Low frequency vibrational spectra, barrier to internal rotation, and RHF/STO-3G* ab initio calculations of 3-bromopropene

Conformational stability from rare gas solutions, r0 structural parameters, barriers to internal rotation, and ab initio calculations for vinyl silyl fluoride

The Raman (3300-10 cm(-1)) and infrared (3300-40 cm(-1)) spectra of gaseous and solid vinyl silyl fluoride, CH(2)=CHSiH(2)F, have been recorded. Raman spectrum of the liquid has also been recorded and depolarization values obtained. Variable-temperature studies of the infrared spectra of the sample dissolved in liquid krypton (-110 to -150 degrees C) and liquid xenon (-60 to -100 degrees C) have been carried out. From these studies, the enthalpy difference has been determined to be 76 +/- 7 cm(-1) (0.91 +/- 0.08 kJ/mol) from the krypton solutions and 69 +/- 7 cm(-1) (0.82 +/- 0.08 kJ/mol) from the xenon solutions, with the gauche conformer the more stable form. From the far-infrared spectrum of the gas, the asymmetric torsional fundamentals for the cis and gauche conformers have been observed at 102.34 and 86.56 cm(-1), respectively, with each having several "hot bands" falling to lower frequencies. From these frequencies along with the experimentally determined conformational enthalpy difference, as well as the gauche skeletal dihedral angle, the potential function governing the conformational interchange has been determined with the following Fourier cosine potential coefficients: V(1) = -80 +/- 11, V(2) = -42 +/- 15, V(3) = 622 +/- 5, V(4) = 34 +/- 5, and V(6) = -31 +/- 2 cm(-1). The gauche-to-cis and gauche-to-gauche barriers are 664 cm(-1) (7.94 kJ/mol) and 608 cm(-1) (7.27 kJ/mol), respectively. Complete vibrational assignments are provided for both conformers. In addition, equilibrium geometries and electronic energies have been determined for both rotamers from ab initio calculations using restricted Hartree-Fock and Møller-Plesset perturbation method to the second order (MP2), as well as density functional theory by the B3LYP methods, employing a number of basis sets up to 6-311+G(2df,2pd). All levels of calculation predict the gauche conformer to be the more stable form. By systematically adjusting the ab initio predicted structural values to fit the previously reported microwave rotational constants, adjusted r(0) parameters have been obtained for both conformers. These values are compared to those for the corresponding chloride and methyl compounds. The spectroscopic and theoretical results are discussed and compared to the corresponding quantities for some similar molecules.

Infrared and Raman spectra, conformational stability, vibrational assignment and ab initio calculations of methylvinyl silyl chloride

The infrared spectra (3200-50 cm(-1)) of gaseous and solid and Raman spectra (3200-10 cm(-1)) of the liquid and solid methylvinyl silyl chloride, CH(2)=CHSiH(CH(3))Cl, and the Si-d isotopomer have been recorded. The three expected stable conformers (the three different groups eclipsing the double bond) have been identified in the fluid phase, but it was not possible to obtain an annealed solid with a single conformer. Variable temperature (-105 to -150 degrees C) studies of the infrared spectra of the sample dissolved in liquid krypton has been carried out. From these data the enthalpy differences between the most stable conformer with the hydrogen atom (HE) eclipsing the double bond to that with the chlorine atom (ClE) and the methyl group (ME) eclipsing the double bond have been determined to be 17+/-4 cm(-1) (203+/-48 Jmol(-1)) and 80+/-12 cm(-1) (957+/-144 Jmol(-1)), respectively. However in the liquid state the ME conformer is the most stable form with enthalpy differences of 13+/-4 and 27+/-7 cm(-1) to the HE and ClE rotamers, respectively. It is estimated that there is 39% of the HE conformer, 35% of the ClE conformer, and 26% of the ME conformer present at ambient temperature. A complete vibration assignment is proposed for the HE conformer which is based on infrared band contours and group frequencies, which is supported by normal coordinate calculations utilizing the force constants from ab initio MP2/6-31G(d) calculations. Additionally, several of the fundamentals for the other two conformers have been assigned. The optimal geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies are reported for all three conformers from MP2/6-31G(d,p) ab initio calculations with full electron correlation. Optimized geometrical parameters and conformational stabilities have been obtained from MP2/6-311+G(d,p) calculations. At this highest level of calculations, the HE conformer is predicted to be more stable by 62 and 84 cm(-1) than the ME and ClE conformers, respectively. The coefficients from the potential function governing the conformational interchange have been obtained from the MP2/6-31G(d) ab initio calculations. By utilizing the frequency of the SiH stretching mode, the r(0)-H distance has been determined to be 1.481 A for the HE conformer. The ab initio calculated quantities are compared to the experimentally determined values where applicable, as well as to some corresponding results for some similar molecules.

Conformational stability from variable temperature infrared spectra of rare gas solutions of 2-chloro-3-fluoropropene

Variable temperature (-55 to -155 degrees C) infrared spectra of rare gas solutions of 2-chloro-3-fluoropropene, H2C=C(Cl)CH2F, have been recorded from 3500 to 400 cm(-1). The relative intensities of 16 conformer pairs at ten different temperatures of a krypton solution have been measured and from these data an enthalpy difference of 271+/-27 cm(-1) (3.17+/-0.32 kJ x mol(-1)) has been obtained with the cis conformer the more stable form. Similar studies were also carried out in xenon and an enthalpy difference of 334+/-33 cm(-1) (4.00+/-0.39 kJ x mol(-1)) was obtained again with the cis conformer the more stable form. It is estimated that there is 35+/-2% of the gauche conformer present at ambient temperature. However, in the crystalline solid the gauche conformer is the stable rotamer. Extensive ab initio calculations with full electron correlation by the perturbation method at the MP2 level with a variety of basis sets as well as density functional theory calculations (DFT) by the B3LYP method have been carried out. Several of these calculations predict an energy difference in the range of 400 cm(-1) with the cis form the more stable conformer but most of the predicted energy differences are significantly larger than the experimentally determined value. The spectroscopic and theoretical results are discussed and compared with the corresponding quantities for some similar molecules.

Infrared and Raman spectra, conformational stability, barriers to internal rotation, normal-coordinate calculations and vibrational assignments for vinyl silyl bromide

The infrared (3200-30 cm(-1) spectra of gaseous and solid, the Raman spectra (3200-30 cm(-1)) of the liquid and solid vinyl silyl bromide, CH2CHSiH2Br, have been recorded. Additionally, quantitative depolarization values have been obtained. Both the gauche and cis conformers have been identified in the fluid phases but only the gauche conformer remains in the solid. Variable temperature studies from 0 to -87 degrees C of the Raman spectrum of the liquid was carried out. From these data, the enthalpy difference has been determined to be 22 +/- 6 cm(-1) (0.26 +/- 0.08 kJ/mol), with the gauche conformer being the more stable form. The predictions from the ab initio calculations up to MP2/6-311 + + G(2d,2p) basis set favor the gauche as the more stable form. A complete vibrational assignment is proposed for both the gauche and cis conformers based on infrared band contours, relative intensities, depolarization values and group frequencies. The vibrational assignments are supported by normal coordinate calculations utilizing the force constants from ab initio MP2/6-31G(d) calculations and the potential energy terms for the conformer interconversion have been obtained from the same calculations. Complete equilibrium geometries have been determined for both rotamers by ab initio calculations employing a variety of basis sets up to 6-311 + + G(2d,2p) at levels of restricted Hartree-Fock (RHF) and/or Moller-Plesset (MP) to second order. The results are discussed and compared to those obtained for some similar molecules.