Photodissociation of methyl iodide

A. T. J. B. Eppink and D. H. Parker. Energy partitioning following photodissociation of methyl iodide in the A band a velocity mapping study. J. Chem. Phys., 110(2) 832-844 (1999). 

Lee, S.-Y. and Heller, E.J. (1982). Exact time-dependent wave packet propagation Application to the photodissociation of methyl iodide, J. Chem. Phys. 76, 3035-3044. 

U. Manthe and A.D. Hammerich, Wave packet dynamics in five dimensions. Photodissociation of methyl iodide, Chem. Phys. Lett., 211 (1993) 7. 

A.D. Hammerich, U. Manthe, R. Kosloff, H.-D. Meyer and L.S. Cederbaum, Time dependent photodissociation of methyl iodide with fife active modes, J. Chem. Phys., 101 (1994) 5623. 

Photoeliminations from Organohalogen Compounds. The 4-band photodissociation of methyl iodide to CH3 and I fragments has been studied in detail... 

Reactions of Halo Compounds. - Calculations have been carried out to investigate the decomposition paths for methyl fluoride and methyl chloride. Methyl chloride undergoes photodissociation on irradiation at 157.6 nm. Photodissociation of methyl iodide at 266 nm has been studied. The methyl radical recombination has been followed by time-resolved photothermal spectroscopy. Methyl iodide also undergoes photochemical decomposition on a GaAs(llO) surface. " Photolysis of methyl iodide at 236 nm in the gas phase brings about liberation of iodine atoms with a quantum yield of 0.69. ... 

Figure 23.10 shows the first snapshot obtained by the ion-imaging technique for the photodissociation of methyl iodide (see the paper by Chandler and Houston (1987)). The product detected is the methyl radical. The dissociation laser operated at 266 nm. At this energy it is known that the I fragment is formed in its first electronically excited state, the state. The... 

The photodissociation of methyl iodide, CH3I CH3 - - I, in the A band with iodine produced either in its ground state Ps/2 or the excited state Pi/2 is a prototypical system for molecular photodissociation.Brief overviews of the many experimental and theoretical studies are found in... 

The general principle of coherent control based on quantum interference between various photoexcitation pathways, including CW-laser weak excitation, is illustrated in Fig. 12.4. This quantum interference can be constructive or destructive, which allows control of the final state, that is, the control of a given reaction product. As an explicit example, Brumer and Shapiro (1986) have considered the process of photodissociation of methyl iodide, where the following two product channels are possible at an excitation energy of E ... 

The photodissociation of trifluoromethyl iodide, CF3I —> CF3 + I/I, which was briefly discussed in Section 6.4, seems to illustrate case (a) of Figure 9.4 while the photo dissociation of methyl iodide, CH3I —> CH3 + I/I, appears more to represent case (b). In both examples, the 1/2 umbrella mode, in which the C atom oscillates relative to the Irrespectively F3-plane, is predominantly excited. Following Shapiro and Bersohn (1980) the dissociation of CH3I and CF3I may be approximately treated in a two-dimensional, pseudo-linear model in which the vibrational coordinate r describes the displacement of the C atom from the H3-/F3-plane and the dissociation coordinate R is the distance from iodine to the center-of-mass of CH3/CF3 (see Figure 9.6).t... 

Guo, H. and Schatz, G.C. (1990b). Time-dependent dynamics of methyl iodide photodissociation in the first continuum, J. Chem. Phys. 93, 393-402. 

Both neutral and charge-transfer photochemistry is involved in the photodissociation at 248 nm of methyl iodide as a monolayer (1 ML) or up to 10 ML on a silverfl 11) surface. Such reactivity involves the fission of the C—I bond and the formation of methyl radicals and iodide adsorbed at the surface12,13a. Dissociation also occurs during X-ray photoelectron spectroscopy136. Multilayers of methyl iodide on silverfl 11) surfaces undergo C—I bond fission on irradiation at 248 nm. Several products such as ethane, ethyl iodide and iodoform are formed but the principal reaction path, which is somewhat time-dependent, yields methane and methylene di-iodide14. 

One should not be left with the impression that electronically nonadiabatic processes are limited to predissociation. Figure 1(a) shows a crossing between two excited repulsive curves in the photolysis of methyl iodide. If the surface hopping process is efficient enough, it can even influence a dissociating molecule that passes the curve crossing within a few femtoseconds, as is the case for methyl iodide photodissociation. 

