The electronic structure of heteroaromatic molecules; ab initio calculations and photoelectron spectra for the isomeric-oxazoles and some -oxadiazoles

The UV photoelectron spectra are reported for isoxazole and 1,3,4-oxadiazole (He(I) region), oxazole and 1,2,5-oxadiazole (He(I) and He(II) regions). Marked fine structure is apparent on some of the spectra. The spectra have been assigned on the basis of ab initio calculations, and by comparison with the azoles and thiazoles. Variations in the lone pairs levels at O-, S- and N- across the azoles, oxazoles, thiazoles and azines are discussed. It is suggested that “through-bond” interactions are more widespread in the azines than previously noted.     

The electronic structure of 6a-thiothiophthen and related molecules: non-empirical calculations and reassignment of the photoelectron spectra

Non-empirical calculations of the electronic structure of the 2,5-dimethyl derivative of the title compound, and the isosteric molecules (1b–4b; 5a, b) formed by replacement of the 1- and/or 6-sulphur atoms by O or NMe are reported. A reassignment of the photoelectron spectra of the molecules in the light of these calculations, and using intensity changes between the unsubstituted and dimethyl derivatives, and substituent effects, shows substantial differences from those recently reported.     

A computer-based storage and retrieval system for electronic absorption spectra

A computer-based system for storage and retrieval of spectra in the ultra-violet region, with absorbance as the observed variable, is described. Differences in concentration between solutions relating to observed and stored spectra are automatically adjusted. The entire spectral curve is stored, not only selected points such as maxima etc.; this makes it possible to investigate the spectral range continuously during the retrieval procedure. The increment between two observations (absorbance values) is 2 nm on the wavelength axis.The input medium is punched cards containing values taken directly from observed (catalogued) spectra. The output is a printed list, giving information about observed and retrieved spectra, system parameters used during the retrieval procedure, etc. An important feature of the system is that displacements between observed and stored spectra are allowed for, in two dimensions during comparison, thus taking into account 'chromic' effects. Information about partial identity between observed and stored (retrieved) spectra is given, even if this partial identity exists in dislocated wavelength regions. To test the program, constructed (test) curves as well as real spectra were used. The system is capable of collecting in groups substances with common spectral characteristics, and thus substances belonging to the same class of chemical compounds. This makes the system a valuable tool not only for identification work but also for the study of spectral behaviour in general.     

UV photoelectron spectra of 5- and 6-azauracil

The He(I) photoelectron spectra of 5-azauracil (1,3,5-triazine-2,4(1H,3H)-dione) and 6-azauracil (1,2,4-triazine-3, 5(2H,4H)-dione) are presented. The spectra are assigned by using INDO/S calculations and comparison arguments with uracil. The first ionization energy in the title compounds is higher than in the usual nucleic acid bases.     

The electronic structure of heteroaromatic molecules; ab initio calculations and photoelectron spectra for the aza derivatives of benzo[c] furan,benzo [c] thiophen, N-methyl isoindole and related compounds

The HeI and II photoelectron spectra obtained for 2,1-benzisothiazole and 2,1,3-benzothiadiazole, and the HeI spectra for the corresponding O- and NMe-heterocycles have been satisfactorily interpreted, by means of a combination of variations between compounds and ab initio MO calculations; a comparison has been made with both the parent systems benzo[c]thiophen etc., and with the monocyclic compounds.     

Calorimetric and spectroscopic studies of the critical phase transition in (CH3NH3)2[SnCl6]

(CH₃NH₃)₂[SnCl₆] undergoes a critical phase transition at 154.32 ± 0.06 K with the critical exponents α = 0.36 ± 0.04, α' = 0.19 ± 0.02, the critical entropy ratio 0.062 and the total transition entropy (1.60 ± 0.18) R. Infrared spectral change favors the order-disorder mechanism of the transition proposed from the thermodynamic data.     

