ChelpG and QTAIM atomic charge and dipole models for the infrared fundamental intensities of the fluorochloromethanes

ChelpG atomic charges and dipoles and the charge–charge flux–dipole flux (CCFDF) model have been used to quantitatively estimate the fundamental infrared intensities of the fluorochloromethanes. Since the ChelpG calculational procedure includes the constraint that the atomic charges and dipoles reproduce the equilibrium dipole moments the model results in accurate intensity values that have a root mean square error of 0.7 km mol^?1 compared to those determined directly from the MP2/6-311G++(3d,3p) electronic density and 23.1 km mol^?1 relative to the experimental intensities. Although these ChelpG results for total dipole moment derivatives are almost the same as those obtained previously using QTAIM (Quantum Theory of Atoms in Molecules) atomic charges and dipoles in the CCFDF model, their charge, charge flux and dipole flux contributions are completely different. Whereas the contributions calculated using the QTAIM parameters have values following expectations based on electronegativity concepts this is not true for those obtained from the ChelpG parameters. Mean dipole moment derivatives determined from experimental fundamental infrared intensities are compared with the ChelpG and QTAIM atomic charges. Furthermore, Generalized Atomic Polar Tensor Charges (GAPT) are found to need correction for their dynamic contributions if they are to be used as static atomic charges.     

Theoretical studies on the electronic structures and absorption spectra of substituted benzenes with one- and two-dimensional charge transfer character

The electronic structures and absorption spectra of one- and two-dimensional charge transfer (CT) molecules based on para-nitroaniline (pNA) and 1,3-diamino-4,6-dinitro- benzene (DADB) have been studied theoretically via semi-empirical and ab initio methods. It is found that the behaviors of optical absorption are strongly influenced by the dimension of CT. Different from the well-known one-dimensional CT molecule of pNA, which shows one intense absorption related to the π → π^* CT transition, two-dimensional CT molecule of DADB exhibits more absorption peaks associated with various low-lying CT transitions in near ultraviolet range. In addition, the relative orientations of transition dipole moment and ground state dipole moment in one- and two-dimensional charge transfer molecules were also discussed.     

Electric dipole polarity of diatomic molecules

The restricted SCF (single-configuration SCF) and MCSCF (multiconfiguration SCF) calculations are performed to compute the ground-state electric dipole moments of four pairs of diatomic molecules—(1) CO and BF; (2) SiO and AlF; (3) CS and BCl; and (4) SiS and AlCl—at a number of internuclear distances on both sides of the equilibrium position. Near Hartree–Fock accuracy is obtained in the SCF calculations. All eight molecules have a range of internuclear distance in which electric dipole moments are of the polarity of A^?B⁺. The shapes of computed electric dipole moment functions are discussed in the language of the molecular orbital method and in relationship to electronegativities of atoms. The present study gives us deeper understanding of electron transfer inside molecules and consequently of the apparent contradiction between electronegativity and the dipole polarity of some molecules. © John Wiley & Sons, Inc.     

Unusual effect of carborane ligands on the electronic properties of <Emphasis Type="Italic">d</Emphasis><Superscript>0</Superscript>-metal complexes

Modern quantum-chemical and photophysical methods have been used to study the structure of the frontier molecular orbitals and the nature of ligand-to-metal charge transfer (LMCT) transitions of structurally complex d ⁰-metallocenes. It has been shown that such metal complexes with carboranyl ligands have emissive LMCT states with preferential charge transfer from aromatic π-ligands to the metal and a large electric dipole moment. The electronic excitation and absorption spectra were simulated for the first time, and dipole moments of metal complexes containing metal–carbon σ- and π-bonds were estimated, which is of fundamental importance for the development of molecular photonics.     

