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Photolysis of Cr

Flash photolysis of metal carbonyls in solution was pioneered by Kelly and Koerner von Gustorf 30). The most complete of these studies has been carried out on the photolysis of Cr(CO)6 30). The historical development of these experiments, which forms an intriguing story in its own right, has been recently retold (2). The salient features are as follows ... [Pg.281]

Relatively little work has been done on the flash photolysis of gas phase metal carbonyls, partly because of the low volatility of many of the compounds. Early work by Callear (41,42) provided some evidence for Ni(CO)3 generated from Ni(CO)4 in the gas phase (41) and Fe atoms produced from Fe(CO)5 (42). This latter process has even been used as the basis of an Fe atom laser (43). More recently Breckenridge and Sinai (44) studied the flash photolysis of Cr(CO)6. Their results, interpreted largely on the basis of data from matrix isolation experiments, were in broad agreement with Kelly and Bonneau s solution work (JJ). In particular, they found no evidence for loss of more than one CO group [Eqs. (4) and (5)]. [Pg.283]

The photolysis of Cr(CO)6 also provides evidence for the formation of both CO (69) and Cr(CO) species (91,92) in vibrationally excited states. Since CO lasers operate on vibrational transitions of CO, they are particularly sensitive method for detecting vibrationally excited CO. It is still not clear in detail how these vibrationally excited molecules are formed during uv photolysis. For Cr(CO)6 (69,92), more CO appeared to be formed in the ground state than in the first vibrational excited state, and excited CO continued to be formed after the end of the uv laser pulse. Similarly, Fe(CO) and Cr(CO) fragments were initially generated with IR absorptions that were shifted to long wavelength (75,91). This shift was apparently due to rotationally-vibrationally excited molecules which relaxed at a rate dependent on the pressure of added buffer gas. [Pg.304]

It had already been established by uv-vis flash photolysis (35) that Cr(CO)5 (solvent) was the first observable intermediate in the photolysis of Cr(CO)6. Figure 9 shows the IR spectrum (96) of the photoproduct Cr(CO)5(C6Hi2) in cyclohexane solution. The spectra were obtained using Cr(CO)5(13CO) (96). The extra spectroscopic information provided by the 13CO group was sufficient to show that the spectrum was consistent... [Pg.304]

Figure 5. Transient time resolved spectrum following KrF photolysis of Cr(C0)6 with 5.0 torr Ar and 0.5 torr CO. The spectrum is displayed over a 10 fis range which is segmented into 10 equal time intervals. The first 3 intervals are labelled. (Reproduced with permission from reference 9. Copyright 1986 American Chemical Society.)... Figure 5. Transient time resolved spectrum following KrF photolysis of Cr(C0)6 with 5.0 torr Ar and 0.5 torr CO. The spectrum is displayed over a 10 fis range which is segmented into 10 equal time intervals. The first 3 intervals are labelled. (Reproduced with permission from reference 9. Copyright 1986 American Chemical Society.)...
Photochemistry of Cr(CO) at 248 nm. We have previously shown8 that the 248 nm photolysis of Cr(CO)6 yields Cr(CO)4 as the principal organometallic product via (7)-(8). Time-resolved laser absorption methods can be used to record the... [Pg.107]

Figure 3. Vibrational energy distribution of the CO product formed via the 351 nm photolysis of Cr(CO) . Experimental data are indicated as A. The lines correspond to results obtained by phase space calculations with an available energy of 25 and 20 Kcal/mole. Figure 3. Vibrational energy distribution of the CO product formed via the 351 nm photolysis of Cr(CO) . Experimental data are indicated as A. The lines correspond to results obtained by phase space calculations with an available energy of 25 and 20 Kcal/mole.
Figure 4. Transient infrared absorption spectrum obtained at 400 ns following the 351 nm photolysis of Cr(CO)6- [Cr(CO)6] = 0.020 torr, [CO] = 0.400 torr, [He] = 20.0 torr. Figure 4. Transient infrared absorption spectrum obtained at 400 ns following the 351 nm photolysis of Cr(CO)6- [Cr(CO)6] = 0.020 torr, [CO] = 0.400 torr, [He] = 20.0 torr.
It is worth noting that one of the great advantages of the matrix technique is that trace impurities are rarely a problem because there is little or no diffusion in the matrix. Recently Bonneau and Kelly Q ) have obtained definitive results on this solution system, partly by reference to data obtained in solid matrices (21), which suggest that saturated perfluorocarbons should interact with Cr(C0)5 less than other practicable room temperature solvents. Thus, Bonneau and Kelly have investigated the laser flash photolysis of Cr(C0>5 in perfluoromethylcyclohexane (C7F14) at room-temperature. A transient species 620 nm) formed... [Pg.45]

Figure 6. Summary of the results of flash photolysis of Cr(CO)6 in C7FH in solution. (Reproduced from Ref. 36. Copyright 1980, American Chemical Society.)... Figure 6. Summary of the results of flash photolysis of Cr(CO)6 in C7FH in solution. (Reproduced from Ref. 36. Copyright 1980, American Chemical Society.)...
Note added in typing In a very recent paper (81) Vaida and co-workers have used picosecond laser photolysis to show that, in cyclohexane solution, Cr(CO)5...cyclohexane (Amax 497 nm) is formed within 25 ps of the photolysis of Cr(C0)5 This suggests that, in solution, the primary photoproduct is Cr(C0)5 and that there is essentially no activation energy for the reaction of Cr(C0)5 with the solvent. Clearly, experiments with pulsed KrF lasers on carbonyls in solution and matrix may be very revealing. [Pg.48]

