The microwave spectrum and dipole moment of pentafluorobenzene

Journal of Molecular Structure, 3 19 (1994) 297-299 0022-2860/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved

297

Short Communication

The microwave spectrum and dipole moment of pentafluorobenzene Masao Onda*, Hiroaki Yamada, Masayoshi Mori, Hideya Miyazaki, Ichiro Yamaguchi Department

of Chemistry,

Faculty of Science and Technology, Sophia University, Tokyo 102, Japan

(Received 26 October 1993)

The microwave spectrum of pentafluorobenzene was reported by Doraiswamy and Sharma in 1974 [l]. They determined its dipole moment as 144(5)D from the Stark effect of only one component, M = 1, of the 3(3,1)-2(2,0) transition. They found that most of the low J lines of the R-branch were in congested regions of the spectrum (8-12.4 GHz). This situation is often encountered for rather large molecules, such as substituted benzenes, due to their small rotational constants and many low lying vibrational states. In 1982, Doraiswamy also reported the centrifugal distortion constants and structure of this molecule

t21. Here, we present the redetermination of the dipole moment with improved accuracy from the Stark coefficient of low-J transitions measured using a molecular beam Fourier transform spectrometer. The rotational transitions were measured using a home-made Balle-Flygare type molecular beam-FT-microwave spectrometer [3]. A single microwave source system was adapted similarly to the one constructed by Suenram et al. [4]. The frequency region of the spectrometer in this investigation

was 6- 14GHz.

* Corresponding author. SSDI

0022-2860(93)07961-U

The sample of pentafluorobenzene was purchased from Aldrich Chemicals and was used without further purification. The sample gas was a mixture of a few per cent of ~n~~uorobenzene with argon. The stagnation pressure was approximately 1 atm. The Stark electric field was chosen to be parallel to the electric field of the microwave radiation (AM = 0 selection rule). The Stark electrode consisted of two aluminum plates (230x 230 x 1 mm). The Stark field was calibrated against the J = 1-O M = 0 transition of OCS (0.71521D) [5]. The observed frequencies and the rotational constants are shown in Table 1. The Stark effect data are summarized in Table 2. The Stark shifts for the measured M components at maximum field (about 300 Vcm-‘) were at most 50 kHz. We could not obtain the coefficients for the components of A4 = 0 and A4 = 1 for the 4(4,1)-3(3,0) and 4(4,0)-3(3,1) transitions with the same precision as for the 3(3,1)2(2,0) transition. The dipole moment obtained from ,six M components was 1.43 D and its uncertainty was estimated to be 0.03D as one standard deviation from least squares fitting. This coincides with the previous value of 1.44(5)D

VI*

298

M. Onda et al./J. Mol. Struct. 319 (1994) 297-299

Table 1 Observed frequencies and rotational

constants of ~ntafluoro~n~n~

Transition

Frequencies

3(3,1)-2(W) 3(W)-2(U) 4(2,3)-3(1,2) 4(3,2)-X2,1) 4(3,1)-3(X2) 4(4,1)-3(3,0) 4(4,0)-363,l)

(MHz)

Observed

Obs. - Calc.

8134.7616 8382.9665 7214.3525 9441.5356 1058 1.0279 11179.3005 11254.0059

-0.0009 0.0034 0.0005 0.0036 0.0013 -0.0034 -0.0023

Rotational

A B C

(MHz)

constants

(MHz)

This work

Ref. 1

1480.8641(2)’ 1030.0685(4) 607.5025(3)

1480.856(3) 1030.066(3) 607.49612)

a One standard deviation.

of only a few large molecules, substituted benzene, have been such as determined with a Stark modulated microwave spectrometer. For such molecules, most of the transitions that appear in the x-band region are of large J; they have many A4 comDipole

moments

ponents and small Stark coefficients. Another factor making the measurement of M components difficult is the collision broadening of the spectral lines and many satellites due to vibrational excited states. In this study, however, the band width of a component is

Table 2 Stark coefficients’ and dipole moment of pentafluorobenzene Transition

M

3(W)-2&O) 4(4,0)-3(3,1) 4(4,1)-3(3,0) Dipole

mOment,~b

(D)

This work

Ref. 1

1.43(3)

1.44(5)

’ x 1O-6 MHz V-’ cm2.

Stark coefficients Observed

Calculated

3.33 -12.40 -3.62 -6.79 -7.39 -18.03

3.28 -12.32 -3.51 -7.02 -8.12 -18.71

M. Onak et al.1.J. Mol. Siruct. 319 (1994) 297-299

reduced to 15 kHz due to the low vibrational temperature of the molecules in the molecular beam. We determined a rather large Stark coefficient above 3 x 10d6 MHzVT2 cm2 for a low J transition precisely by our Stark system. We will continue to measure the dipole moments of tetraf?uorobenzenes(l,2,3,4and 1,2,3,5-) with our present FT system.

299

References 1 S. Doraiswamy and S.D. Sharma, Pramana, 2 (1974) 219. S. Doraiswamy, Pramana, 18 (1982) 303. : T.J. Balle and W.H. Flygare, Rev. Sci. Instrum., 52 (1981) 33. 4 R.D. Suenram, F.J. Lovas, G.T. Fraser, J.Z. Gillies, C.W. Gillies and M. Onda, J. Mol. Spectrosc., 137 (1989) 127. J.S. Muenter, J. Chem. Phys., 48 (1968) 4544.