Theoretical Investigation of the Infrared Spectrum of 5-Bromo-2,4-pentadiynenitrile from a CCSD(T)/B3LYP Anharmonic Potential.

On the grounds of a hybrid CCSD(T)/B3LYP/aug-cc-pVTZ anharmonic potential and the use of a variational and variational-perturbational methods, the IR spectra of 5-bromo-2,4-pentadiynenitrile (BrC5 N) is revisited in the mid-infrared region up to 4500 cm-1 . A position and intensity analysis of our theoretical results allow us to assign the fundamental bands together with their combinations and overtones, in the aforementioned range of frequencies. The main objective of this work is to give an "a priori" complete IR spectrum of BrC5 N, which can be used as a guide for the low-intensity bands in areas not completely studied so far.

Establishing the pivotal role of local aromaticity in the electronic properties of boron-nitride graphene lateral hybrids.

Within an attempt to unravel the conundrum of irregular bandgap variations in hybrids of white-graphene (hBN) and graphene (G) observed in both experiment and theory, strong proofs about the decisive role of aromaticity in their electronic properties are brought to light. Sound numerical experiments conducted on zero-, one- and two-dimensional hBNG hybrids demonstrate that upon structural and/or electronic perturbation caused by foreign doping agents, the uniformity in local cyclic electron delocalization of ideal graphene restructures locally creating carbon hexagons of contrasting cyclic electron delocalization (c.c. local aromatic patterns) which may dominate the bandgap size of the resulting systems. In addition, relying on the quantum chemical aspect of aromaticity in terms of quantitative computations of cyclic electron delocalization together with pictorial intrinsic polarizability density representations, this work provides a solid and handy rule-of-thumb to be used in qualitative and intuitive predictions. According to this empirical rule, the origin of any nonmonotonic bandgap variation observed in stoichiometric 0D (BN)n/graphene hybrids with increasing hBN segment lies in instabilities caused by partially substituted benzenoid rings formed locally at the hBNG interfaces. This relationship, established in 0D graphene flakes and extended to 1D periodic ribbons, can be used to understand and qualitatively predict conflicting bandgap variations of vacancy-free 2D periodic lattices, pointing at the property of aromaticity as the missing link needed to solve the puzzle of conflicting bandgap variations in hBNG hybrids observed in experiment.

Ab Initio Modeling of the IR Spectra of Dicyanoacetylene in the Region 100-4800 cm(-1).

On the grounds of a hybrid CCSD(T)/B3LYP/aug-cc-pVTZ anharmonic potential and the use of a variational-perturbational method, the IR spectrum of dicyanoacetylene is revisited in the region 100-4800 cm(-1), comparing our results with previous experimental data. A position and intensity analysis of our theoretical results allows us to assign fundamentals, combinations, and overtone bands in the aforementioned range of frequencies. Moreover, the outcomes show a good agreement with the most reliable experimental values and predict several unobserved or unassigned overtones and combinations in the mid-infrared region.

A Computational Study of the Interaction and Polarization Effects of Complexes Involving Molecular Graphene and C60 or a Nucleobases.

A systematic analysis of the molecular structure, energetics, electronic (hyper)polarizabilities and their interaction-induced counterparts of C60 with a series of molecular graphene (MG) models, CmHn, where m = 24, 84, 114, 222, 366, 546 and n = 12, 24, 30, 42, 54, 66, was performed. All the reported data were computed by employing density functional theory and a series of basis sets. The main goal of the study is to investigate how alteration of the size of the MG model affects the strength of the interaction, charge rearrangement, and polarization and interaction-induced polarization of the complex, C60-MG. A Hirshfeld-based scheme has been employed in order to provide information on the intrinsic polarizability density representations of the reported complexes. It was found that the interaction energy increases approaching a limit of -26.98 kcal/mol for m = 366 and 546; the polarizability and second hyperpolarizability increase with increasing the size of MG. An opposite trend was observed for the dipole moment. Interestingly, the variation of the first hyperpolarizability is relatively small with m. Since polarizability is a key factor for the stability of molecular graphene with nucleobases (NB), a study of the magnitude of the interaction-induced polarizability of C84H24-NB complexes is also reported, aiming to reveal changes of its magnitude with the type of NB. The binding strength of C84H24-NB complexes is also computed and found to be in agreement with available theoretical and experimental data. The interaction involved in C60 B12N12H24-NB complexes has also been considered, featuring the effect of contamination on the binding strength between MG and NBs.

