Theoretical Electronic Spectra Research Articles

Electronic Spectrum of Tryptophan-Phenylalanine. A Correlated Ab Initio and Time-Dependent Density Functional Theory Study

The theoretical electronic spectrum of the tryptophan-phenylalanine bichromophoric dipeptide was obtained for one of the lowest-energy conformer by various high-level computational methods such as complete active space with second order perturbation theory, second-order approximate coupled-cluster theory, and time-dependent density functional theory. The results show that the first excited state is located on the tryptophan residue and called L(b) state in the amino-acid. The second and third excited states correspond respectively to the L(a) state of Trp and the excited state in the Phe residue. Time-dependent density functional methods appeared to be not efficient to calculate the excited states of such a peptide (except the first one) due to the inclusion of charge transfer states.

Electronic Absorption Spectroscopy of H2X (X = O, Te, Po): Theoretical Treatment of Spin-Orbit Effects

The electronic spectroscopy of H2X (X = O, Te, Po) was investigated by means of spin-orbit configuration interaction (EPCISO) and restricted active space state interaction (SO-RASSI). The transition energies to the low-lying singlet and triplet states of H2O, in which the SO interaction is zero, compare rather well with the experimental data as well as to other theoretical values. The theoretical electronic absorption spectrum is characterized by three allowed transitions A1B1 (2px(O)*g/3s(O)), B1A1(g*g/3s(O)) and A1B2(g*u) calculated at 7.68, 9.94, and 11.72 eV, respectively. The theoretical absorption spectra of H2X (X = Te, Po) are shifted to the red with the A1B1 (npx(X)*g) states calculated at 5.06 eV (H2Te) and 4.40 eV (H2Po) and the A1B2 (g*u) states calculated at 7.89 eV (H2Te) and 7.77 eV (H2Po). The largest SO splitting amounts to 0.34 eV and is found for the lowest a3A1 of H2Po. In H2Te the SO effects are still negligible with a maximum splitting of 0.04 eV for the lowest a3B2. The two methods lead to comparable results but the EPCISO approach depends strongly on the reference wavefunction.

Excitation and decay dynamics of 1s2s inner-shell double-vacancy states of neon atoms

The photo-excitation and Auger decay processes of inner-shell double vacancy states 1s2s2p6(1,3S)3s3p of neutral neon atoms have been studied theoretically. Multi-configuration Dirac-Fock (MCDF) calculations have been carried out, with electron correlation effects taken into consideration. The relaxation of core and excited orbitals and configuration interaction are found to be crucial to creating the double vacancy states by single photo-absorption. The predominant decay paths for the double vacancy states turn out to be of the LLM Auger decay to 1s 2s22p53s(3p), KLL Auger decay to 1s22s2p43s3p, and KLM Auger decay to 1s22p63s(3p). They lead to further Auger decay, creating the neon ions of multiple charge states. For both double and single vacancy states the spectator type of Auger process is dominated in all the Auger decay processes. Theoretical Auger electron spectra are presented for further investigations, experimental and theoretical.

Excitation and circular dichroism spectra of (−)‐(3aS, 7aS)‐2‐chalcogena‐trans‐hydrindans(Ch = S, Se, Te): SAC and SAC‐CI calculations

The ground and several electronic excited states of (3aS,7aS)-2-chalcogena-trans-hydrindans were calculated by the symmetry adapted cluster (SAC) and SAC-configuration interaction (SAC-CI) methods. Theoretical electronic excitation spectra and natural circular dichroism (CD) spectra were obtained for these compounds, and the calculated spectra showed good agreement with the experimental ones reported by Laur (Proceedings of the Third International Symposium on Organic Selenium and Tellurium compounds, Metz, France, 1979, pp. 219-299). For all the chalcogen compounds, the first singlet excited states are assigned to n-sigma* and the other states are assigned to n-Rydberg in our calculations. It indicates that the spectra for the sulfide, selenide, and telluride are almost regarded as the analogues except for the red shifts of the band positions from the sulfide to the telluride. For the telluride, however, the experimental spectra have shapes that cannot be interpreted by the singlet excitations solely. Our calculations predict the triplet states that account for the spectral shapes, indicating importance of the spin-orbit interaction effects for the accurate reproduction of the experimental spectra of the telluride.

