title: 13C longitudinal relaxation time measurements and DFT-GIAO NMR computations for two ammonium ions of a tetraazamacrocyclicscorpiandsystem.
abstract: Spin-lattice relaxation times,T1s, for13C nuclei in two cations Hn1n+(n= 1, 5) ofN-(2-aminoethyl)-cyclam (1,scorpiand) were determined by means of13C{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 tautomeriso-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 amine1in 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-speciesiso-H414+Dhydrunder such conditions can be proposed.
  A small contribution ofiso-H414+Dhydr(see figure) to an ionic mixture of pentamine1was proposed to explain the 'abnormal'13C NMR shifts observed for atoms C11 and C12 in its side-chain arm, at pH <1.
authors.affiliation: Laboratory of Molecular Spectroscopy, Faculty of Chemistry, University of Łódź, ul. Tamka 12, 91-403 Łódź, Poland.
authors.forename: Ryszard B
authors.initials: RB
authors.lastname: Nazarski
keywords: Amino-pendant cyclams; DFT-D3 dispersion correction; Dipolar relaxation; NMR pH-titration; Nitrate receptors; OPLS-AA force field; Protonated polyamines; ‘Wrong-way’ protonation shift