Molecular elimination in photolysis of fluorobenzene at 193 nm: Internal energy of HF determined with time-resolved Fourier-transform spectroscopy

Abstract

Following photodissociation of fluorobenzene (C(6)H(5)F) at 193 nm, rotationally resolved emission spectra of HF(1<or= v <or= 4) in the spectral region 2800-4000 cm(-1) are detected with a step-scan Fourier-transform spectrometer. In the period 0.1-1.1 mus after photolysis, HF(v <or= 4) shows similar Boltzmann-type rotational distributions corresponding to a temperature approximately 1830 K; a short extrapolation from data in the period 0.1-4.1 mus leads to a nascent rotational temperature of 1920+/-140 K with an average rotational energy of 15+/-3 kJ mol(-1). The observed vibrational distribution of (v = 1):(v = 2):(v = 3):(v = 4) = (60+/-7):(24+/-3):(10.5+/-1.2):(5.3+/-0.5) corresponds to a vibrational temperature of 6400 +/- 180 K. An average vibrational energy of 33 +/- 9/3 kJ mol(-1) is derived based on the observed population of HF(1 <or= v <or= 4) and an estimate of the population of HF(v = 0) by extrapolation. The observed internal energy distribution of HF is consistent with that expected for the four-center (alpha,beta) elimination channel. A modified impulse model taking into account geometries and displacement vectors of transition states during bond breaking predicts satisfactorily the rotational excitation of HF. We also compare internal energies of HF observed in this work with those from photolysis of vinyl fluoride (CH(2)CHF) and 2-chloro-1,1-difluoroethene (CF(2)CHCl) at 193 nm.

Homo‐elimination of HF—An Efficient Approach for Intramolecular Aryl–Aryl Coupling

“…The DFT calculations were carried out with the B3LYP hybrid functional and the 6-311GA C H T U N G T R E N N U N G (d,p) standard basis set, [26] which previously had been applied successfully for the investigation of HF elimination in fluorobenzene. [23] Frequency calculations were then performed to characterise stationary points as minima or saddle points and to evaluate zero point vibrational energies. The transition states were found by following the minimum energy path from the reactant to the expected product.…”

“…The activation barrier in the latter case is higher and amounts to 92 kcal mol À1 (Scheme 1). [23] Considering the pyrolysis of 1 and 2, the formation of benzyne 3 as an intermediate through 1,2-HF elimination appears to be most probable (Scheme 2). The highly reactive intermediate 3 could relax by ring closure and result in the target benzoA C H T U N G T R E N N U N G [ghi]fluoranthene (4).…”

Femtosecond photoelectron diffraction on laser-aligned molecules: Towards time-resolved imaging of molecular structure

“…1), with photoelectrons at five different kinetic energies between 31 and 62 eV. pFAB was chosen as a prototype system, because the photochemistry of several related phenyl halides has been studied previously and the existence of several shortlived transition states has been suggested, for example for fluorobenzene [26,27].…”

Photochemistry of aryl halides: Photodissociation dynamics

“…Experimentally, the photodissociation dynamics of aryl halides have widely been studied in the gas phase by photofragment translational spectroscopy (PTS) [50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65], in real time by ultrafast pump-probe spectroscopy combined with timeof-flight (TOF) [66][67][68][69][70][71], by time-resolved Fourier-transform spectroscopy [35,71,72], by resonance enhanced multiphoton ionization (REMPI) technique [73][74][75][76], and by multimass ion imaging techniques [36,48,49,[77][78][79][80]. Fox example, Bersohn and coworkers [50,51] were firstly to study the photodissociation dynamics of the aryl halides at 193 and 248 nm.…”