Spin–orbit ab initio study of alkyl halide dissociation via electronic curve crossing

Melatonin photoreactivity: phosphorescence formation and quenching processes

Studies of melatonin photoreactivity in water solutions: An effect of an external heavy atom I− on UV/Vis absorption, fluorescence and phosphorescence spectra is explored. The data allowed determination of relevant energetics for the system.The heavy atom effect (HAE) of I− on melatonin is clearly found to induce an intersystem crossing from the lowest energy singlet state to the lowest energy triplet state (T1) by a state mixing. Lifetime for the first excited triplet states of melatonin in association with I− and quenching rates for halomethanes (CH2X2, CHX3, CY4, X = Cl, Br, Y = Cl) are determined from Time-Correlated Single-Photon Counting decay curves for the phosphorescence. Dramatic alterations in quenching rate constants with quenchers as CH2X2 < CHX3 < CX4 and Cl < Br are attributed to energy transfer from an I−…Me*(T1) complex to low-lying electronic states of the halomethanes followed by dissociation to form R and X fragments. Relevance of the melatonin photoreactivity to photosensitizer properties in organic media is discussed.

Graphical abstract

Ab initio surface hopping excited-state molecular dynamics approach on the basis of spin-orbit coupled states: An application to the A-band photodissociation of CH3 I

Ab initio molecular dynamics approach has been extended to multi-state dynamics on the basis of the spin-orbit coupled electronic states that are obtained through diagonalization of the spin-orbit coupling matrix with the multi-state second-order multireference perturbation theory energies in diagonal elements and the spin-orbit coupling terms at the state-averaged complete active space self-consistent field level in off-diagonal elements. Nonadiabatic transitions over the spin-orbit coupled states were taken into account explicitly by a surface hopping scheme with utilizing the nonadiabatic coupling terms calculated by numerical differentiation of the spin-orbit coupled wavefunctions and analytical nonadiabatic coupling terms. The present method was applied to the A-band photodissociation of methyl iodide, CH₃ I + hv → CH₃ + I (² P_3/2 )/I* (² P_1/2 ), for which a pioneering theoretical work was reported by Amatatsu, Yabushita, and Morokuma. The present results reproduced well the experimental branching ratio and energy distributions in the dissociative products. © 2018 Wiley Periodicals, Inc.

Resolved (v1, v2 = 1) Combination Vibrational States of CF3 Fragments in the Photofragment Translational Spectra of CF3I

The photodissociation of CF₃I → CF₃(v₁,v₂) + I*/I has been investigated at 248, 266, and 277 nm with our high resolution mini-TOF photofragment translational spectrometer. Based on the theoretical calculations of Clary and of Bowman et al., now in this manuscript, we assign 701 cm^-1 to the CF symmetric stretch (breathing) ν₁ mode, and 1086 cm^-1 to the umbrella ν₂ mode of the CF₃ fragment. In the obtained TOF spectra of I⁺ from the I* channel, situated in the 701 cm^-1 gaps between the original series of (v₁, 0) vibrational peaks, a new series of weaker (v₁, 1) vibrational peaks are partially resolved. These observed new peaks with 1086 cm^-1 ν₂ mode excitation have never been reported in previous literature. In the TOF spectra of I⁺ from the I channel, the new series of (v₁, 1) peaks are also partially resolved. However, these spectra of I channel are less satisfactory, because for higher E_avl and higher E_T, the higher resolution of PTS is required. The potential energy at the curve crossing point and the excitation of CF₃ (v₁, 2) and (v₁, 3) vibrational states have been also analyzed.

Toward spin-orbit coupled diabatic potential energy surfaces for methyl iodide using effective relativistic coupling by asymptotic representation

The theoretical treatment of state-state interactions and the development of coupled multidimensional potential energy surfaces (PESs) is of fundamental importance for the theoretical investigation of nonadiabatic processes. Usually, only derivative or vibronic coupling is considered, but the presence of heavy atoms in a system can render spin-orbit (SO) coupling important as well. In the present study, we apply a new method recently developed by us (J. Chem. Phys. 2012, 136, 034103, and J. Chem. Phys. 2012, 137, 064101) to generate SO coupled diabatic PESs along the C-I dissociation coordinate for methyl iodide (CH3I). This is the first and mandatory step toward the development of fully coupled full-dimensional PESs to describe the multistate photodynamics of this benchmark system. The method we use here is based on the diabatic asymptotic representation of the molecular fine structure states and an effective relativistic coupling operator. It therefore is called effective relativistic coupling by asymptotic representation (ERCAR). This approach allows the efficient and accurate generation of fully coupled PESs including derivative and SO coupling based on high-level ab initio calculations. In this study we develop a specific ERCAR model for CH3I that so far accounts only for the C-I bond cleavage. Details of the diabatization and the accuracy of the results are investigated in comparison to reference ab initio calculations and experiments. The energies of the adiabatic fine structure states are reproduced in excellent agreement with ab initio SO-CI data. The model is also compared to available literature data, and its performance is evaluated critically. This shows that the new method is very promising for the construction of fully coupled full-dimensional PESs for CH3I to be used in future quantum dynamics studies.

