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In order to explore the angular strain role on the ability of Be to form strong beryllium bonds, a theoretical study of the complexes of four beryllium derivatives of ortho closo -carboranes with eight molecules (CO, N 2 , NCH, CNH, OH 2 , SH 2 , NH 3 , and PH 3 ) acting as Lewis bases has been carried out at the G4 computational level. The results for these complexes, which contain besides Be other electron-deficient elements, such as B, have been compared with the analogous ones formed by three beryllium salts (BeCl 2 , CO 3 Be and SO 4 Be) with the same set of Lewis bases. The results show the presence of large and positive values of the electrostatic potential associated to the beryllium atoms in the isolated four beryllium derivatives of ortho -carboranes, evidencing an intrinsically strong acidic nature. In addition, the LUMO orbital in these systems is also associated to the beryllium atom. These features led to short intermolecular distances and large dissociation energies in the complexes of the beryllium derivatives of ortho -carboranes with the Lewis bases. Notably, as a consequence of the special framework provided by the ortho -carboranes, some of these dissociation energies are larger than the corresponding beryllium bonds in the already strongly bound SO 4 Be complexes, in particular for N 2 and CO bases. The localized molecular orbital energy decomposition analysis (LMOEDA) shows that among the attractive terms associated with the dissociation energy, the electrostatic term is the most important one, except for the complexes with the two previously mentioned weakest bases (N 2 and CO), where the polarization term dominates. Hence, these results contribute to further confirm the importance of bending on the beryllium environment leading to strong interactions through the formation of beryllium bonds.

The attribution of 1 H and 13 C NMR signals of a library of 5-, 6- and 7-substituted 2,2-dimethylchroman-4-one derivatives is reported. Substituent effects were interpreted in terms of the Hammett equation, showing a good correlation for carbons para - to the substituent group, not for the meta - ones. Similarly, the Lynch correlation shows the additivity of the substituent chemical shifts in the case of both H and C nuclei, again with the exception of the carbons in the meta - position. Density Functional Theory (DFT)-predicted 1 H and 13 C chemical shifts correspond closely with experimentally observed values, with some exceptions for C NMR data; however, the correlation is valid only for the aromatic moiety and cannot be extended to the heterocyclic ring of the chroman-4-one scaffold.

Co(II) complexes and Co(H2O)6(2+) are used as catalysts in advanced oxidation processes. Therefore it was decided to study the kinetics of reaction of Co(H2O)6(2+) with H2O2. Surprisingly, the kinetic results point out that the process involves three consecutive reactions, each of them requiring an H2O2 molecule, i.e. three H2O2 molecules ligate to the central cobalt cation prior to the formation of radicals. DFT analysis suggests that the transient (H2O)3Co(II)(OOH)2(H2O2) decomposes via: (H2O)3Co(II)(OOH)2(H2O2) → (H2O)3Co(II)(OOH)(˙OOH)(OH) + ˙OH ΔG(0) = -5.975 kcal mol(-1), with no evidence for the formation of a Co(III) transient. It is proposed that analogous mechanisms are involved whenever the redox potential of the central cation is too high to enable the reaction: M(H2O)6(n+) + H2O2 → M((n+1)+)aq + ˙OH + OH(-).

This review documents the development of high-bunch charge electron pulses with sufficient combined spatiotemporal resolution and intensity to literally light up atomic motions. This development holds promise in coming to a first-principles understanding of diverse problems, ranging from molecular reaction dynamics and structure-function correlations in biology to cooperativity in strongly correlated electron-lattice systems. It is now possible to directly observe the key modes involved in propagating structural changes and the enormous reduction in dimensionality that occurs in barrier crossing regions, which is central to chemistry and makes reaction mechanisms transferrable concepts. This information will help direct theoretical advances that will undoubtedly lead to generalized principles with respect to scaling relations in structural dynamics that will bridge chemistry to biology. In this quest, the limitations and future directions for further development are discussed to give an overview of the present status of the field.

Compressed sensing is a method that allows a significant reduction in the number of samples required for accurate measurements in many applications in experimental sciences and engineering. In this work, we show that compressed sensing can also be used to speed up numerical simulations. We apply compressed sensing to extract information from the real-time simulation of atomic and molecular systems, including electronic and nuclear dynamics. We find that, compared to the standard discrete Fourier transform approach, for the calculation of vibrational and optical spectra the total propagation time, and hence the computational cost, can be reduced by approximately a factor of five.

