On the Wavelength-Dependent Photochemistry of the Atmospheric Molecule CF3COCl.

The wavelength control of photochemistry usually results from ultrafast dynamics following the excitation of different electronic states. Here, we investigate the CF3COCl molecule, exhibiting wavelength-dependent photochemistry both via (i) depositing increasing internal energy into a single state and (ii) populating different electronic states. We reveal the mechanism behind the photon-energy dependence by combining nonadiabatic ab initio molecular dynamics techniques with the velocity map imaging experiment. We describe a consecutive mechanism of photodissociation where an immediate release of Cl taking place in an excited electronic state is followed by a slower ground-state dissociation of the CO fragment. The CO release is subject to an activation barrier and is controlled by excess internal energy via the excitation wavelength. Therefore, a selective release of CO along with Cl can be achieved. The mechanism is fully supported by both the measured kinetic energy distributions and anisotropies of the angular distributions. Interestingly, the kinetic energy of the released Cl atom is sensitively modified by accounting for spin-orbit coupling. Given the atmospheric importance of CF3COCl, we discuss the consequences of our findings for atmospheric photochemistry.

Threshold photoelectron spectroscopy of trimethylborane and its pyrolysis products.

Trimethylborane (TMB) and its chemistry upon pyrolysis have been investigated by threshold photoelectron spectroscopy. TMB shows an unstructured spectrum and its adiabatic ionization energy (IEad) has been determined to be 9.93 ± 0.1 eV. Dissociative photoionization induces a methyl radical loss in TMB and the barrier to dissociation in the cation is measured to be 0.65 ± 0.1 eV. Upon pyrolysis methane loss dominates, leading to C2H5B, which can exist in five different isomeric structures. Quantum chemical calculations were used to investigate possible methane loss mechanisms as well as the isomerization pathways on the C2H5B potential energy surface. Through isomer-selective photoion mass-selected threshold photoelectron spectroscopy (ms-TPES) the two isomers CH3BCH2 and CH3CHBH were identified by their ms-TPE spectra and IEad values of 8.55 ± 0.02 eV and 8.73 ± 0.02 eV were determined, respectively. A second channel leading to the loss of ethene from TMB forms CH2BH, which exhibits an IEad value of 9.37 ± 0.03 eV. The reaction mechanism in the literature needs to be expanded by an additional methane loss from the intermediately formed ethyl methyl borane.

Photoelectron spectroscopy of low valent organophosphorus compounds, P-CH3, H-PCH2 and PCH2.

We report the mass-selected slow photoelectron spectra of three reactive organophosphorus species, PCH2, and the two isomers, methylenephosphine or phosphaethylene, HPCH2 and methylphosphinidine, P-CH3. All spectra were recorded by double imaging photoelectron-photoion coincidence spectroscopy (i2PEPICO) using synchrotron radiation and all species were generated in a flow reactor by the reaction of trimethyl phosphine with fluorine atoms. Adiabatic ionisation energies of 8.80 ± 0.02 eV (PCH2), 10.07 ± 0.03 eV (H-PCH2) and 8.91 ± 0.04 eV (P-CH3) were determined and the vibronic structure was simulated by calculating Franck-Condon factors from optimised structures based on quantum chemical methods. Observation of biradicalic P-CH3 isomer with its triplet ground state is surprising because it is less stable than H-PCH2.

Photodissociation of the trichloromethyl radical: photofragment imaging and femtosecond photoelectron spectroscopy.

Halogen-containing radicals play a key role in catalytic reactions leading to stratospheric ozone destruction, thus their photochemistry is of considerable interest. Here we investigate the photodissociation dynamics of the trichloromethyl radical, CCl3 after excitation in the ultraviolet. While the primary processes directly after light absorption are followed by femtosecond-time resolved photoionisation and photoelectron spectroscopy, the reaction products are monitored by photofragment imaging using nanosecond-lasers. The dominant reaction is loss of a Cl atom, associated with a CCl2 fragment. However, the detection of Cl atoms is of limited value, because in the pyrolysis CCl2 is formed as a side product, which in turn dissociates to CCl + Cl. We therefore additionally monitored the molecular fragments CCl2 and CCl by photoionisation at 118.2 nm and disentangled the contributions from various processes. A comparison of the CCl images with control experiments on CCl2 suggest that the dissociation to CCl + Cl2 contributes to the photochemistry of CCl3.

Photodissociation of Benzoyl Chloride: A Velocity Map Imaging Study Using VUV Detection of Chlorine Atoms.

