Loss Of CH Research Articles

Site of gas-phase ethyl ion attachment

The [C2D5]+ species forms an adduct in the gas phase with ethyl acetate, ethylbenzene, and p-ethyltoluene, which subsequently unimolecularly eliminates C2H4 and C2D4 in approximately a 2:1 ratio. These results indicate that the adduct is not a weakly bound ion–molecule complex but a bonded species in which the C2D5 group has become equivalent to the C2H5 group present in the molecule; the preference for elimination of C2H4 is due to an isotope effect. From observations of the relative loss of C2H4 and C2D4 from [C2D5]+ adducts with other molecules containing a C2H5 group, it is concluded that the [C2D5]+ species attaches to the carboxyl group in ethyl benzoate, primarily to the ring in substituted phenetoles and in ethylphenols, primarily to the nitrogen in ethylpyridines, and primarily to the ring for N-ethyl- and p-ethyl-aniline.

Is the collision induced loss of ethene from the (M – H+)− ion of butyrophenone a γ-hydrogen rearrangement?

The (M – H+)− ion of butyrophenone undergoes the following reactions on collisional activation: losses of CH3•, CH4, (C,H5•), C2H4, C3H7•, (CO + CH4), together with formation of C6H5− and C4H5O−. Labelling studies (13C and 2H) show that the losses of CH3•, C3H7• and the formation of C6H5− and C4H5O− are specific and occur without hydrogen scrambling. All other reactions involve prior or accompanying hydrogen rearrangement. In particular, the loss of C2H4 is very complex: it involves loss of ethyl carbon atoms, but all hydrogen atoms are involved via specific rearrangement reactions. The phenyl–alkyl H rearrangements which are noted for this process occur after collisional activation of the (M – H+)− ion.

Electron impact induced fragmentations of 2‐chlorovinyl methyl sulphides

AbstractA series of 2‐chlorovinyl methyl sulphides, CH3SCR1CR2Cl (R1, R2 = H, Me, Et, t‐Bu, Ph; R1 = H, R2 = CH3) has been analysed under electron impact conditions. The most significant common fragmentations involve primary loss of Cl˙ and methyl radicals. Analysis of the phenyl‐substituted derivative suggests that loss of CH3˙ is a lower activation energy process than loss of Cl˙. The characteristic loss of CH3SCl from the same derivative has been examined with respect to the structure of the resulting fragment by means of suitable model compounds.

Carbanion rearrangements. Collision-induced dissociations of enolate ions derived from 3-ethylpentan-2-one

Reaction of HO– with MeCOCHEt2 produces two enolate ions, MeCOEt2 and –CH2COCHEt2. The primary carbanion competitively eliminates C2H4 and C4H8, and forms C2HO–. The elimination of C2H4 is a stepwise reaction proceeding through a six-membered transition state; the first step (deprotonation) is rate-determining. The loss of C4H8 is a rearrangement reaction –CH2COCHEt2 [graphic omitted] –CH2COMe + EtCHCH2. The tertiary carbanion competitively eliminates H2, CH4, and C3H8. The losses of CH4 and C3H8 are stepwise processes occurring through six- and five-membered transition states, respectively. A double isotope fractionation experiment (2H, 13C) shows that both steps of the CH4 elimination are rate-determining.

Partially fluorinated heterocyclic compounds. Part 22. The preparation of allyl 2,5,6-trifluoropyrimidin-4-yl ether and related compounds and a study of their Claisen rearrangement reactions. A new route to 5-fluorouracil and barbituric acid derivatives