The intermolecular hydroaminoalkylation of unactivated alkenes and vinyl arenes with secondary amines occurs readily in the presence of tantalum and niobium binaphtholate catalysts with high regio- and enantioselectivity (up to 98% ee). Mechanistic studies have been conducted in order to determine the kinetic order of the reaction in all reagents and elucidate the rate- and stereodetermining steps. The effects of substrate steric and electronic properties on the overall reaction rate have been evaluated. The reaction is first order in amine and the catalyst, while exhibiting saturation in alkene at high alkene concentration. Unproductive reaction events including reversible amine binding and arene C-H activation have been observed. The formation of the metallaaziridine is a fast reversible nondissociative process and the overall reaction rate is limited either by amide exchange or alkene insertion, as supported by reaction kinetics, kinetic isotope effects, and isotopic labeling studies. These results suggest that the catalytic activity can be enhanced by employing a more electron-deficient ligand backbone.

A highly regioselective base-mediated ring expansion of 2,4-diiminoazetidines via cleavage of C-N and C(sp(3))-H bonds is achieved for the first time to afford efficiently 2,3-dihydropyrimidinesulfonamides. The mechanism of the ring expansion via tandem 4π electrocyclic ring-opening/1,5-H shift/6π electrocyclic ring-closing is well confirmed by the trapping experiments of two key intermediates and deuterium labeling studies.

In modern methods for the preparation of small molecules and polymers, the insertion of substrate carbon-carbon double bonds into metal-carbon bonds is a fundamental step of paramount importance. This issue is illustrated by Mizoroki-Heck coupling as the most prominent example in organic synthesis and also by catalytic insertion polymerization. For unsymmetric substrates H(2)C = CHX the regioselectivity of insertion is decisive for the nature of the product formed. Electron-deficient olefins insert selectively in a 2,1-fashion for electronic reasons. A means for controlling this regioselectivity is lacking to date. In a combined experimental and theoretical study, we now report that, by destabilizing the transition state of 2,1-insertion via steric interactions, the regioselectivity of methyl acrylate insertion into palladium-methyl and phenyl bonds can be inverted entirely to yield the opposite "regioirregular" products in stoichiometric reactions. Insights from these experiments will aid the rational design of complexes which enable a catalytic and regioirregular Mizoroki-Heck reaction of electron-deficient olefins.

The [NiFe] membrane-bound hydrogenase from the microaerophilic, hyperthermophilic Aquifex aeolicus bacterium (Aa Hase) presents oxygen, carbon monoxide, and temperature resistances. Since it oxidizes hydrogen with high turnover, this enzyme is thus of particular interest for biotechnological applications, such as biofuel cells. Efficient immobilization of the enzyme onto electrodes is however a mandatory step. To gain further insight into the parameters governing the interfacial electron process, cyclic voltammetry was performed combining the use of a phenothiazine dye with a membrane electrode design where the enzyme is entrapped in a thin layer. In the absence of the phenothiazine dye, direct electron transfer (DET) for H(2) oxidation is observed due to Aa Hase adsorbed onto the PG electrode. An unexpected loss of the catalytic current with time is however observed. The effect of toluidine blue O (TBO) on the catalytic process is first studied with TBO in solution. In addition to the expected mediated electron transfer process (MET), TBO is demonstrated to reconnect directly some Aa Hase molecules possibly released from the electrode but still entrapped in the thin layer. On adsorbed TBO the two same processes occur demonstrating the ability of the TBO film to connect Aa Hase via a DET process. Loss of activity is however observed due to the poor stability of adsorbed TBO at high temperatures. Aa Hase immobilization is then studied on electropolymerized TBO (pTBO). The effect of film thickness, temperature, presence of inhibitors and pH is evaluated. Given a film thickness less than 20 nm, H(2) oxidation proceeds via a mixed DET/MET process through the pTBO film. A high and very stable H(2) oxidation activity is reached, showing the potential applicability of the bioelectrode for biotechnologies. Finally, the multifunctional roles of TBO-based matrix are underlined, including redox mediator, Aa Hase anchor, but also buffering and ROS scavenger capabilities to drive pH local changes and avoid oxidative damage.

