bluuebunny/crossref_metadata_2025_split huggingface dataset:
Standard values of vicinal coupling constants in the assignment of relative configurations and conformational analysis of acyclic chiral carbinols and hydrocarbons
Authors: Ibarra C. Alvarez , Romo M. T. Garcia , Perez M. S. Arias 
Published: May 2005

AbstractStandard values of vicinal coupling constants are proposed in order to assign relative configuratations and analyse the conformational distribution of acyclic chiral carbinols and hydrocarbons. Thus, the assignment of relative configurations has been achieved for the diastereomeric carbinols, (2R4R, 2S4S)‐ and (2R4S, 2S4R)‐5,5‐dimethyl‐4‐phenyl‐2‐hexanol and (1R3R, 1S3S)‐ and (1R3S, 1S3R)‐1‐mesityl‐4,4‐dimethyl‐3‐phenyl‐1‐pentanol, through the analysis of the respective vicinal coupling constants and the corresponding conformational distribution. The analysis of the vicinal coupling constants allows the establishment of the monoconformational character of the (2R4S, 2S4R) diastereomer in the first case and the (1R3R, 1S3S) diastereomer in the second case.
Conformational and long‐range low field steric deuterium isotope effects on carbon chemical shifts
Authors: Pehk T. , Laht A. , Lippmaa E. 
Published: May 2005

AbstractComparison of the room and low temperature (203 K) 13C NMR spectra of 3‐cis‐bicyclo[4.4.0]decanone and its 2,2,4,4‐tetradeuterio isotopomer provides information about the deuterium isotope effect on the conformational equilibrium in this compound. Four‐bond low field deuterium isotope effects on some carbon chemical shifts are observed. These effects can be used for unambiguous assignment of the 13C lines to carbon atoms in alicyclic compounds.
Carbon‐13 NMR studies of 1‐aryl‐3‐phenyl‐2‐thioxo‐4‐imidazolidinones. Conformational isomerism and substituent effects
Authors: Colebrook Lawrence D. , Khadim Mohammad A. 
Published: May 2005

AbstractCharacteristic 13C chemical shift ranges and substituent shifts of heterocyclic ring carbon atoms have been identified for a number of 1‐aryl‐3‐phenyl‐2‐thioxo‐4‐imidazolidinones. 13CNMR spectra may be used to detect slow internal rotation about the aryl CN‐1 bond in compounds with diastereomeric rotational isomers; many corresponding carbon atoms in the rotamers have distinctly different chemical shifts. The δ‐effects originating from aryl ortho substituents are both electronic and steric in origin.
Carbon‐13 NMR studies of naturally occurring maytansinoids
Authors: Wallace William A. , Sneden Albert T. 
Published: May 2005

AbstractThe 13C NMR spectra of the antileukemic ansa macrolide maytansine and eight naturally occurring homologues were recorded. Signals were assigned to specific carbon atoms of the ansa macrolide ring and C‐3 ester side chain using data from off‐resonance decoupling, single frequency decoupling, and 180°–τ–907° double pulse experiments. The assignment of each carbon is discussed for maytansine and tabulated for the other eight maytansinoids.
Analysis and applications of 13C NMR lanthanide induced shifts of 1,4‐benzodiazepines
Authors: Paul H.‐H. , Sapper H. , Lohmann W. , Kalinowski H.‐O. 
Published: May 2005

AbstractLanthanide induced shifts using Yb(fod)3 and Pr(fod)3 were used for the signal assignments of the 13C spectra of the 1,4‐benzodiazepines diazepam, desmethyldiazepam, prazepam and flurazepam, and for characterizing their solution conformation with respect to the substituents at N‐1 and C‐5. The phenyl substituent at C‐5 is found to be nearly coplanar with the π‐bond between C‐5 and N‐4, and the substituents at N‐1 seem to be orientated towards the carbonyl group with dihedral angles of either 60° or 120° between the bonds N‐1 and C‐2 and C‐12 and C‐13. The metal ions seem to bind to O‐2 with metal‐oxygen distances between 0.19 and 0.26 nm and bond angles between 117° and 167°. Contact shifts induced by Pr(fod)3 are slightly larger than obtained for Yb(fod)3, whereas the latter reagent causes a stronger broadening.
1H and 13C NMR studies on 1‐methoxy‐endo‐tetracyclo[6.3.0.02,11.03,7]undec‐9‐ene
Authors: Jans A. W. H. , Lugtenburg J. , Cornelisse J. , Kruk C. 
Published: May 2005

