Simulating Infrared Spectra of Matrix Isolation Experiments: What Works Best?

Matrix isolation is an experimental technique which allows spectroscopists to observe highly reactive or weakly bound systems by trapping them in frozen noble gas matrices. In this work, we studied the viability of using ab initio molecular dynamics (AIMD) to simulate several H2S·amine complexes isolated in solid argon crystals, comparing the results with conventional density functional theory (DFT) harmonic and anharmonic calculations to assess the most practical and accurate method to calculate the vibrational spectrum of such systems. While AIMD accurately predicted the intensity ratios of certain peaks, the frequencies themselves were no more accurate than conventional harmonic DFT calculations. Modeling an explicit argon matrix did not improve the results. Based on the computational cost of each method, a gas phase anharmonic or scaled harmonic frequency calculation at an appropriate level of theory was determined to be the most accurate and efficient way to simulate matrix isolation spectra.