Assignment of the 6,6 '-Dioxo-3,3 '-biverdazyl Ground State by Using Broken Symmetry and Spectroscopy Oriented Configuration Interaction Techniques

作者：Mattar Saba M^*; Dokainish Hisham M

来源：Journal of Physical Chemistry A, 2010, 114(4): 2010-2021.

DOI：10.1021/jp910643e

The singlet-triplet energy differences (Delta E-ST) and the Heisenberg-Dirac-van-Vleck exchange parameter (J) of 6,6'-dioxo-3,3'-biverdazyl (BVD) have been studied by hybrid density functional (HDF), broken symmetry (BS), and spectroscopy oriented Configuration interaction (SORCI) methods. Energy Surface scans as a function of the dihedral angle between the two verdazyl rings (phi(N2C3C3'N2')) have been performed. The BS computations predict an anti ferromagnetic ground state. However, the diradical index (R-BS) ranges from 97.5 to 99.9%, implying that the interactions between the two Unpaired electrons are very weak. To calculate J and Delta E-ST, the multireference character introduced by these weak spin-spin interactions must be taken into account. Consequently, multireference difference dedicated configuration interaction (MRDDCI) methods, as implemented in the SORCI procedure, are used. The in-plane pi (IP pi), out-of-plane pi (OP pi), and sigma configurations arc included in the CI expansions in a balanced fashion. The OP pi-OP pi and OP pi-IP pi overlaps are the predominant factors that influence the J and Delta E-ST as a function phi(N2C3C3'N2') and cause them to peak around 40 and 140 degrees. In these regions, the antiferromagnetic interactions arc minimal, and the MRDDCI methods predict a triplet ground state. At phi(N2C3C3'N2') = 0, Delta E-ST[MRDDCI3(14,12)] is in excellent agreement with that of 1,1',5,5'-tetramethyl-6.6'-dioxo-3,3'-biverdazyl determined experimentally from electron paramagnetic resonance (EPR) spectroscopy and differs only by 2.3%. Furthermore. Delta E-ST[MRDDCI3(14,12)] is consistently smaller than J(Y) as the verdazyl rings rotate with respect to each other. This corroborates the theory that the HDF-BS technique increases the singlet-triplet energy gap and favors the singlet state. Because the SORCI method is specifically designed for large molecules, the present very good results open the way for the computation of the magnetic properties of larger molecules by the SORCI method. To the best of our knowledge, this is the first time that Delta E-ST has been computed by the MRDDCI3 method by utilizing such a large CI reference space for a molecule of this size.

• 出版日期2010-2-4