Photochemical properties of 1-(9-Anthryl)-2-(2-Quinolyl)ethylene

Spectral and photochemical properties of 1-(9-anthryl)-2-(2-quinolyl)ethylene (9A2QE) in neutral and protonated forms have been studied experimentally and by quantum-chemical methods. It has been found that the quantum yield of trans-cis photoisomerization (φ_tc) has values of φ_tc < 0.5 typical of the diabatic photoisomerization for both forms of 9A2QE. A comparison of this data with the results of the study of other aza-diarylethylenes containing the 2-styrylquinoline (2SQ) moiety has led to the general conclusion that the increase in the π-system in 2SQ upon fusion of the benzene rings results in the disappearance of the α-effect, which lies in the fact that the quantum yield increases upon going from the neutral to protonated form up to the values φ_tc > 0.5, which exceeds the limiting value for the diabatic photoisomerization.     

Structure of 2,3- and 2,5-dihydroxypyridines

On the basis of an analysis of the UV and IR spectra, it was established that 2,3- and 2,5-dihydroxypyridines exist in different forms in neutral, acid, and alkaline media. The electronic absorption spectra of the neutral forms of the molecules and the distribution of the -electron charges in them were calculated by means of the Pariser—Parr—Pople (PPP) MO method. A comparison of the results of the calculations with the experimental UV spectroscopic data demonstrated that the calculations correctly predict the number of ^* absorption bands in the accessible region of the UV spectra and their positions and relative intensities.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 60–63, January, 1973.     

Experimental and quantum-chemical studies of the 3-pyridine-7-hydroxy-2-iminocoumarin and 3-(2-methylthiazole)-7-hydroxy-2-iminocoumarin molecules

The luminescence spectral properties of 3-pyridyl-7-hydroxy-2-iminocoumarin and 3-(2-methylthiazole)-7-hydroxy-2-iminocoumarin were studied in ethanol solutions at different values of pH and also in other solvents differing in polarity. In their ethanol solutions, the compounds can exist as neutral molecules, anions, cations, and zwitterions. The spectral properties and geometry were studied for all possible forms of these compounds by the INDO/S quantum-chemical method.     

Photochemical Properties of 1-(9-Phehanthryl)-2-(2-Quinolyl)ethylene

Spectral and photochemical properties of 1-(9-phehanthryl)-2-(2-quinolyl)ethylene (9Ph2QE) in neutral and protonated forms have been investigated. It has been found that both isomers of 9Ph2QE are photoactive. The quantum yield of trans–cis photoisomerization (? _tc = 0.47) in the neutral form is typical of the diabatic photoisomerization; on passing to the protonated form, ? _tc increases up to 0.70. Thus, the double annelation of the 2-styrylquinoline phenyl group to form 9Ph2QE makes it possible to conserve the α-effect, which consists in an increase in the quantum yield to ? _tc > 0.5 on passing from the neutral to protonated form, whereas the effect disappears for other types of annelation (naphthylquinolylethylenes, 1-(9-anthryl)-2-(2-quinolyl)ethylene).     

Stability of the normal, zwitterionic neutral and anionic forms of aspartic acid in gas phase and aqueous media

Aspartic acid has a place of special importance among amino acids in view of its property to racemize from the - to the -form in living and nonliving systems which can be used to determine age. Molecular geometries of two isomeric normal neutral forms (called A and B forms), isomeric zwitterionic neutral forms (called A(Z) and B(Z) forms) and anions of these forms of aspartic acid were optimized at the RHF/6-31+G^* level. The geometries of hydrogen bonded complexes of each of the above species with one, two and three water molecules were also optimized at the same level. All these systems were solvated in bulk water using the polarized continuum model (PCM) of the self-consistent reaction field (SCRF) theory at the RHF/6-31+G^* level using the gas phase optimized geometries. It is found that in gas phase, the normal neutral A and B forms of aspartic acid would coexist while the zwitterionic A(Z) and B(Z) forms of the same would be absent. On the other hand, in bulk aqueous media, the normal neutral A and B forms of aspartic acid would not occur while the zwitterionic A(Z) and B(Z) forms would be present. Further, the abundance of the A(Z) form would be much more than that of the B(Z) form in bulk water. If the solution pH is adjusted appropriately so that a monoanion is formed, the anion of aspartic acid obtained from the neutral zwitterionic form by deprotonating O₈ would occur dominantly.     