This study aims to present a detailed theoretical investigation of noncovalent intermolecular interactions between different π-π stacking phenothiazine derivatives and between different alkane chains varying from propane to decane. Second-order Møller-Plesset perturbation (MP2), coupled cluster (CC), and density functional (DFT) theories were the quantum chemistry methods used in our calculation. For MP2 and CC methods, the density-fitting and local approximations were taken into account, while in the case of DFT, the M06 and M06-2x hybrid meta-GGA exchange-correlation functionals as well as the semiempirical correction to the DFT functional for dispersion (BLYP-D) was considered. The results obtained with the aforementioned methods were compared with the potential energy curve given by the DF-SCSN-LMP2 theory considered as benchmark. For all these calculations, the correlation-consistent basis sets of cc-pVNZ (where N = D, T, Q) were used. In addition, potential energy curves built using the semiempirical PM6-D2 and the MM3 molecular force field methods were also compared with the benchmark curve and their efficiency was discussed. As the next step, several geometry conformations were investigated for both phenothiazine derivatives and alkane chain dimers. It was found that the conformational stability of these molecular systems is exclusively given by the dispersion-type electron correlation effects. The density functional tight-binding (DFTB) method applied for dimer structures was compared with the results obtained by the higher level local perturbation theory method, and based on these conclusions larger phenothiazine derivative oligomers structures were investigated. Finally, the optimal configuration of the complex molecular systems built by phenothiazine derivative, alkane chain fragments, and thiol groups was determined, and their self-assembling properties were discussed.

Ferrate(VI) (Fe(VI)O(4)(-), Fe(VI)) and ferrate(V) (Fe(V)O(4)(3-), Fe(V)) have a high oxidizing power and upon decomposition form a nontoxic byproduct, Fe(III), which makes them environmentally friendly oxidants in water and wastewater treatment. The kinetics of the reaction between Fe(VI) and I(-) was determined using a stopped-flow technique. The second-order rate constants (k, M(-1) s(-1)) for the oxidation of I(-) and other inorganic compounds by protonated ferrate(VI) (HFe(VI)O(4)(-)) and ferrate(V) (Fe(V)O(4)(3-)) ions were correlated with thermodynamic reduction potentials to understand the reaction mechanisms. A linear relationship was found between log k and 1-e(-) reduction potentials for iodide, cyanides, and superoxide while oxy-compounds of nitrogen, sulfur, selenium, and arsenic demonstrated a linear relationship for 2-e(-) reduction potentials. The estimated limits for the reduction potentials of the couple are E(o)(Fe(VI)/Fe(V)) >or= 0.76 V and E(o)(Fe(VI)/Fe(IV)) >or= -0.18 V. Conclusions drawn from the correlations are consistent with the mechanisms postulated from stoichiometries, intermediates, and products of the reactions. Implication of the kinetic results in treatment using ferrate(VI) is briefly discussed.

Computer-based and theoretical approaches to chemical problems can provide atomistic understanding of complex processes at the molecular level. Examples ranging from rates of ligand-binding reactions in proteins to structural and energetic investigations of diastereomers relevant to organo-catalysis are discussed in the following. They highlight the range of application of theoretical and computational methods to current questions in chemical research.

This work presents a new approach to predict thermal stability of nitroaromatic compounds based on quantum chemical calculations and on quantitative structure-property relationship (QSPR) methods. The data set consists of 22 nitroaromatic compounds of known decomposition enthalpy (taken as a macroscopic property related to explosibility) obtained from differential scanning calorimetry. Geometric, electronic and energetic descriptors have been selected and computed using density functional theory (DFT) calculation to describe the 22 molecules. First approach consisted in looking at their linear correlations with the experimental decomposition enthalpy. Molecular weight, electrophilicity index, electron affinity and oxygen balance appeared as the most correlated descriptors (respectively R(2)=0.76, 0.75, 0.71 and 0.64). Then multilinear regression was computed with these descriptors. The obtained model is a six-parameter equation containing descriptors all issued from quantum chemical calculations. The prediction is satisfactory with a correlation coefficient R(2) of 0.91 and a predictivity coefficient R(cv)(2) of 0.84 using a cross validation method.

