Nitric oxide dimer

A second illustrative example of the utility of TRPES and TRCIS for studying complex molecular photodissociation dynamics that involve multiple electronic state is the case of the weakly bound cis-planar C2V nitric oxide dimer [174], The weak (Do = 710cm-1) 1 A ground-state covalent bond is formed by the pairing of two singly occupied ji orbitals, one from each NO(X2II) monomer. The very intense UV absorption spectrum of the NO dimer appears... 

Nitric oxide dimerizes in the solid state (Fig. 9-2). Dimers persist in the vapor at the boiling point where the structure is similar but the N—N distance is longer (2.237 A) and the ONN angle is 99.6°. The binding energy of the dimer, —16 kJ mol-1, is consistent with the long N—N bond. The dimer has no unpaired spins but feeble intrinsic temperature-independent paramagnetism. Unstable forms can be isolated in matrices. 

The nitric oxide dimer, (NO)2, in Ar matrices exists as the cis form (1862 and 1768cm ) or the trans form (1740cm ), with the v(NO) shown in brackets [268a]. The IR spectrum of the cis dimer in the gaseous state has been assigned [268b]. 

RCMC has been used to compute nitric oxide dimerization in the liquid and vapor phases [2]. The simulation results were found to be in good agreement with experimental data for both the liquid phase, where the association is very strong (fraction of monomers 0.05), and the vapor phase, which shows a very low amount of dimerization (fraction of monomers 0.99). 

Fig. 2.22 (a) 3-pulse echo signals corrected for echo decay and (b) the ESEEM spectrum after FT for nitric oxide dimer with 5=1 trapped in Na-A zeolite. The figure is adapted from [D. Biglino et al. Chem. Phys. Lett. 349, 511 (2001)] with permission from Elsevier... 

The neutral nitric oxide dimer, N2O2, is a diamagnetic molecule and is formed from nitric oxide in the condensed phase at low temperatures as shown in Equation 9 ... 

I. I. Zakharov, Z. R. Ismagilov, S. Ph. Ruzankin, V. F. Anufrienko, S. A. Yashnik and O. I. Zakharova. Density functional theory molecular cluster study of copper interaction with nitric oxide dimer in Cu-ZSM-5 catalysts. J. Phys. Chem. C 111, 2007, 3080-3089. 

The molecular orbital description of the bonding in NO is similar to that in N2 or CO (p. 927) but with an extra electron in one of the tt antibonding orbitals. This effectively reduces the bond order from 3 to 2.5 and accounts for the fact that the interatomic N 0 distance (115 pm) is intermediate between that in the triple-bonded NO+ (106 pm) and values typical of double-bonded NO species ( 120 pm). It also interprets the very low ionization energy of the molecule (9.25 eV, compared with 15.6 eV for N2, 14.0 eV for CO, and 12.1 eV for O2). Similarly, the notable reluctance of NO to dimerize can be related both to the geometrical distribution of the unpaired electron over the entire molecule and to the fact that dimerization to 0=N—N=0 leaves the total bond order unchanged (2 x 2.5 = 5). When NO condenses to a liquid, partial dimerization occurs, the cis-form being more stable than the trans-. The pure liquid is colourless, not blue as sometimes stated blue samples owe their colour to traces of the intensely coloured N2O3.6O ) Crystalline nitric oxide is also colourless (not blue) when pure, ° and X-ray diffraction data are best interpreted in terms of weak association into... 

Nitric oxide exists as a dimer in the solid state. This dimer can occupy two positions as follows ... 

The nitro compound is isomerized to the nitrite, which can decompose either photochemically or thermally to the 9-anthroxyl radical. This radical can then dimerize or add nitric oxide to the 10 position to form the nitrosoketone. Isomerization of the nitrosoketone yields the monooxime, which then yields anthraquinone photochemically in the presence of NO. 

Soluble forms of guanylyl cyclase are activated by nitric oxide. These enzymes are homologous to the catalytic domains of the membrane-bound forms of GC. They are considered heterodimers because they appear to exist, under physiological conditions, as complexes of a and P subunits, each with Mr of 70-80 kDa. Both types of soluble GC contain three primary domains an amino-terminus heme domain responsible for binding nitric oxide (NO), a dimerization domain and a carboxy terminus catalytic domain. The aP heterodimer is required for enzyme activity [35]. This can be seen as similar to the situation for AC, which contains two catalytic entities within a single polypeptide chain (Fig. 21-5). 

Li, H., Raman, C. S., Glaser, C. B., Blasko, E., Young,T. A., Parkinson, J. F., Whitlow, M., Poulos. T. L., Crystal structures of zinc-free and -bound heme domain of human inducible nitric-oxide synthase. Implications for dimer stability and comparison with endothelial nitric-oxide synthase, ]. Biol.Chem. 

A very remote secondary H/D isotope effect has been measured for the 2 + 2-cycloaddition of TCNE to 2,7-dimethylocta-2,fran -4,6-triene. The reaction of nitric oxide with iV-benzylidene-4-methoxyaniline to produce 4-methoxybenzenediazonium nitrate and benzaldehyde is thought to proceed via a 2 + 2-cycloaddition between nitric oxide and the imine double bond. A novel mechanism for the stepwise dimerization of the parent silaethylene to 1,3-disilacyclobutane involves a low-barrier [1,2]-sigmatropic shift. Density functional, correlated ab initio calculations, and frontier MO analysis support a concerted 2 + 2-pathway for the addition of SO3 to alkenes. " The enone cycloaddition reactions of dienones and quinones have been reviewed. The 2 + 2-photocycloadditions of homochiral 2(5H)-furanones to vinylene carbonate are highly diastereoisomeric. ... 

Kaplan and Shechter found that certain oxidants react with the nitronate salts of secondary nitroalkanes to yield vic-dinitroalkanes (111) in a reaction referred to as oxidative dimerization. These reactions are believed to involve transfer of an electron from the secondary alkyl nitronate to the oxidant with the production of a nitroalkyl radical. The radical can then dimerize to the corresponding vtc-dinitroalkane (111) (Equation 1.2) or lose nitric oxide to form a ketone via the Nef reaction (Equation 1.3). Unfortunately, formation of the ketone is a major side-reaction during oxidative dimerization and is often the major product. 

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