The Rotation–Torsion Structure in the ν11/ν15(Gs) Methyl Rocking Fundamental Band of Dimethylacetylene

Ubiquitous T-shaped isomers of OCS-hydrocarbon van der Waals complexes

“…We interpret these two bands in terms of distinct internal rotation states, as expected since dimethylacetylene itself exhibits rather free internal rotation ͑barrier height Ϸ6 cm −1 ͒. 13 Line strengths in the main band are consistent with a 3:5 intensity alternation between K a = even: odd levels as expected for the lowest A internal rotor state. Full details of the spectrum remain to be worked out, but we still obtain precise vibrational shift and intermolecular separation parameters ͑Table I͒.…”

Internal rotation, spin–orbit coupling, and low-frequency vibrations in the ground state of CH3–CC–CH+3and CD3–CC–CD+3

“…In the ground vibronic state of 2-butyne, the internal rotation of the two methyl groups is nearly free [13], with an estimated barrier along the internal-rotation coordinate of only 5.98 cm −1 [14]. Excited vibrational levels of 2-butyne are well known from studies by high-resolution infrared [3][4][5] and Raman spectroscopy [2,6], so that the current understanding of the structure and dynamics in the electronic ground state of neutral 2-butyne can be regarded as excellent.…”

“…These weighting factors were chosen to best reproduce the observed intensity distributions. [14]. c From an ab initio geometry optimisation performed with the TPSSH method and a def2-QZVPP basis set [38].…”

“…The rotational constants A = A i and B of the ground state of the neutral molecule were kept fixed at the values reported in Ref. [14]. The rotational constants A + and B + were kept fixed at the value obtained from an ab initio geometry optimisation performed with the TPSSH method and a def2-QZVPP basis set [38].…”

Ab initio determination of spectroscopic parameters for ethane-like molecules in the ground vibrational state

“…Hindered internal rotation is of particular interest because it has been shown to dramatically accelerate intramolecular vibrational energy redistribution (IVR) [1]. Furthermore, it occurs in a number of molecules in their ground electronic state, ranging from the small (e.g., hydrogen peroxide) [2] to the very large (e.g., carbohydrates) [3], with a barrier to internal rotation that varies from the rigid case (e.g., ethene) [4] to the nearly free rotor (e.g., dimethylacetylene) [5][6][7]. A complete understanding of the phenomena associated with hindered internal rotation requires both the development of a successful model Hamiltonian and an accurate physical interpretation of the terms in that Hamiltonian.…”