Paul Brint

The electronic spectra and magnetic circular dichroism of the uranyl ion

The MCD and absorption spectra of (Bu4N)UO2(NO3)3 have been measured over the range 18 000—33 000 cm-1 in a rigid polymer matrix at ∼10 K. A large number of sharp peaks are observed including structure in the high energy region previously referred to as the ‘structureless continuum’. It is shown that group theory places some rather strict limitations on the electronic states and the possible transitions between them, and we conclude that the most probable orbital ordering is e 1g < a 1g < a 2u < e 1u and that the low energy bands arise from transitions to the triplet states 3 E 1g and 3 E 2g . The higher energy band structure we assign as due to transitions to 3 E 1u . We show that second order spin-orbit interaction is necessary to account for the unsymmetric splitting of the triplets. Furthermore it is demonstrated that the symmetry D ∞h will not account for the intensity of absorption and MCD of the observed transitions and that the effective molecular symmetry is D 3.

Crystal structure and electronic properties of ammine[tris(2-aminoethyl)amine]copper(II) diperchlorate and potassium penta-amminecopper(II) tris(hexafluorophosphate)

The crystal structure of the title compounds [Cu(tren)(NH3)][ClO4]2(1) and K[Cu(NH3)5][PF6]3(2) have been determined by X-ray diffraction methods using three-dimensional diffractometer data; the structures were solved by heavy-atom techniques and successive Fourier synthesis. Compound (1) crystallises in a cubic unit cell, space group P213, with dimensions a= 11.626(3)A and Z= 4, and (2) crystallises in the orthorhombic unit cell, space group Imma with dimensions, a= 14.90(1), b= 11.79(1), c= 10.57(1)A, and Z= 4. The cation in (1) has a trigonal-bipyramidal molecular structure with strict C3 symmetry while that in (2) has a square-based pyramidal molecular structure with strict C2v symmetry. The polycrystalline and single-crystal electronic properties for these two σ-bonding CuN5 chromophores are reported and related to the one-electron energy levels for the two stereochemistries, using extended-Huckel molecular-orbital calculations, and correlated with earlier data on high-symmetry five-co-ordinate CuN5 chromophores.

Vacuum-ultraviolet absorption spectra of propanone, butanone and the cyclic ketones CnH2n–2O (n= 4, 5, 6, 7)

The high-resolution vacuum-ultraviolet absorption spectrum of propanone is reported and analysed in terms of Rydberg series of s, p and d type. The longest series is the s-series observed to n= 17 allowing the ionisation potential to be accurately determined at 78 295 ± 5 cm–1. A number of extensions and corrections to previous work are reported. The spectra of butanone and the cyclic ketones are similarly analysed and extensive Rydberg structure is identified in all spectra except that of cyclobutanone. For cyclopentanone and cyclohexanone long d-series allow accurate determination of the ionisation potentials at 74 800 and 73 794 ± 5 cm–1, respectively. Comparison of the spectra exposes systematic trends in the number of series of p- and d-types and in their relative intensities. The trends are related to the symmetry of the molecules and the location of the geometric origin of the Rydberg orbital set at the centre of charge of the molecular cation. The n= 3 transitions are distinct from all others in showing extensive vibronic structure. The absolute intensities of these transitions are discussed and the vibronic structure analysed. The spectra are discussed in terms of a general description of molecular Rydberg states.

A Gaussian 80 (6-21G) study of the species SiHn (n = 1–4) and SiHm+(m = 1–5): Some comments on the electron impact mass spectrum of silane

Abstract Gaussian 80 (6-21G) calculations were performed in both HF and CI modes on the species SiHn (n = 1–4) and SiHm+ (m = 1–5). In general the agreement between calculated and experimentally determined properties such as the structures of SiHn molecules and the ionisation potentials of SiH4 were very good. The present results were compared with those previously published. The production of the ions SiH3+ and SiH2+ in the electron impact mass spectrum of silane was also analysed.

