Photodissociation Mechanism Research Articles

Calibration of velocity map imaging system and photodissociation dynamics of 1, 4-C4H8BrCl

Depletion of atmospheric ozone layers is more and more serious. Alkyl halides dissociate under the solar UV radiation with the product of free halogen atoms, which greatly damages the ozone layer and is the main culprit for the depletion of ozone layers. In this paper, methyl iodide is chosen as a calibration system of velocity map imaging. Velocity map images of iodine atom I (2P3/2) at different focus voltages are obtained in the dissociation of methyl iodine under an UV radiation of ～266 nm by techniques of velocity map imaging and REMPI (Resonance Enhanced Multiphoton Ionization). The magnification factor N of velocity map imaging system is measured to be 1.13. Photodissociation dynamics of 1, 4-C4H8BrCl under an UV radiation of ～234 nm is investigated on this velocity map imaging system. The speed and angular distributions of the fragments Br(2P3/2) and Br* (2P1/2) atoms in the dissociation are obtained and analyzed. Experimental results suggest that the dissociation of 1, 4-C4H8BrCl to both Br(2P3/2) and Br* (2P1/2) atoms under an UV radiation of ～234 nm happens promptly along the C-Br bond via repulsive surfaces after excitation. The anisotropy coefficient values are obtained from angular distributions of imaging of the fragments Br (2P3/2) and Br* (2P1/2) atoms, by which the ratio between perpendicular transition and parallel transition for those two dissociation channels are calculated. In addition, photodissociation mechanisms of CH2BrCl, 1, 2-C2H4BrCl, 1, 3-C3H6BrCl and 1, 4-C4H8BrCl at an UV radiation of ～234 nm are compared, and the dependences of dissociation mechanisms of dihalogen alkyl compounds on size of the alkyl radical are obtained.

Mechanism of NO Photodissociation in Photolabile Manganese–NO Complexes with Pentadentate N5 Ligands

The Mn-nitrosyl complexes [Mn(PaPy(3))(NO)](ClO(4)) (1; PaPy(3)(-) = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide) and [Mn(PaPy(2)Q)(NO)](ClO(4)) (2, PaPy(2)Q(-) = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-quinoline-2-carboxamide) show a remarkable photolability of the NO ligand upon irradiation of the complexes with UV-vis-NIR light [Eroy-Reveles, A. A.; Leung, Y.; Beavers, C. M.; Olmstead, M. M.; Mascharak, P. K. J. Am. Chem. Soc. 2008, 130, 4447]. Here we report detailed spectroscopic and theoretical studies on complexes 1 and 2 that provide key insight into the mechanism of NO photolabilization in these compounds. IR- and FT-Raman spectroscopy show N-O and Mn-NO stretching frequencies in the 1720-1750 and 630-650 cm(-1) range, respectively, for these Mn-nitrosyls. The latter value for ν(Mn-NO) is one of the highest transition-metal-NO stretching frequencies reported to this date, indicating that the Mn-NO bond is very strong in these complexes. The electronic structure of 1 and 2 is best described as Mn(I)-NO(+), where the Mn(I) center is in the diamagnetic low-spin state and the NO(+) ligand forms two very strong π backbonds with the d(xz) and d(yz) orbitals of the metal. This explains the very strong Mn-NO bonds observed in these complexes, which even supersede the strengths of the Fe- and Ru-NO bonds in analogous (isoelectronic) Fe/Ru(II)-NO(+) complexes. Using time-dependent density functional theory (TD-DFT) calculations, we were able to assign the electronic spectra of 1 and 2, and to gain key insight into the mechanism of NO photorelease in these complexes. Upon irradiation in the UV region, NO is released because of the direct excitation of d(π)_π* → π*_d(π) charge transfer (CT) states (direct mechanism), which is similar to analogous NO adducts of Ru(III) and Fe(III) complexes. These are transitions from the Mn-NO bonding (d(π)_π*) into the Mn-NO antibonding (π*_d(π)) orbitals within the Mn-NO π backbond. Since these transitions lead to the population of Mn-NO antibonding orbitals, they promote the photorelease of NO. In the case of 1 and 2, further transitions with distinct d(π)_π* → π*_d(π) CT character are observed in the 450-500 nm spectral range, again promoting photorelease of NO. This is confirmed by resonance Raman spectroscopy, showing strong resonance enhancement of the Mn-NO stretch at 450-500 nm excitation. The extraordinary photolability of the Mn-nitrosyls upon irradiation in the vis-NIR region is due to the presence of low-lying d(xy) → π*_d(π) singlet and triplet excited states. These have zero oscillator strengths, but can be populated by initial excitation into d(xy) → L(Py/Q_π*) CT transitions between Mn and the coligand, followed by interconversion into the d(xy) → π*_d(π) singlet excited states. These show strong spin-orbit coupling with the analogous d(xy) → π*_d(π) triplet excited states, which promotes intersystem crossing. TD-DFT shows that the d(xy) → π*_d(π) triplet excited states are indeed found at very low energy. These states are strongly Mn-NO antibonding in nature, and hence, promote dissociation of the NO ligand (indirect mechanism). The Mn-nitrosyls therefore show the long sought-after potential for easy tunability of the NO photorelease properties by simple changes in the coligand.

