Solvent dependence of conformational stability and analysis of vibrational spectra of 2,2,3,3,3-pentafluoro-1-propanol

The conformational stability of 2,2,3,3,3-pentafluoro-1-propanol was investigated by the DFT-B3LYP/6-311+G** and the ab initio MP2/6-311+G** calculations. The calculated potential energy curves of 2,2,3,3,3-pentafluoro-1-propanol at both levels of theory were consistent with three distinct minima that correspond to Trans-gauche-gauche (Tgg), trans-trans-gauche (Ttg) and trans-gauche-gauche(-) (Tgg1) conformers in the order of decreasing relative stability. The equilibrium constants for the conformational interconversion of 2,2,3,3,3-pentafluoro-1-propanol were calculated and found to correspond to an equilibrium mixture of about 46% Tgg, 43% Ttg and 11% Tgg1 conformations at 298.15K. The calculated (%Ttg/%Tgg) ratio of 0.93 is consistent with the 0.85 ratio of the observed intensities of the 772 and 794 cm(-1) lines in the Raman spectrum of the liquid. The nature of the high energy Ttg conformation was verified by solvent experiments using formamide (epsilon = 109.5) and acetonitrile (epsilon = 37). The vibrational frequencies of the molecule in its stable forms were computed at B3LYP level and complete vibrational assignments were made based on normal coordinate calculations and comparison with experimental data of the molecule.     

Investigation of conformational stability and vibrational spectra of halomethylsulfonyl isocyanates

The conformational behavior and structural stability of chloro- and fluoromethylsulfonyl isocyanates were investigated by quantum mechanical DFT and ab initio MP2 calculations. The 6-311++G^** basis set was employed to include polarization and diffuse functions in the calculations. The molecules were found to exist in a mixture of two stable gauche conformations. The potential scans were calculated from which the rotational barriers could be estimated. The vibrational frequencies and spectra were computed at B3LYP/6-311++G^** level. The potential energy distributions were then calculated to provide tentative vibrational assignment for the normal modes of the stable conformers of both molecules.     

Infrared and Raman spectra, conformational stability and vibrational assignment of 1-chloro-1-silacyclopentane

Infrared and Raman spectra (3500-60 cm⁻¹) of gas and/or liquid and solid 1-chloro-1-silacyclopentane (c-C₄H₈SiClH) have been recorded and the vibrational data indicate the presence of a single conformer with no symmetry which is consistent with the twisted form. Ab initio calculations with a variety of basis sets up to MP2(full)/aug-cc-pVTZ predict the envelope-axial and envelope-equatorial conformers are saddle points with nearly the same energies but much lower in energy than the planar conformer. Density functional theory calculations by the B3LYP method predicts slightly lower energies for the two envelope forms and considerably lower for the planar form. By utilizing the MP2(full)/6-31G(d) calculations the force constants, frequencies, infrared intensities, band contours, Raman activities, and depolarization values have been obtained to support the vibrational assignment. Estimated r₀ structural parameters have been obtained from adjusted MP2(full)/6-311 + G(d, p) calculations. These experimental and theoretical results are compared to the corresponding quantities of some other five-membered rings.     

Conformations and vibrational spectra of 2,3-dichloro and 2,3-dibromo-1-propene

The infrared spectra of 2,3-dichloro- and 2,3-dibromo-1-propene in the region 4000-200 cm^?1 were recorded of the liquids and of the crystalline solids at ?180 °C. Raman spectra, including semiquantitative polarization measurements were obtained of the liquids. Additional spectra were recorded of the samples dissolved in polar and non-polar solvents and of unannealed as well as of annealed crystalline solids at ?180 °C.Approximately 13 vibrational bands present in the spectra of the liquids and solutions as well as of the amorphous solid vanished in the crystal spectra. From the intensity variations with changing solvent polarity it was concluded that the conformer present in the crystal was more polar (gauche) than the other, simultaneously present in the liquid (cis). A striking similarity between the spectra of the two compounds was observed. All the fundamentals for the gauche conformers have tentatively been assigned.     

