Carbon-13 N.M.R. Studies of Carbocations. III. An Investigation of the Variation of ipso-Substituent Chemical Shifts with Electron Demand in 1,4-Disubstituted Benzenes

1979 ◽  
Vol 32 (7) ◽  
pp. 1511 ◽  
Author(s):  
HM Hugel ◽  
DP Kelly ◽  
RJ Spear ◽  
J Bromilow ◽  
RTC Brownlee ◽  
...  
Keyword(s):  
Chemical Shift ◽  
Large Range ◽  
Chemical Shifts ◽  
Substituent Effects ◽  
Mechanism Of Interaction ◽  
Electron Demand

13C n.m.r. spectra have been obtained of a large range of 1(X),4(Y)-disubstituted benzenes in which X has been varied over a range of 25 substituents from NMe2 to +CHMe for each of the compounds where Y = H, OMe, Me, F, Cl, Br and CF3. The ipso-substituent chemical shifts (ipso-SCS) for each of the latter (that is, the change in chemical shift (Δδ) of C4 by replacement of H by Y) have been shown to vary dramatically with the electron demand of the X substituent as measured by δ(C4). When plotted against ?(C4), the ipso-SCS of F and OMe both decrease linearly with increasing electron demand whilst those of Br and Cl show linear increases. Those of Me and CF3 show discontinuities which indicate changes in the mechanism of interaction of these groups with the attached ipso-carbon. The variations in the ipso-SCS with electron demand of X are considered to be due to Y-induced variations in the sensitivity of the ipso-carbon to the effect of the para (X) substituent and not to through-conjugation effects. The results clearly show the fallacy of assuming that 13C substituent effects are constant.

Carbon magnetic resonance studies of some phenyl, furyl and thienyl organometallics. Some comments on carbon-metal scalar coupling

1974 ◽  
Vol 27 (2) ◽  
pp. 417 ◽  
Author(s):  
D Doddrell ◽  
KG Lewis ◽  
CE Mulquiney ◽  
W Adcock ◽  
W Kitching ◽  
...  
Keyword(s):  
Chemical Shift ◽  
Chemical Shifts ◽  
Substituent Effects ◽  
Coupling Constants ◽  
Scalar Coupling ◽  
Coupling Constant ◽  
Aryl Substituent ◽  
Magnetic Resonance Studies ◽  
Thienyl Group ◽  
13C Chemical Shift

13C chemical shift variations within a series of phenyl, furyl and thienyl Group IVB organometallics appear to be best understood in terms of the usual alkyl and aryl substituent effects on 13C chemical shifts and not variations in dπ ?pπ metal-aryl interactions. Large changes in 13C-metal scalar coupling constants have been observed suggesting that other factors besides the s-character of the carbon-metal bond is responsible in determining the coupling constant.

Carbon-13 N.M.R. chemical shifts and rotational barriers of para-substituted N,N-dimethylbenzamides

1978 ◽  
Vol 31 (12) ◽  
pp. 2615 ◽  
Author(s):  
CW Fong ◽  
SF Lincoln ◽  
EH Williams
Keyword(s):  
Chemical Shift ◽  
Chemical Shifts ◽  
Substituent Effects ◽  
Rotational Barrier ◽  
Rotational Barriers

The carbon-13 N.M.R. chemical shifts for a series of para-substituted N,N-dimethylbenzamides have been measured. The substituent induced 13C shifts have been examined by a dual substituent parameter (DSP) method using Hammett-type constants. The barriers to rotation have also been correlated with Hammett-type constants by the DSP method and related to 13C substituent induced shifts. Substituent effects of the bromomethyl, dibromomethyl and tribromomethyl groups have been examined by using the chemical shift and rotational barrier probes.

An investigation into the origins of polar substituent effects upon 19F chemical shifts, using 4-substituted β,β-difluorostyrenes

1980 ◽  
Vol 58 (8) ◽  
pp. 839-845 ◽  
Author(s):  
William F. Reynolds ◽  
Victoria G. Gibb ◽  
Nick Plavac
Keyword(s):  
Chemical Shift ◽  
Electron Density ◽  
Molecular Orbital ◽  
Excellent Agreement ◽  
Field Effect ◽  
Chemical Shifts ◽  
Substituent Effects ◽  
Molecular Orbital Calculations ◽  
Chemical Shift Difference ◽  
Polar Substituent

19F, 13C, and 1H chemical shifts have been determined for β,β-difluorostyrene and eight 4-substituted derivatives. The β-fluorine chemical shift difference, ΔδF, is used to evaluate the constant in the Buckingham equation. A = 3.0 × 10−11 esu for C—F bonds which is in excellent agreement with the value derived by Adcock and Khor. This allows accurate estimates of direct field effect contributions to 19F chemical shifts in aryl fluorides. Substituent parameter correlations demonstrate that the primary polar effect on 19F chemical shifts is field-induced π polarization. Abinitio molecular orbital calculations confirm that the substituent-induced 19F chemical shifts reflect changes in fluorine π electron density.

