High resolution measurements supported by electronic structure calculations of two naphthalene derivatives: [1,5]- and [1,6]-naphthyridine—Estimation of the zero point inertial defect for planar polycyclic aromatic compounds

Abstract

Polycyclic aromatic hydrocarbons (PAHs) molecules are suspected to be present in the interstellar medium and to participate to the broad and unresolved emissions features, the so-called unidentified infrared bands. In the laboratory, very few studies report the rotationally resolved structure of such important class of molecules. In the present work, both experimental and theoretical approaches provide the first accurate determination of the rotational energy levels of two diazanaphthalene: [1,5]- and [1,6]-naphthyridine. [1,6]-naphthyridine has been studied at high resolution, in the microwave (MW) region using a Fourier transform microwave spectrometer and in the far-infrared (FIR) region using synchrotron-based Fourier transform spectroscopy. The very accurate set of ground state (GS) constants deduced from the analysis of the MW spectrum allowed the analysis of the most intense modes in the FIR (ν38-GS centered at about 483 cm(-1) and ν34-GS centered at about 842 cm(-1)). In contrast with [1,6]-naphthyridine, pure rotation spectroscopy of [1,5]-naphthyridine cannot be performed for symmetry reasons so the combined study of the two intense FIR modes (ν22-GS centered at about 166 cm(-1) and ν18-GS centered at about 818 cm(-1)) provided the GS and the excited states constants. Although the analysis of the very dense rotational patterns for such large molecules remains very challenging, relatively accurate anharmonic density functional theory calculations appeared as a highly relevant supporting tool to the analysis for both molecules. In addition, the good agreement between the experimental and calculated infrared spectrum shows that the present theoretical approach should provide useful data for the astrophysical models. Moreover, inertial defects calculated in the GS (ΔGS) of both molecules exhibit slightly negative values as previously observed for planar species of this molecular family. We adjusted the semi-empirical relations to estimate the zero-point inertial defect (Δ0) of polycyclic aromatic molecules and confirmed the contribution of low frequency out-of-plane vibrational modes to the GS inertial defects of PAHs, which is indeed a key parameter to validate the analysis of such large molecules.

Rotation-vibration interactions in the spectra of polycyclic aromatic hydrocarbons: Quinoline as a test-case species

“…In Table V, we show that the slightly negative GS and ES inertial defects are well reproduced by the calculations, as already discussed for doubly N-substituted naphthalene derivatives. 32 Noticeably, the ES inertial defect is overestimated for ν 45 and slightly underestimated for ν 44 .…”

“…The theoretical approach (at the B97-1/cc-pVTZ//ANO-DZP level of computation) has already been described elsewhere and resulted in accurate predictions of the spectroscopic constants for naphthalene 19 and 18 others PAHs and PANHs. 32,37 The rotational analysis of the two ES of quinoline presented hereafter is another validation of this approach.…”

Structural Changes Induced by Quinones: High‐Resolution Microwave Study of 1,4‐Naphthoquinone

“…Anharmonic frequency calculations of PAHs can be modelled at a relatively low computational cost and considerable accuracy by using density functional theory methods. [37,40] Using the B3LYP/6-311 + + G(d,p) level of theory, which was found to perform well in related molecules, [35] we have predicted the low-frequency modes of 1,4-NQ to be at 78 cm À 1 and 119 cm À 1 (harmonic) and 79 cm À 1 and 120 cm À 1 (anharmonic). These values are similar to those measured experimentally for 1-cis-naphthaldehyde and 2-transnaphthaldehyde, [37] specially the lowest-frequency mode.…”

“…[33][34][35] The expression has been recently refined for polycyclic aromatic molecules. Gruet et al [40] proposed to use a different value of α in Oka's empirical formula, maintaining the dependence on the lowest out-of-plane vibration. Jahn et al [35] proposed another value of α, and to include as many low-frequency out-of-plane vibrations as the number of rings in the PAH, or the number of rings plus one [35] for those PAHs with very low out-of-plane vibrations.…”

Synchrotron-based Fourier transform spectra of the ν23 and ν24 IR b

“…After the pioneering work of Hewett et al [1] using a molecular beam, the advent of synchrotron-based FT spectroscopy permitted to extend the study of rotationnally resolved IR bands of small Polycyclic Aromatic Hydrocarbons (PAHs) such as naphthalene and its derivatives (see e.g. [2,3] and references therein) and diamondoid-like molecules [4,5]. Hexamethylenetetramine (or HMT), C 6 N 4 H 12 , is strongly suspected to be present in several astrophysical objects such as e.g.…”