From these data two of the four possible stable conformers have been confidently identified and their order of stabilities has been experimentally determined where the first indicator is for the position of attachment of the hydroxyl group on the bent cyclobutyl ring (Eq=equatorial or Ax=axial) and the second one (t=trans, g=gauche) is the relative position of the hydroxyl rotor, i.e. rotation around the ring C-O bond. The enthalpy difference between the most stable Eq-t conformer and the second most stable rotamer, Eq-g, has been determined to be 200+/-50 cm(-1) (2.39+/-0.60 kJ/mol). This experimentally determined order is consistent with the order of stability predicted by ab initio calculations Eq-t>Eq-g>Ax-g>Ax-t. Evidence was obtained for the third conformer Ax-g which is predicted by ab initio calculations to be less stable by more than 650cm(-1) than the Eq-t form. The percentage of each conformer at ambient temperature is estimated to be Eq-t (50%), Eq-g (47%) and Ax-g (3%). The conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios and vibrational frequencies have been obtained for all of the conformers from MP2(full)/6-31G(d) ab initio calculations. The optimized geometries and conformational stabilities have been obtained from ab initio calculations utilizing several different basis sets up to MP2(full)/aug-cc-pVTZ and from density functional theory calculations by the B3LYP method. By utilizing previously reported microwave rotational constants for the Eq-t conformer combined with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r0 parameters have been obtained. The determined heavy atom structural parameters for the Eq-t conformer are: the distances C1-C4=1.547(5) angstroms, C4-C6=1.552(5)angstroms, C-O=1.416(5) angstroms and angles angleC6C4C1=86.6(5) degrees , angleC4C1C5=88.9(5) degrees and angleC6C5C1C4=22.8(5) degrees . The results are discussed and compared to the corresponding properties of some similar molecules.
Journal of Molecular Structure, Mar 1, 1998
The Raman (3100-10 cm -1) and i.r. (3200-400 cm -1) spectra of difluoroacetyl fluoride, CHF 2CFO,... more The Raman (3100-10 cm -1) and i.r. (3200-400 cm -1) spectra of difluoroacetyl fluoride, CHF 2CFO, have been recorded for the gaseous and solid phases. Additionally, the Raman spectrum of the liquid phase has been recorded and qualitative depolarization values have been obtained. Based on these data, it is shown that both the gauche and trans (hydrogen atom trans to the fluorine atom) conformers exist in the fluid phases. For the sample dissolved in liquified xenon, a variable temperature study gave an enthalpy difference of 88 ± 18 cm -1 (251 ± 50 cal mol -1) with the gauche conformer the more stable rotamer. This result is at variance with the ab initio calculations where the trans conformer is predicted to be more stable when electron correlation to second order is utilized. At ambient temperature the gauche conformer is the predominant rotamer in the liquid phase and is the only conformer present in the solid. Complete assignments of observed bands to fundamental vibrations of the gauche conformer are reported and a nearly complete assignment is obtained for the trans conformer. The C-H distance ( r0 has been obtained from the isolated carbon-hydrogen stretching frequencies for the trans and gauche forms with values of 1.090 and 1.093 Å, respectively. Additionally, other r0 parameters have been obtained from the previously reported rotational constants for five isotopomers of difluoroacetyl fluoride. All the experimental data are compared with the corresponding quantities obtained from ab initio RHF/3-21G, RHF/6-31G∗, MP2/6-31G∗ and MP2/6-31++G∗ calculations. From the RHF/3-21G calculation the gauche conformer is predicted to be the more stable conformer by 222 cm -1, but the trans conformer is predicted to be more stable by 105, 19 and 56 cm - from the RHF/6-31G∗, MP2/6-31G∗ and MP2/6-31++G∗ calculations, respectively. The structural parameters, vibrational frequencies and harmonic force field have been calculated for both conformers and the values are compared to the experimental data where appropriate.
