Felice Grandinetti

Review: gas-phase ion chemistry of the noble gases: recent advances and future perspectives.

This review article surveys recent experimental and theoretical advances in the gas-phase ion chemistry of the noble gases. Covered issues include the interaction of the noble gases with metal and non-metal cations, the conceivable existence of covalent noble-gas anions, the occurrence of ion–molecule reactions involving singly-charged xenon cations and the occurrence of bond-forming reactions involving doubly-charged cations. Research themes are also highlighted, which are expected to attract further interest in the nearfuture.

Ionic Fluorination of Carbon Monoxide as a Route to Gasphase Carbonylation of Inert CH and NH Bonds

Gaseous FCO+ ions from the ionization of mixtures of nitrogen trifluoride and carbon monoxide execute selective and efficient CO-functionalization of the C-H bonds of benzene and toluene and of the N-H bond of ammonia. The occurrence of these carbonylation reactions has been unambiguously ascertained by Fourier-transform ion cyclotron resonance (FT-ICR) spectrometry, and the details of the structure and the mechanism of formation of the precursor FCO+ ions have been investigated. FT-ICR experiments show that these ions, structurally assigned as F-C-O+ by collisionally activated dissociation (CAD) spectrometry, arise from the reaction of CO.+ with NF3 and of NF+2 with CO. Combining the latter F+ transfer with the independently observed fluoride-ion abstraction by FCO+ from NF3 results in a catalytic cycle in which gaseous NF+2 ions promote the conversion of carbon monoxide into carbonic difluoride, F2 CO, with nitrogen trifluoride as the source of F.

Xenon–Nitrogen Chemistry: Gas‐Phase Generation and Theoretical Investigation of the Xenon–Difluoronitrenium Ion F2NXe+

The xenon-difluoronitrenium ion F(2)N-Xe(+) , a novel xenon-nitrogen species, was obtained in the gas phase by the nucleophilic displacement of HF from protonated NF(3) by Xe. According to Møller-Plesset (MP2) and CCSD(T) theoretical calculations, the enthalpy and Gibbs energy changes (ΔH and ΔG) of this process are predicted to be -3 kcal mol(-1) . The conceivable alternative formation of the inserted isomers FN-XeF(+) is instead endothermic by approximately 40-60 kcal mol(-1) and is not attainable under the employed ion-trap mass spectrometric conditions. F(2)N-Xe(+) is theoretically characterized as a weak electrostatic complex between NF(2)(+) and Xe, with a Xe-N bond length of 2.4-2.5 Å, and a dissociation enthalpy and free energy into its constituting fragments of 15 and 8 kcal mol(-1), respectively. F(2)N-Xe(+) is more fragile than the xenon-nitrenium ions (FO(2)S)(2)NXe(+), F(5)SN(H)Xe(+), and F(5)TeN(H)Xe(+) observed in the condensed phase, but it is still stable enough to be observed in the gas phase. Other otherwise elusive xenon-nitrogen species could be obtained under these experimental conditions.

Cationic Noble Gas Hydrides: A Theoretical Investigation of Dinuclear HNgFNgH+ (Ng = He―Xe)

Theoretical calculations at the B3LYP, MP2, and CCSD(T) levels of theory disclose the conceivable existence of cationic noble gas hydrides containing two Ng atoms. These species have a general formula of HNgFNgH(+) (Ng = He-Xe), and are the cationic counterparts of the neutral HNgF. The optimized geometries, harmonic frequencies, and bonding properties point to ion-dipole complexes between a fluoride anion and two covalent H-Ng(+) cations, best formulated as (H-Ng(+))(2)F(-). The HXeFXeH(+) is also isoelectronic with the recently experimentally observed HXeOXeH (Khriachtchev et al. J. Am. Chem. Soc. 2008, 130, 6114-6118). The resulting HNgFNgH(+) are thermochemically stable with respect to dissociation into HNg(+) + HNgF and HNg(+) + H + Ng + F, but are largely unstable with respect to both the loss of HF (with formation of HNg(+) + Ng) and H(2)F(+) (with formation of two Ng atoms). These decompositions pass through bent transition structures, and only the heaviest HArFArH(+), HKrFKrH(+), and HXeFXeH(+) are protected by energy barriers large enough (ca. 10-15 kcal mol(-1)) to support their conceivable metastability. In line with other series of noble gas compounds, the neon cation HNeFNeH(+) is the least stable among the various HNgFNgH(+).

Ionic Lewis superacids in the gas phase. Part 1. Ionic intermediates from the attack of gaseous SiF+3 on n-bases

The reactivity of the SiF+3 cation towards oxygen bases (H2O, CH3OH, and CH3CH2OH) and nitrogen bases (NH3 and CH3NH2) has been studied using Fourier-transform ion cyclotron resonance mass spectrometry. The SiF+3 ion exclusively attacks the n-centre of the selected bases, yielding excited onium intermediates that undergo fragmentation by elimination of either an alkyl cation (CH3OH, CH3CH2OH, and CH3NH2) or an HF molecule (H2O, NH3, and CH3NH2). In H2O, various protonated fluorosilicic and silicic acids are formed which can be readily converted into their esters and amides by reaction with alcohols and ammonia, respectively. The structure and the reactivity of several such species have been investigated by mass-analyzed ion kinetic energy-collision induced dissociation spectroscopy and ab initio calculations. The extreme affinity of SiF+3 toward n-type electrons ranks it as a powerful gaseous “Lewis superacid”, suitable for generating long-lived, highly reactive ions, e.g. CH+3, in “non nucleophilic” gaseous media, such as, for instance, SiF4.

