Knowledge Bank

Ab initio single- and multi-reference CISD correlation-energy calculations have been performed on the ground states and the lowest-energy singlet excited states of $H2CO$, $H2CNH$, trans-HNNH, and $trans-(CHO)2$, and complexes of these molecules with HF, to investigate the effect of hydrogen bonding on $n\to \pi \ast$ transition energies. The ground state geometries of all monomers and complexes were optimized at MP2/6-31+G(d,p), and vertical $n\to \pi \ast$ transition energies were computed at these geometries. For $H2CO$ and $H2CNH$, which contain only a single chromophore, the blue shift of the $n\to \pi \ast$ transition energy in the complex is similar to the ground state hydrogen bond energy, suggesting that the hydrogen bond is broken or severely weakened in the excited state. In contrast, for trans-HNNH and $trans-(CHO)2$, which have two chromophoric groups, the blue shift of the $n\to \pi \ast$ band in the complex is significantly less than the ground- state hydrogen bond energy, suggesting that the hydrogen bond remains essentially intact in the excited state. In these complexes, the $n\to \pi \ast$ excitation occurs primarily in the nonhydrogen-bonded chromophoric group.