Showing papers on "Pi interaction published in 1980"

Effect of substituents on the electron densities in methane. .sigma. and .pi. Interactions in saturated systems

Abstract: Wave functions for a series of methyl derivatives have been converted into electron-density distributions. The space about the carbon has been separated into regions which may be attributed to each of the atoms, and the electron populations have been obtained for each region by numerical integration of the electron densities. Although it is not readily possible to assign a unique electron population to the methyl hydrogens, because of the dependence on the size chosen for the volume element about carbon, the charge shifts at hydrogen have been found to be relatively unaffected by the carbon size. The charge shifts have been determined and have been separated into sigma and ..pi.. components. Substitutents may be sigma acceptors or donors, and ..pi.. acceptors or donors, and affect methane in much the same fashion as has been found for benzene derivatives.

Weak intramolecular π coordination to methylmercury(II). Structure of (2-benzylpyridine)methylmercury(II) nitrate

Abstract: The title complex, [Hg(CH3)(C12HllN)]N03,C13HI4HgN+.N03-, has a linear C-Hg-N group with Hg-C 2·07(3) and Hg-N 2·10(2) A; there is a weak intramolecular pi interaction between Hg and a C=C bond of the phenyl ring with Hg-C distances of 3·23(2) and 3·33(3) A.

Molecular orbital theory of the hydrogen bond. n → π* transitions in monosubstituted by pyridines and their complexes with H2O

Abstract: Ab initio SCF-CI calculations have been performed to investigate substituent and hydrogen bonding effects on n → π* transitions in 2- and 4-R-pyridines, with the substituents including CH 3 , NH 2 , OH, F, C 2 H 3 , CHO, and CN. Substitution of the saturated π donating and σ withdrawing groups CH 3 , NH 2 , OH, and F in the 2 or 4 position of pyridine increases the n → π* transition energy, while substitution of the σ and π electron withdrawing groups CHO and CN decreases the n → π* transition energy. Orbital energy differences (ϵ π* – ϵ n ) and virtual excitation energies tend to correlate with CI n → π* transition energies when the substituents are saturated, but these correlations tend to be masked by other effects, including π conjugation, n orbital interaction, and configurational mixing, when the substituents are unsaturated. Hydrogen bonding with water leads to an increase in the n → π* transition energy of monosubstituted pyridines when the pyridine ring is the chromophore. In all hydrogen-bonded dimers except water : 2-aminopyridine, this increase is comparable to, but somewhat greater than, the ground state hydrogen bond energy, suggesting that the hydrogen bond is broken in the excited state, and these complexes dissociate upon vibrational relaxation. For the cyclic water : 2-aminopyridine dimer, the predicted increase in the n → π* transition energy is less than the ground state stabilization energy, suggesting that even upon relaxation, this dimer may still remain bound in the excited state. The first excited n → π* state of 2- and 4-pyridinecarboxaldehyde, unlike that of the other 2- and 4-R-pyridines, arises when the carbonyl group is the chromophore. Hydrogen bonding of these molecules to water through the pyridine nitrogen leads to a slight decrease in the n → π* transition energy, suggesting that these bases form stronger hydrogen bonds in the first excited state than in the

β-Thioxoketones. Part 6. Electronic absorption spectra of aromatic β-thioxoketones. A study of enol–enethiol tautomerism

Abstract: Aromatic β-thioxoketones exist in solution as mixtures of rapidly interconverting Z-enol and Z-enethiol tautomers The electronic absorption spectra exhibit in general four absorption bands in the uv–visible region at ca 265 (ArCO, π,π*; ArCC π,π*), 330 (ArCS π,π*; OCCCS π,π*; CO n,π*), 415 (OCCCS π,π*), and 520 nm (CS n,π*), respectively The β-thioxoketones are converted by sodium hydroxide into the corresponding anions CNDO/B Calculations predict that the negative charge in the β-thioxoketonates is delocalized over the OCCCS system, suggesting simultaneously sickles or W shaped conformations Two characteristic absorption bands found for the β-thioxoketonates at ca 275 and 400 nm are assigned to π,π* transitions involving the Ar–CCC–Ar′ and SCCCO chromophores, respectively The enol–enethiol tautomeric equilibrium has been studied by means of low temperature spectroscopy At room temperature equilibrium constants (K293) of 3–5 have been found corresponding to a 4 : 1 enol–enethiol concentration ratio The reaction entropy (ΔSr) has been found to be negative for the enethiol→enol conversion, reflecting the intramolecular O–H ⋯ S hydrogen bond to be considerably stronger than the corresponding O ⋯ H–S hydrogen bond Variations in ΔSr and K293 as functions of substitution in the aryl group next to oxygen are discussed

The imidazole π cation and barbital π anion trapped in a cocrystalline complex x‐irradiated at 12 K: An ESR–ENDOR study

Abstract: The predominant free radicals trapped in single crystals of the 1:1 intermolecular complex of imidazole and 5,5‐diethylbarbituric acid (barbital) x‐irradiated at 12 K have been identified by ESR and ENDOR. The electron abstraction and electron addition products are found to be the imidazole π cation and the barbital π anion, respectively. The π cation provides experimental evidence of evenly distributed unpaired electron density at positions C2, C4, and C5 of the five membered imidazole ring. In the π anion the unpaired electron density is localized primarily on C4 of barbital. It is suggested that π anions are trapped in barbital in preference to imidazole because barbital has a higher cross section for electron capture than imidazole. On the other hand, π cations are trapped in imidazole in preference to barbital because the barbital π cation has a higher cross section for destruction than the imidazole π cation.