The Electronic Determinants of Spin
Crossover Described by Density
Functional Theory
Kasper Planeta Kepp
Abstract Spin crossover (SCO) plays a vital role in living systems and in many
emerging technologies, and the accurate prediction and design of SCO systems is of
high current priority. Density functional theory (DFT) is the state-of-the-art tool for
this purpose due to its ability to describe large molecular electronic systems with an
accuracy that can be predictive if carried out correctly. However, the SCO tendency,
i.e., the free-energy balance of high- and low-spin states, is extremely sensitive to
the theoretical description and physical effects such as dispersion, relativistic effects,
and vibrational entropy. This chapter summarizes the recent fundamental insight into
SCO gained from DFT and efforts that approach the accuracy needed (~10 kJ/mol)
for rational design of SCO to become reality.
1 Introduction
Spin crossover (SCO) is the process, whereby two electronic states of different
quantum-mechanical spins interconvert upon perturbation such as chemical bonding, temperature, light, or applied pressure [1–9]. Since its discovery in coordination
complexes [10], it has emerged as one of the most important chemical processes
[1–3, 11]. If this importance is not immediately appreciated, please consider that
without SCO, the reader would suffocate within 2–3 min due to the absence of O 2
binding to hemoglobin within the honorable reader’s lung arteria.
Some chemical systems can undergo transition between the two spin states without
any change of chemical composition, whereas others only do so upon interaction
with other molecules, such as ligand binding to a metal. The term “SCO system”
or “SCO complex” is mainly used if the transition occurs relatively near standard
conditions of temperature and pressure. Thus, thermal SCO systems are particularly
notable as those where SCO occurs close to atmospheric pressure and within the
range of room temperature by a few hundred Kelvin. Such systems are central to life
K. P. Kepp (B)
DTU Chemistry, Technical University of Denmark, 2800 Kongens Lyngby, Denmark
e-mail: kpj@kemi.dtu.dk
© Springer Nature Switzerland AG 2019
E. Broclawik et al. (eds.), Transition Metals in Coordination Environments,
Challenges and Advances in Computational Chemistry and Physics 29,
https://doi.org/10.1007/978-3-030-11714-6_1
1
Crossover Described by Density
Functional Theory
Kasper Planeta Kepp
Abstract Spin crossover (SCO) plays a vital role in living systems and in many
emerging technologies, and the accurate prediction and design of SCO systems is of
high current priority. Density functional theory (DFT) is the state-of-the-art tool for
this purpose due to its ability to describe large molecular electronic systems with an
accuracy that can be predictive if carried out correctly. However, the SCO tendency,
i.e., the free-energy balance of high- and low-spin states, is extremely sensitive to
the theoretical description and physical effects such as dispersion, relativistic effects,
and vibrational entropy. This chapter summarizes the recent fundamental insight into
SCO gained from DFT and efforts that approach the accuracy needed (~10 kJ/mol)
for rational design of SCO to become reality.
1 Introduction
Spin crossover (SCO) is the process, whereby two electronic states of different
quantum-mechanical spins interconvert upon perturbation such as chemical bonding, temperature, light, or applied pressure [1–9]. Since its discovery in coordination
complexes [10], it has emerged as one of the most important chemical processes
[1–3, 11]. If this importance is not immediately appreciated, please consider that
without SCO, the reader would suffocate within 2–3 min due to the absence of O 2
binding to hemoglobin within the honorable reader’s lung arteria.
Some chemical systems can undergo transition between the two spin states without
any change of chemical composition, whereas others only do so upon interaction
with other molecules, such as ligand binding to a metal. The term “SCO system”
or “SCO complex” is mainly used if the transition occurs relatively near standard
conditions of temperature and pressure. Thus, thermal SCO systems are particularly
notable as those where SCO occurs close to atmospheric pressure and within the
range of room temperature by a few hundred Kelvin. Such systems are central to life
K. P. Kepp (B)
DTU Chemistry, Technical University of Denmark, 2800 Kongens Lyngby, Denmark
e-mail: kpj@kemi.dtu.dk
© Springer Nature Switzerland AG 2019
E. Broclawik et al. (eds.), Transition Metals in Coordination Environments,
Challenges and Advances in Computational Chemistry and Physics 29,
https://doi.org/10.1007/978-3-030-11714-6_1
1