Substituent effects on carbon-13 chemical shifts of para-substituted benzylbenzenes

Carbon-13 chemical shifts of fourteen para-substituted benzylbenzenes have been determined. The relative substituent chemical shifts (SCS) of the methylene carbons and the aromatic ring carbons (C-4, C-1′ and C-4′) correlated well with the Hammett substituent effects using the dual substituent parameter method. The transmission of substituent effects through the benzylbenzene system is briefly discussed.     

Substituent effects in the 13C NMR chemical shifts of alpha-mono-substituted acetonitriles

13C chemical shifts empirical calculations, through a very simple additivity relationship, for the alpha-methylene carbon of some alpha-mono-substituted acetonitriles, Y-CH(2)-CN (Y=H, F, Cl, Br, I, OMe, OEt, SMe, SEt, NMe(2), NEt(2), Me and Et), lead to similar, or even better, results in comparison to the reported values obtained through Quantum Mechanics methods. The observed deviations, for some substituents, are very similar for both approaches. This divergence between experimental and calculated, either empirically or theoretically, values are smaller than for the corresponding acetones, amides, acetic acids and methyl esters, which had been named non-additivity effects (or intramolecular interaction chemical shifts, ICS) and attributed to some orbital interactions. Here, these orbital interactions do not seem to be the main reason for the non-additivity effects in the empirical calculations, which must be due solely to the magnetic anisotropy of the heavy atom present in the substituent. These deviations, which were also observed in the theoretical calculations, were attributed in that case to the non-inclusion of relativistic effects and spin-orbit coupling in the Hamiltonian. Some divergence is also observed for the cyano carbon chemical shifts, probably due to the same reasons.     

Substituent effects on 13C NMR chemical shifts in the saturated framework of primary aliphatic derivatives

A general equation describing the effect of substituents on α-carbons in a saturated framework was developed from ¹³C chemical shifts obtained under uniform conditions for selected aliphatic compounds. Experimental correlations for β- and γ-carbons and a discussion of the results are presented.     

Reversed substituent effect for the 13C NMR chemical shifts in the unsubstituted ring of para-substituted benzophenones

The ¹³C NMR chemical shifts of the C-1′ carbon atom of fifteen para-substituted benzophenones correlate with the ? and ? substituent constants in a reversed manner.     

Substituent effects on the P NMR chemical shifts of arylphosphorothionates

Six tris(aryloxy)phosphorothionates substituted in the para position of the aromatic rings were synthesized and studied by ³¹P NMR, X-ray diffraction techniques and ab initio calculations at a RHF/6-31G** level of theory, in order to find the main structural factors associated with the δ³¹P in these compounds. As the electron-withdrawing (EW) ability of the substituents was increased, an ‘abnormal’ shielding effect on δ³¹P of the arylphosphorothionates was observed. The analyses of the geometrical properties obtained through both experimental and theoretical methods showed that a propeller-type conformation is preferred for the arylphosphorothionates, except in the case of the tris(O-4-methylphenyl)phosphorothionate, since one of the aromatic rings is not rotated in the same direction as the other two in the solid state. The main features associated with the δ³¹P NMR of compounds 1-6 were a decrease of the averaged O-P-O angle and mainly the shortening of the PS bond length, which is consistent with an increase of the thiophosphoryl bond order as δ³¹P values go upfield. On the other hand, comparison of the experimental and calculated bond lengths and bond angles involving α bonded atoms to phosphorus of the six compounds suggested that stereoelectronic interactions of the type n_πO-σ*_PS, n_πO-σ*_P-OAr and n_πS-σ*_P-OAr could be present in the arylphosphorothionates 1-6.     

Substituent effects on 33S NMR chemical shifts in sulphones

The ³³S NMR spectra of some selected sulphones demonstrate additive substituent-induced chemical shift (SCS) effects. In dimethyl sulphone (1), replacement of a methyl group by a vinyl or a phenyl group causes an SCS effect of ?7 to ?8.5 ppm or ?4 to ?5 ppm, respectively. The ³³S chemical shift in 1 is also sensitive to substitution of methyl protons. The ß-substituent effect for methyl and phenyl groups is in the range +7 to +8 ppm and +5.5 to +6 ppm, respectively.     

