Photochemistry of Benzylallene: Ring-Closing Reactions to Form Naphthalene

Abstract

Conformer-specific, vibrationally resolved electronic spectroscopy of benzylallene (4-phenyl-1,2-butadiene) is presented along with a detailed analysis of the products formed via its ultraviolet photoexcitation. Benzylallene is the minor product of the recombination of benzyl and propargyl radicals. The mass-selective resonant two-photon ionization spectrum of benzylallene was recorded under jet-cooled conditions, with its S(0)-S(1) origin at 37,483 cm(-1). UV-UV holeburning spectroscopy was used to show that only one conformer was present in the expansion. Rotational band contour analysis provided rotational constants and transition dipole moment direction consistent with a conformation in which the allene side chain is in the anti position, pointing away from the phenyl ring. The photochemistry of benzylallene was studied in a pump-probe geometry in which photoexcitation occurred by counter-propagating the expansion with a photoexcitation laser. The laser was timed to interact with the gas pulse in a short tube that extended the collisional region of the expansion. The products were cooled during expansion of the gas mixture into vacuum, before being interrogated using mass-selective resonant two-photon ionization. The UV-vis spectra of the photochemical products were compared to literature spectra for identification. Several wavelengths were chosen for photoexcitation, ranging from the S(0)-S(1) origin transition (266.79 nm) to 193 nm. Comparison of the product spectral intensities as a function of photoexcitation wavelength provides information on the wavelength dependence of the product yields. Photoexcitation at 266.79 nm yielded five products (benzyl radical, benzylallenyl radical, 1-phenyl-1,3-butadiene, 1,2-dihydronaphthalene, and naphthalene), with naphthalene and benzylallenyl radicals dominant. At 193 nm, the benzylallenyl radical signal was greatly reduced in intensity, while three additional C(10)H(8) isomeric products were observed. An extensive set of calculations of key stationary points on the ground state C(10)H(10) and C(10)H(9) potential energy surfaces were carried out at the DFT B3LYP/6-311G(d,p) level of theory. Mechanisms for formation of the observed products are proposed based on these potential energy surfaces, constrained by the results of cursory studies of the photochemistry of 1-phenyl-1,3-butadiene and 4-phenyl-1-butyne. A role for tunneling on the excited state surface in the formation of naphthalene is suggested by studies of partially deuterated benzylallene, which blocked naphthalene formation.

Low-Temperature Mechanisms for the Formation of Substituted Azanaphthalenes through Consecutive CN and C2H Additions to Styrene and N-Methylenebenzenamine: A Theoretical Study

“…Recently, Sebree et al used ultraviolet photoexcitation to probe the isomerization channels of benzylallene. 38 Their results supplemented with DFT calculations revealed isomerization pathways from benzylallene which yield naphthalene. In particular, one of the mechanisms identified in their work involved photoisomerization of benzylallene to 1-phenyl-1,3-butadiene and then to 1,2-dihydronaphthalene, which loses two hydrogen atoms to ultimately produce naphthalene.…”

Spectroscopy and ionization thresholds of π-isoelectronic 1-phenylallyl and benzylallenyl resonance stabilized radicals

“…It can be produced by removing one hydrogen from benzylallene (C 6 H 5 -CH 2 -CH=C=CH 2 ), which is the head to tail product of benzyl and propargyl radicals, two RSRs. We are currently pursuing a photochemical study of benzylallene that has led us to consider 30 the possibility that the benzylallenyl radical could be an intermediate in pathways that lead to the formation of naphthalene from benzylallene, 11 motivating its spectroscopic characterization. The out-of-plane !-systems of 1-phenylallyl and benzylallenyl radicals are isoelectronic with one another, since 35 the allene moiety of benzylallene lies in the plane of the ring, with its terminal double bond also in-plane where it is not directly conjugated to the radical site.…”

Calculation of excitation energies from the CC2 linear response theory using Cholesky decomposition

“…Since there is a chance that this molecule acts as photochemical precursor in the formation of bicyclic rings-for instance in the atmosphere of Titan-it is important to study the spectroscopic behavior of both isomers. 32 We chose the benzylallene conformers to perform a second test of our implementation. No exploitation of symmetry point group can be done and calculation of excitation energies can rapidly become intractable.…”

“…32 For this purpose, we used the hierarchy of correlation consistent basis set of Dunning, aug-cc-pVXZ with X := {D, T, Q}. 29 Geometries of both isomers were optimized at the B3LYP/6-311G** level 36, 37 using GAUSSIAN09.…”