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Our simulation demonstrates the robustness of the Coulomb explosion imaging technique in studying methyl iodide photodissociation, and shows that it can be effectively used for imaging non-adiabatic transitions in coordinate space.
The photodissociation dynamics of astrophysically relevant propyl derivatives (C 3 H 7 X; X = CN, OH, HCO) at 157 nm exploiting an ultracompact velocity map imaging (UVMIS) setup has been reported. The successful operation of UVMIS allowed the exploration of the 157 nm photo dissociation of six (iso)propyl systems - n/i-propyl cyanide (C 3 H 7 CN), n/i-propyl alcohol (C 3 H 7 OH), and (iso)butanal (C 3 H 7 CHO) – to explore the C 3 H 7 loss channel. The distinct center-of-mass translational energy distributions for the i-C 3 H 7 X (X= CN, OH, HCO) could be explained through preferential excitation of the low frequency C-H bending modes of the formyl moiety compared to the higher frequency stretchings of the cyano and hydroxy moieties. Although the ionization energy of the n-C 3 H 7 radical exceeds the energy of a 157 nm photon, C 3 H 7 + was observed in the n-C 3 H 7 X (X= CN, OH, HCO) systems as a result of photoionization of vibrationally "hot" n-C 3 H 7 fragments, photoionization of i-C 3 H 7 after a hydrogen shift in vibrationally "hot" n-C 3 H 7 radicals, and/or two-photon ionization. Our experiments reveal that at least the isopropyl radical (i-C 3 H 7 ) and possibly the normal propyl radical (n-C 3 H 7 ) should be present in the interstellar medium and hence searched for by radio telescopes.
In the gas phase, potential energy surfaces can be used to provide insight into the details of photochemical reaction dynamics. In solution, however, it is unclear what potential energy surfaces, if any, can be used to describe even simple chemical reactions such as the photodissociation of a diatomic solute. In this paper, we use mixed quantum/classical (MQC) molecular dynamics (MD) to study the photodissociation of $Na$ $^+_2$ in both liquid Ar and liquid tetrahydrofuran (THF). We examine both the gas-phase potential surfaces and potentials of mean force (PMF), which assume that the solvent remains at equilibrium with the solute throughout the photodissociation process and show that neither resemble a nonequilibrium dynamical energy surface that is generated by taking the time integral of work. For the photodissociation of $Na$ $^+_2$ in liquid Ar, the dynamical energy surface shows clear signatures of solvent caging, and the degree of caging is directly related to the mass of the solvent atoms. For $Na$ $^+_2$ in liquid THF, local specific interactions between the solute and solvent lead to changes in chemical identity that create a kinetic trap that effectively prevents the molecule from dissociating. Here, the results show that nonequilibrium effects play an important role even in simple solution-phase reactions, requiring the use of dynamical energy surface to understand such chemical events.
Laser vaporization of uranium in a pulsed supersonic expansion of nitrogen is used to produce complexes of the form U + (N 2 ) n (n = 1–8). These ions are mass selected in a reflectron time-of-flight spectrometer and studied with visible and UV laser fixed-frequency photodissociation and with tunable infrared laser photodissociation spectroscopy. The dissociation patterns and spectroscopy of U + (N 2 ) n indicate that N 2 ligands are intact molecules and that there is no insertion chemistry resulting in UN + or NUN + . Fixed frequency photodissociation at 532 and 355 nm indicate that the U + –N 2 bond dissociation energy varies little with changing coordination. The photon energy and the number of ligands eliminated allow an estimate of the average U + –N 2 dissociation energy of 12 kcal/mol. Infrared bands are observed for these complexes near the N–N stretch vibration via elimination of N 2 molecules. These resonances are observed to be shifted about 130 cm–1 to the red from the free-N 2 frequency for complexes with n = 3–8. Density functional theory indicates that U + is most stable in the sextet state in these complexes and that N 2 molecules bind in end-on configurations. Furthermore, the fully coordinated complex is predicted to be U + (N 2 ) 8 , which has a cubic structure. The vibrational frequencies predicted by theory are consistently lower than those in the experiment, independent of the isomeric structure or spin state of the complexes. Despite its failure to reproduce the infrared spectra, theory provides an average U + –N 2 dissociation energy of 11.8 ± 0.5 kcal/mol, in good agreement with the value from the experiments.
