Theoretical infrared and electronic absorption spectra of C₁₆H₁₀isomers, their ions and doubly ions

Abstract

Polycyclic aromatic hydrocarbons (PAHs) or PAH-related molecules are considered to be responsible for the unidentified infrared (UIR) emission features at 3.3, 6.2, 7.7, 8.6 and 11.2 µm. However, the exact identification of PAH or PAH-related molecules is difficult. There have been several investigations on the spectroscopic characterization of PAH molecules. But none of them compared the spectra of isomers of PAHs, which might have help in the identification of the UIR emission features. This work presents the infrared and electronic absorption spectra of isomers of C 16 H 10. The aim of the present work is to compare infrared and electronic absorption spectra of four isomers of C 16 H 10 PAH viz. pyrene, aceanthrylene, acephenanthrylene and fluoranthene, their ions and doubly ions. We also compare the spectra of pyrene in the gas-phase and in H 2 O ice. We have used the density functional theory with B3LYP exchange and correlation functional and 6-311++g** basis set to study the infrared spectra. The time-dependent density functional theory (TDDFT) has been used to obtain the electronic absorption spectra. Significant difference in the CC stretching, CH in-plane bending and CH out-of-plane bending vibration modes is observed for the isomers of C 16 H 10 whereas there is no large difference in the CH stretching vibration band. A significant change in the vibrational band is observed for pyrene in H 2 O ice compared to gas-phase pyrene. Though isomers of C 16 H 10 PAH have the same number of carbon and hydrogen atoms, their spectroscopic characteristics are different. This study should help in identifying the isomers of C 16 H 10 , their ions and doubly cation in the interstellar medium.

Ab initio simulation of pyrene spectra in water matrices

“…The electronic spectra of PAHs are most frequently calculated by TDDFT. 3,[61][62][63] Thus, in ref. 3 the conclusions about presence or absence of certain PAHs and their ions in the interstellar clouds were made on the basis of their TD-B3LYP calculated spectra.…”

Infrared Spectrum of the Pyrene Anion in the CH Stretching Region

“…This red shift of the vibrational features is a general observation for PAHs. 53 We note that the transitions observed here are more than 200 cm −1 below the detachment threshold of bare Py − (0.406 eV), 68 and Ar solvation will increase the electron binding energy further. 70 As a result, we do not expect any coupling of the C−H stretching features with the detachment continuum.…”

Isomer Differentiation of Trapped C16H10+ Using Low-Energy Collisions and Visible/VUV Photons

“…Regarding the criterion on the parent breakdown curve, the visible MPD scheme offers the best way to differentiate the Flu + regardless of the laser parameter one varies: the excitation wavelength (Figure ), the pulse energy (Figure and Figure S9), and the number of delayed pulses (Figure a,c). This is due to the much lower absorption cross section of Flu + compared to the other isomers in the spectral range studied from λ OPO ∼432 nm to nm. , In the MPD experiments, we also found two suitable conditions for differentiation between EtP + and Pyr + . First, at λ OPO = 455 nm and high values of E OPO , strong nonlinear effects amplify the difference in dissociation yield between the two isomers (Figure ).…”

“…This is due to the much lower absorption cross section of Flu + compared to the other isomers in the spectral range studied from λ OPO ∼432 nm to 460 nm. 67,68 In the MPD experiments, we also found two suitable conditions for differentiation between EtP + and Pyr + . First, at λ OPO = 455 nm and high values of E OPO , strong nonlinear effects amplify the difference in dissociation yield between the two isomers (Figure 7).…”