Hirshfeld-based intrinsic polarizability density representations as a tool to analyze molecular polarizability.

In this work, a general scheme to visualize polarizability density distributions is proposed and implemented in a Hirshfeld-based partitioning scheme. This allows us to obtain easy-to-interpret pictorial representations of both total and intrinsic polarizabilities where each point of the density is formed by the contribution of any atom or group of atoms in the molecule. In addition, the procedure used here permits the possibility of removing the size dependence of the electric-dipole polarizability. Such a development opens new horizons in exploring new applications for the analysis of the molecular polarizability tensor. For instance, this visualization shows which atoms or regions are more polarizable distinguishing, moreover, the fine structure of atoms affected by the vicinity, and might extend the dipole polarizability as a tool for aromaticity studies in polycyclic aromatic hydrocarbons. Additionally, this approach can serve us to assess the methods performance in describing the interaction of electric fields with a molecule and local electron correlation effects in intrinsic polarizabilities.

Intramolecular vibrational redistribution in the non-radiative excited state decay of uracil in the gas phase: an ab initio molecular dynamics study.

We report a study of intramolecular vibrational distribution (IVR) occurring in the electronic ground state of uracil (S0) in the gas phase, following photoexcitation in the lowest energy bright excited state (Sπ) and decay through the ethylene-like Sπ/S0 Conical Intersection (CI-0π). To this aim we have performed 20 independent ab initio molecular dynamics simulations starting from CI-0π (ten of them with 1 eV kinetic energy randomly distributed over the different molecular degrees of freedom) and 10 starting from the ground state minimum (Franck-Condon, FC, point), with the excess kinetic energy equal to the energy gap between CI-0π and the FC point. The simulations, exploiting PBE0/6-31G(d) calculations, were performed over an overall period of 10 ps. A thorough statistical analysis of the variation of the geometrical parameters of uracil during the simulation time and of the distribution of the kinetic energy among the different vibrational degrees of freedom provides a consistent picture of the IVR process. In the first 0-200 fs the structural dynamics involve mainly the recovery of the average planarity. In the 200-600 fs time range, a substantial activation of CO and NH degrees of freedom is observed. After 500-600 fs most of the geometrical parameters reach average values similar to those found after 10 ps, though the system cannot be considered to be in equilibrium yet.

Gas-phase infrared spectra of three compounds of astrochemical interest: vinyl, allenyl, and propargyl isocyanides.

Isocyanides, isomers of the cyanides detected in the interstellar medium, are also possible components of this medium. The infrared spectra (5000-500 cm(-1) ) of gaseous vinyl isocyanide, allenyl isocyanide, and propargyl isocyanide have been recorded at 0.1 cm(-1) resolution. When prepared on a gram scale to produce a partial pressure of 10 mbar after evaporation in the cell, these three isocyanides, which have previously been reported to be kinetically unstable, do not display any sign of decomposition when recording the spectra. Geometry optimizations and harmonic and anharmonic vibrational frequencies were calculated using the LCCSD(T) method with the cc-pVTZ basis set. Anharmonic frequencies of fundamental, overtone, and combination transitions were calculated using a variational approach implemented in the P Anhar.v2.0 code, to assign the experimental data for each compound. These results improve our knowledge of these under-investigated compounds and pave the way for other physicochemical studies on functionalized isocyanides.

Unleashing the quadratic nonlinear optical responses of graphene by confining white-graphene (h-BN) sections in its framework.

