N–H⋯π interactions in pyrroles: systematic trends from the vibrational spectroscopy of clusters

Abstract

Pyrrole and some of its methylated derivatives are aggregated in a controlled way in pulsed supersonic jet expansions. The cluster N–H stretching dynamics is studied using FTIR and Raman spectroscopy. Dimers, trimers and tetramers can be differentiated. Systematic trends in the dimer N–H⋯π interaction as a function of methyl substitution are identified and explored for predictions. Overtone jet absorption spectroscopy is used to extract anharmonicities for the N–H bond in different environments. The N–H anharmonicity constant increases by 10% upon dimerization. Bulk matrix shifts can be emulated by the formation of Ar-decorated clusters. The experimental results are expected to serve as benchmarks for an accurate ab initio characterization of the N–H⋯π hydrogen bond.