Notes on Kuhn's ω″ molecular orbital method

(1) It is shown that, contrary to statements sometimes made, it is perfectly possible to obtain consistent sets of charges in Kuhn's ω″ molecular-orbital method, whatever the values of the parameters ω, ω′, ω″.

(2) Particular applications are made to the benzyl cation, and to the non-alternants fulvene, heptafulvene and azulene. It appears that including the ω-terms decreases the magnitude of the largest of the net atomic charges as calculated by the simple Hückel method, and results in an overall smoothing-out process. Further inclusion of the ω′-terms continues this smoothing-out process, but inclusion of the ω″-terms may sometimes slightly reverse this process.

(3) The charge distributions obtained for fulvene and azulene in (1) and (2) lead to molecular dipole moments which are still much too large.

In view of (3) it is not clear that the additional work involved in the inclusion of ω′ and ω″ is justified by the greater accuracy thus obtained.     

Preparation of ω-tetrahydropyran-2-ylsulfanylalkylmagnesium chlorides: Useful reagents for the synthesis of 1-(ω-mercaptoalkyl)-1,2-dihydrobuckminsterfullerenes (C60)

ω-Tetrahydropyran-2-ylsulfanylalkylmagnesium chlorides are prepared as Grignard reagents containing protected mercaptoalkyl chains, which are useful for the direct introduction of ω-mercaptoalkyl chains to electrophiles. In order to prove the usefulness of these reagents, they were reacted with buckminsterfullerene (C₆₀) to give 1-(ω-mercaptoalkyl)-1,2-dihydrobuck minsterfullerenes after the deprotection of THP group with TFA.     

Phase characterization of LC (meth)acrylic monomers based on ω-hexyloxy- and ω-undecyloxy-salicylaldimine groups with different alkoxy tail substitutions

The synthesis and phase characterization of three homologous series of liquid crystalline acrylic and methacrylic monomers, consisting of 21 new compounds are presented. They are based on ω-hexyloxy- and ω-undecyloxysalicylaldimine groups with different alkoxy tail substitutions. The liquid crystalline materials were characterized by polarizing optical microscopy and differential thermal analysis. Smectic A and tilted smectic C phases were observed in the compounds. Near the transition to the isotropic, a narrow nematic phase, coexisting with the smectic A phase, was detected for the pentyloxy and hexyloxy derivatives in the M11 and A11 series. In case of M11R11 and M11R12 only a tilted smectic C phase was detected. The clearing point was comparable for all series, around 100°C.     

Mittlere ringe-XIII Die intramolekulare acylierung der ω-(1-naphthyl)-fettsäuren

The intramolecular Friedel-Crafts cyclisation of ω-arylalkanoic acids, which is in the benzene series a valuable synthetic route to bicyclic ketones with medium-sized and large rings, has been investigated with ω-(1-naphthyl)alkanoyl chlorides. The lower members, including naphthyl-caproic acid, close the ring at position 2 to form IV. The higher homologous acids show a preference for annellation at 7, making available a new type of heteronuclear naphthalene cyclic ketones (VII). During cyclisation of ω-(1-naphthyl) decanoic acid, the 1,4 ring closure competes with the 1,7 type.

Steric hindrance of resonance as a consequence of “medium ring torsion” was studied in different classes of naphthocyclenones.     

Catalytic carbopalladation of ω-methylenebicyclo[n.1.0]alkanes

ω-Methylenebicyclo[n.1.0]alkanes undergo carbopalladation via cleavage of the C(1)---C(2) cyclopropane bond. The resultant σ-butenyl complexes decompose via β-hydride elimination to afford the 1-cycloalkenylstyrene products.     

Bounds for total π-electron energy of polymethines

The introduction of a -CH=CH- fragment into an arbitrary alternant ,ω-disubstituted polymethine increases the total Hückel π-electron energy at least by 2(√5 − 1) = 2.472 β and at most by 2(√3 + 1 − √2) = 2.636 β. These bounds are the best possible. With increasing length of the polymethine chain, the energy difference tends to 8/π = 2.546 β.     

