Abstract
Time-resolved infra-red (IR) absorption spectroscopy is used to follow the production of HF from the reaction of fluorine atoms in liquid acetonitrile (CH3CN). Photolysis of dissolved XeF2 using ∼50 fs duration, 267 nm laser pulses generates F atoms and XeF on prompt (sub-ps) timescales, as verified by broadband transient electronic absorption spectroscopy. The fundamental vibrational band of HF in solution spans more than 400 cm(-1) around the band centre at 3300 cm(-1), and analysis of portions of the time-resolved spectra reveals time constants for the rise in HF absorption that become longer to lower wavenumber. The time constants for growth of 40 cm(-1) wide portions of the IR spectra centred at 3420, 3320 and 3240 cm(-1) are, respectively, 3.04 ± 0.26, 5.48 ± 0.24 and 7.47 ± 0.74 ps (1 SD uncertainties). The shift to lower wavenumber with time that causes these changes to the time constants is attributed to evolution of the micro-solvation environment of HF following the chemical reaction. The initial growth of the high-wavenumber portion of the band may contain a contribution from relaxation of initially vibrationally excited HF, for which a time constant of 2.4 ± 0.2 ps is deduced from IR pump and probe spectroscopy of a dilute HF solution in acetonitrile.
Highlights
The near-UV absorption band is assigned to the XeF (B–X)
This trend is similar to that we reported for a shift to lower wavenumber of the DF fundamental band with a time constant of 10 ps following F-atom reaction in CD3CN, which we attributed to a reorganization of the solvent environment to accommodate reaction products.[13]
Ultraviolet photolysis of XeF2 in acetonitrile generates fluorine atoms that react with an B3 ps time constant to produce HF
Summary
Gas-phase reactions of fluorine atoms with molecular hydrogen and with various organic molecules are exothermic by approximately 130–150 kJ molÀ1 and favour production of vibrationally excited HF.[1,2,3,4] These H-atom transfer reactions exhibit a wealth of fascinating dynamics, including tunnelling through low energy barriers associated with the transition state,[5,6] Feshbach resonances that enhance reaction cross sections at specific collision energies,[7,8] and non-adiabatic transitions between the ground and low lying potential energy surfaces.[9,10,11] Many features of the gas-phase dynamics persist for scattering of F-atoms from liquid hydrocarbon surfaces.[12]. We report a time constant for vibrational relaxation of HF(v = 1) using IR pump-and-probe experiments that examine the coupling of the vibrational motions of HF solute to the solvent bath. TEAS spectra were obtained following 267 nm one-photon photolysis of 0.52 M solutions of XeF2 (99.99%, Sigma Aldrich) in dry CH3CN using a white-light continuum probe dispersed onto a 512-pixel array detector. TVAS experiments combined the 267 nm photolysis of XeF2 with the broadband IR probing and detection to monitor the HF products of the F + CH3CN reaction. The TEAS and TVAS experiments used 5 ml samples of the XeF2–CH3CN solutions, circulated through a Harrick cell with CaF2 windows by a peristaltic pump. Water contamination of samples was monitored periodically by FTIR spectroscopy, and only the outcomes of those experiments with minimal water contamination are reported here
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