Isobutane Chemical Ionization Mass Spectra Research Articles

Proton transfer via strained transition states in the elimination of alcohols from MH+ ions of stereoisomeric diethers and hydroxy esters upon chemical ionization and collision‐induced dissociation

AbstractThe isobutane chemical ionization (CI) mass spectra of cis‐ and trans‐1,4‐di(alkoxymethyl)cyclohexanes, with a tertiary alkoxy group ROH and a primary group R′OH, are identical, and they exhibit exclusive elimination of the alcohol ROH originating from the tertiary alkoxyl. The high abundance of the [MH − ROH]+ ion and the absence of [MH − R′OH]+ and MH+ ions is unexpected in the case of trans‐diethers, and it suggests proton transfer from the primary alkoxyl OR′ to the tertiary OR group prior to the elimination of ROH, despite the large distance between them in the trans configuration. Various isomerization pathways were explored in order to account for the similar behavior of the cis‐ and trans‐isomers upon chemical ionization. The results show that the hydrogens at positions 1 and 4 are not involved in the elimination of ROH. Methyl substitution at positions 1 and 4 leads to the competitive elimination of ROH and R′OH, indicating suppression of the proton transfer. Methyl substitution at position 1 adjacent to the primary alkoxy group has a minor effect on the chemical ionization behavior of the diethers. On the other hand, methyl substitution at position 4, adjacent to the tertiary alkoxy group, suppresses the proton transfer, (i.e. both [MH − ROH]+ and [MH − R′OH]+ are abundant in the CI mass spectra of the trans‐isomers), indicating an effect of steric hindrance. The results suggest that direct proton transfer via a strained proton‐bound transition‐state occurs, and methyl substitution at positions 1 and 4 can affect the relative stability of the transition‐state structures involved. Similar behavior has been observed in the CI and collision‐induced dissociation spectra of 1,4‐hydroxy ester cyclohexanes. Copyright © 2001 John Wiley & Sons, Ltd.

Intramolecular proton transfers in stereoisomeric gas-phase ions and the kinetic nature of the protonation process upon chemical ionization

The isobutane chemical ionization (CI) mass spectra of cis- and trans-1-butyl-3- and -4-dimethylaminocyclohexanols and of their methyl ethers exhibit abundant [MH - H(2)O](+) and [MH - MeOH](+) ions respectively. On the other hand, only the MH(+) ions of the cis-isomers exhibit significant [MH - H(2)O](+) and [MH - MeOH](+) ions under collision-induced dissociation (CID) conditions. The non-occurrence of water and methanol elimination in the CID spectra of the trans-isomers indicates retention of the external proton at the dimethylamino group in the MH(+) ions that survive after leaving the ion source and the first quadrupole of the triple-stage quadrupole ion separating system, and the trans-orientation of the two basic sites does not allow proton transfer from the dimethylamino group to the hydroxyl or methoxyl. Such transfer is allowed in the cis-amino alcohols and amino ethers via internal hydrogen-bonded (proton-bridged) structures, resulting in the elimination of water and methanol from the surviving MH(+) ions of these particular stereoisomers upon CID. The abundant [MH - ROH](+) ions in the isobutane-CI mass spectra of the trans-isomers indicates protonation at both basic sites, affording two isomeric MH(+) ions in each case, one protonated at the dimethylamino group and the other at the less basic oxygen function. These results show that the isobutane-CI protonation of the amino ethers and amino alcohols is a kinetically controlled process, occurring competitively at both basic sites of the molecules, despite the large difference between their proton affinities ( approximately 25 and approximately 35 kcal mol(-1); 1 kcal = 4.184 kJ). Copyright 1999 John Wiley & Sons, Ltd.

