13C NMR chemical shift

Although perhaps not strictly a conformational study, the structure of the gem-disulfoxide 184 was determined by X-ray crystallography169. A comparison of the structural parameters so measured with 1H and 13C NMR chemical shifts led the authors to conclude that chemical shifts should allow configurational assignments to be made to related sulfoxides. 

Table 2. 13C NMR chemical shifts and relaxation data T1 of cellulose acetate in solid state and a in acetone-d6 soution at 298 K. Spectrometer JEOL FX-200, frequency 50 MHz...

The formation of alkyl shifted products H and 14 can be explained in terms of the formation of endo-intermediate 21 formed by endo attack of bromine to 2 (Scheme 4). The determined endo-configuration of the bromine atom at the bridge carbon is also in agreement with endo-attack. Endo-Intermediate 21 is probably also responsible for the formation of cyclopropane products 12 and 15. The existence of cyclopropane ring in 12 and 15 has been determined by and 13c NMR chemical shifts and especially by analysis of cyclopropane J cH coupling constants (168 and 181 Hz). On the basis of the symmetry in the molecule 12 we have distinguished easily between isomers 12 and 15. Aryl and alkyl shift products IQ, H, and 14 contain benzylic and allylic bromine atoms which can be hydrolized easily on column material. 

Table 9. 13C-NMR Chemical Shifts (5) of Methyl-Substituted Tetrazoliums 130-135...

Total assignment of the H and 13C NMR chemical shifts as well as the relative configuration of the Diels-Alder adducts 33-35 was accomplished with the help of 2D (111-111 COSY, H-111 NOESY (NOESY = nuclear Overhauser enhancement spectroscopy), H- C XHCORR (XHCORR = nucleus X-hydrogen correlation), H-13C COLOC) and NOE difference spectroscopy <1996JHC697>. 

Perrin et al. probed the structures of 174 and 175 by using the 13C NMR method of isotopic perturbation of equilibrium <2001PCA11383>. The goal of this study was to measure the 13C NMR chemical-shift difference between C5 of a-deuterated molecule and C2-(5) of undeuterated molecule (see Equation 48). If 174 or 175 is a mixture of valence tautomers 180-r/ia and 180-r/]b or 181-equilibrium isotope shift (Aeq) will be observed in addition to the intrinsic isotope shift (5A0). In contrast, if 174 or 175 has C2v symmetry, then only (5A0) will contribute. 

Table 2 Calculated and experimental 13C NMR chemical shifts of 1 -boraadamantane framework in isomeric 4-methyl-1 -boradamantane pyridine adducts...

Figure 7 1H and 13C NMR chemical shifts of some benzo- and dibenzocycl[3.2.2]azines. 

C-NMR chemical shifts are a useful diagnostic tool for carbene complexes, with Ccarb being substantially deshielded. The Ccarb resonance is seen to shift to higher fields as the electron deficiency of the metal center is increased (i.e., with electron-withdrawing ligands and in cationic complexes). 

For the methyl-substituted ethylenes, i.e. in the absence of any steric effects, there is a roughly linear relationship between the chemoselectivity and the 13C nmr chemical shift of the most substituted carbon atom of the bromonium ions (Dubois and Chretien, 1978). This selectivity is therefore discussed in terms of the magnitude of the charge on the carbon atom and the relative hardness of the competing nucleophiles, according to Pearson s theory (Ho, 1977). However, this interpretation does not take into account the substituent dependence of the nucleophilic solvent assistance, which must play a role in determining this chemoselectivity. 

Table 5 13C NMR chemical shifts (in ppm) for the benzotriazol-1 -yl and a-carbon atoms in the spectra of derivatives 27-29 and 39-43 taken in CDCI3...

C NMR spectral data has appeared more widely in the literature. The 13C NMR chemical shifts for a variety of... 

C NMR Chemical Shifts and Coupling Constants of Organometallic Compounds, 12, 135... 

