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Density functional studies of EPR and NMR parameters of paramagnetic systems
KTH, School of Biotechnology (BIO).
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Experimental methods based on the magnetic resonance phenomenon belong to the most widely used experimental techniques for investigations of molecular and electronic structure. The difficulty with such experiments, usually a proper interpretation of data obtained from high-resolution spectra, opens new challenges for pure theoretical methods. One of these methods is density functional theory (DFT), that now has an advanced position among a whole variety of computational techniques. This thesis constitutes an effort in this respect, as it presents theory and discusses calculations of electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) parameters of paramagnetic molecules. It is known that the experimental determination of the magnetic resonance parameters of such molecules, especially in the case of NMR, is quite complicated and requires special techniques of spectral detection. On the other hand, paramagnetics play an important role in many areas, such as molecular magnets, active centers in biological systems, and defects in inorganic conductive materials. Therefore, they have spurred great interest among experimentalists, motivating us to facilitate the interpretation of spectral data through theoretical calculations. This thesis describes new methodologies for the determination of magnetic properties of paramagnetic molecules in the framework of DFT, which have been developed in our laboratory, and their applications in calculations of a wide range of molecular systems.

The first two papers of this thesis deal with the theoretical determination of NMRparameters, such as nuclear shielding tensors and chemical shifts, in paramagnetic nitroxides that form core units in molecular magnets. The developed methodology is aimed to realize a high calculational accuracy for these systems. The effects of hydrogen bonding are also described in that context. Our theory for the evaluation of nuclear shielding tensors in paramagnetic molecules is consistent up to second order in the fine structure constant and considers orbital, fully anisotropic dipolar, and isotropic contact contributions to the shielding tensor.

The next projects concern electron paramagnetic resonance. The well-known EPR parameters, such as the g-tensors and the hyperfine coupling constants are explored. Calculations of electronic g-tensors were carried out in the framework of a spin-restricted open-shell Kohn-Sham method combined with the linear response theory recently developed in our laboratory and allowing us to avoid by definition the spin-contamination problem. The inclusion of solvent effects, described by the polarizable continuum model, extends the possibility to treat molecular systems often investigated in solution. For calculations of the hyperfine coupling constants a so-called restricted-unrestricted approach to account for the spin polarization effect has been developed in the context of DFT. To examine the validity of the approximations implicit in this scheme, the neglect ii of singlet operators, a generalized RU methodology was implemented, which includes a fully unrestricted treatment with both singlet and triplet operators. The small magnitude of the changes in hyperfine coupling constants confirms the validity of the original scheme.

Place, publisher, year, edition, pages
Stockholm: KTH , 2006. , 50 p.
Keyword [en]
spin-restricted DFT, restricted-unrestricted approach, EPR spin Hamiltonian parameters, NMR spin Hamiltonian parameters
National Category
Organic Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-4116ISBN: 91-7178-450-0 (print)OAI: oai:DiVA.org:kth-4116DiVA: diva2:10812
Public defence
2006-10-06, FD5, AlbaNova, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100923Available from: 2006-09-22 Created: 2006-09-22 Last updated: 2012-03-21Bibliographically approved
List of papers
1. Calculations of nuclear magnetic shielding in paramagnetic molecules
Open this publication in new window or tab >>Calculations of nuclear magnetic shielding in paramagnetic molecules
2003 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 118, no 6, 2550-2561 p.Article in journal (Refereed) Published
Abstract [en]

We propose and evaluate first principles methods for calculating the nuclear shielding tensor in open-shell, paramagnetic molecules, dealing with the case of small spin-orbit coupling that, in turn, implies the best applicability to light, organic compounds. The formalism is consistent up to second order in the fine structure constant, and includes orbital, fully anisotropic dipolar, and isotropic contact contributions to the tensor. The proposed method is implemented within the ab initio single- and multiconfiguration self-consistent field as well as density functional theory frameworks. The applications include small main-group radicals and larger nitroxide radicals. The analysis of the results and comparison with the experimental nuclear magnetic resonance data, which are available for the latter compounds, indicate promising accuracy and applicability of the density functional theory method to chemically interesting problems.

