Kinetic studies in mass spectrometry—VI: The [M ? CH3] reaction in substituted ethylbenzenes

The [M] → [M ? CH₃] reaction in a series of m- and p-X substituted ethylbenzenes has been studied by wide range electron energy kinetics and metastable ion characteristics techniques. By this approach, qualitative measures of activation evergy differences between [XC₆H₄CH₂]⁺ ions derived from m- and p-X isomer substrates have been secured, for both their formation and further decomposition. These evergy differences are consistent with (but do not prove) ion structures that have been suggested by previous work in this area, involving the use of isotope labeling, and ionization and appearance potential methods.     

High resolution mass spectrometry—I. Some substituted δ-lactones

The mass spectra of some δ-lactones substituted in various positions by methyl groups have been studied both by accurate mass measurements at high resulution and by deuterium labelling. The loss of carbon dioxide has been shown to be important only for a mono-substituted lactone. A fragmentation mode common to all the lactones studied is the elimination of the ring oxygen atom, plus the adjacent carbon atom with its substituents, as a neutral carbonyl molecule such as formaldehyde, acetaldehyde or acetone. Deuterium labelling has uncovered the existence of a rearrangement involving hydrogen transfer in an even-electron ion, and a mechanism involving a six-membered transition state is proposed for this. The bond fissions, common to all the lactones, by which the principal ions in the mass spectra arise, are summarised.     

Kinetic studies in mass spectrometry–VII: Competing cleavage and rearrangement processes in molecular ion decomposition reactions

Apparently competing cleavage and rearrangement reactions in a series of molecular ions have been studied by ionization and appearance potential methods, and by determination of the electron energy dependence of both normal and metastable daughter ion peak intensities. The processes investigated were (i) [M ? CH₃] vs. [M ? CH₂O] in anisole; (ii) [M ? OC₆H₅] vs. [M ? CO] in phenyl ether; (iii) [M ? NO₂] vs. [M ? NO] in nitrobenzene; (iv) [M ? C₃H₇] vs. [M ? C₂H₄] in butyrophenone: (v) [M ? C₃H₇] vs. [M ? C₃H₆] in n-butylbenzene; (vi) [M ? CH₂OH] vs. [M ? CH₂O] in 2-phenylethanol; (vii) [M ? CH₃CO₂] vs. [M ? CH₂CO] in benzyl acetate; and (viii) [M ? C₄H₉O] vs. [M ? C₄H₇] in n-butylbenzoate. The results are interpreted in terms of k vs. E curves with very different frequency factors for the two reaction types. Appearance potentials of metastable ions for the rearrangement reactions have also been measured.     

Metastable ion studies. 4—mass spectral fragmentation and isomerization in the esters of chlorinated propenoic acids

Metastable peaks have been used to study the fragmentation pathways of the methyl and trideuteriomethyl chloropropenoates and chloromethyl propenoate. The molecular ion peaks of the unsaturated esters are more intense than those of the saturated esters, α-Cleavage, [M? OCH₃]⁺, produces the base peak in almost all compounds, the relative abundances of the additional peaks being low for chloromethyl propenoate. The losses of H₂O, CH₃^. and COOH^. indicate the isomerization of some ionized chloro esters to the chlorinated 2-butenoic acid molecular ions. An intense loss of H₂O observed for methyl 2-chloropropenoate indicates its most facile isomerization, [ester]^+˙ → [acid]^+˙, whereas the isomerization in methyl trichloropropenoate could not be observed. The molecular ion of chloromethyl propenoate, however, also seems to partly rearrange to the chlorinated 3-butenoic acid ion, since the first field free region metastable peak shows a weak loss of CO. The new reaction pathways, i.e. the losses of CHO^˙, CH₂O and CH₂CO from ionized chloromethyl propenoate, were detected.     

Negative ion mass spectrometry—I: The spectra of some stabilised alkylidenetriphenyl-phosphoranes

The 70 eV negative ion mass spectra of a series of phosphoranes exhibit significant peaks for ions resulting from skeletal rearrangements. The primary fragmentation reactions of the molecular anions, which involve reduction of the phosphorus from the penta- to the ter-valent state, have been rationalised in terms of the relative stabilities of the products formed. The processes occurring are in some cases analogous to and in other cases complementary to, those previously observed in the positive ion spectra of similar compounds.     

