Stereoisomeric effects on mass spectra—I. A Review

The unimolecular reactions that give rise to mass spectra are controlled by spatial relationships and energy considerations. In molecules that contain a heteroatom, elimination reactions, involving bond-making as well as breaking, are often prominent, e.g. loss of water from alcohols. The ease of such reactions depends on spatial relationships in the molecule, and the resultant ion intensities in the spectra of geometric isomers can consequently be correlated with differences in geometry and hence furnish a basis for assigning structures. Processes that do not involve bond-making do not have such rigorous geometric requirements, but depend rather on attainment of a transition state defined in terms of a minimum energy content. Common product ions from stereoisomeric hydrocarbons seem often to arise via a common transition state. When this condition holds, the difference between the enthalpies of the isomers is reflected in the relative appearance potentials and–though the cause-and-effect relationship here is less direct and more readily obscured by other factors–relative intensities of the common product derived from the isomers. In both classes of processes, the spectra of stereoisomers can be simplified and made more distinctive by lowering source temperature and ionizing voltage.     

The elimination of small neutral molecules and fragments in mass spectra—I. Substituted salicylic acids and their derivatives

The mass spectra of various ring-substituted salicylic acids and derivatives thereof have been examined. Fragmentation pathways for derivatives of salicylic acid; 3-NO₂ salicylic acid; 5-I salicylic acid; 3-NO₂, 5-I salicylic acid; and 3-NH₂, 5-I salicylic acid are suggested. The major dissociations of these compounds are by elimination of the elements of neutral molecules, e.g. water, methanol, ketene (from the acetylated compounds) carbon monoxide, nitric oxide and nitrogen dioxide, leading to (iodo) phenoxy and (iodo) quinone type ion structures. These ions further decompose to yield the corresponding substituted cyclopentadienyl and cyclopropylium ions.     

Stereochemical effects in mass spectra—Studies on mass spectra of epimers of podophyllotoxin and its derivatives

Mass spectra of thirteen epimers of podophyllotoxin and its derivatives have been examined. By means of high resolution mass spectrometry, metastable ion scanning and isotopelabeling techniques, the effects of configuration of C₁-OH and condensed lactone ring on the fragmentation pattern and the relative abundances of characteristic ions in the mass spectra of these compounds have been studied and rationalized.     

Tautomerism of azine derivatives. 6. Tautomerism of 2-pyrimidinylmethane derivatives

The existence of a pyrimidinyl-pyrimidinylidene tautomeric equilibrium in solutions of 2-pyrimidinylcyanoacetic acid esters in CDCl₃ was observed. Unsymmetrically substituted compounds form two types of ylidene tautomers that differ with respect to the position of the NH proton, the ratio between which is controlled by the substituents in the 4 (6) position. The introduction of both donor and acceptor substituents into the 5 position of the pyrimidine ring increases the amount of the pyrimidine form. The same thing occurs when the polarity of the solvent is decreased. The addition of DMSO or DMF to CDCl₃ leads to conversion of the intrachelate ylidene tautomers to unchelated tautomers. Protonation (CF₃COOH) shifts the equilibrium to favor the ylidene tautomer that has higher basicity.See [1] for communication 5.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 827–831, June, 1984.     

Tautomerism of azine derivatives

The tautomeric properties of 4-pyrimidinylmethanes were studied in the case of 2-CH₃- and 2-CF₃-4-pyrimidinylcyanoacetic esters, 2-CH₃-4-pyrimidinylnitromethane, and 4-pyrimidinylnitromethane. It was shown by ¹H and ¹³C NMR spectroscopy that an equilibrium with the participation of three tautomeric forms — pyrimidine form A and pyrimidinylidene forms B and C with o- and p-quinoid orientations of the double bonds in the heteroring — may be realized in aprotic dipolar solvents (dimethyl sulfoxide).See [1, 2] for communications I and II.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1544–1548, November, 1978.     

