Quantum-Chemical Calculations of the Excited Electronic States of the Prodan Molecule and Its Complexes in Water

A quantum-chemical calculation of the prodan molecule and its complexes in water for the geometry of the ground and fluorescent states is carried out. To describe the fluorescent state, changes in the electronic state (population) on bonds and atoms during transition of the molecule into an excited state are taken into account. A model of interaction of the prodan molecule with a polar proton-donor solvent (water) is suggested. It is shown that interaction with the ionic forms (H₃O₊ provides an explanation for the sensitivity of prodan to the solvent (displacement of the fluorescence bands). The nature of the electronic excited states of the prodan molecule and its complexes has been investigated. The constants of the rates of radiative and nonradiative processes and the fluorescence quantum yields have been calculated. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 3, pp. 330–334, May–June, 2005.     

The π ← n transition of formic acid

New sharp bands of formic acid have been observed in the near ultraviolet at the long wave-length end of the previously observed diffuse band system (2250–2500 Å) by considerably extending the absorption path length. Both the diffuse and sharp bands belong to the same vibrational system which is assigned to the π^* ← n electronic transition in the carbonyl group. Extensive progressions are observed in the carbonyl stretching frequency which is greatly reduced in the excited state (fundamental ν₃′ ≈ 1080 cm⁻¹) and many intervals of about 400 cm⁻¹ are assigned to the OCO bending frequency ν₇′.A band contour analysis of the 2593 Å band shows that the molecule is nonplanar in the excited state because of the magnitude and sign of the inertial defect. From this analysis, the rotational constants for the excited state are S^‘=1.8755, B^‘0.4042, C^‘=0.3378cm⁻¹ By the plausible assumption that the important changes in the molecule are in the C=0 bond length, the OCO angle, and the nonplanarity due to the formyl hydrogen, the following excited state parameters are derived.rC=0 = 1.407A.The changes in formic acid are closely analogous to the changes in formyl fluoride as a result of the π^* ← n transition.     

Photophysics of the Lanthanide Complexes with Conjugated Carboxylic Acids by Low Temperature Fluorescent Spectroscopy

In the context, some lanthanide (Eu³⁺, Tb³⁺ and Sm³⁺) complexes with conjugated carboxylic acids (pyridine-carboxylic acids derivatives) have been synthesized and characterized. The low temperature fluorescent spectra for these complexes have been measured at nitrogen atmosphere (77 K), indicating that the central Ln³⁺ ions locate in an equivalent coordination environment with low symmetry for most of these lanthanide complexes belonging to dimeric or polymeric structure. Therefore, the electronic dipole transition (supersensitive transition) (⁵D₀ → ⁷F₂ for Eu³⁺, ⁵D₄ → ⁷F₆ for Tb³⁺, ⁴G_5/2 → ⁶H_9/2 for Sm³⁺) and magnetic dipole transition (⁵D₀ → ⁷F₁ for Eu³⁺, ⁵D₄ → ⁷F₅ for Tb³⁺, ⁴G_5/2 → ⁶H_5/2 for Sm³⁺) show the regular change in the corresponding split number of fluorescent spectra, which can be realized to predict the fine structure of lanthanide complexes.     

Double Fluorescence Conversion in Ultraviolet and Visible Region for Some Praseodymium Complexes of Aromatic Carboxyates

Four praseodymium complexes of aromatic carboxylates (benzoate, 4-tert-butylbenzoate, 2-benzoylbe-noate, and benzimidazole-5-carboxylate) have been synthesized and characterized, whose photophysical properties have been studied with ultraviolet spectra, phosphorescence spectra, and fluorescence spectra. The fluorescent emission spectra of all praseodymium complexes show two emission peaks under the excitation band of 245 nm at about 395 and 595 nm respectively, while one peak under 415 nm at about 595 nm, which attributed to be ¹S₀ → ¹I₆ (395 nm) transition and the characteristic emission ¹D₂ → ³H₄ (595 nm) transition of Pr³⁺ ion. The ¹S₀ → ¹I₆ transition can be ascribed as the transition of charge transfer state, and the ¹D₂ → ³H₄ can be further proved that there exists an antenna effect in the fluorescence of praseodymium with aromatic carboxylic acids. In conclusion, the praseodymium complexes systems can realize the double fluorescent conversion in both ultraviolet and visible region and can be further studied the application of this conversion.     

