Rotational analysis of the electronic spectrum of the allyl radical

Abstract

The rotationally resolvedà A 2 B 1 X X 2 A 2 electronic spectra of the allyl radical C 3 H 5 and its isotopic derivative C 3 D 5 have been recorded in the gas phase by cavity ring down spectroscopy through a supersonic planar plasma. A rotational analysis yields accurate molecular constants for the first excited electronic state with T 00 ¼ 24 491:229 2ð80Þ for C 3 H 5 . A C 2v planar structure in the zero-point level is indicated. The obtained rotational constants were employed to determine the C-C bond length 1.48 Å and the CCC bond angle 123 in the 2 B 1 electronic state. Two other bands observed are assigned to the excitation of the 7 (339 cm À1 ) and 9 (101 cm À1 ) vibrational modes in the upper electronic state.

Electronic spectrum of the hydrocarbon cation

“…Ten P-branch lines (N ¼ 5-6 to 9-10) observed with the cw CRD spectrometer clearly match the five lines observed previously as listed in [12]. Each of these peaks appears to split into two when scanned with higher resolution.…”

“…Electronic transition of CCCCH 1 3 A rotationally resolved molecular absorption band has been detected in the 408 nm region. The initial aim was to record the previously observedà 2 B 1 ÁX 2 A 2 band of the allyl radical at higher resolution [12]. However, the observed pattern of rotational lines, especially the presence of many doublets of comparable intensity (figure 1), indicates that another molecule is the absorber.…”

“…The experiment (discussed in section 3.2) consisted of a pulsed CRD set-up sampling a plasma generated in a pulsed supersonic jet with a 3 cm  300 mm multilayer slit nozzle [12]. The C 3 D 5 radicals were synthesized in the discharge of a gas pulse (30 Hz repetition rate, 10 bar backing pressure, 100 mA at À600 V applied voltage) of 0.3% C 2 D 2 in an Ar mixture.…”

“…The previously presented assignment and derived molecular structure for allyl radical are incorrect [12]. A re-analysis is difficult due to the overlap with the rotational lines from CCCCH þ 3 .…”

“…The doublets clearly indicate that these lines observed at lower resolution cannot be due to the allyl radical as previously assigned; the unresolved lines are actually due to CCCCH þ 3 . However, one can be sure some of the other absorptions also observed in [12] are due to the allyl radical because of the vibrational assignments and the shifts observed upon deuteration.…”