bluuebunny/crossref_metadata_2025_split huggingface dataset:
NMR–Untersuchungen an Dicyclopentadienderivaten: I—1H‐NMR: Anwendung von paramagnetischen Verschiebungsreagenzien (Eu(fod)3) zur Vereinfachung der Spektren exo/endo‐isomerer Diole
Authors: Kleinpeter E. , Kühn H. , Mühlstädt M. 
Published: May 2005

AbstractDie Bestimmung der Stereochemie einer Reihe exo/endo‐isomerer 9.10‐Dihydro‐ bzw. Tetrahydrodicyclopentadien‐9.10‐diole (exo/exo bzw. endo/endo) unter Verwendung von Eu(fod)3 zur Auftrennung ihrer 1H‐NMR‐Spektren wird beschrieben. Die für halbmolares Verhältnis Eu(fod)3/Diol bestimmten Verschiebungswerte ΔEu gestatten dabei eine eindeutige stereochemische Zuordnung der Verbindungen. Die ΔEu‐Werte werden mit dem Abstand Hi…O‐Atom (Ri) korreliert.
Sur la tétrabromo‐2, 2, 6, 6 méthyl‐4 cyclohexanone: Mise en evidence d'un equilibre conformationnel par RMN à haut champ
Authors: Grenier‐Loustalot M. F. , Iratçabal P. , Lectard A. , Forchioni A. 
Published: May 2005

AbstractLa comparaison après simulation des spectres RMN à haut champ des tétrabromo‐2,2,6,6 méthyl‐4 et tertiobutyl‐4 cyclohexanones (1 et 2) nous a permis de mettre en évidence par l'étude des constantes de couplage, en particulier Jgem, Jea, Jaa et 4J et des déplacements chimiques que 1 présente à l'état dissous une isomérie de rotation.Ce travail confirme les conclusions auxquelles nous avait conduits une étude par spectroscopie d'absorption dans l'infrarouge de 1 dans la région des vibrations de déformation δ(CH3).
Nuclear spin–spin coupling between 15N and 13C in an azomethine ylide and related compounds
Authors: Fritz Hans , Clerin Daniel , Fleury Jean‐Pierre 
Published: May 2005

Abstract13C15N coupling constants of a stable azomethine ylide, a 2‐aza‐1,3‐diene and an O‐tosyloxime have been measured. The large one‐bond coupling constants (21·3 and 21·6 Hz) observed for the azomethine ylide prove the dipolar structure of this compound. For the other two compounds, two‐bond coupling constants are used to derive the stereochemistry at the C,N double bond.
a priori Calculations of molecular physical constants: II—an improvement in chemical shift calculations using a modified version of the CNDO/S method
Authors: Pouzard G. , Rajzmann M. 
Published: May 2005

AbstractA modification is introduced into the CNDO/S method which takes into account electronegativity differences between atoms in a molecule. Results of 13C chemical shift calculations are largely improved by such a procedure with respect to those obtained using conventional CNDO/S wave functions.
Protonenresonanzuntersuchungen an Aminoboranen—II: Einflüsse von para‐Phenylsubstituenten auf die Rotationsbarriere um die NB‐Bindung
Authors: Friebolin H. , Rensch R. , Wendel H. 
Published: May 2005

AbstractUm den elektronischen Einfluß von Substituenten auf die freie Aktivierungsenthalpie der Rotation um die NB‐Bindung in Aminoboranen zu studieren, synthetisierten wir zwei Reihen von Verbindungen: (a) (CH3)2BCl(Phenyl‐p‐X) mit para‐Phenylsubstituenten am Boratom und (b) (p‐X‐Phenyl)CH3NB(CH3)2 mit para‐Phenylsubstituenten am Stickstoffatom der NB‐Bindung (X = NR2, OCH3, C(CH3)3, Si(CH3)3, H, F, Cl, Br, J, CF3 und NO2). Beim Vergleich der Rotationsbarrieren entsprechender Verbindungen beider Reihen ist ein entgegengesetzter Effekt des Substituenteneinflusses festzustellen:Elektronenziehende p‐Phenylsubstituenten erhöhen ΔGc≠, wenn der Phenylkern am Bor steht, senken ΔGc≠ jedoch, wenn er am Stickstoff der NB‐Bindung gebunden ist. Elektronenspendende p‐Phenylsubstituenten zeigen das entgegengesetzte Verhalten.Die Unterschiede der ΔGc≠‐Werte von substituierten und unsubstituierten Verbindungen [δ(ΔGc≠) = ΔGc≠(X) − ΔGc≠(X = H)] innerhalb jeder Reihe lassen sich gut mit den Hammett'schen σ‐Konstanten der entsprechenden Substituenten korrelieren, d.h. mit Hilfe induktiver und mesomerer Substituenteneinflüsse erklären.Ein Vergleich der Steigungen der Korrelationsgeraden zeigt, daß diese Substituenteneinflüsse bei Verbindungen mit N‐Phenyl‐p‐X wesentlich stärker sind als bei solchen mit B‐Phenyl‐p‐X.
Aziridines N‐non substituées: XI—Inversion de l'azote dans des composes C‐substitués symetriquement ou non de part et d'autre du cycle
Authors: Martino R. , Lopez A. , Lattes A. 
Published: May 2005

