Solid acidity chemical shift

Key Words Solid acid catalysts. Phosphorus-containing molecules. Solid-state NMR, Acidity, chemical shift... 

Table 2. Carbon-13 Solid-State Chemical Shifts of Tartaric Acids (25)...

The probe molecules of greatest historical interest in catalysis are the Hammett indicators [13]. The difficulty of making reliable visual or spectrophotometric observations of the state of protonation of these species on solids is well known. We have recently carried out the first NMR studies of Hanunett indicators on solid acids [ 14]. This was also the occasion of the first detailed collaboration between the authors of this article, and theoretical methods proved to strongly compliment the NMR experiments. The Hanunett story is told after first reviewing the application of theoretical chemistry to such problems. Central to the application of any physical method in chemistry is the process of modeling the relationship between the observables and molecular structure. However often one does this, it is rarely an exact process. One can rationalize almost any trend in isotropic chemical shift as a function of some variation in molecular structure - after the fact, but the quantitative prediction of such trends in advance defies intuition in most nontrivial cases. Even though the NMR spectrum is a function... 

Solid state 13C CPMAS NMR spectra of Wheat High Molecular Weight (W.HMW) subunits show well resolved resonances identical with spectra of dry protein and peptide samples [24], Most of the amino acids side-chain resonances are found in the 0-35 ppm region followed by the alpha resonances of the most abundant amino acids glycine, glutamine and proline at chemical shifts of 42, 52 and 60 ppm, respectively, and the carbonyl carbons show a broad peak in 172-177 ppm region. The CPMAS spectra of hydrated whole HMW provides important information on the structural characteristics. 

Shi et al.71 have assigned the backbone and side-chain chemical shifts for 103 of 238 residues of proteorhodopsin using solid state NMR spectroscopy. Analysis of the chemical shifts has allowed determination of protonation states of several carboxylic acids as well as boundaries and distortions of trans-membrane a-helices and secondary structure elements in the loops. It has been shown that internal Asp227, making a part of the counterion, is ionised, while Glul42 located close to the extracellular surface is neutral. 

Table 1. Structural carbon distribution (%) of the humic acids extracted from soil horizons, adopted from Xing (2001). The distribution was calculated from solid state 13C Cross-Polarization Magic-Angle-Spinning (CP/MAS) NMR spectra. Chemical shift assignment for carbon functional groups alkyl 0-50 ppm O-alkyl 50-117 ppm aromatic 107-165 ppm.

Tab. 6.1 Calculated and experimental solid-state principle values of the chemical shifts of selected nitrogen atoms in nucleic acid bases...

The experimental data closely resemble the simulation based on the all-trans PSB values. The discrepancy between the two solid-state NMR studies on rhodopsin arises in part from a difference, in signal-to-noise ratio and in part from possible problems associated with a fatty acid resonance which overlaps with the centerband in the previous study. The simulations illustrate the sensitivity of the sideband intensities to changes in the chemical shift tensor, as well as the quality of data necessary to accurately determine the shift tensor values. 

Biodiesel can be produced by a sustainable continuous process based on catalytic reactive distillation. The integrated design ensures the removal of water byproduct that shifts the chemical equilibrium to completion and preserves the catalyst activity. The novel alternative proposed here replaces the liquid catalysts with solid acids, thus dramatically improving the economics of current biodiesel synthesis and reducing the number of downstream steps. The key benefits of this approach are ... 

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