Matrix isolation FTIR spectroscopic and theoretical study of dimethyl sulfite

The preferred conformations of dimethyl sulfite and their vibrational spectra were studied by matrix-isolation Fourier transform infrared spectroscopy and theoretical methods (density functional theory (DFT) and Moller-Plesset (MP2), with basis sets of different sizes, including the quadruple-zeta, aug-cc-pVQZ basis). Five minima were found at these levels of theory. At the MP2/6-31++G(d,p) and DFT/B3LYP/aug-cc-pVQPZ levels, the GG conformer (where the O-S-O-C dihedral angles are 73.2 and 70.8 degrees ) resulted in the conformational ground state. At the highest level of theory used, the GT conformer (O-S-O-C = +68.5 and -173.2 degrees ) is 0.83 kJ mol(-1) higher in energy than the GG form, while conformer GG' (O-S-O-C = +85.7 and -85.7 degrees ) has a relative energy of 1.18 kJ mol(-1). The remaining two conformers (G'T and TT) are high-energy forms and not experimentally relevant. In consonance with the theoretical predictions, conformer GG was found to be the most stable conformer in the gaseous phase as well as in the low-temperature matrices. Annealing of the argon matrices first promotes the GG'-->GT isomerization, which is followed by conversion of GT into the most stable conformer. There is no evidence of occurrence of GG'-->GG direct conversion in the low-temperature matrices. On the other hand, during deposition of the xenon matrices conformer GG' totally converts to conformer GT. Two observations demonstrated this fact: no evidence of bands corresponding to GG' were observed in xenon matrices and the GG/GT intensity ratio became similar to the GG/(GT + GG') intensity ratio observed in argon matrices. All these results could be explained by taking into account the relative values of the theoretically predicted energy barriers for the different isomerization processes: GG'-->GT, 1.90 kJ mol(-1); GT-->GG, 9.64 kJ mol(-1); and GG'-->GG, 19.46 kJ mol(-1).     

Vibrational spectrum, ab initio calculation, conformational equilibria and torsional modes of 1,3-dibromopropane

The infrared and Raman spectrum of 1,3-dibromopropane is reported in the crystalline, liquid and gaseous states. These measurements are compared to the results of ab initio calculations carried out using the 6-31+g* Gaussian basis set for a restricted Hartree-Fock computation. The calculation was repeated using second order Moeller-Ploesset perturbation theory to accommodate electron correlation using the 6-31 g* basis set. The three most stable conformers are GG (C2), AG (C1) and AA (C2v), where A and G stand for anti and gauche orientations of the bromomethyl group relative to the plane of the carbon atoms. The point group symmetry of each structure is given in parentheses. The fourth conformer, G'G (Cs) is of such high energy that it is not observed experimentally in isotropic media in either the infrared or Raman spectrum. In the crystalline state, comparison of the infrared and Raman spectrum with that calculated for the C2 conformer shows that only the GG (C2) conformer survives, and the doublet structure of many of the bands in the spectrum indicates at least two molecules per unit cell. The ab initio calculations predict and the temperature dependence of the Raman spectrum of the liquid confirms that the stability order is C2相似文献   

Vibrational spectrum, ab initio calculations, conformational equilibria and torsional modes of 1,3-dichloropropane

Ab initio calculations are reported for three of four possible conformers of 1,3-dichloropropane. The fourth conformer, with Cs symmetry, has a predicted enthalpy difference of more than 1500 cm(-1) from the most stable conformer from each calculation regardless of the basis set used, so there is little chance of observing it. Thus, there is no evidence in the infrared or Raman spectrum of the presence of a fourth conformer. The order of stability given by the ab initio calculations is C2(GG)>C1(AG)>C2v(AA)>Cs(GG'), where A indicates the anti form for one of the CH2Cl groups and G indicates the gauche conformation for the other CH2Cl group relative to the plane of the carbon atoms. Almost every band observed can be confidently assigned to one or another of the conformers. Many observed bands proved to be of a composite nature, with several nearly coincident vibrations of different conformers contributing to the band contour. Nonetheless, a complete assignment of fundamentals is possible for the most stable C2 conformer, and 5 of the fundamentals of the C2v conformer and 13 those of the C1 conformer can be confidently assigned.     

