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We report a thorough theoretical and computational investigation of the effect of dye adsorption on the TiO2 conduction band energy in dye-sensitized solar cells that is aimed at assessing the origin of the shifts induced by surface adsorbed species in the position of the TiO2 conduction band. We thus investigate a series of working dye sensitizers and prototypical surface adsorbers and apply an innovative approach to disentangle electrostatic and charge-transfer effects occurring at the crucial dye-TiO2 interface. We clearly demonstrate that an extensive charge rearrangement accompanies the dye-TiO2 interaction, which amounts to transfer of up to 0.3-0.4 electrons from the dyes bound in a dissociative mode to the semiconductor. Molecular monodentate adsorption leads to a much smaller CT. We also find that the amount of CT is modulated by the dye donor groups, with the coumarin dyes showing a stronger CT. A subtle modulation of the semiconductor conduction band edge energy is found by varying the nature of the dye, in line with the experimental data from the literature obtained by capacitance and open circuit voltage measurements. We then decompose the total conduction band shift into contributions directly related to the sensitizer properties, considering the effect of the electric field generated by the dye on the semiconductor conduction band. This effect, which amounts to ca. 40% of the total shift, shows a linear correlation with the TiO2 conduction band shifts. A direct correlation between the dye dipole and the observed conduction band shift is retrieved only for dyes of similar structure and dimensions. We finally found a near-exact proportionality between the amount of charge transfer and the residual contribution to the conduction band shift, which may be as large as 60% of the total shift. The present findings constitute the basis for obtaining a deeper understanding of the crucial interactions taking place at the dye-semiconductor interface, and establish new design rules for dyes with improved DSC functionality.

We propose a new complete memory-distributed algorithm, which significantly improves the parallel implementation of the all-electron four-component Dirac-Kohn-Sham (DKS) module of BERTHA (J. Chem. Theory Comput. 2010, 6, 384). We devised an original procedure for mapping the DKS matrix between an efficient integral-driven distribution, guided by the structure of specific G-spinor basis sets and by density fitting algorithms, and the two-dimensional block-cyclic distribution scheme required by the ScaLAPACK library employed for the linear algebra operations. This implementation, because of the efficiency in the memory distribution, represents a leap forward in the applicability of the DKS procedure to arbitrarily large molecular systems and its porting on last-generation massively parallel systems. The performance of the code is illustrated by some test calculations on several gold clusters of increasing size. The DKS self-consistent procedure has been explicitly converged for two representative clusters, namely Au20 and Au34, for which the density of electronic states is reported and discussed. The largest gold cluster uses more than 39k basis functions and DKS matrices of the order of 23 GB. © 2013 American Chemical Society.

We critically review recent experimental and theoretical investigations into some key aspects of the chemistry of gold(I) complexes of the type [L-Au-S]X-+(-) (L = NHC carbenes and phosphanes, S = alkenes and alkynes, and X- = weakly coordinating counterion). These systems are important intermediates formed during gold-catalyzed nucleophilic additions to an unsaturated substrate, and their specific activity is largely governed by two fundamental factors: the nature of the gold-substrate bond and the role of the ion-pair structure in solution. Both are crucially influenced by the nature and properties of the auxiliary ligand L, and on this interplay we focus our discussion. The relative anion-cation orientation, investigated by NOE NMR spectroscopy and DFT calculations, shows that the exact position of the counterion is determined by the natures of the ancillary ligand and substrate: the counterion is located near the substrate in the phosphane complexes, while for the NHC complexes the preferred position of the counterion is near the ligand. This tunable interionic structure opens the way to greater control over the properties and activity of these catalysts. The bond between Au-I and the unsaturated substrate is investigated using an original and powerful theoretical method of analysis. Our approach permits a rigorous definition and assessment of the charge-displacement (CD) components at the heart of the Dewar-Chatt-Duncanson model: substrate-to-metal (sigma donation) and metal-to-substrate (back-donation) and how these change with different ligands. The results consistently reveal that back-donation is a large and crucially important component of the Au-I-substrate bond in all systems: back-donation penetrates the external side of coordinated alkynes, where nucleophile attack is directed, thus partially mitigating the electron depletion caused by sigma donation.

