Conformational and structural studies of 1-fluorosilacyclobutane from temperature dependent FT-IR spectra of krypton solutions and ab initio calculations

The infrared spectra (3500 to 40 cm⁻¹) of gaseous and solid and the Raman spectra (3500 to 30 cm⁻¹) of liquid and solid 1-fluorosilacyclobutane, c-C₃H₆SiFH, have been obtained. Both the axial and equatorial conformers with respect to the fluorine atom have been identified in the fluid phases. Variable temperature (−105 to −150 °C) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out. From these data, the enthalpy difference has been determined to be 282 ± 27 cm⁻¹ (3.37 ± 0.32 kJ/mol), with the equatorial conformer the more stable form and the only conformer remaining in the annealed solid. At ambient temperature there is approximately 21 ± 2% of the axial conformer present in the vapor phase. From isolated Si–H stretching frequencies the Si–H (r₀) distances are calculated to be 1.484 and 1.485 Å for the equatorial and axial conformers, respectively. Structural parameters have been predicted from MP2/6-311 + G(d,p) ab initio calculations and the adjusted r₀ parameters for both conformers were obtained from a combination of the ab initio predicted values and the six previously reported microwave rotational constants. Along with the Si–H bond distance, the Si–C, and C–C distances of 1.865(5), and 1.571(5) Å, respectively, for the equatorial conformer are significantly different from the values for these parameters previously reported from an election diffraction study. Both the SiC and CC distances and the SiF distance are longer by 0.002 and 0.004 Å, respectively, for the axial conformer. Structural parameters have also been obtained for silacyclobutane, c-C₃H₆SiH₂ and ethylsilylfluoride, CH₃CH₂SiH₂F, from combined ab initio predicted values and previously reported rotational constants. Several of these newly determined parameters are significantly different from those previously reported for both molecules. Complete equilibrium geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, and depolarization ratios have been determined for both rotamers by ab initio calculations employing the 6-31G(d) basis set at the level of Moller–Plesset (MP) to second order. A complete vibrational assignment supported by normal coordinate calculations is proposed for the equatorial conformer, and several of the fundamentals of the axial conformer have also been identified. The results are discussed and compared to corresponding quantities for some similar molecules.     

Conformational and structural studies of ethyl fluorosilane from temperature dependent FT-IR spectra of krypton solutions and ab initio calculations

The infrared spectra (3200–30 cm⁻¹) of gaseous and solid ethyl fluorosilane, CH₃CH₂SiH₂F, have been recorded. Additionally, the Raman spectra (3200–30 cm⁻¹) of the liquid and solid have been recorded and quantitative depolarization values obtained. Both the gauche and trans conformers have been identified in the fluid phases but only the gauche conformer remains in the solid. Variable temperature (−105 to −150°C) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out. From these data, the enthalpy difference has been determined to be 54±16 cm⁻¹ (646±191 J/mol) with the gauche conformer the more stable form. This is consistent with the predictions from ab initio, MP2/6-311+G(2d,2p), calculation as well as those with smaller basis sets with full electron correlations. 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 are supported by normal-coordinate calculations utilizing the force constants from MP2/6-31G(d) ab initio 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 to the second order (MP2) with full electron correlation. The adjusted r₀ parameters have been obtained for both conformers from a combination of the previously reported rotational constants with ab initio predicted values. All results are compared to similar quantities of some corresponding molecules.     

Conformational studies of cyclopropylmethyl isocyanate from temperature dependent FT-IR spectra of xenon solutions and ab initio calculations

