Amines decoupling

Rao and Singh32 calculated relative solvation free energies for normal alkanes, tetra-alkylmethanes, amines and aromatic compounds using AMBER 3.1. Each system was solvated with 216 TIP3P water molecules. The atomic charges were uniformly scaled down by a factor of 0.87 to correct the overestimation of dipole moment by 6-31G basis set. During the perturbation runs, the periodic boundary conditions were applied only for solute-solvent and solvent-solvent interactions with a non-bonded interaction cutoff of 8.5 A. All solute-solute non-bonded interactions were included. Electrostatic decoupling was applied where electrostatic run was completed in 21 windows. Each window included 1 ps of equilibration and 1 ps of data... 

This receptor shows a remarkable selectivity for Mg2+ over Ca2+ under physiological conditions and has found applications in 19F NMR probes and ratiometric fluorescent sensors based on wavelength shifts.[62] In high concentrations, however, both Ca2+ and Mg2+ can be bound. The similarity of fluorescence enhancements with both ions is the result of essentially identical conformational changes produced upon complexation. Each ion-bound state effectively decouples the amine substituent from the oxybenzene unit, so that PET is similarly suppressed. This means that the charge density difference between the two cations is of secondary importance in these conformationally switchable systems. 

NH proton(s) is decoupled from the nitrogen atom and from protons on adjacent carbon atoms. The NH proton peak is therefore a sharp singlet, and the adjacent CH protons are not split by NH. Such is the case for most aliphatic amines. ... 

Other nuclei besides H or have been used to monitor enantiomeric discrimination with chiral lanthanide chelates. This includes the NMR spectra of chiral 2-thiabicyclo[4.3.0]nonane 2,2-dioxides (14) and 8,8-dioxides (15) with Pr(hfc)3 . The Si NMR spectrum of a-C-silylated amines and alcohols (38) in the presence of Eu(tfc)3 was used to monitor the optical purity of these compounds . A refocused-decoupled INEPT (insensitive nuclei enhanced by polarization transfer) pulse sequence was used to circumvent the long spin-lattice relaxation times of the silicon. 

FIGURE 7.3. The proton-decoupled, 5N spectrum (30.4 MHz) of diisopropyl amine in CDCl3. 

The reactions between MeP(S)Cl2 and amines have been utilized in a novel way the products from chiral amines, e.g. aminocarboxylic acids, consist of mixtures of dia-stereoisomeric methylphosphonothioic diamides, two of meso structure, 102 and its mirror image, together with the racemic modification 103 the proton-decoupled P NMR spectra of such mixtures display three well separated singlets from which the enantiomeric purity of the amines can be determined. ... 

Under these conditions, the predominant species in solution is the protonated amine, and intermole-cular proton exchange is slowed, often allowing us to observe spin-spin coupling interactions that are decoupled and masked by exchange in the free amine. 

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