UV photodissociation of benzoyl chloride, Ph-CO-Cl, is associated with the loss of a chlorine atom. Here we excite benzoyl chloride to the S1, S2, and S3 excited states at 237, 253, 265, and 279.6 nm and detect the Cl photofragment by [1 + 1'] photoionization using 118.9 nm VUV radiation. The translational energy distribution of the Cl atom is measured by velocity map ion imaging. An isotropic image and a unimodal translational energy distribution are observed at all dissociation wavelengths, and a fraction of 18-20% of the excess energy is released into translation. The results indicate a dissociation that predominately proceeds from the vibrationally hot S0 ground state, although the observed translational energy release deviates significantly from a prior distribution. However, the impulsive model does also not represent the translational energy release. As a Cl/Cl* branching ratio of 9:1 or more is observed in one-color experiments at 235 nm, we conclude that direct dissociation from excited electronic states contributes only to a minor extent.

Kinetics of 1- and 2-methylallyl + O2 reaction, investigated by photoionisation using synchrotron radiation.

The reaction kinetics of the isomers of the methylallyl radical with molecular oxygen has been studied in a flow tube reactor at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source storage ring. The radicals were generated by direct photodissociation of bromides or iodides at 213 nm. Experiments were conducted at room temperature and low pressures between 1 and 3 mbar using He as the buffer gas. Oxygen was employed in excess to maintain near pseudo-first-order reaction conditions. Concentration-time profiles of the radical were monitored by photoionisation. For the oxidation of 2-methylallyl (2-MA) and with k(2-MA + O2) = (5.1 ± 1.0) × 1011 cm3 mol-1 s-1, the rate constant was found to be in the high-pressure limit already at 1 mbar. In contrast, 1-methylallyl exists in two isomers, E- and Z-1-methylallyl. We selectively detected the E-conformer as well as a mixture of both isomers and observed almost identical rate constants within the uncertainty of the experiment. A small pressure dependence is observed with the rate constant increasing from k(1-MA + O2) = (3.5 ± 0.7) × 1011 cm3 mol-1 s-1 at 1 mbar to k(1-MA + O2) = (4.6 ± 0.9) × 1011 cm3 mol-1 s-1 at 3 mbar. While for 2-methylallyl + O2 no previous experimental data are available, the rate constants for 1-methylallyl are in agreement with previous work. A comparison is drawn for the trends of the high-pressure limiting rate constants and pressure dependences observed for the O2 recombination of allylic radicals with the corresponding reactions of alkyl radicals.

Methylbismuth: an organometallic bismuthinidene biradical.

We report the generation, spectroscopic characterization, and computational analysis of the first free (non-stabilized) organometallic bismuthinidene, BiMe. The title compound was generated in situ from BiMe3 by controlled homolytic Bi-C bond cleavage in the gas phase. Its electronic structure was characterized by a combination of photoion mass-selected threshold photoelectron spectroscopy and DFT as well as multi-reference computations. A triplet ground state was identified and an ionization energy (IE) of 7.88 eV was experimentally determined. Methyl abstraction from BiMe3 to give [BiMe2]• is a key step in the generation of BiMe. We reaveal a bond dissociation energy of 210 ± 7 kJ mol-1, which is substantially higher than the previously accepted value. Nevertheless, the homolytic cleavage of Me-BiMe2 bonds could be achieved at moderate temperatures (60-120 °C) in the condensed phase, suggesting that [BiMe2]• and BiMe are accessible as reactive intermediates under these conditions.

Conformational Preference Determined by C-H···π Interaction of an O-H···O Hydrogen-Bonded Binary Complex of p-Fluorophenol with 2,5-Dihydrofuran: A Laser-Induced Fluorescence Spectroscopy Study.

Conformational preferences of a binary hydrogen-bonded complex between p-fluorophenol (pFP) and 2,5-dihydrofuran (DHF) have been studied by means of laser induced fluorescence (LIF) spectroscopy in a supersonic jet expansion. Calculation predicts two major conformers for this complex, one having a nearly linear geometry in which the two molecular moieties are bound only by an O-H···O H-bond, but in the other an additional C-H···π type interaction between an ortho C-H group of pFP and ethylene group of DHF contributes to the binding stabilization and results in a folded geometry for the complex with respect to a global view, although the H-bond angle of the latter is relatively larger. This prediction is realized experimentally by identifying transitions corresponding to the two discrete conformers in a vibrationally resolved LIF excitation spectrum of the complex, and the red shifts of S1-S0 origin band of pFP moiety of the two conformers are 542 and 659 cm-1, respectively. The assignments are corroborated by means of dispersed fluorescence (DF) spectroscopy. In comparison, the LIF spectral bands for the pFP-tetrahydrofuran complex can be corresponded to only one conformer, whose S1-S0 origin transition shows a red shift (563 cm-1) somewhat similar to the linear conformer of pFP-DHF complex. Such similarities in spectral shifting behavior is consistent with the predictions of electronic structure calculations. The DF spectra also reveal that the energy threshold and pathways of vibrational dynamics in S1 of the two conformers show different behavior. Excitation to 6a1 level of pFP moiety of the folded conformer displays signatures of restricted intramolecular vibrational energy redistribution (IVR), whereas the linear form displays the emission feature for dissipative IVR.

