Gas phase analogues of solvolysis reactions

Can a gas phase contact ion pair containing a hydrocarbon carbocation be formed in the ground state?

So far, no conclusive evidence of a ground-state contact ion-pair containing a hydrocarbon carbocation has been given in the gas phase.

Peter J Derrick and the Grand Scale 'Magnificent Mass Machine' mass spectrometer at Warwick

The value of the Grand Scale 'Magnificent Mass Machine' mass spectrometer in investigating the reactivity of ions in the gas phase is illustrated by a brief analysis of previously unpublished work on metastable ionised n-pentyl methyl ether, which loses predominantly methanol and an ethyl radical, with very minor contributions for elimination of ethane and water. Expulsion of an ethyl radical is interpreted in terms of isomerisation to ionised 3-pentyl methyl ether, via distonic ions and, possibly, an ion-neutral complex comprising ionised ethylcyclopropane and methanol. This explanation is consistent with the closely similar behaviour of the labelled analogues, C₃H₇CH₂CD₂OCH₃^+. and C₃H₇CD₂CH₂OCH₃^+., and is supported by the greater kinetic energy release associated with loss of ethane from ionised n-propyl methyl ether compared to that starting from directly generated ionised 3-pentyl methyl ether.

How does a CC double bond cleave in the gas phase? Fragmentation of protonated ketotifen in mass spectrometry

In the literature, it is reported that the protonated ketotifen mainly undergoes CC double bond cleavage in electrospray ionization tandem mass spectrometry (ESI-MS/MS); however, there is no explanation on the mechanism of this fragmentation reaction. Therefore, we carried out a combined experimental and theoretical study on this interesting fragmentation reaction. The fragmentation of protonated ketotifen (m/z 310) always generated a dominant fragment ion at m/z 96 in different electrospray ionization mass spectrometers (ion trap, triple quadrupole and linear trap quadrupole (LTQ)-orbitrap). The mechanism of the generation of this product ion (m/z 96) through the CC double bond cleavage was proposed to be a sequential hydrogen migration process (including proton transfer, continuous two-step 1,2-hydride transfer and ion-neutral complex-mediated hydride transfer). This mechanism was supported by density functional theory (DFT) calculations and a deuterium labeling experiment. DFT calculations also showed that the formation of the product ion m/z 96 was most favorable in terms of energy. This study provides a reasonable explanation for the fragmentation of protonated ketotifen in ESI-MS/MS, and the fragmentation mechanism is suitable to explain other CC double bond cleavage reactions in mass spectrometry. Copyright © 2016 John Wiley & Sons, Ltd.

Ortho-hydroxyl effect and proton transfer via ion-neutral complex: the fragmentation study of protonated imine resveratrol analogues in mass spectrometry

The fragmentation pathways of protonated imine resveratrol analogues in the gas-phase were investigated by electrospray ionization-tandem mass spectrometry. Benzyl cations were formed in the imine resveratrol analogues that had an ortho-hydroxyl group on the benzene ring A. The specific elimination of the quinomethane neutral, CH2  = C6 H4  = O, from the two isomeric ions [M1 + H](+) and [M3 + H](+) via the corresponding ion-neutral complexes was observed. The fragmentation pathway for the related meta-isomer, ion [M2 + H](+) and the other congeners was not observed. Accurate mass measurements and additional experiments carried out with a chlorinated analogue and the trideuterated isotopolog of M1 supported the overall interpretation of the fragmentation phenomena observed. It is very helpful for understanding the intriguing roles of ortho-hydroxyl effect and ion-neutral complexes in fragmentation reactions and enriching the knowledge of the gas-phase chemistry of the benzyl cation. Copyright © 2016 John Wiley & Sons, Ltd.

