Anthralin: primary products of its redox reactions

One-electron reduction significantly enhances the ability of anthralin, 1, to act as a hydrogen atom donor. On annealing of an MTHF glass in which the radical anion of anthralin, 1*-, is generated radiolytically, this species decays mainly by loss of H* to give the anthralyl anion, 2- . On the other hand, radicals formed on radiolysis of matrices that are suitable for the generation of radical anions or cations are capable to abstract H* from anthralin to give the anthralyl radical, 2* . Both 2- and 2* are obtained simultaneously by mesolytic cleavage of the radical anion of the anthralin dimer. Contrary to general assumptions, the anthralyl radical is found to be much more reactive toward oxygen than the anion. All intermediates are characterized spectroscopically and by reference to quantum chemical calculations. Attempts to generate the radical cation of anthralin by X-irradiation of an Ar matrix containing anthralin led also to significant formation of its radical anion, i.e., anthralin acts apparently as an efficient electron trap in such experiments.     

Ultrafast spectroscopic and matrix isolation studies of p-biphenylyl, o-biphenylyl, and 1-naphthylnitrenium cations

p-biphenylyl, o-biphenylyl, and 1-naphthyl azides were deposited in argon at low temperature in the presence and absence of HCl. In the absence of HCl, the known electronic and vibrational spectra of the corresponding triplet nitrenes, azirines, and didehydroazepines were observed, whereas in the presence of HCl, photolysis of these azides produces new electronic spectra assigned to the corresponding nitrenium cations. For p-biphenylyl azide the resulting spectrum of the nitrenium ion is very similar to the previously observed solution-phase spectrum of this species. The vibrational spectrum of this cation was recorded for the first time. Spectroscopic evidence for the previously unknown o-biphenylylnitrenium cation and 1-naphthylnitrenium cation are provided. The spectra of p- and o-biphenylylnitrenium cations and 1-naphthylnitrenium cation are well reproduced by CASSCF and CASPT2 calculations. The same nitrenium cations were detected in solution by femtosecond time-resolved laser flash photolysis of the appropriate azides in 88% formic acid. The transient spectra of the nitrenium cations recorded in solution are in good agreement with the spectra obtained in HCl matrices. The rates of formation of these cations equal the rates of decay of the singlet nitrenes in 88% formic acid and are as follows: p-biphenylyl (taugrowth = 11.5 ps), o-biphenylyl (taugrowth = 7.7 ps), and 1-naphthylnitrenium cations (taugrowth = 8.4 ps). The decay lifetimes of p- and o-biphenylylnitrenium cations are 50 and 27 ns, respectively. The decay lifetimes of 1-naphthylnitrenium cation is 860 ps in 88% formic acid.     

Denitration of nitroaromatic compounds by arylnitrile radical cations

Substituted nitrobenzenes react with substituted benzonitrile radical cations in an ion trap mass spectrometer by a novel ion/molecule reaction involving NO2 elimination. Formation of an arylated nitrile, Ar1+N identical to CAr2 (where Ar1, Ar2 = aryl), is indicated by collision induced dissociation and comparison with the behavior of the authentic ion. Ab initio calculations (MP2/6-31G*/ /HF/6-31G*) show the reaction of the unsubstituted compounds (Ar1, Ar2 = phenyl) to be exothermic by 48 kcal/mol, consistent with the experimental observation that the reaction rate decreases as the collision energy is increased. Electron withdrawing and donating substituents on either the ionic or the neutral reagent have little effect on the relative amount of product observed, pointing to a radical mechanism. Related denitration reactions were found to occur, between nitrobenzene and its radical cation and between phenylisonitrile and ionized nitrobenzene. These reactions are suggested to yield Ar1+N(= O)OAr2 and Ar2+N identical to CAr1, respectively. The denitration reaction was applied to trinitrotoluene (TNT) as a possible diagnostic reaction for the presence of nitroaromatic explosives.     