Infrared and Raman spectra, ab initio calculations, conformational stability and vibrational assignment of 1-bromo-1-silacyclopentane

Infrared and Raman spectra (3500-60 cm(-1)) of gas and/or liquid and solid 1-bromo-1-silacyclopentane (c-C4H8SiBrH) have been recorded and the vibrational data indicate the presence of a single conformer with no symmetry which is consistent with the twisted form. Ab initio calculations with a variety of basis sets up to MP2(full)/6-311+G(2df,2pd) predict the envelope-axial and envelope-equatorial conformers to be saddle points with nearly the same energies but approximately 900 cm(-1) (5.98 kJ/mol) lower in energy than the planar conformer. Density functional theory calculations by the B3LYP method predict slightly lower energies for the two envelope forms and considerably lower energy for the planar form compared to the MP2 predictions. By utilizing the MP2(full)/6-31G(d) calculations the force constants, frequencies, infrared intensities, band contours, Raman activities, and depolarization values have been obtained to support the vibrational assignment. Estimated r0 structural parameters have been obtained from adjusted MP2(full)/6-311+G(d,p) calculations. These experimental and theoretical results are compared to the corresponding quantities of some other five-membered rings.     

Vibrational spectra and conformational equilibrium of 1,1-dichloro-1-nitrosoethane and its deuterium analog

Conformational equilibrium has been detected in 1,1-dichloro-1-nitrosoethane, and the structures of the conformers have been determined. H/D isotope exchange has been used in assigning and calculating the normal vibrations.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2536–2540, November, 1989.     

Density functional theory investigations on the conformational stability and vibrational spectra of the model compound of vitamin K

Becke 3-Lee–Yang–Parr density functional theory (DFT/B3LYP) using 6-31G(d) and 6-311G(d) basis sets and the scaled quantum mechanics (SQM) force field method are used to study molecular conformations and vibrational spectra of a model compound of vitamin K (VK). In this molecule, there are six conformers on the torsional potential energy map of the dihedral angles C₈C₁₄C₁₅C₁₆ () and C₇C₈C₁₄C₁₅ (β). It is shown that the VK_1 conformer ( = 237.7° and β = 274.2°) is the most stable form. For this lowest energy conformer, the harmonic force fields calculated by B3LYP/6-311G(d) and B3LYP/6-31G(d) methods are scaled with one scale factor of 0.9623 and a set of different scale factors transferred from the previous studies for the other similar molecules, respectively. The vibrational frequencies, IR intensities, and Raman activities are obtained for the lowest energy conformer. On the basis of B3LYP calculations, the normal mode analysis is performed to assign the vibrational fundamental frequencies according to the potential energy distributions. The computational frequencies are in good agreement with the observed results.     

On the vibrational spectra and conformational stability of 1,1-dimethylhydrazine from temperature dependent FT-IR spectra of krypton solutions and ab initio calculations

Variable temperature (−105 to −150 °C) studies of the infrared spectra (3500–400 cm⁻¹) of 1,1-dimethylhydrazine, (CH₃)₂NNH₂, in liquid krypton have been carried out. No convincing spectral evidence could be found for the trans conformer which is expected to be at least 600 cm⁻¹ less stable than the gauche form. The structural parameters, dipole moments, conformational stability, vibrational frequencies, and infrared and Raman intensities have been predicted from MP2/6-31G(d) ab initio calculations. The predicted infrared and Raman spectra are compared to the experimental ones. The adjusted r₀ parameters from MP2/6-311+G(d,p) calculations are compared to those reported from an electron diffraction study. The energy differences between the gauche and trans conformers have been obtained from MP2 ab initio calculations as well as from density functional theory by the B3LYP method calculations from a variety of basis sets. All of these calculations indicate an energy difference of 650–900 cm⁻¹ with the B3LYP calculations predicted the larger values. The potential function governing the conformational interchange has been predicting from both types of calculations and comparisons have been made. The barrier to internal rotation by the independent rotor model of the inner methyl group is predicted to have a value of 1812 cm⁻¹ and that of the outer one of 1662 cm⁻¹ from ab initio MP2/6-31G(d) calculations. These values agree well with the experimentally determined values of 1852±16 and 1558±12 cm⁻¹, respectively, from a fit of the torsional transitions with the coupled rotor model. For the coupled rotor model the predicted V′₃₃ (sin 3τ₀ sin 3τ₁ term) value which ranged from 190 to 232 cm⁻¹ is in reasonable agreement with the experimental value of 268±3 cm⁻¹ but the predicted V₃₃ (cos 3τ₀ cos 3τ₁ term) value of −73 to −139 cm⁻¹ is 25% smaller and of the opposite sign of the experimental value of 333±22 cm⁻¹. These theoretical and spectroscopy results are compared to similar quantities of some corresponding molecules.     