The 1H and 13C chemical shifts for some tetrakis(para-substituted phenyl)ethylenes

The proton and carbon chemical shifts for a series of tetrakis(p-substituted phenyl)ethylenes are described. Assignments followed routine substituent chemical shift trends. Both proton and carbon chemical shifts ortho to the varying substituent follow the empirical parameter, Q. The ethylene carbon chemical shifts are proportional to those at the position para to the varying substituent.     

Substituent effects on 15N and 13C NMR chemical shifts of 3-phenylisoxazoles: a theoretical and spectroscopic study

The synthesis and assignment of 15N and 13C NMR signals of the isoxazole ring in a series of para-substituted 3-phenyl derivatives are reported. DFT calculations of 15N and 13C chemical shifts are presented and compared to observed values. Substituent effects are interpreted in terms of the Hammett correlation and calculated bond orders.     

Substituent effects on 13C-15N spin couplings and 13C chemical shifts in benzenediazonium ions

Similar trends are observed for ¹J(¹³C-¹⁵N) values for a series of aryldiazonium tetrafluoroborates and comparable anilinium fluorosulfonates. Contributions from resonance structures involving double-bond character in the CN bond are judged to be relatively unimportant in the ground state structure of aryldiazonium ions.     

Substituent effects on 13C and 1H chemical shifts in 3-benzylidene-2,4-pentanediones

Substituent effects on the ¹³C and ¹H chemical shifts have been studied for derivatives of 3-benzylidene-2, 4-pentanedione. A significant correlation has been found between chemical shifts of the Z carbonyl group (C-2) and Hammett constants, while no correlation has been found for the E carbonyl group (C-4). Attempts have been made to determine the structural factors which influence these effects. The conformation of 3-benzylidene-2, 4-pentanediones has been determined by ¹³C and ¹H NMR spectroscopy.     

Substituent effects on 13C NMR. 2—chemical shifts in the saturated framework of secondary aliphatic derivatives

¹³C chemical shifts obtained under uniform conditions for selected compounds containing secondary aliphatic fragments were employed in a linear regression analysis. Two-parameter relationships describing the substituent effects in the saturated framework were calculated, and the usefulness of such calculations is discussed. Finally, coefficients for linear relationships in primary and secondary alkyl derivatives are compared.     

Substituent effects on (15)N and (13)C NMR chemical shifts of 5-phenyl-1,3,4-oxathiazol-2-ones: a theoretical and spectroscopic study

The synthesis and assignment of (15)N and (13)C NMR signals of the 1,3,4-oxathiazol-2-one ring in a series of para-substituted 5-phenyl derivatives are reported. DFT calculations of (15)N and (13)C chemical shifts correspond closely to observed values. Substituent effects are interpreted in terms of the Hammett correlation and calculated bond orders.     

Substituent effects on carbon-13 chemical shifts in 2,6-disubstituted adamantanes

Carbon-13 NMR spectral data for a series of symmetrical 2,6-disubstituted adamantanes (O?, CH₂?, CH₃, OH, OCOCH₃) are presented. The substituent effects on ¹³C chemical shifts are additive, except for carbons 2 and 6 in 2,6-adamantanedione. The non-additivity of the substituent effect in the diketone can be interpreted in terms of a through-space interaction of the carbonyl π-electrons; the additivity in the other derivatives indicates that there is no appreciable interaction between the substituents.     

13C NMR study of ortho-, meta- and para- substituted phenyldiphenylamines: Substituent effect correlations

The ¹³C chemical shifts of 11 substituted triphenylamines have been determined and the assignment of these resonances made using intensities, ¹H and ¹⁹F couplings and predictions from bond additivity relationships. ¹³C chemical shifts at carbons bearing the substituent and at carbons ortho to the substituent correlated reasonably well with the Q parameter. A multiple regression analysis of chemical shifts with the field and resonance parameters of Swain and Lupton and the Q parameter produced significantly better correlations than those obtained when Q was omitted for these positions. ¹³C chemical shift correlations for carbons meta and para to the substituent were not significantly better than when Q was omitted. Significant correlations were obtained between field and resonance parameters and ¹³C chemical shifts of C-o and C-p, and C-i, C-o, C-m and C-p of the non-substituent bearing phenyl rings in ortho- and para-substituted phenyldiphenylamines, respectively.     

