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1. Overcoming the out-of-plane bending issue in an aromatic hydrocarbon: the anharmonic vibrational frequencies of c-(CH)C 3 H 2 +

   https://doi.org/10.1039/D0CP01889A  

   Westbrook, Brent R. ; Del Rio, Weston A. ; Lee, Timothy J. ; Fortenberry, Ryan C. ( June 2020 , Physical Chemistry Chemical Physics)

   null (Ed.)

   The challenges associated with the out-of-plane bending problem in multiply-bonded hydrocarbon molecules can be mitigated in quartic force field analyses by varying the step size in the out-of-plane coordinates. Carbon is a highly prevalent element in astronomical and terrestrial environments, but this major piece of its spectra has eluded theoretical examinations for decades. Earlier explanations for this problem focused on method and basis set issues, while this work seeks to corroborate the recent diagnosis as a numerical instability problem related to the generation of the potential energy surface. Explicit anharmonic frequencies for c-(CH)C 3 H 2 + are computed using a quartic force field and the CCSD(T)-F12b method with cc-pVDZ-F12, cc-pVTZ-F12, and aug-cc-pVTZ basis sets. The first of these is shown to offer accuracy comparable to that of the latter two with a substantial reduction in computational time. Additionally, c-(CH)C 3 H 2 + is shown to have two fundamental frequencies at the onset of the interstellar unidentified infrared bands, at 5.134 and 6.088 μm or 1947.9 and 1642.6 cm −1 , respectively. This suggests that the results in the present study should assist in the attribution of parts of these aromatic bands, as well as provide data in support of the laboratory or astronomical detection of c-(CH)C 3 H 2 + . 

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2. The spectral features and detectability of small, cyclic silicon carbide clusters

   https://doi.org/10.3389/fspas.2022.1074879  

   Sehring, Christopher M. ; Palmer, C. Zachary ; Westbrook, Brent R. ; Fortenberry, Ryan C. ( December 2022 , Frontiers in Astronomy and Space Sciences)

   Rovibrational spectral data for several tetra-atomic silicon carbide clusters (TASCCs) are computed in this work using a CCSD(T)-F12b/cc-pCVTZ-F12 quartic force field. Accurate theoretical spectroscopic data may facilitate the observation of TASCCs in the interstellar medium which may lead to a more complete understanding of how the smallest silicon carbide (SiC) solids are formed. Such processes are essential for understanding SiC dust grain formation. Due to SiC dust prevalence in the interstellar medium, this may also shed light on subsequent planetary formation. Rhomboidal Si₂C₂is shown here to have a notably intense (247 km mol⁻¹) anharmonic vibrational frequency at 988.1 cm⁻¹(10.1 μm) forν₂, falling into one of the spectral emission features typically associated with unknown infrared bands of various astronomical regions. Notable intensities are also present for several of the computed anharmonic vibrational frequencies including the cyclic forms of C₄, SiC₃, Si₃C, and Si₄. These features in the 6–10 μm range are natural targets for infrared observation with theJames Webb Space Telescope(JWST)’s MIRI instrument. Additionally,t-Si₂C₂,d-Si₃C, andr-SiC₃each possess dipole moments of greater than 2.0 D making them interesting targets for radioastronomical searches especially sinced-SiC₃is already known in astrophysical media.

    

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3. A Fermi resonance and a parallel-proton-transfer overtone in the Raman spectrum of linear centrosymmetric N4H+: A polarizability-driven first principles molecular dynamics study

   https://doi.org/10.1063/5.0119251  

   Omodemi, Oluwaseun ; Revennaugh, Ramsay ; Riley, Janiyah ; Kaledin, Alexey L. ; Kaledin, Martina ( October 2022 , The Journal of Chemical Physics)

