Nitrogen, chemical exchange

Consider a nucleus that can partition between two magnetically nonequivalent sites. Examples would be protons or carbon atoms involved in cis-trans isomerization, rotation about the carbon—nitrogen atom in amides, proton exchange between solute and solvent or between two conjugate acid-base pairs, or molecular complex formation. In the NMR context the nucleus is said to undergo chemical exchange between the sites. Chemical exchange is a relaxation mechanism, because it is a means by which the nucleus in one site (state) is enabled to leave that state. 

Fractional distillation of NO is an effective method for concentrating N. Chemical exchange processes for nitrogen isotope separation include the following ... 

Preparation of 99.5% Nitrogen-15 by Chemical Exchange between Oxides of Nitrogen in a Solvent Carrier System... 

Resonance line broadening due to chemical exchange and quadrupole-induced relaxation in the H and nB n.m.r. spectra of some boron-nitrogen adducts ArNMe2,BY3 (Y = halogen) has been observed and used to determine the mechanism of amine scrambling in these adducts.165 This is thought to occur via a unimolecular ionization rather than a B—N bond-rupture process. 

T2. Taylor, T. I., and W. Spindel Preparation of Hi y Enriched Nitrogen-15 by Chemical Exchange of NO with HNO3 > Proceedings of the International Sympomm on Isotope Separation, Interscience, New Yoric, 1958, p. 158. 

Saraceno, A. J., and C. F. Trivisonno (eds.) Uranium Isotope Separation by Chemical Exchange Reactions Between UF and UFj-Nitrogen Oxide, Report GAT-674, Feb. 4, 1972. 

N-H V=50 Hz In cases in which this coupling is seen (rare) it is quite large, typically 50 Hz or more. In most cases either the quadrupole moment of the nitrogen atom or chemical exchange decouples this interaction. 

What are some of the rapid processes that occur in organic molecules Two processes that we shall mention are chemical exchange of hydrogen atoms bonded to heteroatoms (such as oxygen and nitrogen), and conformational changes. 

Protons attached to electronegative atoms with lone pairs such as oxygen (or nitrogen) can undergo rapid chemical exchange. That is, they can be transferred rapidly from one molecule to another and are therefore called exchangeable protons. 

Protons that undergo rapid chemical exchange (i.e., those attached to oxygen or nitrogen) can be easily detected by placing the compound in D2O. The protons are rapidly replaced by deuterons, and the proton signal disappears from the spectrum. 

Other chemical exchange reactions that have been used for the fractionation of nitrogen isotopes include ammonia and anunonium nitrate solution [45], ammonia and ammonium carbonate [46], and ammonia and its complexes with aliphatic alcohols [47]. 

Chemical exchange of the protons attached to the nitrogen of eunlno groups with water protons occurs at just the proper rate to affect the %- H scalau coupling (2o). 

---