Mass spectrometry studies of nitrene anions.

Nitrene anions are a class of reactive intermediates that provide a means for studying the corresponding neutral molecules via electron photodetachment spectroscopy and photoelectron spectroscopy. The added electron makes it possible for protected nitrene anions to be manipulated by external electric and magnetic fields of a mass spectrometer. Nitrene anions also display their own unique reactivities as reagents, which have been investigated using ion/molecule reactions. Mass spectrometry of negative ions has thereby provided information on the electronic states, reactivities, and thermochemical properties of nitrene intermediates. This review also includes a discussion of condensed-phase nitrene anions.

Probing the Pyrolysis of Guaiacol and Dimethoxybenzenes Using Collision-Induced Dissociation Charge-Remote Fragmentation Mass Spectrometry.

The dissociation of lignin model compounds has been examined using mass spectrometry and collision-induced dissociation charge-remote fragmentation (CID-CRF). The model compounds guaiacol and o- and m-dimethoxybenzene containing a remote sulfonate (SO3-) charge group undergo CID by dissociation without the involvement of the anionic group. The first dissociation for all three compounds is loss of methyl radical to form phenoxy radicals. Subsequent dissociation pathways depend on the specific structures being examined The dissociation pathways are compared to those observed upon gas-phase pyrolysis that have been reported previously. While the pathways are largely similar, there are some important differences that are explained by changes in dissociation barriers due to the effect of adding the charged group. This work shows that CID-CRF is an effective approach for tracking the thermolysis of lignin model compounds while eliminating secondary reactions that normally convolute such studies.

Investigation of the Substituent Effects of the Azide Functional Group Using the Gas-Phase Acidities of 3- and 4-Azidophenols.

The electronic effect of the azide functional group on an aromatic system has been investigated using Hammett-Taft parameters obtained from the effect of azide substitution on the gas-phase acidity of phenol. Gas-phase acidities of 3- and 4-azidophenol have been measured using mass spectrometry and the kinetic method and found to be 340.8 ± 2.2 and 340.3 ± 2.0 kcal/mol, respectively. The relative electronic effects of the azide substituent on an aromatic system have been measured using Hammett-Taft parameters. The σF and σR values are determined to be 0.38 and 0.02, respectively, consistent with predictions based on electronic structure calculations. The values of σF and σR demonstrate that azide acts as an inductively withdrawing group but has negligible resonance contribution on the phenol. In contrast, acidity values calculated for azide-substituted benzoic acids give values of σF = 0.69 and σR = -0.39, indicating that the azide is a strong π donor, comparable to that of a hydroxyl group. The difference is explained as being the result of "chimeric", or, alternatively, "chameleonic" electronic behavior of the azide, similar to that observed previously for the N-oxide moiety, which can be more or less resonance donating in response to the environment.

Nucleophilic Addition to Singlet Diradicals: Homosymmetric Diradicals.

Experiments have demonstrated that nucleophiles can attack singlet diradicals to generate bonded, closed-shell addition products. Here, we present a molecular orbital analysis for this reaction, focusing on the addition of nucleophiles to homosymmetric diradicals. We show that beginning with the Salem-Rowland molecular orbital description of homosymmetric diradicals, a continuous progression from open-shell diradical to closed-shell addition product occurs during the reaction via a gradual evolution of orbital and configuration interaction coefficients. This theoretical framework is supported by high-level multireference computations (CASPT2, EOM-SF-CCSD(dT)) using the addition of chloride to p-benzyne to generate a p-chlorophenyl anion as a case study. When using levels of theory that include dynamic correlation, the reaction is predicted to be barrierless. No abrupt switch from diradical to closed-shell species happens during the mechanism, but rather a gradual decrease in diradical character occurs as the nucleophile approaches the radical center before ultimately transforming into the closed-shell anion. The overarching conclusion from this work is that there are no electronic impediments of any kind, deriving from orbital symmetry or from any other source, that exist for the addition of nucleophiles to homosymmetric singlet diradicals.

Nucleophilic Addition to Singlet Diradicals: Heterosymmetric Diradicals.

