Unraveling the effect of fluorine substitution on the hydrogen bonding interaction in the complexes of fluorosubstituted pyridines and acetic acid

Abstract

AbstractThe effect of fluorine substitution on the hydrogen bonding interaction in the complexes of fluorosubstituted pyridines and acetic acid was examined through theoretical method. Two types of double hydrogen bonding cyclic structures have been predicted, one with O–H···N and C–H···O interactions and the other with O–H···F and C–H···O interactions. The changes of the hydrogen bonding interactions are visualized by the reduced density gradient (RDG) and bond critical point (BCP) parameters analysis. With increasing in fluorination, the C–H···O bond strength increases, and the O–H···N and O–H···F bond strengths decrease. Although the O–H···N and O–H···F hydrogen bond energies decrease, the bond type has not changed in all of the fluorinated complexes. For the same degree of fluorination complexes, fluorine favors the C–H···O hydrogen bonding interaction through the cooperating of O–H···F and C–H···O hydrogen bonds, although fluorine is a weak hydrogen bond acceptor. The natural population analysis (NPA) and molecular electrostatic potentials (ESP) were adopted to further explore the variation characteristics of hydrogen bond strength. We found that the substituent fluorine atom acts as a role of electron withdrawing. The pyridine ring nitrogen atom and 2‐position fluorine atom become less negative, and the remaining H atom(s) become more electropositive with increasing degree of fluorination. The hydrogen bond strength is correlated with the charge redistribution of the proton H and acceptor (N and F) atoms.