Electric conductance and semi-empirical studies on two thiophene derivatives/metal cation complexation

The formation constants for 1:1 Stoichiometric complexes of 2,4-diamino-3,5-dicyano thiophene (DADCT) and 2-amino cyclohexane thiophene-3-carbonitrile (ACTC) with transition metal cations (Mn⁺² , Ni^+ 2 , Cu^+ 2 , Zn^+ 2 , Cd^+ 2 , UO₂^+ 2 , La^+ 3 and Zr^+ 4 ) in 50% (V./V.) ethanol–water and methanol–water solvents have been determined conductometrically at different temperatures. A semi-empirical PM3 calculations were also used to predict the structure of the metal complex by calculating the enthalpy of formation, the geometrical parameters and Mulliken charges of the free ligands and the suggested structures of the formed complexes. The values of the different thermodynamic parameters (ΔG, ΔH and ΔS) have been obtained. The results show that the complexation reactions are all exothermic except in the case of La^+ 3-DADCT, which is endothermic reaction. The formation constant for the transition metal cations-ACTC complexes were larger than that for the transition metal cations-DADCT complexes. Also, the formation constants for all studied complexes in ethanol–water solvent were higher than that in methanol–water solvent. Using the SPSS computer program, a second order relation was found between Log k and the ionic radius (r) of the cations under investigation. The semi-empirical PM3 calculations show that there are two suggested structures of the complexation of (DADCT) with the studied metal ions.     

Mössbauer and magnetic studies on the spin states of diiron(III)-porphyrin dicopper(II) complexes

Several biological model complexes of cytochrome c oxidase analogues, such as X₂(TPP)₂Fe₂(Apen)Cu₂Cl₄, where TPP=5, 10, 15, 20-tetraphenylporphine, Apen=bis(acetylpyrazine)-ethylenediimine, X=Cl^–, N ₃ ^– , Im, 1-Me-Im, 2-Me-Im, 4-Me-Im and OCH ₃ ^– , were prepared. The electronic spin states of these complexes in solid state were studied by means of Mössbauer and EPR spectroscopies and magnetic susceptibility measurements. The iron(III) atom of these complexes is present in different spin states, depending upon the nature of the axial ligand X of Fe(III)-porphyrin; the N ₃ ^– , Cl^–, OCH ₃ ^– , 2-Me-Im axial groups lead to complexes in a pure high-spin state independent of temperature. In contrast, the imidazole axial groups (e.g. Im, 4-Me-Im) behave differently and all show a temperature dependence of the⁶A₁ ²T₂ spin transition. The magnetic exchange behavior between Fe and Cu atoms in the present complexes is also discussed.     

Study of Preferential Solvation in Binary Mixtures by Means of Frequency-Domain Fluorescence Spectroscopy

The preferential solvation of 8-N,N-(dimethylamino)-11H-indeno[2,1-a]pyrene, Py(S)DMA, in its transient charge transfer (CT) state in binary solvents such as toluene/DMSO liquid mixtures was studied by means of frequency-domain fluorometry. The data obtained were considered within the following kinetic scheme: the preferential solvation was described by the system of consecutive reversible reactions of which each step is associated with the absorption of one DMSO molecule in the first solvation shell of the fluoresent Py(S)DMA dipolar CT molecule. The rate constants of the first two reversible elementary processes (i.e., the decay of solvation complexes of Py(S)DMA with one and two polar molecules, k _–1 = 1.1 10⁹ s^–1 and k _–2 = 1.4 10⁹ s^–1) were determined.     

Contribution to the Analysis of theA2←XA1“Wulf” Transition of Ozone by High-Resolution Fourier Transform Spectrometry

The analysis of the rotational structure of the high-resolution Fourier transform 0⁰₀absorption spectrum of the³A₂←X¹A₁band system of the “Wulf” transition of the isotopomer¹⁶O₃of ozone is reported for the first time. With a near pure case (b) coupling model for the upper triplet state, we have assigned a significant portion of the spectrum, mainly theF₁(J=N+ 1) andF₂(J=N) spin components, primarily in the lower frequency region of the band. The lines corresponding to theF₃(J=N− 1) component are weak at lower frequencies and heavily congested in the central and higher frequency regions of the spectrum. Perturbations and predissociation phenomena have reduced the effective lifetime of the metastable³A₂state and have also limited the number of transitions included in the least-squares fit of the band. Approximately 100 lines have been assigned in the range from 9100–9550 cm⁻¹. Three rotational, three centrifugal distortion, three spin–rotation, and one spin–spin constant were varied. The geometry of the molecule in the³A₂state, as determined from these constants, isr= 1.345 Å and θ = 98.9°, in good agreement withab initioresults.     

