Potential energy surfaces of small molecules and cations
Speakman, Lucas Daniel
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Precise thermochemical properties of benzaldehyde, gallium pentahydride, boron pentahydride, aluminum pentahydride, ozone, and silicon dicarbide have been determined through systematic extrapolations of ab initio energies within the Coupled Cluster framework of higher order excitation corrections. The discrepancy between experiment and theory regarding benzaldehyde’s internal barrier to rotation has been resolved, with a recommended barrier of 7.7 kcal mol-1. Gallium pentahydride may exist at low temperatures, as a weak complex between gallane and molecular hydrogen, with a D0 of 0.11 kcal mol-1. The deprotonation energies of group thirteen pentahydrides follow an unusual pattern: 326.3 (AlH5), 331.0 (GaH5), and 332.4 (BH5) kcal mol-1. The gap in observed properties usually falls between boron and aluminum, with gallium’s properties often very similar to those of aluminum. Several ionization and excitation pathways to the quartet state of ozone radical cation were investigated to aid in synthesis. From the ground state of ozone, vertical ionizations to 4A2 O3+, 4B2 O3+, and 4A1 O3+ are possible at 13.91, 14.39, and 14.90 eV, respectively. Other possible pathways to the quartet states are 4A1 O3+ ← 3A2 O3, 4A2 O3+ ← 3A2 O3, 4A1 O3+ ← 3B2 O3, 4A2 O3+ ← 3B1 O3, 4B2 O3+ ← 3B1 O3, 4A1 O3+ ← 2B2 O3+, and 4A2 O3+ ← 2B2 O3+ with vertical IPs of 12.46, 12.85, 12.82, 12.46, 12.65, 1.36, and 1.26 eV, respectively. One of the most accurate potential energy surfaces in literature was developed for SiC2 by implementing a composite method, c-CBS CCSDT. This method includes extrapolation to the complete basis set limit, CCSD(T), with additional CCSDT, relativistic, and core-valence corrections. It yields a barrier to linearity for SiC2 of 5.45 ± 0.1 kcal mol-1, fundamental vibrational frequencies for the “T-shaped” ground state of 1752, 846, and 15 cm-1, and ΔfH0°(SiC2) of 152.45 ± 0.20 kcal mol-1.