The C-H and alpha(C-X) bond dissociation enthalpies of toluene, C6H5-CH2X (X = F, Cl), and their substituted derivatives: a DFT study

The homolytic C-H bond dissociation enthalpies (BDEs) of toluene and its para- and meta-substituted derivatives have been estimated by using the (RO)B3LYP/6-311++G(2df,2p)//(U)B3LYP/6-311G(d,p) procedure. The performance of two other hybrid functionals of DFT, namely, B3PWP91 and O3LYP, has also been evaluated using the same basis sets and molecules. Our computed results are compared with the available experimental values and are found to be in good agreement. The (RO)B3LYP and (RO)O3LYP procedures are found to produce reliable BDEs for the C-H bonds in toluene and the C-X (X = F, Cl) bond in alpha-substituted toluene (C6H5-CH2X) and their substituted derivatives. The substituent effect on the BDE values has been analyzed in terms of the ground-state effect and the radical effect. The effect of polarization of the C-H bond on the substituent effect is also analyzed. The BDE(C-H) and BDE(C-X) values for alpha-substituted (X = F and Cl) toluenes with a set of para substituents are presented for the first time.     

Theoretical approaches to estimating homolytic bond dissociation energies of organocopper and organosilver compounds

Although organocopper and organosilver compounds are known to decompose by homolytic pathways among others, surprisingly little is known about their bond dissociation energies (BDEs). In order to address this deficiency, the performance of the DFT functionals BLYP, B3LYP, BP86, TPSSTPSS, BHandHLYP, M06L, M06, M06-2X, B97D, and PBEPBE, along with the double hybrids, mPW2-PLYP, B2-PLYP, and the ab initio methods, MP2 and CCSD(T), have been benchmarked against the thermochemistry for the M-C homolytic BDEs (D(0)) of Cu-CH(3) and Ag-CH(3), derived from guided ion beam experiments and CBS limit calculations (D(0)(Cu-CH(3)) = 223 kJ·mol(-1); D(0)(Ag-CH(3)) = 169 kJ·mol(-1)). Of the tested methods, in terms of chemical accuracy, error margin, and computational expense, M06 and BLYP were found to perform best for homolytic dissociation of methylcopper and methylsilver, compared with the CBS limit gold standard. Thus the M06 functional was used to evaluate the M-C homolytic bond dissociation energies of Cu-R and Ag-R, R = Et, Pr, iPr, tBu, allyl, CH(2)Ph, and Ph. It was found that D(0)(Ag-R) was always lower (～50 kJ·mol(-1)) than that of D(0)(Cu-R). The trends in BDE when changing the R ligand reflected the H-R bond energy trends for the alkyl ligands, while for R = allyl, CH(2)Ph, and Ph, some differences in bond energy trends arose. These trends in homolytic bond dissociation energy help rationalize the previously reported (Rijs, N. J.; O'Hair, R. A. J. Organometallics2010, 29, 2282-2291) fragmentation pathways of the organometallate anions, [CH(3)MR](-).     

S=O homolytic bond dissociation enthalpies in sulfoxides

The S=O bond dissociation enthalpies (BDE) were calculated using high-level ab initio methods including G3, G3B3, CBS-Q, CBS-4M, CCSD(T), and MP2. Based on the comparison of these theoretical values and experimental ones, the performances of a number of density functional theory (DFT) methods were then assessed. It was found that the B3P86 method gave the lowest root of mean square error. We therefore used this method to calculate the S=O BDEs of a number of substituted sulfoxides. The electronic effect of the substituents and the remote substituents effect of aryl-substituted sulfoxides on S=O BDE were investigated. In addition, a molecular orbital analysis of typical molecules was conducted in order to investigate the electronic effect on S=O BDEs. We also predicted several S=O BDE values of heteroaromatic substituted sulfoxides using the B3P86 method.     