The interionic structure of four gold(i) [small pi]-alkyne ion pairs{,} with general formula [(PR12R2)Au([small eta]2-3-hexyne)]BF4 (R1 = R2 = tBu{,} 1a; R1 = tBu{,} R2 = o-diphenyl{,} 2a; R1 = R2 = 2{,}4{,}6-trimethoxyphenyl{,} 3a; R1 = R2 = 2{,}4-di-tert-butylphenoxy{,} 4a){,} was studied by 19F{,} 1H-HOESY NMR spectroscopy. In all the cases the anion locates mainly close to the alkyne{,} but the degree of specificity of location strongly depends on the P-ligand and it is high for ion pairs bearing poorly electron donating PR12R2 ligands (particularly{,} 4a) and low for ion pairs with strongly electron donating PR12R2 ones (such as 3a). This result is rationalized through relativistic DFT calculations{,} showing that the electronic properties of the P-ligand finely tune the charge accumulation on the alkyne and{,} consequently{,} its ability of attracting the anion.

An understanding of the interactions involving water and other small hydrogenated molecules such as H(2)S and NH(3) at the molecular level is an important and elusive scientific goal with potential implications for fields ranging from biochemistry to astrochemistry. One longstanding question about water's intermolecular interactions, and notably hydrogen bonding, is the extent and importance of charge transfer (CT) , which can have important implications for the development of reliable model potentials for water chemistry, among other applications. The weakly bound adducts, commonly regarded as pure van der Waals systems, formed by H(2)O, H(2)S, and NH(3) with noble gases or simple molecules such as H(2), provide an interesting case study for these interactions. Their binding energies are approximately 1 or 2 kJ/mol at most, and CT effects in these systems are thought to be negligible. Our laboratory has performed high-resolution molecular-beam scattering experiments that probe the (absolute scale) intermolecular potential of various types of these gas-phase binary complexes with extreme sensitivity. These experiments have yielded surprising and intriguing quantitative results. The key experimental measurable is the "glory" quantum interference shift that shows a systematic, anomalous energy stabilization for the water complexes and clearly points to a significant role for CT effects. To investigate these findings, we have performed very accurate theoretical calculations and devised a simple approach to study the electron displacement that accompanies gas-phase binary intermolecular interactions in extreme detail. These calculations are based on a partial progressive integration of the electron density changes. The results unambiguously show that water's intermolecular interactions are not typical van der Waals complexes. Instead, these interactions possess a definite, strongly stereospecific CT component, even when very weak, where a water molecule may act as electron donor or acceptor depending on its orientation. CT is mediated by an asymmetric role played by the two hydrogen atoms, which causes strong orientation effects. The careful comparison of these calculations with the experimental results shows that the stabilization energy associated to CT is approximately 2-3 eV per electron transferred and may make up for a large portion of the total interaction energy. A simple electron delocalization model helps to validate and explain these findings.

Molecular beam scattering experiments are presented for the first time in order to characterize the nature and strength of the intermolecular interaction of the gas phase H2O-CF4 and H2O-CCl4 weakly bound complexes. Simultaneous measurements of the collision cross section of water, molecular oxygen, and argon atoms scattered by the same target (CF4 or CCl4) have been performed in the thermal energy range. The experimental results show unambiguously that H2O-CCl4 is similar to 0.5 kJ/mol more stable than O-2-CCl4 and Ar-CCl4, which is incompatible with a fully noncovalent nature of the interaction. Interestingly, this is not the case for the analogous CF4 complexes which all show the same binding energy, within the experimental uncertainty. These results are supported by state-of-the-art ab initio calculation of the dimer structures and of the charge displacement upon the formation of the complex. The focus is on the possible stabilization effects due to halogen bond formation and on the extent of the related charge transfer. The latter has been shown to be large from H2O toward CCl4 and negligible from H2O toward CF4.

We review recent theoretical and computational advances in the full relativistic four-component Dirac–Kohn–Sham (DKS) approach and its application to the calculation of the electronic structure of chemical systems containing many heavy atoms. We describe our implementation of an all-electron DKS approach based on the use of G-spinor basis sets, Hermite Gaussian functions, state-of-the-art density-fitting techniques and memory distributed parallelism. This approach has enormously extended the applicability of the DKS method, including for example large clusters of heavy atoms, and opens the way for future key developments. We examine the current limitations and future possible applications of the DKS approach, including the implementation of four-current density functionals and real-time propagation schemes. This would make possible to describe molecules in strong fields, accurately accounting for relativistic kinematic effects and spin–orbit coupling.

