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

摘要

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 C-13 NMR pH-profiles determined for the acidic H2O/D2O solution. These NMR trends were reproduced quite well in delta(c)s 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 H(5)2(5+).