Effects of Ring Fluorination on the Ultraviolet Photodissociation Dynamics of Phenol

Abstract

The dynamics of photoinduced O-H bond fission in five fluorinated phenols (2-fluorophenol, 3-fluorophenol, 2,6-difluorophenol, 3,4,5-trifluorophenol, and pentafluorophenol) have been investigated by H Rydberg atom photofragment translational spectroscopy following excitation at many wavelengths in the range 220    275 nm. The presence of multiple fluorine substituents reduces the efficiency of O-H bond fission (by tunneling) from the first excited (1 1 *) electronic state, whereas all bar the perfluorinated species undergo O-H bond fission when excited at shorter wavelengths (to the 2 1 * state). As in bare phenol, O-H bond fission is deduced to occur by nonadiabatic coupling at conical intersections between the photoprepared 'bright' * states and the 1 1 * potential energy surface. In all cases, the fluorophenoxyl photoproducts are found to be formed in a range of vibrational levels, all of which include an odd number of quanta (typically one) in an out-of-plane (a″) vibrational mode; this product vibration is viewed as a legacy of the parent out-of-plane motions that promote non-adiabatic coupling to the dissociative 1 1 * potential. The radical products also show activity in in-plane vibrations involving coupled (both in-and out-of-phase) C-O and C-F wagging motions, which can be traced to the impulse between the recoiling O and H atoms and, in detail, are sensitive to the presence (or not) of an intramolecular F‧‧‧H-O hydrogen bond. Upper limit values for the O-H bond dissociation energies are reported for all molecules studied apart from pentafluorophenol.

Infrared Spectra and Phototransformations of meta-Fluorophenol Isolated in Argon and Nitrogen Matrices

“…The irradiations were performed at λ = 280, 275, and 270 nm. This set of wavelengths falls within the absorption band observed in the ~260–280 nm region of the UV spectrum of m FP in cyclohexane solution [ 53 ]. After each irradiation, an infrared spectrum was collected to monitor the possible phototransformations taking place in the sample.…”

Tunneling dynamics dictated by the multidimensional conical intersection seam in the πσ*‐mediated photochemistry of heteroaromatic molecules

“…Depending on the ortho ( o ), meta ( m ) or para ( p ) position of the substituent on the phenol ring with respect to the OH moiety, the associated dynamics is apparently quite different. In general, the substitution on the meta ‐ or para ‐position of phenol does not seem to be much influential on the OH bond cleavage dynamic in terms of the tunneling rate and the excess energy disposal dynamics, 116,120,138,142,143,145–151 except the presence of the additional chemical bond cleavage channels introduced especially for the bromo‐ or iodo‐phenol, for example 137,141 . The ortho ‐substitution, on the other hand, gives rise to the substantial changes in the whole reaction dynamics.…”

“…Chemical derivatives of phenol including fluorophenol, chlorophenol, bromophenol, iodophenol, or methylphenol (cresol) have been much studied to date 100,116–121,136–151 . Depending on the ortho ( o ), meta ( m ) or para ( p ) position of the substituent on the phenol ring with respect to the OH moiety, the associated dynamics is apparently quite different.…”

Virtual Issue on Photodissociation: From Fundamental Dynamics and Spectroscopy to Photochemistry in Planetary Atmospheres and in Space

“…Halogenated organic molecules are popular subjects for studies of these types, , but the photodissociation of other organic molecules has also been intensively investigated, ,,− providing a wealth of insights into fundamental photochemistry. As these studies illustrate, it is not always the thermodynamically most favorable outcome that is observed, corresponding to breaking the weakest bond in the molecule, because the dynamical timescales of bond breaking often outpace the flow of energy within the molecule.…”

“…Additionally, analysis of the asymptotic kinetic energy release to the photofragments provides direct access to bond dissociation energies, which are valuable thermochemical data. Regardless of the experimental approach adopted, the outcomes are enriched by exploring the dependence of the photodissociation dynamics on the wavelength of light used to excite the molecules; 20 changing this wavelength can lead to initial excitation of a different electronic state, and scanning the wavelength can locate the energetic threshold for each bond dissociation pathway.…”