Chapter 8
Models of Chemical Bonding
and “Empirical” Methods
Abstract This chapter defines the different atomic radii and their use to predict a
bond length. The valence-shell electron-pair repulsion (VSEPR) model and the ligand
close-packing (LCP) model are reviewed. Finally, different empirical correlations are
reported.
8.1 Introduction
It is possible to obtain very accurate structures either by ab initio optimization or
with the help of experimental methods such as spectroscopy or gas-phase electron
diffraction. However, these methods (described in this book) are extremely time
consuming (up to a few months for an ab initio optimization and up to a few years
by the spectroscopic methods including the synthesis of the isotopologues) and,
furthermore, limited to small molecules (perhaps one hundred atoms for the simplest
ab initio methods and still less for spectroscopy: about 30 atoms). It would be useful
to have methods allowing us to quickly predict an approximate structure (see also
molecular mechanics, Sect. 2.16).
Another advantage is that it would allow us to have a quick estimate before
starting time-consuming ab initio calculations. The first method described in this
chapter is a very rough way to estimate the order of magnitude of the bond lengths.
In the following section, the valence-shell electron-pair repulsion (VSEPR) model
is discussed. It permits making a qualitative prediction of the bond angles. Then, the
ligand close-packing (LCP) model is introduced (a ligand is an atom or a functional
group that donates at least one of its electrons through a covalent bond to one central
atom). It is more recent than the VSEPR model and can be considered as its extension.
Finally, in the last section, some useful empirical correlations are presented.
© Springer Nature Switzerland AG 2020
J. Demaison and N. Vogt, Accurate Structure Determination of Free
Molecules, Lecture Notes in Chemistry 105,
https://doi.org/10.1007/978-3-030-60492-9_8
205
Models of Chemical Bonding
and “Empirical” Methods
Abstract This chapter defines the different atomic radii and their use to predict a
bond length. The valence-shell electron-pair repulsion (VSEPR) model and the ligand
close-packing (LCP) model are reviewed. Finally, different empirical correlations are
reported.
8.1 Introduction
It is possible to obtain very accurate structures either by ab initio optimization or
with the help of experimental methods such as spectroscopy or gas-phase electron
diffraction. However, these methods (described in this book) are extremely time
consuming (up to a few months for an ab initio optimization and up to a few years
by the spectroscopic methods including the synthesis of the isotopologues) and,
furthermore, limited to small molecules (perhaps one hundred atoms for the simplest
ab initio methods and still less for spectroscopy: about 30 atoms). It would be useful
to have methods allowing us to quickly predict an approximate structure (see also
molecular mechanics, Sect. 2.16).
Another advantage is that it would allow us to have a quick estimate before
starting time-consuming ab initio calculations. The first method described in this
chapter is a very rough way to estimate the order of magnitude of the bond lengths.
In the following section, the valence-shell electron-pair repulsion (VSEPR) model
is discussed. It permits making a qualitative prediction of the bond angles. Then, the
ligand close-packing (LCP) model is introduced (a ligand is an atom or a functional
group that donates at least one of its electrons through a covalent bond to one central
atom). It is more recent than the VSEPR model and can be considered as its extension.
Finally, in the last section, some useful empirical correlations are presented.
© Springer Nature Switzerland AG 2020
J. Demaison and N. Vogt, Accurate Structure Determination of Free
Molecules, Lecture Notes in Chemistry 105,
https://doi.org/10.1007/978-3-030-60492-9_8
205