Jerzy W. Wiench

The tetrazole–azide tautomerism of some nitrotetrazolo [1,5-a]pyridines studied by NMR, IR spectroscopy and molecular orbital calculations

Abstract The 6-nitro- and 8-nitro- groups in the tetrazolo[1,5- a ]pyridine molecule exhibit completely different influences on the tetrazole–azide equilibrium. Introduction of the methyl-, nitro-, azido groups or a bromine atom in positions 5, 6, 7 or 8 of the nitrotetrazolopyridine produce changes in the equilibrium constants. Based on the IR spectra taken in the solid state, the tetrazole structure was assigned for almost all the compounds studied. Only one of them, 2,6-diazido-3-nitropyridine, exists in the diazido-form in the solid. The 1 H, 13 C, 15 N, and 17 O NMR spectral parameters (coupling constants, chemical shifts) as well as ab initio molecular orbital calculations were used to describe the tetrazole–azide tautomerism in solutions. The differences in the NMR parameters between the neutral compound (6,8-dinitrotetrazolo[1,5- a ]pyridine) and its σ-adducts are also included as data for distinguishing between both molecules.

Structure and protonation of some indolizine derivatives studied by ab initio MO calculations

Abstract Some gauge invariant atomic orbitals-coupled-perturbed Hartree–Fock (GIAO-CPHF) calculations were performed for seven indolizine derivatives and their monoprotonated forms. Chemical shift, molecular geometry, and charge distribution data are reported for each molecule. The calculations support the results of nuclear magnetic resonance (NMR) spectroscopy measurements showing that protonation occurs preferentially at N1. The good agreement between the calculated and observed 13C and 15N chemical shifts show that such calculations can be used for chemical shift assignment purposes. Cation structures and probable sites for electrophilic reaction or second protonation are also discussed.

An NMR study and ab initio molecular orbital calculation of substituted benzofuroxans and the salt of 4,6-dinitrobenzofuroxan

Abstract 13 C, 15 N and 17 O NMR data are reported for a series of substituted benzofuroxans in aprotic and acidic solutions and for a potassium salt of a substituted benzofuroxan. Some of the title compounds can exhibit fast furoxan valence equilibrium at room temperature regardless of a solvent used, whereas for the others no evidence of above-mentioned process exists. The NMR parameters most sensitive to salt formation are the chemical shifts of the C7, N1, N3 and all of the oxygen nuclei. Hence these are reported as the most satisfactory chemical shifts to be used in distinguishing between the salt and non-ionic forms of the substituted benzofuroxans studied. Calculated energies at the self-consistent field (SCF) level of theory for both tautomeric forms (N1- and N3-oxide) of some compounds studied are used for predicting the tautomeric equilibrium constants. Absolute 17 O shieldings are employed in the reversal of the assignments of 17 O NMR signals existing in the literature.

Alkylation of some substituted tetrazolo[1,5-a]pyridines studied by 1H, 13C, 15N NMR and ab initio molecular orbital calculations

Abstract 1 H, 13 C and 15 N NMR parameters and ab initio molecular orbital calculations were used to characterise some alkylated and arylated tetrazolo[1,5- a ]pyridines. We have found that tetrazolo[1,5- a ]pyridine ( 1 ) and its substituted derivatives undergo alkylation and this reaction results in mixtures of the N1- and N2-alkyl compounds ( a , c and b , d ) in different ratios, depending on the position and nature of substituents. NMR spectral parameters for N3-aryltetrazolo[1,5- a ]pyridine ( e ) obtained by cyclization of 2-pyridyl-aryltriazenes are also presented.

A multinuclear NMR investigation of some benzotriazoles in solution and the solid state

Abstract Four potentially tautomeric benzotriazoles were studied in solution both before and following protonation, and in the solid state using 1H, 13C and 15N NMR measurements. Ab intio GIAO-CHF shielding calculations were performed to assist in 1H, 13C and 15N signal assignments. At room temperature rapid prototropic exchange is observed in solutions of all four compounds. Protonation by trifluoroacetic acid occurs at N-3 (N-1) in all cases. In the solid samples only non-exchanging stable structures were observed on the NMR time scale. In two cases non-exchanging stable structures were observed for solutions at temperatures below 230 K.

Nitrogen NMR study of some mesoionic oxadiazoles and thiadiazoles

15N chemical shifts are reported for 10 mesoionic oxadiazoles and thiadiazoles. Some supporting 14N and 13C NMR data are also reported, together with some ab initio molecular orbital calculations and x‐ray diffraction data. The relation between compound structure and 15N chemical shifts is discussed. 14N NMR measurements and ab initio molecular orbital calculations are employed to identify the charge distributions within the molecules studied. Some 1J(C4,C5), 2J(15N,13C) and 1J(15N,13C) spin coupling data for mesoionic oxadiazoles, thiadiazoles and acetylosydnonimines are given. X‐ray diffraction data for the picrate of acetylsydnonimine and a 3,1,2,‐thiadiazole are reported. The bond lengths within the mesoionic backbone are intermediate between the values for single and double bonds, suggesting a conjugated bond system. The arrangement of the exocyclic group observed in the solid state for acetylosydnonimine corresponds to the arrangement predicted by solution NMR studies. Copyright

Multinuclear Magnetic Resonance Study of Some Mesoionic 1,2,3‐Triazoles and Related Compounds

1H, 13C, 14N and 15N NMR results are reported for some mesoionic 1,2,3‐triazoles and related compounds. It is demonstrated that 14N NMR linewidths are dependent on the charge present on the nitrogen atom. In addition, it is shown that nitrogen NMR is of great value in the determination of the site of protonation, the ability to distinguish readily between mesoionic cyclic and non‐cyclic structures and to follow the transformation between such structures.

NMR study of some sydnones, isosydnones, and isothiosydnones

Multinuclear1H,13C,14N,15N, and17O NMR data are presented for some sydnones, isosydnones and isothiosydnones. The type of valence tautomerism shown in (Fig. 1) is not observed for the compounds studied. At high pH compounds2 and12 are found to undergo transformations. The more suitable NMR parameters are reported for establishing the structures of mesoionic compounds containing three heteroatoms in the five-membered conjugated ring. Someab initio GIAO calculations on a model structure of sydnones and related compounds have been performed.

Two amidine derivatives studied by 1H, 13C, 14N, 15N NMR and GIAO-CHF calculations

1 H, 13C, 14N, and 15N NMR data are presented for two amidine derivatives and their complexes with TFA in 1:1 and 1∶2 molar ratios. The protonation site at the N5 atom was established based upon the observed coupling constants, NOE interactions, and chemical shift change. The GIAO-CHF calculations were also performed for the free bases and their N5 cations.

An NMR study of hydrogen bonding in some azo dyestuffs

Abstract The 1 H, 13 C, and 15 N NMR data reported for compounds 1 – 4 show that in DMSO solutions all of them exist in the azo form only and do not participate in the azo–hydrazoimine equilibrium. The NMR data for compounds 1 and 2 show the presence of a weak hydrogen bond for the non-protonated forms, between N10 and the 2-N H CH 3 proton. All compounds have also been studied in TFA solutions in which they are protonated. The site of protonation of 1 , 2 and 3 is determined to be at N10 in TFA solutions. These results are supported by some ab initio GIAO-CHF molecular orbital calculations.