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

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 H2 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. (C) 2011 Elsevier B.V. All rights reserved.