Determination of inorganic cations and anions by ion-exchange chromatography with evaporative light-scattering detection

Evaporative light-scattering detection (ELSD) was investigated for the direct determination of alkali and alkaline-earth cations by cation-exchange chromatography. Successful single run analysis of Na+, K+, Mg2+ and Ca2+ was achieved in 11 min on the Hamilton PRP-X200 column using an aqueous solution of ammonium formate as mobile phase under a salt concentration step gradient mode (20 mM and 100 mM). Surprisingly the use of ELSD reveals a weak retention of inorganic anions (Cl-, NO3-, SO4(2-)) onto the polymeric cation exchanger, which enables the simultaneous determination of inorganic anions (C1- and NO3-) associated with the cations analysed (Na+ and K+).     

Vibrational spectroscopy of nitroalkane chains using electron autodetachment and ar predissociation

If the binding energy of an excess electron is lower than some of the vibrational levels of its host anion, vibrational excitation can lead to autodetachment. We use excitation of CH stretching modes in nitroalkane anions (2700-3000 cm(-1)), where the excess electron is localized predominantly on the NO2 group. We present data on nitroalkane anions of various chain lengths, showing that this technique is a valid approach to the vibrational spectroscopy of such systems extending to nitroalkane anions at least the size of nitropentane. We compare spectra taken by using vibrational autodetachment with spectra obtained by monitoring Ar evaporation from Ar solvated nitroalkane anions. The spectra of nitromethane and nitroethane are assigned on the basis of ab initio calculations with a detailed analysis of Fermi resonances of CH stretching fundamentals with overtones and combination bands of HCH bending modes.     

Infrared spectra of cis- and trans-peroxynitrite anion, OONO-, in solid argon

The peroxynitrite anion, of vast importance in biochemistry, is formed in vivo from the reaction of NO and O(-2). Laser ablation of 10 different metal targets with concurrent 7 K codeposition of NO/Ar and O(2)/Ar mixtures gives new metal-independent infrared bands at 1458.3 and 806.1 cm(-)(1), and at 1433.3 and 983.2 cm(-1), in addition to known O(-4) and (NO)(-2) absorptions. The new bands are not observed with CCl(4) added to capture electrons or in O(2) and NO experiments without laser ablation to produce electrons, which identifies new product anions. Based on (15)NO and (18)O(2) isotopic shifts, splitting patterns in mixed isotopic experiments, and comparison with DFT isotopic frequency calculations, the former absorptions are assigned to cis-OONO-, and the latter pair to trans-OONO-, which are isolated from metal cations trapped elsewhere in the matrix. The cis- and trans-peroxynitrite anion isomers are probably formed via the ion-molecule reaction between O(-2)and NO: the O(-2) anion, made by the capture of ablated electrons, is attested by the observation of O(-4). cis- and trans-OONO- are reversibly photoisomerized by visible and near-UV radiation. Collisional stabilization of the OONO- ion-molecule dimer complex during formation of the solid argon matrix appears to be crucial.     

Characterization of the activity of L-ascorbic acid 2-[3,4-dihydro-2,5,7,8-tetramethyl-2-(4,8,12-trimethyltridecyl)-2H-1-be nzopyran-6-yl-hydrogen phosphate] potassium salt in hydroxyl radical elimination

The effect of L-ascorbic acid 2-[3,4-dihydro-2,5,7,8-tetramethyl-2-(4,8,12-trimethyltridecyl)-2H -1-benzopyran-6-yl-hydrogen phosphate] potassium salt (EPC-K1) on hydroxyl radical (*OH) elimination was studied using electron spin resonance (ESR) and spectrophotometric experiments. The addition of EPC-K, and *OH scavengers eliminated the *OH generated from Cu2+/H2O2, Fe2+/H2O2 and H2O2/UV-irradiation reaction systems. However, in competitive reactions using different concentrations of a spin-trap agent, the addition of the *OH scavenger altered the IC50 values, whereas the addition of EPC-K1 and a metal chelater did not change the value in the Cu2+/H2O2 and Fe2+/H2O2 reaction systems. The addition of EPC-K1 and metal chelater changed the ESR signal for free Cu2+. The spectrophotometric experiments confirmed that the addition of EPC-K1 and metal chelater altered the absorption spectra due to CuCl2 and FeSO4, whereas the *OH scavenger did not alter the spectra. Therefore, it was demonstrated that EPC-K, has the ability both to scavenge *OH directly and to inhibit the generation of *OH by the chelation of Cu2+ and Fe2+.     

