O2 Binding Energy Research Articles

Coordination of semiquinone and superoxide radical anions to the zinc ion in SOD model complexes that act as the key step in disproportionation of the radical anions.

Reactions of imidazolate-bridged CuII-ZnII heterodinuclear and CuII-CuII homodinuclear complexes, [CuIIZnII(bdpi)(CH3CN)2](ClO4)3 x 2CH3CN (1) and [CuII2(bdpi)(CH3CN)2](ClO4)3 x CH3CN x 3H2O (2) (Hbdpi = 4,5-bis(di(2-pyridylmethyl)aminomethyl)imidazole), with the p-benzosemiquinone radical anion (Q*-) have been examined to provide mechanistic insight into the role of the ZnII ion in copper-zinc superoxide dismutase (Cu,Zn-SOD). The addition of less than one equivalent of Q*- to a deaerated solution of 1 or 2 in propionitrile at -80 degrees C results in the oxidation of Q*- accompanied by the appearance of a new absorption band at 585 nm due to the CuI-Q complex (3 or 4), the absorbance of which increases linearly with the increase in Q*- concentration. Both the resonance Raman spectra of 3 and 4 exhibit a strong resonance-enhanced Raman band at 1580 cm(-1), which can be assigned to a CO stretching vibration in the CuI-Q complexes. Further addition of Q*- to a deaerated solution of 3 in propionitrile results in the reduction of Q*-, whereas no reduction of Q*- occurs with 4, which does not contain the ZnII ion. Thus, the coordination of Q*- to the ZnII ion is essential for the reduction of Q*- by the CuI ion in 3. The coordination of O2*- and Q*- to the ZnII ion has been confirmed by the electronic and ESR spectra of the O2*- and Q*- complexes with mononuclear ZnII complexes, [ZnII[MeIm(Py)2](CH3CN)](ClO4)2 (5) and [ZnII[MeIm(Me)2](H2O)](ClO4)2 (6) (MeIm(Py)2 = (1-methyl-4-imidazolylmethyl)bis(2-pyridylmethyl)amine, MeIm(Me)2 = (1-methyl-4-imidazolylmethyl)bis(6-methyl-2-pyridylmethyl)amine). The binding energies of O2*- with the ZnII ion in 5 and 6 have been evaluated from the deviation of the g values of the ESR spectra from the free spin value.

A b i n i t i o study of O, O2, and C2H4 interactions with Ag2 and Ag+2. A model for surface–adsorbate interactions

A b initio calculations of silver metal–adsorbate interactions were carried out by using a 36-electron relativistic effective core potential (RECP) for the core electrons of Ag and a 3s3p4d→2s2p2d basis for the valence electrons. Unpromoted surface interactions were modeled by a silver dimer while promoted interactions were modeled by a silver dimer cation. Molecular oxygen is predicted to bind as a π complex while the di-σ and π complexes are predicted to bind similarly for ethylene. The binding energy of O2 decreases on a promoted surface while the binding energy of C2H4 increases compared to an unpromoted surface. The dissociative adsorption of O2 on a promoted surface is found to be inhibited by the formation of a high energy intermediate. The binding energy of O on a silver surface is well reproduced when corrections are made for the electron affinity of the adsorbate and the ionization energy of the dimer.

Theoretical studies of O2−:(H2O)n clusters

AbstractThe interaction of superoxide ion O2− with up to four water molecules [O2−: (H2O)n, n = 1, 2, 4] has been investigated using ab initio molecular orbital theory. The binding energy of O2−: H2O is calculated to be −20.6 kcal/mol in good agreement with gas phase experimental data. At the MP3/6‐31G* level the O2−:H2O complex has a C2v structure with a double (cyclic) hydrogen bond between O2− and H2O. A Cs structure with a single hydrogen bond is only 0.7 kcal/mol less stable. Interaction of H2O with the doubly occupied π* orbital of O2− is preferred slightly over interaction with the singly occupied π* orbital. Natural bond orbital analysis suggests that both electrostatic and charge transfer interactions are important in anionic complexes. The charge transfer occurs predominantly in the O2− → H2O direction and is important in determining the relative stabilities of the different structures and states. Singly and doubly hydrogen‐bonded structures for the O2−: (H2O)2 and O2−: (H2O)4 clusters were found to be similar in stability and the increase in binding of the cluster becomes smaller as each additional water molecule is added to the cluster.

Electron–Ion and Ion–Ion Dissociative Recombination of Oxygen. II. Ion–Ion Recombination

The rate coefficient and the thermal cross section for the oxygen ion–ion dissociative recombination are calculated using the semiclassical formalism. The motion of the heavy particles (positive and negative ions) in their mutual Coulomb field is treated classically. At certain ranges of the ion–ion separation (determined from energy conservation), the electron tunnels from the negative ion to the positive ion. Using the experimental data on the electron binding energy of O2−, this tunneling effect is calculated in the WKB approximation. It is found that the linewidth of the levels due to the dissociative effect is important for the electron tunneling and the molecular dissociation. The rate coefficients and the thermal cross sections have been calculated between 200° and 500°K, and have the orders of magnitude 10−8 − 10−7 cm3/sec and 10−12 cm2, respectively. We find that they both decrease as the temperature increases, and have the following approximate relations: α∝T−1/2, σ(th) ∝T−1. No direct laboratory data is available, but the theoretical results are in agreement with indirect observations from the ionosphere.