Cyril Párkányi

Experimental and theoretical dipole moments of purines in their ground and lowest excited singlet states

Abstract The ground-state dipole moments of seven biologically important purines (purine, 6-chloropurine, 6-mercaptopurine, hypoxanthine, theobromine, theophylline and caffeine) were determined at 25°C in acetic acid (all the above compounds with the exception of purine) and in ethyl acetate (purine, theophylline and caffeine). Because of its low solubility, it was not possible to measure the dipole moment of uric acid. The first excited singlet-state dipole moments were obtained on the basis of the Bakhshiev and Chamma—Viallet equations using the variation of the Stokes shift with the solvent dielectric constant-refractive index term. The theoretical dipole moments for all the purines listed above and including uric acid were calculated by combining the use of the PPP (π-LCI-SCF-MO) method for the π-contribution to the overall dipole moment with the σ-contribution obtained as a vector sum of the σbond moments and group moments. The experimental and theoretical values were compared with the data available in the literature for some of the purines under study. For several purines, the calculations were carried out for different tautomeric forms. Excited singlet-state dipole moments are smaller than the ground-state values by 0.8 to 2.2 Debye units for all purines under study with the exception of 6-chloropurine. The effects of the structure upon the ground- and excited-state dipole moments of the purines are discussed.

Perturbation-graph theory—I: Resonance energies of heteroannulene π systems

Abstract Perturbation theory and the graph-theoretical definition of resonance energy are combined and applied to π systems substituted with heteroatoms and external substituents. The systems investigated are monocyclic and contain 3–6 atoms with 2–8 π electrons. In general, heteroatoms are calculated to exert their effects on resonance energy through two separable mechanisms, due either to the electronegativity (h parameter) or the conjugative ability (k parameter) of the substituent atom. There is no first-order effect of electronegativity in species containing 4n+2 (n= integer) π electrons. All electronegative or electropositive substituents are predicted to stabilize 4n π electron systems. Enhanced conjugation stabilizes 4n+2 systems and destabilizes 4n systems. Several examples of ions and molecules are given that qualitatively conform to these rules.

Dipole moments of indoles in their ground and the first excited singlet states

Abstract The ground-state dipole moments of seven substituted indoles (3-acetylindole, 5- bromoindole, 3-formylindole, 2-indolecarboxylic acid, 5- indolecarboxylic acid, 5-methoxy-2-indolecarboxylic acid, and 5-nitroindole) have been measured at room temperature (298 K) in several solvents (benzene, dioxane, n-butyl ether). Their first excited singlet-state dipole moments have been determined by means of the equations of Bakhshiev and Chamma-Viallet, using the variation of the Stokes shift with the solvent dielectric constant-refractive index term. A comparison of the two sets of dipole moments indicates that the first excited singlet state of indoles is more polar than the ground state. The effect of substituents is discussed. The experimental values of dipole moments of selected indoles are compared with those calculated by the PPP (LCI-SCF-MO) method.

Synthesis of silyl and stannyl pyrimidines

Abstract 2,4-Dimethoxy- and 2,4-dibenzyloxy-5-lithiopyrimidines, obtained from the corresponding substituted 5-bromopyrimidines, react with trimethylsilyl chloride or trimethylstannyl chloride to give the appropriate 5-trimethylsilyl or 5-trimethylstannyl derivatives. The following compounds were synthesized by this reaction sequence: 2,4-dimethoxy-5-trimethylsilylpyrimidine (2), 2,4-dibenzyloxy-5-trimethylsilylpyrimidine (3), 2,4-dimethoxy-5-trimethylstannylpyrimidine (4), and 2,4-dibenzyloxy-5-trimethylstannylpyrimidine (5). Similarly, 6-chloro-2,4-dimethoxypyrimidine was converted into the 5-lithio derivative which subsequently gave 6-chloro-2,4-dimethoxy-5-trimethylsilylpyrimidine (6) and 6-chloro-2,4-dimethoxy-5-trimethylstannylpyrimidine (7). In a new modified synthesis of 5-trimethylsilyluracil (1), 2,4-dibenzyloxy-5-trimethylsilylpyrimidine (3) was smoothly converted into 1 by reaction with trimethylsilyl iodide.

