# Showing papers in "Journal of Chemical Physics in 1985"

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TL;DR: In this article, effective core potentials (ECP) have been derived to replace the innermost core electron for third row (K), fourth row (Rb-Ag), and fifth row (Cs-Au) atoms.

Abstract: Ab initio effective core potentials (ECP’s) have been generated to replace the innermost core electron for third‐row (K–Au), fourth‐row (Rb–Ag), and fifth‐row (Cs–Au) atoms The outermost core orbitals—corresponding to the ns2np6 configuration for the three rows here—are not replaced by the ECP but are treated on an equal footing with the nd, (n+1)s and (n+1)p valence orbitals These ECP’s have been derived for use in molecular calculations where these outer core orbitals need to be treated explicitly rather than to be replaced by an ECP The ECP’s for the forth and fifth rows also incorporate the mass–velocity and Darwin relativistic effects into the potentials Analytic fits to the potentials are presented for use in multicenter integral evaluation Gaussian orbital valence basis sets are developed for the (3s, 3p, 3d, 4s, 4p), (4s, 4p, 4d, 5s, 5p), and (5s, 5p, 5d, 6s, 6p) ortibals of the three respective rows

13,717 citations

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TL;DR: In this article, the Coulomb, exchange, and core-orthogonality effects of the chemically inert core electron in the transition metal atoms Sc to Hg have been replaced by the ab initio effective core potentials (ECP).

Abstract: Ab initio effective core potentials (ECP’s) have been generated to replace the Coulomb, exchange, and core‐orthogonality effects of the chemically inert core electron in the transition metal atoms Sc to Hg. For the second and third transition series relative ECP’s have been generated which also incorporate the mass–velocity and Darwin relativistic effects into the potential. The ab initio ECP’s should facilitate valence electron calculations on molecules containing transition‐metal atoms with accuracies approaching all‐electron calculations at a fraction of the computational cost. Analytic fits to the potentials are presented for use in multicenter integral evaluation. Gaussian orbital valence basis sets are developed for the (3d,4s,4p), (4d,5s,5p), and (5d,6s,6p) orbitals of the first, second, and third transition series atoms, respectively. All‐electron and valence‐electron atomic excitation energies are also compared for the low‐lying states of Sc–Hg, and the valence‐electron calculations are found to reproduce the all‐electron excitation energies (typically within a few tenths of an eV).

12,141 citations

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TL;DR: In this article, a consistent set of ab initio effective core potentials (ECP) has been generated for the main group elements from Na to Bi using the procedure originally developed by Kahn.

Abstract: A consistent set of ab initio effective core potentials (ECP) has been generated for the main group elements from Na to Bi using the procedure originally developed by Kahn. The ECP’s are derived from all‐electron numerical Hartree–Fock atomic wave functions and fit to analytical representations for use in molecular calculations. For Rb to Bi the ECP’s are generated from the relativistic Hartree–Fock atomic wave functions of Cowan which incorporate the Darwin and mass–velocity terms. Energy‐optimized valence basis sets of (3s3p) primitive Gaussians are presented for use with the ECP’s. Comparisons between all‐electron and valence‐electron ECP calculations are presented for NaF, NaCl, Cl2, Cl2−, Br2, Br2−, and Xe2+. The results show that the average errors introduced by the ECP’s are generally only a few percent.

8,952 citations

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TL;DR: In this paper, a method of "natural population analysis" was developed to calculate atomic charges and orbital populations of molecular wave functions in general atomic orbital basis sets, which seems to exhibit improved numerical stability and to better describe the electron distribution in compounds of high ionic character.

Abstract: A method of ‘‘natural population analysis’’ has been developed to calculate atomic charges and orbital populations of molecular wave functions in general atomic orbital basis sets. The natural analysis is an alternative to conventional Mulliken population analysis, and seems to exhibit improved numerical stability and to better describe the electron distribution in compounds of high ionic character, such as those containing metal atoms. We calculated ab initio SCF‐MO wave functions for compounds of type CH3X and LiX (X=F, OH, NH2, CH3, BH2, BeH, Li, H) in a variety of basis sets to illustrate the generality of the method, and to compare the natural populations with results of Mulliken analysis, density integration, and empirical measures of ionic character. Natural populations are found to give a satisfactory description of these molecules, providing a unified treatment of covalent and extreme ionic limits at modest computational cost.