The mechanism of alkene elimination from protonated toluenesulphonamides generated by electrospray ionisation

The positive ion electrospray mass spectra of a range of sulphonamides of general structure CH₃C₆H₄SO₂NHR₁ [R₁ = C_nH_2n⁺¹ (n = 1-7), C_nH_2n-1 (n = 3, 4), C₆H₅, C₆H₅CH₂ and C₆H₅CH(CH₃)] and CH₃C₆H₄SO₂NR₁R² [R¹, R² = C_nH_2n+1 (_n = 1-8)] are reported and discussed. The protonated sulphonamides derived from saturated primary and secondary aliphatic amines generally fragment to only a limited extent unless energised by collision. Two general fragmentations are observed: firstly, elimination of an alkene, C_nH_2n, obtained by hydrogen abstraction from one of the C_nH_2n+1 alkyl groups on nitrogen; secondly, cleavage to form CH₃C₆H₄SO₂₊. The mechanism by which an alkene is lost has been probed by studying the variation of the intensity of the [M + H - C_nH_2n]⁺ signal with the structure of the alkyl substituent(s) on nitrogen and by monitoring the competition between the loss of different alkenes from protonated unsymmetrical sulphonamides in which two different alkyl groups are attached to nitrogen. This fragmentation is favoured by branching of the alkyl group at the carbon atom directly attached to nitrogen, thus suggesting that it involves a mechanism in which the stability of the cation obtained by stretching the bond connecting the nitrogen atom to the alkyl group is critical. This interpretation also explains the competition between alkene elimination and cleavage to form CH₃C₆H₄SO₂⁺ (and, in some cases, cleavage to form C₆H₅CH₂⁺ or [C₆H₅CHCH₃]⁺).

Application of positive mode atmospheric chemical ionisation to distinguish epimeric oleanolic and ursolic acids

A new and more reliable method is reported for distinguishing the equatorial and axial epimers of oleanolic and ursolic acids and related triterpenoids based primarily on the relative abundance of the [M+H](+) and [M+-H(2)O](+) signals in their positive mode atmospheric pressure chemical ionisation mass spectra. The rate of elimination of water, which is the principal primary fragmentation of protonated oleanolic and ursolic acids, depends systematically on the stereochemistry of the hydroxyl group in the 3 position. For the b-epimer, in which the 3-hydroxyl substituent is in an equatorial position,[M+-H(2)O](+) is the base peak. In contrast, for the α-epimer, where the 3-hydroxyl group is axial, [M + H](+) is the base peak. This trend, which is general for a range of derivatives of oleanolic and ursolic acids, including the corresponding methyl esters, allows epimeric triterpenoids in these series to be securely differentiated. Confirmatory information is available from the collision-induced dissociation of the [M+-H(2)O](+) primary fragment ions, which follow different pathways for the species derived from axial and equatorial epimers of oleanolic and ursolic acids. These two pieces of independent spectral information permit the stereochemistry of epimeric oleanolic and ursolic acids (and selected derivatives) to be assigned with confidence without relying either on chromatographic retention times or referring to the spectra or other properties of authentic samples of these triterpenoids.

Novel formation of [2M-H](+) species in positive electrospray mass spectra of indoles

RATIONALE

When subjected to positive ion electrospray ionisation (ESI+) mass spectrometry (MS), indoles with a 3-alkyl substituent show a propensity to form novel [2M-H](+) 'covalently bound dimers'. This process, which appears to be initiated in the nebuliser of the instrument, is mechanistically interesting, analytically useful and potentially significant in organic synthesis.

METHODS

A selection of 2- and 3-substituted indoles have been synthesised and analysed by ESI-MS. The formation of the 'homo' and 'hetero' dimers of these compounds has been investigated using ESI+ mode. The mechanism of formation of the observed 'dimeric' species has been probed by synthesising authentic samples of the dimeric compounds.

RESULTS

'Dimeric' species corresponding to [2M-H](+) have been observed for all 3-substituted indoles studied, but not for indoles substituted in just the 2-position. By infusing equimolar mixtures of labelled and unlabelled indoles through the instrument, the expected approximately statistical mixture of homo- and heterodimeric species has been observed. Further experiments have established that this novel dimerisation occurs in the droplets formed in the nebuliser of the instrument.

CONCLUSIONS

It has been shown that 3-substituted indoles form [2M-H](+) dimers in high abundance in the spray obtained from the nebiliser of an ESI+ instrument. The mechanism for the dimerisation does not involve the known 2M dimeric species that is readily formed in the solution-phase chemistry of indoles.

