Shiyu Shen

Spectra and Structure of Silicon-Containing Compounds. XXVIII1 Infrared and Raman Spectra, Vibrational Assignment, and Ab Initio Calculations of Vibrational Spectrum and Structural Parameters of Vinyltrichlorosilane

The infrared spectra (3200-50 cm−1) of gaseous and solid vinyltrichlorosilane, CH2=CH-SiCl3, have been recorded. In addition, the Raman spectrum (3200-10 cm−1) of the liquid has been recorded and quantitative depolarization values obtained. The infrared spectrum of the sample dissolved in liquid xenon (−80°C) has also been recorded. Using the experimental data and normal coordinate calculations with scaled ab initio force constants, the complete vibrational assignment is proposed. The torsional mode was observed in the infrared spectrum of the gas at 69 cm−1 and the threefold barrier of internal rotation was calculated to be 500 cm−1 (5.98 kJ/mol). Ab initio calculations have been carried out at the restricted Hartree–Fock level of the theory as well as with full electron correlation by the perturbation method to second order with different basis sets up to 6-311+G(d,p) to obtain the optimized geometries, harmonic force constants, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies. The ab initio predicted structural parameters are compared with those obtained from a previous electron diffraction study.

Conformational stability of ethylcyclopropane from Raman spectra, temperature dependent FT-IR spectra of xenon solutions and ab initio calculations

Abstract Infrared spectra (3500 to 400 cm −1 ) of xenon solutions of ethylcyclopropane, c -C 3 H 5 C 2 H 5 , at temperatures from −60 to −100°C have been recorded. In addition, Raman spectra of the liquid and polycrystalline phases have been obtained as well as infrared spectra of the gaseous phase. Spectroscopic evidence for the coexistence of the gauche and cis conformers of the title compound is found in the fluid phases. From the spectroscopic data, the enthalpy difference between the gauche and cis conformers has been estimated to be 385 cm −1 (1.10 kcal mol −1 ), with the gauche rotamer the more stable form. In the annealed solid phase, only the gauche conformer is found. Ab initio calculations have been carried with different basis sets up to MP 2 6-31 G , from which structural parameters, conformational stabilities, force constants, and vibrational frequencies have been determined. These calculations support the experimental conclusions and aid in the vibrational assignment of the normal modes. These results are compared to the corresponding quantities for some similar molecules.

On the vibrational spectra and structural parameters of some XN3 molecules (X = H, F, Cl, Br)

Abstract Ab initio calculations with full electron correlation by the perturbation method to second order and hybrid density functional theory calculations by the B3LYP method utilizing the 6-31G* and 6-311+G** basis sets have been carried out for the XN 3 molecules (X=H, F, Cl, Br). From these calculations, force constants, infrared intensities, Raman activities, depolarization ratios, vibrational frequencies, and structural parameters have been determined. These calculations support the assignment of the HN 3 and DN 3 out-of-plane bending modes (ν 6 , A″) at 602 and 591 cm −1 , respectively, which was proposed [J. Chem. Phys. 44 (1966) 4108] earlier and show the recent assignments at 637 and 638 cm −1 [J. Mol. Struct. (Theochem) 434 (1998) 1], respectively, for these modes are in error. Also the usually used frequency of 588 cm −1 for ν 6 for the HN 3 from nitrogen matrix studies for comparison to ab initio predicted values is also clearly in error. The predicted intensity of ν 6 is only 0.2–0.6 km/mol which is 10 3 times less than the stronger bands which explains why it is so weak in the nitrogen matrix compared to its intensity in the gas phase where it drastically increased by Coriolis coupling with ν 5 . For ClN 3 there is extensive coupling of the N 3 deformation at 719 cm −1 with the Cl–N stretch at 545 cm −1 where the latter mode is 71% Cl–N stretch. For BrN 3 the coupling is mainly between the N 3 deformation (682 cm −1 ) and the BrNN bend which is predicted in the 180 cm −1 region. The predicted force constants for FN 3 are compared to those obtained earlier from frequency and distortion data. By combining previously reported rotational constants for HN 3 , FN 3 and ClN 3 with the ab initio MP2/6-311+G** predicted parameters, adjusted r 0 parameters have been obtained for all three molecules. The structural parameters for hydrazoic acid are: r (H–N), 1.015(5); r (N 1 N 2 ), 1.243(5); r (N 2 N 3 ), 1.134(2)A; ∠HN 1 N 2 , 108.8(5); ∠N 1 N 2 N 3 , 171.8(5)°. It is believed that these reported distances and angles have much lower uncertainties than those previously reported from the microwave data alone.

Conformational Stability of Chloromethyl Silyl Chloride from Temperature-Dependent FT-IR Spectra of Krypton Solutions

The Raman spectra (3200–30 cm−1) of liquid and solid, and infrared spectra of gaseous and solid chloromethyl silyl chloride, ClCH2SiH2Cl, have been recorded. Variable temperature (−105–−150°C) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out. From these data, the enthalpy difference is 177 ± 35 cm−1 (2.12 ± 0.42 kJ/mol), with the more stable form being the trans 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 56% of the sample is in the trans form at ambient temperature. A complete vibrational assignment is proposed for both the trans and gauche 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 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+G(2d,2p). The results are discussed and compared to some corresponding results for several related molecules.

Spectra and structure of silicon containing compounds XXXVII, comparison of experimentally determined enthalpy differences of the conformers of CH2CHSiClnY3−n (Y=H or CH3, n=1 or 2) with theoretical values☆

Abstract The infrared spectra (3500–400 cm −1 ) of methylvinyldichlorosilane, CH 2 CHSi(CH 3 )Cl 2 , and dimethylvinylchlorosilane, CH 2 CHSi(CH 3 ) 2 Cl, 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 2 CHSiCl n Y 3− n (Y=H or CH 3 , n =1 or 2) molecules. Only for the CH 2 CHSiH 2 Cl 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.

