Spectra and Structures of Silicon-Containing Compounds. XXIV.* Raman and Infrared Spectra,r 0 Structural Parameters,Vibrational Assignment,Barriers to Internal Rotation,and Ab Initio Calculations of Ethylsilane

The infrared (3200 to 400 cm^–1) and Raman (3200 to 20 cm^–1) spectra of gaseous and solid ethylsilane, CH₃CH₂SiH₃, have been recorded. Additionally, the Raman spectrum of the liquid has been obtained with quantitative depolarization values. The SiH₃ torsional mode has been observed as sum and difference bands with the silicon-hydrogen stretching vibration. Utilizing the torsional fundamental frequency of 132 cm^–1 the threefold periodic barrier of 590 cm^–1 (7.06 kJ/mol) has been obtained. Utilizing the frequencies of the silicon-hydrogen stretches, Si-H bond distances of 1.485 and 1.484 Å have been obtained for the bonds gauche and trans to the methyl group, respectively. Using previously reported rotational constants from seven different isotopomers, the r ₀ parameters have been calculated and are compared to the corresponding r _s parameters. A complete vibrational assignment is proposed that is consistent with the predicted frequencies utilizing the force constants from ab initio MP2/6-31G(d) calculations. Both the infrared intensities as well as the Raman activities and depolarization values have been obtained from the ab initio calculations. Complete equilibrium geometries have been determined by ab initio calculations employing the 6-31G(d), 6-311 + G(d,p), and 6-311+G(2d,2p) basis sets at levels of restricted Hartree–Fock (RHF) and/or Moller–Plesset (MP) to second order. The results are discussed and the theoretical values are compared to the experimental values when appropriate.     

Infrared and Raman Spectra,Conformational Stability,Ab Initio Calculations,and Vibrational Assignments for Cyclopropyldifluorosilane

The infrared (3200–30 cm^–1) spectra of gaseous and solid Cyclopropyldifluorosilane, c-C₃H₅SiF₂H, and the Raman spectra (3200–20 cm^–1) of the liquid with quantitative depolarization values and the solid have been recorded. Both the syn (cis) and skew (gauche) conformers have been identified in the fluid phases, but only the syn conformer remains in the solid. Variable temperature (–55 to –100°C) studies of the infrared spectra of the sample dissolved in liquid xenon have been carried out. From these data, the enthalpy difference has been determined to be 73 ± 10 cm^–1 (209 ± 29 cal mol^–1), with the syn conformer being the more stable rotamer, which is at variance with the predictions from ab initio calculations. A complete vibrational assignment is proposed for both conformers based on infared 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* calculations. Utilizing the frequencies of the silicon–hydrogen sketch, the rm Si—H bond distances of 1.474 and 1.472 Å have been obtained for the syn and skew conformers, respectively. Complete equilibrium geometries have been determined for both rotamers by ab initio calculations employing the 6-31G* and 6-311 +G** basis sets at levels of restricted Hartree-Fock (RHF) and/or Moller–Plesset (MP) to second order. The potential energy terms for the conformer interconversion have been obtained from the MP2/6-31G* calculation. The results are discussed and compared to those obtained for some similar molecules.     

Conformational Stability, Raman and Infrared Spectra, Vibrational Assignment, and Ab Initio Calculations of Allyltrifluorosilane

