Complete Assignment of the Infrared Spectrum of the Gas-Phase Protonated Ammonia Dimer

The
infrared (IR) spectrum of the ammoniated ammonium dimer is
more complex than those of the larger protonated ammonia clusters
due to close-lying fundamental and combination bands and possible
Fermi resonances (FR). To date, the only theoretical analysis involved
partial dimensionality quantum nuclear dynamic simulations, assuming
a symmetric structure (D3d) with the proton midway between the two nitrogen atoms. Here we
report an extensive study of the less symmetric (C3v) dimer, utilizing both second order
vibrational perturbation theory (VPT2) and ab initio molecular dynamics (AIMD), from which we calculated the Fourier
transform (FT) of the dipole-moment autocorrelation function (DACF).
The resultant IR spectrum was assigned using FTed velocity autocorrelation
functions (VACFs) of several interatomic distances and angles. At
50 K, we have been able to assign all 21 AIMD fundamentals, in reasonable
agreement with MP2-based VPT2, about 30 AIMD combination bands, and
a difference band. The combinations involve a wag or the NN stretch
as one of the components, and appear to follow symmetry selection
rules. On this basis, we suggest possible assignments of the experimental
spectrum. The VACF-analysis revealed two possible FR bands, one of
which is the strongest peak in the computed spectrum. Raising the
temperature to 180 K eliminated the “proton transfer mode”
(PTM) fundamental, and reduced the number of observed combination
bands and FRs. With increasing temperature, fundamentals red-shift,
and the doubly degenerate wags exhibit larger anharmonic splittings
in their VACF bending spectra. We have repeated the analysis for the
H3ND+NH3 isotopologue, finding that
it has a simplified spectrum, with all the strong peaks being fundamentals.
Experimental study of this isotopologue may thus provide a good starting
point for disentangling the N2H7+ spectrum.