Todor K. Gounev

CONFORMATIONAL STABILITY AND VIBRATIONAL ASSIGNMENT OF PROPANAL

Abstract Variable temperature (−52 to −100°C) studies of the infrared spectra (3500–400 cm −1 ) of propanal, CH 3 CH 2 CHO, dissolved in liquid xenon have been recorded and bands due to both the cis and gauche conformers have been identified. From these data, the enthalpy difference has been determined to be 370±21 cm −1 (4.43±0.25 kJ mol −1 ) with the cis conformer (methyl group eclipsing the oxygen atom) the more stable form. Utilizing the new infrared data from the xenon solution, along with some additional Raman data, and ab initio predictions from MP2/6-31G* calculations, several reassignments of the fundamentals have been made for the cis conformer. Additionally, several bands which have been identified as arising from the gauche conformer have also been assigned. Infrared (3500–80 cm −1 ) and Raman (3500–30 cm −1 ) spectra of gaseous, liquid and/or solid CD 3 CD 2 CHO are also reported and the fundamentals assigned for the cis conformer. The spectroscopic and theoretical results are compared to the corresponding quantities for some similar molecules.

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.

Conformational stability, r0 structural parameters, ab initio calculations, and vibrational assignment for fluorocyclopentane.

The infrared spectra (3200-50 cm(-1)) of the gas and solid and the Raman spectrum (3200-30 cm(-1)) of liquid and solid fluorocyclopentane, c-C5H9F, have been recorded. Additionally the infrared spectra (3200-400 cm(-1)) of liquid xenon solutions have been recorded at -65 and -95 degrees C. In all of the physical states, only the twisted C(1) conformer was detected. Ab initio calculations utilizing various basis sets up to MP2(full)/6-311+G(2df,2pd) with and without diffuse functions have been used to predict the conformational stabilities. These calculations predict only the twisted C1 conformer as the stable form. The two envelope (C(s) symmetry) forms with axial and equatorial structures were predicted to be first order saddle points with average higher energies of 75 +/- 33 and 683 +/- 44 cm(-1), respectively, from the C1 conformer but lower energies of 2442 and 1812 cm(-1), respectively, than the planar form by MP2 calculations. Similar values were obtained from the corresponding density functional theory calculations by the B3LYP method. A complete vibrational assignment is given for the twisted (C1) conformer which is supported by normal coordinate calculations with scaled force constants from MP2(full)/6-31G(d) calculations. The adjusted r0 structural parameters have been obtained by systematically fitting the MP2(full)/6-311+G(d,p) predicted values with the rotational constants obtained from a microwave study. The determined heavy atom r0 distances in A are (C1C2) = 1.531(3), (C1C3) = 1.519(3), (C2C4) = 1.553(3), (C3C5) = 1.533(3), (C4C5) = 1.540(3), and (C1F6) = 1.411(3) and the angles in degrees are angle C3C1C2 = 105.5(5), angle C1C2C4 = 106.2(5), angle C1C3C5 = 102.9(5), angle F6C1C2 = 108.9(5), and angle F6C1C3 = 107.6(5) with a dihedral angle angle C2C4C5C3 = 25.3(3). These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.

Conformational stability of cyclobutanol from temperature dependent infrared spectra of xenon solutions, r0 structural parameters, ab initio calculations and vibrational assignment

Variable temperature (-55 to -100 degrees C) studies of the infrared spectra (4000-400 cm(-1)) of cyclobutanol, c-C4H7OH dissolved in liquid xenon have been carried out. The infrared spectrum (4000-100 cm(-1)) of the gas has also been recorded. From these data two of the four possible stable conformers have been confidently identified and their order of stabilities has been experimentally determined where the first indicator is for the position of attachment of the hydroxyl group on the bent cyclobutyl ring (Eq=equatorial or Ax=axial) and the second one (t=trans, g=gauche) is the relative position of the hydroxyl rotor, i.e. rotation around the ring C-O bond. The enthalpy difference between the most stable Eq-t conformer and the second most stable rotamer, Eq-g, has been determined to be 200+/-50 cm(-1) (2.39+/-0.60 kJ/mol). This experimentally determined order is consistent with the order of stability predicted by ab initio calculations Eq-t>Eq-g>Ax-g>Ax-t. Evidence was obtained for the third conformer Ax-g which is predicted by ab initio calculations to be less stable by more than 650cm(-1) than the Eq-t form. The percentage of each conformer at ambient temperature is estimated to be Eq-t (50%), Eq-g (47%) and Ax-g (3%). The conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios and vibrational frequencies have been obtained for all of the conformers from MP2(full)/6-31G(d) ab initio calculations. The optimized geometries and conformational stabilities have been obtained from ab initio calculations utilizing several different basis sets up to MP2(full)/aug-cc-pVTZ and from density functional theory calculations by the B3LYP method. By utilizing previously reported microwave rotational constants for the Eq-t conformer combined with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r0 parameters have been obtained. The determined heavy atom structural parameters for the Eq-t conformer are: the distances C1-C4=1.547(5) angstroms, C4-C6=1.552(5)angstroms, C-O=1.416(5) angstroms and angles angleC6C4C1=86.6(5) degrees , angleC4C1C5=88.9(5) degrees and angleC6C5C1C4=22.8(5) degrees . The results are discussed and compared to the corresponding properties of some similar molecules.