Effect of pH on the fluorescence and phosphorescence spectra of indolecarboxylic acids. Evidence for the existence of a singlet excited-state indole acid dianion☆

Abstract Room-temperature electronic absorption, fluorescence, and low-temperature (77 K) phosphorescence spectra of a series of indolecarboxylic acids have been measured in predominantly aqueous solutions at pH 1.2, 7.0, 12.5, and in 5 N NaOH. The 1 L a and 1 L b π, π ∗ states have been assigned to the electronic transitions for all the compounds under study. The quenching of fluorescence, the appearance of a new fluorescence emission band, red-shifted by 40 nm, in very alkaline media, and the disappearance of the indole carboxylate short-wavelength band have been attributed to the formation of an excited singlet-state indolate carboxylate (dianion) stabilized by hydrogen bonding with the solvent. The existence of two successive excited singlet-state prototropic equilibria is discussed.

GC-MS-SPECMA bank analysis of JUniperus communis needles and berries

Abstract Using a combination of gas chromatography-mass spectrometry (electron impact) as well as the headspace technique, more than 70 compounds were found in the juniper needle oil obtained from plants growing in southeastern France (Provence). A comparison between the headspace of fresh and bottled black berries shows a significant loss of the monoterpenoid fraction with time. In the green berries (unripe) this loss is more marked. A sesquiterpenoid fraction of the Juniperus communis needle oil was also investigated by means of the GC (FID) and GC-MS (EI, PCI). Among more than 100 isolated constituents, 64 were sesquiterpenoids. The presence of several oxygen-containing derivatives based on the calamene and calacorene isomers was established but their structures were not fully determined. The various compounds were identified on the basis of our SPECMA data bank (mass spectra and Kovats indices) and our own files.

Organometallic heterocyclic chemistry I. some group VI transition metal complexes containing thiazole, isothiazole, and related ligands

Abstract The heterocycles thiazole, isothiazole, and 4(5) methyl-isothiazole have been used as ligands in reaction with the group VI metal hexacarbonyls. The resulting pentacarbonyl derivatives are coordinated to the metal via the nitrogen atom exclusively; however, it is clear from mass spectral data that migration of the metal from nitrogen to sulfur is an important process in the fragmentation of the complexes as well as formation of pi-bonded species. The electronic effects of the ligands are very similar and related to those of pyridine.

Inductive effects on molecular ionization potentials—VI: Alkenes

Abstract The experimental ionization potentials, E I of fourteen alkenes have been correlated with the sum of the inductive substituent constants, Σσ I ,, of the corresponding alkyl substituents attached to the CC group of the alkene, and a good correlation obtained. Models of alkenes were treated by the simple HMO method using the heteroatom model for the alkyl groups. A good correlation between the experimental ionization potentials, E I of the alkenes and the corresponding energies of their highest occupied π-molecular orbitais was found.

Synthesis and chemical characterization of biologically important complexes of vanillin thiosemicarbazone with manganese(II), iron(II), cobalt(II), nickel(II), copper(II), zinc(II), cadmium(II), and mercury(II)

SummaryVanillin thiosemicarbazone (VTSC) has been used to isolate the complexes of the types [M(VTSC)2(H2O)2]X2 (M=MnII, FeII, CoII, or NiII and X=Cl) and [M(VTSC)X2]H2O (M=CuII, ZnII, CdII or HgII and X=Cl). Probable structures of these complexes are suggested on the basis of elemental analysis, molar conductance, magnetic moment and electronic and i.r. spectral data. The fungicidal activity of VTSC and the isolated complexes has been evaluated on pathogenic fungi,Alternaria (Sp.),Paecilomyces (Sp.) andPestalotia (Sp.).

The MCS model of chemical initiation of cancer: ppp calculations on methylated and N-heteroaromatic polycycles☆

Abstract The MCS model of chemical initiation of cancer concentrates on three important aspects of carcinogenesis by aromatic compounds: M: epoxidation of the M-region, in competition with deactivating reactions; C: intermediate ion formation: bay-region carbocation during the reaction of the diolepoxide with DNA, and/or zwitterion during the M-region epoxide hydration; S: an optimum size for solubility, enzyme specificity and intercalation. PPP (LCI-SCF-π) calculations are presented for the most important methylsubstituted and N -heteroaromatic derivatives of benz[ a ] anthracene and dibenz[ a, h ]anthracene. For comparison, a number of polycyclic aromatic hydrocarbons are included. A good correlation ( r = 0.940) is obtained between the calculated and experimental Iball indices using a linear equation containing the three variables of the model, viz., M , C and Δ. The standard error S.E. = ± 10.4 is within the uncertainty range of the experimental index. The analysis of the data supports the formation of a biradical tetrahedral intermediate during the M-region epoxidation.