LIF Spectroscopy of p-Fluorophenol···Water Complex: Hydrogen Bond Vibrations, Fermi Resonance, and Vibrational Relaxation in the Excited State.

Laser-induced fluorescence (FE) and vibrationally resolved dispersed fluorescence (DF) spectra of a 1:1 water complex of p-fluorophenol (pFP) have been measured in a supersonic jet expansion. The hydrogen bond stretching fundamental (σ01) of the complex appears in the FE spectrum as a doublet with band maxima at 155 and 161 cm-1. Emission spectra recorded upon excitations of the two components reveal that a Fermi resonance between σ1 and a combination involving a low-frequency intramolecular mode of pFP (mode 11) and a bending mode of water at the hydrogen bonded interface (mode ρ1) is responsible for the observed splitting. The DF spectra of the Franck-Condon active 6a01 band (000 + 427 cm-1) of pFP reveals signatures of hydrogen bond induced vibrational energy relaxation (VER) predominantly from the bright (6a01) to a dark (9b1) level in S1. The relative intensities of the emission bands from the locally excited and relaxed levels indicate that VER for excitation up to this level occurs at a time scale similar to the fluorescence decay time of the complex. However, complete VER at a much faster time scale occurs for excitation beyond 822 cm-1 above S1 origin.

On the origin of donor O-H bond weakening in phenol-water complexes.

Matrix isolation infrared spectroscopy has been used to investigate intermolecular interactions in a series of binary O-H⋯O hydrogen bonded phenol-water complexes where water is the common acceptor. The interaction at the binding site has been tuned by incorporating multiple fluorine substitutions at different aromatic ring sites of the phenol moiety. The spectral effects for the aforesaid chemical changes are manifested in the infrared spectra of the complexes as systematic increase in spectral shift of the phenolic O-H stretching fundamental (ΔνO-H). While νO-H bands of the monomers of all the fluorophenols appear within a very narrow frequency range, the increase in ΔνO-H of the complexes from phenol to pentafluorophenol is very large, nearly 90%. The observed values of ΔνO-H do not show a linear correlation with the total binding energies (ΔEb) of the complexes, expected according to Badger-Bauer rule. However, in the same ΔνO-H vs ΔEb plot, nice linear correlations are revealed if the complexes of ortho-fluorophenols are treated separately from their meta/para-substituted analogues. The observations imply that in spite of having the same binding site (O-H⋯O) and the same chemical identities (phenolic), the complexes of ortho and non-ortho fluorophenols do not belong, from the viewpoint of detailed molecular interactions, to a homologous series. Linear correlations of ΔνO-H are, however, observed with respect to the electrostatic component of ΔEb as well as the quantum mechanical charge transfer interaction energy (ECT). From quantitative viewpoint, the latter correlation along with the associated electronic structure parameters appears more satisfactory. It has also been noted that the observed ΔνO-H values of the complexes display a linear relationship with the aqueous phase pKa values of the respective phenol derivatives.

Electronic spectra and excited state dynamics of pentafluorophenol: Effects of low-lying πσ(∗) states.

Multiple fluorine atom substitution effect on photophysics of an aromatic chromophore has been investigated using phenol as the reference system. It has been noticed that the discrete vibronic structure of the S1←S0 absorption system of phenol vapor is completely washed out for pentafluorophenol (PFP), and the latter also shows very large Stokes shift in the fluorescence spectrum. For excitations beyond S1 origin, the emission yield of PFP is reduced sharply with increase in excess vibronic energy. However, in a collisional environment like liquid hydrocarbon, the underlying dynamical process that drives the non-radiative decay is hindered drastically. Electronic structure theory predicts a number of low-lying dark electronic states of πσ(∗) character in the vicinity of the lowest valence ππ(∗) state of this molecule. Tentatively, we have attributed the excitation energy dependent non-radiative decay of the molecule observed only in the gas phase to an interplay between the lowest ππ(∗) and a nearby dissociative πσ(∗) state. Measurements in different liquids reveal that some of the dark excited states light up with appreciable intensity only in protic liquids like methanol and water due to hydrogen bonding between solute and solvents. Electronic structure theory methods indeed predict that for PFP-(H2O)n clusters (n = 1-11), intensities of a number of πσ(∗) states are enhanced with increase in cluster size. In contrast with emitting behavior of the molecule in the gas phase and solutions of nonpolar and polar aprotic liquids, the fluorescence is completely switched off in polar protic liquids. This behavior is a chemically significant manifestation of perfluoro effect, because a very opposite effect occurs in the case of unsubstituted phenol for which fluorescence yield undergoes a very large enhancement in protic liquids. Several dynamical mechanisms have been suggested to interpret the observed photophysical behavior.