Measurement of atomic L shell fluorescence (ω1, ω2, ω3) and Auger (a1, a2 and a3) yields for some elements in the atomic number range 59≤Z≤85

A comprehensive set of theoretical Coster–Kronig and fluorescence yields are presented for atomic numbers 18≤Z≤100. These quantities are based on ab initio relativistic calculations. Agreement with experimental values is fair for ω₁ and generally good for ω₂, ω₃ (Z≥54) [1]. Therefore, atomic L shell fluorescence (ω₁, ω₂, ω₃) and Auger yields (a₁, a₂ and a₃) for some elements in the atomic number range 59≤Z≤85 were determined. These selected measured semi-empirical values were also fitted by least squares to polynomials in the Z of the form ∑_na_nZⁿ and compared with theoretical and with earlier fitted values.     

Pros and cons of σ -aromaticity

A discussion of σ-aromaticity requires a distinction between σ-conjugation, σ-electron delocalization, and σ-bond delocalization, all of which can be considered as prerequisites of σ-aromatic character. All molecules with three or more atoms encounter σ-conjugative interactions. Also, all σ-electrons are delocalized if the term delocalization is taken in its quantum theoretical meaning. However, σ-conjugation and σ-electron delocalization do not necessarily imply σ-bond delocalization. - One can distinguish between three different modes of σ-delocalization: ribbon delocalization in acyclic molecules and larger rings, surface delocalization in small rings, and volume delocalization in cage compounds. Surface delocalization of σ-electrons is found to lead to σ-bond delocalization. An example is cyclopropane. Bonding in cyclopropane can only be described in terms of nonclassical 2-electron 3-center and 4-electron 3-center bonds. Application of the criteria used to define π-aromaticity reveals that the properties of cyclopropane are in line with these criteria and that the term σ-aromaticity cannot be rejected on the grounds that aromaticity is restricted to π-electrons. The pros and cons of using the term σ-aromaticity in chemical discussions are presented.     

Subcellular localization of docosahexaenoic acid and arachidonic acid ω-hydroxylation activity in the brain, liver and colonic adenocarcinoma

A homogenate of rat brain, rat liver or human colonic well differentiated adenocarcinoma was prepared in 250 mM sucrose isoosmolaric buffer (pH 7.6) and fractionated by differential centrifugation at 10³, 10⁴ and 10⁵ g. Each precipitate or supernatant was incubated with NADPH and docosahexaenoic acid or arachidonic acid as a substrate for 30 min at 37°C under aerobic conditions. ω-Hydroxydocosahexaenoic acid or ω-hydroxyeicosatetraenoic acid from an incubation mixture was detected by reversed-phase high-performance liquid chromatography-thermospray mass spectrometry with selected-ion monitoring. ω-Hydroxy polyunsaturated fatty acids were characterized by high intensity of the molecular ion (MH⁺) although common hydroxy polyunsaturated fatty acids were characterized by high intensity of the MH⁺ - H₂O ion. For the rat brain, ω-hydroxylation activity (the amount of ω-hydroxy product produced in 30 min) was concentrated to a 103 g precipitate although the specific activity (the activity per 1 mg of protein) in the 10³ g precipitate did not indicate superiority over other fractions. However, the specific activity of the rat brain increased on addition of a 10⁴ or 10⁵ g precipitate. For the rat liver, although ω-hydroxylation activity was concentrated to a 10³ g precipitate, the specific activity was concentrated to a 10⁵ g precipitate and the Subcellular localization differed from that of rat brain. In the human colonic well differentiated adenocarcinoma, although ω-hydroxylation activity was relatively high in the 10³ g supernatant, the specific activity was relatively high in the 10³ and 10⁵ g precipitates. These results suggest that there is a difference regarding subcellular localization of the ω-hydroxylation activity depending on the species of the organs.     

The effect of a positive charge on a π → π-transition

The position of the maximum for the π → π^*-transition of a bicyclic ketone containing a quaternary nitrogen atom at 3·1 Å from an :β-unsaturated ketone system is shown to depend largely upon electrostatic (repulsion) destabilization in both ground and excited states, the latter being affected more strongly.     

