3-chloro-1-butene: gas-phase molecular structure and conformations as determined by electron diffraction and by molecular mechanics and ab initio calculations

Gaseous 3-chloro-1-butene has been studied experimentally by electron diffraction (ED) at 20 and 180°C, and at these temperatures, 76(10)% and 62(10)%, respectively, of the most stable conformer i.e. the one having a hydrogen atom eclipsing the double bond, were found. The conformer with the chlorine atom eclipsing the C=C bond was also present. However, from the experimental data it was not possible to establish conclusive evidence for the conformer with an eclipsed CH₃ group. Molecular mechanics (MM) calculations and ab initio calculations using a 4-21 basis set were carried out with complete geometry optimization, and calculated parameters from each of the methods were used in combination with the ED data. Such calculations indicated the existence of all three conformers mentioned above. Least-squares analysis including constraints from the ab initio calculation gave as a result the following molecular structure (r_a distances and ??? angles) for the predominant conformer: r(C=C) = 1.337(6) Å, r(=C---C) = 1.503(4) Å, r(C---CH₃) = 1.522 Å, R(C---Cl) = 1.813(4) Å, <r(C---H)> = 1.089(18) Å, ???C=C---C = 122.9(2.1)°, ???C---C---C = 112.6(2.2)°, ???=C---C---Cl = 109.9(0.2)°, ???Cl---C---CH₃ = 109.3°. = 121.9° and = 110.0(1.3)°. The torsional angles were then τ(C=C---C---Cl> = −119.4° and τ(C=C---C---CH₃) = 120.3(2.1)°. Error limits are 2σ (σ includes estimates of systematic errors and correlations), parameters without quoted uncertainties are dependent or were constrained relative to another parameter. Combining the ED data with MM results yielded parameters consistent with those given above.     

Structure cristalline et conformation en solution aqueuse de la carnosine (β-alanyl-L-histidine)

Carnosine (β-alanyl-L-histidine) is a biologically active molecule involved in muscular metabolism. It crystallises in the C; space group with a = 24.725 Å b = 5,427 Å c = 8,004 Å β = 100,2° (Z = 4)

In the crystal, acid and basic groups are engaged in hydrogen bonds whose strength is evaluated through IR frequencies. Molecular conformation in the solid state is defined by τ₁ = /t-177° τ₂ = −38° φ = −96° ψ = +131° χ₁ = 181° χ₂₁ = 62°

NMR study of carnosine in aqueous solution indicates that rotation about CH₂-CH₂ is free and that the other angles take the following values: Ø −150° or −90° and X₁ = 165° or 315°. Infrared and Raman spectra suggest that τ₂ undergoes small changes when going from crystal to solution while ψ is close to +150°.     

(Trifluoromethyl)sulfanyl sulfinylimine, CF3SNSO. Structure and conformation in the electronic ground and excited states

The molecular structure of (trifluoromethyl)sulfanyl sulfinylimine, CF₃---S---N=S=O, was determined by gas electron diffraction (GED) and ab initio calculations (HF/3–21G*, HF/6–31+G* and MP2/6–31+G*). Experimental and theoretical methods result in a structure with planar C---S---N=S=O skeleton (C_s symmetry), anti orientation of the S---C bond relative to the N=S bond and syn orientation of the S---N bond relative to the S=O double bond (anti-syn structure). The following skeletal parameters (r_a values with 3σ uncertainties) were derived in the GED analysis: S---C, 1.831(4) Å; S---N, 1.684(5) Å; N=S, 1.538(6) Å; S=O, 1.453(6) Å; C---S---N, 94.6(8)°; S---N=S, 120.6(6)°; N=S=O: 116.5(8)°. A normal coordinate analysis based on FTIR (gas), FTIR (matrix) and Raman (liquid) spectra was performed. The UV (gas) spectrum was recorded and interpretation of the resonance Raman effect leads to the conclusion that the molecular symmetry (C_s) is retained upon electronic excitation.     

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.     