Vapour phase thermolyses of allyl 2,5,6-trifluoropyrimidin-4-yl, (1), 2,5-difluoro-6-methoxypyrimidin-4-yl, (5), 2,5-difluoropyrimidin-4-yl, (6), and 5,6-difluoro-2-methoxypyrimidin-4-yl, (7), ethers gave the Claisen rearrangement products (2), (17), (18), and (20) respectively, in which N-3 is the migration terminus, but the 2-allyl derivative (10) was inert. In other thermolyses with allyl 5-fluoropyrimidin-4-yl ethers where a preliminary reaction results in the localisation of the C-4 to C-5 double bond, C-5 is the migration terminus: barbituric acid derivatives (28) and (26) are formed from (5) and allyl 2,5-difluoro- 6-hydroxypyrimidin-4-yl ether (13) respectively after hydrolysis of each rearrangement product and (14) also gave (26); 2,4-diallyl-5-fluoro-6-methoxypyrimidine (11) gave (29)(with the loss of CH3) and 2,4,6-triallyloxy-5-fluoropyrimidine (12) was isomerised to (30). Allyl 2,6-dimethoxy-5-fluoro- pyrimidin-4-yl ether (8) gave both N-3 and C-5 migration termini products (21) and (33) respectively. Hydrolysis of (2), (17), and (18) gave the 5-fluorouracil derivatives (22), (23), and (24) respectively.

Theoretical study on the isomerization and dissociation of C4H4O+? radical cation

A MINDO/3 theoretical study has been performed on the potential surface of the C4H4O+˙ radical cation. Fragmentations yielding C3H4+˙(CO loss), C3H3+(CHO loss), and C2H2O+˙(C2H2 loss) ions have been studied. A wide number of pseudostable species have been found. Very complex rearrangements occur previous to the fragmentations. Kinetic aspects are discussed.

Cis/trans stereochemical effects in the negative chemical ionization/OH− mass spectra of strained‐ring azabicycloalkanes using MIKE and CA/MIKE spectrometry

AbstractStereospecific decomposition reactions of isomeric (cis and trans) deprotonated molecules from azabicycloalkane derivatives as azetidinols generated under negative chemical ionization (NCI)/OH− have been examined using mass‐analysed ion kinetic energy (MIKE) and collisional activation (CA)/MIKE spectra. These measurements together with the ones obtained on specifically labelled compounds enabled us to determine the origin of the stereochemical effects. The results indicate that the hydroxylic proton constitutes the preferential (≃90%) site for the deprotonation process. Subsequent fragmentations of the deprotonated species observed in the second field‐free region of a reversed geometry instrument are affected by the stereochemistry of the hydroxylic group. The isomer with the hydroxyl group in the cis position relative to the hydrogen at the ring junction mainly loses H2O, while the trans isomer eliminates CH3˙, both processes occurring with high specificity. Labelling studies indicate that two major pathways exist for the elimination of H2O from the cis isomer and the loss of CH3˙ from the trans isomer. The course of the reaction is determined by the ability of the stereoisomers to transfer a proton during the first decomposition step. When the size of the lactam ring is increased from a five‐membered ring to a six‐ or seven‐membered ring, these stereochemical effects tend to become less pronounced.

Gas Chromatography–Mass Spectrometry of N-Trifluoroacetyl Trimethylsilyl Esters of Some Iminodicarboxylic Acids

Abstract The simultaneous N-trifluoroacetylation and O-trimethylsilylation of some iminodicarboxylic acids were studied by the use of a mixture of α,α,α,α′,α′,α′-hexafluoro-N-methyldiacetamide and N,O-bis(trimethylsilyl)trifluoroacetamide, as a derivatizing reagent. The resulting N-trifluoroacetyl trimethylsilyl ester derivatives gave rather complicated mass spectra with molecular (M+) and M–15 (loss of CH3) ions upon electron impact at 20 eV.

Gas Chromatography–Mass Spectrometry of Trimethylsilylated Imino Derivatives of Alanine

Abstract Trimethylsilylation of seven imino derivatives of alanine with N,O-bis(trimethylsilyl)trifluoroacetamide in acetonitrile was studied. The imino derivatives include 2,2′-iminodipropionitrile (1), 2,2′-iminodipropionamide (2), 2,2′-iminodipropionic acid (3), 2-(1-cyanoethylamino)propionamide (4), 2-(1-cyanoethylamino)propionic acid (5), 2-(1-carbamoylethylamino)propionic acid (6), and 3,5-dimethyl-2,6-piperazinedione (7). The reaction products were identified by gas chromatography–mass spectrometry. Under the present reaction conditions (at 100 °C for 30 min), hydrogen atoms of carboxyl and carbamoyl groups and an imide hydrogen were readily replaced by the trimethylsilyl (TMS) group, but imino hydrogens were not replaced because of the steric hindrance of N-substituent group. For the carbamoyl group, only one hydrogen was replaced. Consequently, 1 was not trimethylsilylated. Furthermore, no definite trimethylsilylation products were obtained for 4. Upon electron impact at 70 eV, 2, 3, and 5–7 gave rather simple spectra with molecular (M+) and M −15 (loss of CH3) ions characteristic of TMS derivatives. A fragment, M −117 (loss of COOTMS) or M −116 (loss of CONHTMS), was very prominent for 2, 3, 5, and 6. Other important ions such as m/z 70 were noted. Preparations and properties of some of the imino derivatives are also described.