Variable temperature (−55 to −100 °C) studies of the infrared spectra (3200 to 100 cm⁻¹) of cyclopropylmethyl isocyanate, c-C₃H₅CH₂NCO, dissolved in liquefied xenon, have been carried out. The infrared spectra (gas and solid) as well as the Raman spectrum of the liquid have been recorded from 3200 to 100 cm⁻¹. By analyzing six conformer pairs in xenon solutions, an enthalpy difference of 193 ± 19 cm⁻¹ (2.31 ± 0.23 kJ/mol) was obtained with the gauche–cis rotamer (the first designation indicates the orientation of the CNCO group with respect to the three-membered ring, the second designation indicates the relative orientation of the NCO group with respect to the bridging CC bond) the more stable form and the only form present in polycrystalline solid. The abundance of the cis–trans conformer present at ambient temperature is 16 ± 1%. The potential function governing the conformational interchange has been obtained from B3LYP/6-31G(d) calculations and the two-dimensional potential has been obtained. From MP2 ab initio calculations utilizing various basis sets with diffuse functions, the gauche–cis conformer is predicted to be more stable by 223 to 269 cm⁻¹, which is consistent with the experimental results. However, without diffuse functions the predicted conformational energy differences are much smaller (77–166 cm⁻¹). Similar diffuse function dependency affects density functional theory calculations by the B3LYP method to a lesser extent. A complete vibrational assignment for the gauche–cis conformer is proposed and several fundamentals for the cis–trans conformer have been identified. The structural parameters, dipole moments, conformational stability, vibrational frequencies, infrared intensities and Raman activities have been predicted from ab initio calculations and r₀ structural parameters are estimated. These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.     

Infrared and Raman spectra, conformational stability, ab initio calculations of structure, and vibrational assignment of 2-hexyne

The infrared spectra (3500–50 cm⁻¹) of the gas and solid and the Raman spectra (3500–50 cm⁻¹) of the liquid and solid have been recorded for 2-hexyne, CH₃–CC–CH₂CH₂CH₃. Variable temperature studies of the infrared spectrum (3500–400 cm⁻¹) of 2-hexyne dissolved in liquid krypton have also been recorded. Utilizing four anti/gauche conformer pairs, the anti(trans) conformer is found to be the lower energy form with an enthalpy difference of 74±8 cm⁻¹ (0.88±0.10 kJ/mol) determined from krypton solutions over the temperature range −105 to −150 °C. At room temperature it is estimated that there is 42% of the anti conformer present. Equilibrium geometries and energies of the two conformers have been determined by ab initio (HF and MP2) and hybrid DFT (B3LYP) methods using a number of basis sets. Only the HF and DFT methods predict the anti conformer as the more stable form as found experimentally. A vibrational assignment is proposed based on the force constants, relative intensities, depolarization ratios from the ab initio and DFT calculations and on rotational band contours obtained using the calculated equilibrium geometries. From calculated energies it is shown that the CH₃ group exhibits almost completely free rotation which is in agreement with the observation of sub-band structure for the degenerate methyl vibrations from which values of the Coriolis coupling constants, ζ, have been determined. The results are compared to similar properties of some corresponding molecules.     

Conformational stability of 1-pentyne from temperature dependent FT-IR spectra of liquid rare gas solutions and ab initio calculations

Variable temperature studies of the infrared spectra (3500–400 cm⁻¹) of 1-pentyne, CH₃CH₂CH₂CCH, dissolved in liquid xenon (−55 to −100°C) and liquid krypton (−105 to −150°C) have been recorded. These data indicate that the anti (methyl group trans to the acetylenic group) and gauche conformers have relative concentrations that vary with the temperature, i.e. enthalpy nonzero. Utilizing seven sets of conformer pairs for the xenon solution and ten sets of conformer pairs for the krypton solution, the enthalpy difference has been determined to be 50±6 cm⁻¹ (0.60±0.07 kJ/mol) and 45±4 cm⁻¹ (0.54±0.05 kJ/mol), respectively, with the anti conformer the more stable form. Because of two equivalent gauche forms, this conformer is estimated to be in higher abundance at 61±1% in the xenon solution and 62±1% in the krypton solution. Optimized geometries and conformational stabilities have been obtained from ab initio calculations with basis sets 6-31G(d), 6-311+G(d,p), 6-311+G(2d,2p) and 6-311+G(2df,2pd) with full electron correlation by the perturbation method to second order (MP2). All of the calculations predict the gauche rotamer to be the more stable form with a high value of 181 cm⁻¹ from the MP2/6-311+G(d,p) calculations and a low value of 107 cm⁻¹ from the MP2/6-311+G(2d,2p) calculation. The r_o adjusted structural parameters have been obtained from a combination of the microwave rotational constants and ab initio predicted parameters. The values are compared to the recently reported values from an electron diffraction study where the value for the CC bond distance appears to be too long. The results are discussed and the conformational stability is compared to those obtained for some similar molecules.     