A few qualitative experiments were made on the photolysis of Cr(en)3+3, on which Nikolaiski has reported in detail (11). Chromatographic analysis showed that both cis-and /raws-bisethylenediamine complexes were formed, as well as some of the blue aquo species. Thus, a complete investigation of this system would be quite complex. [Pg.241]

Ultrafast Photolysis of Cr(CO)6 with Mass-Selective Detection. 47... [Pg.37]

The remarkable xenon complex Cr(CO)5(Xe) was also obtained by UV photolysis of Cr(CO)6 in liquefied xenon and found to have a lifetime of ca. 2 s at -98°C [117]. Later, the corresponding molybdenum and tungsten complexes Mo(CO)5(Xe) and W(CO)5(Xe) were generated in a similar way and characterized by time-resolved IR spectroscopy [118]. The bond energies Cr-Xe, Mo-Xe, and W-Xe were not as low as previously anticipated and determined to be 8-9 kcal/mol, independent of the group VI metal. [Pg.103]

The first transition metal-nohle gas complex to be observed in liquefied noble gas solution was CrCCOlsXe (38). Continuous UV photolysis of Cr(CO)6 dissolved in liquid Xe at 175 K or liquid Kr doped with 5% Xe at 151K produced a new species. This new species had v(C—O) IR bands (Fig. 7) which could be assigned to CrlCOsXe by comparison with those of CrCCOlsXe in a Xe matrix [Eq. (2)]. When the photolysis was halted, Cr(CO)5Xe decayed Avith a lifetime of 2 s. The activation energy for the thermal decay of CrCCOlsXe in hquid Kr + 5% Xe was determined to be Ea = 15 2 kJ mol . The surprisingly long lifetime of CrCCOlsXe in solution at this temperature was attributed to the high concentration of Xe and the extremely low concentration of the other reactants. [Pg.125]

A new technique of investigating short-lived species is fast time-resolved TR spectroscopy. With the help of this method the first gas phase spectrum of naked Cr(CO)5, generated by photolysis of Cr(CO)6, was obtained (Seder et al., 1985). The structure is similar to that shown by matrix experiments C4,. [Pg.250]

The reactivity of a number of alkane complexes has been examined and this field has been reviewed through 1996 by Hall and Perutz. Flash photolysis of Cr(CO)6 in cyclohexane showed that solvation occurs within the first picosecond after photolysis, a fact that appears to rule out spin crossing as an important component in the dissociation of CO from Cr(CO)6. The stability of CpRe(CO)2(alkane) is particularly striking. Comparison of the rate constants for heptane solvated metal complexes with CO, Table 1, reveals that the rate constant for CpRe(CO)2(heptane) is five orders of magnitude slower than that of CpV(CO)3 (heptane). In fact, the stability of the CpRe(CO)2(alkane) complexes is so high that it has been possible to carry out low-temperature NMR on the cyclopentane complex generated by continuous photolysis of... [Pg.3766]

These findings indicate general difficnlties in interpreting the results of flash photolysis experiments with UV visible detection. First, no detailed structural information can be obtained. Second, minnte traces of imparity in the solvent interfere strongly with the spectral behavior. It has now become clear that Cr(CO)s (and other unsaturated carbonyls) are almost always complexed by solvent or matrix molecnles or atoms. Even in a neon matrix there is some interaction to produce the species Ne- Cr(CO)5. Naked Cr(CO)s can be made in the gas phase, and by some rather cunning time-resolved solution experiments. Here, solvated Cr(CO)s is formed by UV photolysis of Cr(CO)6, then the solvating molecule is removed by visible photolysis. Fast time-resolved spectroscopy allows the decay of the naked Cr(CO)s, as it resolvates, to be moiutored. It is clear that this decay is extremely rapid. [Pg.4384]

The mechanism of the photochemical reaction has been the subject of some elegant experiments involving polarized light both for photolysis and for spectroscopic measurements. Unpolarized UV photolysis of Cr(CO)6 in pure methane matrices yields randomly oriented Cr(CO)s- CH4 molecules. Polarized visible photolysis leads to specifically oriented Cr(CO)s- CH4 molecules, as evidenced by the development of linear dichroism in the visible absorption band. The direction of the spectral dichroism depends upon the direction of polarization of the irradiating beam (equation 22). [Pg.4387]

Perutz and Downs characterised [Cr(CO)5(COj)] following photolysis of Cr(CO)u in a low temperature Ar matrix doped with COj and suggested that the mode of co-ordination was ri -O [15]. Recent calculations have supported this assignment [16]. [Pg.256]

The results of early flash studies were irreproducible and led to various conclusions concerning the nature of the primary photoproduct. For example, flash photolysis of Cr(CO)g in room temperature degassed cyclohexane solutions and in polystyrene film was reported to produce a transient having a maximum absorbance in the visible at 450 nm and which decayed with second-order (equal concentration) kinetics (83). The following scheme was proposed for the overall mechanism ... [Pg.239]


See other pages where Photolysis of Cr is mentioned: [Pg.282]    [Pg.107]    [Pg.38]    [Pg.39]    [Pg.42]    [Pg.42]    [Pg.47]    [Pg.47]    [Pg.152]    [Pg.173]    [Pg.188]    [Pg.90]    [Pg.143]    [Pg.143]    [Pg.81]    [Pg.832]    [Pg.131]    [Pg.295]    [Pg.256]    [Pg.207]    [Pg.70]    [Pg.130]    [Pg.648]    [Pg.3768]    [Pg.11]    [Pg.79]    [Pg.105]    [Pg.239]   
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Flash photolysis of [Cr

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