Three series of thioglycosyl donors differing only in their respective aglycon substituents within each series have been prepared as representatives of typical glycosyl donors. The relative anomeric reactivities of these donors were quantified under competitive glycosylation conditions with various reaction time, promoters, solvents and acceptors. Over three orders of magnitude reactivity difference were generated by simple transformation of the para-substituent on the aglycon with methanol as the acceptor, while chemoselectivities became lower with carbohydrate acceptors. Excellent linear correlations were attained between relative reactivity values of donors and sigma(p) values of the substituents in the Hammett plots. This indicates that the glycosylation mechanism remains the same over a wide range of reactivities and glycosylation conditions. The negative slopes of the Hammett plots suggested that electron donating substituents expedite the reactions and the magnitudes of slopes can be rationalized by neighboring group participation as well as electronic properties of the glycon protective groups. Within the same series of donors, less nucleophilic acceptors gave smaller slopes in their Hammett plots. This is consistent with the notion that acceptor nucleophilic attack onto the reactive intermediate is part of the rate limiting step of the glycosylation reaction.

A systematic study of various derivatives of malonaldehyde has been carried out to explore very short hydrogen bonds (r(OO) < 2.450 A). Various electron-withdrawing groups, including CN, NO(2), and BH(2), have been attached to the central carbon atom, C(2). To C(1) and C(3), strong electron donors and/or sterically hindered substituents were used to strengthen the intramolecular hydrogen bond, including but not limited to NH(2), N(CH(3))(2), and C(CH(3))(3). Seven molecules (Figure 2 ) were found to have extremely short intramolecular hydrogen bonds. The chemical systems investigated are intriguing due to their low energetic barriers for the intramolecular hydrogen atom transfers. Classical barriers were predicted using correlated methods including second-order perturbation theory and coupled cluster theory in conjunction with the Dunning hierarchy of correlation consistent basis sets, cc-pVXZ (X = D, T, Q, 5). Focal point analyses allowed for the barriers to be evaluated at the CBS limit including core correlation and zero-point vibrational energy corrections. B3LYP energies are benchmarked against highly accurate correlated energies for intramolecular hydrogen bonded systems. The focal point extrapolated method, including coupled cluster full triple excitation contributions, gives a hydrogen transfer barrier for malonaldehyde of approximately 4 kcal mol(-1). We describe two compounds with extremely low classical barriers, nitromalonamide (0.43 kcal mol(-1)) and 2-borylmalonamide (0.60 kcal mol(-1)). An empirical relationship was drawn between the B3LYP energetic barriers and the predicted coupled cluster barriers at the CBS limit. By relating these two quantities, barrier heights may be estimated for systems too large to presently use highly correlated electronic structure methods.

Co(III) complexes of N(3)O-donor tripodal ligands, 2,4-di(tert-butyl)-6-{[bis(2-pyridyl)methyl]aminomethyl}phenolate (tbuL), 2,4-di(tert-butyl)-6-{[bis(6-methyl-2-pyridyl)methyl]aminomethyl}phenolate (tbuL(Mepy)(2)), were prepared, and precursor Co(II) complexes, [Co(tbuL)Cl] (1) and [Co(tbuL(Mepy)(2))Cl] (2), and ternary Co(III) complexes, [Co(tbuL)(acac)]ClO(4) (3), [Co(tbuL)(tbu-cat)] (4), and [Co(tbuL(Mepy)(2))(tbu-SQ)]ClO(4) (5), where acac, tbu-cat, and tbu-SQ refer to pentane-2,4-dionate, 3,5-di(tert-butyl)catecholate, and 3,5-di(tert-butyl)semiquinonate, respectively, were structurally characterized by the X-ray diffraction method. Complexes 3 and 5 have a mononuclear structure with a fac-N(3)O(3) donor set, while 4 has a mer-N(3)O(3) structure. The cyclic voltammogram (CV) of complex 3 exhibited one reversible redox wave centered at 0.93 V (vs Ag/AgCl) in CH(3)CN. Complex 5 was converted to a phenoxyl radical species upon oxidation with Ce(IV), showing a characteristic pi-pi* transition band at 412 nm. The ESR spectrum at low temperature and the resonance Raman spectrum of 3 established that the radical species has a Co(III)-phenoxyl radical bond. On the other hand, the CVs showed two oxidation processes at E(1/2) = 0.01 and E(pa) = 0.92 V for 4 and E(1/2a) = 0.05 and E(1/2b) = 0.69 V for 5. The rest potential of 4 (-0.11 V) was lower than the E(1/2) value, whereas that of 5 (0.18 V) was higher, indicating that the first redox wave of 4 and 5 is assigned to the tbu-cat and the tbu-SQ redox process, respectively. One-electron oxidized 4 showed absorption, resonance Raman, and ESR spectra which are similar to those of 5, suggesting formation of a stable Co(III)-semiquinonate species, which has the same oxidation level of 5. The resonance Raman spectrum of two-electron oxidized 4 showed the nu(8a) bands of the semiquinonate and phenoxyl radical, which were absent in the spectrum of one-electron oxidized 5. Since both oxidized species were ESR inactive at 5 K, the former was concluded to be a biradical species containing semiquinonate and phenoxyl radicals coupled antiferromagnetically and the latter to a species with a coordinated quinone.

