Loss of hydrogen cyanide from monocyanopyridines upon electron impact: Dewar pyridine structures and ring opening—Ring closure reactions revealed by 13C and 15N Labelling

Extensive ¹³C and ¹⁵N labelling has shown that the molecular ions of 2-, 3- and 4-cyanopyridine with lifetimes up to 10^?6 s eliminate hydrogen cyanide originating predominantly from the ring (?65%). Moreover, this hydrogen cyanide loss occurs after an equilibrated positional interchange of the ring carbon atoms at positions interchange of the ring carbon atoms at positions 2, 4 and 6 via Dewar pyridine structures. In molecular ions with lifetimes of 10^?6–10^?5 s skeletal rearrangements have taken place in such a way that both nitrogen atoms have become equivalent prior to the loss of hydrogen cyanide. Arguments are put forward that this equivalence of nitrogen atoms is caused by the intermediacy of ions with a 1,4-dicyanobuta-1,3-diene structure. About 60% of these intermediate ions eliminate hydrogen cyanide in a fast process. The remaining 40% of these ions undergo ring closure again to a pyridine ring in which the carbon atoms of positions 2, 4 and 6 are positionally interchanged rapidly via Dewar pyridine structures followed by ring opening again and eventual loss of hydrogen cyanide. This interpretation of the ¹³C and ¹⁵N labelling results is further corroborated by a study of the loss of hydrogen cyanide from molecular ions of 1,4-dicyanobuta-1,3-diene labelled with ¹³C in both cyano groups.     

An electron impact study of HCN elimination from indole by use of 13C labelling

The study of the loss of HCN from the molecular ions of [2-¹³C]indole and [3-¹³C]indole shows that, to a good approximation, only the two carbon atoms of the pentagonal ring are involved in this fragmentation process, contrary to the behaviour of the H atoms; the C-2 atom is eliminated predominantly, chiefly in the ion source (85–90%) and a little less in the metastable energy range (75–80%). The losses of ¹³CCH₃^˙ and C₂H₃^˙ from the [M? H¹²CN]^+˙ ions of the two compounds suggest the occurrence of different structures, providing evidence for several mechanisms of HCN elimination.     

An electron impact study of HCN elimination from indolizine by use of 13C labelling

The study of the loss of HCN from the molecular ions of [1-¹³C]-, [2-¹³C]- and [3-¹³C]-indolizine shows that, if the C-3 atom is eliminated predominantly, as may be expected, the C-2 atom, and (a) carbon atom(s) of the hexagonal ring are also involved. The losses of ¹³CCH₃^. and C₂H₃^. from the [M? H¹²CN]^+˙ ions of the three compounds point to the interference of distinct mechanisms of HCN elimination, leading to different structures for the [C₇H₆]^+˙ ions.     

Nitrile elimination and hydrogen rearrangement upon electron impact in schiff base peptide esters

A series of Schiff base peptide esters were examined by gas chromatography mass spectrometry. Electron impact induced elimination of aryl nitriles (ArCN) from Schiff bases , where R, R′ …are amino acid sidechains formed from aromatic aldehydes and peptide esters was shown to be associated with formation of C-terminal peptide fragments. Schiff base derivatives prepared from benzaldehyde and α-deuterobenzaldehyde with several peptide esters indicated that aryl nitrile elimination was accompanied by H(D) transfer to the α-C of the N-terminal amino acid residue. Of the many Schiff base peptide esters of different aldehydes studied, HCN elimination of the type observed in some nonpeptide Schiff bases could only be found among N-methyl pyrrolemethylidene derivatives.     

Insight into the structure of silver cyanide from (13)C and (15)N solid-state NMR spectroscopy

The structure of silver cyanide has been investigated by solid-state multinuclear magnetic resonance spectroscopy. Carbon-13 and nitrogen-15 NMR spectra of magic-angle-spinning (MAS) and stationary powder samples of isotopically enriched Ag(13)CN, Ag(13)C(15)N, and AgC(15)N have been acquired at the external applied magnetic field strengths 4.7, 7.05, and 9.4 T. Axially symmetric carbon and nitrogen chemical shift (CS) tensors provide evidence for linearity of the polymeric (-Ag-CN-)(n)() chains. A two-site model is required to successfully simulate the (13)C MAS NMR line shape, which is dominated by indirect nuclear spin-spin coupling between (109/107)Ag and (13)C nuclei. In combination with relativistic zeroth-order regular approximation density functional theory (ZORA-DFT) calculations on model AgCN fragments, the (13)C MAS NMR results show that 30 +/- 10% of the silver sites are disordered, that is, either -NC-Ag-CN- or -CN-Ag-NC-, and 70 +/- 10% of the silver sites are ordered, that is, -NC-Ag-NC-. Effective dipolar coupling data extracted from (13)C NMR spectra of stationary samples allow an upper limit of 1.194 A to be placed on the carbon-nitrogen internuclear distance. After incorporation of the effects of anisotropic indirect nuclear spin-spin coupling and motional averaging on the NMR-derived distance, a corrected value of r(CN) = 1.16 +/- 0.03 A is obtained. This work provides an example of the type of information which may be obtained from solid-state NMR studies of disordered materials and how such information may complement that available from diffraction studies.     

