IR Evidences for N-9 and N-7 Regioisomers of the Acyclic Purine Nucleoside Analogues

The IR spectra have been recorded in the solid state for the parent molecule, 6-(N-pyrrolyl)purine (1) and its N-9-and N-7-substituted derivatives: 9-and 7-(2-hydroxyprop-1-yl)-6-(N-pyrrol-1-yl)purine (2 and 3), 9-and 7-(2-acetoxyprop-1-yl)-6-(N-pyrrol-1-yl)purine (4 and 5), 9-and 7-(2,3-dihydroxyprop-1-yl)-6-(N-pyrrol-1-yl)purine (6 and 7) as well as 9-and 7-(2,3-diacetoxyprop-1-yl)-6-(N-pyrrol-1-yl)purine (8 and 9). Analysis of the characteristic bands has proved to be useful in differentiating between N-9 and N-7 regioisomers.     

Structure-toxicity relationships of thiono and seleno phosphoramidate compounds: new type of acetylcholinesterase inhibitors

Novel thiono and seleno phosphoramidate compounds with the general formula (X)(Y)P(C₆H₅)₂; (X = NMe₂ & Y = S, 1a; X = NEt₂ & Y = S, 2a; X = NMe(CH₂Ph) & Y = S, 3a; X = NH(CH₂Ph) & Y = S, 4a; X = NEt(CH₂Ph) & Y = S, 5a; X = N(C(Me)₃) (CH₂Ph) & Y = S, 6a; X = N(CH₂Ph)₂ & Y = S, 7a; X = NMe₂ & Y = Se, 1b; X = NEt₂ & Y = Se, 2b; X = NMe(CH₂Ph) & Y = Se, 3b; X = NH (CH₂Ph) & Y = Se, 4b; X = NEt(CH₂Ph) & Y = Se, 5b; X = N(C(Me)₃)(CH₂Ph) & Y = Se, 6b and X = N(CH₂Ph)₂ & Y = Se, 7b) were prepared and characterized by ¹H, ³¹P and ¹³C NMR and IR spectroscopy and elemental analysis. ³¹P chemical shift of thiono and seleno derivatives didn’t show significant different because of their little difference in electronegativity sulfur and selenium. Hydrophobic parameter of compounds was determined by measurement of octanol-water partition coefficient by shake-flask technique. Determination of human erythrocyte acetylcholinesterase (hAChE) activity was carried out according to the Ellman’s modified kinetic method. IC₅₀ values of the selected thiono and seleno compounds varied from 3.4 to 0.11 and 9.9 to 5.1 mM, respectively. The seleno compounds show lower affinities for hAChE relative to the thino compounds. These results demonstrate that hydrophobic and electronic factors of the organophosphorus compounds play a key role on the inhibitory potency.     

Photophysics and Electrochemistry of some Thione Far-Red/Near-IR Triplet Emitters

The photophysics and cyclic voltammetry of two novel phosphorescent thiones, 2,2-dimethyl-indan-1-one-3-thione (DMIKT) and 2,2-dimethyl-indan-1,3-dithione (DMIDT), and three known phosphorescent thiones, 4H-pyran-4-thione (PT), 4H-1-benzopyran-4-thione (BPT) and 2,2-dimethylindan-1-thione (DMIT), have been characterised and compared. The phosphorescence emission of DMIT, DMIKT and DMIDT extends from the red into the near-IR spectral region. The additional carbonyl or thione group of DMIKT and DMIDT causes a significant shift in the emission maxima to 680 nm compared to that of DMIT, at 637 nm, in perfluorinated hydrocarbons. In acetonitrile the emission maxima of DMIKT and DMIDT are at 696 and 706 nm, respectively, and the spectra show vibronic bands which extend out beyond 850 nm. There is a significant reduction in triplet lifetime along this series (from 44 (±2) μs (DMIT) to 10 ((±0.8) μs (DMIKT) in perfluorinated solvents, and 8.6 (±0.5) (DMIT), 1.3 (±0.5) (DMIKT) and 0.35 (±0.07) μs (DMIDT) in acetonitrile), as well as a reduction in the rate constant for ground-state quenching of the triplet, (from 9.8 ((±0.9) to 3.5 ((±0.6) and 1.3 ((±0.2) × 10⁹ mol⁻¹ dm³ s⁻¹ for the same compounds). The addition of the C=O or C=S groups also causes a decrease in phosphorescence quantum yield with the highest emission quantum yield obtained for DMIT (Φ _P = 0.149 (±0.015)). Electrochemical studies show that while PT and BPT exhibit irreversible redox behaviour, DMIT, DMIKT and DMIDT all show at least one reversible reduction wave attributed to a one-electron process centred on the C=S moiety. The suitability of these lumophores for use in OLEDs is discussed.     