The Raman spectra (3500 to 30 cm^–1) of allyltrifluorosilane, CH₂CHCH₂SiF₃, in the liquid with quantitative depolarization ratios and solid states, and the infrared spectra (3500 to 30 cm^–1) of the gas and solid have been recorded. Additionally, the mid-infrared spectra of the sample dissolved in liquified xenon as a function of temperature (–100° to –55°C) have been recorded. All of these data indicate there are two conformers, the more stable gauche rotamer and a very small amount of the cis conformer in the fluid states, but only the gauche form remains in the polycrystalline solid. The variable temperature studies of the infrared spectrum of the xenon solution indicate a relatively large enthalpy difference of 354±30 cm^–1 (4.23±0.36 kJ/mol) between the conformers. The fundamental frequencies for the asymmetric (54 cm^–1) and SiF₃ (48 cm^–1) torsions for the gauche conformer were observed in the far infrared spectrum, and from the SiF₃ torsional frequency the barrier to internal rotation is calculated to have a value of 525 cm^–1 (6.28 kJ/mol). A complete vibrational assignment is presented for the gauche conformer that is consistent with the predicted wavenumbers utilizing the force constants from ab initio MP2/6-31G* calculations. The optimized geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational wavenumbers have been obtained from RHF/6-31G* and/or MP2/6-31G* ab initio calculations. These quantities are compared to the corresponding experimental quantities when appropriate as well as with corresponding results for some similar molecules.Taken in part from the dissertation of Y. E. Nashed, which will be submitted to the Department of Chemistry in partial fulfillment of the Ph.D. degree     

Conformational and Structural Studies of 1-Fluoropropane from Temperature Dependant FT-IR Spectra of Rare Gas Solutions and Ab Initio Calculations

Variable temperature (–55 to –150°C) studies of the infrared spectra (3500 to 400 cm^–1) of 1-fluoropropane, CH₃CH₂CH₂F, dissolved in liquid krypton and xenon have been recorded. Utilizing three conformer pairs in the krypton solution and four conformer pairs in the xenon solution, enthalpy differences of 104±6 cm^–1 (1.24±0.07 kJ/mol) and 99±5 cm^–1 (1.16±0.06 kJ/mol) were obtained from the krypton and xenon solutions, respectively, with the gauche form the more stable conformer. From these data it is estimated that 24% of the trans forms is present at ambient temperature. The conformational stabilities, harmonic force constants, fundamental frequencies, infrared intensities and Raman activities have been obtained from RHF/6-31G(d) and/or MP2/6-31G(d) ab initio calculations and these quantities have been compared to the experimental values when appropriate. The optimized geometries have also been obtained with several different ab initio basis sets up to MP2/6-311+G(2d,2p). The r₀ structural parameters have been obtained by combining the ab initio data with the previously reported rotational constants for both conformers. The results are compared to the corresponding results for some similar molecules.     

Vibrational Spectra, Assignment, Conformational Stability and Ab Initio/DFT Calculations for 1-Nitropropane

The infrared spectra (4000–400 cm^{– 1}) of solid and the Raman spectra (3500–30 cm^{– 1}) of liquid and solid 1-nitropropane, CH₃CH₂CH₂NO₂, have been registered. Both the trans and gauche conformers have been identified in the fluid phase, while the trans form remains in the stable solid. Temperature dependence (190–230K) of the liquid 1-nitropropane Raman spectra has been carried out. From these data, the enthalpy difference was determined to be 870 ± 105 J-mol^–1, with the gauche conformer being the more stable rotamer. Ab initio and DFT calculations at different levels of approximation (HF, MP2, B3LYP, B3PW91) gave optimized geometries, harmonic force fields, and vibrational frequencies for the trans and gauche conformers. All the calculations (except the B3PW91/6-31G* level) predicted gauche as the low-energy conformer. Theoretical force constants are analyzed for formulating constraints in the molecular force field model of 1-nitropropane.     

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.     

Far-Infrared Spectra, Conformational Stability, Barriers to Internal Rotation, Ab Initio Calculations, r0 Structural Parameters, and Vibrational Assignment of Ethyl Methyl Ether

