Three types of noncovalent interactions studied between pyrazine and XF

Three types noncovalent interactions (type I, II and III) between pyrazine (C 4 H 4 N 2 ) and XF (X = F, Cl, Br, and I) have been discovered at the MP2/aug-cc-pVTZ level. TypeI is σ-hole interaction between the positive site on the halogen X of XF and the negative site on one of the pyrazine nitrogens. Type II is counterintuitive σ-hole interaction driven by polarization between the positive site on the halogen X of XF and a portion of the pyrazine ring. Type III is an interaction between the lateral regions of the halogen X of XF and the position of the pyrazine ring. Through comparing the calculated interaction energy, we can know that the type II and type III interactions are weaker than the corresponding type I interactions, and type III interactions are weaker than the corresponding type II interactions in C 4 H 4 N 2 -XF complexes. SAPT analysis shows that the electrostatic energy are the major source of the attraction for the type I (σ-hole) interactions while the type III interactions are mainly dispersion energy. For the type II (counterintuitive σ-hole) interactions in C 4 H 4 N 2 -XF (X = F and Cl) complexes, electrostatic energy are the major source of the attraction, while in C 4 H 4 N 2 -XF (X = Br and I) complexes, the electrostatic term, induction and dispersion play equally important role in the total attractive interaction. NBO analysis, AIM theory, and conceptual DFT are also being utilized. Graphical abstract