One-Electron Pseudo-Potential Determination of Stable Isomers of the Li*Ar n Excited Clusters: Absorption Spectra

We present pseudo-potential calculations of geometrical structures of stable isomers of LiAr _n clusters with both an electronic ground state and excited states of the lithium atom. The Li atom is perturbed by argon atoms in LiAr _n clusters. Its electronic structure obtained as the eigenfunctions of a single-electron operator describing the electron in the field of a Li⁺Ar _n core, the Li⁺ and Ar atoms are replaced by pseudo-potentials. These pseudo-potentials include core-polarization operators to account for the polarization and correlation of the inert core with the valence Lithium electron [J Chem Phys 116, 1839 1]. The geometry optimization of the ground and excited states of LiAr _n (n = 1–12) clusters is carried out via the Basin-Hopping method of Wales et al. [J Phys Chem 101, 5111 2; J Chem Phys 285, 1368 3]. The geometries of the ground and ionic states of LiAr _n clusters were used to determine the energy of the high excited states of the neutral LiAr _n clusters. The variation of the excited state energies of LiAr _n clusters as a function of the number of argon atoms shows an approximate Rydberg character, corresponding to the picture of an excited electron surrounding an ionic cluster core, is already reached for the 3s state. The result of optical transitions calculations shows that the absorption spectral features are sensitive to isomer structure. It is clearly the case for transitions close to the 2p levels of Li which are distorted by the cluster environment.     

Theoretical study on S<Subscript>1</Subscript>(<Superscript>1</Superscript>B<Subscript>3u</Subscript>) state electronic structure and absorption spectrum of pyrazine

Making use of a set of quantum chemistry methods, the harmonic potential surfaces of the ground state (S₀(¹ A _g)) and the first (S₁(¹ B _3u)) excited state of pyrazine are investigated, and the electronic structures of the two states are characterized. In the present study, the conventional quantum mechanical method, taking account of the Born-Oppenheimer adiabatic approximation, is adopted to simulate the absorption spectrum of S₁(¹ B _3u) state of pyrazine. The assignment of main vibronic transitions is made for S₁(¹ B _3u) state. It is found that the spectral profile is mainly described by the Franck-Condon progression of totally symmetric mode ν_6a. For the five totally symmetric modes, the present calculations show that the frequency differences between the ground and the S₁(¹ B _3u) state are small. Therefore the displaced harmonic oscillator approximation along with Franck-Condon transition is used to simulate S₁(¹ B _3u) absorption spectra. The distortion effect due to the so-called quadratic coupling is demonstrated to be unimportant for the absorption spectrum, except the coupling mode ν_10a. The calculated S₁(¹ B _3u) absorption spectrum is in reasonable agreement with the experimental spectra. Supported by Taiwan National Science Council (Grant Nos. NSC 96-2113-M-009-021 and NSC 96-2811-M-009-023)     

Rearrangement Pathways of the <Emphasis Type="BoldItalic">a</Emphasis><Subscript><Emphasis Type="BoldItalic">4</Emphasis></Subscript> Ion of Protonated YGGFL Characterized by IR Spectroscopy and Modeling

The structure of the a ₄ ion from protonated YGGFL was studied in a quadrupole ion trap mass spectrometer by ‘action’ infrared spectroscopy in the 1000–2000 cm^–1 (‘fingerprint’) range using the CLIO Free Electron Laser. The potential energy surface (PES) of this ion was characterized by detailed molecular dynamics scans and density functional theory calculations exploring a large number of isomers and protonation sites. IR and theory indicate the a ₄ ion population is primarily populated by the rearranged, linear structure proposed recently (Bythell et al., J. Am. Chem. Soc. 2010, 132, 14766). This structure contains an imine group at the N- terminus and an amide group –CO–NH₂ at the C-terminus. Our data also indicate that the originally proposed N-terminally protonated linear structure and macrocyclic structures (Polfer et al., J. Am. Chem. Soc. 2007, 129, 5887) are also present as minor populations. The clear differences between the present and previous IR spectra are discussed in detail. This mixture of gas-phase structures is also in agreement with the ion mobility spectrum published by Clemmer and co-workers recently (J. Phys. Chem. A 2008, 112, 1286). Additionally, the calculated cross-sections for the rearranged structures indicate these correspond to the most abundant (and previously unassigned) feature in Clemmer’s work.     

