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International audience; Aromatic substitution reactions of selected ionized benzenes derivatives (chlorobenzene, nitrobenzene, dimethylphtalates) and phenoxy cation using neutral methyl isocyanide as a nucleophile are shown to efficiently occur in the gas phase. Nitrilium ions are produced in high abundance during these processes. These reactions have been performed in the hexapole collision cell of a large-scale hybrid tandem mass spectrometer. Computed 298 K enthalpy diagrams at the B3LYP/6-31+G(d,p) level of theory confirm the exothermic formation of the N-methylbenzonitrilium ions starting with ionized chloro- and nitrobenzene molecular ions. In this last case, two other exothermic processes are also detected: (i) an oxygen atom transfer yielding ionized nitrosobenzene and neutral methyl isocyanate and (ii) a loss of carbon monoxide from the ion/molecule reaction product generated when metastably generated phenoxy cations (produced in the hexapole collision cell) react with methyl isocyanide. Using extended theoretical calculations, several reaction pathways have been derived. The behavior of the three isomeric dimethyl phthalates has been investigated in the same way.

International audience; The chemistry leading to the competitive eliminations of H, CH(3), and OCOCH(3) from adducts of ionized methyl benzoate and neutral methyl isocyanide has been explored using density functional theory molecular orbital calculations. The energies of the various reactants and transition structures were estimated at the B3LYP/6-31+G(d,p) level of theory. Nucleophilic aromatic substitution is proposed to account for the H and OCOCH(3) eliminations. The corresponding sigma-complex intermediates, B(1ipso) and B(1ortho), are stable species lying in deep energy wells situated 70 and 120 kJ/mol, respectively, below the reactants, ionized methyl benzoate and methyl isocyanide. The latter complex, B(1ortho), may be also at the origin of a multistep rearrangement involving hydrogen migrations and methyl elimination from the original methoxy group of the benzoate moiety.

The elusive hydrogen atom adduct to the N-1 position in adenine, which is thought to be the initial intermediate of chemical damage, was specifically generated in the gas phase and characterized by neutralization-reionization mass spectrometry. The N-1 adduct, 1,2-dihydroaden-2-yl radical (1), was generated by femtosecond electron transfer to N-1-protonated adenine that was selectively produced by electrospray ionization of adenine in aqueous-methanol solution. Radical 1 is an intrinsically stable species in the gas phase that undergoes specific loss of the N-1-hydrogen atom to form adenine, but does not isomerize to the more stable C-2 adduct, 1,2-dihydroaden-1-yl radical (5). Radicals 1 that are formed in the fifth and higher electronically excited states of DeltaE > or = 2.5 eV can also undergo ring-cleavage dissociations resulting in expulsion of HCN. The relative stabilities, dissociation, and transition state energies for several hydrogen atom adducts to adenine have been established computationally at highly correlated levels of theory. Transition state theory calculations of 298 K rate constants in the gas phase, including quantum tunnel corrections, indicate the branching ratios for H-atom additions to C-8, C-2, N-3, N-1, and N-7 positions in adenine as 0.68, 0.20, 0.08, 0.03, and 0.01, respectively. The relative free energies of adenine radicals in aqueous solution point to the C-8 adduct as the most stable tautomer, which is predicted to be the predominating (>99.9%) product at thermal equilibrium in solution at 298 K.

The gas-phase protonation of adenine is predicted to occur preferentially at N 1 , N 3 , and N 7 , which have the highest proton affinities PA = 939, 932, and 904 kJ mol - 1 , respectively, as calculated by coupled-cluster abinitio theory and combined density functional and Moller-Plesset theory, B3-MP2, with the 6-311++G-(3df,2p) basis set. The collisionally activated dissociation of protonated adenine tautomers at kiloelectronvolt energies in the gas phase proceeds by the loss of a hydrogen atom, which is accompanied by substantial hydrogen migration in the dissociating ion. The product is the adenine cation-radical (1 + .), which is the most stable C 5 H 5 N 5 + . isomer. The adiabatic and vertical ionization energies of adenine have been calculated to be 8.28 and 8.43 eV, respectively, in excellent agreement with previous photoionization and photoelectron spectroscopy measurements. Structures and relative energies of several isomers of 1 + . (e.g., three imine tautomers and six distonic ions) have been obtained computationally. The imine tautomers are 16-79 kJ mol - 1 less stable than 1 + . at 0 K in the gas phase. The distonic ions, in which a hydrogen atom has been shifted from C 2 or C 8 to N 1 , N 3 , or N 7 , are 51-142 kJ mol - 1 less stable than 1 + .. Dissociations of protonated adenine tautomers by the loss of H require threshold energies in the range of 381-415 kJ mol - 1 and proceed at or close to the thermochemical thresholds. The energy differences between 1 + . and its isomers are diminished to ΔG 2 9 8 (w) = 14-121 kJ mol - 1 in the polar dielectric corresponding to aqueous solution. However, 1 + . remains the most stable isomer in water. H 9 and the protons of the amino group are calculated to be the most acidic protons in 1 + . (pK a = 3.2-3.3). The deprotonation of the amino group in the ionized adenine moiety in DNA is expected to produce a highly reactive radical that is able to abstract a hydrogen atom from thymine and amino acid residues exothermically.

