An ab initio study of water–oxygen complexes (O 2–W n, n=1–6) in the ground and lowest singlet excited states

Abstract

Geometries of the O 2–W n ( n=1–4) complexes with triplet multiplicity were optimized at the UMP2/6-311++G** level of theory. The triplet ground and lowest singlet excited state geometries of O 2 and the O 2–W n ( n=1–6) complexes were optimized at the B3LYP/6-31+G* level of theory. Various methods were tested to study the lowest singlet excited states of O 2 and the O 2–W n ( n=1–6) complexes, and it was found that the B3LYP/6-31+G* method worked well and was economical enough to permit a detailed study of O 2–W n ( n=1–6) complexes. The CCD/AUG-cc-pVDZ and B3LYP/6-31+G* methods were found to yield similar results for the triplet ground and lowest singlet excited states of O 2 and the O 2–W 1 complex. Microsolvation of O 2 in water clusters ( n=1–5) in the ground state does not affect the structures of the latter. However, microsolvation of O 2 in the ground state modifies the structure of the cage form of water hexamer appreciably such that the latter acquires a non-planar cyclic structure. The structure of the O 2–W 1 complex is strongly modified in going from the triplet ground state to the singlet excited state. Thus W 1 in the singlet excited state gets dissociated into the H and OH fragments both of which are bound to the same oxygen atom of the O 2 molecule. This structure of the O 2–W 1 complex in the singlet excited state is retained in the O 2–W n ( n=2,3) complexes also but not in the O 2–W n ( n=4–6) complexes where none of the molecules is dissociated and are bound to one another as in the triplet ground state. The singlet O 2–W 2 complex is an isomer of a hydrogen bonded complex of 2H 2O 2 and the former represents an activated state of the latter by about 1.06 eV. An appreciable amount of electronic charge is transferred to O 2 in the O 2–W n ( n=1–3) complexes from the water molecules, particularly from the hydrogen atoms. Thus O 2 in the singlet excited state of each of these complexes would behave somewhat like O 2 −.