In an attempt to diversify the options in designing graphene-based systems bearing large second order nonlinear optical (NLO) responses of octupolar and/or dipolar character, the subject of the quadratic NLO properties of hybrid boron nitride (BN) graphene flakes is opened up. State of the art ab initio and density functional theory methods applied on a toolbox of book-text octupolar and arbitrary dipolar planar hybrid h-BN-graphene nanosized systems reveal that by confining finite h-BN sections in the internal network of graphene, the capacity of the π-electron network of graphene species in delivering giant second order NLO responses could be fully exploited. Configuration interaction (CIS) and time-dependent density functional (TD) computations, within the sum-overstate (SOS) perturbational approach, expose that the prevailing (hyper)polarization mechanism, lying under the sizable computed octupolar hyperpolarizabilities, is fueled by alternating positive and negative atomic charges located in the internal part of the hybrid flakes, and more precisely at the BN/graphene intersections. This type of charge transfer mechanism distinguishes, in fact, the elemental graphene dipoles/octupoles we report here from other conventional NLO dipoles or octupoles. More interestingly, it is shown that by controlling the shape, size, and covering area of the h-BN domain (or domains), one can effectively regulate "à volonté" both the magnitudes and types of the second order NLO responses switching from dipolar to octupolar and vice versa. Especially in the context of the latter class of NLO properties, this communication brings into surface novel, graphene-based, octupolar planar or quasiplanar motifs. The take home message of this communication is summarized as follows: When the right BN segment is incorporated in the right section of the right graphene flake, systems of giant quadratic NLO octupolar and/or dipolar responses may emerge.

Structural investigation of microhydrated thymine clusters and vibrational study of isolated and aqueous forms of thymine using DFT level of theory.

This theoretical study provides the physically reasonable structures of the microhydrated thymine clusters, from the mono- to the penta-hydrated species, by the exploration of their B3LYP and B3LYP-D potential energy surfaces using a global search algorithm of minima (GSAM). The anharmonic vibrational computations of the isolated and aqueous thymine are also reported. They were performed from B3LYP and B3LYP-D potential electronic surfaces followed by a second order perturbative treatment of the anharmonicity. On that point, the computational strategy to properly take into account the effect of the polar protic solvent consists in considering a micro-hydrated thymine cluster [T,nH2O] surrounded by a polarizable continuum model (PCM). The number of solvent molecules was chosen in such a way that the micro-hydrated cluster presents only one dominant stable conformer at 298 K. All the VPT2 fundamental transitions obtained from the B3LYP and B3LYP-D quartic force fields are reported for the isolated form ([T,0H2O]) and for the aqueous form ([T,nH2O + PCM]). The theoretical results are compared to the available experimental data, which are for some of them reassigned, in order to assess the reliability of the B3LYP and B3LYP-D methods for the anharmonic treatment of such organic species in a polar protic solvent.

The use of the GSAM approach for the structural investigation of low-lying isomers of molecular clusters from density-functional-theory-based potential energy surfaces: the structures of microhydrated nucleic acid bases.

This study presents structural properties of microhydrated nucleic acid bases (NABs) - uracil (U), thymine (T), guanine (G), adenine (A), and cytosine (C) - investigated by theoretical computations at the B3LYP level of theory. To obtain the different representations of these microhydrated species, we applied the GSAM procedure: the most stable conformers labeled X,nH2O (X = U, T, G, A and n = 1...5) for which the Boltzmann population is higher than 2% at 298K are calculated at the B3LYP and B3LYP-D levels of theory. At the B3LYP level, our calculated geometries are compared with those obtained in the literature. New physically relevant isomers are found with the GSAM algorithm, especially for the tetra- and pentahydrated species. The use of DFT-D functional does not strongly modify the relative energies of the isomers for the monohydrated species. When the number of water molecules increases, the results become extremely sensitive to the consideration of dispersion contributions.

Theoretical strategy to build structural models of microhydrated inorganic systems for the knowledge of their vibrational properties: the case of the hydrated nitrate aerosols.