Fluorescent Property of the Gd<Superscript>3+</Superscript>-Doped Terbium Complexes and Crystal Structure of [Tb(TPTZ)(H<Subscript>2</Subscript>O)<Subscript>6</Subscript>]Cl<Subscript>3</Subscript>·3H<Subscript>2</Subscript>O

The complex of Tb(TPTZ)Cl₃·3H₂O was synthesized by adding the ethyl alcohol solution of TbCl₃ (1 mmol) to the solution of 2,4,6-tris-(2-pyridyl)-s-triazine(TPTZ,1 mmol) with constant stirring. The solution which had been filtered was kept at the room temperature for 4 weeks, and then a kind of transparent crystal was formed. Besides, nine kinds of solid complexes in the different molar proportion of terbium to gadolinium had been synthesized by adopting the similar method mentioned above. It was inferred from the elemental analysis and rare earth complexometry that the composition of these complexes is (Tb_xGd_y)(TPTZ)Cl₃·3H₂O (x : y = 0.9 : 0.1, 0.8 : 0.2, 0.7 : 0.3, 0.6 : 0.4, 0.5 : 0.5, 0.4 : 0.6, 0.3 : 0.7, 0.2 : 0.8, 0.1 : 0.9). The absorption spectra and photoluminescence of the complexes were determined in dimethylsulfoxide (DMF), which showed that the excitation of the complexes is mostly ligand based. The triplet state energy level of TPTZ was measured, indicating that the lowest excitation state energy level of Tb(III) and the triplet state energy level of TPTZ match well each other. The fluorescent data indicated that the fluorescent emission intensity of Tb³⁺ ions would be enhanced in the complexes after terbium was doped with Gd³⁺ ion. When x : y was 0.5 : 0.5, the fluorescent emission intensity was the largest. The result obtained by testing the X-ray diffraction of the monocrystal revealed that the molecular formula of the mono-crystal complex is [Tb(TPTZ)(H₂O)₆]Cl₃·3H₂O. The number of metal ion coordinates is nine, and the tridentate TPTZ and six water molecules are bonded with terbium respectively. Besides, it also revealed that the monocrystal belongs to the monoclinic system, and space group Cc with the following unit cell parameters is a = 1.4785 (3) nm, b = 1.0547 (2) nm, c = 1.7385 (4) nm, β = 94.42 (3)°, V = 2.7028 (9) nm³ and Z = 4.     

Oxalyl halides : Analysis of the 3480 Å absorption system of oxalyl chloridefluoride

Oxalyl chloridefluoride (COCl)(COF) exhibits moderately strong discrete absorption in the 3050–3540Å region. The band spectrum has been analyzed as an allowed electronic transition of the planar trans molecule. The most active vibrations are the carbonyl stretching modes ν₁′ and ν₂′ and the in-plane bending mode ν₉. Various other fundamental frequencies in the combining electronic states have been identified. The 0₀⁰ band is at 28 724.5 cm⁻¹; partial rotational analysis confirms that this band is type C. The appearance of “line” structure in the wings of the band is discussed and an explanation offered. The vibrational and rotational analyses confirm that the transition is under the C_s point group, as expected for a singlet-singlet n → π^* type of excitation.     

Photoluminescence study of new lanthanide complexes with benzeneseleninic acids

New lanthanide complexes with benzeneseleninic (ABSe) and 4-chloro-benzeneseleninic (ABSeCl) acids have been synthesized and characterized by elemental analysis, infrared and UV–visible spectroscopies. The emission spectra of the trivalent europium complexes presented the typical electronic ⁵D₀→⁷F_J transitions of the ion (J=0–4). The ground-state geometries of the europium complexes have been calculated by using the Sparkle/AM1 model. From these results, the 4f–4f intensity parameters and energies of the ligand singlet and triplet excited states have been obtained. The lower emission quantum yield for the [Eu(ABSe)₃(H₂O)₂](H₂O)₂ compound, as compared to the [Eu(ABSeCl)₃(H₂O)₂] one, can be associated to the higher numbers of water molecules, in the first and second coordination spheres, that contribute to the luminescence quenching. The [Eu(ABSe)₃(H₂O)₂](H₂O)₂ complex presents an intermediate state whose energy difference with respect to the first excited singlet state is resonant with three phonons from the water molecules, favouring a multiphonon relaxation process from the singlet state followed by a fast internal conversion process; this effect is less pronounced in the complex with the ABSeCl ligand. The luminescence decay curves of the gadolinium complexes indicate that the level responsible for the intramolecular energy transfer process has a triplet character for both compounds. The nephelauxetic effect in these compounds was investigated under the light of a recently proposed covalency scale based on the concept of overlap polarizability of the chemical bond.     

