Signs of 1H, 19F spin—spin couplings in pentafluorobenzaldehyde

The signs of the spin–spin coupling constants between the aldehydic proton and the fluorine nuclei in pentafluorobenzaldehyde are negative over four and six formal bonds and positive over five bonds. The opposite signs had been assumed in previous studies of this molecule. The correct signs are necessary for the determination of the conformational and substituent dependence of these coupling parameters.     

Theoretical study of bifurcated hydrogen bonding effects on the 1J(N,H), 1hJ(N,H), 2hJ(N,N) couplings and 1H, 15N shieldings in model pyrroles

According to the density functional theory calculations, the X···H···N (X?N, O) intramolecular bifurcated (three‐centered) hydrogen bond with one hydrogen donor and two hydrogen acceptors causes a significant decrease of the ^1hJ(N,H) and ^2hJ(N,N) coupling constants across the N? H···N hydrogen bond and an increase of the ¹J(N,H) coupling constant across the N? H covalent bond in the 2,5‐disubsituted pyrroles. This occurs due to a weakening of the N? H···N hydrogen bridge resulting in a lengthening of the N···H distance and a decrease of the hydrogen bond angle at the bifurcated hydrogen bond formation. The gauge‐independent atomic orbital calculations of the shielding constants suggest that a weakening of the N? H···N hydrogen bridge in case of the three‐centered hydrogen bond yields a shielding of the bridge proton and deshielding of the acceptor nitrogen atom. The atoms‐in‐molecules analysis shows that an attenuation of the ^1hJ(N,H) and ^2hJ(N,N) couplings in the compounds with bifurcated hydrogen bond is connected with a decrease of the electron density ρ_H···N at the hydrogen bond critical point and Laplacian of this electron density ?²ρ_H···N. The natural bond orbital analysis suggests that the additional N? H···X interaction partly inhibits the charge transfer from the nitrogen lone pair to the σ*_{N? H} antibonding orbital across hydrogen bond weakening of the ^1hJ(N,H) and ^2hJ(N,N) trans‐hydrogen bond couplings through Fermi‐contact mechanism. An increase of the nitrogen s‐character percentage of the N? H bond in consequence of the bifurcated hydrogen bonding leads to an increase of the ¹J(N,H) coupling constant across the N? H covalent bond and deshielding of the hydrogen donor nitrogen atom. Copyright © 2010 John Wiley & Sons, Ltd.     

Ground state 14N quadrupole couplings in the microwave spectra of N,N'-dimethylaniline and 4,4'-dimethylaminobenzonitrile

Microwave spectra of N,N'-dimethylaniline and 4,4'-dimethylaminobenzonitrile have been recorded in a pulsed supersonic jet using chirped pulse techniques. Experimental substitution structures have been determined for both molecules by detection of the spectra of all (13)C and (15)N isotopomers in natural abundance using a broadband spectrometer. Additionally, a narrowband spectrometer has been used to reveal the (14)N quadrupole splittings at high resolution, from which the orbital occupancy numbers of the amino- and cyano-nitrogen atoms have been determined. An apparent direct relationship between these values and the barriers to inversion of the amino groups is discussed.     

Spin–spin couplings in doubly 15N labelled urea and 1,3-dimethylurea

The ¹H n.m.r. spectra of doubly ¹⁵N labelled urea and 1,3-dimethylurea have been analysed with the aid of computer simulation. ¹H? ¹H, ¹H? ¹⁵N, ¹⁵N? ¹⁵N and ¹³C? ¹⁵N (from ¹³C Ft spectra) couplings are reported.     

Magnetic relaxation characteristics of Gd(III) in N,N-dimethylformamide according to 1H and 14N NMR data

The solvation of Gd³⁺ is studied in N,N-dimethylformamide by ¹H and ¹⁴N NMR in the temperature range of 298–373 K.     

Total synthesis of N14-desacetoxytubulysin H

[reaction: see text] The N14-desacetoxy analogue of tubulysin H was prepared in 20 steps and 2.1% overall yield. Our strategy features a thiazole anion addition to assemble the tubuvaline residue at the C(10)-C(11) bond, as well as acylations at N5, N14, and N17. This iterative coupling approach, as well as the removal of the labile N,O-acetal at N14, enables the synthesis of analogues for detailed studies of structure-activity relationships in this family of potent tubulin disrupters.     

Synthesis,1H,13C,14N,and15N NMR spectra,and cathodic behavior of N-chloro-1,2,4-triazoles

Syntheses are reported for N-chloro-1,2,4-triazoles and the NMR spectra of these compounds were studied. An investigation was also carried out on a platinum electrode in 0.1 N Bu₄ClO₄ in acetonitrile. The reduction proceeds with the transfer of two electrons and the formation of triazole and chloride ions. The half-wave potentials of the reduction of N-chlorotriazoles correlates well with the pK_a values of the corresponding NH-acids. The electrochemical reduction of N-chlorotriazoles is accompanied by the reaction of N-chlorotriazole with the chloride ion formed on the cathode, which leads to the NH form of 1,2,4-triazole through a series of chemical steps.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2814–2821, December, 1990.     

Differentiation between isomeric structures of benzazoles by 14N, 13C and 1H NMR

Nitrogen chemical shifts constitute an effective means of distinguishing between isomeric benzazole ring systems. There is a large difference in nitrogen shielding, usually more than 20 ppm, between isomeric benzenoid- and quinonoid-like structures. ¹³C and ¹H NMR is shown to provide unambiguous structure assignment only in cases of symmetric quinonoid ring systems of azoles.     

Substituent effects on 13C-15N spin couplings and 13C chemical shifts in benzenediazonium ions

Similar trends are observed for ¹J(¹³C-¹⁵N) values for a series of aryldiazonium tetrafluoroborates and comparable anilinium fluorosulfonates. Contributions from resonance structures involving double-bond character in the CN bond are judged to be relatively unimportant in the ground state structure of aryldiazonium ions.     

Measurement of long-range 1H-1H dipolar couplings in weakly aligned proteins

Measurement of 1H-1H dipolar couplings in macromolecules, weakly oriented by a dilute liquid crystalline medium, is generally limited to the largest such interactions. By removing dipolar couplings to nearest neighbors, either by decoupling, deuteration, or both, more remote interactions become accessible. The approach is demonstrated for measurement of amide-amide interactions in the proteins calmodulin and ubiquitin and permits observation of direct dipolar couplings between protons up to 7 A apart. Quantitative evaluation of 1H-1H dipolar couplings measured in ubiquitin shows excellent agreement with its solution structure.     

13C, 1H spin coupling constants of dimethylacetylene

¹³C, ¹H spin coupling constants of dimethylacetylene have been determined by the complete analysis of the proton coupled ¹³C NMR spectrum. For the methyl carbon ¹J(CH) = + 130.6₄ Hz and ⁴J(CH) = + 1.5₈ Hz, and for the acetylenic carbon ²J(CH) = ? 10.34 Hz and ³J(CH) = +4.30 Hz. The ⁵J(HH) long-range coupling constant (+2.79 Hz) between the methyl protons was also determined.