PMID- 20050593
OWN - NLM
STAT- PubMed-not-MEDLINE
DCOM- 20100427
LR  - 20100128
IS  - 1520-5215 (Electronic)
IS  - 1089-5639 (Linking)
VI  - 114
IP  - 4
DP  - 2010 Feb 4
TI  - Assignment of the 6,6'-dioxo-3,3'-biverdazyl ground state by using broken 
      symmetry and spectroscopy oriented configuration interaction techniques.
PG  - 2010-21
LID - 10.1021/jp910643e [doi]
AB  - The singlet-triplet energy differences (DeltaE(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 
      antiferromagnetic 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 DeltaE(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 (IPpi), out-of-plane pi (OPpi), and sigma configurations are included in the 
      CI expansions in a balanced fashion. The OPpi-OPpi and OPpi-IPpi overlaps are the 
      predominant factors that influence the J and DeltaE(ST) as a function of 
      phi(N2C3C3'N2') and cause them to peak around 40 and 140 degrees. In these 
      regions, the antiferromagnetic interactions are minimal, and the MRDDCI methods 
      predict a triplet ground state. At phi(N2C3C3'N2') = 0, 
      DeltaE(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, DeltaE(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 DeltaE(ST) has been computed by the 
      MRDDCI3 method by utilizing such a large CI reference space for a molecule of 
      this size.
FAU - Mattar, Saba M
AU  - Mattar SM
AD  - University of New Brunswick, Department of Chemistry and Centre for Laser, 
      Atomic, and Molecular Sciences, Fredericton, New Brunswick, Canada E3B 6E2. 
      mattar@unb.ca
FAU - Dokainish, Hisham M
AU  - Dokainish HM
LA  - eng
PT  - Journal Article
PL  - United States
TA  - J Phys Chem A
JT  - The journal of physical chemistry. A
JID - 9890903
EDAT- 2010/01/07 06:00
MHDA- 2010/01/07 06:01
CRDT- 2010/01/07 06:00
PHST- 2010/01/07 06:00 [entrez]
PHST- 2010/01/07 06:00 [pubmed]
PHST- 2010/01/07 06:01 [medline]
AID - 10.1021/jp910643e [doi]
PST - ppublish
SO  - J Phys Chem A. 2010 Feb 4;114(4):2010-21. doi: 10.1021/jp910643e.