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 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 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 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.     

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     

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.     

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.     

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.     

Substituent effects on carbon-13 NMR chemical shifts of side chain carbonyl carbons of 4-substituted 1-naphthamides

Substituent induced¹³C NMR chemical shifts of side chain carbonyl carbons of several 4-substituted 1-naphthamides have been measured in DMSO-d ₆ solvent. Analysis of the substituent induced chemical shifts by the DSP equation gave the regression equation. Both {ie207-1} and {ie207-2} values were negative. The negative sign on {ie207-3} term indicates the operation of a reverse substituent effect and that π-polarisation is the important mechanism for the transmission of substituent effects by inductive effect. Theperi-hydrogen interaction in naphthamides forces the amide group out of the plane of the naphthalene ring.     

Substituent effects on 13C?13C coupling constants and chemical shifts in 13C labelled polycyclic aromatic hydroxy compounds

The effects of an hydroxy substituent on ¹³C? ¹³C coupling constants and ¹³C chemical shifts have been measured in 1-hydroxynaphthalene-2-¹³C and 1-hydroxypyrene-1-¹³C. The changes observed in the ¹³C? ¹³C couplings show the effect of a substituent attached directly to the labelled carbon or to a carbon ortho to this. In both situations the effect is a decrease in the numerical magnitude of most of the long range ¹³C? ¹³C coupling constants.     

Computational studies of 13C NMR chemical shifts of saccharides

The (13)C NMR chemical shifts for alpha-D-lyxofuranose, alpha-D-lyxopyranose (1)C(4), alpha-D-lyxopyranose (4)C(1), alpha-D-glucopyranose (4)C(1), and alpha-D-glucofuranose have been studied at ab initio and density-functional theory levels using TZVP quality basis set. The methods were tested by calculating the nuclear magnetic shieldings for tetramethylsilane (TMS) at different levels of theory using large basis sets. Test calculations on the monosaccharides showed B3LYP(TZVP) and BP86(TZVP) to be cost-efficient levels of theory for calculation of NMR chemical shifts of carbohydrates. The accuracy of the molecular structures and chemical shifts calculated at the B3LYP(TZVP) level is comparable to those obtained at the MP2(TZVP) level. Solvent effects were considered by surrounding the saccharides by water molecules and also by employing a continuum solvent model. None of the applied methods to consider solvent effects was successful. The B3LYP(TZVP) and MP2(TZVP)(13)C NMR chemical shift calculations yielded without solvent and rovibrational corrections an average deviation of 5.4 ppm and 5.0 ppm between calculated and measured shifts. A closer agreement between calculated and measured chemical shifts can be obtained by using a reference compound that is structurally reminiscent of saccharides such as neat methanol. An accurate shielding reference for carbohydrates can be constructed by adding an empirical constant shift to the calculated chemical shifts, deduced from comparisons of B3LYP(TZVP) or BP86(TZVP) and measured chemical shifts of monosaccharides. The systematic deviation of about 3 ppm for O(1)H chemical shifts can be designed to hydrogen bonding, whereas solvent effects on the (1)H NMR chemical shifts of C(1)H were found to be small. At the B3LYP(TZVP) level, the barrier for the torsional motion of the hydroxyl group at C(6) in alpha-D-glucofuranose was calculated to 7.5 kcal mol(-1). The torsional displacement was found to introduce large changes of up to 10 ppm to the (13)C NMR chemical shifts yielding uncertainties of about +/-2 ppm in the chemical shifts.     

13C NMR chemical shifts of some highly-branched acyclic compounds

A study has been made of the ¹³C chemical shifts of a number of acyclic alkanes, alkenes, nitriles and ketones which contain quaternary carbon atoms. Similar data have also been obtained for the series of compounds involved in the synthesis of triisopropylacetic acid. Substituent effects and steric factors in these highly substituted compounds are discussed in relation to the chemical shifts.