Magnetic Resonance in Chemistry

doi: 10.1002/omr.1270210801pmid: N/A

Klessinger, Martin; Cho, Jung‐Hyuck

doi: 10.1002/omr.1270210802pmid: N/A

The dependence of three‐bond 13C‐13C couplings of cis‐butane and cis‐butene on the valence angle, the torsional angle of the methyl groups and methyl and methylene substituents is discussed on the basis of INDO‐SCPT calculations. The results support the interpretation of the experimental couplings between the bridgehead carbons of bicyclic hydrocarbons based on a multiple‐path mechanism.

Dando, Neal R.; Gold, Harvey S.; Dyboski, Cecil

doi: 10.1002/omr.1270210803pmid: N/A

The assignment of 13C chemical shifts of several symmetric tetraalkylammonium salts in acetonitrile is based on alkyl chain carbon spin–lattice relaxation rates (T1−1), and on concentration‐dependent changes in these rates.

Leon, V.; Bolívar, R. A.; Tasayco, M. L.; Gonzalez, R.; Rivas, C.

doi: 10.1002/omr.1270210804pmid: N/A

The 13C NMR spectra of a representative series of photoadducts derived from methylmaleic anhydrides and thiophene, containing the same basic carbon skeleton, were studied in order to establish a model for future structure elucidation of similar compounds. Selective heteronuclear 13C{1H} NOE was applied to stereochemical studies on the same compounds with the dual purpose of confirming previous assignments and to gain experience in the use of this technique, rarely applied in organic chemistry. The technique was employed in two different ways, by irradiating the selected proton resonance with and without modulation [νm=½J(CH)].

Balasubrahmanyam, S. N.; Bharathi, S. Narasimha; Usha, G.

doi: 10.1002/omr.1270210805pmid: N/A

The currently accepted geometry of carbonyl magnetic anisotropic effects, if regarded as extendable to the thione group, imply that the more deshielded of the two H‐α lines in the 1H NMR spectra of α,α‐di‐tert‐butylthioacetic esters should be assigned to the ‘180° form’ in which CS is antiperiplanar to C‐αH. CNDO results, however, indicate the opposite; the line should be assigned to the ‘O° form,’ in which CS eclipses C‐αH, if the predicted considerable increase in 1s density at H‐α on going from the 0° form to the 180° form is taken into account. A change in 1s density at H‐α as a result of increased branching of alkylation at C‐α is also found. These specific effects must be taken into account when discussing the anisotropic effects of CO or CS on H‐α shifts.

Gibson, Thomas

doi: 10.1002/omr.1270210806pmid: N/A

Using selective deuteriation and broad‐band and off‐resonance proton decoupling techniques, complete signal assignments are made for the 13C NMR spectra of a number of ketones and unsaturated esters with the bicyclo[2.1.1]hexane ring systems.

Uriac, Ph.; Huet, J.

doi: 10.1002/omr.1270210807pmid: N/A

The 13C NMR spectra of 23 2,3‐dihydro‐1H‐benzo[b]azepines, including nine pairs of diastereomers separated by chromatography, [(2R*, 3R*) and (2R*), 3S*)] are hereby assigned and discussed. The relative configurations of the diastereomers were assigned by two methods. The first, based on the chemical shifts of the asymmetric carbons C‐2 and C‐3 (with regression analysis), shows that the values for (R*, R*) are approximately 1 ppm lower than those for (R*, S*) diastereomers. The second method uses the chemical shifts, δ3, of the R3(CH3) substituents. When these δ3 values are compared by means of the δ3‐δm difference (δm is the mean value obtained from compounds where R2=H), the difference is always negative for (R*, R*) and positive for (R*, S*). This is attributed to a γ‐gauche effect between R2 and R3 in the case of (R*, R*) diastereomers (R2 and R3 are cis). The results corroborate those already obtained by 1H NMR [J(23)(R*, R*)<J(23)(R*, S*)] and are a confirmation of the results of a radiocrystallographic examination carried out on two nitrogen acetylated diastereomers.

Bock, Paul L.; Mosbo, John A.; Redmon, John L.

doi: 10.1002/omr.1270210808pmid: N/A

Oxygen‐17 NMR spectra were obtained from the four pairs of isomeric 2‐R‐2‐oxo‐1,3,2‐dioxaphosphorinanes, where R=OMe (2), NMe2 (3), H (4) or Me (5). The isomerism has been previously shown to be configurational at phosphorus, with one isomer of each pair having an equatorial phosphoryl oxygen (a isomer), and the other an axial orientation for phosphoryl oxygen (b isomer). Only data for the phosphoryl oxygen are reported. Substitution of OMe or NMe2 for H or Me produced upfield shifts of 27.9‐41.8 ppm. In all cases, the chemical shifts of the a isomers were upfield of the b analogs, with differences of 7.9, 18.0, 20.3 and 8.6 ppm for 2—5, respectively. The absolute values of 1J(31P17O) were 5–9 Hz larger for the a isomers.

Goldwhite, Harold; Rowsell, David; Vertal, Lawrence E.; Bowers, Michael T.; Cooper, M. Ashley; Manatt, Stanley L.

doi: 10.1002/omr.1270210809pmid: N/A

The 100 MHz proton and 40.4 MHz 31P NMR spectra of phosphirane (1) have been recorded at −20 °C and analysed iteratively to yield coupling constants and chemical shifts. The 22.6 MHz 13C spectrum of 1 was recorded at 0 °C and analyzed. The 31P chemical shift of 1 was measured as 40 467 515.97 ± 0.08 Hz relative to TMS as 100 000 000.00 Hz. The geminal P‐C‐H couplings in 1 are opposite in sign and of different magnitudes (+16.14 and −2.64 Hz); the PH coupling (+158.34 Hz) is smaller than that in any other organic phosphine. These observations are discussed and correlated with the geometry of 1. The electronic structure of the strained ring of 1 is discussed in terms of a localized valence bond approach.

Lamphun, B. Na; Webb, G. A.; Witanowski, M.

doi: 10.1002/omr.1270210810pmid: N/A

Some INDO/S parameterized SOS shielding calculations are reported for the nitrogen nuclei of some N‐heterocycles. Hydrogen bonding with H2O and CF3CH2OH is incorporated in a supermolecule approach. The calculated effects of hydrogen bonding are found to be in satisfactory agreement with observed nitrogen shielding variations upon a change of solvent.