Knowledge Bank

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 ($\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi$) at the linear geometry and split into a bent $12A\prime \prime$ ground state and a linear $12\Pi$ excited state which becomes $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}A\prime$ 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, $22A\prime$ and $22\Pi$, 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.""