Synthesis of substituted pyrazolo [1,5 — a] pyrimidines

Condensation of 3-amino-4-cyanopyrazole (1) with ethylacetoacetate, ethyl cyanoacetate, diethyl malonate and acetylacetone afforded pyrazolo[1,5-a]pyrimidine derivatives (2—8a). Other compounds (8b—h) of this ring system were obtained by treatment of 1 with arylidenemalononitrile and ethylarylidenecyanoacetate. And the reaction of compound (1) with activated acetylenes yeilded pyrazolo[1,5-a]pyrimidine derivatives (11a—b).     

Substituent effects in mass spectrometry—III: Substituent effects in the dissociation of the molecular ions of para and meta substituted benzoic acids

The effect of substituents on the electron-impact-induced fragmentation of the molecular ions of para and meta substituted benzoic acids has been examined. The substituent is observed to exert an effect on the ionisation potential of the molecular ion, on the appearance potentials of the primary daughter ions and on the amount of H/D scrambling in the molecular ion of the carboxyl-d₁ analogues prior to the loss of hydroxyl therefrom. The energy of activation for the loss of hydroxyl from the molecular ion is in general dependent upon the nature but not the position of the substituent, while the amount of H/D scrambling in the molecular ion of the carboxyl-d₁ derivative is dependent upon both the nature and the position of the substituent. No correlation of the relative ion abundances with σ⁺ constants was observed. The results are consistent with the molecular ions of each compound having a dissimilar energy distribution, which could arise either by different energy transfers from the electron beam to the molecule or by the participation of different isolated electronic excited states (or similar states but to varying extents) in the dissociation of the molecular ions.     

The reaction of 2‐phenoxy‐3,5‐dinitropyridine with substituted anilines in the presence of 1,4‐diazabicyclo[2.2.2]octane in dimethyl sulfoxide: Kinetic and equilibrium studies

The reactions of 2‐phenoxy‐3,5‐dinitropyridine ( 1 ) with a series of substituted anilines ( 4a–d ) in dimethyl sulfoxide (DMSO) in the presence of 1,4‐diazabicyclo[2.2.2]octane (DABCO) yield the 2‐anilino derivatives without the accumulation of intermediates. The kinetics is compatible with a two‐step reaction involving initial nucleophilic attack followed by either base‐catalyzed or uncatalyzed conversion to the product. The base‐catalyzed pathway is likely to involve rate‐limiting proton transfer from the zwitterionic intermediate to base. The results are compared with those for reactions of 1,3,5‐trinitrobenzene (2) and phenyl 2,4,6‐trinitrophenyl ether ( 3 , R = Ph ) with anilines. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 198–203, 2009     

The negative ion mass spectra of mixtures of sulphur and organic compounds. I—The formation of [M+Sn]− from the mixture of sulphur and dinitriles

The negative ion mass spectra of sulphur, dinitriles and their mixtures were studied. The abundance of negative ions of sulphur was almost comparable to that of positive ions. In the negative ion mass spectrum of a mixture of sulphur and dinitrile, intense and characteristic peaks of [M + S_n]^? (n = 2, 3, 4, etc.) were observed resulting from ion–molecule association of dinitrile with the S_n^? anions. As a standard sample of a negative ion mass spectrum, sulphur itself has proved useful in the lower mass region (below m/e 256: S₈^?).     

Kinetic studies in mass spectrometry—V: Evaluation of wide range electron energy kinetics and metastable ion relative abundance techniques,and the structures of [XC6H4CH2]+ ions generated from substituted benzyl phenyl ethers and toluenes

The problem of ion structure and energy is discussed briefly and attention directed to the existence of two types of structural information. Studies of ion formation reactions bear on the initial structure of the ion, whereas decomposition reactions relate to the ion at a later time, after structural rearrangement has had chance to occur. The desirability of conducting both types of investigation concurrently is emphasized. Of the available experimental methods at this time, kinetic measurements for formation studies and metastablel ion relative abundances for decomposition studies have been utilized, although other techniques may eventually prove preferable in the future. The [M – Y] reaction of a series of substituted benzyl substrates XC₆H₄CH₂Y, where Y = OC₆H₅ and H, has been examined, and the present results are compared with previous work on ion structure and energy in this area, with the aim of evaluating the potential of the newer techniques.     

