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.     

Excellent correlation between substituent constants and pyridinium N-methyl chemical shifts

Substituents on the pyridinium ring of N-methylpyridinium derivatives, especially those on the 2- or 4-position, have a large effect on the ¹H and ¹³C NMR chemical shifts of the N-methyl group. Reasonable correlations between the chemical shift changes and the resonance substituent constants are observed. The dual substituent parameter approach provides an excellent correlation when a combination of polar and resonance substituent constants is employed.     

Determination of substituent chemical shifts in the proton resonance spectra of the porphyrins

Reproducible proton chemical shifts in the porphyrin series are obtained when the spectrum is measured in chloroform using the zinc (II) complex of the porphyrin in the presence of an excess of pyrrolidine. This method is specifically demonstrated for the case of 2,4-dicyanodeuteroporphyrin-IX dimethyl ester which, as the free base, shows dramatic effects due to aggregation phenomena. The shifts obtained using the zinc (II) complex plus pyrrolidine method, which allow compilation of substituent chemical shifts in the porphyrin series, are shown to be the same as the less easily accessible infinite dilution shifts of the porphyrin free bases.     

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.     

A correlation of substituent effects with proton chemical shifts in aromatic tetrazolic acids

The effect of substituents on the proton chemical shifts and spin–spin coupling constants in ortho-, meta- and para-substituted 5-phenyltetrazoles (tetrazolic acids) in DMSO–CH₃CN (1:1, v/v) was studied. With the meta- and para- substituted compounds the additivity rule of chemical shifts was obeyed, thereby enabling increments characterizing the effects of individual substituents in monosubstituted benzenes to be determined. By employing the Smith and Proulx equation, the chemical shifts of the aromatic protons were correlated with the F, R and Q substituent constants. The values of these constants are 1.02, ?0.004 and 5.49, respectively, for the tetrazolyl substituent.     

Correlations of chemical shifts of saturated carbon atoms that have a directly bonded substituent

The dependence of the chemical shifts, δ, of saturated carbon atoms upon the nature of the directly bonded substituent is attributed to three factors (1) electronegativity E, (2) field effect, W, and (3) heavy atom ettect, Q. Ten series of compounds (five aliphatic and five cyclic) have been correlated with the equation δ = aE + W + Q+ c where a and c are constants.     

Hydroxy group substituent effects on 1H NMR chemical shifts in the structural elucidation of spirostanes

¹H NMR chemical shifts in spirostanes have been determined by spin-decoupling difference spectroscopy. Substituent effects of 7β-OH on 15-methylene proton shifts, 1β-OH on 11α- and 2α-proton shifts and 6α-OH on 4α-proton shifts have been determined and used in the structural assignment of spirostanes.     

The use of mid-points or average NMR chemical shifts in stereochemical assignments

A comparison of the mid-points or average chemical shifts of mirror-symmetrical spin patterns in the NMR spectra of structural isomers can be used in a straightforward manner to obtain stereochemical information. This result is anticipated from analysis of substituent contributions to chemical shifts and has been observed in a variety of chemical systems, especially cyclobutane derivatives, which comprise a group of compounds for which appreciable data is available and whose structure assignments have often entailed difficulty and even controversy. The method of mid-point comparison may also be useful for conformational analysis.     

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.     

Calculations and assignments of endohedral helium-3 chemical shifts of open-cage fullerenes and higher fullerenes

The endohedral ³He NMR chemical shifts of open-cage fullerene compounds and higher fullerenes ³He@C _n (n = 82, 84, 86) have been calculated at the GIAO-B3LYP/3-21G//AM1 level. The predicted ³He NMR chemical shifts of open-cage fullerene compounds agree well with the experimental data. More importantly, the challenging peak assignments in the two ³He NMR spectra of higher fullerenes have been successfully achieved by our computed endohedral ³He chemical shifts in combination with experimental results.     

Carbon-13 substituent chemical shifts of 4-substituted bicyclo[2.2.2]oct-1-yl cyanides

The carbon-13 substituent chemical shifts of 4-substituted bicyclo[2.2.2]oct-1-yl cyanides are a linear function of the substituent inductive parameters.     

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.     

13C NMR spectra of polyiodosubstituted compounds and structural increments in the chemical shifts for carbon in polyiodoalkanes

Conclusions We propose a topological scheme for empirical estimates of ¹³C in polyiodoalkanes containing both individual substituent increments and also structural increments characterizing two-particle steric interactions of substituents, and also including terms taking into account the asymmetry of the total heterograph.Deceased.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2719–2724, December, 1987.     

Stereochemical aspects of proton chemical shifts. VI—increments due to geminal hydroxyl,methyl substitution

In geminal methyl, hydroxyl substitution an axial hydroxyl, equatorial methyl arrangement has a downfield effect on a syn axial hydrogen atom which is larger by 0.3 ppm than an axial methyl, equatorial hydroxyl arrangement. It is proposed that these observations may constitute a basis for configurational assignment.