bluuebunny/crossref_metadata_2025_split huggingface dataset:
Applications of NMR spectroscopy of chiral association complexes 5. Stereodynamics and diastereotopism in chiral 1,3‐dienes \documentclass{article}\pagestyle{empty}\begin{document}${\rm HOCMe}_{\rm 2} \rlap{--} ({\rm CCl =\!= CCl\rlap{--})}_{\rm 2} {\rm X}$\end{document}
Authors: Becher Georg , Burgemeister Thomas , Henschel Hans‐Helmut , Mannschreck Albrecht 
Published: May 2005

AbstractThe barriers to partial rotation around the central single bond in chiral dienes \documentclass{article}\pagestyle{empty}\begin{document}${\rm HOCMe}_{\rm 2} \rlap{--} ({\rm CCl =\!= CCl\rlap{--})}_{\rm 2} {\rm X}$\end{document} have been determined by coalescence of either 1H NMR signals (X = CH2OCH3) or 13C NMR signals (X = H). In the presence of the optically active shift reagent (+) − Eu(hfbc)3 all 1H signals were split at temperatures where the interconversion of enantiomers is slow. The temperature dependence of these spectra also yielded free activation enthalpies for the enantiomerizations which were in agreement with the ones obtained without Eu(hfbc)3. The assignment of the four methyl resonances appearing in the presence of (+) − Eu(hfbc)3 at low temperature was possible by gradually increasing the rate of enantiomerization or gradually replacing the optically active auxiliary compound by the racemic one.
Cage compounds IV. 1H and 13C NMR study of 3,6‐epoxypentacyclo‐[6.2.2.02,7.04,10.05,9]dodecane and 3,6‐epoxypentacyclo‐[6.2.1.02,7.04,10.05,9]undecane
Authors: Dekker T. G. , Pachler K. G. R. , Wessels P. L. , Dekker J. , Dekker J. J. , Fourie L. 
Published: May 2005

AbstractThe complete 1H and 13C NMR assignments for 3,6‐epoxypentacyclo[6.2.2.02,7.04,10.05,9]dodecane and 3,6‐epoxypentacyclo[6.2.1.02,7.04,10.05,9]undecane are reported. The difference between the 1H and 13C chemical shifts and one‐bond proton‐carbon coupling constants of these two compounds are adequately explained by the difference in the hybridization of the CH bonds.
Effects of solute–solvent interactions and acid catalysis on the rotational barriers in polarized ethylenes. The significance of large entropies of activation as measured by dynamic NMR
Authors: Berg Ulf , Sjöstrand Ulf 
Published: May 2005

AbstractComplete 1H NMR bandshape analysis has been used to study the rotational barrier around the carbon–carbon double bond in two types of polarized ethylenes; on the one hand for planar ketene mercaptals (A), for which the hindered rotation over the 90 degrees twisted transition state is measured and, on the other hand, for sterically hindered ketene aminals (B and C) which are twisted in the ground state, and have measurable barriers over a planar, sterically crowded transition state. For compounds A activation entropies of the order of −45 to −76 J mol−1 K−1 (−11 to −8 cal mol−1 K−1), and for compounds B and C +28 to +42 J mol−1 K−1 (+7 to +10 cal mol−1 K−1) were obtained. This important difference was interpreted in terms of opposite charge development during the course of the rotation in the two cases, with concomitant reversed changes in the orientations of the solvent. The effects of solvent polarity and the presence of acid are shown to be reversed for the two types of compounds. A more polar solvent, and, in particular, addition of small amounts of acid decrease the barrier in system A but increase the barrier in systems B and C. A large negative entropy of activation was found for the acid‐catalyzed process, ΔS≠ = −107 J mol−1 K−1 (−25.5 cal mol−1 K−1). This low value is briefly discussed in relation to the mechanism of the acid‐catalyzed rotation.
Konformationsuntersuchungen mit Hilfe der 13C‐NMR‐Spektroskopie. III. 13C‐NMR‐spektroskopische Verschiebungen durch sterisch gehinderte Methylgruppen
Authors: Mann G. , Kleinpeter E. , Werner H. 
Published: May 2005

