Investigating the C−Cl Antibonding Character in the ππ* Excited State of Vinyl, Allyl, and Propargyl Chloride:  Emission Spectra and ab Initio Calculations

Abstract

We report the emission spectra of dissociating vinyl, allyl, and propargyl chloride upon photoexcitation at 199 nm. To provide a better understanding of the mixed electronic character in the Franck-Condon region of the excited states accessed, we also present ab initio calculations at the configuration interaction level for these three molecules. These experimental and theoretical results indicate large differences in the contribution of nσ* C-Cl and πσ* C-Cl character to these predominantly ππ* CdC/CtC transitions. We present arguments based on the symmetry of the pertinent molecular orbitals to explain this observed variation. We conclude by considering how the differing electronic character of the excited state in these molecules may influence the branching ratios to C-Cl fission and HCl elimination products observed in the photodissociation of vinyl, allyl, and propargyl chloride.

Competing isomeric product channels in the 193 nm photodissociation of 2-chloropropene and in the unimolecular dissociation of the 2-propenyl radical

“…The spectrum is similar to the emission spectrum of vinyl chloride. 35 We assign the strongest feature, appearing near 1625 cm Ϫ1 , to 5 ͑the CvC stretch͒ with a literature value of 1640 cm Ϫ1 . 7 The peaks at 3256 and 4880 cm Ϫ1 thus correspond to 2 5 and 3 5 , respectively.…”

“…[34][35][36] The laser crosses a free-jet expansion of 41% 2-chloropropene in He through a pulsed valve with 0.5 mm diameter orifice. The gas mix is formed by bubbling He through 2-chloropropene maintained at 0°C, where its vapor pressure is ϳ255 Torr; the total pressure of the gas mixture is 615 Torr.…”

Chemical Reaction Dynamics Beyond the Born-Oppenheimer Approximation

“…The experiments and supporting ab initio calculations on the competing photodissociation pathways for allyl chloride upon excitation of the nominally ππ * C=C transition at 193 nm further explored how configuration interaction matrix elements can change with molecular conformation, influencing both nonadiabatic recrossing and product branching (61,62). The C-Cl fission reaction channel is formally Woodward-Hoffmann forbidden both in vinyl chloride and in the minor conformer of allyl chloride, which has a plane of symmetry, but not in gauche allyl chloride.…”

“…Analysis of the experimental results (61,62,64; Y-R Lee & S-M Lin, submitted for publication) along with supporting ab initio calculations on both conformers of allyl chloride and on vinyl chloride (65-67a) reveals two things. Figure 8 shows cuts along the excited state C-Cl fission reaction coordinate in vinyl chloride (top), cis allyl chloride (middle), and gauche allyl chloride (bottom).…”

“…This indicates that even before the avoided crossing, the large configuration interaction matrix elements in gauche allyl chloride result in an admixture of the nσ * C-Cl configuration into the nominal ππ * C=C excited state. The partial σ * C-Cl character is easily seen in Figure 9, which compares the "nominal" π * C=C orbital reached at the equilibrium geometry of the ground state for cis allyl chloride and gauche allyl chloride; this admixture in the Franck-Condon region also reveals itself in the emission from the dissociating molecules, which shows intensity in the C-Cl stretch in allyl chloride but not in vinyl chloride (62). An analogous conformation dependence to the π * C=O σ * C-Cl mixing is observed in the β-chloroketones (67b).…”