Conformational equilibrium and potential energy surface of 1-fluorobutane by microwave spectroscopy and Ab initio calculations

The rotational spectra of four (GT, TT, TG, and GG) of the five possible conformers of 1-fluorobutane have been assigned by combining free jet and conventional microwave spectroscopy. The geometry optimization was performed at the MP2 (full) level of theory with the 6-31G (d) and 6-311G (d, p) basis sets and by using the B3LYP (3df, 3pd) density functional method. The relative stability of the five rotamers is calculated at the QCISD (T)/6-311G (d, p) level of theory. In spite of the fact that ab initio calculations indicated the unobserved GG' conformer to be more stable than at least one of the observed conformers it was not possible to detect its rotational spectrum. GT and TG are the most and the least stable species, respectively. The rotational spectra of several vibrational satellites of the four conformers have been studied by conventional microwave spectroscopy. The overall conformational equilibrium is governed by the two-dimensional potential energy surface of the skeletal torsions MeC-CC and FC-CC, which have been evaluated by a flexible model analysis, based on the experimental values of the relative conformational and vibrational energy spacings, and on the shifts of second moments of inertia upon conformational change and vibrational excitation. The relative energy of the fifth stable conformer (GG') was determined to be 333 cm(-1) from flexible model calculations, and to be 271 cm(-1) from the most accurate ab initio calculations.     

Gas-phase structure of N,N-dimethylglycine.

Three conformers of the neutral amino acid N,N-dimethylglycine [(CH3)2NCH2COOH] were detected in a supersonic expansion by a combination of laser ablation (LA) and molecular-beam Fourier transform microwave (MB-FTMW) spectroscopy. A bifurcated methyl-to-carbonyl (C--HO==C) weak intramolecular hydrogen bond might stabilise the most stable conformer of C(s) symmetry. The second most stable conformer of C1 symmetry has a hydrogen bond between the hydroxyl group and the lone pair at the nitrogen atom (NH--O). The r(s) and r0 structures were derived for this conformer from the rotational data for the parent and six minor 13C, 15N and OD isotopomers. A third conformer exhibits a cis-carboxyl functional group and C1 symmetry. Ab initio MP2/6-311++G(d,p) predictions of the spectroscopic parameters were useful in analysing the spectra. In particular, the agreement of the nuclear quadrupole coupling constants with those calculated was conclusive in identifying the different conformers.     

Conformers, infrared spectrum and UV-induced photochemistry of matrix-isolated furfuryl alcohol

The infrared spectra of furfuryl alcohol (2-furanmethanol, FFA) were investigated for FFA monomers isolated in low-temperature argon matrices. The structural interpretation of the obtained experimental spectra was assisted by analysis of the molecule's conformational landscape. According to the DFT(B3LYP)/6-311++G(d,p) calculations, five different minimum energy structures were found on the potential energy surface of the molecule. They can be defined by the orientation of the OCCO and CCOH dihedral angles: GG', GG, TG, TT, GT (G = +gauche, G' = -gauche, T = trans) and have a symmetry equivalent configuration: GG' = G'G, GG = G'G', TG = TG', GT = G'T. When zero-point energies are taken into account, only three (GG', GG, and TT) out of the five unique minima correspond to stable structures. The most stable conformer GG' (OCCO, 72.7°; CCOH, -59.3°), which in gas phase at room temperature accounts for ～65% of the total population, was the only form isolated in the argon matrices at 14 K. The other two relevant forms convert into conformer GG' during matrix deposition. The low temperature glassy and crystalline states of FFA were also obtained and their infrared spectra assigned, suggesting the sole existence of the GG' conformer also in these phases. The photochemical behavior of FFA induced in situ, by tunable UV-laser, was also studied. The longest wavelength resulting in photochemical changes in the structure of the irradiated sample was found to be λ = 229 nm. Such UV irradiation of the matrix-isolated FFA led to production of formaldehyde and different isomeric C(4)H(4)O species. Cycloprop-2-ene-1-carbaldehyde and buta-2,3-dienal (two conformers) are the main initial C(4)H(4)O photoproducts formed upon short-time excitation at λ = 229 nm. But-3-ynal (two conformers) was the principal photoproduct resulting from prolonged excitation at λ= 229 nm, being consumed upon irradiation at shorter wavelengths (λ < 227.5 nm). Vinyl ketene is produced from FFA in the trans conformation and undergoes isomerization to the cis form upon irradiation at λ < 227.5 nm. Cyclopropene, propyne, allene, and CO were also identified in the irradiated matrices (in particular at the later stages of irradiation), suggesting that the photoproduced aldehydes partially decarbonylate during the performed photochemical experiments.     

