Theoretical study of bifurcated hydrogen bonding effects on the 1 J (N,H), 1h J (N,H), 2h J (N,N) couplings and 1 H, 15 N shieldings in model pyrroles

According to the density functional theory calculations, the X···H···N (XN, O) intramolecular bifurcated (three‐centered) hydrogen bond with one hydrogen donor and two hydrogen acceptors causes a significant decrease of the 1h J (N,H) and 2h J (N,N) coupling constants across the NH···N hydrogen bond and an increase of the 1 J (N,H) coupling constant across the NH covalent bond in the 2,5‐disubsituted pyrroles. This occurs due to a weakening of the NH···N hydrogen bridge resulting in a lengthening of the N···H distance and a decrease of the hydrogen bond angle at the bifurcated hydrogen bond formation. The gauge‐independent atomic orbital calculations of the shielding constants suggest that a weakening of the NH···N hydrogen bridge in case of the three‐centered hydrogen bond yields a shielding of the bridge proton and deshielding of the acceptor nitrogen atom. The atoms‐in‐molecules analysis shows that an attenuation of the 1h J (N,H) and 2h J (N,N) couplings in the compounds with bifurcated hydrogen bond is connected with a decrease of the electron density ρ H···N at the hydrogen bond critical point and Laplacian of this electron density ∇ 2 ρ H···N . The natural bond orbital analysis suggests that the additional NH···X interaction partly inhibits the charge transfer from the nitrogen lone pair to the σ* NH antibonding orbital across hydrogen bond weakening of the 1h J (N,H) and 2h J (N,N) trans ‐hydrogen bond couplings through Fermi‐contact mechanism. An increase of the nitrogen s ‐character percentage of the NH bond in consequence of the bifurcated hydrogen bonding leads to an increase of the 1 J (N,H) coupling constant across the NH covalent bond and deshielding of the hydrogen donor nitrogen atom. Copyright © 2010 John Wiley & Sons, Ltd.