Ab initio study on the radical anions SO−3 and CO−2 and on the charge-transfer complexes MSO3 and MCO2 (M = Li, Na)

Abstract The structures of the MSO 3 ion pairs (M = Li, Na) and the effects of coordination on the vibrational modes of the radical anion SO − 3 were examined by means of ab initio molecular orbital calculations. Both the bidentate and tridentate structures of LiSO 3 and NaSO 3 were predicted to be stable isomers and bidentate binding was inferred as the preferred coordination model. The stability of the lowest energy structures was tested against frozen-core UMP2, UMP3 and UMP4 calculations. Harmonic frequencies were calculated for the bidentate and tridentate LiSO 3 and NaSO 3 complexes and 34 S and 18 O frequency shifts reported for the two stable structures of NaSO 3 . The study was extended to the radical anion CO − 2 and to the bidentate and monodentate isomers LiCO 2 and NaCO 2 . The geometry, stability and vibrational spectra of the stable bidentate and monodentate isomers of LiCO 2 and NaCO 2 are discussed.

Unimolecular decay of the thiomethoxy cation, CH3S+: A computational study on the detailed mechanistic aspects

The unimolecular decay of the triplet thiomethoxy cation CH3S+, ion 1, has been investigated by density functional theory, ab initio, and Phase–space/Rice Ramsperger Kassel Marcus (PST/RRKM) calculations. We have first located on the singlet and triplet B3LYP/6-311+G(d,p) [C,H3,S]+ potential energy surfaces the energy minima and transition structures involved in the lowest energy decompositions of 1, including the loss of H, H2, and S. We have subsequently located the minimum energy points lying on the B3LYP/6-311+G(d,p) hyperline of intersection between the singlet and triplet surfaces, using a recently described steepest descent-based method [Theor. Chem. Acc. 99, 95 (1998)]. The total energies of all these species were refined by CCSD(T)/cc-pVTZ single-point calculations. The obtained potential energy surface has been used to outline the full kinetic scheme for the unimolecular decay of ion 1. The rate constants of the various elementary steps have been calculated by the PST and the RRKM theory. We use...

Methylated NF3. A G2MS theoretical study on the structure, stability, and interconversion of the CH3–NF3+ and CH3F–NF2+ isomers

Abstract The G2MS theory by Morokuma and coworkers [J. Phys. Chem. 101 (1997) 227] has been used to perform a comparative study on protonated and methylated NF 3 . The thermochemistry of protonated NF 3 is reproduced within 10 kJ mol −1 and F 2 N–FH + results in the thermodynamically favoured protomer. The CH 3 –NF 3 + isomer is predicted to be more stable than CH 3 F–NF 2 + by 23.0 kcal mol −1 , and the theoretical data are consistent with the results of gas-phase experiments on methylated NF 3 thus far reported by Kebarle and coworkers.

Neutral Compounds with Xenon–Germanium Bonds: A Theoretical Investigation on FXeGeF and FXeGeF3

The structure and stability of FXeGeF and FXeGeF3 were investigated by MP2, CCSD(T), and B3LYP calculations, and their bonding situation was examined by NBO and AIM analysis. These molecules are thermochemically stable with respect to dissociation into F + Xe + GeF(n) (n = 1, 3), and kinetically stable with respect to dissociation into Xe + GeF(n+1), thus suggesting their conceivable existence as metastable species. FXeGeF and FXeGeF3 are best described by the resonance structures F(-)(Xe-GeF(+)) and F(-)(Xe-GeF3(+)), and feature essentially ionic xenon-fluorine interactions. The xenon-germanium bonds have instead a significant contribution of covalency. The comparison with XeGeF(+) and XeGeF3(+) suggests that the stability of FXeGeF and FXeGeF3 arises from the F(-)-induced stabilization of these ionic moieties. This structural motif resembles that encountered in other noble-gas neutral and ionic species.

The geometries and vibrational patterns of LiClO3 and NaClO3 ion pairs: an ab initio SCF study

Abstract Ab initio SCF calculations at the 3-21G* and 6-31G* HF-SCF level were performed on LiClO 3 and NaClO 3 . Tridentate, bidentate and monodentate coordination models were examined and the bidentate and tridentate structures were characterized as the two lowest-energy minima on the corresponding potential energy surfaces. In particular, bidentate LiClO 3 is favoured by ≈ 24 kJ mol −1 at the 6-31G* level with respect to the tridentate isomer. On the contrary, the tridentate NaClO 3 isomer is more stable than the bidentate by ≈ 7 kJ mol −1 . Correlation energies were calculated via Moller—Plesset perturbation theory carried to the second and third order on the bidentate and tridentate energy minima of the two molecules and the bidentate structures of LiClO 3 and NaClO 3 were determined to be more stable by ≈ 17 and ≈ 3 kJ mol −1 , respectively with regard to the corresponding tridentate models. The results of MP2/6-31G * and MP3/6-31G* calculations and the effect of zero-point energy on the stability of the bidentate and tridentate molecules are discussed. Calculated harmonic frequencies of the stable isomers of LiClO 3 and NaClO 3 and 37 Cl and 18 O frequency shifts are reported.