Gas-phase ion chemistry of BF(3)/NH(3) mixtures

The gas-phase ion chemistry of BF(3)/NH(3) mixtures was investigated by the joint application of mass-spectrometric techniques and theoretical methods. The addition of BF(2)(+) to NH(3) led to the first observation of [BF(2),NH(3)](+) and [BF,NH(2)](+) ions. Diamidoboron cation B(NH(2))(2)(+) was also detected. Consistent with collisionally activated dissociation (CAD) mass spectrometric results, theoretical calculations performed at the B3LYP and CCSD(T) levels identified the F(2)B-NH(3)(+), FB-NH(2)(+), and NH(2)-B-NH(2)(+) ions as the most stable isomers on the corresponding potential energy surfaces. The F(2)B-NH(3)(+) ion represents the protonated form of aminodifluoroborane, BF(2)NH(2), and consequently behaves as a Br?nsted acid under FT-ICR conditions. The FBNH(2)(+) ion is able to add Lewis bases such as water, ammonia, and hydrazoic acid. These species, containing the BN moiety, may represent new promising projectile ions in the boron nitride deposition techniques involving high-energy ion beams.     

Sigma bond activation by cooperative interaction with s2 atoms: B+ + nCH4, n = 1, 2

Ab initio investigations at the MP2 and CCSD(T) level with augmented double and triple zeta basis sets have identified various stationary points on the B+/nCH4, n = 1, 2 hypersurfaces. The electrostatic complexes show a strong variation in the sequential binding energy with De for the loss of one CH4 molecule calculated to be 16.5 and 6.8 kcal mol-1 for the n = 1 and n = 2 complexes, respectively. The covalent molecular ion, CH3BH+, is found to have the expected C3 nu geometry and to be strongly bound by 84.0 kcal mol-1 with respect to B+ + CH4. The interaction of CH4 with CH3BH+ is qualitatively very similar to the interaction of CH4 with HBH+, however, the binding is only about 50% as strong due to the electron donating characteristic of the methyl group. Of particular interest are the insertion transition states which adopt geometries allowing the B+ ion to interact with multiple sigma bonds. In the n = 1 case, the interaction with two CH bonds lowers the insertion activation energy by about 25 kcal mol-1 from that expected for a mechanism involving only one sigma bond. For n = 2, B+ interacts with two CH sigma bonds from one CH4 and one CH sigma bond from the other CH4 leading to an additional activation energy decrease of about 15.7 kcal mol-1 relative to B+ + nCH4.     

Selective anion encapsulation by a metalated cryptophane with a pi-acidic interior

Metalation of the exterior arene faces of the molecular capsule (+/-)-cryptophane-E with [Cp*Ru]+ moieties results in a pi-acidic cavity capable of encapsulating anions. The [CF3SO3]- and [SbF6]- salts have been crystallographically characterized and demonstrate the encapsulation of these anions by the metalated cryptophane. 1H and 19F NMR spectroscopy establish the binding of anions in NO2CD3 solution and reveal the relative affinity of the cavity for different anions (KX-/KOTf-): [BF4]- approximately 0, [PF6]- = 1.18, [CF3SO3]- identical with 1, [SbF6]- = 0.30. Variable temperature rate studies reveal the activation barrier for triflate encapsulation to be DeltaG298K = 18.0(8) kcal.mol-1 (DeltaH = 17.5(4) kcal.mol-1 and DeltaS = 2(1) cal.mol-1.K-1).     

A mass spectrometry study of XCO+, X=Si, Ge: is SiCO+ a main group carbonyl? Comments on the bonding in ground state SiCO and the