An electron-transfer chain reaction between 2-nitropropane anion and alpha-bromoketones derived from nitrobenzene and nitrothiophene was demonstrated by mechanistic study and a specific convenient synthetic protocol. Thus, 2-bromo-1-(5-nitrothiophen-2-yl)ethanone or 2-bromo-1-(4-nitrophenyl)ethanone were reacted with several cyclic nitronate anions to form alpha,beta-unsaturated ketones via a S(RN)1 mechanism. This new method can be used to synthesize a wide variety of chalcone analogues.

Two modifications of the resolution of the identity (RI)/density fitting (DF) approximations are presented. First, we apply linear scaling and J-engine techniques to speed up traditional DF. Second, we develop an algorithm that produces local, accurate fits with effort that scales linearly with system size. The fits produced are continuous, differentiable, well-defined, and do not require preset fitting domains. This metric-independent technique for producing a priori local fits is shown to be accurate and robust even for large systems. Timings are presented for linear scaling RI/DF calculations on large one-, two-, and three-dimensional carbon systems.

Mixed quantum-classical statistical mechanics is employed to calculate dipole moment correlation function and linear absorption spectra. A quantum two-level subsystem interacting with quantum vibrations (primary oscillators) which in turn are coupled to a classical bath composed of infinite set of harmonic oscillators is used as a dissipative system. Starting with mixed quantum-classical Liouville equation for the evaluation of the mixed quantum-classical dipole moment correlation function and using coherent states and the inverse of Baker-Campbell-Hausdorf formula to evaluate the trace over the primary oscillators, whereby, a closed analytical expression for the electronic dipole moment correlation function is obtained. Illustrations of several absorption spectra at different temperatures are provided. An approximate optical four-point correlation is obtained in the high temperature limit. A strategy for deriving an exact optical four-point correlation is suggested.

Operations on maps are well-known theoretical tools for transforming a given polyhedral tessellation. Several theoretical investigations of fullerenes, such as their pi-electronic structure and stability, need information on the original map which was transformed into a larger molecular structure. In this respect, retro-operations, particularly those of the most used leapfrog, chamfering, and capra operations, appear particularly useful in searching the associate graphs of fullerenes. A series of analyzed cages proved to be leapfrog transforms of smaller cages. This information was useful in understanding their closed pi-electronic structure and related properties including the local aromaticity. An index based on the optimized geometries enabled the evaluation of aromaticity of their various substructures. Pictorial images of the pi-electron distribution as the main Kekulé valence structures have been performed by the aid of the JSCHEM software package.

The syntheses and electron paramagnetic resonance (EPR) spectral characterizations of porphyrins (1-3) substituted with two radical groups bound to trans-meso positions are described. One of these compounds, 3, has been studied by variable-temperature magnetic susceptibility and has been structurally characterized. Biradical porphyrin 3 is monoclinic, space group P2(1)/n, with a = 12.239(2) A, b = 17.819(3) A, c = 34.445(7) A, alpha = 90 degrees , beta = 97.466(3) degrees , gamma = 90 degrees , and Z = 2. The bis(nitroxide) porphyrins 1 and 2 exhibit fluid solution EPR spectra consistent with |J| >> |a|. No evidence was observed for conformational modulation of J by rotation about single bonds as shown by the lack of change of the EPR spectra as a function of temperature. The bis(semiquinone) porphyrin 3 exhibits frozen-solution EPR spectra with zero-field splitting and a Deltam(s) = 2 transition characteristic of a triplet state. The intensity of the Deltam(s) = 2 transition of 3 was measured as a function of temperature, and the data fit according to a singlet-triplet model to yield J(3,solution) = -75 cm(-1) (H = - 2Jŝ1.ŝ2). Polycrystalline samples of porphryin 3 were examined by variable-temperature magnetometry. The paramagnetic susceptibility data were fit using a modified Bleaney-Bowers equation to give J(3,solid) = -29 cm(-1) (H = - 2Jŝ(1).ŝ(2)). The antiferromagnetic J values are consistent with the pi topology of the porphyrin ring.

