UV photolysis of 4-iodo-, 4-bromo-, and 4-chlorophenol: Competition between C–Y (Y = halogen) and O–H bond fission

Abstract

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 λ ≲ 240 nm, on repulsive ((n∕π)σ∗) potential energy surfaces (PESs), yielding fast H atoms with mean kinetic energies ∼11,000 cm(-1). For Y = I and Br, this process occurs in competition with prompt C-I and C-Br bond cleavage on another (n∕π)σ∗ 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 λ ≤ 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∕π)σ∗ 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.

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).…”

The near ultraviolet photodissociation dynamics of 2- and 3-substituted thiophenols: Geometric vs. electronic structure effects

“…Previous studies of ways in which ring-substituents affect the dynamics and energetics of photoinduced O-H bond fission in phenols 16,17,[31][32][33][34][35][36] and S-H bond fission in thiophenols [37][38][39] have been extended to the cases of 2-and 3-YPhSH (with Y = Me and F). Using a combination of experiment-H (Rydberg) atom PTS, at many different near UV excitation wavelengths-and complementary electronic structure methods, we have been able to gain further insights into the interplay between substituent-induced geometric (i.e., steric effect/intramolecular hydrogen bonding) and electronic (i.e., π (resonance) or σ (inductive)) effects.…”

“…Such fragmentation dynamics have been demonstrated in phenol [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] and thiophenol, [24][25][26][27][28][29][30] in a range of substituted phenols [31][32][33][34][35][36] and thiophenols, [37][38][39] and in the methylated analogues anisole 40,41 and thioanisole, [42][43][44][45][46] in both the gas and condensed [47][48][49][50] phase. The present study focusses on thiophenols and, particularly, how the much-studied S-H bond fission process is affected by asymmetric substitution (i.e., in the 2-and 3-positions) of the aromatic ring.…”