experiments [15]. DFT-based microkinetic simulations are described in
Chap. 4 [16].
Finally, it has to be borne in mind that DFT calculations inform on the position of
all nuclei at the minima and transition states but do not directly inform on the
movement of the electrons which push the nuclei to change places while chemists
have used the arrow-pushing description to describe and predict change of structures
along a chemical transformation. In this regard, some of us [17] and other authors
[18] have recently devised a simple analysis, based on the movement of localized
orbitals along a reaction pathway, that allows to extract from computed energy
profiles an arrow-pushing description of the electron rearrangements taken place in
a reaction mechanism.
This introductory chapter is not intended to be a revision on DFT calculations of
reaction mechanisms. Several excellent reviews illustrate the relevance acquired by
computational methods to unravel reaction mechanism involving transition metal
systems [19–22]. The main goal of this chapter is to discuss factors influencing the
quality of computational studies on reaction mechanisms, illustrated by selected
examples. From the computational side the emphasis has been usually put on what is
called the level of theory, which is the technical aspects of the quantum mechanical
model (basically the choice of the functional and basis set), but as we will show
along this chapter this is not the only aspect to take into account when assessing
computational studies on reaction mechanisms. Recent articles have also addressed
the scope and challenges [23–25] and pitfalls [26] of computational methods for
studying mechanism and reactivity in homogeneous catalysis, evidencing the current
practical interest of these issues.
2 Reality and Models
What makes computational methods fundamentally different from the common
techniques employed in the organometallic lab is that calculations always deal
with models [27]. The chemical complexity inside the glass vessel where reaction
occurs should be reduced and translated to something that a computer can process. In
fact, as outlined in Fig. 2, any quantum mechanical calculation of a chemical
reaction system implies the choice of two different models: a chemical model and
a theoretical model. The chemical model is the chemical system selected to be
computed, in other words, the computer representation of the real chemical world,
and the theoretical model defines the level of theory to be employed and thus the
computational methods to be used. As there is a whole panoply of possibilities for
both, the chemical and the theoretical models, the big modelers’ dilemma is which
models to choose, as illustrated in Fig. 3.
When dealing with chemical models, some general considerations have to be
taken into account: (1) a model is a simple representation of a system that is used to
describe and simulate a more complex reality; (2) by definition, a model is always an
incomplete description of the real system (Fig. 3). Regarding theoretical models, it
What Makes a Good (Computed) Energy Profile?
5
Chap. 4 [16].
Finally, it has to be borne in mind that DFT calculations inform on the position of
all nuclei at the minima and transition states but do not directly inform on the
movement of the electrons which push the nuclei to change places while chemists
have used the arrow-pushing description to describe and predict change of structures
along a chemical transformation. In this regard, some of us [17] and other authors
[18] have recently devised a simple analysis, based on the movement of localized
orbitals along a reaction pathway, that allows to extract from computed energy
profiles an arrow-pushing description of the electron rearrangements taken place in
a reaction mechanism.
This introductory chapter is not intended to be a revision on DFT calculations of
reaction mechanisms. Several excellent reviews illustrate the relevance acquired by
computational methods to unravel reaction mechanism involving transition metal
systems [19–22]. The main goal of this chapter is to discuss factors influencing the
quality of computational studies on reaction mechanisms, illustrated by selected
examples. From the computational side the emphasis has been usually put on what is
called the level of theory, which is the technical aspects of the quantum mechanical
model (basically the choice of the functional and basis set), but as we will show
along this chapter this is not the only aspect to take into account when assessing
computational studies on reaction mechanisms. Recent articles have also addressed
the scope and challenges [23–25] and pitfalls [26] of computational methods for
studying mechanism and reactivity in homogeneous catalysis, evidencing the current
practical interest of these issues.
2 Reality and Models
What makes computational methods fundamentally different from the common
techniques employed in the organometallic lab is that calculations always deal
with models [27]. The chemical complexity inside the glass vessel where reaction
occurs should be reduced and translated to something that a computer can process. In
fact, as outlined in Fig. 2, any quantum mechanical calculation of a chemical
reaction system implies the choice of two different models: a chemical model and
a theoretical model. The chemical model is the chemical system selected to be
computed, in other words, the computer representation of the real chemical world,
and the theoretical model defines the level of theory to be employed and thus the
computational methods to be used. As there is a whole panoply of possibilities for
both, the chemical and the theoretical models, the big modelers’ dilemma is which
models to choose, as illustrated in Fig. 3.
When dealing with chemical models, some general considerations have to be
taken into account: (1) a model is a simple representation of a system that is used to
describe and simulate a more complex reality; (2) by definition, a model is always an
incomplete description of the real system (Fig. 3). Regarding theoretical models, it
What Makes a Good (Computed) Energy Profile?
5