The gas-phase iodmolecule reaction of N H 2 -with S02F2 proceeds rapidly to yield NS02-(85%) and HNSOzF-(15%). The NS02-, a long-sought species, was studied in a number of gas-phase acid-base bracketing experiments by FT-ICR leading to a AHoacid of 1381 21 kJ/mol for HNS02 at a temperature of 333 K. A lower limit of 3.49 eV has been set for the electron affinity of NS02'. Ab initio calculations using basis sets corrected in the valence region by the generator coordinate method yield a m a c i d of 1386 kJ/mol for HNSO2 at 298 K while the isomeric HOS(0)N is predicted to have a A H o a c i d of 1281 kJ/mol. The electron affinity of the N S O i radical is calculated to be 3.76 eV. Comparison has also been made with the previously identified NSO-ion for which our calculations predict a proton affinity of 1452 kJ/mol which compares very favorably with the experimentally derived value of 1439 f 21 kJ/mol. The optimized molecular structures and harmonic vibrational frequencies obtained at the MP2 level provide a valuable guide toward distinguishing these species.Gas-phase ion chemistry has played an important role over the years in unveiling the intrinsic chemical reactivity of simple molecules under solvent-free conditions. At the same time, considerable insight has been gained on the structure and stability of ionic species from two sources: (1) the continuous development and improvement of experimental techniques in mass spectrometry, ion cyclotron resonance, flowing afterglow, photoionization, and ion beams; (2) theoretical calculations of varying degrees of sophistication which have recently provided a lively interplay with experiments to an extent almost unparalleled in other branches of chemistry.The study of anions has been a particularly fruitful and challenging area of gas-phase ion chemistry despite the fact that few negative ions can be produced directly by conventional ionization or electron attachment processes. Yet, the use of carefully tailored iodmolecule reactions has considerably broadened the horizon of the synthesis of anions in the gas phase.' Likewise, the theoretical description of anions has witnessed substantial progress in ways of dealing with the diffuse nature of the electronic cloud for these systems.2 A particularly interesting situation arises for simple stable anions which can been characterized in the gas-phase but which are not readily isolated in condensed media. Over the years, species such as XeCl-,3 N b -$ H30-,5 NPF-,6 HSO-,' and FzAsS-,* to name a few, represent good examples.During the investigation of the mechanism of nucleophilic iodmolecule reactions in simple inorganic sulfur compound^,^ an easy way has been found to generate the sulfonyl imide ion, NS02-, a heretofore elusive anionic species isoelectronic with SO3. While the synthesis of N-sulfonylamines at low temperatures was reported in the late 1960s,I0 these compounds were known to undergo facile reactions at room temperature'' or to yield cyclic species.I2 Despite its simplicity, the successful isolation of NSO2-had ...

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Gas-phase [C2H5S]+ ions obtained by electron impact ionization from CH3SC2H5 at 13 eV undergo three distinct low-pressure ion/molecule reactions with the parent neutral:  proton transfer, charge transfer, and hydride abstraction. The kinetics of these reactions studied by FT-ICR techniques clearly suggests the [C2H5S]+ species to be a mixture of isomeric ions. While proton transfer and hydride abstraction are consistent with CH3CHSH+ and CH3SCH2 + reagent ions, the observed charge transfer strongly argues for the presence of thioethoxy cation, CH3CH2S+, predicted to be stable only in the triplet state. Charge transfer reactions only occur with substrates having an IE below 8.8 eV and thus yield an upper limit for the recombination energy of the CH3CH2S+ ions. Studies using CD3SC2H5 show that charge-transfer reactions are promoted by cations originating from a sulfur−methyl carbon bond cleavage. Ab initio calculations at several levels of theory predict that CH3CH2S+ ions are only stable in the triplet state. Calculations along the fragmentation pathway of the molecular ion reveal the tendency to generate triplet CH3CH2S+ ions upon cleavage of the sulfur−methyl carbon bond. Calculations were also carried out to determine the lifetime of triplet CH3CH2S+ using nonadiabatic RRKM theory. The exothermic or near thermoneutral spin-forbidden unimolecular isomerizations and dissociations were first characterized at different levels of theory, and the minimum energy crossing points (MECPs) for all the channels were identified at the CCSD(T) level. The probability for surface hopping was then estimated from the spin−orbit matrix elements. The calculated unimolecular dissociation rate constants predict that triplet CH3CH2S+ ions with less than 10 kcal mol-1 of internal energy and at any level of rotational excitation should be long-lived, and strongly support the experimental observations.