Fragmentation reactions of N-benzyltetrahydroquinolines in electrospray ionization mass spectrometry: the roles of ion/neutral complex intermediates

RATIONALE

Electrospray ionization mass spectrometry (ESI-MS) combined with the collision-induced dissociation (CID) technique has assumed increasing importance as an invaluable tool for the structural analysis of organic and biological molecules. However, general rules for elucidating the fragmentation behaviors of charged molecules in the gas phase are still lacking. Therefore, explorations on the mechanistic information are desirable at all times.

METHODS

CID experiments of protonated N-benzyltetrahydroquinolines were carried out on ESI ion trap mass spectrometer and accurate mass measurements were performed on a high-resolution ESI quadrupole time-of-flight (Q-TOF) mass spectrometer in positive ion mode.

RESULTS

An ion/neutral complex, [RC6H4CH2(+)/tetrahydroquinoline], resulting from cleavage of the C-N bond induced by the positive charge brought in by protonation, was proposed to be the intermediate to elucidate the fragmentation reactions. For all the compounds investigated, benzyl cation transfer, electron transfer and hydride transfer reactions mediated by the complex were observed. Moreover, for the compound substituted by a methyl group at the para-position of the benzylic phenyl ring, proton transfer reaction via the complex also occurs.

CONCLUSIONS

This study is a case for better understanding the intriguing roles of ion/neutral complexes in gas-phase fragmentation reactions and enriching the knowledge about the gas-phase chemistry of the benzyl cation. In addition, it provides useful information for researchers working on analysis or structural elucidation of complicated compounds which contain the N-benzyltetrahydroquinoline substructure.

Gas phase retro-Michael reaction resulting from dissociative protonation: fragmentation of protonated warfarin in mass spectrometry

A mass spectrometric study of protonated warfarin and its derivatives (compounds 1 to 5) has been performed. Losses of a substituted benzylideneacetone and a 4-hydroxycoumarin have been observed as a result of retro-Michael reaction. The added proton is initially localized between the two carbonyl oxygens through hydrogen bonding in the most thermodynamically favorable tautomer. Upon collisional activation, the added proton migrates to the C-3 of 4-hydroxycoumarin, which is called the dissociative protonation site, leading to the formation of the intermediate ion-neutral complex (INC). Within the INC, further proton transfer gives rise to a proton-bound complex. The cleavage of one hydrogen bond of the proton-bound complex produces the protonated 4-hydroxycoumarin, while the separation of the other hydrogen bond gives rise to the protonated benzylideneacetone. Theoretical calculations indicate that the 1, 5-proton transfer pathway is most thermodynamically favorable and support the existence of the INC. Both substituent effect and the kinetic method were utilized for explaining the relative abundances of protonated 4-hydroxycoumarin and protonated benzylideneacetone derivative. For monosubstituted warfarins, the electron-donating substituents favor the generation of protonated substituted benzylideneacetone, whereas the electron-withdrawing groups favor the formation of protonated 4-hydroxycoumarin.

Gas-phase chemistry of benzyl cations in dissociation of N-benzylammonium and N-benzyliminium ions studied by mass spectrometry

In this study, the fragmentation reactions of various N-benzylammonium and N-benzyliminium ions were investigated by electrospray ionization mass spectrometry. In general, the dissociation of N-benzylated cations generates benzyl cations easily. Formation of ion/neutral complex intermediates consisting of the benzyl cations and the neutral fragments was observed. The intra-complex reactions included electrophilic aromatic substitution, hydride transfer, electron transfer, proton transfer, and nucleophilic aromatic substitution. These five types of reactions almost covered all the potential reactivities of benzyl cations in chemical reactions. Benzyl cations are well-known as Lewis acid and electrophile in reactions, but the present study showed that the gas-phase reactivities of some suitably ring-substituted benzyl cations were far richer. The 4-methylbenzyl cation was found to react as a Brønsted acid, benzyl cations bearing a strong electron-withdrawing group were found to react as electron acceptors, and para-halogen-substituted benzyl cations could react as substrates for nucleophilic attack at the phenyl ring. The reactions of benzyl cations were also related to the neutral counterparts. For example, in electron transfer reaction, the neutral counterpart should have low ionization energy and in nucleophilic aromatic substitution reaction, the neutral counterpart should be piperazine or analogues. This study provided a panoramic view of the reactions of benzyl cations with neutral N-containing species in the gas phase.

