Photoinduced multi-mode quantum dynamics of pyrrole at the – conical intersections

The photoinduced dynamics of pyrrole at the – S₀ and – S₀ conical intersections has been investigated by multi-mode time-dependent quantum wave-packet calculations. Diabatic potential-energy surfaces have been constructed for both conical intersection using accurate multi-reference ab initio electronic-structure calculations. In addition to the NH stretching coordinate, the three (four) symmetry-allowed coupling modes of A₂ (B₁) symmetry have been considered for the – S₀ (– S₀) conical intersections. Wave-packet dynamics calculations have been performed for three-dimensional models, taking account of the two dominant coupling modes of each conical intersection. The electronic population-transfer processes at the conical intersections, the branching ratio for the dissociation to the ground and excited states of the pyrrolyl radical, and their dependence on the initial preparation of the system have been investigated. It is shown that the excitation of the NH stretching mode strongly enhances the photodissociation rate, while the excitation of the strongest coupling mode has a pronounced effect on the branching ratio of the photodissociation process. Although the inclusion of the second (weaker) coupling mode has little effect on the electronic population dynamics, it leads to interesting changes of the nodal pattern of the wave packet at the conical intersections. The calculations provide insight into the effect of the multiple coupling modes on the process of direct photodissociation through a conical intersection.     

Franck–Condon simulation of photoelectron spectroscopy of HOO: Including Duschinsky effects

Geometry optimization and harmonic vibrational frequency calculations were performed on the and states of HOO and state of HOO⁻. The electron affinity and the term energy () of HOO were calculated at various theory levels. Franck–Condon analyses and spectral simulations were carried out on the and photodetachment processes. The spectral simulations of vibrational structures based on the computed Franck–Condon factors are in excellent agreement with the observed spectra. In addition, the equilibrium geometrical parameters of the state of HOO⁻ and state of HOO were obtained in the spectral simulations.     

Is Cd2 truly a van der Waals molecule? Analysis of rotational profiles recorded at the , transitions

Rotational profiles of the ²²⁸Cd₂ isotopomer recorded in the (υ′, υ″) = (26, 0), (27, 0), (42, 0), (45, 0), (46, 0), (48, 0) vibrational bands of the transition were analysed. As a result, the , , , , and excited- as well as the ground-state rotational constants of the (¹¹⁴Cd)₂ were determined. The analysis allowed determining the absolute values for the and excited- and ground-state bond lengths, respectively. The obtained result – the – distinctly shorter than that obtained with assumption of pure ground-state van der Waals bonding, supports a theoretical prediction of a covalent admixture to the bonding. Analysis of the partially-resolved rotational profile recorded in the (υ′, υ″) = (38, 0) band of the same isotopomer recorded at the transition allowed estimating the rotational constant in the B1_u state.     

DFT study of electronic structure and geometry of anionic silver clusters (n = 11, 12, 17)

Based on both total energy calculations and comparison of experimental and calculated characteristics of photoelectron spectra (PHES), the structural assignment of clusters and has been made using DFT model with recently developed S2LYP functional. The calculated characteristics of PHES for the assigned structures are in excellent agreement with the experimental ones. The electronic structure, geometry and energetic characteristics of low-lying isomers have also been studied. The calculated geometrical parameters of and clusters as well as the geometries of earlier established clusters have been compared with the geometrical characteristics of anionic sodium clusters. The structures of anionic silver and sodium clusters have been found to be very similar. The difference has been observed only for . Based on similarity of the geometries of silver and sodium clusters as well as on the comparison of calculated and experimental characteristics of PHES, the geometry of cluster has been assigned.     

Structures and properties of the tin-doped carbon clusters

A systemic density functional theory study of the tin-doped carbon clusters has been carried out using B3LYP method with TZP+ basis set. For each species, the electronic states, relative energies and geometries of various isomers are reported. Except for smaller SnC₂ and the largest , the Sn-terminated linear or quasi-linear isomer is the most stable structure for clusters. The electronic ground state is alternate between ³Σ (for n-odd member) and ¹Σ (for the n-even member) for linear SnC_n and invariably ²Π for linear and , except for SnC/SnC⁺/SnC⁻,, and . The incremental binding energy diagrams show that strong even–odd alternations in the cluster stability exist for both neutral SnC_n and anionic , with their n-even members being much more stable than the corresponding odd n − 1 and n + 1 ones, while for cationic , the alternation effect is less pronounced. These parity effects also reflect in the ionization potential and electron affinity curves. By comparing with the fragmentation energies accompanying various channels, the most favorable dissociation channel for each kind of the clusters are given. All these results are very similar to those obtained previously for the clusters.     

