Femtosecond degenerate four-wave mixing of carbon disulfide: High-accuracy rotational constants

Femtosecond degenerate four-wave mixing (fs-DFWM) rotational coherence spectroscopy (RCS) has been used to determine the rotational and centrifugal distortion constants of the 00 (0)0 ground and 01 (1)0 vibrationally excited states of gas-phase CS2. RCS transients were recorded over the 0-3300 ps optical delay range, allowing the observation of 87 recurrences. The fits yield rotational constants B-00 (0)0=3.271 549 2(18) GHz for (CS2)-C-12-S-32 and B-00 (0)0=3.175 06(21) GHz for the (CSS)-C-12-S-32-S-34 isotopomer. The rotational constants of the degenerate 01 (1)0 bending level of (CS2)-C-12-S-32 are B-01 (1)0=3.276 72(40) and 3.279 03(40) GHz for the e and f substrates, respectively. These fs-DFWM rotational constants are ten times more accurate than those obtained by CO2 laser/microwave heterodyne measurements and are comparable to those obtained by high-resolution Fourier transform infrared spectroscopy. Ab initio calculations were performed at two levels, second-order Moller-Plesset theory and coupled-cluster singles, doubles, and iterative triples [CCSD(T)]. The equilibrium and vibrationally averaged C = S distances were calculated using large Dunning basis sets. An extrapolation procedure combining the ab initio rotational constants with the experiment yields an equilibrium C = S bond length of 155.448 pm to an accuracy of +/- 20 fm. The theoretical C = S bond length obtained by a complete basis set extrapolation at the CCSD(T) level is r(e)(C = S)=155.579 pm, or 0.13 pm longer than that in the experiment. (c) 2006 American Institute of Physics.