Chemistry A Tool for Spectral Interpretation

The method proceeds as follows a series of spectroscopic results are first collected using appropriate pulse sequences for spectral assignments and control experiments. The spectral features are then assigned and critical 

There has been much recent progress in the application of density functional theory (DFT) to the calculation of shift tensors, and several methods are presently available. The sum-over-state (SOS) DFT method developed by Malkin et al. (70) does not explicitly include the current density, but it has been parametrized to improve numerical accuracy. Ziegler and coworkers have described a GIAO-DFT method (71) that is available as part of the Amsterdam density functional package (72). An alternate method developed by Cheeseman and co-workers (73) is implemented in Gaussian 94 (74). 

For other details on chemical shift calculations and their application to chemical problems, the reader is directed to a symposium volume (58) and papers by Jameson and co-workers (75), Oldfield and co-workers (76), Grant and co-workers (77), Ellis and co-workers (78), and Farrar and coworkers (79). 

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. 

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. 

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