AbstractHigh field NMR spectroscopy, including nuclear Overhauser effect experiments and homonuclear decoupling techniques, provide conclusive evidence for the structure of 1‐methoxy‐endo‐tetracyclo[6.3.0.02,11.03,7]undec‐9‐enundec‐9‐ene. The 13C NMR spectrum has been recorded and the carbon shifts have been assigned by using SFORD spectra and selective decoupling experiments.
NMR study of bicyclopropylidene in the nematic phase at 400 MHz
Authors: Courtieu J. , Gounelle Y. , Jullien J. , Barnier J. P. , Conia J. M. , Kan S. K. 
Published: May 2005

AbstractThe eight spin NMR spectrum (400 MHz) of bicyclopropylidene oriented in a nematic phase has been analysed. The absolute signs and values of 5J(HH) long‐range spin‐spin coupling constants have been determined for the first time for this molecule. The ratios of interatomic HH distances have been deduced from the direct coupling constants.
Equilibria and barriers to interconversion between the syn and anti conformers in 9‐(o‐substituted phenyl)fluorenes obtained by photorearrangement of triptycenes
Authors: Tukada Hideyuki , Iwamura Michiko , Sugawara Tadashi , Iwamura Hiizu 
Published: May 2005

AbstractThe population ratios of the syn to the anti conformers in 9‐(2‐substituted phenyl)fluorenes have been determined by 1H NMR spectroscopy to range from 0.25 (for 2′;‐NH2) to >20 (2′;‐CHNNHC6H3(NO2)‐2,4). They appear to increase, firstly, with the steric bulk of the 2′;‐substituents and, secondly, with the electronic repulsion between the substituent and the π electrons on the fluorene ring. The unusually low population of the syn isomer in the 2′;‐NH2 compound was found by IR absorption to be due to stabilization of the anti form by NH …︁ π hydrogen bonding. Whereas the substituents on the fluorene ring do not affect the population of the two isomers in 9‐(2‐tolyl)fluorenes, the same series of substituents exhibit a positive slope in the Hammett plots of the equilibrium constants in 9‐(2‐methoxymethylphenyl)fluorenes. Through‐space interaction between the methoxymethyl group and the π‐donating fluorene ring is postulated. The free energy of activation (ΔG‡) for the interconversion between the two conformers in 9‐(2‐substitued phenyl)fluorenes have been obtained from the line‐shape depindence on temperatures. The ΔG‡ values ranged from 56.8 (for 2′‐NH2) ot 78.6kJ mol−1 when there is no fluorene ring substituent. IN three typical examples, a full line‐shape analysis gave, additionally, the ΔH‡ and ΔS‡ data of the interconversion. Introduction of a methyl group at position 1 of the fluorene ring raised. ΔG‡ by c. 13kJ mol−1.
Assignment of 13c chemical shifts in α‐pinene and pinanes following catalytic deuterium exchange
Authors: Forsyth David A. , Mahmoud Sabri , Giessen Bill C. 
Published: May 2005

AbstractThe 13C NMR chemical shifts of α‐pinene and cis‐ and trans‐pinanes have been reassigned on the basis of deuterium exchange experiments. Hydrogen at allylic positions in α‐pinene is exchanged for deuterium by treatment with D2 over an amorphous Pd‐Si alloy. Further reaction gives a mixture of pinanes labelled at four positions. The newly assigned shifts correspond well with other examples of confomationally dependent shifts in bicyclo[3.1.1]heptane derivatives.
Carbon‐13 NMR of oxygenated derivatives of polyoxyethylenes
Authors: Barrelle Michel , Béguin Claude , Tessier Sylvie 
Published: May 2005

AbstractThe 13C NMR spectra of several oligo(oxyethylene) glycols and of their derivatives obtained from oxidative cleavage were studied. Assignments are proposed and the 13C chemical shifts recalculated from sets of parameters obtained for the substituents on the parent structure, CH2CH2O.
Selective excitation of 13c resonances depending on the proton multiplicity
Authors: Beloeil Jean‐Claude , Le Cocq Christian , Lallemand Jean‐Yves 
Published: May 2005

