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Spin-Orbit and Spin-Spin Coupling in the Triplet State
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The underlying theory of “Spin” of an electron and its associated inter-actions causing internal fields and spectral shift to bulk-magnetism iswell established now. Our understanding of spin properties is significant andmore useful than ever before. In recent years there seems to be an enormousinterest towards application oriented materials that harness those spin prop-erties. Theoretical simulations remain in a position to “assist or pilot” theexperimental discovery of new materials.In this work, we have outlined available methodologies for spin coupling inmulti-reference and DFT techniques. We have benchmarked multi-referencespin-Hamiltonian computation in isoelectronic diradicals - Trimethylenemethane(TMM) and Oxyallyl. Also with DFT, parameters are predicted with anewly discovered TMM-like stable diradicals, reported to have large positiveexchange interactions. Excellent agreement were obtained and our findingsemphasize that the dipole-dipole interactions alone can predict the splittingof triplet states and that DFT spin procedures hold well in organic species.We have extended our spin-studies to a highly application oriented ma-terial - nanographene. Using our novel spin-parameter arguments we haveexplained the magnetism of graphene. Our studies highlight a few signifi-cant aspects - first there seems to be a size dependency with respect to thespin-Hamiltonian; second, there is a negligible contribution of spin-orbit cou-pling in these systems; third, we give a theoretical account of spin restrictedand unrestricted schemes for the DFT method and their consequences forthe spin and spatial symmetry of the molecules; and, finally, we highlightthe importance of impurities and defects for magnetism in graphene. Wepredict triplet-singlet transitions through linear response TDDFT for thetris(8-hydroxyquinoline) aluminium complex, an organic molecule shown tohave spintronics applications in recent experiments. Our spin studies werein line with those observations and could help to understand the role of thespin-coupling phenomena.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , 67 p.
Series
Trita-BIO-Report, ISSN 1654-2312 ; 2012:17
Keyword [en]
Spin-spin, Spin-Orbit, D and E parameters, ZFS, graphene, TMM, OXA, diradicals, tris(8-hydroxyquinonline) aluminium, magnetic anisotropy, magnetism, triplet
National Category
Theoretical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-95761ISBN: 978-91-7501-366-4 (print)OAI: oai:DiVA.org:kth-95761DiVA: diva2:529247
Public defence
2012-06-07, FB42, AlbaNova Universitetscentrum, Stockholm, 14:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council
Note
QC 20120531Available from: 2012-05-31 Created: 2012-05-29 Last updated: 2012-05-31Bibliographically approved
List of papers
1. Spin-spin coupling in (3)b(2) state of oxyallyl - A comparative study with trimethylenemethane
Open this publication in new window or tab >>Spin-spin coupling in (3)b(2) state of oxyallyl - A comparative study with trimethylenemethane
2011 (English)In: Computational and Theoretical Chemistry, ISSN 2210-271X, Vol. 963, no 1, 51-54 p.Article in journal (Refereed) Published
Abstract [en]

Internal splittings in the spectra are due to quantized angular momentum field of the molecule or atom, which couples through spin–spin and spin–orbit operators. These operators are subjected to recently observed close lying states of oxyallyl biradical and compared with the iso-electronic structure trimethylenemethane. With the multiconfigurational treatment of spin–spin coupling operator we predict reversal of the splitting parameter Dss of oxyallyl with reference to isoelectronic TMM spectra.

 

Keyword
Spin–spin, Biradical, Oxyallyl, Trimethylenemethane(TMM)
National Category
Theoretical Chemistry Atom and Molecular Physics and Optics Condensed Matter Physics
Research subject
SRA - E-Science (SeRC)
Identifiers
urn:nbn:se:kth:diva-29589 (URN)10.1016/j.comptc.2010.09.006 (DOI)000289649100009 ()2-s2.0-79958101545 (Scopus ID)
Funder
Swedish Research CouncilSwedish e‐Science Research Center
Note
QC 20110210. Uppdaterad från submitted till published 20110307Available from: 2011-02-10 Created: 2011-02-10 Last updated: 2012-08-22Bibliographically approved
2. Zero-field splitting of compact trimethylenemethane analogue radicals with density functional theory
Open this publication in new window or tab >>Zero-field splitting of compact trimethylenemethane analogue radicals with density functional theory
2011 (English)In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 501, no 4-6, 608-611 p.Article in journal (Refereed) Published
Abstract [en]

