196
V. Schünemann
to the difference of the isomer shifts of the two species present in the sample. The
simulation yields |δ HS –δ LS | = 0.7 ± 0.15 mms
−1 which is in line with the isomer
shifts reported for 6 N coordinated HS iron(II) centers (0.9–1.05 mms
−1 ) and for
6 N coordinated LS iron(II) centers (0.3–0.4 mms
−1 ) reported at ~300 K. In order
to drive the system into the full HS state, the temperature needs to be increased even
more. At 400 K the low Lamb-Mößbauer factor of chemical complexes in general
leads to an almost vanishing NFS-Signal (4.14c). In terms of conventional Mössbauer
spectroscopy that means that the signal disappears in the experimental noise of the
background signal.
However, this is not the case for the NIS signal, which gives direct access to the
iron-ligand modes. During the transition from a t 2g
6 electron configuration in the low
spin state to a t 2g
4 e g
2 electron configuration in the excited high spin state not only
the spin state is changed, but also a significant increase of the metal-ligand bond
length by ~0.2 Å occurs. This is caused by the occupation of the σ-antibonding e g
*
d-orbitals which leads to a reduced bond strength in comparison to the t 2g
6 LS state.
This reduced bond strength can be ideally detected e.g. by vibrational spectroscopy
techniques like NIS.
At T = 80 K the pDOS of the complex exhibits a complex band structure in the
range of 400–550 cm
−1 typical for iron(II) LS iron stretching and bending modes.
These so called LS marker bands are also visible at 250 K, but now in addition features
occur in the pDOS ranging from 180 to 320 cm
−1 reflecting HS marker bands. At
an even higher temperature of 400 K, the HS marker bands dominate the pDOS.
This indicates that SCO has occurred to almost but not exact 100%, since residual
LS marker bands are still visible. In order to learn something about the character
of the iron-ligand modes, the experimental pDOS can be simulated by DFT-based
normal mode analysis as mentioned in Sect. 2.5. The simulation of the pDOS of the
molecule in its LS state performed using the software package Gaussian 09 is shown
in Fig. 4.14 g–i. The height of the block bars is proportional to the mean square
displacement of the iron in a particular mode. The position of every bar represents
the energy of a particular normal mode of the molecule expressed in wavenumber
units. In this way the whole vibrational features of the molecule can be addressed. Of
course, this holds only for the assumption of harmonic potentials, when anharmonic
effects have to be taken into account life becomes more difficult.
4.4 Electronic and Vibrational Properties of a Heme
Protein: The NO Transporter Protein Nitrophorin
Proteins containing heme are among the most well-known proteins. The oxygen (O 2 )
carrier protein hemoglobin occurring in the blood of all higher organisms contains
4 subunits each having one heme unit. The heme unit contains an iron(II) center
which serves to bind O 2 and transports this molecule to the cells of the organisms.
In recent decades it has become known that also other small molecules like nitric
V. Schünemann
to the difference of the isomer shifts of the two species present in the sample. The
simulation yields |δ HS –δ LS | = 0.7 ± 0.15 mms
−1 which is in line with the isomer
shifts reported for 6 N coordinated HS iron(II) centers (0.9–1.05 mms
−1 ) and for
6 N coordinated LS iron(II) centers (0.3–0.4 mms
−1 ) reported at ~300 K. In order
to drive the system into the full HS state, the temperature needs to be increased even
more. At 400 K the low Lamb-Mößbauer factor of chemical complexes in general
leads to an almost vanishing NFS-Signal (4.14c). In terms of conventional Mössbauer
spectroscopy that means that the signal disappears in the experimental noise of the
background signal.
However, this is not the case for the NIS signal, which gives direct access to the
iron-ligand modes. During the transition from a t 2g
6 electron configuration in the low
spin state to a t 2g
4 e g
2 electron configuration in the excited high spin state not only
the spin state is changed, but also a significant increase of the metal-ligand bond
length by ~0.2 Å occurs. This is caused by the occupation of the σ-antibonding e g
*
d-orbitals which leads to a reduced bond strength in comparison to the t 2g
6 LS state.
This reduced bond strength can be ideally detected e.g. by vibrational spectroscopy
techniques like NIS.
At T = 80 K the pDOS of the complex exhibits a complex band structure in the
range of 400–550 cm
−1 typical for iron(II) LS iron stretching and bending modes.
These so called LS marker bands are also visible at 250 K, but now in addition features
occur in the pDOS ranging from 180 to 320 cm
−1 reflecting HS marker bands. At
an even higher temperature of 400 K, the HS marker bands dominate the pDOS.
This indicates that SCO has occurred to almost but not exact 100%, since residual
LS marker bands are still visible. In order to learn something about the character
of the iron-ligand modes, the experimental pDOS can be simulated by DFT-based
normal mode analysis as mentioned in Sect. 2.5. The simulation of the pDOS of the
molecule in its LS state performed using the software package Gaussian 09 is shown
in Fig. 4.14 g–i. The height of the block bars is proportional to the mean square
displacement of the iron in a particular mode. The position of every bar represents
the energy of a particular normal mode of the molecule expressed in wavenumber
units. In this way the whole vibrational features of the molecule can be addressed. Of
course, this holds only for the assumption of harmonic potentials, when anharmonic
effects have to be taken into account life becomes more difficult.
4.4 Electronic and Vibrational Properties of a Heme
Protein: The NO Transporter Protein Nitrophorin
Proteins containing heme are among the most well-known proteins. The oxygen (O 2 )
carrier protein hemoglobin occurring in the blood of all higher organisms contains
4 subunits each having one heme unit. The heme unit contains an iron(II) center
which serves to bind O 2 and transports this molecule to the cells of the organisms.
In recent decades it has become known that also other small molecules like nitric