In the preceding paper, we examined the addition of nucleophiles to homosymmetric diradicals and showed that the reaction occurs with no symmetry restrictions or other electronic impediments. In this work, we examine the addition of nucleophiles to heterosymmetric diradicals, by using the addition of chloride to m-dehydrotoluene as a case study. Using CASPT2 and density functional theory calculations, we show that the addition of chloride to m-dehydrotoluene is predicted to be barrierless at the asymptotic limit if Cs symmetry is broken, and the reaction is allowed to proceed through a nonplanar geometry. A nonplanar cyclic allene acts as the transitioning structure between open-shell and closed-shell species for the addition of chloride, with a continuous and smooth changing of the wave function by the evolution of orbital and configuration interaction coefficients, such that there is no abrupt switch from diradical to closed-shell species along the reaction coordinate. The overall conclusion from our analysis is that both homosymmetric and heterosymmetric diradicals can undergo reaction with closed-shell reagents without a barrier, and one cannot rule out the direct addition of nucleophiles to diradicals when considering the reaction mechanism.

Mass spectrometric detection of the Gibbs reaction for phenol analysis.

This paper describes a new method for detecting phenols, by reaction with Gibbs reagent to form indophenols, followed by mass spectrometric detection. Unlike the standard Gibbs reaction, which uses a colorometric approach, the use of mass spectrometry allows for simultaneous detection of differently substituted phenols. The procedure is demonstrated to work for a large variety of phenols without para-substitution. With para-substituted phenols, Gibbs products are still often observed, but the specific product depends on the substituent. For para groups with high electronegativity, such as methoxy or halogens, the reaction proceeds by displacement of the substituent. For groups with lower electronegativity, such as amino or alkyl groups, Gibbs products are observed that retain the substituent, indicating that the reaction occurs at the ortho or meta position. In mixtures of phenols, the relative intensities of the Gibbs products are proportional to the relative concentrations, and concentrations as low as 1 µmol/L can be detected. The method is applied to the qualitative analysis of commercial liquid smoke, and it is found that hickory and mesquite flavors have significantly different phenolic composition.

Photoelectron Spectroscopy Study of Quinonimides.

Structures and energetics of o-, m-, and p-quinonimide anions (OC6H4N-) and quinoniminyl radicals have been investigated by using negative ion photoelectron spectroscopy. Modeling of the photoelectron spectrum of the ortho isomer shows that the ground state of the anion is a triplet, while the quinoniminyl radical has a doublet ground state with a doublet-quartet splitting of 35.5 kcal/mol. The para radical has doublet ground state, but a band for a quartet state is missing from the photoelectron spectrum indicating that the anion has a singlet ground state, in contrast to previously reported calculations. The theoretical modeling is revisited here, and it is shown that accurate predictions for the electronic structure of the para-quinonimide anion require both an accurate account of electron correlation and a sufficiently diffuse basis set. Electron affinities of o- and p-quinoniminyl radicals are measured to be 1.715 ± 0.010 and 1.675 ± 0.010 eV, respectively. The photoelectron spectrum of the m-quinonimide anion shows that the ion undergoes several different rearrangements, including a rearrangement to the energetically favorable para isomer. Such rearrangements preclude a meaningful analysis of the experimental spectrum.

Participation of C-H Protons in the Dissociation of a Proton Deficient Dipeptide.

The dissociation of anionic dipeptides Phe*Gly and GlyPhe*, where Phe* refers to sulfonated phenyl alanine, has been investigated by using ion trap mass spectrometry. The dipeptides undergo collision-induced dissociation (CID) to give the same products, indicating that they rearrange to a common structure before dissociation. The rearrangement does not occur with the dipeptide methyl esters. The structures of the b2 ions were investigated to determine the effect that having a remote, anionic site has on product formation. Comparison with the CID spectra for authentic structures shows that the b2 ion obtained from GlyPhe* has predominantly a diketopiperazine structure. The CID spectra for the Phe*Gly b2 ion and the authentic oxazolone are similar, but differences in intensity suggest a two-component mixture. Isotopic labeling studies are consistent with the formation of two products, with one resulting from loss of a non-mobile proton on the Gly α-carbon. The results are attributed to the formation of an oxazole and oxazolone enol product. Electronic structure calculations predict that the enol structure of the Phe*Gly b2 ion is lower in energy than the keto version due to intramolecular hydrogen bonding with the sulfonate group. Graphical Abstract ᅟ.

Anionic substituent control of the electronic structure of aromatic nitrenes.