Addition of water to Al5O4- determined by anion photoelectron spectroscopy and quantum chemical calculations

The anion photoelectron spectra of Al5O4- and Al5O5H2- are presented and interpreted within the context of quantum chemical calculations on these species. Experimentally, the electron affinities of these two molecules are determined to be 3.50(5) eV and 3.10(10) eV for the bare and hydrated cluster, respectively. The spectra show at least three electronic transitions crowded into a 1 eV energy window. Calculations on Al5O4- predict a highly symmetric near-planar structure with a singlet ground state. The neutral structure calculated to be most structurally similar to the ground state structure of the anion is predicted to lie 0.15 eV above the ground state structure of the neutral. The lowest energy neutral isomer does not have significant Franck-Condon overlap with the ground state of the anion. Dissociative addition of water to Al5O4- is energetically favored over physisorption. The ground state structure for the Al5O4- +H(2)O product forms when water adds to the central Al atom in Al5O4- with -H migration to one of the neighboring O atoms. Again, the ground state structures for the anion and neutral are very different, and the PE spectrum represents transitions to a higher-lying neutral structure from the ground state anion structure.     

Spectrophotometric studies on the formation of adducts involved in synergistic extraction of uranium(IV) by mixtures of HTTA and neutral donors

Formation of adducts between U(TTA)₄ and several neutral donors was studied by spectrophotometry and it was shown that each of the neutral donors forms only 1∶1 adduct with U(TTA)₄. The adduct formation constants with some neutral donors were determined for benzene and chloroform media. As these adducts are involved in the synergistic extraction of U(IV) from aqueous media by mixtures of HTTA and neutral donors dissolved in organic solvents, the extraction equilibrium constants were estimated, The adduct formation was found to result in an increase of the co-ordination number of U(IV) from 8 in U(TTA)₄ to 9 in the adducts it forms with the neutral donors. Similar absorption spectral studies with U(DBM)₄ revealed that it forms much weaker adducts than the corresponding ones with U(TTA)₄.     

Spectroscopic and quantum chemical study of the structure of 4-aminopyrimidinoanthrones

4-Amino derivatives of pyrimidinoanthrone exist in the form of an aminoketone isomer in the crystalline state in neutral organic solutions, but in acid and alkaline media the tautomeric equilibrium is shifted toward formation of ionic forms of the iminohydroxyl tautomer. We present the characteristics of the IR and electronic absorption spectra and quantum chemical calculations for the neutral and ionic forms of the tautomers indicated above.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1109–1116, August, 1994.     

Synthesis,Spectroscopic Properties and Redox Behavior Kinetics of Rare-Earth Bistetrakis-4-[3-(3,4-dicyanophenoxy)phenoxy]phthalocyaninato Metal Complexes with Er,Lu and Yb

Novel bistetrakis-4-[3-(3,4-dicyanophenoxy)phenoxy]phthalocyaninato of complexes erbium, lutetium and ytterbium were synthesized using a template fusion method to prevent any polymerization process. The complexes were separated from the reaction mixtures and characterized by NMR, IR and electron absorption spectroscopy. The spectroscopic properties of the metal phthalocyaninates in chloroform, acetone and tetrahydrofuran were studied. The regular bathochromic shift in the Er–Yb–Lu series was determined. In acetone medium all the complexes obtained were found to exist in an equilibrium state between neutral and reduced forms. The linearity of Lambert-Bouger-Beer curves makes it possible to study the kinetics of redox processes in the presence of phenylhydrazine and bromine. The lutetium complex showed better reducing properties and turned fully into the reduced form, while the erbium and ytterbium ones changed only partially. Upon oxidizing all the phthalocyaninates transformed into a mixture of oxidized and neutral-radical forms. The extinction coefficients and effective redox constants were calculated.     

Structure of pyridoxine solvates in aqueous solution from quantum-chemical calculations and NMR spectroscopy

A geometric optimization is performed for the neutral, zwitterionic, and protonated forms of pyridoxine in vacuum and in water with a solvent within the polarizable continuum model (PCM). The structural parameters are optimized for pyridoxine complexes in the neutral and zwitterionic forms with 4-10 water molecules. An analysis is performed of how the number of molecules of the solvent set by the model affects the agreement between the calculated and experimental NMR spectra.     

Acid-base equilibriums of lumichrome and its 1-methyl, 3-methyl, and 1,3-dimethyl derivatives

Lumichrome photophysical properties at different pH were characterized by UV-vis spectroscopy and steady-state and time-resolved fluorescence techniques, in four forms of protonation/deprotonation: neutral form, two monoanions, and dianion. The excited-state lifetimes of these forms of lumichrome were measured and discussed. The results were compared to those obtained for similar forms of alloxazine and/or isoalloxazine, and also to those of 1-methyl- and 3-methyllumichrome and 1,3-dimethyllumichrome. The absorption, emission, and synchronous spectra of lumichrome, 1-methyl- and 3-methyllumichrome, and 1,3-dimethyllumichrome at different pH were measured and used in discussion of fluorescence of neutral and deprotonated forms of lumichrome. The analysis of steady-state and time-resolved spectra and the DFT calculations both predict that the N(1) monoanion and the N(1,3) dianion of lumichrome have predominantly isoalloxazinic structures. Additionally, we confirmed that neutral lumichrome exists in its alloxazinic form only, in both the ground and the excited state. We also confirmed the existence and the alloxazinic structure of a second N(3) monoanion. The estimated values of pK(a) = 8.2 are for the equilibrium between neutral lumichrome and alloxazinic and isoalloxazinic monoanions, with proton dissociation from N(1)-H and N(3)-H groups proceeding at the almost the same pH, while the second value pK(a) = 11.4 refers to the formation of the isoalloxazinic dianion in the ground state.     

