Evidence for the formation of a triple radiative exciplex between excited aqua-uranyl(VI) and thallous ion

Results from steady-state and transient photokinetic and spectroscopic measurements, show the formation in aqueous solution of a triple radiative exciplex between excited UO²⁺₂ and TI⁺. Rate constants of exciplex formation and deactivation are evaluated The nature of bonding and the emittive properties of this species, of probable ¹(Tl·OUO-Tl)⁴⁺ structure, are discussed.     

Structure and stability of the HFF and FHF radicals

The structure and stability of the HFF and FHF isomers in their two lowest electronic states is studied by means of ab initio SCF and CI calculations employing a double-zeta basis plus various polarization functions. Both complexes can be essentially described by an HF dipole (exhibiting a standard HF bond length) attracting a fluorine atom, with an FF distance of 5.14 bohr calculated for the HF··F ground state and an H··F bond of 4.3 bohr for the most stable FHF species. The HF₂ radical is found to prefer a bent structure in its ²Σ⁺ - ²A' ground state with an extremely flat potential curve, while the first very low-lying excited state of HF₂ as well as both components of the ground (in this case ²Π) state of the FH··F complex are calculated to be linear. The symmetric FHF structure exhibits optimal bond lengths at approximately 2.1 bohr but is considerably less stable than the asymmetrical arrangement of nuclei. With respect to dissociation into HF + F the HF₂ form is found to be stable in both (²Σ⁺ - A') and ²Π states while the conformer FH··F shows a minimum only for the ²Π ground state; the exothermicity of the H + F₂ → HF + F reaction is calculated to be 104.6 kcal mol^?1 in good agreement with the experimental quantity of 102.5 ± 2.8 kcal mol^?1. Comparison with the negative ions HF₂^?and FHF^? is made whenever appropriate.     

The molecular structure of cyclic methylsiloxanes

A series of methylcyclosiloxanes from the trimer to the hexamer has been investigated by gas phase electron diffraction. The Si-O bond distance varies between 1.635 and 1.620 Å, the SiOSi angle has values from 132° to 149° within the series. The trimer is substantially planar, the best agreement for the tetramer was obtained with a model of S₄. symmetry. As a consequence of large amplitude vibrations, the larger rings do not possess a well-defined conformation. The results are discussed in relation to previous spectroscopic investigations.     

Studies of gas-phase clusters: the solvation of HNO3 in microscopic aqueous clusters

The formation and stability of aqueous clusters of nitric acid were studied utilizing a free-jet expansion technique, coupled with electron impact ionization mass spectrometry. Evidence for the onset of solvation at very small cluster sizes is presented.     

On m-changes in atom-rigid-rotor collissions: influence of an initial rotation

Reorientation of the angular momentum of a diatomic molecule in collisions with atoms is investigated using classical mechanics. A factorisation formula for cross sections for rotational excitation is given. The factorisation allows the calculation of the state-specific cross section d σ (j′ m′ ← jm)/dΩ once the m-averaged cross sections d σ (j″← 0 )/dΩ for all possible j″ are known. The approach is applied to the Na₂-He system.     

Surface-enhanced raman scattering as a function of molecule-surface separation

The dependence of Raman enhancement on molecule-surface separation is measured for pyridine on roughened Ag surfaces in UHV. Results follow predictions for local field enhancement by small Ag spheroid resonances by McCall, Platzman and Wolff, showing multi-layer enhancement. Isotopic studies confirm that several layers contribute to the enhanced Raman signal.     

Opening of 3-membered rings to 4π-electron systems : State correlation diagrams including excited states

For the cyclopropyl allyl anion conversion 1 and analog reactions of isoelectronics 2–4 state correlation diagrams are constructed assuming a two step process (ring opening yielding a biradical and a following rotation resulting in the 4π system). The conclusions are supported by MINDO/2 calculations of reaction paths and potential curves. The photochemical reaction possibilities are discussed.     

Kinetics of evaporation of some liquids by thermogravimetry

Latent heat of evaporation is one of the decisive factors in the design and operation of chemical plants [1], Brennan et al. [2] have shown that the heat of evaporation of a liquid and the activation energy of evaporation, E^evap_act are found, as expected, to be numerically similar. We present here some results on the kinetics of evaporation of five liquids (selected at random) based on TG data.     