Mass spectrometry of aralkyl compounds with a functional group—XII: Unorthodox elimination of hydrogen cyanide from hydrogen randomized molecular ions of benzylcyanide and of o-, m- and p-cyanobenzylcyanides,revealed by D-, 13C- acid 15N-labelling

Specific D-labelling in the side-chain and in the phenyl ring, ¹³C-labelling in the benzylic position and in the cyano group and ¹⁵N-labelling in the cyano group of benzylcyanide show, that the molecular ion, decomposing in the second field free region, i.e. having a low internal energy, loses hydrogen cyanide with participation of both side-chain carbon atoms (22% benzylic carbon and 78% cyano carbon) after a complete randomization of all hydrogens. This sharply contrasts with the loss of hydrogen cyanide from the hydrogen randomized molecular ion, decomposing in the ion source, where the original cyano group is involved exclusively. The molecular ions of (o, m, p)-cyanobenzylcyanides, decomposing in the ion source as well as in the second field free region, also lose hydrogen cyanide, involving to some extent (6 to 15%) a carbon atom, different from that of the side-chain cyano group, after an extensive randomization of hydrogens as shown by specific D-labelling in several positions and by ¹³C- and ¹⁵N-labelling in the side-chain cyano group. Furthermore, the molecule of hydrogen cyanide, eliminated in the ion source and in the second field free region, appears to contain predominantly the side-chain cyano group (±70%), thus suggesting that few or none of the molecular ions have rearranged to a seven membered ring.     

An extraction system for low-energy hydrogen ions formed by electron impact

A system has been developed for extracting near-zero kinetic energy H⁻ and D⁻ ions formed by dissociative electron attachment. It is the essential part of a new set-up for vibrational spectroscopy of hydrogen molecules. A magnetic field is used to collimate the probing electron beam. Ions produced by electron collision with the target molecules are collected by the combined action of this field and an electrostatic field penetrating into the interaction region. Highly effective extraction is achieved by taking into account the correct out-of plane displacement of ion trajectories which is usually neglected in similar arrangements. The extraction conditions are mass dependent so that by proper tuning, mass selection of detected ions is achieved. The new system is also used for detecting positive ions created by electron collisions with hydrogen atoms and molecules.     

An improved Born approximation for the electron impact ionization of atomic hydrogen

A two-electron continuum wave function satisfying exact asymptotic boundary conditions (see [1]) is simplified in the high energy limit for asymmetric kinematics. The long range correlation between the fast escaping electron and the target atom in a low continuum state is properly taken into account, and corrects therefore the major shortcoming of the ordinary Born approximation. This improved Born approximation has been employed to calculate triply differential ionization cross sections at intermediate energies.     

Hydrogen cyanide elimination and rearrangement upon electron-impact from schiff bases of benzaldehyde and aliphatic amines

The Schiff bases of benzaldehyde and aliphatic amines form immonium ions on electron-impact, which rearrange and eliminate hydrogen cyanide in a different way than the corresponding derivatives of pyridine-2-aldehyde. The presence of at least one hydrogen on the α-carbon atom in the amine moiety is a necessary condition for this rearrangement. A mechanism is suggested for this process.     

Stereospecific alcohol elimination from dialkyl mesaconates under electron impact: Another example of a hidden hydrogen transfer

Under electron impact dimethyl and diethyl mesaconates give rise to abundant [M ? MeOH]⁺ and [M ? EtOH]⁺ ions, respectively. The geometrically isomeric citraconates yield [M ? MeO]⁺ and [M ? EtO]⁺ ions. Mixed methyl ethyl mesaconates eliminate both methanol and ethanol. These findings, together with the results of a deuterium labelling study, indicate that the elimination of alcohol from the molecular ions of the mesaconates is partially preceded by a hidden hydrogen transfer step.     

