Unexpected structural integrity of gas phase isoquinoline cations that eliminate HCN

¹³C labelling has been used to study isoquinoline molecular ions undergoing breakdown by HCN elimination in a mass spectrometer. For otherwise stable ions caused to fragment by collisional activation, there is no skeletal rearrangement prior to HCN loss. Of the ions formed by 70 eV electron impact, 69% of those which fragment in the ion source by HCN loss retain their structural integrity, as do 44% of the metastable ions. Of the ions that eliminate HCN without prior arrangement, approximately two-thirds eliminate C-1 and one-third eliminate C-3. Critical energies are reported for the elimination of HCN from pyridine and isoquinoline molecular ions.     

Mass-spectrometric behavior of isomeric nitro- and nitroaminoindolizines and indoles

Peaks of [M — NO]⁺ and [M — NO₂]⁺ ions are characteristic for the mass spectra of nitroindolizines, whereas peaks of ions of the indole type, viz., [M — HCN]⁺ and [M- H,- HCN]⁺ (for alkylindoles), are not characteristic. In the mass spectra of nitroindoles the latter ions give more intense peaks, while the loss of a nitro group or its rearrangement is a considerably less significant process. When a dialkylamino group is introduced in the nitroindolizine molecule, the primary processes in the fragmentation of such compounds are due to fragmentation of the alkylamino group.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 765–768, June, 1982.     

Mass spectra of the aminoisoquinolines and 5-amino-15N-isoquinoline

The mass spectra of the isomeric aminoisoquinolines and 5-amino-¹⁵N-isoquinoline are reported. In the aminoisoquinolines, the major fragmentation pathway was the loss of two molecules of HCN and a hydrogen atom to give the m/e 89 fragment ion. Two additional pathways culminating with this ion were observed. The mass spectrum of 5-amino-¹⁵N-isoquinoline showed a preference of 4 to 1 for loss of HC¹⁵N (benzenoid amine group) over HC¹⁴N (heterocyclic nitrogen) from the molecular ion.     

Preference factors and isotope effects in the loss of H. (D.) and HCN (DCN) from deuterated pyrazoles upon electron-impact

The mass spectra of deuterated pyrazoles show that loss of H^. and of HCN from the molecular ion occurs with a very high specificity from the 3(5)-position. For the two processes isotope effects and preference factors have been determined. Metastable ion decompositions involving the loss of HCN from the [M - H] -fragment indicate that the identity of the hydrogen atoms in this fragment is lost to a large extent.     

Mass spectral fragmentation pattern of 2,2′-bipyridyls. Part VIII. 2,2′-Bipyridyl-5-carboxylic Acid and 2,2′-bipyridyl-5-sulphonic acid

The mass spectra of 2,2′-bipyridyl-5-carboxylic acid and 2,2′-bipyridyl-5-sulphonic acid obtained by electron impact are described. The principal initial fragmentation routes from the molecular ion of the carboxylic acid involve loss of CO, CN^˙, HCN, CO₂, OH^˙ and H₂O. From the molecular ion of the sulphonic acid the principal fragmentations are accompanied by loss of HCN, O₃, SO₂ and SO₃.     

Heterocyclic studies—XXII. Mass spectra of some 3-hydroxypteridin-4-ones

Mass spectra of 3-hydroxypteridin-4-one and five of its mono-and di-methyl derivatives are recorded. Fragmentation of the non-methyl compound (I; R¹  R²  R³  H) occurred mainly by successive losses of NO, CO, HCN and HCN. Changes in m/e values of the major peaks with methyl-substitution pattern in the derivatives were consistent with the proposed fragmentation pattern. Several minor fragmentation pathways were also observed but all involved an initial loss from the oxygen containing ring.     

Mass spectra of some aminonaphthyridines and aminoquinolines

The mass spectra of all the aminoquinolines, the 2–, 3– and 4-amino-1,5-naphthyridines, some amino-1,6-naphthyridines, and two amino-1,8-naphthyridines with methyl substituents are reported. The major fragment in the aminoquinolines is formed by the loss of HCN from the molecular ion. The most abundant fragment in the aminonaphthyridines is formed by the loss of HCN from the molecular ion except in the 2-amino-1,5-naphthyridine isomer. In both 1,8-naphthyridine isomers investigated, the loss of C₂H₂ is an alternate fragmentation pathway of significance. In all of the compounds investigated, the loss of the primary amino group from the molecular ion was found to be an insignificant fragmentation.     

