Microwave spectrum and rotational isomerism of ethyl nitrate

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Abstract
The existence of two rotational isomers of ethyl nitrate (CH3CH2ONO2) has been confirmed by gas phase microwave spectroscopy. The isomers are designated trans and gauche to indicate the relative positions of the CH3- and -NO2 groups. In the trans form all the heavy atoms lie in a plane. The gauche form differs from the trans by a rotation about the C–O bond of 95° and by an opening of the CON angle by ≈ 3°; both changes appear to indicate considerable nonbonded interaction of the CH3- and -NO2 groups. Stark effect measurements determined the dipole moments to be: μtrans = 3.39 D and μgauche = 3.23 D. The gauche form is less stable by 143 ± 70 cm−1. Several low-lying excited states of the torsional vibration have been assigned for each form. A four-term potential function for internal rotation about the C–O bond was determined. The uncertainties and error limits of a four-term potential function are discussed.