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 ⁰0 ground and 01 ¹0 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 ⁰0}=3.271 549 2(18) GHz for ¹²C³²S₂ and B_{00 ⁰0}=3.175 06(21) GHz for the ¹²C³²S³⁴S isotopomer. The rotational constants of the degenerate 01 ¹0 bending level of ¹²C³²S₂ are B_{01 ¹0}=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 Mo\ller-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. [ABSTRACT FROM AUTHOR]