Dynamical insights into {sup 1}{pi}{sigma}* state mediated photodissociation of aniline
- School of Chemistry, University of Bristol, Bristol BS8 1TS (United Kingdom)
This article reports a comprehensive study of the mechanisms of H atom loss in aniline (C{sub 6}H{sub 5}NH{sub 2}) following ultraviolet excitation, using H (Rydberg) atom photofragment translational spectroscopy. N-H bond fission via the low lying {sup 1}{pi}{sigma}* electronic state of aniline is experimentally demonstrated. The {sup 1}{pi}{sigma}* potential energy surface (PES) of this prototypical aromatic amine is essentially repulsive along the N-H stretch coordinate, but possesses a shallow potential well in the vertical Franck-Condon region, supporting quasibound vibrational levels. Photoexcitation at wavelengths ({lambda}{sub phot}) in the range 293.859 nm{>=}{lambda}{sub phot}{>=}193.3 nm yields H atom loss via a range of mechanisms. With {lambda}{sub phot} resonant with the 1{sup 1}{pi}{pi}*<-S{sub 0} origin (293.859 nm), H atom loss proceeds via, predominantly, multiphoton excitation processes, resonantly enhanced at the one photon energy by the first {sup 1}{pi}{pi}* excited state (the 1{sup 1}{pi}{pi}* state). Direct excitation to the first few quasibound vibrational levels of the {sup 1}{pi}{sigma}* state (at wavelengths in the range 269.513 nm{>=}{lambda}{sub phot}{>=}260 nm) induces N-H bond fission via H atom tunneling through an exit barrier into the repulsive region of the {sup 1{pi}{sigma}*} PES, forming anilino (C{sub 6}H{sub 5}NH) radical products in their ground electronic state, and with very limited vibrational excitation; the photo-prepared vibrational mode in the {sup 1}{pi}{sigma}* state generally evolves adiabatically into the corresponding mode of the anilino radical upon dissociation. However, as the excitation wavelength is reduced ({lambda}{sub phot}<260 nm), N-H bond fission yields fragments with substantially greater vibrational excitation, rationalized in terms of direct excitation to 1{sup 1}{pi}{pi}* levels, followed by coupling to the {sup 1}{pi}{sigma}* PES via a 1{sup 1}{pi}{pi}*/{sup 1}{pi}{sigma}* conical intersection. Changes in product kinetic energy disposal once {lambda}{sub phot} approaches {approx}230 nm likely indicate that the photodissociation pathways of aniline proceed via direct excitation to the (higher) 2{sup 1}{pi}{pi}* state. Analysis of the anilino fragment vibrational energy disposal--and thus the concomitant dynamics of {sup 1}{pi}{sigma}* state mediated photodissociation--provides a particularly interesting study of competing {sigma}*<-{pi} and {pi}*<-{pi} absorption processes and develops our appreciation of the photochemistry of aromatic amines. It also allows revealing comparisons with simple amines (such as ammonia and methylamine) as well as the isoelectronic species, phenol. This study yields a value for the N-H bond strength in aniline, D{sub 0}(H-anilino)=31630{+-}40 cm{sup -1}.
- OSTI ID:
- 21559860
- Journal Information:
- Journal of Chemical Physics, Vol. 132, Issue 21; Other Information: DOI: 10.1063/1.3427544; (c) 2010 American Institute of Physics; ISSN 0021-9606
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
ABSORPTION
AMMONIA
ANILINE
ATOMS
DISSOCIATION
EXCITATION
FISSION
FISSION YIELD
KINETIC ENERGY
METHYLAMINE
PHENOL
PHOTOLYSIS
POTENTIAL ENERGY
REACTION KINETICS
RYDBERG STATES
TUNNEL EFFECT
ULTRAVIOLET RADIATION
VIBRATIONAL STATES
WAVELENGTHS
AMINES
AROMATICS
CHEMICAL REACTIONS
DECOMPOSITION
ELECTROMAGNETIC RADIATION
ENERGY
ENERGY LEVELS
ENERGY-LEVEL TRANSITIONS
EXCITED STATES
HYDRIDES
HYDROGEN COMPOUNDS
HYDROXY COMPOUNDS
KINETICS
NITROGEN COMPOUNDS
NITROGEN HYDRIDES
NUCLEAR REACTION YIELD
NUCLEAR REACTIONS
ORGANIC COMPOUNDS
PHENOLS
PHOTOCHEMICAL REACTIONS
RADIATIONS
SORPTION
YIELDS