Deuterium/hydrogen isotope ratio

Schmidt H-L, Werner RA, Eisenreich W (2003) Systematics of patterns in natural compounds and its importance for the elucidation of biosynthetic pathways. Phytochem Rev 2 61 Culp RA, Noakes JE (1992) Determination of synthetic components in flavor by deuterium/ hydrogen isotopic ratios. J Agric Food Chem 40 1892... 

D/H Deuterium/hydrogen isotope ratio, or ratio of -H/ H in a substance. 

The NMR determinations of the site-specific hydrogen isotope ratios at natural deuterium abundance permitted one to assess primary and secondary thermodynamic fractionation factors in exchange reactions avoiding the synthesis of selectively labelled reagents and their degradations691. 

These reactions proceed through symmetrical transition states [H H H] and with rate constants kn,HH and kH,DH, respectively. The ratio of rate constants, kH,HH/kH,DH> defines a primary hydrogen kinetic isotope effect. More precisely it should be regarded as a primary deuterium kinetic isotope effect because for hydrogen there is also the possibility of a tritium isotope effect. The term primary indicates that bonds at the site of isotopic substitution the isotopic atom are being made or broken in the course of reaction. Within the limits of TST such isotope effects are typically in the range of 4 to 8 (i.e. 4 < kH,HH/kH,DH < 8). 

The D/H ratio of the sun is essentially zero all the primordial deuterium originally present has been converted into He dnring thermonuclear reactions. Analysis of primitive meteorites is the next best approach of estimating the hydrogen isotope composition of the solar system. 

A different experimental approach to the relative importance of one-center and two-center epimerizations in cyclopropane itself was based on the isomeric l-13C-l,2,3-d3-cyclopropanes165"169. Here each carbon has the same substituents, one hydrogen and one deuterium, and should be equally involved in stereomutation events secondary carbon-13 kinetic isotope effects or diastereotopically distinct secondary deuterium kinetic isotope effects may be safely presumed to be inconsequential. Unlike the isomeric 1,2,3-d3-cyclo-propanes (two isomers, only one phenomenological rate constant, for approach to syn, anti equilibrium), the l-13C-l,2,3-d3-cyclopropanes provide four isomers and two distinct observables since there are two chiral forms as well as two meso structures (Scheme 4). Both chiral isomers were synthesized, and the phenomenological rate constants at 407 °C were found to be k, = (4 l2 + 8, ) = (4.63 0.19)x 10 5s l and ka = (4kl2 + 4, ) = (3.10 0.07) x 10 5 s 1. The ratio of rate constants k, kl2 is thus 1.0 0.2 both one-center and two-center... 

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