Infrared and Raman spectra, conformational stability, ab initio calculations, and vibrational assignment of 5-chloropent-2-yne☆

The infrared (3400– 50 cm 21 ) and/or Raman (3400 –10 cm 21 ) spectra of gaseous, xenon solution, liquid and solid 5chloropent-2-yne, CH2ClCH2CCCH3, 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), MP2/6311G(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 2002 Elsevier Science B.V. All rights reserved.

Conformational Studies of Cyclopropylmethyl Ketone from Temperature-Dependent FT–IR Spectra of Xenon Solutions

Variable temperature (−60 to −100°C) studies of the infrared spectra (3500–400 cm−1) of cyclopropylmethyl ketone, c-C3H5C(CH3)O, dissolved in liquid xenon have been recorded. Utilizing several doublets due to the cis and near-trans conformers, the enthalpy difference has been determined to be 269 ± 26 cm−1 (3.22 ± 0.31 kJ/mol) with the cis conformer (oxygen atom cis to the three-membered ring) the more stable rotamer. From these data it is estimated that 79 ± 3% of the cis form is present at ambient temperature. Ab initio calculations have been carried out with different basis sets up to 6-311+G(2df,2pd) at the restricted Hartree–Fock and/or with full electron correlation by the perturbation method to second order (MP2) from which structural parameters and conformation stabilities have been determined. These calculations support the experimental conformational conclusions that the cis form is the more stable conformer. A complete vibrational assignment is given for the cis conformer, which is supported from a normal coordinate calculation utilizing ab initio force constants. Several of the fundamentals of the near-trans conformer have been identified and assigned. Adjusted r0 structural parameters have been obtained from combined ab initio predicted values and previously reported rotational constants from the microwave investigation. The spectroscopic and theoretical results are compared to the corresponding quantities for some similar molecules.

Conformational behavior of ethyloxirane from infrared and Raman spectra, temperature dependent FT-IR spectra of xenon solutions, and ab initio calculations

Abstract Infrared spectra (3500-400 cm −1 ) of gaseous ethyloxirane, c -OC 2 H 3 C 2 H 5 , (1,2-epoxybutane) and xenon solutions of this molecule at temperatures from −60 to −100°C have been recorded. In addition, Raman spectra of the liquid and polycrystalline phases have been obtained, and variable temperature Raman spectra (25 to −89°C) of the liquid were also recorded. Spectroscopic evidence for the co-existence of the gauche-1 and gauche-2 conformers of the title compound is found in the fluid phases, along with small amounts of the cis rotameric form. In xenon solutions, the enthalpy difference between the gauche-1 and gauche-2 conformers has been determined to be 217 ± 34 cm −1 (620 ± 97 cal mol −1 ), with the gauche-1 rotamer the more stable form, whereas in the liquid phase this quantity is estimated to be 95 ± 6 cm −1 (272 ± 17 cal mol −1 ). In the annealed solid phase, only the gauche-1 conformer is found. Ab initio calculations, have been carried with different basis sets up to MP 2 6-31 G ∗ from which structural parameters and conformation stabilities have been determined. These calculations support the experimental conformational conclusions. A force field from these calculations is used to support the vibrational assignments and simulate the observed infrared and Raman spectra. These results are compared to the corresponding quantities for some similar molecules.

Conformational Stability of Propenoyl Bromide from Temperature-Dependent Infrared Spectra of Krypton Solutions, Ab Initio Calculations, and r0 Structural Parameters of the Propenoyl Halides (F, Cl, Br)

Variable temperature (−100 to −150°C) studies of the infrared spectra (3500–400 cm−1) of propenoyl bromide, CH2=CHCBrO, dissolved in liquid krypton, have been carried out. Utilizing six different conformer pairs, an enthalpy difference of 204 ± 20 cm−1 (2.44 ± 0.24 kJ/mol) was obtained, with the anti conformer (carbonyl bond trans to C=C bond) the more stable form. At ambient temperature, there is approximately 28 ± 2% of the syn conformer present. The anti conformer also remains in the infrared and Raman spectra of the polycrystalline solid. The optimal geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies, are reported for both conformers from MP2/6-31G(d) ab initio calculations. The potential function governing the conformational interchange has been obtained from the MP2/6-31G(d) ab initio calculations. The conformational stabilities were calculated from a variety of basis sets and at the highest level of calculations, MP2/6-311 + (2df,2pd), the anti conformer is predicted to be more stable by 178 cm−1, which is in excellent agreement with the experimental results. The r0 adjusted structural parameters have been obtained for propenoyl fluoride and chloride from a combination of the previously reported microwave rotational constants and ab initio predicted parameters. Several of the parameters for the chloride are significantly different than those proposed from an electron diffraction investigation. The results of these spectroscopic, structural, and theoretical studies are discussed and compared to the corresponding results for some similar molecules.

FT-IR Spectra of Xenon Solutions of c-C3H5CFO and CH2CHCFO

The infrared (3500−400 cm−1) spectra of cyclopropylcarbonyl fluoride, c-C3H5CFO, and propenoyl fluoride, CH2CHCFO, in xenon solutions have been recorded at various temperatures (from −60 to −100°C). The data show that there is an equilibrium between the cis (oxygen atom of the carbonyl bond cis with respect to the ring or carbon-carbon double bond) and trans conformers in the fluid phases. Analysis of the temperature-dependent spectra shows the cis conformer is the more stable rotamer for the cyclopropyl compound whereas the trans rotamer is the more stable conformer for propenoyl fluoride.