The cation [Si,C,O]+ has been generated by 1) the electron ionisation (EI) of tetramethoxysilane and 2) chemical ionisation (CI) of a mixture of silane and carbon monoxide. Collisional activation (CA) experiments performed for mass-selected [Si,C,O]+, generated by using both methods, indicate that the structure is not inserted OSiC+; however, a definitive structural assignment as Si(+)-CO, Si(+)-OC or some cyclic variant is impossible based on these results alone. Neutralisation-reionisation (+NR+) experiments for EI-generated [Si,C,O]+ reveal a small peak corresponding to SiC+, but no detectable SiO+ signal, and thus establishes the existence of the Si(+)-CO isomer. CCSD(T)@B3LYP calculations employing a triple-zeta basis set have been used to explore the doublet and quartet potential-energy surfaces of the cation, as well as some important neutral states. The results suggest that both Si(+)-CO and Si(+)-OC isomers are feasible; however, the global minimum is 2 pi SiCO+. Isomeric 2 pi SiOC+ is 12.1 kcal mol-1 less stable than 2 pi SiCO+, and all quartet isomers are much higher in energy. The corresponding neutrals Si-CO and Si-OC are also feasible, but the lowest energy Si-OC isomer (3A") is bound by only 1.5 kcal mol-1. We attribute most, if not all, of the recovery signal in the +NR+ experiment to SiCO+ survivor ions. The nature of the bonding in the lowest energy isomers of Si(+)-(CO,OC) is interpreted with the aid of natural bond order analyses, and the ground state bonding of SiCO+ is discussed in relation to classical analogues such as metal carbonyls and ketenes.     

Theoretical investigation of small polyatomic ions observed in inductively coupled plasma mass spectrometry: H(x)CO+ and H(x)N2(+) (x = 1, 2, 3)

Two series of small polyatomic ions, HxCO+ and HxN2(+) (x = 1, 2, 3), were systematically characterized using three correlated theoretical techniques: density functional theory using the B3LYP functional, spin-restricted second-order perturbation theory, and singles + doubles coupled cluster theory with perturbative triples. On the basis of thermodynamic data, the existence of these ions in inductively coupled plasma mass spectrometry (ICP-MS) experiments is not surprising since the ions are predicted to be considerably more stable than their corresponding dissociation products (by 30-170 kcal/mol). While each pair of isoelectronic ions exhibit very similar thermodynamic and kinetic characteristics, there are significant differences within each series. While the mechanism for dissociation of the larger ions occurs through hydrogen abstraction, the triatomic ions (HCO+ and HN2(+)) appear to dissociate by proton abstraction. These differing mechanisms help to explain large differences in the abundances of HN2(+) and HCO+ observed in ICP-MS experiments.     

Gas-phase positive ion chemistry of 1-bromo-1-chloro-2,2,2-trifluoroethane (halothane) upon electron ionization within an ion trap mass spectrometer

The positive ion chemistry occurring within an ion trap mass spectrometer upon electron ionization of 1-bromo-1-chloro-2,2,2-trifluoroethane, the important anaesthetic halothane, has been mapped by means of collision-induced decomposition and ion/molecule self-reaction experiments. Ionized halothane (M+*) reacts with neutral halothane to form the ionized olefin [ClBrC=CF2]+*. via HF elimination. Among the ionic fragments, [M-Br]+ and [M-F]+ react with halothane via chloride abstraction while [M-Cl]+ is unreactive under the same experimental conditions. Substituted methyl cations CHFX+ and CF2X+ (X = F, Cl, Br) undergo halide transfer processes, their reactivity being highest for X = F. Ionized carbenes CXY+ (X,Y = F,F; H,Br; H,Cl; H,F) react with halothane to form CClXY+ and CBrXY+, whereas CF+ inserts into the C-Cl bond to form CF3+ and CClF2+. Finally, Br+ and Cl+ react with halothane by charge transfer. Collision-induced dissociation experiments disclosed interesting rearrangements involved in the dissociations of +CHX-CF3 ions (X = Br, Cl), which undergo fluorine migration and elimination of CF2, as already observed for +CCl2-CF3 in a previous investigation.     

Generation and collision-induced dissociation of ammonium tetrafluoroborate cluster ions

Singly and doubly charged cluster ions of ammonium tetrafluoroborate (NH4BF4) with general formula [(NH4BF4)nNH4]+ and [(NH4BF4)m(NH4)2]2+, respectively, were generated by electrospray ionization (ESI) and their fragmentation examined using collision-induced dissociation (CID) and ion-trap tandem mass spectrometry. CID of [(NH4BF4)nNH4]+ caused the loss of one or more neutral NH4BF4 units. The n = 2 cluster, [(NH4BF4)2NH4]+, was unique in that it also exhibited a dissociation pathway in which HBF4 was eliminated to create [(NH4BF4)(NH3)NH4]+. Dissociation of [(NH4BF4)m(NH4)2]2+ occurred through two general pathways: (a) 'fission' to produce singly charged cluster ions and (b) elimination of one or more neutral NH4BF4 units to leave doubly charged product ions. CID profiles, and measurements of changing precursor and product ion signal intensity as a function of applied collision voltage, were collected for [(NH4BF4)nNH4]+ and compared with those for analogous [(NaBF4)nNa]+ and [(KBF4)nK]+ ions to determine the influence of the cation on the relative stability of cluster ions. In general, the [(NH4BF4)nNH4]+ clusters were found to be easier to dissociate than both the sodium and potassium clusters of comparable size, with [(KBF4)nK]+ ions the most difficult to dissociate.     

