JOURNAL OF MOLECULAR SPECTROSCOPY 181, 67–77 (1997) ARTICLE NO. MS967155 Rotational Spectra, Structure, and Electric Dipole Moments of Methyl and Ethyl tert -Butyl Ether (MTBE and ETBE) R. D. Suenram, F. J. Lovas, W. Pereyra, 1 G. T. Fraser, and A. R. Hight Walker Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received July 19, 1996 The rotational spectra of methyl and ethyl tertiary butyl ether (MTBE and ETBE) have been observed and analyzed using a pulsed-molecular-beam Fabry – Perot cavity microwave spectrometer. Both a- and b-type transitions are observed for each compound. Small CH 3 internal rotation splittings from the OCH 3 group of Ç1 MHz are observed for MTBE. The high sensitivity of the Fourier transform microwave (FTMW) technique allows observation and analysis of the rotational spectra of all 13 C isotopes and the 18 O isotopic species in natural abundance. For ETBE, the structural analysis indicates that the lowest energy form of the molecule is the completely extended conformer with C s symmetry. For both compounds, the bulkiness of the t-butyl group causes the COC angle to be 6° to 7° larger than that found in most other ethers.  1997 Academic Press, Inc. II. EXPERIMENTAL I. INTRODUCTION The pulsed-molecular-beam, Fabry – Perot cavity micro- wave spectrometers employed at NIST are modified versions of the original Balle – Flygare design (6). The current config- Methyl and ethyl tertiary butyl ethers (MTBE and ETBE) uration is shown in the block diagram of Fig. 1. A number of and tertiary amyl methyl ether (TAME) are among the oxy- the instrument modifications have been previously described genated hydrocarbons that are added to gasoline to improve (7–9). The salient features of the instrument which collec- octane performance and lower emissions especially during tively make it an extremely sensitive device for trace-gas the winter months (1). analysis include: the use of one microwave source with sin- For MTBE, two independent electron diffraction studies gle-sideband modulation (8), phase-synchronization of all have been performed (2, 3). Subsequently Konaka et al. (4) electronic components (10), coaxial orientation of the pulsed performed a molecular orbital constrained electron diffrac- nozzle with the Fabry–Perot cavity axis (11), and the use tion study using the previously published data. This work of a low-noise microwave amplifier in the detection circuit combined the results of a low-level Hartree–Fock calcula- (8). In addition, two different software packages are cur- tion at the 4–21G level with the electron diffraction work rently in use (9, 12) for automated, unattended frequency and led to a more refined set of structural parameters. scans. Figure 2 shows a scan of the normal species of ETBE Egawa et al. have carried out a structural analysis of ETBE in the vicinity of 12 GHz. This scan was obtained using 0.4 using gas-phase electron diffraction, ab initio techniques, MHz/step and 10 nozzle pulses/step (a 2 Hz nozzle repetition and vibrational spectroscopy (5). This work indicated that rate was used Å 5 sec of integration/step). When searching the lowest energy conformation was the fully extended form for the weaker 13 C and 18 O isotopomers, 4096 points were and resulted in a full set of structural parameters. digitized at 125 ns/pt but only the first 1024 points were In efforts to more fully characterize the structural proper- included in the fast Fourier transform into the frequency ties of these important industrial compounds, we have under- domain. This degrades the resolution element from 2 kHz/ taken an investigation of their rotational spectra. The results pt to 8 kHz/pt but effectively increases the S/N ratio by a obtained here can then be compared with the previous struc- factor of 4. Once transitions had been observed, the frequen- tural analysis. This work also was performed as part of a cies were measured at high resolution (2 kHz/pt) by em- larger project aimed at developing new spectroscopic meth- ploying the full 4096 points for the Fourier transform. All ods for analyzing automotive emissions for oxygenated hy- frequency measurements are believed accurate to {2 kHz. drocarbons. Commercially available liquid samples of the ethers were transferred to a vacuum line and degassed. During the course of this work, samples were prepared at a variety of concen- 1 Current address: City College of New York, New York, New York. 67 0022-2852/97 $25.00 Copyright  1997 by Academic Press, Inc. All rights of reproduction in any form reserved.