The far-infrared spectra (350–35 cm^–1) of gaseous ethyl methyl ether-d ₀ and ethyl methyl-d ₃-ether have been recorded at a resolution of 0.10 cm^–1. For the d ₀ species, the fundamental asymmetric torsion of the more stable trans conformer (two methyl moieties are trans to one another) has been observed at 115.40 cm^–1 with four upper state transitions falling to lower frequency, whereas, for the gauche form, it has been observed at 93.56 cm^–1 with two excited states falling to lower frequency. the corresponding series for the d ₃ species start from 106.00 and 87.10 cm^–1, respectively. From these data, the asymmetric torsional potential coefficients for the d ₀ species have been determined to be: V ₁ = 572 ± 30; V ₂ = 85 ± 8; V ₃ = 619 ± 30; V ₄ = 175 ± 18, and V ₆ = –28 ± 3 cm^–1. The trans to gauche and gauche to gauche barriers were calculated to be 958 cm^–1 (11.5 kJ/mol) and 631 cm^–1 (7.55 kJ/mol), respectively, with an energy difference of 550 ± 6 cm^–1 (6.58 ± 0.07 kJ/mol). Utilizing three conformer pairs, variable temperature studies (–105 to –150°C) of the infrared spectra of the d ₀ sample dissolved in liquid krypton gave an enthalpy difference of 547 ± 28 cm^–1 (6.54 ± 0.33 kJ/mol) with the trans conformer the more stable rotamer. It is estimated that there is only 4% of the gauche conformer present at ambient temperatures. The structural parameters, conformational stabilities, barriers to internal rotation, and fundamental vibrational frequencies, which have been determined experimentally, are compared to those obtained from ab initio gradient predictions from RHF/6-31G* and with full electron correlation at the MP2 level with three different basis sets. The adjusted r ₀ structural parameters have been obtained for the trans conformer from combined ab initio MP2/6-311+G** predictions and previously reported microwave rotational constants. The reported distances should be accurate to 0.003 Å and the angles to 0.5°. These results are compared to the corresponding quantities obtained for some similar molecules.     

Conformational Studies of Methyl Vinyl Silane from Temperature-Dependent FT–IR Spectra of Xenon and Krypton Solutions

Variable-temperature (–55 to –155°C) studies of the infrared spectra (3500–400 cm^–1) of methyl vinyl silane, CH₂CHSiH₂CH₃, dissolved in liquid xenon and krypton have been recorded. Utilizing three sets of conformer doublets due to the cis and gauche rotamers, the enthalpy difference has been determined to be 133 ± 11 cm^–1 (1.59 ± 0.13 kJ/mol) with the gauche conformer the more stable form in the krypton solution. In the xenon solution, the enthalpy difference could not be determined because the infrared bands become so broad and the overlap was so extensive that meaningful areas could not be determined. Ab initio calculations have been carried out with several different basis sets up to MP2/6-311+G(2d,2p) from which structural parameters and conformational stabilities have been determined. With the largest basis set, the cis conformer is predicted to be the more stable conformer, which is inconsistent with the experimental results. Utilizing previously reported microwave rotational constants for both conformers along with the ab initio predicted distances and angles, r ₀ parameters have been obtained for both the cis and gauche conformers. The spectroscopic and theoretical 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 r 0 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 r ₀ 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.     

Raman and Infrared Spectra, Ab Initio Calculations, Normal Coordinate Analysis, and Conformational Stability of 2-Methoxypropene

The Raman (3500–40 cm^–1) and infrared (3500–70 cm^–1) spectra of gaseous and solid 2-methoxypropene, CH₃O(CH₃)C=CH₂, and the isotopomers, CD₃O(CH₃)C=CH₂ and CH₃O(CD₃)C=CD₂ have been recorded. In addition, the Raman spectra of the liquids have been recorded with qualitative depolarization measurements. All of these data indicate that only one conformer is present in the fluid phases at ambient temperature and this form is the cis conformer, which remains in the solid. Assignments are provided for the fundamentals of all three isotopomers for the cis conformer with C_s symmetry. The far-infrared spectra of all three isotopic species have been recorded at a resolution of 0.1 cm^–1 in the gas and 1.0 cm^–1 in the solid. The parameters of the potential function governing the asymmetric torsion are determined to be V₃ = 1485 ± 9 cm^–1 and V₆ = –55 ± 4 cm^–1 for the d₀ compound, where only two terms were determined, since a second conformer was not evident. The barrier to internal rotation for the methyl group attached to the oxygen atom is 1370 ± 8 cm^–1 and the C—CH₃ barrier is 772 ± 5 cm^–1. Ab initio calculations with full electron correlation have been carried out by the perturbation method to second order to obtain the equilibrium structural parameters, harmonic force constants, fundamental frequencies, infrared intensities, Raman activities, depolarization values, and conformational stability. The predicted values have been compared to the experimental values where appropriate.     