AbstractDie 13C‐NMR‐Spektren einer Reihe von Cyclohexanen mit 1,3‐diaxialen Methylgruppen wurden bestimmt. Die Resonanzsignale der Methyl‐Kohlenstoffatome 1 und 3 sind in diesen Verbindungen um durchschnittlich 4,5 ppm nach tieferem Feld verschoben (δ‐Effekt). Auch die Ringkohlenstoffatome 1 und 3 zeigen Verschiebungen um durchschnittlich 0,7 ppm nach tieferem Feld (γ‐Effekt). In offenkettigen Kohlenwasser‐stoffen werden analoge Verschiebungseffekte beobachtet, wenn in den betreffenden Verbindungen die Geometrie des gPgM‐Konformeren des n‐Pentans vorkommt.
Dynamique Moléculaire des Chloro‐4 et Bromo‐4 Méthylènecyclohexanes par Résonance Magnétique Nucléaire du 13C
Authors: Iratçlabal P. , Grenier‐Loustalot M. F. , Lichanot A. , Métras F. 
Published: May 2005

AbstractL'étude RMN 13C et 1H à basse température des chloro‐4 et bromo‐4 méthylènecyclohexanes nous a permis de déterminer les paramètres d'activation associés aux équilibres conformationnels de ces composés. Les résultats obtenus montrent que d'un point de vue dynamique les substituants Cl et Br ont la měme influence en séries cyclohexanique et méthylènecyclohexanique.
Effects of impurity on 35Cl and 14N NQR in p‐chloraniline
Authors: Buck K. R. 
Published: May 2005

AbstractThe effect of impurity (p‐toluidine) on the nuclear quadrupole resonance of two different nuclei (35Cl and 14N) in the same molecule has been studied. The 14N resonance is unaffected whereas the 35Cl resonance is greatly reduced.
13C NMR of methyl N‐methylthiobenzimidates. A novel approach to the interpretation of the relative 13C‐E/Z shifts in amides and related compounds
Authors: Walter Wolfgang , Ruback Wulf , Meese Claus O. 
Published: May 2005

AbstractThe 13C NMR shifts of the E/Z mixtures of twelve, mainly ortho substituted, methyl N‐methylthio‐benzimidates have been completely assigned. The angles of twist about the aryl‐imino‐carbon bond were determined by means of the C‐1 aryl shifts. Depending on the bulk of the ortho substituents the values vary from 18° in methyl 2‐fluoro‐N‐methylthiobenzibenzimidate (Z) to 73° in methyl 4‐bromo‐N,2,6‐trimethylthio‐benzimidate (E). An explanation for the 13C‐E/Z‐N‐alkyl and ‐S‐alkyl resonances is facilitated by a new model which is initially based on the influence of electric fields. As this model is derived from amides, thioamides etc. it is possible to interpret the 13C‐E/Z shifts in all related classes of compounds. The role of steric compression does not seem to be as important as was previously assumed.
A multinuclear NMR (13C, 15N and 29Si) study of the SiN bond in silylamines: (p – d) π interaction
Authors: Filleux‐Blanchard M. L. , An Nguyen Dinh 
Published: May 2005

AbstractA series of variously substituted aminosilanes was investigated by 15N NMR spectroscopy to obtain further information on the controversial problem of pπ‐dπ interaction in these systems. The 15N NMR data are consistent with the 13C and 29Si results and suggest that the (p‐d)π backbonding is not negligible in these systems. The values of the 15N chemical shifts and the 13C parameters [δ13C and J(13CH)] are discussed in terms of nitrogen lone‐pair delocalization and provide a good basis for explaining the variations of the 29Si chemical shifts with the nature of the nitrogen atom substituents.
A 13C NMR study of tautomerism in phenoxymethylisoxazol‐5‐ones
Authors: Auzou Gilles , Rips Richard , Likforman Joseph , Hazebroucq Georges , Aji Marc 
Published: May 2005

AbstractA 13C NMR study of new isoxazol‐5‐ones has been carried out in different solvents. Mathematical analysis of the tautomeric ring proton exchange in pyridine solvent indicates that the relative contributions of the NH, CH and OH tautomers are respectively 10 ± 5, 15 ± 3, 10 ± 8%, while the anionic form represents the remaining 65 ± 8%.
195Pt NMR studies on monomer and dimer complexes. Observation of PtPt coupling constants
Authors: Soulié J. , Lallemand J. Y. , Rao R. C. 
Published: May 2005

