Magnetic Resonance in Chemistry

doi: 10.1002/mrc.1260230201pmid: N/A

Lambert, Joseph B.; Simpson, Sharon Vlasak

doi: 10.1002/mrc.1260230202pmid: N/A

The effect of lithium chloride on the relaxation mechanisms of simple acid and alcohols has been examined. Dipolar relaxation is predominant for acetic acid and ethanol. Lithium chloride (approximately 10%) significantly increases the rate of dipolar relaxation for all carbons and protons of acetic acid, probably through an increase in aggregate size or viscosity. Lithium chloride (approximately 1.5%) has a similar but smaller effect on the protons of ethanol. The methyl carbons and fluorines of trifluoroacetic acid and 2,2,2‐trifluoroethanol relax primarily by spin‐rotation. With about 10% lithium chloride, dipolar relaxation becomes more important for trifluoroacetic acid. With about 1.5% lithium chloride, trifluoroethanol possibly shows an enhancement of spin‐rotation relaxation for the CF3 nuclei.

Newmark, Richard A.; Siedle, Allen R.

doi: 10.1002/mrc.1260230203pmid: N/A

Proton and 13C NMR spectra are reported for cis‐(amine)Rh(CO)2Cl and trans‐(amine)PtCl2(C2H4), where amine = tetramethylpyrazine, phenazine, phenazine N‐oxide and quinoxaline. Relative labilities of the amine and ethylene ligands were determined.

Davies, David B.; Rajani, Pramila; MacCoss, Malcolm; Danyluk, Steven S.

doi: 10.1002/mrc.1260230204pmid: N/A

A number of uracil cyclo‐nucleosides with known glycosidic bond conformations have been synthesized as model compounds for evaluating the Karplus parameters of vicinal carbon‐proton coupling across the glycosidic bond. The magnitudes of 3J(C‐2, H‐1′) and 3J(C‐6, H‐1′) were determined from proton‐coupled 13C NMR measurements, and both coupling constants showed an approximate Karplus dependence with dihedral angle. Careful analysis of the results on model compounds reveals that the C‐2, H‐1′ and C‐6, H‐1′ coupling paths are not equivalent, and so the magnitudes of Karplus parameters have been determined for each coupling path for the Karplus relationship in the form 3J=A cos2 θ + B cos θ + C, i.e. A6 = 6.2, B6 = −2.4 and C6 = 0.1 Hz and A2 = 5.0, B2 = 2.1 and C2 = 0.1 Hz. Comparison is made between proton‐carbon coupling magnitudes in nucleoside and peptide coupling paths.

Gate, E. N.; Hooper, D. L.; Stevens, M. F. G.; Threadgill, M. D.; Vaughan, K.

doi: 10.1002/mrc.1260230205pmid: N/A

A series of N‐hydroxymethylamides, RCONR′CH2OH, and their O‐methyl and O‐acetyl derivatives, have been studied by 13C and 1H magnetic resonance spectroscopy. Signals have been assigned to the E‐ and Z‐isomers on the basis of the analysis of the fully coupled spectra, and by comparison of the chemical shifts with those of model compounds. The introduction of the hydroxy, alkoxy or acetoxy groups at the α‐position of the N‐alkyl moiety causes a significant shift in the equilibrium towards the E‐rotamer compared with the unsubstituted N‐alkylamide. The predominant effect in determining the E: Z ratio appears to be the steric interaction between the carbonyl oxygen and the α‐oxygen in the alkyl moiety; intramolecular hydrogen bonding does not play a significant role in determining the rotamer populations of these molecules.

Foster, N.; Pestel, B.

doi: 10.1002/mrc.1260230206pmid: N/A

The temperature‐dependent 13C NMR spectra of a series of para‐substituted 1‐phenyl‐3,3‐tetramethylenetriazenes have been analyzed by the band width method. Electron‐donating substituents clearly accelerate the rate of rotation about the N‐2N‐3 bond in the triazenes; electron withdrawers retard that motion. The exchange averaging process observed in the carbon spectra obeys a linear free energy relationship when Hammett σ* parameters are used in the correlation. These results corroborate those already obtained by analysis of 1H NMR spectra of similar triazenes, regarding the likely contribution of a 1,3‐dipolar resonance form to the ground state of the molecule.

Chimichi, Stefano; Tedeschi, Piero; Nesi, Rodolfo; Ponticelli, Fabio

doi: 10.1002/mrc.1260230207pmid: N/A

The 13C NMR spectra of oxazolopyridine systems are discussed, and a simple method for distinguishing between 2‐methyloxazolo‐ and 3‐methylisoxazolo‐pyridines is suggested, based on the chemical shift of the methyl group. The effect of the 2‐methyloxazole fusion on the pyridine ring is comparable to that of 3‐methylisoxazole, judging by 1J(CH) values determined from line splittings.

Kövér, Katalim E.; Borbély, J.

doi: 10.1002/mrc.1260230208pmid: N/A

The methoxy group conformation of chromone derivatives was studied by 1H NMR methods. For the qualitative determination of the conformational preferences of the model compounds, proton relaxation rates, spin‐spin decoupling difference spectroscopy and 2D COSY experiments were applied. NOE data were used in the calculations of conforrmer population ratios in these six‐spin (ABCX3) systems.

Cho, Jung‐hyuck; Klessinger, Martin; Tecklenborg, Ute; Wilhelm, Konrad

doi: 10.1002/mrc.1260230209pmid: N/A

CC coupling constants of ten bicyclic hydrocarbons and ten cyclic ethers or carbonyl compounds were determined from the satellites of broad band decoupled spectra or from INADEQUATE measurements. The influences of valence angles, dihedral angles, hybridization and heterosubstitution on geminal and vicinal CC coupling constants are discussed in terms of the algebraic additivity of the contributions from individual coupling paths within the framework of a multiple path mechanism.

Inouye, Yoshinobu; Takaya, Kazuharu; Kakisawa, Hiroshi

doi: 10.1002/mrc.1260230210pmid: N/A

The 1H and 13C NMR spectra were assigned for a series of N‐alkyl‐α‐methoxycarbonylmethanimine N‐oxides and N‐methyl‐α‐phenyl‐,N‐methyl‐α‐(pentamethylphenyl)‐ and N‐phenyl‐α‐benzoyl‐methanimine N‐oxides. Data are presented which allow the assignment of their configurations.