The intra- and interionic structures of a mononuclear phosphine gold(I) alkyne complex [(PAr(3)(F))Au(2-hexyne)]BF(4) [1BF4; Ar(F) = 3,5-bis(trifluoromethyl)phenyl] and its analogous complex [(NHC)Au- (2-hexyne)]BF(4) [2BF(4); NHC = 1,3-bis(dilsopropylphenyl)imidazol-2-ylidene] have been investigated by combining 1D and 20 multinuclear NMR spectroscopy and density functional theory calculations. It has been found that alkyne in 1BF(4) is depleted of its electron density to a greater extent than that in 2BF(4). This correlates with the Delta delta((13)C) NMR of the carbon carbon triple bond. Instead, 2BF(4) is much more "kinetically stable" than 1BF(4). (19)F-(1)H HOESY NMR experiments indicate that the counterion locates close to the gold atom in 1BF(4) (differently from that previously observed in the few other gold(I) ion pairs studied), exactly where the computed Coulomb potential indicates that partial positive charge accumulates.

We describe a theoretical investigation of a prototype beta-elimination reaction in systems activated by the pyridine ring. The reactions investigated, the acetohydroxamic-induced and OH--induced beta-elimination with the nitrogen protonated N-[2-(2-pyridyl)ethyl]quinuclidinium Substrate, offers a unique opportunity to test Computational methodologies for the study of beta-elimination reactions in solution since for this system detailed kinetic experimental data have been obtained. We calculated the pK(a)s of the aceto-hydroxamic acid and of the substrate, for which experimental estimates are available. We then thoroughly characterized the reactive free-energy profile. Our study establishes that the reaction proceeds via a quasi reversible Etch mechanism for the reaction induced by acetohydroxamate base, and irreversible E1cb mechanism for the reaction induce by OH- base, involving a stable carbanion intermediate. Except for a discrepancy in the reproduction of the pK(a) of the acetohydroxamic acid and of the H2O, the calculated free-energy profile is in excellent agreement with the experiment, showing the general reliability of the present approach.

We provide a rigorous model-free definition and a detailed theoretical analysis of the electron-charge displacements making up the donation and back-donation components of the Dewar-Chatt-Duncanson model in some realistic catalytic intermediates of formula L-Au(I)-S in which L is an N-heterocyclic carbene or Cl(-) and S is an eta(2)-coordinated substrate containing a C-C multiple bond. We thus show, contrary to a widely held view, that the gold-substrate bond is characterized by a large pi back-donation component that is comparable to, and often as large as, the sigma donation. The back-donation is found to be a highly tunable bond component and we analyze its relationship with the nature of the auxiliary ligand L and with structural (interdependent) factors such as metal-substrate bond lengths and carbon pyramidalization.

We report the definitive assessment of a measurable (half a kJ/mol) charge-transfer (CT) component in the interaction of water with the noble gases. The CT is found to be strongly stereoselective and is mediated by a highly asymmetric, donor/acceptor concerted role of the two hydrogen atoms, which can in fact dictate the equilibrium geometry of the adducts. This finding may be of general relevance in explaining the known peculiar orientational features of water's interactions. By using an original method of partial integration of the change in electron density occurring upon the formation of Ng-water complexes (computed with accurate ab initio methods and basis sets), we map the local charge displacement along the whole intermolecular region and are thus able to appraise CT effects free of the inherent ambiguity of charge decomposition models. With this, it was possible to prove that a small CT (of the order of a couple of millielectrons at most) takes place from Ng to water. Most importantly, this CT correlates quantitatively with some unexpected "glory'' quantum interference effects observed in high-resolution molecular-beam Ng-water scattering experiments that indicate a stronger bond than a pure van der Waals force. The energy stabilization associated with the observed CT does not exceed half a kJ mol(-1). The Ng-water CT is found to be strongly stereoselective and is mediated by a highly asymmetric, donor-acceptor concerted role of the two hydrogen atoms, which can in fact dictate the equilibrium geometry of the adducts. This finding may be of general relevance in explaining the known peculiar orientational features of water's interactions.