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 (r0) are calculated to be 1.484 Å for the gauche conformer. The other r0 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.

Vibrational spectra, conformational stability and ab initio calculations of chlorosulfonyl isocyanate

Abstract The infrared (2500–3500 cm−1) and Raman (2500–10000 cm−1) spectra of gaseous and solid chlorosulfonyl isocyanate, ClSO2NCO, have been recorded. Additionally, the Raman spectrum of the liquid has been obtained with qualitative depolarization ratios and the infrared spectrum of xenon solutions at low temperatures (−60–−100°C) have been recorded. All of these data have been interpreted on the basis of a single gauche conformer (ClSNC ≅ 98°) present in all three physical states. A complete vibrational assignment is provided based on infrared and Raman relative intensities, depolarization values and normal coordinate calculations. Ab initio calculations have been carried out at the RHF 3–21G ∗ , RHF 6–31G ∗ and MP2 6–31G ∗ levels from which structural parameters, conformational stabilities and vibrational data have been obtained. The theoretical predictions are compared with the experimental results where appropriate.

Vibrational spectra, conformational stability and ab initio calculations of fluorosulfonyl isocyanate

Abstract The infrared (2500 to 30 cm−1 and Raman (2500 to 10 cm−1) spectra of gaseous and solid fluorosulfonyl isocyanate, FSO2NCO, have been recorded. Additionally, the Raman spectrum of the liquid has been obtained with qualitative depolarization ratios and the infrared spectrum of xenon solutions at low temperatures (−60 to −100°C) have been recorded. All of these data have been interpreted on the basis of a single gauche conformer ( FSNC ≊ 108° ) present in all three physical states. A complete vibrational assignment is provided based on infrared band contours, infrared and Raman relative intensities, and depolarization values which is supported by normal coordinate calculations. Ab initio calculations have been carried out at the RHF/3-21G, RHF/6-31G∗ and MP2/6-31G∗ levels from which structural parameters, conformational stabilities and vibrational data have been obtained. The theoretical predictions are compared to the experimental results where appropriate, as well as to the corresponding results for the CISO2NCO molecule.

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.

Raman and infrared spectra, conformational stability, normal coordinate analysis, ab initio calculations and vibrational assignment of 1‐chloro‐1‐methylsilacyclobutane

The Raman spectrum (3500–30 cm−1) of liquid 1-chloro-1-methylsilacyclobutane, c-C3H6SiCl(CH3), was recorded and quantitative depolarization values were obtained. Additionally, the infrared (3500–40 cm−1) spectra of the gas and solid were recorded. Both the axial and equatorial (with respect to the methyl group) conformers were identified in the fluid phases. Variable temperature (−55 to −100 °C) studies of the infrared spectra of the sample dissolved in liquid xenon were carried out. From these data, the enthalpy difference was determined as 178 ± 15 cm−1 (2.13 ± 0.18 kJ mol−1), with the axial conformer being the more stable form and the only conformer remaining in the polycrystalline solid. A complete vibrational assignment is proposed for the axial conformer and many of the fundamentals of the equatorial conformer were also identified. The vibrational assignments are supported by normal coordinate calculations utilizing ab initio force constants. Complete equilibrium geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities and depolarization ratios were determined for both rotamers by ab initio calculations employing the 6–31G* and 6–311++G** basis sets at the levels of restricted Hartree–Fock (RHF) and/or Moller–Plesset (MP) to second order. The results are discussed and compared with those obtained for some similar molecules. Copyright

VIBRATIONAL SPECTRA, CONFORMATIONAL STABILITY AND AB INITIO CALCULATIONS OF TRIFLUOROMETHYLSULFONYL ISOCYANATE

Abstract The infrared (2500-30 cm −1 ) spectra of gaseous and solid trifluoromethylsulfonyl isocyanate, CF 3 SO 2 NCO, and the Raman (2500-10 cm −1 ) spectrum of the solid have been recorded. Additionally the Raman spectrum of the liquid with quantitative depolarization ratios and the infrared spectra of xenon solutions at low temperatures (− 60 to − 100°C) have been recorded. All of these data have been interpreted on the basis of a single gauche conformer (CSNC ≅ 108°) present in all three physical states. A complete vibrational assignment is provided on the basis of infrared and Raman relative intensities, depolarization values and group frequencies. The proposed assignment is supported by normal coordinate calculations utilizing force constants from ab initio calculations. The ab initio calculations have been carried out at the RHF/6–31G∗, MP2/3–21G∗ and/or MP2/6–31G∗ levels from which structural parameters, conformational stabilities and vibrational data have been obtained. The theoretical predictions are compared to the experimental results where appropriate, as well as to the corresponding results for the FSO 2 NCO and CISO 2 NCO molecules.