The nature of the C-Br ... Br-C intermolecular interactions found in molecular crystals: a general theoretical-database study

The nature of the C-Br ... Br-C intermolecular interactions has been evaluated by doing first-principles calculations on model dimers and on full-sized dimers extracted from crystals deposited in the Cambridge Structural Database presenting short-distance C-Br ... Br-C contacts. First of all, the strength of the C(sp(n))Br ... BrC(sp(n)) interaction was determined at the MP2/CBS level on model dimers placed at their most stable orientation, getting values of -1.72, -1.83, and -1.91 kcal mol(-1) for n = 3, 2 (non-aromatic), and 1, respectively. SAPT analyses of the interaction energy showed that it is dominated by the dispersion term, although a strong electrostatic component is also present, which depends on the fragment dipole moment (induced by the substituents attached to the two C atoms involved in the C-Br ... Br-C interaction) and the s-hole of the halogen atoms (induced by the asymmetry of the electron density around each bromine atom). The angular dependence of the C-Br ... Br-C interactions was determined by computing the E-int(theta(1),theta(2)) surface (where theta(1) and theta(2) are the angle CBr ... Br and angle Br ... BrC angles) for the (CH3Br)(2) dimer. The most stable orientations were found at theta(1) ... theta(2) = 90 degrees (a case of Type I orientation) and at theta(i) approximate to 180 degrees and theta(j) approximate to 90 degrees (Type II orientation). The E-int(theta(1),theta(2)) surface also showed that the theta(1) = theta(2) = 150 degrees orientation has the lowest energy among all Type I, but rather than a minimum, it should be considered as a saddle point between both Type II minimum energy orientations. Finally, in order to gain information on the properties of the C-Br ... Br-C interactions in real cases, the interaction energy was evaluated for 39 dimers extracted from the Cambridge Structural Database that present C-Br ... Br-C interactions smoothly distributed over the 3.04.5 angstrom range. This allowed establishment of the overall stabilizing nature of these interactions in complex molecules (all have interaction energies that range from -2.35 to -0.38 kcal mol(-1), with an average value of -1.26 kcal mol(-1)). The correlation between stronger interaction energies and higher electron density values at the Br ... Br bond critical point was shown to be incorrect for sub-van der Waals C-Br ... Br-C interactions.