Unimolecular reactions of ionized methyl acetate and its hydrogen‐rearranged isomers

AbstractThe proposed formation of [CH3C(OH)OCH2]+˙ (b) as the intermediate in the isomerization [CH2C(OH)OCH3]+˙ (c)⇄b⇄[CH3COOCH3]+˙ (c has been confirmed by preparation of b from CH3COOCH2OCH3. For the three isomers a–c the dominant metastable ion (MI) dissociation, CH3O˙ loss, involves identical kinetic energy release values. The kinetic barriers for a⇄b and b⇄c must be nearly as high as that for CH3O˙ loss from c, as shown by the insensitivity of the mass spectra from collisionally activated dissociation (CAD) of a–c to ionizing electron energy. The H/D scrambling of metastable [CH2C(OD)OCH3]+˙ and c–D3 ions confirm this, indicating that the barrier for a⇄b is slightly below that for b⇄c. Minor low‐energy dissociations include losses of CH4 and CH3OH from a and losses of ˙CHO and CH2O from b. Comparison of MI and CAD spectra of a–c with those from [CH3(OH)CH2O]+˙ (d) and [CH3COCH2OH]+˙ (e) give no evidence for skeletal rearrangement of a–c to d or e.

A study of gaseous benzenium and toluenium ions generated from 1,4- dihydro- and 1-methyl-1,4-dihydro-benzoic acids

Gaseous benzenium C6H7+(1) and toluenium C7H9+(2) ions have been generated by mass spectrometric loss of ˙CO2H from the corresponding 1,4-dihydrobenzoic acids (3) and (4), and their fragmentations after ca. 10 µs have been investigated by means of mass-analysed ion kinetic energy (MIKE) spectrometry of some 2H and 13C labelled analogues. Metastable C6H7+ ions eliminate H2 after proton randomization, whereas metastable C7H9+ ions expel both H2 and CH4 after incomplete proton equilibration. In particular, 40% of C7H9+ ions randomize all their carbon and hydrogen atoms prior to loss of CH4, and 60% of C7H9+ ions lose the original methyl group along with a hydrogen atom from the (proton-equilibrated) benzenium ring, accompanied by a slow and incomplete exchange between the hydrogen atoms of the ring and the methyl group. It is suggested that loss of both CH4 and H2 occur via the (ipso-)toluenium ion (2). The role of a non-classical C7H9+ isomer, phenylmethonium ion (6), is discussed since striking similarities are found compared with [C6H6·CH3+]* adducts from ion–molecule reactions described in the literature.

Gas Chromatography–Mass Spectrometry of Trimethylsilyl Derivatives of Some Iminodicarboxylic Acids