Matrix isolation FT-IR, FT-Raman spectroscopy, conformational ab initio calculations, and vibrational frequencies of meso and racemic-2,4-pentanediol

The infrared spectra of meso-2,4-pentanediol and racemic-2,4-pentanediol were measured in an argon matrix at 20 K. The Raman spectra of the pure liquids (meso and racemic) were measured at room temperature. The spectra were obtained using a Fourier transform spectrophotometer and a cryostat for the low temperature matrix. The meso and racemic forms of the diol were separated by means of a spinning band distillation column. The energies of nine possible conformers of the meso form and nine conformers of the racemic form were calculated. Extensive ab initio calculations using B3LYP, MP2 and HF methods with several basis sets consistently gave the lowest energy for the TT conformer of the meso form and the G⁻T (=TG⁻) conformer of the racemic form. Ab initio calculations at the B3LYP/6-31G** level were performed for the lowest energy conformer of meso and racemic pentanediol to obtain the equilibrium geometry, vibrational frequencies, and infrared and Raman intensities. Calculated and experimental frequencies were compared to make vibrational assignments.     

Infrared, and Raman spectra, conformational stability, normal coordinate analysis, ab initio calculations, and vibrational assignment of 1-chlorosilacyclobutane

The infrared spectra (3500–40 cm⁻¹) of gaseous and solid and the Raman spectra (3500–30 cm⁻¹) of liquid and solid 1-chlorosilacyclobutane, c-C₃H₆SiClH, have been obtained. Both the axial and equatorial conformers with respect to the chlorine atom have been identified in the fluid phases. Variable temperature (−105 to −150°C) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out. From these data, the enthalpy difference has been determined to be 211±17 cm⁻¹ (2.53±0.21 kJ/mol), with the equatorial conformer being the more stable form and the only conformer remaining in the annealed solid. At ambient temperatures, approximately 26% of the axial conformers are present in the vapor phase. A complete vibrational assignment is proposed for the equatorial conformer, and many of the fundamentals of the axial conformers have also been 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 have been determined for both rotamers by ab initio calculations employing the 6-31G(d) basis set at the levels of restricted Hartree–Fock (RHF) and/or Moller–Plesset (MP) to second order. Structural parameters have also been obtained using MP2/6-311+G(d,p) ab initio calculations. The r₀ parameters for both conformers are obtained from a combination of the ab initio predicted values and the twelve previously reported microwave rotational constants. The results are discussed and compared to those obtained for some similar molecules.     

Vibrational spectra, conformations, ab initio calculations and vibrational assignments of 3-pentyn-2-ol

The infrared spectra of 3-pentyn-2-ol, CH₃CCCH(OH)CH₃, have been recorded as a vapour and liquid at ambient temperature, as a solid at 78 K in the 4000–50 cm⁻¹ range and isolated in an argon matrix at ca. 5 K. Infrared spectra of the solid phase at 78 K were obtained before and after annealing to temperatures of 120 and 130 K. The IR spectra of the solid were quite similar to that of the liquid.

Raman spectra of the liquid were recorded at room temperature and at various temperatures between 295 and 153 K. Spectra of an amorphous and annealed solid were recorded at 78 K. In the variable temperature Raman spectra, some bands changed in relative intensity and were interpreted in terms of conformational equilibria between the three possible conformers. Complete assignments were made for all the bands of the most stable conformer in which OH is oriented anti to C₁(a_Me). From various bands assigned to a second conformer in which OH is oriented anti to H_gem(a_H), the conformational enthalpy differences was found to be between 0.4 and 0.8 kJ mol⁻¹. The highest energy conformer with OH anti to C₃(a_C) was not detected.