Finestructure, hyperfine structure and isotope shift of 4f 6s7s in Er I

The isotope shift (IS) and hyperfine structure (hfs) of nine levels (31720 to 38921 cm^-1) assigned to the configuration 4 f ¹² 6 s 7 s in neutral erbium have been determined experimentally using Doppler-reduced saturation absorption spectroscopy in a gas discharge. We performed a fine structure analysis in the SL-coupling scheme of the single configuration 4 f ¹² 6 s 7 s, confirming and extending the classification of even parity Er I levels. We discriminated the different hfs contributions of the 4f¹² core and the (6 s +7 s) outer electrons of the shell in a non-relativistic JJ-coupling approach and in the relativistic effective tensor operator formalism in SL-coupling. The relativistic one-electron parameters of the hfs for ¹⁶⁷Er were fitted to the experimental data by a least squares fit procedure: [0pt] a ⁰¹ _4f =-147(3) MHz, [0pt] a ¹⁰ _6s + a ¹⁰ _7s =-1840(30) MHz, [0pt] b ⁰² _4f =6560(80) MHz. The level dependencies of the isotope shift were evaluated based on crossed second order (CSO) effects. We obtained the following results for the CSO parameters for the isotope pairs ^170-168Er: d _6s7s =-740(30) MHz, z _4f = 0(5) MHz, ( g _3,6s ( f , 6 s )+ g _{3, 7s} ( f , 7 s ))= -24(15) MHz and for ^170-166Er: d _6s7s =-1500(50) MHz, z _4f =0(10) MHz, ( g _3,6s ( f ,6 s )+ g _3,7s ( f +7 s ))=-50(29) MHz. The resulting parameters for the hfs are compared with those known for other configurations of the Er atom and ion. Received 16 May 1999 and Received in final form 31 January 2000     

DFT/TDDFT investigation of the electronic structures and optoelectronic properties of phosphorescent iridium (III) complexes with non-conjugated cyclometalated carbene ligands

This work investigates the electronic structures and spectroscopic properties of several cyclometalated iridium carbene complexes possessing at least one functionalized methylene moiety, (dfpmb)(dfbmb)Ir(mptz) (2), (dfbmb)₂Ir(mptz) (3), and (dfbmb)₂Ir(mpmtz) (4) (dfpmbH =1-difluorophenyl-3-methyl-benzimidazoline-2-ylidene; dfbmbH?=?1-(2,4-difluorobenzyl)-3-(methylbenzimidazolium; mptzH?=?4-methyl-2-(5-(trifluoromethyl)-2H-1,2,4-triazol-3-yl)pyridine); mpmtzH?=?4-methyl-2-(3-methylene-5-(trifluoromethyl)- 2H-1,2,4-triazole-3-yl)pyridine) on the basis of their prototype (dfpmb)₂Ir(mptz) (1) via DFT and TDDFT methods. A careful examination of results shows: (1) the patterns of the occupied orbitals for 1–4 are almost the same, with the HOMO being an admixture of the Ir atom and benzyl part, whereas the LUMO is predominately delocalized over the ancillary chelate mptz or mpmtz; (2) complexes 1–3, and especially 4, all exhibit blue emission with maximum wavelengths at 506, 495, 486, and 478?nm, respectively; (3) complex 4 with the highest (relative) component of ³MLCT can be reasonably expected to have a higher radiative transition rate constant (k_r ) among 1–4. From the highest absorption peaks, eventually, the molar extinction coefficient discrepancy between experiment and calculations might be tentatively attributable to the synergism of the intrinsic imperfection of the PCM model and the simplification of chemical computations.     

