# Neutron Diffraction with Isotopic Substitution

A quick reference table for setting up a Neutron Diffraction experiment with Hydrogen/Deuterium Isotopic Substitution (NDIS).

An good introduction to the technique of Neutron Diffraction with Isotopic Substitution is given in the following reference paper: Solvent structure and perturbations in solutions of chemical and biological importance, J. L. Finney, A. K. Soper, Chem. Soc. Rev., 1994, (1),1-10, DOI: 10.1039/CS9942300001

Here is a practical rule of thumb is given for deciding what sort of H/D substitution is indicated in your case. This is only indicative and single cases should always be discussed with your local contact at ISIS.

## Ideal case

Calculate the ratio of hydrogens in your solute over the total number of atoms in the solution. E.g. for benzene in water 1:10 solution, this number would be 6 hydrogen divided by 12+30=42 atoms giving 14% substituted atoms. You need about 10% in order to go for solute-solute interactions.

Ideal case, 7 substitutions:

 Solute (non-exchangeable hydrogens) Solvent (nad exchangeable hydrogens of the solute) Solvent- solvent (first order difference) 1 D D 2 D HD 3 D H Solute-solute (first order difference) Same as 1 D D 4 HD D 5 H D Solute-solvent (second order difference) Same as 1 D D 6 HD HD 7 H H

HD =  50% molar mixture of the fully H and fully D

## Less-than-ideal case

Very low concentration of solute (1-2%)

In this case you can get away without deuteration of the solute and measuring just 1b, 2b and 3b (see below). The signal of the solute will be very low anyway.

 Solute (non-exchangeable hydrogens) Solvent (nad exchangeable hydrogens of the solute) Solvent- solvent (first order difference) 1b H D 2b H HD 3b H H

## Non-ideal case

It’s not possible to obtain a fully deuterated solute

Then you will have to go for 1b, 2b and 3b and rely on the simulation for helping you solving the rest of the structure (with all the attached caveat).

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