MASS SPECTRA OF ROUSSINATE ANIONS AND THEIR FRAGMENTATIONS

<正> Mass spectra of Roussinate anions in K+, Me4N+ and Bu4N+ salt have been obtained under appropriate operating conditions and the fragmentation pathways are proposed based on linked scan information of metastable transition in 1st FFR. and precise mass determinations. It was found that the skeleton structure of the red Roussinate anion is more stable than that of black Roussinate anion.It is interesting to note that rearrangement reaction with transfering of methyl radical in tetramethyl amonium to bridged sulfur atom occurs during evaporation of [(CH3)4N]2 [Fe2S2(NO)4] , forming Roussin's red methylester:In this paper we will provide some significant information in regard to the rationalization of the interconversion reaction between Roussinate anions under certain condition.     

Methyl and silyl mesolytic dissociations in the radical cations and radical anions of but-1-ene,allylsilane, hexa-1,3-diene,and penta-2,4-dienylsilane. CAS-MCSCF and coupled cluster theoretical study

Methyl or silyl dissociation in the CH(2)=CHCH(2)-XH(3) (a-XH(3)(*)(+)) and CH(2)=CHCH=CHCH(2)-XH(3) (p-XH(3)(*) (+)) radical cations (X = C, Si) yields a(+) or p(+) and XH(3)(*). Similarly, the radical anions a-CH(3)(*) (-) and p-CH(3)(*) (-) give the pi-delocalized anion and CH(3)(*) preferentially. In contrast, a-SiH(3)(*) (-) and p-SiH(3)(*-) prefer to dissociate into the pi-delocalized radical and silide. All reactions are endoergic: by 43-50 kcal mol(-)(1) in the radical cations, and easier to some extent in the radical anions, that require 29-33 (X = C) and 13-14 kcal mol(-)(1) (X = Si). The fragmentation energy profiles do not present significant barriers for the backward process in the case of the radical cations. All radical anions exhibit an energy maximum along the dissociation pathway, but the barrier is lower than the dissociation limit. Fragmentation is "activated" more in the anions than in the cations with respect to homolysis in the corresponding neutrals (that requires 72-81 kcal mol(-)(1)). Wave function analysis indicates that the C-X bond cleavage in the hydrocarbon radical ions, although formally comparable to a homolytic process, is at variance with this model, due to the spin recoupling of one of the two C-X bond electrons with the originally unpaired electron. This is basically true also for the silyl-substituted radical anions, in which the initial more delocalized charge distribution might suggest some heterolytic character of the bond cleavage.     

Reactions of O−. with methyl benzoate: A negative ion chemical ionization and Fourier transform ion cyclotron resonance study

The reactions of O*- with methyl benzoate have been examined by the measurement of negative ion chemical ionization (NICI) mass spectra using a CI source, with confirmatory studies carried out on a Fourier transform ion cyclotron resonance mass spectrometer. Reaction mechanisms have been elucidated using isotopically labeled esters. Nucleophilic attack at the carbonyl carbon and the aromatic ring were important reaction pathways. Nucleophilic attack at the carbonyl carbon was followed by the production of products (C6HsCO2- and CH3OCO2-) characteristic of radical, beta-fragmentation. Using 18O-labeled methyl benzoate, the SN2 reaction was found to account for a smaller percentage, 21(+/-1)%, of the benzoate product. Aromatic ring attack resulted in formation of [M + O - H]- and [M - 2H]*- ions. Although aryl hydrogens accounted for most H2*+ abstracted by O*-, evidence for abstraction of HarylH*+alkyl and HalkylH*+alkyl was also found. Although present at much lower abundance, dehydrobenzoate, dehydrophenoxy, and C7H6*- ([M - 2H - CO2]*-) radical anions were also observed. An Haryl/Halkyl exchange associated with formation of the benzoate anion was attributed to an Halkyl abstraction that occurred within the methanol/dehydrobenzoate ion-dipole complex. The [M - 2H]*-, dehydrobenzoate, dehydrophenoxy, and [M - 2H - CO2]*- ion signals were quenched by reaction with O2. Conditions required for production of O*- spectra under NICI conditions were also examined.     

