Rotational Spectrum, Structure, and Electric Dipole Moment of Bis(difluoromethyl) Ether R. D. Suenram, F. J. Lovas, A. R. Hight Walker, and D. A. Dixon Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899; and Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999/MS K1-83, Richmond, Washington 99352 Received June 16, 1998 The rotational spectrum of bis(difluoromethyl) ether (CF 2 HOCF 2 H) has been observed and analyzed using both a conventional Stark-modulated microwave spectrometer and a pulsed molecular beam Fabry–Perot cavity microwave (FTMW) spectrometer. The lowest energy conformer studied here has a (hydrogen) syn–anti conformation. The high sensitivity of the FTMW spectrometer permits observation and analysis of the 13 C, 18 O, and 2 H isotopomers in natural abundance. The electric dipole moment was measured for the normal species and found to be  a = 5.020(7)  10 -30 C  m [1.505(2) D],  b = 0.19(4)  10 -30 C  m [0.056(11) D],  c = 0.48(2)  10 -30 C  m [0.144(7) D], and  T = 5.047(10)  10 -30 C  m [1.513(3) D]. INTRODUCTION The depletion of the Earth’s ozone layer by chlorofluorocar- bons (CFCs) has led to a search for new compounds that can be used as alternative refrigerants. For the most part, the new class of compounds being considered consists of various fluorinated hy- drocarbons; however, several compounds from a similar class of compounds containing the ether linkage are also being considered (1). In particular, two ethers with the designation of E134 and E245 have physical properties similar to some of the CFCs currently in use so they have received a major portion of the attention from the thermodynamic researchers. The “E” designa- tions for ether-based compounds were first suggested by Kopko (2) and follow a numbering scheme that was originally adopted for ethane- and propane-based refrigerants. These designations have been adopted by the American Society of Heating, Refrig- erating, and Air Conditioning Engineers (3). Since many of these compounds are new, not much is known about their physical and chemical properties. Research aimed at measuring the OH reac- tivity (4), and their thermodynamic properties (1, 5, 6), have recently been published. Since many of these compounds contain a carbon chain back- bone with three or more atoms connected with single bonds, they tend to exhibit multiple conformations. Multiple conformations add an additional uncertainty to experiments where thermody- namic properties are determined or calculated, since these prop- erties often depend upon a knowledge of the conformers and their relative energies in order to describe the thermodynamic proper- ties (7). In an effort to establish the lowest energy conformation, to limit the number of conformers present, and to measure the electric dipole moments, we have undertaken microwave spec- tral investigations of several members of this series of ethers: (bis(difluoromethyl) ether (CF 2 HOCF 2 H) [E134], 2-(difluoro- methoxy)-1,1,1,trifluoroethane (CF 2 HOCH 2 CF 3 ) [E245], and 2-(difluoromethoxy)-1,1,1,2,tetrafluoroethane (CF 2 HOCHFCF 3 ) [desflurane] (8)). EXPERIMENTAL Detailed descriptions of the pulsed molecular beam and Fabry–Perot cavity microwave spectrometers employed at NIST have been previously published (9). The Stark-modu- lated spectrometer used was essentially the same as described by Suenram and Lovas (10) except that the frequency source was a 2–18 GHz Hewlett–Packard 1 8671-B synthesized signal generator driving an active frequency doubler. The Stark cell was a 1-m stainless steel absorption cell which was cooled to -80°C with dry ice (11). The sample used was obtained from W. R. Grace and was the same sample used by Zhang and coworkers in their OH reactivity experiments (4). The sample was stated to be 99.35% pure. The major impurity was CHF 2 CH 2 F as determined by chromatographic analysis (4). Experiments were started simultaneously with both spec- trometers. The Stark spectrometer provides a broadbanded picture and should indicate the presence of additional low energy isomers if they are present. The spectrum from 24 to 36 GHz from the Stark spectrometer is shown in Fig. 1. The spectrum shows the a-type, high-K -1 , R-branch pileups typical of a prolate rotor. As shown in Fig. 1, the spacing is equal to 3461 MHz  (B + C). The sharp transitions in the spectrum arise from some of the lower K -1 transitions that fall outside of the bandheads. Figure 2 shows an automated scan from the FTMW spec- trometer which is a 4 GHz scan spanning the region from 11.5 to 15.5 GHz. The scan was obtained using a sample consisting of 6.6 kPa of E134 in 0.66 MPa of an 80/20% Ne/He mixture 1 Trade names are used only to adequately describe the apparatus and are not to be construed as an endorsement of specific equipment by NIST. JOURNAL OF MOLECULAR SPECTROSCOPY 192, 441– 448 (1998) ARTICLE NO. MS987713 441 0022-2852/98 $25.00