Properties of water water isotopes. Naturally occurring water is almost completely composed of the neutron-less hydrogen isotope protium. Only 155 ppm include deuterium ( or D), a hydrogen isotope with one neutron, and fewer than 20 parts per quintillion include tritium ( or T), which has two neutrons. Oxygen also has three stable isotopes, with present in 99.76%, in 0.04%, and in 0.2% of water molecules. Deuterium oxide, , is also known as heavy water because of its higher density. It is used in nuclear reactors as a neutron moderator. Tritium is radioactive, decaying with a half-life of 4500 days; exists in nature
Kinetic isotope effect now relatively routine. Moreover, several qualitative and semi-quantitative models allow rough estimates of deuterium isotope effects to be made without calculations, often providing enough information to rationalize experimental data or even support or refute different mechanistic possibilities. Starting materials containing deuterium are often commercially available, making the synthesis of isotopically enriched starting materials relatively straightforward. Also, due to the large relative difference in the mass of deuterium and protium and the attendant differences in vibrational frequencies, the magnitude of the isotope effect is larger than any other pair of isotopes except protium and tritium, allowing both primary and secondary isotope