Ion-dipole Complex Research Articles

Tunneling in the Loss of Hydrogen Chloride from Isopropyl Chloride Cation

The unimolecular dissociation of isopropyl chloride cation has been investigated using mass-analyzed ion kinetic energy spectrometry. The C3H6*+ ion was the only product ion in the metastable dissociation. The kinetic energy release distribution for the HCl loss was determined. Ab initio molecular orbital calculations were performed at the MP2/6-311++G(d,p) level together with single point energy calculations at the QCSID(T)/6-311++G(2d,2p) level. The calculations show that the molecular ion rearranges to an ion-dipole complex prior to loss of HCl via a transition state containing a four-membered ring. The rearrangement involves H atom transfer. On the basis of the potential energy surface obtained for the loss of HCl and Cl*, the rate constants were calculated by transition-state statistical theories with considering tunneling effect. From the calculated result, it is proposed that the observed HCl loss would occur via tunneling through the barrier for isomerization to the ion-dipole complex, CH3CHCH2*+...HCl.

Gram-negative bacterial lipid A analysis by negative electrospray ion trap mass spectrometry: Stepwise dissociations of deprotonated species under low energy CID conditions

After mild acid hydrolysis of the lipopolysaccharide (LPS) from Escherichia coli J5, the recovered underivatized lipid A was analyzed using an electrospray source coupled with an ion trap mass analyzer (ESI-ITMS). Our work deals with the study of these complex endotoxic biomolecules under CID conditions. Sequential MS n experiments with the ion trap instrument allowed the characterization of the substituting side chain. Mono- and diphosphorylated singly or doubly deprotonated lipid A molecules have been studied and provided complementary information. The experimental results obtained, from these negatively charged species, under low energy collision conditions motivated us to propose stepwise fragmentation pathways involving intermediate ion–dipole complexes. In this way charge-driven processes are considered instead of the charge-remote mechanisms usually proposed in the literature.

Ligation of Be + and Mg + to NF 3: Structure, stability, and thermochemistry of the Be +–(NF 3) and Mg +–(NF 3) complexes

The structure, stability, and thermochemistry of the still experimentally unknown Be +–(NF 3) and Mg +–(NF 3) complexes have been theoretically investigated at the B3LYP, RCCSD, and RCCSD(T) levels of theory in conjunction with the 6-311G(d), 6-311 + G(2d), and 6-311 + G(3df) basis sets. Compared with the previously investigated H +–(NF 3), Li +–(NF 3), Na +–(NF 3), and K +–(NF 3), the study of M +–(NF 3) (M = Be or Mg) has revealed novel and somewhat unexpected features of the interaction of NF 3 with monoatomic ions. Thus, irrespective of the employed theoretical level, the ligation of Be + and Mg + to NF 3 leads to the formation of two distinct isomers, namely the M +–NF 3 nitrogen-coordinated complexes 1 (M = Be) and 5 (M = Mg) and the insertion products of F 2N–MF + connectivity 2 (M = Be) and 6 (M = Mg). On the other hand, at variance with M +–(NF 3) (M = Li, Na, and K), neither the fluorine-monocoordinated nor the fluorine-dicoordinated structure were located as minima on the potential energy surface. The geometry and bonding analysis of the M +–NF 3 isomers 1 and 5 and of the F 2N–MF + isomers 2 and 6 point to ion–dipole complexes between M + and NF 3 and MF + and NF 2, respectively. At the RCCSD(T)/6-311 + G(2d)//RCCSD/6-311G(d) level of theory and 298.15 K, the insertion products 2 and 6 are more stable than the nitrogen-coordinated isomers 1 and 5 by 109.2 and 59.2 kcal mol −1, respectively. In addition, assuming the formation of 2 and 6, the Be + and Mg + ion affinities of NF 3 result as high as 127.3 and 62.5 kcal mol −1, respectively. Our results suggest that the Be +–(NF 3) and Mg +–(NF 3) complexes could be actually observed as stable species in the gas phase, and support the proposal that techniques such as Mg + ion attachment mass spectrometry could be used to quantify the emissions of NF 3, one of the perfluorocompounds most extensively used in semiconductor technology.

