Β-distonic Ion Research Articles

Hydrogen radical abstraction by small ionized molecules, distonic ions and ionized carbenes in the gas phase

In the gas phase and within ion–neutral complexes, H abstraction by the ion from the neutral moiety was studied by using FT-ICR experiments and molecular orbital calculations. Ionized methanol abstracts rapidly H from methane and other alkanes while its α-distonic counterpart, CH 2OH 2 +, is completely unreactive. On the other hand, β-distonic ions, such as CH 2CH 2XH + (X=OCH 3, NH 2), are also unreactive towards methane but can abstract H from ketones and ethers. Finally, ionized carbenes, such as HOCNH 2 +, react with methane by a slow H abstraction. Ab initio molecular orbital calculations at the G3(MP2) level were performed in order to understand these behaviors. For ionized methanol and its α-distonic counterpart, the reacting structure that could lead to H abstraction is the highly stabilized complex between protonated methanol and a methyl radical, which yields the final state (CH 3OH 2 ++ CH 3) by simple cleavage. In the case of methanol the encounter complex with methane leads easily by rotation of the methane molecule to this reacting structure. In contrast, in the case of the α-distonic ion, the almost linear structure of the encounter complex [ CH 2OH 2 +⋯CH 4] and the high energy required for its isomerization into the reacting structure prevent the reaction. Two factors are required to observe H abstraction in β-distonic ions: the interaction energy of the encounter complex and the distance between the hydrogen to be transferred and the carbon radical. Reaction of the HOCNH 2 + carbene with methane lies between these two extreme cases. The encounter complex is poorly stabilized (−8.7 kcal mol −1) and the transition state for H abstraction is very close to the reactants energy.

Proton affinity of some radicals of alcohols, ethers and amines

Calculated proton affinities (PA) of the CH 3 CHX (X=OH, OCH 3, and NH 2) α-radicals and of four β-radicals CH 2CH 2X (X=OH, OCH 3, OCH 2CH 3, and NH 2) confirm that PA of α-radicals lie far below those of the corresponding saturated molecules and indicate, as a first and rough evaluation, that a radical in the β-position, has a negligible influence on the PA at the heteroatom. Experiments were performed to measure the PA of β-radicals both by measuring the rates of deprotonation of the corresponding β-distonic ions in a FT-ICR cell and by determining the Δ H f[ CH 2CH 2XH +], itself obtained by appearance energy (AE) measurements, using synchrotron radiation. From these results and those of literature, the following values are proposed: PA[ CH 2CH 2OH]=185.8±0.5 kcal/mol, PA[ CH 2CH 2OCH 3]=193.3±0.5 kcal/mol and PA[ CH 2CH 2NH 2]=116±1 kcal/mol.

Ambident reactivity and characterization of small ionized carbenes

The gas phase reactions of five ionized carbenes, HCOH + 1, HCNH 2 + 2, CH 3COH + 3, HOCOH + 4 and HOCNH 2 + 5 with different molecules are studied by FT-ICR mass spectrometry. Interaction between an ionized carbene and a molecule can yield two kinds of stable adducts, as expected from the electronic structure of the carbene radical cations, explaining the ambident reactivity of these ions. The first kind of adduct corresponds to H-bonded species (hydrogen-bridged radical cations), the second to covalent structures. Since interconversion between these adducts is generally slow, each kind of adduct leads to a particular set of reactions. The H-bonded species can be involved in the protonation of the neutral as well as in the catalyzed interconversion between the carbene and its conventional radical cation counterpart. The covalent adducts, formed by reaction of ionized carbenes with methanal and alkenes, are β-distonic ions. Reactions with labeled propene show that the so formed distonic ions either dissociate by simple cleavage or undergo rearrangements and H-exchange after isomerization into conventional ions by 1,4-H transfers. Cyclopropane gives a characteristic reaction of the carbene structure: addition yields a γ-distonic ion which loses ethylene. Finally, H , I and SCH 3 abstraction from appropriate neutrals confirms a radical reactivity of the carbenic carbon.

Electron Impact Induced Fragmentation of Aromatic Alkoxyimines II [5]. Formation and Transformation of Heterocyclic Radical Cations in the Gas Phase a

The molecular ion 1 of N-(n-propoxy)benzaldimine I rearranges by an 1,5-H-shift to the δ-distonic ion 2 which subsequently cyclizes to the α-distonic ion 3. Homolytic cleavage of the N–O bond in 3 results in the δ-distonic ion 4 which expels CH2O leading to the β-distonic ion 5. Ion 5 is also formed from the molecular ions of tetrahydrooxazines II and III and from M+• of phenylazetidine IVa. In a subsequent step, ion 5 cyclizes to the N-protonated 3,4-dihydroisoquinolinium ion 6. The syntheses of II–IV and their derivatives are described.