By combining the results of kinetic measurements with DFT calculations we provide a clear-cut evidence of the merging between the E2 and E1cb reaction mechanisms for a large series of leaving groups. Our results solve a long-debated issue in chemical reactivity with profound implications both from a fundamental and biological point of view, thus paving the way to further investigations with different substrates.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

We provide a unified view of our recent work on TDDFT calculations on the class of Iridium(III) cyclometallated complexes, of relevance for their extensive use in OLED and LEEC devices. Our results, obtained for vastly different systems, allow us to trace some general conclusions concerning the modeling of these transition metal complexes. The effect of relativistic effects on both geometrical structures and emission properties is analyzed considering scalar relativistic corrections and quadratic response theory. Absorption spectra for these compounds can be satisfactorily reproduced by TDDFT calculations neglecting spin–orbit coupling, because of the strong intensity of singlet–singlet transitions which hinders the weaker singlet–triplet transitions. We also underline the importance of being able to simulate the emission spectra line-shapes, thus allowing for a direct comparison of calculated and experimental quantities and to estimate the emission color perceived by the human eye. Approximate treatments of spin–orbit coupling based on excited state analysis are in some case useful to rationalize experimental trends, but more rigorous and quantitative approaches are required for future applications. Finally, we stress the absolute relevance of solvation effects in the description of the excited states of this class of systems, with very large differences in the excited state properties calculated in vacuo and as a function of different solvents. These differences have profound consequences for the comprehension of the photophysical properties of this important class of systems and should always be considered in their effective modeling. On overall, our results indicate that electronic structure-driven tuning of the excited state properties of Iridium(III) cyclometallated complexes is possible, thus opening the way to a theoretical and computational strategy for the design of new phosphorescent compounds with specific target characteristics.

In exploiting state-of-the-art density fitting algorithms, we show that relativistic four-component Dirac-Kohn-Sham computations using G -spinor basis sets and Hermite Gaussian fitting functions can readily be applied to large molecular systems, including large clusters of heavy atoms. The inherent accuracy of the fitting procedure is discussed and the techniques to ensure high accuracy are mentioned. The power of the method, with its reduced scaling with respect to size and a prefactor shrunk by several orders of magnitude, is illustrated through some all-electron test calculations on gold clusters up to Au16. © 2008 The American Physical Society.

Commonly used exchange-correlations functionals are known to produce inaccurate electric field gradient (EFG) values at the nuclei of transition metals and heavy atoms in molecular calculations. This makes density functional theory (DFT) essentially inapplicable for the determination of nuclear quadrupole moments (NQM) from absolute EFG estimates. However, in a recently proposed indirect approach, the NQM is determined from the changes in the EFG along a series of molecules. We investigate this indirect approach within four-component relativistic DFT, showing that, at least in a series of chemically strictly related molecules, EFG variations can be computed quite accurately. This leads to surprisingly stable and reliable estimates of the NQM, even in notoriously ‘difficult’ cases such as 197 Au.

Spectroscopic constants, including dissociation energies, harmonic and anharmonic vibrational frequencies, and dipole moments, are calculated for the complete alkali auride series (LiAu, NaAu, KAu, RbAu, CsAu). The four-component formulation of relativistic density functional theory has been employed in this study, using the G-spinor basis sets implemented recently in the program BERTHA. The performance of four standard nonrelativistic density functionals employed is investigated by comparing the results with the best available theoretical and experimental data. The present work provides the first theoretical predictions on the molecular properties of RbAu. The intermetallic bond that occurs in the alkali auride series is highly polar and is characterized by a large charge transfer from the alkali metals to gold. The extent of this electron transfer has been investigated using several different charge analysis methods, enabling us to reach some general conclusions on their relative performance. We further report a detailed analysis of the topological properties of relativistic electron density in the bonding region, discussing the features of this approach which characterize the nature of the chemical bond. We have also computed the fully relativistic density for the alkali halides MBr and MI (M = Li, Na, K, Rb, and Cs). The comparative study shows that, on the basis of several topological properties and the variation in bond lengths, the gold atom behaves similarly to a halogen intermediate between Br and I.

