The radio spectra of planar aromatic heterocycles: how to quantify and predict the negative inertial defects

Abstract

The simplest tricyclic aromatic nitrogen heterocyclic molecules 5,6-benzoquinoline and 7,8-benzoquinoline are possible candidates for detection of aromatic systems in the interstellar medium. Therefore the pure rotational spectra have been recorded using frequency-scanned Stark modulated, jet-cooled millimetre wave absorption spectroscopy (48-87 GHz) and Fourier Transform Microwave (FT-MW) spectroscopy (2-26 GHz) of a supersonic rotationally cold molecular jet. Guided by theoretical molecular orbital predictions, spectral analysis of mm-wave spectra, and higher resolution FT-MW spectroscopy provided accurate rotational and centrifugal distortion constants together with N nuclear quadrupole coupling constants for both species. The tricyclic frames of these species undergo low energy out-of-plane zero-point vibrations resulting in deviations from the moments of inertia that the rigid structure would exhibit. The determined inertial defects, along with those of similar species are used to develop an empirical formula for calculation of inertial defects of aromatic ring systems. The predictive ability of the formula is shown to be excellent in general for planar species with a number of pronounced out-of-plane vibrations. The resultant constants for the benzoquinolines are of sufficient accuracy to be used in astrophysical searches for planar aromatic heterocycles.

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

“…However, it has been observed that PAHs usually have a small negative value of the inertial defect due to the dominant effect of low-frequency out-of-plane vibrations. [33][34][35] The value of 1,4-NQ inertial defect is Δ = I c À I a À I b = À 0.4268(2) uÅ 2 . The value obtained for 1,4-NQ can be compared with some of similar systems such as naphthalene (À 0.137 uÅ 2 ), [35] azulene (À 0.152 uÅ 2 ), [36] trans-1-hydroxynaphthalene (À 0.212 uÅ 2 ), [37] cis-2-hydroxynaphthalene (À 0.237 uÅ 2 ), [37] cis-1-naphthaldehyde Table 1.…”

“…[33][34][35] The value of 1,4-NQ inertial defect is Δ = I c À I a À I b = À 0.4268(2) uÅ 2 . The value obtained for 1,4-NQ can be compared with some of similar systems such as naphthalene (À 0.137 uÅ 2 ), [35] azulene (À 0.152 uÅ 2 ), [36] trans-1-hydroxynaphthalene (À 0.212 uÅ 2 ), [37] cis-2-hydroxynaphthalene (À 0.237 uÅ 2 ), [37] cis-1-naphthaldehyde Table 1. Theoretical spectroscopic parameters of 1,4-NQ using the 6-311 [c] (D) y/n/n y/n/n y/n/n y/n/n y/n/n y/n/n y/n/n N [d] 44 21 1.9 0.7 0.7 0.6 1.0 0.7 1.5…”

“…Oka proposed an empirical expression to calculate the inertial defect of planar molecules based on their lowest out-of-plane vibration, predicted theoretically. [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.…”

“…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.…”

“…Including the second lowerfrequency mode improves the prediction of 1,4-NQ inertial defect, bringing it within 5 % of the experimental value, which is comparable to predictions of related PAHs using harmonic frequencies calculated with the same computational method. [35]…”