This study provides theoretical anharmonic calculations for microhydrated NaNO3-labeled (NaNO3, nH2O)x with a water-to-solute ratio (n) ranging from 1 to 3. A representative geometrical model of these forms was first investigated by simulating the molecular clusters as (NaNO3,1H2O)x with x = 1 to 4. The comparison between the calculated time independent anharmonic frequencies using the B3LYP-D/6-311+G(d,p) method and their experimental counterparts led to the choice of a supercluster model. The most probable structures of (NaNO3,nH2O)3 molecular system were investigated by using our global search algorithm we developed recently (GSAM code) both at the B3LYP/6-311+G(d,p) and the B3LYP-D/6-311+G(d,p) levels of theory. The quality of the structural model is illustrated by comparing the B3LYP/6-311+G(d,p) and B3LYP-D/6-311+G(d,p) anharmonic vibrational signatures with those obtained from IR experiments. While an average deviation of 16 cm(-1) is observed in the case of the B3LYP computations, the deviation is reduced to 7 cm(-1) for the B3LYP-D computations.

Structural and static electric response properties of highly symmetric lithiated silicon cages: theoretical predictions.

It is shown by density functional theory calculations that high symmetry silicon cages can be designed by coating with Li atoms. The resulting highly symmetric lithiated silicon cages (up to D(5d) symmetry) are low-lying true minima of the energy hypersurface with binding energies of the order of 4.6 eV per Si atom and moderate highest occupied molecular orbital-lowest unoccupied molecular orbital gaps. Moreover, relying on a systematic study of the electric response properties obtained by ab initio (Hartree-Fock, MP2, and configuration interaction singles (CIS)) and density functional (B3LYP, B2PLYP, and CAM-B3LYP) methods, it is shown that lithium coating has a large impact on the magnitude of their second hyperpolarizabilities resulting to highly hyperpolarizable species. Such hyperpolarizable character is directly connected to the increase in the density of the low-lying excited states triggered by the interaction between the Si cage and the surrounding Li atoms.

Doping-enhanced hyperpolarizabilities of silicon clusters: a global ab initio and density functional theory study of Si10 (Li, Na, K)n (n=1, 2) clusters.

A global theoretical study of the (hyper)polarizabilities of alkali doped Si(10) is presented and discussed. First, a detailed picture about the low lying isomers of Si(10)Li, Si(10)Na, Si(10)K, Si(10)Li(2), Si(10)Na(2), and Si(10)K(2) has been obtained in a global manner. Then, the microscopic first (hyper)polarizabilities of the most stable configurations have been determined by means of ab initio methods of high predictive capability such as those based on the Møller-Plesset perturbation and coupled cluster theory, paying extra attention to the (hyper)polarizabilities of the open shell mono-doped systems Si(10)Li, Si(10)Na, Si(10)K, and the influence of spin contamination. These results were used to assess the performance of methods of low computational cost based on density functional theory (DFT) in the reliable computation of these properties in order to proceed with an in-depth study of their evolution as a function of the alkali metal, the cluster composition, and the cluster structure. The most interesting outcomes of the performed (hyper)polarizability study indicate that while alkali doping leaves the per atom polarizability practically unaffected, influences dramatically the hyperpolarizabilities of Si(10). The lowest energy structures of the mono-doped clusters are characterized by significantly enhanced hyperpolarizabilities as compared to the analogue neutral or charged bare silicon clusters Si(10) and Si(11), while, certain patterns governed by the type and the number of the doping agents are followed. The observed hyperpolarizability increase is found to be in close connection with specific cluster to alkali metal charge transfer excited states and to the cluster structures. Moreover, an interesting correlation between the anisotropy of the electron density, and the hyperpolarizabilities of these systems has been observed. Finally, it is important to note that the presented method assessment points out that among the various DFT functionals used in this work, (B3LYP, B3PW91, BhandHLYP, PBE0, CAM-B3LYP, LC-BLYP, LC-BPW91) only B3PW91 and PBE0 out of the seven provided a consistent quantitative performance for both polarizabilities and hyperpolarizabilities with respect to the ab initio methods utilized here. On the other hand, the long range corrected functionals LC-(U)BLYP and LC-(U)BPW91 (µ = 0.47) failed to supply quantitatively accurate hyperpolarizability results in all the studied clusters while the CAM-(U)B3LYP functional performs satisfactory only in the case of the Na and K doped systems.