Abstract Trimethylsilylation of eight iminodicarboxylic acids (IDCAs) with N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) or N,O-bis(trimethylsilyl)acetamide was studied under various reaction conditions. The IDCAs include iminodiacetic acid (1), 2- and 3-(carboxymethylamino)propionic acids (2 and 3), 2,2′-, 2,3′-, and 3,3′-iminodipropionic acids (4–6), N-methyliminodiacetic acid (7), and nitrilotriacetic acid (8). For 1–6, both bis- and tris(trimethylsilyl) (TMS) derivatives were formed under usual silylating conditions, although the proportion of amounts of the two derivatives varied depending on reaction conditions. This double derivatization was due to the steric hindrance of N-substituent group. However, under mild silylating conditions (BSTFA alone, 120 °C, 30 min), 1–6 yielded only the di-TMS derivatives. Upon electron impact at 20 eV, both the di- and tri-TMS derivatives showed simple spectra with a molecular (M+) ion and M −15 (loss of CH3). The α,α′-IDCAs (1, 2, 4, 7, and 8) are characterized by fragments M −43 (loss of CH3 and CO) and M −117 (loss of COOTMS) (base peak). On the other hand, the β,β′-IDCA (6) is characterized by fragments M −57 (loss of CH3, CH2, and CO) and prominent M −131 (loss of CH2COOTMS). The α,β′-IDCAs (3 and 5) exhibit both the fragments characteristic of α,α′- and β,β′-IDCAs, M −43, M −57, M −117, and M −131. However, base beaks are usually the fragment M −117 and not M −131.

Fragmentation of selected C6H10O isomers: Evidence for a common [C5H7O]+ ion generated by methyl loss from cyclohexene oxide and 5,6‐dihydro‐4‐methyl‐2H‐pyran

AbstractThe loss of CH3˙ from the molecular ions of cyclohexene oxide and 5,6‐dihydro‐4‐methyl‐2H‐pyran has been investigated. On the basis of metastable peak shape analysis, collision‐induced dissociation/mass‐analysed ion kinetic energy spectra and thermochemical data it is concluded that the same [C5H7O]+ ion is formed in both cases.

Ab initio calculations relevant to the mechanism of chemical carcinogenesis byN-nitrosamines. I. The nitrosation of amines

Ab initio calculations have been carried out on the reaction of NO+ with amines, using a 4-21G basis set. The influence of solvation was investigated using one to three molecules of H2O. Geometry optimizations were carried out on reactants, products, and intermediates. The results show that loss of CH3+ is energetically favorable and this fact has implications with respect to the mechanism of carcinogenesis by dimethylnitrosamine.

Novel gas phase ions. II. [CH2=CHOH/CH3OH]+• and [CH2=CHOH/H2O]+•

The title ions are generated by the loss of C2H4 from ionised 4-methoxybutan-1-ol and butan-1,4-diol, respectively, and are best described as complexes between [CH2=CHOH]+• and CH3OH (dissociation energy 21 kcal mol−1) and between ionised vinyl alcohol and H2O (dissociation energy ≥ 13 kcal mol−1). [Formula: see text] of the ions [CH2=CHOH/H2O]+• and [CH2=CHOH/CH3OH]+• were measured as ≤ 110 and 112 kcal mol−1, respectively.

Étude de la Décarbonylation du 1‐Acétylimidazole et du 1‐Acétylpyrazole Ionises

AbstractCollisionally activated dissociations (CAD) spectra have been used to investigate the structures of ions produced by decarbonylation of 1‐acetylimidazole [1] and acetylpyrazole [12] ionized by electron impact. The results are best explained by the formation of non classical methylene‐azolium ions. The loss of carbon dioxide from l‐methoxycarbonylimidazole [7] does not produce these ions but instead leads to ionized C‐methyl‐2(or 4)H‐imidazole. A mixture of structures is produced by electron impact induced fragmentation (loss of C2H4) of 1‐(2‐hydroxypropyl)imidazole [9]

Structure and fragmentation of [C3H7O]+ ions formed by chemical ionization

AbstractThe unimolecular metastable and collision‐induced fragmentation reactions of [C3H7O]+ ions produced by gas‐phase protonation of acetone, propanal, propylene oxide, oxetan and allyl alcohol have been studied. The CID studies show that protonation of acetone and allyl alcohol yield different stable ions with distinct structures while protonation of propanal or propylene oxide yield [C3H7O]+ ions of the same structure. Protonated oxetan rearranges less readily to give the same structure(s) as protonated propanal and propylene oxide. The [C3H7O]+ ions fragmenting as metastable ions after formation by CI have a higher internal energy than the same ions fragmenting after formation by EI. Deuteronation of the C3H6O isomers using CD4 reagent gas shows that loss of C2H3D proceeds by a different mechanism than loss of C2H4. The results are discussed in terms of potential energy profile for the [C3H7O]+˙ system proposed earlier.