First estimation of C4-H bond dissociation energies of NADH and its radical cation in aqueous solution

The heterolytic and homolytic C4-H bond dissociation energies of NADH and its radical cation (NADH*+) in aqueous solution were estimated according to the reaction of NADH with N,N,N',N'-tetramethyl-p-phenylenediamine radical cation perchlorate (TMPA*+) in aqueous solution. The results show that the values of the heterolytic and homolytic C4-H bond dissociation energies of NADH in aqueous solution are 53.6 and 79.3 kcal/mol, respectively; the values of the heterolytic and homolytic C4-H bond dissociation energies of NADH*+*+ in aqueous solution are 5.1 and 36.3 kcal/mol, respectively, which, to our knowledge, is first reported. This energetic information disclosed in the present work should be believed to furnish hints to the understanding of the mechanisms for the redox interconversions of coenzyme couple NADH/NAD+ in vivo.     

Investigation of correlation between impact sensitivities and bond dissociation energies in some triazole energetic compounds

In this article, density functional theory has been utilized to study on the correlation between impact sensitivities h _50% and the bond dissociation energies (BDEs) of nine triazole energetic explosives. By employing B3LYP and B3P86 method with the 6-311G** basis set, all the molecules have been fully optimized. The BDEs for removal of the NO₂ group in these compounds have also been calculated at the same level. Computed results show that BDEs calculated by B3LYP method are all less than those by B3P86 method. The relationship between the impact sensitivities and the weakest C–NO₂ bond dissociation energy (BDE) values have been investigated. The results indicate a good linear correlation between the impact sensitivity h _50% and the ratio (BDE/E) of the weakest BDE to the total energy E.     

Energetic differences between the five- and six-membered ring hydrocarbons: strain energies in the parent and radical molecules

The C-H bond dissociation enthalpies (BDEs) for the five- and six-membered ring alkanes, alkenes, and dienes were investigated and discussed in terms of conventional strain energies (SEs). New determinations are reported for cyclopentane and cyclohexane by time-resolved photoacoustic calorimetry and quantum chemistry methods. The C-H BDEs for the alkenes yielding the alkyl radicals cyclopenten-4-yl and cyclohexen-4-yl and the alpha-C-H BDE in cyclopentene were also calculated. The s-homodesmotic model was used to determine SEs for both the parent molecules and the radicals. When the appropriate s-homodesmotic model is chosen, the obtained SEs are in good agreement with the ones derived from group additivity schemes. The different BDEs in the title molecules are explained by the calculated SEs in the parent molecules and their radicals: (1) BDEs leading to alkyl radicals are ca. 10 kJ mol (-1) lower in cyclopentane and cyclopentene than in cyclohexane and cyclohexene, due to a smaller eclipsing strain in the five-membered radicals relative to the parent molecules (six-membered hydrocarbons and their radicals are essentially strain free). (2) C-H BDEs in cyclopentene and cyclohexene leading to the allyl radicals are similar because cyclopenten-3-yl has almost as much strain as its parent molecule, due to a synperiplanar configuration. (3) The C-H BDE in 1,3-cyclopentadiene is 27 kJ mol (-1) higher than in 1,4-cyclohexadiene due to the stabilizing effect of the conjugated double bond in 1,3-cyclopentadiene and not to a destabilization of the cyclopentadienyl radical. The chemical insight afforded by group additivity methods in choosing the correct model for SE estimation is highlighted.     

A comparative computational study of the homolytic and heterolytic bond dissociation energies involved in the activation step of ATRP and SET‐LRP of vinyl monomers

A quantum‐chemical calculation of the homolytic and heterolytic bond dissociation energies of the model compounds of the monomer and dimer is reported. These model compounds include the dormant chloride, bromide, and iodide species for representative activated and nonactivated monomers containing electron‐withdrawing groups as well as for a nonactivated monomer containing an electron‐donor group. Two examples of sulfonyl and N‐halide initiators are also reported. The homolytic inner‐sphere electron‐transfer bond dissociation is known as atom transfer and is responsible for the activation step in ATRP. The heterolytic outer sphere single electron transfer bond dissociation is responsible for the activation step in single electron transfer mediated living radical polymerization (SET‐LRP). The results of this study demonstrated much lower bond dissociation energies for the outer sphere single electron transfer processes. These results explain the higher rate constant of activation, the higher apparent rate constant of propagation, and the lower polymerization temperature for both activated and nonactivated monomers containing electron‐withdrawing groups in SET‐LRP. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1607–1618, 2007     

A quantum chemistry study on C–H homolytic bond dissociation enthalpies of five-membered and six-membered heterocyclic compounds

