Combination giving vibrational excitation

The dichloromethyl radicals combine to give a vibrationally excited state of 1,1,2,2-tetrachloroethane [2186],... 

IR spectra would be difficult enough to interpret completely if all bands observed were fundamentals, but unfortunately they are not. Many peaks are due to overtones (the first one occurring at slightly less than twice the frequency of the fundamental), combinations (two vibrations can become excited simultaneously, giving a band at about the sum of their separate frequencies), and Fermi resonance (an insidious phenomenon Where a fundamental and an overtone or combination band are expected to occur at very nearly the same frequency, one sometimes finds instead two about equally intense bands, one shifted to higher and one to lower fi-equency). 

Both linear and nonlinear Raman spectroscopy can be combined with time-resolved detection techniques when pumping with short laser pulses [8.781. Since Raman spectroscopy allows the determination of molecular parameters from measurements of frequencies and populations of vibrational and rotational energy levels, time-resolved techniques give information on energy transfer between vibrational levels or on structural changes of short-lived intermediate species in chemical reactions. One example is the vibrational excitation of molecules in liquids and the collisional energy transfer from the excited vibrational modes into other levels or into translational energy of the collision partners. These processes proceed on picosecond to femtosecond time scales [8.77,8.79]. 

Vibrational excitation can occur, and usually does, in conduction with rotational excitations, resulting in vibration-rotation spectra. For straightforward IR vibrational spectra, the simultaneous rotational excitations result in broardening of the vibrational bands. Bands are additionally broadened if two (or more) normal modes have frequencies close to one another, as well as by instrument resolution and other experimental artifacts. Additional bands, other than those caused by vibrational fundamentals, can arise in the spectrum by the absorption of two or more quanta of energy simultaneously, giving rise to combination and overtone bands, and by further excitations from already vibrationaUy excited states which give rise to what are called hot bands. 

In multiplying by we use, again, examples of the vibrations of NH3. The result depends on whether we require when (a) one quantum of each of two different e vibrations is excited (i.e. a combination level) or (b) two quanta of the same e vibration are excited (i.e. an overtone level). In case (a), such as for the combination V3 - - V4, the product is written E x E and the result is obtained by first squaring the characters under each operation, giving... 

Tables for all degenerate point groups, giving the symmetry species of vibrational combination states resulting from the excitation of one quantum of each of two different degenerate vibrations and of vibrational overtone states resulting from the excitation of two quanta of the same degenerate vibration, are given in the books by Herzberg and by Hollas, referred to in the bibliography.

Absorption of ultraviolet and visible radiation in organic molecules is restricted to certain functional groups (chromophores) that contain valence electrons of low excitation energy (Figure 4). The spectrum of a molecule containing these chromophores is complex. This is because the superposition of rotational and vibrational transitions on the electronic transitions gives a combination of overlapping lines. This appears as a continuous absorption band. 

A look at Table IV will show the general simplicity of the spectra. This is due to the fact that ionization occurs by electron tunneling, which does not excite the ions vibrationally, as is the case with impact ionization. Thus the combination of field ionization and mass spectrometry has considerable analytical potentialities, when it is considered that acetone, for example, gives rise to 19 peaks of comparable intensity by impact ionization and shows no peaks over 0.1% except the parent in field ionization. 

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