An unusual intramolecular transfer of the fluorobenzyl cation between two remote amidic nitrogen atoms induced by collision in the gas phase

A highly unusual rearrangement in collision-induced dissociation mass spectrometry is reported that involves intramolecular transfer of the fluorobenzyl cation between two remote amidic nitrogen atoms separated by five chemical bonds. The same intramolecular transfer was also observed for two related analogs. It is postulated that the ionic reactions are initiated by protonation of the first amidic nitrogen, resulting in formation of the fluorobenzyl cation and a neutral partner that are maintained together in the gas phase by electrostatic interactions as an intermediate ion-neutral complex. In the ion-neutral complex, the nascent fluorobenzyl cation approaches geometrically to the second amidic nitrogen atom on the neutral partner, and subsequently forms a new C-N bond and an isomeric precursor ion as the charge is retained on the amidic nitrogen. The newly formed isomeric precursor ion eventually undergoes the final fragmentation by amide bond cleavage. Alternatively, the ionic reactions proceed through a direct intramolecular transfer mechanism by which the molecular ion adopts to a ring-like configuration in the gas phase, so that both the donor and recipient nitrogens are geometrically close to each other within a bonding distance to permit a direct transfer between two sites even though they are separated by multiple chemical bonds.

Concomitant hydride and proton transfer: an essay on competing and consecutive key reactions occurring in gaseousion/neutral complexes

The interplay of proton transfer and hydride transfer reactions in alkylbenzenium ions and related protonated di- and oligophenylalkanes is presented and discussed. While intra- and interannular proton exchange has been recognised to be an ubiquitous feature in protonated arenes, hydride abstraction is much less obvious but can become a dominating fragmentation channel in metastable ions of tert-butyl-substituted alkylbenzenium ions and related carbocations. In such cases, proton-induced release of the tert-butyl cation gives rise to ion/neutral complexes as reactive intermediates, for example, [(CH(3))(3)C(+)...arylCH(2)(α)(CH(2))(n)CH(2)(ω)aryl '] with n ≥ 0, and highly regioselective intra-complex hydride transfer occurs from all of the benzylic methylene hydride ion donor groups (α-CH(2) and ω-CH(2)) to the tert-butyl cation acting as a Lewis acid. Substituent effects on the individual contributions to the overall hydride transfer from different donor sites, including ortho-methyl groups, in particular, and the concomitant intra- complex proton transfer from the tert-butyl cation to the neutral diarylalkane constituent corroborate the view of "bisolvated" complexes as the central intermediates, in which the carbenium ion is coordinated to both of the aromatic π-electron systems. The role of cyclisation processes converting the benzylic, [M - H](+) type, ions into the isomeric benzenium, [M + H](+)-type, ions prior to fragmentation is demonstrated for several cases. This overall scenario, consisting of consecutive and/or competing intra-complex hydride abstraction and proton transfer, intraannular proton shifts (H+ ring walk) and interannular proton transfer, hydrogen exchange ("scrambling") processes, and cyclisation and other electrophilic substitution reactions, is of general importance in this field of gas-phase ion chemistry, and more recent examples concerning protonated ethers, benzylpyridinium and benzylammmonium ions are discussed in which these recurring features play central and concerted mechanistic roles as well.

Hydride transfer reactions via ion-neutral complex: fragmentation of protonated N-benzylpiperidines and protonated N-benzylpiperazines in mass spectrometry

An ion-neutral complex (INC)-mediated hydride transfer reaction was observed in the fragmentation of protonated N-benzylpiperidines and protonated N-benzylpiperazines in electrospray ionization mass spectrometry. Upon protonation at the nitrogen atom, these compounds initially dissociated to an INC consisting of [RC(6)H(4)CH(2)](+) (R = substituent) and piperidine or piperazine. Although this INC was unstable, it did exist and was supported by both experiments and density functional theory (DFT) calculations. In the subsequent fragmentation, hydride transfer from the neutral partner to the cation species competed with the direct separation. The distribution of the two corresponding product ions was found to depend on the stabilization energy of this INC, and it was also approved by the study of substituent effects. For monosubstituted N-benzylpiperidines, strong electron-donating substituents favored the formation of [RC(6)H(4)CH(2)](+), whereas strong electron-withdrawing substituents favored the competing hydride transfer reaction leading to a loss of toluene. The logarithmic values of the abundance ratios of the two ions were well correlated with the nature of the substituents, or rather, the stabilization energy of this INC.

Ion-neutral complexes resulting from dissociative protonation: fragmentation of alpha-furanylmethyl benzyl ethers and 4-N,N-dimethylbenzyl benzyl ethers

The loss of CH(2)O during mass spectrometry in two series of alpha-aromaticmethyl benzyl ether compounds, namely, alpha-furanylmethyl p-substituted-benzyl ethers and 4-N,N-dimethylbenzyl p-substituted-benzyl ethers, is particularly interesting. The fragmentation mechanism is proposed to involve an ion-neutral complex-mediated pathway. Specifically, before the formation of an ion-neutral intermediate, the proton is transferred from the thermodynamically favored site at either the ether oxygen atom or the nitrogen atom to the dissociative protonation site at C(alpha) position in either the furyl group or the 4-N,N-dimethylphenyl group. This transfer has been clarified via computational studies and isotopically labeled experiments. In addition, the decomposition of the intermediate may be affected by the substituent groups on the phenyl ring. This conclusion is indicated by the reasonably good correlation between ln[([M + H - CH(2)O](+))/([M + H - CH(2)O - C(6)H(5)R](+))] and the substituent constants.

Intramolecular transacylation: fragmentation of protonated molecules via ion-neutral complexes in mass spectrometry

An intramolecular transacylation reaction was observed in the mass spectrometry of molecules containing both benzoyl and carboxymethyl groups on an aromatic heterocyclic core. The reaction is triggered by a dissociative protonation on the heterocyclic ring at the atom (carbon or nitrogen) that bonds to the benzoyl group, leading to an intermediate ion-neutral complex. The incipient benzoyl cation in the complex migrates to attack the carboxyl group of the neutral partner at the carbonyl or hydroxyl oxygen under thermodynamic or kinetic control, respectively. Elimination of benzoic acid followed by loss of carbon monoxide takes place as a result of the transacylation.

Host-guest hydrogen atom transfer induced by electron capture

1,n-Alkanediammonium cations in noncovalent complexes with two dibenzo-18-crown-6-ether (DBCE) ligands undergo an unusual intramolecular tandem hydrogen atom and proton transfer to the crown ether ligand upon charge reduction by electron capture. Deuterium labeling established that both migrating hydrogens originated from the ammonium groups. The double hydrogen transfer was found to depend on the length of the alkane chain connecting the ammonium groups. Ab initio calculations provided structures for select alkanediammonium.dibenzo-18-crown-6-ether complexes and dissociation products. This first observation of an intra-complex hydrogen transfer is explained by the unusual electronic properties of the complexes and the substantial hydrogen atom affinity of the aromatic rings in the crown ligand.

Direct intramolecular gas-phase transfer reactions during fragmentation of sildenafil and thiosildenafil analogs in electrospray ionization mass spectrometry

A study is made of the mass spectral fragmentation pathways of sildenafil, thiosildenafil, and analogous compounds. A prominent gas-phase reaction that occurs during collision-induced dissociation (CID) of thiosildenafil compounds is the transfer of an alkyl group from the piperazine nitrogen atom to the sulfur atom of the thiocarbonyl group. This phenomenon is clearly demonstrated through a comparison of electrospray ionization mass spectral fragmentation patterns of four sildenafil-type compounds and three related thiosildenafil derivatives. Molecular modeling and fragmentation patterns support a direct intramolecular alkyl transfer mechanism rather than an ion-neutral complex mechanism. CID of thiohydroxyhomosildenafil results in a facile hydroxyethyl migration to the sulfur atom followed by a second intramolecular reaction to form a spiro-1,3-oxathiolane ring, which fragments in two directions to generate both carbonyl and thiocarbonyl product ions from this thiocarbonyl compound. While methyl migration to the thiocarbonyl sulfur atom of thiosildenafil is dominant, methyl migration to the carbonyl oxygen atom of sildenafil may occur to a small extent.

