Kao MH, Orr-Ewing AJ. Charge-Separated Reactive Intermediates from the UV Photodissociation of Chlorobenzene in Solution.
J Phys Chem A 2022;
126:6934-6943. [PMID:
36148486 PMCID:
PMC9549464 DOI:
10.1021/acs.jpca.2c05327]
[Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
![]()
Although ultraviolet
(UV)-induced photochemical cleavage of carbon–halogen
bonds in gaseous halocarbons is mostly homolytic, the photolysis of
chlorobenzene in solution has been proposed to produce a phenyl cation,
c-C6H5+, which is a highly reactive
intermediate of potential use in chemical synthesis and N2 activation. Any evidence for such a route to phenyl cations is indirect,
with uncertainty remaining about the possible mechanism. Here, ultrafast
transient absorption spectroscopy of UV-excited (λ = 240 and
270 nm) chlorobenzene solutions in fluorinated (perfluorohexane) and
protic (ethanol and 2,2,2-trifluoroethanol) solvents reveals a broad
electronic absorption band centered at 540 nm that is assigned to
an isomer of chlorobenzene with both charge-separated and triplet-spin
carbene character. This spectroscopic feature is weaker, or absent,
when experiments are conducted in cyclohexane. The intermediate isomer
of chlorobenzene has a solvent-dependent lifetime of 30–110
ps, determined by reaction with the solvent or quenching to a lower-lying
singlet state. Evidence is presented for dissociation to ortho-benzyne, but the intermediate could also be a precursor to phenyl
cation formation.
Collapse