Electronic structure and reactivity of 3-hydroxyquinoline

The reactivity indexes of the neutral, dipolar, cationic, and anionic forms of 3-hydroxy-quinoline were calculated by the simple MO LCAO method using dynamic and statistical approximations. The predicted (on the basis of the localization energies) charge distributions, boundary densities, free valence indexes, and orientations of electrophilic substituents for the cationic and anionic forms of 3-hydroxyquinoline are in good agreement with the experimental data. The orientations of nucleophilic and radical substituents for the four forms of 3-hydroxyquinoline are predicted. The reactivity indexes of the neutral form of 3-hydroxyquinoline were calculated by means of the Pariser-Parr-Pople method.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 191–196, February, 1972.     

Matrix-isolation FT-IR spectroscopic study and theoretical DFT(B3LYP)/6-31++G** calculations of the vibrational and conformational properties of tyrosine

The complicated conformational isomerism of tyrosine is studied by experimental matrix-isolation FT-IR spectroscopy combined with theoretical DFT(B3LYP)/6-31++G** calculations. Not less than 18 possible conformations of tyrosine have been considered theoretically. The results revealed that the most and the less stable forms of neutral tyrosine have the same conformation of the main part of amino acid (conformation II) but they differ in orientation of the phenyl ring. The calculated values of the relative energies suggest that all conformations would be detectable in the experimental spectrum. However, it appeared that it is not possible to distinguish in the experimental spectrum between the bands due to the forms with the same conformation of the main part of amino acid but a different orientation of the phenol ring.     

Influence of a Hydrophobic Environment on the Structure of Arginine—Carboxylate Salt Bridge

The exact structure of an arginine-carboxylate salt bridge in different chemical environments remains a controversial problem.In the present work,the zwitterionic and neutral forms of arginine-carboxylate salt bridge were studied by the B3LYP/6-311G(d,p)//PM3 method.It turns out that the neutral forms are more stable than the zwitterionic coumterparts in gas phase.However,whnen bound by α-cyclodextrin,the zwitterionic forms become more stable than the corresponding neutral ones.It is suggested that the hydrophobic environment provided by the cyclodextrin cavity leads to such behavior.Therefore,the salt bridge still could be in a zwitterionic form in the hydrophobic interior of the real proteins.     

A theoretical investigation of the relative stability of hydrated glycine and methylcarbamic acid--from water clusters to interstellar ices

We have theoretically investigated how the low-energy conformers of the neutral and the zwitterionic forms of glycine as well as methylcarbamic acid are stabilized by the presence water. The MP2/6-311++G(d,p) method was utilized to conduct calculations on glycine and methylcarbamic acid in both isolated clusters and in clusters embedded in the conductor-like polarizable continuum model (C-PCM), where the clusters explicitly contain between one and ten water molecules. The neutral forms of glycine and methylcarbamic acid were found to have similar hydration energies, whereas the neutral methylcarbamic acid was determined to be approximately 32 kJ mol(-1) more stable than the neutral glycine in the isolated clusters and 30 kJ mol(-1) more stable in the C-PCM embedded clusters. Both the number and strength of the hydrogen bonding interactions between water and the zwitterions drive the stability. This lowers the relative energy of the glycine zwitterion from 50 kJ mol(-1) above neutral glycine, when there are two water molecules in the clusters to 11 kJ mol(-1) below for the clusters containing ten water molecules. For the methylcarbamic acid clusters with two water molecules, the zwitterion is 51 kJ mol(-1) higher in energy than the neutral form, but it remains 13 kJ mol(-1) above the neutral methylcarbamic acid in the clusters containing ten water molecules. When the bulk water environment is simulated by the C-PCM calculations, we find both the methylcarbamic acid and glycine zwitterionic forms have similar energies at 20 kJ mol(-1) above the neutral methylcarbamic acid energy and 10 kJ mol(-1) lower than the neutral glycine energy. Although neither methylcarbamic acid nor glycine have been detected in the interstellar medium yet, our findings indicate that methylcarbamic acid is the more stable product from methylamine and carbon dioxide reactions in a water ice. This suggests that methylcarbamic acid likely plays a role in the intermediate steps if glycine is formed in the interstellar medium.     