Spectra and structure of silicon containing compounds

The Raman (50 to 3200 cm^–1) and infrared (50 to 3500 cm^–1) spectra of chlorodimethylmethoxysilane, Cl(CH₃)₂SiOCH₃, in the vapor and solid phases have been recorded. Raman spectra of the liquid including depolarization ratios have also been recorded. Optimized geometries and conformational stabilities have been obtained from ab initio calculations utilizing the RHF/3–21G* and RHF/6–31G* basis sets. The calculations from both of these basis sets indicated the gauche conformer to be significantly more stable than the trans conformer. Since the gauche has twice the multiplicity of the trans form it is unlikely that the trans conformer will be detected in the fluid phases at room temperature. This is supported by the fact that no infrared or Raman bands were found to vanish in the spectra of the crystalline solid. The vibrational frequencies have been calculated using appropriate scaling factors, and the vibrational spectra are interpreted in detail. The results have been compared with those obtained for some related molecules.Dedicated to Professor Dr. H. Kriegsmann on the occasion of his 70th birthdayFor part XX, see J Raman Spectrosc 26:in press (1995)Analytical R/D Department, Organic Products Division, Miles Inc., Bushy Park Plant. Charleston, SC 9411, USAChemistry Department, Mu'tah University, P.O.Box 7, Mu'tah-Karak, JordanDepartment of Chemistry, Moscow State University, Moscow, B-234, RussiaDepartment of Ceramic Engineering, Inha University, Nam-Ku, Incheon 160, KoreaDepartment of Chemistry, University of Oslo, P.O.Box 1033, 0315 Oslo, Norway     

Conformational Stability,Structural Parameters,Vibrational Frequencies,and Raman and Infrared Intensities of Allylsilane

The Raman spectra (3500 to 30 cm^–1) of allylsilane, CH₂CHCH₂SiH₃, in the liquid with quantitative depolarization ratios and solid states and the infrared spectra (3500 to 30 cm^–1) of the gas and solid have been recorded. Similar data have also been recorded for the Si-d₃ isotopomer. Additionally, the mid-infrared spectra of the normal sample dissolved in liquified xenon as a function of temperature (–100 to –50°C) have been recorded. All these data indicate there is a single conformer, the gauche rotamer, in all three physical states. Utilizing the Si-H stretching frequencies from the infrared spectrum of the gaseous CH₂CHCH₂SiD₂H isotopomer, the three Si-H bond distances (r ₀) are calculated to be 1.484 Å for the gauche conformer. The other r ₀ parameters are estimated from the previously reported rotational constants. The fundamental frequencies for the asymmetric (78 cm^–1) and SiH₃ (137 cm^–1) torsions were obtained from sum and difference bands with the SiH₃ stretches. From the SiH₃ torsional frequency the barrier to internal rotation is calculated to have a value of 731 cm^–1 (8.74 kJ/mol). The optimized geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies have been obtained from RHF/6-31G* and/or MP2/6-31G* ab initio calculations. These quantities are compared to the corresponding experimental quantities when appropriate as well as with some corresponding results for some similar molecules.     

Conformational Stability from Variable-Temperature Infrared Spectra of Krypton Solutions, Ab Initio Calculations, and r o Structural Parameters of Chlorocyclopentane