Boron dipyrromethene dyes bearing nitro, amino, isocyanate and isothiocyanate functions were readily prepared under mild conditions. Various combinations allow to produce urea, diurea, thiourea, dithiourea in the 3, 4 and 5-substitution positions of the appended phenyl group. Condensation of the 3,4-substituted diamino derivative with 1,10-phenanthroline-5,6-dione and 6-formyl-2-methylpyridine allow to prepare dipyridophenazine and indole derivatives. The 3,5-dinitro-substituted indacene dye was characterized by an X-ray molecular structure showing a pronounced tilt angle of the dinitrophenyl group relative to the indacene core (approximately 84 degrees) whereas one nitro groups is basically coplanar with the phenyl ring and the second titled by approximately 21 degrees. The optical properties of these dyes reveals on/off switching of the fluorescence from the nitro to the amino compounds and further to the urea likely understood in the framework of an photoinduced electron transfer process.

We report a theoretical study based on density functional theory (DFT) and time-dependent DFT (TDDFT) calculations on the nature and role of the absorption bands involved in the nonlinear optical response of the complexes [Ru(CF3CO2)3T] (T = T1, T2; T1 = 4'-(C6H4-p-NBu2)-2,2':6',2''-terpyridine, T2 = 4'-(C6H4-p-NMe2)-2,2':6',2''-terpyridine). Geometry optimizations, performed without any symmetry constraints, confirm a twisting of the -C6H4-p-NBu2 moiety with respect to the plane of the chelated terpyridine. Despite this lack of strong pi interaction, TDDFT excited states calculations of the electronic spectrum in solution provide evidence of a relevant role of the NBu2 donor group in the low-energy LMCT band at 911 nm. Calculations also show that the two bands at higher energy (508 and 455 nm) are not attributable only to LMCT and ILCT transitions but to a mixing of ILCT/MLCT and ILCT/pi-pi* transitions, respectively. The 911 nm LMCT band, appearing at lower wavelength of the second harmonic (670 nm) of the EFISH experiment, controls the negative value of the second-order NLO response. This is confirmed by our calculations of the static component beta0(zzz) of the quadratic hyperpolarizability tensor, showing a large positive value. In addition we have found that the increase of the dipole moment upon excitation occurs, in all the characterized transitions, along the dipole moment axis, thus explaining why the EFISH and solvatochromic experimental values of the quadratic hyperpolarizability agree as sign and value.

Recently, we reported that a calix[6]arene carboxylic acid derivative can selectively extract the lysine-rich protein cytochrome c by interacting with amino groups on the protein surface. In the present article, quantitative extraction and recovery of cytochrome c using this calix[6]arene carboxylic acid derivative are described. Both adjustment of the pH under acidic conditions and addition of an alcohol are necessary to strip the extracted protein from an organic solution to an aqueous solution. Separation of cytochrome c and lysozyme using the calix[6]arene was achieved under the optimal conditions. In the forward extraction stage, 93% of the cytochrome c was extracted, while lysozyme remained in the solution. In the subsequent stripping stage, the extracted cytochrome c was quantitatively recovered in an aqueous solution. Finally, separation of these proteins, which have similar molecular weights and isoelectric points, was accomplished.

