Abstract
The key feature of matrix-isolation infrared (MI-IR) spectroscopy is the isolation of single guest molecules in a host system at cryogenic conditions. The matrix mostly hinders rotation of the guest molecule, providing access to pure vibrational features. Vibrational self-consistent field (VSCF) and configuration interaction computations (VCI) on ab initio multimode potential energy surfaces (PES) give rise to anharmonic vibrational spectra. In a single-sourced combination of these experimental and computational approaches, we have established an iterative spectroscopic characterization procedure. The present article reviews the scope of this procedure by highlighting the strengths and limitations based on the examples of water, carbon dioxide, methane, methanol, and fluoroethane. An assessment of setups for the construction of the multimode PES on the example of methanol demonstrates that CCSD(T)-F12 level of theory is preferable to compute (a) accurate vibrational frequencies and (b) equilibrium or vibrationally averaged structural parameters. Our procedure has allowed us to uniquely assign unknown or disputed bands and enabled us to clarify problematic spectral regions that are crowded with combination bands and overtones. Besides spectroscopic assignment, the excellent agreement between theory and experiment paves the way to tackle questions of rather fundamental nature as to whether or not matrix effects are systematic, and it shows the limits of conventional notations used by spectroscopists.
Highlights
Ever since the matrix-isolation (MI) technique has been established for IR spectroscopy [1], it has played an increasingly important role in the characterization of molecular vibration and has been closely linked to computational investigations
We have pointed out that the accuracy of the potential energy surface (PES) is at the center of each Vibrational self-consistent field (VSCF)/vibrational configuration interaction (VCI) calculation
It is reasonable to assume that an accurate PES has a favorable effect on the overall accuracy of the VSCF/VCI calculation
Summary
Ever since the matrix-isolation (MI) technique has been established for IR spectroscopy [1], it has played an increasingly important role in the characterization of molecular vibration and has been closely linked to computational investigations. At the example of methanol, we compare our computational results to our experimental MI-IR data as well as the literature data from the experiment [28, 56,57,58,59] and other computational approaches [60,61,62,63,64] Apart from this assessment, we present results from VSCF/VCI calculations as a support in the conceptualization of MI-IR spectra. We will show that the quantitative discrepancies are small enough to guarantee that the qualitative conclusions drawn from the calculations are sensible This combined investigation with a focus on the synergy of MI-IR spectroscopy and VCI computations is the central aspect of our work
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