Anharmonicity Effects in IR Spectra of [Re(X)(CO)<sub>3</sub>(α-diimine)]
(α-diimine = 2,2′-bipyridine
or pyridylimidazo[1,5‑<i>a</i>]pyridine; X = Cl or
NCS) Complexes in Ground and Excited Electronic States
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Abstract
Infrared spectra of [Re(X)(CO)<sub><b>3</b></sub>(α-diimine)]
(α-diimine = 2,2′-bipyridine, X = Cl, NCS, or pyridylimidazo[1,5-<i>a</i>]pyridine, X = Cl) in the ground and the lowest triplet
electronic states were calculated by a global hybrid density functional
going beyond the harmonic level by means of second-order vibrational
perturbation theory (VPT2) and including bulk solvent effects by the
polarizable continuum model (PCM). The full-dimensionality (FD) VPT2
is compared with the reduced-dimensionality (RD) model, where only
selected vibrational modes are calculated anharmonically. The simulated
difference IR spectra (excited state minus ground state) in the ν(CO)
region closely match experimental time-resolved infrared (TRIR) spectra.
Very good agreement was also obtained for ground-state spectra in
the fingerprint region. In comparison with the harmonic simulated
spectra, the calculated anharmonic frequencies are closer to experimental
values and do not require scaling when the B3LYP functional is used.
Several spectral features due to combination bands have been identified
by VPT2 simulations in the ν(CO) spectral region, which are
of importance for a correct interpretation of TRIR experiments