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Vibronically resolved laser-induced fluorescence/dispersed fluorescence (LIF/DF) and cavity ring-down (CRD) spectra of the electronic transition of the calcium isopropoxide [CaOCH(CH 3 ) 2 ] radical have been obtained under jet-cooled conditions. An essentially constant energy separation of 68 cm −1 has been observed for the vibrational ground levels and all fundamental vibrational levels accessed in the LIF measurement. To simulate the experimental spectra and assign the recorded vibronic bands, Franck–Condon (FC) factors and vibrational branching ratios (VBRs) are predicted from vibrational modes and their frequencies calculated using the complete-active-space self-consistent field (CASSCF) and equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) methods. Combined with the calculated electronic transition energy, the computational results, especially those from the EOM-CCSD calculations, reproduced the experimental spectra with considerable accuracy. The experimental and computational results suggest that the FC matrix for the studied electronic transition is largely diagonal, but transitions from the vibrationless levels of the à state to the X̃-state levels of the CCC bending ( ν 14 and ν 15 ), CaO stretch ( ν 13 ), and CaOC asymmetric stretch ( ν 9 and ν 11 ) modes also have considerable intensities. Transitions to low-frequency in-plane [ ν 17 ( a ′)] and out-of-plane [ ν 30 ( a ′′)] CaOC bending modes were observed in the experimental LIF/DF spectra, the latter being FC-forbidden but induced by the pseudo-Jahn–Teller (pJT) effect. Both bending modes are coupled to the CaOC asymmetric stretch mode via the Duschinsky rotation, as demonstrated in the DF spectra obtained by pumping non-origin vibronic transitions. The pJT interaction also induces transitions to the ground-state vibrational level of the ν 10 ( a ′) mode, which has the CaOC bending character. Our combined experimental and computational results provide critical information for future direct laser cooling of the target molecule and other alkaline earth monoalkoxide radicals.
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Diborane anharmonic vibrational frequencies and Intensities: Experiment and theory
The vibrational dynamics of diborane have been extensively studied both theoretically and experimentally ever since the bridge structure of diborane was established in the 1950s. Numerous infrared and several Raman spectroscopic studies have followed in the ensuing years at ever increasing levels of spectral resolution. In parallel, ab initio computations of the underlying potential energy surface have progressed as well as the methods to calculate the anharmonic vibration dynamics beyond the double harmonic approximation. Nevertheless, even 70 years after the bridge structure of diborane was established, there are still significant discrepancies between experiment and theory for the fundamental vibrational frequencies of diborane. In this work we use para-hydrogen (pH2) matrix isolation infrared spectroscopy to characterize six fundamental vibrations of B2H6 and B2D6 and compare them with results from configuration-selective vibrational configuration interaction theory. The calculated frequencies and intensities are in very good agreement with the pH2 matrix isolation spectra, even several combination bands are well reproduced. We believe that the reason discrepancies have existed for so long is related to the large amount of anharmonicity that is associated with the bridge BH stretching modes. However, the calculated frequencies and intensities reported here for the vibrational modes of all three boron isotopologues of B2H6 and B2D6 are within ± 2.00 cm− 1 and ± 1.44 cm− 1, respectively, of the experimental frequencies and therefore a refined vibrational assignment of diborane has been achieved.
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- Award ID(s):
- 2101719
- PAR ID:
- 10525441
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Journal of Molecular Spectroscopy
- Volume:
- 400
- Issue:
- C
- ISSN:
- 0022-2852
- Page Range / eLocation ID:
- 111887
- Subject(s) / Keyword(s):
- Matrix Isolation Spectroscopy, Anharmonic Vibrational Dynamics, Infrared Spectroscopy, Computational Spectroscopy, Diborane, Fermi Resonance, Darling-Dennison Resonance
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.jms.2024.111887
