The C=O Stretching Frequency in the S1 (π* - n) State of Acetaldehyde and its Deuterated Derivatives Determined With the Photofragment Excitation Spectroscopy

Takumi Kono,Masao Takayanagi,Ichiro Hanazaki

doi:10.1155/1995/75278

Abstract

The vibrational structure was recorded for the S1 (π* - n) state of acetaldehyde (CH3CHO) and its deuterated derivatives (CH3CDO, CD3CHO and CD3CDO up to ∼3500 cm-1above the 0-0 bands with the LIF (laser induced fluorescence) and PHOFEX (photofragment excitation) techniques. In the PHOFEX spectroscopy, the yield of the formyl radical, HCO(X ˜) or DCO(X ˜), produced in the photodissociation of acetaldehyde is measured against the excitation frequency. The yield was determined by monitoring one of the rotational lines in the B˜ ← X˜ transition of the formyl radical by the LIF technique. This is the first measurement of the vibronic structure of acetaldehydes in supersonic jets above the dissociation threshold where they are non-fluorescent. The results have made it possible to locate v4 (the C=O stretching) and 2v4 bands unequivocally; the v4 fundamentals at 1217, 1189, 1210 and 1178 cm-1 above the 0-0 band for CH3CHO, CH3CDO, CD3CHO and CD3CDO, respectively. The corresponding overtone (2v4) bands are observed, respectively, at 2414, 2350, 2399 and 2334cm-1. All the vibronic bands newly observed in the PHOFEX spectra are found to be interpreted as the combinations of v4 and 2v4 with the modes appearing in the lower frequency (< 1200 cm-1) region, which consist of v10 (the C—C—O in-plane bending), v14 (the C=O out-of-plane wagging) and v15 (the methyl torsion).

Highlights

The spectroscopy and dynamics of acetaldehyde in the gas phase have been studied extensively, since it is considered to be one of the typical carbonyl compounds whose photochemical behaviour attracts profound interest, Recently, we have reported detailed studies on the photodissociation of jet-cooled acetaldehyde in the lowest excited singlet electronic (S) state,[2,3,4] where the rotational and vibrational excitation of the product, HCO, have been determined as a function of dissociating photon energy
The rotational excitation of HCO was found to be determined solely by the energy released at the exit channel below the barrier, while the vibrational excitation is determined by the excess energy above the reaction barrier.[4]
It has been found that most of the vibronic bands appearing up to 1000 cm-1 from the 0-0 band are assigned to the fundamental, overtone and combination bands of the Vo, VI4 and v15 modes.5,9

Summary

Introduction

The spectroscopy and dynamics of acetaldehyde in the gas phase have been studied extensively, since it is considered to be one of the typical carbonyl compounds whose photochemical behaviour attracts profound interest, Recently, we have reported detailed studies on the photodissociation of jet-cooled acetaldehyde in the lowest excited singlet electronic (S) state,[2,3,4] where the rotational and vibrational excitation of the product, HCO, have been determined as a function of dissociating photon energy. We present the PHOFEX spectra of acetaldehyde and the deuterated derivatives

Results

Conclusion