Urea Protons Research Articles

Novel Strategies for the Complexation of Neutral Guest Molecules by Synthetic Macrocyclic Hosts

Abstract Complexes of macrocyclic polyethers with neutral guests have been studied in solution. The first part, a systematic study of complexes of malononitrile with a variety of macrocyclic polyethers, allowed the evaluation of the thermo-dynamic parameters of complexation. The second part deals with the complexation of urea. Complexes with simple 18-membered macrocycles were prepared. Protonation of urea allowed the phase transfer of urea from aqueous to chloroform solution. Finally the complexation of urea by 2-carboxy-1,3-xylyl crown ethers is described.

A 17O NMR study of the protonation of urea

Abstract17O NMR has been applied to the study of urea over a wide pH range, in fluorosulfonic acid and in magic acid (HFSO3SbF5). A 96 ppm diamagnetic chemical shift was observed on modifying the medium from an aqueous solution at pH 3 to a fluorosulfonic acid solvent. We attributed this shift to a protonation reaction at the urea oxygen having a pKa of 0.1. A 36 ppm paramagnetic shift was observed in magic acid relative to the resonance position in fluorosulfonic acid, which is consistent with the formation of a diprotonated form of urea. Values of transverse and longitudinal relaxation rates were found to be equal as a function of pH and to double on lowering the pH from 2 to −1.

A semiempirical - SCF approach to the protonation of urea and thiourea

A semiempirical - SCF approach to the protonation of urea and thiourea

NUCLEAR MAGNETIC RESONANCE STUDIES OF THE PROTONATION OF WEAK BASES IN FLUOROSULPHURIC ACID: II. AMIDES, THIOAMIDES, AND SULPHONAMIDES

The proton magnetic resonance spectra of solutions of amides in fluorosulphuric acid confirm previous results which indicated that protonation of amides occurs on the carbonyl oxygen. A similar study of solutions of thioacetamide, thioacetanilide, thiourea, and N-methylthiourea provides conclusive evidence that the sulphur atom is protonated in these bases. In the case of N-methyl- and N,N′-dimethyl-p-toluenesulphonamide and sulphamide protonation occurs on nitrogen rather than on oxygen. It was not possible to obtain any conclusive evidence on the site of protonation of urea and N,N′-dimethylurea. Evidence is presented which indicates that thiourea and N-methylthiourea are diprotonated in solution in strong acids such as fluorosulphuric acid, and diprotonation of urea and N,N′-dimethylurea under the same conditions also seems very likely.