A catalyzed rearrangement of gaseous 1,1-dichloroethene radical cations

The reaction of the radical cation of 1,1-dichloroethene 1 with methanol has been investigated by FT-ICR spectrometry and compared with that of the radical cations of 1,2-dichloroethene 2 and of the 1,1- and 1,2-dibromoethenes 3 and 4 with methanol and ethanol, respectively. The 1,1-dichloro derivative 1 + is special because it exhibits besides a slow hydride transfer also a slow isomerization into an unreactive isomer, which was proven by separate experiments to be the 1,2-disubstituted isomer 2 +. This unprecedented isomerization of a radical cation by a reciprocal 1,2-shift of Cl and H is catalyzed by methanol. The mechanism of this isomerization and the competing reactions of 1 + with methanol have been modeled by ab initio calculation of the corresponding minimum energy reaction pathway (MERP). The results show that the isomerization takes place by a 1,2-Cl shift in a β-distonic onium ion 1distb + which is formed by a fast addition of methanol to the ionized CC double bond. The resulting α-distonic ion 1αdistb + displays the structure of the addition product of methanol to the carbene center of chloromethyl chlorocarbene 5 +. However, the 1,2-H shift within 1αdistb +, which completes the exothermic isomerization 1 +→ 2 + , requires a very large critical energy and blocks this reaction pathway. This large barrier is encompassed by a reaction route on which 1αdistb + starts to dissociate into methanol and 5 +, and the incipient chloromethyl chlorocarbene radical cation 5 + isomerizes with the aid of methanol by some kind of an internal proton shuttle. Thus, the catalyst methanol acts in this system in two different modes. Firstly, it is covalently bonded to the 1,1-dichloroethene radical cation and enables an easy 1,2-Cl shift within the resulting β-distonic ion. Secondly, during the process of reconstituting methanol it lowers the energy of the transition state of the 1,2-H shift in the incipient chloromethyl chlorocarbene radical cation 5 + to the 1,2-dichloroethene radical cation 2 +.

Hydrogen-shift isomers of ionic and neutral hydroxypyridines: a combined experimental and computational investigation

Apart from pyridine N-oxide ( 1a), the C 5H 5NO family of stable molecules comprises, 2-, 3- and 4-hydroxypyridine ( 2a, 3a and 4a) as well as their keto counterparts 2-, 3- and 4(1 H)-pyridone ( 2b, 3b and 4b). This study focuses on the characterisation of their radical cations and a number of stable H-shift isomers, which are α- or β-distonic ions. This was done by using a combination of mass spectrometric experiments and computational chemistry, at the B3LYP/CBSB7 level of theory. The ionic species were identified on the basis of both their collision-induced dissociation (CID) characteristics and specific associative ion–molecule reactions with dimethyl disulfide and tert-butyl isocyanide as substrates. The distonic ions ( 1b +, 2c +, 2d + and 3c +) were obtained by dissociative electron impact ionisation and subjected to neutralisation–reionisation mass spectrometry (NRMS). From CID spectra of the intense survivor ions, it follows that the neutral counterparts of the α-distonic ions 2c + and 3c + are viable chemical species in the rarefied gas phase. The energy-rich ylide type neutrals 1b, on the other hand, readily isomerise into pyridine N-oxide, 1a, or else dissociate. The neutral counterpart of the β-distonic ion 2d + only has a marginal stability and part of these neutrals are proposed to isomerise into energy-rich 2-pyridone molecules 2b. This is in agreement with the computational results. However, ionised 2-pyridone cannot readily be differentiated from its enol isomer 2-hydroxypyridine. In contrast, the keto isomers of ionised 3- and 4-hydroxypyridine display characteristically different CID spectra.

A density functional theory study of methyl shifts and methylcyclopropanol ion formation in C 4H 8O +· ions

C 4H 8O +· ions undergo a number of skeletal isomerizations, including rearrangement to the 2-butanone ion before metastable dissociation. However, the mechanisms whereby these rearrangements take place are not all firmly resolved. To remedy this, we characterized potential steps in the skeletal isomerizations of C 4H 8O +· ions using density functional theory. We established that branched and straight chain C 4H 8O +· ions with the oxygen on the terminal carbon interconvert through methylcyclopropanol ions and β-distonic ions. The cyclic isomers were found to be more stable than their ring-opened β-distonic forms. Transformation of ions with the O on a terminal carbon to ions with O on the second carbon occurs by methyl shifts that convert CH 2 ·CH(CH 3)CHOH + and CH 3CH 2CHCHOH +· to CH 3CH(OH)CHCH 2 +·. The second methyl shift takes place by inversion of the methyl configuration, possibly in accord with predictions of Woodward–Hoffmann rules.

The Chemistry of Solvated Distonic Ions: Preparation, Isomerization, and Fragmentation

It has been shown that solvated distonic ions can be prepared by selected collision-induced radical removal from the proton-bound dimer of appropriate molecules. The α- and β-distonic ethanol ions solvated by methanol or ethanol, [CH3•CHOH2+/ROH] and [•CH2CH2OH2+/ROH] (ROH = CH3OH and C2H5OH), have been prepared by selected loss of a designated radical from the proton-bound dimer of labeled alcohols, and their reactions studied in the metastable ion time frame. It is found that isomerization of the α-distonic ion to its conventional structure can be catalyzed by both methanol and ethanol, such that the energy barrier to this process is reduced to 25 ± 2 kcal mol-1, lower than that in the uncatalyzed reaction by 10 kcal mo1-1. The β-distonic ions do not isomerize to either the α-form or the conventional form when solvated by the alcohol molecule, and their fragmentation is dominated by water loss, similar to the isolated ion.