The C–H homolytic bond dissociation enthalpies (BDEs) of 27 heterocyclic compounds of small systems (less than 8 non-hydrogen atoms) were evaluated by the composite ab initio methods of G4 and CBS-Q. In addition, the C–H BDEs of an extended database including 60 heterocyclic compounds were assessed by 16 DFT functionals. The correlation between the theoretical and experimental values reveals that the BMK functional provided the lowest root mean square error (RMSE) of 10.2 kJ/mol, and the correlation coefficient (R²) was 0.955. The mean deviation (MD), mean absolute deviation (MAD) of BMK are 0.1 kJ/mol and 7.9 kJ/mol separately. Therefore, we utilized BMK to research C–H BDEs together with the substituent effects of five-membered and six-membered heterocyclic compounds including large systems. The nature of C–H BDE change pattern was analyzed by the natural bond orbital (NBO).     

BAC-MP4 predictions of thermochemistry for gas-phase antimony compounds in the Sb-H-C-O-Cl system

Calibrated by both experimental data and high-level coupled-cluster calculations, the BAC-MP4 methodology was applied to 51 SbL(n) (L = H, CH(3), C(2)H(5), Cl, and OH, n = 1-5) molecules, providing calculated heats of formation and associated thermodynamic parameters. These data identify a linear variation in heats of formation with ligand substitution, trends in bond dissociation energies (BDEs) with ligand identity [BDE(Sb-C(2)H(5)) < BDE(Sb-CH(3)) < BDE(Sb-H) < BDE(Sb-Cl) < BDE(Sb-OH)], and a monotonic decrease in BDE upon successive ligand elimination. The linear variation in BDE is consistent with the behavior of other group V elements, in contrast to the characteristic high-low-high trend of adjacent group III (In) and group IV (Sn) elements. Additionally, these data complement those of previous studies of metal-organic species and provide a foundation of thermochemical data that can aid in the selection of CVD precursors and deposition conditions for the growth of antimony-containing materials.     

Electronic structures and reactivities of iodinating agents in the gas phase and in solutions: A density functional study

The electronic and spatial structures of a broad spectrum of neutral compounds with X-Hal (X = N, O, Cl; Hal = Cl, Br, I) bonds and their protonated forms and of different electronic states of triiodide cation, I₃ ⁺, were determined from density functional B3LYP/6 311G* quantum chemical calculations. The effects of the structure of these compounds on the parameters of electrophilic reactivity were revealed and the thermochemical characteristics of homolytic and heterolytic X-Hal bond dissociation and of iodine transfer in hydroxyl-containing solvents were calculated. Due to low homolytic bond dissociation energies of X-I, the formation of molecular iodine and triiodide cation I₃ ⁺ becomes thermodynamically favorable and the cation should act as iodinating agent alternative to acylhypoiodites and N-iodoimides. The solvation effects of MeOH and CH₂Cl₂ on the X-Hal bond homolysis and heterolysis were determined using the PCM model. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1280–1288, August, 2006.     

Homolytic versus Heterolytic Dissociation of Alkalimetal Halides: The Effect of Microsolvation

Herein we report density functional calculations of homolytic and heterolytic dissociation energies of the diatomic alkalimetal halides MX (M=Li, Na, K, Rb, and Cs and X=F, Cl, Br, I, and At) and their corresponding microsolvated structures MX?(H₂O)_n (n=1 to 4). Our results show that the homolytic dissociation energy of the MX?(H₂O)_n species increases with the number of water molecules involved in the microsolvated salts. On the other hand, the heterolytic dissociation energy follows exactly the opposite trend. As a result, while for the isolated diatomic alkalimetal halides, homolytic dissociation is always favored over heterolytic dissociation, the latter is preferred for CsF and CsCl already for n=2, and for n=4 it is the preferential mode of dissociation for more than half of the species studied.     

Stability of carbon-centered radicals: effect of functional groups on the energetics of addition of molecular oxygen

In this paper we examine a series of hydrocarbons with structural features which cause a weakening of the C-H bond. We use theoretical calculations to explore whether the carbon-centered radicals R(*) which are created after breaking the bond can be stabilized enough so that they resist the addition of molecular oxygen, i.e. where the reaction R(*) + O(2) --> ROO(*) becomes energetically unfavorable. Calculations using a B3LYP-based method provide accurate bond dissociation enthalpies (BDEs) for R-H and R-OO(*) bonds, as well as Gibbs free energy changes for the addition reaction. The data show strong correlations between R-OO(*) and R-H BDEs for a wide variety of structures. They also show an equally strong correlation between the R-OO(*) BDE and the unpaired spin density at the site of addition. Using these data we examine the major functional group categories proposed in several experimental studies, and assess their relative importance. Finally, we combine effects to try to optimize resistance to the addition of molecular oxygen, an important factor in designing carbon-based antioxidants.     