A two‐dimensional photochemical model of the atmosphere: 2. The tropospheric budgets of the anthropogenic chlorocarbons CO, CH4, CH3Cl and the effect of various NOx sources on tropospheric ozone

This paper presents two‐dimensional photochemical model simulations that show the influence of the various NOx sources from industry, lightning, the stratosphere, and aircraft on the tropospheric distributions of NOx, HNO3, and O3. We found that, by far, the best agreement with the global observations is obtained if the industrial sources are included in the calculations. Industrial activities have led to substantial increases of ozone concentrations in the lower troposphere of the northern hemisphere. Emissions of NOx by high‐flying aircraft have only a small effect on ozone concentrations in the troposphere. The ability of the model to simulate the global distributions of the long‐lived chlorocarbons CFCl3 and CF2Cl2 indicates that the interhemispheric exchange is rather well described. The model confirms an earlier finding by Rowland et al. (1982) that the CF2Cl2 emission rates estimated by the Chemical Manufacturers Association (CMA) may be 35–40% lower for the period 1976 through 1980. The model also supports the earlier finding of Hyson et al. (1980) that CFCl3 is apparently increasing roughly 10% faster than the CMA emission rates can account for during the period 1976 through 1980. Although this disparity could be explained by assuming 10% higher emission rates for CFCl3 during the period, the behavior of the interhemispheric gradient indicates that errors in the determination of the absolute concentrations could also be an explanation. The ability of the model to simultaneously simulate measured distributions of the shorter‐lived chlorocarbons CH3CCl3, CH2Cl2, C2HCl3, and C2Cl4 indicates that its average OH concentrations are also about correct. Based on these OH concentrations, the model tropospheric budgets of CO, CH4, and CH3Cl are calculated. The computed losses of CO, CH4, and CH3Cl by reaction with OH are 2.1×1015 g CO yr−1, 3.2×1014 g CH4 yr−1, and 1.9× 012 g CH3Cl yr−1, respectively.

Further examples of skeletal rearrangements of the Wagner‐Meerwein type in chemical ionization mass spectrometry: The case of [C6H9]+ ions

AbstractThe [C6H9]+ ions produced either via unimolecular H2O loss from 13 [C6H11O]+ precursors or direct protonation of 1,3‐ and 1,4‐cyclohexadiene have identical collisional activation mass spectra. The kinetic energy release data for the process [C6H11O]+→[C6H9]++H2O are also very similar (on average T0.5=24 meV) irrespective of the constitution of the precursor. From the proton affinities of 1,3‐cyclohexadiene (PA=837.2 kJ mol−1) using ion cyclotron resonance mass spectrometry the heat of formation of the [C6H9]+ ion is determined to 804.6 kJ mol−1. This value taken together with the results of molecular orbital calculations (MNDO) and the structure indicative losses of CH3. and C2H4 upon collisional activation suggest that the [C6H9]+ ion has the structure of the 1‐methylcyclopentenylium ion f and not that of the slightly less stable cyclohexenylium ion g. The generator of an easily interconverting system of isomeric [C6H9]+ ions is unlikely to be due to the high barrier separating the various isomers.

Ortho, Effects in organic molecules on electron impact: X—unusual ortho effects of the methyl group in o‐picolinotoluidide

AbstractThe mass spectrum of o‐picolinotoluidide gives rise to three major fragments at m/z 184, m/z 169 and m/z 168, corresponding to the loss of CO from the molecular ion followed by the loss of ṄH2 and ĊH3 by independent pathways. It has been shown that the ortho methyl group and the nitrogen of the pyridine ring in the 2‐position are involved in the formation of these three major fragments observed in the mass spectrum of o‐picolinotoluidide. The mass spectrum of 2‐(o‐toluidino) pyridine, the molecular ion of which can resemble the [MCO]+ ion in o‐picolinotoluidide, also shows loss of CH3 and NH2 radicals from the molecular ions. Based on these observations coupled with the high resolution data, the mass analysed ion kinetic energy spectrometry and high voltage scans of these fragments in both the compounds, two mechanistic pathways have been proposed for the formation of these ions in o‐picolinotoluidide.