Note: Single-ultraviolet-photon dissociation dynamics of ${\rm CS}_{\rm 2}^ + (\tilde X{}^2\Pi _g)$ CS 2+(X̃2Πg) in 227–243 nm revealed by time-sliced velocity map imaging

The single-UV-photon (227–243 nm) dissociation of ${\rm CS}_2^ + (\tilde X{}^2\Pi _{g,3/2,1/2})$ CS 2+(X̃2Πg,3/2,1/2) into the S+(4S)CS(X1)Σ+ channel was investigated by means of time-sliced velocity map imaging technique. In the high-energy region of the $\tilde B{}^2\Sigma _u^ +$B̃2Σu+ state, a rather subtle photodissociation dynamics was observed, i.e., a slight variation in the excitation photon energy may lead to a drastic change in the vibronic distributions of the CS(X1Σ+) fragments originating from the two spin-orbit $\tilde X{}^{\rm 2}\Pi _g$X̃2Πg states. The photodissociation mechanisms involved here may be interpreted as being due to the complex interplay among various vibronic and spin-orbit interactions in this energy region of ${\rm CS}_{\rm 2}^ +$ CS 2+. It is quite impressive that, for such a simple triatomic cation species, ${\rm CS}_{\rm 2}^ +$ CS 2+, there exhibits such complexity and subtlety in its photodissociation dynamics.

Ultraviolet photodissociation of bromopropane: Energy partitioning and curve potential crossing analysis

The photodissociation mechanism of bromopropane (C3H7Br) molecule in the UV region is treated within time-dependent numerical methods. The results and our ab initio calculations indicated that the absorption of C3H7Br in the investigated region primarily originated from the excitations to the lowest three repulsive states, as assigned as 2A 2, 4A 1, and 3A 1 in C s symmetry. Dissociations occurred on the PES surfaces of the three states, giving C3H7 + Br (2 P 3/2) or C3H7 + Br* (2 P 1/2) as two final products, and being impacted by an avoided crossing between the PES surfaces of the 3A 1 and 4A 1 states. The transition dipole to the 2A 2 state was perpendicular to the symmetry plane, so perpendicular to the C-Br bond. The transitions to the 3A 1 state was polarized parallel to the symmetry plane, and also parallel to the C-Br bond. We have also determined the avoided crossing probabilities, which affected the relative fractions of the individual pathways and angular distributions of photofragments, for the photolysis of C3H7Br in the UV region.

Mechanistic Photodissociation of Glycolaldehyde: Insights from Ab Initio and RRKM Calculations

Herein we report a theoretical study on mechanistic photodissociation of glycolaldehyde, HOCH(2) CHO. Equilibrium structures, transition states, and intersection structures for the α-C-C and -C-H bond fissions and the β-C-O bond fission in the excited states are determined by the complete active space self-consistent field (CASSCF) method. Based on the CASSCF optimized structures, the potential energy profiles for the dissociations are refined by performing single-point calculations using the multi-state multi-reference CASSCF second order perturbation (MS-MR-CASPT2) method. With a low excitation energy of 280-340 nm, the T(1) α-C-C and β-C-O bond fissions following intersystem crossing from the S(1) state are the predominant and comparable channels, whereas the α-C-H bond fissions both in the S(1) and in the T(1) states are nearly prohibited due to the relevant high barriers. The rate constants for the T(1) α-C-C and β-C-O bond fissions are also calculated by RRKM theory. Furthermore, the S(0) reactions can occur as a consequence of intersystem crossing via T(1)/S(0) intersection points resulting from the T(1) C-C and C-O bond cleavages. This photodissociation mechanism is consistent with recent experimental studies.

