Conformations and Low-Frequency Intramolecular Motions of 1-Butanol, 1-Butanethiol, -butanol, and -butanethiol Investigated by Fourier Transform Microwave Spectroscopy Combined with Quantum Chemical Calculations.

The rotational spectra of 1-butanol (1-BuOH), 1-butanethiol (1-BuSH), 2-methyl-1-propanol (-BuOH), and 2-methyl-1-propanethiol (-BuSH) were measured by Fourier transform microwave spectroscopy in the frequency region from 3.7 up to 25 GHz. The observed spectral lines were assigned by observation of the deuterium substitution effect and by ab initio or density functional theory calculations at the levels of MP2/6-311++G(d,p) or B3LYP and -B3LYP, respectively. For 1-BuOH and 1-BuSH, seven of the 14 conformations, anticipated to exist as stable, were detected, whereas four and three among the five possible conformations were identified for -BuOH and -BuSH, respectively. We further found that, of the seven conformers of 1-BuOH, five were and two , with respect to the internal rotation axis: the C2-C3 bond, while three of -BuOH existed in and one in . The most stable conformer of the two BuOH molecules was with respect to the C-O bond, while all the sulfur analogues were to the C-S axis. The rare isotopomers examined included C and OD of 1-BuOH and OD of -BuOH, S, C, and SD of the two sulfur molecules, and the rotational constants obtained on these isotopomers were employed in the molecular structure derivation. The potential barrier to CH internal rotation and the deuterium quadrupole coupling constant, where available, were also derived from the spectral analysis, and the molecular parameters thus obtained were compared with those derived using quantum-chemical calculations; the values derived using -B3LYP/6-311++G(d,p) were in better agreement with the observed than those derived using MP2/6-311++G(d,p) and B3LYP/6-311++G(d,p). The form of 1-BuOH and of 1-BuSH and the form of -BuSH exhibited additional spectral splittings, which were interpreted as caused by the OH or SH group tunneling between the symmetric and antisymmetric states. Some of the = 8 rotational levels of 1-BuSH happened to be near-degenerate with others, and the splittings in them caused by mutual repulsion could be precisely determined by the observation of the transitions involving those split levels. Such splittings were determined for 1-BuSH, 1-BuSD, and -BuSH to be 1694.1731 (22), 56.3174 (16), and 6.4678 (14) MHz, respectively. A natural bond orbital analysis was performed to show that the most stable conformation of the primary and secondary alcohols is because of the charge transfer from the lone-pair electron of the oxygen atom to the antibonding orbital of the C-H bond in 1-BuOH, whereas in -BuOH, the charge transfer to the antibonding orbital of the C1-C2 bond.