Cation-π complexes of the form UO x + (benzene) (x = 0, 1, 2) are produced by laser vaporization and cooled in a supersonic molecular beam. These ions are mass selected and studied with UV–visible laser photodissociation spectroscopy. Each of these complexes photodissociates by elimination of the benzene ligand. Above an energetic threshold, the absorption and photodissociation are continuous, indicating a high density of strongly coupled electronic states. The thresholds for the dissociation of each of these three complexes are measured and assigned as their respective bond dissociation energies. The bond energies determined [U + –(benzene): 42.5 ± 0.3 kcal/mol; UO + –(benzene): 41.0 ± 0.3 kcal/mol; UO 2 + –(benzene): 39.7 ± 0.3 kcal/mol] are comparable to those of transition metal ion-benzene complexes. Computational studies at the DFT/B3LYP level complement the experiments, predicting dissociation energies in reasonably good agreement with the experiments. Experiments and theory agree that the U+(benzene) complex is more strongly bound than its corresponding oxide ions. This new thermochemistry on actinide cation-π bonding should stimulate higher-level computational studies on these systems.
Here, the photodissociation dynamics of the dimethyl-substituted acetone oxide Criegee intermediate [(CH 3 ) 2 COO] is characterized following electronic excitation to the bright 1 ππ* state, which leads to O ( 1 D) + acetone [(CH 3 ) 2 CO, S 0 ] products. The UV action spectrum of (CH 3 ) 2 COO recorded with O ( 1 D) detection under jet-cooled conditions is broad, unstructured, and essentially unchanged from the corresponding electronic absorption spectrum obtained using a UV-induced depletion method. This indicates that UV excitation of (CH 3 ) 2 COO leads predominantly to the O ( 1 D) product channel. A higher energy O ( 3 P) + (CH 3 ) 2 CO (T 1 ) product channel is not observed, although it is energetically accessible. In addition, complementary MS-CASPT2 trajectory surface-hopping (TSH) simulations indicate minimal population leading to the O ( 3 P) channel and non-unity overall probability for dissociation (within 100 fs). Velocity map imaging of the O ( 1 D) products is utilized to reveal the total kinetic energy release (TKER) distribution upon photodissociation of (CH 3 ) 2 COO at various UV excitation energies. Simulation of the TKER distributions is performed using a hybrid model that combines an impulsive model with a statistical component, the latter reflecting the longer-lived (>100 fs) trajectories identified in the TSH calculations. The impulsive model accounts for vibrational activation of (CH 3 ) 2 CO arising from geometrical changes between the Criegee intermediate and the carbonyl product, indicating the importance of CO stretch, CCO bend, and CC stretch along with activation of hindered rotation and rock of the methyl groups in the (CH 3 ) 2 CO product. Detailed comparison is also made with the TKER distribution arising from photodissociation dynamics of CH 2 OO upon UV excitation.
It is well known that ultraviolet photoexcitation of iron pentacarbonyl results in rapid loss of carbonyl ligands leading to the formation of coordinatively unsaturated iron carbonyl compounds. Here we employ ultrafast mid-infrared transient absorption spectroscopy to probe the photodissociation dynamics of gas-phase iron pentacarbonyl following ultraviolet excitation at 265 and 199 nm. After photoexcitation at 265 nm, our results show evidence for sequential dissociation of iron pentacarbonyl to form iron tricarbonyl via a short-lived iron tetracarbonyl intermediate. Photodissociation at 199 nm results in the prompt production of Fe(CO) 3 within 0.25 ps via several energetically accessible pathways. An additional 15 ps time constant extracted from the data is tentatively assigned to intersystem crossing to the triplet manifold of iron tricarbonyl or iron dicarbonyl. Mechanisms for formation of iron tetracarbonyl, iron tricarbonyl, and iron dicarbonyl are proposed and theoretically validated with one-dimensional cuts through the potential energy surface as well as bond dissociation energies. Ground state calculations are computed at the CCSD(T) level of theory and excited states are computed with EOM-EE-CCSD(dT).
