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.     

Temperature effects on the carbon-13 NMR shifts of selected compounds

The effects of temperature on the ¹³C NMR shifts of benzene, cyclohexane, pyridine, cyclohexene, neopentane, hexane, and heptane have been determined from +10 to +70°C. In addition, the temperature shift of ²D in D₂O was found to be +10.4 × 10^?3 ppm/°C. All shifts appear to be linear functions of temperature within experimental error.     

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.     

35Cl/37Cl isotope effects on 13C chemical shifts. Assignment of carbon-13 NMR lines of polychlorinated hydrocarbons

An isotope effect of about 0·1 Hz at 25 MHz (0·004 ppm) on the ¹³C chemical shifts due to directly bonded chlorine has been observed at ?50°C for hexachloropropene, hexachlorobutadiene and tetrachlorobutenyne. This effect yields an unambiguous assignment of ¹³C NMR lines to specific carbon atoms in these polychlorinated hydrocarbons.     

Proton and carbon-13 chemical shifts: Comparison between theory and experiment

A series of ab initio ¹H and ¹³C NMR chemical shifts are presented for all molecules for which gas-phase experimental measurements exist. Quantitative agreement with this large set of data is achieved by the use of gauge-invariant atomic orbitals in an SCF perturbation theory approach. The effect of basis set completeness on these ¹H and ¹³C chemical shifts is also examined. The 4-31G basis set is found to provide internally consistent results and give satisfactory agreement with gas-phase experimental data. Errors within 6% for ¹H shifts and 3% for ¹³C shifts result. Increasing the basis set to the 6-31G^* level does not significantly improve the agreement. For ¹H shifts only, the 3-21G basis set is adequate. The validity of the particular computational approach employed here is further substantiated by comparison to another ab initio magnetic shielding method.     

Effects of some phosphorus substituents on the carbon-13 chemical shifts of alkyl chains

The carbon-13 NMR spectra of 16 aliphatic phosphorus compounds with -PCl₂, -P(OCH₃)₂, -P(CH₃)₂ or -PS(CH₃)₂ groups were determined and interpreted on the basis of the usual α, β-deshielding and γ-shielding effects. The α-effects of all the groups were quite large (15 to 30 ppm), though the β-effects were small (0 to 3 ppm). The γ-effects were also small (0·2 to 1·6 ppm), but of significance in suggesting reduced steric compression relative to that found for methyl and most of the common organic functional groups. This may be attributed to the greater length of the C? P bond. The phosphorus-31 NMR spectra of these compounds also show the existence of interaction with γ-carbons by exhibiting small additive upfield shifts.     

Principal component analysis of carbon-13 substituent-induced chemical shifts of some unsaturated compounds

Principal component analysis (PCA) is applied to 32 disubstituted unsaturated compounds (Y–CH₂–X): cyanides, oximes and propenes; bearing 12 -substituents: F, Cl, Br, I, OMe, OEt, SMe, SEt, NMe₂, NEt₂, Me, and Et. The experimental ¹³C chemical shifts for the -carbon and functional carbon atoms are correlated with theoretically derived molecular properties, i.e. partial charges, electronegativity, hardness, dipole moments and the nuclear repulsion energies. In the first PCA, the clustering of these three classes of organic compounds occurred mostly because of the chemical shifts and partial charges, and also of the dipole moments, hardness and electronegativity parameters as confirmed by loading graph. A strong grouping is observed in the second PCA, showing the chemical shift dependence on the type of heteroatom substituents. Therefore, sulfur, nitrogen, oxygen and neutral groups exhibit four types of C-13 SCS influences, indicating that the heteroatom (Y) properties play a significant role on the effects on chemical shifts. The -halogenated compounds represent a very heterogeneous group due to possible orbital interactions between the functional group and the substituent. The third PCA shows the grouping of F, Cl, Br and I derivatives, confirming the second PCA results that same halogen presents the same or very similar effects on the chemical shifts.     

Intrinsic carbon-13 NMR solvent shifts in hydrocarbons. Relevance of long range substituent parameters

Carbon-13 NMR chemical shifts have been measured in a number of binary mixtures of normal alkanes. Intrinsic solvent shifts are deduced from the shifts and the relevance of some small substituent effects in alkanes is discussed. A comparison is made between solvent effects and thermally induced chemical shift differences in alkanes.     

Phosphorus-containing groups in analyses of carbon-13 chemical shifts of 1-hetera-4-cyclohexanones

The ¹³C NMR chemical shifts of a series of phosphorinanes and the corresponding 4-phosphorinanones have been evaluated using ‘double difference’ plots. Most of the compounds fit the same linear relationships for the carbons alpha and beta to the hetera group previously found for the analogous sulfur, oxygen and nitrogen six-membered heterocycles, suggesting that these phosphorus compounds possess similar chair conformations. For the carbons gamma to the hetera group, the plots suggest that there are transannular interactions between the trivalent phosphine groups and the carbonyl group, similar to that previously observed for the sulfide group. In these 1-hetera-4-cyclohexanones, transannular interactions appear whenever a lone pair of electrons on a second-row heteroatom is present in the proper conformation for interaction with the carbonyl group.     

A simplex optimized INDO calculation of 13C chemical shifts in hydrocarbons

The variable-size simplex optimization method is used to reparametrize the I + A and β parameters of an INDO approximation to the perturbed Hartree–Fock calculation of ¹³C chemical shifts in hydrocarbons. The absolute shifts for 39 nuclei in a set of molecules containing up to four carbons are reproduced within a standard error of 9.9 ppm for an unconstrained optimization and to a standard error of 10.0 ppm for an optimization constrained to yield gross atomic charges in agreement with double-zeta ab initio calculations.     

Assignment of configurations of conjugated dienic pheromones by fuzzy classification of carbon-13 chemical shifts

The configurations of conjugated dienes, and some sex pheromones and analogous compounds, can be characterized by a fuzzy classification method applied to the ¹³C chemical shifts of the dienic carbons. A fuzzy relation matrix of 12 model dienes is set up, and a fuzzy parameter (λ) is established. If λ ≥ 0.95 in the similarity test of two dienes, then both dienes have the same configuration. Some empirical rules dealing with the influence of terminal groups and assignment of the order of dienic carbons are presented. Configurations of 14 compounds prepared by different methods were established reliably. After correct assignments of dienic carbons, the method was used to quantify the isomeric composition of (10,12)-pentadecadienyl acetate, (9,11)-tridecadienyl acetate and (9,11)-tetradecadienyl acetate by the inverted gated decoupling technique; the results agreed well with those obtained by gas chromatography.     

The determination of the COOH substiuent effect for branched carboxylic acids from carbon-13 chemical shifts

The carboxyl substituent effects for branched carboxylic acids have been determined by carbon-13 Fourier ransform nuclear magnetic resonance of eighteen measured and reported branched acids. For α-branched systems the substituent effects are α = 15.3 ppm, β = 2.6 ppm, γ = ?1.9 ppm, δ = 0.9 ppm and ? = 0.5 ppm. For other branched acids, the determined carboxyl substituent effects are α = 19.2 ppm, β = 2.1 ppm, γ = ?1.4 ppm, δ = 0.6 ppm, and ? = 0.6 ppm.