Competitive bond rupture in the photodissociation of bromoacetyl chloride and 2- and 3-bromopropionyl chloride: adiabatic versus diabatic dissociation.

Competitive bond dissociation mechanisms for bromoacetyl chloride and 2- and 3-bromopropionyl chloride following the (1) [n(O)→π*(C=O)] transition at 234-235 nm are investigated. Branching ratios for C−Br/C−Cl bond fission are found by using the (2+1) resonance-enhanced multiphoton ionization (REMPI) technique coupled with velocity ion imaging. The fragment branching ratios depend mainly on the dissociation pathways and the distances between the orbitals of Br and the C=O chromophore. C−Cl bond fission is anticipated to follow an adiabatic potential surface for a strong diabatic coupling between the n(O)π*(C=O) and np (Cl)σ*(C−Cl) bands. In contrast, C−Br bond fission is subject to much weaker coupling between n(O)π*(C=O) and np (Br)σ*(C−Br). Thus, a diabatic pathway is preferred for bromoacetyl chloride and 2-bromopropionyl chloride, which leads to excited-state products. For 3-bromopropionyl chloride, the available energy is not high enough to reach the excited-state products such that C−Br bond fission must proceed through an adiabatic pathway with severe suppression by nonadiabatic coupling. The fragment translational energies and anisotropy parameters for the three molecules are also analyzed and appropriately interpreted.

Ultrafast dynamics of o-bromofluorobenzene studied by time-resolved photoelectron imaging.

Photodissociation dynamics and rotational wave packet coherences of o-bromofluorobenzene are studied by femtosecond time-resolved photoelectron imaging [figure: see text]. The decay of different photoelectron rings shows the population decay of states from which the lifetimes of different states are determined. The variation of photoelectron angular distributions reflects the evolution of rotational coherences.Photodissociation dynamics and rotational wave packet coherences of o-bromofluorobenzene are studied by femtosecond time-resolved photoelectron imaging (TR-PEI) spectroscopy combined with the (1+2') resonance-enhanced multiphoton ionization (REMPI). Photoelectron kinetic energy and angular distributions indicate ionization dynamics from some Rydberg states at the (1+1') photon energy. The lifetimes of the S(1) (A') and T(2) (A') states are determined from the decay of the photoelectron signals to be 38 ps and 27 ps. The electron population decay of the two states is attributed to predissociation and tunneling dissociation. The variation of time-dependent anisotropy parameters in the first 5 ps shows the rotational wave coherences of molecule.

Photodissociation dynamics of propionyl chloride in the ultraviolet region.

Velocity imaging technique combined with (2+1) resonance-enhanced multiphoton ionization (REMPI) is used to detect primary photodissociation of propionyl chloride. In one-color experiments at 235 nm, the Cl and Cl(*) fragments are produced rapidly, leading to a fraction of translational energy release of 0.37 and 0.35, anisotropy parameters of 1.1 and 0.8, and quantum yield of 0.67 and 0.33, respectively, when initial excitation of the (1)(n, pi(*))(CO) band is coupled to the (1)(n(Cl), sigma(C-Cl)(*)) repulsive configuration. The resulting propionyl radical with sufficient internal energy may undergo secondary dissociation to yield CO that is characteristic of an isotropic distribution. The REMPI spectra of the CO (0,0) and (1,1) bands are measured, giving rise to a Boltzmann rotational temperature of 1200 and 770 K, respectively, and a Boltzmann vibrational temperature of 2800 K. A minor channel of HCl elimination is not detected, probably because of predissociation in two-photon absorption at 235 nm. In two-color experiments comprising an additional 248 nm photolyzing laser, Cl and Cl(*) are produced with a fraction of translational energy release of 0.43 and 0.40 and anisotropy parameters of 1.0 and 0.6, respectively. The secondary production of CO is not observed although the internal energy partitioned in the propionyl radical is in the proximity of the dissociation barrier. In either experimental scheme, a small component appearing in the center of the Cl and Cl(*) images is proposed to stem from ground state dissociation via internal conversion.

Photodissociation of dibromobenzenes at 266 nm by the velocity imaging technique.

A velocity imaging technique combined with (2+1) resonance-enhanced multiphoton ionization (REMPI) is used to detect the primary Br((2)P(3/2)) fragment in the photodissociation of o-, m-, and p-dibromobenzene at 266 nm. The obtained translational energy distributions suggest that the Br fragments are produced via two dissociation channels. For o- and m-dibromobenzene, the slow channel that yields an anisotropy parameter close to zero is proposed to stem from excitation of the lowest excited singlet (pi,pi*) state followed by predissociation along a repulsive triplet (n,sigma*) state localized on the C-Br bond. The fast channel that gives rise to an anisotropy parameter of 0.53-0.73 is attributed to a bound triplet state with smaller dissociation barrier. For p-dibromobenzene, the dissociation rates are reversed, because the barrier for the bound triplet state becomes higher than the singlet-triplet crossing energy. The fractions of translational energy release are determined to be 6-8 and 29-40 % for the slow and fast channels, respectively; the quantum yields are 0.2 and 0.8, and are insensitive to the position of the substituent. The Br fragmentation from bromobenzene and bromofluorobenzenes at the same photolyzing wavelength is also compared to understand the effect of the number of halogen atoms on the phenyl ring.

Halogen effect on the photodissociation mechanism for gas-phase bromobenzene and iodobenzene.

The velocity imaging technique combined with (2+1) resonance-enhanced multiphoton ionization (REMPI) is used to detect the halogen fragments in the photodissociation of bromobenzene and iodobenzene at 266 nm. With the aid of potential energy curve calculations by Lunell (Y. J. Liu, P. Persson, S. Lunell, J. Phys. Chem. A 2004, 108, 2339-2345.), the Br fragmentation is proposed to stem from excitation of the lowest excited singlet (pi-pi*) state followed by predissociation along a repulsive triplet (n-sigma*) state. The slowed dissociation rate leads to production of the isotropic Br fragments and 93 % internal energy deposition. Only the ground state Br((2)P(3/2)) is detectable. In contrast, when iodine is substituted, the iodine effect stabilizes the repulsive states associated with the I-C6H5 bond rupture and the subsequent dissociation channels become more complicated. 84 % of the iodobenzene molecules obtained follow a direct dissociation channel, while the remaining undergo a predissociative process. Both routes result in rapid dissociation with anisotropy parameters of 0.7+/-0.2 and 0.9+/-0.2 as well as 70 % and 26 % in the fractions of translational energy deposition, respectively. The relative quantum yields of I* and I are 0.35 and 0.65 and their related photodissociation pathways are discussed in detail.