Effect of Methyl Substitution on the N–H···O Interaction in Complexes of Pyrrole with Water, Methanol, and Dimethyl Ether: Matrix Isolation Infrared Spectroscopy and ab Initio Computational Studies

Hydrogen-bonded interactions of pyrrole with water and methanol have been studied using matrix isolation infrared spectroscopy and compared with the calculation performed on dimethyl ether. Computations carried out at MP2/aug-cc-pVDZ level of theory yielded two minima for the pyrrole–water and pyrrole–methanol complexes. The global and local minima correspond to the N–H···O and O–H···π complexes, respectively, where the N–H group of pyrrole interacts with oxygen of water/methanol and O–H of water and methanol interacts with the π cloud of pyrrole. Computations performed on the pyrrole–dimethyl ether gave only N–H···O type complex. From the experimental vibrational wavenumber shifts in the N–H stretching and N–H bending modes of pyrrole, as well as in the O–H stretching modes of water and methanol, the 1:1 N–H···O complexes were discerned. The strength of the N–H···O hydrogen bond and the corresponding shift in the N–H stretching vibrational wavenumbers increases in the order pyrrole–water < pyrrole–methanol < pyrrole–dimethyl ether, where a proton is successively replaced by a methyl group. Apart from the 1:1 complexes, higher clusters of 2:1 and 1:2 pyrrole–water and pyrrole–methanol complexes were also generated in N₂ matrix. Atoms in molecules and natural bond orbital analyses were carried out at the MP2/aug-cc-pVDZ level to understand the nature of interaction in the 1:1 pyrrole–water, pyrrole–methanol and pyrrole–dimethyl ether complexes