AbstractLe phénomène d'inversion de l'azote a pu être mis en évidence par RMN du proton dans un certain nombre d'aziridines NH diversement C‐substituées. Lorsque l'encombrement des deux faces du petit cycle est trop différent, un seul invertomère existe. A partir des différents déplacements chimiques et des constantes de couplages HCNH, on peut constater que l'invertomère privilégié, ou unique, est toujours celui pour lequel le proton NH est du côté du cycle le moins encombré. Pour les aziridines symétriques, la barrière d'inversion a été mesurée: elle est abaissée lorsque l'encombrement stérique augmente. Dans les dérivés C‐phénylés un facteur supplémentaire de stabilisation des invertomères apparaît, sans doute dû à des conformations préférentielles du cycle aromatique.
Proton magnetic resonance spectra of some 6‐ and 7‐substituted 2‐amino‐4‐hydroxypteridines
Authors: Storm Carlyle B. , Chung Hyang S. 
Published: May 2005

AbstractThe conformations in solution of 6‐phenyltetrahydropterin, 6,7‐diphenyltetrahydropterin and 6‐7, dimethyltetrahydropterin have been determined from their proton magnetic resonance spectra. The 6‐phenyltetrahydropterin has the reduced pyrazine ring in a half chair conformation with the phenyl ring in the equatorial position. Both the 6,7‐diphenyl‐ and 6,7‐dimethyltetrahydropterins are of the cis configuration and have one substituent in an axial position. The enzymatic activity of these cofactors may be related to these conformations.
Spin–spin couplings in doubly 15N labelled urea and 1,3‐dimethylurea
Authors: Stilbs P. , Forsén S. 
Published: May 2005

AbstractThe 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.
Hétérocycles contenant du phosphore: XXVII—Etude par Résonance Magnétique Nucléaire de quelques dérivés des dioxa‐et dithiaphospholanes‐1,3,2 monosubstitués en 4
Authors: Revel Monique , Roussel Jean , Navech Jacques , Mathis Ferdinand 
Published: May 2005

AbstractLes spectres de RMN du proton d'une série de dérivés des méthyl‐4 (ou phényl‐4) dioxa‐ et dithiaphospholanes‐1,3,2 sont analysés. La configuration de ces composés est précisée. L'angle de torsion de la liaison C4C5 est calculée et la conformation du cycle à l'état dissous est proposée.
Study of the intermolecular Overhauser effect {1H}13C
Authors: Vandenbosch J. C. , Zimmermann D. , Reisse J. 
Published: May 2005

AbstractThe intermolecular Overhauser effect {1H}13C is studied on two binary systems chloroform/carbon disulfide and chloroform/acetone. Some comments are made concerning the use of these measurements for the study of intermolecular interactions in solution.
Carbon‐13 n.m.r. and CNDO/S study of 1,3‐indandione and heterocyclic analogues
Authors: Galasso Vinicio , Pellizer Giorgio , Le Bail Henri , Pappalardo Giuseppe C. 
Published: May 2005

Abstract13C n.m.r. spectra are reported for 1,3‐indandione, phthalic anhydride, thiophthalic anhydride, and phthalimide in chloroform‐d solution. The 13C chemical shifts have also been calculated by means of Pople's perturbation theory using CNDO/S wave functions; the agreement between theoretical and experimental values is improved when an empirical charge‐dependent term is added to the calculated diamagnetic and paramagneitc contributions.
Déplacements chimiques induits en RMN par un réactif lanthanidique: I—Application á l'analyse conformationnelle de sulfites cycliques
Authors: Cazaux Louis , Chassaing Gérard , Maroni Pierre 
Published: May 2005

