Mass spectrometry of thio analogues of pyrimidine bases: Correlation of the intensities of the M+˙ and the [M – ˙SH]+ ions of 2-alkylthiouracils

The mass spectral fragmentations of eight 2-alkylthiouracils with C₁₀, C₁₂, C₁₆ and C₁₈ in the alkylthio group were investigated. Fragmentation pathways are proposed on the basis of accurate mass and matastable transition measurements. The correlation between the intensities of the M⁺˙ and [M - ˙SH]⁺ ions of 36 2-alkylthiouracils is discussed. The data obtained create the basis for distinguishing structural isomers.     

Formation of [M − H]+ ions in electron impact mass spectra of nitrosocarbonyl compounds α- and γ-nitrosocarbonyl compounds

The presence of the [M + H]⁺ ions and the absence of the monomer molecular ions M^+· in the mass spectra of some tertiary α- and γ-nitrosocarbonyl compounds is reported. This effect is caused by the rearrangement of the mobile hydrogen in the α-carbonyl position in the fragmentation pattern of the dimer molecular ions 2M^+·.     

On the CO elimination from [M – CH3]+ ions in substituted phenyl methyl ethers

It was inferred from the collisional activation spectra that CO loss from [M – CH₃]⁺ ions generated from o-,m- and p-cyanoanisole yields a common ion, presumably the cyanocyclopentadienyl cation. A similar product ion is found to be generated in the three dimethoxybenzene isomers. In case of o-dimethoxybenzene loss of CO was also found to occur via an important additional route, which leads to the formation of protonated phenol.     

Mass spectra of aliphatic dicarboxylic acids and their dimethyl esters: Cyclic structures for the [M − H2O]+˙ ions from the diacids and [M − MeOH]+˙ ions from the dimethyl esters

Methyl 2-oxocycIoalkane carboxylate structures are proposed lor the [M ? MeOH]^+˙ ions from dimethyl adipate, pimelate, suberate and azelate. This proposal is based on a comparison of the metastable ion mass spectra and the kinetic energy releases for the major fragmentation reaction of these species with the same data for the molecular ions of authentic cyclic β-keto esters. The mass spectra of α,α,α′,α′-d₄-pimelic acid and its dimethyl ester indicate that the α-hydrogens are involved only to a minor extent in the formation of [M ? ROH]^+˙ and [M ? 2ROH]^+˙ ions, while these α-hydrogens are involved almost exclusively in the loss of ROH from the [M ? RO˙]⁺ ions (R = H or CH₃). The molecules XCO(CH₂)₇COOMe (X = OH, Cl) form abundant ions in their mass spectra with the same structure as the [M ? 2MeOH]^+˙ ions from dimethyl azelate.     

Mass spectrometry of pyridine compounds–IV: The formation and decomposition of the [M – methyl]+ ion [C8H10N]+ from 4-t-butylpyridine in comparison with that of the [M – methyl]+ ion [C9H11]+ from t-butylbenzene,as studied by D- and 13C-labelling

A strong secondary isotope effect is observed in the preferred loss of methyl vs. trideutero-methyl from the molecular ions of appropriately labelled 4-t-butylpyridine and t-butylbenzene decomposing in the first and second field free regions of a double focusing mass spectrometer. This has been rationalised by invoking the theory of radiationless transitions², which can account for the higher population of activated states responsible for loss of methyl vs. that for trideuteromethyl. ¹³C-Labelling at the central carbon atom of the t-butyl group indicates that the [M – methyl]⁺ ions, decomposing further by elimination of ethylene, cannot be represented exclusively by a pyridylated (or phenylated) cyclopropane ion if present at all. It is concluded that ions with structures generated by 1,2-hydrogen-, 1,2-pyridyl- (or 1,2-phenyl-) and 1,2-methyl shifts must also play a role. D-labelling further shows an extensive randomisation of side-chain hydrogen atoms in the [M-methyl]⁺ ions of 4-t-butylbenzene; in this case, however, the expelled ethylene also contains ring hydrogen atoms (≤2). Presumably this is caused by exchange between the side-chain and ortho-hydrogen atoms in the initially generated phenyldimethylcarbinyl carbenium ion.     

