“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

Nuclear inelastic scattering on spin crossover complexes

H. Paulsen and A.X. Trautwein

Institut für PhysikUniversität zu Lübeck

Ratzeburger Allee 160, D-23538 Lübeck, GermanyE-mail: paulsen@physik.uni-luebeck.de

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

NIS experiments on SCO compounds

Iron containing spin crossover (SCO) compounds (see for instance the contribution of P. Gütlich in this volume) have been investigated since long by conventional Mössbauer spectroscopy and by vibrational spectroscopies like IR or Raman. Those molecular vibrations that are connected with the iron center exhibit a considerable change of frequency upon spin crossover. For this reason such complexes are well suited for investigations with nuclear inelastic scattering (NIS) of synchrotron radiation, a vibrational spectroscopy that is based on the Mössbauer effect.

In the following slides three examples are shown for [Fe(tpa)(NCS)2], [Fe(tptMetame)](ClO4)2 and [Fe(phen)2(NCS)2]. Together with the experimental NIS spectra simulated curves obtained by density functional theory (DFT) calculations are presented. In the case of [Fe(phen)2(NCS)2] the complete spectrum of frequencies and normal modes of molecular vibrations could be assigned by a combination of NIS, IR, Raman experiments and DFT calculations.

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

Scheme of spin crossover

Taken from: Gütlich P, Hauser A,Spiering H (1994) Angew ChemInt Ed Engl 33: 2024

LS (S=0)

HS (S=2)

The driving force for a temperature dependent spin crossover is the increase of entropy in the high spin (HS) state, that is mostly due to a softening of the iron-ligand bond stretching modes. This softening can be observed very well by NIS.

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

U n d u la to r

P re m o n o c h ro m a to r

D e te c to r (A P D )

D e te c to r (A P D )

S a m p le

H ig h re s o lu t io nm o n o c h ro m a to r

S i(4 2 2 )

S i(12 2 2 )S i(1 1 1 )

Simple scheme of NIS

ESRF, GrenobleTypical NIS spectrum

NFS spectrum (see textbook)

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

NIS spectrum of molecular crystals

Paulsen H, Winkler H, Trautwein AX, Grünsteudel H, Rusanov V, Toftlund H (1999) Phys Rev B 59: 975

L

lllEEkEeES

1

22 )()()ˆ,(2

ukkuk

Approximations: low temperature, isolated molecule

Absorption probability mean square displacement

k: wavevector of the X-rays

E: energy of the X-rays (after subtraction of the resonance energy)

El: energy of the lth normal mode

ul: displacement vector of the iron nucleus

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

Paulsen H, Benda R, Herta C, Schünemann V, Chumakov AI, Duelund L, Winkler H, Toftlund H, Trautwein AX (2001) Phys Rev Lett 86: 1351

[Fe(tptMetame)](ClO4)2

tptMetame = 1,1,1-tris{[N-(2-pyridylmethyl)-N-methyl-amino]-methyl}ethane)

NIS spectra of a single crystal (T=30 K)of Fe(tptMetame)

(click on the picture to start the animation)

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

Paulsen H, Benda R, Herta C, Schünemann V, Chumakov AI, Duelund L, Winkler H, Toftlund H, Trautwein AX (2001) Phys Rev Lett 86: 1351

NIS spectra of a single crystal (T=30 K)of Fe(tptMetame)

(click on the picture to start the animation)

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

Paulsen H, Benda R, Herta C, Schünemann V, Chumakov AI, Duelund L, Winkler H, Toftlund H, Trautwein AX (2001) Phys Rev Lett 86: 1351

NIS spectra of a single crystal of Fe(tptMetame) (room temperature)

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

Complete assignment of vibrational modes for [Fe(phen)2(NCS)2]

E (cm-1)

Abs

orpt

ion

prob

abil

ity

200 400

Ronayne KL, Paulsen H, Höfer A, Dennis AC, Wolny JA, Chumakov AI, Schünemann V,Winkler H, Spiering H, Bousseksou A,Gütlich P, Trautwein AX, McGarvey JJ (2006) Phys Chem Chem Phys 8: 4685-4693

Measured (symbols) and simulated (solid lines) NIS spectra at 30 K (blue curves) and at room temperature (red curves).

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

Spectral distribution of entropy difference

0 10 20 30 40

0

10

20

30

40

0

1

2

3

4

b

bb

b

b bs

s

s

s

s

b

bb

Svi

b(i)

(J K

-1 m

ol-1)

s

(breathing mode)

i 1 S

vib(i

) (J

K-1 m

ol-1)

Mode number i

Ronayne KL, Paulsen H, Höfer A, Dennis AC, Wolny JA, Chumakov AI, Schünemann V,Winkler H, Spiering H, Bousseksou A, Gütlich P, Trautwein AX, McGarvey JJ (2006) Phys Chem Chem Phys 8: 4685-4693

Only the first 30 vibrations out of 147 contribute to the vibrational entropy difference.

Also low frequency modes below 10 meV contribute significantly.

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

NIS on SCO compounds - summary

In the NIS spectra of iron-containing spin crossover compounds one can observe predominant peaks that are shifted to lower energies upon crossover from the LS to the HS state. Comparison of the measured spectra with simulated spectra obtained by density functional calculations reveals that these peaks can be attributed to iron-ligand stretching modes.

In this way NIS allows a direct view on the driving force of the temperature dependent spin crossover.