This work has been digitalized and published in 2013 by Verlag Zeitschrift für Naturforschung in cooperation with the Max Planck Society for the Advancement of Science under a Creative Commons Attribution 4.0 International License. Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Creative Commons Namensnennung 4.0 Lizenz. Microwave Spectrum of 1-Butene Oxide S. 0. Ljunggren and P. J. Mjöberg Department of Physical Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden and J. E. Bäckvall Department of Organic Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden Z. Naturforsch. 33a, 1312— 1322 (1978); received July 18, 1978 The microwave spectrum of 1-butene oxide in the gas phase has been studied in the frequency region 18.0—39.0 GHz. The spectrum observed arose from a rotamer with a dihedral H-C2 -C3 -C4 angle of 59° ± 1°. In addition to several ^-branch progressions the spectrum contained several long perpendicular RP and PR progressions. However, of the ground state lines, only the inter mediate PR transitions showed internal rotation splittings that could be resolved to yield a barrier height of 3.02 kcal mol-1. The value derived from the line splittings of the first excited methyl torsional state was slightly higher (3.17 kcal mol-1) but must be regarded as being less reliable. The components of the dipole moment, the rotational constants, and the quartic and sextic centrifugal distortion coefficients for the ground state and three vibrationally excited states were determined. Introduction The preferred conformation of 1,2-disubstituted ethanes and analogous ethane fragments often differ from what would be predicted on steric grounds [1]. On the basis of steric bulk and size these molecules would be expected to be more stable in an anti rather than in a gauche conforma tion, which is true for a large number of molecules. However, when electron pairs or polar bonds are present there is usually a strong preference for the gauche conformation. Examples of this include 2-haloethanols [2], propyl halides [3], 1,2-di- methoxyethane [4], and many other compounds. Microwave studies have been performed on cyclopropyl methyl ether [5] and on epifluorohydrin [ 6 ], molecules with a structure similar to that of 1-butene oxide. As expected, epifluorohydrin prefers a conformation that is gauche with respect to the oxygen atom, while the cyclopropyl methyl ether molecule may assume two equivalent con formations. The purpose of the present study was to determine the preferred conformation of 1-butene oxide and to study the barrier to internal rotation of the methyl group in this molecule. Experimental The sample of 1-butene oxide was obtained from Fluka AG and was further purified by distillation. Microwave spectra were recorded using a Hewlett- Packard Model 8460 A spectrometer equipped with a phase-stabilised source oscillator and with a Stark cell modulation frequency of 33.33 kHz. The measurements were carried out at — 20 °C (253 K) in the frequency region 18.0—39.0 GHz with sample pressures of 9 to 55 mTorr (1.2 to 7.3 Pa). Microwave Spectrum The 1-butene oxide molecule is a prolate asym metric rotor with x — — 0.936 and with the main component of the dipole moment along the b axis. The strongest lines in the spectrum are 6-type Q-branch progressions of the type J k , j -k <- J k -i , j -k + i with K= 1,2 or 3 (Figure 1). These lines were easily assigned using current methods. In addition to the ground state transitions, two other complete sets of Q-branch transitions with the approximate relative intensities 0.56 and 0.31 (in relation to the ground state) at — 20 °C could be assigned to the first and second excited states of the heavy atom torsion, respectively. Another set of Q-branch transitions with a relative intensity of 0.28 was observed rather close to the ground state lines. These lines appeared to be single during a rapid scan (1 MHz s-1) at 55 mTorr (7.3 Pa) but were found to be split upon slow scanning at 9 mTorr (1.2 Pa). The splittings ranged from 0.5 to 1.1 MHz