AbstractLes déplacements chimiques induits par Eu(fod)3 sur plusieurs séries de sulfites cycliques à six chaînons permettent de déterminer les paramètres Kc et ΔSR de l'équilibre de complexation pour une stoechiométrie supposée 1:1. La constante d'équilibre apparente Kc décroît lorsque la substitution en 4 et 6 et la polarité du substituant en 5 augmentent en relation avec une augmentation de la fréquence d'élongation vSO en infrarouge. Ainsi, le groupe SO ax s'associera plus fortement que SO éq. Lorsqu'un équilibre conformationnel existe en l'absence de réactif lanthanidique, on peut prévoir qu'il évoluera avec lui vers une forme à SO ax. L'exploitation des ΔSR à l'aide de l'équation de pseudocontact confirme ces prévisions: (i) les formes chaises à SO ax sont stabilisées; (ii) les chaises à SO éq avec deux substituants éq en 4 et 6 présentent un équilibre avec quelques pourcents de conformation flexible à SO ax, surtout en l'absence de substituant ax en 5; (iii) les formes croisées d'axe 2–5 à SO intermédiaire et t‐Bu trans en 4 et 6 évoluent vers un bateau à O en proue et SO ax; (iv) le t‐Bu‐5 oxo‐2 dioxathianne‐1,3,2 trans, chaise à t‐Bu et SO axiaux pour ⋍70%, a son équilibre conformationnel complètement déplacé vers cette forme; (v) le triméthyl‐4,4,6 oxo‐2 dioxathianne‐1,3,2, cis, équilibre où interviennent les trois types de SO, s'associe essentiellement en conformation croisée d'axe 1–4 et à groupe SO ax. La plupart de ces résultats ont été étayés par l'analyse des constantes de couplage pour le rapport R0/S0 = 1.
Calculation of carbon‐13 n.m.r. shielding constants of carbocations
Authors: Bernstein Thomas , Geschke Dieter 
Published: May 2005

AbstractCarbon‐13 n.m.r. shielding constants of isobutene molecules and t‐butyl carbenium ions are calculated using the NDDO quantum chemical approximations. The theoretical resonance shifts between corresponding nuclei of the two structures are compared with experimental values published by Olah et al. The results are discussed with respect to excitation energies which are obtained including Fischer‐Kollmar‐parametrization.
Etude par RMN 1H d'hétérocycles voisins de l'oxazaphospholane‐1,3,2: III—Le phényl‐1 dioxa‐2,8 aza‐5 phosphav‐1 bicyclo[3.3.0]octane et le dioxa‐1,6 diaza‐4,9 phosphav‐5 spir[4.4]nonane
Authors: Devillers Jean , Houalla Doureid , Roussel Jean , Wolf Robert 
Published: May 2005

AbstractL'interprétation, en termes d'analyse conformationnelle, des paramètres de RMN 1H obtenus pour le phényl‐l dioxa‐2,8 aza‐5 phosphav‐1 bicyclo[3.3.0]octane, conduit à proposer pour chacun des cycles pentagonaux une forme enveloppe bloquée, l'atome d'azote étant plan. La même analyse appliquée au dioxa‐1,6 diaza‐4,9 phosphav‐5 spiro[4.4]nonane, montre que le cycle oxazaphospholane est le siège de mouvements de libration autour de la liaison CC.
Lanthanide reagent effects on 3‐(2′,4′‐dimethylphenyl)‐1H,3H‐quinazoline‐2,4‐dione
Authors: Buti M. , Vigne C. , Montginoul C. , Torreilles E. , Giral L. 
Published: May 2005

AbstractWe report an analysis of the europium‐shifted n.m.r. spectrum of 3‐(2′,4′‐dimethylphenyl)‐1H,3H‐quinazoline‐2,4‐dione and the resonance assignment of most of its protons.
Analyse RMN (1H, 13C, 31P) de méthoxy‐2 dioxaphospholanes‐1,3,2 di‐ et tétraméthylés en position 4 et 5
Authors: Pouchoulin G. , Llinas J. R. , Buono G. , Vincent E. J. 
Published: May 2005