A density fitting approach for the Coulomb matrix representation within the four-component formulation of relativistic density-functional theory is presented. Our implementation, which uses G-spinor basis sets, shares all the advantages of those found in nonrelativistic quantum chemistry. We show that very accurate Coulomb energies may be obtained using a modest number of scalar auxiliary basis functions for molecules containing heavy atoms. The efficiency of this new implementation is demonstrated in a detailed study of the spectroscopic properties of the gold dimer, and its scaling behavior has been tested by calculations of some closed-shell gold clusters (Au-2,Au-3(+),Au-4,Au-5(+)). The algorithm is found to scale as O(N-3), just as it does in the nonrelativistic case, and represents a dramatic improvement in efficiency over the conventional approach in the calculation of the Coulomb matrix, with computation times that are reduced to less than 3% for Au-2 and up to 1% in the case of Au-5(+).

An implementation of the generalized gradient approximation within the four-component formulation of relativistic density-functional theory using G-spinor basis sets is presented. This approach is based on the direct evaluation of the relativistic density and its gradient from the G-spinor amplitudes and gradients without explicit reference to the total density matrix. This proves to be a particularly efficient scheme, with an intrinsic computational cost that scales linearly with the number of G-spinor basis functions. In order to validate this new implementation, incorporated in the parallel version of the program BERTHA, a detailed study of the diatomic system CsAu is also reported. The spectroscopic constants D(e),r(e),omega(e), and x(e)omega(e) and the dipole moment mu have been calculated and compared with the best available theoretical and experimental data. The sensitivity of our results to the details of the numerical schemes used to evaluate the matrix elements is analyzed in detail. Also presented is a comparative study of molecular properties in the alkali auride series which have been obtained using several standard non-relativistic density functionals.

We present recent developments in the implementation of the density fitting approach for the Coulomb interaction within the four-component formulation of relativistic density functional theory [Belpassi et al., J. Chem. Phys. 124, 124104 (2006)]. In particular, we make use of the Poisson equation to generate suitable auxiliary basis sets and simplify the electron repulsion integrals [Manby and Knowles, Phys. Rev. Lett. 87, 163001 (2001)]. We propose a particularly simple and efficient method for the generation of accurate Poisson auxiliary basis sets, based on already available standard Coulomb fitting sets. Just as is found in the nonrelativistic case, we show that the number of standard auxiliary fitting functions that need to be added to the Poisson-generated functions in order to achieve a fitting accuracy equal or, in some cases, better than that of the standard procedure is remarkably small. The efficiency of the present implementation is demonstrated in a detailed study of the spectroscopic properties and energetics of several gold containing systems, including the Au dimer and the CsAu molecule. The extraction reaction of a H(2)O molecule from a Au(H(2)O)(9) (+) cluster is also calculated as an example of mixed heavy-light-atom molecular systems. The scaling behavior of the algorithm implemented is illustrated for some closed shell gold clusters up to Au(5) (+). The increased sparsity of the Coulomb matrices involved in the Poisson fitting is identified, as are potential computational applications and the use of the Poisson fitting for the relativistic exchange-correlation problem.

We recently devised a simple scheme for analyzing on quantitative grounds the Dewar-Chatt-Duncanson donation and back-donation in symmetric coordination complexes. Our approach is based on a symmetry decomposition of the so called Charge-Displacement (CD) function quantifying the charge flow, upon formation of a metal (M)-substrate (S) bond, along the M-S interaction axis and provides clear-cut measures of donation and back-donation charges in correlation with experimental observables [G. Bistoni et al., Angew. Chem., Int. Ed. 52, 11599 (2013)]. The symmetry constraints exclude of course from the analysis most systems of interest in coordination chemistry. In this paper, we show how to entirely overcome this limitation by taking advantage of the properties of the natural orbitals for chemical valence [M. Mitoraj and A. Michalak, J. Mol. Model. 13, 347 (2007)]. A general scheme for disentangling donation and back-donation in the CD function of both symmetric and non-symmetric systems is presented and illustrated through applications to M-ethyne (M = Au, Ni and W) coordination bonds, including an explicative study on substrate activation in a model reaction mechanism. (C) 2015 AIP Publishing LLC.