Towards the understanding of the spectroscopic behaviour of the C–H oscillator in C–HO hydrogen bonds: the effect of solvent polarity

The effect of solvent polarity versus specific C–HO contacts on the vibrational νC–H mode is studied using CHCl₃ as a model system. Ab initio SCI–PCM calculations show that the overall shift of the νC–H band, sometimes ascribed to the C–HO hydrogen bonding, can in fact be explained by the electrostatic interaction with a dielectric environment. The presence of a new νC–H band – assigned to the C–HO bonded forms – remains as the most reliable evidence of C–HO hydrogen bonding.     

Synthesis and catalytic properties in olefin epoxidation of dioxomolybdenum(vi) complexes bearing a bidentate or tetradentate salen-type ligand

The monomeric cis-dioxomolybdenum(VI) complexes [MoO₂(oep-saldpen)] and [MoO₂Cl₂(oep-H₂saldpen)], with a tetradentate [N₂(imine)O₂] and bidentate [N₂(imine)] salen-type ligand functionalised with two pyrrole derivative pendant arms [oep-H₂saldpen = 1,2-diphenylethylenebis(3-oxyethylpyrrole)salicylideneimine], were synthesised and characterised by ¹H NMR, IR and Raman spectroscopy. The solid-state structure of the free ligand oep-H₂saldpen was determined by single crystal X-ray diffraction. Assignment of the vibrational spectra of the molybdenum complexes was supported by carrying out ab initio calculations for the possible isomers using [MoO₂(salen)] and [MoO₂Cl₂(H₂salen)] as model compounds [H₂salen = N,N′-ethylenebis(salicylideneimine)]. The oep-saldpen complexes were examined as catalysts for the epoxidation of cyclooctene, (R)-(+)-limonene, styrene, α-pinene, and cis and trans-β-methylstyrene, with tert-butyl hydroperoxide as the oxidant. Both complexes exhibited high selectivity for the epoxidation reaction, with the bis(chloro) complex being always the more active of the two.     

Synthesis and catalytic properties in olefin epoxidation of chiral oxazoline dioxomolybdenum(VI) complexes

Dioxomolybdenum(VI) complexes with the general formula [MoO₂X₂(N,N)] (X = Cl, OSiPh₃) containing a chiral bidentate oxazoline ligand (N,N = 2,2′-bis[(4S)-4-benzyl-2-oxazoline]) have been prepared and characterised by ¹H NMR, IR spectroscopy and thermogravimetric analysis. The bis(chloro) complex was heterogenised in the ordered mesoporous silica MCM-41 by direct grafting in dichloromethane. Elemental analysis and ²⁹Si MAS NMR spectroscopy of the derivatised material indicated the presence of monopodally anchored species of the type MoO₂[(–O)₃SiO]Cl(N,N). The complex [MoO₂Cl₂(N,N)] and the derivatised material exhibited initial activities of 147 and , respectively, in the catalytic epoxidation of cyclooctene using tert-butylhydroperoxide (tBuOOH) as the oxidant, both yielding 1,2-epoxycyclooctane quantitatively within 24 h at 55 °C. The MCM-41 grafted catalyst could be recycled with no loss in performance with respect to the epoxide yields obtained for reaction times above 2 h. With trans-β-methylstyrene as the substrate, the bis(chloro) complex and the derivatised material gave epoxides as the only products with yields in the range of 56–64% after 24 h, but no catalytic asymmetric induction was observed. The triphenylsiloxy complex was more active than the bis(chloro) complex for the epoxidation of trans-β-methylstyrene, but the enantiomeric excess was negligible and the corresponding diols were also formed. For the reaction catalysed by the supported material, changing the oxidant from tBuOOH to cumene hydroperoxide greatly improved the catalytic activity but the enantiomeric excess continued very low and the corresponding diol was the main product.     

β-Cyclodextrin Complexes of Benzaldehyde,Vanillin and Cinnamaldehyde: A Raman Spectroscopic Study1

Abstract

Raman spectra of benzaldehyde, vanillin and trans-cinnamaldehyde, and of their complexes in β-cyclodextrin are presented and discussed. The guest molecules provide both an aromatic ring for privileged interaction with the cyclodextrin cavity, and characteristic vibrations (e.g., vC=O, vC=C, vCC? (?=phenyl), vC-H?) with group frequencies in spectral regions free from cyclodextrin bands which are useful for probing the guest perturbed by complex formation with β-cyclodextrin. In particular, the C=O bond is shown to be a good vibrational spectroscopic probe to monitor the effects of both the medium, through its dielectric constant, and of specific interactions, namely of the hydrogen bonding type. The first of these effects is evaluated theoretically and a Kirkwood-Bauer-Magat plot for benzaldehyde is used to estimate the effective dielectric constant for the guest in the benzaldehyde-β-cyclodextrin complex. The above mentioned Raman spectroscopic results are interpreted and discussed.     

New high-energy luminescence bands from Co2+ in ZnSe

Three sharp absorption features in the energy range 2.36–2.55 eV have been detected in the transmission spectrum of Co-diffused ZnSe, and a number of luminescence transitions originating from the lowest of these states at 2.361 eV have been observed. Photoluminescence excitation spectra prove that these are high energy excited states of the Co²⁺_Zn impurity, a conclusion confirmed by comparison of measured and predicted luminescence energies. This represents the first identification of luminescence branching from a higher excited state of a transition metal ion in any semiconductor. The sharp, weakly phonon-coupled transitions involve either intra-impurity excitation or transitions from the impurity to localised states split off from a minimum in the conduction band. The implications of these observations for the mechanism of host-impurity energy transfer and for the nature of the excited state wavefunctions are discussed.     

Z4-Kerdock Codes, Orthogonal Spreads, and Extremal Euclidean Line-Sets

When m is odd, spreads in an orthogonal vector space of type⁺(2m + 2,2) are related to binary Kerdock codes and extremalline-sets in 2^{m + 1} with prescribed angles. Spreads in a 2m-dimensionalbinary symplectic vector space are related to Kerdock codesover Z₄ and extremal line-sets in with prescribed angles. These connections involve binary, realand complex geometry associated with extraspecial 2-groups.A geometric map from symplectic to orthogonal spreads is shownto induce the Gray map from a corresponding Z₄-Kerdock codeto its binary image. These geometric considerations lead tothe construction, for any odd composite m, of large numbersof Z₄-Kerdock codes. They also produce new Z₄-linear Kerdockand Preparata codes. 1991 Mathematics Subject Classification:primary 94B60; secondary 51M15, 20C99.