Accurate rotational constant and bond lengths of hexafluorobenzene by femtosecond rotational Raman coherence spectroscopy and ab initio calculations.

The gas-phase rotational motion of hexafluorobenzene has been measured in real time using femtosecond (fs) time-resolved rotational Raman coherence spectroscopy (RR-RCS) at T = 100 and 295 K. This four-wave mixing method allows to probe the rotation of non-polar gasphase molecules with fs time resolution over times up to ~5 ns. The ground state rotational constant of hexafluorobenzene is determined as B₀ = 1029.740(28) MHz (2σ uncertainty) from RR-RCS transients measured in a pulsed seeded supersonic jet, where essentially only the v = 0 state is populated. Using this B₀ value, RR-RCS measurements in a room temperature gas cell give the rotational constants B_v of the five lowest-lying thermally populated vibrationally excited states v_7/8, v₉, v_11/12, v₁₃, and v_14/15. Their B_v constants differ from B₀ by between -1.02 MHz and +2.23 MHz. Combining the B₀ with the results of all-electron coupled-cluster CCSD(T) calculations of Demaison et al. [Mol. Phys. 111, 1539 (2013)] and of our own allow to determine the C-C and C-F semi-experimental equilibrium bond lengths r_e(C-C) = 1.3866(3) Å and r_e(C-F) = 1.3244(4) Å. These agree with the CCSD(T)/wCVQZ r_e bond lengths calculated by Demaison et al. within ±0.0005 Å. We also calculate the semi-experimental thermally averaged bond lengths r_g(C-C)=1.3907(3) Å and r_g(C-F)=1.3250(4) Å. These are at least ten times more accurate than two sets of experimental gas-phase electron diffraction r_g bond lengths measured in the 1960s. [ABSTRACT FROM AUTHOR]