The importance of dihydrogen complexes HnGe(H2)+ (n=0,1) to the chemistry of cationic germanium hydrides: advanced theoretical and mass spectrometric analysis

Investigations of [Ge,Hn]-/0/- (n = 2,3) have been performed using a four-sector mass spectrometer. The results reveal that the complexes HnGe(H2)+ (n = 0,1) play an important role in the unimolecular dissociation of the metastable cations. Theoretical calculations support the experimental observations in most instances, and the established view that the global minimum of [Ge,H2]+ is an inserted structure may need reexamination; CCSD(T,full)/cc-pVTZ//CCSD(T)/6-311 ++ G(d,p) and B3LYP/cc-pVTZ studies of three low-lying cation states (2A1 HGeH+, 2B2 Ge(H2)+ and 2B1 Ge(H2)+) indicate a very small energy difference (ca. 4 kcal mol(-1)) between 2A1 HGeH+ and 2B2 Ge(H2)+; B3LYP favours the ion-molecule complex, whereas coupled-cluster calculations favour the inserted structure for the global minimum. Single-point multireference (MR) averaged coupled-pair functional and MR-configuration interaction calculations give conflicting results regarding the global minimum. We also present theoretical evidence indicating that the orbital-crossing point implicated in the spin-allowed metastable dissociation HGeH+* --> Ge(H2)+* --> Ge+ + H2 lies above the H-loss asymptote. Thus, a quantum-mechanical tunneling mechanism is invoked to explain the preponderance of the H2-loss signal for the metastable ion.     

Selected ion flow tube study of the reactions between gas phase cations and CHCl2F, CHClF2, and CH2ClF

The branching ratios and rate coefficients have been measured at 298 K for the reactions between CHCl2F, CHClF2, and CH2ClF and the following cations (with recombination energies in the range 6.3-21.6 eV); H3O+, SFx+ (x = 1-5), CFy+ (y = 1-3), NO+, NO2+, O2+, Xe+, N2O+, O+, CO2+, Kr+, CO+, N+, N2+, Ar+, F+, and Ne+. The majority of the reactions proceed at the calculated collisional rate, but the reagent ions SF3+, NO+, NO2+, and SF2+ do not react. Surprisingly, although all of the observed product channels are calculated to be endothermic, H3O+ does react with CHCl2F. On thermochemical grounds, Xe+ appears to react with these molecules only when it is in its higher-energy 2P1/2 spin-orbit state. In general, most of the reactions form products by dissociative charge transfer, but some of the reactions of CH2ClF with the lower-energy cations produce the parent cation in significant abundance. The branching ratios produced in this study and by threshold photoelectron-photoion coincidence spectroscopy agree reasonably well over the energy range 11-22 eV. In about one-fifth of the large number of reactions studied, the branching ratios are in excellent agreement and appreciable energy resonance between an excited state and the ground state of the ionized neutral exists, suggesting that these reactions proceed exclusively by a long-range charge-transfer mechanism. Upper limits for the enthalpy of formation at 298 K of SF4Cl (-637 kJ mol-1), SClF (-28 kJ mol-1), and SHF (-7 kJ mol-1) are determined.     

