Structural and isotopomeric isomerism of hydrogen- bonded complexes

Two kinds of hydrogen-bonded complex isomerism, structural and isotopomeric, are the subject of contemporary research. Structural isomerism represents an extension of the phenomenon known from organic chemistry. Isotopomeric isomerism can be considered with any complex containing, in addition to the hydrogen involved in the hydrogen bond, at least one other hydrogen atom. Then, two different, isomeric species can be created by mono-deuteration of the complex. An example is given by the water-dimer gas-phase isotopomers HOD·OHD and DOH·OHD. The isomeric systems of both types can exhibit interchanges in the relative stabilities of the structures with changing temperature. Moreover, the temperature changes can be quite pronounced so that values of temperature derivatives are obtained which are not frequently observed with other types of isomeric systems. It can be well manifested in thermodynamic terms (standard changes of enthalpy, entropy and heat capacity upon cluster formation), or at least in isomerism contributions to the latter terms, especially so for heat capacity.