Structural, energetic and vibrational properties of oxidized mercury in the gas and aqueous phases

Elemental mercury emitted by anthropogenic processes is oxidized in the atmosphere by halogens, ozone or nitro species. The oxidized mercury then deposits in the surface environment or reduces back to elemental mercury. The aqueous phase atmospheric photoreduction of oxidized mercury can also take place in the clouds. Thus, the oxidation, reduction and surface deposition control the overall mercury budget and its distribution in the atmosphere. In this study, the structural and vibrational properties of all oxidized mercury molecules both in gas and aqueous phases are studied using the first principle density-functional method. A correlation of bond distance, vibrational frequency, stability and solvation free energy with the electronegativity of non-metal atoms has emerged from the calculations. The bond distances between mercury and first elements depend on the electronegativity of the second elements; the more electronegative the second element is, the shorter the bond between mercury and the first element. The wave number of stretching frequency is inversely related to the bond distances. Importantly, the mercury – halogen bond distances lengthen in the aqueous phase from the corresponding gas phase distances in all molecules. Interestingly, the increment of distances in halide molecules is higher than molecules that contain oxygen and nitrogen atoms. For isomers, the molecules with mercury-oxygen bonds are more stable than those with mercury-halogen bonds both in the gas and aqueous phases. The relative stability, however, decreases in the aqueous phase. Solvation free energy is higher for molecules where oxygen is a terminal atom than that for molecules where oxygen is a bridging atom.