The thermodynamic properties of aqueous nitroxyl (HNO) and its anion (NO ؊ ) have been revised to show that the ground state of NO ؊ is triplet and that HNO in its singlet ground state has much lower acidity, pKa( 1 HNO͞ 3 NO ؊ ) Ϸ 11.4, than previously believed. These conclusions are in accord with the observed large differences between 1 HNO and 3 NO ؊ in their reactivities toward O2 and NO. Laser flash photolysis was used to generate 1 HNO and 3 NO ؊ by photochemical cleavage of trioxodinitrate (Angeli's anion). The spin-allowed addition of 3 O2 to 3 NO ؊ produced peroxynitrite with nearly diffusion-controlled rate (k ‫؍‬ 2.7 ؋ 10 9 M ؊1 ⅐s ؊1 ). In contrast, the spin-forbidden addition of 3 O2 to 1 HNO was not detected (k Ͻ Ͻ 3 ؋ 10 5 M ؊1 ⅐s ؊1 N itroxyl (HNO, also known as nitrosyl hydride) and its anion, NO Ϫ , are the simplest molecules with nitrogen in the ϩ1 oxidation state and yet their aqueous chemistry is not well understood. Recent suggestions that these redox neighbors of the biologically important NO radical may play a role in cellular metabolism (1-4) and in aerobic environments may be precursors to cytotoxic peroxynitrite, ONOO Ϫ , (5, 6) have engendered considerable interest in the chemistry of HNO͞NO Ϫ . The characterization of these species is complicated by their instability with respect to formation of nitrous oxide (7,8). In most cases where nitroxyl has been invoked as an intermediate, the rate-determining step was its generation, a situation that allows little insight into the properties and reactivities of HNO͞NO Ϫ themselves. The NO Ϫ anion is isoelectronic with O 2 and, like O 2 , should have a triplet ground state, whereas the ground state of HNO should be a singlet. Indeed, these ground state assignments have been well established for HNO͞NO Ϫ in the gas phase (9, 10).A frequently used source for aqueous HNO͞NO Ϫ is trioxodinitrate (N 2 O 3 2Ϫ , also known as Angeli's anion), whose conjugate acid (H 2 N 2 O 3 ) has consecutive pKa values of 2.5 and 9.7 (11). It is widely accepted (7, 8) that slow decomposition of the monoprotonated anion occurs through heterolytic NON bond cleavageSubsequent addition of O 2 could yield peroxynitriteHowever, nitrate, which is the peroxynitrite decomposition product, was not detected among the end products of HN 2 O 3 Ϫ decay (12). This result was interpreted as evidence against the occurrence of reaction 2. On the other hand, the same researchers reported peroxynitrite formation during N 2 O 3 2Ϫ photolysis in alkaline solution (13). To reconcile the data, it was suggested that thermal reaction 1 followed by deprotonation of HNO produces singlet NO Ϫ , which is the ground state in water, and that 1 NO Ϫ is unreactive toward O 2 . In contrast, photochemical cleavage of N 2 O 3 2Ϫ was thought to generate the long-lived triplet excited state of NO Ϫ , which reacted with O 2 . However, it seems unlikely that hydration can reverse a gas-phase energy gap of about 70 kJ͞mol between the ground state 3 NO Ϫ and the excited state 1 NO Ϫ (10). Moreover, by ana...

The mechanism and dynamics of photoinduced charge separation and charge recombination have been investigated in synthetic DNA hairpins possessing donor and acceptor stilbenes separated by one to seven A:T base pairs. The application of femtosecond broadband pump-probe spectroscopy, nanosecond transient absorption spectroscopy, and picosecond fluorescence decay measurements permits detailed analysis of the formation and decay of the stilbene acceptor singlet state and of the charge-separated intermediates. When the donor and acceptor are separated by a single A:T base pair, charge separation occurs via a single-step superexchange mechanism. However, when the donor and acceptor are separated by two or more A:T base pairs, charge separation occurs via a multistep process consisting of hole injection, hole transport, and hole trapping. In such cases, hole arrival at the electron donor is slower than hole injection into the bridging A-tract. Rate constants for charge separation (hole arrival) and charge recombination are dependent upon the donor-acceptor distance; however, the rate constant for hole injection is independent of the donor-acceptor distance. The observation of crossover from a superexchange to a hopping mechanism provides a "missing link" in the analysis of DNA electron transfer and requires reevaluation of the existing literature for photoinduced electron transfer in DNA.