skip to main content

Title: Photodissociation dynamics of acetone studied by time-resolved ion imaging and photofragment excitation spectroscopy
The photodissociation dynamics of acetone has been investigated using velocity-map ion imaging and photofragment excitation (PHOFEX) spectroscopy across a range of wavelengths spanning the first absorption band (236–308 nm). The radical products of the Norrish Type I dissociation, methyl and acetyl, as well as the molecular product ketene have been detected by single-photon VUV ionization at 118 nm. Ketene appears to be formed with non-negligible yield at all wavelengths, with a maximum value of Φ ≈ 0.3 at 280 nm. The modest translational energy release is inconsistent with dissociation over high barriers on the S 0 surface, and ketene formation is tentatively assigned to a roaming pathway involving frustrated dissociation to the radical products. Fast-moving radical products are detected at λ ≤ 305 nm with total translational energy distributions that extend to the energetic limit, consistent with dissociation occurring near-exclusively on the T 1 surface following intersystem crossing. At energies below the T 1 barrier a statistical component indicative of S 0 dissociation is observed, although dissociation via the S 1 /S 0 conical intersection is absent at shorter wavelengths, in contrast to acetaldehyde. The methyl radical yield is enhanced over that of acetyl in PHOFEX spectra at λ ≤ more » 260 nm due to the onset of secondary dissociation of internally excited acetyl radicals. Time-resolved ion imaging experiments using picosecond duration pulses at 266 nm find an appearance time constant of τ = 1490 ± 140 ps for CH 3 radicals formed on T 1 . The associated rate is representative of S 1 → T 1 intersystem crossing. At 284 nm, CH 3 is formed on T 1 with two distinct timescales: a fast <10 ns component is accompanied by a slower component with τ = 42 ± 7 ns. A two-step mechanism involving fast internal conversion, followed by slower intersystem crossing (S 1 → S 0 → T 1 ) is proposed to explain the slow component. « less
; ;
Award ID(s):
Publication Date:
Journal Name:
Physical Chemistry Chemical Physics
Page Range or eLocation-ID:
2457 to 2469
Sponsoring Org:
National Science Foundation
More Like this
  1. Acetaldehyde cations (CH 3 CHO + ) were prepared using single-photon vacuum ultraviolet ionization of CH 3 CHO in a molecular beam and the fragmentation dynamics explored over the photolysis wavelength range 390–210 nm using velocity-map ion imaging and photofragment yield (PHOFY) spectroscopy. Four fragmentation channels are characterized: CH 3 CHO + → C 2 H 3 O + + H (I), CH 3 CHO + → HCO + + CH 3 (II), CH 3 CHO + → CH 3 + + HCO (III), CH 3 CHO + → CH 4 + + CO (IV). Channels (I), (II), and (IV) are observed across the full photolysis wavelength range while channel (III) is observed only at λ < 317 nm. Maximum fragment ion yields are obtained at ∼250 nm. Ion images were recorded over the range 316–228 nm, which corresponds to initial excitation to the B̃ 2 A′ and C̃ 2 A′ states of CH 3 CHO + . The speed and angular distributions are distinctly different for each detected ion and show evidence of both statistical and dynamical fragmentation pathways. At longer wavelengths, fragmentation via channel (I) leads to modest translational energies ( E T ), consistent with dissociation overmore »a small barrier and production of highly internally excited CH 3 CO + . Additional components with E INT greater than the CH 3 CO + secondary dissociation threshold appear at shorter wavelengths and are assigned to fragmentation products of vinyl alcohol cation or oxirane cation formed by isomerization of energized CH 3 CHO + . The E T distribution observed for channel (III) products peaks at zero but is notably colder than that predicted by phase space theory, particularly at longer photolysis wavelengths. The colder-than-statistical E T distributions are attributed to contributions from secondary fragmentation of energized CH 3 CO + formed via channel (I), which are attenuated by CH 3 CHO + isomerization at shorter wavelengths. Fragmentation via channels (II) and (IV) results in qualitatively similar outcomes, with evidence of isotropic statistical components at low- E T and anisotropic components due to excited state dynamics at higher E T .« less
  2. The dissociative photoionization processes of methyl hydroperoxide (CH 3 OOH) have been studied by imaging Photoelectron Photoion Coincidence (iPEPICO) spectroscopy experiments as well as quantum-chemical and statistical rate calculations. Energy selected CH 3 OOH + ions dissociate into CH 2 OOH + , HCO + , CH 3 + , and H 3 O + ions in the 11.4–14.0 eV photon energy range. The lowest-energy dissociation channel is the formation of the cation of the smallest “QOOH” radical, CH 2 OOH + . An extended RRKM model fitted to the experimental data yields a 0 K appearance energy of 11.647 ± 0.005 eV for the CH 2 OOH + ion, and a 74.2 ± 2.6 kJ mol –1 mixed experimental-theoretical 0 K heat of formation for the CH 2 OOH radical. The proton affinity of the Criegee intermediate, CH 2 OO, was also obtained from the heat of formation of CH 2 OOH + (792.8 ± 0.9 kJ mol –1 ) to be 847.7 ± 1.1 kJ mol –1 , reducing the uncertainty of the previously available computational value by a factor of 4. RRKM modeling of the complex web of possible rearrangement-dissociation processes were used to model the higher-energymore »fragmentation. Supported by Born–Oppenheimer molecular dynamics simulations, we found that the HCO + fragment ion is produced through a roaming transition state followed by a low barrier. H 3 O + is formed in a consecutive process from the CH 2 OOH + fragment ion, while direct C–O fission of the molecular ion leads to the methyl cation.« less
