Vincent Brites

Titan's ionic species: theoretical treatment of N2H+ and related ions.

We use different ab initio methods to compute the three-dimensional potential energy surface (3D-PES) of the ground state of N(2)H(+). This includes the standard coupled cluster, the complete active space self-consistent field, the internally contacted multi reference configuration interaction, and the newly developed CCSD(T)-F12 methods. For the description of H and N atoms, several basis sets are tested. Then, we incorporate the 3D-PES analytical representations into variational calculations of the rovibrational spectrum of N(2)H(+)(X(1)Sigma(+)) up to 7200 cm(-1) above the zero point vibrational energy. Our data show that the CCSD(T)-F12/aug-cc-pVTZ approach represents a compromise for good description of the PES and computation cost. This technique is recommended for full dimensional PES generation of atmospheric and astrophysical relevant polyatomic systems. We applied this method to derive the rovibrational spectra of N(2)H(+)(X(1)Sigma(+)) and of N(2)H(++)(X(2)Sigma(+)). Finally, we discuss the existence of the N(2)H(++)(X(2)Sigma(+)) in Titans atmosphere.

Infrared Predissociation Vibrational Spectroscopy of Li+(H2O)3–4Ar0,1 Reanalyzed Using Density Functional Theory Molecular Dynamics

The experimental IR-PD (infrared predissociation) spectra of Li(+)(H2O)(3-4)Ar and Li(+)(H2O)(3-4) clusters, monitoring two different loss channels and thus different temperatures, have been reanalyzed using DFT-MD (density functional theory based molecular dynamics) simulations for finite temperature and anharmonic theoretical spectroscopy. The use of DFT-MD to calculate IR-PD spectra at low and elevated temperatures was found remarkably accurate and useful in precise structural characterization. The dynamical spectra have in particular provided the opportunity to estimate the clusters temperatures in the IR-PD experiments. The temperatures for Li(+)(H2O)(3-4)Ar are estimated at 50-60 K whereas Li(+)(H2O)3 and Li(+)(H2O)4 have been estimated at around 500-600 and 400 K, respectively.

Stalking Higher Energy Conformers on the Potential Energy Surface of Charged Species.

Combined theoretical DFT-MD and RRKM methodologies and experimental spectroscopic infrared predissociation (IRPD) strategies to map potential energy surfaces (PES) of complex ionic clusters are presented, providing lowest and high energy conformers, thresholds to isomerization, and cluster formation pathways. We believe this association not only represents a significant advance in the field of mapping minima and transition states on the PES but also directly measures dynamical pathways for the formation of structural conformers and isomers. Pathways are unraveled over picosecond (DFT-MD) and microsecond (RRKM) time scales while changing the amount of internal energy is experimentally achieved by changing the loss channel for the IRPD measurements, thus directly probing different kinetic and isomerization pathways. Demonstration is provided for Li(+)(H2O)3,4 ionic clusters. Nonstatistical formation of these ionic clusters by both direct and cascade processes, involving isomerization processes that can lead to trapping of high energy conformers along the paths due to evaporative cooling, has been unraveled.

Theoretical Investigations on CaO Ions: Vibronic States and Photoelectron Spectroscopy

The low-lying electronic states, X(2)Π and A(2)Σ(+) of CaO(+) and X(2)Σ(+) and A(2)Π of CaO(-), have been determined at the MRCI+Q level of theory with the aug-cc-pV5Z(O) and cc-pCV5Z(Ca) basis sets. The two states of CaO(+) are close within <0.1 eV and coupled via spin-orbit effect. The X(2)Σ(+) and A(2)Π states of CaO(-) are energetically separated by <1 eV such that the first excited state is close to the electronic ground state of neutral CaO and unstable with respect to electron detachment. Using the potential energy curves and the spin-orbit coupling terms, the vibronic energy levels of these ions have been determined. The ionization energy and the electron affinity of CaO are calculated at 6.79 and 0.79 eV, respectively. The photoelectron spectra of CaO(-) and CaO have also been simulated.

Multiscale Study of Gas Slip Flows in Nanochannels

The slip velocity effect at the wall interface becomes important when the Knudsen number is above 0.01. In most problems, the Maxwell slip model is used based on the Tangential Momentum Accommodation Coefficient (TMAC), a gas-wall couple constant. The original Maxwell slip theory is isotropic which is not suitable for strongly anisotropic surfaces. The present work presents a multi-scale analysis of the anisotropic slip phenomenon which comprises three stages: i) the ab-initio study of the gas-wall interaction potential ii) Molecular Dynamic (MD) computation of the isotropic/anisotropic TMAC coefficients on different surfaces iii) MD simulation of gas flows using an anisotropic surface model and comparison with the slip theory. The interaction between an Ar gas atom and a solid Pt fcc (111) slab is carried out using CRYSTAL 09 software and PBE functional for solids (PBEsol). The ab-initio based results including equilibrium distance and adsorption energy are in good agreement with empirical results in literature. The gas-wall potential is then decomposed to pair-wise potentials for Molecular Dynamics simulation. Next, the TMAC coefficients are computed using MD method with the pair-wise potential. The gas atoms are projected onto the solid slabs with different arriving angle and relative momentum changes are measured to determine the TMAC coefficients. Different types of surfaces are considered in this paper including perfectly smooth crystalline surface, randomly rough surfaces obtained from atom deposition simulations and, anisotropic surfaces with stripes. The phantom layer technique is used to maintain the bulk solid atoms at constant temperature allowing the study of the temperature effect. The orientation dependency of TMACs is computed and analyzed in comparison with isotropic/anisotropic scattering kernel models. Finally, we use MD method to simulate gas flows in nano channel. Instead of describing explicitly the solid atomic wall, an effective anisotropic gas wall collision mechanism with TMAC coefficients determined previously is adopted. A special MD wall boundary condition is proposed to mimic the mechanism. Both pressure and acceleration driven methods are used to simulate gas flows in slip and transitional regimes. In the former method, a constant gravity-like force is applied to the gas atoms. The latter method controls the kinetic pressure difference between the inlet and the outlet. Numerical results are then compared with analytical solutions issued from the anisotropic slip theory. It is shown that the extension of the Maxwells model using two TMAC parameters can describe quite well the anistropic slip effect in the slip regime.