Quantum-chemical calculations have been carried out at the MP2 and B3LYP levels with a variety of basis sets. Except for small basis set calculations for which the a_H conformer had slightly lower energy, all the calculations revealed that a_Me was the low energy conformer. The B3LYP/cc-pVTZ calculations suggested the a_Me conformer as more stable by 0.8 and 8.3 kJ mol⁻¹ relative to a_H an a_C, respectively. Vibrational wavenumbers and infrared and Raman band intensities for two of the three conformers are reported from B3LYP/cc-pVTZ calculations.     

Raman and infrared spectra, conformational stability, barriers to internal rotation, vibrational assignment, and ab initio calculations of 3-methyl-3-butenenitrile

The Raman (3500-30 cm⁻¹) spectra of liquid and solid and the infrared (3500-40 cm⁻¹) spectra of gaseous and solid 3-methyl-3-butenenitrile, CH₂C(CH₃)CH₂CN, have been recorded. Both cis and gauche conformers have been identified in the fluid phases but only the cis form 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 163±16 cm⁻¹ (1.20±0.19 kJ mol⁻¹), with the cis conformer the more stable rotamer. It is estimated that there is 48±2% of the gauche conformer present at  25°C. A complete vibrational assignment is proposed for the cis conformer based on infrared band contours, relative intensities, depolarization ratios and group frequencies. Several of the fundamentals for the gauche conformer have also been identified. The vibrational assignments are supported by normal coordinate calculations utilizing ab initio force constants. Complete equilibrium geometries have been obtained for both rotamers by ab initio calculations employing the 6-31G(d), 6-311G(d,p), 6-311+G(d,p) and 6-311+G(2d,2p) basis sets at the levels of restricted Hartree-Fock (HF) and/or Møller-Plesset perturbation theory to the second order (MP2). Only with the 6-311G(2d,2p) and 6-311G(2df,2pd) basis sets with or without diffuse functions is the cis conformer predicted to be more stable than the gauche form. The potential energy terms for the conformational interchange have been obtained at the MP2(full)/6-311+G(2d,2p) level, and compared to those obtained from the experimental data. The results are discussed and compared to the corresponding quantities obtained for some similar molecules.     

Infrared and Raman spectra, conformational stability, normal coordinate analysis, ab initio calculations, and vibrational assignment of 1-fluoro-1-methylsilacyclobutane

The infrared (3500–40 cm⁻¹) spectra of gaseous and solid 1-fluoro-1-methylsilacyclobutane, c-C₃H₆SiF(CH₃), have been recorded. Additionally, the Raman spectrum (3500–30 cm⁻¹) of the liquid has been recorded and quantitative depolarization values have been obtained. Both the axial and equatorial (with respect to the methyl group) conformers have been identified in the fluid phases. Variable temperature (−55–−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 267±10 cm⁻¹ (3.19±0.12 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 for the equatorial conformer have also been identified. The vibrational assignments are supported by normal coordinate calculations utilizing ab initio force constants. Complete equilibrium geometries have been 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 to those obtained for some similar molecules.     

Conformational stability of 1-bromo-2-fluoroethane from temperature dependent FT-IR spectra of rare gas solutions

Variable temperature (−55 to −135°C) studies of the infrared spectra (3500–400 cm⁻¹) of 1-bromo-2-fluoroethane, BrCH₂CH₂F, dissolved in liquid krypton and xenon have been recorded. From these data, the enthalpy difference has been determined to be 108±9 cm⁻¹ (1.296±0.113 kJ/mol) and 112±8 cm⁻¹ (1.346±0.098 kJ/mol) from the krypton and xenon solutions, respectively, with the trans conformer the more stable rotamer. Complete vibrational assignments are presented for both conformers which are consistent with the predicted frequencies obtained from the ab initio MP2/6-31G^* calculations. The optimized geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, and depolarization ratios 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. Structural parameters and conformational stability have also been obtained from MP2/6-311+G^** calculations. Combining the ab initio predicted structural parameters with the microwave rotational constants, r_o parameters have been obtained for the gauche conformer.     