The infrared spectra (3500–400 cm^–1) of krypton solutions of chlorocyclopentane, c-C₅H₉Cl, at variable temperatures (–101 to –150°C) have been recorded and the fundamental vibrations of the axial conformer and several of those for the equatorial form have been assigned. Utilizing two pairs of fundamentals for the two conformers in the krypton solution, an enthalpy difference of 145±15 cm^–1 (1.73±0.18 kJ-mol^–1) has been obtained with the axial conformer the more stable form. It is estimated that there is 67±2% of the axial conformer present at ambient temperature. Convincing spectroscopic evidence shows that a significant percentage of the chlorocyclopentane molecules are undergoing pseudorotation at ambient temperature. The conformational stabilities, harmonic force constants, fundamental frequencies, infrared intensities, and Raman activities have been obtained from MP2/6-31G(d) calculations with full electron correlation and these quantities have been compared to the experimental values when appropriate. The optimized geometries and conformational stabilities have also been obtained from ab initio MP2 calculations as well as by density functional theory (DFT) by the B3LYP method with several different basis sets. The adjusted r ₀ structural parameters have been obtained for both conformers by combining the ab initio data with the previously reported microwave rotational constants. These new values of the structural parameters for both conformers are compared to those previously reported from electron diffraction and microwave studies. These results are compared to the corresponding quantities of some similar molecules.     

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.     

Infrared and Raman spectra,conformational stability,barriers to internal rotation,ab initio calculations and vibrational assignment of ethyl methyl selenide

The Raman spectra (3200–10 cm⁻¹) of ethyl methyl selenide in the gas, liquid and solid phases and the infrared spectra (3200–30 cm⁻¹) of the gas and solid have been recorded. Qualitative depolarization ratios have been obtained for the lines in the Raman spectrum of the liquid. By a variable temperature Raman study of the liquid, it has been determined that the gauche conformer is more stable than the trans rotamer by 158±16 cm⁻¹ (452±46 cal mol⁻¹), and the gauche conformer is the rotamer present in the solid. A complete vibrational assignment for the gauche conformer is presented. All of these data are compared to the corresponding quantities obtained from ab initio Hartree—Fock gradient calculations employing the STO-3G* and 4–31G*/MIDI-4* basis sets. Complete equilibrium geometries have been calculated for both rotamers and the results are discussed and compared with the corresponding quantities for some similar molecules.     

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 r ₀ 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.     

Raman spectra,conformational stability,structural parameters,vibrational assignment,and ab initio calculations for epifluorohydrin

The Raman spectra (3200–100 cm⁻¹) of epifluorohydrin, OCH₂CH(CH₂F), in variable solvents, as well as that of the gas have been recorded and several of the bands due to the two less stable conformers have been identified. The variable solvent studies were inconclusive on the relative conformer stabilities. The conformational energy differences and optimized geometries for all three conformers have been obtained from ab initio calculations with the 3–21G, 4–31G and 6–31G* basis sets. A normal coordinate analysis has also been performed for each conformer with a force field determined from the 3–21G basis set. Assignment of the vibrational fundamentals observed in the Raman spectra of the fluid phases is proposed based on the normal coordinate calculations. In the liquid phase, one of the conformers with a large dipole moment predominates and it appears to be the gauche-I form which is the only one found in the solid. Utilizing the three rotational constants previously reported for each conformer, along with restricted relative distances for several of the structural parameters among the conformers from ab initio calculations, r₀ structural parameters for the heavy atoms have been determined.     

Conformational analysis,barriers to internal rotation,vibrational assignment and ab initio calculations of chloroacetone

The infrared (3500-20 cm⁻¹) and Raman (3200-10 cm⁻¹) spectra have been recorded for gaseous and solid chloroacetone (1-chloro-2-propanone), CH₂ClC(O)CH₃. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. These data have been interpreted on the basis that the molecule exists predominantly in a gauche conformation having a “near cis” structure of C₁ symmetry (dih ClCCO=142°C) in the vapor but for the liquid a second conformer having a trans structure (chlorine atom oriented trans to the methyl group) with C_s point group symmetry is present. From a study of the Raman spectrum of the liquid at variable temperatures, the trans conformation has been determined to be more stable than the gauche form by 1042±203 cm⁻¹ (2.98±0.6 kcal mol⁻¹ and is the only conformer present in the spectrum of the annealed solid. From ab initio calculations at the 3-21G* and 6-31G* basis set levels optimized geometries for both the gauche and trans conformers have been obtained and the potential surfaces governing internal rotation of the symmetric and asymmetric rotors have been obtained. The observed vibrational frequencies and assignments to the fundamental vibrations for both the gauche and trans conformers are compared to those calculated with the 3-21G* basis set. The results are discussed and compared with the corresponding quantities obtained for some similar molecules.     