Reaction of 3 with Ph3C+PF6" resulted in the formation of methylidene complex [(n-C5H5)Re(N0)(PPh3)(CH2)]+ PF6 (8) in 88-100% spectroscopic yields, as shown in Figure 11. Although 8 decomposes in solution slowly at -10 °C and rapidly at 25 °C (She decomposition is second order in 8), it can be isolated as an off-white powder (pure by H NMR) when the reaction is worked up at -23 °C. The methylidene H and 13C NMR chemical shifts are similar to those observed previously for carbene complexes [28]. However, the multiplicity of the H NMR spectrum indicates the two methylidene protons to be non-equivalent (Figure 11). Since no coalescence is.observed below the decomposition point of 8, a lower limit of AG >15 kcal/mol can be set for the rotational barrier about the rhenium-methylidene bond. 

Substituted allenyl cations 47 have been generated from propargyl alcohols 48 under stable carbocation conditions (Sbf s/f SOsII in SO2CIF) (equation 17). On the basis of 13C-NMR chemical shifts, the positive charge has been found to be extensively delocalized with the mesomeric allenyl cations contributing highly to the total ion structure36,37. 

TABLE 1. 13C-NMR Chemical Shifts of Tris(ethynyl)methyl Cations (49a-d) and Their Precursor Alcohols 50a-d°... 

For the CH with a Cs symmetrical structure 1 a 13C NMR chemical shift of 11.5 ppm is calculated. This is 7.1 ppm more shielded than that calculated for the hydrocarbon CH4 at the GIAO-MP2 level.19 The shielding effect in 1 is as expected for a hypercoordinated carbocation carbon.20 The calculated minimum energy structure for diprotonated methane (CHi+) 2 has C2v symmetry with two 3c-2e... 

A comparison of IGLO/DZ//MP2/6-31G(d) calculated and measured 13C NMR chemical shifts demonstrates that the partially methyl bridged isomer is the preferred species.36 It was demonstrated that the calculated 13C chemical shifts are highly sensitive toward hyperconjugational distortion, i.e. the degree of bridging (ca. 6ppm/deg from 68° to 98°). 

Electron density methods such as GIAO-DFT methods require much less computational resources in terms of cpu time, memory and disk space compared to wave-function methods such as GIAO-MP2. A systematic study of a set of 16 alkyl- and cycloalkyl cation (Scheme 1) was performed to investigate the performance of GIAO-B3LYP methods for prediction of 13C NMR chemical shifts for these types of carbocations.37... 

The comparative study of the experimental NMR spectra of Zs-1-cyclopropyl -2-(triisopropylsilyl)ethyl cation (17) and the computational model structure E-1-cyclopropyl-2-(trimethylsilyl)ethyl cation (18) demonstrates another application of calculations of 3H and 13C NMR chemical shifts and nuclear spin-spin coupling constants. In particular vicinal3./(11,11) spin-spin coupling constants are useful for... 

An experimental and calculational NMR investigation of dicyclopropyl substituted cyclobutylmethyl cation (19)45 has shown that IGLO/DZ//B3LYP/6-31G(d) calculated 13C NMR chemical shifts facilitate the assignment of the spectra. 

C NMR chemical shifts of a series of higher substituted a-vinyl substituted vinyl cations 24-27 were calculated to explore the sensitivity of the predicted isotropic shifts to electron correlation, basis set and geometry effects in differently substituted l,3-dienyl-2-cations.51... 

The GIAO-MP2/TZP calculated 13C NMR chemical shifts of the cyclopropylidene substituted dienyl cation 27 show for almost all carbon positions larger deviations from the experimental shifts than the other cations 22-26. The GIAO-MP2/TZP method overestimates the influence of cr-delocalization of the positive charge into the cyclopropane subunit on the chemical shifts. Electron correlation corrections for cyclopropylidenemethyl cations such as 27 and 28 are too large to be adequately described by the GIAO-MP2 perturbation theory method and higher hierarchies of approximations such as coupled cluster models are required to rectify the problem. 

Scheme 2 13C NMR chemical shifts of cyclopropylcyclopropylidenmethyl cation 28. bottom to top calculated GIAO-HF, GIAO-DFT (B3LYP), GIAO-MP2, GIAO-CCSD, GIAO-CCSD(T) with TZP/DZ basis for MP2/6-31G(d,p) geometry and (top) experimental chemical shifts.

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