The two-conformer hypothesis: 2,3,4,6-tetra- O-methyl-mannopyranosyl and -glucopyranosyl oxacarbenium ions

Computational chemistry can give information about the probable conformations of reactive intermediates that are difficult to determine experimentally. Based on density functional theory (DFT) calculations of tetra- O-methyl- d-mannopyranosyl and -glucopyranosyl oxacarbenium ions, two families of conformations, which we call B0 and B1, were found. For the manno configuration, a 4 H 3 and 3 E almost isoenergetic pair were found, whereas for the gluco-configuration a 4 H 3 and 5 S 1 pair favouring 4 H 3 were calculated. These results corroborate earlier results and suggest that this two or more conformer hypothesis is general. Nucleophilic attack on these pairs of cations was modelled with methanol and led to four cases to consider namely α- or β-attack on B0 or B1. The resulting complexes ( G0, G1 and F0, F1) demonstrate facial selectivity. The relative energies of these complexes are dominated by intramolecular hydrogen bonding and the conformational consequences to the pyranose ring of changes in the C-5–O-5–C-1–C-2 torsion angle. Constrained variation of the nucleophilic oxygen (methanol) to C-1 distance shows that these ion dipole complexes are the only minima with this constraint.

The Decarbonylation of the Acetamide Radical Cation and the Enolization of its Dimer by Self-Catalysis

The acetamide radical cation, CH3C(=O)NH2•+, and its enol, CH2=C(OH)NH2•+, undergo several unimolecular reactions in the μs time-frame of which decarbonylation is predominant. This reaction produces the ylid ion CH2NH3•+, rather than CH3NH3•+ [ J. Am. Chem. Soc. 109, 4819 (1987)]. A previously proposed mechanism via ion–dipole complexes is confirmed by the present CBS-QB3 calculations. These calculations reveal the existence of a second mechanism which proceeds via the enol ion and the distonic ion •CH2C(=O)NH3+. Both mechanisms can account for previously reported isotopic labeling experiments. Tandem mass spectrometry based experiments do not provide evidence that the non-decomposing acetamide ions rearrange to any significant extent to the more stable enol form. However, this transformation occurs smoothly by interaction with a neutral acetamide molecule (“self-catalysis”). By integration of experimental data (MS/MS/MS and labeling experiments) and ab initio calculations [CBS-Q (RHF/DZP)] three mechanisms for this assisted tautomerization have been traced. In the first mechanism the neutral acetamide component of the dimer ion accepts a C–H proton from its ionic partner and then donates the proton back to the oxygen atom. This is an example of “proton-transport catalysis”. In the second mechanism, isomerization takes place within the ionic partner via a conventional 1,3-H shift. The neutral partner serves only to lower the energy of the transition state by ion–dipole attractions. This is an example of the “Spectator” mechanism. In the third mechanism, proton transfer from the ionic partner to its neutral counterpart is followed by back-donation of a hydrogen atom. This is an example of the “Quid-pro-Quo” mechanism. The behavior of the acetamide dimer ion is compared to that of the acetone dimer ion which undergoes only proton-transport catalysis.

Computational studies of nucleophilic substitution at carbonyl carbon: the S(N)2 mechanism versus the tetrahedral intermediate in organic synthesis.

A theoretical study specifically addresses the question of whether nucleophilic addition to the carbonyl groups of acid chlorides, esters, and anhydrides involves an addition-elimination pathway or proceeds by a concerted S(N)2-like mechanism in the absence of the generally assumed tetrahedral intermediate. Density functional calculations [B3LYP/6-31+G(d,p)] establish that chloride ion exchange reactions with both formyl and acetyl chloride proceed by a pi attack on the C=O bond. No discernible tetrahedral intermediate typical of an addition-elimination pathway was found in either case. While a tetrahedral intermediate does exist for the addition of fluoride ion to (Cl)(2)C=O, halide exchange of LiCl with both ClFC=O and (Cl)(2)C=O also proceeds by a concerted S(N)2-like pathway. The formation of a tetrahedral intermediate from the addition of methanol to acetyl chloride is slightly exothermic (4.4 kcal/mol). The ion-dipole complex of methanol weakly bonded to the carbonyl carbon of protonated acetyl chloride is stabilized by 13.8 kcal/mol but does not collapse to a tetrahedral intermediate. When four CH(3)OH molecules are H-bonded to protonated acetyl chloride, a tetrahedral intermediate is not completely formed and this solvated complex more closely resembles the precursor to an S(N)1-type ionization of Cl(-). With six H-bonding methanol molecules, a methanol adds to the carbonyl carbon and a proton relay occurs with formation of a tetrahedral-like structure that immediately loses chloride ion in an S(N)1-like solvolysis. These results corroborate earlier suggestions (Bentley et al. J. Org. Chem. 1996, 61, 7927) that the methanolysis of acetyl chloride does not proceed through the generally assumed addition-elimination pathway with a discrete tetrahedral intermediate but is consistent with ionization of Cl(-). The reaction of methoxide ion with methyl acetate proceeds via a multiple-well energy surface and involves the intermediacy of an asymmetrical species with differing C-OMe bond lengths. Models of synthetic applications of acyl transfer reactions involving anhydrides that form N-acyloxazolidinones also proceed by a concerted S(N)2-type pathway even with the carboxylate leaving group. Concerted transition states were observed for the reactions of each enantiomer of a 1,3-diphenylcycloprop-2-ene carboxylic anhydride by S-3-lithio-4-phenyloxazolidinone. Despite close structural similarities between the diastereomeric transition states, the relative energies correlated closely with the experimental results.

An ab initio and DFT study of the fragmentation and isomerisation of MeP(O)(OMe)+

The fragmentation behaviour of the ion MeP(O)OMe+ has been investigated using quantum mechanical calculations at the B3LYP and MP2 levels to support experiments made with an Ion Trap Mass Spectrometer. Two mechanisms for the loss of CH2O are found, one involving a 1,3-H migration to phosphorus and the other a 1,2-methyl migration to give P(OMe)2+ followed by a 1,3-H migration. In each case an ion-dipole complex is formed that rapidly dissociates to yield CH2O. The relative importance of each route has been previously determined experimentally via isotopic labelling experiments, and the theoretical results are found to be consistent with these experimental results. The mechanisms suggested in the earlier work involving a 1,4 H migration to O are shown to be energetically unfavourable.

Gas-phase activation and reaction dynamics of chiral ion-dipole complexes.

A family of enantiomerically pure oxonium ions, that is O-protonated 1-aryl-1-methoxyethanes, has been generated in the gas phase by the (CH(3))(2)Cl(+) methylation of the corresponding 1-arylethanols. Some information on their reaction dynamics was obtained from a detailed kinetic study of their inversion of configuration and dissociation. The activation parameters of the inversion reaction are found to obey two different isokinetic relationships depending upon the nature and the position of the substituents in the oxonium ions. In contrast, the activation parameters of the dissociation reaction obey a single isokinetic relationship. The inversion and dissociation rate constants do not follow simple linear free-energy relationships. This complicated kinetic picture has been rationalized in terms of different activation dynamics in gaseous CH(3)Cl, which, in turn, determine the reaction dynamics of the oxonium ion. When the predominant activation of the oxonium ion involves resonant energy exchange from the 1015 cm(-1) CH(3) rocking mode of unperturbed CH(3)Cl, the inversion reaction proceeds through the dynamically most favored TS, characterized by the unassisted C(alpha)bond;O bond elongation. When, instead, the activation of the oxonium ions requires the formation of an intimate encounter complex with CH(3)Cl, the inversion reaction takes place via the energetically most favored TS, characterized by multiple coordination of the CH(3)OH moiety with the H(alpha) and H(ortho) atoms of the benzylic residue. The activation dynamics operating in the intimate encounter complex with CH(3)Cl is also responsible for the dissociation of most selected oxonium ions.

