No abstract

No abstract

Traditional spectroscopic analysis of the complex and irregular absorption spectrum of NO2 has provided a relatively small amount of information concerning the nature of the excited states. An extensive ab initio investigation has been undertaken, therefore, to provide a basis for interpretation of the experimental results. Multiconfiguration self-consistent-field (MC–SCF) wavefunctions have been computed for the low-lying X̃ 2A1, Ã 2B2, B̃ 2B1, C̃ 2A2, 4B2, 4A2, and 2Σ+g electronic states of NO2. The minima of the Ã 2B2, B̃ 2B1, and C̃ 2A2 states have all been found to be within 2 eV of the minimum of the X̃ 2A1 ground state; for these states, C2v potential surfaces have been constructed for purposes of a spectral interpretation. The 4B2, 4A2, and 2Σ+g states are all more than 4 eV above the minimum of the ground state and have been examined in less detail. The study described here significantly improves on previous NO2 ab initio calculations in three important areas: (1) The double-zeta-plus-polarization quality basis set is larger and more flexible; (2) the treatment of molecular correlation is more extensive; and (3) the electronic energies have been calculated for several different bond lengths and bond angles in each state. For the four lowest doublet states the following spectral data have been obtained:     The ground state experimental constants are included in parentheses. The estimated accuracy of the various parameters is ±0.02 Å for bond length, ±2° for bond angle, ±10% for the vibrational frequencies, ±0.10 D for dipole moments, and ±0.3 eV for the adiabatic excitation energies. An unusual feature has been found for the 2Σ+g state. The equilibrium geometry of this linear state has two unequal bond lengths of 1.20 Å and 1.42 Å and the inversion barrier is approximately 800 cm−1.

The quenching of triplet-state molecules by molecular oxygen is examined theoretically. Two different quenching processes are considered: namely, (i) quenching by transfer of electronic excitation energy from the triplet state of the donor molecule to oxygen, resulting in a ground-state donor molecule and an electronically excited singlet-state oxygen molecule; and (ii) quenching by enhancement of intersystem crossing in the donor molecule, resulting in a vibrationally excited donor molecule in its ground electronic state and a ground-state molecule. The relative importance of these two quenching processes is determined almost entirely by Franck—Condon factors, rather than by electronic factors. From our analysis we conclude that quenching by transfer of electronic excitation energy from the triplet-state molecule to oxygen is usually 100–1000 times faster than quenching by enhanced intersystem crossing. Exchange interactions appear to be less important in promoting the radiationless transitions than do interactions involving charge-transfer virtual states. The absolute value of the quenching rate calculated for the energy transfer process satisfactorily accounts for the experimental observations on the oxygen quenching of triplet-state molecules in solution. These theoretical results give strong support to the previous suggestion that quenching of triplet-state molecules by oxygen leads to the production of electronically excited singlet-state (1Σg+ or 1Δg) oxygen molecules.

Characteristic chemiluminescence emission of singlet ('Ag) molecular oxygen at 1268 nm is reported from a Haber-Weiss reaction. The reaction consists of mixing aqueous hydrogen peroxide with a solution of potassium superoxide, solubilized by 18-crown-6 ether in carbon tetrachloride or in dry acetonitrile at room temperature. Since the discovery of the enzyme superoxide dismutase by J. M. McCord and I. Fridovich [(1968) J. Biol. Chem. 243, 5733-5760], the identity of the reactive oxidant in superoxide-generating systems in biology has remained a chemical mystery. The results presented here suggest strongly that the reactive species is singlet oxygen generated via the Haber-Weiss reaction and not, as usually assumed, the hydroxyl radical, -OH, generated by the same reaction.matic systems, the proposal has met with almost universal acceptance and forms the foundation of this very active research field. The spectroscopic data presented here, however, show the characteristic 1268-nm chemiluminescence emission spectrum of singlet molecular oxygen, in the reaction of superoxide anion with hydrogen peroxide. This experiment constitutes direct evidence of the generation of singlet dioxygen in the Haber-Weiss reaction and suggests this species as a possible reactive oxidant in the reaction. This result strongly suggests singlet oxygen could be the sought-after in vivo reactive species. Experimental Results in Carbon Tetrachloride Solutions Superoxide Dismutase (SOD) MechanismsThe enzyme SOD has come to be regarded as a major participant in the antioxidative defense of the host system. The enzyme is both constitutive and inducible under oxidative stress. SOD catalyzes the conversion of superoxide anion to H202 and the dioxygen molecule (1). The enzyme is a strong catalyst and is ubiquitous, even though the uncatalyzed dismutation of O2 is already a fast reaction (107 M-1s-1) under physiological conditions. The superoxide anion, moreover, has proven to be relatively unreactive toward most biological components (2). The question becomes, What then is the biological oxidant generated from superoxide anion in vivo that this enzyme protects against? Based on chemical product and luminescence sensitization studies using potassium superoxide, it was proposed (3) that SOD protects the host system by preventing the generation of the highly reactive long-lived diffusible singlet oxygen molecule O2(1Ag) from superoxide anion. Spectroscopic evidence has established the generation of singlet oxygen in the (i) water-induced dismutation of superoxide anions (4, 5) and (ii) in the electron transfer reactions of superoxide anion with metal redox systems (6, 7). A different interpretation was offered by Beauchamp and Fridovich (8), who in an attempt to rationalize the in vivo role of SOD, proposed that the reactive intermediate was the highly reactive hydroxyl radical generated by the reaction of superoxide anion with hydrogen peroxide-the classic Haber-Weiss reaction (9). They based their suggestion on the inhibitory effects of the...