Origin and Nature of Transmission Modes of Anomalous Effects of meta-Substituents on the 13C Chemical Shift of the Carboxyl Carbon (δCO) of Benzoic Acid

2010 ◽  
Vol 63 (2) ◽  
pp. 321 ◽  
Author(s):  
Susanta K. Sen Gupta ◽  
Rajendra Prasad
Keyword(s):  
Chemical Shift ◽  
Benzoic Acid ◽  
Chemical Shifts ◽  
Substituent Effects ◽  
Basis Set ◽  
Benzoic Acids ◽  
Anomalous Effect ◽  
Atomic Orbitals ◽  
13C Chemical Shift ◽  
Carboxyl Carbon

Studies of substituent effects on NMR chemical shifts are of great benefit in determining fine details of electron distribution in molecules. Interestingly, NMR substituent effects are often different and even opposite to those associated with chemical reactivity. Among molecules exhibiting anomalous (reverse) substituent effects is benzoic acid, the standard model for studying substituent effects. The substituent effect on the 13C chemical shift of its carboxyl carbon (δ CO) is just the opposite of that on its acid strength or reactivity. To develop insights into the origin of the anomalous effect of a substituent on δ CO, occupancies of natural atomic orbitals at the carboxyl and ring carbons of a set of 10 meta-substituted benzoic acids have been calculated at the density functional theory level using the B3LYP function with split valance 6–311G++** basis set. Statistical correlations obtained for the 13C chemical shifts, δ CO and δ C-ring of these benzoic acids with the natural atomic orbital occupancies calculated for respective carbon atoms on one hand and with Taft’s inductive and resonance parameters (σ I and σ R BA ) of the substituents on the other hand have been critically analyzed. The findings have established firmly that a meta-substituent’s anomalous effect on δ CO is caused by the substituent-induced changes in the total occupancy of only the p z natural atomic orbitals at the carboxyl carbon. The study has demonstrated further that the transmission of the anomalous effect can be successfully interpreted by a 5.5:–2.5:1 combination of the localized, extended, and resonance-induced π-polarization effects.

29Si NMR chemical shifts in pertrimethylsilylated 1,6-anhydro-β-D-glucopyranose derivatives. Empirical assignment from Hammett type dependence of the chemical shifts

1983 ◽  
Vol 48 (9) ◽  
pp. 2503-2508 ◽  
Author(s):  
Jan Schraml ◽  
Jaroslav Včelák ◽  
Miloslav Černý ◽  
Václav Chvalovský
Keyword(s):  
Chemical Shift ◽  
Carbon Atom ◽  
Silicon Atom ◽  
Chemical Shifts ◽  
Substituent Effects ◽  
Polar Effect ◽  
29Si Nmr ◽  
Nmr Chemical Shifts ◽  
Trimethylsilyl Groups

29Si chemical shift of Si-3 silicon atom of the trimethylsiloxy group attached to C(3) carbon atom in 1,6-anhydro-β-D-glucopyranose derivatives correlates linearly with the sum of Taft polar constants σσ of substituents R2 and R4 on C(2) and C(4) carbon atoms. Quality of this correlation allows assignment of Si-3 line in the spectra of derivatives containing two or three trimethylsilyl groups in the molecule. The shielding order δ(Si-4) < δ(Si-3) found in 1,6-anhydro-2,3,4-O-tris(trimethylsilyl)-β-D-glucopyranose is the same as recently found in other pyranose derivatives with the same configuration of substituents but the order is reversed by strong polar effect of the substituent in 1,6-anhydro-2-O-p-toluenesulphonyl-3,4-O-bis(trimethylsilyl)-β-D-glucopyranose. This finding warns against indiscriminate use of empirical assignment rules based on simple shielding order without considering possible substituent effects.

Substituent effects on the 13C chemical shifts of monosubstituted twistanes

1977 ◽  
Vol 55 (17) ◽  
pp. 3161-3165 ◽  
Author(s):  
Helmut Beierbeck ◽  
John K. Saunders
Keyword(s):  
Chemical Shift ◽  
Substituent Effect ◽  
Chemical Shifts ◽  
Substituent Effects ◽  
13C Chemical Shifts ◽  
Bond Interaction ◽  
Chemical Shift Data ◽  
Shift Data ◽  
Deshielding Effect

The 13C chemical shift data for some hydroxy, chloro, bromo, and oxo twistane derivatives are presented and compared to the shifts observed in corresponding adamantanes. The substituent effect at the α and β carbons is more pronounced in twistanes than in adamantanes. The substituent shift induced at an antiperiplanar γ carbon is shown to depend on the presence or absence of 1,3-diaxial hydrogen–hydrogen interactions between the substituted and γ carbons. If such an interaction is present the effect is of shielding whereas if it is absent, the effect is of deshielding. The deshielding effect appears to occur via a through bond interaction.

