Rotational constants and structure of para-difluorobenzene determined by femtosecond Raman coherence spectroscopy: A new transient type

摘要

Femtosecond Raman rotational coherence spectroscopy (RCS) detected by degenerate four-wave mixing is a background-free method that allows to determine accurate gas-phase rotational constants of non-polar molecules. Raman RCS has so far mostly been applied to the regular coherence patterns of symmetric-top molecules, while its application to nonpolar asymmetric tops has been hampered by the large number of RCS transient types, the resulting variability of the RCS patterns, and the 10(3)-10(4) times larger computational effort to simulate and fit rotational Raman RCS transients. We present the rotational Raman RCS spectra of the nonpolar asymmetric top 1,4-difluorobenzene (para-difluorobenzene, p-DFB) measured in a pulsed Ar supersonic jet and in a gas cell over delay times up to similar to 2.5 ns. p-DFB exhibits rotational Raman transitions with Delta J = 0,1,2 and Delta K = 0, 2, leading to the observation of J-, K-, A-, and C-type transients, as well as a novel transient (S-type) that has not been characterized so far. The jet and gas cell RCS measurements were fully analyzed and yield the ground-state (v = 0) rotational constants A(0) = 5637.68(20) MHz, B-0 = 1428.23(37) MHz, and C-0 = 1138.90(48) MHz (1 sigma uncertainties). Combining the A(0), B-0, and C-0 constants with coupled-cluster with single-, double-and perturbatively corrected triple-excitation calculations using large basis sets allows to determine the semi-experimental equilibrium bond lengths r(e)(C-1-C-2) = 1.3849(4) angstrom, r(e)(C-2-C-3) = 1.3917(4) angstrom, r(e)(C-F) = 1.3422(3) angstrom, and r(e)(C-2-H-2) = 1.0791(5) angstrom. (C) 2015 AIP Publishing LLC.