AbstractL'étude en RMN du 1H, 13C et 31P d'hétérocycles dioxaphospholaniques a permis d'identifier sans ambiguité la position des substituants sur le cycle et aussi d'accéder à un certain nombre d'informations relatives à la stéréochimie de la molécule.
Radikale aus Polymethin‐Farbstoffen: III—Polymethinoxonole
Authors: Oehling H. , Baer F. 
Published: May 2005

AbstractDie ESR‐Spektren von Neutralradikalen in Lösung, erzeugt durch elektrolytische Reduktion einer Reihe dinuclearer Polymethinoxonole, werden interpretiert. Auf Grund der Werte der ESR‐Kopplungskonstanten, von 1H‐NMR‐Messungen und HMO‐Rechnungen werden Konformation, π‐Elektronendichte‐verteilung und CC‐Bindungslängen bei diesen Radikalen diskutiert.
19F n.m.r spectra of heptafluoroisoquinoline and hexafluoro‐1‐methoxyisoquinoline. Long‐range inter‐ring coupling constants
Authors: Matthews R. S. 
Published: May 2005

AbstractAn analysis of the 19F n.m.r. spectra of heptafluoroisoquinoline and hexafluoro‐1‐methoxyisoquinoline is presented. As in heptafluoroquinoline, the inter‐ring FF coupling constants alternate in sign and magnitude and are positive over an odd number of bonds. They correlate with SCFMO carbon–carbon polarizabilities and this infers that the long‐range coupling mechanism is dominated by the contribution from the π‐electron system.
A comment on the interpretation of 13C n.m.r. δ‐syn‐axial substituent effects in cyclic molecules
Authors: Engelhardt G. , Jancke H. , Zeigan D. 
Published: May 2005

AbstractAn alternative explanation for δ‐syn‐axial substituent effects of 13C chemical shifts is given using the concept that the δ‐syn‐axial interaction in the substituted compound replaces the γ‐gauche interaction in the parent molecule. The application of this principle to substituted norbornanes and steroids leads to the conclusion that the ‘net steric’ δ‐syn‐axial shift is upfield rather than downfield.
Long range 13C shift effects (δ and ϵ) in methylcyclohexanes
Authors: Vierhapper Friedrich W. , Willer Rodney L. 
Published: May 2005

Abstract13C n.m.r. spectra of a number of methyl substituted cyclohexanes, some of them conformationally homogeneous, have been recorded in CDCl3 and used to determine shift effects engendered by the introduction of methyl groups on carbon atoms remote from the site of substitution. Sizeable changes in shifts are found, including a substantial effect of an equatorial methyl group on an axial methyl group δ to it (+0.67 ppm, ‘δea’). The effects reported are of consequence in investigations of conformational problems by 13C n.m.r. techniques.
13C n.m.r. spectra of some pyrylium salts and related compounds
Authors: Balaban A. T. , Wray V. 
Published: May 2005

AbstractThe 13C chemical shifts of several alkyl and phenyl substituted pyrylium perchlorates, together with related pyridine and pyridinium salts, are reported. The shifts in the isoelectronic series benzene, pyridine, pyrylium cation correlate well with charge densities calculated by INDO MO theory. Charge densities also account for the shift changes found at C‐3, C‐4 and C‐5 for protonation of pyridine and 2,4,6‐trimethylpyridine. The shift changes observed on protonation for C‐2 and C‐6, along the series pyridine, 2,4,6‐trimethylpyridine and 2,4,6‐triphenylpyridine can only be rationalized by consideration of both charge density and π‐bond order changes. The effects of alkyl substitution on the shifts of the pyrylium cations are not accounted for by charge density changes. Empirical correlations of these shifts with literature data for the alkylbenzenes and the shifts of the phenyl substituted 6‐membered heterocycles are discussed.
Peak suppression in Fourier transform n.m.r.: Application of the selective excitation technique to the problem of 13C satellites in 31P resonance
Authors: Harris R. K. , Newman R. H. , Okruszek A. 
Published: May 2005

AbstractSelective pulsed excitation was used in a technique to suppress the resonance due to a per‐12C‐isotopomer so as to overcome dynamic range problems when observing ‘13C satellite’ lines. The technique was used to obtain nuclear spin parameters from the 13C satellites in 31P resonance for the racemic isomer of P,P′‐dimethyl‐P,P′‐diphenyldiphosphine disulphide, [MePhP(:S)]2.
1H n.m.r. investigation on the site of hydration in the asymmetric diazanaphthalenes
Authors: van de Weijer Peter , Meer Douwe van der 
Published: May 2005