AbstractVarious platinum complexes have been studied by 195Pt FT NMR. Long range J(31P195Pt) and J(195Pt195Pt) are observed in dinuclear complexes. The value of 1J(31P195Pt) in monomeric and dimeric complexes is shown to depend mainly upon the Pt—P bond length.
Sels de Pyrylium. XVIII. Etude par RMN du Carbone‐13 de Sels d'Aminopyrylium et des Barrières de Rotation dans quelques Cations N,N‐Diméthylaminopyrylium
Authors: Chenon M. Th. , Sib S. , Simalty M. 
Published: May 2005

AbstractDans les cations N,N‐diméthylamino‐2 pyrylium, la conjugaison du doublet de l'azote avec le cycle confère à la liaison C‐2N un caractère partiel de double liaison. Les valeurs des paramètres ΔG≠, ΔH≠ ΔS≠ relatifs à l'empěchement de rotation correspondant ont été déterminées par analyse de la forme des bandes. Cette conjugaison diminue le caractère électrophile du carbone C‐4, ce qui rend impossible les réactions de transalcoylation. Dans les cations N,N‐diméthylamino‐4 pyrylium il existe de měme une barrière de rotation autour de la liaison C‐4N, dont les paramètres ΔG≠ ont été évalués. La comparaison de ces deux types de cations montre que l'empěchement de rotation autour de la liaison CN est plus important en position 4 qu'en position 2. Par ailleurs, plus le substituant en position 4 est électrodonneur, moins la barrière de rotation autour de la liaison C‐2N est élevée: la valeur du paramètre ΔG≠ décroǐt de 19,1 kcal mol−1 (79,9 kJ mol−1) à 12,6 kcal mol−1 (52,7 kJ mol−1) selon l'ordre suivant des substituants R‐4: ‐C6H5, ‐CH3, –OCH3, –N(CH3)2.
Conformation et Spectrometrie de RMN. III. Spectres 13C de Diméthylamino Cyclanols et Dérivés
Authors: Laffite Colette , Wylde Renée , Mavel Gérard 
Published: May 2005

AbstractLes spectres RMN du 13C de diméthylamino‐2 cyclohexanols, des quatre diméthylamino‐3 décahydronaphtols‐2 trans et de leurs iodures d'ammonium quaternaires sont décrits. Le calcul des séplacements chimiques de chaque carbone en tenant compte des interactions gauches permet d'attribuer correctement chaque signal. Dans le cas des composés cis ou trans diéquatorial, on a pu non seulement vérifier des modifications conformationelles portant sur les carbones substitués, mais on a pu mettre également en évidence des effects portant sur les carbones voisins, effets que les méthodes IR et RMN du proton ne pouvaient pas permettre d'observer.
Some sum‐over‐states perturbation calculations of 19F13C coupling constants for some carbocations and related fluorobenzenes
Authors: Duangthai S. , Webb G. A. 
Published: May 2005

AbstractSum‐over‐states perturbation calculations within the INDO framework are reported for 24 1J(FC) and 34 3J(FC) couplings. In general, satisfactory agreement with the experimental data is obtained when the integral products SF2(O) SC2(O) and 〈r−3〉F 〈r−3〉C take the values of 136.543 au−6 and 58.352 au−6, respectively, for the 1J(FC) couplings. The corresponding values for the 3J(FC) couplings are 29.520 au−6 and 44.340 au−6, respectively. All of the 1J(FC) values are predicted to be negative, whereas all of the 3J(FC) values are calculated with positive signs. The results indicate the importance of including the contact, orbital and dipolar contributions in the calculations.
Some self‐consistent perturbation calculations of 1J(CC)
Authors: Khin Tun , Webb G. A. 
Published: May 2005

AbstractSelf‐consistent perturbation calculations of 70 1J(CC) values are reported within the INDO framework. A least‐squares agreement between the calculated and observed couplings, for a variety of bond multiplicities, provides values of 13.503 au−6 and 5.072 au−6 for (SC2(O))2 and 〈r−3〉C2, respectively. The non‐contact terms are found to be important in cases of multiple bonding.
Carbon‐13 NMR spectra of nucleoside 3′,5′‐cyclic phosphates. Proposition for revised signal assignments
Authors: Uesugi Seiichi , Tanaka Sumie , Ikehara Morio 
Published: May 2005