Chemistry and Stereochemistry of Benzyl-Benzyl Interactions in MH+ Ions of Dibenzyl Esters upon Chemical Ionization and Collision-induced Dissociation Conditions

Isobutane chemical ionization mass spectra of dibenzyl esters of a wide variety of aliphatic, olefinic, alicyclic and aromatic dicarboxylic acids exhibit abundant m/z 181 C 14 H + 13 ions, indicating a highly general rearrangement process involving the formation of a new bond between the two benzyl groups. An extensive collision-induced dissociation and deuterium labeling study suggested that these ions are an almost equimolar mixture of isomeric α-o-tolylbenzyl, α-p-tolylhenzyJ and p-benzylbenzyl cation structures, and this composition is identical for all the diesters examined. This structural assignment of the C 14 H 13 + ions suggests a mechanistic pathway for their generation, based on the formation of the new bond between the benzyl methylene group of the protonated benzoxycarb-onyl and the phenyl ring of the other ester moiety via π- (and/or ion-neutral) and α-complexe. Stereoisomeric diesters show an unusual steric effect: trans-isomers give rise to much more abundant C 14 H + 13 ions than the cis counterparts. This behavior is explained by stabilized proton-bridged structures of the MH+ ions of the cis-isomers.

Configurational effects on internal proton transfers and ion–neutral complex formation in stereoisomeric 1,4‐di(alkoxymethyl)cyclohexanes on chemical ionization

AbstractCis‐ and trans‐1,4‐dimethyl‐1‐d3‐methoxymethyl‐4‐(2‐methoxy‐2‐propyl)cyclohexane (3a‐c and 3a‐t), the isomeric 1,4‐dimethyl‐1‐methoxymethyl‐4‐(2‐d3‐methoxy‐2‐propyl)cyclohexanes (3b‐c and 3b‐t) and 1,4‐dimethyl‐1‐methoxymethyl‐4‐(2‐d3‐methoxy‐2‐propyl)cyclohexanes (3b‐c and 3b‐t) and 1,4‐dimethyl‐1‐ethoxymethyl‐4‐(2‐methoxy‐2‐propyl)cyclohexanes (3c‐c and 3c‐t) give rise to different isobutane chemical ionization (CI) mass spectra. The cis isomers exhibit abundant [MHROH]+ ions (100%, ROH originating from the tertiary alkoxy group), while no MH+ or [MHR′OH]+ ions (R′OH originating from the primary alkoxyl) were detected in the mass spectra. This behaviour indicates an efficient proton transfer between the two ether functions in the transient MH+ ions of the cis isomers, resulting in the total and exclusive elimination of methanol from the tertiary position. In contrast to the cis diethers, the trans isomers 3a‐t, 3b‐t and 3c‐t afford relatively abundant [MHROH]+ and [MHR′OH]+ ions. This behaviour is consistent with protonation on each of the two distant non‐interacting ether groups, resulting in two isomeric MH+ ions, each of which eliminates the corresponding alcohol. The highly stereospecific behaviour of the cis and trans diethers 3c and 3t is dependent on the presence of the methyl substituents at positions 1 and 4: identical mass spectra were obtained for cis‐ and trans‐1‐ alkoxymethyl‐4‐(2‐alkoxy‐2‐propyl)cyclohexanes (4‐c and 4‐t) (four pairs with different 1‐ and 4‐alkoxy groups), and both stereoisomers exhibit exclusive elimination of ROH originating from the tertiary methoxy group in each pair. The absence of [MHR′OH]+ and MH+ ions in the isobutane CI mass spectra of 4‐t requires proton transfer from the primary OR′ to the tertiary OR group prior to the elimination of ROH, despite the large distance between them in the trans configuration. These results indicate isomerization of the fragmenting MH+ ions of 4‐t to structures which allow a facile proton transfer from the primary the tertiary alkoxyl. The similarity of the mass spectra of 4‐c and 4‐t suggests trans → cis isomerization, which may occur via an ion–neutral complex.

Positive electron impact and chemical ionization mass spectra of some nitramine nitrates

AbstractThe positive electron impact (EI) and isobutane chemical ionization (CI) mass spectra of six nitramine nitrates were studied with the aid of some accurate mass measurements. In the EI spectra, β fission relative to both the nitramine and nitrate ester is important. In the CI spectra a major ion occurs at [MH – 45]+ and was found to be mainly due to [M + 2H  NO2]+. All of the compounds except N‐(2 hydroxyethyl)‐N‐(2′,4′,6′‐trinitrophenyl)nitramine nitrate gave an [MH]+ ion. The [MH – 45]+ ion in the isobutane CI mass spectra of tetryl is also due to [M + 2H  NO2]+.