Ring-opening protonolysis of sila[1]ferrocenophanes as a route to stabilized silylium ions

The ring-opening reactions of a series of sila[1]ferrocenophanes with protic acids of anions with various degrees of noncoordinating character have been explored. Ferrocenyl-substituted silyl triflates FcSiMe2OTf (5 a) and Fc(3)SiOTf (5 b) (Fc=(eta5-C5H4)Fe(eta5-C5H5)) were synthesized by means of HOTf-induced ring-opening protonolysis of strained sila[1]ferrocenophanes fcSiMe2 (3 a) and fcSiFc2 (3 b) (fc=(eta5-C5H4)2Fe). Reaction of 3 a and 3 b with HBF4 yielded fluorosubstituted ferrocenylsilanes FcSiMe2F (6 a) and Fc3SiF (6 b) and suggested the intermediacy of a highly reactive silylium ion capable of abstracting F- from the [BF4]- ion. Generation of the solvated silylium ions [FcSiMe2THF]+ (7a+), [Fc3SiTHF]+ (7b+) and [FcSiiPr2OEt2]+ (7c+) at low temperatures, by reaction of the corresponding sila[1]ferrocenophanes (3 a, 3 b, and fcSiiPr2 (3 c), respectively) with H(OEt2)(S)TFPB (S=Et2O or THF; TFPB=tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) was monitored by using low-temperature 1H, 13C, and 29Si NMR spectroscopy. In situ reaction of 7a+, 7b+, and 7c+ with excess pyridine generated [FcSiMe2py]+ (8a+), [Fc3Sipy]+ (8b+), and [FcSiiPr2py]+ (8c+), respectively, as observed by 1H, 13C, and 29Si NMR spectroscopy. A preparative-scale reaction of 3 b with H(OEt2)(THF)TFPB at -60 degrees C and subsequent addition of excess pyridine gave isolable red crystals of 8b-[TFPB]CHCl3, which were characterized by 1H and 29Si NMR spectroscopy as well as by single-crystal X-ray diffraction.     

CS2O+ and CS2O in the gas phase: an experimental and computational study

The CS2O+ ion and CS2O molecule were prepared and structurally characterized by mass spectrometric techniques as isolated species in the gas phase. The theoretical analysis, performed by B3LYP and CCSD(T) computational methods, predicted different CS2O+ isomers, SSCO+, O(CS2)+, SCSO+, SCOS+ and S(COS)+, and structurally related singlet and triplet CS2O. Experiment and theory agree in identifying the obtained CS2O+ ions as a mixture of SCSO+ and SCOS+ isomers. CS2O neutral species, prepared by neutralization-reionization mass spectrometry, were directly characterized as intact, long-lived species with a lifetime tau > or =2 micros.     

Definitive ab initio studies of model SN2 reactions CH(3)X+F- (X=F,Cl, CN,OH, SH,NH(2), PH(2))

The energetics of the stationary points of the gas-phase reactions CH(3)X+F(-)-->CH(3)F+X(-) (X=F, Cl, CN, OH, SH, NH(2) and PH(2)) have been definitively computed using focal point analyses. These analyses entailed extrapolation to the one-particle limit for the Hartree-Fock and MP2 energies using basis sets of up to aug-cc-pV5Z quality, inclusion of higher-order electron correlation [CCSD and CCSD(T)] with basis sets of aug-cc-pVTZ quality, and addition of auxiliary terms for core correlation and scalar relativistic effects. The final net activation barriers for the forward reactions are: E (b/F,F)=-0.8, E (b/F, Cl)=-12.2, E (b/F,OH)=+13.6, E b/F,OH=+16.1, E b/F,SH=+2.8, Eb/F, NH=+32.8, and E b/F,PH =+19.7 kcal x mol(-1). For the reverse reactions E b/F,F= -0.8, Eb/Cl,F =+18.3, E b/CN,F=+12.2, E b/OH,F =-1.8, E b/SH,F =+13.2, E b/NH(2),=-1.5, and E b/PH(2) =+9.6 kcal x mol(-1). The change in energetics between the CCSD(T)/aug-cc-pVTZ reference prediction and the final extrapolated focal point value is generally 0.5-1.0 kcal mol(-1). The inclusion of a tight d function in the basis sets for second-row atoms, that is, utilizing the aug-cc-pV(X+d)Z series, appears to change the relative energies by only 0.2 kcal x mol(-1). Additionally, several decomposition schemes have been utilized to partition the ion-molecule complexation energies, namely the Morokuma-Kitaura (MK), reduced variational space (RVS), and symmetry adapted perturbation theory (SAPT) techniques. The reactant complexes fall into two groups, mostly electrostatic complexes (FCH(3).F(-) and ClCH(3).F(-)), and those with substantial covalent character (NCCH(3).F(-), CH(3)OH.F(-), CH(3)SH.F(-), CH(3)NH(2).F(-) and CH(3)PH(2).F(-)). All of the product complexes are of the form FCH(3).X(-) and are primarily electrostatic.     