POSITIONAL EFFECT OF PHENYLENE MOIETY ON A CERTAIN PERYLENE CHROMOPHORE: A SEMIEMPIRICAL TREATMENT

A benzimidazole derivative of perylenetetracarboxylic-3,4,9,10 anhydride has been considered as the base system. Then, by means of annellation (phenylene fusion) a series of nonexistent structures have been constructed. The positional effect of annellation on these systems has been analyzed theoretically by performing PM3/RHF type quantum chemical calculations. The geometries and stabilities are discussed. The theoretical electronic spectra have been produced by using ZINDO/S (CI) type calculations and discussed.

Excited state intramolecular proton transfer (ESIPT) in a dioxotetraamine derived schiff base and its complexation with Fe(III) and Cr(III)

The excited state intramolecular proton transfer (ESIPT) process in a symmetric Schiff base derived from dioxotetraamine and salicylaldehyde, N, N′-Bis{2-[(2-hydroxybenzylidine)amino]ethyl}malonamide (BHAEM) has been investigated experimentally by the fluorescent method and a two step proton transfer mechanism from dienol to ketoenol and then to diketo tautomer has been proposed on the basis of the theoretical evidence obtained through semi-empirical MO/CI calculations using the AM1 Hamiltonian. BHAEM on interaction with Fe(III) and Cr(III), in an aqueous medium of 0.1N ionic strength and 25 ± 1 °C forms monomeric metal complexes of the type ML and MLH −2 with both metal ions, whereas with Cr(III) it forms two more additional species, MLH and MLH −1. The stabilities of these metal complexes have been evaluated using potentiometric and spectrophotometric methods and the relative stabilities of the complexes formed with this ligand are rationalized. At low pH, the ligand coordinates to the metal ions through bis-imine nitrogen and bis-phenolate oxygen atoms but with an increase of pH, further coordination occurs through two amide nitrogen atoms of the ionized amide groups. The structures of the metal complexes were predicted from minimum strain energy calculations employing molecular mechanics using the MM3 force field and from their theoretical electronic spectra obtained through semi-empirical AM1/ZINDO method.

Characterization of the Al(III) binding site of protocatechuic acid by electronic spectroscopy and quantum chemical calculations

In acidic aqueous medium and in methanol, the formation of a 1:1 complex between Al(III) and protocatechuic acid (PCAH) was highlighted by electronic absorption spectroscopy. The ground and low-lying excited electronic states of the possible structures of the 1:1 complex were studied using DFT approach. Molecular and electronic properties were calculated using the VSXC functional, while excited singlet states were examined using TD-DFT methodology. A very good agreement in wavelengths was found between the theoretical electronic spectrum of a chelate involving the catechol group and the 1:1 complex experimental spectrum.

Measurement and theoretical characterization of electronic absorption spectrum of neutral chrysene (C 18H 12) and its positive ion in H 3BO 3 matrix

The ultraviolet and visible spectrum of chrysene and its radical cation formed by ultraviolet irradiation were measured in boric acid glass at room temperature. The theoretical electronic absorption spectrum of any polycyclic aromatic hydrocarbon (PAH) in boric acid matrix is calculated for the first time using semi empirical methods. Earlier reported theoretical results of electronic spectrum are calculated in free state and the results are compared with the spectrum of aromatic systems in glassy or other matrices. The interaction between the trapped PAHs (neutral and ions) and its environment induces strong perturbations of the energy levels which results in large shifts of the electronic transitions as compared to the ideal case of a free, isolated PAH molecule. This shifting due to perturbation has largely been ignored in earlier calculations, while comparing the calculations with the experimentally measured spectrum, in other matrices. The spectrum of singlet and doublet state of chrysene are computed in aqueous medium and also in free state to estimate the spectral shift. Several other geometric (bond length and bond angles) and spectroscopic parameters of chrysene like difference of HOMO–LUMO, ionization potential, dipole moment and polarizability are calculated using semi empirical methods, namely Austin Model 1 (AM1) and Parametric Method 3 (PM3). To get an idea about how the symmetry of chrysene molecule varies upon ionization, the mean polarizability ( α) as well as its tensor components α xx , α yy and α zz are calculated within a field of 0.005 a.u. The lasing action in neutral chrysene and in its cationic form is also discussed for the first time.

