The microwave spectroscopy study of 1,2-dimethoxyethane (original) (raw)
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Fourier transform microwave study on 2-methyloxetane and 3-methyloxetane
Chemical Physics, 1996
The rotational spectra of 2-methyloxetane and 3-methyloxetane have been reinvestigated using Fourier transform microwave spectroscopy (FTMW). A,E splittings due to internal rotation of the methyl group have been observed in the ground and several ring-puckering and methyl torsion excited states for both molecules• Internal rotation barriers of V 3 = 3.332(14) kcal mo1-1 (13.941(58) El mo1-1) for 2-methyloxetane and V3= 3.284(12) kcal mol-l (13.740(50) kJ mol-~) for 3-methyloxetane have been determined from the ground vibrational state spectra. Furthermore, the rotational, quartic and some higher-order centrifugal distortion constants are reported. The experimental setup of a waveguide Fourier transform microwave spectrometer is also presented.
Journal of Molecular Structure, 2006
The rotational spectrum of 13 C 1 methyl formate (H 13 COOCH 3 ) has been observed in the frequency range 7-610 GHz. Two hundred and ninety one transitions up to JZ58 (K max Z24) were assigned to the A-species of the ground torsional state. They could be fitted to a standard Watsonian involving 19 parameters (up to one decic centrifugal distortion constant). About 260 E-transitions were also assigned. A global analysis of all these transitions using the internal axis method gave a satisfactory fit permitting to determine the three internal rotation parameters (I a Z 3.132(4) mÅ 2 , V 3 Z4912(6) J/mol and :(i,a)Z52.30(5)8, which is the angle between the principal axis a and the internal rotation axis i) and allowing us to make an accurate prediction of the rotational spectrum. q
Fourier-transform microwave and submillimeter-wave spectroscopy of chloroiodomethane, CH 2 ICl
Guided by a previous microwave study , the rotational spectrum of both chlorine isotopologues of chloroiodomethane in its vibrational and electronic ground state has been re-investigated in the microwave region and extended to the millimeter/submillimeter-wave region. Weak a-type transitions have been recorded by Fourier transform microwave spectroscopy below 20 GHz whilst strong btype rotational transitions have been recorded between 15 and 646 GHz, corresponding to energy levels with J 00 6 108 and K 00 a 6 12. Molecular constants including those describing the hyperfine structures owing to the two halogen atoms were accurately determined for both species from the least-squares analysis of a total of 1475 distinct transition frequencies (of which 906 belong to the CH 2 I 35 Cl isotopologue). The two sets of rotational constants allowed us to derive an r 0 structure of CH 2 ICl.
The microwave spectrum of the mono deuterated species of methyl formate
Journal of Molecular Spectroscopy, 2009
The microwave spectrum of mono deuterated methyl formate ðHCOOCH 2 DÞ was measured in three regions by Fourier transform microwave spectroscopy, conventional Stark modulation techniques, and submillimeter wave spectroscopy. Effects due to the large amplitude torsional motion of the partially deuterated CH 2 D methyl group were evidenced and lead to two sets of transitions. Both sets display a rigid-rotator behavior. In one of them, a small tunneling splitting ranging from 0 to 20 MHz was observed. This splitting was accounted for using a phenomenological IAM-like treatment accounting for its rotational dependence. This model was used to analyze line frequencies of the 1388 measured microwave lines. The unitless standard deviation of the fit is 1.5 and the spectroscopic parameters obtained are consistent with the structure of the molecule and the nature of the large amplitude tunneling motion.
Journal of Molecular Structure, 2010
The FT-microwave spectrum of ethynylcyclohexane, cC 6 H 11 CCH has been investigated from 11,000 to 21,000 MHz and 32 transitions for the chair-equatorial conformer and 22 transitions for the chair-axial form have been observed and assigned from which the rotational constants and five centrifugal distortion constants have been obtained. By utilizing the six microwave rotational constants along with ab initio MP2(full)/6-311 + G(d,p) predicted structural values, adjusted r 0 parameters have been obtained for the two conformers. The determined heavy atom structural parameters for the equatorial[axial] conformer are: the distances (Å) C 1 AC 9,10 = 1.532(3)[1.537(3)], C 9,10 AC 15,16 = 1.544(3)[1.534(3)], C 4 AC 15,16 = 1.541(3)[1.545(3)], C 6 "C 7 = 1.210(3)[1.212(3)] and angles in degrees: \C 1 C 9,10 C 15,16 = 111.0(5) [111.2(5)], \C 6 C 4 C 15,16 = 110.7(5)[110.3(5)] and sC 1 C 9,10 C 15,16 C 4 = 56.4(10)[55.3(10)]. Variable temperature (À60 to À100°C) studies of a xenon solution were carried out and by using 10 conformer pairs an enthalpy difference of 124 ± 13 cm À1 (1.48 ± 0.16 kJ/mol) was obtained with the more stable form the chair-equatorial conformer. At ambient temperature, the abundance of the chair-equatorial conformer is 55 ± 3%. By measuring the temperature dependency of the Raman spectrum of the liquid of the conformer pair at 822 (axial)/842 (equatorial) cm À1 , a standard enthalpy difference of 172 ± 32 cm À1 (2.06 ± 0.38 kJ/mol) was obtained again with the chair-equatorial conformer the more stable form. From MP2(full)/6-311 + G(d,p) ab initio calculations the chair-axial conformer is predicted to be more stable by 135 cm À1 (1.62 kJ/mol) whereas in contrast the density functional theory calculations by the B3LYP method with the same basis set gives the chair-equatorial conformer more stable by 289 cm À1 (3.46 kJ/mol). A complete vibrational assignment is given for the chair-equatorial conformer and most of the fundamentals for the chair-axial form have been assigned. To support the vibrational assignments, normal coordinate calculations with two scaling factors for the force constants from MP2(full)/6-31G(d) calculations were carried out to predict the fundamental vibrational frequencies, infrared intensities, Raman activities, depolarization values and infrared band contours. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules.