Spectrochimica Acta Part a Molecular and Biomolecular Spectroscopy, Oct 1, 1999
The infrared spectra (3500-50 cm -1) of the gas and solid and the Raman spectrum (3500-30 cm -1) ... more The infrared spectra (3500-50 cm -1) of the gas and solid and the Raman spectrum (3500-30 cm -1) of solid 2-pentyne, CH 3CH 2CCCH 3, have been recorded. Additionally, the infrared spectrum (3500-400 cm -1) of a xenon solution has been recorded. A complete vibrational assignment is proposed based on infrared band contours, relative intensities, depolarization values, and group frequencies. The assignment is supported by normal coordinate calculations utilizing ab initio force constants. The internal rotational barrier for the CH 3 rotor of the ethyl group was determined to be 1285 cm -1 from the torsional transitions whereas that for the CH 3 rotor attached to the carbon of the triple bond has nearly free rotation. Complete equilibrium geometries have been determined employing several basis sets at the levels of restricted Hartree-Fock (HF), and/or with full electron correlation by the perturbation method to second order (MP2) as well as with a hybrid density functional theory (B3LYP). The results are discussed and compared to those obtained for some similar molecules.
Journal of Raman Spectroscopy, 1992
ABSTRACT
Struct Chem, 1999
The infrared (3200 to 400 cm-1 ) spectra of gaseous and solid and Raman (3200 to 20 cm-1) spectra... more The infrared (3200 to 400 cm-1 ) spectra of gaseous and solid and Raman (3200 to 20 cm-1) spectra of liquid and solid ethyl chlorosilane-Si-d2, CH3CH2SiD2Cl, have been recorded. Both the gauche and transconformers have been identified in the fluid phases, but only the ...
Journal of Molecular Structure, 2008
The infrared spectra of 3-butyn-1-ol, HC tbnd CCH 2CH 2OH, have been recorded as a vapour in the ... more The infrared spectra of 3-butyn-1-ol, HC tbnd CCH 2CH 2OH, have been recorded as a vapour in the range 3600-50 cm -1 and as a liquid between 3600 and 400 cm -1. Additional spectra of the alcohol isolated in an argon matrix at ca. 5 K were obtained and spectra were recorded after annealing to various temperatures between 10 and 35 K. Raman spectra of the liquid were recorded at room temperature and at various temperatures between 295 and 143 K. Spectra of an amorphous solid were recorded at 78 K. In spite of several attempts and many different annealing temperatures, the sample crystallized neither in the IR nor in the Raman cryostats. In the variable temperature Raman spectra, some bands of the liquid changed in relative intensity and were interpreted in terms of conformational equilibria between two of the five possible conformers. Complete assignments were made for all the bands of the most stable conformer gg, in which the OH group is approaching the triple bond, forming an intramolecular hydrogen bond. From various bands assigned to a second conformer aa, in which OH is oriented anti to the sbnd C tbnd C sbnd bond, or a third conformer ag, the conformational enthalpy difference was found to be Delta confH( ag-gg) = 0.9 kJ mol -1 in the liquid. The two highest energy conformers g'g and ag were not detected. Quantum-chemical calculations have been carried out at the MP2 and B3LYP levels with a variety of basis sets. The calculations revealed that gg was the low energy conformer and CBS-QB3 calculations suggested the gg conformer was more stable by 5.4 and 4.2 kJ mol -1 relative to ag and aa, respectively, in the vapour. Vibrational wavenumbers and infrared and Raman band intensities for the three low energy conformers are reported from B3LYP/cc-pVTZ calculations.
The Journal of Physical Chemistry a, Apr 1, 2008
Infrared spectra (4000 to 400 cm(-1)) of the gas and variable temperature xenon solutions, and th... more Infrared spectra (4000 to 400 cm(-1)) of the gas and variable temperature xenon solutions, and the Raman spectrum of the liquid have been recorded for cyclopropylisocyanate. The enthalpy difference has been determined to be 77 ± 8 cm(-1) (0.92 ± 0.10 kJ/mol) with the trans form more stable than the cis conformer with 59 ± 2% present at ambient temperature. By utilizing three rotational constants for each conformer, combined with structural parameters predicted from MP2(full)/6-311+G(d,p) calculations, the adjusted r(0) parameters have been obtained. Heavy atom structural parameters for the trans [cis] conformers are the following: distances (Å) (C-C(2,3)) = 1.509(3) [1.509(3)], (C(2)-C(3)) = 1.523(3) [1.521(3)], (C-N) = 1.412(3) [1.411(3)], (N═C) =1.214(3) [1.212(3)], (C═O) = 1.163(3) [1.164(3)]; angles (°) ∠CCN = 116.7(5) [120.1(5)], ∠CNC = 136.3(5) [137.6(5)]. The centrifugal distortion constants have been predicted from ab initio and DFT calculations and are compared to the experimentally determined values.
