Bending mode progression

For a bending mode progression in a harmonic oscillator, the shifts in the band head locations upon sulfur isotope substitution are given by [25]... 

A progression of up to four components in this mode can be observed, the second component being usually the most intense. The information obtained thus on the vibrational structure of the first transition can be applied also to those molecules in which the vibrational structure of this transition cannot be resolved. This situation is the result of the excitation of lower frequency quanta (probably skeletal out of plane bending modes) in combination with the stretching mode. In sterically hindered molecules which show such resolved spectra, the excitation of these low frequency modes would be less probable because of their steeper potential curve. 

In accordance with the selection rules, only excitation of full-symmetric vibrational modes can accompany the electronic transitions. One can conclude that the vibrational progressions for electronic transitions in the triatomic CAs more often result from excitation of bending modes. It is caused by significant change in the bond angle at the excitation. 

In common with other aza-naphthalenes, 1,6-naphthyridine displays a progression in the angle-bending mode. Along the a axis this progression displays a normal Franck-Condon envelope but when based on the false origins 347 and 484/8 polarized parallel to the b and c axes, this is certainly not the case. 

The intermolecular frequencies in Table 3.63 indicate that the larger sets predict progressively smaller frequencies for the H-bond stretch, but the trends are more erratic for the two bending modes. The frequencies seem to be calculated rather well, even with the smallest 4-31G basis set, surprising in light of the low order of theory and the neglect of anhar-monic effects. 

Fig. 65. Spectrum of water (bending mode) at various sili-con/electrolyte interfaces (a) nitromethane (b) acetonitrile (c) dimethyl sulphoxide. Note the progressive shift of the frequency upon increasing basicity of the solvent. (After [180]). Reprinted by permission of Elsevier Science.

The vertical electronic spectrum of acetylene shows many similarities with that of the isoelectronic molecule HCN. Both are linear molecules in their electronic ground states, but their lowest-valence excited states are bent. Photoexcitation of acetylene in the near-UV (190-240 nm) induces a electron promotion, resulting in population of the planar, trans bent A A state ( S in the linear limit) [312], The A A —transition is dominated by long progressions involving v 3 (the excited state trans bending mode), as would be expected on Franck-Condon grounds. The A—X... 

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