QTAIM charge-charge flux-dipole flux interpretation of electronegativity and potential models of the fluorochloromethane mean dipole moment derivatives

Infrared fundamental vibrational intensities and quantum theory atoms in molecules (QTAIM) charge-charge flux-dipole flux (CCFDF) contributions to the polar tensors of the fluorochloromethanes have been calculated at the QCISD/cc-pVTZ level. A root-mean-square error of 20.0 km mol(-1) has been found compared to an experimental error estimate of 14.4 and 21.1 km mol(-1) for MP2/6-311++G(3d,3p) results. The errors in the QCISD polar tensor elements and mean dipole moment derivatives are 0.059 e when compared with the experimental values. Both theoretical levels provide results showing that the dynamical charge and dipole fluxes provide significant contributions to the mean dipole moment derivatives and tend to be of opposite signs canceling one another. Although the experimental mean dipole moment derivative values suggest that all the fluorochloromethane molecules have electronic structures consistent with a simple electronegativity model with transferable atomic charges for their terminal atoms, the QTAIM/CCFDF models confirm this only for the fluoromethanes. Whereas the fluorine atom does not suffer a saturation effect in its capacity to drain electronic charge from carbon atoms that are attached to other fluorine and chlorine atoms, the zero flux electronic charge of the chlorine atom depends on the number and kind of the other substituent atoms. Both the QTAIM carbon charges (r = 0.990) and mean dipole moment derivatives (r = 0.996) are found to obey Siegbahn's potential model for carbon 1s electron ionization energies at the QCISD/cc-pVTZ level. The latter is a consequence of the carbon mean derivatives obeying the electronegativity model and not necessarily to their similarities with atomic charges. Atomic dipole contributions to the neighboring atom electrostatic potentials of the fluorochloromethanes are found to be of comparable size to the atomic charge contributions and increase the accuracy of Siegbahn's model for the QTAIM charge model results. Substitution effects of the hydrogen, fluorine, and chlorine atoms on the charge and dipole flux QTAIM contributions are found to be additive for the mean dipole derivatives of the fluorochloromethanes.     

Partial Charge Transfer in the Shortest Possible Metallofullerene Peapod,La@C82⊂[11]Cycloparaphenylene

[11]Cycloparaphenylene ([11]CPP) selectively encapsulates La@C₈₂ to form the shortest possible metallofullerene–carbon nanotube (CNT) peapod, La@C₈₂?[11]CPP, in solution and in the solid state. Complexation in solution was affected by the polarity of the solvent and was 16 times stronger in the polar solvent nitrobenzene than in the nonpolar solvent 1,2‐dichlorobenzene. Electrochemical analysis revealed that the redox potentials of La@C₈₂ were negatively shifted upon complexation from free La@C₈₂. Furthermore, the shifts in the redox potentials increased with polarity of the solvent. These results are consistent with formation of a polar complex, (La@C₈₂)^δ??[11]CPP^δ+, by partial electron transfer from [11]CPP to La@C₈₂. This is the first observation of such an electronic interaction between a fullerene pea and CPP pod. Theoretical calculations also supported partial charge transfer (0.07) from [11]CPP to La@C₈₂. The structure of the complex was unambiguously determined by X‐ray crystallographic analysis, which showed the La atom inside the C₈₂ near the periphery of the [11]CPP. The dipole moment of La@C₈₂ was projected toward the CPP pea, nearly perpendicular to the CPP axis. The position of the La atom and the direction of the dipole moment in La@C₈₂?[11]CPP were significantly different from those observed in La@C₈₂?CNT, thus indicating a difference in orientation of the fullerene peas between fullerene–CPP and fullerene–CNT peapods. These results highlight the importance of pea–pea interactions in determining the orientation of the metallofullerene in metallofullerene–CNT peapods.     

Molecular electric properties in electronic excited states: multipole moments and polarizabilities of H2O in the lowest1 B 1 and3 B 1 excited states

Summary The dipole and quadrupole moments and the dipole polarizability tensor components are calculated for the¹ B ₁ and³ B ₁ excited states of the water molecule by using the complete active space (CAS) SCF method and an extended basis set of atomic natural orbitals. The dipole moment in the lowest¹ B ₁ (0.640 a.u.) and³ B ₁ (0.416 a.u.) states is found to be antiparallel to that in the ground electronic state of H₂O. The shape of the quadrupole moment ellipsoid is significantly modified by the electronic excitation to both states investigated in this paper. All components of the excited state dipole polarizability tensor increase by about an order of magnitude compared to their values in the ground electronic state. The present results are used to discuss some aspects of intermolecular interactions involving molecules in their excited electronic states.     

