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

ABSTRACT

Spin-lattice relaxation times, s, for C nuclei in two cations H ( = 1, 5) of -(2-aminoethyl)-cyclam (, ) were determined by means of C{H} NMR experiments in aqueous solution at pH 11.5 and 0.2. The theoretical study [modeling with OPLS-AA, B3LYP/6-31G() 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 -H 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' C chemical shift changes found previously for the side-chain carbons of amine in strongly acidic solution (HNO). In conclusion, an earlier proposal of its association with NO at pH <1 was rejected. Instead, the participation of small amounts of a micro-species -H under such conditions can be proposed.

GRAPHICAL ABSTRACT

A small contribution of -H (see figure) to an ionic mixture of pentamine was proposed to explain the 'abnormal' C NMR shifts observed for atoms C11 and C12 in its side-chain arm, at pH <1.