On the relative intensities of the Raman active fundamentals, r₀ structural parameters, and pathway of chair–boat interconversion of cyclohexane and cyclohexane‐d₁₂

Abstract

Raman spectra of liquid cyclohexane, C 6 H 12 , and deuterated cyclohexane, C 6 D 12 , were recorded with both parallel and perpendicular polarizations. The observed vibrational wavenumbers, depolarization ratios, and their intensities were measured and compared with the corresponding predicted values as well as the experimental values previously reported. The conformational energetics were obtained with the Møller-Plesset perturbation method to the second order [MP2(full)] as well as with density functional theory by the B3LYP method utilizing a variety of basis sets. The average ab initio predicted difference in energy between the more stable chair form (D 3d ) and the less stable twisted-boat form (D 2 ) is 2213 cm −1 (26.47 kJ/mol), with a similar value of 2223 cm −1 (26.59 kJ/mol) from the density function theory calculations. By using two dihedral angles as variables, we calculated the chair-boat interconversion pathway for cyclohexane at the MP2(full)/6-31G(d) level. The harmonic force constants, Raman intensities, depolarization values, and the potential energy distribution were predicted from both MP2(full) and B3LYP calculations with the 6-31G(d) basis set and compared with the experimental values for the chair form when available. The 'adjusted' r 0 structural parameters were obtained from MP2/6-311+G(d,p) calculations and previously reported microwave rotational constants of five isotopomers of cyclohexane: (equatorial and axial). The determined distances in Å are: r(CC) = 1.536(3), r(CH) ax = 1.098(1); r(CH) eq = 1.095(1); and the angles in degrees: ∠CCH ax = 108.8(3); ∠CCH eq = 110.2(3); ∠CCC = 111.1(3); and ∠HCH = 107.6(3) with dihedral angle ∠CCCC = 55 .7(3). These values are compared with those previously reported and it is found that the difference in the r 0 distances (0.003 Å) between the two CH values is much smaller than the difference (0.008 Å) previously reported for the r s values.

Conformational stability from variable temperature infrared spectra of xenon solution, r0 structural parameters, and vibrational assignment of fluorocyclohexane

“…It should be noted that the assignments are only approximate descriptions and the important point is that the fundamental frequencies for each conformer in the fingerprint region could be confidently assigned so that the bands chosen for the enthalpy determination corresponded to the conformer for which they were being utilized. The assignments for the six C-C stretches in the fingerprint region can be made with a comparison to their assignment for cyclohexane [1]. For the assignment of these modes in cyclohexane there is mixing of CH 2 which has resulted in different descriptions for the two Raman lines at 798 and 1037 cm À1 (E g ) as well as a similar problem with the 800 and 1166 cm À1 bands in the A 1g vibrations.…”

“…The most stable conformation of cyclohexane is the chair form with a very large energy difference from the intermediate twisted forms by predicted ab initio MP2 with full electron correlation to have an energy difference [1] of 2213 ± 16 cm À1 (6.33 ± 0.05 kcal/ mol) and for the boat form an energy difference of 2768 ± 16 cm À1 (7.91 ± 0.04 kcal/mol). Thus, most substitution on cyclohexane results in the formation of equatorial and axial conformers and there has been a significant amount of research carried out for the determination of the energy difference between these two conformers.…”

Conformational stability from Raman spectra, r0 structural parameters, and vibrational assignment of methylcyclohexane

“…We have found that good structural parameters for hydrocarbons and many substituted ones can be determined by adjusting the structural parameters obtained from the ab initio MP2/6-311+G(d,p) calculations to fit the rotational constants obtained from microwave experimental data by using a computer program ''A&M'' (Ab initio and microwave) developed [20] in our laboratory. Therefore, we recently obtained [1] the structural parameters for cyclohexane by combining the 15 rotational constants previously reported from the microwave with the predicted parameters from ab initio MP2(full)/6-311+G(d,p) calculations with the fit of Table 4 and the difference between the predicted CC bond distance of 1.530 Å and the adjusted r 0 value of 1.536(3) Å is 0.006 Å. The r 0 distance is larger than the corresponding r s distance of 1.530(3) Å, but it is in excellent agreement with the value (1.535(1) Å) for this parameter from the Raman spectrum.…”

“…The most stable conformation of cyclohexane is the chair form with a very large energy difference from the boat form by predicted ab initio MP2 calculations of 2768 ± 16 cm −1 (33.16 ± 0.21 kJ/mol) and with the intermediate twisted form predicted to have an energy difference [1] of 2213 ± 18 cm −1 (26.50 ± 0.21 kJ/mol). Therefore, most monosubstitution on cyclohexane results in the formation of equatorial and axial forms.…”

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

“…Additionally, we were interested in the structural parameters for comparison to those of cyclohexane which we have obtained [4] from combining ab initio calculations with microwave rotational constants.…”