Keyword
HYPERFINE COUPLING-CONSTANTS, DENSITY-FUNCTIONAL CALCULATIONS, GENERALIZED GRADIENT APPROXIMATION, CONSISTENT-FIELD CALCULATIONS, SPIN-ORBIT PSEUDOPOTENTIALS, INHOMOGENEOUS ELECTRON-GAS, CORRELATION-ENERGY, EXCHANGE-ENERGY, WAVE-FUNCTIONS, RADICALS
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-13644 (URN)10.1063/1.1535904 (DOI)000180564800012 ()
Note
QC 20100624Available from: 2010-06-24 Created: 2010-06-22 Last updated: 2017-12-12Bibliographically approved
2. Influence of hydrogen bonding in the paramagnetic NMR shieldings of nitronylnitroxide derivative molecules
Open this publication in new window or tab >>Influence of hydrogen bonding in the paramagnetic NMR shieldings of nitronylnitroxide derivative molecules
2004 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 108, no 4, 1197-1206 p.Article in journal (Refereed) Published
Abstract [en]

We apply our recently developed methodology for first-principles computations of paramagnetic NMR shieldings and explore the shieldings in a selected set of radicals that form core units in molecular magnets. The influence on these parameters of hydrogen bonding, that corresponds to the crystal environment, is the prime objective of the study. Nitronylnitroxide radicals with the hydroxyphenyl group in the ortho, meta, and para positions, as well as p-methoxyphenyl derivatives, are chosen for this purpose. The strong (NOHO)-H-... hydrogen bonding in the real crystal structure has been simulated by adding water molecules in the twelve molecular complexes investigated. Comparison of calculated and experimental data is made by taking the special features of the solid state and solution environments into account. The observed change in the shielding constant due to hydrogen bonding is explained by the spin delocalization picture; the dominating contribution is the temperature-dependent contact shielding containing the isotropic hyperfine coupling constant.

Keyword
NITRONYL NITROXIDE RADICALS, POLARIZED NEUTRON-DIFFRACTION, SPIN-ORBIT PSEUDOPOTENTIALS, DENSITY-FUNCTIONAL THEORY, AB-INITIO, FERROMAGNETIC INTERACTIONS, ORGANIC FERROMAGNET, MAGNETIC-PROPERTIES, AMINOXYL RADICALS, CRYSTAL PACKING
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-13641 (URN)10.1021/jp030693g (DOI)000188535700008 ()2-s2.0-0442295459 (Scopus ID)
Note
QC 20100624Available from: 2010-06-24 Created: 2010-06-22 Last updated: 2017-12-12Bibliographically approved
3. Restricted density functional response theory for open-shell systems
Open this publication in new window or tab >>Restricted density functional response theory for open-shell systems
2005 (English)In: ADVANCES IN QUANTUM CHEMISTRY, VOL 50: Book Series: ADVANCES IN QUANTUM CHEMISTRY / [ed] Sabin, JR; Brandas, E, SAN DIEGO: ELSEVIER ACADEMIC PRESS INC , 2005, Vol. 50, 271-288 p.Conference paper, Published paper (Refereed)
Abstract [en]

This work presents development, implementation and applications of density functional theory (DFT) methods for calculation molecular properties of open-shell molecules. The theory of restricted open-shell density functional response theory is briefly summarized and the advantages and disadvantages of a spin-restricted formulation is discussed. Sample calculations are presented and discussed for excitation energies, polarizabilities and ESR spectral parameters: g-tensors and A-tensors (hyperfine coupling constants). For the A-tensors a recent generalization of the restricted-unrestricted approach [Fernandez, et al., J. Chem. Phys. 97 (1992) 3412] has been used. It is found that the additional complexity of spin-restricted methods is motivated by the quality of our results.