Collision-induced decompositions of [M – H]− and [M + Li]+ ions from fast atom bombardment of dinucleoside phenylphosphonates

The collision-induced decompositions of the [M – H]^? and [M + Li]⁺ ions of a few dinucleoside phenylphosphonates were studied using fast atom bombardment and linked scanning at constant B/E. Deprotonation takes place on the base or sugar moieties. The [M – H]^? ion decomposes mainly by cleavage on either side of the phosphonate linkage, leading to the formation of mononucleotide fragment ions and also by cleavage of the basesugar bond. Rupture of the 3′-phosphonate bond is preferred. Unlike the normal charged nucleotides, these neutral nucleotides do not eliminate a neutral base from the [M – H]^? ion. However, the mononucleotide fragment ions which can have the charge on the phosphorus oxygen eliminate neutral bases by charge-remote fragmentation. The 4,4′-dimethoxytrityl (DMT)-protected nucleotides show the additional fragmentation of loss of DMT. Li⁺ attachment can occur at several sites in the molecule. As observed for the [M – H]^? ion, the major cleavage occurs on either side of the phosphonate bond in the fully deprotected nucleotides, cleavage of the ester bond on C(3′) being preferred. Cleavage of the 5′-phosphonate bond is not observed in the DMT-protected nucleotides. Many of the fragmentations observed can be explained as arising from charge-remote reactions.     

Theoretical study in [C2H4–Tl]+ and [C2H2–Tl]+ complexes

We studied the attraction between [C₂H_n] and Tl(I) in the hypothetical [C₂H_n–Tl]⁺ complexes (n = 2,4) using ab initio methodology. We found that the changes around the equilibrium distance C–Tl and in the interaction energies are sensitive to the electron correlation potential. We evaluated these effects using several levels of theory, including Hartree–Fock (HF), second‐order Møller–Plesset (MP2), MP4, coupled cluster singles and doubles CCSD(T), and local density approximation augmented by nonlocal corrections for exchange and correlation due to Becke and Perdew (LDA/BP). The obtained interaction energies differences at the equilibrium distance R_e (C–Tl) range from 33 and 46 kJ/mol at the different levels used. These results indicate that the interaction between olefinic systems and Tl(I) are a real minimum on the potential energy surfaces (PES). We can predict that these new complexes are viable for synthesizing. At long distances, the behavior of the [C₂H_n]–Tl⁺ interaction may be related mainly to charge‐induced dipole and dispersion terms, both involving the individual properties of the olefinic π‐system and thallium ion. However, the charge‐induced dipole term (R^?4) is found as the principal contribution in the stability at long and short distances. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Mass spectrometry of onium compounds. XXIII—ionisation potentials in the structural assignment of [M – N2] ions from diazo-oxides

The ionisation potential shows that fulven-6one is formed pyrolytically from o-benzo-diazo-oxide. Appearance potential measurements show increasing pyrolytic contribution to the [M – N₂] species with increase in ion source temperature.     

Consecutive reactions arising from the molecular ions of 14α-equilenin and 14β-isoequilenin

It is shown that two configurational steroidal isomers, 14α-equilenin and 14β-isoequilenin, may be easily distinguished by the unimolecular mass analysed ion kinetic energy spectra of their corresponding molecular ions. Furthermore, many of the peaks in these spectra are found to be due to consecutive reactions, whereby product ions of an initial reaction are themselves reactive and can undergo further fragmentation. In each of these cases, the individual steps of the reaction sequence have been separated in the different field free regions of a reversed geometry, double focusing mass spectrometer. Isotopic labelling with ¹⁸O in the 17-position has also helped to clarify reaction sequences.     

Rearrangement of molecular ions following electron impact: IV—Origin of the [M—28]+˙ ions in the mass spectra of α-methyl-2-decalone

The transposition of the molecular ions (ring contraction) of 2-decalones is demonstrated by a study of the [M–28]⁺˙ peak and its homologue in labelled products using ionization and appearance energy measurements, and mass analysed ion kinetic energy and collision induced dissociation spectra.     

