The Electronic Determinants of Spin Crossover Described …
25
Many of the catalytic processes that are of fundamental and technological interest
involve changes in the spin states. The widely used RPBE functional [168] has a
substantially better spin-state balance [94] than the original PBE functional [140]
from which it was made, because it is less biased toward LS; this tendency as usual
agrees well with the reduced over-binding tendency of RPBE, since HS bias and
under-binding goes together [28].
5 Conclusions
This chapter has outlined the basic machinery of thermal SCO in single molecules
with a particular focus on the achievements in modeling this process using DFT.
The contribution of dispersion forces already in a single molecule undergoing SCO
has been discussed; the generic dispersion effect on SCO arising from the first coordination sphere expansion favors the LS state by typical 10 kJ/mol; this generic
dispersion term is supplemented by additional contributions from the type and bulkiness of the ligands that can favor either LS or HS. There is also a generic relativistic
SCO effect with a surprisingly large contribution to the spin-state balance, typically
also 10 kJ/mol in favor of LS but much less variable because the relativistic contribution is dominated by the metal ion and not the different ligands of SCO systems.
Considering that typical SCO energies are of the order of 10–20 kJ/mol in favor
of LS, these two effects combined favor LS more than the total energy gap. Thus,
any conclusion on the accuracy of a theoretical method compared to the “observed
ground-state spin” or the experimental enthalpy of SCO should consider these terms.
Put another way, a functional without these two terms that gives 20 kJ/mol too much
HS compared to experimental H SCO is excellent. If the calculation is compared to
observed ground states, which reflect free energies, then one cannot ignore vibrational entropy, which largely determines the spin transition and SCO process. This
entropy can be estimated decently but not very precisely from frequency analysis
using standard quantum-chemical programs and is less sensitive to DFT functional
used but more sensitive to the intermolecular interactions and explicit solvent effects
not generally accounted for in current theoretical models of DFT. Once we master the
ability to predict quantitatively the single-molecule SCO energetics, which requires
an accuracy of about 10 kJ/mol, we can hope to continue to these challenges of real
systems in condense states in the near future.
References
1. Halcrow MA (2013) Spin-crossover materials: properties and applications. Wiley
2. Gütlich P, Goodwin HA (2004) Spin crossover—an overall perspective. In: Spin crossover in
transition metal compounds I. Springer, pp 1–47
3. Létard J-F, Guionneau P, Goux-Capes L (2004) Towards spin crossover applications. Spin
Crossover Transit Met Compd III 1–19
25
Many of the catalytic processes that are of fundamental and technological interest
involve changes in the spin states. The widely used RPBE functional [168] has a
substantially better spin-state balance [94] than the original PBE functional [140]
from which it was made, because it is less biased toward LS; this tendency as usual
agrees well with the reduced over-binding tendency of RPBE, since HS bias and
under-binding goes together [28].
5 Conclusions
This chapter has outlined the basic machinery of thermal SCO in single molecules
with a particular focus on the achievements in modeling this process using DFT.
The contribution of dispersion forces already in a single molecule undergoing SCO
has been discussed; the generic dispersion effect on SCO arising from the first coordination sphere expansion favors the LS state by typical 10 kJ/mol; this generic
dispersion term is supplemented by additional contributions from the type and bulkiness of the ligands that can favor either LS or HS. There is also a generic relativistic
SCO effect with a surprisingly large contribution to the spin-state balance, typically
also 10 kJ/mol in favor of LS but much less variable because the relativistic contribution is dominated by the metal ion and not the different ligands of SCO systems.
Considering that typical SCO energies are of the order of 10–20 kJ/mol in favor
of LS, these two effects combined favor LS more than the total energy gap. Thus,
any conclusion on the accuracy of a theoretical method compared to the “observed
ground-state spin” or the experimental enthalpy of SCO should consider these terms.
Put another way, a functional without these two terms that gives 20 kJ/mol too much
HS compared to experimental H SCO is excellent. If the calculation is compared to
observed ground states, which reflect free energies, then one cannot ignore vibrational entropy, which largely determines the spin transition and SCO process. This
entropy can be estimated decently but not very precisely from frequency analysis
using standard quantum-chemical programs and is less sensitive to DFT functional
used but more sensitive to the intermolecular interactions and explicit solvent effects
not generally accounted for in current theoretical models of DFT. Once we master the
ability to predict quantitatively the single-molecule SCO energetics, which requires
an accuracy of about 10 kJ/mol, we can hope to continue to these challenges of real
systems in condense states in the near future.
References
1. Halcrow MA (2013) Spin-crossover materials: properties and applications. Wiley
2. Gütlich P, Goodwin HA (2004) Spin crossover—an overall perspective. In: Spin crossover in
transition metal compounds I. Springer, pp 1–47
3. Létard J-F, Guionneau P, Goux-Capes L (2004) Towards spin crossover applications. Spin
Crossover Transit Met Compd III 1–19