AbstractA new pulse sequence which allows selective excitations of carbons according to their multiplicity is described. The method is tested with an ionophorous antibioticthe sodium salt of lysocellin, C34H60O10.
Application of NMR spectroscopy of chiral association complexes 11—Rotation about the C(sp2)—C(aryl) Bond in 2,6‐Difluorobenzamides Studied by 19F NMR
Authors: Holǐk Miroslav , Turečková Milena , Mannschreck Albrecht , Stühler Georgine 
Published: May 2005

AbstractThe energy barrier to rotation about the C(sp2)C(aryl) single bond in non‐planar N,N‐dimethyl‐, N,N‐tetramethylene‐, N,N‐diisopropyl‐2,6‐difluorobenzamides and in N,N‐dimethyl‐2‐chloro‐6‐fluorobenzamide was investigated by 19F{1H} NMR in the presence of an optically active shift reagent. The free energy of activation was calculated from the coalescence temperature of the fluorine signals; the calculation was accomplished using the simple, approximative equation and, also, by the graphical procedure including the (eigen) line width, i.e. without the broadening caused by two‐site exchange. A distinct effect of the size of the N‐substituents on the ΔGc‡values was observed, in accordance with expectation.
Ordering in the nematic mesophase of a rapidly spinning sample
Authors: Teeäär R. , Alla M. , Lippmaa E. 
Published: May 2005

AbstractThe high resolution 13C NMR spectrum of an eutectic mixture of N‐(p‐methoxybenzylidene)‐p‐n‐butylaniline and N‐(p‐ethoxybenzylidene)‐p‐n‐butylaniline has been measured as a function of the spinning angle θ between the rotor axis and the magnetic field. The influence of a strong magnetic field and rapid sample spinning gives rise to a specific orientation of this nematic mesophase; if 0 < θ < 54°44′; the director of the nematic phase is oriented in parallel with the rotor axis, and if 54°44′; < θ <90° it is perpendicular to the rotor axis. At the exact magic angle a nematic sample displays the chemical shifts of the corresponding isotropic liquid. Nematic order is retained at all other director‐field angle values, thus allowing chemical shift anisotropy measurements of the nematic sample or of compounds dissolved therein.
The use of partially restricted molecular orbitals to investigate transmission mechanisms of spin–spin coupling constants. III—an INDO study of long‐range FH couplings in fluorinated derivatives of toluene
Authors: Facelli J. C. , Giribet C. G. , Contreras R. H. 
Published: May 2005

AbstractTransmission mechanisms of long‐range FH couplings in several fluorinated derivatives of toluene are analysed carrying out partially restricted molecular orbital calculations at the INDO level of approximation. All π‐components follow the A+B sin2θ dependence, with the dihedral angle defined by the exocyclic CH (or CF) bond and the aromatic ring plane. Four‐ and five‐bond couplings show an important σ‐component, which follows the known zig‐zag rule. Both σ‐ and π‐contributions show a conspicuous insensitivity to fluorine ring substitution. Moreover, four‐bond couplings present an important through‐space component, which is negative for a cis arrangement. A sign inversion is found at a dihedral angle between 70° and 80°, which is interpreted as indicating that several through‐space mechanisms are operating.
N‐Nitrosodecahydroquinolines 3‐Nitrogen‐15 Nuclear Magnetic Resonance Spectra
Authors: Vierhapper Friedrich W. , Srinivasan Puliyur R. 
Published: May 2005

Abstract15N Chemical shifts of N‐nitroso‐trans‐decahydroquinoline, N‐nitroso‐cis‐decahydroquinoline, of 12 methyl‐or t‐butyl‐substituted N‐nitrosodecahydroquinolines and of N‐nitroso‐trans‐syn‐trans‐perhydroacridine are reported. Shift effects for N‐nitrosation were calculated by comparison with the parent amines. Shift effects for ring substitution, obtained by comparison with the data of N‐nitrosopiperidine, are of similar direction and magnitude as in the parent amines for the ring nitrogens (N‐1). Methyl substitution nearly always causes deshielding for the nitroso nitrogens (N‐2), with a very large deshielding if the planarity of the (C‐α)2NNO region is disturbed.
Carbon‐13 chemical shifts and 1J(CH) coupling constants in 3,5 ‐ dehydrodiamantane
Authors: Hájek Milan , Třiska Jan , Isajev Sergej D. , Vodička Luděk 
Published: May 2005