The electronic structures of phenylnitrenes with anionic π-donating substituents are investigated by using mass spectrometry and electronic structure calculations. Reactions of para-CH(2)(-)-substituted phenylnitrene, formed by dissociative deprotonation of p-azidotoluene, with CS(2) and NO indicate that it has a closed-shell singlet ground state, whereas reactions of p-oxidophenylnitrene formed by dissociative deprotonation of p-azidophenol indicate either a triplet ground state or a singlet with a small singlet-triplet splitting. The ground electronic state assignments based on ion reactivity are consistent with electronic structure calculations. The stability of the closed-shell singlet states in nitrenes is shown by Natural Resonance Theory to be very sensitive to the amount of deprotonated-imine character in the wave function, such that large changes in state energies can be achieved by small modifications of the electronic structure.

Hydration energies of aromatic ions in the gas phase.

The hydration energies of aromatic ions, measured by using energy-resolved collision-induced dissociation measurements, are reported. The hydration energies of protonated acetophenone, aniline, anisole, benzene, benzonitrile, phenol, and toluene are 0.67 ± 0.04, 0.62 ± 0.04, 0.86 ± 0.04, 0.46 ± 0.06, 0.84 ± 0.04, 0.75 ± 0.07, and 0.42 ± 0.04 eV, respectively. The measured values are in good agreement with those predicted by using coupled cluster theory, provided the proper geometries of the ions and sufficient basis set were used.

Spin-state dependent radical stabilization in nitrenes: the unusually small singlet-triplet splitting in 2-furanylnitrene.

Geometries and energies of the triplet and singlet states of 2-furanylnitrene and 3-furanylnitrene have been calculated by using spin-flip coupled-cluster methods. Calculations with triple-ζ basis sets predict a singlet-triplet splitting of 10.9 kcal/mol for 2-furanylnitrene, 4.5 kcal/mol smaller than that in phenylnitrene. In contrast, the singlet-triplet splitting in 3-furanylnitrene is computed to be 1.9 kcal/mol larger than that in phenylnitrene. The differences in the singlet-triplet splittings for the furanylnitrenes are attributed to the differences in the radical stabilizing abilities of the 2-furanyl- and 3-furanyl-groups compared to a phenyl ring. Comparison of the singlet-triplet splittings of more than 20 substituted aromatic nitrenes and the radical stabilizing ability of the aromatic systems reveals a high degree of correlation between the singlet-triplet splitting and the radical stabilizing ability, indicating that singlet states of aromatic nitrenes are preferentially stabilized by radical stabilizing substituents. The preferential stabilization of the singlet states is attributed to the decrease in electron pair repulsion resulting from increased delocalization of the radical electron.

Experimental investigation of the absolute enthalpies of formation of 2,3-, 2,4-, and 3,4-pyridynes.

The absolute enthalpies of formation of 3,4-, 2,3-, and/or 2,4-didehydropyridines (3,4-, 2,3- and 2,4-pyridynes) have been determined by using energy-resolved collision-induced dissociation of deprotonated 2- and 3-chloropyridines. Bracketing experiments find the gas-phase acidities of 2- and 3-chloropyridines to be 383 ± 2 and 378 ± 2 kcal/mol, respectively. Whereas deprotonation of 3-chloropyridine leads to formation of a single ion isomer, deprotonation of the 2-chloro isomer results in a nearly 60:40 mixture of regioisomers. The enthalpy of formation of 3,4-pyridyne is measured to be 121 ± 3 kcal/mol by using the chloride dissociation energy for deprotonated 3-chloropyridine. The structure of the product formed upon dissociation of the ion from 2-chloropyridine cannot be unequivocally assigned because of the isomeric mixture of reactant ions and the fact that the potential neutral products (2,3-pyridyne and 2,4-pyridyne) are predicted by high level spin-flip coupled-cluster calculations to be nearly the same in energy. Consequently, the enthalpies of formation for both neutral products are assigned to be 130 ± 3 kcal/mol. Comparison of the enthalpies of dehydrogenation of benzene and pyridine indicates that the nitrogen in the pyridine ring does not have any effect on the stability of the aryne triple bond in 3,4-pyridyne, destabilizes the aryne triple bond in 2,3-pyridyne, and stabilizes the 1,3-interaction in 2,4-pyridyne compared to that in m-benzyne. Natural bond order calculations show that the effects on the 2,3- and 2,4-pyridynes result from polarization of the electrons caused by interaction with the lone pair. The polarization in 2,4-pyridyne is stabilizing because it creates a 1,2-interaction between the nitrogen and dehydrocarbons that is stronger than the 1,3-interaction between the dehydrocarbons.

Photoelectron imaging of NCCCN(-): The triplet ground state and the singlet-triplet splitting of dicyanocarbene.

