Complexes of acetylene-fluoroform: A matrix isolation and computational study

Abstract

Hydrogen-bonded complexes of C2H2 and CHF3 have been investigated using matrix isolation infrared spectroscopy and ab initio computations. The complexes were trapped in both solid argon and nitrogen matrices at 12 K. The structure of the complexes and the energies were computed at the B3LYP and MP2 levels of theory using a 6-311++G(d,p) basis set and at the MP2/aug-cc-pvdz level. Our computations indicated two minima for the 1:1 C2H2–CHF3 complex, with the C–H…π complex being the global minimum, where CHF3 is the proton donor. The second minimum corresponded to a relatively less exothermic C–H…F complex, in which C2H2 is the proton donor. Experimentally, we observed only the C–H…π complex in our matrix, which was evidenced by the shifts in the vibrational frequencies of the modes involving the C2H2 and CHF3 sub-molecules. The increase in the blue shift of the C–H stretching frequency in going from CHCl3–acetylene complex to CHF3–acetylene complex with corresponding increase in the interaction energy helps to place these two complexes on the left hand end of the qualitative diagram (Fig. 1). We also performed computations to study the higher complexes of C2H2 and CHF3. One minimum was found for the 1:2 C2H2–CHF3 complexes and two minima for the 2:1 C2H2–CHF3 complexes, at all levels of theory. Experimentally we observed the features corresponding to the 1:2 and 2:1 C2H2–CHF3 complexes in the N2 matrix. The computed vibrational frequencies of C2H2–CHF3 complexes at B3LYP and MP2/6-311++G(d,p) level corroborated well with the experimental frequencies. Interestingly, no experimental evidence for the formation of higher complexes was observed in the Ar matrix.