Dimer dissociation energies of small odd-size clusters

The dimer dissociation energies of gold cluster ions Au ⁺ _n , n = 9, 11, 13, 15 have been determined with an extension of a recently developed model-independent method. Monomer-dimer decay pathway branching ratios provide the energy dependent process which is needed in this method. The measured values are D ₂ ( Au ⁺ ₉ ) = 3.66(8)(9) eV, D ₂ ( Au ⁺ ₁₁ ) = 4.27(11)(8) eV, D ₂ ( Au ⁺ ₁₃ ) = 4.50(9)(7) eV and D ₂ ( Au ⁺ ₁₅ ) = 4.29(10)(6) eV. Received 13 May 2002 / Received in final form 22 July 2002 Published online 24 September 2002 RID="a" ID="a"e-mail: manuel.vogel@uni-mainz.de     

A Fluorescence Study of Novel Styrylindoles in Homogenous and Microhetrogeneous Media

In this paper are presented absorption and fluorescence emission properties of 3-styrylindoles viz. 3-(2-phenylethenyl-E)-NH-indole (1), 3-[2-(4-nitrophenyl)ethenyl-E)-NH-indole (2), 3-[2-(4-cyanophenyl)ethenyl-E]-N-ethylindole (3) and 3-[2-(4-cyanophenyl)ethenyl-E]-NH-indole (4) in organic solvents, 1,4-dioxane-water binary mixtures and micelles (SDS, CTAB and Triton-X-100). The fluorescence properties of 2-4 have been utilized to probe the microenvironment (binding constant, CMC, micropolarity and solubilization site) of the micelles.     

Potential use of small basis set on the calculations of electronic properties of some four-membered heterocycles: a conformational study

The potential use of small basis sets upon a low level of theory was studied on the calculations of electronic properties (dipole moment, static polarisability and static hyperpolarisabilities) of a series of 16 four-membered heterocycles with an exocyclic double bond at the position 3 (1–16). First, the calculations were performed within the Hartree–Fock (HF) approximation using 6-31G, 6-31G(d,p) and 6-31+G(d,p) as basis sets for the different conformational states of each molecule, and the results obtained were compared with the MP2/6-31+G(d,p) results reported. In the second place, in order to know the real potential of HF calculations, these were compared with those calculated using larger approaches such as MP2/6-311+G(d,p), MP2/aug-cc-pVDZ, CCS/6-311+G(d,p), CCS/aug-cc-pVDZ, CCS/aug-cc-pVTZ, CCSD/6-31+G(d) and CCSD/cc-pVDZ, taken into account only the planar and equilibrium geometries of each molecules. The HF approaches permit us to obtain a good qualitative representation of the dipole moment as a function of puckering angle in comparison with MP2, CCS and CCSD levels for all tested molecules. However, only HF/6-31+G(d,p) provides quantitative values of dipole moment for the heterocycles 1, 5 and 13 in comparison with MP2, CCS and CCSD levels. On the other hand, the polarisability and hyperpolarisabilities were quite sensitive to the quality of level of theory and basis sets. In particular, HF/6-31+G(d,p) predicted a representative approximation of alpha for the molecule 16 in comparison with larger methods as MP2/aug-cc-pVDZ, CCS/aug-cc-pVDZ and CCS/aug-cc-pVTZ, while a detailed analysis showed that HF can be used in the calculation of alpha for the molecules 3, 7, 11 and 15, but it requires the use of extended basis sets. Also, HF/6-31+G(d,p) predicted values of beta very similar to those calculated at the MP2 and CCSD levels used, for the planar and equilibrium geometry of the molecules 10 and 14. Furthermore, HF/6-31+G(d,p) described a representative dependence of this property with the puckering angle for the heterocycles 9, 10 and 14, compared with the MP2 curves.     

