Theoretical study of beryllium borohydride Be(BH₄)₂ Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/6q182q54v

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  • Ab initio calculations using the B3LYP and CCSD(T) methods were employed in this work to explore the most stable structure of the intriguing compound Be(BH₄)₂, for which as many as seven different structures have been proposed in the literature. Use of the B3LYP density functional method with several large basis sets predicts the classic D[subscript 2d] structure to be the lowest energy form, but with a D[subscript 3d] configuration having an energy minimum only 353 cm⁻¹ higher in energy. Potential energy surfaces were calculated using BH₄ torsional and tumbling coordinates and these showed a low energy barrier of 553 cm⁻¹ for interconversion of the two forms. Similar calculations were done using the CCSD(T) method, which is usually considered the "gold standard" of quantum calculations. These were much more time intensive and gave the reverse ordering, with the D[subscript 3d] configuration 90 cm⁻¹ lower in energy than the D[subscript 2d] form. In all cases, inclusion of zero point energy calculated for both isomers tended to lower the ground state energy of the D[subscript 3d] structure relative to that of the D[subscript 2d] form. It is concluded that the D[subscript 3d] structure is the most stable form of Be(BH₄)₂ but that a substantial fraction of the molecules exist in the D[subscript 2d] form in the room temperature vapor. Recent versions of the Gaussian program used in this work allow calculations of anharmonic properties of a molecule and these were done to predict more accurate vibration-rotation parameters as well as infrared and Raman intensities. These were employed in a reexamination of several earlier experimental infrared spectra. Comparison of the predicted and experimental spectra of Be(BH₄)₂ vapor shows the clear presence of both forms in the room temperature vapor. When Be(BH₄)₂ is isolated in an argon matrix at 20 K, only infrared features attributable to the D[subscript 3d] form remain, a result consistent with the predictions of the CCSD(T) calculations. Previous infrared spectra of the vapor at 0.0015 cm⁻¹ resolution showed no resolvable vibration-rotation transitions, but concurrent studies in a cold jet revealed many sharp lines with width comparable to the resolution. The jet spectra were quite limited in range however, and efforts to simulate these using theoretical vibration-rotation parameters were unsuccessful. Possible reasons for the extreme congestion of features in both the jet and room temperature spectra are discussed.
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