In order to better understand the reaction mechanism during
the substitution reaction process, topological analysis has been
done for reactants and transition states at MP2/6-311CG(d,p)
level. Table 2 lists the electron density (r) and Laplacian of
electron density (P2r) at bond critical point (BCP). According to
topological analysis of electron density distribution in the theory
of AIM, r is used to describe the strength of a bond and P2r
describes the characteristic of a bond. According to topological
analysis, the bond C–F in TSa3 is weakened and the bond C–H is
partially formed (the r are only 0.1146 and 0.1759, respectively).
The bonds C–Cl andC–Hin TSb3 andC–Br andC–HinTSc3 are
the same case. In addition, the results listed in Table 2 show that r
of the C–X (XZF, Cl, Br) bond decreases from CH3F toCH3Br.
The following facts may be deduced that the strength of the C–X
bond for CH3X decreases according to the sequence of CH3F,
CH3Cl and CH3Br. As can be seen in Table 2, the results are in
reasonable agreement with the experimental C–X bond dissociation
enthalpies [25].