Formation of molecular hydrogen from protonated 9,10-dihydroanthracene: Is the ejected H 2 rotationally and vibrationally excited?

Abstract

The infrared multiple-photon dissociation (IRMPD) spectrum of protonated 9,10-dihydroanthracene ([DHA+H] +, m/ z 181) has been recorded using an infrared free electron laser. Protonation was accomplished by electrospray ionization with subsequent mass-selection and trapping in a Fourier transform ion cyclotron mass spectrometer. IR-induced fragment ions were observed at m/ z 179, 166, and 165. Density functional calculations (B3LYP/6-311++G(d,p)) of the infrared spectra of the two possible protonated isomers of DHA showed that the observed IRMPD spectrum is best fit to a mixture of the two isomers. Potential energy surfaces for the loss of atomic and molecular hydrogen from the aliphatic carbons of [DHA+H] + have been calculated. The lowest energy barriers are for loss of H 2. After H 2 ejection, stabilization of the remaining fragment occurs by hydrogen migration from one of the aliphatic carbons to the bare ejection site. In all cases the stabilized fragment is computed to be 9-hydroanthracene. The IRMPD spectrum of the m/ z 179 fragment has been recorded and is shown to correspond closely both to the calculated and previously recorded IRMPD spectrum of ionic 9-hydroanthracene. The highly asymmetric transition state conformation of the to-be-formed H 2 and the remaining fragment is highly suggestive of rotational, vibrational, and, possibly, translational excitation of the ejected H 2. Evidence for such excitation from astronomical observations that show the close proximity of PAHs and H 2 in certain interstellar objects and that show H 2 rotational excitation, which has been difficult to explain via either collisional activation or UV pumping, is reviewed.