Magnetic Resonance in Chemistry

doi: 10.1002/mrc.1260230701pmid: N/A

Sidorkin, V. F.; Pestunovich, V. A.; Voronkov, M. G.

doi: 10.1002/mrc.1260230702pmid: N/A

The diamagnetic and paramagnetic terms of the 29Si nuclear shielding constant of silatranes, XSi(OCH2CH2)3N, and the related four‐coordinated silicon compounds, triethoxysilanes, XSi(OC2H5)3, have been estimated to explain the experimental peculiarities of their 29Si NMR spectra. High‐field shifts of the 29Si NMR signals of silatranes relative to triethoxysilanes are of diamagnetic origin, arising mainly from the existence of the Si←N bond and pentacoordination of the silicon atom in silatranes. At the same time, the decrease in the positive charge on the silicon atom upon Si←N interaction increases the absolute value of the negative paramagnetic term. When the Si←N bond in silatrane molecules becomes stronger through introduction of a more electronegative substituent X, the absolute value of the difference in the paramagnetic terms between silatranes and the related triethoxysilanes increases to a greater extent than that in the diamagnetic terms. This reduces the difference in 29Si chemical shifts of the corresponding four‐ and five‐coordinated silicon compounds with increasing electronegativity of the substituent X.

Díaz, E.; Domínguez, G. G.; Mannino, A.; Negrón, G.; Jankowski, K.

doi: 10.1002/mrc.1260230703pmid: N/A

The assignment of the proton and carbon spectra of some cyclopropane sesquiterpene lactone derivatives of helenalin and anhydroparthenin is presented. The interpretation of the spectra was achieved using 1H‐13C two‐dimensional heteronuclear chemical shift correlation, proton two‐dimensional homonuclear correlated 2D and 2D‐J, and some classical techniques. In particular, the spectra of halocyclopropane derivatives are discussed and compared with the spectra of the parent sesquiterpenes.

Mahendroo, Prem P.; Dean Sherry, A.

doi: 10.1002/mrc.1260230704pmid: N/A

A series of sulfur compounds have been examined by 33S NMR at 15.3 MHz. The sharpest resonances were found for the inorganic sulfates, with line widths varying from 3.6 Hz for ammonium sulfate to 9.0 Hz for sodium sulfate. Increasing the viscosity of the solvent did not affect the line widths appreciably. Other anions and molecules examined, and their corresponding line widths, included cysteic acid (30 Hz), sodium thiosulfate (60 Hz), ammonium sulfamate (300 Hz), carbon disulfide (400 Hz), concentrated sulfuric acid (1200 Hz) and dimethyl sulfate (2000 Hz). A comparison of the measured line widths and the spin‐lattice relaxation times, T1, for five of these samples indicated that exchange processes may contribute to and often dominate the 33S resonance line widths in aqueous samples. No signals were observed in concentrated aqueous solutions of cysteine, adenosine monosulfate or chondroitin sulfate after overnight scans.

Sohár, P.; Pelczer, I.; Váczi, I.; Tombor, A.; Matolcsy, G.

doi: 10.1002/mrc.1260230705pmid: N/A

N‐Acyl derivatives of 1,3,3‐trimethyl‐6‐azabicyclo[3.2.1]octane were synthesized as compounds with potential pesticidal activity. The conformation of the starting compound and the substituent dependence of the amide rotamers of the acyl derivatives were studied by 1 and 13C NMR spectroscopy, making use of solvent effects (ASIS) and DR, DNOE and DEPT measurements.

Kerschl, Susanna; Sebald, Angelika; Wrackmeyer, Bernd

doi: 10.1002/mrc.1260230706pmid: N/A

Secondary 13C/12C isotope effects (Δδi) over one bond on the 119Sn nuclear shielding have been determined for 42 organotin compounds. Changes in the magnitude and sign of Δδi are related to the nature of the carbon atom and the nature of the other substituents at the tin atom. In addition to the rotational and vibrational effects connected with the Sn13C bond itself, Δδi is affected by the bonding of tin to the other substituents. In the case of polar bonds this leads to deshielding of the tin atom in the heavy isotopomer, which compensates the shielding effect normally associated with heavy isotopic substitution. Therefore, the magnitude of Δδi values and of the SnC bond distances are not related in a simple manner. A relationship exists between J(119Sn13C) and Δδi in alkynyltin compounds. The Δδi (13C/12C) values for 29Si and 207Pb shielding in some representative organo‐silanes and ‐plumbanes are reported for comparison.

Kamounah, Fadhil S.; Al‐Sheibani, Ikbal. S.; Shibaldain, Nazar L.; Salman, Salman R.

doi: 10.1002/mrc.1260230707pmid: N/A

Proton NMR data for a number of 1‐X‐4‐Y‐3,5‐dimethylbenzenes and 1‐X‐2‐Y‐4‐Z‐3,5‐dimethylbenzenes are presented. The proton chemical shifts of the methyl groups at positions 3 and 5 are mainly affected by the ring current effect of the substituents. This conclusion was strongly supported by the proton chemical shifts calculated using additivity rules.

Spraul, M.; Keul, H.; Pfeffer, B.; Hähnle, J.; Griesbaum, K.

doi: 10.1002/mrc.1260230708pmid: N/A

The 13C NMR data of 42 ring‐halogenated oxiranes containing fluorine, chlorine, bromine and, in one case, iodine as substituents are reported and discussed with respect to the influence of the halogen and of other substituents on the chemical shifts of the ring carbon atoms. For monocyclic mono‐ and dichlorooxiranes increments have been determined which allow the calculation of the chemical shifts of the ring carbon atoms. Comparison of the 13C NMR data of substituted 1,2‐dihaloethylenes with those of the corresponding epoxides showed that the signals of the ring carbon atoms of the halogenated oxiranes always appeared at considerably higher field than those of the sp2 hybridized vinylic carbons in the respective precursors.

Dimitrov, V. S.; Ladd, J. A.

doi: 10.1002/mrc.1260230709pmid: N/A

The field dependence of the 1H and 13C coalescence temperatures in N,N‐dimethyltrichloroacetamide has been used to determine the most reliable activation parameters for internal rotation of the amide moiety. The results were found to be in excellent agreement with those obtained earlier by the double‐fitting procedure and demonstrate its validity. Direct measurements of the T2 relaxation times of the methyl protons show that T2 depends exponentially on temperature.

Duddeck, Helmut; Hanna, Atef G.

doi: 10.1002/mrc.1260230710pmid: N/A

The 17O chemical shifts and one‐bond phosphorus‐oxygen coupling constants of a series of 1‐adamantylphosphoryl derivatives are reported and discussed. It was found that a significant 2p electron transfer towards the phosphoryl oxygen in the phosphonic esters is clearly reflected by the 17O NMR parameters.