Angular distribution for the major photoelectron bands of nitrogen below 45 eV binding energy

High-resolution optically detected magnetic resonance (ODMR) and optically detected electron—electron double resonance (ODEEDOR) have been performed on the lowest excited triplet states of pyrazine-h₄ and pyrazine-d₄, as dilute guests in polycrystalline benzene and cyclohexane host matrices. Computer simulated spectra have been fitted to those experimentally observed, using a spin hamiltonian which takes into account the ¹⁴N and ¹H (or ²D) nuclei of the pyrazine guest molecule. Good agreement is obtained when one ¹⁴N nuclear quadrupole energy difference, |ε_y^N - ε_z^N| ? 1.3 MHz, and the remaining component, |ε_x^N| = 0.6 ± 0.2 MHz for pyrazine-d₄ in benzene-d₆.     

The Nature of the vibronic intensity borrowing mechanism in the 1B3u(n)1Ag(X) transition of pyrazine

The transition intensities of the S₀S₁ gas phase absorption spectra of pyrazine-d₀ and -d₄ were carefully measured. A discussion of the various aspects of vibronic intensity borrowing for this unique case is presented using our new experimental results.     

List of subjects

The quantum yields of fluorescence and intersystem crossing in pyrazine-h₄ and -d₄ and pyrimidine vapors have been studied at moderately high pressures (50–100 Torr) in the presence of a foreign gas as a function of excitation energy in the S₁ (n, π*) ← S_o absorption region. The fluorescence quantum yield of pyrazine fluctuates in the lower energy region, but on the average is constant over the range of the excess vibrational energy 0–4000 cm^?1, whereas the fluorescence yield of pyrimidine decreases smoothly with increasing excess energy. For each of the three molecules, the intersystem crossing yield is approximately constant with a value near unity in a wide range of the excess energy, indicating that the intramolecular nonradiative transition from S₁ is governed by the crossing to the triplet state. From a calculation based on the first-order vibronic coupling (Herzberg-Teller) theory the radiative rate constant of the pyrazine fluorescence is found to be relatively large for the vibronic levels involving a non-totally symmetric vibration, ν₁₀₂. This accounts for the fluctuation of the fluorescence yields of pyrazine-h₄ and -d₄.     

Calculation of the in-plane force constants and vibrational spectra of pyridine and its deuterated derivatives by the CNDO/2 force method

The in-plane vibrations of pyridine and its deuterated derivatives (4-d₁, 2,6-d₂, 3,5-d₂, and d₅) have been studied with the CNDO/2 force method. The calculated values of the force field scaled by five empirical parameters yield good agreement with the experimental spectra. The results seem to permit the reassignment of some bands.     

Force field calculations and reassigments of Raman and IR frequencies of pyrazine-N,N'-dioxide

Force field calculations have been carried out for the planar and non-planar modes of pyrazine-N,N′-O₂ using the observed vibrational frequencies obtained from the IR and Raman spectral studies on pyrazine-N,N′-O₂-h₄ and pyrazine-N,N′-O₂-d₄ reported in the literature [D.A. Thornton, P.F.M. Verhoeven, G.M. Watkins, Herman O. Desseyn, Benjamin J. Van der Veken, Spectrochim. Acta 46A (1990) 1439]. The purpose of the present work is to determine force fields for the pyrazine-N,N′-O₂ molecule and to present vibrational assignments for the observed IR and Raman frequencies to the fundamental modes, combination bands and overtones. The planar force field determined in the present case is expected to be better than that reported earlier [S. Szöke, G. Varsanyi, E. Baitz, Acta Chim. 53 (1967) 345] because of the inclusion of the observed frequencies due to pyrazine-N,N′-O₂-d₄ isotopomer. In addition, the non-planar force field for this molecule is reported for the first time.     

