Deciphering the Rotational Spectrum of the First Excited Torsional State of Propylene Oxide

Abstract The first excited torsional state of the chiral molecule propylene oxide, CH 3 C 2 H 3 O , is investigated from millimeter up to sub-millimeter wavelengths (75-950 GHz). The first excited vibrational mode of propylene oxide, υ 24 , is analysed using the programs ERHAM and XIAM. Rotational constants and tunneling parameters are provided, and a description of the A-E splittings due to internal rotation is given. Furthermore, the potential barrier height to internal rotation V 3 is determined to be V 3 = 898.6611 ( 894 ) cm - 1 . Our results are compared with quantum chemical calculations and literature values. We present a line list of the dense spectrum of the first excited torsional state of propylene oxide in the (sub-)millimeter range. Our results will be useful for further studies of chiral molecules in vibrationally excited states and will enable astronomers to search for rotational transitions originating from υ 24 of propylene oxide in interstellar space.