Characterization of weak NH-pi intermolecular interactions of ammonia with various substituted pi-systems

Among the several weak intermolecular interactions pervading chemistry and biology, the NH-pi interaction is one of the most widely known. Nevertheless its weak nature makes it one of the most poorly understood and characterized interactions. The present study details the results obtained on gas-phase complexes of ammonia with various substituted pi systems using both laser vibrational spectroscopy and ab initio calculations. The spectroscopic measurements carried out by applying one-color resonant two-photon ionization (R2PI) and IR-vibrational predissociation spectroscopy in the region of the NH stretches yield the first experimental NH stretching shifts of ammonia upon its interaction with various kinds of pi-systems. The experiments were complemented by ab initio calculations and energy decompositions, carried out at the second-order Møller-Plesset (MP2) level of theory. The observed complexes show characteristic vibrational spectra which are very similar to the calculated ones, thereby allowing an in-depth analysis of the interaction forces and energies. The interaction energy of the conformers responsible for the observed vibrational spectra has the maximum contribution from dispersion energies. This implies that polarizabilities of the pi-electron systems play a very important role in governing the nature and geometry of the NH-pi interaction. The larger polarizability of ammonia as compared to water and the tendency to maximize the dispersion energy implies that the characteristics of the NH-pi interactions are markedly different from that of the corresponding OH-pi interactions.