A predictive tool for assessing (13)C NMR chemical shifts of flavonoids

Herein are presented the (1)H and (13)C NMR data for seven monohydroxyflavones (3-, 5-, 6-, 7-, 2'-, 3'-, and 4'-hydroxyflavone), five dihydroxyflavones (3,2'-, 3,3'-, 3,4'-, 3,6-, 2',3'-dihydroxyflavone), a trihydroxyflavone (apigenin; 5,7,4'-trihydroxyflavone), a tetrahydroxyflavone (luteolin; 5,7,3',4'-tetrahydroxyflavone), and three glycosylated hydroxyflavones (orientin; luteolin-6C-beta-D-glucoside, homoorientin; luteolin-8C-beta-D-glucoside, vitexin; apigenin-8C-beta-D-glucoside). When these NMR spectra are compared, it is possible to assess the impact of flavone modification and to elucidate detailed structural and electronic information for these flavonoids. A simple predictive tool for assigning flavonoid (13)C chemical shifts, which is based on the cumulative differences between the monohydroxyflavones and flavone (13)C chemical shifts, is demonstrated. The tool can be used to accurately predict (13)C flavonoid chemical shifts and it is expected to be useful for rapid assessment of flavonoid (13)C NMR spectra and for assigning substitution patterns in newly isolated flavonoids.     

Substituent effects on ionisation and (13)C NMR properties of some monosubstituted phenols A potentiometric, spectrophotometric and (13)C NMR study

The pK(a) values of ionisation of a set of phenols ortho, meta and para substituted are studied by spectrophotometry and (13)C NMR spectroscopy. A dual substituent analysis of equilibrium and NMR results, according to the Swain and Lupton procedure, is presented. The results of this analysis allow the assignment of the contribution of field and resonance contributions, both on equilibrium constants and chemical shifts.     

13C NMR chemical shifts of N-unsubstituted- and N-methyl-pyrazole derivatives

¹³C shielding data for 100 derivatives of pyrazole are reported. These include methyl, ethyl, n-propyl, tert-butyl, phenyl, hydroxymethyl, carboxyl, ethoxycarbonyl, cyano, amino, hydrazino, nitro, azido, chloro, bromo and iodo groups as substituents on the ring carbon atoms.     

Prediction of 13C NMR chemical shifts in substituted naphthalenes

The prediction of the ¹³C NMR signals for derivatives of naphthalene has been investigated using mathematical modeling techniques. Two empirical multiple regression models which utilize the field, resonance, and Charton's steric parameters together with molar refractivity were developed, one for α- and the other for β-substituted naphthalene derivatives. In the α case the model had a correlation coefficient of observed versus predicted line positions of r = 0.973 with a standard deviation of 2.2 ppm while in the β case r = 0.979 with the standard deviation being 2.3 ppm. The database consisted of 3152 signals from 394 naphthalene derivatives. We also report the use of the Taft steric parameter in place of the Charton steric parameter in the above- mentioned prediction equations. Received: 19 June 1998 / Accepted: 20 October 1998 / Published online: 16 March 1999     

Substituent effects on 13C chemical shifts of ketenimines: a GIAO/HF and DFT study

GIAO/HF and DFT methods were utilized to predict the ¹³C chemical shifts of substituted ketenimines. GIAO HF/6–311+G(2d,p) and B3LYP/6–311+G(2d,p) methods were applied on the optimized B3LYP/6–31G(d) geometries and ¹³C chemical shifts of C_α and C_β of substituted ketenimines were correlated with group electronegativities. HF and DFT calculations indicated that increasing substituent group electronegativity leads to increasing chemical shift of C_β of substituted ketenimines, whereas the C_α values decrease. Copyright © 2003 John Wiley & Sons, Ltd.