More Electropositive is More Electronegative: Atom Size Determines C=X Group Electronegativity

Abstract

Opposite to what one might expect, we find that the C=X group can become effectively more, not less, electronegative when the Pauling electronegativity of atom X decreases down Groups 16, 15, and 14 of the Periodic Table. Our quantum-chemical analyses, show that, and why, this phenomenon is a direct consequence of the increasing size of atom X down a group. These findings can be applied to tuning and improving the hydrogen-bond donor strength of amides H2NC(=X)R by increasingly withdrawing density from the NH2 group. A striking example is that H2NC(=SiR2)R is a stronger hydrogen-bond donor than H2NC(=CR2)R.