Matrix isolation and low temperature solid state FTIR spectroscopic study of alpha-furil

Alpha-furil [C(4)H(3)O-C(=O)-C(=O)-C(4)H(3)O] has been isolated in argon and xenon matrices and studied by FTIR spectroscopy, supported by DFT(B3LYP)/6-311++G(d,p) calculations. The obtained spectra were fully assigned and revealed the presence in the matrices of three different conformers, all of them exhibiting skewed conformations around the intercarbonyl bond with the two C(4)H(3)O-C(=O) fragments nearly planar. The three conformers differ in the orientation of the furan rings relative to the carbonyl groups: the most stable conformer, I (C(2) symmetry; O=C-C=O intercarbonyl dihedral equal to 153.1 degrees), has both furan rings orientated in such a way that one of their beta-hydrogen atoms approaches the oxygen atom of the most distant carbonyl group, forming two H-C=C-C-C=O six-membered rings; the second most stable conformer, II (C(1) symmetry; O=C-C=O intercarbonyl dihedral equal to 126.9 degrees ), has one furan ring orientated as in I, while the second furan group is rotated by ca. 180 degrees (resulting in an energetically less favourable H-C=C-C=O five-membered ring); in the third conformer, III (C(2) symmetry; O=C-C=O dihedral equal to 106.2 degrees ), both furan rings assume the latter orientation relative to the dicarbonyl group. The theoretical calculations predicted the two higher energy forms being 5.85 and 6.22 kJ mol(-1) higher in energy than the most stable form, respectively, and energy barriers for conformational interconversion higher than 40 kJ mol(-1). These barriers are high enough to prevent observation of conformational isomerization for the matrix isolated compound. The three possible conformers of alpha-furil were also found to be present in CCl(4) solution, as well as in a low temperature neat amorphous phase of the compound prepared from fast condensation of its vapour onto a suitable 10 K cooled substrate. On the other hand, in agreement with the available X-ray data [S. C. Biswas, S. Ray and A. Podder, Chem. Phys. Lett., 1987, 134, 541], the IR spectra obtained for the neat low temperature crystalline state reveals that, in this phase, alpha-furil exists uniquely in its most stable conformational state, I.     

Conformational stability from variable temperature infrared spectra of rare gas solutions of 2-chloro-3-fluoropropene

Variable temperature (-55 to -155 degrees C) infrared spectra of rare gas solutions of 2-chloro-3-fluoropropene, H2C=C(Cl)CH2F, have been recorded from 3500 to 400 cm(-1). The relative intensities of 16 conformer pairs at ten different temperatures of a krypton solution have been measured and from these data an enthalpy difference of 271+/-27 cm(-1) (3.17+/-0.32 kJ x mol(-1)) has been obtained with the cis conformer the more stable form. Similar studies were also carried out in xenon and an enthalpy difference of 334+/-33 cm(-1) (4.00+/-0.39 kJ x mol(-1)) was obtained again with the cis conformer the more stable form. It is estimated that there is 35+/-2% of the gauche conformer present at ambient temperature. However, in the crystalline solid the gauche conformer is the stable rotamer. Extensive ab initio calculations with full electron correlation by the perturbation method at the MP2 level with a variety of basis sets as well as density functional theory calculations (DFT) by the B3LYP method have been carried out. Several of these calculations predict an energy difference in the range of 400 cm(-1) with the cis form the more stable conformer but most of the predicted energy differences are significantly larger than the experimentally determined value. The spectroscopic and theoretical results are discussed and compared with the corresponding quantities for some similar molecules.     