  3. Hydrogen-rich cation radicals (GATT + 2H) + ˙ and (AGTT + 2H) + ˙ represent oligonucleotide models of charged hydrogen atom adducts to DNA. These tetranucleotide cation radicals were generated in the gas phase by one-electron reduction of the respective (GATT + 2H) 2+ and (AGTT + 2H) 2+ dications in which the charging protons were placed on the guanine and adenine nucleobases. We used wavelength-dependent UV/Vis photodissociation in the valence-electron excitation region of 210–700 nm to produce action spectra of (GATT + 2H) + ˙ and (AGTT + 2H) + ˙ that showed radical-associated absorption bands in the near-UV (330 nm) and visible (400–440 nm) regions. Born–Oppenheimer molecular dynamics and density-functional theory calculations were used to obtain and rank by energy multiple (GATT + 2H) dication and cation-radical structures. Time-dependent density functional theory (TD-DFT) calculations of excited-state energies and electronic transitions in (GATT + 2H) + ˙ were augmented by vibronic spectra calculations at 310 K for selected low-energy cation radicals to provide a match with the action spectrum. The stable product of one-electron reduction was identified as having a 7,8-dihydroguanine cation radical moiety, formed by intramolecular hydrogen atom migration from adenine N-1–H. The hydrogen migration was calculated tomore »have a transition state with a low activation energy, E a = 96.5 kJ mol −1 , and positive activation entropy, Δ S ‡ = 75 J mol −1 K −1 . This allowed for a fast isomerization of the primary reduction products on the ion-trap time scale of 150 ms that was substantially accelerated by highly exothermic electron transfer.« less
  4. Laser-induced fluorescence (LIF) excitation, dispersed fluorescence (DFL), UV–UV-hole burning, and UV-depletion spectra have been collected on methyl anthranilate (MA, methyl 2-aminobenzoate) and its water-containing complex (MA–H 2 O), under jet-cooled conditions in the gas phase. As a close structural analog of a sunscreen agent, MA has a strong absorption due to the S 0 –S 1 transition that begins in the UV-A region, with the electronic origin at 28 852 cm −1 (346.6 nm). Unlike most sunscreens that have fast non-radiative pathways back to the ground state, MA fluoresces efficiently, with an excited state lifetime of 27 ns. Relative to methyl benzoate, inter-system crossing to the triplet manifold is shut off in MA by the strong intramolecular NH⋯OC H-bond, which shifts the 3 nπ* state well above the 1 ππ* S 1 state. Single vibronic level DFL spectra are used to obtain a near-complete assignment of the vibronic structure in the excited state. Much of the vibrational structure in the excitation spectrum is Franck–Condon activity due to three in-plane vibrations that modulate the distance between the NH 2 and CO 2 Me groups, ν 33 (421 cm −1 ), ν 34 (366 cm −1 ), and ν 36 (179 cm −1more »). Based on the close correspondence between experiment and theory at the TD-DFT B3LYP-D3BJ/def2TZVP level of theory, the major structural changes associated with electronic excitation are evaluated, leading to the conclusion that the major motion is a reorientation and constriction of the 6-membered H-bonded ring closed by the intramolecular NH⋯OC H-bond. This leads to a shortening of the NH⋯OC H-bond distance from 1.926 Å to 1.723 Å, equivalent to about a 25% reduction in the H⋯O distance compared to full H-atom transfer. As a result, the excited state process near the S 1 origin is a hydrogen atom dislocation that is brought about primarily by heavy atom motion, since the shortened H-bond distance results from extensive heavy-atom motion, with only a 0.03 Å increase in the NH bond length relative to its ground state value.« less
  5. The T 1 excited state relaxation in thiophosgene has attracted much attention as a relatively simple model for the intersystem crossing (ISC) transitions in polyatomic molecules. The very short (20–40 ps) T 1 lifetime predicted in several theoretical studies strongly disagrees with the experimental values (∼20 ns) indicating that the kinetics of T 1 → S 0 ISC is not well understood. We use the nonadiabatic transition state theory (NA-TST) with the Zhu–Nakamura transition probability and the multireference perturbation theory (CASPT2) to show that the T 1 → S 0 ISC occurs in the quantum tunneling regime. We also introduce a new zero-point vibrational energy correction scheme that improves the accuracy of the predicted ISC rate constants at low internal energies. The predicted lifetimes of the T 1 vibrational states are between one and two orders of magnitude larger than the experimental values. This overestimation is attributed to the multidimensional nature of quantum tunneling that facilitates ISC transitions along the non-minimum energy path and is not accounted for in the one-dimensional NA-TST.