Knowledge Bank

“The ultraviolet spectrum of the nitrate ion has been measured in several different solvents. Two bands appear, one near 3000 \AA., the other near 2000 \AA. The 3000 \AA. hand may be assigned as a forbidden $n\to \pi \ast$ transition of electronic symmetry $A^\{\backslash prime\}\_\{1\}\; \backslash to\; A\_\{1\}^\{\backslash prime\}^\{\backslash prime\}$. This is of particular interest because it requires coupling with two vibrations to make it appear, according to the Herzberg-Teller selection rules, Experimentally the band shows both a blue shift and a considerable increase in intensity in going to solvents of increasing hydrogen bonding power. These solvent effects are in accord with theory for a. band of this nature. The band near 2000 \AA. is attributed, to an allowed $A1\prime \to E\prime$ transition of a $\pi \to \pi \ast$ type. It shows a blue shift in hydrogen bonding solvents similar to that of the $n\to \pi \ast$ transition. This is explained in terms of the nature of the $\pi$-orbitals of the molecule.”