The triad in the region of the lowest-frequency parallel fundamental band (v5=1←0) of CH3SiH3: Fermi-type interactions and giant torsional splittings

Abstract

The lowest frequency parallel fundamental band ν5 of CH3SiH3 near 700 cm−1 has been measured at a resolution of 0.004 cm−1 with Fourier transform spectroscopy to investigate vibration–torsion–rotation interactions in symmetric tops. The torsional splittings in the spectrum are increased from ∼0.005 cm−1 to ∼1 cm−1 by Fermi-type vibration–torsion interactions between the torsional stack (v6=0,1,2,…) in the ground vibrational state and the corresponding stack for v5=1. Resonant interactions were observed between the states (v5=1,v6=0) and (v5=0,v6=5) for the rotational series with (k=±1,σ=∓1), where σ labels the torsional sublevels. In this resonance, the two unperturbed states are near opposite limits for torsional motion: (v5=0,v6=5) involves nearly free rotation, while (v5=1,v6=0) involves small amplitude torsional oscillation. For the (k=±1,σ=∓1) rotational series, perturbation-allowed transitions in the high overtone (v6=5←0) were observed. Over 750 frequencies measured here have been analyzed together with more than 2500 measurements involved in the recent analysis of the lowest-lying degenerate fundamental band ν12 given by Moazzen-Ahmadi et al. [J. Mol. Spectrosc. 175, 54 (1996)]. A fit to within experimental error was achieved using 41 parameters, an increase of only 4 when the new band is added. The analysis shows that the inclusion of the Fermi-type interactions leads to a considerable simplification of the Hamiltonian for the ground vibrational state. For example, both the second and third terms (V0,6,V0,9) in the Fourier expansion of the hindering potential as well as the torsional flexing term (F0,m) vanish in the ground state. The changes in the leading terms in the torsional Hamiltonian have been quantitatively explained by a contact transformation. The large perturbations produced by the interaction matrix elements off-diagonal by 5 units in v6 have serious implications for vibrational relaxation in molecules undergoing internal rotation.

A Combined Frequency Analysis of the ν3, ν9, 3ν4 and the Far-Infrared Bands of Ethane: A Reassessment of the Torsional Parameters for the Ground Vibrational State

“…The parametrization of the Hamiltonian was very similar to that in methyl silane (5). As with methyl silane, it was necessary to eliminate V 0,6 , V 0,9 and some of the higher order parameters from the fit when the Fermi-coupling parameters were included.…”

“…Ethane is a benchmark molecule exhibiting internal rotation about a single bond (1,2). In addition, it has been demonstrated that the torsional potential is a sensitive probe of the electronic structure in different electronic states (3) and that the torsional motion plays a key role in intramolecular vibrational redistribution (IVR) (4,5). The significance of IVR lies in the fact that many larger molecules that are active in biological environments possess methyl groups which undergo internal motions.…”

Torsion–rotation global analysis of the first three torsional states (νt=0, 1, 2) and terahertz database for methanol

“…[15], where m unfortunately has a different meaning from the subscript m of Refs. [34,35], as explained in a footnote to Table 2) torsional parameters F and V 3 , which multiply P 2 c and (1 À cos 3c)/2, respectively, leads to new names for one of the {440} parameters and eight of the {422} parameters, as shown in the fourth column of Table 2. (Names previously used in the literature are given in parentheses following the name proposed here.)…”

“…[15], used in the first column of this table, does not have the same meaning as the subscript m of Refs. [34,35], used in the fourth column of this table. b {A, B} AB + BA.…”

“…The main principle of our presently favored scheme is to agree on a letter for a low-order term, and then add one or more of the six subscripts m, J, K, ab, bc, and ac to indicate higher-order corrections to these terms. The m, J, and K subscripts have proved convenient for symmetric-top internal rotor molecules [34,35], and we have simply added the ab, bc, and ac subscripts to allow for terms occurring in asymmetric-top internal rotor molecules.…”