Prediction of anharmonic, condensed-phase IR spectra using a composite approach: Discrete encapsulated chloride hydrates
Wonanke, A.D.D. / Crittenden, D.L. (2022)
Journal of Molecular Spectroscopy, 2022, 387, 111660
- Date: Juni 2022
Composite approaches in which clustering and encapsulation effects are modelled at different levels of theory are capable of reproducing experimental infrared (IR) spectroscopic band centres to within 23 cm−1 (mean absolute deviation), with maximum absolute errors less than 65 cm−1. Anharmonic fundamentals for water stretching modes within isolated clusters may be computed by applying complexation shifts to experimental gas phase water fundamentals, or by empirically scaling harmonic fundamentals computed using “medium accuracy” quantum chemical methods such as dispersion-corrected generalised gradient approximation density functional theories (DFT) or second-order Møller–Plesset perturbation theory (MP2). Environmental effects are modelled using density functional tight binding (DFTB) theories as the difference between harmonic fundamentals in the crystalline environment and in the gas phase. This approach affords a significant improvement in accuracy over conventional approaches in which IR band centres are modelled at a single level of theory. DFT or MP2 calculations on isolated chloride hydrate clusters yield mean and maximum absolute errors of 36 cm−1 and 144 cm−1, respectively. Directly predicting vibrational frequencies in the condensed phase using DFTB models is less accurate again, incurring mean and maximum absolute errors of 93 cm−1 and 204 cm−1, respectively.