Reactivity and acid-base behavior of ring-methoxylated arylalkanoic acid radical cations and radical zwitterions in aqueous solution. Influence of structural effects and pH on the benzylic C-H deprotonation pathway

A product and time-resolved kinetic study of the one-electron oxidation of ring-methoxylated phenylpropanoic and phenylbutanoic acids (Ar(CH2)nCO2H, n = 2, 3) has been carried out at different pH values. Oxidation leads to the formation of aromatic radical cations (Ar.+(CH2)nCO2H) or radical zwitterions (Ar.+(CH2)nCO2-) depending on pH, and pKa values for the corresponding acid-base equilibria have been measured. In the radical cation, the acidity of the carboxylic proton decreases by increasing the number of methoxy ring substituents and by increasing the distance between the carboxylic group and the aromatic ring. At pH 1.7 or 6.7, the radical cations or radical zwitterions undergo benzylic C-H deprotonation as the exclusive side-chain fragmentation pathway, as clearly shown by product analysis results. At pH 1.7, the first-order deprotonation rate constants measured for the ring-methoxylated arylalkanoic acid radical cations are similar to those measured previously in acidic aqueous solution for the alpha-C-H deprotonation of structurally related ring-methoxylated alkylaromatic radical cations. In basic solution, the second-order rate constants for reaction of the radical zwitterions with (-)OH (k-OH)) have been obtained. These values are similar to those obtained previously for the (-)OH-induced alpha-C-H deprotonation of structurally related ring-methoxylated alkylaromatic radical cations, indicating that under these conditions the radical zwitterions undergo benzylic C-H deprotonation. Very interestingly, with 3,4-dimethoxyphenylethanoic acid radical zwitterion, that was previously observed to undergo exclusive decarboxylation up to pH 10, competition between decarboxylation and benzylic C-H deprotonation is observed above pH 11.     

Competitive coupling of amide anion over menthyl propionate anion with aryl radical

A competitive coupling of amide anion over menthyl propionate anion with aryl radicalin photo-S_(RN) 1 mechanism was encountered. The rcaction afforded N-aryl propionic amide in excel-lent yield. In contrast, the expected nucleophilic photo-S_(RN) 1 substitution originating from the carb-anion was observed in the case of t-butyl propionate. According to the proposed mechanisms and MOcorrelation diagrams of the coupling step of nucleophiles with aryl radical, the interesting con-trast is reasonably attributed to the variation in energy gap between π~*c-o and π~*Ar of (ArNu)-Usually, the odd electron of (ArNu)- is weightly populated at π~*c-o, however, the diminished priv-ilege of π~*c-o in menthyl propionate promotes a dominant population of the odd electron at π~*Ar,which leads to the fragmentation of (ArNu)- into the starting carbanion and aryl radical.     

Comparative DFT study of the spin trapping of methyl, mercapto, hydroperoxy, superoxide, and nitric oxide radicals by various substituted cyclic nitrones