Computational study toward understanding the photodissociation mechanism of sarin

AbstractThe potential energy surface for sarin (C4H10FO2P) dissociation into (CH3)2CHO+PO(F)(CH3), CH3+(CH3)2CHOPO(F), and F+(CH3)2CHOPO(CH3) in the T1 and S1 states were investigated at the complete‐active‐space self‐consistent field (CASSCF) with the 6‐31G** and aug‐cc‐PVDZ basis sets. The different reaction pathways are characterized on the basis of the computed potential energy surface and surface crossing point, the time‐dependent density functional theory (TD‐DFT) was used to calculate the vertical energies based on the CAS(8,7)/6‐31G** optimized excited structures, which may provide some new insights into the mechanism of the ultraviolet photo‐degradation of sarin molecules. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Photodissociation dynamics of ClOOCl at 248.4 and 308.4 nm

The dynamics of ClOOCl photodissociation at 248.4 and 308.4 nm was studied with photofragment translational spectroscopy. At 248.4 nm photoexcitation, the observed products are Cl, O(2), ClO and O. Product translational energy distributions P(E) and anisotropy parameters β were deduced from the measured time-of-flight spectra of the Cl, O(2), and ClO photoproducts. The photodissociation mechanisms have been discussed and compared with available theoretical results. Synchronous and fast sequential breaking of the two Cl-O bonds may both contribute to the dissociation. The relative product yields for [ClO]: [Cl] was measured to be 0.15 ± 0.04:1. The relative amounts of [O]:[O(2)] products were estimated to be 0.12:1. The branching ratios among the Cl + O(2) + Cl:ClO + ClO:ClO + Cl + O product channels were estimated to be 0.82:0.08:0.10. At 308.4 nm excitation, time-of-flight spectra of the O(2) and ClO photoproducts were recorded while there was interference from Cl(2) impurity in detecting the Cl product. Nonetheless, the observed ClO yield relative to the O(2) yield at 308.4 nm is 1.5 times that at 248.4 nm. The branching ratio between the Cl + O(2) + Cl:ClO + ClO product channels was estimated to be 0.81:0.19 at 308.4 nm. This result suggests that the ClO product may contribute a noticeable yield in the photolysis of ClOOCl at the atmospherically important wavelengths above 300 nm.

Photochemistry of HI on argon and waternanoparticles: Hydronium radical generation in HI·(H2O)n

Photochemistry of HI molecules on large Ar(n) and (H(2)O)(n), n ∼ 100-500, clusters was investigated after excitation with 243 nm and 193 nm laser radiation. The measured H-fragment kinetic energy distributions pointed to a completely different photodissociation mechanism of HI on water than on argon clusters. Distinct features corresponding to the fragment caging (slow fragments) and direct exit (fast fragments) were observed in the spectra from HI photodissociation on Ar(n) clusters. On the other hand, the fast fragments were entirely missing in the spectrum from HI·(H(2)O)(n) and the slow-fragment part of the spectrum had a different shape from HI·Ar(n). The HI·(H(2)O)(n) spectrum was interpreted in terms of the acidic dissociation of HI on (H(2)O)(n) in the ground state, and hydronium radical H(3)O formation following the UV excitation of the ionically dissociated species into states of a charge-transfer-to-solvent character. The H(3)O generation was proved by experiments with deuterated species DI and D(2)O. The experiment was complemented by ab initio calculations of structures and absorption spectra for small HI·(H(2)O)(n) clusters, n = 0-5, supporting the proposed model.

