Millimetre-wave spectroscopy and ab initio calculations for fluorophosphaethyne (FCP)

Abstract

The semistable FCP molecule has been produced in the gas phase by a high temperature reaction between phosphine and trichlorofluoromethane. Its rotational spectrum has been observed in the millimetre- and submillimetre-wave regions, from 80 to 650 GHz, for the ground and various vibrationally excited states of F12CP, and for the ground state of F13CP. All the excited states that lie below 1200 cm−1, which are 1000, 0110, 0200, 0220, 0310, 0330, and 1110, have been investigated for the most abundant isotopologue. The analysis of the spectra has been performed taking simultaneously into account the strong Fermi resonance that couples the states v 1, v 2, v 3 with v 1 − 1, v 2 + 2, v 3, and l-type resonances between different sublevels of a given bending state, in order to obtain directly deperturbed rotational parameters. CCSD(T) calculations with a series of correlation consistent basis sets have been also performed, which provided a reliable estimate of the quartic force field of fluorophosphaethyne. Effects due to truncation of the basis and to core correlation have been accounted for. The theoretical force field has been partly refined by a least-squares fit to the available vibrational and rotational experimental data. A mixed experimental–theoretical equilibrium structure has been finally calculated combining experimental ground-state rotational constants of F12CP and F13CP with computed zero-point contributions. The best estimate of the equilibrium structure of fluorophosphaethyne can be established as re (F−C) = 1.2759 ± 0.0004 Å and re (C−P) = 1.5445 ± 0.0002 Å.