Temperature dependence of the spin-lattice relaxation rates in the triplet state of pyrazine at low temperatures

The electron spin-Lattice relaxation, (SLR) rate constants have been measured for the triplet state at pyrazine-d₄ in a cyclohexane matrix from 1 to 8 K by resolving the phosphorescent decay curve into three exponential components. A matrix kinetics is presented that gives the relationship between the SLR rate constants and the observed decay component rate constants and the decay component intensities of the three zero-field levels. An exact numerical procedure based on the Newton-Raphson technique is described in detail for systems of two and three coupled levels. This procedure was used to find the SLR rate constants starting from either the decay component rate constants or the decay component intensities. Both the matrix kinetics and the numerical procedure should be useful in many other studies involving multilevel systems whose sublevels are connected by some combination of SLR and microwaves. The observed SLR rate constants are best explained as a function of temperature as the sum of a direct process (linear in T) and a Raman process (T² dependence). Their relative magnitudes indicate that the Raman process is probably antharmonic. Vancus mechanisms for SLR in this system are examined, and a short review of SLR theory and experiment is given. The nature of the zero-field sublevels in pyrazine-J₄ is also discussed as is the effect of the choice of matrix solvent upon SLR rate constants and the phosphorescent spectrum.     

Charge localization: The ion kinetic energy spectra of pyrazine,pyrimidine and pyridazine

The ion kinetic energy (IKE) spectra of pyrazine, pyrimidine, pyridazine, pyrazine-d₄, pyrimidine-d₄ and pyridazine-d₄ have shown that these compounds can more readily be distinguished on the basis of IKE spectra than mass spectra. Studies of the fragmentation of various doublycharged ions have given information about the localization of charge on the heteroatoms and on the structures of the ions themselves. Transition states from which fragmentation occurs for doublycharged ions with loss of either [H]⁺ or [D]⁺ have shown large differences of the order 1 Å between the intercharge distances in the two cases.     

Synthesis and photophysical properties of oligoarylenes with a pyrrolo[2,3-d]pyrimidine core

The palladium-catalyzed Suzuki-Miyaura reaction of 2,4-dichloropyrrolo[2,3-d]pyrimidine with aryl boronic acids has been studied. Pd(OAc)₂/dicyclohexyl(2-biphenyl)phosphine/K₃PO₄ was found to be an efficient catalyst system to prepare 4-aryl-2-chloro- and 2,4-diarylpyrrolo[2,3-d]pyrimidines. Novel non-linear molecules consisting of a pyrrolo[2,3-d]pyrimidine core and aryl branches have been elucidated as blue light-emitters with fluorescence quantum yields ranging from 4% to 67% in THF solution. The impact of an electron-withdrawing t-BuOCO group attached to the pyrrole ring of pyrrolopyrimidine derivatives on optical properties is discussed.     

Electrophoretic determination of phenyl and p-bromophenyl substituted 1H,2H,3H,4H-pyrido[4,3-d]pyrimidinium diiodobromides

Phenyl and p-bromophenyl substituted 1H,2H,3H,4H-pyrido[4,3-d]pyrimidinium diiodobromides have been identified and determined by capillary zone electrophoresis on an unmodified quartz capillary. It has been found that an increase in the number of bromine atoms in the structure of 1H,2H,3H,4H-pyrido[4,3-d]pyrimidinium derivatives consecutively decreases the electrophoretic mobility of the cations. The developed method makes possible the determination pyrido[4,3-d]pyrimidinium derivatives in the concentration range (0.03–0.25) mM with the c _min (3.1–10.0) μM.     

Vibrational spectra of 1,3-dideuterioallene and normal mode calculations for allene

Infrared and Raman spectra of 1,3-d₂-allene were measured and fundamental frequencies were assigned. The frequencies were used with previously published data for the d₀, d₄, d₁ and 1,1,-d₂ isotopomers of allene in force field calculations. A new ab initio quantum mechanical force field calculated at the 6-31G** level was scaled to fit the anharmonic fundamental frequencies and used to provide some of the interaction constants for a second, conventionally refined force field.     

