Analysis of torsional spectra of molecules with two internal C3v rotors. 12. Low frequency vibrational spectra, methyl torsional potential function, and internal rotation of n-butane

Abstract

The vibrational spectrum of gaseous n-butane has been investigated below 500 cm"1. A vibrational assignment for the observed bands in this region has been given for both s-trans and the high-energy gauche conformers. The asymmetric potential function has been calculated from three observed transitions, leading to a value of the enthalpy difference between the conformers of 0.89 kcal/mol and a gauche dihedral angle of 62°. The values of the potential coefficients were V1 = 418 ± 6 cm"1 (1.19 kcal/mol); V3 = 639 ± 67 cm"1 (1.83 kcal/mol); VT6 = 136 ± 23 cm"1 (0.40 kcal/mol). Bands have been assigned to the methyl torsions of both conformers and from these assignments the barriers to internal rotation of the methyl groups for both the s-trans and gauche conformers have been calculated to be 3.21

Conformational statistics of n-butane in the condensed phase and the effects of temperature

“…E tg is the sum of E gas tg and E ext tg and the dashed line in Figure 2 represents the former term. Both models yield E tg values below the range of 788-884 cal/mol obtained from experiments in the gas phase [3][4][5][6][7] consistent with the lower experimental values found in earlier experiments on condensed phases. However, it is interesting to note that the CI (2k) values are at the opposite end of the experimental E tg range of values in liquids compared to the CCd values which are for the most part below this range of 502-681 cal/mol [8][9][10][11].…”

“…Flory suggested the effect was likely negligible [2]; however, this was not borne out in experiment. In particular, for the simplest flexible alkane n-butane, experiments report a trans-gauche energy difference, E tg , range of 788-884 cal/mol in the gas phase [3][4][5][6][7] and 502-681 cal/mol [8][9][10][11] in the condensed phase.…”

Prediction of protein side-chain rotamers from a backbone-dependent rotamer library: a new homology modeling tool

“…Examples of conformations which are inconsistent with a rotamer model are particularly numerous for long polar side-chains. These conformations, with w 1 within 20 of 0 , 120 , and À120 , for example, are unlikely to be correct, since the strain energy involved is greater than 4 kcal/mole (Durig & Compton, 1979;Compton et al, 1980;Wiberg & Murcko, 1988). Some of these cases may be due to crystallographic averaging of two rotamers.…”

The butane condensed matter conformational problem

“…This view was later challenged by Chandler et al whose rigorous theory of hydrocarbons predicted an increase in the gauche population resulting from short-range packing in the liquid phase [3,4]. Experiments have given credence to the latter view with gas-phase studies typically reporting the trans-gauche energy, E tg , to be 788-884 cal/mol [5][6][7][8][9] while studies of n-butane, both as a liquid and dissolved in other isotropic liquid solvents, consistently report a lower range of 502-597 cal/mol [10][11][12].…”