3.2. Structural Dynamics of Hydrogen Bonding
Table 1 compares averaged calculated geometrical parameters of the radicals in aqueous solution and in gas phase. The listing in the table demonstrates substantial changes in bond lengths and angles upon transferring from the gas to the solution phase. In Table 2, the magnetic moments (MM) of the, and radicals in gas phase and solution phase are compared. Because of combination of hydrogen bonds associated with the polar -NH2, -NH3 and -COO(H) groups, solvent configuration around the radical and leads to the changes of the geometry.
<Table 1 >
<Table 2 >
A good descriptor of the hydrogen bonded structure with water molecules around a solute is the radial distribution function (RDF) of the hydrogen bonds around the O1 and O2oxygens displayed graphically in Fig. 3 while the average numbers of the hydrogen bonds with water molecules are listed in Table 3. The listings in the table suggest that , and radicals have approximately similar number of hydrogen bonds around the carboxyl group and that the number of hydrogen bonds around the amino group forradical is more than that around the and radicals.
The average observed number of water molecules in the first hydration shell for , and radicals are 6.21, 6.96 and 5.92, respectively. The solvation structure for and radicals shows that O1 and O2 oxygens have hydrogen bonds of approximately the same strength centered at 1.74 Å and 1.62 Å for the radical and at 1.65 Å and 1.74 Å for radical, respectively. However, the solvation structure for the radical shows that O1 oxygen has a stronger hydrogen bond than O2 oxygen, centered, respectively, at 1.86 Å and 4.38 Å.
<Table 3 >
<Fig. 3 >