Probing the Compound I-like reactivity of a bare high-valent oxo iron porphyrin complex: the oxidation of tertiary amines

The mechanisms of oxidative N-dealkylation of amines by heme enzymes including peroxidases and cytochromes P450 and by functional models for the active Compound I species have long been studied. A debated issue has concerned in particular the character of the primary step initiating the oxidation sequence, either a hydrogen atom transfer (HAT) or an electron transfer (ET) event, facing problems such as the possible contribution of multiple oxidants and complex environmental effects. In the present study, an oxo iron(IV) porphyrin radical cation intermediate 1, [(TPFPP)*+ Fe(IV)=O]+ (TPFPP = meso-tetrakis (pentafluorophenyl)porphinato dianion), functional model of Compound I, has been produced as a bare species. The gas-phase reaction with amines (A) studied by ESI-FT-ICR mass spectrometry has revealed for the first time the elementary steps and the ionic intermediates involved in the oxidative activation. Ionic products are formed involving ET (A*+, the amine radical cation), formal hydride transfer (HT) from the amine ([A(-H)]+, an iminium ion), and oxygen atom transfer (OAT) to the amine (A(O), likely a carbinolamine product), whereas an ionic product involving a net initial HAT event is never observed. The reaction appears to be initiated by an ET event for the majority of the tested amines which included tertiary aliphatic and aromatic amines as well as a cyclic and a secondary amine. For a series of N,N-dimethylanilines the reaction efficiency for the ET activated pathways was found to correlate with the ionization energy of the amine. A stepwise pathway accounts for the C-H bond activation resulting in the formal HT product, namely a primary ET process forming A*+, which is deprotonated at the alpha-C-H bond forming an N-methyl-N-arylaminomethyl radical, A(-H)*, readily oxidized to the iminium ion, [A(-H)]+. The kinetic isotope effect (KIE) for proton transfer (PT) increases as the acidity of the amine radical cation increases and the PT reaction to the base, the ferryl group of (TPFPP)Fe(IV)=O, approaches thermoneutrality. The ET reaction displayed by 1 with gaseous N,N-dimethylaniline finds a counterpart in the ET reactivity of FeO+, reportedly a potent oxidant in the gas phase, and with the barrierless ET process for a model (P)*+ Fe(IV)=O species (where P is the porphine dianion) as found by theoretical calculations. Finally, the remarkable OAT reactivity of 1 with C6F5N(CH3)2 may hint to a mechanism along a route of diverse spin multiplicity.

Four decades of joy in mass spectrometry

Tremendous developments in mass spectrometry have taken place in the last 40 years. This holds for both the science and the instrumental revolutions in this field. In chemistry the research was heavily focused on organic molecules that upon electron ionization fragmented via complex mechanistic pathways as shown by isotopic labeling experiments. These studies, including ion structure determinations, were performed with use of double focusing mass spectrometers of both conventional and reversed geometry, and equipped with various types of metastable ion scanning and collision-induced dissociation techniques developed by physical and analytical chemists. Time-resolved mass spectrometry by use of the field ionization kinetics method, developed by physical chemists, was another powerful way to unravel details of unimolecular gas phase ion dissociations. Then the development of new ionization methods, such as desorption chemical ionization, field desorption, and fast atom bombardment permitted not only to analyze unvolatile, thermally labile and higher molecular weight compounds, but also to study their chemical behavior in the gas phase, initially with use of double focusing instruments and later on with multisector and hybrid mass spectrometers. These ionization methods also enabled to study organometallic compounds and increasingly the field of medium-sized to large biomolecules, the latter being exploded in the last decade by the development of electrospray- and matrix-assisted laser desorption ionization/time-of-flight mass spectrometry. Another area of research concerned the bimolecular chemistry of organic ions with organic molecules in the gas phase. Initially this was performed with use of among others drift-cell ion cyclotron resonance spectroscopy, that later on was replaced by the developed method of ion trapping and Fourier transform ion cyclotron resonance. Combination of the latter with the afore-mentioned ionization methods has shifted also in this case the research on organic molecules to organometallic/inorganic systems, and predominantly to biomolecules in the last decade. This invited review will describe the research efforts made by the author's group over the last 40 years together with some personal experiences during his career.

