Isotope effect of hydrated clusters of hydrogen chloride, HCl(H2O) n and DCl(H2O) n (n = 0–4): application of dynamic extended molecular orbital method

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The recently proposed dynamic extended molecular orbital (DEMO) method is applied to the HCl(H2O) n and DCl(H2O) n (n = 0–4) clusters in order to explore the isotope effect on their structures, wavefunctions, and energies, theoretically. Since the DEMO method determines both electronic and nuclear wavefunctions simultaneously by optimizing all parameters including basis sets and their centres variationally, we can get the different nuclear orbitals for proton and deuteron as well as their electronic wavefunctions. The positions of the centres of nuclear orbitals show that the deuteron has weaker hydrogen bonding than the proton. There are three isomers in the case of n = 3 clusters, and less stable isomers have hydrogen transferred and non-transferred structures. In the conventional MO calculation, both hydrogen transferred and non-transferred isomers are calculated to be energy minima. When we have applied the DEMO method, only the hydrogen transferred structure is obtained for HCl(H2O)3, while both structures are optimized for DCl(H2O)3. Such strong H/D dependence on the structures of the HCl(H2O) n and DCl(H2O) n clusters can be expressed directly by using the DEMO method. The present application demonstrates that the DEMO method is a useful tool for analysing the anharmonicity and vibronic effects of a hydrogen bonding system.