Crystals of isomeric tritolylamines: embrace motifs in crystals, and their thermochemical properties
Calculations of intermolecular potentials Calculations of intermolecular potentials have been made by two independent methods, density functional and empirical van der Waals/coulombic methods. The density functional calculations were made with the program DMol, 1,2 with care to employ the pwc functional of Perdew and Wang 3 because it behaves correctly at large distances. 4 In addition, basis set superposition errors were avoided by using numerical basis sets 1 calculated to a large 20Å cutoff.
... a large 20Å cutoff. This methodology for intermolecular potentials has been tested for benzene pairs (for which there are experimental data and accurate calculations), with the conclusion that the actual intermolecular energy is ca 70% of that calculated. 5 Calculations for the neutral pairs of TTA isomers used symmetry C i . The empirical calculations used the 12-6 potential for van der Waals interactions, and a coulombic potential for electrostatic interactions, within the force-field cvff of the program Discover_3. 6 The atom parameters (r*, ε, charge) used had been refined 7 for use with molecules such as these: C, 1.94, 0.093, -0.10; N 1.85, 0.15, -0.21; H 1.62, 0.02, +0.11. The permittivity was set to be numerically equal to interatomic distance. The results are shown in the Figure. The minima in the density functional calculated potentials for 333TTA and 334TTA occur at distances ca 0.3Å shorter than those observed in the crystals, while the empirical potential wells are ca 0.2Å longer than observed. An isolated molecule pair is expected to be tighter than the same molecule pair in a condensed phase. The differences between the potentials calculated by the two methods are consistent with the expected overbinding of the pwc density functional methodology. The two methods yield similar energies at the crystal intermolecular distances. Notice that the embrace with the smallest N--N separation is not the most stable: the N--N separation reflects instead the canting of the tolyl rings to the molecular plane. This is another example of the principle for intermolecular interactions that 'shorter is not necessarily stronger'. 8