Ammonia vibrational stretching modes

Direct ionization produces a staircaselike structure in the plot of ion current as a function of photon energy, where the height of each step is proportional to the probability of production of a certain vibronic state of the ion. Such favorable cases of staircaselike structure have been observed for ammonia87 and acetylene.88 The structure in ammonia is attributable to excitation of successive vibrational levels of the out-of-plane bending mode of the ion and in acetylene, to excitation of the C-C stretching mode. As a result, these molecules are favorable candidates for studying the effects of vibrational excitation on the cross sections for ion-molecule reactions. 

IR spectra of ammonia adsorbed on oxovanadium species supported on porous sihca are shown in Figure 2.33. Two kinds of interactions with hydroxyl groups are evident (282). The broad band at about 1470 cm corresponds to the bending vibrations of ammonium ions formed by reaction with addic vanadia hydroxyls. The band at 1635 cm is ascribed to the bending mode of ammonia molecules H-bonded to silanols. Simultaneously, a shift of the OH stretching modes of the silanol groups (external silanols at 3745 cm and terminal and H-bonded silanols in hydroxyl chains at 3710 and 3547 cm, respectively) by about —800 cm is observed. The bands at 3390 and 3330 cm are assigned to N-H modes. 

We will see later that the irreducible representations found here provide descriptions of the molecular vibrations that involve the N—H stretching modes of the ammonia molecule. The vibration of the molecule is a collective motion of all the atoms that make it up, and these irreducible representations are describing these collective motions. The vibrations fall into three patterns two are a doubly degenerate pair ( ) and the third a separate single vibration (Ai). 

As an example we will look at the N—H stretching modes in ammonia. The basis of N—H bonds in ammonia was used in Chapter 4 as part of the development of matrices in symmetry. We can now use this basis (defined in Figure 4.7) to consider degenerate vibrations of a molecule. The reducible representation and the application of the reduction formula for the three-vector basis is shown in Table 6.12. Once the totals are divided by the order of the group (/t = 6 in Ci ), we find... 

The analysis of the /flc-isomer is identical to the ammonia N—H stretching modes example of Section 6.6.2, so that the three basis vectors give rise to three vibrational modes with irreducible representations ... 

In the present work low temperature adsoi ption of fluoroform and CO, were used to characterize surface basicity of silica, both pure and exposed to bases. It was found that adsorption of deuterated ammonia results in appearance of a new CH stretching vibration band of adsorbed CHF, with the position typical of strong basic sites, absent on the surface of pure silica. Low-frequency shift of mode of adsorbed CO, supports the conclusion about such basicity induced by the presence of H-bonded bases. 

Inelastic tunnelling electrons can also be used to selectively induce either the translation over a metallic surface or desorption from the metallic surface of individual molecules, as has been shown for NH3 on Cu(lOO) surfaces (Pascual et al, 2003). Activation of either the stretching vibration of ammonia ( 408 meV) leading to lateral translation on the surface, or the inversion of its pyramidal structure (umbrella mode s(NH3) 139 meV) leading to desorption, can be achieved by adjusting 7t and Vt. 

In isolation, the BHT ion is tetrahedral, and consequently only two fundamentals, the asymmetric BH stretch (V3) and asymmetric BH4 deformation (V4) are IR active for the isolated ion, whereas all four fundamentals are Raman active. The Raman active fundamentals were characterized in liquid ammonia solutions, whereas IR spectra of thin films of NaBH4 on alkali halide crystals or diluted in an alkali halide host crystal have been reported ". Raman and IR studies focused specifically on NaBH4 and LiBH4 have also been reported. The vibrational modes in borohydrides are of three distinct types librational (below 1000 cm ), B-H bending (1127 cm ) and B-H stretching (2200-2400 cm ). The overtone of the deformation mode (2V4) occurs around 2228 cm ... 

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