Spin-symmetrised structures and vibrational frequencies of iron-sulfur clusters.

Calculations of relaxed geometries of multi-centre transition metal compounds are routinely carried out using Broken Symmetry Density Functional Theory. The resulting low-spin open shell electronic state is described by one single Slater determinant and is affected by spin contamination. To alleviate this symmetry breaking, the Extended Broken Symmetry (EBS) approach can be applied to complexes with an arbitrary number of local high-spin metal ions. The actual symmetry is therefore reconstructed through minimization of an effective Hamiltonian leading to a relaxed geometry consistent with the magnetic couplings. In the present work we extend the approach already introduced by [Chu et al., J. Chem. Theory Comput., 2017, 13, 4675] to the calculation of vibrational frequencies. As prototypes we have considered the iron-sulfur clusters Fe2S2Cl42- and Fe4S4Cl4. We have compared the results obtained for different spin states (high spin, broken symmetry and extended broken symmetry) and by using different DFT functionals (B3LYP, OPBE, BP, M06 and B2PLYP) and a post-HF method (SCS-MP2). The data have shown that for specific vibrational modes the EBS technique produces shifts up to 40 cm-1 with respect to the routinely used Broken Symmetry approach, indicating that the use of a consistent spin-symmetrised state is a crucial ingredient for an accurate description of vibrational properties, as certified by the comparison with the experimental data for the Fe2S2Cl42- cluster.

New Experimental Evidences Regarding Conformational Equilibrium in Ammonium-Bis(trifluoromethanesulfonyl)imide Ionic Liquids.

The X-ray scattering patterns of the two ionic liquids, N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide (TMPA-TFSI) and N-trimethyl-N-hexylammonium bis(trifluoromethanesulfonyl)imide (TMHA-TFSI), sharing a common anion and differing in the length of the alkyl chain of the cation, were measured at room temperature. Molecular dynamics calculations supported the interpretation of the data. The two force-field models, GAFF and DLPOLY 4, were adopted to simulate the scattering patterns. Both of them are able to reproduce the main peaks of the experimental data; however, the DLPOLY model seems to better reproduce the finer details. Moreover, from the simulations, the concentration of the two conformers of TFSI are derived. The comparison with previously reported infrared spectroscopy measurements suggests that also under this aspect the DPOLY model has a better agreement with the experiments.

Two different models to predict ionic-liquid diffraction patterns: fixed-charge versus polarizable potentials.

This study reports the performance of classical molecular dynamics (MD) in predicting the X-ray diffraction patterns of butylammonium nitrate (BAN) and two derivatives, 4-hydroxybutan-1-ammonium nitrate (4-HOBAN) and 4-methoxybutan-1-ammonium nitrate (4-MeOBAN). The structure functions and radial distribution functions obtained from energy-dispersive X-ray diffraction spectra, recorded newly for BAN and for the first time for 4-MeOBAN and 4-HOBAN, are compared with the corresponding quantities calculated from MD trajectories, to access information on the morphology of these liquids. The different behavior of two force fields, a polarizable multipole force field and a fixed-charge one supplemented by an explicit three-body term, is shown. The three-body force field proves to be superior in reproducing the intermediate q range, for which the polarizable force field gives the wrong peak position and intensities. In addition, both models can correctly account for the presence or absence of a low q peak in the scattering patterns.

Overcoming the inadequacy of X-ray powder diffraction in reliable hydrogen location with the aid of first principles calculations: crystal structure determination of orotaldehyde monohydrate.

First principle calculations of periodic crystal structure were successfully combined with powder X-ray diffraction measures to determine the structure of orotaldehyde monohydrate. This approach was particularly helpful to overcome the inadequacy of powder X-ray diffraction to reliably locate the hydrogen atoms of the intermolecular bond network of the crystal molecules. Density functional calculations were accomplished for the free molecule and its cyclic dimers showing that the most stable centrosymmetric dimer is the building block of the molecular crystal.

Comprehensive Infrared Study of Tetryl, Dinitrotoluene, and Trinitrotoluene Compounds.

The present work describes an experimental and theoretical study of energetic materials used for detecting explosives in order to prevent terrorist actions, as well as for de-mining projects. Particular attention was devoted to examining the infrared absorption spectroscopy of classic explosives in order to create a useful mobile apparatus for on-field detection of explosives. This paper reports the vibrational absorption spectra of tetryl, dinitrotoluene, and trinitrotoluene molecules approached using two different spectroscopic techniques, Fourier transform infrared spectroscopy (FT-IR) and laser photoacoustic spectroscopy (LPAS). Diffuse reflectance Fourier transform infrared spectra of all samples were analyzed in a very wide spectral range (400-7500 cm(-1)) showing for the first time the existence of weak absorption bands attributable to overtones or combination bands, while laser photoacoustic spectroscopy spectra have been investigated in the fingerprint region of organic compounds that share the CO2 laser emission range (~920-1100 cm(-1)). The Fourier transform infrared spectra of both matrix isolated dinitrotoluenes have been also investigated. The theoretical treatment of tetryl is reported for the first time.

Dimerisation of urea in water solution: a quantum mechanical investigation.

The effect of water solvation on the structure and stability of cyclic dimers of urea has been investigated with the aid of density functional theory at the B3LYP/6-311++G** level. Several hydration models have been discussed. Specific solvent effects have been simulated through single and multiple water-urea interactions involving all the hydration sites of urea. The bulk solvent effects have been estimated through polarised continuum models. Under all the hydration patterns cyclic dimers continue to be stable structures although the solvent weakens the urea-urea interaction. Single and multiple specific urea-water interactions are competitive with urea dimerisation. The anticooperative nature of the two intermolecular interactions is largely due to the changes on sigma- and pi-electron density of urea caused by hydrogen bonding with water. The stability of the dimer is however, lost within a few ps when the hydrated dimer is described by a quantum mechanical molecular dynamics approach (ADMP). The cyclic dimer evolves towards structures where urea molecules are linked not more directly but through water molecules which have a bridge function.