Structures and properties of π Br‐bond in complexes C 2 H 4− n F n BrF ( n = 0–2)

Using the counterpoise‐corrected potential energy surface method, the stationary structures of the π Br‐bond complexes C 2 H 4‐ n F n BrF ( n = 0–2) with all real frequencies have been obtained at MP2/aug‐cc‐pVDZ level. The order of the π Br‐bond length is 2.625 Å (C 2 H 4 BrF) < 2.714 Å (C 2 H 3 FBrF) < 2.751 Å ( g ‐C 2 H 2 F 2 BrF) < 2.771 Å ( trans ‐C 2 H 2 F 2 BrF) < 2.778 Å ( cis ‐C 2 H 2 F 2 BrF). The interaction energies ( E int ) are, respectively,‐5.9 (C 2 H 4 BrF),‐4.4 (C 2 H 3 FBrF),‐3.7 ( g ‐C 2 H 2 F 2 BrF),‐3.1 ( cis ‐C 2 H 2 F 2 BrF),‐2.8 kcal/mol ( trans ‐C 2 H 2 F 2 BrF), at the CCSD (T)/aug‐cc‐pVDZ level, which include larger electron correlation contributions ( E corre ). The order of E corre is‐3.40 (C 2 H 4 BrF),‐3.60 (C 2 H 3 FBrF),‐3.85 ( g ‐C 2 H 2 F 2 BrF),‐3.86 ( cis ‐C 2 H 2 F 2 BrF),‐3.88 kcal/mol ( trans ‐C 2 H 2 F 2 BrF). The earlier results show above that the F substituent effect elongates the π Br‐bond, reduces the E int , and increases the E corre contribution of the interaction energy. Interestingly, the interaction energy of the cis ‐C 2 H 2 F 2 BrF structure with longer interaction distance is larger than that of the corresponding trans ‐C 2 H 2 F 2 BrF structure with shorter interaction distance. This reason comes from a special secondary interaction between lone pairs of Br atom with positive charge and some atoms (H, C) with positive charges of C 2 H 2 F 2 in the cis ‐C 2 H 2 F 2 BrF structure. Comparing with corresponding C 2 H 4‐ n F n ClF and C 2 H 4‐ n F n HF, the C 2 H 4‐ n F n BrF system has the larger E int in which main contribution comes from the larger E corre , representing the larger dispersion interaction. The larger E corre contribution of the E int of π Br‐bond can be used to understand that the π Br‐bond is shorter and stronger than corresponding π Cl‐bond. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008