The photoelectron spectra of NCCCN(-) have been measured at 355 and 266 nm by means of photoelectron imaging. The spectra show two distinct features, corresponding to the ground and first excited states of dycianocarbene. With support from theoretical calculations using the spin-flip coupled-cluster methods, the ground electronic state of HCCCN is assigned as a triplet state, while the first excited state is a closed-shell singlet. The photoelectron band corresponding to the triplet is broad and congested, indicating a large geometry change between the anion and neutral. A single sharp feature of the singlet band suggests that the geometry of the excited neutral is similar to that of the anion. In agreement with these observations, theoretical calculations show that the neutral triplet state is either linear or quasilinear (X (3)B(1) or (3)Sigma(g) (-)), while the closed-shell singlet (a (1)A(1)) geometry is strongly bent, similar to the anion structure. The adiabatic electron binding energy of the closed-shell singlet is measured to be 3.72+/-0.02 eV. The best estimate of the origin of the triplet band gives an experimental upper bound of the adiabatic electron affinity of NCCCN, EA

Topological control of spin states in disjoint diradicals.

The meta- and para-bis-allylbenzene radical anions have been generated and investigated using mass spectrometry. The ions are formed by reaction of the corresponding bis-2-propenylbenzenes with atomic oxygen anion. Reactivity of the ions indicates that the ions most likely have a bis-allylbenzene structure. Reaction of the ions with carbon disulfide creates CS(2) adducts, which, upon collision-induced dissociation, decompose to regenerate the bis-allylbenzene anion or carbon disulfide radical anion. The branching ratios for the two products indicate differences in the electronic structures of the neutral bis-allylbenzene diradicals. The difference in branching ratios and corresponding estimated electron affinities is interpreted in terms of different electronic states being formed, with the para diradical a singlet and the meta diradical either a ground-state triplet or a singlet with a very small singlet-triplet splitting. The difference in electron affinities is used to estimate a singlet-triplet splitting of 0.06 eV for the para diradical. The studies show that topology can be used to control the electronic properties of disjoint, tetramethyleneethane (TME)-like diradicals.

Photoelectron spectroscopy of chloro-substituted phenylnitrene anions.

The 355 nm time-of-flight negative ion photoelectron spectra of (o-, m-, and p-chlorophenyl)nitrene radical anions are reported. Electron affinities are obtained from the photoelectron spectra, and are 1.79 +/- 0.05, 1.82 +/- 0.05, and 1.72 +/- 0.05 eV for the (o-, m-, and p-chlorophenyl)nitrenes, respectively. Singlet-triplet splittings are determined to be 14 +/- 2, 15 +/- 1, and 14 +/- 2 kcal/mol, respectively. The shapes of the photoelectron bands indicate resonance interactions in the singlet states for the ortho- and para-substituted isomers, which is attributed to quinoidal structures of the open-shell singlet states. Reanalysis of the photoelectron spectrum of phenylnitrene anion leads to a revised experimental singlet-triplet splitting of 14.8 kcal/mol in the unsubstituted phenylnitrene.

The Curtin-Hammett principle in mass spectrometry.

The Curtin-Hammett principle (CHP) is an important concept in physical organic chemistry and is often utilized in the investigation of reaction mechanisms. Two reactants, A and B, in rapid equilibrium, react to form products P(A) and P(B) with rates k(A) and k(B), respectively. If the reaction is under kinetic control and the rate of equilibration between the two reactants is much faster than the reactions to form products, then the branching ratio of products P(A) and P(B) depends solely on the difference in barrier heights for the two product channels. The CHP is based on the fact that the ratio of products formed is not determined by the reactant population ratio. However, the CHP also applies to studies in other areas of chemistry, including mass spectrometry. This Account describes work from our groups in which the results must be interpreted in light of the CHP. These studies illustrate two important implications of the CHP. First, they demonstrate how product distributions cannot be used to assess reactant structure in mechanistic studies in Curtin-Hammett systems. A recent investigation of the structure of hydroxysiliconate anions demonstrated that it was not possible to distinguish between the possible reactant ion structures. A second important implication of the CHP is that the structure of the reactant does not affect the product branching ratio and therefore does not need to be a consideration if the CHP applies. We address this aspect of the discussion through kinetic method studies of the acidities of amino acids and proton affinities of bifunctional compounds. Recently reported mass spectrometric studies illustrate how the CHP puts limitations on what conclusions can be drawn from product distribution studies but also allows experimental methods, such as the kinetic method, to be carried out for complicated systems without having to know all the details of the reactant ion structures. These studies show that although the CHP is most commonly applied in mechanistic studies in physical organic chemistry, it also applies to other areas of chemistry, including mass spectrometry. Although the CHP in some cases limits the conclusions that can be drawn from an experimental study, its proper application can often be used to greatly simplify very complicated chemical systems. Therefore, it is important in mass spectrometry, and indeed, in all areas of chemistry, to recognize those systems in which the CHP should and should not apply.

