Universidad de Zaragoza
:: UNIVERSIDAD :: ESTUDIOS :: INVESTIGACION :: REL.INTERNACIONALES ::
decorativo Instituto de Ciencia de Materiales de Aragón | Tfno: 976 761 231
 - Fax: 976 762 453
decorativo Foto Foto Foto Foto Foto Foto
decorativo
decorativo
decorativo
decorativo Estás en: Inicio >
decorativo
decorativo
decorativo
 Buscar por:
 
decorativo
decorativo
decorativodecorativoInformación generaldecorativo
decorativo
decorativo*Iniciodecorativo
--------
decorativo*El institutodecorativo
--------
decorativo*Cómo llegardecorativo
--------
decorativo*Contactodecorativo
--------
decorativo*Organigramadecorativo
--------
decorativo*Memoriadecorativo
--------
decorativo*Galería de imágenesdecorativo
--------
decorativodecorativoInvestigacióndecorativo
decorativo
decorativo*Departamentos. Gruposdecorativo
--------
decorativo*Unidades de apoyodecorativo
--------
decorativo*Colaboracionesdecorativo
--------
decorativo*Oferta científicadecorativo
--------
decorativo*Selección de temasdecorativo
--------
decorativodecorativoEmpresadecorativo
decorativo
decorativo*Proyectos industrialesdecorativo
--------
decorativo*Oferta tecnológicadecorativo
--------
decorativodecorativoFormacióndecorativo
decorativo
decorativo*Tercer ciclodecorativo
--------
decorativo*Postgradosdecorativo
--------
decorativo*Marie Curiedecorativo
--------
decorativodecorativoDivulgacióndecorativo
decorativo
decorativo*Temasdecorativo
--------
decorativo*Actividadesdecorativo
--------
decorativo*Proyectosdecorativo
--------
decorativo*Ven a conocernosdecorativo
--------
decorativodecorativoUtilidadesdecorativo
decorativo
decorativo*Directoriodecorativo
--------
decorativo*Webmaildecorativo
--------
decorativo*Enlacesdecorativo
--------
decorativo*Intranetdecorativo
--------

decorativo
A Thiazyl-Based Organic Ferromagnet

Magnetism and spin-density distribution byPolarised Neutron Driffraction experiments and ab initio calculations.

Javier Campo and Fernando Palacio (Instituto de Ciencia de Materiales de Aragón, Zaragoza, Spain)Javier Luzón and Garry J. McIntyre (ILL, Grenoble, France)Antonio Alberola and Jeremy M. Rawson (University of Cambridge, Cambridge, UK)Carley Paulsen and Robert D. Farley (CRTBT – CNRS, Grenoble, France)

In 1928 Heisenberg proposed [1] that bulkferromagnetic order would only ever beachieved in systems containing heavy atoms,i.e. metals, their oxides and related derivatives.Indeed it was not until 1991 that the first organicferromagnet was reported; the b-polymorph ofthe radical p-NPNN was shown to order below0.6K [2]. Since then a number of other organicradicals have been found to undergo bulkferromagnetic order, although they almostinvariably order below 1K. The nitroxide-baseddiradical DOTMDAA is the sole exception,ordering at 1.6K [3]. We have previously shownthat the thiazyl radical p-NCC6F4CNSSN• ordersas a canted antiferromagnet at 36K underambient pressure [4], the highest reportedmagnetic ordering temperature for an organicradical. By replacing the CN group by Br aparamagnetic system possessing very weakantiferromagnetic interactions is obtained. [5]We have recently reported that if the CN groupis substituted by NO2 to give the p-O2N·C6F4CNSSN• radical, ferromagneticordering is observed below Tc = 1.3 K [6]. This isthe first S-based organic radical, entirely metal-free, to undergo ferromagnetic ordering and onlythe second organic radical to order as aferromagnet above 1K.These different magnetic behaviors are dueto the change in the molecular packing (see Fig.1 for the p-O2N·C6F4CNSSN•, and consequentlya substantial modification of the magneticinteraction pathways. Knowledge of the spin-density distribution in the dithiadiazolyl radicalring allows us a better understanding of the richmagnetism of DTDA derivatives and hencehelps in the design of new organic ferromagnetsbased on the DTDA ring with higher TC.

Figure 1. Crystal structure of p-NO2.C6F4CNSSN•

Here we report on the magnetism of the p-O2N·C6F4CNSSN• radical and on the study of itsspin-density distribution using polarized-neutrondiffraction and ab initio calculations techniques.Low temperature ac susceptibilitymeasurements on a single crystal of p-O2N·C6F4CNSSN• were made between 0.6 and2.5K on a SQUID magnetometer in a field of 2G.The crystal, when oriented both perpendicularand parallel to the [001] axis, showed an abruptincrease in the out-of-phase susceptibility below1.4 K, reaching a maximum at 1.3K, consistentwith the onset of bulk ferromagnetic order at Tc= 1.32±0.02K [Fig. 2]. Little anisotropy in M vs H or c vs T data was observed. The difference inthe data is attributed to demagnetising effectsarising from the sample geometry.

Figure 2. Single-crystal ac susceptibility of the p-O2N·C6F4CNSSN• radical in an applied field of 2 G andat 1.11 Hz.