Tautomerism and 1H and 13C NMR assignment of methyl derivatives of 9-hydroxyphenalenone

9-Hydroxyphenalenone is a planar multicyclic beta-keto-enol that demonstrates C2V symmetry on the NMR timescale. Off-axis substitution breaks the molecular symmetry and results in tautomers. 1H and 13C NMR assignments were made for 9-hydroxyphenalenone and three methyl derivatives, and the solution-phase tautomers were determined.     

Stereoisomeric effects on mass spectra—II: The isomeric methyldecalins

Mass spectra of the four stereoisomeric 1-methyldecalins and the four 2-methyldecalins have been measured. For each set of stereoisomers, the order of relative intensity of the parent-less-CH₃ ion, but not of the parent ion, parallels the order of relative stability of the molecules. The correlation suggests that loss of the methyl group from the members of each set of stereoisomers leads to a common product, with the differing conformational energies converting into differences in vibrational energy, which in turn lead to differing extents of further decomposition.     

19F spectra—structure correlations of fluorinated quaternary phosphonium salts

The ¹⁹F NMR chemical shifts and geminal FCP and FCH coupling constants are reported for a number of fluorinated quaternary phosphonium salts of the type \documentclass{article}\pagestyle{empty}\begin{document}$ [{\rm R}_3 \mathop {\rm P}\limits^ \oplus {\rm CFXY}]{\rm Z}^ \ominus $\end{document}, where generally R = alkyl or aryl; X, Y = hydrogen or halogen and Z = halogen. Spectral parameters remain relatively unaffected upon variance of R, Z^?, and/or solvent, except when R is a bulky alkyl group. Linear relationships accurately describe the correlation between the summation of the electronegativities of X and Y, or simply of Y if X = F, and the ¹⁹F NMR chemical shift and geminal FCP coupling constant; geminal FCH coupling constants show no such dependence. These relationships are of demonstrated analytical value, particularly for the purpose of spectral prediction.     

Identification of 4,5-asymmetrically substituted 1-(N-isoimido)-1,2,3-triazoles by benzene induced shifts in NMR spectra—VII

Proton chemical shifts, as well as solvent shifts induced by benzene, can be used for the structure determination of 4,5-asymmetrically substituted 1-(N-isoimido)-1,2,3-triazoles, derived from the oxidation of α-diketone bisaroylhydrazones. The geometry of the postulated ‘collision complex’ is discussed and an attempt is made to estimate theoretically the induced shifts of some of the protons.     

Tautomerism of derivatives of azines. 16. Tautomerism of acylmethylpyrazines and -quinoxalines

The tautomeric equilibria of acylmethylpyrazines and -quinoxalines in chloroform were studied by ¹H, ¹⁴N, and ¹⁷O NMR spectroscopy. It was shown that keto-enol tautomerism is realized in the acylmethylpyrazine series. Annelation leads to the development of an ylidene tautomer in the acylmethylquinoxaline series. A marked dependence of the position of the intrachelate equilibrium on the character of the solvent was observed.See [1] for Communication 15.Deceased.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1663–1667, December, 1987.     

Tautomerism in malonic ester derivatives of s-triazine

Pmr and uv spectra of tris-, bis- and monomalonic ester derivatives of s-triazine were studied in chloroform, dioxane, methanol, water, acetonitrile, dimethylsulfoxide, dimethylformamide, and cyclohexylamine. The results indicate that the enaminic structures (I, II and III) predominate in chloroform, dioxane, methanol and water. A small amount of enolate is present in acetonitrile and a larger amount of enolate is present in dimethylsulfoxide, dimethylformamide and cyclohexylamine in the case of the tris- and bismalonic ester derivatives of s-triazine. The conclusion is reached that for these latter compounds in basic solvent, a tautomeric equilibrium exists between the enaminic form and resonance-stabilized enolate ions.     

