Incremental NH stretching downshift through stepwise nitrogen complexation of pyrrole: a combined jet expansion and matrix isolation study.

Aggregates of pyrrole with nitrogen are studied by Fourier transform infrared spectroscopy in supersonic jet expansions as well as in neon, argon and nitrogen cryomatrices. The NH stretching vibration undergoes a significant downshift upon switching from isolated gas phase conditions to bulk nitrogen matrices, which can be reconstructed incrementally by stepwise cluster formation with an increasing number of nitrogen molecules both in supersonic expansions and neon or argon matrices. The modelling of the bulk matrix shift by finite cluster theory remains an interesting challenge. Self-aggregation of pyrrole also yields the first spectra of the homodimer and -trimer in a neon matrix, showing particularly small (up to 10 cm) deviations from the isolated gas phase values.