Gas-phase protonation of carbonic acid is predicted to occur preferentially at the carbonyl oxygen with the 298 K proton affinity that was calculated as PA = 741 and 775 kJ mol-1 for the syn- and anti-conformer, respectively. The hydroxyl groups are less basic with a topical PA = 660 kJ mol-1. The standard enthalpy of formation of gas-phase carbonic acid was calculated as = −614 kJ mol-1. Collisional neutralization of protonated carbonic acid, anti-1+, yields transient trihydroxymethyl radicals that dissociate rapidly by loss of a hydrogen atom, so that no survivor species are observed on a 360 ns time scale. High-level ab initio calculations with G2(MP2), G2, B3-MP2, and QCISD(T)/6-311+G(3df,2p) find two stable conformers of trihydroxymethyl radicals, anti-1 and syn-1, that interconvert rapidly by O−H bond rotations. The lowest-energy dissociation of anti-1 is loss of a hydrogen atom which requires 93 kJ mol-1 in the transition state and forms the anti-conformer of carbonic acid with an overall reaction ...

The dehydrobenzonitrilium distonic radical cations a−c, isomers of benzonitrile radical cation d, were prepared by collisional dehalogenation of protonated halogenobenzonitriles and characterized by tandem mass spectrometry (MS/MS/MS experiments and ion−molecule reactions) making use of a hybrid mass spectrometer of sector−quadrupole−sector configuration. These experimental results were further supported by density functional theory calculations at the B3LYP/6-311++G(3df,2p) level, indicating that although the distonic ions are less stable than ion d by ca. 45−50 kJ mol-1, they are protected against isomerization by relatively large energy barriers. Relative energies of benzonitrile and its isomers as well as their proton affinities and ionization energies were also evaluated. N-protonation in benzonitrile is about 120 kJ mol-1 more exothermic than ring protonation.

Collisional activation (high or low translational energy) and neutralization−reionization mass spectrometries appear inefficient for the characterization of the connectivity of isomeric C2S3•+ ions generated by dissociative ionization of 1,2-dithiolo[4,3-c][1,2]dithiole-3,6-dione (1), 1,2-dithiolo[4,3-c][1,2]dithiole-3-one-6-thione (2), and 1,3,4,6-tetrapentalene-2,5-dione (3). In contrast, ion−molecule reactions (particularly with nitric oxide) readily differentiate the C-sulfide ethenedithione ion, SCCS2•+ (4), and the S-sulfide ethenedithione ion, SCCSS•+ (5). The collisional activation spectra of these ion−molecule reaction products have been recorded on a new type of hybrid tandem mass spectrometer of sectors−quadrupole−sectors configuration. The characterization of the isomeric C2S3•+ ions is supported by ab initio calculations at the G2(MP2,SVP) level. The C-sulfide ethenedithione ion is predicted to be more stable than the S-sulfide form by 91 kJ mol-1. The calculated reaction enthalpies for the t...

A theoretical study of the [C3,H5,N]•+ potential energy surface is presented. Ab initio molecular orbital calculations at the QCISD(T)/UMP2 level with the 6-31G(d,p) basis set show that acetonitrile N-methylide [CH3CNCH2]•+, a•+, and N-methylketenimine [CH3NCCH2]•+, b•+, are the most stable species among the 15 isomers considered, and a heat of formation of ΔfH°298 = 970 kJ/mol is proposed for both species. Detailed examination of the [C3,H5,N]•+ potential energy surface indicates that a•+, b•+, and related isomers are stable and distinct species in the gas phase, isolated by energy barriers as high as 300 kJ/mol. Their neutral equivalent, a and b, have also been studied, thus allowing their adiabatic ionization energies to be estimated: IEa(a) = 7.1 eV and IEa(b) = 8.0 eV. Finally, the theoretical study of a•+ and b•+ fragmentations provides an explanation for the similarity in their high-energy CID spectra by showing a possible isomerization of these species prior to dissociation.

Acetonitrile N-methylide radical cations [CH3CNCH2]•+, a•+, and N-methylketeneimine [CH3NCCH2]•+, b•+, have been prepared by ion−molecule reactions between ionized cyclobutanone or ionized ketene a...

Electron impact ionization of carbon disulfide at a pressure of ca. 0.5−1 Torr generates CS2•+ together with a variety of other sulfur-containing ions dominated by CS3•+ and C2S2•+ radical cations (5 and 1% of the intensity of the CS2 (m/z 76) base peak, respectively). The CS3•+ ions were found to be efficient agents for S•+ transfer to the nitrogen atom of nitriles, thus yielding nitrile N-sulfide radical cations, RCNS•+ (R = H, CH3, C2H5, C3H7, C6H5, Cl, Br, and I). These reactions were performed in a chemical ionization source and also in an RF-only quadrupole collision cell installed in the middle of a six-sector tandem mass spectrometer (EBEEBE). The connectivity of the RCNS•+ ions is established by collisional activation, and the stability of the corresponding neutral molecules is evaluated by the neutralization−reionization technique. The structures and energies of CS3•+, RCNS•+, and neutral RCNS (R = H, Br, CH3 and C6H5) were examined by G2(MP2,SVP) theory. The calculated stabilities of the RCNS n...