[Show abstract][Hide abstract] ABSTRACT: In an attempt to diversify the options in designing graphene based systems bearing large second order non-linear 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 nano-sized systems reveal that by confining finite h-BN sections in the internal network of graphene, the capacity of the -electron network of graphene species in delivering giant second order NLO responses could be fully exploited. Configuration interaction (CIS) and time depended density functional (TD) computations, within the sum-over-state (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 reported 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 motifs. The take home message of this communication is summarized in the following statement: 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.
Journal of the American Chemical Society 04/2014; · 10.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This theoretical study provides the anharmonic vibrational wavenumbers of isolated and aqueous guanine. They were performed at the DFT B3LYP/6-31+G(d,p) level of theory using two different ways for the treatment of anharmonicity: time-independent (VPT2) and time-dependent (molecular dynamics) approaches. The wavenumbers obtained are compared to experimental data for isolated and aqueous forms: the VPT2 approach is slightly better than MD, especially for the determination of stretching and wagging (NH) motions. Finally, the structural model of solvatation used for aqueous guanine which combines an explicit solvent model with a polarizable continuum model (PCM) was validated.
[Show abstract][Hide abstract] ABSTRACT: a b s t r a c t A systematic analysis of the dipole polarizabilities and first hyperpolarizabilities of a cluster series based on the magic Al 13 cluster is presented. Reliable values of the dipole polarizabilities and first hyperpolar-izabilities of Al 13 X 0/À1 (X = Li, Na, K) by means of ab initio and density functional theory clusters are for the first time provided and discussed. In addition, useful information involving the partitioning of the dipole polarizabilities in terms of intrinsic polarizabilities computed with the fractional occupation iter-ative Hirshfeld method (FOHI) are illustrated. Finally, a step by step study of the polarization mechanism dominating the hyperpolarizabilities of these species is illustrated and explained in a comprehensive manner. The presented outcomes point out a monotonic (hyper)polarizability evolution going from Li to K for both neutral and charged cluster families. Also, it is clearly shown that all neutral clusters become largely more (hyper)polarizable and more anisotropic after receiving one electron. Concerning the prin-cipal hyperpolarization mechanism in these species, the presented analysis suggests that is mainly uni-directional in character, involving intense charge transfer processes from the metal cluster to the alkali metal. Ó 2013 Elsevier B.V. All rights reserved.
Computational and Theoretical Chemistry 06/2013; · 1.14 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
Physical Chemistry Chemical Physics 06/2013; · 3.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This study presents structural properties of microhydrated nucleic acid bases (NAB's) -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, the GSAM procedure was applied: 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 to 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. On the other hand, when the number of water molecules increases, the results become extremely sensitive to the consideration of dispersion contributions.
The Journal of Physical Chemistry A 04/2013; · 2.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A discussion about the negative sign of the sizable static second hyperpolarizability of a short zigzag graphene ribbon (C60H20) reported in G.-L. Chai, C.-S. Lina and W.-D. Cheng, J. Mater. Chem., 2012, 22, 11303 is presented. As shown, the closed-shell singlet configuration of the specific ribbon as described by approximate methods as the HF and DFT causes serious side effects on the computations of electric response properties such as the dipole polarizabilities and the second hyperpolarizability.
[Show abstract][Hide abstract] ABSTRACT: 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.
The Journal of Physical Chemistry A 04/2013; · 2.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A study about the size-dependence of the hyperpolarizability-enhancement observed in metal-functionalized finite graphene species is presented. The reported ab initio and density functional results suggest that edge-passivation of graphene fragments with metallic agents (in this case Li) triggers an impressive enhancement of the second hyperpolarizability at the static limit. However, such a trend holds only in small graphene-type fragments. Strong evidence is provided showing that the specific effect drastically weakens with increasing the size of the graphene fragments regardless of their shape. The observed hyperpolarizability-enhancement in small systems and its severe decrease with increasing the size of the graphene species is qualitatively explained in terms of their charge transfer polarization mechanism.
