Theoretical Electronic Spectra Research Articles

Theoretical analysis of the oxocarbons: structure and spectroscopic properties of croconate ion and its coordination compound with lithium

Ab initio methods were used in conjunction with Monte Carlo simulation to analyze the structure and spectroscopic properties of the croconate ion in the gas phase and in aqueous solution. The infrared and Raman spectra were calculated and band assignments were made showing a good agreement with experiment. The electronic spectrum of the croconate ion was calculated in the gas phase and in aqueous solution, using a sequential Monte Carlo/quantum mechanical approach, taking into account the solvent and counter ion effects. The electronic spectrum for the free croconate ion in aqueous solution showed two transitions at 479 and 468 nm when the first solvation shell is considered. These transitions were not sensitive to additional solvent molecules beyond the first solvation shell. The experimental electronic spectrum was only reproduced when the combined effects of the solvent and counter ion were taken into account. The calculated spectrum for the cis-[Li2(C5O5)(H2O)21] complex showed two transitions at 383 and 365 nm, in agreement with the experimental observations of 372 and 351 nm. These results strongly suggest that in order to reproduce the experimental electronic spectrum of the oxocarbons in solution, we must take into account the combined effects of the solvent and the counter ions. A new proposal for the interaction of Li+ with the croconate anion in solution, based on the theoretical electronic spectra, is also discussed.

Molecular orbital calculations, experimental and theoretical UV spectra of granulatimides and didemnimides, biologically active polycyclic heteroaromatic alkaloids from the ascidian Didemnum granulatum

A detailed computational study was performed for compounds granulatimide, isogranulatimide, and didemnimides A, D, and E, using the semiempirical Austin model 1 quantum chemical method. The electronic features and structural parameters were confronted with the inhibition of the G2 cell cycle checkpoint of mammalian cancer cells. All compounds were submitted to a rigorous conformational analysis using the Tripos 5.2 force field implemented in the Spartan 5.01 program. The electronic density in specific regions of the molecules appears to play a pivotal role towards activity. The molecular planarity creates a broad negative electrostatic potential on the two sides of the active compounds (granulatimide and isogralulatimide) and a positive potential in their central core, while the non-planar compounds (didemnimides A, D, and E, which are inactive) present an asymmetric potential scattered over the molecules. These electrostatic potential features are likely to be the modulator of hydrophobicity or lipophilicity of the compounds, which appear correlated with activity. The hydrogen attached to the N atom of the pyrrole moiety of indole is more positive for active compounds than for the inactive molecules. The theoretical electronic spectra were obtained for all compounds using the configuration interaction method, with the AM1 routine. All transitions present π→π∗ nature. The theoretical results are in good agreement with experimental values.

Theoretical investigation of the molecular structure of the isoquercitrin molecule

Isoquercitrin is a glycosilated flavonoid that has received a great deal of attention because of its numerous biological effects. We present a theoretical study on isoquercitrin using both empirical (Molecular Mechanics (MM), with MMX force field) and quantum chemical (AM1 semiempirical method) techniques. The most stable structures of the molecule obtained by MM calculations have been used as input data for the semiempirical treatment. The position and orientation of the glucose moiety with regard to the remainder of the molecule have been investigated. The flexibility of isoquercitrin principally lies in rotations around the inter-ring bond and the sugar link. In order to know the structural modifications generated by the substitution by a sugar, geometrical parameters of quercetin (aglycon) and isoquercitrin have been compared. The good accordance between theoretical and experimental electronic spectra permits to confirm the reliability of the structural model.

Theoretical near UV and VIS electronic spectra for the ZnII–anhydrotetracycline complex

The UV–VIS electronic spectra for the free ligand and complexed forms of the ZnII–AHTC system have been analyzed using the configuration interaction (CI) procedure as implemented in the ZINDO program. The main transitions in the UV and VIS region are assigned and compared with the experimental data available. The electronic spectrum for the free ligand, the tautomeric form ionized at O11 [LH–(O11)], is very close to that observed experimentally, with the calculated absorption band centered at 429 nm and the experimental one at 428 nm. This absorption is attributed to a π→π* transition of the BCD ring system. Comparing the visible spectra for the complexed forms with that obtained for the free ligand, a blue shift is observed for all structures analyzed, except for the complex II-B, where a red shift from 429 nm (free ligand) to 436 nm (complexed form) was calculated. Experimentally, a bathochromic shift from 428 nm to 440 nm was observed with the [ZnII]/[AHTC] ratio ranging from 0 to 3. These values are very close to those calculated for the complex II-B, in which the AHTC acts as a tridentate ligand. The possibility of the formation of the binuclear complex (M2L) was discarded, once it was observed that the second metal complexation shifts the VIS band to lower wavelength values. This effect is not observed in the experimental spectrum at high ZnII concentration.

Electronic structure of phosphate glasses with a complex oxygen sublattice structure

Calculations of the electronic structure of phosphate glasses are performed in the MO LCAO xα approximation discrete variation method. On the basis of an analysis of theoretical and experimental electronic spectra of the system BeO-P2O5 regularities are found in the formation of the valence band of alkaline-earth phosphate glasses with different types of anion sublattice. Data on the electronic structure are used to refine the models of short-range order; in particular, the possibility of oxygen in the threefold coordination state is confirmed. With the features of the spectrum of electronic states taken into account, localization of charge carriers, the nature of the optical transitions, and hole-transport phenomena are discussed.