AbstractThe site of hydration for 1,6‐ and 1,7‐naphthyridine has been determined by specific line broadening in the 1H n.m.r. spectra on the addition of water. The site of hydration appears to be the same as the site of protonation (β‐nitrogen atom). The site of hydration in quinazoline has been shown to be the α‐nitrogen atom. This strongly indicates that the site of protonation in this compound will also be at that nitrogen.
The rα‐structure of partially oriented m‐dichlorobenzene as determined from its proton magnetic resonance spectrum including the carbon–13 satellites
Authors: Diehl P. , Bösiger H. 
Published: May 2005

AbstractThe proton spectrum, including 13C satellites at natural abundance, of partially oriented m‐dichlorobenzene was analysed. Carbon–carbon, carbon–hydrogen and hydrogen–hydrogen internuclear distance ratios, as well as bond angles were derived and corrected for harmonic vibrations (rα‐structure).
13C shieldings of several diacetylenic alcohols
Authors: Hearn Milton T. W. 
Published: May 2005

AbstractThe 13C spectra of several conjugated diynols and symmetrical diynediols, represented by the parent compounds 2,4‐pentadiyn‐1‐ol and 2,4‐hexadiyne 1,6‐diol, respectively, have been determined and individual resonances assigned. The data show close similarities to those observed with related 2‐yn‐1‐ols. Mutual shielding interactions between the conjugated triple bonds result in upfield shifts of both the α‐ and β‐sp‐hybridized carbon nuclei by c. 4.5 ppm, relative to the ethynyl prototype. The deshielding γ effect induced by the hydroxyl group at the interior sp‐hybridized carbons is discussed.
Nuclear magnetic resonance of oriented molecules in lyotropic liquid crystalline media: Pyridine, 4‐cyanopyridine and benzonitrile
Authors: Long Robert C. , Goldstein J. H. 
Published: May 2005

AbstractProton distance ratios for pyridine, benzonitrile and 4‐cyanopyridine have been obtained in a potassium laurate lyotropic mesophase and compare favorably with values for different thermotropic solvents. Order matrices indicate a preference for the C2v symmetry axis of the molecules to be aligned along the optical axis of the mesophase.
Unusual temperature effects on the non‐equivalence of geminal methyl groups
Authors: Jennings W. Brian , Al‐Showiman Salim , Tolley Malcolm S. , Boyd Derek R. 
Published: May 2005

AbstractThe geminal anisochronism (Δδ) of the isopropyl methyl groups in the 1H n.m.r. spectrum of N‐[2‐methyl‐1‐(1‐naphthyl)propylidene]benzylamine showed unusually pronounced temperature and solvent effects. It is noteworthy that both accidental and dynamic equivalences were encountered. Slow rotation around the naphthyl–imino bond is responsible for the diastereotopic nature of the geminal methyl groups, and possible reasons for the temperature and solvent effects are considered. Surprisingly, the temperature effect was not reflected in the 13C anisochronism of the methyl carbon nuclei. These observations underline the need for caution in drawing firm conclusions from variable temperature studies on geminal non‐equivalence.
Aufspaltungseffekte höherer Ordnung in 1H‐CW‐off‐resonance‐entkoppelten 13C‐NMR‐spektren
Authors: Radeglia R. 
Published: May 2005

AbstractZusätzliche Aufspaltungen, die durch Effekte höherer Ordnung in 1H‐off‐resonance‐entkoppelten 13C‐NMR‐Spektren hervorgerufen werden, können mit Hilfe eines einfachen Modells abgeschätzt werden.
Effects of simultaneous 13C1H and 13C11B decoupling on the carbon‐13 n.m.r. spectra of some organoboron compounds
Authors: Mazurek M. , Mallard T. M. , Gorin P. A. J. 
Published: May 2005

Abstract13C FT n.m.r. spectra were obtained from borates of methyl α‐D‐glucopyranoside, triethylboron, sodium tetraphenylboron and 1‐butaneboronic acid employing a simultaneous 1H and 11B decoupling network. The effectiveness of the system was evident using the three latter organoboron compounds. Since no diminution in line broadening of signals occurred in the spectrum of the sugar borate, the absence of 13CO11B coupling was indicated. This finding was confirmed by comparing the effects of varying the temperatures of sugar borates and triethylboron on their conventional 13C n.m.r. spectra.
Carbon‐13 nuclear magnetic resonance spectroscopy of polychlorinated diphenyl ethers
Authors: Edlund Ulf , Norström Åke 
Published: May 2005