Conformational and structural studies of 1-chloropropane and 1-bromopropane 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 (3500–400 cm⁻¹) of 1-chloropropane (CH₃CH₂CH₂Cl) and 1-bromopropane (CH₃CH₂CH₂Br) dissolved in liquid krypton and xenon, respectively, have been recorded. Utilizing two conformer pairs in krypton solution for chloride and three conformer pairs in xenon solution for bromide, enthalpy differences of 52±3 cm⁻¹ (0.62±0.06 kJ/mol) and 72±7 cm⁻¹ (0.86±0.08 kJ/mol) were obtained for the chloride and bromide, respectively, with the gauche form being the more stable conformer for both molecules. From these data, it is estimated that 28 and 26% of trans form are present at ambient temperature for the chloride and bromide, respectively. The conformation 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 for both halopropanes 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 with full electron correlation by the perturbation method up to MP2/6-311+G(2d,2p). The r₀ structural parameters of both halopropanes have been obtained by combining the ab initio data with the previously reported microwave rotational constants for both conformers. The quantities are compared to the corresponding results for some similar molecules.     

Far infrared spectra, conformational equilibria, vibrational assignments, ab initio calculations and structural parameters for 2-bromoethanol

The far infrared spectrum from 370 to 50 cm⁻¹ of gaseous 2-bromoethanol, BrCH₂CH₂OH, was recorded at a resolution of 0.10 cm⁻¹. The fundamental O–H torsion of the more stable gauche (Gg′) conformer, where the capital G refers to internal rotation around the C–C bond and the lower case g to the internal rotation around the C–O bond, was observed as a series of Q-branch transitions beginning at 340 cm⁻¹. The corresponding O–H torsional modes were observed for two of the other high energy conformers, Tg (285 cm⁻¹) and Tt (234 cm⁻¹). The heavy atom asymmetric torsion (rotation around C–C bond) for the Gg′ conformer has been observed at 140 cm⁻¹. Variable temperature (−63 to −100°C) studies of the infrared spectra (4000–400 cm⁻¹) of the sample dissolved in liquid xenon have been recorded. From these data the enthalpy differences have been determined to be 411±40 cm⁻¹ (4.92±0.48 kJ/mol) for the Gg′/Tt and 315±40 cm⁻¹ (3.76±0.48 kJ/mol) for the Gg′/Tg, with the Gg′ conformer the most stable form. Additionally, the infrared spectrum of the gas, and Raman spectrum of the liquid phase are reported. The structural parameters, conformational stabilities, barriers to internal rotation and fundamental frequencies have been obtained from ab initio calculations utilizing different basis sets at the restricted Hartree–Fock or with full electron correlation by the perturbation method to second order. The theoretical results are compared to the experimental results when appropriate. Combining the ab initio calculations with the microwave rotational constants, r₀ adjusted parameters have been obtained for the three 2-haloethanols (F, Cl and Br) for the Gg′ conformers.     

Raman and infrared spectra, conformational stability, ab initio calculations and vibrational assignments for 2-chloroethyl silane

The infrared (3500–30 cm⁻¹) spectra of gaseous and solid and the Raman (3500–10 cm⁻¹) spectra of liquid with quantitative depolarization ratios and solid 2-chloroethyl silane, ClCH₂CH₂SiH₃, have been recorded. Similar data have been recorded for the Si–d₃ isotopomer. These data indicate that two conformers, trans and gauche, are present in the fluid states but only one conformer, trans, is present in the solid. The mid-infrared spectra of the sample dissolved in liquified xenon as a function of temperature (−55 to −100°C) has been recorded. The enthalpy difference between the conformers has been determined to be 181±12 cm⁻¹ (2.17±0.14 kJ/mol) with the trans rotamer the more stable form. From the isolated Si–H frequencies from the Si–d₂ isotopomer the r_o Si–H distances of 1.484 and 1.483 Å for the trans and 1.481 for the gauche conformers have been obtained. Ab initio calculations have been carried out with several different basis sets up to MP2/6-311+G** from which structural parameters and conformational stabilities have been determined. With all the basis sets the trans form is predicted to be the more stable conformer which is consistent with the experimental results. These results are compared to the corresponding quantities for the carbon analogue.     