Theoretical electronic spectra of 2‐aminopurine in vapor and in water

AbstractThe accurate quantum chemical CASSCF and CASPT2 methods combined with a Monte Carlo procedure to mimic solvation effects have been used in the calculation of the spectroscopic properties of two tautomers of 2‐aminopurine (2AP). Absorption and emission spectra have been simulated both in vacuum and in aqueous environment. State and transition energies and properties have been obtained with high accuracy, leading to the assignment of the most important spectroscopic features. The lowest‐lying 1(π,π*) (1La) state has been determined as responsible for the first band in the absorption spectrum and also for the strong fluorescence observed for the system in water. The combined approach used in the present work gives quantitatively accurate results. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

DFT and TD-DFT investigation and spectroscopic characterization of the molecular and electronic structure of the Zn(II)–3-hydroxyflavone complex

The ground and low-lying excited electronic states of the 1:1 complex of Zn with 3-hydroxyflavone (3HF) in methanol solution are studied using DFT approach. Molecular and electronic properties are calculated using B3LYP, while excited singlet states are examined using TD-DFT methodology. A very good agreement was found between the theoretical absorption electronic spectrum of [Zn(3HF)] + and the experimental one. A structural and spectroscopic comparison is made with the [Pb(3HF)] + chelate.

Spectral and theoretical studies of N- p-(ethylbenzene)thiobenzohydroxamic acid metal chelates

The preparation of N- p-(ethylbenzene)thiobenzohydroxamic acid chelates with several metal ions and their spectral properties are described in this paper. This is followed by a theoretical study of metal complexes of some thiohydroxamic acids, as well as the prepared chelates. The electronic properties of the metal complexes are discussed. The experimental and theoretical electronic spectra are also compared. A possible reason for the smaller p K a values of thiohydroxamic acid complexes than those of the corresponding hydroxamic acids is given.

Characteristic low-energy electrons emitted in proton-atom collisions at intermediate impact velocities

We report experimental and theoretical electron spectra within the very low-electron-energy range below 30 eV. Doubly differential cross sections for ionization of different noble gases (He, Ne, Ar, Kr, Xe) by impact of 100-keV and 225-keV protons are reported. A pronounced structure in the low-energy region of emitted-electron spectra is observed with different intensities for the different targets. We compare our experimental findings with two theoretical models: the first-order Born approximation and the ``continuum-distorted-wave--eikonal-initial-state'' model using different optimized central potentials to calculate the bound and continuum states of the target. We attribute our experimental observation to a signature of the predicted low-energy binary peak.

Theoretical X-ray photoelectron and Auger electron spectra of polymers by density functional theory calculations using model dimers

We present theoretical X-ray photoelectron spectra (XPS) and Auger electron spectra (AES) for polymers by density functional theory (DFT) calculations with the Slater's transition-state concept. The simulated XPS and AES of three polymers [poly(ethylene) (PE), poly( cis-butadiene) (PcBD), and poly(styrene) (PS)] by DFT calculations using model molecules are in good accordance with the experimental ones. The combined analysis of AES and XPS enable us to clarify the electronic structure of single and double excitation for polymers from the theoretical viewpoint.

Complexes of aluminium(III) with isoquercitrin: spectroscopic characterization and quantum chemical calculations

The composition and conformation of complexes of aluminium(III) with isoquercitrin (Iso) have been studied in methanol solution. This molecule presents two potential chelating sites in competition. UV–Vis spectroscopy provides evidence of three different species in neutral solution: Al(Iso) 2+ Al(Iso) 2 + and Al 2(Iso) 3+. This last one is formed only if a great quantity of Al(III) is presented in the medium. The first site involved in complexation is the 5-hydroxy-4-keto group. FT-Raman spectroscopy has allowed to confirm this mechanism. The stability constants of this complexes have been determined using the spectfit program. In acidic condition, only the first complex is obtained. In alkaline medium, Al(Iso) + and Al(Iso) 2 − complexes are formed, the catechol group is then the chelating site involved in these species To propose a molecular conformation for the free isoquercitrin and its complexes in neutral medium, both semiempirical molecular orbital and DFT calculations have been performed. It has been showed that the participation of cynnamoyl and benzoyle mesomeric forms stabilise the structure of Al(Iso) 2+. The structural models have been validated by the good agreement between theoretical and experimental electronic spectra.

Theoretical electronic spectra for models having ferredoxin-core

Ferredoxin-core model [Fe 4S 4] type iron–sulfur cluster having four cysteine moieties in the amide form, has been considered theoretically for electronic excitation spectrum. The neutral as well as the ∓2 charged forms of the model have been investigated within the restrictions of ZINDO/S (RHF) approach involving configuration interaction considerations. The charged forms, especially the negative one exhibited some bathochromic shift and hyperchromic effect.