Isobutane chemical ionization mass spectra of unsaturated alcohols

AbstractAnalysis of the isobutane chemical ionization mass spectra of hexenols, cyclohexenols and various syn/anti pairs of bicyclic and tricyclic homoallylic alcohols shows that: (i) the spectra of the allylic alcohols are dominated by [M + H – H2O]+ and [M + C4H9–H2O]+ ions and contain traces of [M + H]+ ions; (ii) [M + H]+ ions are prominent in the spectra of acyclic and certain cyclic homoallylic alcohols; and (iii) [M + H]+ ions dominate the spectra of other acyclic unsaturated alcohols. The [M + H]+ ions may result from either: (a) protonation of the hydroxyl group, followed by a very rapid intramolecular proton transfer from the protonated hydroxyl group to the carbon–carbon double bond or internal solvation of the protonated hydroxyl group by the carbon–carbon double bond; and/or (b) direct protonation of the carbon–carbon double bond with significant internal solvation of the resulting carbocation by the hydroxyl group, which may lead to carbon–oxygen bond formation to give a protonated cyclic ether. The consequences of placing various geometric constraints on the possible intramolecular interactions between the hydroxyl group and the carbon–carbon double bond in unsaturated alcohols are explored.

Fused-silica capillary gas chromatography—mass spectrometry of some dicarboxylic acids present in condensation-type polymers : I. Dimethyl esters

Isothermal retention indices for some aliphatic saturated, aliphatic unsaturated, alicyclic and aromatic dicarboxylic acid dimethyl esters on a cross-linked polymethylsiloxane stationary phase at six column temperatures are listed. The characteristic features of their electron impact and methane and isobutane chemical ionization mass spectra and their usefulness for identification purposes are discussed.

Chromatography of monomers : V. Temperature-programmed glass capillary gas chromatographic separation and gas chromatography-mass spectrometric identification of a mixture of C 1C 18 alkyl esters of acrylic and methacrylic acids

The possibilities for the separation of a mixture of aliphatic C 1C 18 alkyl esters of acrylic and methacrylic acids have been studied on glass capillary columns with non-polar (OV-101 and SE-54) and polar (SP-1000) phases under temperature-programmed conditions. The electron impact and methane and isobutane chemical ionization mass spectra of 38 acrylate and methacrylate monomers are reported and their use for identification purposes is discussed.

Electron impact and chemical ionization mass spectra of some tetra‐acetylated anomeric glycosides

AbstractElectron impact and chemical ionization (CH4, iso‐C4H10 and NH3) mass spectra of some tetra‐acetylated anomeric glycosides have been examined with a view to the characterization of anomeric pairs. Minor differences observed in the relative intensities of common ions in the anomeric pairs in the electron impact mass spectra are found to be enhanced in the methane and isobutane chemical ionization mass spectra. In the absence of thermal decomposition, the β‐anomers show greater ion abundances of common glycosyl ions than the α‐anomers. Ammonia chemical ionization mass spectra show complementary behaviour indicating strong adduct ions and practically no fragmentation.

Fused-silica capillary gas chromatography—mass spectrometry of some dicarboxylic acids present in condensation-type polymers : II. Bis(trimethylsilyl) esters

Isothermal retention indices of some dicarboxylic acid bis(trimethylsilyl) esters at four temperatures of a non-polar capillary column with a cross-linked polymethylsiloxane stationary phase are presented. The separation of particular esters is discussed and the chromatographic behaviour of bis(trimethylsilyl) esters is compared with that of dimethyl esters. The characteristic features of electron impact and methane and isobutane chemical ionization mass spectra and their usefulness for the identification of the trimethylsilyl derivatives are discussed.