FT-IR, NIR-FT-Raman and gas phase infrared spectra of 3-aminoacetophenone by density functional theory and ab initio Hartree-Fock calculations

The gas phase infrared spectrum of 3-aminoacetophenone (3AAP) was measured in the range 5000-500cm(-1) and with a resolution of 0.5cm(-1). The Fourier transform Raman (FT-Raman) and Fourier transform infrared (FT-IR) spectra of 3AAP were recorded in the solid phase. Geometry optimizations were done without any constraint and several thermodynamic parameters were calculated for the minimum energy conformer at ab initio and density functional theory (DFT) levels invoking 6-311G(2df 2p) basis set and the results are compared with the experimental values. Harmonic-vibrational wavenumber was also calculated for the minimum energy conformer at ab initio and DFT levels using 6-31G(d,p) basis set and the results are compared with related molecules. With the help of specific scaling procedures, the observed vibrational wavenumbers in gas phase, FT-IR and FT-Raman spectra were analyzed and assigned to different normal modes of the molecule. Most of the modes have wavenumbers in the expected range, the error obtained was in general very low. The appropriate theoretical spectrogram for the FT-IR spectra of the title molecule is also constructed.     

Conformational stability of chlorocyclohexane from temperature-dependent FT-IR spectra of xenon solutions, r 0 structural parameters, and vibrational assignment

Variable temperature (?55 to ?105 °C) studies of the infrared spectra (4000–400 cm^?1) of chlorocyclohexane (c-C₆H₁₁Cl) dissolved in liquefied xenon have been carried out. The infrared spectra of the gas and solid have also been recorded from 4000–100 cm^?1. By analyzing six conformer pairs in the xenon solution, a standard enthalpy difference of 132 ± 13 cm^?1 (1.58 ± 0.16 kJ/mol) was obtained with the equatorial conformer the more stable form. At ambient temperature, the abundance of the axial conformer is 34 ± 1%. The potential surface describing the conformational interchange has been determined and the Fourier coefficients were obtained. From MP2 ab initio calculations utilizing various basis sets with and without diffuse functions, the equatorial conformer is predicted to be more stable by 161 ± 18 cm^?1 from the four largest basis set calculations, which is consistent with the experimental results. However, the average from the corresponding B3LYP density functional theory calculations is 274 ± 15 cm^?1 which is certainly too large. By utilizing the previously reported microwave rotational constants for two isotopomers (³⁵Cl, ³⁷Cl) combined with the structural parameters predicted from the MP2(full)/6-311+G(d,p) calculations, adjusted r ₀ structural parameters have been obtained. The determined heavy atom distances for the most stable chair-equatorial conformer in Å are: r ₀(C₁–C₇,₈) = 1.532(3); r ₀(C₇,₈–C₁₃,₁₄) = 1.536(3); r ₀(C₄–C₁₃,₁₄) = 1.524(3); and r ₀(C₄–Cl₆) = 1.802(5) and the angles in degrees: ∠C₁C₇,₈C₁₃,₁₄ = 111.3(5)º; ∠Cl₆C₄C₁₃,₁₄ = 109.7(5)º with the two dihedral angles ∠C₈C₁C₇C₁₃ = 56.3(10)º and ∠C₁₄C₄C₁₃C₇ = 56.7(10)º. These parameters are in good agreement with those reported earlier from microwave and electron diffraction studies where the CC and CH distances were all assumed to be equal. A few of the previously reported vibrational assignments have been corrected. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules.     

Infrared and Raman spectra, ab initio calculations and vibrational assignment for 3-fluoro-1-butyne

The infrared (3500-80 cm⁻¹) and Raman (3500-20 cm⁻¹) spectra of 3-fluoro-1-butyne, CH₃CHFCCH, have been recorded for the gas and solid. Additionally, the Raman spectrum of the liquid has also been recorded to aid in the vibrational assignment. Ab initio electronic structure calculations of energies, geometrical structures, vibrational frequencies, infrared intensities, Raman activities and the potential energy function for the methyl torsion have been calculated to assist in the interpretation of the spectra. The fundamental torsional mode is observed at 251 cm⁻¹ with a series of sequence peaks falling to lower frequency. The three-fold methyl torsional barrier is calculated to be 1441 ± 20 cm⁻¹ (4.12 ± 0.06 kcal mol⁻¹) where the uncertainty is partly due to the uncertainty in values of the V₆ term. A complete vibrational assignment is proposed based on band contours, relative intensities, and ab initio predicted frequencies. Several fundamentals are significantly shifted in the condensed phases compared to values in the vapor state.     

