Thermochemical degradation solution

Figure 1. Effect of time on thermochemical degradation of starch in alkaline solution according to Equation 3.

Figure 4. Effect of time on organic acid production from thermochemical degradation of starch and cellulose in alkaline solution.

Starch and cellulose can both be thermochemically degraded in alkaline solution to water-soluble compounds of relatively low molecular weight. (A parallel study examines more completely the nature of these compounds (15).) Both starch and cellulose degradation processes can be described by second-order kinetics, with the hydroxide ion concentration determined by the stoichiometry of polysaccharide conversion to organic acids. The thermochemical degradation activation energy in alkaline solution for both starch and cellulose is 39,500 calories/mole. 

From the standard thermochemical data ArG° = (—371.3 — 379.9 + 733.9) kJ mol-1 = —17.3 kJmol-1, corresponding to an equilibrium constant K = 1.1 x 103 M-1. This is a worrying result because all peptides in solution at 298 K should spontaneously fall apart to the monomers and hence all proteins are subject to degradation due to spontaneous hydrolysis. Fortunately, the reaction is kinetically hindered, which means that it occurs very slowly. Kinetics always control the rate at which equilibrium is achieved, relating the ratio of the forward and backward rate constants to the equilibrium constant ... 

If we use B3LYP/VTZ+1 harmonics scaled by 0.985 for the Ezpv rather than the actual anharmonic values, mean absolute error at the W1 level deteriorates from 0.37 to 0.40 kcal/mol, which most users would regard as insignificant. At the W2 level, however, we see a somewhat more noticeable degradation from 0.23 to 0.30 kcal/mol - if kJ/mol accuracy is required, literally every little bit counts . If one is primarily concerned with keeping the maximum absolute error down, rather than getting sub-kJ/mol accuracy for individual molecules, the use of B3LYP/VTZ+1 harmonic values of Ezpv scaled by 0.985 is an acceptable fallback solution . The same would appear to be true for thermochemical properties to which the Ezpv contribution is smaller than for the TAE (e.g. ionization potentials, electron affinities, proton affinities, and the like). 

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