Which Triatomic Monohalosilylenes, Monohalogermylenes, and Monohalostannylenes (HMX) Fluoresce or Phosphoresce and Why? An Ab Initio Investigation

Exploring the structure and photodissociation mechanism of the electronic states of monochlorogermylene including the spin-orbit-coupling

The electronic states of monochlorogermene (HGeCl) are computed by using the explicitly correlated internally contracted multireference configuration interaction (icMRCI-F12) method with Davidson correction. A total of 23 spin-free states with energy up to 9.4 eV as well as 55 spin-orbit coupled states generated by these spin-free states after considering the spin-orbit coupling (SOC) effect are investigated. The geometries and harmonic vibrational frequencies of the lowest electronic states are calculated, and the spectroscopic constants, oscillator strength, vertical transition energy and potential energy curves (PECs) of the spin-free states and spin-coupled states are presented. Based on the calculation of the electronic states, the photodissociation dynamics in the ultraviolet region is discussed. Our studies will shed light on the electronic structure and decomposition processes of HGeCl radicals.

Hydroxysilylene (HSi-OH) in the gas phase

The hydroxysilylene (HSiOH) molecule has been spectroscopically identified in the gas phase for the first time. This highly reactive species was produced in a twin electric discharge jet using separate precursor streams of 16O2/18O2 and Si2H6/Si2D6, both diluted in high pressure argon. The strongest and most stable laser induced fluorescence (LIF) signals were obtained by applying an electric discharge to each of the precursor streams and then merging the discharge products just prior to expansion into vacuum. Bands of the Ã1A-X~1A' electronic transition of HSiOH were found in the 455-420 nm region, and single vibronic level emission spectra showed only transitions attributable to the trans-hydroxysilylene ground state isomer. High resolution, rotationally resolved spectra were obtained for the 0-0 bands of HSi16OH and HSi18OH. The rotational constants were used to obtain ground and excited state molecular structures of HSiOH, with some necessary constraints. The derived ground state structure is trans-HSiOH, with geometric parameters similar to theoretical predictions from the literature. In the excited state, a skew-HSiOH structure was obtained with a dihedral angle of 102°. Our own CASSCF/aug-cc-pVTZ calculations predict a similar excited state skew geometry. The lack of odd quantum number changes in the torsional mode in emission and our difficulties in obtaining DSiOD spectra, despite considerable effort, all suggest that further experimental and theoretical efforts will be necessary to thoroughly understand the electronic spectrum of hydroxysilylene.

Ab Initio Study on Electronic Excited States of Monochlorosilylene

We perform a high-level ab initio study on 20 electronic states of monochlorosilylene (HSiCl) using an internally contracted multireference configuration interaction method including Davidson correction (icMRCI+Q). The spin-orbit coupling (SOC) effect is investigated, leading to splitting of the 20 spin-orbit-free states into 50 spin-orbit-coupled states. Vertical transition energies, oscillator strengths, and potential energy curves are presented with and without considering the SOC effect. Analysis indicates that the SOC effect plays an important role, especially for the high-lying excited states of HSiCl. The state interaction and the dynamics of the electronic states of HSiCl in the ultraviolet region are discussed based on our calculation results. Our study paves the way to understanding the behavior of electronic excited states of monochlorosilylene.

Performance of EOM-CCSD(T)(a)*-Based Quartic Force Fields in Computing Fundamental, Anharmonic Vibrational Frequencies of Molecular Electronically Excited States with Application to the Ã1A″ State of :CCH2 (Vinylidene)

Highly accurate anharmonic vibrational frequencies of electronically excited states are not as easily computed as their ground electronic state counterparts, but recently developed approximate triple excited state methods may be changing that. One emerging excited state method is equation of motion coupled cluster theory at the singles and doubles level with perturbative triples computed via the (a)* formalism, EOMEE-CCSD(T)(a)*. One of the most employed means for the ready computation of vibrational anharmonic frequencies for ground electronic states is second-order vibrational perturbation theory (VPT2), a theory based on quartic force fields (QFFs),fourth-order Taylor series expansions of the potential portion of the internuclear Watson Hamiltonian. The combination of these two is herein benchmarked for its performance for use as a means of computing rovibrational spectra of electronically excited states. Specifically, the EOMEE-CCSD(T)(a)* approach employing a complete basis set extrapolation along with core electron inclusion and relativity (the so-called "CcCR" approach) defining the QFF produces anharmonic fundamental vibrational frequencies within 2.83%, on the average, of reported gas-phase experimentally assigned values for the test set including the A ~ 1 A ″ states of HCF, HCCl, HSiF, HNO, and HPO. However, some states have exceptional accuracy in the fundamentals, most notably for ν2 of A ~ 1 A ″ HCCl in which the CcCR QFF value is within 1.8 cm-1 at 927.9 cm-1 (or 0.2%) of the experiment. Additionally, this approach produces rotational constants to, on the absolute average, within 0.41% of available experimental data, showcasing notable accuracy in the computation of rovibronic spectral data. Furthermore, utilizing a hybrid approach composed of harmonic CcCR force constants along with a set of simple EOMEE-CCSD(T)(a)*/aug-cc-pVQZ QFF cubic and quartic force constants is faster than using pure CcCR and better represents those modes that suffer from numerical instability in the anharmonic portion of the QFF, implying that this so-called "CcCR + QZ" QFF approach may be the best for future applications. Finally, complete, rovibrational spectral data are provided for A ~ 1 A 2 :CCH2, a molecule of potential astrochemical interest, in order to aid in its potential future experimental rovibronic characterization.

Electronic excited states of monobromosilylene molecules including the spin-orbit-coupling

We employ the internally contracted multireference configuration interaction (icMRCI-F12) with Davidson corrections to explore the electronic states of monobromosilylene molecules (HSiBr). A total of 20 states with energy up to 8.7 eV and the corresponding 50 states after taking the spin-orbit coupling (SOC) effects into account are investigated. The spectroscopic constants of the low-lying states, as well as oscillator strengths, vertical transition energies and potential energy curves (PECs) for all the 20 spin-free states and the 50 spin-orbit-coupled states of HSiBr are presented. The results indicate that the SOC effect significantly affects the dissociation pathways and the PECs of electronic excited states of HSiBr. Based on our calculation results, the interactions between the states and the dissociation of HSiBr in the UV region are discussed. Our study sheds some light on the complex interactions and dynamics of the electronic excited states of HSiBr, which would provide valuable information for future experimental investigations.