Details of theoretical calculations
The crystal structure geometry was taken after normalizing the distances involving H atoms to their typical neutron values (C–H= 1.083, N–H = 1.009 and O–H = 0.983 Å). The normalized model was used for Hirshfeld surface analysis29 and PIXEL energy calculation.30-32 The Hirshfeld surface analysis was performed with CrystalExplorer program33 to qualitatively analyze the intermolecular interactions. Further, the intermolecular interaction energy (E tot) was calculated for various dimers using the PIXEL method. TheE tot value decomposed into the Coulombic (E coul), polarization (E pol), dispersion (E disp), and repulsion ((E rep) contributions. The PIXEL code handles up to two-component system, but the title complex is a four-component system (lysinium, mandalate and two water molecules) which cannot be handled by PIXEL code. Therefore, the title complex was separated into different combinations of two-component system which yielded five different pairs. These pairs were: pair1: L-lysinium-L-mandelate, pair2: L-lysinium-water1, pair3: L-lysinium-water2, pair4: L-mandelate-water1 and pair5: L-mandelate-water2. We followed this procedure as suggested for the dihydropyrimidinium hydrochloride derivatives in the literature.34 For each pair, the electron densities have been obtained using the program GAUSSIAN 0935 at MP2/6-31G** level of theory. The intermolecular interaction energies (E tot) of different dimers were computed by the PIXEL method were further compared with those of dimeric energies (ΔE cp) computed using the M06-2X/6-311++G(d,p) level of theory with Grimme’s empirical dispersion D3.36 Further, the value of ΔE cp was corrected for basis set superposition error (BSSE) using the counterpoise method.37
The topological analysis within the framework of Bader’s QTAIM approach was performed for different dimers identified in the title complex to characterize the nature of bonding. To obtain wave function, single point energy calculation (with the normalized X-ray structure geometry) was carried out for these dimers at M06-2X-D3/6-311++G(d,p) level of theory. From the calculation, we computed topological properties like electron density ρ (r), Laplacian of the electron density ∇2ρ (r), potential energy density V (r), kinetic energy density G (r) and total energy density H (r) for intermolecular interactions at their bond critical points (BCPs) using the AIMALL package.38 Further, the bond dissociation energies (D e =\(\frac{-V(r)}{2}\)) for all the intermolecular interactions were calculated using the relationship of Espinosa et al .39