Abstract13C NMR spectra of 3,5‐dehydrodiamantane were compared with those of diamantanoid compounds with and without a cyclopropane ring. The effect of the cyclopropane ring in 3,5‐dehydrodiamantane is similar to that for the carbon atoms of 2,4‐dehydroadamantane and 2,4‐methano‐2,4‐dehydroadamantane, but not for 1,3‐dehydroadamantane. One‐bond coupling constants [1J(CH)] are discussed in terms of ring size and compared with bond angles. The observed difference between 1J(CH) of CH and CH2 groups of diamantanoid skeletons is connected with a change of the number of equatorial or axial positions of CH bonds in the cyclohexane ring.
Substituent effects on one‐bond 15N13C couplings in N,N‐dimethylanilines and nitrobenzenes. Effect of steric inhibition of conjugation
Authors: Axenrod T. , Watnick C. M. , Wieder M. J. , Duangthai S. , Webb G. A. , Yeh H. J. C. , Bulusu S. , King M. M. 
Published: May 2005

AbstractThe 1J(15N13C) values for a series of ring‐substituted N,N‐dimethylaniline‐15N derivatives and a series of nitrobenzene‐15N derivatives were measured from the 13C spectra. In the nitrobenzenes, small changes in 1J(15N13C) are attributed to the inductive effect of the substituents, since steric inhibition of conjugation has little effect on the magnitude of the coupling. In contrast, steric inhibition of nitrogen lone‐pair delocalization in N,N‐dimethylaniline derivatives markedly reduces the value of 1J(15N13C). Theoretical calculations of 1J(15N13C) values for the two series of compounds were made using standard INDO parameters and a ‘sum‐over‐states’ perturbation approach. Fair agreement between the calculated and experimental values is found.
Transmission of substituent effects—a CNDO approach. The influence of the internal electric field on the 29Si NMR substituent chemical shifts of phenyltrimethylsilanes and phenoxytrimethylsilanes
Authors: Wolff Renate , Radeglia Reiner , Ponec Robert 
Published: May 2005

AbstractThe 29Si NMR subtituent chemical shifts of 14 meta‐and para‐substituted derivatives of phenyltrimethyl‐and phenoxytrimethyl‐silanes have been interpreted in a uniform manner. They are shown to depend not only on the local but also on the non‐local parts of the charge distribution around the resonating nucleus. While the local part is taken into account by the paramagnetic screening constant, the influence of the non‐local part can be described by the induced internal electric field and additional higher order terms.
13C Chemical shifts and conformational equilibria in quaternary piperidinium salts
Authors: Sugiura Makiko , Takao Narao 
Published: May 2005

AbstractCarbon‐13 nuclear magnetic resonance spectra of a series of N‐methyl‐N‐alkylpiperidinium salts have been measured, and the observed chemical shifts analysed in terms of the stereochemical and conformational properties of the molecules. Furthermore, the differences of the free energy (ΔG°) between two conformers on ring inversion have been estimated.
1H and 13C NMR spectral studies of C‐2 substituted isomeric exo‐ and endo‐5‐methyl‐bicyclo[3.2.1]octane‐6,8‐diones
Authors: Kasturi T. R. , Reddy S. Madhava , Murthy Parvathi S. 
Published: May 2005

AbstractA detailed analysis of the 1H and 13C NMR spectra of C‐2 aryl and alkyl/desalkyl substituted isomeric exo‐ and endo‐5‐methylbicyclo[3.2.1]octane‐6,8‐diones is presented. The chemical shift of the C‐5 angular methyl, the C‐2 alkyl/olefinic (C‐10)/C‐2 methine protons, the aromatic proton shieldings and the characteristic AMX and ABX spectral pattern of the ketomethylene and bridgehead protons were found to be sensitive to the phenyl ring orientation (anisotropy). These distinctive features could be used for configurational distinction for this class of compounds. With increasing ortho‐methoxy substitution on the phenyl ring, considerable deshilelding of the bridgehead proton was observed (ca. 0.6 ppm). Absence of the C‐2 alkyl group in the desalkyl isomers resulted in substantial changes in the chemical shifts of different protons. A study of the NMR spectra of the corresponding bicyclic compounds with C‐2 methoxy/hydroxy substitution instead of the aryl group revealed that the anisotropy of the phenyl ring and the electronegative oxygen substituents have opposite effects.The 13C NMR spectral assignment of each carbon resonance of C‐2 aryl and alkyl/desalkyl substituted isomeric exo‐ and endo‐5‐methylbicyclo[3.2.1]octane‐6,8‐diones and the corresponding C‐2 methoxy/hydroxy/chloro and methyl bicyclic compounds are reported. Additional ortho‐methoxy substitution on the phenyl ring was found to produce considerable high field shifts of the C‐10 and C‐1 carbon resonances. A high‐field shift was observed for the C‐6 and C‐8 carbonyl carbons, presumably due to 1,3‐dicarbonyl interactions. The chemical shifts of C‐1′ aromatic, C‐10 alkyl and C‐2 carbons, which are sensitive to exo/endo isomerism, could be utilized in differentiating a pair of isomers.
A convenient method of observing relatively broad nuclear magnetic resonances in the Fourier transform mode
Authors: Canet D. , Brondeau J. , Marchal J. P. , Robin‐Lherbier B. 
Published: May 2005