Ab initio structural trends and torsional sensitivity in n-hexane and comparisons with crystallographic structural results

The molecular structures of n-hexane were determined by RHF/4-21G ab initio geometry optimization at 30° grid points in its three-dimensional τ₁(C11–C8–C5–C1), τ₂(C14–C11–C8–C5), τ₃(C17–C14–C11–C8) conformational space. Of the resulting 12×12×12=1728 grid structures, 468 are symmetrically non-equivalent and were optimized constraining the torsions τ₁, τ₂, and τ₃ to the respective grid points, while all other structural parameters were relaxed without any constraints. From the results, complete parameter surfaces were constructed using natural cubic spline functions, which make it possible to calculate parameter gradients, |P|=[(∂P/∂τ_i)²+(∂P/∂τ_j)²]^1/2, where P is a C–C bond length or C–C–C angle. The parameter gradients provide an effective measure of the torsional sensitivity of the system and indicate that dynamic activities in one part of the molecule can significantly affect the density of states, and thus the contributions to vibrational entropy, in another part. This opens the possibility of dynamic entropic conformational steering in complex molecules; i.e. the generation of free energy contributions from dynamic effects of one part of a molecule on another. When the conformational trends in the calculated C–C bond lengths and C–C–C angles are compared with average parameters taken from some 900 crystallographic structures containing n-hexyl fragments or longer C–C bond sequences, some correlation between calculated and experimental trends in angles is found, in contrast to the bond lengths for which the two sets of data are in complete disagreement. The results confirm experiences often made in crystallography. That is, effects of temperature, crystal structure and packing, and molecular volume effects are manifested more clearly in bond lengths than bond angles which depend mainly on intramolecular properties. Frequency analyses of the τ₁, τ₂ and τ₃ torsional angles in the crystal structures show conformational steering in the sense that, if τ₁ is trans peri-planar (170°≤τ₁≤180°; −180°≤τ₁≤−170°), the values of τ₂ and τ₃ are clustered closely around the ideal gauche (±60°) and trans (±180°) positions. In contrast, when τ₁ is in the region (50°≤τ₁≤70°), there is a definite increase in the populations of τ₂ and τ₃ at −90 and −150°.     

Molecular structure and conformational stability of allylisocyanate—post-Hartree–Fock and density functional theory studies

The molecular structure and conformational stability of allylisocyanate (CH₂CHCH₂NCO) molecule was studied using the ab initio and DFT methods. The geometries of possible conformers, C-gauche (δ=120°, θ=0°) (δ=C=C–C–N and θ=C–C–N=C) and C-cis N-trans (δ=0° and θ=180°) were optimized employing HF/6-31G^*, MP2/6-31G^* levels of theory of ab initio and BLYP, B3LYP, BPW91 and B3PW91 methods of DFT implementing the atomic basis set 6-311+G(d,p). The structural and physical parameters of the above conformers were discussed with the experimental and theoretical values of the related molecules, methylisocyanate and 3-fluoropropene. It has been found that the N=C=O bond angle is not linear as the experimental result for both the conformers and the theoretical bond angle is 173°. The rotational potential energy surfaces have been performed at the HF/6-31G^*, and MP2/6-31G^* levels of theory. The Fourier decomposition potentials were analysed at the HF/6-31G^*, and MP2/6-31G^* levels of theory. The HF/6-31G^* level of theory predicted that the C-gauche conformer is more stable than the C-cis N-trans conformer by 0.41 kJ/mol, but the MP2 and DFT methods predicted the C-cis N-trans conformer is found to be more stable than the C-gauche conformer. The calculated chemical hardness value at the HF/6-31G^* level of theory predicted the C-cis N-trans form is more stable than C-gauche form, whereas the chemical hardness value at the MP2/6-31G^* level of theory favours the slight preference towards the C-gauge conformer.     

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 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.     

