Physical image versus structure relation. Part 14—an attempt to rationalize some acidic region 13C NMR‐pH titration shifts for tetraaza macrocycles throughout the conformational GIAO DFT computational results: a pendant‐arm cyclam case

The most probable time‐averaged conformations of three polyammonium cations H_n 2 ⁿ⁺ (n = 3–5) formed from the macrocyclic pentamine ligand ( 2 , scorpiand) derivative of 1,4,8,11‐tetraazacyclotetradecane (cyclam)] were analyzed in order to elucidate an origin of ‘wrong‐way’ amine‐protonation shifts found in some ¹³C NMR pH‐profiles determined for the acidic H₂O/D₂O solution. These NMR trends were reproduced quite well in δ_Cs computed for multicomponent shapes of related cations, which were in turn elucidated by the best fitting experimental data to those predicted by the gauge‐independent atomic orbital (GIAO) B3LYP/6‐31G* method, including the IEF‐PCM approach. A consistent DFT methodology of the treatment of such equilibrated cationic mixtures is proposed. Moreover, a few novel ONIOM2‐GIAO B3LYP/6‐31G*:STO‐3G type supermolecular calculations were performed for a simulated presence of bulk water molecules surrounding H₅ 2 ⁵⁺. Copyright © 2009 John Wiley & Sons, Ltd.