Utilisation of CO2 as "Structure Modifier" of Inorganic Solids.

Utilisation of CO2 as a chemical reagent is challenging, due to the molecule's inherent chemical stability. However, CO2 reacts promptly at high temperature (∼1000 °C) with alkaline-earth oxides to form carbonates and such reactions are used towards capture and re-utilisation. In this work, this concept is extended and CO2 is utilised as a reagent to modify the crystal structure of mixed-metal inorganic solids. Modification of the crystal structure is a "tool" used by materials scientists to tailor the physical property of solids. CO2 gas was reacted with several isostructural mixed-metal oxides Sr2 CuO3 , Sr1.8 Ba0.2 CuO3 and Ba2 PdO3 . These oxides are carefully selected to show anion vacancies in their crystal structure, to act as host sites for CO2 molecules, leading to the formation of carbonate anions, (CO3 )2- . The corresponding oxide carbonates were formed successfully and the favourable formation of SrCO3 as secondary phase was minimised via an innovative, yet simple synthetic procedure involving alternating of CO2 and air. We also derived a simple model to predict the kinetics of the reactions for the cuprates, using first-principles density functional theory and assimilating the reaction to a gas-surface process.

Periodic Dispersion-Corrected Approach for Isolation Spectroscopy of N2 in an Argon Environment: Clusters, Surfaces, and Matrices.

Ab initio and Perdew, Burke, and Ernzerhof (PBE) density functional theory with dispersion correction (PBE-D3) calculations are performed to study N2-Arn (n ≤ 3) complexes and N2 trapped in Ar matrix (i.e., N2@Ar). For cluster computations, we used both Møller-Plesset (MP2) and PBE-D3 methods. For N2@Ar, we used a periodic-dispersion corrected model for Ar matrix, which consists on a slab of four layers of Ar atoms. We determined the equilibrium structures and binding energies of N2 interacting with these entities. We also deduced the N2 vibrational frequency shifts caused by clustering or embedding compared to an isolated N2 molecule. Upon complexation or embedding, the vibrational frequency of N2 is slightly shifted, while its equilibrium distance remains unchanged. This is due to the weak interactions between N2 and Ar within these compounds. Our calculations show the importance of inclusion of dispersion effects for the accurate description of geometrical and spectroscopic parameters of N2 isolated, in interaction with Ar surfaces, or trapped in Ar matrices.

Understanding of matrix embedding: a theoretical spectroscopic study of CO interacting with Ar clusters, surfaces and matrices.

Through benchmark studies, we explore the performance of PBE density functional theory, with and without Grimme's dispersion correction (DFT-D3), in predicting spectroscopic properties for molecules interacting with rare gas matrices. Here, a periodic-dispersion corrected model of matrix embedding is used for the first time. We use PBE-D3 to determine the equilibrium structures and harmonic vibrational frequencies of carbon monoxide in interaction with small Ar clusters (CO-Arn, n = 1, 2, 3), with an Ar surface and embedded in an Ar matrix. Our results show a converging trend for both the vibrational frequencies and binding energies when going from the gas-phase to a fully periodic approach describing CO embedding in Ar. This trend is explained in terms of solvation effects, as CO is expected to alter the structure of the Ar matrix. Due to a competition between CO-Ar interactions and Ar-Ar interactions, perturbations caused by the presence of CO are found to extend over several Šin the matrix. Accordingly, it is mandatory to fully relax rare gas matrices when studying their interaction with embedded molecules. Moreover, we show that the binding energy per Ar is almost constant (∼-130 cm(-1) atom(-1)) regardless of the environment of the CO molecule. Finally, we show that the concentration of the solute into the cold matrix influences the spectroscopic parameters of molecules embedded into cold matrices. We suggest hence that several cautions should be taken before comparing these parameters to gas phase measurements and to theoretical data of isolated species.

Vibrations of a single adsorbed organic molecule: anharmonicity matters!

Vibrational spectroscopy is a powerful tool to identify molecules and to characterise their chemical state. Inelastic electron tunnelling spectroscopy (IETS) combined with scanning tunnelling microscopy (STM) allows the application of vibrational analysis to a single molecule. Up to now, IETS was restricted to small species due to the complexity of vibration spectra for larger molecules. We extend the horizon of IETS for both experiment and theory by measuring the STM-IETS spectra of mercaptopyridine adsorbed on the (111) surface of gold and comparing it to theoretical spectra. Such complex spectra with more than 20 lines can be reliably determined and computed leading to completely new insights. Experimentally, the vibrational spectra exhibit a dependence on the specific adsorption site of the molecules. Theoretically, this dependence is only accessible if anharmonic contributions to the interaction potentials are included. These joint experimental and theoretical advances open new perspectives for structure determination of organic adlayers.

