The classical and nonclassical forms of protonated acetylene, C2H+3. Structures, vibrational frequencies, and infrared intensities from explicitly correlated wave functions

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Abstract
The vibrational frequencies and IR intensities of C2H+3 and C2D+3 have been predicted using the methods of ab initio molecular quantum mechanics. Specifically, configuration interaction (CI) wave functions have been constructed from double zeta plus polarization (DZ+P) basis sets. In addition, a much larger basis set, triple zeta plus diffuse plus double polarization, has been used at the self‐consistent‐field (SCF) level of theory. The results are qualitatively similar to those (at much lower levels of theory) reported earlier by Raine and Schaefer. At the highest level of theory, CI including all single, double, and triple excitations (213 780 configurations for the classical structure), the nonclassical structure is the lower by only 0.7 kcal/mol.