A widely-used model in this class is the direct-interaction with product repulsion (DIPR) model [173—175], which assumes that a generalised force produces a known total impulse between B and C. The final translational energy of the products is determined by the initial orientation of BC, the repulsive energy released into BC and the form of the repulsive force as the products separate. This latter can be obtained from experiment or may be assumed to take some simple form such as an exponential decay with distance. Another method is to calculate this distribution from the quasi-diatomic reflection approximation often used for photodissociation [176]. This is called the DIPR—DIP model ( distributed as in photodissociation ) and has given good agreement for the product translational and rotational energy distributions from the reactions of alkali atoms with methyl iodide. 

The lack of laser action in the photolysis of isopropyl iodide raises intriguing questions. As Husain and Donovan point out, this does not necessarily indicate the absence of population inversion, since under the laser experimental conditions there could instead be an insufficient absolute concentration of I atoms. Spectroscopic studies show that excited iodine atoms are produced from isopropyl iodide photodissociation, but at lower relative concentrations than for n-propyl iodide under similar conditions. Since the two propyl iodides show similar I quenching rates, it would appear most likely that a decreased I /I ratio is the reason stimulated emission is not seen. The present experiments, unfortunately, cannot provide a more quantitative explanation. The distinct broadness of the isopropyl iodide distribution in fig. 2 indicates a departure from the methyl- ethyln-propyl trend, and might represent comparable amounts of P and I atom production, with overlapping translational energy distributions, at least when viewed with our present... 

With the advent of the suprasensitive EPR method of radical detection, it was natural that it should be used to observe and identify the radicals produced by the photodissociation of adsorbed gas molecules. Thus, silica gel with adsorbed methyl iodide subjected to a prolonged irradiation by ultraviolet light at 77°K and transferred to the resonance cavity of a EPR spectrometer displays the pattern, reproduced in Fig. 20a 131), which belongs to adsorbed methyl radicals, CHj split... 

The simplest way to model a photodissociation reaction is by using a one-dimensional picture, such as the dissociation of a diatomic molecule into two atoms. The molecule starts out in the bound ground-state potential with the bond distance constrained near the equilibrium separation. One can also consider the dissociation of a polyatomic molecule in this way instead of using an interatomic distance, one can use either the length of the breaking bond or the distance between the centers of mass (the line of centers) of the two fragments as the dissociation coordinate. Two examples of such a model are shown in Figure 1 for methyl iodide and ketene photodissociation. 

Careful determination of the internal state distributions of one of the photoproducts can lead to an understanding of the nature of the forces that act upon the molecule as it breaks apart. One of the nicest examples of this kind of work is water photolysis through the A state. Photolysis via the A state produces little rotational excitation as has been observed by LIF of the OH product [19,20], Ab initio studies of the A state have shown that it is purely repulsive, that is, a direct dissociation similar to methyl iodide in Figure 1(a). There is calculated to be little or no variation in the PES as a function of the HOH bending angle [21]. This result means that the OH recoils from the dissociation as if from a central force field and little angular momentum is imparted. This example of photodissociation is an example that nearly perfectly follows the simple one-dimensional model. 

Xic, D., Guo, H., Amatatsu, Y. and Kosloff, R. (2000) Three-dimensional photodissociation dynamics of rotational state selected methyl iodide,, 7. Phys. Chem. 104, 1009-1019. ... 

The photodissociation quantum yield of iodomethylate APA-Mel is more than two orders of magnitude less than that of APA (Table 8). Low photoactivity of the N-methylated salt is not connected with possible competitive process of the photoinduced electron transfer from iodide anion to quatemized heteroaromatic cation, as has been shown by replacement of the iodide counter-ion in the salt by the CH3S04 ion [61]. Therefore, the N-methylated APA cation indeed displays low photochemical activity relative to dissociation of the azide group. 

Table 8. The photodissociation quantum yields of APA and N-methyl-9-(4 -azidophenyl)acridinium iodide APA-Mel [61]...

According to these data, l-methyl-4-(4 -azidostyiyl)quinolinium iodide (azido hemicyanine AHC) has the longest-wavelength light sensitivity among all known aromatic azides on irradiation at 485 nm, the quantum yield of its photodissociation is 0.85. 

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