The Magnitude and Mechanism of Charge Enhancement of CH∙∙O H-bonds

Quantum calculations find that neutral methylamines and thioethers form complexes, with N-methylacetamide (NMA) as proton acceptor, with binding energies of 2–5 kcal/mol. This interaction is magnified by a factor of 4–9, bringing the binding energy up to as much as 20 kcal/mol, when a CH3+ group is added to the proton donor. Complexes prefer trifurcated arrangements, wherein three separate methyl groups donate a proton to the O acceptor. Binding energies lessen when the systems are immersed in solvents of increasing polarity, but the ionic complexes retain their favored status even in water. The binding energy is reduced when the methyl groups are replaced by longer alkyl chains. The proton acceptor prefers to associate with those CH groups that are as close as possible to the S/N center of the formal positive charge. A single linear CH··O hydrogen bond (H-bond) is less favorable than is trifurcation with three separate methyl groups. A trifurcated arrangement with three H atoms of the same methyl group is even less favorable. Various means of analysis, including NBO, SAPT, NMR, and electron density shifts, all identify the +CH··O interaction as a true H-bond

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

Spin-lattice relaxation times, T1s, for 13C nuclei in two cations Hn1n+ (n = 1, 5)of N-(2-amino-ethyl)-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 microspecies iso-H414+Dhydr under such conditions can be proposed.Publikacja w ramach programu Springer Open Choice/Open Access finansowanego przez Ministerstwo Nauki i Szkolnictwa Wyższego i realizowanego w ramach umowy na narodową licencję akademicką na czasopisma Springer w latach 2010-2013