High energy conformers of M+(APE)(H2O)0–1Ar0–1 clusters revealed by combined IR-PD and DFT-MD anharmonic vibrational spectroscopy

IR-PD vibrational spectroscopy and DFT-based molecular dynamics simulations are combined in order to unravel the structures of M(+)(APE)(H2O)0-1 ionic clusters (M = Na, K), where APE (2-amino-1-phenyl ethanol) is commonly used as an analogue for the noradrenaline neurotransmitter. The strength of the synergy between experiments and simulations presented here is that DFT-MD provides anharmonic vibrational spectra that unambiguously help assign the ionic clusters structures. Depending on the interacting cation, we have found that the lowest energy conformers of K(+)(APE)(H2O)0-1 clusters are formed, while the lowest energy conformers of Na(+)(APE)(H2O)0-1 clusters can only be observed through water loss channel (i.e. without argon tagged to the clusters). Trapping of higher energy conformers is observed when the argon loss channel is recorded in the experiment. This has been rationalized by transition state energies. The dynamical anharmonic vibrational spectra unambiguously provide the prominent OH stretch due to the OH···NH2 H-bond, within 10 cm(-1) of the experiment, hence reproducing the 240-300 cm(-1) red-shift (depending on the interacting cation) from bare neutral APE. When this H-bond is not present, the dynamical anharmonic spectra provide the water O-H stretches as well as the rotational motion of the water molecule at finite temperature, as observed in the experiment.

Theoretical spectroscopy of the HNCl− anion

Ab initio computations have been performed to investigate the stability and the spectroscopy of the HNCl− anion. Using explicitly correlated coupled-cluster methodology and core-valence basis set, the linear–bent path of the neutral and anionic molecules have been computed, and the adiabatic electron affinity of HNCl has been deduced at 1.29 eV. The three-dimensional potential energy surface of the neutral HNCl ( ) and of the HNCl− ( ) anion have also been generated using the same computational approach, and their analytical representations have been used for the variational determination of their rovibrational energy levels. Our computations reveal the localization of the additional electron into an anti-bounding molecular orbital associated with the N–Cl bond. The modelling of the photoelectron spectrum of HNCl− and DNCl− is also presented, showing the vibrational progression associated with the N–Cl stretching mode.

Mg2H2: New insight on the Mg–Mg bonding and spectroscopic study

The six dimensional potential energy surface of the electronic ground state X̃(1)Σ(g)(+) of Mg(2)H(2) has been generated by the coupled-cluster approach with single, double and perturbative triple excitations [CCSD(T)] combined with the aug-cc-pCVTZ basis set for Mg atoms and the aug-cc-pVTZ basis set for the H atoms. The analytical representation of this surface was used in variational calculations of the rovibrational energies of Mg(2)H(2), Mg(2)D(2), and HMg(2)D for J = 0 and 1. For Mg(2)H(2), the rotational constant B(0) is computed to be 0.1438 cm(-1), and the fundamental anharmonic wavenumbers are calculated to be ν(1) = 1527.3 cm(-1) (Σ(g)(+)), ν(2) = 275.3 cm(-1) (Σ(g)(+)), ν(3) = 1503.6 cm(-1) (Σ(u)(+)), ν(4) = 312.9 cm(-1) (Π(g)), and ν(5) = 256.5 cm(-1) (Π(u)). In addition, the electronic ground states of Mg(2)H, MgH(2), Mg(2), and MgH have been investigated in order to compute the bonding energies of Mg(2)H(2) and to explain the strength of the Mg-Mg bond in this tetra-atomic molecule. The nature of the low-lying excited states of Mg(2)H(2) is also studied.

Ionic Chemistry of Tetravinylsilane Cation (TVS + ) Formed by Electron Impact: Theory and Experiment

We performed experimental and ab initio studies on tetravinylsilane cation (TVS(+)) and its ionic and neutral fragmentation products. The aim of the study is the assignment of the products formed in electron impact ionization reaction of TVS. The experimental data were compared with ab initio data calculated at the MP2/cc-pVDZ level of theory. We found good agreement between the calculated reaction enthalpies and experimental appearance energies of the ions. More generally, our calculations reveal that there is a competition between intramolecular isomerization and fragmentation processes occurring after ionization of TVS, leading to the formation of a multitude of neutral and ionic species important for characterizing the silicon-carbon-containing plasma and media. New routes for the synthesis of bearing silicon molecules are suggested.

Simulation of the – absorption and emission spectra of the SiCCl radical

The potential energy surface of the state and the transition dipole moments of the SiCCl radical have been calculated ab initio using multireference configuration interaction approaches. The rovibrational states of the state have been computed with the EVEREST code and, together with the previously calculated rovibronic states of the electronic ground state, have been used to produce absorption and emission spectra. The simulated emission spectra compare very well with the experimental laser-induced fluorescence spectra. The assignment of the rovibronic energies of the Renner–Teller electronic ground state has been completed.