Chlorodifluoroacetyl Isothiocyanate, ClF₂CC(O)NCS: Preparation and Structural and Spectroscopic Studies

Chlorodifluoroacetyl isothiocyanate, ClF₂CC­(O)­NCS, was synthesized by the reaction of ClF₂CC­(O)Cl with an excess of AgNCS. The colorless product melts at −85 °C, and its vapor pressure follows the equation ln p = −4471.1 (1/T) + 11.35 (p [atm], T [K]) in the range −38 to 22 °C. The compound has been characterized by IR (gas phase, Ar matrix, and matrix photochemistry), by liquid Raman, by ¹⁹F and ¹³C NMR, gas UV–vis, and photoelectron spectroscopy (PES), by photoionization mass spectrometry (PIMS), and by gas electron diffraction (GED). The conformational properties of ClF₂CC­(O)­NCS have been analyzed by joint application of vibrational spectroscopy, GED and quantum chemical calculations. The existence of two conformers has been detected in the gas and liquid phases, in which the C–Cl bond adopts a gauche orientation with respect to the CO group; the CO group is in syn- or anti-position with respect to the NC double bond of the NCS group. The computed ΔG° difference between these two gauche–syn and gauche–anti forms is ΔG° = 0.63 kcal mol^–1 in the B3LYP/6-31G­(d) approximation. The most significant gas-phase structural parameters for gauche–syn ClF₂CC­(O)­NCS are r_e(NCS) 1.559(2) Å, r_e(NCS) 1.213(2) Å, r_e(NC) 1.399(7) Å, r_e(CO) 1.199(2) Å, and ∠_e(CNC) 134.7(13)°. Photolysis of ClF₂CC­(O)­NCS using an ArF excimer laser (193 nm) mainly yields ClF₂CNCS, CO, and ClC­(O)­CF₂NCS. The valence electronic properties of the title compound were studied using PES and PIMS. The experimental first vertical ionization energy of 10.43 eV corresponds to the ejection primarily of the sulfur lone-pair electrons of the in-plane nonbonding orbital on the NCS group.