Journal of Molecular Spectroscopy, 1976
Rotational spectra of six isotopic species of ethylene ozonide-Da have been assigned. The 0,-O, distance was calculated six independent ways using Kraitchman's substitution equations, and values between 1.458 and 1.462 d were obtained. This is in contrast to a similar analysis for normal ethylene ozonide where the 0,-O, distance varied between 1.455 and 1.502 A. This difference is associated with the greater value of la-I. for the Dg species which markedly decreases the effect of axes rotations upon isotopic substitution. The structural parameters derived from the Da data set are in excellent agreement with those extracted from the Hp analyses after minimization of axes rotation effects.
Journal of Molecular Spectroscopy, 2004
The jet-cooled Fourier-transform microwave spectrum of N-methylacetamide (CH 3 ANHAC(@O)ACH 3 ), a molecule containing two methyl tops with relatively low barriers to internal rotation, has been recorded and fit to nearly experimental uncertainty. Measurements were carried out between 10 and 26 GHz, with the nitrogen quadrupole splittings resolved for many transitions. The permutation-inversion group for this molecule is G 18 (not isomorphic to any point group), with irreducible representations A 1 , A 2 , E 1 , E 2 , E 3 , and E 4 . One of these symmetry species and the usual three asymmetric rotor quantum numbers J KaKc were assigned to each torsion-rotation level involved in the observed transitions. F values were assigned to hyperfine components, where F ¼ J þ I N . Transitions involving levels of A 1 and A 2 species could be fit to an asymmetric rotor Hamiltonian. The other transitions were first fit separately for each symmetry species using a Pickett-like effective rotational Hamiltonian. Constants from these fits show a number of additive properties which can be correlated with sums and differences of effects involving the two tops. A final global fit to 48 molecular parameters for 839 hyperfine components of 216 torsion-rotation transitions involving 152 torsion-rotation levels was carried out using a newly written two-top computer program, giving a root-mean-square deviation of observed-minus-calculated residuals of 4 kHz. This program was written in the principal axis system of the molecule and uses a free-rotor basis set for each top, a symmetric-top basis set for the rotational functions, and a single-step diagonalization procedure. Such an approach requires quite long computation times, but it is much less prone to subtle programming errors (a consideration felt to be important since checking the new program against precise fits of low-barrier two-top molecules in the literature was not possible). The two internal rotation angles in this molecule correspond to the Ramachandran angles w and / often defined to describe polypeptide folding. Barriers to internal rotation about these two angles were found to be 73 and 79 cm À1 , respectively. Top-top coupling in both the kinetic and potential energy part of the Hamiltonian is relatively small in this molecule.
Chemphyschem : a European journal of chemical physics and physical chemistry, 2016
The molecular beam Fourier transform microwave spectra of 2-acetyl-5-methylfuran were recorded in the frequency range 2-26.5 GHz. Quantum chemical calculations calculated two conformers with a trans and a cis configuration of the acetyl group, both of which were assigned in the experimental spectrum. All rotational transitions split into quintets due to the internal rotations of two non-equivalent methyl groups. Using the program XIAM, the experimental spectra can be simulated with standard deviations within the measurement accuracy, yielding well-determined rotational and internal rotation parameters, inter alia the V3 potentials. While the V3 barrier height of the ring methyl rotor does not change for both conformers, that of the acetyl methyl rotor differs by about 100 cm1. The predicted values from quantum chemistry are only in the correct order of magnitude.
Journal of Molecular Structure, 2001
The microwave (MW) spectrum of methyl vinyl sul®de (CH 2 yCHSCH 3 ) has been investigated in the 20.0±60.5 GHz spectral region at dry ice temperature. The most stable rotamer is planar with the methyl and vinyl groups in the syn conformation. A less stable skew rotamer has been assigned for the ®rst time. Its CyC±S±C torsional angle is approximately 1548 from syn. Isotopic species of all the heavy atoms of the skeleton of the syn conformation have been assigned. This allowed the following structure parameters to be calculated as: C1±C2 133.15 (18) pm, C1±S3 174.11 (9) pm, S3±C4 179.70 (9) pm, C2±C1±S3 130.23 (11)8 and C1±S3±C4 101.99 (9)8. The uncertainties are one standard deviation. Ten vibrationally excited states of the syn conformer were assigned, while the ground state and two excited states of the skew conformer were assigned. The frequencies of several of these excited states have been determined by relative intensity measurements. The syn form was found to be 5.0 (3) kJ mol 21 more stable than the skew conformer by relative intensity measurements. The uncertainty is one standard deviation. Quantum chemical calculations at HF, MP2 (full), DFT and QCISD levels of theory using the 6-31111G pp , cc-pVTZ and AUG-cc-pVTZ basis sets have been made to assist the experimental work. q