Development of an ONIOM-G3B3 method to accurately predict C-H and N-H bond dissociation enthalpies of ribonucleosides and deoxyribonucleosides

The roles of ribonucleoside and deoxyribonucleoside radicals in DNA and RNA damage cannot be properly understood in the absence of knowledge of the C-H and N-H bond dissociation enthalpies (BDEs) depicting the energy cost to generate each of these radicals. However, because the nucleoside radicals tend to be extremely short-lived and it is very difficult to separate and identify different nucleoside radicals, experimental BDEs for nucleosides have remained elusive. Herein, we developed an ONIOM-G3B3 method in order to reliably predict the BDEs of nucleosides and we carefully benchmarked this new method against over 60 experimental BDEs of diverse sizable molecules. It was found that the accuracy of the ONIOM-G3B3 method was about 1.4 kcal/mol for BDE calculations. Using the ONIOM-G3B3 method, a full scale of C-H and N-H BDEs were obtained for the first time for ribonucleosides and deoxyribonucleosides with an estimated error bar of +/-1.4 kcal/mol. Discussions were then made about the interesting connections between these BDE values and previously reported experimental observations concerning radical-mediated DNA and RNA lesions. The significance of the work is twofold: (i) Nucleosides represent one of the most important groups of compounds in science. A full scale of reliable bond dissociation enthalpies for nucleosides is of fundamental importance. (ii) This work demonstrates the feasibility to accurately predict the bond strength of various sizable molecules ranging from nanosize molecular devices to biologically significant compounds.     

Equilibrium acidities and homolytic bond dissociation energies of acidic C-H bonds in alpha-arylacetophenones and related compounds

The equilibrium acidities (pK(AH)s) and the oxidation potentials of the congugate anions [E(ox)(A(-))s] were determined in dimethyl sulfoxide (DMSO) for eight ketones of the structure GCOCH(3) and 20 of the structure RCOCH(2)G, (where R = alkyl, phenyl and G = alkyl, aryl). The homolytic bond dissociation energies (BDEs) for the acidic C-H bonds of the ketones were estimated using the equation BDE(AH) = 1.37pK(AH) + 23.1E(ox)(A(-)) + 73.3. While the equilibrium acidities of GCOCH(3) were found to be dependent on the remote substituent G, the BDE values for the C-H bonds remained essentially invariant (93.5 +/- 0.5 kcal/mol). A linear correlation between pK(AH) values and [E(ox)(A(-))s] was found for the ketones. For RCOCH(2)G ketones, both pK(AH) and BDE values for the adjacent C-H bonds are sensitive to the nature of the substituent G. However, the steric bulk of the aryl group tends to exert a leveling effect on BDEs. The BDE of alpha-9-anthracenylacetophenone is higher than that of alpha-2-anthracenylacetophenone by 3 kcal/mol, reflecting significant steric inhibition of resonance in the 9-substituted system. A range of 80.7-84.4 kcal/mol is observed for RCOCH(2)G ketones. The results are discussed in terms of solvation, steric, and resonance effects. Ab initio density functional theory (DFT) calculations are employed to illustrate the effect of steric interactions on radical and anion geometries. The DFT results parallel the trends in the experimental BDEs of alpha-arylacetophenones.     

Triradical thermochemistry from collision-induced dissociation threshold energy measurements. The heat of formation of 1,3,5-trimethylenebenzene.