AbstractOn the basis of the data obtained from 13C NMR spectra of 8,2′‐S‐cycloadenosine 3′,5′‐cyclic phosphate and other nucleoside 3′,5′‐cyclic phosphate analogues, it is suggested that the published assignments of the C‐3′ and C‐4′ signals in nucleoside 3′,5′‐cyclic phosphates should be reversed. According to the revised assignments, C‐4′, which is fixed very closely to the diesterified phosphate group is markedly shielded (−12.5 to −15 ppm), and the C‐3′ signal shows a downfield shift (+6 to +8 ppm) which is comparable to that for the C‐5′ signal, for all compounds so far measured when compared with the chemical shifts for the corresponding nucleosides. The 3′,5′‐cyclic phosphates of thymidine and 8,2′‐S‐cycloadenosine, which have no α‐OH group on C‐2′, show similar chemical shift changes for the corresponding sugar carbons which are different from those observed for ribonucleoside derivatives.
Les Amines Biogènes (Phénol‐ et Catéchol‐Amines): Attribution des Spectres de Résonance 13C et Etude de la Deprotonation
Authors: Haran Raymond , Nepveu‐Juras Françoise , Laurent Jean‐pierre 
Published: May 2005

AbstractUn certain nombre d'amines biogènes (phénol‐ et catéchol‐ amines) ont été étudiées en solution aqueuse par RMN du 13C. Les attributions ont été réalisées sur les formes protonées et l'influence de la nature de la chaǐne latérale sur les conditions de résonance des noyaux du cycle a été étudiée. Les différences de comportement observées à la déprotonation entre phénol‐ et catéchol‐ amines sont discutées en fonction de la répartition géométrique des centres chargés. Il est établi que, dans les deux séries, le groupement amino protoné est le site le moins acide.
1H and 13C NMR studies on 3‐aryl‐5‐alkyl‐2,4‐oxazolidinediones. The magnetic non‐equivalence of isomeric nuclei
Authors: Içli Siddik 
Published: May 2005

AbstractMagnetic non‐equivalence of diastereotopically related proton and 13C nuclei in enantiomeric and diastereomeric rotational isomers of 3‐aryl‐5‐alkyl‐2,4‐oxazolidinedlones has been investigated. Steric interactions between the aryl and the heterocyclic moieties of these compounds produce sufficient restriction to rotation about the aryl CN bond that the presence of torsional isomers may be detected at normal temperatures. Free energies of activation for rotation have been calculated. Chemical shift differences, associated with rotational isomerism, may be detected for both the proton and the 13C nuclei on the hetero ring, but only in the 13C spectra of the nuclei on the aryl moiety.
Investigation by dynamic NMR spectroscopy of the mechanism of rotation about the carbondouble bond in nitroenamines
Authors: Bakhmutov V. I. , Burmistrov V. A. 
Published: May 2005

AbstractRapid Z,E‐isomerization of nitroenamines RNHCHC(NO2)COOCH3 in solutions was investigated by dynamic NMR spectroscopy. In weakly basic solvents all the studied nitroenamines isomerize by a thermal mechanism. In pyridine the isomerization proceeds by the thermal mechanism in aliphatic enamines with RCH3, COCH3 and H, but involves the formation of an intermediate mesomeric ion with kinetically controlled ionization of the NH bond in the case of aromatic nitroenamines where R = X—Ph‐.
Synthesis of new crown ethers and their metal complexes: 1H and 13C NMR study
Authors: Torizuka Koichi , Sato Takeo 
Published: May 2005

AbstractA new type of crown ethers containing a diphenyl ether unit has been prepared, the ring size ranging from 12 to 36. 1H and 13C NMR spectra of both free ligands and their metal‐ion complexes have been recorded. For 18‐ and 21‐membered compounds a general downfield shift was observed for both methylene and aromatic proton resonances on metal‐ion complexation. The stoichiometry of K+ and Na+ complexes was deduced from chemical shift dependence on metal‐ion concentration. The K+ and Na+ complexes of 18‐ and 21‐membered rings have a guest to host ratio of 1:1, whereas the K+ salt of the 15‐membered ring exists as a 1:2 complex in solution. The 1H shift observed on salt formation was attributed to electric‐field and conformational effects. The 13C resonances for the aryl carbons, C‐1, C‐2 and C‐3, and the α‐methylene carbon in 15‐ and 18‐membered rings were shifted upfield when an equivalent amount of KSCN was added in CDCI3−DMSO‐d6. The shift changes were independent of the anion, and similar results were obtained for SCN−, Br−, and I− salts. The upfield shift is explained by conformational factors. The spectral changes were slight for 12‐ and 36‐membered rings. In 15‐ and 18‐membered rings, complexation induces conformational changes which force the C‐α carbon into the plane of the benzene ring. The solution conformation of these molecules is discussed.
Psi‐angle dependence of geminal proton–carbonyl‐13C coupling constants in peptides. A theoretical investigation
Authors: Mohanakrishnan P. , Easwaran K. R. K. 
Published: May 2005

