Carbenium ions chemical shifts

Although there have been many published reports of chemical shift calculations neglecting electron correlation, it is well established that electron correlation is important for carbenium ions, triple bonds, and other systems sensitive to correlation. Currently, Gauss MP2 method (distributed as part of ACES II) (85) is the most extensively validated methodology for carbenium ion chemical shift calculations. 

Figure 3 shows 13c MAS spectra of acetone-2-13c on various materials. Two isotropic peaks at 231 and 227 ppm were observed for acetone on ZnCl2 powder, and appreciable chemical shift anisotropy was reflected in the sideband patterns at 193 K. The 231 ppm peak was in complete agreement with the shift observed for acetone diffused into ZnY zeolite. A much greater shift, 245 ppm, was observed on AICI3 powder. For comparison, acetone has chemical shifts of 205 ppm in CDCI3 solution, 244 ppm in concentrated H2SO4 and 249 ppm in superacid solutions. The resonance structures 5 for acetone on metal halide salts underscore the similarity of the acetone complex to carbenium ions. The relative contributions of the two canonical forms rationalizes the dependence of the observed isotropic 13c shift on the Lewis acidity of the metal halide. 

Carbenium ions, 42 115, 143 acid catalysis, 41 336 chemical shift tensors, 42 124-125 fragments in zeolites, 42 92-93 history, 42 116 superacids, 42 117 Carbide catalysts, 34 37 Carbidic carbon, 37 138, 146-147 Carbidic intermediates, 30 189-190, 194 Fischer-Tropsch synthesis, 30 196-197, 206-212... 

Until the year 2002 no experimental data existed on the structures of unperturbed RsE" cations, the exact analogues of the carbenium ions. Computational data combined with NMR chemical shift calculations, which could be compared to experiment, were the only source of reliable structural information for silylium... 

The conversion of ethylene on a fresh zeolite HZSM-5 catalyst, which had not been used beforehand for methanol conversion, led to the spectra shown in Fig. 37c. The MAS NMR spectrum consists of signals at 14, 24, and 34 ppm caused by alkyl groups of cyclic compounds. Furthermore, a broad signal in the chemical shift range of alkenic and aromatic compounds appeared at ca. 120 ppm. The UV/Vis spectrum consists of bands similar to those shown in Fig. 37b and an additional weak band at ca. 450 nm. The latter may be attributed to condensed aromatics or trienylic carbenium ions (301). A weak shoulder observed at ca. 400 nm is an indication for the formation of hexamethylbenzenium ions (302). 

Since many NMR studies of solid acids entail the observation of a chemical shift change in a probe molecule, reactant, intermediate, or product upon complexation with an acid site, there is an opportunity to fundamentally impact the application of NMR to solid acids by making better use of this information. We therefore review the salient parts of the chemical shift in some detail. Since some of the more visible controversies in NMR studies of solid acids regard carbenium ions and related electrophilic species, this treatment will use such ions as examples wherever possible. 

In chemical shift calculations for acylium ions, it was not necessary to model the ionic lattice to obtain accurate values. These ions have tetravalent carbons with no formally empty orbitals, as verified by natural bond orbital calculations (89). Shift calculations for simple carbenium ions with formally empty orbitals may require treatment of the medium. We prepared the isopropyl cation by the adsorption of 2-bromopropane-2-13C onto frozen SbF5 at 223 K and obtained a 13C CP/MAS spectrum at 83 K (53). Analysis of the spinning sidebands yielded experimental values of = 497 ppm, 822 = 385 ppm, and (%3 = 77 ppm. The isotropic 13C shift, 320 ppm, is within 1 ppm of the value in magic acid solution (17). Other NMR evidence includes dipolar dephasing experiments and observation at higher temperature of a scalar doublet ( c-h = 165 Hz) for the cation center. 

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... 