A method for measuring the heats of formation of triradicals using energy-resolved collision-induced dissociation (CID) of chloro-substituted biradical negative ions is described. This method is applied to the determination of the heat of formation of 1,3,5-trimethylenebenzene, which was generated by CID of the 5-chloromethyl-m-xylylene ion. The measured CID threshold energy for chloride loss (0.83 +/- 0.07 eV) is combined with the electron affinity of the 5-chloromethyl-m-xylylene biradical (1.120 +/- 0.059 eV) to give a heat of formation of the triradical of 111.0 +/- 4.1 kcal/mol that agrees with the bond additivity value of 109.3 +/- 2.1 kcal/mol. The measured heat of formation indicates a third C-H bond dissociation energy (BDE) in 1,3,5-trimethylbenzene of 88.2 +/- 5.0 kcal/mol, indistinguishable from the C-H BDE in toluene or the first or second C-H BDEs in m-xylene. The results are in agreement with the predictions made on the basis of simple qualitative and high-level molecular orbital theories that predict negligible interaction between the unpaired electrons in the high-spin triradical.     

Homolysis of N-alkoxyamines: a computational study.

During nitroxide-mediated polymerization (NMP) in the presence of a nitroxide R2(R1)NO*, the reversible formation of N-alkoxyamines [P-ON(R1)R2] reduces significantly the concentration of polymer radicals (P*) and their involvement in termination reactions. The control of the livingness and polydispersity of the resulting polymer depends strongly on the magnitude of the bond dissociation energy (BDE) of the C-ON(R1)R2 bond. In this study, theoretical BDEs of a large series of model N-alkoxyamines are calculated with the PM3 method. In order to provide a predictive tool, correlations between the calculated BDEs and the cleavage temperature (Tc), and the dissociation rate constant (k(d)), of the N-alkoxyamines are established. The homolytic cleavage of the N-OC bond is also investigated at the B3P86/6-311++G(d,p)//B3LYP/6-31G(d), level. Furthermore, a natural bond orbital analysis is carried out for some N-alkoxyamines with a O-C-ON(R1)R2 fragment, and the strengthening of their C-ON(R1)R2 bond is interpreted in terms of stabilizing anomeric interactions.     

Noncovalent Modulation of Chemoselectivity in the Gas Phase Leads to a Switchover in Reaction Type from Heterolytic to Homolytic to Electrocyclic Cleavage

In the gas phase, thermal activation of supramolecular assemblies such as host–guest complexes leads commonly to noncovalent dissociation into the individual components. Chemical reactions, for example of encapsulated guest molecules, are only found in exceptional cases. As observed by mass spectrometry, when 1-amino-methyl-2,3-diazabicyclo[2.2.2]oct-2-ene (DBOA) is complexed by the macrocycle β-cyclodextrin, its protonated complex undergoes collision-induced dissociation into its components, the conventional reaction pathway. Inside the macrocyclic cavity of cucurbit[7]uril (CB7), a competitive chemical reaction of monoprotonated DBOA takes place upon thermal activation, namely a stepwise homolytic covalent bond cleavage with the elimination of N₂, while the doubly protonated CB7⋅DBOA complex undergoes an inner-phase elimination of ethylene, a concerted, electrocyclic ring-opening reaction. These chemical reaction pathways stand in contrast to the gas-phase chemistry of uncomplexed monoprotonated DBOA, for which an elimination of NH₃ predominates upon collision-induced activation, as a heterolytic bond cleavage reaction. The combined results, which can be rationalized in terms of organic-chemical reaction mechanisms and density-function theoretical calculations, demonstrate that chemical reactions in the gas phase can be steered chemoselectively through noncovalent interactions.     

烷烃中碳氢键离解能的估算及其应用

将烷烃中的C－H键看成氢原子H与烷基Ri相连接而成的Ri－H键，以烷基的 HOMO能级和氢原子的轨道能来关联Ri－H键的离解能BDE。研究表明，烷烃分子中 Ri－H键的离能BDE与烷基Ri的极化效应指数PEI（Ri）有良好的线性关系：BDE＝ c+dPEI(Ri)。所得方程具有良好的估算精度。烷基Ri极化效应指数PEI（Ri）在羟 基自由基与烷烃反应速度常数的定量相关中，也得到良好的应用。     

Determination of the C4-H bond dissociation energies of NADH models and their radical cations in acetonitrile

