13C isotropic chemical shift

C Isotropic Chemical Shifts of Mesityl Oxide in Various Media at 298 K... 

Figure 2. The MP2/6-311+G optimized geometry of the 1-3-dimethylcyclopentyl carbenium ion. Selected bond distances in A are shown. The GIAO-MP2/tzp/dz values of the 13C isotropic chemical shifts are underlined.

To compare with the experimental NMR data, the 13C isotropic chemical shifts of all of the acetylide and vinylidene complexes were calculated. These systems were too large for us to be able to do chemical shift calculations at the MP2 level. We thus used the GIAO method at the RHF level. Fortunately, the RHF level of theory appears to be adequate for these complexes. We used a mixed basis set for the GIAO, with tzplarge+ on C, tzplarge on H, tzp on the Mg and O associated with the adsorbed species, and dzp on the remaining Mg and O (26). 

The calculated 13C shielding contour map reproduce well the experimental results. This means that the change of the main-chain dihedral angles dominates the 13C isotropic chemical shift behavior of the L-alanine residue Cp-carbon. 

Fig. 15. The pulse sequence for the 13C 2H correlation experiment.38 This two-dimensional experiment, conducted under magic-angle spinning, separates 2H line-shapes according to the 13C isotropic chemical shift of nearby l3C spins, i.e. the bonded l3C in practice. The narrow black pulses are 90° pulses wide ones are 180° pulses. The 2H pulses are placed symmetrically within the rotor period.

Finally, solution-state chemical shifts often are weighted time-averages arising from equilibria between low-energy conformations. The combined cp/mas NMR/ X-ray crystallography approach permits one to obtain chemical shifts from nuclei in known conformations. These solid-state 13C isotropic chemical shifts of nuclei in conformational polymorphs have enabled deconvolution of solution-state weighted time averaged values to ascertain the predominant conformation in a particular solvent.20... 

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... 

In these spectra, the protein has been regenerated with retinal specifically 13 C labeled at positions 11,12 and 13, and in each case the retinal resonance exhibits a sharp centerband at the isotropic chemical shift and is flanked by rotational sidebands. Other lines in the spectrum are the natural-abundance 13C resonances of the protein carbonyls ca 175 ppm) and aliphatic carbons (0-100 ppm). Contributions from the Ammonyx-LO detergent in these spectra are seen in the different intensities in the 0-100 ppm region. Ammonyx-LO does not exhibit NMR resonances above 100 ppm. Spectra of the 9-cis pigment isorhodopsin are similar. Table 38 summarizes the isotropic chemical shifts from the solid-state NMR spectra of rhodopsin regenerated with retinal13 C labeled at each position along... 

Table 39 summarizes the isotropic chemical shifts from the 13C NMR spectra of isorhodopsin along with chemical shift data from the 9-cis PSB chloride salt. The difference in chemical shifts between isorhodopsin and the 9-cis PSB are qualitatively similar to the differences observed between rhodopsin and the 11 -cis PSB with the exception of C7, where a 4.3 ppm chemical shift difference is observed, and C12, where the difference, amounting to about 3 ppm in rhodopsin, has now vanished. 

Fig, 16. 50.1-MHz 13C MAS spectra of benzaldehyde-a-13C and benzene reacting on zeolite HY. The spectrum acquired at 120 K after the sample was heated at 448 K clearly shows an isotropic chemical shift at 207 ppm, consistent with the chemical shift of the trityl cation. Furthermore, the Herzfeld-Berger analysis of the sideband intensities reveals an axially symmetric tensor, thus providing unambiguous evidence for the trityl cation 16. 

The principal components of the trityl cation in zeolite HY are <5 = 282 ppm and <5j = 55 ppm. It is instructive to tabulate all of the 13C principal component data measured for free carbenium ions in zeolites as well as for a few carbenium ions characterized in other solid acid media (Table III). The zeolitic species, in addition to the trityl cation (119), are the substituted cyclopentenyl cation 8 (102), the phenylindanyl cation 13, and the methylindanyl cation 12 (113). Values for the rert-butyl cation 2 and methylcyclopentyl cation 17 (prepared on metal halides) (43, 45) are included for comparison. Note that the ordering of isotropic chemical shifts is reasonably consistent with one s intuition from resonance structures i.e., the more delocalized the positive charge, the smaller the isotropic shift. This effect is even more apparent in the magnitudes of the CSA. Since... 

However, it is found that a combination of techniques, such as proton dipolar decoupling (removes the dipolar interactions), magic angle spinning (reduces the chemical shift tensor to the isotropic chemical shift value), and cross-polarization (increases the sensitivity of rare spins, like 13C) applied to a solid state material, results in sharp lines for 13C nuclei in the solid state10). Thus, the observation of narrow lines or high resolution NMR in the solid state is possible. 

