Internal rotation in 1,4-dimethylnaphthalene studied by high resolution laser spectroscopy

Abstract

The high resolution laser induced fluorescence spectrum of the 0₀ ⁰ S ₁(¹ A ₁) ← S ₀(¹ A ₁) electronic transition in 1,4-dimethylnaphthalene has been studied in a molecular beam. The residual Doppler width of 12 MHz allowed rotational resolution and the study of the effects of the internal rotation of the two methyl groups. All strong lines were assigned and the rotational constants in both the ground and excited electronic states were determined. The internal rotation of the two methyl groups manifests itself in the spectrum by a splitting of each rotational transition into three lines. The splitting of the lines is 40 ± 1 MHz and constant up to J = 11 and K ₊₁ = 11. The intensity ratio of the lines is 1:2:1 within 10 per cent. No further splittings were observed in the investigated frequency range. It is shown that the spectrum is totally explained by the simple model of two independent internal rotors attached to an asymmetric rotor frame. The effect of the interaction of the two rotors is negligible with the present resolution as is the effect of the interaction between internal rotation and overall rotation. The measured splitting of 40 MHz yields information about the barriers to internal rotation in the two electronic states. With some assumptions a barrier height of 570 ± 10 cm^-1 in the excited S ₁ state is deduced.

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Notes on contributors

Jon T. Hougen

Nederlandse organisatie voor Wetenschappelijk Onderzoek, visiting scientist University of Nijmegen, spring 1988.