Pi-donation and stabilizing effects of pnicogens in carbenium and silicenium ions: a theoretical study of [C(XH2)3]+ and [Si(XH2)3]+ (X = N, P, As, Sb, Bi)

Quantum chemical calculations at the MP2 and CCSD(T) levels of theory are reported for cations of the general type [A(XH2)3]+ with A = C, Si and X = N, P, As, Sb, Bi. Population analysis, methyl stabilization energies (MSEs), and structural criteria were used to predict the p(pi)-donor ability of and the pi-stabilization energy exerted by this series of pnicogens. All of the substituents XH2 considered in these studies invariably stabilize the triply substituted carbenium as well as the silicenium ions. The calculated data show that the intrinsic p(pi)-donation of the group 15 atoms follows the order N < P < As < Sb < Bi. However, the trend of the stabilization energies is fully reversed. The intrinsic stabilization energies of the planar carbenium ions decrease monotonically from 161.2 kcal mol(-1) for X = NH2 to 98.0 kcal mol(-1) for X = BiH2. The effective stabilization of the pnicogens in the equilibrium structures, which also includes the energy-demanding pyramidalization of the XH2 substituents, follows the same trend, although the absolute numbers are reduced to 145.6 kcalmol(-1) for X = NH2 and 53.2 kcalmol(-1) for X = BiH2. This seemingly contrasting behavior of increasing p(pi) charge donation and decreasing stabilization has already been found for other substituents. Previous studies have shown that carbenium ions substituted by chalcogens up to the fourth row also stabilize C+ less effectively with respect to heavier substituents. Of the ions investigated in this study, only the silicenium ions that are stabilized by pnicogens from the third to the sixth row of the periodic system yield increased stabilizing energies that follow the corresponding intrinsic p(pi)-donor abilities of the respective substituent.     

Homolytic dissociation of the vulcanization accelerator tetramethylthiuram disulfide (TMTD) and structures and stabilities of the related radicals Me2NCSn* (n = 1-4)

The homolytic dissociation of the important vulcanization accelerator tetramethylthiuram disulfide (TMTD) has been studied by ab initio calculations according to the G3X(MP2) and G3X(MP2)-RAD theories. Homolytic cleavage of the SS bond requires a low enthalpy of 150.0 kJ mol-1, whereas 268.0 kJ mol-1 is needed for the dissociation of one of the C-S single bonds. To cleave one of the SS bonds of the corresponding trisulfide (TMTT) requires 191.1 kJ mol-1. Me2NCS2* is a particularly stable sulfur radical as reflected in the low S-H bond dissociation enthalpy of the corresponding acid Me2NC(=S)SH (301.7 kJ mol-1). Me2NCS2* (2B2) is a sigma radical characterized by the unpaired spin density shared equally between the two sulfur atoms and by a 4-center (NCS2) delocalized pi system. The ESR g-tensors of the radicals Me2NCSn* (n = 1-3) have been calculated. Both TMTD and the mentioned radicals form stable chelate complexes with a Li+ cation, which here serves as a model for the zinc ions used in accelerated rubber vulcanization. Although the binding energy of the complex [Li(TMTD)]+ is larger than that of the isomeric species [Li(S2CNMe2)2]+ (12), the dissociation enthalpy of TMTD as a ligand is smaller (125.5 kJ mol-1) than that of free TMTD. In other words, the homolytic dissociation of the SS bonds of TMTD is facilitated by the presence of Li+ ions. The sulfurization of TMTD in the presence of Li+ to give the paramagnetic complex [Li(S3CNMe2)2]+ is strongly exothermic. These results suggest that TMTD reacts with naked zinc ions as well as with the surface atoms of solid zinc oxide particles in an analogous manner producing highly reactive complexes, which probably initiate the crosslinking process during vulcanization reactions of natural or synthetic rubber accelerated by TMTD/ZnO.     

