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. Author manuscript; available in PMC: 2020 Jul 1.
Published in final edited form as: Acc Chem Res. 2019 Jul 18;52(10):2870–2880. doi: 10.1021/acs.accounts.9b00189

Figure 7.

Figure 7.

Quantum mechanics (MP2) calculated energies (EMP2) of XBs from chlorobenzene to the carbonyl oxygen of N-acetylamide (NMA, a model for a peptide bond), and effects from adjacent hydroxyl groups. The Cl-XB is fairly weak, and addition of a hydroxyl to an adjacent (ortho) carbon weakens the interaction further. Rotation of the OH to form an HB to the Cl, however, significantly increases the stabilizing potential of the Cl-XB (with EMP2 becoming more negative by ~1.5 kcal/mol). The inset shows the MP2 calculated inductive effects of a hydroxyl (OH) substituent on charges at the carbons of benzene (phenol). The carbons of benzene carry a charge of −0.15e, determined through an MP2 calculation. The charge at the ortho- and para-carbons become more negative, reflecting the electron donating effect, while that of the meta-carbon becomes more positive, indicative of the electron withdrawing effect of the hydroxyl group to these positions. The Hammett constants58 for hydroxyl substituents are −0.37 for the para- and +0.12 for the meta-positions, consistent with the quantum calculated effects on the carbon charges.