Effects of Ca substitution on the phonon anomalies in the Raman scattering of YBa<Subscript>2</Subscript>Cu<Subscript>4</Subscript>O<Subscript>8</Subscript>

We report on the temperature dependence of the frequencies and linewidths in the phonon Raman scattering for Y _1-y Ca y Ba ₂ Cu ₄ O ₈ system ( y = 0 - 0.15). The phonon anomalies above T _c of the frequencies are observed for the out-of-phase O(2)-O(3) A _g and O(4) A _g modes, and these onset temperatures decrease with increasing Ca content. These features are consistent with the spin-gap behaviors associated with Ca doping reported previously. Furthermore, we find that the smaller gap exists near or just above the Ba phonon frequency at in the undoped samples and its energy increases with Ca doping. Received 9 June 1999     

Synthesis and Fluorescence Studies of Some New Fluorophores and Their Effect on Hybridization of Oligodeoxyribonucleotides

Some novel fluorophores, viz. 6-(6-isobutyrylamino-1,3-dioxo-1H,3H-benzo[de]isoquinolin-2-yl)-hexanoic acid (1), 6-(6-dimethylamino-1,3-dioxo-1H, 3H-benzo[de]isoquinolin-2-yl)-hexanoic acid (2), 6-(6-benzoylamino-1, 3-dioxo-1H, 3H-benzo[de]isoquinolin-2-yl)-hexanoic acid (3), 6-(6-amino-1-oxo-1H, 3H-benzo[de]isoquinolin-2-yl)-hexanoic acid (4) and 6-(6-amino-1H,3H-benzo[de] isoquinolin-2-yl)-hexanoic acid (5) have been designed, synthesized and characterized. Their comparative fluorescence has been studied in different organic solvents and aqueous solutions containing inorganic ions. Out of these, two fluorophores, 1 and 2 have been used for labelling of nucleosides which were finally converted into their phosphoramidites, and used for labelling of oligodeoxyribonucleotides through covalent attachment. These fluorophores after attachment on oligodeoxyribonucleotides showed good fluorescence signals and higher hybridisation affinity than unlabelled oligodeoxyribonucleotides.     

Energy loss of 18 keV electrons in gaseous T and quench condensed D films

Measurements of the energy loss of fast electrons at an energy of 18 keV have been performed on molecules of hydrogen isotopes, gaseous T ₂ and frozen D ₂ . Whereas in the case of gaseous T ₂ the values of total inelastic cross-section ( cm² for E = 18.6 keV), average energy loss ( eV) and peak position of the energy loss spectra ( eV) agree well with the expectations, the corresponding values for quench condensed D ₂ differ significantly from the ones for gaseous T ₂ . We observe a significant lower total inelastic cross-section ( cm², for E = 18.6 keV) larger average energy loss ( eV) and higher peak position ( eV). These differences may be interpreted in terms of changes of the final state spectrum. A CI calculation for a D ₂ cluster shows indeed a clear shift of the excited states in agreement with the observation. Received 24 August 1999     

Precise measurement of hyperfine structure in the state of Rb

We demonstrate a technique to measure hyperfine structure using a frequency-stabilized diode laser and an acousto-optic modulator locked to the frequency difference between two hyperfine peaks. We use this technique to measure hyperfine intervals in the 5 P _3/2 state of ⁸⁵Rb and obtain a precision of 20 kHz. We extract values for the magnetic-dipole coupling constant A = 25.038(5) MHz and the electric-quadrupole coupling constant B = 26.011(22) MHz. These values are a significant improvement over previous results. Received 6 March 2003 Published online 15 April 2003     