How does hybrid bridging core modification enhance the nonlinear optical properties in donor‐π‐acceptor configuration? A case study of dinitrophenol derivatives

This study spotlights the fundamental insights about the structure and static first hyperpolarizability (β) of a series of 2,4‐dinitrophenol derivatives (1–5), which are designed by novel bridging core modifications. The central bridging core modifications show noteworthy effects to modulate the optical and nonlinear optical properties in these derivatives. The derivative systems show significantly large amplitudes of first hyperpolarizability as compared to parent system 1 , which are 4, 46, 66, and 90% larger for systems 2, 3, 4 , and 5 , respectively, at Moller–Plesset (MP2)/6‐31G* level of theory. The static first hyperpolarizability and frequency dependent coupled‐perturbed Kohn–Sham first hyperpolarizability are calculated by means of MP2 and density functional theory methods and compared with respective experimental values wherever possible. Using two‐level model with full‐set of parameters dependence of transition energy (ΔΕ), transition dipole moment (μ⁰) as well as change in dipole moment from ground to excited state (Δμ), the origin of increase in β amplitudes is traced from the change in dipole moment from ground to excited state. The causes of change in dipole moments are further discovered through sum of Mulliken atomic charges and intermolecular charge transfer spotted in frontier molecular orbitals analysis. Additionally, analysis of conformational isomers and UV‐Visible spectra has been also performed for all designed derivatives. Thus, our present investigation provides novel and explanatory insights on the chemical nature and origin of intrinsic nonlinear optical (NLO) properties of 2,4‐dinitrophenol derivatives. © 2014 Wiley Periodicals, Inc.     

Dielectric classification of <Emphasis Type="Italic">D</Emphasis>-and <Emphasis Type="Italic">L</Emphasis>-amino acids by thermal and analytical methods

Dielectric analysis (DEA), supported by thermogravimetric analysis (TG), differential scanning calorimetry (DSC), powder X-ray diffraction analysis (PXRD) and photomicrography, reveal the chiral difference in the amino acids. The acids are classified as dielectric materials based on their structure, relating chirality to the vector sum of the average dipole moment, composed of the constant optical (electronic) and infra-red (atomic) polarizabilities, as well as dipole orientation. This study encompasses 14 L-and D-amino acid isomers. Physical properties recorded include AC electrical conductivity, charge transfer complexes, melting, recrystallization, amorphous and crystalline phases, and relaxation spectra, activation energies and polarization times for the electrical charging process.     

A Valence Bond Study of the Low‐Lying States of the NF Molecule

The electronic structures of the three lowest‐lying states of NF are investigated by means of modern valence bond (VB) methods such as the VB self‐consistent field (VBSCF), breathing orbital VB (BOVB), and VB configuration interaction (VBCI) methods. The wave functions for the three states are expressed in terms of 9–12 VB structures, which can be further condensed into three or four classical Lewis structures, whose weights are quantitatively estimated. Despite the compactness of the wave functions, the BOVB and VBCI methods reproduce the spectroscopic properties and dipole moments of the three states well, in good agreement with previous computational studies and experimental values. By analogy to the isoelectronic O₂ molecule, the ground state ³Σ^? possesses both a σ bond and 3‐electron π bonds. However, here the polar σ bond contributes the most to the overall bonding. It is augmented by a fractional (19 %) contribution of three‐electron π bonding that arises from π charge transfer from fluorine to nitrogen. In the singlet ¹Δ and ¹Σ⁺ excited states the π‐bonding component is classically covalent, and it contributes 28 % and 37 % to the overall bonding picture for the two states, respectively. The resonance energies are calculated and reveal that π bonding contributes at least 24, 35 and 42 kcal mol^?1 to the total bonding energies of the ³Σ^?, ¹Δ and ¹Σ⁺ states, respectively. Some unusual properties of the NF molecule, like the equilibrium distance shortening and bonding energy increasing upon excitation, the counterintuitive values of the dipole moments and the reversal of the dipole moments as the bond is stretched, are interpreted in the light of the simple valence bond picture. The overall polarity of the molecule is very small in the ground state, and is opposite to the relative electronegativity of N vs F in the singlet excited states. The values of the dipole moments in the three states are quantitatively accounted for by the calculated weights of the VB structures.     