AbstractTheoretical calculations of the geminal carbonyl‐13C‐ proton coupling constant, 2J(C′H), in α‐amino acids have been carried out using Dirac Vector model and Penney‐Dirac bond order formulations. The results indicate that the couplings are dependent on the backbone torsion angle psi (ψ) of the amino acid residues in peptides. The meagre available experimental data seem to support the theoretical findings.
IR, 1H NMR and 13C NMR spectra of isomeric 1‐aryl‐4‐methyl‐3‐ and‐5‐pyrazolecarboxylic esters
Authors: Sohár Pál , Fehér Ödön , Tihanyi Endre 
Published: May 2005

AbstractThe IR, 1H and 13C NMR spectra of six esters of 1‐aryl‐4‐methyl‐3‐ and 5‐pyrazolecarboxylic acids are reported. On the basis of their spectra the 3‐ or 5‐position of the substituent in such structural isomeric pairs can be established without recourse to the spectrum of the other member of the pair. The spectral data also give information on the conformation, which depends on the relative position and size of the hetero ring substituents.
Etude par RMN du 13C d'Epoxydes en Série Diterpénique
Authors: Delmond Bernard , Papillaud Bernard , Valade Jacques , Petraud Michel , Barbe Bernard 
Published: May 2005

AbstractLes auteurs étudient au moyen de la RMN du 13C divers époxydes préparés à partir d'esters méthyliques dérivés d'acides résiniques diterpéniques appartenant au groupe pimarique. Cette analyse permet de dégager en fonction de la configuration du cycle époxydique, certains effets sur les déplacements chimiques des carbones en α, β, γ et δ par rapport à l'oxygène époxydique.
13C NMR of α‐haloketones
Authors: Yalpani M. , Modarai B. , Khoshdel E. 
Published: May 2005

AbstractThe carbon chemical shifts of several series of α‐haloketones have been measured. Whereas the carbon carrying the halogens shows a regular downfield shift with increasing electronegativity of the substituents, the adjacent carbonyl carbon is shifted upfield by chlorine, bromine and iodine, and is little affected by fluorine substitution. The conformational implications of these results are discussed.
Nitroxide biradicals from substituted bis(hydroxy‐ureas)
Authors: Everard John. E. , Parlett Graham R. 
Published: May 2005

AbstractThe EPR spectra of several nitroxide biradicals derived from substituted bis(hydroxy‐ureas) have been determined. The main differences in these spectra arose dur to intramolecular interaction of the nitroxide groups. Interaction was only observed for the case where the nitroxide group was bonded to an aliphatic group.
13C‐NMR‐Untersuchungen zur Azido‐Tetrazol‐Valenzisomerie. Tetrazolo[1,5‐α]pyrimidine
Authors: Könnecke Andreas , Kleinpeter Erich , Lippmann Eberhard 
Published: May 2005

AbstractDie 13C‐NMR‐Spektroskopie wurde zum Studium der Azido‐Tetrazol‐Valenzisomerie von drei Tetrazolo‐[1,5‐a]pyrimidinen herangezogen. Große chemische Verschiebungsänderungen der C‐4‐Kohlenstoffatome eignen sich zur Unterscheidung von Azido‐ und Tetrazoloisomeren. Daneben wurde die Struktur neuer Heterocyclen gesichert und gezeigt, daß die Arbeit von LaNoce und Giuliani bezüglich 9‐Methyl‐s‐triazolo‐[4,3‐c]tetrazolo[1,5‐a]pyrimidin unkorrekt ist.
Lanthanide shift reagents. XII Equilibrium binding constants for aromatic aldehydes and ketones with tris‐(tetramethylheptanedionato)europium, eu(thd)3
Authors: Rackham David M. 
Published: May 2005