Predissociation via Conformational Change: Photodissociation of N,N-Dimethylnitrosamine in the S1 State

We investigated the photodissociation mechanism of N,N-dimethylnitrosamine (CH(3))(2)NNO (DMN) by ab intio quantum chemical methods. Inspired by an earlier study we calculated two-dimensional potential energy surfaces of the S(1) state of DMN in its planar and pyramidal conformations. While the planar molecular geometry appears to possess no direct dissociation channel, the pyramidal configuration is dissociative yielding the products NO + (CH(3))(2)N. Using wave packet dynamics on the planar S(1) potential energy surface the experimental absorption spectrum was well reproduced which gives indirect but strong support for the nondissociative nature of this surface. The transition from the planar to the pyramidal conformation of DMN was then investigated by an ab initio molecular dynamics method which revealed the time evolution of the geometrical parameters of the molecule up to the dissociation of the N-N bond. This occurs about 90 fs after photon excitation. The calculated minimum energy path along the N-N coordinate and the structural changes of the molecule along this coordinate provided a detailed picture of this indirect dissociation or, more specific, predissociation process via conformational change.

On the role of conical intersections and their local topography in the photodissociation of the 1-hydroxyethyl radical

The role of conical intersections in the photodissociation of the 1-hydroxyethyl radical is investigated. Two local minima on the 1 2A–2 2A seam of the conical intersection, which correspond to significant stretching of either the O–H or the nonmethyl C–H bond are located, as is an energy minimized intersection on the 2 2A–3 2A seam. The local topography for these conical intersections is determined from the average energy gradient, energy difference gradient and interstate coupling gradient and used to discuss the mechanism of photodissociation. The results are compared with a theoretical study of the hydroxymethyl radical, and are used to explain recent REMPI and photofragment yield spectra. The absence of a REMPI spectrum for the 2 2A state, nominally the 3s Rydberg state, is attributed to a 1 2A–2 2A conical intersection. Time-of-flight spectra in the region of the 2 2A state are discussed, as are spectra in the region of the 3 2A, a 3p Rydberg, state.

Thin Film Deposition and Photodissociation Mechanisms for Lanthanide Oxide Production from Tris(2,2,6,6-tetramethyl-3,5-heptanedionato)Ln(III) in Laser-Assisted MOCVD

Photoionization mass spectrometry reveals details of the multistep unimolecular mechanism, whereby the 2,2,6,6-tetramethyl-3,5-heptanedionato (thd−) anionic ligand decomposes, while still bound in the metal complex, to yield a gas-phase metal oxide product in metal−organic chemical vapor deposition (MOCVD) of lanthanide oxides from Ln(thd)3 precursors. The decomposition occurs with stepwise elimination of small closed-shell hydrocarbon fragments and carbon monoxide up to a penultimate Ln(OC2H) ethyneoxide, from which both LnO (dominant) and LnC2 (minor) products are derived. Formation of the metal oxide and carbide occurs in competition with a previously described mechanism(1-3) wherein sequential dissociation of ligand radicals produces the reduced metal Ln0. Evidence for gas-phase formation of a Ln2(thd)6 dimer as a result of expansion-cooling in the precursor source assembly is also given. Laser-assisted MOCVD of Eu(thd)3 on silica, with subsequent exposure to atmosphere, produces amorphous Eu2O3 with small areas of crystallinity attributed to reaction of the oxide with atmospheric carbon dioxide and water.

Photochemistry of Methyl Ethyl Ketone: Quantum Yields and S1/S0‐Diradical Mechanism of Photodissociation

Pulsed laser photolysis (PLP) at λ=248 and 308 nm coupled with gas-chromatographic analysis is applied to determine the photodissociation quantum yield (QY) of methyl ethyl ketone (MEK). Temperature dependent UV absorption cross-sections [σ(MEK)(λ,T)] are also determined. At 308 nm, the QY decreases with decreasing temperature (T=323-233 K) and with increasing pressure (P=67-998 mbar synthetic air). Stern-Volmer (SV) analysis of the T and P dependent QYs provides the experimental estimate of E(S1)=398±9 kJ mol(-1) (=300±6 nm) for the barrier of the first excited singlet state (S(1)). The QY at 248 nm is close to unity and independent of pressure (T=298 K). Theoretical reaction pathways are examined systematically on the basis of CASPT2/6-31+G* calculations. Among three possible pathways, a S(1)/S(0)-diradical mechanism, which involves H atom transfer on the S(1) surface, followed by a nonadiabatic transition at a diradical isomer of MEK, explains the experimental data very well. Therefore, this unusual mechanism, which is not seen in any smaller carbonyl compounds, is proposed as an important pathway for the MEK dissociation. Our study supports the view that both the absorption cross-sections and the QYs of carbonyls have significant temperature dependences that should be taken into account for accurate modelling of atmospheric chemistry.

