The Unrestricted Hartree-Fock Theory of Chemical Reactions. IV: Singlet Radical States with "Antiferromagnetic" Spin Orderings in Four-Center Exchange Reaction of Hydrogen Molecules

A UHF theoretical analysis is carried out on four-center exchange reaction of two hydrogen molecules in reaction paths with D2 symmetry including D2h, D2d and Td conformations as special cases. It is shown that the UHF singlet ground state of the H4 system consists of ten distinct UHF configurations which appear at different regions of nuclear configuration space. Three among the ten configurations are closed shell type and the other seven correspond to singlet radicals with distinct “antiferromagnetically” ordered spin structures. Three among the seven singlet radical configurations have spin structures modulated one dimensionally, the other three have those modulated two dimensionally and are very similar to Overhauser's helical spin density wave. Another configuration has tera-hedrally modulated spin structure. The adiabatic potentials of the UHF configurations are calculated and their interconnection relation is studied. The electronic correlation effects underlying these singlet radical states are discussed in connection with orbital correlation diagrams.