Theoretical auger electron spectra of polymers by density functional theory calculations using model dimers

We propose a new approach for analysis of Auger electron spectra (AES) of polymers by density functional theory (DFT) calculations with the Slater's transition-state concept. Simulated AES and X-ray photoelectron spectra (XPS) of four polymers [(CH2CH2)n (PE), (CH2CH(CH3))n (PP), (CH2CH(OCH3))n (PVME), and (CH2CH(COCH3))n (PVMK)] by DFT calculations using model dimers are in a good accordance with the experimental ones. The experimental AES of the polymers can be classified in each range of 1s–2p2p, 1s–2s2p, and 1s–2s2s transitions for C KVV and O KVV spectra, and in individual contributions of the functional groups from the theoretical analysis. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 394–401, 2002

Theoretical Auger electron and X-ray emission spectra of CO and H 2O by density functional theory calculations

We propose a new method for analysis of X-ray emission and Auger electron spectra (XES and AES) of molecules involving the valence spectra using density functional theory (DFT) calculations. To obtain the more accurate transition energies and the relative intensities, we use the total-energy difference procedure ( ΔE-KS) for all transition energies, and transform the coefficients in the LCGTO-MO scheme in the DFT to those for the linear combination of the LCGTO-AO scheme. The method is applied to the analysis of valence spectra, XES and AES for CO and H 2O molecules. The simulated spectra are in a good agreement with the experimental results.

Theoretical Auger electron spectra of polymers by density functional theory calculations using model dimers.

We propose a new approach for analysis of Auger electron spectra (AES) of polymers by density functional theory (DFT) calculations with the Slater's transition-state concept. Simulated AES and X-ray photoelectron spectra (XPS) of four polymers [(CH2CH2)n (PE), (CH2CH(CH3))n (PP), (CH2CH(OCH3))n (PVME), and (CH2CH(COCH3))n (PVMK)] by DFT calculations using model dimers are in a good accordance with the experimental ones. The experimental AES of the polymers can be classified in each range of 1s-2p2p, 1s-2s2p, and 1s-2s2s transitions for C KVV and O KVV spectra, and in individual contributions of the functional groups from the theoretical analysis.

A critical comparison of theoretical and experimental electronic spectrum and potential energy curves of HF molecule and its positive and negative ions

Although ab initio many-body methods can provide accurate predictions for most small systems, substantial effort is devoted to design and develop fairly accurate and computationally efficient ‘packageable’ methods for treating large molecular systems, where packageable implies a method that can be incorporated into widely distributed suites of electronic structure programs. Here, we provide further tests and illustrations of our recently developed improved virtual orbital-complete active space configuration interaction (IVO-CASCI) method, which is designed to supplant CIS and CASSCF approaches in electronic structure ‘packages’ because of greater computational efficiency without sacrificing accuracy. The IVO-CASCI method is used here to compute the ground, excited, positive, and negative ion potential energy curve of hydrogen fluoride. Additional dynamical correlation is incorporated by using IVO-CASCI wavefunctions as the initial approximation in selected further effective valence shell Hamiltonian computations for HF and its ions. The excellent agreement between the theory, experiment, and some benchmark calculations for several ground, excited, and ion states of HF reinforces the claim that the IVO-CASCI method is a viable alternate packageable many-body method for ground, excited, and other open-shell states of small to large molecular systems.

Complexes of Al(III) with 3′4′-dihydroxy-flavone: characterization, theoretical and spectroscopic study

Complex formation between aluminium chloride and 3′4′-dihydroxyflavone (3′4′diOHF) in methanol has been studied by UV–visible and Raman spectroscopies combined with quantum chemical calculations. Job's method of continuous variation and the molar ratio method were applied to ascertain the stoichiometry composition of the chelate in pure methanol. A 1:1 complex was indicated by both the methods. Geometry optimizations of free and complexed molecules by AM1 and DFT methods show that structural modifications of the ligand, induced by complexation, are minor, and are localized on the chelating site. The good agreement between experimental and theoretical electronic spectra of both 3′4′diOHF and complex confirm the structural models. The great similarities between Raman spectra of the free and complexed form constitute an another proof of the absence of pronounced electronic and geometric changes, and notably demonstrate that the quinoidal form induced by the deprotonation of the two hydroxyl groups does not participate in the 3′4′diOHF complex structure. Whereas no complexation occurs in acidic medium, complexes of high stoichiometry are formed in alkaline medium. (Al(3′4′diOHF) 2) − and (Al(3′4′diOHF) 3) 3− species are observed in methanol in the presence of sodium acetate or sodium methanoate.