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

✍ Scribed by Andrei V. Afonin; Alexander V. Vashchenko

• Publisher: John Wiley and Sons
• Year: 2010
• Tongue: English
• Weight: 196 KB
• Volume: 48
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.2579

✦ Synopsis

Abstract

According to the density functional theory calculations, the X···H···N (XN, 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 NH···N hydrogen bond and an increase of the ^1^J(N,H) coupling constant across the NH covalent bond in the 2,5‐disubsituted pyrroles. This occurs due to a weakening of the NH···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 NH···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 NH···X interaction partly inhibits the charge transfer from the nitrogen lone pair to the σ*~NH~ 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 NH bond in consequence of the bifurcated hydrogen bonding leads to an increase of the ^1^J(N,H) coupling constant across the NH covalent bond and deshielding of the hydrogen donor nitrogen atom. Copyright © 2010 John Wiley & Sons, Ltd.