On the synergy of matrix-isolation infrared spectroscopy and vibrational configuration interaction computations

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.

Increase of Radiative Forcing through Midinfrared Absorption by Stable CO2 Dimers?

“…The mixing ratio of carbon dioxide in neon, from now on denoted as ρ, was adjusted and quantified by barometric monitoring. This setup is detailed elsewhere. , In our main experiments, matrix isolation and IR spectroscopy were performed at 6 K. We accumulated the IR spectra (resolution, 0.3 cm –1 ; scans, 512) within roughly 30 min. To study the influence of initial gas temperature during mixing on the dimerization, the CO 2 /Ne mixtures were equilibrated at either 25 °C (cf.…”

Determining internal coordinate sets for optimal representation of molecular vibration

“…b Assignment is formulated in the so-called chemist's notation and based on earlier studies. [149][150][151] symmetry-equivalent: the H2CH3 and H2CH4 angles, the H3CO and H4CO angles, and two dihedral angles, i.e., H3COH1 and H4COH1. To preserve symmetry in intrinsic frequencies, only IC sets, including all symmetrical counterparts, are allowed.…”

“…We will discuss this circumstance in an upcoming paper. 152 If a general (non-planar) molecule has atom(s) X with multiplicities greater than two and all incident bonds are co-planar, these atoms form planar subunits. From topology (cf.…”