The AuNP terminal group contacts with water and
counterions are visualized in Figures 7 and 8. The average
number of H-bonds and ion contacts per 0.5 ps time frame and
their lifetimes are shown in Table 1. The analysis of H-bonds
and ion contacts with the terminal groups reveals differences
between the two AuNP systems. The terminal amine groups of
AuNP+ form three contacts with the solvent each. There are
two possible configurations: First, one hydrogen of NH3
+
connects to a counterion Cl− and the other two form Hbonds
with water oxygens (Figure 7b), and second, the amine
hydrogens make three H-bonds with water oxygens (Figure
7c). The solvent configurations around the terminal carboxyl
groups of AuNP− appear more complicated, as they form six or
seven contacts (Figure 8). One frequent case is a configuration
of seven H-bonds between the carboxyl group and seven
waters. Another relevant configuration involves contacts
between Na+, COO−, and water in such a way that the two
carboxylic oxygens form H-bonds with four waters and two
(ionic) Na−O bonds with the counterion, and in addition, Na+
forms four ionic Na−O bonds with the nearby waters (Figure
8b,c).