10–100 eV Ar+ ion induced damage to d-ribose and 2-deoxy-d-ribose molecules in condensed phase

We report that 10-100 eV Ar+ ion irradiation induces severe damage to the biologically relevant sugar molecules D-ribose and 2-deoxy-D-ribose in the condensed phase on a polycrystalline Pt substrate. Ar+ ions with kinetic energies down to 15 eV induce effective decomposition of both sugar molecules, leading to the desorption of abundant cation and anion fragments, including CH3+, C2H3+, C3H3+, H3O+, CHO+, CH3O+, C2H3O+, H-, O-, and OH-, etc. Use of isotopically labelled molecules (5- 13C D-ribose and 1-D D-ribose) reveals the site specificity for some of the fragment origins, and thus the nature of the chemical bond breaking. It is found that all of the chemical bonds in both molecules are vulnerable to ion impact at energies down to 15 eV, particularly both the endo- and exocyclic C-O bonds. In addition to molecular fragmentation, several chemical reactions are also observed. A small amount of O-/O fragments abstract hydrogen to form OH-. It is found that the formation of the H3O+ ion is related to the hydroxyl groups of the sugar molecules, and is associated with additional hydrogen loss from the parent or adjacent molecules via hydrogen abstraction or proton transfer. The formation of several other cation fragments also requires hydrogen abstraction from its parent or an adjacent molecule. These fragmentations and reactions are likely to occur in a real biomedium during ionizing radiation treatment of tumors and thus bear significant radiobiological relevance.

New Selective Haloform-type Reaction Yielding 3-Hydroxy-2,2-difluoroacids: Theoretical Study of the Mechanism

Experimental results of an unprecedented haloform-type reaction in which 4-alkyl-4-hydroxy-3,3-difluoromethyl trifluoromethyl ketones undergo base-promoted selective cleavage of the CO-CF(3) bond, yielding 3-hydroxy-2,2-difluoroacids and fluoroform, are rationalized using DFT (B3LYP) calculations. The gas-phase addition of hydroxide ion to 1,1,1,3,3-pentafluoro-4-hydroxypentan-2-one (R) is found to be a barrierless process, yielding a tetrahedral intermediate (INT), involving a DeltaG(r)(298 K) of -61.4 kcal/mol. The CO-CF(3) bond cleavage in INT leads to a hydrogen-bonded [CH(3)CHOHCF(2)CO(2)H...CF(3)](-) complex by passage through a transition structure (TS1) with a DeltaG()(298 K) of 20.8 kcal/mol and a DeltaG(r)(298 K) of 9.8 kcal/mol. This complex undergoes a proton transfer between its components, yielding a hydrogen-bonded [CH(3)CHOHCF(2)CO(2)...CHF(3)](-) complex. This process has associated with it a DeltaG()(298 K) of only 3.1 kcal/mol and a DeltaG(r)(298 K) of -43.3 kcal/mol. The CO-CF(2) bond cleavage in INT leads to a hydrogen-bonded [CH(3)CHOHCF(2)...CF(3)CO(2)H](-) complex by passage through a transition structure (TS3) with a DeltaG()(298 K) of 29.2 kcal/mol and a DeltaG(r)(298 K) of 25.1 kcal/mol. The lower energy barrier found for CO-CF(3) bond cleavage in INT is ascribed to the larger number of fluorine atoms stabilizing the negative charge accumulated on the CF(3) moiety of TS1, as compared to the number of fluorine atoms stabilizing the negative charge on the CH(3)CHOHCF(2) moiety of TS3. The solvent-induced effects on the two pathways, introduced within the SCRF formalism through PCM calculations, do not reverse the predicted preference of the CO-CF(3) over the CO-CF(2) bond cleavage of R in the gas phase.