Structure and reactivity of benzoylnitrene radical anion in the gas phase.

The open-shell benzoylnitrene radical anion, readily generated by electron ionization of benzoylazide, undergoes unique chemical reactivity with radical reagents and Lewis acids in the gas phase. Reaction with nitric oxide, NO, proceeds by loss of N2 and formation of benzoate ion. This novel reaction is also observed to occur with phenylnitrene anion, forming phenoxide. Similar reactivity was observed in the reaction between benzoylnitrene radical anion and NO2, forming benzoate ion and nitrous oxide. Electronic structure calculations indicate that the reaction has a high-energy barrier that is overcome by the energy released by bond formation. Benzoylnitrene radical anion also transfers oxygen anion to NO and NO2 as well as to CS2 and SO2. In contrast, phenylnitrene anion reacts with carbon disulfide by C+ or CS+ abstraction, forming S- or S2-. Electronic structure calculations indicate that benzoylnitrene in the ground state resembles a slightly polarized benzoate anion, but with a free radical localized on the nitrogen.

Benzoylnitrene radical anion: a new reagent for the generation of M-2H anions.

Benzoylnitrene radical anion, formed in high abundance by electron ionization of benzoylazide, is found to be a useful reagent for the formation of ionized reactive intermediates, such as diradicals and carbenes. The reactivity of the ion is similar to what is observed with atomic oxygen anion, occurring in many instances by H(2)(+) transfer. However, because benzoylnitrene radical anion is less basic than oxygen anion, it undergoes H(2)(+) transfer with substrates that react with oxygen anion by only proton transfer and therefore can be used to generate reactive ions not easily prepared by other methodologies.

Thermochemical studies of benzoylnitrene radical anion: the N-H bond dissociation energy in benzamide in the gas phase.

The thermochemical properties of benzoylnitrene radical anion, C6H5CON-, were determined by using a combination of energy-resolved collision-induced dissociation (CID) and proton affinity bracketing. Benzoylnitrene radical anion dissociates upon CID to give NCO- and phenyl radical with a dissociation enthalpy of 0.85 +/- 0.09 eV, which is used to derive an enthalpy of formation of 33 +/- 9 kJ/mol for the nitrene radical anion. Bracketing studies with the anion indicate a proton affinity of 1453 +/- 10 kJ/mol, indicating that the acidity of benzamidyl radical, C6H5CONH, is between those of benzamide and benzoic acid. Combining the measurements gives an enthalpy of formation for benzamidyl radical of 110 +/- 14 kJ/mol and a homolytic N-H bond dissociation energy in benzamide of 429 +/- 14 kJ/mol. Additional thermochemical properties obtained include the electron affinity of benzamidyl radical, the hydrogen atom affinity of benzoylnitrene radical anion, and the oxygen anion affinity of benzonitrile.

Regioselectivity of pyridine deprotonation in the gas phase.

The regioselective deprotonation of pyridine in the gas phase has been investigated by using chemical reactivity studies. The mixture of regioisomers, trapped as carboxylates, formed in an equilibrium mixture is determined to result from 70-80% deprotonation in the 4-position, and 20-30% deprotonation at the 3-position. The ion formed by deprotonation in the 2-position is not measurably deprotonated at equilibrium because the ion is destabilized by lone-pair repulsion. From the composition of the mixture, the gas-phase acidities (DeltaH degrees acid) at the 4-, 3-, and 2-positions are determined to be 389.9 +/- 2.0, 391.2-391.5, and >391.5 kcal/mol, respectively. The relative acidities of the 4- and 3-positions are explained by using Hammett-Taft parameters, derived by using the measured gas-phase acidities of pyridine carboxylic acids. The values of sigmaF and sigmaR are -0.18 and 0.74, respectively, showing the infused nitrogen in pyridine to have a strong pi electron-withdrawing effect, but with little sigma-inductive effect.