Probing the effects of heterogeneity on delocalized ππ interaction energies

Dimers composed of benzene (Bz), 1,3,5-triazine (Tz), cyanogen (Cy) and diacetylene (Di) are used to examine the effects of heterogeneity at the molecular level and at the cluster level on ππ stacking energies. The MP2 complete basis set (CBS) limits for the interaction energies (E_int) of these model systems were determined with extrapolation techniques designed for correlation consistent basis sets. CCSD(T) calculations were used to correct for higher-order correlation effects (δE^CCSD(T)_MP2) which were as large as +2.81 kcal mol⁻¹. The introduction of nitrogen atoms into the parallel-slipped dimers of the aforementioned molecules causes significant changes to E_int. The CCSD(T)/CBS E_int for Di–Cy is −2.47 kcal mol⁻¹ which is substantially larger than either Cy–Cy (−1.69 kcal mol⁻¹) or Di–Di (−1.42 kcal mol⁻¹). Similarly, the heteroaromatic Bz–Tz dimer has an E_int of −3.75 kcal mol⁻¹ which is much larger than either Tz–Tz (−3.03 kcal mol⁻¹) or Bz–Bz (−2.78 kcal mol⁻¹). Symmetry-adapted perturbation theory calculations reveal a correlation between the electrostatic component of E_int and the large increase in the interaction energy for the mixed dimers. However, all components (exchange, induction, dispersion) must be considered to rationalize the observed trend. Another significant conclusion of this work is that basis-set superposition error has a negligible impact on the popular δE^CCSD(T)_MP2 correction, which indicates that counterpoise corrections are not necessary when computing higher-order correlation effects on E_int. Spin-component-scaled MP2 (SCS-MP2 and SCSN-MP2) calculations with a correlation-consistent triple-ζ basis set reproduce the trends in the interaction energies despite overestimating the CCSD(T)/CBS E_int of Bz–Tz by 20–30%.