13C NMR and ESR spectroscopic investigations on tert-butyl-substituted cyclopentadienones

¹³C NMR spectra of 11 di-, tri- and tetra-tert-butyl-substituted cyclopentadienones are discussed with respect to charge distribution and substituent effects. The chemical shifts of the unsubstituted cyclopentadienone, calculated from substituent increments, are in good agreement with published CNDO calculations and can be rationalized by the inductive effect of the carbonyl group. In addition, the ESR signals shown by some of the cyclopentadienones are described, and possible reasons for their appearance are discussed.     

13C NMR and force field investigations of hydrindane conformations

¹³C NMR shifts of trans- and cis-annelated bicyclo[4.3.0]nonanes with substituents R in position 8 (R ? H, OH, Cl, Br) and 1-hydroxy derivatives were analysed on the basis of force field calculated torsional angles using Allinger's MM1 program. Shielding increments for the 6 membered ring agree with corresponding cyclohexane values within ± 0.8 ppm maximal deviation. ¹³C NMR line shape analysis with cis-hydrindane between 148 and 180 K yielded ΔH* = 37.0 ± 0.4 kJ mol^?1 and ΔS* = 28 J mol^?1 K^?1 for the topomerization. The force field calculated reaction profile showed ΔH* = 37 kJ mol^?1, in close agreement.     

13C CPMAS NMR investigations of cellulose polymorphs in different pulps

In order to obtain information about the crystallinity and polymorphs of cellulose, and the occurrence of hemicelluloses in pulp fibers, wood cellulose, bacterial cellulose, cotton linters, viscose, and celluloses in different pulps were investigated by solid state ¹³C CPMAS NMR spectroscopy. A mixed softwood kraft pulp and a dissolving-grade pulp were treated under strongly alkaline and acidic conditions and the effect on cellulose crystallinity was studied. The presence of different crystalline polymorphs of cellulose and the amounts of hemicelluloses are considered.     

77Se, 13C and 1H NMR investigations on ortho-carbonyl benzeneselenenyl derivatives

o-Carbonyl benzeneselenenyl compounds with COCH₃, CHO and COOCH₃ as carbonyl functions and SeCl, SeBr, SeSCN, SeSeCN, SeCN and SeCH₃ as selenium-containing groups, have been studied by ¹H, ¹³C and ⁷⁷Se NMR spectroscopy. The IR CO stretching frequencies of these compounds are also reported. If the SeCH₃ derivatives are excluded, the compounds mainly adopt a planar ‘cis’ conformation, due to an interaction between the CO group and the selenium atom. The range of over 800 ppm for the observed ⁷⁷Se chemical shifts makes ⁷⁷Se NMR spectroscopy a powerful tool for physical organic chemists.     

13C NMR investigations of molecular order of rod-like,bent-core,and thiophene mesogens

In this review, methods to obtain the orientational order of topologically variant molecular mesogens using by one- and two-dimensional (2D) solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy are described. Besides ¹³C chemical shifts, the ¹³C─¹H dipolar couplings measured from 2D-separated local field (SLF) technique are used for computing the order parameters of a variety of mesogens. The investigated molecules are composed of a variable number of rings in the core, that is, core ranging from simply one ring to five rings. Among the mesogens investigated, a special focus has been placed on mesogens with thiophene rings, which are gaining popularity as liquid crystalline organic semiconductors. The replacement of a phenyl ring by thiophene in the core has a dramatic influence on molecular topology, as observed from the measured order parameters. The review highlights the advantages of the 2D SLF method for understanding the local dynamics and for mapping the topology of mesogens through the measured order parameters. SLF NMR studies of as many as 24 molecular mesogens that vary in terms of the molecular structure as well as topology are covered in the review. Order parameters of the rings have been estimated from the ¹³C─¹H dipolar couplings in the nematic, smectic A, smectic C, and tilted hexatic phases as well as in B₁ and B₂ mesophases of various mesogens. It is anticipated that, in the years to come, the 2D SLF method would provide advanced molecular information on structurally complex mesogens that are emerging in liquid crystal science through the incessant efforts of synthetic chemists. The mini review covers the orientational order of topologically variant molecular mesogens determined by 1D and 2D solid-state ¹³C NMR spectroscopy. Accordingly, rod-like, bent-core, and thiophene mesogens were subjected to 2D SLF measurements to get the order parameters from which the topology was established. The replacement of phenyl ring by thiophene and its influence on order parameters as well as on molecular topology is also discussed.     

