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

ABSTRACT Spin-lattice relaxation times, T1s, for 13C nuclei in two cations H n1n+ (n = 1, 5) of N-(2-aminoethyl)-cyclam (1, scorpiand) were determined by means of 13C{1H} NMR experiments in aqueous solution at pH 11.5 and 0.2. The theoretical study [modeling with OPLS-AA, B3LYP/6-31G(d) geometry optimizations, dispersion-corrected energies (DFT-D3), and DFT-GIAO predictions of the NMR chemical shifts (including an IEF-PCM simulation of hydration)] was also done for several conformers of the tautomer iso-H414+ not investigated before. The binding directions in protonated polyamino receptors necessary for efficient complexation of the nitrate anion(s) were briefly outlined, as well. All these results were discussed in terms of 'abnormal' 13C chemical shift changes found previously for the side-chain carbons of amine 1 in strongly acidic solution (HNO3). In conclusion, an earlier proposal of its association with NO3- at pH <1 was rejected. Instead, the participation of small amounts of a micro-species iso-H414+Dhydr under such conditions can be proposed. GRAPHICAL ABSTRACT A small contribution of iso-H414+Dhydr (see figure) to an ionic mixture of pentamine 1 was proposed to explain the 'abnormal' 13C NMR shifts observed for atoms C11 and C12 in its side-chain arm, at pH <1.