ChemInform Abstract: TEMPERATURE DEPENDENCE OF THE ISOBUTANE CHEMICAL IONIZATION MASS SPECTRA OF OPEN‐CHAIN AND CYCLIC ALCOHOLS; STRUCTURAL, STEREOCHEMICAL AND MOLECULAR SIZE EFFECTS. A REEVALUATION OF THE ISOBUTANE CHEMICAL IONIZATION OF ALCOHOLS

Chemischer InformationsdienstVolume 14, Issue 31 Physical Organic Chemistry ChemInform Abstract: TEMPERATURE DEPENDENCE OF THE ISOBUTANE CHEMICAL IONIZATION MASS SPECTRA OF OPEN-CHAIN AND CYCLIC ALCOHOLS; STRUCTURAL, STEREOCHEMICAL AND MOLECULAR SIZE EFFECTS. A REEVALUATION OF THE ISOBUTANE CHEMICAL IONIZATION OF ALCOHOLS F. J. WINKLER, Search for more papers by this authorF. O. + GUELACAR, Search for more papers by this authorF. MERMOUD, Search for more papers by this authorA. BUCHS, Search for more papers by this author F. J. WINKLER, Search for more papers by this authorF. O. + GUELACAR, Search for more papers by this authorF. MERMOUD, Search for more papers by this authorA. BUCHS, Search for more papers by this author First published: August 2, 1983 https://doi.org/10.1002/chin.198331070Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume14, Issue31August 2, 1983 RelatedInformation

Formation of 6-dimethylcarbamyloxy-dGuo, 6-dimethylamino-dGuo and 4-dimethylamino-dThd following in vitro reaction of dimethylcarbamyl chloride with calf thymus DNA and 6-diethylcarbamyloxy-dGuo following in vitro reaction of diethylcarbamyl chloride with calf thymus DNA

The rodent carcinogens dimethylcarbamyl chloride (DMCC) and diethylcarbamyl chloride (DECC) react with dGuo (pH 7.0–7.5, 37°C, 4 h) to form the O 6-acyl derivatives 6-dimethylcarbamyloxy-2′-deoxyguanosine (6-DMC-dGuo) and 6-diethylcarbamyloxy-2′-deoxyguanosine (6-DEC-dGuo), respectively. Reaction of DMCC with dThd under identical conditions yielded 4-dimethylamino-thymidine (4-DMA-dThd). Compounds 6-DMC-dGuo and 6-DEC-dGuo undergo a nucleophilic aromatic substitution reaction with dimethylamine (DMA) to form 6-dimethylamino-2′-deoxyguanosine (6-DMA-dGuo) via displacement of the C-6 dialkylcarbamyloxy moiety. The substitution reaction did not take place when diethylamine or NH 3 were substituted for DMA. The structures of the new compounds 6-DMC-dGuo, 6-DEC-dGuo, 4-DMA-dThd and 6-DMA-dGuo were deduced from chemical analyses and syntheses, UV and nuclear magnetic resonance (NMR) spectra and electron impact, isobutane chemical ionization and source insertion isobutane chemical ionization mass spectra. It was postulated that 4-DMA-dThd was formed following reaction of the transient intermediate 4-DMC-dThd with DMA formed by hydrolysis of DMCC. Calf thymus DNA was reacted in vitro with DMCC (pH 7.0–7.5, 37°C, 4 h) and the modified DNA hydrolyzed enzymatically to 2′-deoxynucleosides. Compounds 6-DMC-dGuo, 4-DMA-dThd and 6-DMA-dGuo were identified in the hydrolysate by high-pressure liquid chromatography (HPLC). In an indentical manner 6-DEC-dGuo was identified following in vitro reaction of DECC with calf thymus DNA. Compounds 6-DEC-dGuo and 6-DMC-dGuo possess novel structures with respect to the types of adducts known to be formed between carcinogens and bases in DNA. The implications of these findings with respect to chemical mutagenesis and carcinogenesis is discussed. The structural relationship between N 4-dimethyl-5-methylcytosine (4-dimethylamino-Thy) formed in DNA following in vitro reaction with DMCC and 5-methylcytosine, the only modified base found in vertebrate DNA is noted.

