The high symmetry of the MoS 2 structure yields some interesting multiplescattering features. For example, in the amplified Fourier transform there is a
significant feature at 6.18 Å. This arises from multiple scattering along the Mo–
Mo–Mo pathway (Fig. 6.18); the multiple scattering yields additional phase shifts
that shorten the apparent distance from the true value of 6.32 Å. Additional small
multiple-scattering features are also evident, and there is even a small peak at 9.30 Å
that corresponds to a Mo–Mo–Mo–Mo pathway. Notice that the multiple-scattering
peaks are progressively shorter than the true distances, because of the additional
phase shifts.
6.8.3 Nitrogenase
A more complicated example involves nitrogenase (N 2 ase)—an enzyme that catalyzes reduction of dinitrogen to ammonia, along with ATP hydrolysis and H 2
evolution [247–249]:
N 2 þ 8H
þ
þ 8e
À
þ 16MgATP ! 2NH 3 þ H 2 þ 16MgADP þ 16P i
The Mo version of N 2 ase contains an MoFe 7 S 9 cluster knows as the FeMo
cofactor (Fig. 6.19). There is a strong beat in the k-space EXAFS, in this case caused
by Mo–S and Mo–Fe components of comparable intensity. The oblong nature of the
cluster is revealed by a modest long distance Mo-Fe peak at 5.0 Å. Although this
feature is in the earliest EXAFS data [250], unfortunately it was only understood in
light of the crystal structure.
Additional complexity is concealed in the twin peak Fourier transform. The peak
at 2.31 Å turns out to be the combination of the Mo–S shell at 2.36 Å and shorter
Fig. 6.18 Left: key interatomic distances in MoS 2 . Middle: k-space EXAFS for MoS 2 . Note the
strong beat pattern arising from Mo–S and Mo–Mo components of comparable intensity. Right:
EXAFS Fourier transform for MoS 2 , k ¼ 2–22 Å
À1
, 2 Å
À1 damping, with incorporated Mo–S phase
shift
6.8 Some Examples
159
significant feature at 6.18 Å. This arises from multiple scattering along the Mo–
Mo–Mo pathway (Fig. 6.18); the multiple scattering yields additional phase shifts
that shorten the apparent distance from the true value of 6.32 Å. Additional small
multiple-scattering features are also evident, and there is even a small peak at 9.30 Å
that corresponds to a Mo–Mo–Mo–Mo pathway. Notice that the multiple-scattering
peaks are progressively shorter than the true distances, because of the additional
phase shifts.
6.8.3 Nitrogenase
A more complicated example involves nitrogenase (N 2 ase)—an enzyme that catalyzes reduction of dinitrogen to ammonia, along with ATP hydrolysis and H 2
evolution [247–249]:
N 2 þ 8H
þ
þ 8e
À
þ 16MgATP ! 2NH 3 þ H 2 þ 16MgADP þ 16P i
The Mo version of N 2 ase contains an MoFe 7 S 9 cluster knows as the FeMo
cofactor (Fig. 6.19). There is a strong beat in the k-space EXAFS, in this case caused
by Mo–S and Mo–Fe components of comparable intensity. The oblong nature of the
cluster is revealed by a modest long distance Mo-Fe peak at 5.0 Å. Although this
feature is in the earliest EXAFS data [250], unfortunately it was only understood in
light of the crystal structure.
Additional complexity is concealed in the twin peak Fourier transform. The peak
at 2.31 Å turns out to be the combination of the Mo–S shell at 2.36 Å and shorter
Fig. 6.18 Left: key interatomic distances in MoS 2 . Middle: k-space EXAFS for MoS 2 . Note the
strong beat pattern arising from Mo–S and Mo–Mo components of comparable intensity. Right:
EXAFS Fourier transform for MoS 2 , k ¼ 2–22 Å
À1
, 2 Å
À1 damping, with incorporated Mo–S phase
shift
6.8 Some Examples
159