The thermodynamics of the spin trapping of various cyclic nitrones with biologically relevant radicals such as methyl, mercapto, hydroperoxy, superoxide anion, and nitric oxide was investigated using computational methods. A density functional theory (DFT) approach was employed in this study at the B3LYP/6-31+G(d,p)//B3LYP/6-31G(d) level. The order of increasing favorability for Delta G(rxn) (kcal/mol) of the radical reaction with various nitrones, in general, follows a trend similar to their respective experimental reduction potentials as well as their experimental second-order rate constants in aqueous solution: NO (14.57) < O2*- (-7.51) < *O2H (-13.92) < *SH (-16.55) < *CH3 (-32.17) < *OH (-43.66). The same qualitative trend is predicted upon considering the effect of solvation using the polarizable continuum model (PCM): i.e., NO (14.12) < O2*- (9.95) < *O2H (-6.95) < *SH (-13.57) < *CH3 (-32.88) < *OH (-38.91). All radical reactions with these nitrones are exoergic, except for NO (and O2*- in the aqueous phase), which is endoergic, and the free energy of activation (Delta G) for the NO additions ranges from 17.7 to 20.3 kcal/mol. This study also predicts the favorable formation of certain adducts that exhibit intramolecular H-bonding interactions, nucleophilic addition, or H-atom transfer reactions. The spin density on the nitronyl N of the superoxide adducts reveals conformational dependences. The failure of nitrones to trap NO at normal conditions was theoretically rationalized due to the endoergic reaction parameters.     

Koopmans-based analysis of the optical spectra of p-phenylene-bridged intervalence radical ions

[Figure: see text]. The optical spectra of 10 p-phenylene-bridged delocalized intervalence compounds MC6H4M*- or *+ are analyzed using the Koopmans-based method, which considers only transitions from filled orbitals to the singly occupied orbital (SOMO), called Hoijtink type A transitions, and from the SOMO to unoccupied orbitals, Hoijtink type B transitions, and ignores configuration interaction. The radical ions with quinonoid structures, those that form ring-M double bonds with M = C(CN)2, NMe2, 3-oxo-9-azabicyclo[3.3.1], NPPh3, and O when the odd electron of the intervalence oxidation level is removed, are calculated to have the lowest-allowed type B transition lying mostly above the lowest-allowed A transition, with B(i)- A(j) decreasing in the order shown from +14 370 to -1390 cm(-1), and the more intense second-lowest-allowed type B transition B(i) - A(j) from +14 940 to +7070 cm(-1). The five radical anions with benzenoid structures, which form ring-M single bonds with X = CN, CO2Me, CHO, C3HMeBF2O2, and NO2 when the odd electron of the intervalence oxidation level is removed, have a B(i)- A(j) value of the opposite sign that increases in magnitude from -2880 to -17 050 cm(-)(1) in the order shown. Configuration interaction is of course present in the observed spectra, and the predictions ignoring it mostly overestimate transition energies by 1900-2600 cm(-1) for the quinonoid compounds (but by 450 cm(-1) for the M = C(CN)2 radical anion), and by 1000-1400 cm(-1) for the benzenoid compounds (2500 cm(-1) for the M = CN radical anion). The very simple Koopmans-based model is useful for considering the optical spectra of these radical ions.     

Ab initio inter-radical potential for p-phenylenediamine radical cation dimer

Ab initio molecular orbital calculations were carried out to investigate the inter-radical interaction of the paired p-phenylenediamine radical cations in the singlet state. After initial optimization of the dimer in the parallel sandwich (D2h) and parallel displaced (Cs) configurations at the B3LYP/6-31G* theoretical level, the MP2/6-31G* and B3LYP/6-31G* single energies of the dimer were calculated as a function of the inter-radical distance R. The depths of the potential minima near R = 3.2 A were estimated to be in the order of the hydrogen bonding energy, assuming that the electrostatic contribution between the cations is canceled out by the attractive contributions due to the counter anions on the aspect of a simple electrostatic model. This can be related to the indications of the cation dimer formation in solution in the presence of counter anions at a low temperature reported previously in the literature by resonance Raman and electronic absorption spectra. The inter-radical (Raman active) frequencies of the dimer were calculated, one of which corresponds to the reported value at 161 cm(-1) observed in the resonance Raman spectrum in ethanol at 200 K by Yokoyama and Maeda (Chem. Phys. Lett. 48 (1977) 59).     

