Origin of Substituent Effects in Edge-to-Face Aryl-Aryl Interactions

Abstract

Substituent effects in the edge-to-face configuration of the benzene dimer have been studied using modern density functional theory. An accurate interaction potential energy curve has been computed for the unsubstituted dimer using ab initio methods with large basis sets. The recommended binding energy for the edge-to-face benzene dimer is 2.31 kcal mol(-1), estimated at the counterpoise-corrected CCSD(T)/aug-cc-pVTZ level of theory. For both edge-ring and face-ring-substituted dimers, interaction energies correlate with sigma(m) for the substituents, indicating that substituent effects can be understood qualitatively in terms of simple electrostatic effects, although in the latter case dispersion results in some scatter in the data. In contrast to prevailing models of substituent effects in benzene dimers, polarization of the pi-system of the substituted ring does not induce substituent effects. For edge-ring-substituted dimers, substituent effects arise from differential electrostatic interactions between the hydrogens on the substituted ring and the pi-cloud of the face ring and direct interactions of the substituents with the unsubstituted ring. For face-ring-substituted dimers, substituent effects arise from direct electrostatic and dispersion interactions of the substituents with the edge ring. Substituents with sigma(m) > 0.12 favor edge ring substitution while for sigma(m) < 0.12 substitution on the face ring is preferred.

Figure 1

Prototypical benzene dimer configurations.

Figure 2

(a) Monosubstituted edge-to-face benzene dimers and (b) truncated structures used to quantify the role of direct interactions of substituents with the other ring.

Figure 3

Potential energy scans at the MP2/AVTZ, SCS-MP2/AVTZ, CCSD(T)/AVDZ, estimated CCSD(T)/AVTZ, and M05-2X/6-31+G(d) levels of theory.

Figure 4

(a) Relative interaction energies for edge-ring-substituted dimers [E_int(C₆H₅-X^...C₆H₆)] plotted against σ_m^X; and (b) relative interaction energies for face-ring-substituted dimers [E_int(C₆H₆^...C₆H₅-Y)] plotted against σ_m^Y. Obvious outliers (H, BF₂, SCH₃, SH, and SiH₃) are labeled in the lower plot. Removing these outliers from the least-squares-fit yields E_int = 1.09σ_m^Y - 0.38 (r = 0.95).

Figure 5

(a) Relative interaction energies for truncated edge-ring-substituted dimers [E_int(H-X^...C₆H₆)] plotted against E_int(C₆H₅-X^...C₆H₆); and (b) relative interaction energies for truncated face-ring-substituted dimers [E_int(C₆H₆^...H-Y)] plotted against E_int(C₆H₅^...C₆H₅-Y).

Figure 6

Symmetrically disubstituted benzene dimers: (a) edge-ring-disubstituted and (b) face-ring-disubstituted.

Figure 7

(a) Relative interaction energies for edge-ring-disubstituted dimers [E_int(X-C₆H₄-X^...C₆H₆)] plotted against E_int(C₆H₅-X^...C₆H₆); and (b) relative interaction energies for face-ring-disubstituted dimers [E_int(C₆H₆^...H-C₆H₄-Y)] plotted against E_int(C₆H₆^...C₆H₅-Y).

Figure 8

Primary origins of substituent effects in monosubstituted edge-to-face benzene dimers. In edge-ring-substituted dimers, substituent effects arise from the direct, primarily electrostatic, interaction of the substituent with the face ring and differential interactions between the edge ring hydrogens and the π-cloud of the face ring. In face-ring-substituted dimers, substituent effects are dictated by direct electrostatic and dispersion interactions between the substituent and the edge ring hydrogens.