Accessing the Intrinsic Nature of Electronic Transitions from Gas‐Phase Spectroscopy of Molecular Ion/Zwitterion Complexes

A molecule's color is governed by the nature of its electronic transitions. Herein we show that the degree of charge transfer can be assessed by measuring the change in absorption induced by complexation with the betaine zwitterion. Our approach benefits from direct formation of complexes by electrospray of a mixture solution, followed by photodissociation action spectroscopy. We explored two ion groups: 1) No permanent dipole moment due to even charge delocalization (e.g. MnO₄⁻) and 2) Non‐even charge distribution but where the charge according to resonance forms is either delocalized (e.g. oxyluciferin) or located at one site (e.g. m ‐nitrophenolate, m NP). The maximal shift for ions from (1) was <0.05 eV but as large as 0.3 eV and 0.2 eV for m NP and oxyluciferin. Hence our work supports the notion that oxyluciferin undergoes CT, and that the microenvironment can account for large variation in light emission from insects, ranging from green to red (shift of 0.3 eV).     

A MNDO study of the structural and electronic properties of polysilane model compounds

The results of MNDO geometry optimizations on selected H? (SiH₂)_n? H polysilane model compounds are presented. Near energetic degeneracy is indicated for all-trans(T), alternating gauche–trans (GT), and all-gauche (G⁺G⁺) models (n = 10). The most stable (T) and least stable (G⁺G⁺) conformations are separated by only ca. 0.11 eV. The existence of low-energy barriers to moderate structural distortion is also suggested. Orbital localizations and charge density distributions along the “polymer” backbone are found to be sensitive functions of such distortion. The ground-state electronic distribution of the saturated all-trans silane chains are calculated to be considerably more polarizable than the fully conjugated H? (CH)_n? H π-electron framework of comparable length. The one-electron HOMO → LUMO excitation can be viewed essentially as an in-plane Si 3p → Si3s + H1s intramolecular charge transfer transition. The qualitatively different atomic orbital character of the HOMO and LUMO levels yields transition moment components for the separate repeat units which are relatively small. In the case of the rigidly trans conformation, the phase relationships of the transition moment terms are such as to constructively sum to a large net value reflecting strong optical absorption, as is observed experimentally.     

Membrane structure and biologic activity of adrenocorticotropin (ACTH) and melanotropin (MSH) peptides. Estimation of structural parameters including the influence of the helix dipole moment

Estimation of preferred conformation, orientation, and accumulation of adrenocorticotropin (1–24)-peptide at an aqueous-hydrophobic interface produced a model that agreed with that developed from experimental observations with lipid membranes. Thus, the N-terminal message segment (residues 1–11) was incorporated into the hydrophobic phase as an α-helical, perpendicularly oriented domain with an apparent dissociation constant of ca. 5 · 10^?5 M . The C-terminal address segment (residues 12-24) remained in the aqueous phase as a random-coil domain. Three parameters proved sufficient to define the model: the Gibbs free energy of hydrophobic association, the molecular amphiphilic moment, and the molecular electric dipole moment. For estimating interactions with biologic membranes (that carry a net negative charge), the Boltzmann distribution of charged peptides was also considered. The estimations were extended to adrenocorticotropin (1-10)-peptide and α-melanotropin. In the first case, the prediction agreed with the earlier observations, in the second, it awaits its experimental proof. The estimated membrane structures were compared with available biologic data. As for opioid peptides, it appears that the amphiphilic moment is an important new parameter for determining quantitative structure-activity relationships (QSAR) in receptor selection and biologic potency.     