A theoretical study on the ionization of tetrafluoroethylene with analysis of vibrational structure of the photoelectron spectra

Ab initio calculations have been performed to study on the molecular structures and the vibrational levels of the low-lying ionic states (²B_2u,²A_g,²B_2g,²B_3u,²A_u,²B_1g,²B_1u, and²B_3g) of tetrafluoroethylene. The equilibrium molecular structures and vibrational modes of these states are presented. The theoretical ionization intensity curves including the vibrational structures of the low-lying eight ionic states are also presented and compared with the photoelectron spectrum. Some new assignments of the photoelectron spectra are proposed.     

Simplification of the Clegg-Pitzer-Brimblecombe Mole-Fraction Composition Based Model Equations for Binary Solutions,Conversion of the Margules Expansion Terms into a Virial Form,and Comparison with an Extended Ion-Interaction (Pitzer) Model

Clegg, Pitzer, and Brimblecombe (J. Phys. Chem. 96:9470–9479, 1992) described a thermodynamic model for representing the activities of solutes and a solvent, for a single electrolyte and for mixtures of arbitrary complexity, which is valid to very high concentrations including electrolytes approaching complete mutual solubility. This model contains a Debye-Hückel term along with two ionic-strength-dependent virial terms and a Margules expansion in the mole fractions of the components at the four-suffix level, with ionic strengths expressed on the mole-fraction composition scale. This model is an extension of earlier work by Pitzer and Simonson (J. Phys. Chem. 90:3005–3009, 1986). However, Pitzer’s molality-based ion-interaction model (Activity Coefficients in Electrolyte Solutions, 2nd edn., CRC Press, 1991) is more commonly used for thermodynamic modeling calculations. In this paper we recast the Margules expansion terms of the mole-fraction-based model equations for a single electrolyte in a single solvent into simpler virial expansions in powers of the mole-fraction-based ionic strength. We thereby show that these reformulated equations are functionally analogous to those of Pitzer’s standard ion-interaction model with an additional virial term added that is cubic in the ionic strength. By using a series of algebraic transformations among composition scales, we show that the pairs of terms involving the B_M,X⁽¹⁾B_{\mathrm{M,X}}^{(1)} and the B_M,X⁽²⁾B_{\mathrm{M,X}}^{(2)} parameters in the original mole-fraction-based model expression for the natural logarithm of the mean activity coefficient (and consequently for the excess Gibbs energy) differ from each other only by a simple numerical factor of −2 and, therefore, these four terms can be replaced by two terms yielding simpler expressions. Test calculations are presented for several soluble electrolytes to compare the effectiveness of the reformulated mole-fraction- and molality-based models, at the same virial level in powers of ionic strength, for representing activity data over different ionic strength ranges. The molality-based model gives slightly better fits over the ionic strength range 0 mol⋅kg⁻¹≤I≤6 mol⋅kg⁻¹, whereas the mole-fraction-based model is generally better for more extended ranges.     

The Role of the nπ* 1Au State in the Photoabsorption and Relaxation of Pyrazine

The geometric, energetic, and spectroscopic properties of the ground state and the lowest four singlet excited states of pyrazine have been studied by using DFT/TD‐DFT, CASSCF, CASPT2, and related quantum chemical calculations. The second singlet nπ* state, ¹A_u, which is conventionally regarded dark due to the dipole‐forbidden ¹A_u←¹A_g transition, has been investigated in detail. Our new simulation has shown that the state could be visible in the absorption spectrum by intensity borrowing from neighboring nπ* ¹B_3u and ππ* ¹B_2u states through vibronic coupling. The scans on potential‐energy surfaces further indicated that the ¹A_u state intersects with the ¹B_2u states near the equilibrium of the latter, thus implying its participation in the ultrafast relaxation process.     

