Preface
In the frame of the Born–Oppenheimer approximation, the equilibrium structure of
a molecule corresponds to the minimum of the potential hypersurface. Thus, it has a
clear physical definition, and it is the only structure permitting the comparison
between molecules. This is quite important because the molecular geometry is
required to explain molecular behavior (reactivity, electric and magnetic properties,
etc.). As described in the introduction, it was possible since the beginning of the
twentieth century to determine the equilibrium structure of a molecule. However, it
was a difficult and time-consuming task. Furthermore, it was limited to very small
molecules (typically up to three independent parameters; examples are OCS and
SO 2 ), and the result was not always accurate (a nice illustration is HCO
+ discussed
in Chap. 6).
For these reasons, empirical methods trying to approximate the equilibrium
structure have been developed. At first sight, they are appealing and they are still
extensively employed. However, although they often give satisfactory results for
very small molecules, they rapidly become unreliable for medium-sized molecules.
Starting from the end of the seventies, a lot of progress has been made. First,
from the experimental point of view, the introduction of Fourier transform technique and computer-controlled experiments permitted to record spectra much faster
and with a better sensitivity, in particular making possible the measurement of some
important isotopologues (
13 C) in natural abundance.
But the most important advancement was the considerable increase of power
of the computers, and the development of ab initio methods permitting since the
nineties to optimize a molecular geometry with an accuracy sometimes better than
the experimental one (typically, a few thousandth of an Å). Another essential
application of ab initio methods is the calculation of anharmonic force fields, which
allow us to correct experimental internuclear distances determined by gas-phase
electron diffraction and/or high-resolution spectroscopy and which is the basis
of the semiexperimental procedure. This last technique is one of the most powerful
methods to determine an accurate equilibrium structure. Coupled with the mixed
regression, it has recently enabled the determination of accurate equilibrium
ix
In the frame of the Born–Oppenheimer approximation, the equilibrium structure of
a molecule corresponds to the minimum of the potential hypersurface. Thus, it has a
clear physical definition, and it is the only structure permitting the comparison
between molecules. This is quite important because the molecular geometry is
required to explain molecular behavior (reactivity, electric and magnetic properties,
etc.). As described in the introduction, it was possible since the beginning of the
twentieth century to determine the equilibrium structure of a molecule. However, it
was a difficult and time-consuming task. Furthermore, it was limited to very small
molecules (typically up to three independent parameters; examples are OCS and
SO 2 ), and the result was not always accurate (a nice illustration is HCO
+ discussed
in Chap. 6).
For these reasons, empirical methods trying to approximate the equilibrium
structure have been developed. At first sight, they are appealing and they are still
extensively employed. However, although they often give satisfactory results for
very small molecules, they rapidly become unreliable for medium-sized molecules.
Starting from the end of the seventies, a lot of progress has been made. First,
from the experimental point of view, the introduction of Fourier transform technique and computer-controlled experiments permitted to record spectra much faster
and with a better sensitivity, in particular making possible the measurement of some
important isotopologues (
13 C) in natural abundance.
But the most important advancement was the considerable increase of power
of the computers, and the development of ab initio methods permitting since the
nineties to optimize a molecular geometry with an accuracy sometimes better than
the experimental one (typically, a few thousandth of an Å). Another essential
application of ab initio methods is the calculation of anharmonic force fields, which
allow us to correct experimental internuclear distances determined by gas-phase
electron diffraction and/or high-resolution spectroscopy and which is the basis
of the semiexperimental procedure. This last technique is one of the most powerful
methods to determine an accurate equilibrium structure. Coupled with the mixed
regression, it has recently enabled the determination of accurate equilibrium
ix