Another promising approach to the study of microporosity of zeolites involves the measurement of the isotropic 13C NMR chemical shift which, as has been shown in the studies of the tacticity of polymers, is highly sensitive to the environment of the nucleus. In the first study of this kind, Boxhoorn et al. (329) observed that the C-3 carbon resonance from the tetrapropylam-monium cation enclathrated in the framework of zeolite ZSM-5 in the course of synthesis is split into two components of equal intensity. The reason for this is that the cation is located at the cross-section of the two nonequivalent... 

In line with these solution-phase 13C NMR results, the isotropic solid-state 13C NMR chemical shifts of the cage carbons of silatranes appeared to be nearly independent on the Si — N bond length254. 

Also shown in Figure 2 are the GIAO-MP2/tzp/dz values of the isotropic 13C chemical shifts. The predicted chemical shift for C2 and C3 is 255.3 ppm, which compares to the experimental value of 250 ppm. For C2, theory predicts a chemical shift of 152.8 ppm, whereas the experimental value is 148 ppm. The CH2 carbons are predicted to have isotropic chemical shifts of 50.0 ppm which compare to the measured values of 33 ppm. Finally, the methyl carbons have theoretical values of 28.9 ppm, whereas the experimental chemical shifts are 24 ppm. In all cases the theoretical values are downfield of the experimental chemical shifts. The differences are generally =5 ppm, with the exception of C3. The source of this larger difference is not clear. Still, the agreement is sufficient to verify the presence of the 1,3-dimethylcyclopentyl carbenium ion within the zeolite. 

Experimentally, the isopropyl cation was prepared by the low temperature reaction of 2-bromopropane-2-13C with frozen SbF5. The 13C spectrum was measured at 83 K using slow speed magic angle spinning. Analysis of the spectrum using the method of Herzfeld and Berger yielded tensor values of 8n = 497 ppm, 822 = 385 ppm and 833 = 77 ppm for the central carbon, which results in an isotropic chemical shift of 320 ppm. 

Fig. 8. Results of the l3C VACSY experiment on tyrosine ethyl ester.17 The experiment separates 13C (or other) chemical shift anisotropy powder patterns according to isotropic chemical shift. Motional details of each 3C site may then be determined by simulation of the relevant chemical shift anisotropy powder pattern, (a) The complete two-dimensional l3C VACSY spectrum, (b) Slices from the l3C VACSY experiment for the tyrosine phenyl ring at different temperatures. The simulations assume the phenyl ring is undergoing 180° flips at the rates indicated. All spectra are taken from reference 17.

Fig. 6. Top 2D MAT sequence for correlating isotopic chemical shift and CSA with two separate experiments P+ and P . All pulses following CP are 90°. A four-step phase cycling is used with 6 = —y, x, —y, x. and 62 = —y, x, x, -y. The receiver phases are x, -x, — y, -y for the P+ pulse sequence and x, —x,y, y for the P pulse sequence. (The sign of receiver phases with an asterisk depends on the relation between the pulse phase and the receiver phase of the particular spectrometer in use. These receiver phases must be changed in sign when the quadrature phase cycle (x,y, —x, -y) of the excitation pulse and the receiver phase in a single-pulse test experiment result in a null signal.) Phase alternation of the first H 90° pulse and quadrature phase cycling of the last 13C 90° pulse can be added to the above phase cycle. The time period T can be any multiple of a rotor period except for multiples of 3. Bottom 2D isotropic chemical shift versus CSA spectrum of calcium formate powder with a three-fold MAT echo extension. (Taken from Gan and Ernst178 with permission.)...

Solid-state 13C NMR has been shown to be a more effective analytical tool for demonstrating the formation of P-sheets in polypeptides and proteins, because the isotropic 13C NMR chemical shifts of carbon atoms in proteins are sensitive to the P-sheet s secondary structure. It is well established that SF conformations are dependent upon the species of silkworms and conditions of the sample preparation. In particular, has been reported that fibroin from Bomhyx mori adopts two dimorphic structures, silk I and silk II. The silk II form is identified by the C chemical shifts of glycine (Gly), serine (Ser), and alanine (Ala) that are indicative of P-sheets, while the silk I form produces chemical shifts that are associated with a loose helix or distorted P-tum. However, when compared with silk II, the less stable silk I shows a relatively unresolved structure, and the conformation of the soluble form of SF rapidly undergoes a transition to the insoluble silk II conformation. 

Fig. 11. (a) Gain of spectral resolution by MAS spinning of a single crystal (b) schematic behavior of 13C or 170 MAS lines of SQA as a function of temperature, with coexistence region (c) schematic of 13C or 170 isotropic chemical shift, obtained by the average of all the line positions. 

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