strongly contributed to the success of vibrational spectroscopy in the present days.
One has nowadays available experimental data of superior quality, and their
interpretation can be ground in solid theoretical foundations, facilitated by the
comparison of experimental and calculated spectra, and helped by use of modern,
effective data post-processing methods, including multivariate statistical-based
procedures.
If per se vibrational spectroscopy is already an authoritative technique for
structure and reactivity evaluation, when used simultaneously with other experimental methods it strongly increases its predictive and interpretative power. Among
these, X-ray diffraction (XRD), allowing access to details of the structure of
crystalline materials and thermodynamic methods (such as differential scanning
calorimetry (DSC)), providing information on phase transition properties, deserve
here to be mentioned. Vibrational spectroscopy can use structural information
obtained from XRD and extract detailed information on several important properties of the crystalline materials not easy accessible (or not accessible at all) to X-ray
diffraction methods, in particular those that are related with hydrogen atoms, which
are weak X-ray scatters. Hydrogen bonding, above all, has been a relevant subject
for which vibrational spectroscopy has proved to be a superior investigation
method. On the other hand, vibrational spectroscopy, being a molecule-based
technique, magnifies the interpretative capabilities of the thermodynamic approaches on differentiating and characterizing structurally different phases of a compound and interpreting phase transition data at a molecular level.
For the studies on isolated molecules, matrix isolation vibrational spectroscopy
(in particular matrix isolation infrared spectroscopy) has emerged as a very powerful
and elegant method, overcoming some of the difficulties of gas phase spectroscopy
such as the complexity of the spectra resulting from rotational structure, extensive
anharmonic contributions, and the need for the compound under study to have a
substantial vapor pressure in order to acquire good quality spectra.
Matrix isolation is a technique where molecules are trapped from the gas phase
in an environment of solidified inert gases at temperatures close to absolute zero.
The method was first developed almost simultaneously by the groups of Pimentel at
Berkeley (USA) [1], and Porter at Cambridge (UK) [2] and had its name coined in
the historical paper by Whittle et al. [1] “Matrix isolation method for the experimental study of unstable species”.
Though matrix isolation was invented as a tool for study reactive species, the
method has also many advantages over other techniques for the study of stable
molecules and has been used extensively for this purpose too. The main advantages
of the method can be attributed to the very low work temperature, virtual absence of
intermolecular interactions between the molecules under investigation, and rigidity
and chemical inertness of the matrix medium. Because inert gases are generally
used as matrix host, solute–solvent interactions can be neglected for most of the
practical applications. On the other hand, molecular diffusion and rotation are
inhibited (except in the case of very small molecules), allowing registering of pure
vibrational spectra, which can be easily compared with theoretically predicted
spectroscopic data (that are usually obtained for molecules in vacuo). Also, hot
200
R. Fausto et al.
One has nowadays available experimental data of superior quality, and their
interpretation can be ground in solid theoretical foundations, facilitated by the
comparison of experimental and calculated spectra, and helped by use of modern,
effective data post-processing methods, including multivariate statistical-based
procedures.
If per se vibrational spectroscopy is already an authoritative technique for
structure and reactivity evaluation, when used simultaneously with other experimental methods it strongly increases its predictive and interpretative power. Among
these, X-ray diffraction (XRD), allowing access to details of the structure of
crystalline materials and thermodynamic methods (such as differential scanning
calorimetry (DSC)), providing information on phase transition properties, deserve
here to be mentioned. Vibrational spectroscopy can use structural information
obtained from XRD and extract detailed information on several important properties of the crystalline materials not easy accessible (or not accessible at all) to X-ray
diffraction methods, in particular those that are related with hydrogen atoms, which
are weak X-ray scatters. Hydrogen bonding, above all, has been a relevant subject
for which vibrational spectroscopy has proved to be a superior investigation
method. On the other hand, vibrational spectroscopy, being a molecule-based
technique, magnifies the interpretative capabilities of the thermodynamic approaches on differentiating and characterizing structurally different phases of a compound and interpreting phase transition data at a molecular level.
For the studies on isolated molecules, matrix isolation vibrational spectroscopy
(in particular matrix isolation infrared spectroscopy) has emerged as a very powerful
and elegant method, overcoming some of the difficulties of gas phase spectroscopy
such as the complexity of the spectra resulting from rotational structure, extensive
anharmonic contributions, and the need for the compound under study to have a
substantial vapor pressure in order to acquire good quality spectra.
Matrix isolation is a technique where molecules are trapped from the gas phase
in an environment of solidified inert gases at temperatures close to absolute zero.
The method was first developed almost simultaneously by the groups of Pimentel at
Berkeley (USA) [1], and Porter at Cambridge (UK) [2] and had its name coined in
the historical paper by Whittle et al. [1] “Matrix isolation method for the experimental study of unstable species”.
Though matrix isolation was invented as a tool for study reactive species, the
method has also many advantages over other techniques for the study of stable
molecules and has been used extensively for this purpose too. The main advantages
of the method can be attributed to the very low work temperature, virtual absence of
intermolecular interactions between the molecules under investigation, and rigidity
and chemical inertness of the matrix medium. Because inert gases are generally
used as matrix host, solute–solvent interactions can be neglected for most of the
practical applications. On the other hand, molecular diffusion and rotation are
inhibited (except in the case of very small molecules), allowing registering of pure
vibrational spectra, which can be easily compared with theoretically predicted
spectroscopic data (that are usually obtained for molecules in vacuo). Also, hot
200
R. Fausto et al.