Vibrational spectra and rotational isomerism of bis(N-2-chloroethyl)nitramine

The IR and Raman spectra of bis(N-2-chloroethyl)nitramine (BCENA) in the liquid and crystalline states and in CCl₄ and CH₃CN solutions are studied. The spectra are compared, and it is concluded that BCENA exists as a mixture of conformers of different polarities in the liquid state and as one less polar conformer in the crystalline state. To determine the conformations corresponding to the total electron energy minima and interpret the vibrational spectrum of BCENA, we performed an ab initio quantum chemical calculation of the BCENA molecule in the Hartree-Fock approximation using the 3–21G* and 6–31G* basis sets. Out of twelve possible conformations five are stable; the most stable conformer is C₂(GG). The frequencies and forms of normal vibrations of stable conformers are calculated using scaled quantum chemical force fields. The calculated and experimental frequencies are compared, and the relations between the frequencies of skeletal stretching and bending vibrations are analyzed. It is concluded that the BCENA crystal is formed by the C₂ (GG) conformer. The vibrational spectrum is interpreted, and the frequencies are assigned to vibrations of conformers differing in form. Translated fromZhumal Struktumoi Khimii, Vol. 38, No. 2, pp. 303–317, March–April, 1997.     

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.     

Photoelectron imaging of propanal by resonant multiphoton ionization via the 3s Rydberg state

We report the conformationally- and vibrationally-selected photoelectron spectroscopy of propanal obtained by resonance-enhanced multiphoton ionization (REMPI) using photoelectron imaging. These photoelectron spectra, employing (2 + 1) ionization via the (n, 3s) Rydberg transitions in the range from 365 to 371 nm, confirm that there are two stable conformer origins in the lowest ionic state, the cis conformer with a co-planar CCCO geometry and a gauche conformer with a approximately 119 degrees CCCO dihedral angle. From ab initio calculations at the B3LYP/6-311++G** level, we find the gauche conformer is slightly more stable, with the energy difference between two conformers determined to be only 65 cm(-1). In our photoelectron spectra, the vertical ionization potential (IP) for the cis conformer of propanal was then determined to be 9.999 (+/-0.003) eV, while that of the gauche conformer of propanal was estimated to be 9.944 eV. A long vibrational progression in the in-plane CCCO deformation vibrational mode, v, for the cis conformer is systematically observed in all photoelectron spectra in which this mode is excited, suggesting that the geometry of the ground ionic state is significantly different from that of the 3s Rydberg state, particularly along the v(15) coordinates.     

Is 2-cyclopropylpropene really gauche?

Infrared spectra of gaseous and solid 2-cyclopropylpropene (2-CPP, c-C3H5C (CH3)CH2) have been recorded from 3500 to 40 cm-1, and Raman spectra (3200-150 cm-1) of the liquid as well as mid-infrared spectra of 2-CPP in liquid krypton solution (from -105 to -150 degrees C) were also obtained. Ab initio calculations, with basis sets up to 6-311+G(2df, 2pd), were carried out for this molecule, using the restricted Hartree-Fock (RHF) approach, with full electron correlation by the perturbation method to second order (MP2(full)) and density functional theory (DFT) by the B3LYP method. The combination of the experimental and computational results (particularly with the higher basis sets) unequivocally identifies the more stable conformer of 2-CPP as the trans form, with the gauche rotamer higher in energy, but also stable. The cis structure of this compound is not observed experimentally, and is predicted by the computational approaches to be a transition state. By studying the temperature variation of two well-resolved sets of conformational doublets of 2-CPP dissolved in liquid krypton, an average enthalpy difference between conformers of 182+/-18 cm-1 (2.18+/-0.22 kJ mol-1) has been determined, with the trans conformation lower in energy in the fluid states, and the sole conformer present in the polycrystalline solid phase. This enthalpy difference corresponds to an ambient temperature conformational equilibrium in the fluid phases of 2-cyclopropylpropene containing approximately 55+/-2% of the more stable trans rotameric form. A complete vibrational assignment for the trans conformer of 2-CPP is given, and many of the bands of the gauche rotamer have also been assigned. Structural parameters, dipole moments, and rotational constants for this molecule have been calculated at the MP2(full)/6-311+G(d,p) level, and these results--as well as the results from the experimental studies--are compared to similar quantities in related compounds.     