Electron-pair densities of group 2 atoms in their and terms

Electron-pair intracule (relative motion) and extracule (center-of-mass motion) densities are studied in both position and momentum spaces for the ¹ P and ³ P terms of the group 2 atoms Be (atomic number Z =4), Mg (Z =12), Ca (Z =20), Sr (Z =38), Ba (Z =56), and Ra (Z =88). In position space, the ¹ P - ³ P difference in the intracule densities shows that the probability of a small interelectronic distance is larger in the triplet for all the six atoms, as reported for the lightest Be atom in the literature. The position-space extracule density clarifies that the triplet electrons are more likely to be at opposite positions with respect to the nucleus than the singlet electrons for all the atoms. In momentum space, the singlet generally has a larger probability of a small relative momentum between two electrons as a na?ve manifestation of the Fermi hole in the triplet. The extracule density in momentum space shows that the ¹ P term has a distribution larger in a large center-of-mass momentum region than the ³ P term. Received: 26 August 1998 / Received in final form: 1 February 1999     

Interplay of structural, magnetic and transport properties in thelayered Co-based perovskite LnBaCo 2 O 5 (Ln = Tb, Dy, Ho)

The oxygen deficient cobaltites LnBaCo₂O₅ (Ln = Tb, Dy, Ho) exhibit two successive crystallographic transitions at T _N ∼340 K and at T _CO ∼210 K. Whereas the first transition (P4/mmm to Pmmm) is related to the long-range antiferromagnetic ordering of the Co ions (spin ordering), the second transition (Pmmm to Pmmb) corresponds to the long-range ordering of the Co²⁺ and Co³⁺ species (charge ordering) occurring in 1:1 ratio in the structure. The charge ordered (CO) state was directly evidenced by the observation of additional superstructure peaks using neutron and electron diffraction techniques. The CO state was also confirmed indirectly from refinement of high resolution neutron diffraction data as well as from resistivity and DSC measurements. From the refined saturated magnetic moment values only, ∼3.7 and ∼2.7 , the electronic configuration of the Co ions in LnBaCo₂O₅ remains conjectural. Two pictures, with Co³⁺ ions either in intermediate spin state ( t ⁵ _2g e ¹ _g ) or in high spin state ( t ⁴ _2g e ² _g ), describe equally well our experimental data. In both cases, the observed magnetic structure can be explained using the qualitative Goodenough-Kanamori rules for superexchange. Finally, in contrast to the parent Ln = Y compound [Vogt et al. , Phys. Rev. Lett. 84, 2969 (2000)], we do not report any spin transition in LnBaCo₂O₅ (Ln = Tb, Dy, Ho). Received 13 December 2000     

Theoretical study on structures and stability of SiC3S isomers

Various levels of calculations are carried out to explore the potential energy surfaces (PES) of singlet and triplet SiC₃S, a molecule of potential interest in interstellar chemistry. At the DFT/B3LYP/6-311G(d) level, a total of 57 minimum isomers and 92 interconversion transition states are located. The structures of the most relevant isomers and transition states are further optimized at the QCISD/6-311G(d) level followed by CCSD(T)/6-311?+?G(2df) single-point energy calculations. At the QCISD level, the lowest-lying isomer is the chain-like SiCCCS ³ 1 (0.0?kcal/mol) with a great kinetic stability of 54.1?kcal/mol. In addition, ring isomers CC-cCSSi ¹ 9 (19.8?kcal/mol), S-cCCCSi ¹ 12 (30.4?kcal/mol), S-cCCSiC ¹ 18 (9.4?kcal/mol), S-cSiCCC ¹ 21 (34.4?kcal/mol) and cage-like isomer cage-SiSCCC ¹ 23 (51.8?kcal/mol) also possess considerable kinetic stability (more than 10.0?kcal/mol). As a result, these six isomers are predicted to be possible candidates for future experimental and astrophysical detection. The bond natures and possible formation pathways in interstellar space of the SiCCCS are discussed. The predicted structure and spectroscopic properties for it are expected to be informative for the identification of SiC₃S and even larger SiC _n S species either in laboratory or in space.     

