Halogen bond in (CH3)nX (X = N, P, n = 3; X = S, n = 2) and (CH3)nXO (X = N, P, n = 3; X = S, n = 2) adducts with CF3I.: Structural and energy analysis including relativistic zero-order regular approximation approach in a density functional theory framework

Density functional theory (DFT) calculations on the geometry and bonding energy of the X- - -1 intermolecular interaction of CF3I and (CH3)(n)X (X = N, P, n = 3; X = S, n = 2) and (CH3)(n)XO (X = N, P, n = 3; X = S, n = 2) are reported. The effect of the basis set, Effective Core Potential (ECP), and relativistic corrections have been investigated to gain insight into the origin and nature of this interaction. Energy decomposition in terms of different contributions allows us to understand the different donor behavior of second and third row atoms toward perfluorinated iodo hydrocarbon compounds, suggesting that bond energies are mainly due to orbital contribution. TD-DFT calculations of the van der Waals coefficient C-6 for CF3I- - -N(CH3)(3) and CF3I- - -P(CH3)(3) adducts estimate values of -2.02 and -1.30 kcal/mol, respectively, for their dispersion energy, thus representing only a small contribution to the total bonding energy (-5.64 and -5.76 kcal/mol, respectively). The role of competition between intra- and intermolecular donation has been elucidated. All the adducts with oxides as donors show a weaker interaction with CF3I than the relative donor without oxygen except the CF3I- - -ON(CH3) adduct, which has a stronger interaction. Basis set superposition errors (BSSE) are very relevant in describing the relative order of stability of the oxide of trimethyl derivatives of group 15 elements. Explicit scalar relativistic (s.r.) ZORA (zero-order regular approximation) correction,, are essential to reproduce correctly geometrical parameters and bonding energies involving X- - -1 interaction.