Carbon-13 and proton magnetic resonance studies of some carbonyl-bis-(amino acid esters)

A number of carbonyl-bis-(amino acid esters) have been examined by proton and carbon-13 nuclear magnetic resonance techniques. All but one of the compounds were synthesized with two chiral centers of like-configuration. In one series, the diastereotopic nonequivalence of isopropyl methyl groups attached to the asymmetric centers is apparent in both the proton and the carbon spectra, and the relative magnitude of the observed nonequivalence increases slightly with increasing ‘bulk’ of the neighboring ester groups. Carbon-13 chemical shifts are reported, and a linear correlation of Taft σ* inductive constants with ester carbonyl carbon chemical shifts and with amide proton chemical shifts (for a series in which only variation of the ester substituent occurs) is presented. In addition, the effect in terms of chemical shift differences of keeping the same ester group at the asymmetric centers while varying the other substituent group, is examined.     

Nuclear magnetic resonance study of the molecular structure of 1,3-benzodioxole and 1,3-indandione partially oriented in nematic solvents

The molecular structures of 1,3-benzodioxole and 1,3-indandione partially oriented in nematic solvents have been studied by NMR. Results for 1,3-indandione agree well with a planar model. The structural parameters are similar to those obtained by X-ray studies. No keto-enol tautomerism is observed. An anomalous structural result is obtained for 1,3-benzodioxole if the molecule is assumed to be planar. Dipolar couplings agree well with puckered model. The puckering angle is found to be about 15.5° ± 2°, similar to the values observed for TTF, TSeF and 2,5-dihydrofuran.     

Conformational studies of cyclo(L-Phe-L-Pro-Gly-L-Pro)2 by 13C nuclear magnetic resonance

The 13C-NMR spectrum (Fig. 2,1) of cyclooctapeptide cyclo(L-phe-L-Pro-Gly-L-Pro)2 (A) in CDC13 suggested that its conformation involved the coexistence of two kinds of C2-symmetric conformation with trans-trans-trans-trans and cis-trans-trans-trans forms. Adding 0.5 equivalent of CsSCN or one equivalent of DL-Phe-OMe.HCl to the solution of cyclopeptide (A) in CDC13 yielded 13C-NMR spectra (Fig. 2,2 and Table I) which suggested a single C2-symmetric conformation with trans-trans-trans-trans form, resulting from the formation of complexes with CsSCN or DL-Phe-OMe.HCl. The 13C-NMR spectrum of complexes of A with DL-Phe-OMe.HCl displayed separate resonances for C(gamma), C(o), C(m), C(alpha), and C(beta) of D-Phe-OMe.HCl and L-Phe-OMe.HCl (Table I).     

13C nuclear magnetic resonance studies of cellulose acetate

¹³C-NMR spectra of ring carbons and O-acetyl carbonyl carbons of cellulose acetate (CA) in dimethyl sulfoxide-d₆ were analyzed. The CA samples with the degree of substitution (DS) ranging from 0.84 and 1.91 were prepared by homogeneous acetylation of cellulose with acetic anhydride in a 10% LiCl/dimethyl acetamide solvent. It was found that the use of these low DS samples permitted easier assignments not only of the ring carbon but also of the O-acetyl carbonyl carbon signals. The assignments were confirmed by comparing with the ¹H-NMR spectra of the samples obtained by complete acetylation of the corresponding CA samples with acetyl-d₃ chloride. Two methods for determining the distribution of O-acetyl groups of CA, i.e., the relative DS at the three different types of hydroxyl groups, were developed. One is based on the measurements of the relative intensities of the signals for the ring carbons and the other is based on the measurements of the relative intensities of the signals for the O-acetyl carbonyl carbons.     

13C magnetic resonance studies of reduced proaporphine alkaloids

The natural abundance ¹³C magnetic resonance spectra of reduced proaporphines are reported, with complete assignments based on peak multiplicity and empirical calculations of chemical shifts. The chemical shift values of C-8, C-12 and C-7 are found to be strongly diagnostic of the spiro carbon configuration.     

The structure of pyrazine determined from its proton magnetic resonance spectrum,including the carbon-13 satellites

The structure of pyrazine partially oriented in a liquid crystal has been determined from its proton magnetic resonance spectrum utilising the ¹³C satellites (at natural abundance). Carbon–carbon, carbon-hydrogen and hydrogen–hydrogen internuclear distance ratios have been determined.     

13C nuclear magnetic resonance studies on some substituted isoxazoles

¹³C n.m.r. spectra of some substituted isoxazoles have been examined to ascertain the reactive site in the metallation of 4-substituted 3,5-dimethylisoxazoles. The results obtained indicate that metallation occurred exclusively at the C-5 methyl.     