Model identity SN2 reactions CH3X + X- (X = F, Cl, CN, OH, SH, NH2, PH2): Marcus theory analyzed

The structures of seven gas phase identity S(N)2 reactions of the form CH(3)X + X(-) have been characterized with seven distinct theoretical methods: RHF, B3LYP, BLYP, BP86, MP2, CCSD, and CCSD(T), in conjunction with basis sets of double and triple zeta quality. Additionally, the energetics of said reactions have been definitively computed using focal point analyses utilizing extrapolation to the one-particle limit for the Hartree-Fock and MP2 energies using basis sets of up to aug-cc-pV5Z quality, inclusion of higher order correlation effects [CCSD and CCSD(T)] with basis sets of aug-cc-pVTZ quality, and additional auxiliary terms for core correlation and scalar relativistic effects. Final net activation barriers for the reactions are E(b)(F,F)= -0.8, E(b)(Cl,Cl)= 1.6, E(b)(CN,CN)= 28.7, E(b)(OH,OH)= 14.3, E(b)(SH,SH)= 13.8, E(b)(NH2,NH2)= 28.6, and E(b)(PH2,PH2)= 25.7 kcal mol(-1). General trends in the energetics, specifically the performance of the density functionals, and the component energies of the focal point analyses are discussed. The utility of classic Marcus theory as a technique for barrier predictions has been carefully analyzed. The standard Marcus theory results show disparities of up to 9 kcal mol(-1) with respect to explicitly computed results. However, when alternative approaches to Marcus theory, independent of the well-depths, are considered, excellent performance is achieved, with the largest deviations being under 3 kcal mol(-1).     

Reactivity of anomeric 1alpha- and 1beta-pentofuranosyl azide derivatives in ammonia chemical ionization

Electron impact (EI), fast atom bombardment (FAB) and ammonia chemical ionization [CI(NH3)] mass spectrometry were applied with the aim of differentiating between the anomeric 1alpha- and 1beta-azidopentofuranosyl derivatives. Calculated ammonium affinities [AA(M)] and proton affinities [PA(M)] show that beta-anomers have higher affinities for H+ and NH4+ ions than alpha-azides. Protonated molecules, obtained by CI(NH3) of azidofuranosyl derivatives, lose HN3 giving abundant furanosyl (S+) ions. Ammonia solvation of MH+ ions competes with the previous reaction producing the [SNHN2NH3]+ ion, a competitive product to the ammonium-attached [SN3NH4]+ ion. The fragmentation pathways of the stable and metastable [MNH4]+, MH+ ions, and several other important fragment ions, were determined using mass analyzed ion kinetic energy spectrometry (MIKES). The abundance of the [SN3NH4]+ and/or [SNHN2NH3]+ ions was found to correlate inversely with the exothermicity of ammonia solvation of the MH+ ion. The abundance of the fragment ions [SNHNH3]+, [SNH3]+ and SNH+ in some examples correlates with the exothermicity of the corresponding [MNH4]+ and MH+ parent ion formation. The fragment ions SNH3+ and SNHNH3+ can be formed, at least in part, in the ammonia solvation reaction of the S+ and SNH+ ions taking place within the high-pressure region of the CI ion source.     

Mechanisms of the reactions of W and W+ with NOx (x=1, 2): a computational study

The mechanisms of the reactions of W and W+ with NOx (x=1, 2) were studied at the CCSD(T)/[SDD+6-311G(d)]//B3LYP/[SDD+6-31G(d)] level of theory. It was shown that the insertion pathway of the reaction W(7S)+NO2(2A1) is a multistate process, which involves several lower lying electronic states of numerous intermediates and transition states, and leads to oxidation, WO(3Sigma)+NO(2Pi), and/or nitration, WN(4Sigma)+O2(3Sigmag-), of the W-center. Oxidation products WO(3Sigma)+NO(2Pi) lie 87.6 kcal/mol below the reactants, while the nitration channel is only 31.0 kcal/mol exothermic. Furthermore, it was shown that nitration of W with NO2 is kinetically less favorable than its oxidation. The addition-dissociation pathway of the reaction W(7S)+NO2(2A1) proceeds via the octet (ground) state potential energy surface of the reaction, requires 3.3 kcal/mol barrier, and leads exclusively to oxidation products. Calculations show that oxidation of the W+ cation by NO2 is a barrierless process in the gas phase, proceeds exclusively via the insertion pathway, and is exothermic by 82.9 kcal/mol. The nitration of W+ by NO2 is only 14.1 kcal/mol exothermic and could be accessible only under high-temperature conditions. Reactions of M=W/W+ with NO are also barrierless processes in the gas phase and lead to the N-O insertion product NMO, which are 105.4 and 77.4 kcal/mol lower than the reactants for W and W+, respectively.     

