Marina Attinà

Displacement of a nitro-group by [18F] fluoride ion. A new route to aryl flurides of high specific activity

Nucleophilic displacement of activated nitro-group by [18F] fluoride ion is an efficient route to 18F-labelled aromatics; these compounds can be in the no-flurine-carrier-added state if required.

Gas phase protonation of alkyl and phenyl azides

Abstract Gas-phase protonation of azides has been investigated with mass spectrometric and computational techniques. The proton affinities of MeN3, EtN3 and PhN3, derived from bracketing experiments, are respectively 199±3, 210±5, and 196±3 kcal mol−1. The structures and the energetics of MeN3 and MeN3H+ have been investigated by ab initio MO techniques, a proton affinity of MeN3 of 195.3 kcal mol−1 at the 6-31G **//6-31G* level of theory being obtained, in agreement with the experimental value. The loss of N2 from MeN3H+, yielding the MeNH+ nitrenium ion, has also been theoretically estimated (by ab initio techniques) to be endothermic by ca. 15 kcal mol−1, again in accord with experimental observations. In fact, MeN3H+ is fairly stable, being the base peak in the CH4 chemical ionization spectrum of MeN3, while PhN3H+ is but a minor peak in the CH4 chemical ionization spectrum of PhN3, undergoing extensive N2 loss. Collisional Activation (CA) mass spectrometry of the charged fragment formed upon N2 loss from CH3N3H+ suggests rearrangement to CH2=NH+2, while the PhNH+ fragment retains the nitrenium-ion structure, reacting with benzene to give a C12H12N+ derivative whose CA spectrum is closely similar to that of protonated diphenylamine.

A mass spectrometric and computational study of gaseous peroxynitric acid and (HOONO2)H+ protomers

Abstract The positive ion chemistry of peroxynitric acid ( 1 ) was investigated in the gas phase by mass-analyzed ion kinetic, collisionally activated dissociation, and Fourier transform-ion cyclotron resonance mass spectrometric techniques and theoretical methods up to the B3LYP/6-311++g(3 df ,2 pd ) and G2, i.e. QCISD(T)/6-311+g(3 df ,2 pd ), levels. The ion–neutral complex HOOH–NO 2 + ( 1a ) is the only detectable protomer in CI experiments involving the protonation of 1 by H 3 O + , and can also be obtained from the reaction of NO 2 + with H 2 O 2 . 1a behaves as a protonating and nitrating agent toward gaseous nucleophiles. The experimental proton affinity of 1 is estimated to be 176 ± 3 kcal mol −1 , in excellent agreement with the 175 ± 2 kcal mol −1 G2 PA. The theoretical results show that 1a is more stable than the HOONO 2 H + ( 1b ) and the H 2 OONO 2 + ( 1c ) protomers by 13 and 16 kcal mol −1 , respectively, at the B3LYP level of theory, and account for the exclusive formation of 1a in the CI experiments. The experimental and B3LYP theoretical binding energy of NO 2 + to H 2 O 2 amounts to 18 ± 2 kcal mol −1 .

Positive ion chemistry of gaseous boric and polyboric acids

Abstract The positive ion chemistry of H 3 BO 3 has been studied by chemical ionization (CI) and Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. From the results of equilibrium and “bracketing” experiments, the gas-phase basicity (GPB) and the proton affinity (PA) of H 3 BO 3 are estimated to be 167.8 ± 0.5 and 176 ± 2 kcal mol −1 respectively. The latter value is in fair agreement with the PA of 181.2k cal mol −1 from MO SCF calculations at the MP3/6-31G**//6-31G* + ZPVE (6-31G*) level of theory. The PA of HBO 2 calculated at the MP4(SDTQ)/6-31G**//6-31G* + ZPVE (6-31G*) level is 181.2 kcal mol −1 , which falls within the interval set by ICR “bracketing” experiments. The H 4 BO + 3 ion undergoes condensation with H 3 BO 3 yielding protonated polyboric acids containing up to seven B atoms belonging to three different classes, (H n +2 B n O 2 n +1 )H + , (H n B n O 2 n H + and (H n −2 B n O 2 n −1 )H + , formed via sequences established by FT-ICR experiments involving isolation of the ionic reactants by multiple resonance. The structures of the polyions observed are discussed and compared to those of the corresponding anions, also observed in the gas phase, and of the few anions whose structures have been established in crystalline borates. The fast isotope exchange with H 2 18 O undergone by H 4 BO + 3 suggests that the H 6 BO + 4 ion observed in the CI spectra of H 3 BO 3 may contain a tetrahedrally coordinated B atom, as present in many borate and polyborate anions. The reactions of H 3 BO 3 and/or of H 4 BO + 3 with MeOH and HCOOH, yielding protonated esters, respectively anhydrides, of boric and polyboric acids are surveyed.