Spectroscopy of free-base N-confused tetraphenylporphyrin radical anion and radical cation

The radical anions and radical cations of the two tautomers (1e and 1i) of 5,10,15,20-tetraphenyl N-confused free-base porphyrin have been studied using a combination of cyclic voltammetry, steady state absorption spectroscopy, and computational chemistry. N-Confused porphyrins (NCPs), alternatively called 2-aza-21-carba-5,10,15,20-tetraphenylporphyrins or inverted porphyrins, are of great interest for their potential as building blocks in assemblies designed for artificial photosynthesis, and understanding the absorption spectra of the corresponding radical ions is paramount to future studies in multicomponent arrays where electron-transfer reactions are involved. NCP 1e was shown to oxidize at a potential of E(ox) 0.65 V vs Fc(+)|Fc in DMF and reduce at E(red) -1.42 V, while the corresponding values for 1i in toluene were E(ox) 0.60 V and E(red) -1.64 V. The geometries of these radical ions were computed at the B3LYP/6-31+G(d)//B3LYP/6-31G(d) level in the gas phase and in solution using the polarizable continuum model (PCM). From these structures and that of H(2)TPP and its corresponding radical ions, the computed redox potentials for 1e and 1i were calculated using the Born-Haber cycle. While the computed reduction potentials and electron affinities were in excellent agreement with the experimental reduction potentials, the calculated oxidation potentials displayed a somewhat less ideal relationship with experiment. The absorption spectra of the four radical ions were also measured experimentally, with radical cations 1e(?+) and 1i(?+) displaying significant changes in the Soret and Q-band regions as well as new low energy absorption bands in the near-IR region. The changes in the absorption spectra of radical anions 1e(?-) and 1i(?-) were not as dramatic, with the changes occurring only in the Soret and Q-band regions. These results were favorably modeled using time-dependent density functional calculations at the TD-B3LYP/6-31+G(d)//B3LYP/6-31G(d) level. These results were also compared to the existing data of free base tetraphenylporphyrin and free base tetraphenylchlorin.     

Aromatic nucleophilic substitution (S<Subscript>N</Subscript>Ar) Reactions of 1,2- and 1,4-halonitrobenzenes and 1,4-dinitrobenzene with carbanions in the gas phase

In the gas-phase reactions of halonitro- and dinitrophenide anions with X (X = F, Cl, Br, NO(2)) and NO(2) groups in ortho or para position to each other with selected C-H acids: CH(3)CN, CH(3)COCH(3), and CH(3)NO(2), products of the S(N)Ar-type reaction are formed. Nitrophenide anions are generated by decarboxylation of the respective nitrobenzenecarboxylate anions in ESI ion source and the S(N)Ar reaction takes place either in the medium-pressure zone of the ion source or in the collision chamber of the triple quadrupole mass spectrometer. In the case of F, Cl, and NO(2) derivatives, the main ionic product is the respective [NO(2)-Ph-CHR](-) anion (R = CN, COCH(3), NO(2)). In the case of Br derivatives, the main ionic product is Br(-) ion because it has lower proton affinity than the [NO(2)-Ph-CHR](-) anion (for R = CN, COCH(3)). For some halonitrophenide anion C-H acid pairs of reactants, the S(N)Ar reaction is competed by the formation of halophenolate anions. This reaction can be rationalized by the single electron-transfer mechanism or by homolytic C-H bond cleavage in the proton-bound complex, both resulting in the formation of the halonitrobenzene radical anion, which in turn undergoes -NO(2) to -ONO rearrangement followed by the NO(.) elimination.     

二氮双环阳离子游离基中N,N'-三电子键

用从头算HF/3-21+G^*优化了二氮双环[m,n,l]游离基阳离子(m,n,l≥2～5)。分子[4,4,4]和[2,2,2]^+.,[3,3,3]^+.,[4,4,4]^+.游离基阳离子的优势构型有D3对称性,而其它游离基阳离子的优势构型为非对称性。通过比较这些阳离子几何构型,HOMO和NHOMO(即NextHOMO和HOMO-1),和由MNDO计算确定的原子对作用能,表明当二氮双环游离基阳离子的桥链(CH2)n的n≥3时,桥头氮原子通过空间相互用用形成了一个弱的N,N'-三电子σ键。形成的三电子键强度不随环的扩大而增强。而三电子键强度被两个因素影响：一个是桥头氮原子间的p轨道重叠的取向;另一个是它们相应p轨道成分。     