The Journal of Physical Chemistry C 01/2013; 117:3134-3140. · 4.84 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a detailed periodic ab initio quantum-mechanical simulation of two recently proposed systems, namely hydrogenated porous graphene (HPG) and biphenyl carbon (BPC), using hybrid HF-DFT functionals and all-electron Gaussian-type basis sets. The equilibrium geom-etry, the vibrational spectrum (including IR intensities), the full set of components of the polarizability and hyper-polarizability tensors are provided, the latter evaluated through a coupled-perturbed KS/HF scheme. IR and Raman spectra for the two systems are quite different, and differ also from graphene, thus permitting their experimental identification. It is then shown that small defects inserted into the graphene sheet lead to finite values for the in-plane components of the static (hyper)polarizability tensors, spanning a relatively large range of values. By dehydrogenation of porous graphene into biphenyl carbon, a noteworthy enhancement of the nonlinear optical properties through the static second dipole hyperpolarizability can be achieved. Vibrational contributions to the polarizability are negligible for both systems.
The Journal of Physical Chemistry C 01/2013; 117:2222-2229. · 4.84 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: I n a recent paper, Hu et al. 1 used "the linear pentacene as a model of the graphene zigzag edge to investigate the effect of the number and location of Li atoms on its hyperpolariz-abilities". In brief, the authors computed the first dipole hyperpolarizabilities of several isomers of lithiated pentacene (C 22 H 14) with one up to six lithium atoms fixed on one of its two zigzag edges. Thirteen out of the fourteen different systems considered there were built by replacing one up to five hydrogens of pentacene with Li (Li n C 22 H (14−n)), whereas the last one, Li 6 C 22 H 9 , was created from Li 5 C 22 H 9 with the addition of one extra Li atom with no further replacement of any hydrogen atom. For this last open shell system (hereafter Li 6 -pentacene), Hu et al. reported an exceptionally large first hyperpolarizability value of 4 501 764 au 2 at the UBH&HLYP level of theory. According to their text, none of the results concerning the rest of the systems predispose for such a large static hyperpolarizability value. Indeed, out of the 14 systems examined therein by means of density functional theory (DFT), the next larger value belongs to Li 5 C 22 H 9 and amounts to only 14 904 au. Therefore, the authors concluded that when one Li atom is attached on the lithiated side of Li 5 C 22 H 9 its total first hyperpolarizability increases by a factor of 302. Although one could reasonably raise serious doubts about the aptness of the specific model for study purposes concerning graphene, both the reported outsized hyperpolarizability enhancement going from Li 5 C 22 H 9 to Li 6 C 22 H 9 and the magnitude of the reported property itself are intriguingly astonishing. To our knowledge, systems of that size rarely deliver hyperpolarizability values of such a weight. As Hu et al. report, the total first hyperpolarizability of L 6 -pentacene (as presented in their work) lies orders of magnitudes higher than that in other systems, which have attracted considerable attention owing to their exceptional hyperpolarizable character. According to the literature, one might indeed experience such a large hyperpolarizability value in cases where loosely bound electrons are involved. Some of the most representative paradigms of such systems are the so-called electron solvated systems 3 or ionic salts with electride and alkalide characteristics (see ref 4 and references therein). Nonetheless, none of these possibilities is supported by the orbital analysis presented in ref 1. As shown therein, Li 6 -pentacene is closer to a classical donor−acceptor system than to a molecule that implicates loosely bound electrons. Motivated by this unforeseen effect, we made an effort to understand its source by trying first to reproduce the reported values. During this process, we found that the equilibrium geometry used by Hu et al. does not correspond to a true minimum of the potential energy hypersurface. In addition, analytic computations of the first hyperpolarizability carried out with the Gaussian 09 suite of programs, 5 both on this structure and on its true minimum analogue, do not confirm the magnitude of the total first hyperpolarizability presented in ref 1. We choose to start our commentary from the equilibrium geometry of Li 6 -pentacene. Unfortunately, the authors do not provide in their original article sufficient details in their text about its structure. Of course, one can assume from the pictures provided that most likely a planar structure has been taken into account. However, this cannot be claimed with absolute certainty because only some selected bond lengths and angles are reported (see table 1 of ref 1). In addition, no information concerning the symmetry of this trial system is provided. Therefore, to make direct comparisons, we carried out full geometry optimizations starting from the analogue planar structure of L 6 -penacene of an ideal C 2v symmetry in which all Li atoms lie on the plane defined by pentacene. As it is clearly mentioned in ref 1, the geometry of Li 6 -pentacene has been obtained at the UB3LYP level of theory with a Pople's style basis set. More specifically, the authors used the standard augmented and polarized valence triple-ζ 6-311G(3df) and the valence double-ζ 6-31+G(d) basis set for Li and C atoms, respectively (Li:[4s3p3d1f], C:[4s3p1d]). What is not transparent is the basis set used for the H atoms. For this atom, the authors describe the basis set employed as 6-31+G(d). The specific notation, if not a typo, is rather unfortunate and misleading. In Pople's notation for the H atom, the diffuse s-Gaussian type functions are defined as "++". Also, the widely accepted strategy to polarize an atomic set of H consists of adding p-type Gaussian-type functions first instead of d-type, as it is assumed from the description given in ref 1. Hence, the H basis set used has to be either the nonpolarized 6-31G 6 ([2s]) set or the augmented polarized 6-31++G** ([3s1p]) set. For the sake of simplicity, hereafter we shall be referring to these basis sets as A0 and A1, respectively, thus, A0Li:6-311G(3df)/C:6-31+G(d)/H:6-31G and A1Li:6-311G(3df)/C:6-31+G(d)/H:6-31++G**. As seen in Table 1 an absolute match is found between the structural data presented in ref 1 and the optimized geometry
The Journal of Physical Chemistry C 12/2012; 114. · 4.84 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The first and most important step in the study of properties and
dynamics of atomic and molecular clusters is the determination of their
microscopic structures. In that sense, we have developped a new
algorithm, called Global Search Algorithm of Minima exploration
(GSAM), which include two original schemes: the spheroidal generation,
allowing the generation of rings, spheres, m rings cylinders and planar
structures, and the raking optimization process allowing to discard step
by step the conformations that become physically irreasonable during the
optimization process. Using such an algorithm, the microscopic structure
of the well-known Silicon clusters have been revisited as a test case.
The more interesting case of the binary GanAsm will be presented.
[Show abstract][Hide abstract] ABSTRACT: An iterative variation-perturbation algorithm allowing a anharmonic
vibrational treatment of polyatomic molecules is proposed. This state
specific process consists on an iterative construction of small 3N-5
Vibrational Configuration Interation (VCI) matrixes (N being the number
of atoms) with the most pertinent couplings and includes a perturbative
treatment of the weakest contributions. Thus, this scheme allow to
massively reduce the size of the CI matrixes with a minimum loss of
correlation energy. Through the example of H2CO, the results
stemmed from the VCI-P process are compared to their full VCI
counterpart. Moreover, the computations of anharmonic intensities are
also implemented. As illustration, the modelization of the medium
infrared (MIR) spectrum of the glycolaldehyde is reported.
[Show abstract][Hide abstract] ABSTRACT: AlnLn0/2– (n = 4–7, 12), AlnLn+2 (n = 4–7), and Al4L8 clusters were investigated by using the global-search-algorithm-for-minima approach and DFT calculations at the BP86/LANL2DZ level of theory. These investigations indicate that most of the computed species are viable and diamagnetic, although there is no clear general relationship between their shapes and their electron counts. Al4H8 and Al4Cl8 appear to be the most stable compounds in the investigated series.
Berichte der deutschen chemischen Gesellschaft 10/2012; 2012(30). · 2.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: An automatic Born-Oppenheimer potential energy surface (PES) generation method AGAPES is presented designed for the calculation of vibrational spectra of large rigid and semi-rigid polyatomic molecules within the mid-infrared energy range. An adaptive approach guided by information from intermediate vibrational calculations in connection with a multi-mode expansion of the PES in internal valence coordinates is used and its versatility is tested for a selection of molecules: HNO, HClCO, and formaldoxime. Significant computational savings are reported. The possibility of linear scaling of the sampling grid size with the molecular size due to decrease of correlation of remote coordinates in large molecules is examined and finally, possible improvements are suggested.
The Journal of Chemical Physics 06/2012; 136(22):224105. · 3.12 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
Journal of Computational Chemistry 04/2012; 33(10):1068-79. · 3.84 Impact Factor