NMR and UV studies of 4-thio-2'-deoxyuridine and its derivatives

5-Substituted-4-thio-2'-deoxyuridine nucleosides have been chemically synthesized and studied by NMR and UV spectroscopy. The results have been analyzed and discussed in connection with the previous data. The imino proton signal and the carbon signal of the thiocarbonyl group in the 5-substituted-4-thio-2'-deoxyuridines were found to be at much lower field, offering a potential for monitoring these modified bases at the DNA level. All 4-thionucleosides have strong absorptions at around 340 nm and consequently would be useful as potential UVA-induced anticancer agents.     

Lanthanide-induced shifts in proton NMR spectra—IV: Praseodymium-induced shifts to lower applied fields and obscured chemical shifts from lanthanide-induced shift (LIS) data

Praseodymium(III) tris(dipivalomethanato) [Pr(DPM)₃] reduces the complex overlapping aromatic absorptions in the proton NMR spectrum of a monosubstituted naphthalene, cis-3-(1-naphthyl)-1,3,5,5-tetramethlcyclohexan-1-ol( 1 ) to a virtually first-order pattern, whereas Eu(DPM)₃ at the same and higher molar concentrations does not completely resolve the aromatic proton signals. Plots of the Pr(DPM)₃-induced shifts measured in carbon tetrachloride solution versus molar equivalents of Pr(DPM)₃ added may be extrapolated to provide accurate chemical shifts in the absence of added lanthanide shift reagent (LSR). The qualitative conformational conclusions from the Pr(DPM)₃ shifts agree with those from the Eu(DPM)₃-induced shifts, but there are detailed differences in the relative lanthanide-induced shifts (LIS) for different proton types in the molecule.     

13C NMR spectroscopy of cyclopropane derivatives. 1—monocyclic compounds

The ¹³C NMR data of an appreciable number of cyclopropane derivatives have been collected. Most of the spectra were recorded by ourselvesand some were taken from the Literature. With a view to furthering the useof ¹³C NMR spectroscopy as a diagnostic tool in this field, we havemeasured not only the chemical shifts,δ, but also the coupling constants ¹J(CH). It is shown that both embody valuable structural information.     

Pyrophthalone and its derivatives as ligands—I. Mode of coordination—formation of isomeric iminopyrophthalone complexes

The complex formation of Co(II), Ni(II), Zn(II) with aminopyrophthalone was studied by means of electronic, IR, EPR and ¹H-NMR spectroscopy. It was established that with Co(II) and Ni(II) two isomeric complexes are formed in contrast to Zn(II) and Cu(II) complex formation. On the basis of the spectral data obtained the coordination mode and structure of the complex species are assumed.     

29Si and 13C NMR studies of organosilicon chemistry. V—Sugar derivatives

²⁹Si spectra have been obtained for four trimethylsilylated hexopyranoside sugars, and the resonance due to the CH₂OSiMe₃ group at position 5 has been assigned by experiments involving gated decoupling at a single ¹H frequency.     

The elimination of small neutral molecules and fragments in mass spectra—II: 2-substituted 8-hydroxyquinolines

The mass spectra of a series of 2-substituted 8-hydroxyquinolines and related compounds are reported. Fragmentation pathways based upon interpretation of spectra and metastable transitions for 8-quinolinol; 2-methyl-8-quinolinol; 4-methyl-8quinolinol; 2-hydroxymethyl-8-quinolinol; 2-aminomethyl-8-quinolinol; 8-quinolinol-2-carboxaldehyde; 8-quinolinol-2-carb-oxaldehyde oxime; and 8-quinolinol-2-carboxaldehyde tert-butylimine are discussed, as are dissociations in the related compounds 2-hydroxymethyl quinoline and quinoline-2-carboxaldehyde oxime. It is suggested that the molecular ions of 2-substituted-8-quinolinols undergo rearrangement to a substituted azatropylium ion prior to dissociation. An ion of m/e 89 has been observed in these spectra and is considered to be characteristic of the quinoline nucleus bearing an X-substituent in any position, where X ? OR, SR, NH₂ etc. (R ? H, CH₃, etc).