Anharmonic Frequencies of (MO)₂ and Related Hydrides for M = Mg, Al, Si, P, S, Ca, and Ti and Heuristics for Predicting Anharmonic Corrections of Inorganic Oxides

Abstract

The low-frequency vibrational fundamentals of D 2h inorganic oxides are readily modeled by heuristic scaling factors at fractions of the computational cost compared to explicit anharmonic frequency computations. Oxygen and the other elements in the present study are abundant in geochemical environments and have the potential to aggregate into minerals in planet-forming regions or in the remnants of supernovae. Explicit quartic force field computations at the CCSD­(T)-F12b/cc-pVTZ-F12 level of theory generate scaling factors that accurately predict the anharmonic frequencies with an average error of less than 1.0 cm–1 for both the metal–oxygen stretching frequencies and the torsion and antisymmetric stretching frequencies. Inclusion of hydrogen motions is less absolutely accurate but is similarly relatively predictive. The fundamental vibrational frequencies for the seven tetra-atomic inorganic oxides examined presently fall below 876 cm–1 and most of the hydrogenated species do as well. Additionally, ν6 for the SiO dimer is shown to have an intensity of 562 km mol–1, with each of the other molecules having one or more frequencies with intensities greater than 80 km mol–1, again with most in the low-frequency infrared range. These intensities and the frequencies computed in the present study should assist in laboratory characterization and potential interstellar or circumstellar observation.

Fundamental Vibrational Frequencies and Spectroscopic Constants of Substituted Cyclopropenylidene (c-C3HX, X = F, Cl, CN)

“…While experimental values are unavailable to benchmark the c-C 3 HF and c-C 3 HCl molecules directly, their structures and internal coordinate definitions are similar to those of both c-C 3 HCN and the previously studied c-C 3 H 2 , suggesting that the QFF methodology should yield reliable rotational constants for them, too. Similarly, the exceptional performance of the F12-TZ QFF in the rotational constants supports the corresponding vibrational frequencies, which are already expected to be reliable at this level of theory. − …”

“…QFFs are fourth-order Taylor series expansions of the internuclear potential energy portion of the Watson Hamiltonian . When conjoined to coupled cluster theory at the singles, doubles, and perturbative triples level within the F12b explicitly correlated formalism [CCSD­(T)-F12b] , and a triple-ζ basis set, QFFs can achieve agreement to within 5–7 cm –1 of gas-phase experimental vibrational frequencies. − The accuracy of the rotational data is typically less impressive, especially compared to more expensive composite methods, but the availability of experimental rotational data for c-C 3 HCN can serve to benchmark the performance of the QFFs for the other two molecules given the similar structures. Regardless, the fundamental vibrational frequencies provided here will be invaluable in elucidating the spectra gathered by the existing Stratospheric Observatory for Infrared Astronomy (SOFIA) and the upcoming James Webb Space Telescope (JWST).…”

Pathways to Detection of Strongly-Bound Inorganic Species: The Vibrational and Rotational Spectral Data of AlH2OH, HMgOH, AlH2NH2, and HMgNH2

“…Most of the anharmonic vibrational frequencies for the related and previously explored OAlOH molecule fall at 12.82 μm (the internal Al-O stretch) or longer directly in this "uncertain area of spectrum" (Mölster et al, 2001) from high-level quantum chemical computations (Fortenberry et al, 2020). This is also in line with (MO) 2 molecules where M Mg, Al, Si, P, S, Ca, and Ti and also where all of the spectral features of these molecules lie below 12 μm with many of these also above 17 μm (Westbrook and Fortenberry, 2020). Similar spectra behavior has also been noted in the enstatite monomer (Valencia et al, 2020).…”

“…Then, the SPECTRO program (Gaw et al, 1991) utilizes rotational and vibrational perturbation theory at second-order (VPT2) to produce the rotational constants and the fundamental vibrational frequencies (Mills, 1972;Watson, 1977;Papousek and Aliev, 1982). SPECTRO can not only treat Fermi resonances, but polyads of these resonances for more accurate descriptions of the frequencies (Martin and Taylor, 1997 (Møller and Plesset, 1934;Frisch et al, 2009) and have previously been shown to be reliable for semi-quantitative descriptions of the fundamental intensities (Yu et al, 2015;Finney et al, 2016;Westbrook and Fortenberry, 2020). These are also reported for the molecules of interest.…”

Anharmonic Vibrational Frequencies of Water Borane and Associated Molecules

“…QFFs are fourth-order Taylor series expansions of the internuclear potential energy portion of the Watson Hamiltonian [ 20 ]. When coupled with the F12-TZ level of theory, QFFs frequently offer agreement with gas-phase vibrational frequencies of within 5 to 7 cm [ 21 , 22 , 23 , 24 , 25 ]. Other techniques for computing accurate anharmonic spectral data exist [ 26 ], but as a result of the good performance of the F12-TZ QFF on ammonia borane, this same methodology is used herein to investigate water borane (BH OH ), borinic acid (BH OH), HBO, and borane (BH ).…”