ELECTRONIC STRUCTURE OF HCCO RADICAL
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Date
1991
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Ohio State University
Abstract
Several electronic states of the HCCO radical have been studied by ab initio SCF, MCSCF, CI, and UMP2 calculations. Geometry optimizations have been carried out with a TZP basis set at the UMP2 level for the lower two states and at the SDCl level for the upper two states. Generally-contracted Dunning correlation-consistent pVDZ and pVTZ basis sets have been used for the accurate energy determination. The two lowest states form a Renner-Teller pair, degenerate ($^{2}\Pi$) at the linear geometry and split into a bent $1^{2}A^{\prime\prime}$ ground state and a linear $1^{2}\Pi$ excited state which becomes $^{2}A^{\prime}$ upon bending.Davidson-corrected 56-reference SDCI with a Dunning generally-contracted [431/31] basis set gives an energy difference of 981 $cm^{-1}$ for the lower Renner-Teller pair. Two higher states, $2^{2}A^{\prime}$ and $2^{2}\Pi$, also form a Renner-Teller pair with a barrier to the linearity of 311 $cm^{-1}$. The same level of calculation places these states about 32316 $cm^{-1}$ above the ground state minimum. The bonding is mostly $H-C=C=O$ in the two lowest states, and is H-C$\equiv$ C- \.{O} in the upper pair of states. The equilibrium geometries of the various states are given in the following table. [FIGURE] Optimized at the UMP2 and SDCl levels with a Dunning TZP segmented -contraction basis set. The bent structures are in trans conformation.""
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Author Institution: Department of Chemistry, The Ohio State University