Gaseous Ethylenexenonium Ions (C2H4Xe+) and Ethylenefluoroxenonium Ions (C2H4XeF+): A Joint Mass Spectrometric and Theoretical Study

A mechanistic link has been found between the gas-phase preparation of ethylenexenonium ions and the reactions of unsaturated compounds with XeF+ salts, utilized in solution as stereoselective fluorinating agents. Complex C2H4XeF+ (2) is the charged intermediate in the formation process of C2H4Xe+ (1) from XeF+ and ethylene. This was evidenced through FT-ICR and TQ mass spectrometry, complemented by DFT calculations at the BLYP/DZVP//CCSD(T) level of theory.

Lifetimes of Gaseous Ion–Neutral Complexes: The Rate of Isotopic Scrambling within Ethyl Ions as an Internal Clock

The theoretically computed rate of H/D scrambling within labeled ethyl ions is used as a clock to time the lifetime of ion–neutral complexes, previously characterized as the charged intermediates in gas-phase aromatic alkylation. From experiments performed in the temperature interval from 298 to 393 K in CF4 at 720 Torr, the lifetimes of [CF3C6H5⋅C2HD4+] complexes are estimated to range from about 20 to 200 ps, depending on the temperature.

FT-ICR studies of gas-phase ionic nitration of benzene: the role of electron- and proton-transfer processes

Abstract The detailed modes of reaction of (RONO2)H+ (R = H, CH3 or C2H5) with benzene (C6X6, X = H, D) were determined by Fourier transform ion cyclotron resonance mass spectrometry. Irrespective of the structure and energy of the potentially nitrating (RONO2)H+ reactants, benzene nitration and oxidation are found to proceed by two-step mechanisms, involving a fast, preliminary electron and proton transfer between (RONO2)H+ and C6X6, followed by the rate-determining attack of the C6X+6 and C6X6H+ daughters on the nucleophilic centres of RONO2. These reaction pathways, predominant under isolated pair conditions, are discussed and compared with classical acid-induced aromatic nitration patterns both in the gas phase at atmospheric pressure and in solution.

Gas-phase fluorination of acetylene by XeF+: Formation, structure and reactivity of C2H2F+ isomeric ions

Abstract The gas-phase reactivity of XeF+ towards acetylene was investigated by triple quadrupole mass spectrometry. XeF+ promotes both F+ and Xe+ transfer to acetylene, yielding C2H2F+ and C2H2Xe+, respectively. The C2H2F+ ions formed were probed by low-energy collisionally activated dissociation mass spectrometry and characterized as CH2CF+, namely the isomer identified as the most stable by previous theoretical studies. The 1-fluorovinyl cation reacts in the gas-phase with typical nucleophiles (CH3COCH3,CH3CN,CH3OH,C2H4), as a Bronsted acid and/or as a fluorinating agent, depending on the thermochemistry of the processes involved.

Gas-phase ion chemistry of H3BO3. Protonated orthoboric, metaboric and polyboric acids, and their anions in the gas phase

Formation of protonated orthoboric, metaboric and polyboric acids, and of their anions has been demonstrated in the gas phase by mass spectrometric techniques, which have been used to measure the proton affinity of H3BO3, 175.6 kcal mol–1, in fair agreement with the value of 181.2 kcal mol–1 from MO SCF ab initio calculations (1 cal = 4.184 J).

Gas-phase reaction of free isopropyl ions with phenol and anisole

Unsolvated isopropyl ions, obtained in the dilute gas state from the radiolysis of propane, react with PhOH yielding isomeric isopropylphenols and isopropyl phenyl ether, in the ratio of ca. 3 : 1 at 320 Torr. At lower pressures, the ratio is further shifted in favour of ring alkylation, reaching a value in excess of 10 : 1 at 22 Torr. The isomeric composition of isopropylphenols passes from 83%ortho, 3%meta, 14%para at 720 Torr to 61%ortho, 3%meta, 36%para at 22 Torr. A similar pressure dependence characterizes the isomeric composition of products from the alkylation of PhOMe, which passes from 85%ortho, 5%meta, 10%para in C3H8 at 720 Torr to 64%ortho, 21%meta, and 15%para at 20 Torr. The substrate selectivity of isopropyl ion, referred exclusively to the alkylation, is measured by the apparent ratios kPhOH:kPhH= 1.0–1.6 and kPhOMe:kPhMe= 0.6–0.9 at 720 Torr. The results are consistent with a model involving kinetically predominant attack to the oxygen atom, i.e. to the n-type nucleophilic centre of the ambident substrate, and competition between proton transfer and alkylation channels, reflecting respectively the Bronsted acid and the Lewis acid reactivity of the alkyl cation. The role of isomerization processes in determining the orientation has been independently evaluated by protonating isopropyl phenyl ether with gaseous Bronsted acids, such as H3+ and CnH5+(n= 1 or 2), thus obtaining via an independent route the same oxonium ion arising from the attack of isopropyl ion on PhOH. The results demonstrate dealkylation to PhOH and intramolecular isomerization to o- and p-isopropylphenols, whose ratio depends on the nature of the Bronsted acid, and the exothermicity of the proton transfer to the ether.