Vapor Pressure of Molecular Crystals. XIII. Vapor Pressure of α-Glycine Crystal. The Energy of Proton Transfer

Publisher Website
Google Scholar
Abstract
Sublimation pressures of crystalline α-glycine were measured between 138 and 153°C by the effusion method. The pressure equation logP(mmHg)=13.666-(6816.8/T) fits well the experimental data. The heat, entropy and free energy (at 25°C) of sublimation were 31.18±0.49 kcal./mole, 49.33±1.17 cal./deg. mole and 16.47±0.60 kcal./mole, respectively. An energy cycle was constructed which correlates zwitterion and molecule in crystal and vapor states, and hydrated zwitterion and molecule in solutions. The observed heat of sublimation corresponds to the change in heat content in going from the zwitterioncrystal to gaseous molecules. The molecular configuration is more stable in vapor state. By using the experimental values of the heat of sublimation, the heat of solution of crystals into water and the temperature dependence of pKa in solution, with the help of the theoretical values of the lattice energy of the crystal, the following heat values were determined; the heat of proton jump in vapor, NH3+CH2COO− → NH2CH2COOH, −72 kcal./mole, the heat of proton jump in the crystal 7∼10 kcal./mole, hypothetical heat of sublimation of crystals composed of molecules 21∼24 kcal./mole, the heats of hydration of a zwitterion and a molecule −99.8 and −17.8 kcal./mole, respectively, the heat of solution of the hypothetical crystal to give a solution of molecular glycine 3∼6 kcal./mole, and the heat of self-neutralization in aqueous solution 9.7 kcal./mole. The negative quantities show that the processes are exothermic. It was shown that the proton transfer in a medium of large dielectric constant or in a strong crystalline field requires only a small amount of energy and its significance in chemical reactions and biochemical activities was suggested. Each of the derived heat values was examined by independent checks from different sources, except for the energies of proton transfer.

This publication has 11 references indexed in Scilit: