UV photodissociation dynamics of iodobenzene: Effects of fluorination

Abstract

The UV photochemistry of various fluorinated iodobenzenes (4-fluoro-, 2,4-difluoro-, 3,5-difluoro-, and perfluoro-iodobenzene) has been investigated at many wavelengths by velocity map imaging, time-resolved near infrared absorption spectroscopy and (spin-orbit resolved) ab initio calculations of the ground and excited state potentials along the C-I stretch coordinate, R(C-I). The textbook description of the near UV photochemistry of CH(3)I, i.e., σ∗←n excitation to the (3)Q(0+) state, followed by direct dissociation (to yield spin-orbit excited iodine atom (I∗) products) or by non-adiabatic coupling via a conical intersection (CI) with the (1)Q(1) potential (to yield ground state iodine (I) atoms) is shown to provide a good zero-order model for aryl iodide photochemistry also. However, the aryl halides also possess occupied π and low-lying π∗ orbitals, and have lower (C(2v) or C(s)) symmetry than CH(3)I. Both of these factors introduce additional subtleties. For example, excitations to and predissociation of ππ∗ excited states provide additional routes to I products, most obviously at long UV wavelengths. nσ∗∕πσ∗ configuration mixing stabilizes the (analogue of the) (3)Q(0+) potential energy surface (PES), to an extent that scales with the degree of fluorination; the corresponding 4A(1) PES in C(6)F(5)I is actually predicted to exhibit a minimum at extended R(C-I). This has the effect of extending the long wavelength threshold for forming I∗ products. The lowered symmetry enables an additional (sloped) CI with the 5A(2) (9A(") in 2,4-difluorobenzene) PES, which provides an extra non-adiabatic route to (fast) ground state I atoms when populating the 4A(1) PES at shorter UV excitation wavelengths.

Near ultraviolet photochemistry of 2-bromo- and 2-iodothiophene: Revealing photoinduced ring opening in the gas phase?

“…Analogy with previous experimental studies of bare and substituted halobenzenes [4][5][6][7][8] encourages the expectation that near UV excitation of the title halothiophenes should result in eventual CX (X = Br, I) bond fission on one or more (n/π)σ * excited state PESs. A previous one colour ion imaging study of Br atoms resulting from 267 nm photolysis of 2-bromothiophene lends support to this expectation, 9 as do evident similarities in the resonance Raman (RR) spectra measured following excitation of 2-iodothiophene and iodobenzene (in cyclohexane solution).…”

“…6(f) is still discernible at λ = 220.2 nm. As in the cases of iodobenzene, 6 various fluorinated iodobenzenes, 7 and 4-iodophenol, 8 the I * images measured following excitation of 2-iodothiophene at long wavelength show hints of product vibrational structure, as illustrated in Fig. 7(a) (λ = 277.4 nm).…”

“…[6][7][8] Ab initio calculations for the ground state thienyl radical provide clues as to the assignment of the vibrational peak observed in Fig. 7(a).…”

Photodissociation Dynamics of C6F5Br at 234 nm: Fluorination Effects on Br/Br*Formation Pathways

“…It has been unraveled by numerous studies that dissociation pathways are substantially dependent on the type of halogen, 9-11 atomic substituents, 12 and excitation wavelengths. 13 In the early work on the bromobenzene (C 6 H 5 Br) photodissociation near 266 nm, [7][8][9][10][11][12]14 the main dissociation channel is an indirect dissociation involving the bound (π, π * ) and repulsive (π, σ * ) states. In the short wavelength, significant UV absorption of bromobenzene is caused by and transition; the mixing between these states becomes more probable and additional routes can be observed as in the case of photodissociation of iodobezene.…”

“…The A-band of alkyl bromide arises from the C-Br bond localized transition and consists of three overlapping transitions to repulsive states ( Q 1 PESs via a conical intersection along the C-Br bond coordinate. [4][5][6] Compared to the photodissociation of alkyl bromide, the photodissociation dynamics of aryl halides [7][8][9][10][11][12][13][14][15][16] show more complicated because more electronic states are involved and thus making multiple dissociation pathways probable. Multiple dissociation pathways include the predissociation between excited electronic states, internal conversion processes from vibrational excited states, and dissociation processes from repulsive states.…”