The unexpected effect of aqueous ion pairs on the forbidden n →π* transition in nitrate.

Aqueous nitrate is ubiquitous in the environment, found for example in stratospheric clouds, tropospheric particulate matter, rain and snow, fertilized fields, rivers and the ocean. Its photolysis is initiated by absorption into the strongly forbidden n →π* transition. Photolysis reactivates deposited nitrate, releasing nitrogen oxides, and UV light is commonly used to break down nitrate pollution. The transition is doubly forbidden unless its symmetry is broken, giving a powerful means of probing the interactions of nitrate with its environment and of using experiment to validate the results of theory. In this study we demonstrate the remarkably different effects of the addition of a series of mono- and di-valent metal chlorides on the nitrate UV transition. While they all shift the transition to shorter wavelengths, the shift changes significantly from one to another. For the monovalent series Li+, Na+, K+, the blue shift decreases down the column being strongest for Li+ and weakest for K+. For the divalent series Mg2+, Ca2+, Ba2+, the opposite effect is observed with the energy shift of Ba2+ being an order of magnitude larger than for Mg2+. The absorption intensity also changes; the addition of Na+ and K+ decrease intensity whereas Li+ increases intensity. For the divalent cations an increase is seen for all three members of the series Mg2+, Ca2+ and Ba2+. Paradoxically, the effect of addition of CaCl2 to the solution is to decrease the environmental photolysis rate of nitrate; despite the increase in intensity, Ca2+ blue shifts the peak position above the tropospheric photolysis threshold around 300 nm. Using computational chemistry we conclude that the effects are due to the microscopic interactions of the nitrate anion and not continuum effects. Two microscopic mechanisms are investigated in detail, the formation of a nitrate monohydrate cluster and a contact ion pair. The contact ion pair shows the potential for significant impact on the energy and intensity of the transition.

The riddle of the forbidden UV absorption of aqueous nitrate: the oscillator strength of the n →π* transition in NO3- including second order vibronic coupling.

The environmentally relevant n →π* transition in the nitrate anion is doubly forbidden by symmetry, as the Franck-Condon and first order vibronic coupling terms are both zero in the gas phase. Inclusion of the second order vibronic coupling term is therefore essential when calculating the oscillator strength. Here we have calculated an oscillator strength of 5.7 × 10-6. The second order vibronic coupling term is included by manually displacing the ground-state geometry simultaneously along two normal modes, Ql and Qk, in 19 × 19 steps, and calculating the transition dipole moment at each point by TD-DFT/ωB97XD/aug-cc-pVTZ and fitting to a polynomial in order to evaluate the second derivative with respect to Ql and Qk. In the aqueous phase the high symmetry of NO3- is broken and the first order term is no longer forbidden. However, the calculated solvated geometry still resembles the gas phase geometry and the calculated first order term does not contribute significantly to the overall oscillator strength of 1.9 × 10-6. This is a rare example of higher order vibronic coupling being more important than the lowest order term.