Spin–Orbit Contributions in High‐Spin Nitrenes/Carbenes: A Hybrid CASSCF/MRMP2 Study of Zero‐Field Splitting Tensors

Zero‐field splitting (ZFS) tensors ( D tensors) of organic high‐spin oligonitrenes/oligocarbenes up to spin‐septet are quantitatively determined on the basis of quantum chemical calculations. The spin–orbit contributions, D ^SO tensors are calculated in terms of a hybrid CASSCF/MRMP2 approach, which was recently proposed by us. The spin–spin counterparts, D ^SS tensors are computed based on McWeeny–Mizuno’s equation in conjunction with the RODFT spin densities. The present calculations show that more than 10 % of ZFS arises from spin–orbit interactions in the high‐spin nitrenes under study. Contributions of spin‐bearing site–site interactions are estimated with the aid of a semi‐empirical model for the D tensors and found to be ca. 5 % of the D ^SO tensor. The analysis of intermediate states reveal that the largest contributions to the calculated D ^SO tensors are attributed to intra‐site spin flip excitations and delocalized π and π* orbitals play an important role in the inter‐site spin–orbit interactions.     

Experimental evidence for the triplet-like spin state appearing in ground-state singlet biradicals as a key feature for generalized ferrimagnetic spin alignment

The authors have previously proposed a theoretical model for exotic spin alignment in organic molecular assemblages: The alternating chain of organic biradicals in a singlet (Sb=0) ground state and monoradicals with S=1/2 has a ferrimagnetic ground state for the whole chain, which has been termed generalized ferrimagnetism. An important feature of the generalized ferrimagnetic spin alignment has been found in the deviation of the expectation value Sb2 of the biradical spin from zero. Even a triplet-like spin state Sb2=2 (Sb=1) has been predicted in the theoretical calculations. In this study, we have found experimental evidence for the pseudo-triplet state appearing in the ground-state singlet biradical of a real open-shell compound. At first, we have demonstrated from theoretical calculations that the singlet biradical has Sb2=2 (Sb=1) in a molecular pair with an S=1 metal ion as well as with the S=1/2 monoradical. The pseudo-triplet state of the biradical affords a singlet state of the whole system of the biradical-metal ion pair, which is readily detectable in experiments for verifying the theoretical prediction. As a model compound for the biradical-metal ion pair, a transition metal complex, [(bnn)(Ni(hfac)2)1.5(H2O)] (1), has been synthesized from a nitronyl nitroxide-based ground-state singlet biradical bnn and Ni(hfac)2. From X-ray crystallographic analyses, the compound contains a molecular pair of bnn and Ni(hfac)2, which serves as a model system under the above theoretical studies. It has been found from the analysis of the temperature dependence of magnetic susceptibility that the bnn-Ni(hfac)2 pair has the singlet (S=0) ground state. The singlet ground state of the pair results from an antiparallel coupling of the pseudo-triplet of the biradical and the S=1 spin on the Ni ion. The pseudo-triplet state in the ground-state singlet biradical has thus been verified experimentally, which is crucially important to realize the generalized ferrimagnetic spin alignment.     

A quantum and deductive chemical study for all congeners of polybromo/chlorodibenzo-p-dioxin and polybromo/chlorodibenzofuran

We have performed semiempirical quantum chemical calculations to obtain the optimized structure and the free energy (DeltaG) for all congeners (1701) of polybromo/chlorodibenzo-p-dioxins, which include all the isomers of all the homologues, and those for (3321) polybromo/chlorodibenzofurans. Then, to apply the "Quantum and Deductive Chemistry Approach" on the dioxin chemistry, we have carried out the multiple linear regression (MLR) as functions of temperature and the substituted numbers and positions of chlorine and bromine. The optimized structure of dibenzo-p-dioxin and the dibenzofuran ring is significantly influenced by the substitutions of the peri and lateral halogen atoms. The bond length between the aromatic ring and halogen atom also is influenced by the neighboring atoms. The bromine substitution at the 2 and 8 positions of dibenzofuran reduces the steric repulsion between the chlorine atoms at the 1 and 9 positions. The coefficients of the predicting equation of DeltaG derived by MLR suggest that the probabilities of chlorine elimination from the peri and lateral positions for polychlorodibenzo-p-dioxin are nearly equal.     