Role of nonlocal exchange in molecular crystals: the case of two proton-ordered phases of ice.

We present a periodic density functional theory investigation of two proton-ordered phases of ice. Their equilibrium lattice parameters,relative stabilities, formation energies, and densities of states have been evaluated. Nine exchange-correlation functionals, representative of the generalized gradient approximation (GGA), global hybrids,range-separated hybrids, meta-GGA, and hybrid meta-GGA families have been taken into account, considering two oxygen basis sets. Although the hydrogen-bond network of ice is well reproduced at the B3LYP,M06-L, or LC- wPBE levels, formation energies are only correctly evaluated with the two former functionals. Band gaps on the other hand are only quantitatively reproduced at the B3LYP level. These results indicate that this last functional, a de facto reference for molecular calculations, gives in average the most accurate results for the considered ice properties.

Adsorption of cyanodiacetylene on ice: a periodic approach.

The adsorption of cyanodiacetylene (HC5N) on ice has been investigated within a periodic approach at the density functional theory level using the parameter-free PBE0 hybrid functional. Two adsorption models, involving monodentate hydrogen bonding, have been considered. A third model, corresponding to a bridging binding model, has also been taken into account but found unstable. Calculations have been carried out considering two orientations of a proton-ordered model of ice surfaces, using two different unit cells in each case. Eight different cases of the adsorption of HC5N on ice have been investigated in depth at the geometric, energetic and electronic levels. Although HC5N is found to mainly bind to ice by direct hydrogen bonding, densities of states analysis suggest that its pi system plays a relevant role in the adsorption, especially by interaction with dangling surface hydrogen atoms, leading to significantly different adsorption geometries in all cases investigated. Analysis of the infrared spectral signature of HC5N shows the typical large red-shift of the H-C stretching mode frequency in the adsorption model involving hydrogen bonding between the H atom of HC5N and ice, in line with both experimental and previous theoretical findings based on cluster approaches.

A global search algorithm of minima exploration for the investigation of low lying isomers of clusters from density functional theory-based potential energy surfaces: The example of Si(n) (n=3,15) as a test case.

Using an effective generation algorithm coupled with a PBE0/LANL2DZdp level of theory, 905 stable structures of Si(n) (n=3,15) have been found. This global search algorithm of minima exploration includes two original parts: the spheroidal generation, allowing the generation of rings, sphericals, m rings cylinders, and planar structures, and the raking optimization, which discards step by step the conformations that become physically unreasonable during the optimization process. The 142 isomers lying below 1 eV are reported and include the 28 structures reported in the literature. Conformational energies are well reproduced with respect to the values previously published (DeltaE=0,00+/-0,09 eV).

Vibrational analysis of glycine radical: a comparative ab initio static and dynamic study.

We present quantum mechanical (QM) vibrational computations beyond the harmonic approximation for an organic molecule that exhibits both torsional and NH(2) out of plane type modes: the glycine radical. The effective second order perturbative, variational and variation-perturbation treatments-defined as static approaches-as well as vibrational analysis from ab initio molecular dynamics trajectories at 300 K and 600 K were performed using the B3LYP/6-31+G(d,p) description of the electronic structure. Theses schemes are compared in terms of prediction of fundamental transitions, simulation of the corresponding medium infrared (MIR) spectrum and extraction of substantial information for the understanding of chemical problems. The validity of the analyses is checked for a similar molecule, formamide, for which experimental data are available.

Electrostatic interaction schemes for evaluating the polarizability of silicon clusters.

