13C NMR chemical shifts of carbonyl groups in substituted benzaldehydes and acetophenones: substituent chemical shift increments

13C NMR Substituent chemical shift (SCS) increments have been determined for the carbonyl carbon of a variety of substituted benzaldehydes and acetophenones. The 13C NMR chemical shift of the carbonyl carbon can be predicted for many di- and trisubstituted benzaldehydes and acetophenones through simple additivity of the SCS increments. The magnitude and sign of the SCS increments have been explored using Hartree-Fock 6-31G* calculations to determine the natural atomic charges of the carbonyl carbon. When a substituent capable of intermolecular hydrogen bonding is present, deviations from additivity on the order of 2 ppm are observed in dilution experiments; deviations of up to 6 ppm can result from intramolecular hydrogen bonding.     

15N NMR chemical shifts of ring substituted benzonitriles

15N chemical shifts in an extensive series of para (15) and meta (15) as well as ortho (8) substituted benzonitriles, X-C6H4-CN, were measured in deuteriochloroform solutions, using three different methods of referencing. The standard error of the average chemical shift was less than 0.03 ppm in most cases. The results are discussed for both empirical correlations with substituent parameters and quantum chemical calculations. The 15N chemical shifts calculated at the GIAO/B3LYP/6-31 + G*//B3LYP/6-31 + G* level reproduce the experimental values well, and include nitrogen atoms in the substituent groups (range of 300 ppm with slope 0.98 and R = 0.998, n = 43). The 15N shifts in hydroxybenzonitriles are affected by interaction with the OH group. Therefore, these derivatives are excluded from the correlation analysis. The resultant 15N chemical shift correlates well with substituent constants, both in the simple Hammett or DSP relationships and the 13C substituent-induced chemical shifts of the CN carbon.     

Methyl substituent effects for methyltryptophans in 13C nmr

The ¹³C and ¹H nmr spectra of methyltryptophans 2–5 in 0.1 N sodium deuteroxide methanol-d₄ were assigned based on 1-D and 2-D nmr techniques, including COSY, inverse-detected direct (HMQC) and long-range (HMBC) correlation. Methyl substituent effects in chemical shifts (SCS) for the indole ring of tryptophan were calculated and compared with those of indole. The correlations were linear except for 4-methyltryptophan, which suggest structural changes in the indole ring of 4-methyltryptophan and 4-methylindole. The results of molecular modeling and NOE experiments supported that suggestion.     

13C and 15N NMR chemical shifts of alkylsubstituted benzonitriles

¹³C NMR data of 14 and ¹⁵N NMR data of four alkylsubstituted benzonitriles are reported and discussed in relation to substituent effects and stereochemistry.     

The calculation of hydroxyl group C-13 substituent chemical shifts in steroids and other rigid systems

A four parameter equation, for the calculation of one-bond secondary OH group C-13 substituent chemical shifts in steroids and other rigid systems, is deduced from literature data by the use of a multiple regression analysis. The magnitude of the coefficients are shown to be consistent with deviations from simple additivity of C-13 shifts in various systems. Conformational information in non-rigid systems is evident from differences between the calculated and observed SCS values.     

Carbon-13 chemical shifts of substituted nortricyclenes

¹³C NMR spectra of a series of substituted nortricyclene derivatives have been measured, and the ¹³C chemical shifts interpreted in terms of α, β, γ and δ-effects. The nature of these substiuent shifts is discussed together with some analytical possibilities. The substituent shifts provide valuable data about the steric effects in strained molecules and can be used as increments for structural analysis, particularly for the determination of orientations of substituent groups.     

Spatial substituent effects of various fluorinated groups on the 13C chemical shifts in cyclohexanes

The effect of introduction of fluorinated groups (CH(2)F, CHF(2), CF(3), C(2)F(5), OCF(3), SCF(3)) on the (13)C NMR chemical shifts in cyclohexanes is examined. The two main effects are caused by location at the alpha and gamma carbon positions. Comparison of the various data allowed the calculation of increments corresponding to the introduction of fluorinated groups at axial or equatorial positions on the cyclohexane ring. The introduction of fluorine atoms in methoxy and thiomethoxy groups has only a slight effect through the heteroatom on the (13)C chemical shifts.     