Magnetic field strengths used in UC NMR at present usually are between 1.9 and 11.7 Tesla, corresponding to 13C Larmor frequencies between 20 and 500 MHz (Fig. 2.35). The range A of 13C chemical shifts of all organic compounds measured so far and including reactive intermediates such as carbenium ions approaches 400 ppm or 10 kHz at 25 MHz. The average 13C line widths dv1/2 now attainable are about 1 Hz, i.e. of the same order as those observed in H NMR. 

Electron deficiency at a carbon causes drastic deshielding. This is observed for the sp2 carbons typical of carbocations [79], In such systems, the sp2 13C chemical shift range may approach 400 ppm relative to TMS. If the positive charge is dispersed in a carboca-tion, e.g. by resonance, the electron deficient carbon will be more shielded. The following comparison of f-butyl-, dimethylhydroxy- and dimethylphenyl-carbenium ion illustrates this ... 

Table 4.76. 13C Chemical Shifts of Selected Carbenium Ions in Magic Acid (SbF5/FS03H/S02ClF) between -40°C and -80°C [79, 492] (5C in ppm). (Guani-dinium chloride and formamidinium chloride (last entries) are measured in hexa-deuteriodimethyl sulfoxide at room temperature.)... 

Traditionally, the same overall mechanisms of acid catalysis invoking carben-ium ions have been assumed to prevail both in heterogeneous (2) and in liquid homogeneous (3) systems. But these mechanisms do not adequately take into account the fact that adsorbed, rather than free, carbenium ions are formed in the pores of solid catalysts. Consequently, a quantum-chemical model that demonstrates how the interaction of carbenium ions with the sites of their adsorption can influence the reaction mechanism has been formulated by Kazansky (4), taking double-bond-shift reactions in olefins as a particular example. According to this view, adsorbed carbenium ions are best regarded as transition states rather than reaction intermediates, a notion that had also been proposed earlier by Zhidomirov and one of us (5). 

Estimates of the kinetics of methyl loss from energy-selected CztHg" species have been made by calculation.23 The hydride transfer from alkanes to carbenium ions in the gas phase is calculated to involve a species with a symmetric potential well, which is different from the situation in superacid or zeolite media.24 A correlation between the charge on a carbon and the in-plane tensor component of its 13 C chemical shift has been observed for a number of simple cationic and anionic species.25 High-level calculations... 

The H NMR spectrum shows resonances for (5.95 ppm), Hg/H g/ exo (4.01 ppm), H /Hg/ endo (3.12 ppm) and Hy (0.30 ppm). The NMR data are in accord with a bridged, puckered bicyclobutonium ion structure that is static on the NMR time scale. The 29Si NMR chemical shift of 43.1 ppm for ion 428 indicates that the silicon is involved in stabilization of the positive charge. The stabilization occurs by shifting electron density from the Cy—Si cr-bond across the bridging bond to the formal carbenium carbon Ca. This y-silyl- type of interaction may be termed silicon homohyperconjugation. 

Olah et al.532,533 studied trihalomethyl cations (CX3+, X = Cl, Br, I) under stable ion conditions. 13C NMR chemical shift values correlate well with the decreasing order of back-donation (Cl > Br > I). Similar correlation was also found for dimethylhalo-carbenium ions 258. The CF3+ fluoro analog, however, could not be observed under any conditions. This can be attributed to a combination of unfavorable thermodynamics (generation of CF3+ from CF4 is endothermic by about 20lstarting materials and a suitably strong Lewis acid.534... 

As might be expected, NMR calculations that ignore electron correlation often give poor results, especially for molecules which typically require a correlated treatment in order to predict other properties accurately. For example, a good description of multiple bonds and lone pairs generally requires a correlated method. Thus, RHF NMR predictions for molecules such as CO and acetonitrile are poor (20). Furthermore, it has recently been shown that isotropic chemical shift calculations at the RHF level are unreliable for benzenium (21) and related carbenium ions which we often encounter in catalysis. 

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.

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. 

