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doi: 10.1002/mrc.1270220302pmid: N/A
13C and 15N NMR chemical shift and spin–lattice relaxation data have been measured for both meso‐ and racemic‐pentane‐2,4‐diamine. At high pH (12), relaxation is consistent with hindered rotation of the NH2 group due, in part, to the formation of intramolecular hydrogen bonds. At low pH (2), relaxation is consistent with relatively unhindered rotation of the NH3+ group. Rotational jump rates and barriers are reported, determined from the NT1 ratios between 15N and 13C nuclei. In all cases, the ratios for the racemic diastereomer are higher than those of the meso compounds; this is interpreted in terms of conformationally more stable intramolecular hydrogen bond formation in the meso compound. Chemical shifts for the diastereomeric amines show that 15N shifts move downfield on protonation along with methyl and methylene carbons, while the methine carbon resonances move upfield.
doi: 10.1002/mrc.1270220303pmid: N/A
A simple sequence in which 13C precessions are allowed to develop with proton coupling for a period of 1/2 3J(CH) s before initiating decoupling and acquisition allows the determination of vicinal carbon–hydrogen coupling constants to facilitate the characterization of trisubstituted double bonds. The experiment is illustrated by studies of citraconic anhydride and limonene.
Guilleme, Joaquin; Diez, Ernesto; Garrigos, Lluis; Esteban, Angel L.
doi: 10.1002/mrc.1270220304pmid: N/A
Methyl carbon chemical shifts have been assigned for methylbenzoic acids dissolved in CDCl3, and for methylbenzoate anions obtained by dissolving these acids in aqueous NaOH solution. Chemical shifts have been interpreted by means of additive substituent parameters which reflect conformational features existing between adjacent substituents. Barriers to rotation of a methyl group adjacent to a carboxyl or carboxylate group have been estimated to differ by less than 2 kJ mol−1 from the barrier of a methyl group in o‐xylene.
Koole, N. J.; de Bie, M. J. A.; Hansen, P. E.
doi: 10.1002/mrc.1270220305pmid: N/A
Experimental carbon–proton coupling constants are reported for 36 monosubstituted allenes, 20 ethenes and 5 butatrienes. The data for the allenes cover a range of substituents in which either the first atom of the substituent (all Group IV–VII elements of the second and third row are covered) and/or the substitution of that atom is varied. The electronegativity (σI) of the substituent directly attached to the coupled carbon atom is correlated with the 1J(CH) coupling. This is also borne out by the good correlations between 1J(CH) values in the allenes and 1J(CH) values in various other classes of chemical compounds. Theoretically calculated 1J(CH) values correctly reflect the substituent effects on the experimental values in the allenes, ethenes and butatrienes spearately, but fail to give a satisfactory description of the differences in 1J(CH) for these types of compounds. In disubstituted allens and ethenes the substituent effects on 1J(CH) values are additive. The experimental and calculated values differ by less than 1 Hz. Two‐ and three‐bond carbon–proton couplings are also discussed in terms of electronegativity, substituent and hybridization effects and mutual relationships. Large values, up to 6 Hz, are found for 4J(CH) and 5J(CH) in the allenes and butatrienes. These large values are ascribed to σ‐π interactions. For geometrically equivalent couplings a constant ratio of nJ(CH)/nJ(HH) is found in the ethenes (0.65; n = 3) and in the allenes and butatrienes (0.55; n = 4 and 5, respectively). 1J(CH) and 3J(CH) coupling constants are used for the conformational analyses of vinyl, allenyl and butatrienyl ethers and thioethers. At room temperature the methyl and ethyl ethers are predominantly in an s‐cis conformation, whereas the iso‐propyl allenyl ether is a mixture of s‐cis and s‐trans; the tert‐butyl allenyl ether exists mainly in the s‐trans conformation. The thioethers are all in the s‐cis conformation.
doi: 10.1002/mrc.1270220306pmid: N/A
Values of one‐bond J(C35 Cl) and T1(35Cl) were determined for small chlorine‐containing molecules through 13C scalar relaxation in the rotating frame. A method is suggested for evaluating the contribution of scalar relaxation which eliminates experimental imperfections. The magnitude of J(C35Cl) seems to be influenced by the same factors as J(CF) values. The differences in 35Cl relaxation times are rationalized in terms of the molecular reorientation time.
doi: 10.1002/mrc.1270220307pmid: N/A
Glutacononitrile (1,3‐dicyanopropene) (2) was converted to its anion (1) by treatment with sodium hydride in tetrahydrofuran. Both 13C and 1 NMR showed that the sodium salt of 1 in THF existed in three slowly interconverting geometries (U, W and S). These geometries were present in similar amounts but the relative amounts of each proved to be concentration dependent. Irradiation of a 0.01 M solution of 1 at −80°C led to the conversion of a mixture of the U, W and S forms of 1 to only the W and S forms. Upon warming to 0°C equilibrium was re‐established with a half‐life of approximately 8 h.
doi: 10.1002/mrc.1270220308pmid: N/A
The E0 rotamer of c‐Gly‐Met has been found to be almost as equally populated as the F rotamer at low temperatures, whereas in c‐Gly‐D,L‐Nle and c‐Gly‐D,L‐Nvl the F rotamer predominates. The increase of the E0 rotamer population parallels the increase of the boat diketopiperazine (DKP) ring conformation, in which Med side‐chain occupies a pseudoaxial position. Thermodynamic parameters for DKP ring inversion and for conformational transitions in the side‐chains were experimentally determined for each cyclic dipeptide investigated.
doi: 10.1002/mrc.1270220309pmid: N/A
Long‐range couplings were observed between protons attached to C‐2 and C‐5 of the 4‐imidazolidinone ring. Since 4J(25) has a measurable value only for a trans arrangement of H‐2 and H‐5, this coupling can be utilized as an easy method of identification of 4‐imidazolidinone stereoisomers.
Schneider, Hans‐jörg; Agrawal, Pawan Kumar
doi: 10.1002/mrc.1270220310pmid: N/A
Force field (MM2) calculations, 13C NMR substituent‐induced shifts (SIS) and epimeric shift differences (ESD) indicate a preference for equatorial substituents in the 2‐position, but equal eq/ax populations in the 1‐position of tetralines. Similar conclusions are reached from Yb(fod)3‐induced shifts, which are also used for signal assignments, e.g. in 1‐tetralone. Configurational assignments are possible for 1,2‐ and 1,3‐epimers (ESD up to 4 ppm) but, in line with the non‐discriminating eq/ax conformations at C‐I, not for 1,4‐epimers (ESD<0.5 ppm). More than 50 compounds were measured, including functional derivatives which show regular SIS for substituents in the aromatic moiety only for m‐ and p‐carbon atoms. OMe, but not OH or OAc substituents, induce o‐carbon SIS varying from −11 to −19 ppm. Conversion of 1‐hydroxytetralin to esters induces shielding variations at the aromatic carbon atoms which indicate the electrostatic origin of derivatization shifts.
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