Various linear and nonlinear vibrational and electronic spectroscopy experiments in liquids are usually analyzed within the second-cumulant approximation, and therefore the fundamental quantity of interest is the equilibrium time-correlation function of the fluctuating transition frequency. In the usual approach the "bath" variables responsible for the fluctuating frequency are treated classically, leading to a classical time-correlation function. Alternatively, sometimes a quantum correction appropriate for relatively high temperatures is included, which adds an imaginary part to the classical time-correlation function. This approach, although appealing, does not satisfy detailed balance. One can consider a similar correction, but where detailed balance is satisfied, by using the harmonic quantum correction factor. In this article, we compare these approaches for a model system and two realistic examples. Our conclusion is that for linear spectroscopy the classical result is usually adequate, whereas for nonlinear spectroscopy it can be more important to include quantum corrections.

One way to reduce the computational cost of electronic structure calculations is to use auxiliary basis expansions to approximate four-center integrals in terms of two- and three-center integrals, usually by using the variationally optimum Coulomb metric to determine the expansion coefficients. However, the long-range decay behavior of the auxiliary basis expansion coefficients has not been characterized. We find that this decay can be surprisingly slow. Numerical experiments on linear alkanes and a toy model both show that the decay can be as slow as 1/r in the distance between the auxiliary function and the fitted charge distribution. The Coulomb metric fitting equations also involve divergent matrix elements for extended systems treated with periodic boundary conditions. An attenuated Coulomb metric that is short-range can eliminate these oddities without substantially degrading calculated relative energies. The sparsity of the fit coefficients is assessed on simple hydrocarbon molecules and shows quite early onset of linear growth in the number of significant coefficients with system size using the attenuated Coulomb metric. Hence it is possible to design linear scaling auxiliary basis methods without additional approximations to treat large systems.

The ability to manipulate electron spin in organic molecular materials offers a new and extremely tantalizing route towards spin electronics, both from fundamental and technological points of view. This is mainly due to the unquestionable advantage of weak spin-orbit and hyperfine interactions in organic molecules, which leads to the possibility of preserving spin-coherence over times and distances much longer than in conventional metals or semiconductors. Here we demonstrate theoretically that organic spin valves, obtained by sandwiching an organic molecule between magnetic contacts, can show a large bias-dependent magnetoresistance and that this can be engineered by an appropriate choice of molecules and anchoring groups. Our results, obtained through a combination of state-of-the-art non-equilibrium transport methods and density functional theory, show that although the magnitude of the effect varies with the details of the molecule, large magnetoresistance can be found both in the tunnelling and the metallic limit.

Infrared absorbance measurements have been carried out on two liquid crystalline organo-siloxane tetrapodes. Results unambiguously show the existence of a biaxial nematic phase below a uniaxial nematic phase. The three components of IR absorbance are used to calculate the various order parameters. On cooling, a weak first-order transition from isotropic to nematic is observed, followed by a second-order phase transition to biaxial nematic where the biaxiality parameters are found to be significantly large. Results are supported by observations from conoscopy and texture. Temperature dependences of the order parameters are well explained by the mean-field model for a biaxial phase.

A qualitative description of the electronic structure of single-wall carbon nanotubes from a chemical perspective is presented using real-space orbital representations and traditional concepts of aromaticity, orbital symmetry and frontier orbitals. This unusual view of carbon nanotubes allows us to merge the solid-state physics description of band structures with the molecular orbitals framework of reaction mechanisms used in organic chemistry and to predict intriguing chemical selectivity based on electronic structure.

Ahmed Zewail won the 1999 Nobel Prize in chemistry for his visionary work in probing the motions of atoms at the femtosecond level. This pioneering research, a decade earlier, opened up a new frontier of scientific knowledge. Zewail and his multidisciplinary team at Caltech are now pushing further into the realm of molecular complexity, with the ultimate aim of exploring the global dynamics of biological systems at atomic resolution. This requires a new method of 'watching' reactions--ultrafast electron diffraction.

This Account focuses on our recent and systematic effort in the development of generic scanning probe lithography (SPL)-based methodologies to produce nanopatterns of self-assembled monolayers (SAMs). The key to achieving high spatial precision is to keep the tip-surface interactions strong and local. The approaches used include two AFM-based methods, nanoshaving and nanografting, which rely on the local force, and two STM-based techniques, electron-induced diffusion and desorption, which use tunneling electrons for fabrication. In this Account we discuss the principle of these procedures and the critical steps in controlling local tip-surface interactions. The advantages of SPL will be illustrated through various examples of production and modification of SAM nanopatterns and their potential applications.