The polarized-neutron diffraction experimentwas performed on the D3 lifting-counterdiffractometer at the ILL at 1.5 K in an appliedfield of 9 T. A multipolar model of the spindensity (Fig.3) was fitted to the measuredflipping ratios. Almost all the spin density islocalized in the nitrogen and sulfur p orbitalsperpendicular to the DTDA ring (0.281µB and0.247µB, respectively). These orbitals generatethe Single Occupied Molecular Orbital (SOMO).
Some negative density is observed between the sulfur and nitrogen atoms that can be due to covalence and polarization effects. In addition, a small negative density is observed (-0.055µB) on the carbon site of the DTDA ring due to polarization effects. The spin density over the rest of the radical is below the limits of the experimental accuracy. [7]

Figure 3. Spin-density reconstruction of the p- O 2 N·C 6 F 4 CNSSN• radical projected along the perpendicular to the CNSSN mean plane.

To support the experimental modeling, ab initio calculations were performed at the generalized gradient approximation level, using several exchange-correlation functionals (PW91, BLYP,VWM) with a double numerical basis set including functions and numerical integrations, as implemented in the Dmol3 software based on density functional theory. Both periodic (crystal) and molecular calculations were made.

The ab initio calculations (Fig 4 and Table I) confirm the experimental results and, in particular, they show that the negative spin density in the plane of the DTDA ring is not a spurious effect of the multipolar model used. Such a negative spin density plays an important role in the ferromagnetic behavior of the O 2 N·C 6 F 4 CNSSN• radical. The spin populations in the rest of the molecule were also obtained. In the carbon atoms negative and positive low spin density populations alternate due to polarization effects. Only a weak indication of the latter was observed experimentally, which emphasizes the importance of combining experiment and ab initio calculations in this kind of studies.

Acknowledgements

We would like to thank the E.P.S.R.C., the C.I.C.Y.T. (grant MAT2001-3507-C02-02) for financial support, and the European Science Foundation programme “Molecular Magnets” for partial support to the collaborative activities. One of us, JL, thanks the ILL for a PhD research fellowship.

BLYP

PW91

VWM

BLYP

This

µB

molec.

molec.

molec.

crystal

experim.

N2

0.226

0.220

0.225

0.214

0.247

S1

0.308

0.312

0.310

0.317

0.281

C5

0.065

-0.061

-0.062

-0.061

-0.055

C1

-0.003

-0.003

-0.003

-0.003

 

C2

0.001

0.001

0.001

0.002

 

C3

-0.004

-0.004

-0.004

-0.004

 

C4

0.005

0.002

0.002

0.004

 

F2

0.000

0.000

0.000

0.001

 

Table I. Ab initio and experimental spin-density populations in the p-O 2N·C 6F4 CNSSN• radical

Figure 4. Spin density distribution of the p- O 2 N·C 6 F 4 CNSSN• radical calculated with the Dmol3 package (BLYP functional, molecular calculation). The red surface is the contour of 0.0002 m B /Å 3 and the blue one is the contour of –0.0002 m B /Å 3 .

Principal publication

“A thiazyl-based organic ferromagnet”. A. Alberola,R.J. Less, C.M. Pask, J.M. Rawson, F. Palacio, P. Oliete, C. Paulsen, A. Yamaguchi and R.D. Farley. Angew. Chem. Int. Ed ., 42 , 4782-85 (2003) “Spin-Density distribution of the high Tc p -O 2 N· C 6 F 4 · CNSSN free radical studied by polarized neutron diffraction”. J. Luzón, J. Campo, F. Palacio, G.J. McIntyre, A.E. Goeta, C.M. Pask and J.M. Rawson, Polyhedron , 22 , 2301-03 (2003).

References

[1] W. Heisenberg, Z. Phys. 1928 , 49 , 619.

[2] H. Tamura, Chem. Phys. Lett. 1991 , 186 , 401.

[3] R. Chiarelli, M.A. Novak, A. Rassat, J.L. Tholance, Nature 1993 , 363 , 147.

[4] F. Palacio, G. Antorrena, M. Castro, R. Burriel,J.M. Rawson, J.N.B. Smith, N. Bricklebank, J. Novoa and C. Ritter, Phys. Rev. Lett . 79 , 2336-9 (1997).

[5] G. Antorrena, J.E. Davies, M. Hartley, F. Palacio,J.M. Rawson, J.N.B. Smith and A. Steiner. J. Chem. Soc., Chem. Commun . 1999 , 1393-4.

[6] A. Alberola, R.J. Less, C.M. Pask, J.M. Rawson,F. Palacio, P. Oliete, C. Paulsen, A. Yamaguchi and R.D. Farley. Angew. Chem. Int. Ed ., 42 , 4782­85 (2003)

[7] J. Luzón, J. Campo, F. Palacio, G.J. McIntyre,A.E. Goeta, C.M. Pask and J.M. Rawson, Polyhedron , 22 , 2301-03 (2003).

decorativo decorativo
decorativo
©2017 Instituto de Ciencia de Materiales de Aragón | Tfno: 976 761 231 - Fax: 976 762 453
©2017 Universidad de Zaragoza (Pedro Cerbuna 12, 50009 ZARAGOZA-ESPAÑA | Tfno. información: (34) 976-761000)
decorativo