{Insert Figure 2}
As mentioned earlier, relatively weak stabilizing molecular dimers are formed between cation-water and anion-water dimeric complexes in the solid state. Both water molecules (O1W and O2W) interact with lysinium and mandelate ions forming [0···-1] and [0···+1] molecular species with stabilizing in nature. There are five such molecular types observed in which cation-water complex makes four molecular dimers (motifs D5, D7-9 and [0···+1] type) and anion-water complex forms only one dimer (motif D6 and [0···-1] type).
As shown in Fig. 3, water molecules bridge the adjacent cations and adjacent anion in the solid state. We note that the lysinium side chain amino group (via H2A) interacts with water molecule (O2W) through N–H···O interaction and found to be stronger dimer (motif D5,E tot: -86.4 kJ mol-1) among four [0···+1] type molecular dimers. The backbone amino group (via H1B) also interacts with the water molecule (O1W) through intermolecular N–H···O interaction (motif D9,E tot: -21.4 kJ mol-1). This dimer is relatively weaker than that of the dimers formed by water molecules and oxygens of the carboxylate group of the lysinium (motif D7, E tot: -32.8 kJ mol-1 and motif D8, E tot: -22.4 kJ mol-1). The water molecule (O2W) interacts with carboxylate group of the anion forming a molecular dimer D6. The strength of this dimer (E tot: -51.4 kJ mol-1) is found to be stronger than the dimer formed by intermolecular N–H···O interaction involving backbone nitrogen and water molecule (motif D9). From the energetic point of view, one can see that water molecules in the complex make stronger interactions with side chain N of the cation and carboxylate group of the anion.