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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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A correlation between hydroxyl vibrations under compression and anharmonicity: glaucophane as a test case
Abstract The infrared hydroxyl bands and first hydroxyl combination bands of glaucophane are characterized under pressure. In this weakly hydrogen-bonded mineral, the anharmonicity parameter, as determined from the difference between combinations and the fundamentals, is nearly constant with pressure to 15 GPa, indicating that the ambient pressure value of hydroxyl-bond anharmonicity closely reflects its value at high pressures. Given this near-constancy, the Grüneisen parameters of the hydroxyl stretching vibrations of a wide range of minerals, as derived from the pressure dependence of their O–H stretching frequencies, are correlated with the anharmonic parameter of each vibration, as determined from the ambient pressure offset of the summed frequencies of the fundamentaln = 0 to 1 transitions and the frequency of the hydroxyl combination or overtone band corresponding to then = 0 to 2 transition. This correlation is motivated by (1) the anharmonic origin of the Grüneisen parameter; and (2) the grossly similar form of the interatomic potential governing weak- and medium-strength hydrogen bonding in many minerals. This possible correlation provides a means through which the likely pressure-induced hydroxyl mode shifts of phases might be estimated from ambient pressure near-infrared measurements and emphasizes the importance of near-infrared combination/overtone band measurements. In this context, the combination/overtone bands of high-pressure hydrous phases are almost completely uncharacterized, and thus one probe of their anharmonicity has been neglected. Such information directly constrains the nature of hydrogen bonding in these phases, and hence provides possible insights into both their retention of hydrogen and its mobility. Deviations from the anharmonicity-Grüneisen parameter correlation, when observed (as may be the case in prehnite), could provide insights into anomalous effects on the hydroxyl potential well induced by bifurcated H-bonds, pressure-dependent Davydov splitting, or the influence of neighboring cations.
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- Award ID(s):
- 2017294
- PAR ID:
- 10466016
- Publisher / Repository:
- Springer Science + Business Media
- Date Published:
- Journal Name:
- Physics and Chemistry of Minerals
- Volume:
- 50
- Issue:
- 4
- ISSN:
- 0342-1791
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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