Electrostatic interaction schemes have been applied to predict the evolution of the polarizability in Si(n) clusters of increasing size (n=3-19). Both on-site polarization and charge transfer effects have been included in the interaction scheme, of which the values have been compared to B3LYP/6-311G(*) and other first principles results. To reproduce the pattern of the variation of the B3LYP average polarizability per Si atom as a function of the cluster size, the atomic polarizability employed in the interaction scheme should amount to roughly 80% of the bulk atomic polarizability. However, this results in a systematic underestimation of the polarizability per Si atom by about 25%, whereas increasing the atomic polarizability value leads to excessive variations of the polarizability per Si with the cluster size. An improved agreement is obtained when incorporating a charge transfer contribution, at least for sufficiently large clusters, substantiating the fact that in large clusters electrostatic effects are dominant over quantum effects. This charge transfer atomic polarizability term has been modeled by a simple function, which evolves linearly with the difference of Cartesian coordinates between the atom and the center of mass and that has been verified using B3LYP/6-311G(*) calculations. In the case of the prediction of the polarizability anisotropy, a similar atomic polarizability corresponding to 80% of the bulk atomic polarizability has been shown suitable to reproduce the B3LYP results, whereas inclusion of charge transfer effects can slightly improve the agreement, provided the amount of charge transfer increases with the size of the cluster.

Can the hybrid meta GGA and DFT-D methods describe the stacking interactions in conjugated polymers?

Newly developed hybrid meta density functionals and density functionals augmented by a classical London dispersion term have been systematically applied for the description of stacking energy and intermolecular distance of thiophene dimer and substituted thiophene dimer. The performance of the various approaches is compared with the benchmark ab-initio calculations done with CCSD(T) (Tsuzuki et al., JACS 2002, 124, 12200). Our results indicate that, contrary to the previous DFT methods which are not reliable, the new generation of DFT performs better the stacking interactions. These functionals, and especially those with an empirical correction, are suitable for general application in conducting polymers and, in particular, the modeling of solid state in which the overlap of Pi-Pi interactions between the conjugated chains is important.

Electric dipole (hyper)polarizabilities of selected X2Y2 and X3Y3 (X = Al, Ga, In and Y = P, As): III-V semiconductor clusters. An ab initio comparative study.

A systematic ab initio comparative study of the (hyper)polarizabilities of selected III-V stoichiometric semiconductor clusters has been carried out. Our investigation focuses on the ground state structures of the dimers and on two dissimilar trimer configurations of aluminum, gallium, indium phosphide and arsenide. The basis set effect on both the polarizabilities and hyperpolarizabilities of the studied systems has been explicitly taken into account relying on the augmented correlation consistent aug-cc-pVnZ (n = D, T, Q, and 5) basis sets series. In addition, a rough estimation of the effects of the relativistic effects on the investigated properties is provided by extension of the study to include calculations performed with relativistic electron core potentials (or pseudopotentials). Electron correlation effects have been estimated utilizing methods of increasing predictive reliability, e.g., the Møller-Plesset many body perturbation theory and the couple cluster approach. Our results reveal that in the considered semiconductor species the Group III elements (Al, Ga, In) play a vital role on the values of their relative (hyper)polarizability. At all levels of theory employed the most hyperpolarizable clusters are the indium derivatives while the aluminum arsenide clusters also exhibit high, comparable hyperpolarizabilities. The less hyperpolarizable species are those composed of gallium and this is associated with the strong influence of the nuclear charge on the valence electrons of Ga due to the poor shielding that is provided by the semicore d electrons. In addition, the analysis of the electronic structure and the hyperpolarizability magnitudes reveals that clusters, in which their bonding is characterized by strong electron transfer from the electropositive to the electronegative atoms, are less hyperpolarizable than species in which the corresponding electron transfer is weaker. Lastly, from the methodological point of view our results point out that the hyperpolarizabilities of those species converge when an augmented triple-zeta quality basis set is used and, also, that the second order Møller-Plesset approximation (MP2) overestimates considerably their second hyperpolarizabilities with respect to the highest level of coupled cluster theory applied in this study (CCSD(T)).