Infrared and Raman spectra and normal coordinate analysis of trans-1,2-dicyanocyclopropane and its deuterated isotopomers

Infrared and Raman spectra are presented for trans-1,2-dicyanocyclopropane and its 1,2-d₂- and 3,3-d₂-isotopomers. Most of the d₀ assignments agree with previous assignments of Strumpf and Dunker encompassing 26 of the 27 fundamentals. The 1,2-d₂ assignments are similar, but for the 3,3-d₂ species only 11 bands could be assigned. A complete ab initio quantum mechanical force field has been calculated for this molecule at the 6-31G* basis set level. This force-field was scaled and least-squares optimized using eight parameters for functionally related diagonal force constants and their geometric mean for off-diagonals. Both theoretical minimum energy and “experimentally corrected” geometries were used with no significant difference in results. An earlier calculation with a 4-31G basis set gave a similar frequency fit but different scaling factors. As an alternative approach, the scaled ab initio force field, was also used as a starting point for a more conventional refinement of 27 force constants which had a significant influence on the potential energy distribution. The remainder were fixed at the scaled ab initio values. This empirical force field resulted in a fit to 65 fundamentals of d₀, 1,2-d₂ and 3,3-d₂ to < 1% average error.     

The first entry to 5,6-dihydroxy-3-mercaptoindole, 5-hydroxy-3-mercaptoindole and their 2-carbomethoxy derivatives by a mild thiocyanation/reduction methodology

The hitherto unknown 5,6-dihydroxy-3-mercaptoindole (4a) and its 2-carbomethoxy derivative (4b), as well as the analogous 5-hydroxy 3-mercaptoindoles, have been conveniently obtained as O,S-acetyl derivatives 3a-d by thiocyanation of the corresponding acetoxyindoles 1a-d with the NH₄SCN/oxone system followed by SmI₂ reduction and acetylation.     

A new reaction of P4S10 and Lawesson's reagent; a new method for the synthesis of dithieno[3,2-b;2,3-d]thiophenes

A new reaction of P₄S₁₀ and Lawesson's reagent for the synthesis of fused thiophenes has been uncovered. It has given easy access to the synthesis of derivatives of the technologically important heterocycle dithieno[3,2-b;2^′,3^′-d]thiophene, DTT. Electrochemical polymerization of one of the derivatives has been demonstrated.     

Al(OTf)3-catalyzed one-pot synthesis of pyrrolo[3,2-d]pyrimidinedione derivatives

A series of polysubstituted pyrrolo[3,2-d]pyrimidinedione derivatives have been synthesized in excellent yields. This prominent scaffold is obtained via an Al(OTf)₃-catalyzed tandem addition-annulation sequence between propargylic alcohols and aminopyrimidines. The process is simple, facile, inexpensive, and provides a diverse range of substituted pyrrolo [3,2-d]pyrimidinedione derivatives with short reaction times from readily obtainable starting materials.     

Vibrational spectra and a potential function for 3-chlorocyclopropene and its various deuterated modifications

I.r. and Raman spectra have been obtained for 3-chlorocyclopropene and 3-chlorocyclopropene-d₃ under a variety of experimental conditions. For mixtures of the two d₂ isotopic species and of the two d₂ isotopic species, only infrared spectra have been recorded. The pattern of gas-to-condensed-phase frequency shifts is interpreted as evidence of hydrogen bonding for the CH hydrogen atoms. A complete assignment of the vibrational fundamentals for the d₀ and d₃ species is derived from the spectra. For the d₀ species these frequencies (in cm⁻¹) are: (a′) 3171, 3029, 1633, 1266, 1141, 913, 713, 591, 345; (a″) 3140, 1034, 1017, 866, 840, 349. With guidance from normal coordinate calculations 59 of the 60 fundamentals for the d₁ and d₂ species are assigned. The full set of 89 observed frequencies has been used to fit an 18-parameter valence-type potential function. In comparison with the force constants for CC and CC stretching of cyclopropene and 3,3-difluorocyclopropene, the CC force constants of 3-chlorocyclopropene show changes similar to those expected for a substitution of a single fluorine atom. This finding is consistent with a σ-acceptor role for the mechanism of the interaction of fluorine and chlorine atoms with the cyclopropene ring system.     