Theoretical study of the π→π* excited states of linear polyenes: The energy gap between 11Bu+ and 21Ag− states and their character

Multireference perturbation theory with complete active space self-consistent field (CASSCF) reference functions is applied to the study of the valence π→π* excited states of 1,3-butadiene, 1,3,5-hexatriene, 1,3,5,7-octatetraene, and 1,3,5,7,9-decapentaene. Our focus was put on determining the nature of the two lowest-lying singlet excited states, 1¹B_u⁺ and 2¹A_g⁻, and their ordering. The 1¹B_u⁺ state is a singly excited state with an ionic nature originating from the HOMO→LUMO one-electron transition while the covalent 2¹A_g⁻ state is the doubly excited state which comes mainly from the (HOMO)²→(LUMO)² transition. The active-space and basis-set effects are taken into account to estimate the excitation energies of larger polyenes. For butadiene, the 1¹B_u⁺ state is calculated to be slightly lower by 0.1 eV than the doubly excited 2¹A_g⁻ state at the ground-state equilibrium geometry. For hexatriene, our calculations predict the two states to be virtually degenerate. Octatetraene is the first polyene for which we predict that the 2¹A_g⁻ state is the lowest excited singlet state at the ground-state geometry. The present theory also indicates that the 2¹A_g⁻ state lies clearly below the 1¹B_u⁺ state in decapentaene with the energy gap of 0.4 eV. The 0–0 transition and the emission energies are also calculated using the planar C_2h relaxed excited-state geometries. The covalent 2¹A_g⁻ state is much more sensitive to the geometry variation than is the ionic 1¹B_u⁺ state, which places the 2¹A_g⁻ state significantly below the 1¹B_u⁺ state at the relaxed geometry. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 157–175, 1998     

The electronic states of 1,2,5-oxadiazole studied by VUV absorption spectroscopy and CI, CCSD(T) and DFT methods

The 1,2,5-oxadiazole VUV absorption spectrum in the range 5–11.5 eV, shows broad bands centred near 6.2, 7.1, 8.3, 8.8, 10.6 and 11.3 eV. Rydberg states associated with three ionisation energies (IE) were identified in the complex fine structure above 8.7 eV. Electronic vertical excitation energies for singlet and triplet valence, and Rydberg states were computed using ab initio multi-reference multi-root CI methods. There is generally a good correlation between the envelope of the theoretical intensities and the experimental spectrum. The nature of the more intense calculated Rydberg states, and positions of the main valence and Rydberg bands are discussed. The lowest triplet, singlet and Rydberg 3s excited states have equilibrium structures that are non-planar with C_S symmetry, in a chair-like orientation where the O and H atoms lie out of the NCCN plane. This finding is consistent with the doubling of the low energy UV spectral lines [B.J. Forrest, A.W. Richardson, Can. J. Chem., 50 (1972) 2088].The nearly degenerate IE of the UV-photoelectron spectrum (UV–PES, Palmer et al. 1977) makes analysis of the VUV spectrum difficult, leading to the necessity for reinvestigation. Vertical studies (IE_V) using CI, Tamm–Dancoff (TDA) and Green’s Function (GF) methods all gave similar results, with near degeneracy of the first 3IE_V confirming the earlier study. Studies of the adiabatic IE (IE_A) using CCSD(T) and B3LYP methods, showed the energy sequence ²A₂ < ²B₁ < ²B₂, but these states are all saddle points, in contrast to the 4th state (²A₁) which is a minimum. In contrast, MP2 study of the ²B₂ state showed a minimum, with only two saddle points.Complete minima were found after minor twisting of the structures. The lowest energy cationic state is ²A^″ (C_S), which closely resembles the ²B₂ state. The O–N–C–C skeleton is twisted by 8°. The corresponding ²A^′ state (C_S) is effectively identical to the ²B₁ state. Attempts to find minima for other symmetry states were unsuccessful.     

The Low‐Lying Excited States of 2,2′‐Bithiophene: A Theoretical Analysis

The low‐energy regions of the singlet→singlet, singlet→triplet, and triplet→triplet electronic spectra of 2,2′‐bithiophene are studied using multiconfigurational second‐order perturbation theory (CASPT2) and extended atomic natural orbitals (ANO) basis sets. The computed vertical, adiabatic, and emission transition energies are in agreement with the available experimental data. The two lowest singlet excited states, 1¹B_u and 2¹B_u, are computed to be degenerate, a novel feature of the system to be borne in mind during the rationalization of its photophysics. As regards the observed high triplet quantum yield of the molecule, it is concluded that the triplet states 2³A_g and 2³B_u, separated about 0.4 eV from the two lowest singlet excited states, can be populated by intersystem crossing from nonplanar singlet states.     