Place, publisher, year, edition, pages
SAN DIEGO: ELSEVIER ACADEMIC PRESS INC, 2005
Series
ADVANCES IN QUANTUM CHEMISTRY, ISSN 0065-3276 ; 50
Keyword
hyperfine coupling-constants, polarizable continuum model, hydrocarbon radical cations, electronic g-tensors, hartree-fock, wave-functions, molecules, naphthalene, anthracene, hydrogen
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-6178 (URN)10.1016/S0065-3276(05)50013-3 (DOI)000235210900015 ()2-s2.0-33645886942 (Scopus ID)0-12-034850-0 (ISBN)
Conference
Conference on Response Theory and Molecular Properties. Sandbjerg Estate, DENMARK. MAY 05-08, 2004
Note
QC 20111013Available from: 2006-09-22 Created: 2006-09-22 Last updated: 2012-03-21Bibliographically approved
4. Electronic g-tensors of solvated molecules using the polarizable continuum model
Open this publication in new window or tab >>Electronic g-tensors of solvated molecules using the polarizable continuum model
2004 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 121, 5051- p.Article in journal (Refereed) Published
Abstract [en]

We present the implementation of density functional response theory combined with the polarizable continuum model (PCM), enabling first principles calculations of molecular g-tensors of solvated molecules. The calculated g-tensor shifts are compared with experimental g-tensor shifts obtained from electron paramagnetic resonance spectra for a few solvated species. The results indicate qualitative agreement between the calculations and the experimental data for aprotic solvents, whereas PCM fails to reproduce the electronic g-tensor behavior for protic solvents. This failure of PCM for protic solvents can be resolved by including into the model those solvent molecules which are involved in hydrogen bonding with the solute. The results for the protic solvents show that the explicit inclusion of the solvent molecules of the first solvation sphere is not sufficient in order to reproduce the behavior of the electronic g-tensor in protic solvents, and that better agreement with experimental data can be obtained by including the long-range electrostatic effects accounted for by the PCM approach on top of the explicit hydrogen-bonded complexes.

Keyword
hyperfine coupling-constants, density-functional calculations, semiquinone radical-anions, nitroxide spin-label, nitric oxide, correlation-energy, orbit operators, linear-response, epr parameters, hartree-fock
National Category
Organic Chemistry
Identifiers
urn:nbn:se:kth:diva-6174 (URN)10.1063/1.1779568 (DOI)000223720600005 ()2-s2.0-4944231543 (Scopus ID)
Note
QC 20100923 QC 20110916Available from: 2006-09-22 Created: 2006-09-22 Last updated: 2012-03-21Bibliographically approved
5. Density functional theory for hyperfine coupling constants with the restricted-unrestricted approach
Open this publication in new window or tab >>Density functional theory for hyperfine coupling constants with the restricted-unrestricted approach
2004 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 121, no 16, 7614-7623 p.Article in journal (Refereed) Published
Abstract [en]

This work presents derivation, implementation, and the first applications of the restricted-unrestricted approach based on restricted Kohn-Sham formalism for evaluation of hyperfine coupling constants. By using the spin-restricted Kohn-Sham method the well-known spin contamination problem existing in the unrestricted Kohn-Sham formalism is avoided and a proper description of spin polarization is achieved via the restricted-unrestricted approach without introducing spin contamination into the evaluation of the hyperfine coupling constants. The performance of the proposed formalism is evaluated for a set of organic radicals and transition metal compounds. The results of this investigation indicate promising accuracy of the restricted-unrestricted approach for calculation of the isotropic hyperfine coupling constants in organic radicals as well as transition metal compounds.

