The Static Polarizability and Second Hyperpolarizability of Fullerenes and Carbon Nanotubes†

An atomistic model for the charge distribution in layered MoS2

We present an atomistic model for predicting the distribution of doping electric charges in layered molybdenum disulfide (MoS₂). This model mimics the charge around each ion as a net Gaussian-spatially distributed charge plus an induced dipole, and is able to predict the distribution of doping charges in layered MoS₂ in a self-consistent scheme. The profiles of doping charges in monolayer MoS₂ flakes computed by this charge-dipole model are in good agreement with those obtained by density-functional-theory calculations. Using this model, we quantitatively predict the charge enhancement in MoS₂ monolayer nanoribbons, with which strong ionic charge-localization effects are shown.

Effect of alkaline earth metal at the single wall CNT mouth on the electronic structure and second hyperpolarizability

In the present work, electronic structure and second hyperpolarizability of a number of alkaline earth metals (M [Formula: see text] Be, Mg and Ca) complexes with carbon nanotube (CNT) has been studied by using different DFT functional. The complexes have sufficient thermal stability. Significant amount of charge transfer from metal to CNT results in stronger ground state polarization. The second hyperpolarizability obtained at different DFT functional (BHHLYP, CAM-B3LYP, B2PLYP, [Formula: see text]B97XD) showed a consistent trend. The magnitude of second hyperpolarizability of M@CNT[3,0] complexes enhances rather appreciably when a second metal atom is introduced into other mouth position. The longitudinal component of second hyperpolarizability of M@CNT[3,0]@M complexes increases with increasing size of metal atom. The magnitude of second hyperpolarizability of Ca@CNT[3,0]@Ca complex is comparable with Fe([Formula: see text]-C[Formula: see text]B[Formula: see text]. However, widening/lengthening of CNT markedly reduces the cubic responses. The two state model can qualitatively explain the variation of second hyperpolarizability.

Frequency-Dependent Polarizabilities of Amino Acids as Calculated by an Electrostatic Interaction Model

The frequency-dependent polarizability of the 20 essential amino acids has been calculated by an electrostatic interaction model where an Unsöld-type of model has been adopted for the frequency dependence. The interaction model has previously been parametrized from Hartree-Fock calculations on a set of molecules, and the model is in this work extended by sulfur parameters by including a set of 18 small sulfur compounds. The results for the amino acids by using the interaction model compare well with Hartree-Fock calculations with deviations of around 5% for the isotropic polarizability. Furthermore, the intrinsic (or optical) dielectric constant related to the polarizability has been calculated for three small proteins, ribonuclease inhibitor, lysozyme, and green fluorescent protein, adopting the interaction model. The results are consistent with the intrinsic dielectric constants found for proteins in the literature.

The influence of distribution of hydroxyl groups on vibrational spectra of fullerenol C60(OH)24 isomers: DFT study

The infrared and Raman spectra of C60(OH)24 molecule with uniform and non-uniform distribution of hydroxyl groups have been investigated using first principle DFT calculations at the B3LYP/6-31G(d,p) level of theory. The important features of the obtained geometries have been measured and compared to experimental results. The reference calculations of C60 molecule geometry and vibrational spectra have been made and compared to available experimental data. The striking differences of infrared spectra between C60(OH)24 molecule with uniform and non-uniform distribution of hydroxyl groups have been shown and discussed. The OH modes have been identified as the most sensitive to C60(OH)24 isomer configuration. The C-C stretching modes in the Raman spectra of the C60(OH)24 molecule have been found as a potential sensor of OH groups distribution over fullerene C60 surface.

Local electric field factors by a combined charge-transfer and point–dipole interaction model

A model for the local electric field as a linear response to a frequency-dependent external electric field is presented based on a combined charge-transfer and point–dipole interaction force-field model.

Polarizability as a landmark property for fullerene chemistry and materials science

The review summarizes data on dipole polarizability of fullerenes and their derivatives, covering the most widespread classes of fullerene-containing molecules (fullerenes, fullerene exohedral derivatives, fullerene dimers, endofullerenes, fullerene ions, and derivatives with ionic bonds).

POLARIZABILITY AND SECOND HYPERPOLARIZABILITY OF SEMICONDUCTING SINGLE-WALL CARBON NANOTUBES

Considering the exciton effect, the polarizability and second polarizability spectra of semiconducting single-wall carbon nanotubes (S-SWCNTs) have been calculated by PPP (Pariser, Parr, and Pople) approximation and the single-excitation configuration interaction (single-CI) method and sum-over-state method. We have studied the nonlinear optical (NLO) properties of the S-SWCNTs with different lengths and diameters. The highest and first peak positions, bandgap, and the spectra shape of the polarizability and second hyperpolarizability change as N (the number of atoms) increases. Also, the peak values of γzzzz are very bigger than that for γxxxx. Our results indicate that the nonlinear optical properties of fullerenes are similar to S-SWCNTs.