A microwave and quantum chemical study of the conformational properties and intramolecular hydrogen bonding of 1-fluorocyclopropanecarboxylic acid

The structural and conformational properties of 1-fluorocyclopropanecarboxylic acid have been explored by microwave spectroscopy and a series of ab initio (MP2/6-311++G(d,p) level), density functional theory (B3LYP/aug-cc-pVTZ level), and G3 quantum chemical calculations. Four "stable" conformers, denoted conformers I-IV, were found in the quantum chemical calculations, three of which (conformers I -III) were predicted to be low-energy forms. Conformer I was in all the quantum chemical calculations predicted to have the lowest energy, conformer III to have the second lowest energy, and conformer II to have the third lowest energy. Conformers II and III were calculated to have relatively large dipole moments, while conformer I was predicted to have a small dipole moment. The microwave spectrum was investigated in the 18-62 GHz spectral range. The microwave spectra of conformers II and III were assigned. Conformer I was not assigned presumably because its dipole moment is comparatively small. Conformer II is stabilized by an intramolecular hydrogen bond formed between the fluorine atom and the hydrogen atom of the carboxylic acid group. Conformer III has a synperiplanar orientation for the F-C-C=O and H-O-C=O chains of atoms. Its dipole moment is: mua = 3.4(10), mub = 10.1(13), and muc = 0.0 (assumed) and mu(tot) = 10.6(14) x 10(-30) C m [3.2(4) D]. Several vibrationally excited states of the lowest torsional mode of each of II and III were also assigned. The hydrogen-bonded conformer II was found to be 2.7(2) kJ/mol less stable than III by relative intensity measurements. Absolute intensity measurements were used to show that the unassigned conformer I is the most abundant form present at a concentration of roughly 65% at room temperature. Conformer I was estimated to be ca. 5.0 kJ/mol more stable than the hydrogen-bonded rotamer (conformer II) and ca. 2.3 kJ/mol more stable than conformer III. The best agreement with the theoretical calculations is found in the MP2 calculations, which predict conformer I to be 5.1 kJ/mol more stable than III and 1.7 kJ/mol more stable than II.     

Infrared and Raman spectra, conformational stability, ab initio calculations of structure, and vibrational assignment of ethynylmethyl cyclobutane.

The infrared spectra (3200-300 cm(-1)) of the gas and solid and the Raman spectra (3200-30 cm(-1) of the liquid with quantitative depolarization values and the solid have been recorded of ethynylmethyl cyclobutane (cyclobutylmethyl acetylene), c-C4H7CH2C[ triple bond]CH. Both the equatorial and the axial conformers have been identified in the fluid phases and both the gauche and trans conformations of the methyl acetylenic group have been identified for each ring conformer. 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 differences have been determined to the 112 +/- 11 cm(-1) (1.34 +/- 0.13 kJ mol) between the most stable equatorial-trans (Et) conformer and the equatorial-gauche (Eg) conformer which is the second most stable conformer and 327 +/- 35 cm(-1) (3.91 + 0.42 kJ/mol) with the axial-gauche (Ag) conformer which is the least stable conformer. The enthalpy difference between the axial-trans (At) and the equatorial-gauche(Eg) is 56 +/- 6 cm(-1). At ambient temperature there is approximately 33% of the Et conformer, 38% of the Eg form, 15% of the At and 14% of the Ag conformer. For the polycrystalline solid the Eg conformer is the only form present which probably results form the packing in the crystal. A complete vibrational assignment is proposed for both equatorial conformers and additionally a few of the fundamentals of the At and Ag conformers have been assigned. The vibrational assignments are supported by normal coordinate calculations utilizing the force constants from ab initio MP2/6-31G(d) calculations. Complete equilibrium geometries have been determined for all four possible rotamers by ab initio calculations employing the 6-31G(d) and 6-311 + + G(d,p) basis sets at levels of restricted Hartree-Fock (RHF) and /or Moller-Plesset (MP2) with full electron correlation by the perturbation method to second order. The results are discussed and compared to those obtained for some similar molecules.     