Phase characterization of two homologous series of LC methacrylic monomers based on ω-hexyl- and ω-butyl-oxysalicylaldimine groups with different alkoxy tail substitutions

The phase characterization of two homologous series of liquid crystalline methacrylic monomers based on χ -hexyl- and χ -butyloxysalicylaldimine groups, with different alkoxy tails, is presented. The liquid crystalline materials were characterized by polarising optical microscopy, differential scanning calorimetry, differential thermal analysis, and X-ray diffraction. All the monomers exhibit the simultaneous occurrence of smectic A and C phases. When the alkyl chain is short, a narrow nematic phase is observed, leading to an I-N-SmA-SmC phase sequence.     

Simulated T ← S spectra of benzaldehydes as a function of the energy gap between the ππ and nπ levels

The T_1,2 ← S₀ spectra of benzaldehydes have been studied as a function of the energy separation between the vibrationless levels. It is shown that the spectra are very complicated in the region of ΔE[T₂⁰(nπ^*)-T₁⁰(ππ^*)] = 250–400 cm⁻¹, reflecting effective vibronic interferences between T₂⁰(0-0) and each of the ν₃₆-ν₃₃ out-of-plane vibrational levels of T₁⁰(ππ^*). The simulated spectra correspond to the observed spectra. In the case of T₁⁰ = ³nπ^* and T₂⁰ = ³ππ^* the spectral change is not so drastic as in the reverse case loc. cit. because the optical intensity generally concentrates in the longest wavelength band, i.e., the origin band of the T₁(nπ^*) ← S₀ transition. The simulation spectra are useful for interpretation of the absorption spectra in similar electronic structure systems of substituted benzaldehydes.     

Theoretical determination of the ionization potentials of the 1σ and 2σ electrons of CO(Σ)

Large-basis-set calculations of near Hartree-Fock accuracy were performed on CO⁺(1σ-hole ²Σ⁺) and CO⁺)2σ-hole, ²Σ⁺); correlation energies for these systems and for CO were calculated using an atoms-in-molecule approach, relativistic energies and vibrational structure corrections were also considered. The results are: IP(CO, 1σ) = 542.4 (542.57) eV, IP(CO,2σ) = 297.0 (296.24) cV, D_c(CO, ¹Σ⁺) = 10.8 (11.1) Ev, D₃(CO⁺, 1σ, ²Σ⁺) = 11.9 eV, D_e(CO⁺, 2σ, ²Σ⁺) = 9.1 eV, where IP and D_e stand respectively for ionization potential and dissociation energy, and where the numbers in parentheses refer to the most recent experimental values. The electron transfers resulting from the ionization of inner-shell electrons are discussed. Finally a quantitative correlation is developed correlating absolute chemical shifts to charge densities. Agreement between the calculated values and those derived from the correlation is quite satisfactory.     

UV photodissociation of N-methylpyrrole: The role of πσ states in non-hydride heteroaromatic systems

The photophysics of jet-cooled N-methylpyrrole molecules following excitation to their first excited singlet state (the ¹A₂ state, arising from a 3s/σ^*←π electron promotion) has been investigated by resonance enhanced multiphoton ionisation spectroscopy, by measurements of wavelength resolved ‘action’ spectra for forming CH₃ photoproducts, and by velocity map imaging studies of these CH₃ products (in their v = 0 and v₂ = 1 vibrational levels). CH₃ products are observed at all excitation wavelengths within the NMP absorption band. Direct dissociation on the ¹A₂ potential energy surface (PES) yields ‘fast’ CH₃ fragments, with an average total kinetic energy release (TKER) of 6500 cm⁻¹, but this product channel is only observed in a narrow wavelength range near the absorption band origin. All of the measured CH₃ images also show a broad component, peaking at lower TKER (1700 cm⁻¹); this component extends beneath the ‘fast’ feature in images recorded at wavelengths near the origin, and accounts for all of the CH₃ products observed at shorter photolysis wavelengths. These products are attributed to decay of highly vibrationally excited ground state molecules formed by radiationless transfer from the ¹A₂ state. Similarities and differences with the results of previous studies of the H + pyrrolyl products arising in the UV photodissociation of pyrrole are discussed in terms of the likely nuclear motions on the relevant ground and excited PESs (along R_N–CH3/R_N–H), and the possible couplings between these surfaces. The present study confirms that the proposed model of ¹πσ^* state induced bond fission in heteroaromatic molecules [A.L. Sobolewski, W. Domcke, Chem. Phys. 259 (2000) 181] is also applicable to non-hydride substituted heteroaromatics, but that mass effects can have an important influence on the subsequent nuclear dynamics.     

