Matrix Isolation and Computational Studies of the CF2I Radical (original) (raw)
Related papers
The Journal of Physical Chemistry A, 2009
The photochemical reaction path following the promotion of CF 2 I 2 into its lowest-lying excited electronic singlet state has been modeled using ab initio multiconfigurational quantum chemical calculations. It is found that a conical intersection drives the electronically excited CF 2 I 2 * species either to the CF 2 I + I radical pair or back to the starting CF 2 I 2 structure. The structures of the computed relaxation pathways explain the photoproduct selectivity previously observed in the gas phase. Furthermore, the results provide the basis for explaining the condensed-phase photochemistry of CF 2 I 2 .
Journal of Chemical Theory and Computation, 2009
Exploiting the locality of the chemical potential of an excited state when it is evaluated using the ground-state density functional theory (DFT), a new descriptor for excited states has been proposed. This index is based on the assumption that the relaxation of the electronic density drives the chemical reactivity of excited states. The sign of the descriptor characterizes the electrophilic or nucleophilic behavior of the atomic regions. A relation between the new descriptor and the dual descriptor is derived and provides a posteriori justification of its use to rationalize the Woodward-Hoffmann rules for photochemical reactions within the conceptual DFT. Finally, the descriptor is successfully applied to some [2 + 2] photocycloadditions, like Paterno-Bü chi reactions.
Photochemistry by conical intersections: a practical guide for experimentalists
Journal of Photochemistry and Photobiology A: Chemistry, 2001
Many photochemical reactions are believed to proceed through conical intersections. The properties of conical intersections leading to the ground state of a given system are discussed using the phase-change rule: the ground-state total electronic wave function changes its sign when the system is transported along a complete loop around a conical intersection. It is shown that this property may be used to find the conical intersections present in the system, to predict possible products and even the energy disposal. An important corollary is that in a photochemical reaction involving a conical intersection, more than one product is necessarily formed. One of the products is always a 'photochemically allowed' one (Woodward-Hoffmann nomenclature), the second may be a thermally allowed one. A method to qualitatively predict the geometry of a conical intersection is presented and compared with previous calculations. For the 1,4-hexadiene system, the method was shown to help in locating computationally a conical intersection that can lead to the formation of benzene and H 2 , accounting for the 'helicopter-type' motion observed by Lee and coworkers [J. Chem. Phys. 95 (1991) 297].
Photochemistry and Photobiology, 2003
Phototropin is a blue light-activated photoreceptor that plays a dominant role in the phototropism of plants. The protein contains two subunits that bind flavin mononucleotide (FMN), which are responsible for the initial steps of the light-induced reaction. It has been proposed that the photoexcited flavin molecule adds a cysteine residue of the protein backbone, thus activating autophosphorylation of the enzyme. In this study, the electronic properties of several FMN-related compounds in different charge and spin states are characterized by means of ab initio quantum mechanical calculations. The model compounds serve as idealized model chromophores for phototropism. Reaction energies are estimated for simple model reactions, roughly representing the addition of a cysteine residue to the flavin molecule. Excitation energies were calculated with the help of time-dependent density functional theory. On the basis of these calculations we propose the following mechanism for the addition reaction: (1) after photoexcitation of FMN out of the singlet ground state S 0 , excited singlet state(s) are populated; these relax to the lowest excited singlet state S 1 , and subsequently by intersystem crossing FMN in the lowest triplet state, T 1 is formed; (2) the triplet easily removes the neutral hydrogen atom from the H-S group of the cysteine residue; and (3) the resulting thio radical is added.
The Journal of Chemical Physics, 2004
Wave packet simulations of the photodetachment spectrum of OHF Ϫ are performed on several electronic adiabatic states, two triplets and four singlets of neutral OHF. The transition moments to these six states have been approximated using the ab initio electronic wave functions of OHF Ϫ and OHF calculated at the equilibrium configuration of the parent anion. In a first step, two-dimensional simulations of the spectrum are performed on new two-dimensional potential energy surfaces ͑PESs͒ of the neutral in a OHF collinear geometry. The resulting simulated spectrum is in rather good agreement with the experimental one, reproducing all the structures from 0 to 2.5 eV electron kinetic energies. At energies below 0.5 eV, all calculated states, singlets and triplets, contribute to the total spectrum. At higher energies, however, only the triplet states participate. In a second step, to improve the description of the spectrum, three-dimensional wave packet simulations of the spectrum are performed, getting an excellent agreement with the experiment. The collinear 3 ⌺ Ϫ and 3 ⌸ states split in two 3 AЉ and one 3 AЈ. New adiabatic PESs are used in this work for the 2 3 AЉ and 1 3 AЈ states, while the one recently proposed was used for the ground 1 3 AЉ. It is found that the minimum energy paths of the 3 ⌺ Ϫ and 3 ⌸ states cross twice at collinear geometry, so that at the transition state the ground state corresponds to 3 ⌸, while 3 ⌺ Ϫ is the lowest state otherwise. Such conical intersections are expected to give rise to important ⌺-⌸ vibronic effects, requiring a complete three-dimensional model of coupled diabatic states to improve our understanding of the reaction dynamics in this kind of systems.
