Femtosecond degenerate four-wave mixing of cyclopropane.

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 D_J and D_JK. 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 r_e(C–C)=1.5034(3) Å, r_e(C–H)=1.0775(5) Å, and α_e(H–C–H)=115.09(10)°. [ABSTRACT FROM AUTHOR]