Higher-order effects in the vibration–rotation spectra of semirigid molecules

Microwave spectra of deuterated ethanes: Internal rotation potential function and rz structure

Perturbations in the vibration-rotation-torsion energy levels of an ethane molecule exhibiting internal rotation splittings

The perturbation‐allowed torsional spectrum of gaseous CH3CH3 has been measured between 225 and 340 cm−1 at a resolution of 0.015 cm−1 using a Fourier transform spectrometer. The absorption path length was 124 m; the gas temperature and pressure were 295 K and 107 Torr, respectively. The P, Q, and R branches have been observed for all four torsional sublevels in the bands v4=1←0 and 2←1. A torsion–rotation Hamiltonian containing seven parameters was used to analyze 204 frequencies assigned in the P and R branches of the two bands. Effective values were obtained for the reduced rotational constant AR, the two leading coefficients V3 and V6 in the Fourier expansion of the hindering potential, the B rotational constant, and three distortion parameters. Estimates for the three zeroth order torsional parameters free of higher order contributions are: AR =2.6711(50) cm−1, V3=1012.0(1.0) cm−1, and V6=10.71(0.60) cm−1. The torsional energies calculated from the current model are compared to those obtained by earl...

The torsional spectrum of CD3CD3

Diode laser spectra of most of the Q branches of the ν9 band of ethane from RQ8–PQ15 have been recorded. The Q branches RQ0–RQ4 were deconvoluted to yield an effective resolution of (0.5–1.0)×10−3 cm−1 FWHM. Torsional splittings were observed for most lines. In contrast to predictions based on first order theory, the splittings which range from (2–53)×10−3 cm−1, have a marked J and K dependence. A second order theory of torsion‐vibration‐rotation interaction between ν9 and 3ν4 is developed, which fits the splittings with an rms error of 0.0006 cm−1, using only three adjustable parameters: the barrier to internal rotation in ν9, the energy difference between ν9 and 3ν4, and an effective coupling constant. The barrier to internal rotation in ν9 is found to be 1123±10 cm−1.

Diode laser spectra of the torsional splittings in the ν9 band of ethane: Torsion‐vibration‐rotation interactions and the barrier to internal rotation

It is shown that pure rotational transitions in doubly degenerate torsional states of C2H6 (with selection rules Delta K = 0, plus or minus 1) are made allowed by Coriolis interaction between torsion and dipole-allowed vibrations. Expressions are presented for integrated intensities from which strengths of lines in the millimeter region can be calculated.

Theory of pure rotational transitions in doubly degenerate torsional states of ethane

A general expression is derived for the integrated intensity of rotational transitions in the vibronic ground state of tetrahedral molecules, taking into account the nuclear spin statistics. It is ...

Arieh Rosenberg

Papers

Theory of pure rotational transitions in doubly degenerate torsional states of ethane

On nuclear spin statistics in rotational transition intensities in tetrahedral AB4 molecules