Derivatives of pyrazinecarboxylic acid: 1H, 13C and 15N NMR spectroscopic investigations

NMR spectroscopic studies are undertaken with derivatives of 2‐pyrazinecarboxylic acid. Complete and unambiguous assignment of chemical shifts (¹H, ¹³C, ¹⁵N) and coupling constants (¹H,¹H; ¹³C,¹H; ¹⁵N,¹H) is achieved by combined application of various 1D and 2D NMR spectroscopic techniques. Unequivocal mapping of ¹³C,¹H spin coupling constants is accomplished by 2D (δ,J) long‐range INEPT spectra with selective excitation. Phenomena such as the tautomerism of 3‐hydroxy‐2‐pyrazinecarboxylic acid are discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

Applications of1H and13C NMR spectroscopy in structural investigations of Vinca indole alkaloids

This review considers the laws connecting the parameters of the¹H and¹³C NMR spectra with the structure of the substances and the use of these laws for solving structural and stereochemical problems of theVinca indole alkaloids and other compounds of closely related structure. For each type of alkaloids, characteristic features of the PMR and¹³C NMR spectra are given that permit the structures of similar bases to be established and their stereochemical identification to be performed.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 3–15, January–February, 1986.     

13C NMR of indazoles

The carbon-13 chemical shifts and some selected coupling constants of 183 indazoles are reported. The main conclusions of the original references are briefly summarized.Published in Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1159–1179, September, 1995.     

1H and 13C NMR investigations of inclusion complexes between β-cyclodextrin and naphthalenediamines/phenol derivatives

Three inclusion complexes between β-CD and 1,5-naphthalenediamine, 1,8-naphthalenediamine, ethyl p-hydroxylbenzoate are synthesized and identified via ¹H and ¹³C NMR spectra, respectively. The possible conformations of the inclusion complexes are depicted.     

13C NMR spectra of alkenylsilanes

Conclusions The¹³C NMR spectra of 12 alkenylsilanes were measured and the values of the increments of the chemical shifts of the (CH₃)₃Si group were determined for some allylsilanes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 938–939, April, 1978.     

13C NMR spectra of S-methylquinolines

The ¹³C chemical shifts and the increments () of the SCH₃ group in the NMR spectra of S-methylquinolines were measured. It is shown that the chemical shifts of S-methylquinolines correlate satisfactorily with the total charges on the carbon atoms calculated within the CNDO/2 approximation. The coupling of the SCH₃ group with the quinoline ring was examined on the basis of the values.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1665–1671, December, 1979.The authors thank É. É. Liepin' for his discussion of the results.Original article submitted October 1, 1978.     

13C NMR studies of methylnucleosides

The ¹³C chemical shifts of 22 methylated ribonucleosides and deoxyribonucleosides have been measured and assigned. The chemical shift differences between methylated and unmodified nucleosides are correlated with their characteristic modifications of structure. The significance of the chemical shift and tautomeric variations is discussed.     

X-ray crystallographic and (13)C NMR investigations of the effects of electron-withdrawing groups on a series of pyrroles

A series of pyrroles substituted with various electron-withdrawing groups (EWGs) on the N atom have been synthesized and full characterization including X-ray crystal structures obtained. Analysis of (13)C chemical shifts and X-ray crystal structures reveals that a trend between decreased aromaticity and the strength of the EWG exists. Experimental results regarding alternative mechanisms of nucleophilic substitution reactions can thus be rationalized.     

1H and 13C NMR studies on 1,3,2-dioxarsolanes

The ¹H and ¹³C NMR spectra of a series of 1,3,2-dioxarsolanes have been obtained at 2.1 T and some at 9.4 T. The chemical shifts and spin-spin coupling constants have been obtained from complete spectral analyses of the ¹H and proton-coupled ¹³C spectra. The spectral data are interpreted on the basis of two rapidly interconverting half-chair conformers with a pseudoaxial substituent at arsenic. Unique assignment of syn/anti or cis/trans geometries have been made from ¹H or ¹³C NMR spectroscopy alone. The syn and trans isomers of the 4-methyl- and 4,5-dimethyl-1,3,2-dioxarsolanes, respectively, appear to be conformationally biased towards the forms with pseudoequatorial methyl groups. The general trends in the geminal and vicinal ¹H? ¹H and ¹³C? ¹H coupling constants are interpreted in terms of stereospecific, electronegativity and lone-pair effects of the oxygen heteroatoms and conformational factors. The NMR data on the 1,3,2-dioxarsolanes are discussed with reference to related 1,3-dithoxa- and 1,3-dithia- five-membered rings with As, P, S or C at the 2-position.