Analysis of rich inelastic electron tunneling spectra: case study of terthiophene on Au(111).

Even moderately small molecules like 2,2':5',2"-terthiophene exhibit quite rich vibrational spectra. Detection and assignment of vibronic transitions of such a single adsorbed molecule in inelastic electron tunneling spectroscopy (IETS) using scanning tunneling microscopy are notoriously hampered by noise and the low efficiency of inelastic channels of typically well below 1%. We demonstrate by a thorough statistical analysis that detection of almost all predicted transitions can be determined experimentally within the energy range 0-120 meV with an estimated detection limit for the efficiency of inelastic channels of ∼0.15%. The maximum accuracy of our transition energies is 2 meV and thus smaller than the thermal broadening at 5 K. On short time scales up to some hours, that accuracy appears to be limited by tunneling current noise. The present analysis confirms earlier results which showed that IETS obeys propensity rules rather than selection rules as observed for optical transitions. Furthermore, the previous indications that anharmonic components in the interaction potentials are important for calculating properties of molecular vibrations were corroborated.

Mechanism of the S-->N isomerization and aquation of the thiocyanato pentaammine cobalt(III) ion.

All of the stationary points on the potential energy surface of the S-->N isomerization and aquation of the Co(NH3)5SCN2+ ion have been investigated with ab initio quantum chemical methods. Also the corresponding anations of the Co(NH3)5OH2(3+) ion by the N and S ends of SCN- and the substitution of thiocyanate via the D mechanism have been studied. All calculations have been performed by taking into account hydration. The most favorable reaction of Co(NH3)5SCN2+ is the isomerization. It is concerted, follows the I or Id mechanism, depending on the applied criteria, and proceeds via a T-shaped transition state. The aquations of Co(NH3)5SCN2+ and Co-(NH3)5NCS2+ and the corresponding inverse reactions, the anations, all proceed via the Id mechanism. The activation energies, calculated for the isomerization and aquation, agree with experiment, and so does the difference of the activation energies for the anations by the two donors of SCN-. This energy difference reflects the disparate nucleophilicities of the N and S ends of SCN- and shows that bond making in the transition state is significant for the Id mechanism. Isomerization and aquation are two parallel reactions which proceed via two disparate transition states. The computed activation energy for the SCN- substitution via the D mechanism is the highest, and therefore, this pathway is unlikely to operate for the isomerization and aquation of Co(NH3)5SCN2+. The S-->N isomerization and the SCN- substitution via the D mechanism were furthermore computed for the free ions in the gas phase: the isomerization would require a higher activation energy and follow the Ia mechanism. The activation energy for the SCN- substitution via the D mechanism would be very high, because of the large electrostatic work which is required for the removal of an anion from a (formally) 3+ charged cation.

Accurate total energies without self-consistency.

We present a new way of calculating approximate but accurate total energies within the framework of density functional theory. Our technique is based on an expansion of the energy functional to second order and does not require self-consistent iterations of the total density. The functional can be minimized by using the same techniques as developed for variational density functional perturbation theory. The method is ideally suited to systems composed of weakly interacting fragments, but it can also be applied to semiconductors and insulators. We show the versatility of our approach in a variety of examples exhibiting different types of chemical bonding.

H-densities: a new concept for hydrated molecules.

It is common to represent molecules by "ball-and-stick" models that represent static positions of atoms. However, the vibrational states of water molecules involved in hydrogen bonding have wide amplitudes, even in their ground states. Here we introduce a new representation of this wide-amplitude vibrational motion: H-density plots. These plots represent the delocalized zero-point vibrational motion of terminal hydrogen atoms of water molecules weakly bound to other molecules. They are a vibrational analogy to electron densities. Calculations of the H-densities for complexes of water with water, benzene, phenol, and DNA bases are presented. These are obtained using the quantum diffusion Monte Carlo method. Comparisons of measured and calculated rotational constants provide experimental evidence of the new concept.