Predictive abilities of scaled quantum-mechanical molecular force fields: application to 2-methylbuta-1,3-diene (isoprene) (original) (raw)
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Ab initio vibrational analysis of trans- and gauche-2,3-dimethylbuta-1,3-diene
Journal of Molecular Structure, 1990
The harmonic force fields of the s-trans(anti) andgauche conformers of 2,3-dimethylbuta-1,3diene have been calculated ab initio at the RHF/6-31G//RHF/6-31G level. These force fields have been further refined using scale factors transferred from analogous calculations at the RHF/ 6-31G level for s-trans-buta-1,3-diene and ethane. A complete assignment of the experimental vibrational frequencies is given. Regularities in the vibrational spectra of the stable conformers of buta-1,3-diene, isoprene (2-methylbuta-1,3-diene) and 2,3-dimethylbuta-1,3-diene are discussed.
Ab initio structures and vibrational analysis of the isoprene conformers
Journal of Molecular Structure, 1987
The geometrical parameters of the tTt-hexatriene-1,3,5 and tCt-hexatriene-1,3,5 molecules have been computed at the RHF/6-31G level. The structures of both molecules are found to be planar in this approximation. The force fields of these molecules were calculated at the HF/6-31G//HF/6-31G level. The vibrational analyses of both molecules and their 2,3,4,5tetradeuteroanalogues made it possible to refine the assignment of some experimental vibrational frequencies. The calculated mean amplitudes of vibrations of the non-deuterated molecules are compared with the experimental ones.
Scaled ab initio force fields of s-cis and skew conformers of methyl vinyl ether
Journal of Molecular Structure, 1991
The ab initio 4-21G force field of the s-cis methyl vinyl ether calculated earlier was rescaled by using scale factors optimized for CHzOCHz and CH2=CHF molecules. The ab initio force field in the same basis set was computed for the skew conformer and scaled by using the same factors. The theoretical shift of frequencies in going from the s-cis to the skew conformer is in keeping with observed values. Previous assignments of the observed skew form bands are revised by taking into account ab initio results. The complete optimization of geometrical parameters of the s-cis, s-trans and skew conformers was carried out in the 4-21 (0") G set and the local minimum was found at an angle of internal rotation around the =C-O bond equal to 149 o, that is, contrary to the 4-21G optimization, close to 144 ° value obtained from experimental data.
The relationship between intramolecular forces, molecular geometry, and vibrational frequencies as reflected in trirert-butylmethane (TTBM) and isobutane was investigated using empirical force field and ab initio calculations. The strong intramolecular forces in TTBM and the resulting large deviations in its physical properties from normal values make it an ideal system for this study. Isobutane, which consists of a tert-butyl group, but is not as overcrowded as TTBM, serves as a good model for comparison. Studying TTBM with the more recent Urey-Bradley (UBCFF) and valence (VCFF) consistent force fields pointed to the need for the inclusion of a n anharmonic bond stretching potential in both force fields and a stretch--bend interaction term in the VCFF. Ab initio calculations verified the results of the empirical force fields and showed how close nonbonded hydrogen-hydrogen interactions compress the C,-H, bond and raise its effective force constant. They also showed how the increased 1-3 geminal interactions induced by closing the H,-C,-C, bond angle stretch the Ct-Ht bond and lower its effective force constant. The modified force fields leave the calculated structures essentially unchanged, but result in dramatic improvement in the vibrational frcquencies. particularly in the VCFF. Thus, both shifts toward higher frequencies and lower frequencies due to overcrowding were accounted for to within IO cm-l or less. The root mean square deviation of the CH stretching frequencies was rcduced from 26.7 to 7.9 cm-l in isobutane and from 39.0 to 4.7 cm-' in TTBM.