Heterolytic and homolytic bond dissociation energies of the C4-H bonds in ten NADH models (seven 1,4-dihydronicotinamide derivatives, two Hantzsch 1,4-dihydropyridine derivatives, and 9,10-dihydroacridine) and their radical cations in acetonitrile were evaluated by titration calorimetry and electrochemistry, according to the four thermodynamic cycles constructed from the reactions of the NADH models with N,N,N',N'-tetramethyl-p-phenylenediamine radical cation perchlorate in acetonitrile (note: C9-H bond rather than C4-H bond for 9,10-dihydroacridine; however, unless specified, the C9-H bond will be described as a C4-H bond for convenience). The results show that the energetic scales of the heterolytic and homolytic bond dissociation energies of the C4-H bonds cover ranges of 64.2-81.1 and 67.9-73.7 kcal mol(-1) for the neutral NADH models, respectively, and the energetic scales of the heterolytic and homolytic bond dissociation energies of the (C4-H)(.+) bonds cover ranges of 4.1-9.7 and 31.4-43.5 kcal mol(-1) for the radical cations of the NADH models, respectively. Detailed comparison of the two sets of C4-H bond dissociation energies in 1-benzyl-1,4-dihydronicotinamide (BNAH), Hantzsch 1,4-dihydropyridine (HEH), and 9,10-dihydroacridine (AcrH(2)) (as the three most typical NADH models) shows that for BNAH and AcrH(2), the heterolytic C4-H bond dissociation energies are smaller (by 3.62 kcal mol(-1)) and larger (by 7.4 kcal mol(-1)), respectively, than the corresponding homolytic C4-H bond dissociation energy. However, for HEH, the heterolytic C4-H bond dissociation energy (69.3 kcal mol(-1)) is very close to the corresponding homolytic C4-H bond dissociation energy (69.4 kcal mol(-1)). These results suggests that the hydride is released more easily than the corresponding hydrogen atom from BNAH and vice versa for AcrH(2), and that there are two almost equal possibilities for the hydride and the hydrogen atom transfers from HEH. Examination of the two sets of the (C4-H)(.+) bond dissociation energies shows that the homolytic (C4-H)(.+) bond dissociation energies are much larger than the corresponding heterolytic (C4-H)(.+) bond dissociation energies for the ten NADH models by 23.3-34.4 kcal mol(-1); this suggests that if the hydride transfer from the NADH models is initiated by a one-electron transfer, the proton transfer should be more likely to take place than the corresponding hydrogen atom transfer in the second step. In addition, some elusive structural information about the reaction intermediates of the NADH models was obtained by using Hammett-type linear free-energy analysis.     

N-NO bond dissociation energies of N-nitroso diphenylamine derivatives (or analogues) and their radical anions: implications for the effect of reductive electron transfer on N-NO bond activation and for the mechanisms of NO transfer to nitranions

The heterolytic and homolytic N-NO bond dissociation energies [i.e., deltaHhet(N-NO) and deltaHhomo(N-NO)] of 12 N-nitroso-diphenylamine derivatives (1-12) and two N-nitrosoindoles (13 and 14) in acetonitrile were determined by titration calorimetry and from a thermodynamic cycle, respectively. Comparison of these two sets of data indicates that homolysis of the N-NO bonds to generate NO* and nitrogen radical is energetically much more favorable (by 23.3-44.8 kcal/mol) than the corresponding heterolysis to generate a pair of ions, giving hints for the driving force and possible mechanism of NO-initiated chemical and biological transformations. The first (N-NO)-* bond dissociation energies [i.e., deltaH(N-NO)-* and deltaH'(N-NO)-*] of radical anions 1-*-14-* were also derived on the basis of appropriate cycles utilizing the experimentally measured deltaHhet(N-NO) and electrochemical data. Comparisons of these two quantities with those of the neutral N-NO bonds indicate a remarkable bond activation upon a possible one-electron transfer to the N-NO bonds, with an average bond-weakening effect of 48.8 +/- 0.3 kcal/mol for heterolysis and 22.3 +/- 0.3 kcal/mol for homolysis, respectively. The good to excellent linear correlations among the energetics of the related heterolytic processes [deltaHhet(N-NO), deltaH(N-NO)-*, and pKa(N-H)] and the related homolytic processes [deltaHhomo(N-NO), deltaH'(N-NO)-*, and BDE(N-H)] imply that the governing structural factors for these bond scissions are similar. Examples illustrating the use of such bond energetic data jointly with relevant redox potentials for analyzing various mechanistic possibilities for nitrosation of nitranions are presented.