Detection of3T1?3T2 and3T1?3A2 transitions of Ni by conventional Fourier transform infrared FT-IR spectroscopy

A FT-IR spectroscopic study was carried out in the region 4000–400 cm^–1 for ZnSeNi and ZnSNi at room temperature. The data obtained were examined on the basis of the energy states calculations of the (3d)ⁿ configuration, based on the defect molecule approach. The present investigation reveals the transition from the ground state to the first two excited states namely³T₁–³A₂ and³T₁–³T₂ of Ni⁺² (d⁸).     

Judd–Ofelt treatment on luminescence of europium complexes with β-diketone and bis(β-diketone)

Eu(DBM)₃2H₂O and Eu₂(DDBM)₃H₂O were synthesized by reactions between EuCl₃ and chelating regents of β-diketone (dibenzoylmethane, HDBM) and bis(β-diketone) (1,3-bis(3-phenyl-3-oxopropanoyl)benzene, H₂DDBM), respectively, and their luminescence properties were investigated by the fluorescence spectra and metastable state decay spectra. It was found that the relative intensity ratio of ⁵D₀→⁷F₂ to ⁵D₀→⁷F₁transition and the radiative lifetime shows a little change attributing to the different symmetry of europium ions, which Ω₂ of Eu complexes with β-diketone and bis(β-diketone) are 13.08 and 12.24, respectively. Moreover, it was also found that the metastable state lifetime of Eu₂(DDBM)₃H₂O is much longer than that of Eu(DBM)₃2H₂O, due to smaller water quenching and lower triplet level of ligands. The Commission Internacionale d’Éclairage (CIE) chromaticity coordinates calculated from emission spectrum are x=0.637 and y=0.343 for Eu₂(DDBM)₃H₂O, which presents high red color purity near 100%.     

Fourier transform emission spectroscopy of some new bands of ReN

The emission spectrum of ReN has been reinvestigated in the visible region using a Fourier transform spectrometer. Two new bands have been identified with band origins near 22 110 and 22 224 cm⁻¹. These bands have a common lower state and have been assigned as the 0⁺–A1 and 0⁻–A1 transitions. After rotational analysis it was noted that the new 0⁺–A1 transition also has its upper state in common with the upper state of the [24.7]0⁺–X0⁺ transition reported previously [W.J. Balfour, J. Cao, C.X.W. Qian, S.J. Rixon, J. Mol. Spectrosc. 183 (1997) 113–118.]. This observation provides T₀₀ = 2616.26 cm⁻¹ for the A1 state. It is likely that the A1 and X0⁺ states are two spin components of the ³Σ⁻ ground state.     

Vibration-rotation transitions in the CF radical, studied by laser magnetic resonance spectroscopy at 7.8 μm

Several transitions in the vibration-rotation spectrum of the CF radical in its X²Π state have been detected by CO-laser magnetic resonance. In addition to strong resonances for the ²Π_3/2-²Π_3/2 transitions in the fundamental band at 1286 cm⁻¹, weaker “cross” ²Π_3/2-²Π_1/2 and hot band transitions have also been observed. The ¹⁹F nuclear hyperfine splitting is clearly resolved for almost every transition. These observations provide an interesting comparison with the recent study of the same (1, 0) band of CF by diode laser spectroscopy.     

The electronic structure of KMnO4 investigated by photoemission and electron-energy-loss spectroscopy

From photoemission and electron-energy-loss data the following picture of KMnO₄, with Mn^VII (with a formal charge state Mn⁷⁺ (3d ⁰)) tetrahedrally surrounded by four O^2–-ions, is deduced: strong covalent bonding between Mn^VII and O^2– leads to a considerable occupation of the Mn-3 d shell. The ground state of the (MnO₄)^–1 molecule is an orbital and spin singlet as seen by the absence of any multiplet splitting in the Mn core levels. The valence band shows a four peak structure extending form 4 eV to 8 eV below the Fermi energy. The first peak at 4.2 eV has mainly O-2p character. The remaining peaks are of strongly mixed Mn-3d/O-2p character due to the covalent bonding. This mixing decreases with increasing binding energy. The electron energy loss data show a variety of structures between 2 eV and 10 eV independent of the primary electron energy which defines them as dipole allowed charge-transfer transitions. An additional excitation at 1.8 eV decreases quickly in intensity with increasing electron energy which classifies it as a dipole or spin forbidden transition in the compound. This energy is close to the value of 1.6 eV reported for the activation energy observed in electrical transport data. The results are compared to quantum chemical molecular orbital calculations of the (MnO₄)^–1 molecule.Physics Department, Allahabad University Allahabad 211002, India     

