Microwave Spectrum of the Ethylmethyl Ether Molecule

The ethylmethyl ether molecule (CH₃CH₂OCH₃) has internal rotations of two CH₃ groups of threefold symmetry. The molecular structure is shown in Figure 1. This molecule is a slightly asymmetric top molecule and has the dipole moment components along principal inertia a- and b-axis. Since the component along a-axis is so small that the transitions assigned so far are all b-type transitions. The first microwave spectra of this molecule and its isotopic species were observed in the frequency range from 8.5 to 34 GHz by Hayashi and Kuwada, and molecular parameters A, B, C and D_J and components of dipole moment in the ground state were reported. They also reported the potential barrier heights 2554 and 3294 cal/mol for OCH₃ and CH₃C groups, respectively [1].

In this study, we have observed rotational transitions in the 24-110 GHz frequency range and newly assigned 111 transitions according to the prediction from the molecular parameters determined by Hayashi et al. A rotational transition split into two or four components due to the internal rotation of the two methyl groups: a transition split into two components due to the internal rotation of OCH₃ group, and each component further splits into two components due to the internal rotation of CH₃C group. However, for lower J transitions these latter splitting are too small to be observed with a conventional microwave spectrometer. We analyzed the averaged frequencies of the split components on the basis of the Watson A-reduced Hamiltonian [2], neglecting the effect of the internal rotations. A total of 122 transitions including 11 transitions observed by Hayashi [1] were fitted to the Hamiltonian with a 1σ standard deviation of 24 kHz.

The block diagram of spectrometer is shown in Figure 2. [3] The fundamental microwave source is a microwave synthesizer (HP83642A) operating in the frequency range from 2 to 40 GHz. In the frequency range from 40 to 110 GHz, millimeter-wave source modules (Hewlett Packard, HP83556A, HP83557A and HP8358A) were used. In the measurement above 40 GHz, the source frequency was modulated by small amplitude of a 50 kHz sinusoidal-wave, and the detected microwave signal was demodulated by a lock-in amplifier operated in the 2f mode. The second derivative of an absorption line shape was recorded on a personal computer. In the measurement below 40 GHz, the square-wave Stark modulation at 100 kHz was used to prevent distortion of baselines. The accuracy of observed frequencies is estimated to be better than 70 kHz for the Stark modulation measurements and better than 50 kHz for the source modulation measurements. The observation was made at room temperature.

Transitions of Q-branch series for Ka=1 ← 0, 2 ← 1, and 3 ← 2 and those of R-branch series for Ka=1 ← 0, 0 ← 1, 1 ← 2, 2 ← 3, 3 ← 4 and 4 ← 5 were assigned.

(a) Q-branch transition

J_{1 J-1} ← J_{0 J} series: The absorption line for the 26_{1 25} ← 26_{0 26} transition is shown in Figure 3 as a typical rotational line. All the lines belonging to this series show doublet structures due to the internal rotation of the -OCH₃ group. Separations of the components in this series are from 0.36 to 1.03 MHz, which increase with J. The assignment to this series was made with the help of calculated frequencies using the rotational constants reported by Hayashi et al. To find the successive J transitions in the Q-branch series, we used the power series expansion of J (J+1). We assigned the transitions of this series with J=1 to 29.

Q-branch series transitions J_{2 J-2} ← J_{1 J-1} with J = 7 to 9, with 12 to 16, and with J=21 to 37, J_{2 J-1} ← J_1J with J =7 and 12 to 22, and J_{3 J–3} ← J_{2 J-2} with J = 21 to 30 were assigned. Absorption lines for 20 _{2 19} ← 20_{1 20} and 30 _{2 27} ← 30_{2 28} are shown in Figure 4 and Figure 5, respectively. Transitions belonging to J_{2 J-2} ← J_{1 J-1} series show the doublet structures and those belonging to J_{2 J-1} ← J_{1 J} , and J_{3 J–3} ← J_{2 J-2} series show the quartet structures due to the internal rotations of the two methyl groups.

(b) R-branch transition

The absorption line for 9_{1 9} ← 8_{0 8} is shown in Figure 6 and that for 31_{3 28} ← 30_{4 27} is included in Figure 5. The R-branch series transitions (J+1) _{1 J+1} ← J_{0 J} with J = 1 to 10, (J+1) _{0 J+1} ← J_1J with J = 5 to 14, (J+1)_{1 J} ← J_{2 J-1} with J = 10 to 18, (J+1) _{2 J-1} ← J _{3 J-2} with J = 18, 21, 22, 23, and 24, (J+1)_{3 J-2} ← J_{4 J-3} with J =25 to 31, and (J+1)_{4 J-3} ← J_{5 J-4} with J = 34 to 38 were assigned. Transitions belonging to (J+1)_{1 J+1} ← J_{0 J} , (J+1) _{0 J+1} ← J_{1 J} , and (J+1) _{1 J} ← J_{2 J-1} series show the doublet structures and those belonging to (J+1)_{2 J-1} ← J_{3 J-2} , (J+1)_{3 J-2} ← J_4J-3, and (J+1)_{4 J-3} ← J_{5 J-4} series show the quartet structures.

(c) Rotational Constant

A total of 322 lines have been observed and listed in Table 1. 122 b-type transitions in total were assigned. In this study, we obtained the average of frequencies of split components as the transition frequencies. The 122 transition frequencies are also listed in Table 1. We fitted these frequencies to determine the rotational constants using the Watson A-reduced Hamiltonian.

where P is total angular momentum with components P_a, P_b and P_c along the a-, b- and c-axis, respectively.

The determined rotational constants are listed in Table 2. A, B, C and D_J were refined and D_JK, D_K, d_J and d_K were determined for the first time.

The potential barrier heights for the CH₃ torsions are relatively high, and the torsional splittings are small in the ground state. The average frequencies of these split components were determined as transition frequencies. These frequencies were fitted to the asymmetric rotor Hamiltonian with 8 molecular parameters with a 1σ standard deviation of 24 kHz.