ENTHALPY DIFFERENCE BETWEEN CONFORMERS OF N-BUTANE AND THE POTENTIAL FUNCTION GOVERNING CONFORMATIONAL INTERCHANGE

The enthalpy difference between the more stable s-trans and high-energy gauche conformer of n-butane, CH3CH2CH2CH3, has been determined in the vapor state using variable-low-temperature infrared spectra of the gas to be 234 +/- 33 cm(-1) (669 +/- 96 cal/mol). Additionally, the enthalpy difference has been obtained from n-butane dissolved in liquefied argon, krypton, and xenon from variable-temperature studies of the infrared spectrum and the determined values ranged from 243 +/- 8 cm(-1) (693 +/- 19 cal/mol) to 218 +/- 8 cm(-1) (621 +/- 19 cal/mol) in going from liquid argon to xenon, respectively. Utilizing the asymmetric torsional fundamentals of the s-trans and gauche conformers of 121.28 and 116.60 cm(-1), four excited state transitions for the s-trans and two for the gauche conformers, and the new value for the enthalpy, as well as the dihedral angle of 62.8 degrees for the gauche conformer, the potential function governing conformational interchange has been determined. The resulting potential coefficients are V-1 = 242 +/- 7, V-2 = 43 +/- 5, V-3 = 1146 +/- 2, V-4 = 40 +/- 2, V-5 = -6 +/- 2 and V-6 = -36 +/- 1 cm(-1). From this potential function the s-trans to gauche, gauche to gauche, and gauche to s-trans barriers are determined to be 1266 +/- 20 cm(-1) (3.62 +/- 0.06 kcal/mol), 1146 +/- 20 cm(-1) (3.27 +/- 0.06 kcal/mol), and 1032 +/- 10 cm(-1) (2.95 +/- 0.03 kcal/mol), respectively. Additionally, this potential has a ''syn'' barrier, which is the energy difference between the s-trans minimum and the gauche to gauche transition state of 1380 +/- 10 cm(-1) (3.95 +/- 0.03 kcal/mol).