Reduced Nucleophilicities ИB of Lewis Bases B: Is ИB Independent of Whether B is Involved in a Hydrogen Bond or a Halogen Bond?

Reduced nucleophilicities ИB of axially symmetric molecules B were determined from , where De is the equilibrium dissociation energy of the complexes B⋅⋅⋅XY, NB is the nucleophilicity of B, EXY is the electrophilicity of the halogen-bond donor XY and σ min ${{\sigma }_{{\rm { min}}}{\rm \ }}$ is the minimum electrostatic surface potential of B. The series B⋅⋅⋅ClY, B⋅⋅⋅BrY, B⋅⋅⋅IY (Y=F, Cl, Br, I, CN, and CCH) as well as (B⋅⋅⋅XY, XY=F2 , Cl2 , Br2 ,and BrCl) of complexes were investigated. Molecules B were grouped so that the terminal atom involved in the halogen bond was fixed within the group. Groups having N as the terminal atom were RCN (R=CH3 , H, and F) or RN (R=N and P), those with C as the terminal atom were RNC (R=H and F) and RC (R=O, S and Se), and those with a terminal O atom were R=C=O (R=O or S). Graphs of D e ${{D}_{{\rm { e}}}}$ versus EXY for each group were straight lines through the origin, with generally different gradients, hence implying different NB . By contrast, when D e / σ min ${{D}_{{\rm { e}}}/{\sigma }_{{\rm { min}}}}$ was the ordinate the lines conflated to give a single straight line, which then defines a common (reduced) nucleophilicity ИB for that group of B. Hence it was concluded that ИB is an intrinsic property of the terminal atom, independent of the remainder of B, and only weakly dependent on the type (C, N or O) of the terminal atom. Moreover, ИB for each B was the same as determined previously from the hydrogen-bonded series B⋅⋅⋅HX, (X=F, Cl, Br, I, CN, CCH, and CP).