Negative ion chemical ionization mass spectrometry—binding of molecules to bromide and iodide anions

The analytical potential of negative ion chemical ionization (NICI) mass spectrometry utilizing dibromodifluoro-methane (CF₂Br₂) and iodomethane (CH₃I)/methane (CH₄) as reagent gases is examined. The NICI mass spectrum of CF₂Br₂ contains Br^?, [HBr₂]^? and [CF₂Br₃]^? anions. Weak acids (i.e. those acids with approximately ΔH°_(acid) values between 1674 and 1464 kJ mol^?1) react with Br^? to produce minor yields of the hydrogen?bonded bromide attachment [MH + Br]^? anion or are unreactive. Strong acids (i.e. those acids with approximately ΔH°_(acid) > 1464 kJ mol^?1) produce primarily [MH + Br]^? anions with a minor yield of proton transfer [M ? H]^? anion. The NICI spectrum of CH₃I/CH₄ is dominated by I^?. Weak acids react with I^? to yield minor amounts of [MH + 1]^? or are unreactive. Strong acids produce only [MH + l]^? anions. From a consideration of the gas-phase basicity of the halide anion and the binding energy of the hydrogen-bonded halide attachment adduct, thermochemical data are used as a potential guide to rationalize or predict the ions observed in NICI mass spectra.     

Investigation of Large Polychloride Anions: [Cl11]−, [Cl12]2−, and [Cl13]−

For decades the chemistry of polyhalides was dominated by polyiodides and more recently also by an increasing number of polybromides. However, apart from a few structures containing trichloride anions and a single report on an octachloride dianion, [Cl₈]^2?, polychlorine compounds such as polychloride anions are unknown. Herein, we report on the synthesis and investigation of large polychloride monoanions such as [Cl₁₁]^? found in [AsPh₄][Cl₁₁], [PPh₄][Cl₁₁], and [PNP][Cl₁₁]?Cl₂, and [Cl₁₃]^? obtained in [PNP][Cl₁₃]. The polychloride dianion [Cl₁₂]^2? has been obtained in [NMe₃Ph]₂[Cl₁₂]. The novel compounds have been thoroughly characterized by NMR spectroscopy, single‐crystal Raman spectroscopy, and single‐crystal X‐ray diffraction. The assignment of their spectra is supported by molecular and periodic solid‐state quantum‐chemical calculations.     

Organic mass spectrometry—X. Electron impact fragmentations of alkylthio group in 2-alkylthio-5-aminothiazolo[5,4-d] pyrimidines

In order to discuss hydrogen transfer in the skeletal fragmentation of thioethers on electron impact, mass spectra of a series of 2-n-alkylthio-5-aminothiazolo [5,4-d]pyrimidines have been determined. To aid the interpretation of the hydrogen migration, deuterium-labeled compounds which are substituted with deuterium in each position of 2-n-butylthio-5-aminothiazolo-pyrimidines were studied. By correlation of the spectra obtained from such labeled compounds, the initial hydrogen migration in the fragmentation to produce [M ? SH], [MS ? CH₃] and m/e 184 ions is concluded to be as follows: migration of the α-hydrogen atom to the sulfur induces formation of the [M ? SH] ion; migration of the β-hydrogen atom to the sulfur or nitrogen atom by a McLafferty rearrangement induces formation of the m/e 184 ion; and migration of γ-hydrogen atom to the sulfur induces formation of the [M ? SCH₃] ion.     

Studies in organic mass spectrometry—VI. The fragmentation of enol derivatives of acyclic β-diketones

The mass spectra of enol derivatives of β-diketones, such as enol ethers and enaminies are discussed. Their behaviour under electron-impact is in accordance with the fragmentation we suggested for an acyclic β-diketone in the enol form. Rearrangement of the enol functional group is observed in the spectra. This process is a general one, as it show not only migration of oxygen and nitrogen in the enol ethers and enamines, but also migeration of sulphur and chlorine in the thio-ether and in 4-chloro-3-pentene-2-one. Enol derivatives are suitable compounds for determining the branching of alkyl chains in β-diketones.     