For our final example of theoretical NMR in catalysis, we again turn to carbenium ion chemistry. Here we study the formation of the isopropyl cation on frozen SbF5, a strong Lewis acid (27). In contrast to the studies presented earlier, with this system we were able to experimentally measure the chemical shift tensor. Because the full tensor is naturally obtained from NMR calculations, a comparison can readily be made. In addition, for the isopropyl cation we also studied the effect the medium (in this case, the charge balancing anion) had on the chemical shift tensor. 

Theory helps the experimentalists in many ways this volume is on chemical shift calculations, but the other ways in which theoretical chemistry guides NMR studies of catalysis should not be overlooked. Indeed, further theoretical work on two of the cations discussed above has helped us understand why some carbenium ions persist indefinitely in zeolite solid acids as stable species at 298 K, and others do not (25). The three classes of carbenium ions we were most concerned with, the indanyl cation, the dimethylcyclopentenyl cation, and the pentamethylbenzenium cation (Scheme 1), could all be formally generated by protonation of an olefin. We actually synthesized them in the zeolites by other routes, but we suspected that the simplest parent olefins" of these cations must be very basic hydrocarbons, otherwise the carbenium ions might just transfer protons back to the conjugate base site on the zeolite. Experimental values were not available for any of the parent olefins shown below, so we calculated the proton affinities (enthalpies) by first determining the... 

Anisochrony due to axial chirality of the diastereotopic methylene protons in H2C = C=C(Me)COCHBrR (cf. Fig. 24) has been observed the chemical shift differences may be as high as 0.13 ppm.34) Also related to axial chirality are several cases of anisochronous methyl groups in isopropyl moieties which are diastereotopic through being part of a chiral allene of the type Me2CHCR=C=CR R" 35,49). These cases resemble that shown in Fig. 27 where a prochiral center (Me2CH—C. ..) is attached to a chiral ferrocenyl moiety it should be noted that the ferrocenylmethyl-carbenium ion fragment is chiral only if rotation about the Cp—C+ bond (marked a in Fig. 27) is slow on the NMR time scale 42 . 

Until the year 2002 no experimental data existed on the structures of unperturbed R3E+ cations, the exact analogues of the carbenium ions. Computational data combined with NMR chemical shift calculations, which could be compared to experiment, were the only source of reliable structural information for silylium ions6,7,13,77,121 while for germylium, stannylium and plumbylium ions this combined approach was not attractive due to either the non-existence of the experimental data (Ge) or the complexity of the computational problem (Sn, Pb). On the other hand, a series of excellent experimental studies demonstrated, for example, the high coordination tendency of small trialkylsilylium ions either toward the counteranion38,114,127,138 or toward the solvent.36,37,67,116,127 The solid state structures of these silyl cation salts showed clear indications either of cation/anion or cation/ solvent coordination. Thus, the nature of the observed cation, i.e. the degree of silylium ion character remained disputable.10,11,13... 

One of the carbenium ions below exhibits a l3C signal at 5 320.6 for the charged carbon, while the other s occurs at 8 250.3. Which chemical- shift goes with which structure and why ... 

Of course, a NMR chemical shift value of 225 ppm can be interpreted in different ways. One can assume that 61 represents a free silylium cation in solution that because of considerable -conjugation between 3pn(Si) orbital and the three phenyl rings is internally stabilized and, therefore, does not interact with solvent molecules. The question in this case would be to which extent 11-conjugation has reduced the silylium cation character and how much positive charge is spread over the C framework. It could be that 61 represents more a carbenium ion in the same way as the silaguanidinium ion represents more an ammonium ion (see section 4.1). 

A number of stable carbenium ions can be generated in superacid media most NMR data on chemical shifts and coupling constants of carbenium ions have been reported in superacid media [7-10]. However, superacids cannot be used for polymerization studies because excess superacid not only stabilizes the ionic species, but also protonates any alkene which would otherwise be available for oligomerization/polymerization. 

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