Formation and infrared absorption of protonated naphthalenes (1-C ₁₀H₉⁺ 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-C ₁₀H₉⁺, 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-C ₁₀H₉⁺ 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-C₁₀H ₉ 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-C ₁₀H₉ 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.