Direct excitation of higher excited state and kinetics of photoreactions
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2019
Excited-state reactions (ESR) play an essential role in chemical, physical, and biological processes. The mathematical models are usually used to study ESR in kinetics and steady-state regimes. In these models, the excitation pulse populates the first excited state (the first singlet level) of the primary molecular form. Recently, researchers' paid growing attention to the reactions excited via the higher energy levels. We modeled these reactions using the system of linear differential equations. Exact analytical expressions of the kinetics of N* and P* populations were derived for the general case when excitation performed via the higher S n singlet state by the delta pulse. The graphical forms of these expressions were N and P time-dependent pulses. We detected the changes of the pulses' shapes, their maxima locations, the time behavior of the populations, and the total yield of the P* population. The changes occur due to the populating of the product excited state in the kinetic and thermodynamic reaction regimes. Numerical analysis performed for different ESR parameters revealed peculiarities of the N* and P* populations. Kinetics properties of these population characterize systems with varying rates of reversible ESR and various contributions of anti-Kasha (AK) reaction (from the S n state) to P* population. Modeling data presented in graphical form, allowed to understand better (a) the impact of the AK reaction on the kinetic properties of the excited states of the molecular systems operating in various mode of ESR (kinetic, reversible and intermediate); (b) the photochemical processes' mechanisms. Also, this modeling allowed establishing the criteria for revealing the effect of the AK reaction for improving the efficiency of anti-Kasha processes.
Photodissociation of isocyanic acid, HNCO, was studied with high-level ab initio methods. Geometry optimizations of stationary points and surface crossing seams were performed with the complete active space self-consistent-field ͑CASSCF͒ method, and the energetics were re-evaluated with single-point second-order multireference perturbation theory ͑CASPT2͒. The three product channels that participate in the photodissociation process are †1 ‡ HN(X 3 ⌺ Ϫ )ϩCO at 86.0 ͑calculated 79.6͒ kcal/mol, †2 ‡ HϩNCO(X 2 ⌸) at 109.7 ͑108.7͒ kcal/mol, and †3 ‡ HN(a 1 ⌬) ϩCO at 122.2 ͑120.8͒ kcal/mol. The four electronic states, S 0 , S 1 , T 1 , and T 2 , that interconnect these channels were studied in detail. S 1 exhibits dissociation barriers to both, channel †2 ‡ and †3 ‡, whose respective reverse heights are 11.3 and 1.2 kcal/mol, in good agreement with experiment as well as previous theoretical works. The two triplets, T 1 and T 2 , show barriers of similar heights for HN bond fission, while S 0 has no barriers to either channel. Various key isomerization transition states as well as numerous minima on the seam of surface crossings ͑MSX's͒ were also found. At photoexcitation energies near channel †3 ‡ threshold, products to channel †3 ‡ are likely to be formed via S 1˜ † 3 ‡ ͑if enough energy in excitation͒ and S 1˜S 0˜ † 3 ‡. Channel †2 ‡ can be formed via S 1˜S 0˜ † 2 ‡; ͑HN-mode quanta͒ϩS 1˜T 1˜ † 2 ‡; S 1˜T 2˜ † 2 ‡; S 1˜T 2˜T 1˜ † 2 ‡, and channel †1 ‡ via S 1˜S 0˜T 1˜ † 1 ‡, S 1˜T 1˜ † 1 ‡ and S 1˜T 2˜T 1˜ † 1 ‡. At higher photoexcitation energies the S 1˜ † 3 ‡ pathway is expected to be dominant while S 1˜ † 2 ‡, with the higher activation energy, is expected to drop rapidly. Also addressed are such important issues as the impact of a vibrationally excited HN mode on a channel †2 ‡ yield, and the band origin of the S 1 -S 0 excitation spectrum.
The Transfer and Conversion of Electronic Energy in Some ‘Model’ Photochemical Systems
Photochemistry and Photobiology, 1965
Recent studies of the effects of molecular structure and reaction environment on the mechanism of primary photochemical processes involving transfer and conversion of electronic energy in relatively 'simple' organic molecules are presented and discussed. A quantitative i.r. spectroscopic method for studying intramolecular and intermolecular photoprocesses of U.V. irradiated substrates dispersed in solid alkali halide matrices at room temperature is described. Current data for the substrates ortho-nitrobenzaldehyde, anthracene and benzophenonebenzhydrol are presented. A series of 'model' ketones containing cyclopropyl groups have been synthesized and while their absorption spectra are similar, the efficiency of vapor-phase photodissociation into radicals is shown to be strongly dependent on molecular structure. Butyrophenone and a series of ring substituted derivatives have been photolyzed in the liquid phase using the quantum yield of the photoelimination of ethylene (Type I1 split) as a "probe" to determine the effect of substituents on the internal H atom abstracting power of the excited carbonyl chromophore. @ c~R~ is very sensitive to ring substitution, dropping from 0.24 in butyrophenone to 0.20, 0.058 and 0.00 in the p-CH3, p-OCH3 and p-NHz derivatives respectively, and to 0.00 in both ortho and para hydroxy derivatives. This effect is correlated with their absorption spectra in terms of the lowest states of these alkyl aryl ketones being s(n, n*) rather than 3(n, n*) in character. While several 'classic' photochemical reactions, unimolecular and bimolecular, proceed efficiently in solid KBr matrices giving the same product as in liquid systems, the 'model' cyclopropyl compounds and the alkyl aryl ketones did not undergo their usual intramolecular processes. Implications of this molecular environment effect are pointed out. IN ORDER to obtain a 'complete' understanding of the photochemistry or photobiology of a given system, ideally one has to elucidate the entire 'life history' of the photoprocess, starting with the act of absorption and concluding with a detailed analysis of the system in its final physical and chemical state. Clearly this is a monumental (and perhaps impossible) task even for simple organic molecules, much less for complex molecules of biological significance. However, despite the formidable challenges that face us, in recent years, great advances have been made in the understanding of photoprocesses. In part, these have been *Presented at the Rapporteur Session,