Journal of Molecular Structure, Apr 20, 1998
The infrared (3500 to 30 cm -1) spectra of gaseous and solid and the Raman (3500 to 10 cm -1) spe... more The infrared (3500 to 30 cm -1) spectra of gaseous and solid and the Raman (3500 to 10 cm -1) spectra of the liquid with quantitative depolarization ratios and solid chloromethyl methyl silane, ClCH 2SiH 2CH 3, have been recorded. Similar data have also been recorded for the Si-d 2 isotopomer. These data indicate that two conformers are present in the fluid states but only one conformer is present in the annealed crystalline state. The mid-infrared spectra of the sample dissolved in liquified xenon as a function of temperature (- 100 to - 70°C) have been recorded. Utilizing conformer pairs at 738 ( gauche), 685 ( gauche), and 700 ( trans) cm -1 the enthalpy difference has been determined to be 180 ± 18 cm -1 (515 ± 51 cal mol -1) with the gauche conformer the more stable species. However, in the spectrum of the solid, the trans conformer is the stable rotamer remaining after the sample is well annealed. Utilizing the SiH stretching frequencies from the infrared spectrum of the ClCH 2SiHDCH 3 isotopomer, the two SiH bond distances are calculated to be 1.482 and 1.487 Å for the gauche conformer. The optimized geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies are reported for both conformers from {RHF}/{6-31 G∗ } and/or {MP2 }/{6-31 G∗ } ab initio calculations. The gauche conformer is predicted to be the more stable rotamer from both ab initio calculations in agreement with the experimental results. The other calculated quantities are compared to the experimentally determined values where applicable as well as with some corresponding results for some similar molecules.
Journal of Molecular Structure, 2003
The infrared spectra (3500-400 cm -1) of methylvinyldichlorosilane, CH 2CHSi(CH 3)Cl 2, and dime... more The infrared spectra (3500-400 cm -1) of methylvinyldichlorosilane, CH 2CHSi(CH 3)Cl 2, and dimethylvinylchlorosilane, CH 2CHSi(CH 3) 2Cl, dissolved in liquid krypton have been recorded. From temperature (-105 to -150 °C) dependent FT-IR spectra of these solutions, it is shown that the cis conformer (methyl group eclipsing the double bond) of methylvinyldichlorosilane is the more stable form with an enthalpy difference of 41±10 cm -1 (0.48±0.12 kJ/mol) which is at variance with most of the theoretical predictions. Similar studies for dimethylvinylchlorosilane give an enthalpy difference of 25±10 cm -1 (0.30±0.12 kJ/mol) with the gauche conformer (methyl group eclipsing the double bond) the more stable rotamer which is consistent with the ab initio calculations except with the largest basis set MP2/6-311+G(2d,2p) utilized. Both ab initio calculations with full electron correlation by the perturbation method to the second order (MP2) and density functional theory calculations by the B3LYP method have been carried out with a variety of basis sets up to 6-311G(2d,2p) with and without diffuse functions for all four of the CH 2CHSiCl nY 3- n (Y=H or CH 3, n=1 or 2) molecules. Only for the CH 2CHSiH 2Cl molecule are all of the predictions that the gauche conformer is the more stable rotamer consistent with the experimental results whereas for the other three molecules the conformation stability predictions changed with the MP2/6-311G(2d,2p) calculations compared to those with the smaller basis sets. With such small energy differences between conformers, one cannot rely on ab initio calculations to predict the correct conformer stability for these type of molecules.