Multiphoton ionisation spectroscopy of the 3s(2 + 2, 1) and 4s(3 + 1) Rydberg states of acetone: evidence for a molecular valence state at 153 nm

The resonance-enhanced multiphoton ionisation spectra of a molecular beam of acetone seeded in helium in the region of the 3s Rydberg state shows clear evidence of rotational and vibrational cooling of the compound. The very large spectroscopic width of the 3s adsorption band in photoabsorption spectroscopy is entirely due to thermal effects and not to interaction with an underlying molecular valence state. The vibrational modes active in the spectrum are all localised on the methyl groups of the molecule, which is explained in terms of the location of the 3s Rydberg orbital of the molecule. The REMPl spectra of the 4s Rydberg state shows no enhancement of resolution when the sample is cooled and the presence of a valence state centred at 153 nm is established. This is the first valence state found at energies higher than the UV (noπ*) state. Very sharp structure at 185 nm is attributed to a triplet state, probably associated with one of the 3p Rydberg states.

The vacuum ultraviolet absorption spectrum of formaldehyde

The very high resolution vacuum ultraviolet absorption spectrum of formaldehyde is reported. Rydberg series of s-, p-, d-and f-type are assigned and rotational structure (observed only in the f-series) is illustrated. The observation of these series is interpreted in terms of the molecular potential strongly affecting all Rydberg orbitals which makes molecular symmetry (rather than atomic structure) the determinator of allowedness. The rotationally structured, non-predissociated f-series is explained by comparison with similar series in atomic spectra and a related ‘giant resonance’ is assigned in the ionisation continuum. This is the first such resonance assigned in the valence ionisation region of a molecular spectrum.

Bonding in clusters. Part 3. Protonation of nido-pentaborane(9), nido-hexaborane(10), and closo-hexaborate(6)(2–)

The bonding and structures of B5H9, B6H10, and B6H62– and the protonated species B5H10+, B6H11+, B6H7–, and B6H8 are analysed with MNDO, Gaussian-80, and self-consistent charge calculations. The known isomer of B6H10 is shown to be more stable than other isomers for which metalloborane analogues are known. The bonding in B5H10+ can be regarded as a B5H8+⋯ H2 complex but B6H11+ prefers a structure with six B–H terminal and five B–H–B bridged bonds. Protonation of B6H62– gives a face-capped B6H7– structure. Diprotonation of B6H62– produces a molecule in which the B6 octahedral skeleton is entirely disrupted and the molecule is predicted to be highly unstable. Attention is drawn to some of the observed differences in the chemistries of related borane, metalloborane, and metallo-clusters.

Evidence for Rydberg Doorway States in Photoion Pair Formation in Bromomethane

The vacuum ultraviolet laser-excited photoion-pair formation spectrum of CH 3Br has been measured under high resolution in the threshold region. The (2 + 1) and (3 + 1) resonance-enhanced multiphoton ionization spectra in the same energy region are also reported. By comparison of the spectra in this and a more extended region, resonances in the photoion-pair formation spectrum are assigned to p and f Rydberg states. It is concluded that all the structure in the photoion-pair formation spectrum near threshold can be accounted for by members of the Omega = 0 subset of Rydberg states that act as doorway states to the ion channel.

Vacuum ultraviolet absorption spectrum of p-benzoquinone

The vacuum ultraviolet absorption spectrum of p-benzoquinone measured by photometric and photographic techniques is reported. The valence transitions are assigned and the Rydberg absorptions analysed showing that both p and d series are present. This observation is interpreted as showing that the highest occupied molecular orbital of the molecule is localised on the ketone groups and that molecular core structure has to be explicitly considered in accounting for the series. Oscillator strength data are reported and shown to support the ketonic character of the Rydberg series.

An ab initio study of the ground and excited states of HPO

Abstract A systematic theoretical characterization has been performed for the molecular structure, vibrational and rotational constants and electronic propertie