D.G. De Kowalewski

Additivity relation for the proton-proton coupling constants of trisubstituted benzenes

Abstract The NMR proton spectra of 12 trisubstituted benzenes in different solvents have been accurately analyzed. The proton-proton coupling constants are calculated by using an additivity relation from the data for benzene and the monosubstituted benzenes. The values obtained are in good agreement with those determined experimentally except in the case of 2,4-dinitrophenol and 2-nitro-4-chlorophenol.

Additivity of substituent effects on the proton-proton coupling constants of disubstituted pyridines

Abstract From the spin-spin coupling constants of pyridine and monosubstituted pyridines the effects of several substituents have been calculated- Assuming an additivity relationship when two of these substituents are present in the same molecule, the spin-spin coupling constants for 11 disubstituted pyridines have been calculated. The NMR spectra of 13 disubstituted pyridines have been studied to obtain accurate values of their coupling constants. The experimental values of these constants are in very good agreement with those calculated using the additivity relationships.

1H and 13C NMR study of substituted 3-OH pyridines

Abstract n J HH , n J CH and δ 13 C values have been measured for a series of X substituted 3-hydroxypyridines (X = 2-NH 2 , 2-NO 2 , 5-Cl, 6-CH 3 , 2-Cl, 2-Br, 2-I). The results show that the additivity of δ 13 C provides a valuable criterion to differentiate the phenolic from the zwitterion structure. This conclusion is based on the fact that in the first case, for 2-NH 2 -, 2-NO 2 -, 5-Cl- and 6-CH 3 -3-hydroxypyridines, there is agreement between the experimental and the additivity δ 13 C values, while in the three halogen derivatives (2-Cl-, 2-Br- and 2-I-3-hydroxypyridines) the δ exp—add C3 values of −4.95, −7.25 and 9.05 are probably due to the negative charge present on the three position of the zwitterion. Since the additivity of 1 J CH values holds in all substances examined (unlike the case of the 2-pyridone derivatives) it is not possible to use that criterion to differentiate between the phenolic and dipolar structures. The above conclusions are in agreement with IR, p K , NQR, RX, kinetics experiments and quantum chemical calculations of other authors.

Additivity scheme in nmr chemical shifts of disubstituted pyridines

Abstract An additivity relationship for chemical shifts similar to that found in the literature for coupling constants has been tested in eleven disubstituted pyridines. The chemical shifts of these disubstituted pyridines were measured in different solvents at different concentrations and extrapolated to infinite dilution. In order to compute the “effects” of the different substituents, the chemical shifts of the corresponding monosubstituted pyridines and of pyridine were measured in the same solvents at different concentrations. These chemical shifts were extrapolated to infinite dilution. In most cases it was found that the extrapolated chemical shifts for disubstituted pyridines are in good agreement with those calculated using the additivity relationship. Only in a few cases was it found that the additivity scheme does not hold.

Substituent effects on the proton spin-spin coupling of benzenes with side-chain interacting groups

Abstract Sixty MHz NMR proton spectra of 15 2,4-substituted benzaldehydes and 12 methylbenzenes dissolved in different solvents have been accurately analyzed. Substituent effects on the coupling constants have been determined and have been found to be additive in those trisubstituted compounds where there are neither internal hydrogen bonds or strong interacting groups present.

A nuclear magnetic resonance study of 2,5-dihydrofuran partially oriented in a nematic phase

Abstract From the study of the proton magnetic resonance spectra of dihydrofuran dissolved in a nematic phase the ratios of interproton distances have been derived.

13C- and 1H-NMR spectra of 2-pyridone derivatives

Abstract 13 C- and 1 H-NMR spectroscopy was used to obtain n j HH , n J CH and δ C-13 for a series of substituted 2-hydroxypyridines (3-hydroxy; 4-hydroxy; 5-chloro; 3-methyl-5-chloro; 6-chloro; 6-amino; 6-hydroxy). It is shown that the additivity of δ C-13 and 1 J CH values provides a valuable criterion for differentiating between quinoidal and aromatic structures and in defining conformations. The first conclusion is based on the fact that for 3-substituted compounds there is accordance between experimental values and the additivity 1 J CH and δ C-13 values calculated for 2-pyridone. In the 6-substituted compounds there is marked deviation from additivity in 1 J CH . On the other hand, it seems that the substituent effect on δ C-13 in the hypothetical 2-hydroxypyridine and the real 2-pyridone are markedly different for C6, since there is no accordance between additivity and experimental values for the chemical shifts of this carbon. These conclusions are in agreement with results obtained from IR and p K studies and quantum-chemical calculations which indicate that a substituent in position 3 favours a pyridone structure, while a pyridinol structure is favoured when the substituent is at ring-position 6.

A theoretical and experimental study of the electric field and magnetic anisotropy effects on proton chemical shifts

Abstract Proton magnetic shielding constants are divided into different contributions using the IPPP technique (inner projections of the polarization propagator). Total magnetic shielding constants are calculated within the CHF-INDO-GIAO approach (coupled-Hartree-Fock-INDO-gauge-invariant atomic orbitals). In order to compare the electric field and magnetic anisotropy effects of neighbouring groups, two model compounds were chosen, namely, ethyl cyanoformate, I, and ethylformate, II, which show to frozen and unequally populated rotamers each at room temperature. Their proton spectra were measured and the difference in shielding of methylene protons in each pair of rotamers was theoretically analysed with the abovementioned technique. The experimental difference in chemical shifts is quantitatively reproduced with the present analysis.

Conformational analysis in 2,4-dinitrophenol by nuclear Overhauser effect experiments

Abstract The cis conformation of the hydroxyl with respect to the nitro group in 2,4-dinitrophenol under fast exchange conditions and the consequent persistence of the intramolecular hydrogen bond are determined. These facts emerge from the existence of a scalar-coupling-of-the-first-kind relaxation interaction linking the phenolic and the ring protons of the compound which is directly established by a nuclear Overhauser effect experiment. Relaxation information is incidentally obtained. Decoupling experiments in solutions of 2-nitro-4-chlorophenol were made showing similar existence of the cis conformation.

The use of double irradiation methods for the assignment of ABX2 spectra with a very small coupling constant

Abstract Partial decoupling and the INDOR technique are used to measure the nuclear magnetic resonance splittings that are small enough to be obscured by the magnet-field inhomogeneity; J BX = (0.10 ± 0.01) Hz in 2,3-dibromopropene. Double irradiation experiments at certain frequencies which produce sign dependent asymmetries are used to determine the magnitude and sign of the coupling constants in 2,3-dibromopropene.