UV photolysis of 4-iodo-, 4-bromo-, and 4-chlorophenol: Competition between C-Y (Y = halogen) and O-H bond fission
- School of Chemistry, University of Bristol, Bristol BS8 1TS (United Kingdom)
The wavelength dependences of C-Y and O-H bond fission following ultraviolet photoexcitation of 4-halophenols (4-YPhOH) have been investigated using a combination of velocity map imaging, H Rydberg atom photofragment translational spectroscopy, and high level spin-orbit resolved electronic structure calculations, revealing a systematic evolution in fragmentation behaviour across the series Y = I, Br, Cl (and F). All undergo O-H bond fission following excitation at wavelengths {lambda} Less-Than-Or-Equivalent-To 240 nm, on repulsive ((n/{pi}){sigma}*) potential energy surfaces (PESs), yielding fast H atoms with mean kinetic energies {approx}11 000 cm{sup -1}. For Y = I and Br, this process occurs in competition with prompt C-I and C-Br bond cleavage on another (n/{pi}){sigma}* PES, but no Cl/Cl* products unambiguously attributable to one photon induced C-Cl bond fission are observed from 4-ClPhOH. Differences in fragmentation behaviour at longer excitation wavelengths are more marked. Prompt C-I bond fission is observed following excitation of 4-IPhOH at all {lambda}{<=} 330 nm; the wavelength dependent trends in I/I* product branching ratio, kinetic energy release, and recoil anisotropy suggest that (with regard to C-I bond fission) 4-IPhOH behaves like a mildly perturbed iodobenzene. Br atoms are observed when exciting 4-BrPhOH at long wavelengths also, but their velocity distributions suggest that dissociation occurs after internal conversion to the ground state. O-H bond fission, by tunnelling (as in phenol), is observed only in the cases of 4-FPhOH and, more weakly, 4-ClPhOH. These observed differences in behaviour can be understood given due recognition of (i) the differences in the vertical excitation energies of the C-Y centred (n/{pi}){sigma}* potentials across the series Y = I < Br < Cl and the concomitant reduction in C-Y bond strength, cf. that of the rival O-H bond, and (ii) the much increased spin-orbit coupling in, particularly, 4-IPhOH. The present results provide (another) reminder of the risks inherent in extrapolating photochemical behaviour measured for one molecule at one wavelength to other (related) molecules and to other excitation energies.
- OSTI ID:
- 22105493
- Journal Information:
- Journal of Chemical Physics, Vol. 138, Issue 16; Other Information: (c) 2013 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA); ISSN 0021-9606
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ANISOTROPY
BRANCHING RATIO
DISSOCIATION
ELECTRONIC STRUCTURE
EXCITATION
FISSION
FRAGMENTATION
FREQUENCY DEPENDENCE
GROUND STATES
HALOGENS
INTERNAL CONVERSION
KINETIC ENERGY
L-S COUPLING
PHENOL
PHOTOLYSIS
PHOTON-MOLECULE COLLISIONS
POTENTIAL ENERGY
SURFACES
TUNNEL EFFECT
WAVELENGTHS