The direct photodetachment and two-photon photodissociation–photodetachment processes of a series of PtI n - (n = 2–5) anions were systematically studied using cryogenic anion photoelectron spectroscopy and first-principles electronic structure calculations. The adiabatic/vertical detachment energies (ADEs/VDEs) of these anions were determined from their 193 nm photoelectron (PE) spectra, i.e., 3.54/3.63, 4.04/4.09, 4.33/4.36, and 4.37/4.41 eV for n = 2–5, respectively, and well reproduced by B3LYP-D3(BJ)/aug-cc-pVTZ-pp calculations. As the coordination number increases, the electron affinity (EA) of PtI n • (n = 2–5) neutrals (equivalent to the corresponding anion’s ADE) gradually increases, exceeding the EA of Cl at n = 3 and exhibiting superhalogen characteristics for n ≥ 3. Meanwhile, the ground state transition contributed from detaching electrons in the highest occupied molecular orbital gradually evolves from the central metal Pt to the iodine ligands. For the PtI 3 - anion, besides one-photon direct detachment, four distinct two-photon photodissociation–photodetachment channels were identified, and the competition between them was discussed.
We have studied the photodissociation of gas-phase bromocyclopropane by 200 nm wavelength ultraviolet radiation using ultrafast electron diffraction. Bromocyclopropane is a prototypical molecule in the study of organobromides, a class of molecules that have a significant impact on atmospheric ozone depletion through their photochemistry. Here, previous studies have revealed two possible reaction pathways for the photodissociation of bromine from bromocyclopropane; either the C–Br bond dissociates, leaving behind a cyclopropyl ring, or there is a concerted opening of the cyclopropyl ring along with the C–Br bond dissociation. In this work, both our experimental and simulation results indicate that the majority of the UV-photoexcited BCP molecules (88% ± 11% in the experiment) follow the first reaction pathway, in which the cyclopropyl ring remains closed after homolytic C–Br bond cleavage. This direct bond dissociation occurs within the experimental time resolution of 270 fs. In order to differentiate between the possible reaction end-products, both of which have diffraction signals dominated by the bromine atom, a new analysis method has been employed, which is more sensitive to the structure of the end-products.
The dicarbon molecule (C2) is found in flames, comets, stars, and the diffuse interstellar medium. In comets, it is responsible for the green color of the coma, but it is not found in the tail. It has long been held to photodissociate in sunlight with a lifetime precluding observation in the tail, but the mechanism was not known. Here we directly observe photodissociation of C 2 . From the speed of the recoiling carbon atoms, a bond dissociation energy of 602.804(29) kJ∙mol -1 is determined, with an uncertainty comparable to its more experimentally accessible N 2 and O 2 counterparts. The value is within 0.03 kJ∙mol -1 of high-level quantum theory. This work shows that, to break the quadruple bond of C 2 using sunlight, the molecule must absorb two photons and undergo two “forbidden” transitions.
Formamide (NH 2 CHO) is the simplest molecule containing a peptide linkage [-NH-C($=$O)-], and it plays an essential role in the study of prebiotic chemistry. Exposure to UV irradiation allows formamide to decompose and act as a prebiotic feedstock in the formation of nucleobases and other necessary starting materials. The photodissociation mechanism of gaseous formamide at 193 nm is studied using (a) chirped-pulse Fourier transform millimeter-wave spectroscopy in the 260-290 GHz spectral region in a room-temperature flow-tube reactor at 1 mu bar pressure, (b) a combination of electronic structure theory, transition state theory, and quasiclassical trajectories, and (c) the Active Thermochemical Tables. The HCN and HNC photoproducts of hydrogenated (NH 2 CHO) and deuterated (NH 2 CDO and ND 2 CHO) formamide precursors are examined to gain insight into the photodissociation mechanism. Here, the theoretical investigation has characterized the main pathway leading to each of the HCN/HNC isomers from the precursor isotopologues. The theoretical branching ratio [HNC]/[HCN] = 2.1 for nascent photofragments agrees with the experiment. The effect of the postphotolysis HNC ↔ HCN isomerization on the [HNC]/[HCN] ratio is predicted. We report the experimental branching ratio [HNCO]: ([HNC] + [HCN]) = 12 ± 3 and propose that most of HNCO originates from dissociation on the S-1 electronic state of formamide.
