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Deuterium

All matter whether solid, liquid or gas comprise of atoms, and the nucleus of these atoms consist of nuclear particles. These atoms have a central sphere, the nucleus made up of positive charged particles, the protons and uncharged particles, the neutrons. These are of same size and weight within the nucleus while the cloud around it is filled with negatively charged particles, the electrons.

Based on the number of neutrons inside the nucleus, the mass number of atoms are specified. The Hydrogen atom follows the same rule and we get to see three main representation. Hydrogen atom with only proton inside the nucleus is termed as protium, one each of proton and neutron inside the nucleus is termed as Deuterium and finally the third category of isotope Tritium with one proton and two neutrons.

These nucleons are held together by electrostatic attraction of opposite charges and when these Deuterium nuclei are fused together to form heavier elements, they result in the release of huge binding energy.

 

Deuterium Definition

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Around 1942, the work on fissile matter started and the detonation of such materials yields huge amount of energy and this led to the idea of studying in depth of deuterium nuclei which requires the deuterium to overcome the electric repulsion of positively charged particles. 

The idea of collision of deuterium nuclei and fusion to finally form Helium 3 with release of a neutron and huge energy. Another reaction of deuterium fusing with tritium yields a neutron along with an alpha particle along with 17.6 MeV of energy. Deuterium is much easier and cheap to separate from heavy water than uranium 235 but the production of tritium is difficult to obtain till the production of these reactors were possible.

Deuterium formation is expected either through synthesis reactions where free nucleons get together to fuse and release huge energy. These can be carried also by spallation or by photodisintegration reactions. These synthesis models are difficult as the conditions needed for making or formation of deuterium normally which are same as those of rapid destruction. Free neutrons are produced at high temperatures when heavier nuclei thermally dissociate. 

These can survive a severe hot matter is dispersed and then cooled quickly as compared to its destruction. Free nucleons are also produced by passing strong shock wave where the gas cools rapidly which allows the deuterium to escape destruction. The search and study to yield energy also gave impetus to thermonuclear reactions which involve deuterium in a big way:

$D + D \rightarrow 3He + n + 3.27 MeV$
$D + D \rightarrow T + p + 4.03 MeV$
$T + T \rightarrow 4He + 2n + 11.27 MeV$
$3He + D \rightarrow 4He + p + 18.35 MeV$

Here, D is deuterium, n is the neutron, p is proton and T is tritium. We need to remember here that if density of neutrons and protons are nearly equal then there are two possible incidents which follow up here. During this period the deuterium can be formed and survive the destruction as well. The expansion can be rapid enough which do not allow recombination until the gas cools to temperatures which are considered as freezing level. 
These happens when charged particles reactions are found to be slower than the overall expansion. 

Deuterium is formed when the gas is very hot but the expansion is quick enough with the deuterium ejection without getting destroyed. Deuterium which forms at high temperature is also vulnerable to destruction during the thermonuclear reaction. Only a small fraction of the matter in universe can be raised to high temperature and then expanded quickly. The deuterium fraction in ejecta has to be greater than the interstellar value of 10-5.

Models of deuterium formation requires large number of neutrons stars to interact with black holes and throw out neutron rich matter in the nearby stars and the magnitude of such density are always associated with stellar envelopes. The big bang synthesis from nucleus provide the elegant theory deuterium and helium synthesis. If we look into the structure of solid deuterium crystal, it has hexagonal close packed lattice form. Each of these unit cells has two deuterium atoms. There are two specific sets of atoms which form the whole lattice. 

Deuterium Oxide

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The product obtained by treating indole magnesium iodide with ether and deuterium oxide in presence of tetra hydro furan is deuterated to about 50% in both 1 and 3 positions of indole nucleus. 

The reaction of indole Grignard reagent with deuterium oxide has recently shown that similar to the alkaline metal salts indole, the indole Grignard reagent undergoes N deuteration in tetra hydro furan. The optimum conditions for exchange at 3 positions involved the use of a moderate excess of deuterium oxide. Smaller amount as well as excess of $D_{2}O$ gives a low yield of 3 deuterated product. In the presence of small amounts of D2O, the complex is found to be stable and exchanges slowly. 

Hydrogen Deuterium Exchange

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Hydrogen is a naturally occuring element composed of protium, deuterium and tritium and the hydrogen exchange is used for describing the mode of isotopic exchange involving two different isotopes of hydrogen. The hydrogen deuterium exchange is a specific mass spectrometry technique in order to detect the changes for the conformation and dynamics of proteins. The wide application of hydrogen deuterium mass spectrometry shows efficient analysis with the help of large data sets associated with this technique.

The hydrogen exchange phenomenon is based on investigation of protein shape, its motion, the folding and all of its interactions. In all kinds of amino acids, like the peptide and proteins, the hydrogen atoms continuously go through changes in positions of hydrogen atoms in its surrounding solvent. If the solvent surrounding a protein is replaced with a solvent containing an isotopic form of hydrogen like deuterium results in the isotopic exchanges in solvent of the protein.

The exchange can be measured to find the occurrence and rate of occurrence. The technique of amide hydrogen exchange when coupled with mass spectrometry involves many acronym and terms. Hydrogen exchange helps in 3H / 1H experiments developed before to measure hydrogen exchange by nuclear magnetic resonance and mass spectrometry. 

Deuterium Exchange Mechanism

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Many of the organic anions undergo hydrogen / deuterium exchange in presence of deuterated reagents like D2O and CH3OD. These exchanges and reactions shows valuable method of probing mechanism of both ion and molecular reactions along with the structures of ions in gaseous phase.
The general mechanism of this hydrogen / deuterium exchange method is explained on the basis of potential energy reaction. The forward reaction is found to be endothermic which result in organic anion of R2CH- by the proton abstraction from R2CH2 which helps in carrying out the hydrogen / deuterium exchange in the presence of deuterium oxide D2O. 

The reaction steps are as follows:

$R_{2}CH^{-} + D_{2}O \leftrightarrow [R_{2}CH^{-} * D_{2}O]* \leftrightarrow R_{2}CHD.OD-] * \leftrightarrow [R_{2}CD^{-} . HDO] * \leftrightarrow R2CD- + HDO$

  • At first the complex is formed which shows a gain of ion dipole and ion induced dipole energy
  • The energy remains within the complex during the reaction and is further used for converting the complex into second complex after the transfer of deuterium.
  • Proton abstraction from the R2CHD molecule becomes ionised once more as it cannot escape the complex due to endothermic type
  • The exchanged R2CD- can either escape from complex or undergo more exchange with HDO molecule
  • The collisions of R2CH- with other D2O molecules finally results in replacement of all hydrogens by deuterium. 

Let us take another form of this hydrogen exchange mechanism into account. When we come across an aldehyde or ketone molecule with one or more alpha hydrogen dissolved in aqueous solution with D2O in it, it also contains catalytic amounts of either D+ or OD-. 

The incorporation of deuterium by the rate determining acid or base catalysed enolization and then followed up with deuterium incorporation as enol form helps in converting the keto form.

The deuterium exchange has two purposes and they are: 
  • The observance of changes in hydrogen ratios both before and after deuterium exchange, the number of alpha hydrogen in molecule which are exchangeable can be predicted.
  • The exchange of alpha hydrogens in an easy way to introduce the isotopic label for molecules.
$Acetone + 6 D_{2}O \rightarrow D+ or OD^{-} \rightarrow Acetone – d6 + 6 HOD$

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