The molecular ion of allyl bromide has been characterized by ab initio molecular orbital calculations at the MP4(SDTQ) level with optimized geometries at the MP2 level in order to account for experimental data suggesting the presence of two isomers. The calculations predict the existence of an allyl bromide molecular ion with structural parameters resembling the neutral species except for a lengthening of the double bond. This structure is calculated to be more stable than a cyclic bromonium radical cation structure. Rearrangement of the molecular ion of allyl bromide to that of 1-bromopropene is shown to be possible through a transition state represented by the distonic ion, + BrHCCH 2 CH 2 • , lying just below the dissociation limit of the allyl bromide molecular ion. Studies based on ion/molecule reactivity of C 3 H 5 Br •+ ions generated from allyl bromide and 1-bromopropene with ammonia, methanol, allyl bromide, and charge transfer reactions strongly suggest that a small fraction of the molecular ions of allyl bromide isomerize to the 1-bromopropene molecular ion as predicted by the calculation. These experiments cannot establish unequivocally whether the allyl bromide molecular ions retain the structure of the parent molecules as predicted by the calculations or undergo ion/ molecule reactions mediated by a bromonium type complex. Charge transfer experiments also suggest the adiabatic ionization energy of allyl bromide to be 9.83 ( 0.07 eV.

Adiabatic ionization potentials (AIPs) and vertical ionization potentials (VIPs) for all fluorinated, chlorinated, and chlorofluorinated ethylenes have been determined by ab initio computations. The calculated AIPs give a mean absolute deviation of 0.014 eV at G2 and 0.015 eV at G3 theories compared to experimental values. We have estimated AIPs (in eV) for AIP ((E)-CHCl=CFCl) = 9.59, AIP ((Z)-CHCl=CFCl) = 9.60, AIP (CCl2=CFCl) = 9.42, and AIP (CHF=CCl2) = 9.65. Furthermore, our calculated AIPs values of 9.58 eV for (Z)-CFCl=CFCl and 9.56 eV for (E)-CFCl=CFCl are very different from the experimental data of 10.2 eV. VIPs are calculated by Koopmans's theorem with HF methodology and by G2 and G3 theories. Koopmans's theorem fails in giving a good description of the behavior of the VIPs for fluoroethylenes. Furthermore, significant improvement in the results is observed by the mean absolute deviation from experimental data on the computed values (0.242 eV using the 6-311+G(3df,2p) basis set and 0.248 eV using the GTlarge basis set, compared with 0.049 eV at G2 and 0.045 eV at G3 theories) when orbital relaxation and changes in electronic correlation and zero-point energies are taken into account. Our estimated VIPs values calculated by G3 theory (in eV) are VIP ((E)-CHCl=CFCl) = 9.89, VIP ((Z)-CHCl=CFCl) = 9.90, VIP ((E)-CFCl=CHF)= 10.26, VIP ((Z)-CFCl=CHF) = 10.25, VIP ((E)-CFCl=CFCl) = 9.93, VIP ((Z)-CFCl=CFCl) = 9.96, VIP (CCl2=CFCl) = 9.71, VIP (CF2=CHCl) = 10.19, VIP (CHF=CCl2) = 9.96, VIP (CH2=CFCl) = 10.32, VIP ((Z)-CHCl=CHF) = 10.16, and VIP ((E)-CHCl=CHF) = 10.16. Furthermore, the variation of the VIPs and AIPs with the increase in the number of halogen atoms in the molecules presents different patterns to chloroethylenes and fluoroethylenes.