Elastic constant anisotropy,core structure of wedge disclinations and optical texture of main-chain P-4-BCMU liquid crystals

The elastic constant anisotropy and the core structure of wedge disclinations with strengths s = ±½ of lyotropic liquid crystals of a soluble polydiacetylene, P-4-BCMU (M _w = 5.15 × 10⁵, M _n = 2.16 × 10⁵, and M _w/M _n = 2.4), in chloroform was studied. The Frank elastic constant anisotropy defined by ε = (k₁₁ ? k₃₃)/(k₁₁ + k₃₃) for this polymer was determined by three different methods. The results show that the value of ε for this polymer is 0.5 (±0.05) in solid state and/or in liquid crystal state, indicating that the splay constant k₁₁ is three time higher than the bend constant k₃₃. This result further implies that splay is unfavorable in the liquid as it requires greater energy than bend. In the area adjacent to the core of a singularity the value of ε increases with decreasing distance to the core, indicating increasing anisotropy of the elastic constants toward the core. The influence of the elastic constant anisotropy on the optical texture of the LC polymer when viewed by polarizing light microscope is studied. © 1994 John Wiley & Sons, Inc.     

Light scattering from polymer films having optically anisotropic rodlike texture. III. Inter-rod interference

Effects of interparticle interference on depolarized light scattering from polymer films having nonspherulitic (rodlike, disklike, or fibrous) crystalline superstructures are explored, both theoretically and experimentally. Discrepancies between the observed “rodlike scattering” and the predicted scattering based on “single-rod” theory are pointed out and accounted for in terms of various types of aggregation models for polymeric solids and solutions.     

Hexaazaphenalenyl anion revisited: a highly symmetric planar pi system with multiple-networking ability for self-assembled metal complexation

The hexaazaphenalenyl anion (HAP, 3-), a highly symmetric heterocyclic pi system, has been synthesized and characterized. The crystal structures of its potassium salt K+.3- and copper complex Cu2+.(NH3)4.(3-)2 show pi-pi stacking and radially extended hydrogen bonds as well as coordination bonds constructed by the HAP anion. These in-plane and out-of-plane strong interactions demonstrate the multiple-networking ability of the HAP anion.     

Quintet bis(carbene) from triplet diarylcarbenes introduced at the 1,8-position of anthracene

[9-(10-phenyl)anthryl](4-bromo-2,6-dimethylphenyl)diazomethane was found to be stable enough to survive Sonogashira coupling reaction conditions and was converted to [9-(10-phenyl)anthryl](4-trimethylsilylethynyl-2,6-dimethylphenyl)diazomethane, which was reacted with 1,8-diiodoanthracene to give bis(diazo) compound. Bis(carbene) generated by irradiation of the bis(diazo) compound generated a fairly persistent S = 2 quintet state. [structure: see text]     

Phase equilibria and phase separation dynamics in a polymer composite containing a main-chain liquid crystalline polymer

Various topological phase diagrams of blends of main-chain liquid crystalline polymer (MCLCP) and flexible polymer have been established theoretically in the framework of Matsuyama–Kato theory by combining Flory–Huggins (FH) free energy for isotropic mixing, Maier–Saupe (MS) free energy for nematic ordering in the constituent MCLCP, and free energy pertaining to polymer chain-rigidity. As a scouting study, various phase diagrams of binary flexible polymer blends have been solved self-consistently that reveal a combined lower critical solution temperature (LCST) and upper critical solution temperature (UCST), including an hourglass phase diagram. The calculated phase diagrams exhibit liquidus and solidus lines along with a nematic–isotropic (NI) transition of the constituent MCLCP. Depending on the strengths of the FH interaction parameters and the anisotropic (nematic–nematic) interaction parameters, the self-consistent solution reveals an hourglass type phase diagram overlapping with the NI transition of the constituent MCLCP. Subsequently, thermodynamic parameters estimated from the phase diagrams hitherto established have been employed in the numerical computation to elucidate phase separation dynamics and morphology evolution accompanying thermal-quench induced phase separation of the MCLCP/polymer mixture. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3621-3630, 2006