The remarkable decarbonylation of CH 3OPO [rad]+ and its distonic isomer CH 2OPOH [rad]+: an experimental and CBS-QB3 computational study

The dissociation chemistry of the title ions was investigated using tandem mass spectrometry-based experiments in conjunction with isotopic labeling and computational quantum chemistry. Upon collisional activation, ions CH 3OPO + ( 1a) and CH 2OPOH + ( 1b) readily lose CH 2O. This reaction is completely suppressed in the low-energy (metastable) ions, which instead abundantly lose CO. The product ion generated in this decarbonylation reaction is the ylide ion HPOH 2 +, rather than its more stable isomer H 2POH +. Remarkably, the oxygen atoms become equivalent in this reaction: ions CH 3 18 O P O + and CH 2 O P 18 OH + lose C 18 O and C 16 O in a 1:1 ratio. The experiments further show that the dissociating ions have a fairly large internal energy content: up to 40 kcal/mol for the distonic ion 1b, which, along with the “enol” ion CH 2P(OH)O +, represents the global minimum of the CH 3O 2P + potential energy surface. The mechanism of this intriguing decarbonylation was studied, using the CBS-QB3 model chemistry to probe the structure and energetics of potential intermediates and their interconversion barriers. It appears that metastable ions 1a and 1b have sufficient internal energy to communicate with a great many isomers. Of these, the cyclic ion P[OCH 2O(H)] + and ion [CH 2OH⋅⋅⋅OP] +, a hydrogen-bridged radical cation (HBRC), play a key role in the oxygen equilibration. The HBRC’s [HOP⋅⋅⋅HC(H)O] + and [HPO(H)H⋅⋅⋅CO] + are key intermediates in the actual mechanism for the decarbonylation. The latter ion may lose CO to produce HPOH 2 + or else isomerize into the ion–dipole complex [H 2O⋅⋅⋅P(H)CO] +, whose dissociation into H 2O+HPCO + accounts for the observed minor loss of water.

Tautomerization and dissociation of ethylene phosphonate ions [OCH 2CH 2O]P(H)O [rad]+: an experimental and CBS-QB3 computational study

