Studying Reaction Intermediates using Time-resolved Fourier-transform Infrared Spectroscopy and p-H2 Matrix Isolation Technique

Either one or both of two techniques employed in our laboratory for studying transient species in chemical reactions will be introduced.

We developed the time-resolved Fourier-transform infrared (TR-FTIR) absorption spectroscopy to investigate IR absorption of gaseous transient species. A flow reactor with a multipassing UV photolysis beam and a multipassing IR probe beam is coupled to a step-scan FTIR spectrometer with both dc- and ac-detection to record temporal profiles of the infrared absorption of reaction intermediates. IR absorption spectra of several reactive species such as CH₃OO,¹ CH₃SO, CH₃SO₂,² ClCOOH,³ and CH₃C(O)OO were recorded. Spectral assignments were made based on reaction mechanisms and comparison of observed vibrational wavenumbers and rotational contours with those predicted quantum-chemically. Related reaction kinetics will also be discussed. In this talk, the identifications of ClCOOH and CH₃C(O)OO will be discussed.

Para-hydrogen (p-H₂) has recently emerged as a new matrix host. Because of the ‘softness’ associated with the extensive delocalization of the H₂ moieties, new characteristics of molecules isolated in this quantum solid are explored. We demonstrated that the internal rotation of methanol persists in solid p-H₂ by observation of splittings of the E/A torsional doublets in internal-rotation-coupled vibrational modes.⁴ We also provided direct spectral evidence that CH₃F isolated in p-H₂ rotates about only its symmetry axis, and not about the other two axes by observation of two weak absorption lines from the E (K = 1) level and one intense feature from A (K = 0) for degenerate modes ν₄−ν₆ of CH₃F.⁵ We demonstrated another feature of solid p-H₂, the absence of cage effect, by reaction of Cl, produced from in situ photodissociation of Cl₂, with HCOOH; complexes of Cl with HCOOH were observed. This feature might open up a new direction for investigating the entrance channel of bimolecular reactions and their mode selectivity by carrying out reactions without kinetic energy.