Density functional and spin-orbit ab initio study of CF3Br: molecular properties and electronic curve crossing

Quantum chemical calculations of CF(3)Br and the CF(3) radical are performed using density functional theory (DFT) and time-dependent DFT (TDDFT). Molecular structures, vibrational frequencies, dipole moment, bond dissociation energy, and vertical excitation energies of CF(3)Br are calculated and compared with available experimental results. The performance of six hybrid and five hybrid meta functionals in DFT and TDDFT calculations are evaluated. The ωB97X, B3PW91, and M05-2X functionals give very good results for molecular structures, vibrational frequencies, and vertical excitation energies, respectively. The ωB97X functional calculates well the dipole moment of CF(3)Br. B3LYP, one of the most widely used functionals, does not perform well for calculations of the C-Br bond length, bond dissociation energy, and vertical excitation energies. Potential energy curves of the low-lying excited states of CF(3)Br are obtained using the multiconfigurational spin-orbit ab initio method. The crossing point between 2A(1) and 3E states is located near the C-Br bond length of 2.45 Å. Comparison with CH(3)Br shows that fluorination does not alter the location of the crossing point. The relation between the calculated potential energy curves and recent experimental result is briefly discussed.

Spin–orbit ab initio investigation of the UV photoinduced bond cleavage in iodotrimethylstannane

The photoinduced homolytic cleavage of the Sn–I bond in iodotrimethylstannane (CH3)3SnI, observed after UV irradiation, is investigated by means of spin–orbit ab initio calculations based on CASSCF (complete active space self-consistent field) and MS-CASPT2 (multi-state complete active space 2nd order perturbation theory) methods. The absorption electronic spectrum is characterized by ten low-lying 1,3A′ and 1,3A″ spin eigenstates corresponding to py(I),px(I) → σ*SnI; σSnI → σ*SnI and py(Sn), px(Sn) → σ*SnI, where σSnI and σ*SnI are the bonding and anti-bonding orbitals of the Sn–I bond along the pz axis. From the 1A′ electronic ground state and these ten spin eigenstates, 21 spin–orbit states are generated leading to various deactivation channels of (CH3)3SnI, corresponding to the formation of radicals (CH3)3Sn• and •I and to the ionic species (CH3)3Sn+ and I–. Irradiation into the upper band at 175 nm should lead to the heterolytic cleavage of the Sn–I bond to form the ionic primary products exclusively, whereas absorption into the shoulder at 250 nm induces the homolytic breaking with formation of the radical products.

Ab initio study of valence and Rydberg states of CH3Br

We performed configuration interaction ab initio calculations on the valence and 5s, 5p(a(1)), and 5p(e) Rydberg bands of the CH(3)Br molecule as a function of the methyl-bromide distance for frozen C(3v) geometries. The valence state potential energy curves are repulsive, the Rydberg state ones are similar to the one of the CH(3)Br(+) ion with a minimum at short distance. One state emerging from the 5p(e) band has valence and ion-pair characters as distance increases and the corresponding potential curve has a second minimum at large distance. This state has a very strong parallel electric dipole transition moment with the ground state and plays a central role in UV photon absorption spectra. It is also responsible for the parallel character of the anisotropy parameters measured in ion-pair production experiments. In each band, there is a single state, which has a non-negligible transition moment with the ground state, corresponding to a transition perpendicular to the molecular axis of symmetry, except for the 5p(e) band where it is parallel. The perpendicular transition moments between ground and valence states increase sharply as methyl-bromide distance decreases due to a mixing between valence and 5s Rydberg band at short distance. In each band, spin orbit interaction produces a pair of states, which have significant transition moments with the ground one. In the valence band, the mixing between singlet and triplet states is weak and the perpendicular transition to the (1)Q(1) state is dominant. In each Rydberg band, however, spin-orbit interaction is larger than the exchange interaction and the two significant transition moments with the ground state have comparable strengths. The valence band has an additional state ((1)Q(0)) with significant parallel transition moment induced by spin-orbit interaction with the ground state at large distance.

