13C substituent effects in monosubstituted benzenes

A brief analysis is presnted of the current understanding of substituent perturbations in monosubstituted benzenes as determined by substituent induced carbon chemical shifts. A critical tabulation of the four substituent chemical shifts is given for c. 700 monosubstituted benzenes.     

An ab initio study of atomic interactions in monosubstituted benzenes

Molecules of monosubstituted benzenes XC₆H₅ (X = F, Cl, Br, OH, NH₂, CH₃, CH₂CH₃) were studied by the RHF/6-311G(d) method with full geometry optimization. Analysis of the molecular orbitals and contributions made to them by atomic orbitals, and also of the populations of the valence p orbitals of atoms in substituents X directly bonded to the aromatic ring showed that the features of the electron distribution in such molecules should not be attributed to the capability of the lone electron pairs of the heteroatoms in these substituents for p,π conjugation with the π-electron system of the molecule.     

Carbon-13 nuclear magnetic resonance spectroscopy IX—monosubstituted ethylenes

Carbon-13 chemical shifts of sixteen monosubstituted ethylenes were obtained. In order to explain the chemical shifts, σ and π electron densities of these compounds are calculated by the σ-included ω-HMO method.

1 See Ref. 8.

A linear relationship exists between carbon-13 chemical shifts and the calculated electron densities, and also between substituent constants and electron densities. A slope of unity is obtained between the chemical shifts of α carbons of monosubstituted ethylenes and those of carbons adjacent to the substituents in monosubstituted benzenes. On the other hand, a plot of chemical shifts of C_ortho of benzene derivatives against that of the β carbon in ethylene derivatives gives a slope of 3. These slopes can be explained by the calculated electron densities. A slope of 4/3 is obtained between the direct coupling constant ¹J(C? H) of the α carbon in monosubstituted ethylenes and that in the corresponding substituted methanes.     

Relativistic calculation of nuclear magnetic shielding tensor including two-electron spin-orbit interactions

A relativistic calculation of nuclear magnetic shielding tensor including two-electron spin-orbit interactions is performed. In order to reduce the computational load in evaluating the two-electron relativistic integrals, the charge density is approximated by a linear combination of the squares of s-type spatial basis functions. Including the two-electron spin-orbit interaction effect is found to improve the calculation results.     

13C chemical shifts of monosubstituted adamantanes

¹³C chemical shifts are reported for adamantane, nine 1-substituted adamantanes and nine 2-substituted adamantanes. The substituents are F, Cl, Br, I, NH₂, OH, CH², CN and CO₂H. The assignments and results are discussed in terms of chemical shift patterns.     

Deuterium isotope effects on 13C and 15N nuclear shielding in o-hydroxyazo dyes

Deuterium isotope effects on the ¹³C and ¹⁵N nuclear shieldings of o-hydroxyazo compounds are described. Both the direct and the equilibrium contributions were determined. Large direct deuterium isotope effects on ¹⁵N nuclear shieldings for ¹Δ, ²Δ and ⁴Δ and negative Δ¹J(¹⁵NH) [D] values were observed. Isotope effects on ¹⁵N nuclear shieldings, because of their magnitudes, are shown to be very useful in determining changes in the azo–hydrazone equilibrium. Isotope effects on ¹³C nuclear shieldings are smaller, but the effects observed at the carbon resonances of the N-phenyl ring are likewise very useful in determining the shift in the equilibrium. Deuterium substitution leads in all cases to a shift in the equilibrium so that the content of the predominant form of the protio compound is increased.     

Theoretical study of ionization potentials in monosubstituted benzenes

Monosubstituted benzenes, in which the substituents participate in the π-electron system, are studied following a classification in two classes according to the π-electronic structure of the substituent. For this type of molecule, a relation is established between the nature of the substituent and, on the one hand, the energies of the two highest occupied molecular orbitals and, on the other hand, their respective differences. The two orbitals referred to above have π-character and belong to the a₂ and b₁ species if a C_2v point group is assumed. Simple symmetry arguments lead to the conclusion that the a₂ orbitals have, essentially, an intraring character, whereas the π-orbitals of the substituents do give an important contribution to the b₁ orbitals. Therefore, an a₂ electron must have a larger interaction with the benzene ring and a smaller kinetic energy, whereas a b₁ electron must have a larger interaction with the substituent and a larger kinetic energy. It is also expected that the changes in the π-electronic structure of the substituent must much more influence the variations on the b₁ energies and on the components of orbital energies associated with the substituent than the variations on the a₂ energies and on the intraring components of the orbital energies. A modified version of the MOPAC program was prepared to perform the decomposition of the orbital energies in their kinetic and potential energy components and these in their monocentric and bicentric terms. MNDO calculations on nine monosubstituted benzenes, using the modified MOPAC program, give good confirmation of the symmetry predictions and prove the consistency of the classification of the substituents that is introduced. © 1993 John Wiley & Sons, Inc.     

The effect of weak intermolecular interactions on the nuclear magnetic resonance shielding constant in N2

Ab initio calculations are applied to examine the influence of the intermolecular interactions on the shielding constant in gaseous nitrogen. An accurate literature potential energy surface and the nuclear magnetic resonance shielding surface of the N₂–N₂ complex calculated in this work provide results in satisfactory agreement with the available experimental estimates of the effect.     

Deuterium isotope effects on 13C nuclear shielding as a measure of tautomeric equilibria

Large deuterium isotope effects of both signs have been observed on the ¹³C nuclear shielding in proton-chelating tautomeric forms of β-thioxoketones, and small isotope effects of both signs have been found in related forms of Schiff's bases of salicylaldehydes. These effects are interpreted in terms of shifts in the tautomeric equilibria on deuterium substitution of the enolic proton. The observation of deuterium isotope effects is suggested as a useful method for the detection of fast tautomeric equilibria in systems of the following type. .     

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.     

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 nuclear overhauser polarization-magic-angle spinning nuclear magnetic resonance spectroscopy in uniformly 13C-labeled solid proteins

A recently introduced (13)C polarization technique based on the nuclear Overhauser effect in rotating solid (nuclear Overhauser polarization-magic-angle spinning, NOP-MAS) (Takegoshi, K.; Terao, T. J. Chem. Phys. 2002, 117, 1700-1707) is applied to uniformly (13)C, (15)N-labeled proteins. NOP enhancement factors per scan of 1.5 approximately 2.0 are obtained, while that by cross polarization (CP) is less than 1.0. We show that uniform enhancement of all (13)C signals by CP is difficult to attain, while it is easily achieved by NOP, thus enabling quantitative comparison of signal intensities. NOP is easy to carry out under fast MAS and works well even for somewhat mobile molecules, for which CP does not work. Moreover, in labeled protein samples containing nonlabeled additives, NOP can eliminate the latter signals. For these features, NOP is superior to CP in many uniformly (13)C labeled proteins.     

13C NMR spectra of some monosubstituted quinolizinium bromides

Chemical shifts are reported for quinolizinium bromide and 12 monosubstituted derivatives, carrying a bromohydroxy, diethylamino or piperidino group as the substituent. In addition, ¹J(CH) values and long-range coupling constants are given.     

13C-13C spin-spin coupling constants in the spectra of monosubstituted pyridines

The ¹³C- {¹H} NMR spectra of pyridine and a number of monosubstituted pyridines for compounds with the natural percentage of the ¹³C isotope were analyzed. The direct, geminal, and vicinal ¹³C-¹³C spin-spin coupling constants (SSCC) were determined. Linear relationships that link the ¹³C-¹³C SSCC in the spectra of monosubstituted pyridines and benzenes were obtained.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1383–1385, October, 1985.