A novel and efficient synthesis of pyrazolo[3,4-d]pyrimidine derivatives and the study of their anti-bacterial activity

In this work 4-amino-6-aryl-2-phenyl pyrimidine-5-carbonitrile derivatives were synthesized through a one-pot,three-component reaction of an aldehyde,malononitrile and benzamidine hydrochloride,in the presence of magnetic nano Fe₃O₄ particles as a catalyst under solvent-free conditions.3-Amino-6-aryl- 2-phenylpyrazolo[3,4-d]pyrimidine derivatives were prepared through an efficient and environmentally friendly reaction between 4-amino-6-aryl-2-phenylpyrimidine-5-carbonitrile derivatives and hydrazine hydrate and their antibacterial activity has been evaluated.     

Synthesis and antitumor activities of a new series of 4,5-dihydro-1H-thiochromeno[4,3-d]pyrimidine derivatives

A new series of 4,5-dihydro-1H-thiochromeno[4,3-d]pyrimidine derivatives have been designed and synthesized.The antitumor activities of the target compounds have been evaluated in vitro against two human cancer cell lines including A549 (human alveolar adenocarcinoma cell) and H460 (human lung cancer) by MTT assay.Most of the target compounds exhibited significant antitumor activities against A549 and H460 cancer cell lines.The most potent compound 4-(benzo[d][1,3]dioxol5-yl)-8,9-difluoro-2-(4-methylpiperazin-1-yl)-4,5-dihydro-1H-thiochromeno[4,3-d]pyrimidine (CH05) (IC50=0.44 M,3.07 M) was 2.0 and 8.4 times more active than gefitinib (IC50=0.89 M,16.81 M) against A549 and H460 cell lines,respectively.     

Optical properties of CeBO3 and CeB3O6 compounds: first-principles calculations and experimental results

The cerium borates o-CeBO₃, m-CeBO₃ and CeB₃O₆ have been shown to be isostructural to their lanthanum derivatives. From diffuse reflectance, electron energy loss spectroscopy (EELS) and band structure calculations, it has been evidenced that a Ce³⁺ 4f-5d transition is responsible for weak absorption peaks around 3.5 eV while the O2p-Ce5d charge transfer gives rise to a strong absorption around 7 eV. Starting from self-consistent full potential LAPW calculations, the dielectric tensors of the three compounds were computed and compared to experimental data. It results in a satisfactory fit between the observed and the calculated extinction coefficient k and the index of refraction n.     

D-isotope-labelled n-butane cations in CF3CCl3 and in CF2ClCFCl2: Structure and reactions at low temperature

Cations of n-butane, n-butane-1,4-d₆, -d₄, -d₂ and n-butane-2,3-d4 produced by X-irradiation of CF₂ClCFCl₂ and CF₃CCl₃ 3 matrices containing 1–2 mole% of solute have been investigated by ESR spectroscopy. The measurements have been performed at 77–130 K. The results confirm that the two largest couplings are assigned to two protons on the methyl groups. Smaller couplings have been resolved in CF₃CCl₃ and assigned to specific H atoms. A geometry with a non-planar carbon skeleton is proposed. Partly deuterated methyl groups give rise to rotational isomers. Thermal and photoinduced decompositions producing 2-butyl radicals and 2-butane cations have been further studied.     

Vibrational spectra and structure of cyclopropane-d1 and dicyclopropyl ketone

Infrared spectra have been obtained for cyclopropane-d₁, C₃H₅D(CP-d₁), in the vapor phase and for dicyclopropyl ketone (C₃H₅)₂ CO (DCPK) in the liquid state. Raman spectra of the latter compound in the liquid and solid phases have also been recorded. By comparing the experimental with the theoretical spectra of CP-d₁ it has been possible to assign most of the fundamental frequencies of the molecule. Additionably, empirical spectra—structure correlations from the literature for cyclopropyl derivatives containing the COX moiety (X=halogen) have been used to investigate the conformational preference of DCPK. The experimental results confirm that the molecular configuration with the carbonyl group cis to the planes of the two rings is the predominant species in the liquid state, though evidence for a second conformer, which we tentatively identify as the cis—trans conformer, is presented.