Photodissociation of Formyl Fluoride in Rare Gas Matrixes

Abstract

Photodissociation of formyl fluoride (HCOF) is studied in Ar, Kr, and Xe matrixes at 248 and 193 nm excitation by following spectral changes in the infrared absorption spectra. In all matrixes, the main photodissociation products are CO/HF species, including CO-HF and OC-HF complexes and thermally unstable CO/HF species (a distorted CO/HF complex or a reaction intermediate), which indicate negligible cage exit of atoms produced via the C-F and C-H bond cleavage channels. However, the observation of traces of H, F, CO, CO(2), F(2)CO, FCO, and HRg(2)(+) (Rg = Kr or Xe) in Kr and Xe matrixes would imply some importance of other reaction channels too. The analysis of the decay curves of the precursor shows that dissociation efficiency of HCOF increases as Ar < Kr < Xe, the difference being the factor of 10 between Ar and Xe. Moreover, HCOF dissociates 20-50 times faster at 193 nm compared to 248 nm. Interestingly, whereas the CO/HF species are stable with respect to photolysis in Ar, they photobleach in Kr and Xe matrixes at 248 and 193 nm, even though the first excited states of CO and HF are not energetically accessible with 193 and 248 nm photons. In krypton matrix, the photodissociation of CO/HF species at 248 nm is observed to be a single photon process. Quantum chemical calculations of electronic excitation energies of CO-HF and OC-HF complexes show that the electronic states of HF and CO mostly retain their diatomic nature in the pair. This clearly demonstrates that photodissociation of CO/HF complexes is promoted by the surrounding rare gas lattice.

Structure and Matrix Isolation Infrared Spectrum of Formyl Fluoride Dimer: Blue-Shift of the C−H Stretching Frequency

“…The infrared spectra of the HCOF monomer and the known dimers are in agreement with the previous experimental reports, 9,15 and the splitting of the monomer absorptions can be ascribed to well-known matrix site-effects. For formaldehyde, similar site splitting has been observed for all fundamentals except the ν 5 mode in the nitrogen matrix, but interestingly no splitting is observed in the argon matrix.…”

“…The infrared spectra of the HCOF monomer and the known dimers are in agreement with the previous experimental reports, , and the splitting of the monomer absorptions can be ascribed to well-known matrix site-effects. For formaldehyde, similar site splitting has been observed for all fundamentals except the ν 5 mode in the nitrogen matrix, but interestingly no splitting is observed in the argon matrix. − Examples of other small polyatomic molecules showing distinct site splitting include, for example, cis- and trans- formic acid in argon matrix and HONO in krypton matrix. , Typically, the relative intensity of the doublets is sensitive to deposition temperature and annealing of the matrix.…”

“…A closer look at the spectrum shows that the vibrational bands of the HCOF monomer are split into two lines. Gradual annealing of the sample sharpened and slightly decreased the intensities of the absorption lines of the monomer, and after annealing at 30 K, the monomer absorptions consisted of two clearly separated lines as presented in our previous study . It is also worth mentioning that the single absorption line at 1023.0 cm -1 sharpened as well.…”

“…Gradual annealing of the sample sharpened and slightly decreased the intensities of the absorption lines of the monomer, and after annealing at 30 K, the monomer absorptions consisted of two clearly separated lines as presented in our previous study. 9 It is also worth mentioning that the single absorption line at 1023.0 cm -1 sharpened as well. Simultaneously, the broad shoulder bands of the HCOF absorptions were gradually bleached, and the dimer bands at 1818 and 672.7 cm -1 gradually gained intensity upon annealing.…”

“…After the deposition of formyl fluoride in an argon matrix (HCOF/Ar ) 1/500) at 10 K, five of the six normal vibrations of HCOF monomer, two dimer absorptions at 1818.5 and 672 cm -1 , and the absorption of carbon monoxide were observed in the infrared spectrum. 9,15 The appearance of CO in the sample indicates decomposition of HCOF during the deposition. However, there was no evidence of HF in the IR spectrum.…”