Characterization of the three lowest-lying singlet electronic states of AlOH

Two linear (1Σ+ and Π)1 and three bent (1 1A′, 2 1A′, and 1 1A″) lowest-lying electronic singlet states of AlOH have been systematically investigated employing ab initio self-consistent-field, configuration interaction with single and double excitations, coupled cluster with single and double excitations (CCSD), CCSD with perturbative triple excitations [CCSD(T)], and CCSD with iterative partial triple excitations (CCSDT-3 and CC3) quantum mechanical methods with basis sets up to augmented correlation consistent polarized valence quadruple zeta (aug-cc-pVQZ). The linear Σ+1 state is found to be a remarkably low-energy transition state between the two equivalent bent 1 1A′ structures, while the linear Π1 state is a second-order saddle point, which leads to the bent 2 1A′ and 1 1A″ states. The bent ground (X̃ 1A′) state of AlOH is predicted to have a bond angle of 157° at the aug-cc-pVQZ CC3 level of theory and is classified as a quasilinear molecule, confirming previous studies. Employing the equation-of-motion coupled cluster method, the first singlet excited state (à 1A′) is predicted to have a bond angle of 110° and to lie 114 kcal/mol (39 900 cm−1, 4.94 eV) above the ground state, whereas the second singlet excited state (B̃ 1A″) is predicted to have a bond angle of 116° and to be located 119 kcal/mol (41 700 cm−1, 5.17 eV) above the ground state. These theoretical energy separations are in excellent agreement with the experimental values T0 (à 1A′)=114.57 kcal/mol (40 073 cm−1, 4.968 eV) and T0 (B̃ 1A″)=119.36 kcal/mol (41 747 cm−1, 5.176 eV). The barriers to linearity for the two bent singlet excited states are determined to be 11.6 kcal/mol for the à 1A′ state and 6.2 kcal/mol for the B̃ 1A″ state.