Laser vaporization of uranium in a pulsed supersonic expansion of carbon dioxide is used to produce complexes of the form U + (CO 2 ) n , UO + (CO 2 ) n , and UO 2+ (CO 2 ) n . These ions are selected in a reflectron time-offlight mass spectrometer and studied with visible laser photodissociation and tunable infrared laser photodissociation spectroscopy in the region of the CO 2 antisymmetric stretch. The dissociation patterns and spectroscopy of these ions indicate that CO 2 ligands are intact molecules. Although reaction products that form oxide-carbonyl or oxalate species are predicted to be stable, there is no direct evidence in the frequency range studied for the formation of these species. There is no clear indication for the coordination numbers for singly charged uranium and its oxide complexes with CO 2 . However, there is strong support in the vibrational patterns for an eight-coordinate complex of the doubly charged UO 2+ species, i.e., UO 2+ (CO 2 ) 8 .
Excitation of iron pentacarbonyl [Fe(CO) 5 ], a prototypical photocatalyst, at 266 nm causes the sequential loss of two CO ligands in the gas phase, creating catalytically active, unsaturated iron carbonyls. Despite numerous studies, major aspects of its ultrafast photochemistry remain unresolved because the early excited-state dynamics have so far eluded spectroscopic observation. This has led to the long-held assumption that ultrafast dissociation of gas-phase Fe(CO) 5 proceeds exclusively on the singlet manifold. Herein, we present a combined experimental–theoretical study employing ultrafast extreme ultraviolet transient absorption spectroscopy near the Fe M 2,3 -edge, which features spectral evolution on 100 fs and 3 ps time scales, alongside high-level electronic structure theory, which enables characterization of the molecular geometries and electronic states involved in the ultrafast photodissociation of Fe(CO) 5 . We assign the 100 fs evolution to spectroscopic signatures associated with intertwined structural and electronic dynamics on the singlet metal-centered states during the first CO loss and the 3 ps evolution to the competing dissociation of Fe(CO) 4 along the lowest singlet and triplet surfaces to form Fe(CO) 3 . Calculations of transient spectra in both singlet and triplet states as well as spin–orbit coupling constants along key structural pathways provide evidence for intersystem crossing to the triplet ground state of Fe(CO) 4 . Finally, our work presents the first spectroscopic detection of transient excited states during ultrafast photodissociation of gas-phase Fe(CO) 5 and challenges the long-standing assumption that triplet states do not play a role in the ultrafast dynamics.
Abstract The dynamics of photodissociation and recombination in heme proteins represent an archetypical photochemical reaction widely used to understand the interplay between chemical dynamics and reaction environment. We report a study of the photodissociation mechanism for the Fe(II)-S bond between the heme iron and methionine sulfur of ferrous cytochrome c . This bond dissociation is an essential step in the conversion of cytochrome c from an electron transfer protein to a peroxidase enzyme. We use ultrafast X-ray solution scattering to follow the dynamics of Fe(II)-S bond dissociation and 1 s 3 p (Kβ) X-ray emission spectroscopy to follow the dynamics of the iron charge and spin multiplicity during bond dissociation. From these measurements, we conclude that the formation of a triplet metal-centered excited state with anti-bonding Fe(II)-S interactions triggers the bond dissociation and precedes the formation of the metastable Fe high-spin quintet state.
Pyruvic acid, a representative alpha-keto carboxylic acid, is one of the few organic molecules destroyed in the troposphere by solar radiation rather than by reactions with free radicals. To date, only its stable final products were identified, often with contribution from secondary chemistry, making it difficult to elucidate photodissociation mechanisms following excitation to the lowest singlet excited-state (S 1 ) and the role of the internal hydrogen bond in the most-stable Tc conformer. Using multiplexed photoionization mass spectrometry we report the first direct experimental evidence, via the observation of singlet methylhydroxycarbene (MHC) following 351 nm excitation, supporting the decarboxylation mechanism previously proposed. Decarboxylation to MHC + CO 2 represents 97–100% of product branching at 351 nm. We observe vinyl alcohol and acetaldehyde, which we attribute to isomerization of MHC. We also observe a 3 ± 2% yield of the Norrish Type I photoproducts CH 3 CO + DOCO, but only from d 1 -pyruvic acid. At 4 Torr pressure, we measure a photodissociation quantum yield of $1.0^{+0}_{–0.4}$, consistent with IUPAC recommendations. However, our measured product branching fractions disagree with IUPAC. In light of previous calculations, these results support a mechanism in which hydrogen transfer on the S 1 excited state occurs at least partially by tunneling, in competition with intersystem crossing to the T 1 state. Here, we present the first evidence of a bimolecular reaction of MHC in the gas phase, where MHC reacts with pyruvic acid to produce a C 4 H 8 O 2 product. This observation implies that some MHC produced from pyruvic acid in Earth's troposphere will be stabilized and participate in chemical reactions with O 2 and H 2 O, and should be considered in atmospheric modeling.
