Homonuclear versus heteronuclear resonance-assisted hydrogen bonds: tautomerism, aromaticity, and intramolecular hydrogen bonding in heterocyclic systems with different exocyclic proton donor/acceptor.
Tautomerism and resonance-assisted hydrogen bonding have been analyzed on the basis of the results of ab initio calculations at the MP2/AUG-cc-pVDZ level of theory for the series of molecules containing different heterocycles connected with resonance spacer and containing different exocyclic proton donor/acceptor atoms. It is demonstrated that the position of tautomeric equilibrium is controlled mainly by two factors: aromaticity of heterocycle, which could be different for two tautomers, and relative proton affinities of two heteroatoms forming a hydrogen bond. Replacement of exocyclic proton donor/acceptor atom results in change of an aromaticity degree of heterocycle leading to alteration of relative stability of tautomers. Comparison of structure and properties of E and Z conformers of molecules demonstrates resonance-assisted character of intramolecular hydrogen bond. Application of the NBO theory reveals that the pi-component of the electron density within resonant spacer plays the primary role for determination of characteristics of hydrogen bond while sigma-skeleton only reflects the pi-polarization. An analysis of strength of intramolecular hydrogen bond using geometrical, energetic, and AIM and NBO parameters indicates that the homonuclear N...H-N hydrogen bond is considerably weaker than heteronuclear N...H-O and N...H-S hydrogen bonds in the case of the XH tautomers.
Zubatyuk
Roman I
RI
STC Institute for Single Crystals, National Academy of Sciences of Ukraine, 60 Lenina Avenue, Kharkiv 61001, Ukraine.
Shishkin
Oleg V
OV
Gorb
Leonid
L
Leszczynski
Jerzy
J
D006571:Heterocyclic Compounds / Q000737:chemistry*; D006841:Hydrocarbons, Aromatic; D006860:Hydrogen Bonding; D007536:Isomerism; D008958:Models, Molecular; D011522:Protons
D006571:Heterocyclic Compounds; D006841:Hydrocarbons, Aromatic; D011522:Protons