8,332 citations

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TL;DR: In this article, an MCSCF procedure is described which is based on the direct minimization of an approximate energy expression which is periodic and correct to second order in the changes in the orthonormal orbitals.

Abstract: An MCSCF procedure is described which is based on the direct minimization of an approximate energy expression which is periodic and correct to second order in the changes in the orthonormal orbitals Within this approximation, the CI coefficients are fully optimized, thereby accounting for the coupling between orbital rotations and CI coefficients to higher order than in previous treatments Additional transformations among the internal orbitals and their associated one‐ and two‐electron integrals are performed which amounts to treating the rotations among internal orbitals to higher than second order These extra steps are cheap compared to the four index transformation performed in each iteration, but lead to a remarkable enhancement of convergence and overall efficiency In all calculations attempted to date, convergence has been achieved in at most three iterations The energy has been observed to converge better than quadratically from the first iteration even when the initial Hessian matrix has many negative eigenvalues

2,739 citations

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TL;DR: The method of natural localized molecular orbitals (NLMOs) as discussed by the authors is an extension of the previously developed natural atomic orbital (NAO) and natural bond orbital (NBO) methods, and uses only the information contained in the one particle density matrix.

Abstract: The method of natural localized molecular orbitals (NLMOs) is presented as a novel and efficient technique for obtaining LMOs for SCF and CI wave functions. It is an extension of the previously developed natural atomic orbital (NAO) and natural bond orbital (NBO) methods, and uses only the information contained in the one‐particle density matrix. Results are presented for methane and cytosine to indicate that NLMOs closely resemble LMOs obtained by the Boys and Edmiston–Ruedenberg methods, with the exception that the NLMO procedure automatically preserves the MO σ–π separation in planar molecules. The computation time is modest, generally only a small fraction of the SCF computation time. In addition, the derivation of NLMOs from NBOs gives direct insight into the nature of the LMO ‘‘delocalization tails,’’ thus enhancing the role of LMOs as a bridge between chemical intuition and molecular wave functions.

1,948 citations

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TL;DR: In this paper, the authors derived equilibrium fluctuation expressions for the linear response of many body systems thermostated by the Nose-Hoover thermostat and showed that in the thermodynamic limit this response is the same as that of the corresponding Gaussian isothermal system.

Abstract: We derive equilibrium fluctuation expressions for the linear response of many body systems thermostated by the Nose–Hoover thermostat. We show that in the thermodynamic limit this response is the same as that of the corresponding Gaussian isothermal system. Numerical comparisons for shear flow show however that the Gaussian methods provide a significantly more efficient means of calculating the shear viscosity coefficient.

1,904 citations

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TL;DR: The formal definition of the generalized discrete variable representation for quantum mechanics and its connection to the usual variational basis representation (VBR) is given and the DVR is shown to be accurate in itself, and an efficient representation for optimizing basis set parameters.

Abstract: The formal definition of the generalized discrete variable representation (DVR) for quantum mechanics and its connection to the usual variational basis representation (VBR) is given. Using the one dimensional Morse oscillator example, we compare the ‘‘Gaussian quadrature’’ DVR, more general DVR’s, and other ‘‘pointwise’’ representations such as the finite difference method and a Simpson’s rule quadrature. The DVR is shown to be accurate in itself, and an efficient representation for optimizing basis set parameters. Extensions to multidimensional problems are discussed.

1,477 citations

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Abstract: We have examined the response of an exact and an MCSCF reference state to a general time‐dependent field. The time development of both the exact and the MCSCF reference state have been parametrized in terms of explicit exponential time‐dependent transformations. The time development has been determined by requiring the Ehrenfest theorem to be satisfied through each order in the interaction between the molecular system and the field. The response of the exact and the MCSCF reference state has been used to evaluate linear, quadratic, and cubic response functions. It has been shown how a large variety of molecular properties may be expressed in terms of these response functions. It has also been demonstrated that molecular properties containing the electric dipole operator may be expressed in equivalent forms involving the momentum operator both for the exact and the MCSCF state. The MCSCF response functions have been transformed to computationally attractive expressions which do not contain summation indices over intermediate states and which allow direct techniques to be straightforwardly applied.

