Photochemistry of UV-Excited Trifluoroacetylacetone and Hexafluoroacetylacetone I: Infrared Spectra of Fluorinated Methylfuranones Formed by HF Photoelimination

Abstract

The photochemistry of gas-phase 1,1,1-trifluoroacetylacetone (TFAA) excited with ultraviolet (UV) light involves a significant photoelimination channel that produces hydrogen fluoride and a fluorinated methylfuranone, 2,2-difluoro-5-methyl-3(2H)-furanone (2FMF). This pathway is remarkable because it is a gas-phase unimolecular reaction that forms a five-membered ring product. This report is the first of such a TFAA photoelimination channel, which is similar to one observed with 1,1,1,5,5,5-hexafluoroacetylacetone (HFAA), resulting in 2,2-difluoro-5-trifluoromethyl-3(2H)-furanone. We present infrared spectral observations of 2FMF produced by pulsed, UV-laser excitation of TFAA, along with analogous results from HFAA, supported by density functional theory (DFT) computational studies. DFT results for the infrared spectrum of 5-methyl-3(2H)-furanone, the expected comparable acetylacetone photoelimination product, help suggest that UV excitation of acetylacetone fails to follow a similar type of photoelimination. We use a weighted RMS approach as a figure of merit for comparing calculated infrared frequencies with experimental data. Results from the three acetylacetones reveal how the presence of fluorine atoms in acetylacetone influences the gas-phase molecular photochemistry.

Dissociative electron attachment to the complexation ligands hexafluoroacetylacetone, trifluoroacetylacetone and acetylacetone; a comparative experimental and theoretical study

“…We thus expect the reaction path to be similar to that of HFAAc, i.e., rotation to a hydrogen bonded intermediate promoting HF loss and the formation of [M À HF] À in the furanone form. In fact this reaction was also observed for neutral TFAAc in a recent photolysis study by Muyskens et al 37 From TFAAc, we also observe [M À 2HF] À (Fig. 7b) and HF 2 À (Fig.…”

“…This argument was later supported by a tandem-chemical laser spectroscopy study by Pimentel et al 36 who detected the neutral HF formed in the reaction. Recently, Muyskens et al 37 showed that this reaction is also observed in photolysis of triuoroacetylacetone (TFAAc), but the corresponding reaction leading to the formation of H 2 from AAc was not observed. In a sense, electron excitation from a bonding to an antibonding orbital is comparable to electron capture to the same antibonding orbital in DEA.…”

Ultrafast X-ray Spectroscopy of Intersystem Crossing in Hexafluoroacetylacetone: Chromophore Photophysics and Spectral Changes in the Face of Electron-Withdrawing Groups

“…We also note that the TCC rotamer has been suggested as a precursor to the transition state for the proposed photodissociation pathway to form HF + pentafluoromethyl-3-furanone. 17 Despite this, the possible pentafluoromethyl-3-furanone (PF3F) product, is not observed in the experimental spectrum. The PF3F molecule in the S 0 ground state is predicted by ROKS to have a C K-edge spectrum very similar to HfAcAc ground state (as shown in Figure S6).…”

Fluorine atom influence on intramolecular hydrogen bonding, isomerization and methyl group rotation in fluorinated acetylacetones

“…Although the presence of trifluoromethyl groups on the chelated ring weakens the HB, the enol form remains dominant in the gas phase. Recently Muyskens et al [8] showed that both TFAA and HFAA undergo gas-phase HF elimination upon UV excitation, whereas the analogous H 2 elimination pathway in AA is not evident. Further understanding of the role that fluorine atoms play in the HB strength and the HF elimination pathway leads to our interest in 1-fluoro-pentane-2,4-dione (monofluoroacetylacetone, MFAA), which is the focus of this paper.…”

“…The questions then are which isomer with an asymmetric fluorine distribution will be favored, and is the strength of the HB simply related to the number of fluorine atoms? Scheme 2 shows the proposed mechanism for the gas phase photoelimination reaction of TFAA and HFAA reported by Muyskens et al [8] adapted for MFAA. In this scheme, the orientation of the IHB is such that proton transfer is not required before C2-C3 rotation brings the fluorine atom into proximity of the proton leading to elimination.…”