Fragmentation behaviour and ring expansion of 1-methylimidazole and 1-methylpyrazole upon electron-impact

The relative losses of unlabelled vs. labelled HCN from the [M]⁺˙ and [M – 1]⁺ ions of a number of specifically labelled 1-methylimidazoles (I) and 1-methylpyrazoles (II) have been determined. Hydrogen randomisation in the molecular ions prior to fragmentation is insignificant. Expulsion of HCN follows two distinct pathways: elimination involving positions 2 and 3 (predominant in I) and elimination involving the methyl group and the nitrogen atom at position 1 (predominant in II). The molecular ions eject H˙ from the methyl groups to a high degree of specificity. In both cases some contribution by position 5 is observed. The resultant [M – 1]⁺ ions exhibit extensive, but incomplete hydrogen randomisation. Loss of HCN from these ions is consistent with intermediacy of ring-expanded ions, but notably in II a proportion of the HCN is generated from the group. A mechanism for this observation is presented.     

Density-functional calculations of HCN adsorption on the pristine and Si-doped graphynes

Graphyne, a lattice of benzene rings connected by acetylene bonds, is one-atom-thick planar sheet of sp- and sp²-bonded carbons differing from the hybridization of graphene (considered as pure sp²). Here, HCN adsorption on the pristine and Si-doped graphynes was studied using density-functional calculations in terms of geometric, energetic, and electronic properties. It was found that HCN molecule is weakly adsorbed on the pristine graphyne and slightly affects its electronic properties. While, Si-doped graphyne shows high reactivity toward HCN, and, in the most favorable state, the calculated adsorption energy is about ?10.1 kcal/mol. The graphyne, in which sp-carbon was substituted by Si atom, is more favorable for HCN adsorption in comparison with sp²-carbon. It was shown that the electronic properties of Si-doped graphyne are strongly sensitive to the presence of HCN molecule and therefore it may be used in sensor devices.     

Bridgehead nitrogen heterocycles—II: The mass spectra of some pyrazolo[1,5-a]pyridines

Pyrazolo[1,5-a]pyridine undergoes fragmentation upon electron-impact by loss of HCN or C₂H₂N·. In the 2,3-dimethyl derivative loss of HCN and CH₃CN were both observed from the [M – 1]⁺ ion and, in the 3-methyl-2-phenyl derivative, loss of PhCN occurred from the corresponding [M – 1]⁺ ion. Loss of methyl and phenyl radicals was also observed. In 3-acetyl-2-methyl and 3-benzoyl-2-phenyl derivatives, characteristic losses of the CH₃CO· and PhCO· groups were noted. The introduction of a 7-methyl substituent into the six-membered ring had little effect on the fragmentation pattern.     

The electron-impact-induced fragmentation of acridine

Studies of both high and low resolution spectra, and of metastable decompositions occurring in both the first and second field-free regions of the mass spectrometer have led to a postulated scheme for the fragmentation of acridine under electron-impact. There is no specific loss of label from either [9-²H₁]acridine or [4,5-²H₂]acridine in any fragmentation, nor is there any total scrambling of label in either molecular ion prior to loss of HCN. There is certainly some degree of scrambling preceding HCN loss from [M]⁺˙ at 70 eV, but this does not involve the 9-H to any detectable extent. There is no strong evidence for the acridine molecular ion having the same structure as that of four other C₁₃H₉N isomers.     

Fragmentation D'azoles Sous L'impact Electronique. V—Isomerisation Du Benzimidazole Avant Degradation

The mass spectrum of benzimidazole is investigated using deuterium and carbon-13 labelling. The [M – HCN]⁺· ion is the result of two competitive reaction. Isomerisation into a cyanoaniline structure which eliminates HCN from the amino group and also from the nitrile group (by an ortho effect) is confirmed by ions abundance ratios. Hydrogen scrambling does not occur to a significant extent in the ions under consideration.     

Mass spectrometry of nitrogen heterocycles:XI—Loss of HCN from indazole studied by 15N labelling

A small preference for the N-1 nitrogen atom is observed in the loss of HCN from the molecular ion of ¹⁵N labelled indazole. In addition there is a small isotope effect.     

Generation and characterization of ionic and neutral methylene isothiocyanate by a combined tandem mass spectrometry and computational study

The heterocumulene, methyleneisothiocyanate ion, CH₂?N?C?S⁺ (1a⁺), is generated by the dissociative electron ionization of 2‐mercaptoimidazole. This conclusion follows from tandem mass spectrometry experiments and theoretical calculations at the B3LYP/6‐311G** and G2/G2(MP2) levels. The calculations predict that 1a⁺ is separated by high energy barriers from its isomers CHNCHS (1b⁺), CHNCSH (1d⁺), CNCHSH (1e⁺) and CHNHCS (1f⁺). The low energy metastable ions 1a⁺ dissociate by loss of HCN via the pathway 1a⁺ → 1b⁺ → HCS⁺ + HCN. Neutralization‐reionization experiments confirm the theoretical prediction that the hitherto unknown heterocumulene CH₂?N?C?S ^. is a stable species in the rarefied gas phase. Copyright © 2004 John Wiley & Sons, Ltd.     