The unimolecular gas-phase chemistry of the cyclic title ion, [OCH 2CH 2O]P(H)O +, 1a +, and its tautomer ethylene phosphite, [OCH 2CH 2O]POH +, 1b +, was investigated using mass spectrometry-based experiments in conjunction with isotopic labelling and computational quantum chemistry, at the CBS-QB3 level of theory. A facile tautomerization of the “keto” ion 1a + into its more stable (by 34 kcal/mol) “enol” isomer 1b + is prevented by a substantial 1,2-H shift barrier (14 kcal/mol relative to 1a +). In line with this, the collision-induced dissociation (CID) and neutralization-reionization (NR) spectra of the two isomers are characteristically different. Unlike the corresponding acyclic dimethyl phosphonate/phosphite tautomers, (CH 3O) 2P(H)O +/(CH 3O) 2POH +, where the phosphonate isomer rapidly loses its structure identity by a facile distonicization into CH 2O(CH 3O)P(H)OH +, the barrier for this reaction in 1a + is prohibitively high and the cyclic distonic 1,2-H shift isomer [OCH 2CH 2O(H)]PO +, 1c +, is not directly accessible. The 1,2-H shift barrier separating 1a + and 1b + is calculated to lie close to the thermochemical threshold for the formation of C 2H 4 ++HOP(O) 2. This reaction dominates the closely similar metastable ion (MI) spectra of these tautomers. At these elevated energies, the “enol” ion 1b + can undergo ring-opening by CH 2O or CH 2CH 2 cleavage, yielding ion–dipole complexes of the type [C 2H 4] +/HOP(O) 2, 1e +, and H-bridged radical cations CH 2 O ⋯[ H O P OCH 2] + , 1f +, respectively. Moreover, communication of 1b + with the distonic ion 1c + now also becomes feasible. These computational findings account for the similarity of the MI spectra and provide a rationale for the observation that in the losses of CO, HCO and C 2H 3O from metastable ions [ OCH 2 CH 2 O] P( H) 18 O + and [ OCH 2 CH 2 O] P 18 OH + , the 18 O -atom loses its positional identity. Theory and experiment yield a consistent potential energy profile for the cyclic phosphonate/phosphite system showing that non-dissociating ions 1a + retain their structure identity in the microsecond time-frame. However, the interaction of 1a with a benzonitrile (BN) molecule in a chemical ionization type experiment readily yields the more stable “enol” type ion 1b +. Experiments with benzonitrile-d 5 support the proposal that this interaction does not involve the lowering of the 1,2-H shift barrier between the tautomers, via a proton-transport catalysis type mechanism. Rather, a “quid-pro-quo” mechanism is operative, analogous to that proposed for the benzonitrile-assisted enolization of acetamide [Int. J. Mass Spectrom. 210/211 (2001) 489].

Investigation of the regio- and stereo-selectivity of deoxyguanosine linkage to deuterated 2-hydroxyestradiol by using liquid chromatography/ESI-ion trap mass spectrometry

From previous studies on the reactivity of estradiol 2,3-quinone towards deoxyribonucleosides, it was demonstrated that several isomeric adducts were formed. Although adduction on steroid ring A or B has been evidenced using sequential MS n experiments, in some cases attachment positions are difficult to identify unambiguously. In this work, 2-hydroxyestradiol labeled with deuterium at various positions [6β ( 1); 6α-7α ( 2); 6α-6β-7α ( 3)] have been used. Isomeric adduct differentiation could be achieved using LC-ESI-MS n. The m/ z shift of the quasi-molecular ions as well as the fragmentation pathways suggested that adduction could occur on both C6 and C9 sites of the steroid B ring: Nucleophilic attack of the base on the C6 position of the steroid led to major adducts and addition of the base on the activated C9 site gave minor adducts that were found to be unstable. LC-MS n experiments carried out under deuterated medium provided information about some fragmentation processes by studying the m/ z shift of fragment ions: (1) the loss of deoxyribose from the quasi-molecular ions took place according to a process involving a deuterium transfer from the deoxyribose alcohol function; (2) the cleavage of the steroid-base linkage involved a deuterium transfer from the hydroxy group of the catechol and likely occurred via the formation of an ion-dipole complex. The model studies conducted in this work provide new information on the fragmentation mechanisms of covalent adducts formed from estrogen quinones and deoxyguanosine, the most reactive DNA base. Besides, the first unequivocal characterization of adducts involving the steroid C9 position is shown by using deuterium labeled estrogen quinones.

Protonation of gas-phase aromatic molecules: IR spectrum of the fluoronium isomer of protonated fluorobenzene.

