Solvent effects on the photodissociation of formic acid: A theoretical study

Photodissociation of aqueous formic acid has been investigated with the CASSCF, DFT, and MR-CI methods. Solvent effects are considered as a combination of the hydrogen-bonding interaction from explicit H2O molecules and the effects from the bulk surrounding H2O molecules using the polarizable continuum model. It is found that the hydrogen-bonding effect from the explicit water in the complex is the major factor to influence properties of aqueous formic acid, while the bulk surrounding H2O molecules has a noticeable influence on the structures of the complex. The direct C-O bond fission along the S-1 pathway is predicted to be an important channel upon photolysis of aqueous formic acid at 200 nm, which is consistent with experimental observation that aqueous formic acid dissociates predominantly into fragments of HCO and OH. The existence of a dark channel upon photolysis of aqueous formic acid at 200 nm is assigned as fast relaxation from the S-1 Franck-Condon geometry to the T-1/S-1 intersection and subsequent S-1 -> T-1 intersystem crossing process. S-1 -> S-0 internal conversion followed by molecular elimination to CO + H2O is the most probable primary process for formation of carbon monoxide, which was observed with considerable yield upon photolysis of aqueous formic acid at 253.7 nm.