Solvatochromism of Heteroaromatic Compounds: XVIII. Reversible Effect of the Medium on the UV spectra of 2-(2-Benzoyl-1-phenylethenyl)-5-phenylpyrrole

Theoretical (B3LYP/6-31G^*) and experimental (NMR, UV, and IR spectroscopy) data on the electronic and steric structure of 2-(2-benzoyl-1-phenylethenyl)-5-phenylpyrrole have been analyzed in the context of the hypothesis of the zwitter ionic nature of its ground state. The molecule involves a strong NH···O bond, and its ground state can be described as a hybrid of the neutral and zwitter ionic canonical forms; this conclusion was confirmed by the observation of a reversible medium effect on the electronic absorption spectrum of the substrate.     

Cinnamoyl pyrones in proton-donating media: electronic structure and spectral properties of protolytic forms

Spectral properties of cinnamoyl pyrone (CP) and its derivatives were studied in water-alcohol and water solutions within a wide pH/H(0) range. It was found that the most of CP may exist in neutral, anionic and cationic forms, except for alkylamino substituted CP, which can also form dications. The constants characterizing equilibria between all the protolytic forms were obtained. CP anions appears as a result of 4-hydroxy group dissociation. It was found that the acidity of CP decreases upon the excitation, that excepts the photodissociation or intramolecular proton transfer in the excited state. The formation of cations and dications occurs by means of protonation of alkylamino group or/and exocyclic carbonyl group. The theoretical analysis of electronic structure has shown that long-wavelength electronic transitions of neutral and anionic CP forms are of interfragmental charge-transfer (ICT) character. However, in the case of anions, ICT is hindered due to their nonplanar geometrical structure. The band maxima positions in experimental absorption spectra of neutral CP and in absorption and emission spectra of cations correlate linearly with theoretical estimations of charge transfer in CP molecules. The absence of the fluorescence, characteristic for the most of neutral CP, is due to the intersystem crossing, that is proved by the appearance of the phosphorescence of non-fluorescent CP at low temperatures.     

Excited state properties of the chromophore of the asFP595 chromoprotein: 2D and 3D theoretical analyses

The ground and excited state properties (e.g., the intramolecular charge and energy transfer, and electron‐hole coherence) of the chromophore of the asFP595 chromoprotein from Anemonia sulcata in the neutral and anionic forms are theoretically studied with quantum chemistry methods. The ground‐state properties of the asFP595 in the neutral and anionic forms, such as the alternations of the bond lengths and the Mulliken charge distributions, are compared. The calculated transition energies of the asFP595 in the neutral and anionic form are consistent with the experimental results. To study the excited state properties of the asFP595 chromophore, the energies and densities of highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs), as well as the CI main coefficients, are compared between the two forms. The intramolecular charge and energy transfer in the neutral and anionic forms are investigated and compared with the three‐dimensional (3D) real‐space analysis methods, including the strength and orientation of the transition dipoles with transition density, and the orientation and result of the intramolecular charge transfer with charge difference density. The electron‐hole coherence and delocalization on the excitation are studied with the 2D real‐space analysis method of the transition density matrix. In all, the calculated results are remain in good agreement with the experimental data, and the theoretical analysis results supported the proposed models in the experiment. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Direct Visual Evidence for Neutral and Charged Hexaphyrin Aromaticity with and without Keto-Defect

We use density functional theory and time-dependent together with a set of extensive mul-tidimensional visualization techniques to characterize the influence of keto effect on charge distribution at ground state and electronic transitions for neutral and charged hexaphyrin aromaticity with and without keto-defect. It is found that the aromaticity is the key fac-tor to influence the ground state Mulliken charges distribution properties, other than the meso-aryl-substituted effect. But with the enhancement of the keto-defect, the distributionchanges of Mulliken charges on the hexaphyrin groups are larger than those on the pentaflu-orophenyl substituted groups, following with the aromaticity changes from nonaromatic toaromatic. Furthermore, through characterizing by transition density and charge difference density, direct visual evidence for neutral and charged aromaticity with and without keto-defect can be clearly derived, and the ability of charge transfer between units of monoradical (nonaromaticity) and singlet biradical (aromaticity) forms is much stronger than that of neutral forms.     

Rearrangements photochimiques d'α-epoxycetones spiranniques et de composes dicarbonyles; nouvelle etude de la photolyse du benzoyloxy-3-5-cholestene-2; photoepimerisation en 5 de la benzoyl-2-5α-cholestanone-3

The photolysis of epoxides derived from 2 - benzylidène - 5α - cholestan - 3 - one depends considerably on stereochemistry. No photoepimerisation can be detected; expected β diketones are formed. Several tautomeric forms of 2 - benzoyl - 5α - cholestan - 3 - one have been isolated in crystalline form, and some exist in neutral solution. A δ hydrogen abstraction by the benzoyl group is involved in the photolysis of this δ-diketone.