Keyword
TRANSITION-METAL-COMPLEXES, RADICALS, EPR
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-13639 (URN)10.1063/1.1799013 (DOI)000224456500011 ()2-s2.0-8344231616 (Scopus ID)
Note
QC 20100624Available from: 2010-06-24 Created: 2010-06-22 Last updated: 2017-12-12Bibliographically approved
6. Time-dependent density functional theory with the generalized restricted-unrestricted approach
Open this publication in new window or tab >>Time-dependent density functional theory with the generalized restricted-unrestricted approach
Show others...
2006 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 124, no 17, 174103- p.Article in journal (Refereed) Published
Abstract [en]

This work presents the derivation, implementation, and first applications of the generalized restricted-unrestricted method based on the density functional Kohn-Sham formalism. By using a spin-restricted Kohn-Sham representation for the reference state the well-known spin contamination problem is avoided, while the unrestricted representation of the perturbation response retains a proper description of spin polarization. The formulation is a generalization of our previous implementation of the restricted-unrestricted method [Z. Rinkevicius , J. Chem. Phys. 121, 7614 (2004)], as it accounts for the full unrestricted response instead of describing the spin polarization in terms of triplet operators only. The purpose of this paper is to investigate the role of the generalization employed and demonstrate its numerical performance. For this purpose we focus on isotropic hyperfine coupling constants of a set of organic radicals and transition metal compounds. For both classes of molecules we observe that the effect of neglecting singlet excitation operators in the response part of restricted-unrestricted formalism changes the calculated hyperfine coupling parameters by a few MHz. The obtained results confirm the validity of the approximation used in the simplified restricted-unrestricted approach, i.e. spin polarization can, in most cases, be adequately described by an account of only triplet operators in the response term.

Keyword
Approximation theory; Numerical analysis; Polarization; Quantum theory; Transition metal compounds; Isotropic hyperfine coupling; Singlet excitation operators; Spin polarization; Probability density function
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-7071 (URN)10.1063/1.2191501 (DOI)000237321700005 ()2-s2.0-34547554886 (Scopus ID)
Note
QC 20100811Available from: 2007-05-11 Created: 2007-05-11 Last updated: 2012-03-21Bibliographically approved
7. Density functional theory for spin Hamiltonian parameters of azurin: Part I. EPR parameters
Open this publication in new window or tab >>Density functional theory for spin Hamiltonian parameters of azurin: Part I. EPR parameters
(English)Manuscript (Other academic)
National Category
Organic Chemistry
Identifiers
urn:nbn:se:kth:diva-6177 (URN)
Note
QC 20100923Available from: 2006-09-22 Created: 2006-09-22 Last updated: 2010-09-23Bibliographically approved
8. Restricted density-functional linear response theory calculations of electronic g-tensors
Open this publication in new window or tab >>Restricted density-functional linear response theory calculations of electronic g-tensors
Show others...
2003 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 119, no 20, 10489-10496 p.Article in journal (Refereed) Published
Abstract [en]

A method for calculations of electronic g-tensors based on a spin-restricted open-shell Kohn-Sham formalism and linear response theory is described. Test calculations for main group organic radicals and transition metal compounds have been carried out using two different spin-orbit approximations: Scaled spin-orbit and atomic mean-field spin-orbit operators. The results indicate slightly better performance of the proposed spin-restricted approach compared to previous methods based on the unrestricted Kohn-Sham formalism. An exception to this general improvement are the anions, as they show considerable spin-polarization. The results also show the superiority of the atomic mean field spin-orbit approximation of the spin-orbit operator with respect to the scaled approximation. For main group radicals, quantitative agreement with high level ab initio as well as experimental data are achieved, whereas for transition metal compounds the results systematically underestimate experimental values, showing also a considerable dependency on the employed exchange correlation functional, similarly to previous reports on calculations using unrestricted density functional theory. From this work we conclude that these problems are not primarily associated with the spin-contamination problem, and that they must be referred to the current time-dependent density functional theories as such.

Keyword
EFFECTIVE NUCLEAR CHARGES, SPIN-ORBIT CALCULATIONS, CORRELATION-ENERGY, HARTREE-FOCK, APPROXIMATION, EXCHANGE, OPERATORS, MOLECULES, ACCURATE, GAS
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-13642 (URN)10.1063/1.1620497 (DOI)000186554500004 ()
Note
QC 20100624Available from: 2010-06-24 Created: 2010-06-22 Last updated: 2017-12-12Bibliographically approved

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