Infrared and Raman spectra, conformational stability, ab initio calculations and vibrational assignment for 1,2-pentadiene (ethyl allene)

The infrared (3500-50 cm⁻¹) and Raman (3500-20 cm⁻¹) spectra of 1,2-pentadiene, H₂C=C=C(H)CH₂CH₃ (ethyl allene), have been recorded for both the gaseous and solid states. Additionally, the Raman spectrum of the liquid has been obtained with qualitative depolarization values. In the fluid phases both the cis and gauche conformers have been identified, with the gauche rotamer being the predominant form although it may not be the conformer of lowest energy. In the solid state only the cis conformer remains after repeated annealing of the crystal. The asymmetric torsion of the cis conformer is observed as a series of Q-branch transitions beginning at 103.4 cm⁻¹ and falling to lower frequency. An estimate of the potential function governing conformer interconversion is provided. A complete assignment of the normal modes for the cis conformer is given and several of the fundamentals are assigned for the gauche rotamer. Ab initio electronic structure calculations of energies, conformational geometries, vibrational frequencies, and potential energy functions have been made to complement and assist the interpretation of the infrared and Raman spectra. In particular, the transitions among torsional energy levels for both the symmetric (methyl) and asymmetric (ethyl) motions have been calculated. The results are compared to the corresponding quantities for some similar molecules.     

Spectra and structure of small ring compounds. LXVII vibrational spectra, variable temperature FT-IR spectra of krypton solutions, conformational stability and ab initio calculations of 1-bromosilacyclobutane

The infrared (3,200-30 cm(-1) spectra of gaseous and solid 1-bromosilacyclobutane, c-C3H6SiBrH, have been recorded. Additionally, the Raman spectra of the liquid (3,200- 30 cm(-1) with quantitative depolarization values and the solid have been recorded. Both the equatorial and the axial conformers have been identified in the fluid phases, Variable temperature ( - 105 to - 150 degrees C) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out. From these data the enthalpy difference has been determined to be 182 +/- 18 cm(-1) (2.18 +/- 0.22 kJ/mol) with the equatorial conformer the more stable rotamer and only conformer remaining in the annealing solid. At ambient temperature there is approximately 22% of the axial conformer present in the vapor phase. A complete vibrational assignment is proposed for both conformers based on infrared 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. From the frequencies of the Si-H stretch, the Si-H bond distance of 1.483 A has been determined for both the equatorial and the axial conformers. Complete equilibrium geometries have been determined for both rotamers by ab initio calculations employing the 6-31G(d) and 6-311 +/- G(d,p) basis sets at levels of Hartree Fock (RHF) and/or Moller- Plesset with full electron correlation by the perturbation method to the second order (MP2). The results are discussed and compared to those obtained for some similar molecules.     

Matrix-isolation FT-IR study and theoretical calculations of the vibrational, tautomeric and H-bonding properties of hypoxanthine

The neutral compound hypoxanthine is investigated using the technique of matrix-isolation FT-IR spectroscopy combined with density functional theory (DFT) and ab initio methods. Two theoretical methods (RHF and DFT/B3-LYP) are compared for vibrational frequency prediction, and four methods (RHF//RHF, MP2//RHF, DFT//DFT and MP2//DFT) for prediction of the relative energies of the tautomers and the interaction energies of the complexes. All the possible tautomeric forms have been considered theoretically, and the results indicate that two oxo forms (O17 and O19) and one hydroxy form (H9-r1) are the three most stable forms. The experimental FT-IR spectra are consistent with this prediction, and nearly all the characteristic spectral features of these forms have been identified in the spectrum. A theoretical study of the H-bonded complexes of these three tautomers with water is also performed. Several structures have been found for each form and the results demonstrate that the closed complexes with two H-bonds are the most stable systems due to the H-bond cooperative effect.