Experimental and theoretical Auger and autoionization spectra for electron impact on laser-excited Na atoms

An experimental method is described to obtain electron spectra with good resolution and high intensity following L-shell excitation or ionization of laser-excited Na atoms. We report on experimental and theoretical electron spectra in the energy range of 25 - 33 eV for laser excitation to , , and and for 1.5 keV electron impact. Due to the good agreement in line positions and intensities essentially all lines can be assigned. Furthermore, experimental relative cross sections for states excited from , and are determined and compared with theoretical values.

Theoretical Vibrational Spectra of Neutral and Doped Poly(p-phenylene sulfide) Oligomers

The vibrational properties of the first oligomers of poly(p-phenylene sulfide) (PPS) in their neutral, singly and doubly ionized states have been computed at the ab-initio SCF level followed by scaling of the ab-initio force constants with appropriate scale factors. The effect of the basis set on geometries and vibrational spectra has been studied, and the results have been compared with the available experimental data relative to the oligomers and to the polymer. The 3-21G* basis set has been found to be a satisfactory compromise between accuracy and computional effort. In the trimer, the smallest oligomer whose spectra contain all the essential features pertinent to the polymer, ionization has been found to give rise to a strong intensity enhancement of a small number of selected vibrational bands in the infrared and in the Raman spectra, in good agreement with the experimental findings. Combining the experimental and theoretical Raman spectra of the doped trimer with the experimental and theoretical electronic spectra of the same system suggests that dicationsin addition to radical cationsare formed upon doping.

Theoretical study on the electronic spectrum of TcO 4 ?

The theoretical electronic spectrum of TcO 4 − calculated by the SAC(symmetry adapted cluster)/SAC-CI method is presented. The spectrum is in good agreement with the experimental one. The observed peaks are assigned and the existence of several absorptions in the energy region higher than that observed is predicted. The difference and the similarity between the electronic spectra of TcO 4 − and MnO 4 − are clarified. The spectral difference between TcO 4 − and MnO 4 − is due to a remarkably high energy shift of the 31T2 state of TcO 4 − .

Electronic spectra of RMn(CO) 3(α-diimine) (RH, CH 3): a CASSCF/CCI comparative study of the lowest singlet excited states

A theoretical study, based on CASSCF/CCI calculations of the lowest part of the electronic spectra of RMn(CO) 3(α- diimine) (RH, CH 3), is reported. These systems are a model for a rather general class of transition metal complexes with low-lying Metal-to-Ligand Charge Transfer (MLCT) states. The theoretical electronic spectrum of HMn(CO) 3(α- diimine) is characterized by a high density of states below 35000 cm −1 (285 nm). The lowest 1A' and 1A″ singlet states, directly accessible through allowed transitions from the 1A' ground state, correspond to d→π * excitations, between 17110 and 22300 cm −1. These values are in excellent agreement with the vis-UV absorption spectra usually obtained for this family of molecules with a MLCT band at around 500 nm ( 20000 cm −1). The next 1A' and 1A″ states ( above 27800 cm −1 ) correspond to d→d excitations and should contribute to a shoulder on the absorption spectra at around 350 nm. The first excited 1A' σ→π * state, susceptible to play a role in the homolysis of the Metal-R bond, is calculated at 32150 cm −1. This state is not directly accessible upon irradiation at 500 nm, but may interact with the lowest 1MLCT states early on the reaction path corresponding to the homolysis of the Metal-R bond. In the light of these results, the photochemical behavior of other transition metal ( α- diimine) complexes with low-lying MLCT excited states is discussed. The excitation energies for the lowest 1A' states only slightly differ when going from the hydride to the alkyl complex.

Electronic absorption spectrum and π-electronic structure of cryptocyanine

Absorption, fluorescence excitation and APF spectra of cryptocyanine have been measured. Dipole moment of the respective S1 excited state has been estimated from shifts of the marked maximum of the first absorption band in various solvents. On the basis of quantum-chemical calculations carried out by the PPP method in the approximation of quasi-real geometry we have received the optimum model of π-electronic structure of the cryptocyanine molecule and therefrom the theoretical electronic singlet spectrum inclusive character of the S0 - S1 transition.

Peptides from Non-Amino Acid Sources. II. A Quantum-Chemical Study of the Isomers of Formamide

Ab initio molecular-orbital calculations have been carried out on nine isomers corresponding to the empirical formula of CH3NO. The near-Hartree–Fock wavefunctions were used to calculate the ground-state energies as well as dipole moments. The total energy values and transition moments of the first 12 singlet and triplet excited states have been calculated by the virtual orbital technique. These latter results were relevant to some questions in photochemistry and also gave the basis of the theoretical electronic spectra. The computed total energies for the electronic ground states of the nine isomers studied showed that formamide, the model compound for the peptide bond, was much more stable than the other eight isomers.

``Multiple-Scattering'' Model for Polyatomic Molecules

The ``multiple-scattering'' technique of determining energy bands in solids is modified for the problem of calculating bound one-electron eigenstates of polyatomic molecules. The computational simplicity and applicability of this method to molecules of arbitrary complexity are dependent on the adoption of a model Hartree—Fock Hamiltonian based on the Slater approximation to the exchange potential and on the assumption of truncated spherically averaged potentials. The composite wavefunctions are expanded in rapidly convergent partial-wave series around each atomic site. For molecules of moderate size, the calculations can easily be carried to self-consistency within the spherical approximation, and first-order perturbation theory can be applied to correct for the nonspherical components of the potential. Emphasis is placed on the advantages of this technique as a first step in calculating the theoretical electronic spectra of complex polyatomic systems.