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

Abstract

The most probable time‐averaged conformations of three polyammonium cations Hn2n+ (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 δCs 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 H525+. Copyright © 2009 John Wiley & Sons, Ltd.

13C longitudinal relaxation time measurements and DFT-GIAO NMR computations for two ammonium ions of a tetraazamacrocyclic scorpiand system

“…Just such spatial arrangement was only considered for H 4 1 4+ most likely existing in the pH range 1-4 [6 and refs therein], which would make potentially possible H-bonds with the NO 3 − oxygens. The fourth protonation of 1 occurring at N 3 [7, 12] give rise to the formation of an ‘extended’ all-out conformation of its macrocyclic unit, which most likely adopts a virtually planar macroring system, with all exocyclic N + -H bonds in an out configuration defined by Park and Simmons [45].…”

“…The fourth protonation of 1 occurring at N 3 [7, 12] give rise to the formation of an ‘extended’ all-out conformation of its macrocyclic unit, which most likely adopts a virtually planar macroring system, with all exocyclic N + -H bonds in an out configuration defined by Park and Simmons [45]. An outside orientation of the ring NH 2 + groups was found for several polyammoniums of this type [6, 46–48]; see also Figs. 2, S3, and S4.…”

“…These relaxation data afforded valuable information on the molecular mobility of both these species, which were assumed as two ammonium cations mainly existing under such conditions. However, some contribution of the entity H 4 1 4+ not fully protonated even at pH 0.2 can be inferred from the nonzero slope of 13 C NMR pH-titration profiles of 1 in the pH range 0.2–1.0 [6, 7], strongly suggesting not complete protonation. In fact, an exhaustive protonation can be difficult to achievement in some cases due to an unfavorable build-up of positive charges in the macrocycle, especially if its cavity is small [53].…”

“…In our last study on macrocyclic ligands [6],1 the overall composite conformations of some protonated forms of a polyamine 1 , i.e., 1-(2-aminoethyl)-1,4,8,11-tetraazacyclotetradecane commonly called scorpiand , were proposed on the basis of its earlier 13 C NMR pH-titration with nitric acid [7]. These spectroscopic data were analyzed in the light of GIAO (gauge-independent atomic orbitals) [8 and refs therein, 9] based predictions of δ C s made for numerous ammonium ions H n 1 n + coexisting in aqueous media.…”

“…But, according to our recent work [11], large caution must be taken in interpretations of all 13 C NMR data-based results on the shapes of molecules being in dynamic equilibrium between more than two distinct forms easily feasible energetically. Because it was also the case of the title ions H 4 1 4+ (with non-ionized N 1 ) [7, 12] and H 5 1 5+ existing as ensembles of several fast-interconverting forms [6, 13], the three explanations of ‘anomalous’ NMR shifts in question should be considered in details (vide infra).…”