Conformational studies of cyclopropylmethyl isothiocyanate from temperature-dependent FT-IR spectra of rare gas solutions and ab initio calculations

Variable temperature (-55 to -150 degrees C) studies of the infrared spectra (3200-100 cm(-1)) of cyclopropylmethyl isothiocyanate, c-C(3)H(5)CH(2)NCS, dissolved in liquefied rare gases (Xe and Kr), have been carried out. The infrared spectra of the gas and solid, as well as the Raman spectrum of the liquid, have also been recorded from 3200 to 100 cm(-1). By analyzing six conformer pairs in xenon solutions, a standard enthalpy difference of 228 +/- 23 cm(-1) (2.73 +/- 0.27 kJ.mol(-1)) was obtained with the gauche-cis (the first designation indicates the orientation of the CNCS group with respect to the three-membered ring, the second designation indicates the relative orientation of the NCS group with respect to the bridging C-C bond) rotamer the more stable form, and it is also the only form present in polycrystalline solid. Given statistical weights of 2:1 for the gauche-cis and cis-trans forms (the only stable conformers predicted); the abundance of cis-trans conformer present at ambient temperature is 14 +/- 2%. The potential surface describing the conformational interchange has been analyzed, and the corresponding two-dimensional Fourier coefficients were obtained. From MP2 ab initio calculations utilizing various basis sets with diffuse functions, the gauche-cis conformer is predicted to be more stable by 159-302 cm(-1), which is consistent with the experimental results. However, without diffuse functions, the conformational energy differences are nearly zero even with large basis sets. For calculations with density functional theory by the B3LYP method, basis sets without diffuse functions also predict smaller energy differences between the conformers, although not nearly as small as the MP2 results. 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, and infrared and Raman intensities have been predicted from ab initio calculations and compared to the experimental values when applicable; the r(0) structural parameters are also estimated. The energies for the linear CNCS moiety were calculated. These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.     

High resolution millimeter-wave spectroscopy of cyclopropylphosphine-borane

The microwave spectrum of cyclopropylphosphine-borane, C(3)H(5)PH(2)-BH(3), has been investigated in the frequency range 150-195 GHz. The spectral assignment was supported by high level ab initio calculations. Two stable conformations have been predicted: the most stable antiperiplanar form and synclinal form that is higher in energy by 7.3 kJ/mol. In the observed spectra, only the most stable antiperiplanar (ap) form has been assigned. The analysis of the rotational spectra in the lowest excited vibrational states of the ap conformer has enabled determination of the potential function for the C-P torsional mode in the vicinity of equilibrium position. The barrier to internal rotation of the BH(3) top has been determined to be 9.616(15) kJ/mol and agrees well with quantum chemical calculations.     

Spectra and structure of silicon containing compounds. XXXIX. Raman and infrared spectra,conformational stability,vibrational assignment and ab initio calculations of n-propyltrifluorosilane

The infrared (3100-40 cm(-1)) spectra of gaseous and solid and Raman (3200-20 cm(-1)) spectra of liquid with qualitative depolarization values and solid n-propyltrifluorosilane, CH(3)CH(2)CH(2)SiF(3), have been recorded. Additionally the infrared spectra of the sample in nitrogen and argon matrices have been recorded. Both the anti and gauche conformers have been identified in the fluid phases but only the anti conformer remains in the solid. Variable temperature (-105 to -150 degrees C) studies of the infrared spectra of the sample dissolved in liquid krypton have been recorded and the enthalpy difference has been determined to be 135+/-14 cm(-1) (1.62+/-0.17 kJ mol(-1)) with the anti conformer the more stable form. At ambient temperature it is estimated that there is 51+/-2% of the gauche conformer present. Also the enthalpy difference in the liquid was obtained from variable temperature studies of the Raman spectra and from three conformer pairs an average value of 179+/-18 cm(-1) (2.14+/-0.22 kJ mol(-1)) was obtained again with the anti form the more stable conformer. Relatively complete vibrational assignments are proposed for both conformers based on the relative infrared and Raman spectral intensities, infrared band contours, depolarization ratios which are supported by normal coordinate calculations. The geometrical parameters, harmonic force constants, vibrational frequencies, infrared intensities, Raman activities, depolarization ratios, and energy differences have been obtained for the anti and gauche conformers from ab initio MP2/6-31G(d) calculations. Structural parameters and energy differences have also been obtained utilizing the larger 6-311+G(d, p) and 6-311+G(2d, 2p) basis sets. By utilizing the previously reported microwave rotational constants for five isotopomers of CH(3)SiF(3) along with ab initio predicted structural values, r(0) parameters have been obtained for methyltrifluorosilane. Similarly, from the ab initio predicted parameters "adjusted r(0)" parameters have been estimated for both conformers of n-propyltrifluorosilane. The results are discussed and compared with those obtained for some similar molecules.     