This work presents the computation of spin-spin coupling that arise between interactions of spin-centers which eventually leads to zero-field splitting in organic molecules. With DFT implementation spin-spin Hamiltonian is subjected to recently observed compact trimethylenemethane analogue di-radicals -nitroxide-substituted nitronyl nitroxide and iminonitroxide, which exhibits large positive exchange interactions with ZFS at the room temperature. We obtain the parameters defining the ZFS that have an excellent agreement with the experiment, consistent with the perception that spin-orbit coupling contribution to the ZFS is negligible in organic molecule. g-tensor shift also calculated with restricted DFT-linear response formalism (RDFT-LR) and compared with realistic observations.

Keyword
ELECTRONIC G-TENSORS, TRANSITION-METAL-COMPLEXES, AB-INITIO CALCULATIONS, SPIN-ORBIT OPERATORS, KOHN-SHAM THEORY, RESPONSE THEORY, PARAMETERS
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-31359 (URN)10.1016/j.cplett.2010.11.068 (DOI)000285829300087 ()2-s2.0-78650842621 (Scopus ID)
Note
QC 20110316Available from: 2011-03-16 Created: 2011-03-14 Last updated: 2017-12-11Bibliographically approved
3. Spin-spin and spin-orbit interactions in nanographene fragments: A quantum chemistry approach
Open this publication in new window or tab >>Spin-spin and spin-orbit interactions in nanographene fragments: A quantum chemistry approach
2012 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 136, no 10, 104702- p.Article in journal (Refereed) Published
Abstract [en]

The relativistic behavior of graphene structures, starting from the fundamental building blocks - the poly-aromatic hydrocarbons (PAHs) along with other PAH nanographenes - is studied to quantify any associated intrinsic magnetism in the triplet (T) state and subsequently in the ground singlet (S) state with account of possible S-T mixture induced by spin-orbit coupling (SOC). We employ a first principle quantum chemical-based approach and density functional theory (DFT) for a systematic treatment of the spin-Hamiltonian by considering both the spin-orbit and spin-spin interactions as dependent on different numbers of benzene rings. We assess these relativistic spin-coupling phenomena in terms of splitting parameters which cause magnetic anisotropy in absence of external perturbations. Possible routes for changes in the couplings in terms of doping and defects are also simulated and discussed. Accounting for the artificial character of the broken-symmetry solutions for strong spin polarization of the so-called "singlet open-shell" ground state in zigzag graphene nanoribbons predicted by spin-unrestricted DFT approaches, we interpolate results from more sophisticated methods for the S-T gaps and spin-orbit coupling (SOC) integrals and find that these spin interactions become weak as function of size and increasing decoupling of electrons at the edges. This leads to reduced electron spin-spin interaction and hence almost negligible intrinsic magnetism in the carbon-based PAHs and carbon nanographene fragments. Our results are in agreement with the fact that direct experimental evidence of edge magnetism in pristine graphene has been reported so far. We support the notion that magnetism in graphene only can be ascribed to structural defects or impurities.

National Category
Physical Sciences Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-93387 (URN)10.1063/1.3687002 (DOI)000301664600033 ()2-s2.0-84858969231 (Scopus ID)
Funder
Swedish e‐Science Research Center
Note

QC 20120417

Available from: 2012-04-17 Created: 2012-04-16 Last updated: 2017-12-07Bibliographically approved
4. Modelling the triplet states of the tris(8-hydroxyquinoline) aluminium (Alq3) complex
Open this publication in new window or tab >>Modelling the triplet states of the tris(8-hydroxyquinoline) aluminium (Alq3) complex
(English)Manuscript (preprint) (Other academic)
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-96122 (URN)
Note
QS 2012Available from: 2012-05-31 Created: 2012-05-31 Last updated: 2012-05-31Bibliographically approved

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