Photofragment angular momentum distributions in the molecular frame. II. Single state dissociation, multiple state interference, and nonaxial recoil in photodissociation of polyatomic molecules

We present an a(q) (k)(s) polarization-parameter model to describe product angular momentum polarization from the one-photon photodissociation of polyatomic molecules in the molecular frame. We make the approximation that the final photofragment recoil direction is unique and described by the molecular frame polar coordinates (alpha,phi(i)), for which the axial recoil approximation is a special case (e.g., alpha=0). This approximation allows the separation of geometrical and dynamical factors, in particular, the expression of the experimental sensitivities to each of the a(q) (k)(s) in terms of the molecular frame polar angles (chi(i),phi(i)) of the transition dipole moment mu(i). This separation is applied to the linearly polarized photodissociation of polyatomic molecules (asymmetric, symmetric, and spherical top molecules are discussed) and to all dissociation mechanisms that satisfy our recoil approximation, including those with nonaxial recoil and multiple state interference, giving important insight into the geometrical properties of the photodissociation mechanism. For example, we demonstrate that the ratio of polarization parameters A(0) (k)(aniso)/A(0) (k)(iso)=beta (where beta is the spatial anisotropy parameter) is an indication that the dynamics can be explained by a single dissociative state. We also show that for asymmetric top photodissociation, the sensitivity to the a(1) (k)(s) parameters, which can arise either from single-surface or multiple-surface interference mechanisms, is nonzero only for components of the transition dipole moments within the v-d plane of the recoil frame.

Spin-orbit ab initio investigation of photolysis of o-, m-, and p-iodotoluene

The multistate second order multiconfigurational perturbation theory in conjunction with spin-orbit interaction through complete active space state interaction (MS-CASPT2/CASSI-SO) was employed to calculate the potential energy curves for the ground and low-lying excited states of o-, m-, and p-iodotoluene along the assumed photolysis reaction coordinates. The mechanism and channels leading to products I((2)P(3/2)) and I( *)((2)P(3/2)) for o-, m-, and p-iodotoluene photolysis at 266 and 304 nm were elucidated with the computed potential energy curves and the surface crossing points. The effects of methyl substituent and heavy atom on the photodissociation mechanism were discussed by the comparison to related alkyl and aryl halides.

Spectroscopic Investigation of the Multiphoton Photolysis Reactions of Bromomethanes (CHBr3, CHBr2Cl, CHBrCl2, and CH2Br2) at Near-Ultraviolet Wavelengths

Nascent emission and laser-induced dispersed fluorescence spectra of products or intermediates from the multiphoton photolysis reaction of bromomethanes (CHBr(3), CHBr(2)Cl, CHBrCl(2), and CH(2)Br(2)) at 266 nm were recorded in a slow flow cell. Electronically excited species including CH (A(2)Delta, B(2)Sigma(-), and C(2)Sigma(+)), C(2) (d(3)Pi(g)), and atomic Br ((4)D(J) and (4)P(J)) were observed in the nascent emission spectra. Free radicals such CHBr or CHCl were also successfully found using laser-induced dispersed fluorescence spectroscopy. The reactive intermediate, CHBr, was seen only in the photolysis of CHBr(3), whereas CHCl was only discovered when the precursor was CHBr(2)Cl or CHBrCl(2). More experiments including the power dependence and temporal waveform measurements were conducted. The present study reports the first direct measurements of the intermediate products in the multiphoton photodissociation reaction of these bromomethanes at 266 nm. Nascent emission spectra following the photolysis at longer near-ultraviolet wavelengths (280 and 355 nm) were also acquired. On the bassis of these results, the multiphoton photodissociation mechanism of these bromomethanes at 266 nm can be confirmed.

