Vibrational photochemistry

The Range of Photochemical Reactions Vibrational Photochemistry and Radiation Chemistry... 

With this one exception of vibrational photochemistry through multiphoton infrared light absorption, photochemistry is restricted to the chemical reactions of electronic excited states of molecules. Radiation chemistry is outside the scope of this book, so a very short section is devoted to it to conclude this introduction. 

At one stage this vibrational photochemistry held the promise of mode selectivity , the possibility of tuning the excitation wavelength to excite one particular vibrational mode in a polyatomic molecule. It would then be possible to select one particular bond for dissociation, simply by changing the wavelength of the irradiation beam according to the vibrational frequency (mode) of that bond. In practice this mode selectivity has not been observed. No matter which specific vibration is excited by multiphoton IR absorption, the energy is very quickly distributed statistically over all the available vibrational modes, so that the weakest bonds dissociate faster. 

Butenhoff et al. discovered the near-infrared (NIR)-induced tautomerism of H2P embedded in a hexane Shpol skii matrix [90SA(A)519] and gave a detailed description of its vibrational photochemistry (90JPC7847). 

M. J. Berry, Vibrational photochemistry and photophysics, Proc. Robert A. Welch Foundation Conf. on Chemical Research, XXVIII, Welch Foundation, 1984, p. 133. 

Pulsed lasers are used to monitor the time-evolution of excited states, the formation of photochemical products and the kinetics of photoprocesses. The high intensity of the laser beam also enables very weak transitions to be excited, so that vibrational photochemistry and multiphoton transitions, which exhibit different selection rules, can be studied. Chap. 15 provides further details about some of the experiments that may be performed using pulsed laser sources. Table 14.2 gives details of the most commonly used lasers in photochemistry today. The key properties to consider for any particular application are wavelength(s), pulse duration, and power. Simplicity of operation is another important factor. 

A major obstacle to the development of vibrational photochemistry, in general, and to the successful application of single-photon vibrational photochemistry to isotope enrichment, in particular, has been the lack of a powerful laser providing tunable, narrow-linewidth, infrared radiation. There are indications that this lack of an infrared equivalent to the dye laser may not continue for long, but there are other difficulties. One is the rather limited amount of energy acquired by a molecule absorbing a... 

Some of the problems associated with selective vibrational photochemistry can be avoided in electronic photochemistry, and dye lasers are available to provide tunable radiation. To trap the selectively excited species, reactions involving molecules as distinct from radicals are preferred, since then the opportunities for isotopic scrambling are minimized. This approach to isotope... 

Wang Q, Schoenlein R W, Peteanu L A, Mathies R A and Shank C V 1994 Vibrationally coherent photochemistry in the femtosecond primary event of vision Science 266 422... 

Modem photochemistry (IR, UV or VIS) is induced by coherent or incoherent radiative excitation processes [4, 5, 6 and 7]. The first step within a photochemical process is of course a preparation step within our conceptual framework, in which time-dependent states are generated that possibly show IVR. In an ideal scenario, energy from a laser would be deposited in a spatially localized, large amplitude vibrational motion of the reacting molecular system, which would then possibly lead to the cleavage of selected chemical bonds. This is basically the central idea behind the concepts for a mode selective chemistry , introduced in the late 1970s [127], and has continuously received much attention [10, 117. 122. 128. 129. 130. 131. 132. 133. 134... 

Another area of research ia laser photochemistry is the dissociation of molecular species by absorption of many photons (105). The dissociation energy of many molecules is around 4.8 x 10 J (3 eV). If one uses an iafrared laser with a photon energy around 1.6 x 10 ° J (0.1 eV), about 30 photons would have to be absorbed to produce dissociation (Eig. 17). The curve shows the molecular binding energy for a polyatomic molecule as a function of interatomic distance. The horizontal lines iadicate bound excited states of the molecule. These are the vibrational states of the molecule. Eor... 

The assignment of the TR spectra were based on the known photochemistry of the aryl azides and comparison of the TR spectra vibrational frequencies to those predicted by density functional theory calculations for the likely photochemical intermediates. The good agreement between the experimental TR vibrational... 

The photochemistry of a-methylstilbene (5) resembles stilbene photochemistry in many ways. However, as pointed out earlier, both the cis and trcms isomers are nonclassical acceptors of triplet excitation. This suggests that both the cis and trcms triplet states correspond to high-energy vibrational levels of the twisted or phantom triplet. Azulene does not alter the photo-... 

These IR kinetic experiments (75) were the first examples of vibrationally excited metal carbonyls to be observed. More detailed studies on the behavior of hot carbonyls should provide an intriguing insight into the photophysics of these molecules. We now look at metal carbonyl photochemistry in solution. 

The complex photochemistry of CH MCCO) (M = Mn, Re) has already been unravelled by matrix isolation experiments, utilising both IR (V(C-O) and V(N N) vibrations) and UV/visible spectroscopy (12). 

It is well known that y or X photons have energies suitable for excitation of inner electrons. We can use ultraviolet and visible radiation to initiate chemical reactions (photochemistry). Infrared radiation excites bond vibrations only whereas hyperfrequencies excite molecular rotation. In Tab. 1.1 the energies associated with chemical bonds and Brownian motion are compared with the microwave photon corresponding to the frequency used in microwave heating systems such as domestic and industrial ovens (2.45 GHz, 12.22 cm). 

The limitation in all of these flash experiments is that only broad featureless UV/vis bands are observed and hence assignment has to rely on comparison with matrix data and/or kinetic consistency. How much more informative vibrational spectroscopy would be There is good reason to be optimistic as in the recent work of Schaffner (8), where, incidentally, it is shown how important a role is played by traces of H2O in the detailed mechanism of the photochemistry of Cr(C0)6 ... 

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