Effect of matrix on IR frequencies of acetylene and acetylene-methanol complex: infrared matrix isolation and ab initio study

Abstract

Effect of nitrogen and argon matrices on the C-H asymmetric stretching and bending infrared frequencies of the acetylene molecule, C₂H₂, has been studied by matrix isolation experiments as well as by calculations at MP2 level of theory. The complexes of C₂H₂ in nitrogen and argon matrices, viz., C₂H₂(N₂)_m (with m=2-8) and C₂H₂(Ar)n (with n=2-10) are theoretically explored. The computed acetylenic C-H asymmetric stretch in C₂H₂-nitrogen complexes shows a redshift of 3.0 to 11.9 cm^-1 compared with the frequencies of the free acetylene molecule, and a corresponding blueshift of 7.4 to 26.2 cm^-1 when C₂H₂ is complexed with argon atoms. The trends in the computed shifts are in good agreement with the experiments. The molecular electrostatic potential minimum of C₂H₂ becomes more negative when complexed with nitrogen than on complexation with argon. This observation implies a greater basic character for C₂H₂ in the nitrogen matrix, favoring the formation of H-π(C₂H²-MeOH) complex as compared to that in the Ar matrix. Experimentally the preferential formation of H-π(C₂H₂-MeOH) complex in the N₂ matrix has indeed been observed.