M. Okumura

EHF theory of chemical reactions Part 4. UNO CASSCF, UNO CASPT2 and R(U)HF coupled-cluster (CC) wavefunctions

Abstract Spin-restricted and spin-unrestricted post-Hartree-Fock methods are compared by depicting potential energy curves of several hydrogen clusters (H 2 -H 6 ), hydrogen fluoride and fluorine molecules. In the former approach, RHF, RMP, RHF coupled-cluster (CC), UNO CASSCF and UNO CASPT2 are utilized, whereas the latter approach includes UHF MP n (UMP n ) and UHF coupled-cluster (UCC), approximate spin-projected UMP n and UCC (APUMP n and APUCC) and Lowdin-type full spin-projected UMP n (PMP n ). Characteristics (shapes in particular) of the potential curves are discussed in relation to the roles of electron correlation corrections revealed by both methods. The UNO CASSCF followed by PT2 provides reasonable potential curves in the whole region. The spin contamination error is remarkable in the case of the UMP n wavefunction, leading to the necessity of spin projection. The resulting PMP2 and APUMP2 can be used for larger clusters and are regarded as alternatives to UNO CASPT2. The spin contamination error is not so serious for UCCSD(T), at least in the weak and intermediate correlation regime, whereas it is important in the magnetic (dissociation) region where APUCCSD(T) works well in depicting the potential energy curves. It is regarded as an approximate version of UNO CASSCF CC SD(T). Implications of the calculated results are discussed in order to clarify the applicabilities of the post-HF methods to chemical reactions in the ground and lower-excited states.

MO theoretical studies of magnetic interactions in clusters of nitronyl nitroxide and related species

Abstract Ab initio and semi-empirical molecular orbital calculations are carried out for clusters of nitroxide radicals. The intermolecular effective exchange integrals calculated are positive (ferromagnetic) in the case of appropriately stacked forms of these species. Implications of the calculated results are discussed in relation to the ferromagnetic intermolecular interactions observed for nitroxide radicals such as para-nitrophenyl nitronyl nitroxide.

Theoretical calculations of effective exchange integrals between nitronyl nitroxides with donor and acceptor groups

Abstract Semi-empirical calculations on the basis of an extended McConnell model have been carried out for dimers of phenyl nitronyl nitroxides with donor or acceptor groups. The intermolecular effective exchange integrals are positive (ferromagnetic) for appropriately stacked forms of these species because of large spin polarization effects. Implications of these results are discussed in relation to the organic ferromagnetism discovered recently for para-nitrophenyl nitronyl nitroxide.

A theoretical explanation of the organic ferromagnetism in the β-phase of para-nitrophenyl nitronyl nitroxide

Abstract Molecular orbital calculations have been carried out for clusters of para-nitrophenyl nitronyl nitroxide ( p -NPNN) extracted from the experimental crystal structure of its β-phase. Three-dimensional effective exchange integrals ( J ab ) are found to be possitive (ferromagnetic). The ferromagnetic transition temperature is estimated by a generalized Langevin—Weiss model combined with the calculated J ab , being consistent with the observed value. A theoretical explanation is presented for the long-range ferromagnetic ordering in the p -NPNN crystal. Implications of the calculated results are also discussed in relation to a theoretical possibility of organic ferromagnetism of other substituted phenyl nitronyl nitroxides.

A MO-theoretical calculation of the antiferromagnetism in the γ-phase of p-nitrophenyl nitronyl nitroxide

Abstract Semi-empirical and ab initio molecular orbital (MO) calculations have been carried out for clusters of p -nitrophenyl nitronyl nitroxide ( p -NPNN) extracted from the experimental crystal structure of its γ-phase. It is found that the intraplane exchange interactions ( J ab ) are ferromagnetic, whereas the interplane one is antiferromagnetic. The antiferromagnetic transition temperature ( T N ) is estimated by a generalized Langevin-Weiss-Neel (GLWN) model combined with the calculated J ab values by the MO methods. The Neel temperature ( T N ) by the mean-field GLWN model is of the order of the experimental value, although the former is much larger than the latter because of the low dimensionality of the γ-phase. A systematic theoretical explanation is presented for the magnetic long-range orderings in the β- and γ-phase crystals of p -NPNN.

Electronic structures of poly-cations and -anions of C60. Possible mechanisms of organic ferromagnetism

Abstract The ORHF and CAS CI calculations by MNDO and PM3 parametrizations are carried out for poly-cation and -anion states of C 60 in order to elucidate bond alternations and changes of ground spin multiplicities by doping of electrons or holes. The singlet state is calculated to be more stable than the triplet state for the dianion of C 60 in accordance with a recent experiment. A three-band model is constructed of the t 1u orbitals of C 60 in order to examine the itinerant ferro- and antiferro-magnetism. Implications of the calculated results are discussed in relation to the organic ferromagnetism observed in the C 60 ·TDAE complex.

High-spin ion radicals of polyenes and polyamines. A MO theoretical study

Abstract Ab initio and semiempirical molecular orbital calculations have been carried out for model compounds of the high-spin ion radicals of poly( m -phenyleneoctatetraene) derivatives. The calculated results are consistent with previous experiments. The computations have also been performed for high-spin ion radicals of aromatic amine derivatives and other ion radicals formed by doping. It is shown that the spin polarization rule proposed previously for ion radicals is consistent with the existing experimental results.

Theoretical Studies of Magnetic Orderings in the β- and γ-Phases of P-NPNN and Related Nitroxides

Abstract Intermolecular spin alignment rules are derived on the basis of ab initio results obtained for clusters of triplet carbenes, free radicals, etc. Interrelationships between several theoretical models are also clarified. The APUHF calculations are performed for radical pairs in the β and γ-phase crystals of para-nitrophenyl nitronyl nitroxide. The magnetic transition temperatures are calculated by the use of Langevin-Weiss type mean-field theory combined with the calculated effective exchange integrals. The origins of ferromagnetic interactions in both the phases are discussed in relation to the intermolecular spin alignment rules.

Theoretical approaches to molecular magnetisms through-bond couplings between triplet carbenes and related species

Abstract Ab initio post Hartree-Fock calculations were carried out for polyradical species formed by the through-bond couplings between organic radical groups such as triplet carbenes. Detailed numerical results indicate that the dynamic spin polarization (SP) rules proposed previously are applicable to qualitative predictions of the ground spin states of these species. On the other hand, the spin derealization (SD) mechanism is operative in the case of polyradicals linked with hetero atoms such as oxygen and nitrogen atoms. The ionized states of polycarbenes and polyenes are also investigated in relation to possibilities of organic magnetic conductors. Computational results are compatible with experimental results available.

Ferromagnetic intermolecular interaction of the cation radical ofm-N-methylpyridinium nitronyl nitroxide. A CASSCF study

Abstract Semi-empirical APUHF and ab initio UNO CASSCF/4-31G calculations have been carried out for the dimer of m -N-methylpyridinium nitronyl nitroxide cation ( m -MPYNN + ) extracted from its experimental crystal structure in order to elucidate the origin of the large ferromagnetic effective exchange integral | J ab |. It is found that the sign of the calculated J ab is positive, and its magnitudes are 6.51 and 3.96 cm −1 by these methods, which correspond to about 80 and 50% of the experimental value, respectively. The dynamic spin polarization plays an important role in determining the sign of J ab , whereas the short intermolecular contact induced by the Coulombic attraction is reponsible for the large J ab . Implications of the calculated results are discussed in relation to the possibilities of high- T c organic ferromagnets and organic helimagnets.