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Density functional and atoms‐in‐molecules (AIM) and natural bond orbital (NBO) approaches have been applied in the study of energetic (E), geometrical (G) and electronic (AIM and NBO) consequences of H bonding in malonaldehyde (MAE) derivatives and naphthazarin (NZ). AIM parameters and other measures of HB strength were used: (a) for the verification of (i) the reliability of the O···O distance (G consequence) as an indicator of IMHB strength; (ii) the capacity of the classically computed energetic parameters (ΔESEs) to serve as acceptable measures of IMHB strength; and (b) for the separation of the ΔESEs into (i) stabilization (HB) energies (EHSEs) that serve as apparent IMHB energies (EHB,As), and (ii) stabilization (isomerization) energies (ENSEs) that do not (owing to intractable contributions that are not germane to the solitary HB donor(D)‐acceptor(A) interactions). Some of the sources of the anomalies have been rationalized. AIM topological properties were used to study the nature of the IMHB interactions. An exponential parametric model for the correlation of EHSE with the O···O distance, which has asymptotic characteristics at long O···O distances, was obtained. The model (a) has predictive ability, that is, can be used to estimate, in an empirical manner, EHB,As that are otherwise grossly underestimated, and (b) can treat both the MAE derivatives and the NZ systems even though they possess very different resonant spacers connecting the HB D‐A segments. MAE and NZ are also demonstrated to have essentially the same IMHB strength. By contrast, a quadratic model for EHSE‐HB distance correlation was found to be unphysical. Use of electronic consequences of H bonding was shown to be essential for study of IMHBs with intractable interactions. Thus, AIM energy density and NBO second‐order interaction energy parameters were used for the verification of predictions of IMHB strengths made on the bases of energetic and geometrical consequences. Copyright © 2006 John Wiley & Sons, Ltd.

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First-principles density functional theory has been employed to study the composition dependent quantum size effect in a series of silicon-germanium core-shell structured nanowires with the diameters ranging from 0.5 to 3.2 nm. Analysis of the calculated band gap energies in Si-core/Ge-shell and Ge-core/Si-shell structured nanowires shows a nonlinear composition dependence for nanowires with fixed diameter ͑fixed total number N = N Core + N Shell of Si and Ge atoms in the unit cell͒. In contrast, for nanowires with fixed core size and varying shell thickness, our calculation results reveal a striking linear blueshift of the direct band gap with composition. The obtained linear composition effect implies an inverse square relation between the energy of the fundamental band gap and the size of nanowire, in agreement with experimental observations. Our results provide useful guidelines for experimental gap engineering in the core-shell structured nanowire heterostructures.There has been fast-growing interest in nanometer-scale semiconductor materials such as quantum dots ͑0-D͒, quantum wires ͑1-D͒, and quantum wells ͑2-D͒. 1-3 These nanostructures constitute an intriguing class of semiconductor materials due to unique size dependent electronic and optical properties that are intrinsically associated with their low dimensionality and quantum confinement effect. 3,4 It has been established that the optical properties of nanocrystals strongly depend on the ratio of the nanocrystal radius, R, to the Bohr radius of the bulk exciton, R B . 5 Three different regions, R ӷ R B , R ϳ R B , and R Ӷ R B , corresponding to weak, medium, and strong confinement regimes, respectively, have been proposed. The confinement of electrons and holes in nanoscale crystallites leads to an increase in the electronic band gap with decreasing crystallite size and, consequently, a blueshift of the absorption and emission spectra. A qualitative quantum mechanical approach based on the effective mass approximation and particle-in-a-box models 1-5 predicts that the dependence of the band gap on the radius R of crystallites is of the form E g ϳ 1/R 2 . More detailed analysis of the quantum confinement effect in the framework of firstprinciples and semiempirical methods shows that size dependence of band gaps can be described by an inverse power law E g ϳ 1/R n with a smaller parameter n being between 1 and 2. 6-8 The ability to control the physical and chemical properties of low dimensional nanomaterials by variation of their size is of crucial importance for nanotechnology applications. 2,9 With the progress in synthesis and fabrication of low dimensional nanomaterials, semiconductor nanowires have attracted much attention because of their exceptional electronic, optical, and transport properties. 10 In these quasi-onedimensional materials, the motions of electrons and holes are confined in the radial plane, while the charge carriers are free to move along the nanowire's axis. Semiconductor nanowires can be used as building blocks in the engineeri...