Quantifying the photoinduced release of nitric oxide from N,N'-bis(carboxymethyl)-N,N'-dinitroso-1,4-phenylenediamine. Effect of reducing agents on the mechanism of the photoinduced reactions

N,N'-Bis(carboxymethyl)-N,N'-dinitroso-1,4-phenylenediamine (1) fragments to release 1 equiv of NO* and the denitrosated radical of 1 (2), when exposed to a approximately 10 ns, 308 nm laser pulse. Species 2 can fragment to give another equivalent of NO* and the doubly denitrosated quinoimine derivative of 1 (3), it can recombine with NO* to give 1 and ring-nitrosated isomers of 1, or in the presence of a reducing agent, 2 can be reduced (to species 4). Photogenerated NO* can be used to probe fast reactions of biochemical interest, making 1 a valuable research tool. This paper focuses on the chemistry of 2, whose reactivity must be well characterized if 1 is to be used to its full potential. [Ru(NH3)6]2+ (RuII) and [Fe(CN)6]4- (FeII) were both shown to reduce 2, with bimolecular rate constants in the diffusion limit. When solutions initially containing 70 microM of RuII, 20 microM myoglobin (Mb) and varying amounts of 1 were irradiated, the only Mb reaction product was nitrosomyoglobin (MbNO). In contrast, in solutions containing only Mb and 1, Mb is converted to both MbNO and oxidized myoglobin (metMb). When FeII was used in place of RuII, Mb was oxidized to metMb, but approximately 100x more slowly than in solutions containing only Mb and 1. This showed that 2 first oxidized FeII to [Fe(CN)6]3- (FeIII), which then oxidized Mb at the slower rate. The ratio metMb/MbNO obtained in the experiments with FeII was 0.6, whereas the ratio predicted from previously known chemistry of 2 was approximately 1 under the experimental conditions. The result is explained if, upon photolysis, 1 first forms a caged encounter complex [2, NO*], which fragments to give 3 and 2 equiv of NO*, without ever releasing free 2 into solution. This hypothesis was further strengthened by analyzing the amount of NO* generated by photolysis of 1 in the absence of added reductant. The original mechanism underestimates the NO* generated, a problem solved by invoking direct release of NO* and 3 from photolysis of 1.     

Electron transfer to protonated beta-alanine N-methylamide in the gas phase: an experimental and computational study of dissociation energetics and mechanisms

Ammonium radicals derived from protonated beta-alanine N-methyl amide (BANMA) were generated by femtosecond collisional electron transfer to gas-phase cations prepared by chemical ionization and electrospray. Regardless of the mode of precursor ion preparation, the radicals underwent complete dissociation on the time scale of 5.15 micros. Deuterium isotope labeling and product analysis pointed out several competitive and convergent dissociation pathways that were not completely resolved by experiment. Ab initio calculations, which were extrapolated up to the CCSD(T)/6-311++G(3df,2p) level of theory, provided the proton affinity and gas-phase basicity of BANMA as PA = 971 kJ mol-1 and GB = 932 kJ mol-1 to form the most stable ion structure 1c+ in which the protonated ammonium group was internally solvated by hydrogen bonding to the amide carbonyl. Ion 1c+ was calculated to have an adiabatic recombination energy of 3.33 eV to form ammonium radical 1c*. The potential energy surface for competitive and consecutive isomerizations and dissociations of 1c* was investigated at correlated levels of theory and used for Rice-Ramsperger-Kassel-Marcus (RRKM) calculations. RRKM unimolecular rate constants suggested that dissociations starting from the ground electronic state of radical 1c* were dominated by loss of an ammonium hydrogen atom. In contrast, dissociations starting from the B excited state were predicted to proceed by reversible isomerization to an aminoketyl radical (1f*). The latter can in part dissociate by N-Calpha bond cleavage leading to the loss of the amide methyl group. This indicates that apparently competitive dissociations observed for larger amide and peptide radicals, such as backbone cleavages and losses of side-chain groups, may originate from different electronic states and proceed on different potential energy surfaces.