X-ray photoelectron spectra of inorganic molecules: XXX. Organometallic derivatives of tungsten(O) and tungsten(II)

The W 4f binding energies of various carbonyl-containing complexes of tungsten are reported. Comparisons between W^II complexes containing the [(η³-allyl)W(CO)₂] moiety and neutral derivatives of the type W(CO)_6-xL_x are made. The absence of a clear-cut correlation between the W 4f binding energies and the value of x in W(CO)_6-xL_x is discussed and these results compared with those obtained for the analogous molybdenum complexes.     

The relation between structure and reactivity in five-membered heteroaromatic compounds—II: CNDO/2 calculations on azolium cations and on the zwitterions resulting from their deprotonation

Olofson's group and Norris and Henry have reported proton exchange rates for a variety of azolium cations in aqueous solution. They found that ring protons in positions located α- to pyrrole-type N atoms exchange much faster than those located β- to such nitrogens, and that addition of pyridine-type nitrogen to the ring also caused a large increase in rate of exchange. This report describes the results of CNDO/2 calculations on azolium cations representative of those studied experimentally, and on the zwitterions resulting from the deprotonation of these cations. The calculated vapor-phase energies of deprotonation are in agreement with the structure-reactivity trends summarized above, but the calculated effect of added nitrogen is unexpectedly small relative to the effect of interchanging α- and β- pyrrole-type N atoms. The calculated charge distributions and one- and two-atom contributions to the calculated energies of deprotonation are analyzed in terms of classical organic mechanisms for transmission of substituent effects. The results of this analysis suggest that the relative reactivities of isomeric tetrazolium cations are determined primarily by coulombic effects, but that relative reactivities of isomeric positions in the imidazolium and pyrazolium series are apparently determined primarily by inductive and hybridization effects. π-Electron resonance (contributions from carbenoid resonance forms) is apparently not of overriding importance. When nitrogen is added to the ring in an α-position relative to the CH group undergoing exchange, the resulting increase in CH acidity seems to be due to inductive and hybridization effects, partially offset by a coulombic effect due to the negative charge on the added nitrogen. If classical σ-inductive and hybridization effects from an added β-nitrogen are at all influential, they seem much less so than from an added α-nitrogen; thus according to CNDO/2 these effects alone apparently cannot explain the observed large rate increase due to added β-nitrogen. The possible importance of solvent effects, delocalization of the added negative charge into the σ-framework and coulombic effects due to the added nitrogen are discussed.     

The infrared spectra and structure of acetamide and acrylamide

The far-IR vapour phase spectra of acetamide and acrylamide are reported and several transitions in the amido “inversion” vibration are identified. The potential energy curves governing the vibration are deduced from the experimental data and are found to have a single broad minimum for both molecules, implying that the amido hydrogens are copianar with the N-CO atoms, but that the resistance to their out of plane displacement is small. The band contours found in the mid-IR spectrum of acrylamide suggest that the cis orientation of the two double bonds about the C-C single bond is favoured over the trans orientation; indeed no evidence of the latter has been found.     

Non-empirical calculations on the conformational structure of azobenzene and benzylideneaniline

The conformational structures of cis- and trans-azobenzene and benzylideneaniline have been investigated by means of ab initio SCF calculations. Contrary to semiempirical results, the equilibrium molecular geometries are correctly accounted for in the non-empirical SCF-formalism. Trans-azobenzene is found to be planar, or at least peri-planar, while the phenyl rings of the cis-isomer are twisted by 56° out-of-plane. Both isomers of benzylideneaniline are non-planar, with rotational angles θ₁ (C-N) = 48°, θ₂(C-C) = 0° and θ₁ = θ₂ = 75° for the trans and cis form, respectively. Trans-azobenzene is calculated to be more stable by 10.4 kcal mol^?1 than the cis isomer, which is in good accord with the experimental value of 10 kcal mol^?1. The energy of isomerization of benzylideneaniline amounts to 13.0 kcal mol^?1.     

Vibrational spectra and structure of trinitromethane

The vibrational spectrum of trinitromethane was interpreted in terms of the additive interatomic interaction model on the basis of experimental infrared and Raman spectra of HC(NO₂)₃, DC(NO₂)₃, HC(¹⁵NO₂)₃ and normal coordinate analysis. The frequency assignment results were used in discussing its structure. It was shown that the symmetry of trinitromethane is below C₃ in the liquid state.     

