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We use periodic DFT calculations at LDA and PBE level to investigate 3d transition metal dihalide (TMDH) monolayers in H‐ and T‐phase. By analyzing the phonon dispersion, we have obtained a rough overview which combinations may form stable structures. We have focused on identifying and explaining trends in the predicted electronic properties. Although their geometric structures are simple, the associated electronic and magnetic properties are not as easy to understand. At first glance, it seems that there is no clear trend, as even isovalence‐electronic TMDH monolayers formed from the same metal but different halides can feature different magnetic moments. The identification of potential trends is further complicated by the fact that for a significant number of species, LDA results and PBE results predict different ground‐state electronic structures. By rigorously analyzing the potential energy surfaces associated with different magnetic moments, we could show that the apparent inconsistencies can be easily understood as a result of the differences in the relative energy between electronic states of different magnetic moments. We further show that the trends in the band gaps can be easily rationalized by an electron counting rule based on simple symmetry arguments. [ABSTRACT FROM AUTHOR]

Vibrational spectra of the CH stretching vibration of diazine molecules, pyrimidine and pyrazine, were measured by infrared (IR)–vacuum ultraviolet (VUV) spectroscopy under the jet‐cooled gas‐phase condition. The observed IR spectra were analyzed by three anharmonic algorithms to account for the Fermi‐resonance (F‐R). The anharmonic analysis of the F‐R pattern was performed with second‐order vibrational perturbation theory (VPT2) with quartic potentials (QPs) at the DFT level of B2PLYP/6–311++G(d,p), followed by vibrational configuration interaction (VCI) method with the same QP. The VPT2 + QP method reasonably reproduced most of the bands in the observed spectra for all the species, especially for pyrimidine, a decent agreement is obtained with respect to the band positions and relative intensities. The analyses of the spectra show that all the observed spectra can be well interpreted by the F‐R between the CH stretching fundamentals and the first overtone and 1 + 1 combination bands involving the in‐plane CH bending vibrations and the contribution of the higher‐order anharmonic coupling to the observed spectra seems to lead to red‐shift of the F‐R patterns. Discrete variable representation based methods with potential energy surfaces at CCSD/aug‐cc‐pVDZ theory with 10 degrees of freedom were also carried out to assess the quality of QP at DFT level. [ABSTRACT FROM AUTHOR]

Vibrational spectra of the methyl groups in CH3SH, CH3SCH3, CH3OH, and CH3OCH3 monomers were measured to analyze their complex features as a result of bend/umbrella‐stretch Fermi resonance. Multiple bands were recorded between 2800–3000 cm−1 associated with the vibrations of the methyl groups in CH3OH, and CH3OCH3. On the other hand, the corresponding spectra of CH3SH and CH3SCH3 are relatively simpler with main peaks between 2900 to 3000 cm−1. Theoretical ab initio anharmonic algorithm using six normal modes on a methyl group with potential energy surface at CCSD(T)‐F12/aug‐cc‐pVTZ quality is able to account for all the experimentally observed features across these four species. In CH3SCH3 and CH3SH, all three CH stretch fundamentals are above 2900 cm−1, but the fundamental of umbrella mode is lower than 1350 cm−1, thus only the overtone states of two bending modes are able to borrow sufficient intensities. In CH3OCH3 and CH3OH, umbrella modes have frequencies similar to the bending modes and the CH stretch fundamentals are red‐shifted to 2850–2950 cm−1 leading to more complex vibrational features due to better energy‐matching condition. [ABSTRACT FROM AUTHOR]

A model using (H2O) n+, n=3-4, clusters was adopted to study the spectroscopic features of ionized water medium. The finite clusters were characterized using a systematic searching algorithm to locate the stable minimum structures. The electronic transition features of the minimum structures were calculated using Symmetry Adapted Cluster/Configuration Interaction (SAC-CI) and Time-Dependent Density Functional Theory (TDDFT) approaches. A new type of minimum structure - SolCat, a cationic water monomer being directly solvated by two proton-acceptor solvent molecules, was identified in addition to the proton transfer (PT) and hemibonding (Hm) isomers. CCSD(T)/aug-cc-pVTZ//CCSD/aug-cc-pVDZ was adopted to characterize the minimum structures of (H2O)n+, n=2-4, clusters, and this theory level provided the converged energetics for (H2O)2+ clusters and showed consistent results as the early EOM-IP-CC(2,3)//EOM-IP-CCSD calculations reported by Pieniazek et al. [J. Phys. Chem. A, 112, 6159-6170 (2008)]. For describing the larger size of clusters, BH&HLYP and BNL functionals were recommended for the economical geometry searching and sampling. The electronic transition features predicted by BH&HLYP and BNL functional gave the best vertical excitation energy with respect to the SAC-CI method. Taking into account the population probability and the oscillator strength, the electronic transition of the transient but spectroscopically-sensitive Hm and SolCat species could not be neglected at high temperature condition. [ABSTRACT FROM AUTHOR]