Anharmonicities of intermonomer modes

Beyond these ID anharmonicities, the intermonomer modes may also display a multidimensional anharmonicity that couples various modes. It means that the potential 

Intramonomer vibrations are found inside the X H or Y molecules. They consequently exist even in the absence of an H-bond between these two molecules, in opposition to intermonomer bands we have seen above, which exist due to the formation of the H-bond. An H-bond has, however, marked effects on some intramonomer vibrations. It has a spectacular effect on the stretching vibration of the H-atom of X H that establishes the H-bond. 

Studied and has been shown to be extremely sensitive to the presence of H-bonds in its vicinity (12-14). It also pnt into evidence the weak H-bonds N2 molecules accept, a result that may have important consequences in atmospheric chemistry. We cannot sufficiently stress the interest of these resnlts that have been collected nsing these methods, and which eventnally gave us a precise understanding of the original properties of simple H-bonds, snch as their spectroscopic properties we describe in this chapter, but also of their thermodynamic properties, and of the reactivity of H-bonds we describe in Ch. 6. This is crncial, becanse we shall see in aqneons systems the importance of the interactions of H-bonds, more precisely in the H-bond networks that appear with the presence of H2O molecnles, and which have collective properties we conld have hardly nnderstood and could not have defined if we had previonsly not got a precise knowledge of the properties of isolated H-bonds. It could be achieved thanks to the hypersensitivity of IR spectroscopy to H-bonds. Microwave spectroscopy, described in Ch. 3, is also a precise technique, in particular for what concerns the geometrical parameters of these simple H-bonded systems. The sensitivity of IR spectroscopy has more general impact, as IR spectroscopy is a method of a much wider application that may also be nsed with liqnids and solids. Its sensitivity may then be used to study systems where the number of H-bonds is small. 

This shift of is often easily measured with the help of an IR spectrometer that is most common and easily operable in many laboratories. The existence of correlations of this shift with other characteristic quantities of H-bonds, such as their enthalpies A// or their X Y equilibrium distances, allows a rapid estimate of the magnitudes of these quantities that would otherwise require more time and effort to directly measure them. This is the reason why IR spectroscopy is so often used in routine procedures. It should nevertheless be kept in mind that retrieving such quantities from an indirect IR measurement can only be done within the limits of such a procedure that are mentioned in the following subsections. 

More recently, anticooperative effects between the two H-bonds established by H2O molecules have also been displayed (25), by simply showing that an HDO molecule that establishes an H-bond with a pyridine or tetrahydrofuran molecule with its H-atom and none with its D-atom has a I /O-H- ) band centred at 3375cm . When the D-atom also establishes a D-bond with another pyridine molecule, the i, (0-H- ) band centre shifts to 3420cm . This 

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