Ab Initio Calculations on the Tricarbon Monoxide Molecule C3O

The six-dimensional potential energy surface of the ground state X¹Σ⁺ of C₃O has been generated by the CCSD(T) approach. The spectroscopic constants of this molecule are calculated and the vibrational spectrum is derived. The fundamentals are accurate to within 10 cm⁻¹ compared to the available experimental data. The ground state correlates to the closed shell C₂(X¹Σ⁺_g)+CO(X¹Σ⁺) dissociation limit. The bonding in this molecule can be explained by charge transfer from CO to C₂. The lowest electronic excited states are also investigated.     

Influence of Complexing and Excitation Energy on Spectral and Luminescent Properties of 2-Amino-4-Methylphenol

Energy level diagrams of 2-amino-4-methylphenol and its complexes with water are calculated by the method of intermediate neglect of differential overlap (INDO). It is demonstrated that the substitution by the amino group results in the dependence of the quantum fluorescence yield on the excitation energy. The decrease of the quantum fluorescence yield of 2-amino-4-methylphenol in going from hexane to water is explained. Complexing of the 2-amino-4-methylphenol molecule with water with the formation of the H-bond reduces the quantum fluorescence yield compared to the isolated molecule due to the increased efficiency of the S₁ → T₄ conversion.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 71–76, March, 2005.     

Paramagnetic Proton Nuclear Spin Relaxation Theory of Low-Symmetry Complexes for Electron Spin Quantum NumberS=

A generalization of the modified Solomon–Bloembergen–Morgan (MSBM) equations has been derived in order to describe paramagnetic relaxation enhancement (PRE) of paramagnetic complexes characterized by both a transient (Δ^ZFS_t) and a static (Δ^ZFS_s) zero-field splitting (ZFS) interaction. The new theory includes the effects of static ZFS, hyperfine coupling, and angular dependence and is presented for the case of electron spin quantum numberS= , for example, Mn(II) and Fe(III) complexes. The model gives the difference from MSBM theory in terms of a correction term δ which is given in closed analytical form. The theory may be important in analyzing the PRE of proton spin–lattice relaxation dispersion measurements (NMRD profiles) of low-symmetry aqua–metal complexes which are likely to be formed upon transition metal ions associated with charged molecular surfaces of biomacromolecules. The theory has been implemented with a computer program which calculates solvent water protonT₁NMRD profiles using both MSBM and the new theory.     

Thermal decomposition of dichloroketene and its reaction with H atoms

The thermal unimolecular decomposition of dichloroketene CCl₂CO → CCl₂ + CO (reaction 1) was studied experimentally and computationally. Dichloroketene was produced by the pulsed laser photolysis of hexachloroacetone, and the kinetics of its decay due to reaction 1 was monitored using photoionization mass spectrometry. Rate constants of reaction 1 were determined in direct time-resolved experiments as a function of temperature (740–870 K) and bath gas density ([He] = (3–25) × 10¹⁶ atom cm⁻³, [N₂] = 12 × 10¹⁶ molecule cm⁻³). Reaction 1 is in the falloff region under these conditions. The potential energy surface (PES) of reaction 1 was studied using quantum chemical methods. The experimental k₁ (T, P) dependence was reproduced with an RRKM/master equation model based on quantum chemical calculations. Parameterized expressions for the rate constants of reaction 1 and the reverse reaction, that of CCl₂ with CO, were obtained over wide ranges of temperatures and pressures. The enthalpy of formation of CCl₂CO was determined in quantum chemical calculations. The kinetics of the reaction of dichloroketene with hydrogen atoms (reaction 2), an important channel of destruction of CCl₂CO in flames, was studied computationally. The PES of reaction 2 was studied using quantum chemical methods. Temperature and pressure dependences of the rate constants of the four dominant reaction channels were obtained in transition state theory and master equation calculations; the technique of isodesmic reactions for transition states was applied to the channel of Cl atom abstraction. Analysis of the experimental data and the computational models of reactions 1 and 2 demonstrates that thermal decomposition is a major pathway of destruction for dichloroketene in combustion systems.