Journal of Molecular Structure, 2002
The Raman spectra (3200±30 cm 21 ) of liquid and solid, and infrared spectra of gaseous and solid... more The Raman spectra (3200±30 cm 21 ) of liquid and solid, and infrared spectra of gaseous and solid ethyl chlorogermane, CH 3 CH 2 GeH 2 Cl, have been recorded. Variable temperature (2105 to 2150 8C) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out. From these data, the enthalpy difference has been determined to be 143^30 cm 21 (1.71^0.36 kJ mol 21 ), with the more stable form, the gauche conformer, which is consistent with the prediction from ab initio calculations at both the Hartree±Fock level and with electron correlation by the perturbation method to second order. It is estimated that 80% of the sample is in the gauche form at ambient temperature. A complete vibrational assignment is proposed for both the gauche and trans conformers based on infrared band contours, relative intensities, depolarization values, and group frequencies which is supported by normal coordinate calculations utilizing the force constants from the ab initio MP2/6-31G(d) calculations. The optimized geometries and conformational stabilities have also been obtained from ab initio calculations utilizing several different basis sets with full electron correlation by the perturbation method up to MP2/6-311 1 G(2d,2p). The r 0 adjusted structural parameters for methyl chlorogermane and ethyl germane have been obtained from a combination of the previously reported microwave rotational constants and ab initio predicted parameters. The results are discussed and compared to some corresponding results for several related molecules. q
Journal of Raman Spectroscopy, Mar 1, 2010
The Raman (3200-30 cm−1) and/or infrared spectra (3500 to 400 cm−1) of gaseous, liquid and solid ... more The Raman (3200-30 cm−1) and/or infrared spectra (3500 to 400 cm−1) of gaseous, liquid and solid dimethylsilylisocyanate, (CH3)2 Si(H)NCO, have been recorded. The MP2(full) calculations, employing a variety of basis sets with and without diffusion functions, have been used to predict the structural parameters, conformational stability, vibrational fundamental wavenumbers, Raman activities, depolarization values and infrared intensities to support the vibrational assignment. The low wavenumber Raman spectrum of the gas with a significant number of Q-branches for the SiNC(O) bend is consistent with an essentially linear SiNCO moiety. The ab initio calculations supported this conclusion as all possible orientations of the NCO moiety lead to nearly the same energy. This result is at variance with the conclusion from the electron diffraction study that the heavy atom skeleton was bent with an angle of 152(5)° with one stable cis conformer. It is believed that this reported angle difference from 180° is due to the shrinkage effect. The SiH distance of 1.486 Å has been obtained from the isolated SiH stretching wavenumber. From the adjustment of the ab initio MP2(full)/6-311+G(d,p) predicted structural parameters, a proposed structure is reported, which is expected to give rotational constants within a few megahertz of the actual ones. These experimental and theoretical results are compared with the corresponding quantities of similar molecules. Copyright © 2009 John Wiley & Sons, Ltd.
The Journal of Physical Chemistry a, Jun 1, 2010
H) has been investigated for the first time in the 28-80 GHz spectral interval at a temperature o... more H) has been investigated for the first time in the 28-80 GHz spectral interval at a temperature of -30°C. The spectrum of the ground vibrational state of one conformer characterized by an anticlinal orientation for the CdC-C-Si chain of atoms and a synclinal conformation for the CsCsSisH link has been assigned. This rotamer was found to be at least 2 kJ/mol more stable than further rotameric forms. The spectroscopic investigation has been augmented with quantum chemical calculations employing the MP2 and B3LYP methods using the 6-311++G(3df,3pd) basis set. The theoretical predictions are generally in good agreement with the experimental results.
J Mol Struct, 1997
The infrared (3500 to 40 cm−1) and Raman (3500 to 20 cm−1) spectra of gaseous and solid dimethyl ... more The infrared (3500 to 40 cm−1) and Raman (3500 to 20 cm−1) spectra of gaseous and solid dimethyl vinyl chlorosilane, CH2CHSi(CH3)2Cl, have been recorded. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values obtained. Both the cis and gauche conformers have been identified in the fluid phases but only the gauche conformer remains in the solid. Variable temperature (− 60 to − 100°C) studies of the infrared spectra of dimethyl vinyl chlorosilane dissolved in liquid xenon have been recorded. From these data the enthalpy difference has been determined to be 41 ± 9 cm−1 (117 ± 26 cal mol−1) with the gauche conformer the more stable rotamer. A complete vibrational assignment is proposed for the gauche conformer based on infrared and Raman relative intensities, depolarization values and group frequencies. Several of the fundamentals for the cis conformer have also been identified. The vibrational assignments are supported by normal coordinate calculations. Complete equilibrium geometries have been determined for both rotamers by ab initio calculations employing the 3-21G∗ and 6-31G∗ basis sets at the levels of the restricted Hartree-Fock (RHF) and Møller-Plesset (MP) to second order. The frequencies for the normal modes and the potential energy terms for the conformer interconversion have been calculated at the level. The results are discussed and compared to those obtained for some similar molecules.