The electronic structure of some heterocycles with bridgehead nitrogen: photoelectron spectra and ab initio molecular orbital calculations

He(I) and He(II) photoelectron spectra are reported for the cycl[3,3,3]azine (1), cycl[3,2,2]azine (2), indolizine (6) and imidazo[1,2-a] pyridine (7), as well as He(I) spectra for related compounds (3–5). Ab initio molecular orbital calculations have been used to assign the spectra of 1, 2, 3, 6 and 7, and to give information about the nature of the π-electron energy levels. The first IP for 1 is singularly low (5.86 eV), and this has been interpreted in terms of occupancy of the 1a₁'' orbital which is normally vacant in related compounds. In the cyclazines, the nitrogen lone pair seems to be split into two π-levels.     

X-ray photoelectron spectra and electronic structure of rare-earth orthovanadates

Photoelectron spectra of 4d and valence states in RVO₄ (R = Y, Nd, Eu, Gd, Tb, Dy, Yb) have been investigated. The experimental spectra are interpreted using the results of the Xα discrete variational method calculations for orthovanadates. Transformations of electronic structure and covalency in the RVO₄ series are discussed. It is shown that lanthanide 4f orbitals significantly mix with the O 2pAO's and hybridize with the rare-earths 5pAO's. The 5p levels spin-orbital splitting in orthovanadates has been evaluated.     

On the nature of excited electronic states in cyanine dyes: implications for visual pigment spectra

CNDO/S CI calculations are carried out on polyenes and on cyanine dyes. In contrast to polyenes, doubly excited configurations have a strong effect on the first optically allowed excited state in cyanines. Protonated Schiff bases of retinal are closely related to cyanine dyes, with important consequences for models of visual pigment spectra and photochemistry.     

The synthesis of barbaralyl systems via bicyclic[3.2.2] irontricarbonyl cations

Reaction of bicyclo[3.2.2] tertiary alcohols 7b,c and 23b with Fe₂(CO)⁹, resulted in the corresponding dienyl Fe(CO)₃ complexes, which upon HBF₄/Ac₂O treatment yielded the bicyclo [3.2.2] dienyl irontricarbonyl cations. Nucleophilic addition of CN^- to those cations, resulted in the formation of δ,μ-bonded complexes, which were degraded with Me₃NO to give substituted tricydic hydrocarbons (barbaralyl systems) in unusual positions. For purpose of comparison, the trifluoroacetolysis of 2-methyl-endo-6,7-benzobicyclo[3.2.2] nonatriene (22) has been studied.     

Energy ordering of the B and C states in XeCl,XeBr, and KrCl,from temperature dependence of emission spectra

Inert-gas halide emission spectra from a Tesla discharge are studied as a function of pressure and temperature. From the temperature dependence of the infinite pressure ratio of broad-band (C → A and B → A) emission to B → X emission, the energy separations, T_eC - T_eB, are found to be ?130 cm^?1 (XeCl), ?80 cm^?1 (XeBr) and 375 cm^?1 (KrCl). Estimates of the (C → A)/(B → X) spontaneous emission branching ratios agree well with theoretical predictions.     

The orthosomycin family of antibiotics—iii : Mass spectral studies of flambamycin and its degradation products

Mass spectral evidence for the constitution (54) of the antibiotic, flambamycin, is summarised in Schemes 1–10. An analysis of all the evidence given in Parts I, II and III for the location of the two ortho-ester groups flambamycin and its derivatives is presented.     

Preparation and properties of an antibody-selective membrane electrode

A membrane electrode with selective response for the antibodies of the antigenic determinant dinitrophenol (DNP) has been devised by incorporating DNP—ion carrier conjugates in a polyvinyl chloride membrane matrix. Details of the preparative and immobilization procedures are provided along with an evaluation of optimum solution conditions for electrode use. The response mechanism of the antibody electrode is postulated to involve a “selectivity shift” effect.