Mass spectra of 1,6:2,3- and 1,6:3,4-dianhydro-β-D-hexopyranose derivatives: Formation of cluster ions assisted by participation of neighbouring acyloxy groups

Chemical ionization and electron impact mass spectra of 1,6:2,3-and 1,6:3,4-dianhydro-β-D-hexopyranose derivatives are reported. The methane and isobutane chemical ionization mass spectra of both positional and configurational isomers differ significantly, while the electron impact mass spectra of stereoisomers are hard to discern. Examination of the isobutane chemical ionization mass spectra indicates that only those isomers containing the acyloxy group and the oxirane ring in the trans position give stable [M+C4H9]+ ions, otherwise these ions are of negligible abundance. This observation is interpreted in terms of participation of the acyloxy group in oxirane ring opening. This feature of the chemical ionization mass spectra can be used for stereochemical assignment.

The isolation and characterization of 3-(2-carboxyethyl)cytosine following in vitro reaction of β-propiolactone with calf thymus DNA

The new adduct 3-(2-carboxyethyl)cytosine (3-CEC) was isolated following in vitro reaction of the carcinogen β-propiolactone (BPL) with calf thymus DNA. The structure of 3-CEC was confirmed by synthesis from BPL and dCyd. Reaction of BPL with cCyd (pH 7.0–7.5, 37°C) gave 3-(2-carboxyethyl)deoxycytidine (3-CEdCyd) (9% yield) and 3, N 4-bis(2-carboxyethyl)deoxycytidine (3, N 4-BCEdCyd) (0.6% yield). 3-CEdCyd and 3, N 4-BCEdCyd were hydrolyzed (1.5 N HC1, 100°C, 2 h) to 3-CEC and 3, N 4-bis(2-carboxyethyl)cytosine (3, N 4-BCEC), respectively. The structure of 3-CEC was assigned on the basis of UV and NMR spectra and the electron impact (EI) mass spectra of 3-CEC and a tri-trimethylsilyl (TMS) derivative of 3 CEC as well as deuterated (d 27) tri-TMS derivative of 3-CEC. The structure of 3, N 4-BCEC was assigned on the basis of UV spectra and the EI mass spectra of a tri-TMS derivative. EI and isobutane chemical ionization mass spectra of 3-methylcytosine (3-MeCyt) and a di-TMS derivative of 3-MeCyt were obtained and were helpful in deducing the structures of 3-CEC and 3, N 4-BCEC. This is the first report of the alkylation by BPL of an exocyclic atom on a base in DNA. Compound 3, N 4-BCEC was not detected in BPL-reacted calf thymus DNA. The relative amounts of 1-(2-carboxyethyl)adenine (1-CEA), 7-(2-carboxyethyl)guanine (7-CEG), 3-(2-carboxyethyl)thymine (3-CET) and 3-CEC isolated from BPL-reacted DNA following perchloric acid hydrolysis were 0.23, 1.00, 0.39 and 0.41 respectively, when the alkylation reaction was conducted in phosphate buffer at 0–5°C and pH 7.5 and 0.10, 1.00, 0.29 and 0.28 respectively when the reaction was conducted in H 2O at 37°C and pH 7.0–7.5.

Chemical ionization and electron impact mass spectra of alicyclic substituted 2‐aryl‐1,3‐dithianes

AbstractThe methane and isobutane chemical ionization mass spectra of alicyclic substituted 2‐aryl‐1,3‐dithianes were examined by gas chromatography mass spectrometry. The protonated molecular ion was found to be of low abundance in the methane spectra, while a protonated cyclic sulfide cation (m/z 107) appeared as the base peak. A protonated molecular ion was the base peak when isobutane was used as the reagent gas. Electron impact mass spectra displayed weak molecular ions and were characterized by the m/z 106 fragment.

Electron impact and chemical ionization mass spectra of catechol derivatives from insect cuticle

Electron impact and isobutane chemical ionization mass spectra of a series of catechol derivatives isolated from hydrolysates of insect cuticle were studied. Electron impact mass spectrometry resulted in sufficient information for the identification of all the compounds. Furthermore, characteristic fragmentation patterns were demonstrated, which allow classification of the catechol derivatives according to the substituent next to the aromatioring of the aliphatic sidechain. By chemical ionization mass spectrometry [M + H]+ions were the base peak in most spectra, but for some compounds [M + H]+ was absent, and [M + H - H2O]+ the base peak. In many of the CI spectra [2M + H]+ ions were present and in one case [3M + H]+. Chemical ionization mass spectrometry was unsuitable for the identification of the catechol derivatives investigated.

