Acetylene-phenol complexes: A matrix isolation infrared and ab initio study

The infrared spectra of the hydrogen-bonded complexes of acetylene-phenol have been investigated in solid argon and nitrogen matrices. Two types of acetylene-phenol complexes, OH⋯π and CH⋯O, are seen in the infrared spectra. Formation of these complexes was evidenced from the shifts in the vibrational frequencies of the modes involving the acetylene and phenol submolecules. The structure of the complexes and the energies were computed at HF and B3LYP levels of theory using 6-31++G∗∗ basis sets. The geometries optimized at HF level were used to calculate the single point MP2/6-31++G∗∗ energies for the complexes. The global minimum corresponded to the OH⋯π complex, where phenol acts as a proton donor to the acetylene π-cloud. The second minimum corresponded to the CH⋯O complex where acetylene acts as a proton donor to the oxygen of phenol and the third minimum corresponded to the CH⋯π complex where acetylene attacks the π-cloud of phenol. The computed vibrational frequencies of acetylene-phenol complexes at B3LYP/6-31++G∗∗ level corroborated well with the experimental frequencies. No experimental evidence for the formation of CH⋯π complex in Ar/N2 matrix was observed. AIM analysis was also performed to understand the nature of the interactions in these complexes.