The lowest triplet state of benzene: Differences in electronic structure in two crystalline modifications of a cyclohexane host

Microwave induced delayed phosphorescence (MIDP) measurements have been performed in the 00 bands of the phosphorescence spectrum of benzene dissolved in two phases of cyclohexane. From the relative radiative rates for decay of the three zero-field levels it is concluded that on the average the symmetry of the electronic structure is lower than D_2h. In the monoclinic low temperature stable phase of cyclohexane (phase II) the structure is approximately antiquinoidal, and in the metastable phase III it is approximately quinoidal.     

Conversions between ordered and disordered cellulose. Effects of mechanical treatment followed by cyclic wetting and drying

An investigation into the effects of mechanical treatment and hydration on the order of cellulose substrates (microcrystalline cellulose and Cladophora cellulose) was performed by the use of ball milling followed by cyclic wetting and drying. The results, monitored by¹³C-CP/MAS NMR-spectroscopy, were evaluated by calculation of the crystallinity indices and principal component analysis of the NMR data acquired. The results showed that a large part of the disorder induced by the mechanical treatment of cellulose by ball milling is reversible and reordering upon hydration leads to the cellulose I form initially present. The C4 signals corresponding to the reversibly disordered cellulose chains are observed in the amorphous region between 79 and 86 ppm in the¹³C-CP/MAS NMR-spectra together with signals from cellulose chains on the surface of ordered regions. The peak cluster which contains the C2, C3 and C5 ring carbons can be divided into two specific spectral regions; one between 74 and 77 ppm largely originates from ring carbons within disordered cellulose structures, and one between 70 and 74 ppm contains larger contributions from ordered cellulose. The behaviour of the celluloses upon milling is in accordance with a concept of ordered cellulose fibrils containing amorphous cellulose mainly as surface layers and induced reversible lattice distortions.     

Cyclophanes—13: Crystal and molecular structure of [2.2][2,5]furano(1,4)naphthalenophane

The crystal and molecular structure of [2.2](2,5)furano(1,4)naphthalenophane (1) was determined by X-ray crystallography. The molecule exists in the anti-conformation and the study represents the first instance in which the structural features of a naphthalenoid ring within a cyclophane were determined. Crystals of cyclophane 1 are orthorhombic, space group Pbca, with a = 7.859(2). b = 11.482(3) and c = 28.818(8) Å. While the nonbridged portion of the naphthalenoid ring is planar, the portion which is bridged to the furanoid ring through its 1 and 4 C atoms is puckered and boat-shaped. These C atoms are positioned 14° out of the plane of the other four C atoms of this ring. The furanoid ring is essentially planar but is not parallel to the naphthalenoid ring. It is inclined 22° to the least squares plane of the bridged portion of the naphthalenoid ring. This angle of inclination staggers the atoms of the furanoid and bridged naphthalenoid ring and positions the 3 and 4 C atoms, the 2 and 5 C atoms and the 0 atom of the furanoid ring 3.4. 2.9 and 2.6 Å. respectively, from the least squares plane of the bridged portion of the naphthalenoid ring. While the internal angles around the bridging C atoms α- to the naphthalenoid ring are 109°, those α- to the furanoid ring are 113°. In addition unusually large bond angles ($\text{\$}\text{?}$137°) at the 2 and 5 C atoms of the furanoid ring, external to the ring, are also observed. The distortions are considered with respect to the strain within the cyclophane macrocycle and are compared with other similar systems.     

On the question of allene formation from tricyclic cyclopropylidenes

The carbenoid or carbene derived from $\text{8}$ undergoes dimerization or trapping with DPIBF. The allene mechanism previously proposed was made doubtful by the finding that stereoisomeric carbenes

and

give stereoisomeric products without crossover.     

Synthesis and characterization of the non-Kekulé, singlet biradicaloid Ar'Ge(micro-NSiMe(3))(2)GeAr' (Ar' = 2,6-Dipp(2)C(6)H(3), Dipp = 2,6-i-Pr(2)C(6)H(3))

Reaction of Ar'GeGeAr' (1) with an excess of Me3SiN3 gives the non-Kekulé, biradicaloid Ar'Ge(mu-NSiMe3)2GeAr' (3, Ar' = 2,6-Dipp2C6H3, Dipp = 2,6-i-Pr2C6H3) which has a planar Ge2N2Si2 array and pyramidal geometry at the germaniums. DFT calculations for the model MeGe(mu-NSiH3)2GeMe indicate no Ge-Ge bonding and a singlet ground state. The calculated energy difference between the optimized singlet and triplet states is 17.51 kcal/mol.     

The [2 + 4] Diels-Alder cycloadditon product of a probable dialuminene,Ar'AlAlAr' (Ar' = C(6)H(3)- 2,6-dipp(2); dipp = C(6)H(3)-2,6-Pr(i)(2)), with toluene

Reduction of Ar'AlI2 (Ar' = Ar'= C6H3-2,6-Dipp2; Dipp = C6H3-2,6-Pri2) with KC8 in diethyl ether most probably affords the first "dialuminene", Ar'AlAlAr'; it was characterized by its reaction with toluene which yielded a [2 + 4] cycloaddition product incorporating the Ar'AlAlAr' unit.     

Nucleic acid related compounds. 31. Smooth and efficient palladium-copper catalyzed coupling of terminal alkynes with 5-iodouracil nucleosides

Coupling of terminal alkynes with protected 5-iodouracil nucleosides in the presence of dichlorobis(triphenylphosphine)palladium and copper(I) iodide in triethylamine gives the corresponding 5-(alkyn-1-yl)uracil nucleosides in 72–92% yields.     

High frequency titrations involving chelation with ethylenediaminetetra-acetic acid. II. quantitative determination of some divalent metals

The high frequency oscillator provides an excellent means of detecting the end-points of titrations performed with ethylenediaminetetraacetic acid or its salts. The determination of various divalent metals is reported, based on both direct and indirect titrations. Because of the great sensitivity of the method it is possible to determine cobalt, nickel, copper (II), zinc, cadmium, lead and manganese (II) in concentration ranges of 1/1000 to 1/6000M.     

Di‐2‐pyridyl ketone N4,N4‐(butane‐1,4‐diyl)thiosemicarbazone

The title compound, C₁₆H₁₇N₅S, is in the thione form and crystallizes with two independent mol­ecules in the asymmetric unit. In both mol­ecules, the penta­methyl­ene­imine five‐membered ring adopts an envelope conformation, and in one of the molecules this ring shows positional disorder. The thione S and hydrazine N atoms are in the Z configuration with respect to the C—N bond.     

Spherical coordinate representations of solvent composition for liquid chromatography method development using experimental design

One of the major techniques used for the method development of ternary and quaternary high performance liquid chromatography (HPLC) systems has been to use mixture designs, often referred to as "Glajch's Triangle". This technique does not allow for the systematic and simultaneous optimization of other factors such as gradient time, pH and temperature that affect the quality of separations. An alternative approach is to use experimental designs. The condition, however, that the composition of all components of the mobile phase must total 100% presents a problem when trying to mathematically represent ranges of each mobile phase constituent of a ternary or quaternary system. A method is described here, based on spherical coordinate representations, that adheres to the constraints of the mobile phase composition and allows experimental designs, such as central composite and factorial designs, to be applied to the simultaneous optimization of the mobile phase composition. Other factors, in particular temperature and gradient time, can then be included in the design. As a result of applying these designs to the HPLC separation of phenols and corticosteroids, it was found necessary to include three-way interactions between experimental factors in the model. The significance of these interactions shows that they need to be considered in HPLC method development.