We have derived an analytical effective-mass model and employed first-principles density functional theory to study the spatial confinement of carriers in core-shell and multishell structured semiconductor nanowires. The band offset effect is analyzed based on the subband charge density distributions, which is strongly dependent upon the strain relaxation. First-principles calculation results for spatially confined Si/Ge and GaN/GaP nanowires indicate accumulation of a Ge-core hole gas and a GaN-core electron gas, respectively, in agreement with experimental observations.

2-(3-Pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxy, IM-3Py (1), reacts with copper(II) bis(hexafluoroacetylacetonate), Cu(hfac)2, to give three complexes. One of them is a centrosymmetric three-spin species, Cu(hfac)2(IM-3Py)2 (4), where two nitroxide ligands are pyridyl bound to the metal and the spins independent from 300 to 5 K. The two other complexes, [Cu(hfac)2]4(IM-3Py)2 (α-phases, 5, and β-phases, 6) are six-spin clusters comprising four copper(II) ions linked by two tridentate IM-3Py units. Their molecular structures are almost identical, but they crystallize in different space groups. For both complexes, two octahedral endo metal ions are involved in a cyclic structure and the two others are exocyclic in a trigonal-bipyramidal environment. On the one hand, the magnetic properties of these two clusters are similar since, for both, the high-temperature behavior can only be accounted for by large ferromagnetic interactions; on the other hand they are dramatically different because the α-phase has a S = 3 ground spin state, while the β-phase exhibits two transitions resulting in two independent spins below 70 K. The transition temperatures are T C1 ↑ = 211 K, T C1 ↓ = 152 K, T C2 ↑ = 80 K, and T C2 ↓ = 70 K. With the help of ab initio calculations on model fragments, the presence of a ferromagnetic interaction through the pyridyl ring has been tentatively rationalized invoking McConnell's electron transfer mechanism. The high-temperature transition has been assigned to a change of the N-imino coordination from equatorial in a trigonal-pyramidal coordination to axial in a square-planar pyramidal environment at the exocyclic copper center. The low-temperature transition corresponds to a switch from axial to equatorial of the nitroxyl coordination at the endocyclic octahedral metal ion. Relevant crystallographic parameters are as follows:  4, triclinic, P1̄, a (Å) = 6.955(2), b (Å) = 11.485(2), c (Å) = 14.012(3), α (deg) = 112.55(1), β (deg) = 98.79(1), γ (deg) = 90.28(1), Z = 1; 5, monoclinic, C2/c, a (Å) = 25.319(4), b (Å) = 17.653(3), c (Å) = 19.973(4), β (deg) =96.60(2), Z = 4; 6, triclinic, P1̄, a (Å) = 12.200(2), b (Å) = 13.389(3), c (Å) = 15.457(3), α (deg) = 102.54(2), β (deg) = 110.16(2), γ (deg) = 92.57(2), Z = 1.

A systematic investigation aimed at identifying the transition from moderate (M) to strong (S) hydrogen bonds (HBs) and the physical bases of the main geometry-based HB strength classifications reported in the literature has been undertaken using the quantum theory of atoms in molecules (QTAIM). Correlations between the Laplacian of the electron density (rho) at the O...H hydrogen bond critical points (HBCPs), nabla2rhohb, specifically between the more intuitive parameter Lhb = -nabla2rhohb and other QTAIM parameters, have also been explored. The transition from MHBs to SHBs has been identified as the minimum (maximum) in the geometric dependence of Lhb (nabla2rhohb). For O-H...O intramolecular (IM) HBs (including resonance-assisted HBs), the transition is obtained, in a truly remarkable agreement with the existing geometry-based HB strength classifications, when the O...O (O...H) distance is approximately 2.51 ( approximately 1.55) A and when the ratio of the potential energy density (|Vhb|) to the kinetic energy density (Ghb) approximately 1.3. Accordingly, the ranges of the |Vhb|/Ghb ratios are >2-1.3 and 1.3-1 for, respectively, SHBs and MHBs. When the O...O distance is not a genuine indicator of HB strength, the |Vhb|/Ghb ratio and other parameters should be considered to characterize the strength of the HBs. Rationalizations have been provided by way of decoding the physical bases of the transition in terms of the properties of rho and the mechanical characteristics of the interactions that created the HBCPs. Lhb was found to correlate, with a very high degree of fidelity, with at least three parameters (in addition to O...O and O...H distances and the IMHB energy), Vhb/Ghb, Hhb/rhohb (the ratio of the total energy density, Hhb, to the electron density, rhohb (the so-called bond degree parameter)), and deltahb(O,H) (the delocalization index), demonstrating the importance and utility of Lhb (nabla2rhohb) for the study of HB interactions. A new refined energetics-based classification of O-H...O IMHB strengths has been advanced. The approach taken in this investigation can be extended to other HB systems.

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