Gas-phase reactions of chromium and chromium fluoride cations CrFn+ (n = 0-4) with phosphane

The reactions of chromium and chromium fluoride monocations CrFn+ (n = 0-4) with phosphane are investigated by Fourier-transform ion cyclotron resonance mass spectrometry. Besides condensing slowly with phosphane, Cr+ is unreactive. The ionic products of the chromium fluoride cations are as follows: (i) CrF+ yields CrPH2+ and subsequently CrPH3+; (ii) from CrF2+, the ions PH3+, Cr+, and CrF2H+ are generated; and (iii) both CrF3+ and CrF4+ yield PH3+. The structure and formation of [Cr,P,H3]+ are investigated by collision-induced dissociation and isotopic labeling experiments. For the neutral species [P,H3,F2] formed by reaction of CrF2+ with phosphane, the structures are interrogated by quantum-mechanical calculations at the MP2/6-31++G** level of theory.     

Generation of neutrals from ionic precursors in the gas phase. The rearrangement of CCCCCHO to HCCCCCO

The neutrals HCCCCCO and CCCCCHO have been studied by experiment and by molecular modelling at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory. Neutral HCCCCCO has been made by one-electron reduction of [HCCCCCO]+ in the dual collision cell of a VG ZAB 2HF mass spectrometer. The isomer CCCCCHO is also formed in the dual collision cell, but this time by one-electron oxidation of the anion [CCCCCHO]-. Comparison of the CID and +NR+ mass spectra of [HCCCCCO]+ indicates that neutral HCCCCCO, when energised, retains its structural integrity. If the excess energy of HCCCCCO is > or = 170 kJ mol-1, decomposition can occur to give HCCCC and CO (calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory). The situation with the isomer CCCCCHO is different. Comparison of the -CR+ and -NR+ spectra of [CCCCCHO]- shows that both neutral and cationic forms of CCCCCHO partially rearrange to a species which decomposes by loss of CO. The peak corresponding to loss of CO is more pronounced in the -NR+ spectrum, indicating that the rearrangement is more prevalent for the neutral than the cation. Theoretical calculations suggest that the species losing CO could be CCCCHCO or HCCCCCO, but that HCCCCCO is the more likely. The lowest-energy rearrangement pathway occurs by successive H transfers, namely CCCCCHO-->CCCCHCO-->CCCHCCO-->HCCCCCO. The rearrangement of CCCCCHO to HCCCCCO requires CCCCCHO to have an excess energy of > or = 94 kJ mol-1. The species HCCCCCO formed by this exothermic sequence (214 kJ mol-1) has a maximum excess energy of 308 kJ mol-1: this is sufficient to effect decomposition to HCCCC and CO.     

Syntheses, solution multi-NMR characterization, and reactivities of [C6F5Xe]+ salts of weakly coordinating borate anions, [BY4]- (Y = CF3, C6F5, CN, or OTeF5)

New examples of [C6F5Xe]+ salts of the weakly coordinating anions [B(CF3)4]-, [B(C6F5)4]-, [B(CN)4]-, and [B(OTeF5)4]- have been synthesized by metathesis reactions of [C6F5Xe][BF4] with the corresponding MI[BY4] salts (MI = K or Cs; Y = CF3, C6F5, CN, or OTeF5). The salts were characterized in solution by multi-NMR spectroscopy. Their stabilities in prototypic solvents (CH3CN and CH2Cl2) and decomposition products are reported. The influence of the coordinating nature of [BY4]- on the decomposition rate of [C6F5Xe]+ as well as the presence of the weakly nucleophilic [BF4]- ion has been studied. The electrophilic pentafluorophenylation of C6H5F by [C6F5Xe][BY4] in solvents of different coordinating abilities (CH3CN and CH2Cl2) and the effects of stronger nucleophiles (fluoride and water) on the pentafluorophenylation process have been investigated. Simulations of the 19F and 129Xe NMR spectra of [C6F5Xe]+ have provided the complete set of aryl 19F-19F and 129Xe-19F coupling constants and their relative signs. The 19F NMR parameters of the [C6F5Xe]+ cation in the present series of salts are shown to reflect the relative degrees of cation-solvent interactions.