200
M. Lundberg and M. G. Delcey
hole rotates out of the target core orbitals, these can be frozen during the orbital
optimization using supsym. In the RASPT2 program, both imaginary and IPEA shifts
can be specified, with default values being 0.1 and 0.25 eV, respectively. Spin–orbit
coupled states are formed in a RAS state interaction (SI) algorithm using the spin
keyword in the RASSI program. RASSI also calculates transition matrix elements
between all computed states. The default is the electric dipole approximation but a
complete second-order expansion as well as the exact form of the wavevector are
also implemented [81, 82]. With most practical aspects of the modeling covered, the
next sections will describe applications of X-ray spectroscopy in different fields of
chemistry.
4 Electronic Structure from X-ray Spectra
In this section, we will describe how the combination of X-ray experiments and multiconfigurational modeling can be used to extract very detailed information about
electronic structure. The first examples show how modeling can be used to characterize the total spin and oxidation state of a complex. The next examples focus on
the structure of individual orbitals involved in metal–ligand binding, often through
studies of charge-transfer and ligand-field transitions. The examples include both
ground-state electronic structures and time-resolved studies of transient reaction
intermediates at the femtosecond timescale. The final examples describe the electronic structure at even finer detail by looking into splittings between states with the
same formal orbital occupation, in the X-ray field usually called multiplet splittings.
These examples illustrate how multiconfigurational methods give a correct description of these different states, and that this can be used to extract detailed orbital
information.
4.1 Spin and Oxidation State
Reaction mechanisms of redox reactions can at the most basic level be described in
terms of changes of the spin and oxidation states of the metal. X-ray spectroscopy is
ideal to observe these effects because the core hole is a very local probe of the metal
site as shown in Fig. 2. Multiconfigurational calculations accurately predict oxidation
state dependent spectral changes, as has been shown for ferrous (3d
6 ) and ferric (3d
5 )
complexes [10, 32], as well as a series of photocatalytically relevant Mn systems [11].
The predictive power of the simulations makes it possible to identify problems in
the experimental data, which is important as many samples easily photodamage in
intense X-ray beams [50].
In addition to fingerprinting reaction intermediates, calculations can also be used
to understand how oxidation state is reflected in the L-edge XAS spectrum because
both redox and core-excitation processes can be treated at an equal level. One of the
M. Lundberg and M. G. Delcey
hole rotates out of the target core orbitals, these can be frozen during the orbital
optimization using supsym. In the RASPT2 program, both imaginary and IPEA shifts
can be specified, with default values being 0.1 and 0.25 eV, respectively. Spin–orbit
coupled states are formed in a RAS state interaction (SI) algorithm using the spin
keyword in the RASSI program. RASSI also calculates transition matrix elements
between all computed states. The default is the electric dipole approximation but a
complete second-order expansion as well as the exact form of the wavevector are
also implemented [81, 82]. With most practical aspects of the modeling covered, the
next sections will describe applications of X-ray spectroscopy in different fields of
chemistry.
4 Electronic Structure from X-ray Spectra
In this section, we will describe how the combination of X-ray experiments and multiconfigurational modeling can be used to extract very detailed information about
electronic structure. The first examples show how modeling can be used to characterize the total spin and oxidation state of a complex. The next examples focus on
the structure of individual orbitals involved in metal–ligand binding, often through
studies of charge-transfer and ligand-field transitions. The examples include both
ground-state electronic structures and time-resolved studies of transient reaction
intermediates at the femtosecond timescale. The final examples describe the electronic structure at even finer detail by looking into splittings between states with the
same formal orbital occupation, in the X-ray field usually called multiplet splittings.
These examples illustrate how multiconfigurational methods give a correct description of these different states, and that this can be used to extract detailed orbital
information.
4.1 Spin and Oxidation State
Reaction mechanisms of redox reactions can at the most basic level be described in
terms of changes of the spin and oxidation states of the metal. X-ray spectroscopy is
ideal to observe these effects because the core hole is a very local probe of the metal
site as shown in Fig. 2. Multiconfigurational calculations accurately predict oxidation
state dependent spectral changes, as has been shown for ferrous (3d
6 ) and ferric (3d
5 )
complexes [10, 32], as well as a series of photocatalytically relevant Mn systems [11].
The predictive power of the simulations makes it possible to identify problems in
the experimental data, which is important as many samples easily photodamage in
intense X-ray beams [50].
In addition to fingerprinting reaction intermediates, calculations can also be used
to understand how oxidation state is reflected in the L-edge XAS spectrum because
both redox and core-excitation processes can be treated at an equal level. One of the