AbstractCarbon‐13 n.m.r. chemical shifts are reported for diphenyl ether and several of its chlorinated analogs. Carbon shieldings were found to be dependent on the degree of steric interference to conjugative release by the oxygen atom. Substituent effects due to attached chlorine atoms seem to be more easily transmitted into the adjacent aryl moiety if the number of ortho chlorines is reduced. These results were clearly seen by a variable‐by‐variable plot display or by using a pattern recognition computer program. Spin‐lattice relaxation times have been measured for several compounds mainly for assignment purposes, but they are also briefly discussed in view of possible interconversion mechanisms.
Carbon‐13 nuclear magnetic resonance spectra of some aromatic diterpenoids
Authors: Nishida Toshiaki , Wahlberg Inger , Enzell Curt R. 
Published: May 2005

Abstract13C n.m.r. studies of a series of tricarbocyclic ring C aromatic diterpenoids using proton‐noise and single‐frequency off‐resonance decoupling, partially relaxed Fourier transform techniques, shift reagents and specifically labelled derivatives have permitted unequivocal assignments of almost all signals. The shieldings caused by oxygenation of C‐18 and C‐19 and by introduction of various substituents in the aromatic ring are discussed. It is concluded that the 13C n.m.r. data are sufficiently characteristic to allow stereochemical assignments. Some effects of deuterium substitution on the carbon resonances are presented.
The proton magnetic resonance spectra of some disubstituted acetophenones
Authors: Batts B. D. , Pasaribu S. J. , Williams L. R. 
Published: May 2005

AbstractAn accurate measurement of the chemical shifts and the coupling constants of some disubstituted acetophenones has been made. The acetophenones studied contained nitro, bromo or amino groups substituted in either the 3,4 or 2,5 positions. For compounds with no substituent adjacent to the acetyl group, the chemical shifts and the coupling constants estimated by the simple additivity of the substituent increments were found to be in reasonable agreement with the experimental values. The two nitro, bromo derivatives substituted in positions 2 and 5 probably prefer the conformer in which the proton H‐6 is adjacent to the acetyl methyl group whilst the 3,4‐disubstituted bromo, nitro and bromo, amino derivatives prefer the conformer in which the H‐6 proton is adjacent to the carbonyl group.
13C n.m.r. spectra of lysergic acid derivatives: II—dihydrolysergamides
Authors: Zetta L. , Gatti G. 
Published: May 2005

AbstractThe conformations of dihydrolysergamides and 10‐methoxydihydrolysergamides have been inferred from the study of the chemical shifts of the amide hydrogens and their temperature dependence. The 13C chemical shifts have been shown to be sensitive to conformational changes of the piperidine ring. The results indicate that the conformation of the latter depends both on the substituent in position 10 and on the solvent.
Steric effects in 29Si and 13C n.m.r. spectra of trimethylsiloxy‐substituted benzenes
Authors: Schraml J. , Chvalovský V. , Jancke H. , Engelhardt G. 
Published: May 2005

Abstract29Si and 13C NMR spectra of trimethylphenoxysilane and three bis(trimethylsiloxy)benzenes have been investigated. Crowding in the ortho derivative leads to a small but observable deshielding of both silicon and carbon nuclei of the trimethylsiloxy group. In comparison to analogous effects observed in trimethylsilyl‐substituted benzenes the oxygen link appears to increase the susceptibility of silicon to this steric effect but to decrease that of the methyl carbons. It is suggested that the operative steric interaction might not be that between the terminal methyl groups but involves the oxygen atom.
Steric gamma effect on 29Si shielding in (CH3)3SiOCR groups
Authors: Engelhardt G. , Schraml J. 
Published: May 2005

AbstractIt is shown that a γ‐methyl group causes approximately a 3 ppm upfield shift of the silicon resonance in (CH3)3SiOCCH3 fragments. It is estimated that polar effects contribute about 0.8 ppm to this shift leaving a net steric γ upfield shift of 2 ppm. Variations around these average values are interpreted by different conformer populations, the importance of which are illustrated by the shifts in trimethylsiloxy derivatives of adamantane.