The IR spectrum of the fluoronium isomer of protonated fluorobenzene (F-C(6)H(6)F(+), phenylfluoronium) is recorded in the vicinity of the C-H and F-H stretch fundamentals to obtain the first structured spectrum of an isolated protonated aromatic molecule in the gas phase. Stable F-C(6)H(6)F(+) ions are produced via proton transfer from CH(5)(+) to fluorobenzene (C(6)H(5)F) in a supersonic plasma expansion. The F-C(6)H(6)F(+) spectrum recorded between 2,540 and 4,050 cm(-1) is consistent with a weakly bound ion-dipole complex composed of HF and the phenyl cation, HF-C(6)H(5)(+). The strongest transition occurs at 3,645 cm(-1) and is assigned to the F-H stretch (sigma(FH)). The antisymmetric C-H stretch of the two ortho hydrogen atoms, sigma(CH) = 3,125 cm(-1), is nearly unshifted from bare C(6)H(5)(+), indicating that HF complexation has little influence on the C-H bond strength of C(6)H(5)(+). Despite the simultaneous production of the more stable ring protonated carbenium isomers of C(6)H(6)F(+) (fluorobenzenium) in the electron ionization source, F-C(6)H(6)F(+) can selectively be photodissociated into C(6)H(5)(+) and HF under the present experimental conditions, because it has a much lower dissociation energy than all carbenium isomers. Quantum chemical calculations at the B3LYP and MP2 levels of theory using the 6-311G(2df,2pd) basis support the interpretation of the experimental data and provide further details on structural, energetic, and vibrational properties of F-C(6)H(6)F(+), the carbenium isomers of C(6)H(6)F(+), and other weakly bound HF-C(6)H(5)(+) ion-dipole complexes. The dissociation energy of F-C(6)H(6)F(+) with respect to dehydrofluorination is calculated as D(0) = 4521 cm(-1) (approximately 54 kJ/mol). Analysis of the charge distribution in F-C(6)H(6)F(+) supports the notation of a HF-C(6)H(5)(+) ion-dipole complex, with nearly the whole positive charge of the added proton distributed over the C(6)H(5)(+) ring. As a result, protonation at the F atom strongly destabilizes the C-F bond in C(6)H(5)F.

Gas-phase behaviour of negative ions produced from thiazidic diuretics under electrospray conditions.

A systematic mass spectrometric study of 10 thiazidic diuretics and related compounds was undertaken by mass spectrometry (MS) with electrospray ionization in the negative ion mode. Collisional dissociation 'in-source' (CID-MS) and in a low-pressure collision cell (CID-MS/MS) were compared in both excitation regions. Spectra obtained by CID-MS and by CID-MS/MS were matched. Using the two methods, loss of HCl and consecutive dissociations from 2HCl losses were exhibited from compounds such as methyclothiazide and trichlormethiazide but not from other thiazidic diuretics that contain chlorine substituents in the aromatic moiety. However, deprotonated dichlorphenamide gave rise to loss of HCl by CID-MS and CID-MS/MS. For other diuretics such as hydroflumethiazide and hydrochlorothiazide, the loss of HCN and [HCN + SO(2)] was relevant. Reaction mechanisms were checked by means of deuterium-hydrogen exchange, which showed that deprotonation took place regioselectively on the heterocyclic moiety. The cleavage pathways require molecular isomerization forming ion-dipole complexes prior to decompositions, allowing long-distance proton transfer for neutral elimination. Identifications of the most specific fragmentations presented in this paper were applied to the screening and unambiguous identification of diuretics for horse doping control.

Ab Initio GB Study of Methylation Reaction of Adenine, Cytosine, Guanine, and Thymine by Methanediazonium Ion

Methylation reaction at ten nucleophilic sites in four DNA base molecules by methanediazonium ion (N2Me+) was examined by ab initio MO/GB calculation which includes the solvent effect with the continuum model using the generalized Born formula. The stabilization energy of the ion–dipole complex as well as the energy of the transition state are roughly consistent with the experimental fraction of methylation by N -methyl-N -nitrosourea. For the guanine N7 site, which is the principal site of methylation, the stabilization energy is the largest and the energy of the transition state is low. The reactions at the guanine O6 and N7 sites were analysed by drawing the potential energy surface with respect to two parameters, the O6–C(Me) distance and the N7–C(Me) distance. The methylation reactions of the guanine O6 and N7 sites begin from the common geometry, the global minimum in the two-dimensional potential energy surface.

Theoretical study of ammonia and methane activation by first-row transition metal cations M(+) (M = Ti, V, Cr).