Theoretical study on the structures and properties of LiCn, , and (n = 1–10) clusters

The lithium-doped carbon clusters LiC_n, , and (n = 1–10) have been investigated systemically with density functional theory (DFT) method at the B3LYP/6-311+G* level. According to the total energies of different kinds of isomers, the LiC_n, , and (n = 1–10) clusters have Li-terminated linear ground states structures, except for LiC₂, LiC₃, , and (n = 4–6). The incremental binding energies are evaluated to elucidate the stabilities of the clusters with different numbers of carbon atoms for neutral molecules, cations, and anions, respectively. Clear even–odd alternation effects are observed for the stability of the cationic clusters and anionic clusters, while for neutral LiC_n clusters the alternation effect is less pronounced. Similarly, the ionization potentials and electron affinities of LiC_n also express an obvious parity alternation. In addition, the most favorable dissociation channels are acquired according to the fragmentation energies accompanying various pathways.     

Magnetic properties of the spinel-type

The spinel sulphide CuCr₂S₄ is a metallic ferromagnet with a Curie temperature , while CuHf₂S₄ has no magnetic anomaly. Magnetic properties of the quaternary spinel-type Cu(Cr_1-xHf_x)₂S₄ system have been studied. With increasing x the ferromagnetic properties are weakened gradually from a predominant ferromagnetic, a spin-glass, finally to a simple paramagnetic behavior. For the composition of x0.50, a re-entrant spin-glass phase could emerge, even though the Curie temperature is ill-defined as a ferromagnetic phase boundary. Specimens with x≥0.90 remain paramagnetic down to 4.2 K. A spin crossover phenomenon is found around 160 K in the specimens of x=0.50–0.70. A step-like anomaly is manifestly detected in the magnetization, which corresponds with the change of the spin state. This crossover indicates that the spin state converts from high temperature S=2 into low temperature states. In the ordered states in , the magnetic moment originates from only Cr³⁺ ions.     

Theoretical mapping of new L–(N)–L family of species with donor–acceptor bonding between N and ligand L

We perform a computational mapping study of a family of new inorganic species, based on idea of donor–acceptor type bonding between N⁺ and a ligand L with a terminal electron lone pair. The nitrogen ion is seen as being in an atomic ¹D state, with empty 2p acceptor orbitals [I.S.K. Kerkines., A. Papakondylis, A. Mavridis, J. Phys. Chem. A, 2002, 106, 4435]. We consider a series of small ligands, such as PN, CCH⁻, CCCN⁻, , and others. Chemical bonding analysis confirms the suggested bonding picture as characteristic for experimentally known and as well as for most of the predicted species. A number of these new compounds is found to be thermodynamically stable with respect to the existing or . They are candidates for new synthetic targets.     

Theoretical study on the reaction of the Δ ground state of ZrO with CS2 in the gas phase

The mechanisms for the three products ZrS⁺, and ZrOS⁺ of the title reaction have been studied by using B3LYP/6-311+G* and CCSD(T)/SDD+6-311+G* methods. It is found that both ZrS⁺ and formations involve the same O/S exchange process via a four-center transition state TS12 to form an intermediate IM2. Exception of that IM2 can dissociate into the ZrS⁺ product, a favorable intramolecular rearrangement mechanism associated with the formation has been identified, which explains why ZrS⁺ was excluded as a precusor for the formation and why the lower efficiency of the ZrS⁺ formation was observed in experiment. For the formation of ZrOS⁺, two parallel channels (path A and B) yielding their corresponding product isomer have been identified. Path B involving an insertion–elimination mechanism with a calculated barrier underestimated by ca. 25.0 kJ/mol should be attributed to the threshold of 114.8 ± 12.5 kJ/mol assigned in the experiment. But path A should make some contributions to the formation of ZrOS⁺ at elevated energy.     

