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The rotational and centrifugal constants of cyclobutanes, [C.sub.4][H.sub.8] which was measured accurately by employing femtosecond rotational Raman coherence, detected by degenerate four-wave mixing is presented. The vibrational levels of cyclobutane exhibit tunneling splitting due to the ring-puckering interconversion between the symmetry-equivalent minima via a planar barrier.

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 B00 00=3.271 549 2(18) GHz for 12C32S2 and B00 00=3.175 06(21) GHz for the 12C32S34S isotopomer. The rotational constants of the degenerate 01 10 bending level of 12C32S2 are B01 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 re(C==S)=155.579 pm, or 0.13 pm longer than that in the experiment. [ABSTRACT FROM AUTHOR]

Femtosecond degenerate four-wave mixing (fs-DFWM) is applied for the measurement of rotational constants of cyclopropane (C3H6). The rotational coherence method yields a very accurate B0=20 093.322(12) MHz and centrifugal distortion constants DJ and DJK. To exploit the full resolution of the fs-DFWM method, the accuracy of the optical delay measurement was increased by nearly two orders of magnitude, including elimination of effects from the refractive index of air. The fs-DFWM molecular constants are comparable in accuracy to those from high-resolution infrared spectroscopy and are only surpassed by those of dipole distortion microwave spectroscopy. In parallel, the equilibrium structure, vibrationally averaged structure parameters and rotational constants were calculated using high-level ab initio methods and large basis sets. Combining these with the results of previous calculations and the measured rotational constants yields re(C–C)=1.5034(3) Å, re(C–H)=1.0775(5) Å, and αe(H–C–H)=115.09(10)°. [ABSTRACT FROM AUTHOR]