Conformational studies of cyclo-gly-pro(S) by X-ray diffraction and proton magnetic resonance

The X-ray crystal structure and the PMR spectrum in ²H₂O and DMSO-d₆ of cyclo-glycyl-4-thiaprolyl has been determined. In the crystal, the diketopiperazine ring of the molecule adopts a boat conformation and the thiazolidine ring an envelope conformation, very similar to the analogous compound cyclo-glycyl-prolyl. Comparison of proton-proton dihedral angles derived from the crystal structure and from vicinal coupling constants in solution indicates that the conformation of cyclo-glycyl-4-thiaprolyl is nearly the same in crystal and in solution in the limits of applied mehods.     

13C nuclear magnetic resonance spectra of arylthallium(III) bis(trifluoroacetates)

¹³C chemical shifts and ¹³C? ²⁰⁵Tl spin–spin coupling constants are reported for phenylthallium(III) bis(trifluoroacetate) and some of its mono- and dimethyl derivatives. The signs of ⁿJ(¹³C, ²⁰⁵Tl) relative to ⁿ⁺¹J(¹H, ²⁰⁵Tl) are determined by offresonance decoupling of protons.     

The conformation of (μ-butatriene)hexacarbonyldiiron complex from proton NMR studies in three nematic phases

Proton NMR spectroscopy in three different liquid crystals has been used to determine two conformational angles of (μ-butatriene)hexacarbonyldiiron complex, namely the angle between the two CH₂ planes and the dihedral angle between the two planes containing four carbon atoms of the butatriene moiety. The values are 44 and 46°, respectively. The direct and the indirect geminal HH couplings are shown to be of the same sign in the liquid crystals with positive diamagnetic anisotropy.     

A NMR study of tetrathiofulvalene and tetracyanoquinodimethane: relative proton geometries and order parameters using nematic solvents; carbon-proton spin spin coupling constants for TTF

From proton magnetic resonance spectra of TTF and TCNQ partially oriented in nematic liquid crystal solvents the ratio of length to width of the proton rectangle is 3.124 ± 0.03 for TTF and 1.765 ± 0.024 for TCNQ. Values of J_CH obtained for TTF dissolved in d₆-acetone, are J_CH = 187.9 ± 0.3 Hz, J_CCH = 6.7 ±0.1 Hz and J_CSCH = 9.0 ±0.2 Hz.     

13C Nuclear magnetic resonance studies of some substituted thiols and thioanisoles

¹³C NMR spectra of thiophenol, 4-nitrothiophenol, 4-X-thioanisole (X = NO₂, NH₂, SH, H) were examined in CDCl₃ solution. Substituent chemical shift values (additivity parameters) for the −SH and −SCH₃ groups are reported.In para-substituted thioanisoles, while the −NH₂ group caused a deshielding and −NO₂ a shielding of SCH₃ carbon resonance, −SH appeared to have had no effect on the carbon resonance.     

High-resolution proton nuclear magnetic resonance studies of urine from asphyxiated newborn infants

High-resolution proton nuclear magnetic resonance spectroscopy was used to study human urine obtained from 10 normal babies and twenty babies with various degrees of neonatal asphyxia, respiratory distress syndrome (RDS), and meconium aspiration syndrome (MAS). All sick babies showed different degrees of oxygen deficiency, indicated by an obvious increase of the lactate signal level in the urine spectra. Changes in the concentration of other urinary metabolites produced from the citric acid cycle were also observed. In extremely serious cases, the signals of some of the major components, including citrate, α-ketoglutarate, and succinate, simply disappeared. The spectra of urine, serum, and CSF of an infant suffering from SIDS showed common characteristics of the metabolites.     

Carbon-13 nuclear magnetic resonance studies on high spin iron(III) porphyrins

Carbon-13 NMR studies on a series of high spin iron(III) porphyrins, namely tetraphenylporphyrin iron(III) halides [Fe(TPP) X, X=Cl, Br, I] in CDCl₃ solution are reported. As expected the¹³C shifts are found to be an order of magnitude larger than the corresponding proton shifts. The dipolar contribution, which is quite important for the proton NMR, becomes much less significant for the¹³C shifts. No systematic variation in the¹³C shift across the series is observed, except for the meso-carbon which shows a small but gradual decrease in going from the chloro to the iodo complex. The¹³C shift for the various carbon atoms of the porphyrin ligand shows interesting pattern which is discussed in terms of spin delocalisation mechanisms.     

13C nuclear magnetic resonance studies on acetophenones: Barriers to internal rotation

The barrier to internal rotation in a series of p-substituted acetophenones has been determined by means of low temperature carbon-13 n.m.r. and total bandshape analysis, resulting in: ΔG = 5·4 ± 0·1 kcal mol^?1 (22·4 ± 0·4 kJ mol^?1) for the unsubstituted acetophenone. The substituent effects on the barrier are found to be the same as for the corresponding benzaldehydes. The barrier height is discussed in terms of contributions from resonance and steric effects.