Dithia[3.3 ]phanes. 4—mass spectrometry of some dithia[3.3]-naphthalenophanes1

The mass spectral fragmentation of 23 dithia [3.3]naphthalenophanes is discussed. The most characteristic species for these compounds are the M⁺ ion and ions at m/z 115, 135 (or 136), 141, 155 (or 154) and 185.     

Metastable ion studies. VII—loss of water from the molecular ion of cyclopentanol

Metastable peaks for the loss of water from the molecular ion of cyclopentanol and six ²H labelled analogues have been studied. The results, together with some thermochemical date and a detailed examination of metastable peak shapes, have allowed the identification of the two mechanisms for water loss, both of which take place from the α-cleaved molecular ion. Daughter ion structures are proposed.     

Synthesis of substituted pyrimido[4,5‐b] and [5,4‐c]‐[1,8]naphthyridines

Ethyl 1‐ethyl‐7‐methyl‐4‐oxo‐1,4‐dihydro[1,8]naphthyridine‐3‐carboxylate ( 1 ), precursor of nalidixic acid, has been converted in two steps through ([1,8]naphthyridin‐3‐yl)carbonylguanidine derivatives into substituted pyrimido[4,5‐b] and [5,4‐c][1,8]naphthyridines.     

[M + 11]+˙ and [M + 11]−˙ ions in the nitrogen positive and negative ion chemical ionization mass spectra of aromatic amines

The N₂ negative ion chemical ionization (NICI) mass spectra of aniline, aminonaphthalenes, aminobiphenyls and aminoanthracenes show an unexpected addition appearing at [M + 11]^?˙. This addition is also observed in the N₂ positive chemical ionization (PCI) mass spectra. An ion at [M – 15]^? is found in the NICI spectra of aminoaromatics such as aniline, 1- and 2-aminonaphthalene and 1- and 2-aminoanthracene. Ion formation was studied using labeled reagents, variation of ion source pressure and temperature and examination of ion chromatograms. These experiments indicate that the [M + 11]^?˙, [M – 15]^?˙ and [M + 11]^+˙ ions result from the ionization of analytes altered by surface-assisted reactions. Experiments with ¹⁵N₂, [¹⁵N] aniline, [2,3,4,5,6-²H₅] aniline and [¹³C₆] aniline show that the [M + 11]^?˙ ion corresponds to [M + N – 3H]^?˙. The added nitrogen originates from the N₂ buffer gas and the addition occurs with loss of one ring and two amino group hydrogens. Fragmentation patterns in the N₂ PCI mass spectrum of aniline suggest that the neutral product of the surface-assisted reaction is 1,4-dicyanobuta-1,3-diene. Experiments with diamino-substituted aromatics show analogous reactions resulting in the formation of [M – 4H]^?˙ ions for aromatics with ortho-amino groups. Experiments with methylsubstituted aminoaromatics indicate that unsubstituted sites ortho to the amino group facilitate nitrogen addition, and that methyl groups provide additional sites for nitrogen addition.     

The study and characterization of nucleosides by mass spectrometry—I: The mass spectra of pyrimidine 2,2′-anhydronucleosides and their derivatives

The mass spectra of 2,2′-anhydrouridine, 2,2′-anhydrothymidine and 2,2′-anhydro-4-thiouridine are reported. The acetyl, trifluoroacetyl, trityl, pivaloyl and trimethylsilyl ether derivatives were also studied. Deuterium labeling in acetyl and trimethylsilyl groups aided characterization of many ions in the spectra, as well as helping to clarify hydrogen migration processes. The anhydronucleosides and their derivatives are readily distinguished from natural nucleosides by the presence of an ion containing the base moiety plus the anhydro-ring plus one hydrogen atom from the rest of the molecule. As for natural nucleosides the [base + H]⁺ and [base + 2H]⁺ ions are usually prominent, but in contrast to natural nucleosides, ions characteristic of the sugar moiety do not retain the 2′-oxygen atom (i.e. the oxygen atom of the anhydro-ring). The mass spectra of deuterium labeled derivatives suggest a test for the presence of a 3′-O-acetyl function (the O-acetyl group is lost from the molecular ion much more readily from the 3′- than from the 5′-carbon atom). The trimethylsilyl derivatives showed evidence in their mass spectra for migration of trimethylsilyl groups in addition to hydrogen atoms.