Barrier to asymmetric internal rotation, conformational stability, vibrational spectra and assignments, and Ab Initio calculations of n‐butane‐d0, d5 and d10

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Abstract
The asymmetric torsional potential function of n‐butane has been redefined based on a combination of results obtained from experimental and theoretical calculations. From the vibrational spectra of gaseous n‐butane below 500 cm−1 the asymmetric torsional fundamentals of the more stable s‐trans and high‐energy gauche conformations of n‐butane have been assigned at 121.28 and 116.60 cm−1, respectively, each with excited states falling to lower frequencies. Additionally, from variable‐temperature studies of the Raman spectra for n‐butane‐d0 and d10, the conformational enthalpy difference has been determined to be 381 ± 41 cm−1 (1089 ± 117 cal mol−1) and 369 ± 72 cm−1 (1055 ± 206 cal mol−1), respectively. Optimized structural parameters, obtained from ab initio methods, for the s‐trans and gauche conformers have been used to allow for structural relaxation during the asymmetric internal rotation and to place constraints on the determined potential function. The resulting potential coefficients are V1 = 584 ± 7, V2 = ‐96 ± 7, V3 = 1165 ± 3, V4 = 45 ± 3, V5 = ‐3 ± 2 and V6 = ‐40 + 1 and the s‐trans to gauche, gauche to gauche and gauche to s‐trans barriers are determined to be 1274, 1370 and 891 cm−1, respectively, with an enthalpy difference between the conformers of 383 ± 17 cm−1 (1095 ± 49 cal mol−1). This potential function is consistent with a dihedral angle for the gauche conformer of 65.4°. 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 1753 cm−1 (5.01 kcal mol−1), which is in excellent agreement with the most recent value of 1836 cm−1 (5.25 kcal mol−1) obtained from a very high‐level ab initio calculation. In order to provide a more complete description of the normal vibrational modes of n‐butane, normal coordinate analyses have been completed for n‐butane‐d0, ‐d5 and ‐d10 based on force constants from ab initio calculations. The infrared (3500‐40 cm−1) and Raman (3500‐10 cm−1) spectra of n‐butane‐d5 and ‐d10 have been obtained and complete vibrational assignments are provided. The structural parameters, conformational stabilities, barriers to internal rotation and vibrational frequencies which have been determined experientally are compared with those obtained by ab initio methods.