TUFIR of Malonaldehyde

A tunable far-infrared difference frequency spectrometer has been used to examine the fully

protonated form of malonaldehyde in the region near the ground-state tunneling frequency

(21 cm -I). An extremely dense and complex spectrum is observed in which the

strongest features have been assigned as pure rotational lines involving high values of J and

K _ I' These transitions, which occur within the individual rotational manifolds of the

two halves of the ground-state tunneling doublet, have been analyzed simultaneously with

existing microwave data for this species. The value of the ground-state tunneling

splitting, determinable indirectly from analysis of vibration-rotation interactions, is

21.58476(17) cm- I

, and is in close agreement with that similarly obtained in previous

microwave work. A thorough treatment of the centrifugal distortion in this system

significantly extends the range of rotational states whose energies may be reliably calculated,

and should therefore be valuable in the future direct measurement of the tunneling

frequency. Aspects of the far-infrared spectrum of this species, and of the indirect method of

determining the tunneling splitting, are discussed.