   We present molecular dynamics (MD), polarizability driven MD (α-DMD), and pump–probe simulations of Raman spectra of the protonated nitrogen dimer N4H+, and some of its isotopologues, using the explicitly correlated coupled-cluster singles and doubles with perturbative triples [CCSD(T)]-F12b/aug-cc-pVTZ based potential energy surface in permutationally invariant polynomials (PIPs) of Yu et al. [J. Phys. Chem. A 119, 11623 (2015)] and a corresponding PIP-derived CCSD(T)/aug-cc-pVTZ-tr (N:spd, H:sp) polarizability tensor surface (PTS), the latter reported here for the first time. To represent the PTS in terms of a PIP basis, we utilize a recently described formulation for computing the polarizability using a many-body expansion in the orders of dipole–dipole interactions while generating a training set using a novel approach based on linear regression for potential energy distributions. The MD/α-DMD simulations reveal (i) a strong Raman activity at 260 and 2400 cm−1, corresponding to the symmetric N–N⋯H bend and symmetric N–N stretch modes, respectively; (ii) a very broad spectral region in the 500–2000 cm−1 range, assignable to the parallel N⋯H+⋯N proton transfer overtone; and (iii) the presence of a Fermi-like resonance in the Raman spectrum near 2400 cm−1 between the Σg+ N–N stretch fundamental and the Πu overtone corresponding to perpendicular N⋯H+⋯N proton transfer.

    

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4. High-level ab initio composites: thermochemical bond dissociation energies for vanadium species

   https://doi.org/10.1080/00268976.2023.2248292  

   Welch, Bradley ; McLean, Ryan ; Wilson, Angela K. ( December 2023 , Molecular Physics)

   Bond dissociation energies (BDE) are key descriptors for molecules and are among the most sought-after properties in chemistry. Despite their importance, the accurate prediction of BDE’s for transition metal species can be particularly daunting for both experiment and computation. Experimental data has been limited and, when available, often has large error bars, making the critical evaluation and identification of suitable computational methods difficult. However, recent advancements in the experimental determination of BDE’s with techniques such as Velocity Map Imaging and 2 Photon Ionization now provide useful gauges for computational strategies and new methodologies, providing energies with unprecedented accuracies. The vanadium diatomics (VX, X=B, C, N, O, F, Al, Si, P, S, Cl) have been challenging for computational chemistry methods, and, thus, a new experimental gauge enables methods to be reevaluated and developed for these species. Herein, the super-correlation consistent Composite (super-ccCA or s-ccCA), a new thermochemical scheme centered around CCSD(T)/complete basis set (CBS) limit computations with additional contributions that account for scalar-relativistic effects, and coupled cluster contributions beyond CCSD(T) up to quintuple excitations has been considered. The agreement between determinations made by the s-ccCA scheme and by recent experiment is excellent, demonstrating the utility of the new approach in addressing challenging metal systems, even those of multireference nature. In light of recent experimental BDE’s, the longstanding correlation consistent composite approach (ccCA) is also evaluated for the VX species and find that the mean absolute deviation (MAD) is greatly reduced compared to previously used experimental values. 

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5. Accurate Interaction Energies of CO 2 with the 20 Naturally Occurring Amino Acids

   https://doi.org/10.1002/cphc.202300027  

   Sylvanus, Amarachi G. ; Vogiatzis, Konstantinos D. ( May 2023 , ChemPhysChem)

   We have performed a series of highly accurate calculations between CO₂and the 20 naturally occurring amino acids for the investigation of the attractive noncovalent interactions. Different nucleophilic groups present in the amino acid structures were considered (α‐NH₂, COOH, side groups), and the stronger binding sites were identified. A database of accurate reference interactions energies was compiled as computed by explicitly‐correlated coupled‐cluster singles‐and‐doubles, together with perturbative triples extrapolated to the complete‐basis‐set limit. The CCSD(F12)(T)/CBS reference values were used for comparing a variety of popular density functionals with different basis sets. Our results show that most density functionals with the triple‐zeta basis set def2‐TZVPP align with the CCSD(F12)(T)/CBS reference values, but errors range from 0.1 kcal/mol up to 1.0 kcal/mol.

    

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