Twisted amides: X-ray crystallographic and theoretical study of two acylated glycolurils with aromatic substituents

X-ray crystallography and theoretical analysis were applied to explore the molecular basis for the efficient and selective Claisen-like condensations of diacylglycolurils. The crystal structures of 1-acetyl-6-benzoyl-3,4,7,8-tetramethylglycoluril (4b), and of 1-(3′-oxo-3′-phenylpropionyl)-3,4,7,8-tetramethylglycoluril (5b), the product of base-promoted intramolecular condensation of 4b, were obtained by X-ray diffraction. The acetyl (Ac) group in 4b is essentially coplanar with the attached tetrahydroimidazolone ring of the glycoluril core (τ=7°), while the benzoyl (Bz) group is twisted by τ=45° relative to a plane through the ring to which it is bonded. Product 5b contains a flat amide (τ=7°). Ab initio energy optimizations of the experimental structures for 4b and 5b give optimized geometries which are not dramatically altered, suggesting that crystal packing effects are small. An atoms-in-molecules study of the delocalization of the Fermi hole reveals that electrons in the Bz C=O group of 4b are delocalized into the phenyl ring as well as into the urea moiety of the glycoluril core. This effect stabilizes the Bz over the Ac carbonyl group, and accounts for selective twisting of the Bz group. The Laplacian of the electron density reveals a non-bonded valence shell charge concentration at O of the Ac group, corresponding to a lone-pair region, aligned with a charge depletion in the valence shell of the Bz C=O carbon [(C15–O16C18)=113°]. The angle of approach [(O16C18=O19)] is 100°, equal to the angle for ideal nucleophilic attack on a carbonyl group. Oxygen atom O16 is thus poised to attack C18; only the O16C18 distance (3.248 Å) seems to prevent reaction. These results suggest that the same distance restraint may prevent O-acylation in the enolate intermediate 6b derived from4b. By contrast, the transition state for C-acylation, leading from 6b towards product 5b requires a different geometry, which may explain the observed selectivity for C-acylation in this enolate. The results show that, as 4b is converted to 5b, amide torsional strain is relieved, which may account for the high reactivity of 4b and the efficiency and irreversibility of this condensation process. This study provides a starting point for quantitative correlation of substrate structure in diacylglycolurils with kinetic data for the rearrangement reaction.     

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.     

Study of the N2 bΠu state via 1 + 1 multiphoton ionization

Medium-resolution spectra of the N₂ b¹Π_u-X¹Σ_g⁺ band system were recorded by 1 + 1 multiphoton ionization. In the spectra we found different linewidths for transitions to different vibrational levels in the b ¹Π_u state: Δν₀ = 0.50 ± 0.05 cm⁻¹, Δν₁ = 0.28 ± 0.02 cm⁻¹, Δν₂ = 0.65 ± 0.06 cm⁻¹, Δν₃ = 3.2 ± 0.5 cm⁻¹, Δν₄ = 0.60 ± 0.07 cm⁻¹, and Δν₅ = 0.28 ± 0.02 cm⁻¹. From these linewidths, predissociation lifetimes τ_ν were obtained: τ₀ = 16 ± 3 ps, τ₁ > 150 ps, τ₂ = 10 ± 2 ps, τ₃ = 1.6 ± 0.3 ps, τ₄ = 9 ± 2 ps, and τ₅ > 150 ps. Band origins and rotational constants for the b ¹Π_uν = 0 and 1 levels were determined for the ¹⁴N₂ and ¹⁴N¹⁵N molecules.     

The molecular structure of gaseous methyl vinyl ether at room temperature, studied by molecular orbital constrained electron diffraction and microwave spectroscopy

The gas phase molecular structure of methyl vinyl ether at room temperature has been studied by joint analysis of electron diffraction and microwave data. Constraints on geometrical and thermal parameters were derived from the geometry and force field of the s-cis form, obtained by ab-initio calculations (4–21 G basis set) after complete geometry relaxation. A range of models was investigated that fits all available data (infrared, microwave and electron diffraction). The following r_g/r-parameters were obtained: C=C: 1.337 Å, C(sp²)---O: 1.359 Å, C(sp³)---O: 1.427 Å, : 1.102 Å C=C---O : 127.3° and COC: 116.8°. Experimental r_g---r_e (ab initio) corrections are given for C=C, C(sp²)---O and Csp³)---O.

This investigation demonstrates that molecular orbital constrained electron diffraction is sufficiently reliable and in such a manner that it can be applied to more complicated problems.     

