Competition between hydrogen- (HB) and halogen-bonded (XB) 4-ammoniumpyridine and halogenammonium (NH F X ; n=0-3; X=F, Cl, Br, and I) cation-cation complexes are explored by means of DFT calculations. HB and XB minima structures are found for all systems in the gas phase. As the number of fluorine atoms increases, the HB complexes are more favored than those of XB. Proton transfer is generally observed in complexes with two, three, or four halogen atoms. The XB complexes evolve from traditional halogen bonds, to halogen-shared complexes, and to ionic complexes as the number of fluorine atoms increases. The dissociation transition states and their corresponding barriers are also characterized; the barriers increase as the number of fluorine atoms increases. The results if solvent effects are considered indicate that, even in an apolar solvent, such as n-hexane, most of the complexes have favorable binding energies. Atoms-in-molecules theory is used to analyze the complexes, and results in good correlations between electron density and total electron energy density (Η) values with the intermolecular bond length. According to the Η values obtained, the covalency of these interactions starts to manifest at distances around 72-74 % the sum of the van der Waals radii of the interacting atoms.

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Table S1. Calculated gas-phase relative energies (kJ/mol) of 5BrU tautomers U7, U8 and U10.

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In the present work, the hydrogen bond (HB) interactions between substituted syn and anti rotamers of methyl N-(2-pyridyl) carbamate and acetic acid were investigated using quantum mechanical (QM) calculations. The rotamers have two typical active sites to form hydrogen bonds with acetic acid, such that four stable complexes are found on the potential energy surface. The complexes in which the oxygen atom of carbamate acts as proton acceptor are stabilized by EWSs and are destabilized by EDSs. The trend in the effects of substituents is reversed in the other two complexes, in which the nitrogen atom of ring is involved in the interaction. According to energy data, the substituent effects on the interaction energy can be expressed by Hammett constants. The natural resonance theory (NRT) model was used to investigate the charge distribution on the carbamate group and to discuss the interaction energies. The individual HB energies were estimated to evaluate their cooperative contributions on the interaction energies of the complexes. In addition, the localized molecular orbital energy decomposition analyses (LMO-EDA) demonstrate that the electrostatic interactions are the most important stabilizing components of interactions.