Vibrational relaxation on the mixed vibronic levels of the and states in all-trans-neurosporene as revealed by subpicosecond time-resolved, stimulated emission and transient absorption spectroscopy

In all-trans-neurosporene, whose level is lower than the level by one vibrational quantum, the following two steps of vibrational relaxation from the mixed vibronic states were identified, in the order, stimulated emission stimulated emission transient absorption. The stimulated emission patterns were simulated by the use of Franck–Condon factors in the transitions from the pair of mixed vibronic levels down to the ground vibronic levels. The vibronic levels of the mixed and state were characterized theoretically based on the diabatic approximation.     

Solution thermodynamics near the liquid–liquid critical point. II. Excess second-order derivatives

We present a comprehensive study of the behaviour of excess second-order derivatives of binary mixtures near the liquid–liquid critical point. Specifically, excess (isobaric and isochoric) molar heat capacities ( and ), excess (isothermal and isentropic) compressibilities ( and ), and excess isobaric thermal expansivities () have been determined over the whole composition (x) range at atmospheric pressure and in the homogeneous region within (293.15–323.15) K. Results are consistent with the predictions of the current theory of critical phenomena. Remarkably, anomalous and curves in the critical region are observed. Such anomalies, which are reported here for the first time, are of significant size for and very mild in the case of , thereby indicating that volumetric effects at near-liquid–liquid criticality are, as expected, very small.     

Excess thermodynamic properties, and for the binary systems {1,2-dichloropropane + 2-alkoxyethanols} at T = 298.15 K

This article presents the experimental data of and , obtained at T = 298.15 K and atmospheric pressure, for four binary systems composed of 1,2-dichloropropane (1,2-DCP) and four 2-alkoxyethanols. The 2-alkoxyethanols are 2-methoxyethanol (2-ME), 2-ethoxyethanol (2-EE), 2-propoxyethanol (2-PE) and 2-butoxyethanol (2-BE). The of the mixture has been shown positive for 2-ME, ‘s-shaped’ for all remaining systems, being negative at low and positive at high mole fraction of 1,2-DCP. The values for all binary mixtures are also shown both positive at low and negative at high mole fraction of 1,2-DCP. The experimental results of and were fitted to Redlich–Kister equation to correlate the composition dependence of both excess properties. In this work, data were also used to test the suitability of thermodynamic models (Wilson, NRTL, and UNIQUAC equations) based on local-composition theory. The results have been qualitatively discussed in terms of the polarity, self-association, and hydrogen bond among molecules.     

Geometries, stability and aromaticity of , [M(Al2P2)] (M = Li, Na, K, Cu) and N(Al2P2) (N = Be, Mg, Ca, Zn) clusters

Geometrical structure, aromaticity and other properties of , [M(Al₂P₂)]⁻ (M = Li, Na, K, Cu) and N(Al₂P₂) (N = Be, Mg, Ca, Zn) species are theoretically investigated with density functional theory (DFT) methods. Calculation results show that for species, the planar structure, with D_2h symmetry at the ¹A_g state, is the global minimum at the B3LYP/6-311+G_* level. Natural bond orbital (NBO) analysis indicates the existence of delocalization in the most stable species and its pyramidal complexes. Nucleus-independent chemical shift (NICS) and molecular orbital (MO) analysis further reveal that that pyramidal [M(Al₂P₂)]⁻ and N(Al₂P₂) species preserve the aromatic nature of the most stable unit.     

Intense laser-induced decomposition of mass-selected 2-, 3-, and 4-methylaniline cations

Photodecomposition processes of 2-, 3-, and 4-methylaniline cations induced by a moderately intense (10¹¹ W/cm²) visible nanosecond laser field and an intense (10¹⁵ W/cm²) UV femtosecond laser field have been investigated using a tandem mass spectrometer. Highly unsaturated fragment cations such as and are dominantly produced by the nanosecond laser, while less unsaturated smaller fragment cations such as , and are produced mainly by the femtosecond laser. Ab initio calculations have also been performed to estimate the stable geometrical structures of and and those of possible intermediate ring compounds for discussing the photodecomposition pathways in intense laser fields.     

Photoelectron spectroscopic study of iron–benzene cluster anions

Iron–benzene cluster anions, (n = 1–7, m = 1–4), were generated via laser vaporization and studied using mass spectrometry, anion photoelectron spectroscopy and in one case by density functional theory. Based on these studies, we propose that and Fe₁Bz₁ as well as and Fe₂Bz₁ exhibit half-sandwich structures, that and Fe₁Bz₂, and Fe₂Bz₂, as well as Fe₃Bz₂ and Fe₄Bz₂ are sandwich structures, and that and Fe₂Bz₃ and larger species form ‘rice-ball’ structures which in each case consist of benzene molecules surrounding an iron cluster core.     