Ab initio studies of structural features not easily amenable to experiment : Part 40. Structural and conformational investigations of 2-butanone and 2-pentanone

The ab initio gradient refined 4-21G geometries of two conformations of 2-butanone and of six conformations of 2-pentanone are reported. The C---C---C=O torsional energies of both systems were determined with geometry optimization at each point and are compared with those previously calculated for some homologous aldehydes and carboxylic acids. In agreement with the structural trends known for C---H bonds in methyl groups adjacent to C=O, it is found that a C---C bond eclipsing an adjacent C=O bond is more stable and shorter than in a skew position (C---C---C= O = 120°). The sum total of the 4-21G results available for various systems may support the following general rule: in X--- C---C=O systems the C---X bond is relatively short when syn-coplanar with C==O (X---C---C= O = 0°), and relatively long when skew with C=O (X---C---C=O 120°).     

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.     

Structure of gaseous tetramethylsuccinic anhydride

Tetramethylsuccinic anhydride has been studied by gas electron diffraction. A nonplanar ring was obtained, with torsional angles φ(C5O1C2C3) = 10.6 ± 1.5°, φ(01C2C3C4) = 26.5 ± 3.2° and φ(C2C3C4C5) = 30.4 ± 4.0°. Torsional angles calculated by the molecular mechanics method were in fairly good agreement with the experimental values.     

An ab initio comparative structural study of alkenylphosphonic acid derivatives

A comparative study of the electronic structure and conformational properties of alkenylphosphonic acid derivatives with different substituents has been carried out by means of ab initio quantum mechanical methods. The ab initio calculations have been performed using different basis sets. A strongly polarized partial triple bond for the phosphoryl group has been found. A very weak π conjugation has been detected in the C=C/P=O system. Intramolecular hydrogen bonds have been found in 2-cis-carboxyvinylphosphonic acid.     

Conformational stabilities of dimethylmethoxyphosphine and dimethyl(methylthio)phosphine from temperature dependent infrared spectra of rare gas solutions

Variable temperature (−55 to −150°C) studies of the infrared spectra (3500 to 400 cm⁻¹) of dimethylmethoxyphosphine, (CH₃)₂POCH₃ and dimethyl(methylthio)phosphine, (CH₃)₂PSCH₃ dissolved in liquid krypton and/or xenon have been recorded. From these data, the enthalpy differences have been determined to be 393±50 cm⁻¹ (4.71±0.60 kJ/mol), for (CH₃)₂POCH₃ with the near-cis conformer the more stable rotamer and 80±10cm⁻¹ (0.96±0.12 kJ/mol) for (CH₃)₂PSCH₃ with the cis conformer the more stable form. Complete vibrational assignments are presented for both molecules, which are consistent with the predicted frequencies obtained from the ab initio MP2/6-31G(d) calculations. The optimized geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, and depolarization ratios have been obtained from RHF/6-31G(d) and/or MP2/6-31G(d) ab initio calculations. These quantities are compared to the corresponding experimental quantities when appropriate as well as with some corresponding results for some similar molecules.     

Implications of hyperconjugative effects on bond lengths of allylic systems. An NBO investigation

The influence of hyperconjugative interactions on bond lengths of some allylic compounds (H₂CCH–CH₂–M(CH₃)₃; M=C, Si, Ge) has been investigated through NBO calculations using ab initio and density functional methods. The optimized structural parameters, at the B3LYP/6-31+G(d,p) and HF/6-31+G(d,p) levels, showed a good agreement with the resonance theory. Partial geometry optimization with orbital interactions removed confirmed the observations and revealed that σ→σ* interactions, together with the more common σ→π* ones, play an important role in determining the variations in bond lengths on going from C to Ge.     

The molecular structure of N-sulfinyl trimethylsilanamine, (CH3)3SiNSO

The geometric structure of (CH₃)₃Si---NSO in the vapour phase has been determined by gas electron diffraction. The molecule possesses a planar Si---N=S=O skeleton with syn conformation. The Si(CH₃)₃ group staggers the N=S double bond. The following skeletal parameters (r_a distances and angles with 3σ errors limits) were obtained: Si---N 1.750(6)Å, N=S 1.508(5)Å, S=O 1.444(4)Å, Si---N=S 133.9(9)°, N=S=O 122.5(10)°. Ab initio calculations (HF/3−21G*) were performed for H₃Si---NSO and confirm the planar syn structure for sulfinyl silanamines.     

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.