The potential energy surfaces for the reaction of first-row transition metal cations Ti(+)((4)F,(2)F), V(+)((5)D,(3)F), and Cr(+)((6)S,(4)D) with NH(3) and CH(4) have been built up by using density functional theory. In all cases, the high-spin ion-dipole complex, which is the most stable species on the respective potential energy hypersurfaces, is initially formed. In the second step, a hydrogen shift process leads to the formation of the insertion products, which are more stable in a low-spin state. From these intermediates three dissociation channels have been considered. All the results have been compared with existing experimental and theoretical data and our earlier work on the reactivity of Sc(+), to clarify similarities and differences in the behavior of the transition metal ions considered.

Complexes of lithium cation with nitrogen trifluoride: a computational investigation on the structure and stability of Li +–(NF 3) isomers

The structure and stability of the still experimentally unknown Li +–(NF 3) ions have been theoretically investigated at the B3LYP, QCISD, QCISD(T), and CCSD(T) levels of theory in conjunction with the 6-311G(d), 6-311+G(2d), and 6-311+G(3df) basis sets. Irrespective of the employed theoretical level, it was found that the ligation of Li + to the F atoms of NF 3 may occur in two distinct ways, leading to the formation of the monocoordinated isomer 2 and the dicoordinated isomer 3. These two isomeric ions are practically degenerate and more stable than the nitrogen-coordinated isomer 1 by ca. 6 kcal mol −1. In addition, the optimized geometries of the Li +–(NF 3) isomers 1, 2, and 3 and the analysis of their chemical bonding indicate the formation of ion–dipole complexes between Li + and NF 3. The Li + ion affinity of NF 3 at 298.15 K is computed as 15.6 kcal mol −1 at the B3LYP/6-311+G(3df) level of theory, 13.3 kcal mol −1 at the QCISD(T)/6-311+G(2d) level of theory, and 12.8 kcal mol −1 at the CCSD(T)/6-311+G(2d) level of theory. These values are large enough to suggest the possibility that lithiated NF 3 could be actually observed as a stable species in the gas phase. From the applied point of view, this finding supports the proposal that Li + ion attachment mass spectrometry, recently proposed by Fujii [J. Phys. Chem. A 104 (2000) 9613] as a conceivable technique to quantify the emissions of the greenhouse gases CF 4, C 2F 6, C 4F 8, and SF 6 from the electronic industry, could be also employed to quantify the emissions of NF 3, one of the perfluorocompounds most extensively used in semiconductor technology.

Reactions of O −· with methyl benzoate: a negative ion chemical ionization and Fourier transform ion cyclotron resonance study

The reactions of O −· with methyl benzoate have been examined by the measurement of negative ion chemical ionization (NICI) mass spectra using a CI source, with confirmatory studies carried out on a Fourier transform ion cyclotron resonance mass spectrometer. Reaction mechanisms have been elucidated using isotopically labeled esters. Nucleophilic attack at the carbonyl carbon and the aromatic ring were important reaction pathways. Nucleophilic attack at the carbonyl carbon was followed by the production of products (C 6H 5CO 2 − and CH 3OCO 2 −) characteristic of radical, β-fragmentation. Using 18O-labeled methyl benzoate, the S N2 reaction was found to account for a smaller percentage, 21(±1)%, of the benzoate product. Aromatic ring attack resulted in formation of [M + O − H] − and [M − 2H] −· ions. Although aryl hydrogens accounted for most H 2 +· abstracted by O −·, evidence for abstraction of H arylH alkyl +· and H alkylH alkyl +· was also found. Although present at much lower abundance, dehydrobenzoate, dehydrophenoxy, and C 7H 6 −· ([M − 2H − CO 2] −·) radical anions were also observed. An H aryl/H alkyl exchange associated with formation of the benzoate anion was attributed to an H alkyl abstraction that occurred within the methanol/dehydrobenzoate ion–dipole complex. The [M − 2H] −·, dehydrobenzoate, dehydrophenoxy, and [M − 2H − CO 2] −· ion signals were quenched by reaction with O 2. Conditions required for production of O −· spectra under NICI conditions were also examined.