Stereomutations of two-bladed propeller derivatives: ortho-substituted diaryl ethylene and diaryl ketone

Stereochemical analysis, supported by ab initio computations, predicts the existence of three possible stable helical conformers for o,o'-diisopropyl-1,1'-diphenylethylene (1) and o,o'-diisopropylbenzophenone (2). At low temperature the NMR spectra of 1 showed distinct sets of signals for these conformers, thus allowing the measurement of the three barriers involved in the related stereomutation processes to be obtained (DeltaG=6.45, 4.65, and < or =4.0 kcal mol(-1)). The NMR spectra also indicate that the asymmetric conformer (C1 point group) is the most stable one in solution, as anticipated by calculations. X-ray diffraction confirmed that this structure is that adopted in the crystalline state. On the other hand, o,o'-diisopropylbenzophenone (2) is predicted by calculations to exist essentially as a C2-type conformer, a result that was confirmed by the low-temperature NMR spectra. The interconversion barrier for the enantiomeric forms of this conformer was also measured (DeltaG=6.3(5) kcal mol(-1)).     

Conformational stability from variable temperature infrared spectra of krypton solutions, ab initio calculations, vibrational assignment, and r0 structural parameters of 1,3-difluoropropane

The infrared spectra (3200-400 cm(-1)) of krypton solutions of 1,3-difluoropropane, FCH2CH2CH2F, at variable temperatures (-105 to -150 degrees C) have been recorded. Additionally, the infrared spectra (3200-50 cm(-1)) of the gas and solid have been recorded as well as the Raman spectrum of the liquid. From a comparison of the spectra of the fluid phases with that in the solid, all of the fundamental vibrations of the C2 conformer (gauche-gauche) where the first gauche indicates the form for one of the CH2F groups and the second gauche the other CH2F, and many of those for the C1 form (trans-gauche) have been identified. Tentative assignments have been made for a few of the fundamentals of the other two conformers, i.e. C2v (trans-trans) and Cs (gauche-gauche'). By utilizing six pairs of fundamentals for these two conformers in the krypton solutions, an enthalpy difference of 277 +/- 28 cm(-1) (3.31 +/- 0.33 kJ mol(-1)) has been obtained for the C2 versus C1 conformer with the C2 conformer the more stable form. For the C2v conformer, the enthalpy difference has been determined to be 716 +/- 72 cm(-1) (8.57 +/- 0.86 kJ mol(-1)) and for the Cs form 971 +/- 115 cm(-1) (11.6 +/- 1.4 kJ mol(-1)). It is estimated that there is 64 +/- 3% of the C2 form, 34 +/-3% of the C1 form, 1% of the C2v form and 0.6% of the Cs conformer present at ambient temperature. Equilibrium geometries and total energies of the four stable conformers have been determined from ab initio calculations with full electron correlation by the perturbation method to second order as well as by hybrid density functional theory calculations with the B3LYP method using a number of basis sets. The MP2 calculations predict the C1 conformer stability to be slightly higher than the experimentally determined value whereas for the C2v and Cs conformers the predicted energy difference is much larger than the experimental value. The B3LYP calculations predict a better energy difference for both the C1 and C2v as well as for the Cs conformers than the MP2 values. A complete vibrational assignment is proposed for the C2 conformer and many of the fundamentals have been identified for the C1 form based on the force constants, relative intensities and rotational-vibrational band contours obtained from the predicted equilibrium geometry parameters. By combining previously reported rotational constants for the C2 and C1 conformers with ab initio MP2/6-311 + G(d, p) predicted parameters, adjusted r0 parameters have been obtained for both conformers. Comparisons are made with the parameters obtained for some other molecules containing the FCH2 group. The spectroscopic and theoretical results are compared to the corresponding properties for some similar molecules.