Structures and properties of π Br-bond in complexes C2H4−nFnBrF (n = 0–2)

Abstract

Using the counterpoise-corrected potential energy surface method, the stationary structures of the π Br-bond complexes C2H4-nFn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 (C2H4BrF) < 2.714 Å (C2H3FBrF) < 2.751 Å (g-C2H2F2BrF) < 2.771 Å (trans-C2H2F2BrF) < 2.778 Å (cis-C2H2F2BrF). The interaction energies (Eint) are, respectively,-5.9 (C2H4BrF),-4.4 (C2H3FBrF),-3.7 (g-C2H2F2BrF),-3.1 (cis-C2H2F2BrF),-2.8 kcal/mol (trans-C2H2F2BrF), at the CCSD (T)/aug-cc-pVDZ level, which include larger electron correlation contributions (Ecorre). The order of Ecorre is-3.40 (C2H4BrF),-3.60 (C2H3FBrF),-3.85 (g-C2H2F2BrF),-3.86 (cis-C2H2F2BrF),-3.88 kcal/mol (trans-C2H2F2BrF). The earlier results show above that the F substituent effect elongates the π Br-bond, reduces the Eint, and increases the Ecorre contribution of the interaction energy. Interestingly, the interaction energy of the cis-C2H2F2BrF structure with longer interaction distance is larger than that of the corresponding trans-C2H2F2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 C2H2F2 in the cis-C2H2F2BrF structure. Comparing with corresponding C2H4-nFnClF and C2H4-nFnHF, the C2H4-nFnBrF system has the larger Eint in which main contribution comes from the larger Ecorre, representing the larger dispersion interaction. The larger Ecorre contribution of the Eint 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