Journal of Molecular Structure, Feb 20, 1996
The Raman (3200-10 cm -1) spectra of liquid and solid and mid-infrared (3400-400 cm -1) spectra o... more The Raman (3200-10 cm -1) spectra of liquid and solid and mid-infrared (3400-400 cm -1) spectra of gaseous and solid 3-methyl-2-butenoyl chloride (3,3-dimethylacryloyl chloride), (CH 3) 2CC(H)CClO, have been recorded. Additionally, the far-infrared (350-65 cm -1) spectrum of the gas has been recorded. These spectral data have been interpreted on the basis that the syn conformer (the CC double bond is oriented cis to the CO bond) is the only stable conformer present in all three phases. This conclusion is supported by ab initio calculations where a second conformer is predicted to be nearly 5 kcal mol -1 higher in energy. A complete vibrational assignment of the normal modes is provided. The structural parameters, force constants, and vibrational frequencies for the syn conformer have been determined from ab initio calculations employing the {RHF}/{3-21 G*} and {RHF}/{6-31 G*} basis sets, as well as with electron correlation at the {MP2 }/{6-31 G*} level. The theoretical results are compared to the experimental values, as well as with the corresponding quantities obtained for some similar molecules where appropriate.
Journal of Molecular Structure, Nov 9, 1999
The infrared (3500-30 cm -1) spectra of gaseous and solid and the Raman (3500-10 cm -1) spectra o... more The infrared (3500-30 cm -1) spectra of gaseous and solid and the Raman (3500-10 cm -1) spectra of liquid with quantitative depolarization ratios and solid 2-chloroethyl silane, ClCH 2CH 2SiH 3, have been recorded. Similar data have been recorded for the Si-d 3 isotopomer. These data indicate that two conformers, trans and gauche, are present in the fluid states but only one conformer, trans, is present in the solid. The mid-infrared spectra of the sample dissolved in liquified xenon as a function of temperature (-55 to -100°C) has been recorded. The enthalpy difference between the conformers has been determined to be 181±12 cm -1 (2.17±0.14 kJ/mol) with the trans rotamer the more stable form. From the isolated Si-H frequencies from the Si-d 2 isotopomer the ro Si-H distances of 1.484 and 1.483 Å for the trans and 1.481 for the gauche conformers have been obtained. Ab initio calculations have been carried out with several different basis sets up to MP2/6-311+G** from which structural parameters and conformational stabilities have been determined. With all the basis sets the trans form is predicted to be the more stable conformer which is consistent with the experimental results. These results are compared to the corresponding quantities for the carbon analogue.
Journal of Molecular Structure, 1999
Infrared spectra (4000±50 cm 21 ) of the vapor, amorphous and crystalline solids and Raman spectr... more Infrared spectra (4000±50 cm 21 ) of the vapor, amorphous and crystalline solids and Raman spectra (3600±10 cm 21 ) of the liquid with qualitative depolarization data as well as the amorphous and crystalline solids of methylaminothiophosphoryl di¯uoride, CH 3 N(H)P(S)F 2 , and three deuterated species, CD 3 N(H)P(S)F 2 , CH 3 N(D)P(S)F 2 , and CD 3 N(D)P(S)F 2 , have been recorded. The spectra indicate that in the vapor, liquid and amorphous solid a small amount of a second conformer is present, whereas only one conformer remains in the low temperature crystalline phase. The near-infrared spectra of the vapor con®rms the existence of two conformers in the gas phase. Asymmetric top contour simulation of the vapor shows that the trans conformer is the predominant vapor phase conformer. From a temperature study of the Raman spectrum of the liquid the enthalpy difference between the trans and near-cis conformers was determined to be 368^15 cm 21 (4.41^0.2 kJ/mol), with the trans conformer being thermodynamically preferred. Ab Initio calculations with structure optimization using the 6-31G(d) and 6-3111G(d,p) basis sets at the restricted Hartree±Fock (RHF) and/or with full electron correlation by the perturbation method to second order (MP2) support the occurrence of near-trans (,58 from trans) and near-cis (,208 from cis) conformers. From the RHF/6-31G(d) calculation the near-trans conformer is predicted to be the more stable form by 451 cm 21 (5.35 kJ/mol) and from the MP2/6-3111G(d,p) calculation by 387 cm 21 (4.63 kJ/mol). All of the normal modes of the near-trans rotamer have been assigned based on infrared band contours, depolarization values and group frequencies and the assignment is supported by the normal coordinate calculation utilizing harmonic force constants from the MP2/6-31G(d) ab initio calculations. q
Canadian Journal of Analytical Sciences and Spectroscopy, 2003