Vibrational spectra,conformational stability,barriers to internal rotation,r0 structural parameters and ab initio calculations of fluoromethyl methyl ether

The far-infrared spectrum of gaseous fluoromethyl methyl ether, FCH₂OCH₃, along with three of the deuterium isotopes, has been recorded at a resolution of 0.10 cm^–1 in the 350 to 50 cm^–1 region. The fundamental asymmetric torsional and methyl torsional modes are extensively mixed and have been observed at 182 and 132 cm^–1, respectively, for the stablegauche conformer with the lower frequency band having several excited states falling to lower frequency. An estimate is given for the potential function governing the asymmetric rotation. On the basis of a one-dimensional model the barrier to internal rotation of the methyl moiety is determined to be 527±9 cm^–1 (1.51±0.03 kcal/mol). A complete assignment of the vibrational fundamentals for all four isotopic species observed from the infrared (3500 to 50 cm^–1) spectra of the gas and solid and from the Raman (3200 to 10 cm^–1) spectra of the gas, liquid, and solid is proposed. No evidence could be found in any of the spectra for the high-energytrans conformer. All of these data are compared to the corresponding quantities obtained from ab initio Hartree-Fock gradient calculations employing the 3-21G and 6-31G* basis sets along with the 6-31G* basis set with electron correlation at the MP2 level. Additionally, completer ₀ geometries have been determined from the previously reported microwave data and carbon-hydrogen distances determined from infrared studies. The heavy-atom structural parameters (distances in Å, angles in degrees) arer(C₁-F) = 1.395 ± 0.005;r(C₁-O) = 1.368 ± 0.007;r(C₂-O) = 1.426 ±0.003; FC₁O = 111.33 ± 0.25; C₁OC₂ = 113.50 ± 0.18 and dih FC₁OC₂ = 69.12 ± 0.26. All of these results are discussed and compared with the corresponding quantities obtained for some similar molecules.     

Spectra and structure of small ring compounds. LIX

The Raman (3200 to 10 cm^–1) and far infrared (380 to 60 cm^–1) spectra of gaseous fluorocyclobutane,c-C₄H₇F, have been recorded. A series of Q-branches observed in both of these spectra beginning at 166 cm^–1 with successive transitions falling to lower frequencies have been assigned to the ring puckering vibrations of both the low energy equatorial and high energy axial conformers. These data have been fit to an asymmetric potential function of the form:V (cm^–1)=(1.76±0.05)10³X+(4.88±0.28)10⁴X²+(2.12 ±0.16)10³ exp(–5.66±0.41)10X² with a reduced mass function ofg ₄₄= 0.00386–0.00295X+0.03485X²+0.1228X³ +0.3459X⁴, whereX is the ring puckering coordinate. Utilizing this potential, the difference between the puckering angles for the two conformers was calculated to be 4° with the equatorial conformer having the larger value of 28°. This potential function is consistent with an energy difference between the equatorial and axial forms of 447 cm^–1 (1.28 kcal/mol) and a barrier to ring inversion from the equatorial to the axial conformation of 713 cm^–1 (2.04 kcal/mol). Experimental values for the enthalpy difference between the two conformers have been determined for both the liquid (400±30 c^–1) and gas (413±43 cm^–1) from investigations of the Raman spectra at variable temperatures. The conformational stability, enthalpy difference, structural parameters, and fundamental vibrational frequencies, which have been determined experimentally, are compared to those obtained from ab initio Hartree-Fock calculations employing the 3-21G, 6-31 G^*, and 6-31 G^** basis sets.For part LVIII, seeStruct. Chem. 1991,2, 195.Taken in part from the thesis of M. J. Lee, which has been submitted to the Department of Chemistry in partial fulfillment of the Ph.D. degree, May 1991.