Formation and infrared absorption of protonated naphthalenes (1-C10H9+ and 2-C10H9+) and their neutral counterparts in solid para-hydrogen

Protonated naphthalene (C10H9+) and its neutral counterparts (hydronaphthyl radicals, C10H9) are important intermediates in the reactions of aromatic compounds and in understanding the unidentified infrared (IR) emissions from interstellar media. We report the IR spectra of 1-C10H9+, 2-C10H9+, 1-C10H9, and 2-C10H9 trapped in solid para-hydrogen (p-H2); the latter three are new. These species were produced upon electron bombardment of a mixture of naphthalene (C10H8) and p-H2 during matrix deposition. The intensities of IR features of 1-C10H9+ decreased after the matrix was maintained in darkness for 19 h, whereas those of 1-C10H9 and 2-C10H9 increased. Irradiation of this matrix sample with light at 365 nm diminished lines of 1-C10H9+ and 2-C10H9 and enhanced lines of 1-C10H9 and 2-C10H9+; the latter species was unstable and converted to 1-C10H9+ in less than 30 min and 2-C10H9 was converted to 1-C10H9 at 365 nm. Observed wavenumbers and relative intensities of these species agree satisfactorily with the anharmonic vibrational wavenumbers and IR intensities predicted with the B3PW91/6-311++G(2d,2p) method. Compared with spectra recorded previously with IR photodissociation of Ar-tagged C10H9+ or IR multiphoton dissociation of C10H9+, our method has the advantages of producing high-resolution IR spectra with a wide spectral coverage, true IR intensity and excellent ratio of signal to noise; both protonated species and their neutral counterparts are produced with little interference from other fragments. With these advantages, the IR spectra of 1-C10H9+, 2-C10H9+, 1-C10H9, and 2-C10H9 are here clearly characterized.