The emission of α,ω-diphenylpolyenes: A model involving several molecular structures

Available photophysical evidence for the emission of α,ω-diphenylpolyenes is shown to be consistent with a previously reported model [J. Catalán, J.L.G. de Paz, J. Chem. Phys. 124 (2006) 034306] involving two electronically excited molecular structures of 1B_u and C_s symmetry, respectively. The 1B_u structure is produced by direct light absorption from the all-trans form of the α,ω-diphenylpolyene in the ground state and its emission exhibits mirror symmetry with respect to the absorption of the compound. On the other hand, the C_s structure is generated from the 1B_u structure of the α,ω-diphenylpolyene by rotation about a C–C single bond in the polyene chain, its emission being red-shifted with respect to the previous one and exhibiting markedly decreased vibrational structure. At room temperature, both emissions give the excitation spectrum, which are ascribed to the first absorption band for the compound.     

(all-E)-1,3,5,7-Octatetraene: Electron-Energy-Loss and Electron-Transmission Spectra

Electron-energy-loss and electron-transmission spectra of (all-E)-1,3,5,7-octatetraene were recorded with a trochoidal electron spectrometer. The energy-loss spectra reveal two triplet states at 1.73 and 3.25 eV (0,0-transitions), the UV-active 1¹B_u state at 4.40 eV, and a higher-lying singlet state at 6.04 eV. The 2¹A_g state recently reported to be the lowest excited singlet state in this polyene, could not be observed. This failure is probably de to a small excitation cross section under the scattering conditions used and the presence of the second triplet sate in the pertinent energy region. The electron-transmission spectrum revealed three resonances (i.e. short-lived anion states) at 1.5, 2.5, and 4.15 eV.     

Further evidence of the vibronic coupling of pyrazine as revealed by preresonance raman effect

The excitation wavelength dependences of the intensities of the Raman lines of pyrazine have been measured. The intensity enhancement of the non-totally symmetric Raman line at 925 cm^?1 provided firm evidence of the vibronic coupling between the lowest ¹B_3u(n,π^*) and second lowest ¹B_2u(π,π^*) states. The excitation wavelength dependences of other non-totally symmetric Raman lines suggest also the various vibronic coupling schemes.     

Charge and spin correlation structures of conjugated π systems: Analysis of full CI wave functions of the PPP hamiltonian

An analysis of the electronic correlation structures by means of the charge and spin correlation functions is carried out for full CI wave functions of four, five, and six membered conjugated π systems described by the Pariser–Parr–Pople Hamiltonian. The low-lying states of these systems are classified as covalent (CV ) and ionic (IN ) states depending on whether the probability of finding two electrons simultaneously at the same position is small or large. It is found that many of excited CV states, the typical ones of which are the 2¹A_g state of linear π systems, have stronger CV character than the ground CV state, and their spin coupling structures are different from each other as well as from that of the ground CV state. The spin coupling structure in the ground CV state has an “antiferromagnetic” spin arrangement in favor of antiparallel coupling between nearest neighbor spins while in excited CV states the extent of the antiparallel spin coupling between nearest neighbor sites is decreased. IN states, which are less common for low-lying states than CV ones, are also found to have characteristic modulations in the charge correlation. In particular, the charge correlations in the lowest singlet IN states, 1¹B_u of linear π systems, 1¹B_2g of cyclobutadiene and 1¹B_1U of benzene, are alternating.     

1B2u1A1g fluorescence from benzene produced by electron impact (30–1000 eV)

The ¹B_2u → ¹A_1g fluorescence resulting from electron impact (30–1000 eV) on benzene has been studied in the pressure range 10^?4 ?2 × 10^?3 torr. The fluorescence spectrum is compared with the spectrum obtained by other methods. The energy dependence of the absolute emission cross section indicates a small probability for internal conversion from higher singlet states to the ¹B_2u state.     