Mechanisms of UV Photodissociation of Small Protonated Peptides

Photofragmentation of protonated dipeptides by 263 nm photons is investigated with an experimental technique based on the detection in coincidence of the ionic and neutral fragments. With this method, it is possible to determine whether the fragmentation takes place in one or several steps. The timing of these steps can also be evaluated. The interpretation of the various fragmentation pathways is tentatively developed along the same line as that previously proposed for tryptophan. The fragmentation can be explained by two types of mechanisms: internal conversions and direct fragmentations triggered by the migration of the photoactive electron on positive charged sites or on oxygen sites.

Photodissociation and Photoionization Mechanisms in Lanthanide-based Fluorinated β-Diketonate Metal−Organic Chemical-Vapor Deposition Precursors

A detailed photodissociation mechanism for the fluorinated lanthanide (Ln) β-diketonate metal−organic chemical-vapor deposition (MOCVD) precursors Ln(fod)3, Ln(hfac)3, and Ln(hfac)3diglyme was elucidated using photoionization time-of-flight mass spectrometry. The collisionless environment of the molecular beam source revealed a series of unimolecular steps, starting with dissociation of an intact β-diketonate ligand. Dissociation steps for the second and third ligands are associated with the attachment of a fluoride to the metal ion, leading to one of three ultimate products: Ln, LnF, or LnF2. Except in the case of Pr, no LnO is observed. The pattern of ligand fragmentation strongly supports a mechanism for the fluoride-attachment step which is similar to one previously proposed for the thermally activated decomposition of Ce(hfac)3glyme.(1) The detailed mechanistic understanding of the step-by-step fluorination will provide a basis for new ligand design, which maintains the advantageous mass-transport pr...

Ab initio molecular dynamics simulations of excited hydrogen halides and methyl halides

Using excited-state ab initio molecular dynamics simulations employing the complete-active-space self-consistent-field approach, we study the mechanism of photodissociation in terms of time evolution of structure, kinetic energy, charges and potential energy for the first excited state of hydrogen halides and methyl halides. Although the hydrogen halides and methyl halides are similar in the photodissociation mechanism, their dynamics are slightly different. The presence of the methyl group causes delay in photodissociation as compared to hydrogen halides.

An ab initio investigation of the mechanisms of photodissociation in bromobenzene and iodobenzene

In concert with the latest experiment of velocity imaging technique [X. P. Zhang et al., ChemPhysChem 9, 1130 (2008)], quantum chemical calculations with relativistic effect were performed on the photodissociation of bromobenzene (BrPh) and iodobenzene (IPh) at 266 nm. The method of multistate second order multiconfigurational perturbation theory in conjunction with spin-orbit interaction through complete active space state interaction was employed to calculate the potential energy curves for the ground and low-lying excited states of BrPh and IPh along their photodissociation reaction coordinates. The dissociation mechanisms with products of X((2)P(3/2)) and X(*)((2)P(1/2)) (X = Br,I) states were clarified.

A theoretical study on S-loss photodissociation of the [formula omitted] ion

The CASPT2 potential energy curves for S-loss dissociation from the X 2Π g, A 2Π u, B 2 Σ u + , C 2 Σ g + , 1 4Σ −, 1 4Π, 1 2Σ −, and 2 4Σ − states of the CS 2 + ion were calculated. A conical intersection point between the B 2 Σ u + and C 2 Σ g + potential energy surfaces was found. At the CASSCF level we located the minimum energy crossing points (MECPs) for the X 2Π g/1 4Σ −, A 2Π u/1 4Σ −, B 2 Σ u + /1 4Σ −, C 2 Σ g + /1 4Σ −, A 2Π u/1 2Σ −, and B 2 Σ u + /1 2Σ − state-pairs and calculated spin–orbit coupling values. MECP for the C 2 Σ g + /1 2Σ − state-pair was not found. We suggest photodissociation processes via A 2Π u and B 2 Σ u + . Photodissociation mechanisms via A 2Π u, B 2 Σ u + , and C 2 Σ g + proposed in the previous experimental studies are discussed.