Geometry of α-cyclodextrin inclusion-complexes with some substituted benzenes deduced from C-13 NMR chemical shifts

The geometries of inclusion complexes of -cyclodextrin (-CD) with guests, benzoic acid, p-hydroxy benzoic acid, and p-nitrophenol in aqueous solution have been determined by comparing the complexation induced C-13 shifts of guest molecules with quantum chemical predictions. In the calculations, the non-polar environmental effect produced by the -CD cavity on the C-13 shifts of included guest molecule has been formulated by the so-called solvent effect theory. The geometries of the complexes predicted theoretically were consistent with those in aqueous solution determined by H-1 NMR and those in the solid by the X-ray method.Details containing a more complete methodology and complete data will be published elsewhere.     

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.     

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     

Comparison of the substituent effects on the 13C NMR with the 1H NMR chemical shifts of CHN in substituted benzylideneanilines

Fifty‐two samples of substituted benzylideneanilines XPhCH?NPhYs (XBAYs) were synthesized, and their NMR spectra were determined in this paper. Together with the NMR data of other 77 samples of XBAYs quoted from literatures, the ¹H NMR chemical shifts (δ_H(CH?N)) and ¹³C NMR chemical shifts (δ_C(CH?N)) of the CH?N bridging group were investigated for total of 129 samples of XBAYs. The result shows that the δ_H(CH?N) and δ_C(CH?N) have no distinctive linear relationship, which is contrary to the theoretical thought that declared the δ_H(CH?N) values would increase as the δ_C(CH?N) values increase. With the in‐depth analysis, we found that the effects of σ_F and σ_R of X/Y group on the δ_H(CH?N) and the δ_C(CH?N) are opposite; the effects of the substituent specific cross‐interaction effect between X and Y (Δσ²) on the δ_H(CH?N) and the δ_C(CH?N) are different; the contributions of parameters in the regression equations of the δ_H(CH?N) and the δ_C(CH?N) [Eqns 4 and 7), respectively] also have an obvious difference. Copyright © 2015 John Wiley & Sons, Ltd.     

Carbon-13 chemical shift and the conformation of substituted naphthalenes

The carbon-13 chemical shift of 15 substituted naphthalenes has been employed to calculate the stability and population of the various conformers of 1-naphthaldehyde, 2-naphthaldehyde, 1-hydroxynaphthalene, 2-hydroxynaphthalene and 2-methoxynaphthalene. It was found that the results obtained by this method are comparable to that obtained using other methods.     

Interpretation of substituent effects on 13C and 15N NMR chemical shifts in 6-substituted purines

A range of purine derivatives modified at position 6 of the basic purine skeleton exhibit a variety of biological activities. Several derivatives are used or tested nowadays for pharmacological treatments. The present work aims to analyze the effects of substituents on the electron distribution in the purine core as reflected by NMR chemical shifts. We collected a comprehensive set of experimental NMR data for a variety of 6-substituted purines (-NH(2), -NHMe, -NMe(2), -OMe, -Me, -CCH, and -CN) and determined the molecular and crystal structures of three derivatives (-NHMe, -CCH, and -CN) by X-ray diffraction. The density-functional methods calibrated in our recent study (Phys. Chem. Chem. Phys., 2010, 12, 5126) have been employed to enable understanding of the substituent-induced changes in the NMR chemical shifts of the atoms in the purine skeleton. Analyses of the nuclear shielding using localized molecular orbitals (LMOs), specifically the natural LMOs (NLMOs) and Pipek-Mezey LMOs, were used to break down the values of the isotropic (13)C and (15)N NMR chemical shifts and the chemical shift tensors into the contributions of the individual LMOs. The experimental and calculated trends in the chemical shift of the N-3 atom correlate nicely with the Hammett constants (σ(para)) and the calculated natural charges on N-3, whereas the contributions of the LMOs to the N-1 and C-6 chemical shifts are found to be more complex.     

The N-substitution and N-oxidation effects on the carbon-13 chemical shift of some substituted anilines

The N-substitution and the N-oxidation effects on the carbon-13 chemical shift for all ring carbons of some substituted anilines are discussed. Correlation of the N-substitution effect with substituent constants by means of a linear combination of two empirical parameters, σ_I and σ_R has been studied. The best correlation was obtained between the N-methylation effect and the substituent constants.     

Conformational analysis XX—13C NMR studies of saturated heterocycles 5—substituent effects on the 13C chemical shifts of methyl substituted 1,3-dithiolanes

The ¹³C NMR chemical shifts for 1,3-dithiolane and 13 methyl substituted derivatives are reported. Substituent effects are derived and compared with those for cyclopentanes and 1,3-dioxolanes. The magnitude and variety of the substituent effects are best explained with the aid of a half-chair conformation where the S-1? C-2? S-3 plane passes between C-4 and C-5.