Anharmonic Coupling Revealed by the Vibrational Spectra of Solvated Protonated Methanol: Fermi Resonance, Combination Bands, and Isotope Effect

Structures of DMSO clusters and quantum cluster equilibrium (QCE)

Dimethylsulfoxide (DMSO) clusters are crucial for understanding processes in liquid DMSO. Despite its importance, DMSO clusters have received negligible attention due to the complexity of their potential energy surfaces (PESs). In this work, we explored the PESs of the DMSO clusters from dimer to decamer, starting with classical molecular dynamics, followed by full optimizations at the PW6B95-D3/def2-TZVP level of theory. In addition, the binding energies, the binding enthalpy per DMSO, and the quantum theory of atoms in molecules (QTAIM) analysis of the most stable isomers are reported. Temperature effects on the stability of the isomers have also been assessed. After thoroughly exploring the PESs of the DMSO clusters, 159 configurations have been used to apply the quantum cluster equilibrium (QCE) theory to liquid DMSO. The quantum cluster equilibrium theory has been applied to determine the liquid properties of DMSO from DMSO clusters. Thus, using the QCE, the population of the liquid DMSO, its infrared spectrum, and some thermodynamic properties of the liquid DMSO are predicted. The QCE results show that the population of the liquid DMSO is mainly dominated by the DMSO dimer and decamer, with the contribution in trace of the DMSO monomer, trimer, tetramer, pentamer, and octamer. More interestingly, the predicted infrared spectrum of liquid DMSO is in qualitative agreement with the experiment.

Anharmonic IR spectra of solvated ammonium and aminium ions: resemblance between water and bisulfate solvations

In this work, we analyze the vibrational spectra of ammonium, methylammonium, and dimethylammonium ions solvated by either water molecules or bisulfate anions using anharmonic vibrational algorithms. Rich and complicated spectral features in the 2700-3200 cm^-1 region of the experimental spectra of these clusters are attributed to originate from strong Fermi resonance between hydrogen-bonded NH stretching fundamentals and NH bending overtones. Additional weaker bands around 2500-2600 cm^-1 in solvated aminium ions are assigned to the combination tones involving the CH-NH (methyl-amino) rocking modes. Furthermore, the qualitative resemblance in band positions and spectral patterns between two-water-solvated and two-bisulfate-solvated cations suggest a common vibrational coupling scheme beneath the two seemingly different micro-solvation environments.

An ab initio anharmonic approach to IR, Raman and SFG spectra of the solvated methylammonium ion

The methylammonium ion (CH₃NH₃⁺, or noted as MA-H⁺) is one of the smallest organic ammonium ions that play important roles in organic-inorganic halide perovskites. Despite the simple structure, the vibrational spectra of MA-H⁺ exhibit complicated features in the 3 μm region which are sensitive to the solvation environment. In the present work, we have applied the ab initio anharmonic algorithm at the CCSD/aug-cc-pVDZ level to simulate the IR and Raman spectra of the solvated methylammonium ion, MA-H⁺⋯X₃, where X denotes the solvent molecules, to understand the Fermi resonance mechanism in which the overtones of NH bending modes are coupled with the fundamentals of NH stretching modes. The spectral features of the solvated clusters with proper solvent species resemble those observed in the perovskite crystal, indicating that they have similar solvation environments and hydrogen bond interactions. Therefore, a linkage between the gas-phase cluster models and the condensed-phase materials can be established, and our simulations and anharmonic analyses help in interpreting the spectral assignments of the observed IR and Raman spectra of perovskites reliably. Furthermore, we have extended this approach to the SFG spectra to demonstrate the selective appearance of bands depending on both the beam polarization configurations and the symmetry of vibrational modes.