Novel β-Distonic Radical Cations [CnH2n+2S]•+ (n = 2, 3) Formed upon Decarbonylation of Ionized S-Alkyl Thioformates: A Mass Spectrometric and ab Initio Study

Decarbonylation is commonly observed in the 70 eV electron impact mass spectra of a series of S-alkyl thioformates, H−C(O)SR 1−7. Using a combination of collisional activation (CA) and neutralization−reionization (NR) mass spectra, it is shown that the spectra of the so-formed [M−CO]•+ ions differ from the isomeric alkane thiol ions (RSH•+), if R > CH3. These fragment ions are therefore distonic radical cations, and given the fact that the α-distonic •CH2−S+H2 ions are not present under our experimental conditions, it is proposed that they actually are β-distonic ions. Strong evidence for the distonic nature of the fragment ions derived from 2−7•+ has been obtained owing to the use of a new hybrid tandem mass spectrometer presenting a sector−quadrupole−sector configuration. In the case of ethyl derivatives (R = C2H5), both the distonic •CH2CH2S+H2 (2b) and conventional CH3CH2SH•+ (2a) ions are observed. The nonclassical structure 2b reacts indeed with nitric oxide (NO•) giving the production of H2S+−NO io...

The .beta.-distonic ion from the reaction of pyridine radical cation and ethene: a demonstration of high-pressure trapping in Fourier transform mass spectrometry

The adduct ion formed in the reaction of pyridine radical cation and ethene exhibits different chemical reactivities and fragmentation behavior than the isomeric ethylpyridine radical cations. The most characteristic features of the adduct ion are the collisionally activated loss of C 2 H 4 to give back the pyridine radical cation and the second ionization to produce a doubly charged ion. A characteristic reaction is with dioxygen to form a new adduct, which may have the structure of a distonic peroxy radical cation. The reactive site for attachment of C 2 H 4 is the nitrogen atom of the pyridine, giving rise to the β-distonic ion adduct, C 5 H 5 N + -CH 2 C.H 2 , which can be differentiated from the isomeric a-distonic ion, C 5 H 5 N + -C.HCH 3

Gas phase substitution reactions by radical cations. Part 2. Reaction of the CC ring-opened oxirane radical cation with benzonitrile and benzoic acid

Under conditions of chemical ionization in the high pressure source of a mass spectrometer, the CC ring-opened oxirane radical cation transfers a methylene group to benzonitrile and benzoic acid. This transfer reaction leads to the formation of [M + 14] + product ions. The structures of these ions have been established by collisionally induced dissociation of these ions compared with isomeric reference ions. Methylene transfer to benzonitrile yields the N-methylenebenzonitrile radical cation with a high selectivity. In the case of benzoic acid, a β-distonic ion is formed in which the methylene group is attached to the carbonyl O atom.

Reactions of the β‐distonic ion +CH2OCH2CH2˙ with butyronitrile: Evidence for an intermediate three‐body ion‐neutral complex during deprotonation

Reactions of the β‐distonic ion ⁺CH₂OCH₂CH₂˙ with butyronitrile: Evidence for an intermediate three‐body ion‐neutral complex during deprotonation

Unimolecular and bimolecular reactions of the β-distonic ion [rad]CH 2CH 2OCH +2 and its deuterated derivatives

The unimolecular and bimolecular reactions of the β-distonic ion +CH 2OCH 2CH 2 ( a) and its deuterated analogues b and c have been studied by tandem mass spectrometry and by Fourier transform ion cyclotron resonance (FT-ICR) spectrometry. The spontaneous reactions of metastable a are the loss of H and CO respectively. H and D are eliminated from metastable b and c irrespective of the position of the D atoms, suggesting an oxetane radical cation d as an intermediate for this process. However, a careful examination of the collisional activation (CA) spectra of a—c reveals that the CH 2 groups of a do not scramble by a mutual interconversion of a and d, in line with previous results. The reactions of a and the isotopomers b and c with pyridine and acetonitrile respectively have been investigated by FT-ICR spectrometry using an external ion source. Besides the elimination of an H atom which is very probably a collision-induced process, a reacts with pyridine by charge exchange, by protonation, and by the formation of an N-formyl pyridinium ion but not by the transfer of C 2H + 4 . All six H atoms of a take part in the protonation. A mechanism involving a rearrangement of a into CO and C 2H + 6 in the collision complex with pyridine and subsequent transfer of a proton from C 2H + 6 is suggested. The generation of the N-formyl pyridinium ion occurs specifically from the CH 2O moiety of a and corresponds to an electrophilic reaction of the β-distonic ion. In addition to the known transfer of C 2H + 4 from a to acetonitrile a proton transfer has been observed. However, the energetically allowed formation of an N-formyl nitrilium ion is not detected. The mechanistic origin of this diverse behaviour of the two nulceophiles pyridine and acetonitrile towards the β-distonic ion a is discussed.