Stretching and bending vibrations

Van der Waals complexes can be observed spectroscopically by a variety of different teclmiques, including microwave, infrared and ultraviolet/visible spectroscopy. Their existence is perhaps the simplest and most direct demonstration that there are attractive forces between stable molecules. Indeed the spectroscopic properties of Van der Waals complexes provide one of the most detailed sources of infonnation available on intennolecular forces, especially in the region around the potential minimum. The measured rotational constants of Van der Waals complexes provide infonnation on intennolecular distances and orientations, and the frequencies of bending and stretching vibrations provide infonnation on how easily the complex can be distorted from its equilibrium confonnation. In favourable cases, the whole of the potential well can be mapped out from spectroscopic data. 

The intennolecular bending and stretching vibrations of Van der Waals complexes typically have wave-numbers... 

Most infrared spectroscopy of complexes is carried out in tire mid-infrared, which is tire region in which tire monomers usually absorb infrared radiation. Van der Waals complexes can absorb mid-infrared radiation eitlier witli or without simultaneous excitation of intennolecular bending and stretching vibrations. The mid-infrared bands tliat contain tire most infonnation about intennolecular forces are combination bands, in which tire intennolecular vibrations are excited. Such spectra map out tire vibrational and rotational energy levels associated witli monomers in excited vibrational states and, tluis, provide infonnation on interaction potentials involving excited monomers, which may be slightly different from Arose for ground-state molecules. 

It is tlius of great interest to carry out experiments tliat excite tire intennolecular bending and stretching vibrations directly, witliout exciting tire monomers as well. These transitions he deep in tire far infrared, typically in tire 20-... 

Coupling between bending and stretching vibrations can occur if the stretching bond forms one side of the changing angle. 

In this compound there are GdCl - octahedra in which the Gd3 + ion is shielded from the further surroundings by the large Cl" ions. Nevertheless the photo-excited emission spectrum shows not only vibronic transitions due to coupling with the Gd-Cl vibrations, but also due to NH4 bending and stretching vibrations [34]. 

Fig. 1. The bending and stretching vibrations of a planar molecule with the CN = 3, involved in the PJT-type D3h — C3v process due to the presence of a lone pair.

In addition to other bending and stretching vibration bands for the aliphatic moiety of the molecule. 

Qian, C.X.W., Ogai, A., Iwata, L., and Reisler, H. (1988). State-selective photodissociation dynamics of NOC1 The influence of excited state bending and stretching vibrations, J. Chem. Phys. 89, 6547-6548. 

Dramatic differences appear upon Al-bonding to thiophene the frequency region below 500 cm is now populated by a series of strong peaks which correspond to C-Al bending and stretching vibrations, appearing between 160 and 300 cm and 300 and 450 cm , respectively. The remarkable intensity of these peaks is likely related to the strong dipole moment of the Al-C bonds. 

Bending and stretching vibrations characteristic of most functional groups appear in the region from 3500 to 1500 cm ... 

Weber et al have investigated van der Waals vibrational transitions in T Ar and other tetrazine-rare gas complexes. They observed that (i) the frequencies of these transitions in the cluster T X (X = Ar, Kr, Xe) fit fairly well in with harmonic or near harmonic progressions and (ii) the relative intensities of corresponding transitions of stretch vibrational modes are sensibly uncoupled, unless the vibrational mixing in the three different clusters would be very similar. 

It was observed that the surface acidify of the fillers influences the bending and stretching vibrational frequencies of the water physically adsorbed on the filler surface [6, 14]. The conductivity of composite membranes and maximum power density of DMFCs at 145 °C appear to be related to the characteristics of the water adsorbed oti the filler particles [6,14]. Inorganic fillers characterized by acidic properties undergo a strong interaction with water and enhance the DMFC performance at high temperatures. The self-diffusimi coefficients of water and methanol have been determined over... 

---