Rotational spectra and conformational structures of 1-phenyl-2-propanol, methamphetamine, and 1-phenyl-2-propanone

Microwave spectra have been recorded for 1-phenyl-2-propanol, methamphetamine, and 1-phenyl-2-propanone from 11 to 24 GHz using a Fourier-transform microwave spectrometer. Only one spectrum from a single conformational isomer was observed for each species. The rotational transitions in the spectrum of 1-phenyl-2-propanone were split into separate transitions arising from the A- and E-torsional levels of the methyl rotor. The fit of the E-state transitions to a "high-barrier" internal rotation Hamiltonian determines V3 = 238(1) cm-1 and rotor-axis angles of thetaa = 87.7(5) degrees, thetab = 50.0(5) degrees, and thetac = 40.0(5) degrees. Ab initio optimizations (MP2/6-31G**) and single-point calculations (MP2/6-311++G**) were used to model the structures of 1-phenyl-2-propanol, methamphetamine, and 1-phenyl-2-propanone. The lowest energy conformations of these species were found to be stabilized by weak OH-pi, NH-pi, and CH-pi hydrogen-bonding interactions. Moments of inertia, derived from the model structures, were used to assign the spectra to the lowest energy conformation of each species. A series of MP2/6-31G* partial optimizations along the internal rotation pathway were used to estimate the barrier to methyl rotation to be 355 cm-1 for 1-phenyl-2-propanone.     

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

The infrared spectra (3500-50 cm-1) of gas and solid and the Raman spectrum (3500-50 cm-1) of liquid 2-fluorobutane, CH3CHFCH2CH3, have been recorded. Variable temperature studies over the range -105 to -150 degrees C of the infrared spectra (3500-400 cm-1) of the sample dissolved in liquid krypton have also been recorded. By utilizing the relative intensities of six conformer pairs each for both Me-trans/F-trans and Me-trans/H-trans, the Me-trans conformer is found to be the lowest energy form with an enthalpy difference to the F-trans conformer of 102 +/- 10 cm-1 ( 1.21+/- 0.12 kJmol-1) whereas the H-trans conformer is the highest energy form with an enthalpy difference of 208 +/- 21 cm-1 ( 2.49 +/- 0.25 kJmol-1) higher than the Me-trans form. At ambient temperature, it is estimated that there is 50 +/- 2% of the Me-trans form, 31 +/- 1% of the F-trans form, and 19 +/- 1% of the H-trans conformer present. Equilibrium geometries and total energies of the three conformers have been determined by ab initio calculations with full electron correlation by the perturbation method to second order using a number of basis sets. A complete vibrational assignment is proposed for the Me-trans conformer and many of the fundamentals have been identified for the other two forms based on the force constants, relative infrared and Raman intensities, and depolarization ratios obtained from MP2/6-31Gd ab initio calculations. The spectroscopic and theoretical results are compared to the corresponding properties for some similar molecules.     

The vibrational spectra and conformational properties of liquid dimethyl oxalate

The infrared spectra of dimethyl oxalate and dimethyl oxalate-d₆ have been recorded in the liquid phase. These spectra are compared with both the infrared and Raman spectra recorded in solid phase. It is shown that dimethyl oxalate exists as a conformational equilibrium mixture in the liquid phase. The most stable conformer is that present exclusively in the crystal, the planar, cis-ester C_2h structure. The second conformer is probably one formed by rotation about the carbonyl carbon-oxygen bonds of the molecule.     