Fluorescence Study of the Relaxation Process in Excited Hetarylthiazole Cations

The absorption and fluorescence spectra of five cations protonated at the quinolyl nitrogen atom (IH⁺–VH⁺) and one ethylated (IEt⁺) cation were investigated. For these compounds (except VH⁺) both an anomalously large fluorescence Stokes shift (up to 238 nm) and a large short-wavelength fluorescence shift (up to 145 nm) at decreasing temperatures (down to 77 K) were observed. This is not the case for unprotonated molecules. The ground-state conjugation between quinolyl and another molecular fragment was found for II, IH⁺, IIH⁺, and IEt⁺. The relaxation process of excited cations is medium viscosity and temperature dependent. The experimental results are explained in terms of excited-state structural relaxation.     

Theoretical study of the photo-isomerisation reactions of 1,2-dihydro-1,2-phosphaborine and 1,2-dihydro-1,2-alumazaine

The mechanisms of the photochemical isomerisation reactions are investigated theoretically using the model systems, 1,2-dihydro-1,2-phosphaborine (5) and 1,2-dihydro-1,2-alumazaine (6), using the CAS(6,6)/6-311G(d,p) and MP2-CAS-(6,6)/6-311++G(3df,3pd)//CAS(6,6)/6-311G(d,p) methods. For each model reactant, three reaction pathways, which lead to three kinds of photo-isomers, are examined. The structures of the conical intersections, which play a key role in such photo-rearrangements, are determined. The thermal (or dark) reactions of the reactant species are also examined, using the same level of theory, to provide a qualitative explanation of the reaction pathways. These model investigations demonstrate that the preferred reaction route for these two aromatic heterocyclics is as follows: reactant → Franck–Condon region → conical intersection → photoproduct. The theoretical evidences anticipate that after irradiation of 5, the photoproduct yield of the Dewar BP-isomer, 8, should be larger than that of the Dewar BP-isomer, 7, whereas no Dewar BP-isomer 9 can be observed. Moreover, the present theoretical data predict after irradiation of 6, all three Dewar AlN-isomers (10, 11, and 12) and the starting molecule, 6, are produced.     

A reinvestigation of the acid-promoted heterocyclization of 2-(2-oxo-2-arylethyl)malononitriles in the presence of amines

The reinvestigation of the acid-promoted cyclization of 2-(2-oxo-2-arylethyl)malononitriles, in the presence of benzylamine or aniline, in ethanol or acetonitrile, has confirmed that this is a long-time reaction process for a low-yielding synthesis of 2-amino-5-arylfuran-3-carbonitriles (2), or 2-amino-5-aryl-1-phenyl-1H-pyrrole-3-carbonitriles (4), depending on the base used. However, the microwave-assisted synthesis of 2-amino-5-(4′-methoxyphenyl)furan-3(4)-(di)carbonitriles (2c and 3c) proceeds in shorter reaction times and higher yields than does the classical thermal heating protocol. In these reactions we have observed for the first time, and characterized by their spectroscopic data and X-ray analysis, the unexpected formation of 2-amino-5-aryl-3 (4)-(di)carbonitriles (3), whose formation has been rationalized by density functional theory (DFT) analysis of the proposed reaction mechanism.     

Investigations on the Photoreactions of Phenothiazine and Phenoxazine in Presence of 9-cyanoanthracene by Using Steady State and Time Resolved Spectroscopic Techniques

By using electrochemical, steady state and time resolved (fluorescence lifetime and transient absorption) spectroscopic techniques, detailed investigations were made to reveal the mechanisms of charge separation or forward electron transfer reactions within the electron donor phenothiazine (PTZH) or phenoxazine (PXZH) and well known electron acceptor 9-cyanoanthracene (CNA). The transient absorption spectra suggest that the charge separated species formed in the excited singlet state resulted from intermolecular photoinduced electron transfer reactions within the donor PTZH (or PXZH) and CNA acceptor relaxes to the corresponding triplet state. Though alternative mechanisms of via formations of contact neutral radical by H-transfer reaction have been proposed but the observed results obtained from the time resolved measurements indicate that the regeneration of ground state reactants is primarily responsible due to direct recombination of triplet contact ion-pair (CIP) or solvent-separated ion-pair (SSIP).