A “classical” trajectory driven nuclear dynamics by a parallelized quantum‐classical approach to a realistic model Hamiltonian of benzene radical cation

We explore the workability of a parallelized algorithm of time‐dependent discrete variable representation (TDDVR) methodology formulated by involving “classical” trajectories on each DOF of a multi‐mode multi‐state Hamiltonian to reproduce the population dynamics, photoabsorption spectra and nuclear dynamics of the benzene radical cation. To perform such dynamics, we have used a realistic model Hamiltonian consists of five lowest electronic states (X²E_1g, B²E_2g, C²A_2u, D²E_1u, and E²B_2u) which are interconnected through several conical intersections with nine vibrational modes. The calculated nuclear dynamics and photoabsorption spectra with the advent of our parallelized TDDVR approach show excellent agreement with the results obtained by multiconfiguration time‐dependent Hartree method and experimental findings, respectively. The major focus of this article is to demonstrate how the “classical” trajectories for the different modes and the “classical” energy functional for those modes on each surface can enlight the time‐dependent feature of nuclear density and its' nodal structure. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Assignment of the four lowest ionized states of p-benzoquinone and the question of “lone pair splitting” in this system

The lowest ionized states of p-benzoquinone are assigned to be n_/²B_3g, n₄/²B_2u, π₁/²B_3u, and π₂/²B_1g in order of increasing energy. This sequence of states confirms Turner's original assignment and invalidates subsequent proposals. The origin of the difficulties with these assignments are traced back to the non-validity of Koopmans' theorem for the “oxygen lone pair” ionizations (i.e., ionization to the n_/²B_3g and n₊/²B_2u states).     

Excited-state absorption spectroscopy and state ordering in polyenes: 1,3,5,7-Octatetraene

The S_n ← S₁ spectrum of 1,3,5,7-octatetraene has been obtained in cyclohexane. Calculations predict different S_n ← S₁ spectra for the lowest excited ¹Ag^? or ¹B_u⁺ states. The experimental S_n ← S₁ spectrum is consistent with the 2 ¹A_g^? as the lowest excited state. Extension of this technique to smaller polyenes is discussed.     

1 Bu–2 Ag conical intersection in trans-butadiene: ultrafast dynamics and optical spectra

The potential-energy functions of the 1 ¹B_u and 2 ¹A_g excited valence states of trans-butadiene have been characterised by the CASPT2 method. Based on these ab initio data, a vibronic-coupling model describing the conical intersection of the 1 ¹B_u and 2 ¹A_g states has been constructed. UV resonance-Raman and absorption spectra have been calculated, employing the time-dependent approach. The time-dependent wave-packet calculations reproduce the expected ultrafast (≈30 fs) radiationless decay of the optically bright 1 ¹B_u state into the dark 2 ¹A_g state.     

Structure and Reactivity of the <Emphasis Type="BoldItalic">N</Emphasis>-Acetyl-Cysteine Radical Cation and Anion: Does Radical Migration Occur?

The structure and reactivity of the N-acetyl-cysteine radical cation and anion were studied using ion-molecule reactions, infrared multi-photon dissociation (IRMPD) spectroscopy, and density functional theory (DFT) calculations. The radical cation was generated by first nitrosylating the thiol of N-acetyl-cysteine followed by the homolytic cleavage of the S–NO bond in the gas phase. IRMPD spectroscopy coupled with DFT calculations revealed that for the radical cation the radical migrates from its initial position on the sulfur atom to the α-carbon position, which is 2.5 kJ mol^–1 lower in energy. The radical migration was confirmed by time-resolved ion-molecule reactions. These results are in contrast with our previous study on cysteine methyl ester radical cation (Osburn et al., Chem. Eur. J. 2011 , 17, 873–879) and the study by Sinha et al. for cysteine radical cation (Phys. Chem. Chem. Phys. 2010 , 12, 9794–9800) where the radical was found to stay on the sulfur atom as formed. A similar approach allowed us to form a hydrogen-deficient radical anion of N-acetyl-cysteine, (M – 2H) ^•– . IRMPD studies and ion-molecule reactions performed on the radical anion showed that the radical remains on the sulfur, which is the initial and more stable (by 63.6 kJ mol^–1) position, and does not rearrange.