Formalism for the determination of structural isotope effects with neutrons

In general the analysis of neutron isotopic substitution experiments in terms of partial structure factors and partial pair distribution functions is based on the assumption that the structure of isotopic variants of a molecule is identical. This assumption is clearly only an approximation especially in the case of hydrogen bonding molecular liquids like liquid water and structural isotope effects have been measured with X-rays for more than 20 years. An analysis method of neutron isotope data is presented that avoids the necessity to assume structural equality and allows the determination of the isotope effect in the hydrogen-hydrogen partial structure factor of liquid water from neutron data. It is shown that a combination of X-ray and neutron scattering measurements allows in principle the determination of the isotope effects on all partial structure factors of liquid water.