Stereochemical effects in the mass spectra of 2‐hydroxy‐, 5‐hydroxy‐ and 2,5‐dihydroxyprotoadamantanes

AbstractElectron ionization and methane and isobutane chemical ionization mass spectra have been obtained for the four configurationally isomeric 2,5‐protoadamantanediols and the corresponding model monoalcohols. Although the electron impact (both 70 and 15 eV) and methane chemical ionization mass spectra of the 2,5‐protoadamantanediols are distinct, the variations are not large and in general they are difficult to rationalize in terms of specific structural features. In contrast, the isobutane chemical ionization mass spectra of these diols show significant differences which can easily be correlated with sterochemical orientations of the substituents.

Isolation of 3-(2-carboxyethyl)thymine following in vitro reaction of β-propiolactone with calf thymus DNA

3-(2-Carboxyethyl)thymine (3-CET) was synthesized from β-propiolactone (BPL) and dThd5′P at pH 9.0–9.5 via the intermediate 3-(2-carboxyethyl)thymidine-5′-monophosphoric acid (3-CEdThd5′P). 3-CEdThd5′P was converted to 3-CET by hydrolysis in 1.5 N HCl at 100°C for 2 h. The structure of 3-CET was assigned on the basis of UV spectra, electron impact (EI) and isobutane chemical ionization mass spectra and the EI mass spectrum of a trimethylsilyl derivative of 3-CET. BPL was reacted in vitro with calf thymus DNA at pH 7.5. 100 A units of BPL-reacted DNA yielded, following perchloric acid hydrolysis and preparative paper chromatography, 3 A units of 3-CET. Reaction of BPL with the phosphodiester thymidylyl-(3′-5′)thymidine gave 3-(2-carboxyethyl)thymidylyl-(3′-5′)-3-(2-carboxyethyl)thymidine (∼3%). Phosphotriester formation was not detected.

In vitro Dimroth rearrangement of 1-(2-carboxyethyl) adenine to N6-(2-carboxyethyl)adenine in single-stranded calf thymus DNA

The new adduct N 6-(2-carboxyethyl)adenine ( N 6-CEA) was prepared from 1-(2-carboxyethyl)adenine (1-CEA) by base catalyzed (Dimroth) rearrangement of 1-CEA. The structure of N 6-CEA was assigned on the basis of UV spectra and electron impact and isobutane chemical ionization mass spectra. When the carcinogen β-propiolactone was reacted in vitro with calf thymus DNA, 1-CEA but not N 6-CEA was detected on paper chromatograms following acid hydrolysis of the DNA. When BPL-reacted single-stranded DNA was incubated at pH 11.7 (37°C, 18 h) prior to acid hydrolysis, it was found that 1-CEA was completely converted to N 6-CEA in DNA by Dimroth rearrangement, whereas no conversion occurred at pH 7.5. The extent of Dimroth rearrangement at various pHs and temperatures was determined for 1-CEA, 1-methyladenine (1-MeA), 1-(2-carboxyethyl)-deoxyadenosine-5′-monophosphoric acid (1-CEdAdo5′P) and the phosphodiester 5′- O-(2-carboxyethyl)phosphono-1-(2-carboxyethyl)deoxyadenosine (1-CE-dAdo-5′-P-CE).

Cardenolide analogues: 10--characterization of cardiac glycosides by chemical ionization mass spectrometry.

The potential of chemical ionization mass spectrometry for the characterization of naturally occurring and semi-synthetic cardiac glycosides has been investigated. Methane, isobutane and ammonia were used as reactant gases. With the exception of ouabain, the ammonia chemical ionization mass spectra of the cardiac glycosides examined in this work contained abundant [M + NH4]+ions and abundant fragment ions formed by cleavage of glycoside bonds. Ammonia chemical ionization mass spectrometry was found to provide a rapid and sensitive method for the characterization of the products of glycosidation reactions. In contrast, the methane and isobutane chemical ionization mass spectra of the cardiac glycosides, with the exception of ouabain, did not contain protonated molecular ions and did not contain abundant fragment ions above m/z 400.