Computational High-Frequency Overtone Spectra of the Water–Ammonia Complex

Abstract

We have computed vibrational high-frequency overtone spectra of the water-ammonia complex, H(2)O-NH(3), and its isotopomers. The complex has been modeled as two independently vibrating monomer units. The internal coordinate Hamiltonians for each monomer unit have been constructed using exact gas phase kinetic energy operators. The potential energy and dipole moment surfaces have been calculated with the explicitly correlated coupled cluster method CCSD(T)-F12A and the valence triple-ζ VTZ-F12 basis around the equilibrium geometry of the complex. The vibrational eigenvalues have been calculated variationally and the eigenfunctions obtained have been used to compute the intensities of the absorption transitions. In H(2)O-NH(3), the water molecule acts as the proton donor and its symmetry is broken. The hydrogen-bonded OH bond oscillator undergoes a large redshift and intensity enhancement compared to the free hydrogen bond. Broken degeneracy of the asymmetric vibrations, quenched inversion splittings, and blueshift of the symmetric bending mode are the most visible changes in the ammonia unit.

The effect of large amplitude motions on the vibrational intensities in hydrogen bonded complexes

“…28 The torsional coordinate isomerizes the H 2 O· · ·NH 3 complex from the most stable trans-conformation to the cis-conformation (see Figure S3 44 ). Sälli et al 27 showed that the OH b -stretching wavenumber varies by 0.5 cm −1 between the cis-and trans-conformer of the H 2 O· · ·NH 3 complex. Moreover, Lane and Kjaergaard 24 have shown that the barrier between these two conformers is only a few wavenumbers.…”

“…For complexes containing H 2 O as the donor unit, the two OH-stretches and the H f OH b bend of the donor unit have been included. 27,29 The Hamiltonian for this three-dimensional system iŝ…”

“…For comparison, the fourth order contribution to the same transition in the H 2 O· · ·H 2 O and H 2 O· · ·N 2 complexes is 0.0 cm −1 in both cases. Previously, Sälli et al 27 simulated the vibrational spectra of the H 2 O· · ·NH 3 complex using a three-dimensional LM model for the water unit. Similar to our LM model, their results predict a fundamental OH b -stretching transition wavenumber of 3419 cm −1 .…”

“…If these modes are ignored in the vibrational model, it leads to large discrepancies between experimental and calculated vibrational frequencies. [26][27][28] Intensity studies of hydrogen bonded complexes are limited. Kjaergaard et al 29 compared the performance of the second order Vibrational Perturbation Theory (VPT2) method 30 with a three-dimensional harmonically coupled anharmonic oscillator (HCAO) local mode (LM) model 31 on the H 2 O· · ·H 2 O complex.…”

“…However, this by itself can lead to large discrepancies in transition frequencies and oscillator strengths if the effect of the intermolecular modes is ignored. 26,27,36 Recently, we developed a vibrational model to describe the vibrational transitions in hydrogen bonded complexes, where the effect of the intermolecular modes is included as a perturbation on a LM model. 28 This Local Mode Perturbation Theory (LMPT) model was shown to calculate accurately the transition wavenumbers of the donor unit vibrations in water dimer.…”