Magnetic Resonance in Chemistry

doi: 10.1002/mrc.1270090201pmid: N/A

Pobedimskii, D. G.; Nasybullin, Sh. A.; Kirpichnikov, P. A.; Svitych, R. B.; Yablonskii, O. P.; Buchachenko, A. L.

doi: 10.1002/mrc.1270090202pmid: N/A

The vanadium IV ion in vanadyl acetylacetonate (VIV) forms labile paramagnetic complexes with organic phosphites in the first coordination sphere. The enthalpy of complex formation between VIV and triphenyl phosphite was 2.6 kcal mol−1. Complex formation enthalpies ΔH and the activation energies E of ligand (hydroperoxide) escape from the metal ion sphere were determined from the temperature dependence of paramagnetic broadening of the n.m.r. lines of hydroperoxides in the presence of vanadyl acetylacetonate. At low temperatures the phosphite sharply weakens the bond between the metal ion and hydroperoxide in the second coordination sphere (ΔH decreases fivefold). Taken in excess, phosphite displaces the hydroperoxide molecules from the coordination sphere of the VIV ion and thus blocks it. The observed n.m.r. characteristics of the paramagnetic complexes explain, on the model level, the kinetic regularities of the reaction of hydroperoxides with phosphite catalysed by transient metal ions.

Wolff, R.; Radeglia, R.

doi: 10.1002/mrc.1270090203pmid: N/A

Auf der Basis des CNDO/2‐Verfahrens werden paramagnetische Abschirmungskonstanten für das Zentralatom tetrasubstituierter Siliciumverbindungen Me4–nSiXn (X = F, OMe, NMe2, C1) mit und ohne ΔE‐Näherung berechnet und mit den experimentellen 29Si‐NMR‐chemischen Verschiebungen verglichen. Die aus dem Vergleich der berechneten Werte erhaltenen ‘mittleren Anregungsenergien’ ΔE sind von der Ladung am Zentralatom abhängig und können für quantitative Betrachtungen nicht als konstant angesehen werden.

Douglas, Alan W.

doi: 10.1002/mrc.1270090204pmid: N/A

Values of long range 13CH coupling through the double bond in a number of isopropenyl compounds are reported. There is evidence that substituent dependence of 3J(CH) is not related linearly to that of HH couplings in vinyl compounds. The ratio of 3J(CH) to 3J(HH) in analogous pairs of compounds appears to increase with decreasing substituent polarity and increasing steric bulk.

van de Weijer, Peter; Meer, Douwe van der

doi: 10.1002/mrc.1270090205pmid: N/A

The 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.

Albright, Thomas A.; Freeman, Walter J.

doi: 10.1002/mrc.1270090206pmid: N/A

The 13C and 15N n.m.r. results for a series of diazo compounds are reported. It is found that the diazo carbon is shielded by an extraordinary amount compared with normal sp2 hybridized carbons. The 15N chemical shifts reveal that the terminal nitrogen is deshielded relative to the central one. This is contrary to that expected from charge effects but support is found for this phenomenon in other systems. One‐bond 13C14N coupling in diazomethane is also reported for the first time. INDO MO calculations of the charges and finite perturbation calculations of 13C14N and CH couplings are compared with the experimental results.

Graham, Laurine L.; Vanderkooi, Garret; Getz, Joseph A.

doi: 10.1002/mrc.1270090207pmid: N/A

A new method of conformational analysis has been developed, in which energy minimization calculations are combined with lanthanide‐induced shift data. First, exhaustive energy calculations are carried out on the free molecules in order to determine the conformations of lowest energy. Then, the coordinates of all low energy conformations or pairs of conformations are used in the pseudocontact shift equation for lanthanide‐induced shifts in order to find which of the theoretically obtained conformation(s) gives the best agreement with experiment. The molecules complexed to the lanthanide shift reagent were N,N‐diisopropylformamide (DIPF) and N,N‐diisopropylacetamide (DIPA). Two different lanthanide shift reagents were used, Eu(fod)3 Fod is the anion of 1,1,1,2,2,3,3‐heptafluoro‐7,7‐dimethyl‐4,6‐octanedione‐d27. and Pr(fod)3, in order to check the validity of the method. Proton magnetic resonance spectra were taken at 6 °C in carbon tetrachloride solution. The principal conformation found was different for each amide. DIPF was found to exist as a mixture of I (39 mol%) and II (61 mol%) with Eu(fod)3, and a mixture of I (37%) and II (63%) with Pr(fod)3. DIPA was found to exist as a mixture of I (79%) and IV (21%) with Eu(fod)3 and a mixture of I (87%) and IV (13%) with Pr(fod)3. For both molecules, the two conformations of lowest computed energy were also the pair which gave the best fit to the lanthanide shift reagent data. The location of the principal magnetic axis of the complex was found to lie between 0° and 14° from the lanthanide atom–oxygen atom bond axis. The technique of combining lanthanide shift reagent data with energy calculations shows great promise in conformational analysis.

Kleinpeter, E.; Kühn, H.; Mühlstädt, M.

doi: 10.1002/mrc.1270090208pmid: N/A

Die 1H‐NMR‐Spektren einer Reihe substituierter Oxa‐ bzw. Azatetracycloundecane werden aufgezeigt und an ausgewählten Beispielen mit Hilfe von Spin‐Spin‐Entkopplung und Eu(fod)3—paramagnetischen Verschiebungsversuchen eingehend analysiert. Aus den mit angeführten 13C‐NMR‐Spektren werden die einzelnen C‐Resonanzen unter Verwendung bekannter Substituenteneffekte und paramagnetischer Signal‐verschiebungen zugeordnet. Die paramagnetischen Verschiebungswerte ΔEu werden hinsichtlich ihrer Kontakt‐ und Pseudokontaktbeiträge diskutiert.

Diehl, P.; Bösiger, H.

doi: 10.1002/mrc.1270090209pmid: N/A

The 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).

Sørensen, S.; Jakobsen, H. J.

doi: 10.1002/mrc.1270090210pmid: N/A

The 31P31P and 13C31P coupling constants in 1,6‐diphosphatriptycene have been obtained from analysis of its proton decoupled 13C n.m.r. spectra. More accurate data, however, resulted from simultaneous analysis of the proton decoupled 13C spectra and 31P(13C) satellite spectra. The 13C31P couplings are strongly influenced by the proximity and orientation of the phosphorus lone pair electrons. The first 31P31P coupling in an aromatic diphosphine is reported.