Nucleophilic and electrophilic radical attack on maleic and fumaric acids in aqueous solution

Rate coefficients (k) of CH₂OH, , and radical addition to maleic and fumaric acids were investigated between pH 1 and 8. Strong pH dependences observed were attributed to changes in protonation states of acids: H₂X, HX⁻ and X²⁻. k of CH₂OH, , addition to fumaric acid decreased in the order k_H2F>k_HF^->k_F^2- in agreement with the nucleophilic character of reaction. The electrophilic radical showed opposite tendency. With maleic acid the monoanion had the highest reactivity towards nucleophilic and the lowest one towards electrophilic radicals. This is attributed to a prevalence of steric over polar effects for HM⁻.     

Effect of salts on the excited state of pyranine as determined by steady-state fluorescence

Pyranine is a pH-sensitive fluorescent probe useful in the pH range of 4.5–8, and it has been extensively employed to determine pH inside cells, membranes and membrane models. The fluorescent properties of pyranine are a consequence of the excited states ROH^* and RO^−*. The prototropic equilibrium of these excited species has a much lower than that of the ground state. In this paper we determined the (1.42 ± 0.06) and the relative quantum yield of pyranine in the pH range of 1–8 by analyzing the component peaks of the steady-state of the dye's emission spectrum. As pyranine is very sensitive to the medium we studied the influence of salts formed by mono-, di-, and trivalent ions on the apparent . In all cases, the presence of salts reduced the apparent to varying degrees depending on the valence of the cations. The strategy used to obtain this information was a dual emission ratiometric method at 441 and 511 nm after excitation at 350 nm. The results obtained demonstrate that pyranine is suitable to determine the pH of aqueous solutions in the range of 1–3.5.     

Thermochemical properties of the rare earth complexes with pyromellitic acid

Fourteen rare earth complexes with pyromellitic acid were synthesized and characterized by means of chemical and elemental analysis, and TG–DTG. The constant-volume combustion energies of complexes, Δ_cU, were measured by a precise rotating-bomb calorimeter (RBC-type II). Their standard molar enthalpies of combustion, , and standard molar enthalpies of formation, , were calculated at T = 298.15 K. The relationship of and with the atomic numbers of the elements in the lanthanide series was examined. The results show that a certain amount of covalence is present in the chemical bond between rare earth cations and the ligand.     

The photophysics of flavins: What makes the difference between gas phase and aqueous solution?

The ground and low-lying excited electronic states of isoalloxazine, 10-methylisoallox-azine and lumiflavin, three flavin-related compounds, were investigated by means of quantum chemical methods. Minimum structures were determined employing (time-dependent) Kohn–Sham density functional theory. Spectral properties were computed utilizing a combined density functional and multi-reference configuration interaction (DFT/MRCI) method. Solvent effects were mimicked by a conductor like screening model and micro-hydration with four explicit water molecules. At selected points along a linearly interpolated path connecting the Franck–Condon region and the S₁minimum, spin–orbit interaction was computed employing a nonempirical mean-field Hamiltonian. For isoalloxazine, intersystem crossing (ISC) rate constants were computed, taking both direct and vibronic spin–orbit coupling into account.On the basis of these calculations we suggest the following photo relaxation model. In the vacuum, efficient ISC takes place between the primarily excited state (S₁) and the lowest state (T₂). The energetic proximity of the state (S₂) enhances the nonradiative relaxation of S₁ by internal conversion (IC). In aqueous solution these ISC and IC channels are energetically not accessible due to the blue shift of the states. The high triplet quantum yield observed in experiment [J.T.M. Kennis, S. Crosson, M. Gauden, I.H.M. van Stokkum, K. Moffat, R. van Grondelle, Biochemistry 42 (2003) 3385–3392] is explained by the intersection between the state (S₁) potential energy hypersurface (PEH) and the second (T₂) PEH along the relaxation pathway and the strong enhancement of their spin–orbit coupling by vibronic interactions. The calculated ISC rate for this channel is in good agreement with experimental results. According to our model, lack of an efficient IC channel leads to an increased fluorescence quantum yield in aqueous solution.