A DFT study of the vibrational spectra of 1-, and 2-nitrotriphenylene

The infrared (IR) and Raman spectra, and intensities of triphenylene, 1-, and 2-nitrotriphenylene were investigated by the density functional theory (DFT, B3LYP method) with 6-311+G** basis set. Normal mode assignments are proposed with particular emphasis on the nitro group vibrations. Compared to 2-nitrotriphenylene (2-NTRP) 1-nitrotriphenylene (1-NTRP) is predicted to show asymmetric nitro stretches at higher frequencies. Through the vibrational study, the structure–spectroscopic relationships of these nitro polycyclic aromatic hydrocarbons (nitro-PAHs) are made, and possible insights into their differential mutagenic potencies correlated. The geometrical distortions of the TRP structure upon nitro group substitution and correlations between structural parameters and vibrational data as well as structure–function relationships related to the mutagenicity of this important class of polycyclic aromatic hydrocarbons are discussed.     

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.     

DFT studies, vibrational spectra and conformational stability of 4-hydroxy-3-methylacetophenone and 4-hydroxy-3-methoxyacetophenone

The FT-IR and FT-Raman spectra of 4-hydroxy-3-methylacetophenone (HMA) and 4-hydroxy-3-methoxyacetophenone (HMOA) have been recorded. The total energy calculations of HMA and HMOA were tried for various possible conformers. The spectra were interpreted with the aid of normal coordinate analysis based on density functional theory (DFT) using standard B3LYP/6-31G* and B3LYP/6-311+G** methods and basis sets combinations for the most optimized geometries. Normal coordinate calculations were performed with the DFT force field corrected by a recommended set of scaling factors yielding fairly good agreement between observed and calculated frequencies. On the basis of the comparison between calculated and experimental results, assignments of fundamental modes were examined.     

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

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

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.     

Infrared and Raman spectra, conformational stability, ab initio calculations, and vibrational assignments for methylaminothiophosphoryl difluoride

Infrared spectra (4000–50 cm⁻¹) of the vapor, amorphous and crystalline solids and Raman spectra (3600–10 cm⁻¹) of the liquid with qualitative depolarization data as well as the amorphous and crystalline solids of methylaminothiophosphoryl difluoride, CH₃N(H)P(=S)F₂, and three deuterated species, CD₃N(H)P(=S)F₂, CH₃N(D)P(=S)F₂, and CD₃N(D)P(=S)F₂, have been recorded. The spectra indicate that in the vapor, liquid and amorphous solid a small amount of a second conformer is present, whereas only one conformer remains in the low temperature crystalline phase. The near-infrared spectra of the vapor confirms the existence of two conformers in the gas phase. Asymmetric top contour simulation of the vapor shows that the trans conformer is the predominant vapor phase conformer. From a temperature study of the Raman spectrum of the liquid the enthalpy difference between the trans and near-cis conformers was determined to be 368±15 cm⁻¹ (4.41±0.2 kJ/mol), with the trans conformer being thermodynamically preferred. Ab Initio calculations with structure optimization using the 6-31G(d) and 6-311+G(d,p) basis sets at the restricted Hartree–Fock (RHF) and/or with full electron correlation by the perturbation method to second order (MP2) support the occurrence of near-trans (5° from trans) and near-cis (20° from cis) conformers. From the RHF/6-31G(d) calculation the near-trans conformer is predicted to be the more stable form by 451 cm⁻¹ (5.35 kJ/mol) and from the MP2/6-311+G(d,p) calculation by 387 cm⁻¹ (4.63 kJ/mol). All of the normal modes of the near-trans rotamer have been assigned based on infrared band contours, depolarization values and group frequencies and the assignment is supported by the normal coordinate calculation utilizing harmonic force constants from the MP2/6-31G(d) ab initio calculations.     

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.     

N-allenylation of pyrroles by 2,3-dichloro-1-propene

Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, p. 125, January, 1992.     

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.