The elusive C-H•••O complex in the hydrogen bonded systems of Phenylacetylene: A Matrix Isolation Infrared and Ab Initio Study

Hydrogen-bonded complexes of phenylacetylene (PhAc) with methanol (MeOH) and diethylether (DEE) were studied using matrix isolation infrared spectroscopy. This study specifically searched for the C-H center dot center dot center dot O hydrogen bonded complex in these systems, which manifest a n-sigma* interaction and which is a local minimum on the PhAc-MeOH potential surface, as in the case of PhAc-H2O heterodimer. This n-sigma* local minimum eluded observation in gas phase studies and it was therefore thought interesting to look for this isomer in cryogenic matrices. While MeOH can interact with PhAc as both a proton donor (O-H center dot center dot center dot pi complex) or a proton acceptor (n-sigma* complex), DEE can only manifest the n-sigma* isomer. A comparison of the spectral shifts observed in the features of PhAc-MeOH and PhAc-DEE would therefore independently confirm the existence or not of n-sigma* complex in both these systems. In addition to the n-sigma* complex observed in both the above systems, the O-H center dot center dot center dot pi complex was also discerned in the PhAc-MeOH system. These complexes have stabilization energy in the range of 8-25 kJ /mol. The experimental results were corroborated by computations performed at MP2 and M06-2X, levels of theory, using 6-311 ++G(d,p) and aug-cc-pVDZ basis sets. Single point calculations at the CCSD level of theory were also performed. Atoms-in-molecules (AIM), NBO and LMOEDA analysis were also performed to understand the nature of the intermolecular interactions in these complexes.