Journal of Physical Chemistry B vol:114 issue:22 pages:7692-7702
EPR spectroscopy has proven to be an indispensable tool in elucidating the structure of metal sites in proteins.
In recent years, experimental EPR data have been complemented by theoretical calculations, which have
become a standard tool of many quantum chemical packages. However, there have only been a few attempts
to calculate EPR g tensors for exchange-coupled systems with more than two spins. In this work, we present
a quantum chemical study of structural, electronic, and magnetic properties of intermediates in the reaction
cycle of multicopper oxidases and of their inorganic models. All these systems contain three copper(II) ions
bridged by hydroxide or O2- anions and their ground states are antiferromagnetically coupled doublets. We
demonstrate that only multireference methods, such as CASSCF/CASPT2 or MRCI can yield qualitatively
correct results (compared to the experimental values) and consider the accuracy of the calculated EPR g
tensors as the current benchmark of quantum chemical methods. By decomposing the calculated g tensors
into terms arising from interactions of the ground state with the various excited states, the origin of the
zero-field splitting is explained. The results of the study demonstrate that a truly quantitative prediction of
the g tensors of exchange-coupled systems is a great challenge to contemporary theory. The predictions strongly
depend on small energy differences that are difficult to predict with sufficient accuracy by any quantum
chemical method that is applicable to systems of the size of our target systems.