Microwave spectra, r0 structural parameters, and conformational stability from xenon solutions of silylcyclohexane

Microwave, Raman, and Infrared Spectra; Adjusted r0 Structural Parameters; Conformational Stability; and Vibrational Assignment of Germylcyclohexane

“…The experimentally obtained enthalpy difference value of 453 ( 38 cm -1 compares well with the value of 413 ( 22 cm -1 obtained by averaging the results of the four largest basis set calculations using the MP2(full) method. However, as we have seen in other enthalpy difference calculations, 6,10 the B3LYP method produces results that are significantly higher than the experimental value, regardless of basis set, with an average from the four highest basis sets of 627 ( 5 cm -1 (Table 4). The smallest basis sets [6-31G(d) and 6-31+G(d)] gave results that were very different from the higher level calculations and were omitted from the average.…”

“…Additionally, the methyl-and silylcyclohexane studies were performed at low temperatures, and packing effects influence conformational preferences as the freezing point is approached. The difference between gas-phase and liquid-phase experimental determination can be seen 6,9 in the 100 cm -1 difference in the determined ∆H by the Raman study of the liquid and the infrared study of the xenon solution for silylcyclohexane.…”

“…The smallest basis sets [6-31G(d) and 6-31+G(d)] gave results that were very different from the higher level calculations and were omitted from the average. The small number of calculated orbitals, particularly in the case of a period 3 atom, likely contributes to this irregularity, which has been seen to a lesser extent in the study of silylcyclohexane 6 and was nonexistent in the study of methylcyclohexane. 10 As Oulette has postulated 11,12 and as we have found, 6,10 for group 4 monosubstituted cyclohexanes for which the electronegativity of the substituent group is not the dominating factor conformational preference is determined primarily by nonbonded repulsive interactions of axial hydrogens and the "over the plane" hydrogen of the substituent group.…”

“…The small number of calculated orbitals, particularly in the case of a period 3 atom, likely contributes to this irregularity, which has been seen to a lesser extent in the study of silylcyclohexane 6 and was nonexistent in the study of methylcyclohexane. 10 As Oulette has postulated 11,12 and as we have found, 6,10 for group 4 monosubstituted cyclohexanes for which the electronegativity of the substituent group is not the dominating factor conformational preference is determined primarily by nonbonded repulsive interactions of axial hydrogens and the "over the plane" hydrogen of the substituent group. The smaller methyl group has the shortest ring-substituent bond length of the series to the ring carbon and a larger CCH bond angle than both the silane and the germane (107.2 versus 103.2 and 103.4, respectively), placing the axial methyl hydrogen closer to the axial ring hydrogens than is found for the silane and germane in the axial conformation, due to the longer Si-C and Ge-C bond lengths and smaller SiCH and GeCH bond angles.…”

“…There have been several studies of the conformational preferences of monosubstituted cyclohexanes, c-C 6 H 11 X, in the last two decades, and many of these molecules (X = F, Cl, Br, CH 3 , SiH 3 ) were found to exist as chair−equatorial and chair−axial conformers in the gaseous and liquid states. Extensive results have been reported on the determination of the conformational enthalpy of these compounds by utilizing a variety of techniques, such as NMR, electron diffraction, microwave, – infrared, – and Raman ,, spectroscopies, along with theoretical predictions from molecular mechanics , and ab initio – calculations. For some of these molecules there are very large variations in the values, with several reported results with no estimated experimental uncertainties.…”