Scott Brownridge

Efficient calculation of electron paramagnetic resonance g-tensors by multireference configuration interaction sum-over-state expansions, using the atomic mean-field spin–orbit method

Using the multireference configuration interaction method due to Grimme and Waletzke, combined with the atomic mean-field approximations for the efficient calculation of spin–orbit matrix elements, the g-tensors in second-order perturbation theory have been calculated for the main group radicals CO+, CN, BO, BS, MgF, AlO, O2, HCO, H2O+, NO2, CO2−, NF2, NO22−, O3−, ClO2, and H2CO+, and for the transition metal compounds ZnH, ZnF, and TiF3, using explicit sum-over-state expansions for up to 20 excited states. In most cases, a valence triple-zeta basis set with polarization functions has been employed. It is shown that the addition of diffuse functions to this basis set does not improve the g-tensor results, and in several instances leads to slower convergence of the sum-over-state expansion. The calculated g-tensors are in good agreement with experimental values, and with our previous multireference configuration interaction results available for 9 of the 19 radicals. Our results are shown to be equivalent ...

Preparation, structural and spectroscopic characterisation of the salts M3X3AF6(A = As or Sb, M = S or Se, X = Cl or Br) containing the novel sulfur– and selenium–halogen cations (X2MMMX)+

The salts X2MMMX(AF6)(A = As or Sb, M = S or Se, X = Cl or Br) were prepared quantitatively by the reaction of stoichiometric amounts of MX3AF6 and M or from stoichiometric amounts of M, X2 and AsF5(M = S or Se; X = Br) in liquid SO2. They have been characterised by elemental analysis, single-crystal X-ray diffraction, Fourier-transform (FT)-Raman and 77Se FT-NMR spectroscopy. The crystal structures of X2MMMX(AsF6) consist of (X2MMMX)+ cations and AsF6– anions. The structure of the (X2MMMX)+ cation is dominated by an intracationic halogen–chalcogen contact and M–M bond alternation giving rise to a short M–M bond distance indicative of thermodynamically stable npπ–npπ(n= 3 or 4) bonds. Since the structure of these cations is different from those of (YM)2MY+(Y = Me or C6F5), theoretical calculations were performed to understand these differences and the bonding in these cations. In the X2SSSX(AsF6) salts (X = Cl or Br) the structures of the cations are disordered and therefore exact bond distances could be not obtained. However, bond distances were estimated from their FT-Raman spectra and supported by molecular orbital calculations. The FT-Raman spectrum of Se2Br5AsF6 is reported.

Reversible Dissociation of (Se6I2)(AsF6)2 in SO2(I)

The dissociation of (Se6I2)(AsF6)2 in SO2(1) has been investigated by 77Se NMR spectroscopy at –80 °C involving both a natural-abundance sample as well as that containing selenium enriched in the 77Se-isotope (enrichment 92 %). In addition to the previously characterized cations Se6I2 2+, SeI3 +, 1,1,4,4-Se4I4 2+, Se10 2+, Se8 2+, and Se4 2+, the NMR spectra of the equilibrium solution exhibited additional resonances the assignment of which has been carried out by 77Se-77Se COSY, selective irradiation experiments, and spectral simulation. Combining this information with the trends in the chemical shifts as well as with iodine, selenium, and charge balances that were calculated from quantitative integrated intensities, the presence of cyclic cations Se7[+ and 1,4-Se7I2 2+, as well as acyclic Se2I+, and 1,1,6,6-Se6I4 2+(two rotamers) could be inferred. Upon evaporation of the solvent only Se6I22+ was found to be present in the solid state.

Preparation and characterization of 1,3,2-dithiazolidine and 1,4-dithia-7-azabicyclo[2.2.1]heptane cations, and a mechanistic study of the cycloaddition reactions of alkenes with SNS

The cation SNS+(as the AsF6– salt) underwent quantitative concerted symmetry-allowed cycloaddition reactions with alkenes [C2H4, trans- and cis-MeHCCHMe, H2CCMe2, MeHCCH2, Me2CCMe2 and norbornene (bicyclo[2.2.1]hept-2-ene)] to give 1,3,2-dithiazolidine cations 1, which in a second quantitative concerted symmetry-allowed cycloaddition reaction with another alkene molecule gave 1,4-dithia-7-azabicyclo[2.2.1]heptane cations 2(with the exception of Me2CCMe2). The cycloadducts were characterized by elemental analyses and IR and NMR (1H, 13C, 14N) spectroscopies. The vibrational spectra were assigned with the aid of frequencies obtained by ab initio(RHF/6–31G*) calculations. When alkene = C2H4 the calculated geometry of 2 was in good agreement with that obtained from its crystal structure reported previously; that of 1 correlates well with the experimental data (IR, Fourier-transform Raman, NMR). Kinetic studies showed that the rate constants of the first cycloaddition of SNS+ to C2H4 are comparable with those of nitrile and alkyne cycloadditions, indicating that the cycloaddition proceeds via the interaction of the highest occupied molecular orbital of the alkene and the lowest unoccupied one of SNS+ as was previously observed for various nitriles and alkynes. The second cycloaddition leads to stereospecific 2, except for H2CCHMe. Contrary to the prediction of a simple frontier molecular model, the rate of the second cycloaddition was faster than the first for C2H4, cis-MeHCCHMe, and H2CCMe2 and strongly dependent on the steric activity of the alkene. It is proposed that the second cycloaddition likely occurs via a concerted and synchronous pathway.

Recent Developments in the Synthesis and Characterization of the Neutral Diradical Bis(1,3,2,4-dithiadiazolyl)

Abstract It has been shown1 that SNS+ undergoes cycloaddition with various nitriles to give 6π 1,3,2,4-dithiadiazolium salts that give 7π 1,3,2,4- dithiadiazolyl radicals on reduction. SNSAsF6 reacts with cyanogen (NC-CN) to produce which on reduction forms the neutral species (1). The X-ray single crystal structures of the related and show the two rings of each dication to be coplanar. To date, we do not have a crystal structure of the title compound, but spectroscopic and computational evidence indicates that the two rings of the neutral are also coplanar.

Solid-State and Solution FT-Raman Spectra of the Tetraiodine Dication I42+: Implications for the Claimed Existance of ‘T+’ in SO2Solution

Abstract I4 2+ is the only known cyclic homopolyatomic cation or anion of iodine. It has a rectangular planar structure which may be thought of as containing two I2 + units joined by a weak π∗-π∗ 4 centre 2 electron bond.1 In this paper we report our FT-Raman spectra of I4 2+, which conflict with those published by Gillespie et al. 1 and present evidence that the peaks attributed to a species containing iodine in the +1 oxidation state are in fact due to I4 2+.