Diode laser spectroscopy of the fundamental bands of 12C14N, 13C14N, 12C15N, 13C15N free radicals in the ground 2 Sigma+ electronic state

Rotationally resolved spectra of the fundamental band of the CN free radical in four isotopic forms have been measured using tunable diode laser absorption spectroscopy. The source of the radical was a microwave discharge in a mixture of isotopically selected methane and nitrogen diluted with argon. The lines were measured to an accuracy of 5 x 10(-4) cm(-1) and fitted to the formula for the vibration rotation spectrum of a diatomic molecule, including quartic distortion constants. The band origins of each of the isotopomers from the five parameter fits were found to be 12C14N: 2042.42115(38) cm(-1), 13C14N: 2000.08479(23) cm(-1), 12C15N: 2011.25594(25) cm(-1), 13C15N: 1968.22093(33) cm(-1) with one standard deviation from the fit given in parenthesis. Some of the lines showed a resolved splitting due to the spin rotation interaction. This was averaged for fitting purposes. The average equilibrium internuclear distance derived from the upsilon = 0 and 1 rotational constants of the four isotopomers is 1.171800(6) A which is in good agreement with the value determined from microwave spectroscopy.     

Chemical shielding anisotropy of 13C and 15N in acetonitrile

High resolution ¹³C and ¹⁵N NMR spectra have been obtained for powdered CH₃CN. The presence of resolved dipolar structure in the ¹³C spectra permits the conclusion that the symmetry axis of the ¹³C shielding tensor lies along the CN bond direction.     

Investigation of the alkylation of nitroazoles with α-haloketones by13C,15N,and14N NMR

General methods have been worked out for the alkylation of nitroazoles with bromoacetone, bromoacetophenone, and diazoacetone in homogeneous media and by phase-transfer catalysis. The structures of the N-acetonylazoles were established by¹³C,¹⁵N, and¹⁴N high-resolution NMR spectroscopy.Deceased.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1827–1837, August, 1990.     

Synthesis of 13C and 15N labelled (S)-Tryptophan

(R,S)-serine-1-¹³C was incubated in a culture of $\text{Escherichia}$$\text{coli}$ cells to produce (S)-trytophan-1-¹³C. Bromoacetyl bromide-2-¹³C was converted to bromoacetanilide and cyclization of the anilide, followed by reduction and dehydrogenation furnished indole-3-¹³C. Indole-¹⁵N was synthesized by known sequences. These ¹³C and ¹⁵N isotomers of indole were converted by commercially available, lyophilized $\text{E}$. $\text{coli}$ to furnish (S)-tryptophan-γ-¹³C and (S)-tryptophan-indole-¹⁵N, respectively.     

1H, 13C and 15N NMR spectra of ciprofloxacin

1H NMR assignment, including the values of delta(H) and J(H,H) for the cyclopropane moiety, and 13C NMR and 15N NMR spectral data for ciprofloxacin are presented.     

13C and 15N NMR spectra of 4-substituted-1-dimethylphosphonopiperazines

¹³C and ¹⁵N NMR chemical shifts and ¹³C-³¹P and ¹⁵N-³¹P coupling contants are reported for a series of ten 4-substituted-1-dimethylphosphonopiperazines.     

13C and 15N NMR spectra of 2,3-substituted 2H-azirines

The ¹³C and ¹⁵N NMR spectra of a series of 2,3-substituted 2H-azirines have been studied. The ¹⁵N chemical shift for the nitrogen in the azirine ring is found at much higher field than in acyclic imines with a considerable electronic effect for the substituents on the double bond. Cooperative steric and electronic effects associated with substituents on the unsaturated carbon atom of the ring were found to influence the shielding of the ¹³C and ¹⁵N nuclei. Reaction constants have been calculated for 2-alkyl(aryl)-3-phenylazirines. It has been shown that the azirine ring has a powerful —I effect (when compared with the phenyl ring) that exceeds the analogous value for the cyano group.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1181–1183, September, 1986.     

An objective means of diagnosing anorexia nervosa and bulimia nervosa using 15N/14N and 13C/12C ratios in hair

An objective means based on the carbon and nitrogen stable isotope analysis of five hairs per individual is presented for distinguishing between individuals with anorexia nervosa and/or bulimia nervosa from non-clinical individuals (i.e. clinically normal controls). Using discriminant analysis, an algorithm has been developed that provides both sensitivity and specificity of 80% in making diagnoses of individuals with these eating disorders. With further refinements, the results suggest that it may be also possible to distinguish between individuals with anorexia or bulimia. Finally, the study shows the value of conducting blind tests and using larger sample sizes of both control and treatment groups. Both groups are needed to validate the diagnostic value of a method and to provide measures of sensitivity and specificity of any diagnostic test.