Journal of Molecular Structure, 1996
The infrared and Raman spectrum of 1 ,Zdichloropropane has been reinvestigated and is reported in the gas, liquid, amorphous solid and two crystal modifications. All three conformers are present in disordered phases and many bands are composites of overlapping contributors. Assignments of observed bands are substantiated with the aid of ab initio calculations utilizing the RHF/3-21G and RHF/6-31G* basis sets. Electron correlation was approximated by an MP2/6-31G* calculation for all three conformers. From the temperature dependence of the Raman spectrum of the liquid, the bands at 528 and 671 cm-' were uniquely assigned, respectively, to the conformer with the hydrogen atom trans to the chlorine atom (G-) and the chlorine atom trans to the chlorine atom (A), and an average enthalpy difference for the liquid phase was found to be 130 cm-' (370 cal mol-') at -57°C. In contrast, an enthalpy difference of 1038 cm-' was obtained from the ab initio calculations for the isolated molecule using the RHF/3-21G basis set and 561 cm-' (1.605 kcal mol-') from the MP2/6-3 lG* calculation. The enthalpy difference showed experimental variation with temperature, corresponding to a more highly associated liquid at -105°C than at 60°C. The scaled harmonic force fields obtained using the MP2/6-3 lG* calculation are reported for all three conformers together with potential energy distributions and scaled and unscaled frequencies calculated from these force fields. These results are compared to those obtained for some similar molecules.
Transferability of quantum mechanical force field scale factors between conjugated hydrocarbons
Journal of Molecular Structure, 1990
The transferability of scale factors (empirical corrections) for quantum mechanical force fields among closely related molecules is tested for the related hydrocarbons, ethylene, 1,3-butadiene, and 1,3,5-hexatriene and their isotopomers. The agreement demonstrated here between the fundamental vibrational frequencies calculated from the scaled quantum mechanical force fields and the experimental frequencies of these molecules confirms this transferability.
Journal of the American Chemical Society, 1979
The relationship between intramolecular forces, molecular geometry, and vibrational frequencies as reflected in trirert-butylmethane (TTBM) and isobutane was investigated using empirical force field and ab initio calculations. The strong intramolecular forces in TTBM and the resulting large deviations in its physical properties from normal values make it an ideal system for this study. Isobutane, which consists of a tert-butyl group, but is not as overcrowded as TTBM, serves as a good model for comparison. Studying TTBM with the more recent Urey-Bradley (UBCFF) and valence (VCFF) consistent force fields pointed to the need for the inclusion of a n anharmonic bond stretching potential in both force fields and a stretch-bend interaction term in the VCFF. Ab initio calculations verified the results of the empirical force fields and showed how close nonbonded hydrogen-hydrogen interactions compress the C,-H, bond and raise its effective force constant. They also showed how the increased 1-3 geminal interactions induced by closing the H,-C,-C, bond angle stretch the Ct-Ht bond and lower its effective force constant. The modified force fields leave the calculated structures essentially unchanged, but result in dramatic improvement in the vibrational frcquencies. particularly in the VCFF. Thus, both shifts toward higher frequencies and lower frequencies due to overcrowding were accounted for to within IO cm-l or less. The root mean square deviation of the CH stretching frequencies was rcduced from 26.7 to 7.9 cm-l in isobutane and from 39.0 to 4.7 cm-' in TTBM.
Journal of Molecular Structure, 2001
The infrared (3400-50 cm 21 ) and/or Raman (3400 -10 cm 21 ) spectra of gaseous, xenon solution, liquid and solid 5chloropent-2-yne, CH 2 ClCH 2 CCCH 3 , have been recorded. These data indicate that the molecule exists in the anti (the C -Cl bond is trans to the CxC bond) and the gauche conformations in the vapor and liquid but only the anti conformer remains in the solid state. From a variable temperature infrared study of the xenon solution, the anti conformation has been determined to be more stable than the gauche form by 233^23 cm 21 (2.79^0.28 kJ/mol) and it is estimated that 39% of the sample is in the gauche form at ambient temperature. The optimized geometries, conformation stabilities, harmonic force fields, Raman activities, depolarization ratios, and infrared intensities have been obtained from ab initio MP2/6-31G(d) calculations with full electron correlation. These predicted quantities are compared to the corresponding experimental quantities when appropriate. Equilibrium geometries and energies for both conformers have been obtained from ab initio MP2/6-311G(d,p), MP2/6-311G(2d,2p) and MP2/6-311G(2df,2pd) calculations. Vibrational assignments for the 24 normal modes for the anti conformer are proposed and several of the fundamentals for the gauche conformer are assigned. The sub-band structure on the pseudodegenerate vibrations of the methyl group indicates that it is almost free internal rotation. From this fine structure, the Coriolis coupling constants, j, have been determined. These experimental and theoretical results are compared to the corresponding quantities of some similar molecules. q