The Radio Spectrum of the Phenyl Radical

Abstract

The phenyl radical, C 6 H 5 , derived from benzene by removal of one hydrogen atom, was detected at centimeter wavelengths in a pulsed supersonic molecular beam and subsequently at millimeter wavelengths in a low-pressure direct current glow discharge. Fourteen rotational transitions between 9 and 40 GHz and over 50 transitions between 150 and 330 GHz, each split by spin doubling, have been measured for the normal isotopic species, and a comparable number have been measured for the fully deuterated species. The spectrum of both isotopic species at millimeter wavelengths is reproduced to an uncertainty of 0.5 km s 1 or better with seven spectroscopic constants. Rotational constants predicted from high-level molecular structure calculations are in excellent agreement with the measurements. Phenyl is a prime candidate for astronomical detection, because it is the prototypical aromatic hydrocarbon radical and a possible progenitor of other aromatic species. Subject headings: ISM: molecules-line: identification-molecular data-molecular processes-radio lines: ISM Phenyl, the cyclic radical C 6 H 5 shown in Figure 1, is a crucial intermediate in combustion and soot formation (Glassman 1996 and references therein) and may also play an important role in the chemistry of the interstellar gas and circumstellar envelopes. It is believed that phenyl is the first aromatic radical formed along a reaction pathway that begins with ions and molecules containing two or three carbon atoms and leads to polycyclic aromatic molecules (Woods et al. 2002; Kaiser et al. 2000). Although well characterized by electron spin resonance and by UV, visible, Raman, and IR spectroscopy (Fri-derichsen et al. 2001 and references therein), the radio spectrum of phenyl until now has eluded detection; in the absence of accurate rest frequencies, no searches for phenyl to our knowledge have been undertaken in astronomical sources. Here we describe the detection of this fundamental radical in a discharge through benzene by both Fourier transform microwave (FTM) spectroscopy and millimeter-wave absorption spectroscopy, and give the spectroscopic data needed for an astronomical search throughout the radio band. Phenyl is a planar asymmetric rotor with symmetry, a C 2v 2 A 1 electronic ground state, and a calculated electric dipole moment of 0.9 D along its intermediate principal inertial axis b (see Fig. 1). Because of the near equality of its A and B rotational constants, phenyl falls close enough to the oblate symmetric top limit that its rotational spectrum is that of a symmetric top with slightly broken symmetry. Owing to the interaction of the unpaired electron with molecular rotation and with the magnetic moments of the five protons, the lower ro-tational transitions are split into many resolved hyperfine components , but the hyperfine structure (hfs) collapses with increasing frequency, yielding in the millimeter-wave band a fairly simple rotational spectrum with spin doubling that is well described by a small number of spectroscopic constants. To detect phenyl, we first undertook a FTM search for the fundamental transition predicted to occur near 9.2 GHz 1 r 0 1, 1 0, 0 (Tonokura et al. 2002; Vereecken et al. 2002), using experimental conditions that optimized the production of o-benzyne, the closely related C 6 H 4 ring (Brown, Godfrey, & Rodler 1986). Using ben-zene as a precursor gas, a series of closely spaced magnetic lines with approximately the right multiplicity were found within 1% of the literature predictions. Subsequent searches for other transitions yielded additional series of closely spaced magnetic lines of comparable intensity, the centroids of which could be predicted to a few MHz. On the basis of the preliminary rotational constants derived from the FTM measurements, searches for the millimeter-wave lines of phenyl were then undertaken, again at conditions that optimized the production of o-benzyne. Much to our surprise, lines of o-benzyne and phenyl-detected in either case within 50 MHz of those predicted-were remarkably strong when observed with our free space millimeter-wave spectrometer (Ap-poni et al. 1999), as the spectrum in Figure 2 shows. Phenyl was produced in a low-pressure direct current discharge through a flowing mixture of benzene and argon. The flow rate of argon at standard pressure and temperature was 2 cm 3 minute 1 , and the total pressure in the cell with the discharge on was about 4 mtorr. The strongest lines were observed with a fairly low discharge current of 75 mA and the walls of the cell cooled to 220 K. Under these conditions, lines of phenyl are about 5 times weaker than those of o-benzyne, but the mole fraction of the two species is comparable (; i.e., a concentration of 4 1.4 # 10 cm 3), owing to the difference in dipole moments (a 10 3 # 10 factor of 2.5) and the absence of spin doubling (a factor of 2) in o-benzyne. Phenyl is more than 1000 times more abundant than the linear C 6 H radical observed with the same spectrometer in a discharge through acetylene and argon at a lower temperature (120 K) and somewhat higher pressure (20 mtorr). In all, 62 rotational lines, each split by spin doubling from the unpaired electron, were measured between 150 and 330 GHz for the normal isotopic species; a comparable number of lines (50) were measured for C 6 D 5 when fully deuterated benzene was used in place of benzene. When the transition frequencies were analyzed with Watson's A-reduced Hamiltonian, the millimeter-wave spectrum of phenyl was reproduced with seven spectro-scopic constants (Table 1): three rotational constants, the three leading fourth-order centrifugal distortion constants (, , and D D J J K