Toxic fluorine compounds containing the C—F Link—XII Some reactions of ω-fluoroacetophenone

The reaction between ω-fluoroacetophenone and sodium methoxide in methanolic solution has been investigated. The condensation at room temperature results in the formation of 2,3-epoxy-4-fluoro-1,3-diphenylbutan-1-one (I) and 2-fluoro-1,3-diphenyl-butan-1-one-4-al (II). From the latter 3-fluoro-2,4-diphenylfuran (V) has been prepared. A general method for preparing substitued fluorofurans thus becomes possible.     

Reversible excited state metal-to-metal electron transfer across a heterocyclic σ, π-bridge

A laser flash photolysis study of a binuclear methyl (η¹-azaferrocene)cobaloxime is reported. Excitation at 355 nm (4 ns, approximately 60 mJ) results in the reversible electron transfer between cobalt and iron centres CO^III/Fe^II CO^II/Fe^III. This constitutes the first example of photoinduced metal-to-metal electron transfer across a heterocyclic σ, π-bridge     

Construction of a three-dimensional network with open channels via Ag–Ag and π–π interactions between nanoscale sized molecular chains

The reaction of Ag₂O with pybz (pybz=4-(4-pyridyl)benzoate) gave the monomer compound [Ag(pycz)(H₂O)], 1. Using 4,4′-bipyridyl (bpy) as a spacer to increase the length of the monomer resulted in the nanosized molecular chain compound [Ag₂(pybz)₂(bpy)], 2. In 1, two monomers [Ag(pycz)(H₂O)] are combined together through Agπ, ππ and Ag(CC) interactions to form a dimer, with the distances of 3.34, 3.56 and 3.18 Å, respectively. In 2, the [Ag₂(pybz)₂(bpy)] units are held together via ππ (3.4–3.5 Å) interactions resulting in a 3D network with 1D open channels.     

Delocalization of the unpaired spin density in some niobocene complexes with σ-donor, π-acceptors

Extended Hückel calculations and qualitative MO have been used to analyze the EPR data relevant to the localization of the unpaired spin density in several paramagnetic niobocene complexes with σ-donor π-acceptors, acetylene, aldehyde, ketene, ketenimine, and carbon disulfide.     

Effets des substituants sur la conformation du cycle γ-lactonique : I. Dérivés de la γ-butyrolactone

The structural study of some γ-butyrolactones substituted (i) in position 2 (position ): C₄H₄O₂Br₂ (II) and C₄H₅O₂R [R = Oφ (III); R = OCOφ (IV); R = OH (V); R = Br (VI); R = Cl (VII)] or (ii) in position 3 or 4 (β or β′): C₄H₅O₂Cl (VIII and IX) has been carried out by using different techniques of physical chemistry. Crystallographic data analysis demonstrates that in the solid state, 2,2-dibromo-γ-butyrolactone, unlike the 2,2-diphenyl-γ-butyrolactone, adopts an “envelope” structure which is comparable to those of compounds (III) and (IV). Spectroscopic data relative to the methylene bending mode δ(CH₂) are interpreted for the dissolved state in terms of rigid (III, IV, V, IX) or exchanging (VI, VII, VIII) “envelope” forms. For and β halogenated derivatives (VI, VII, VIII), quantitative analysis of infrared spectra shows a pseudo-axial predominance in apolar solvents, as found by application of the PCILO method. Interpretation of NMR spectra recorded at 250 MHz (III, IV, V, VI) confirms the data obtained by vibrational spectroscopy.     

Interactive n→σ delocalizations that control an aqueous organic equilibrium

Hydrated acetaldehydes were condensed in D₂O with substituted alcohols and thiols to determine ΔG of hemiacetalization by ¹H NMR. Specific n→σ^* delocalizations in the alkoxy/alkylthio functionality of the product interact to influence n→σ^* delocalization in the hemiacetal functionality. Delocalization in the latter functionality controls ΔG.