AbstractA method is proposed for eliminating baseline distortions due to spurious ringings in Fourier transform NMR spectra involving broad resonances. It is based on the pulse sequence (π/2)–(Acquistion/Add)–π–τ–(π/2) (Acquisition/Subtract). τ is of the order of, or shorter than, the spectrometer recovery time. The sensitivity of the method is discussed.
Fluoro‐α‐amino acids. 1—use of 19F NMR spectroscopy for the configurational determination of β‐fluoro‐α‐amino acids through complexation by 18‐crown‐6 ether
Authors: Hamman S. , Salon M. C. , Béguin C. 
Published: May 2005

AbstractErythro and threo configurational assignments have been made for 11 β‐fluoro‐α‐amino acids or esters using the effect of complexation of the ammonium group by 18‐crown‐6 ether on the 19F NMR parameters. For the erythro configurations, 3J(HF) increases and a high‐field 19F chemical shift is generally observed; these phenomena are accompanied by a decrease in 3J(HH) and 3J(CF). The opposite effects are observed for the threo configurations. These observations can be explained by a change in the relative population of the conformers around the Cα‐Cβ bond on complexation of the ammonium group. This complexation impairs the interactions between the ammonium and fluorine groups and, concomitantly, the steric hindrance between the ammonium and R (methyl, phenyl or carboxylate) groups is increased.
77Se NMR study of some organic selenium compounds formed in the selenium dioxide oxidation of ketones
Authors: Laitalainen Tarja , Rahkamaa Erkki 
Published: May 2005

Abstract77Se NMR chemical shifts and 1J(SeC) coupling constants were measured for nine organic selenium compounds: 4,5,6,7‐tetrahydro‐,4′,4′,6,6‐tetramethylspiro[1,3‐benzoxaselenole‐2,1′‐cyclohexane]‐2′,4,6′‐trione and closely related derivatives, bis(2‐hydroxy‐4,4,6,6‐tetramethyl‐3‐oxo‐1‐cyclohexenyl) selenide and derivatives, and 1,5,5‐trimethyl‐7‐selenabicyclo[2.2.1]heptane‐2,3‐dione. The chemicalshifts ranged from −107 to 595 ppm from the external dimethyl selenide standard. The bridged selenabicyclic compound showed a small coupling constant (42 Hz).
Compounds with bridgehead nitrogen. 42—1H NMR and stereochemistry of isomeric 6a‐methylperhydroindolo[3,2,1‐i,j]benzoxazines and the position of conformational equilibrium in perhydropyrrolo[1,2‐c][1,3]oxazine
Authors: Crabb Trevor A. , Newton Roger F. , Rouse Janet 
Published: May 2005

AbstractThe 1H NMR parameters of the NCH2O protons in the spectrum of perhydropyrrolo[1,2‐c][1,3]oxazine show its existence in solution at room temperature in the O‐inside cis‐fused conformation. rel‐(3aS,6aS,6bR,10aS,11aS)‐6a‐Methylperhydroindolo[3,2,1‐i,j]benzoxazine and rel‐(3aS,6aS,6bS,10aR,11aS)‐6a‐methylperhydroindolo[3,2,1‐i,j]benzoxazine are shown to adopt cis‐ and trans‐fused conformations, respectively, for the corresponding bicyclic moiety.
119Sn spin–lattice relaxation times and nuclear Overhauser enhancement factors in some organotin compounds
Authors: Blunden Stephen J. , Frangou Andrew , Gillies Duncan G. 
Published: May 2005