Nonmetallic electronegativity equalization and point-dipole interaction model including exchange interactions for molecular dipole moments and polarizabilities

The electronegativity equalization model (EEM) has been combined with a point-dipole interaction model to obtain a molecular mechanics model consisting of atomic charges, atomic dipole moments, and two-atom relay tensors to describe molecular dipole moments and molecular dipole-dipole polarizabilities. The EEM has been phrased as an atom-atom charge-transfer model allowing for a modification of the charge-transfer terms to avoid that the polarizability approaches infinity for two particles at infinite distance and for long chains. In the present work, these shortcomings have been resolved by adding an energy term for transporting charges through individual atoms. A Gaussian distribution is adopted for the atomic charge distributions, resulting in a damping of the electrostatic interactions at short distances. Assuming that an interatomic exchange term may be described as the overlap between two electronic charge distributions, the EEM has also been extended by a short-range exchange term. The result is a molecular mechanics model where the difference of charge transfer in insulating and metallic systems is modeled regarding the difference in bond length between different types of system. For example, the model is capable of modeling charge transfer in both alkanes and alkenes with alternating double bonds with the same set of carbon parameters only relying on the difference in bond length between carbon sigma- and pi-bonds. Analytical results have been obtained for the polarizability of a long linear chain. These results show that the model is capable of describing the polarizability scaling both linearly and nonlinearly with the size of the system. Similarly, a linear chain with an end atom with a high electronegativity has been analyzed analytically. The dipole moment of this model system can either be independent of the length or increase linearly with the length of the chain. In addition, the model has been parametrized for alkane and alkene chains with data from density functional theory calculations, where the polarizability behaves differently with the chain length. For the molecular dipole moment, the same two systems have been studied with an aldehyde end group. Both the molecular polarizability and the dipole moment are well described as a function of the chain length for both alkane and alkene chains demonstrating the power of the presented model.

Enthalpy and entropy effects in hydrogen adsorption on carbon nanotubes

Interaction energies and entropies associated with hydrogen adsorption on the inner and outer surfaces of zigzag single-wall carbon nanotubes (SWCNT) of various diameters are analyzed by means of molecular mechanics, density functional theory, and ab initio calculations. For a single molecule the strongest interaction, which is 3.5 greater than that with the planar graphite sheet, is found inside a (8,0) nanotube. Adsorption on the outer surfaces is weaker than that on graphite. Due to the steric considerations, both processes are accompanied by an extremely strong decline in entropy. Absence of specific adsorption sites and weak attractive interaction between hydrogen molecules within carbon nanotubes results in their close packing at low temperatures. Using the calculated geometric and thermodynamic parameters in Langmuir isotherms we predict the adsorption capacity of SWCNTs at room temperature to be smaller than 1 wt % even at 100 bar.

An unusual feature of end-substituted model carbon (6,0) nanotubes

We have examined the effects of substituents on the computed electrostatic potentials V(S)(r) and average local ionization energies I(S)(r) on the surfaces of model carbon nanotubes of the types (5,5), (6,1) and (6,0). For the (5,5) and the (6,1), the effects upon both V(S)(r) and I(S)(r) of substituting a hydroxyl group at one end are primarily localized to that part of the system. For the (6,0) tube, however, a remarkable change is observed over its entire length, with V(S)(r) showing a marked gradation from strongly positive at the substituted end to strongly negative at the other; I(S)(r) correspondingly goes from higher to lower values. Replacing OH by another resonance- donor, NH2, produces similar results in the (6,0) system, while the resonance withdrawing NO2 does the opposite, but in equally striking fashion. We explain these observations by noting that the arrangement of the C-C bonds in the (6,0) tube facilitates charge delocalization over the full length and entire surface of the tube. Substituting NH2 and NO2 at opposite ends of the (6,0) tube greatly strengthens the gradations in both V(S)(r) and I(S)(r). The first hyperpolarizability of this system was found to be nine times that of para-nitroaniline, suggesting possible nonlinear optical applications. [figure: see text]. HF/STO-5G electrostatic potential on outer surface of open (6,0) C72H10NH2NO2. The nitro group is at the right end of the tube, the amino group at the left. In eV: purple is less than 14, blue is between 14 and 15, green is between 15 and 16.5, yellow is between 16.5 and 17.5, and red is more than 17.5.