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Assignment of Infrared-Active Combination Bands in the Vibrational Spectra of Protonated Molecular Clusters Using Driven Classical Trajectories: Application to N4H+ and N4D+

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journal contribution
posted on 10.06.2019 by Reagan Hooper, Dalton Boutwell, Martina Kaledin
We investigate the utility of the driven molecular dynamics (DMD) approach to complex molecular vibrations by applying it to linear clusters with several degenerate vibrational modes and infrared (IR) intense combination bands. Here, the prominent features in N4H+ and N4D+ IR spectra, reported and described by others previously, have been characterized for the first time by DMD using recently published high-level potential and dipole moment surfaces. Namely, the calculations closely correlate the parallel proton stretch vibration in N4H+, at 750 cm–1, with the one observed experimentally at 743 cm–1. Second, the intense IR-active combination bands found in experimental spectra within 900–1100 cm–1 have been properly recovered by DMD at 950 cm–1 as strongly IR-active and confirmed as consisting of H+ asymmetric stretch and N2···N2 intermolecular symmetric stretch modes. Furthermore, we show that certain combination bands involving overtone transitions may be recovered by DMD using a hard-driving regime, such as the 1409 cm–1 band measured in N4H+, revealed by DMD at 1375 cm–1, and assigned to a progressive combination of the parallel H+ stretch and two quanta of N2···N2 stretch, in agreement with quantum mechanical studies reported previously by others.