AbstractDipole‐dipole relaxation via non‐bonded protons is an important relaxation mechanism for119Sn in tri‐n‐propyltin and tri‐n ‐butyltin compounds. This causes a negative nuclear Overhauser effect, arising from the negative magnetogyric ratio, which in some cases nulls the signal. The relative contributions from the spin‐rotation and dipole‐dipole mechanisms vary: larger molecules have lower spin‐rotation and higher dipolar relaxation rates. The practical significance of large nuclear Overhauser enhancement factors in recording 119Sn spectra and the relation of the dipole‐dipole contribution to the molecular motion and of the spin‐rotation contribution to the absolute shift scale for 119Sn are discussed.
On the Karplus relation for 3J(POCC) of nucleotides
Authors: Davies David B. , Sadikot Husaini 
Published: May 2005

AbstractThe original Karplus parameters for analysing 3J(POCC) magnitudes of nucleotides in terms of conformational properties of the OC bond were taken from results for 3′,5′‐nucleotides and applied to 3′→ 5′‐oligonucleotides; the parameters were later modified to take account of ‘largey’ magnitudes of 3J(POCC) observed in 2′ → 5′‐oligonucleotides. In this work the origin of this discrepancy is explained in terms of substituent electronegativity effects at C‐1′, and quantified using the 13C NMR results of 2′,3′‐cyclic mononucleotides. A new set of Karplus parameters suitable for analysing 3J(POCC) magnitudes in 3′‐ and 5′‐nucleotides and 3′ → 5′‐oligonucleotides is determined from 13C NMR measurements on 3′‐nucleotides and available results for 3′,5′‐cyclic mononucleotides. A method of dealing with J(P, C‐1′) coupling in 2′‐nucleotides, 2′,3′‐cyclic nucleotides and 2′ → 5′‐oligonucleotides using the same Karplus relationship is suggested.
Temperature and solvent variation of the 13C and 1H NMR spectra of r‐1,t‐2,c‐3,t‐4‐tetrachlorotetralin
Authors: Bowmaker Graham A. , Calvert David J. , de la Mare Peter B. D. , Jones Alan J. 
Published: May 2005

AbstractVicinal coupling constants in the 1H and 13C NMR spectra of r‐1,t‐2,c‐3,t‐4‐tetrachlorotetralin change with temperature and solvent in a manner better explained by conformational equilibration than by intramolecular or solvent‐determined distortions.
Investigation of benzimidazoles. III—transmission of the substituent effects in 2‐substituted 1‐methylbenzimidazoles studied by 13C nuclear magnetic resonance
Authors: Lopyrev V. A. , Larina L. I. , Vakul'skaya T. I. , Shibanova E. F. , Titova I. A. , Voronkov M. G. , Liepin'sh E. E. 
Published: May 2005

AbstractThe influence of substituents on the 13C NMR chemical shifts of 2‐substituted 1‐methylbenzimidazoles has been investigated. The electronic effects of the substituents are transmitted to C‐4 and C‐7 mainly by the resonance mechanism, and to C‐5, C‐6 and N‐CH3, by approximately equal contributions of the resonance and inductive components. A critical analysis of the share in the transmission of substituent effects through the ‘pyridine‐type’ and ‘pyrrole‐type’ nitrogen atoms is given.
Nitrogen‐15 nuclear magnetic resonance spectroscopic study of compounds with nitrogennitrogen bonds
Authors: Yavari Issa , Roberts John D. 
Published: May 2005

AbstractThe 15N NMR spectra of several substances with nitrogen—nitrogen bonds have been obtained at the natural‐abundance level by high‐resolution NMR spectroscopy. Azo (NN) nitrogens are 300–500 ppm deshielded compared with hydrazo (NHNH) nitrogens. The sensitivity of the 15N shifts in these types of substances to substituent changes are reported, along with 15N–1H spin–spin couplings for some hydrazo compounds. Hydrogen‐bonding effects arising from solvent changes on the 15N shifts of azoxybenzene are different for its two kinds of nitrogen. The 15N NMR spectrum of N,N'‐dinitrosopiperazine in dimethyl sulfoxide at room temperature is consistent with the presence of two different conformations about the NNO bonds.