AbstractThe binding constants, K, are reported for Eu(thd)3 with 11 ketones and 7 aldehydes related to acetophenone and benzaldehyde, respectively. Variations in K are rationalised in terms of the electronic and steric requirements of the aryl ring substituents.
Détermination par RMN 1H et 13C des Couplages J(CH) et J(CC) dans des Monomères Precurseurs de Lignine: Vanilline, Férulate d'Ethyle et Alcool Coniférylique Sélectivement Enrichis en 2H et 13C
Authors: Gagnaire Didier , Nardin Robert , Robert Daniele 
Published: May 2005

AbstractQuelques monoméres composés modèles de ligning ont été sélectivement enrichis en 2H et 13C: c'est le cas de la vanilline, du férulate 'éthyle, de l'alcool conigérylique et du monoéthyl ester de l'acide malonique. L'effet isotopique dû au deutérium sur les déplacement chimiques 13C dans le cas de la vanilline 2H‐formyle, la vanilline 2H‐5 et l'alcool coniférylique 2H 5, αα, a permis l'attribution non ambifüe des signatux RMN 13C des carbones aromatiques. Les valeurs absolues des constants de couplages 1,2,3 J(CC)ont été déterminées à partir des specters 13C de la vanillin 13C formyle, du férulate d'éthyle et de l'alcool coniférylique, ces deux derniers ayant les positions vinyliques C‐γ et C‐β Sélectivement marquées en 13C. Il a été possible d'observer la valeur des couplages 4 (CO, C‐4) pour la vanilline, et 4 J (C‐γ, C‐4) pour le férulate d'éthyle. La détermination des couplages 1,2,3,4 J (CH) en valeurs absolues a été réalisée sur les spectes 13C en ‘découplage séquential’ des composés non marqés. Lorsque cette technique s'est révélée inutilisable à cause d'effets du second ordre, nous avons déterminé cretaines valeurs J (CH) et leurs signes, soit en analysant le spectre 1H‐RMN à 100 MHZ des alcohols coniféryliques 13C‐γ et 13C‐β comme dans le cas de 2 J (C‐β, H‐γ), 2 J (C‐β, H‐α), 2 (C‐γ, H‐β) et 3 J (C‐γ, H‐α). soit en observant le spectre proton à 250 MHZ avec une expérience de double irradiation du gérulate d'éthyle 13C‐β dans le cas de 2 J (C‐β, H‐γ).
15N NMR spectroscopy acids: 7—solvent effects on α‐and ω‐amino
Authors: Kricheldorf Hans R. 
Published: May 2005

AbstractThe natural abundance 15N NMR spectra of several α‐and ω‐amino acids were measured in various protic solvents. Increasing acidity of the solvents results in an upfield shift in the case of the α‐amino acids, while ω‐amino acids are almost insensitive to solvent effects.
Three‐bond 31P31P coupling and 31P chemical shift effects in dimers of phospholium salts and phosphole oxides
Authors: Quin Louis D. , Mesch Keith A. 
Published: May 2005

AbstractThe chemically nonequivalent 31P nuclei of the Diels‐Alder dimers of phospholium ions and phosphole oxides are coupled through three bonds by as much as 35–45 Hz. The chemical shift for a phosphorus atom in the 2‐phospholene moiety falls in the expected range; the shift for the constricted atom bridging the 6‐membered ring is far downfield, making Δδ as much as 43.5 ppm for one phospholium ion dimer.
Long‐range nuclear spin–spin coupling between selenium‐77 and phosphorus‐31 in biphosphorus compounds
Authors: Colquhoun Ian J. , McFarlane H. Christina E. , McFarlane William , Nash John A. , Keat Rodney , Rycroft David S. , Thompson David G. 
Published: May 2005

AbstractThe coupling constants nJ(77Se31P), n = 1–4, have been measured in the proton‐decoupled 31P NMR spectra of a range of diphosphorus selenides and diselenides. 31P–{1H, 31P} and 31P–{1H, 77Se} triple resonance experiments have been used to establish the signs of the coupling constants, and it is found that both the magnitudes and signs depend upon the stereochemical relationship of the coupled nuclei.