Unearthing the Mechanism of Prebiotic Nitrile Bond Reduction in Hydrogen Cyanide through a Curious Association of Two Molecular Radical Anions

HCN is clearly associated with the prebiotic chemical evolution of life. It has been known for decades that the radiolysis of HCN solutions produces sugars, amino acids and nucleobases. Remarkably, recent experimental studies have shown that the photolytic reduction of aqueous HCN by a photoredox reagent [Cu(CN)₃]^2? specifically yields sugars, which are the essential building blocks of RNA. Although a mechanistic understanding of such reductions with solvated electrons is poor, the general consensus is that they involve neutral free radicals. We show herein through the use of electronic structure studies and molecular simulations that the reduction of the nitrile bond of HCN is initiated through the formation of a molecular dipole‐bound anion from the photoredox reagent. Our theoretical studies show how HCN binds to the photoexcited reagent and then extracts an electron from the reagent and is ultimately detached as a dipole‐bound anion. The dipole‐bound anionic form of [HCN]^? can easily convert into a solvated valence‐bound form of [HCN]^?. After the formation of solvated [HCN]^?, an extraordinary chemical event ensues through a counter‐intuitive coupling of two valence‐bound anions to form a solvated molecular dianionic intermediate, [HCN]₂^2?. Finally, a proton‐coupled electron transfer occurs within the dianionic entity to complete the reduction. This mechanistic scenario is applicable to the reduction of other prebiotic nitrile species and avoids neutral radical‐based pathways, thereby preventing the proliferation of reactive species and preserving chemical selectivity. Furthermore, we show how such similar nitrile reduction pathways operate to yield the sugar precursors.     

Salts of HCN‐Cyanide Aggregates: [CN(HCN)2]− and [CN(HCN)3]−

Although pure hydrogen cyanide can spontaneously polymerize or even explode, when initiated by small amounts of bases (e.g. CN^?), the reaction of liquid HCN with [WCC]CN (WCC=weakly coordinating cation=Ph₄P, Ph₃PNPPh₃=PNP) was investigated. Depending on the cation, it was possible to extract salts containing the formal dihydrogen tricyanide [CN(HCN)₂]^? and trihydrogen tetracyanide ions [CN(HCN)₃]^? from liquid HCN when a fast crystallization was carried out at low temperatures. X‐ray structure elucidation revealed hydrogen‐bridged linear [CN(HCN)₂]^? and Y‐shaped [CN(HCN)₃]^? molecular ions in the crystal. Both anions can be considered members of highly labile cyanide‐HCN solvates of the type [CN(HCN)_n]^? (n=1, 2, 3 …) as well as formal polypseudohalide ions.     

Mass spectrometry and structure of ions of heterocyclic compounds activated by collision: Naphthiridines and benzazines

The dissociation and the structure at the isomeric [M — HCN]⁺ and [M — 2HCN]⁺ ions, formed during the fragmentation of naphthiridines and benzazines, were investigated by the collisionally activated dissociation (CAD) method. It was established that the stable [M — HCN]⁺ ions of 1,5- and 1,8-naphthiridines, 1,6-naphthiridine, quinoxaline, and quinazoline have different structures. The [M — 2HCN]⁺ ions can exist in two isomeric forms, one of which is characteristic of naphthiridines and the other of benzazines.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vo. 25, No. 5, pp. 626–629, September–October, 1989.     

The mass spectra of cinnolines and their N-oxides

The mass spectra of cinnoline and various alkyl derivatives, and their 1- and 2-mono-oxides and 1,2-dioxides and a quaternary salt have been investigated. The spectra are interpreted in the light of experiments with deutero and ¹⁵N labelled derivatives whose syntheses are described. The major fragmentation path for cinnoline is loss of nitrogen molecule while that for its homologues is loss of nitrogen plus a hydrogen atom in some instances. Various initial fragmentations occur with the N-oxides, and deoxygenation appears to be the dominant process; in both 4-methylcinnoline 1-oxide and 1,2-dioxide an important path is formation of a 3-methyl-anthranil cation by loss of HCN and HCNO respectively.     

Formation of CN(B2Σ) by the electron impact dissociation of HCN. measurements near threshold

Emission spectra of the CN violet band system (B²Σ—X²Σ) were observed by the electron impact on HCN with several impact energies near the threshold. The formation of CN(B) by the dissociative excitation of HCN was investigated. The threshold energy agreed essentially with that obtained by the photodissociation measurements by Okabe et al. The excitation function and the dependence of the vibrational populations of CN(B) on the electron energy were obtained. These results suggest that an optically allowed state contributes to the formation of CN(B) from HCN as a main precursor.     

Fragmentation d'azoles sous l'impact electronique VII –N-ethylbenzimidazoles

Carbon-13 and deuterium labelling experiments show that the [M ? CH₃]⁺ ion observed in the mass spectrum of l-ethylbenzimidazole rearranges quantitatively to a quinoxalinium structure prior to HCN loss.