Deuterium isotope effect in the T1 → S0 radiationless transition of propynal in the gas phase

The phosphorescence lifetimes of propynal-h₁ and propynal-d₁ have been measured at room temperature in the 40 mTorr-1 Torr pressure range The reciprocal of the zero-pressure lifetime (k₀) is (3.10 ± 0.05) × 10³ and (1.70 ± 0.04) × 10³ s^?1 for propynal-h₁ and propynal-d¹ For both compounds the rate constant for self-quenching between triplet and ground-state molecules is k_SQ = (1 2±007) × 10³ Torr^?1 s^?1 The deuterium isotope effect is attributed to the T₁ → S₀ radiationlcss decay, for which k^H_ISC/k^D_ISC = 2 4     

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.     

A method for polarizing protons by proton-chlorine rotating frame cross-relaxation in high magnetic fields

A new technique is demonstrated for polarizing protons in high dc magnetic fields. The method has been used successfully on polycrystalline samples at an arbitrary dc magnetic field strength without recourse to rotation of the sample or cycling to low or zero field.     

Additivity scheme in nmr chemical shifts of disubstituted pyridines

An additivity relationship for chemical shifts similar to that found in the literature for coupling constants has been tested in eleven disubstituted pyridines. The chemical shifts of these disubstituted pyridines were measured in different solvents at different concentrations and extrapolated to infinite dilution.In order to compute the “effects” of the different substituents, the chemical shifts of the corresponding monosubstituted pyridines and of pyridine were measured in the same solvents at different concentrations. These chemical shifts were extrapolated to infinite dilution.In most cases it was found that the extrapolated chemical shifts for disubstituted pyridines are in good agreement with those calculated using the additivity relationship. Only in a few cases was it found that the additivity scheme does not hold.     

Electronic absorption spectrum of methyl allenyl sulfide

The ultraviolet absorption spectrum of methyl allenyl sulfide CH₃SCHCCH₂ was recorded in solution from 30000–50000 cm^?1 in order to inquire further into the role of sulfur 3d valence-shell and 4s Rydberg-type atomic orbitals for the low-lying excited singlet states in unsaturated organic sulfides. It is shown that sulfur 3d atomic orbitals cannot be neglected for the interpretation of the low-energy excited singlet states of unsaturated organic sulfides.     

Rearrangement during cyclisation of some 3-(2-pyrrolyl) propionic acids and related compounds

Cyclisation of the title compounds leads to mixtures of the expected 4H-cyclopental[b]pyrrol-4-ones (1), the corresponding 6-ones (3) by a single rearrangement, and the cyclopenta[c]pyrrol-4-ones (4) by a double rearrangement, the proportions depending upon the substituents.The ¹H NMR spectra of 2-methyl-4H-cyctopenta[b]thiophen-6-one (3f) shows 6J long range $\text{CH}{}_3\text{CH}{}_2$ coupling, but this is absent in the corresponding pyrroles (3c,d). The ¹³C spectra of 1 and 3 cannot be interpreted on the basis of substituent chemical shifts in pyrroles and thiophens, and are clearly -CH = CH-X (X = NMe, NPh, S) bridged derivatives of cyclopent-2-enone.     

Limitations on mndo and mndo/ci computations of activation barriers

The ability of MNDO and MNDO/CI for predicting activation barriers of unimolecular reactions is tested for decompositions of methane, ethane, nitromethane, and methyl nitrite. MNDO is not useful for this purpose; MNDO/CI gives energy barriers in agreement with experiment to about ±10 $\text{kcal}\text{mole}$ for the types of reactions considered.     

Normal coordinates and potential energy distributions of methyl acetylene and some halogen substituted analogues

The normal coordinates and potential energy distributions of methyl acetylene, methyl-d₃ acetylene and some halogen substituted analogues (CH₃CCX; X = H, D, Cl, Br, I) have been calculated using previously determined force fields. For methyl chloro-acetylene, the chlorine-35,37 isotope frequency splitting has been calculated. In addition, for methyl acetylene, the harmonic frequencies were obtained and plotted for hypothetical molecules in which the mass of X varied from 1 to 1000 a.m.u. The frequencies for these molecules are compared and discussed.