The rovibrational dynamics of hydroxylamine

The combination of overtone spectroscopy and detailed quantum mechanical modelling of rovibrational dynamics provides an unique tool for investigating many aspects of intramolecular dynamics such as dynamical symmetries, coupling hierarchies, and the separation of time scales, which control the intramolecular energy flow at high excitations relevant for unimolecular reactions. Hydroxylamine NH 2 OH, which plays an important role in N/O/H chemistry in general, provides an intriguing example for the interplay between high frequency stretch‐bend motions and low frequency torsion/inversion tunneling. This paper discusses the dynamical analysis of its high resolution infrared absorption spectrum up to the visible range on the basis of up to five‐dimensional ab initio models using vibrational DVR grids of several million points. The torsion‐inversion process is found to follow a consecutive mechanism with very slow (≥ 1 μs) inversion tunneling below the trans‐cis isomerisation barrier (2673 cm −1 at MP2/TZ2P). The metastable cis isomer shows localised zero point and fundamental level eigenfunctions. From the overtone spectrum fast energy redistribution is deduced within the OH and NH 2 manifolds, respectively, on time scales below 100 fs. A direct coupling between the two moieties leads to a slow decay of OH stretching excitation within 3 ps. Further significant contributions to the effective homogeneous broadening observed in high OH stretch overtones are predicted from the 1,2‐H shift leading to ammonia oxide NH 3 O with a barrier around 18200 cm −1 (CASPT2) and a NH 3 O well depth exceeding 1 eV. The spectroscopic characterisation of the metastable isomers of hydroxyl amine, cis ‐NH 2 OH and NH 3 O, by stimulated emission pumping experiments is suggested on the basis of ab initio calculations for electronically excited states.