Electronic states and nature of bonding in the molecule YN by all-electron ab initio CASSCF calculations

In the present work, we present results of all-electron ab initio CASSCF calculations of nine electronic states of the molecule YN. Also reported are the spectroscopic constants derived on the basis of the calculated potential energies. The predicted electronic ground state is ¹∑⁺, and this state is found to be separated from the excited states ³∑⁺, ³Π, and ¹Π by 5177, 9290, and 9915 cm^?1, respectively. The chemical bond in the YN molecule is polar with charge transfer from Y to N, giving rise to a dipole moment of 8.19 Debye at 3.3 au in the ¹∑⁺ ground state is basically a double bond composed of two π bonds. The dissociation energy of the YN molecule has been derived as 4.59 eV. © 1993 John Wiley & Sons, Inc.     

Theoretical Infrared Radiative Lifetimes for CO a3∏

INDO self-consistent-field method was employed to calculate the potential energy and dipole moment functions for the excited a³∏ state of CO. Vibrationally averaged dipole moments and infrared radiative lifetimes were then obtained from the dipole moment function and vibrational wave functions generated by solving numerically the Schrödinger equation for nuclear motions. The calculated dipole moment is 1.468 (expt'I 1.375 D) for ν=0, and decreases with increasing ν, as found experimentally. Calculated infrared radiative lifetimes, with experimental results in parentheses, are 13.5 (17.3, 19.0±5.9), 7.3 (7.8, 13.1±2.9), and 5.0 (4.7, 5.6±1.0) msec, respectively, for ν=1, 2, and 3. The polarity of calculated dipole moment is C⁺O^?, differing from that for the ground X¹∑⁺ state. The origin of this difference is found to be due to the delocalization of the 5a orbital in the a³∏ state.     

Quantum chemical investigation of linear hydrogen bonding in ONCCN···HX (X = F,Cl, Br) dimers

Linear hydrogen bonding formed between the nitrogen end of cyanogen‐N‐oxide (ONCCN) and hydrogen halides HX (X = F, Cl, Br) has been observed in their ground Σ states. The order of agreement of energetic stabilities between the correlated functionals used in this calculation is: B3LYP < PBE0 < PBE < PW91 in conjunction with the 6–311++G(3df,3pd) basis set. Analysis of various parameters describing the existence of H‐bonds in these dimers follows the conventional trend: ONCCN···HF > ONCCN···HCl > ONCCN···HBr in the series, except H‐bond lengths and static dipole polarizabilities which are in reverse order. The atomic charges obtained from the Mulliken and natural population analysis is used to assess the charge transfer effects that accompany the dimer formation. It is found from the investigation that the dimers having highest binding energy are accompanied by the highest transfer of charge. The ¹⁴N nuclear quadrupole coupling constants of the monomer ON¹CCN² are found to be decreased upon complection and in the series it increases from F through Br. We observed enhancements in the values of the dimer dipole moment and intrinsic dipole polarizabilities compared with the sum of the monomer values by intermolecular electrical interaction. Investigation reveals vibrational spectral shifts of HX and CN stretching modes similar to the conventional red‐shifted H‐bonded dimers; for the former case, the infrared band intensity increases significantly. Finally, the new vibrational modes originated from the intermolecular interaction are outlined. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Second-harmonic generation and effect of adsorbates for free-electron metal and semiconductor surfaces

We discuss aspects of a developing microscopic theory of SHG from simple metal and semiconductor surfaces. For semiconductors calculations of the dynamical nonlinear susceptibility on the basis of realistic tight-binding parametrizations of the electronic Hamiltonian provide a practical scheme. In the resulting spectra the effect of the dangling bonds on SHG is clearly seen together with a strong decrease upon saturation with H atoms. In the metal case the adsorbate induced changes of the static nonlinear electron density can be calculated self-consistently by applying density functional theory to the jellium model. The second-order dipole moment determines the effect of adsorbates on the SHG intensity in the adiabatic limit. Quite general a correlation with the nature of the adsorbate expressed by its electronegativity and the characteristic charge transfer, adsorption dipole and polarizabilities in first and second order is found.