Theoretical study of the water activation by a cobalt cation: Ab initio multireference theory versus density functional theory

The reaction mechanism of Co+ (5F,3F) with H2O has been studied by the ab initio multireference-based theory (MR–SDCI and MC–QDPT) and the density functional theory (B3LYP and BLYP). In the energetics derived by the MR–SDCI(+Q) plus the B3LYP zero-point vibrational energy, the ion–dipole complex, CoOH2+, is initially formed with the binding energy of 38.2 (triplet) and 34.1 (quintet) kcal/mol, which is the most stable complex in the respective potential energy surfaces. Then, Co+ activates one O–H bond of H2O, leading to the insertion complex, HCoOH+. There are three possible dissociation channels from HCoOH+, i.e., →CoOH++H, →CoH++OH, and →CoO++H2. The third dissociation is expected to occur through the transition state of a four-centered structure, with the activation barrier of 61.6 (triplet) and 49.2 (quintet) kcal/mol, although this dissociation has not been detected in the experiment. The ground state of CoO+ is predicted to be Δ,5 and the lowest triplet state is Γ3 with the energy level of 20.8 kcal/mol above. The B3LYP provides the energetics qualitatively similar to the MR–SDCI(+Q) ones through the reactions, with the maximum deviation of 13 kcal/mol. The calculated results are consistent with experimental observations.

Comparison of gas and solution phase intrinsic rate constants for electron transfer of tetraalkylhydrazines†

Rate constants for 40 gas-phase electron transfer reactions at 550 K between 22 tetraalkylhydrazines are fitted using Marcus cross rate theory, and compared with previous studies in acetonitrile solution. The relative reactivities are surprisingly similar in the gas phase and in solution. Strong cation–neutral association in the gas phase appears to lessen striking electronic effects previously reported for similar reactions in solution. The relative reactivities of 1,2-dimethylhexahydropyridazine ([6]Me2) and 1,2-dimethyl-1,2,3,6-tetrahydropyridazine ([u6]Me2) switch between solution and the gas phase. It is suggested that this may occur because gas-phase ion–dipole complexes are much more tightly bound than are encounter complexes in solution, allowing a conformational change to the more reactive conformation of [u6]Me2 in the gas phase, but not in solution.

Location of the Negative Charge(s) on the Backbone of Single-Stranded Deoxyribonucleic Acid in the Gas Phase

Multiply-deprotonated, single-strand products of mono and heteronucleotides, prepared in a nanospray external source coupled to an ion trap mass spectrometer, were studied under low-energy collision conditions. Fragmentations enabled the negative charge carried by the selected anions to be located. Comparison of multi-deprotonated 5′-dephosphorylated and 5′-phosphorylated nucleotides showed that the former led to competitive BH and B− losses via isomerization into ion–dipole complexes prior to dissociation, which contrasts with the 5′-phosphorylated nucleoside yielding the competitive HPO3 and nucleobase eliminations. The proposed mechanisms are discussed according to: (i) the charge state of the precursor studied and (ii) the thermochemical gas-phase properties such as the relative acidities of neutrals, which formally constitute the ion–dipole. The formation of various fragment ions from doubly- and triply-deprotonated precursor ions enabled the negative charge to be located, specifically at the terminal phosphodiester groups of the single-strand oligonucleotides. The potential energy is, therefore, minimal for distant charged phosphorylated groups. These findings suggest that a specific conformation is favored in order to minimize coulomb repulsion during the desolvation process. This hypothesis is consistent with the assumed behavior of triply- and quadruply-deprotonated, mono-cationized oligonucleotides, which involves a specific location of the sodium cation, as previously shown.