Experimental and theoretical study of the photodissociation of bromo-3-fluorobenzene

The UV photodissociation of bromo-3-fluorobenzene under collisionless conditions has been studied as a function of the excitation wavelength between 255 and 265 nm. The experiments were performed using ultrafast pump-probe laser spectroscopy. To aid in the interpretation of the results, it was necessary to extend the theoretical framework substantially compared to previous studies, to also include quantum dynamical simulations employing a two-dimensional nuclear Hamiltonian. The nonadiabatic potential energy surfaces (PES) were parameterized against high-level MS-CASTP2 quantum chemical calculations, using both the C-Br distance and the out-of-plane bending of the bromine as nuclear parameters. We show that the wavelength dependence of the photodissociation via the S0-->1pipi*-->1pisigma* channel, accessible with a approximately 260 nm pulse, is captured in this model. We thereby present the first correlation between experiments and theory within the quantitative regime.

A Diabatic Representation Including Both Valence Nonadiabatic Interactions and Spin−Orbit Effects for Reaction Dynamics

A diabatic representation is convenient in the study of electronically nonadiabatic chemical reactions because the diabatic energies and couplings are smooth functions of the nuclear coordinates and the couplings are scalar quantities. A method called the fourfold way was devised in our group to generate diabatic representations for spin-free electronic states. One drawback of diabatic states computed from the spin-free Hamiltonian, called a valence diabatic representation, for systems in which spin-orbit coupling cannot be ignored is that the couplings between the states are not zero in asymptotic regions, leading to difficulties in the calculation of reaction probabilities and other properties by semiclassical dynamics methods. Here we report an extension of the fourfold way to construct diabatic representations suitable for spin-coupled systems. In this article we formulate the method for the case of even-electron systems that yield pairs of fragments with doublet spin multiplicity. For this type of system, we introduce the further simplification of calculating the triplet diabatic energies in terms of the singlet diabatic energies via Slater's rules and assuming constant ratios of Coulomb to exchange integrals. Furthermore, the valence diabatic couplings in the triplet manifold are taken equal to the singlet ones. An important feature of the method is the introduction of scaling functions, as they allow one to deal with multibond reactions without having to include high-energy diabatic states. The global transformation matrix to the new diabatic representation, called the spin-valence diabatic representation, is constructed as the product of channel-specific transformation matrices, each one taken as the product of an asymptotic transformation matrix and a scaling function that depends on ratios of the spin-orbit splitting and the valence splittings. Thus the underlying basis functions are recoupled into suitable diabatic basis functions in a manner that provides a multibond generalization of the switch between Hund's cases in diatomic spectroscopy. The spin-orbit matrix elements in this representation are taken equal to their atomic values times a scaling function that depends on the internuclear distances. The spin-valence diabatic potential energy matrix is suitable for semiclassical dynamics simulations. Diagonalization of this matrix produces the spin-coupled adiabatic energies. For the sake of illustration, diabatic potential energy matrices are constructed along bond-fission coordinates for the HBr and the BrCH(2)Cl molecules. Comparison of the spin-coupled adiabatic energies obtained from the spin-valence diabatics with those obtained by ab initio calculations with geometry-dependent spin-orbit matrix elements shows that the new method is sufficiently accurate for practical purposes. The method formulated here should be most useful for systems with a large number of atoms, especially heavy atoms, and/or a large number of spin-coupled electronic states.

Anab initiostudy of the CH3I photodissociation. I. Potential energy surfaces

The multireference spin-orbit (SO) configuration interaction (CI) method in its Lambda-S contracted SO-CI version is employed to calculate two-dimensional potential energy surfaces for the ground and low-lying excited states of CH3I relevant to the photodissociation process in its A absorption band. The computed equilibrium geometry for the X A1 ground state, as well as vibrational frequencies for the nu2 umbrella and nu3 symmetric stretch modes, are found to be in good agreement with available experimental data. The 3Q0+ state converging to the excited I(2P1/2o) limit is found to possess a shallow minimum of 850 cm(-1) strongly shifted to larger internuclear distances (RC-I approximately 6.5a0) relative to the ground state. This makes a commonly employed single-exponent approximation for analysis of the CH3I fragmentation dynamics unsuitable. The 4E(3A1) state dissociating to the same atomic limit is calculated to lie too high in the Franck-Condon region to have any significant impact on the A-band absorption. The computed vertical excitation energies for the 3Q1, 3Q0+, and 1Q states indicate that the A-band spectrum must lie approximately between 33,000 and 44,300 cm(-1), i.e., between 225 and 300 nm. This result is in very good agreement with the experimental findings. The lowest Rydberg states are computed to lie at >or=49,000 cm(-1) and correspond to the ...a(1)2n3a1(6sI) leading configuration. They are responsible for the vacuum ultraviolet absorption lines found experimentally beyond the A-band spectrum at 201.1 nm (49,722 cm(-1)) and higher.