New helerocyclic ring systems from α-llydroxymethylenekelones. VIII. Polyuzasteroidal analogues of equilenin

Some 6, 15,16-triazasteroidal analogues of Equilenin carrying a 7-methyl substituent have been synthesized from 4-methyl-7-methoxy-10,11]-dihydroisoxazolo[5,4-i]phenanthridine (III).     

Mass spectrometry and organic analysis—XII αβ-Unsaturated secondary alcohols

The fact that some αβ-unsaturated alcohols give mass spectra that bear some similarity with the corresponding saturated ketones is discussed, and the double hydrogen transfer necessary to accommodate this is examined by deuterium labelling.     

MNDO study of fragmentations in mass spectrometry. Part I. The geometry and electron structure of [C3H6O] + ˙ systems and [C2H3O]+ ions

MNDO calculations of [C₃H₆O]^{+ ˙} predict the parallel existence of both structures of radical cations of acetone (1) and propen-2-ol (2) in electron ionization spectra. The calculated heats of formation of 1^{+ ˙} (ΔH_f^MNDO = 783.2 kj mol^?1) and of 2^{+ ˙} (ΔH_f^MNDO = 649.8 kJ mol^?1) are in very good agreement with the experimental results. A comparison with the results of ab initio calculations (3–21 G and 6–31 G) and experimental data for the individual structures of the main fragment [C₂H₃O]⁺ demonstrates a sufficient accuracy of MNDO results, suggesting the possibility of applying the method also in other cationic systems, especially in larger ones.     

Mass spectrometry characterization of 3‐OH butyrated β‐cyclodextrin

Oligo(3‐OH butyrate)‐β‐cyclodextrin esters (PHB‐CD) were obtained through ring opening of β‐butyrolactone (β‐BL) in the presence of β‐cyclodextrin (CD) and (‐)‐sparteine (SP) as nucleophilic activator. The resulted reaction mixture was first separated in two fractions and then investigated through a deep mass spectrometry (MS) study performed on a liquid chromatography‐electrospray ionization‐quadrupole time of flight (LC‐ESI‐QTOF) instrument. LC MS and tandem MS structural assignment of the reaction products was completed by NMR. The performed analysis revealed that poly(3‐OH butyrate) homopolymers (PHB) are formed together with the PHB‐CD products. Secondary reactions resulting in the formation of crotonates were also proved to occur. A comparison between MS and NMR results demonstrated that more than one PHB oligomer is attached to the CD in the PHB‐CD product. The tandem MS fragmentation studies validated the proposed structure of CD derivatives. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

A mass spectrometry/mass spectrometry investigation of the nature of [C10H10]+˙, [C9H7]+ and [C10H8]+˙ gas phase ions

Additional evidence for the rearrangement of the 1- and 3-phenylcyclobutene radical cations, their corresponding ring-opened 1,3-butadiene ions and 1,2-dihydronaphthalene radical cations to methylindenetype ions has been obtained for the decomposing ions by mass analysed ion kinetic energy spectroscopy (MIKES). The nature of the [C₉H₇]⁺ and [C₁₀H₈]^+˙ daughter ions arising from the electron ionization induced fragmentation of these [C₁₀H₁₀]^+˙ precursors has been investigated by collisionally activated dissociation (CAD), collisional ionization and ion kinetic energy spectroscopy. The [C₉H₇]⁺ produced from the various C₁₀H₁₀ hydrocarbons are of identical structure or an identical mixture of interconverting structures. These ions are similar in nature to the [C₉H₇]⁺ generated from indene by low energy electron ionization. The [C₁₀H₈]^+˙ ions also possess a common structure, which is presumably that of the maphthalene radical cation.     

Mass spectrometry in the investigation of β-lysine-containing peptides

Mass spectrometry can be used for amino acid sequence determination in β-lysine-containing peptides and for the identification of α- and β-lysine residues in peptides, as well as for amide bond type determination in β-lysine peptides. The peptide bond in streptothricin D is shown to be formed through the participation of an ε-amino group of the L-β-lysine residue.