Intimate interactions with carbonyl groups: dipole-dipole or n→π*?

Abstract

Amide carbonyl groups in proteins can engage in C═O···C═O and C-X···C═O interactions, where X is a halogen. The putative involvement of four poles suggests that these interactions are primarily dipolar. Our survey of crystal structures with a C-X···C═O contact that is short (i.e., within the sum of the X and C van der Waals radii) revealed no preferred C-X···C═O dihedral angle. Moreover, we found that structures with a short X(-)···C═O contact display the signatures of an n→π* interaction. We conclude that intimate interactions with carbonyl groups do not require a dipole.

Figure 1

Values of C–X···C=O dihedral angles. The abscissa is used to distribute the values randomly. (A) X = F. (B) X = Cl. (C) X = Br. (D) X = I.

Figure 2

Definition of distance d, and angles θ and Θ.

Figure 3

Plots of X⁻···C=O distance (d) and angle (θ). (A) X = F. (B) X = Cl. (C) X = Br. (D) X = I.

Figure 4

Values of carbonyl group pyramidalization induced by a sub-van der Waals X⁻ ···C=O interaction. The abscissa is used to distribute the values randomly. (A) X = F. (B) X = Cl. (C) X = Br. (D) X = I.

Figure 5

Parameters for structures with a long Cl⁻···C=O interaction. (A) Plot of Cl⁻···C=O distance (d) and angle (θ). (B) Values of carbonyl group pyramidalization. The abscissa is used to distribute the values randomly.

Figure 6

Orbital energy diagrams for two distinct electronic phenomena of carbonyl groups. (A) n→π* electronic transition of a single carbonyl group. The inset depicts the Zimmerman “circle-dot-y” notation for the ground state and excited state in which “o” represents an s-rich electron, “●” represents a π electron, and “y” represents a p-rich electron.( B) n→π* interaction between two carbonyl groups.