Chemometrical Analysis of Substituent Effects. XIII. Comparison of Substituent Effects on Dissociation and Chemical Shift in 13C NMR Spectra of Mono- and Disubstituted Benzoic Acids

2000 ◽  
Vol 65 (1) ◽  
pp. 106-116 ◽  
Author(s):  
Jiří Kulhánek ◽  
Oldřich Pytela ◽  
Antonín Lyčka
Keyword(s):  
Chemical Shift ◽  
Latent Variable ◽  
Chemical Shifts ◽  
Substituent Effects ◽  
Principal Component ◽  
Data Matrix ◽  
Steric Effects ◽  
Benzoic Acids ◽  
Indicator Variable ◽  
Substituted Benzoic Acids

The 13C chemical shifts have been measured of the carboxyl carbon atoms for all the 2-, 3-, and 4-substituted benzoic acids with H, CH3, CH3O, F, Cl, Br, I, and NO2 substituents, as well as for all 3,4-, 3,5-, and 2,6-disubstituted benzoic acids with combinations of CH3, CH3O, Cl (or Br), NO2 substituents and for symmetrically 2,6-disubstituted derivatives with Et, EtO, PrO, i-PrO, and BuO substituents. The chemical shifts of carboxylic group carbon atoms of the 3- and 4-substituted derivatives show correlation only with the substituent constants σI. For the 2-substituted derivatives was found the dependence only on σI and on the υ constant describing steric effects (s = 0.122, R = 0.996, without the CH3 derivative which has a distinct anisotropic effect). The substituent effects on the carboxylic carbon chemical shift show additivity with 3,4-, 3,5-, and 2,6-substituents, and the 2,6-disubstituted derivatives show a linear synergic effect of substituents due obviously to the steric hindrance to resonance. Application of the principal component analysis to the data matrix involving all the combinations of mono- and disubstitution involving the above-mentioned substituents has proved an identical substituent effect from all the positions on the chemical shift described by one latent variable, steric effects and anisotropic behaviour of methyl at the 2 and 2,6 positions being predominantly described by the second latent variable (with the total explained variability of 99.5%). Comparison of substituent effects on the chemical shift of carboxylic carbon with that on the dissociation constant measured in the same solvent has confirmed the anisotropy due to ortho methyl group, the ortho halogen substituents in monosubstituted derivatives also having a different effect. The dependence of chemical shift on pKa was not very close for the derivatives studied (s = 1.005, R = 0.690). The inclusion of anisotropy of ortho alkyl group by means of an indicator variable improved the correlation (s = 0.533, R = 0.925), and omitting of 2-F, 2-Cl, 2-Br, and 2-I substituents gave a regression without deviating points (s = 0.352, R = 0.968).

13C NMR study of 3-(X-benzal)phthalides and 2-(X-benzal)-1,3-indanediones

1980 ◽  
Vol 45 (10) ◽  
pp. 2772-2778 ◽  
Author(s):  
Eva Solčániová ◽  
Pavol Hrnčiar ◽  
Tibor Liptaj
Keyword(s):  
Chemical Shift ◽  
Nmr Spectra ◽  
Electronic Effect ◽  
Chemical Shifts ◽  
Substituent Effects ◽  
Carbonyl Groups ◽  
Aryl Group ◽  
Nmr Study ◽  
Derivatives Of ◽  
C Nmr

13C NMR spectra of 14 derivatives of 3-(X-benzal)phthalides (I) and 10 derivatives of 2-(X-benzal)-1,3-indanediones (II) were investigated. The correlation of 13C chemical shifts of carbon atoms of the phthalide ring with σ-constants showed that the electronic effect of substituents was transmitted from the benzylidene group of 3-(X-benzal)phthalides on the chemical shift of the carbonyl group not only through oxygen, but also through the aromatic ring of the phthalide moiety. The transmission of substituent effects in 2-(X-benzal)-1,3-indanediones on the chemical shift of the carbonyl groups was more pronounced on the carbonyl, which is in the trans-arrangement with respect to the aryl group. This phenomenon was also observed at carbon atoms of the benzene ring of the indanedione moiety closer to the trans-CO group.

Principal Component Regression with Chemical Shift Increments. I. p-Disubstituted Benzenes and 2-Naphthyl Derivatives

1996 ◽  
Vol 61 (5) ◽  
pp. 713-725 ◽  
Author(s):  
Miroslav Holík
Keyword(s):  
Chemical Shift ◽  
Chemical Shifts ◽  
Principal Component Regression ◽  
Principal Component ◽  
Regression Coefficients ◽  
Regression Parameters ◽  
Monosubstituted Benzenes ◽  
Value Decomposition ◽  
Electron Demand ◽  
Better Than

Prediction of 13C substituent chemical shifts in 14 series of para-disubstituted benzenes and in 2-substituted naphthalenes was based on principal component regression with chemical shift increments for the ipso, ortho, meta and para position of monosubstituted benzenes. Mean-centered matrix of shift increments was submitted to singular value decomposition and principal component regression was used for the projection of the investigated substituent chemical shifts and for the calculation of regression coefficients. Residual standard deviation between experimental and fitted values in para-disubstituted benzenes was in agreement with absolute values of "an electron demand" of substituents. Inspection of the regression parameters revealed that for the prediction of chemical shifts in 2-substituted naphthalenes the combination of chemical shift increments was better than the use of single increments. It is believed that the presented procedure is general and can be used for other aromatic or heteroaromatic systems.

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