Here, we investigated the photodissociation dynamics of three xylene isomers C 6 H 4 (CH 3 ) 2 at 157nm. The center-of-mass translational energy distributions of C 7 H 7 radicals were found to peak at 26kJmol −1 . Although the ionization energy of the C 7 H 7 tolyl fragment exceeds the energy of a 157nm photon, C 7 H 7 + was observed as a result of the photoionization of vibrationally ‘hot’ tolyl (C 7 H 7 ) radicals and/or two-photon ionization. The formation of rovibrationally excited tolyl fragments was discussed. Our experiments suggest the presence of tolyl radicals in the interstellar medium as a precursor to methylated polycyclic aromatic hydrocarbons upon reaction with vinylacetylene (C 4 H 4 ).
The photodissociation of jet-cooled cyclohexyl was studied by exciting the radicals to their 3p Rydberg state by using 248 nm laser light and detecting photoproducts by photofragment translational spectroscopy. Both H atom loss and dissociation to heavy fragment pairs are observed. The H atom loss channel exhibits a two-component translational energy distribution. The fast photoproduct component is attributed to impulsive cleavage directly from an excited state, likely the Rydberg 3s state, forming cyclohexene. The slow component is due to statistical decomposition of hot cyclohexyl radicals that internally convert to the ground electronic state prior to H atom loss. The fast and slow components are present in an ~0.7:1 ratio, similar to findings in other alkyl radicals. Internal conversion to the ground state also leads to ring-opening followed by dissociation to 1-buten-4-yl + ethene in comparable yield to H-loss, with the C 4 H 7 fragment containing enough internal energy to dissociate further to butadiene via H atom loss. A very minor ground-state C 5 H 8 + CH 3 channel is observed, attributed predominantly to 1,3-pentadiene formation. Lastly, the ground-state branching ratios agree well with RRKM calculations, which also predict C 4 H 6 + C 2 H 5 and C 3 H 6 + C 3 H 5 channels with similar yield to C 5 H 8 + CH 3 . If these channels were active, it was at levels too low to be observed.
UV excitation of the CH 2 OO Criegee intermediate across most of the broad span of the (B 1A') – (X 1A') spectrum results in prompt dissociation to two energetically accessible asymptotes: O ( 1 D) + H 2 CO (X 1 A 1 ) and O ( 3 P) + H 2 CO (a 3A"). Dissociation proceeds on multiple singlet potential energy surfaces that are coupled by two regions of conical intersection (CoIn). Velocity map imaging (VMI) studies reveal a bimodal total kinetic energy (TKER) distribution for the O ( 1 D) + H 2 CO (X 1 A 1 ) products with the major and minor components accounting for ca. 40% and ca. 20% on average of the available energy (E avl ), respectively. The unexpected low TKER component corresponds to highly internally excited H 2 CO (X 1 A 1 ) products accommodating ca. 80% of E avl . Full dimensional trajectory calculations suggest that the bimodal TKER distribution of the O ( 1 D) + H 2 CO (X 1 A 1 ) products originates from two different dynamical pathways: a primary pathway (69%) evolving through one CoIn region to products and a smaller component (20%) sampling both CoIn regions enroute to products. Those that access both CoIn regions likely give rise to the more highly internally excited H 2 CO (X 1 A 1 ) products. The remaining trajectories (11%) dissociate to O ( 3 P) + H 2 CO (a 3 A") products after traversing through both CoIn regions. Here, the complementary experimental and theoretical investigation provides insight on the photodissociation of CH 2 OO via multiple dissociation pathways through two regions of CoIn that control the branching and energy distributions of products.