1,003 citations

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TL;DR: In this article, a time-dependent formulation of two-photon spectroscopy is employed to show that selectivity of reactivity can be achieved via coherent twophoton processes, and the problem of finding the optimum waveform (i.e., coherent pulse sequence) that will maximize the formation of a desired chemical species is formulated as a problem in the calculus of variations.

Abstract: A time‐dependent formulation of two‐photon spectroscopy is employed to show that selectivity of reactivity can be achieved via coherent two‐photon processes. The problem of finding the optimum waveform (i.e., coherent pulse sequence) that will maximize the formation of a desired chemical species is formulated as a problem in the calculus of variations, and solved for two different cases.

866 citations

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TL;DR: In this article, the effects of single, double, and triple excitation operators on electron correlation were analyzed and an alternate version of the approximate CCSDT•1 method was implemented.

Abstract: Coupled cluster models for electron correlation which include the effects of single, double, and triple excitation operators are analyzed. An alternate version of the approximate CCSDT‐1 method is implemented. In this version, the full CCSDT cluster operator eT1+T2+T3 is preserved in the creation of single and double excitation coefficients, but in calculation of triple excitation coefficients only the T2 operator is used. We also present a theoretical analysis of the simplest improvement for the evaluation of the contribution of triples beyond that obtained with fourth‐order MBPT. In this approximation, an MBPT(4)‐like calculation of the triples energy is evaluated with converged CCSD T2 coefficients. This is found to offer a good approximation to the converged CCSDT‐1 results.

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TL;DR: In this article, the authors measured cross sections for the reactions of Ar+ with H2, D2, and HD to form ArH+ and ArD+ using a new guided ion beam tandem mass spectrometer which affords an experimental energy range from 0.05 to 500 eV laboratory.

Abstract: Cross sections for the reactions of Ar+ with H2, D2, and HD to form ArH+ and ArD+ are measured using a new guided ion beam tandem mass spectrometer which affords an experimental energy range from 0.05 to 500 eV laboratory. The apparatus and experimental techniques are described in detail. Cross sections for H2 and D2 are found to be nearly identical over this entire energy range when compared at the same barycentric energy. The total HD cross section is the same as H2 and D2 at low energies, but differs significantly above 4 eV c.m., where product dissociation becomes important. The intramolecular isotope effect for reaction with HD exhibits a reversal at low energy, favoring the deuteride product below ∼0.14 eV c.m., and surprising nonmonotonic behavior at energies above 5 eV c.m. In all these systems, a new feature at higher energies is observed. This is interpreted as the onset of a product channel having an energy barrier of 8±1 eV. The room temperature rate constant derived from the data for the reac...

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TL;DR: In this paper, a simple, fully quantum mechanical model for electron transfer using a one mode treatment which incorporates this coupling is studied, and the limits of the moderate and the high friction are analyzed in detail.

Abstract: In biological and chemical electron transfer, a nuclear reaction coordinate is coupled to other nuclear and/or ‘‘solvent’’ coordinates. This coupling, or friction, if strong enough, may substantially slow down motion along the reaction coordinate, and thus vitiate the assumption of electron transfer being nonadiabatic with respect to the nuclei. Here, a simple, fully quantum mechanical model for electron transfer using a one mode treatment which incorporates this coupling is studied. Path integral methods are used to study the dependence of the reaction rate on friction, and the limits of the moderate and the high friction are analyzed in detail. The first limit will prevail if the reaction coordinate is, e.g., an underdamped nuclear vibration, whereas the second limit will prevail if it corresponds to a slow or diffusive degree of freedom. In the high‐friction limit, the reaction rate is explicitly shown to vary between the nonadiabatic and adiabatic expressions as the tunneling matrix element and/or the friction are varied. Starting from a path integral expression for the time evolution of the reduced density matrix for the electron and reaction coordinate, a Fokker–Planck equation is obtained which reduces in the high‐friction limit to a Smoluchowski equation similar to one solved by Zusman.

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TL;DR: In this paper, the authors measured the thickness as a function of time of liquid films as they are squeezed between molecularly smooth mica surfaces and determined the film thickness with an accuracy of 0.2 nm as they drain from ∼1 μm to a few molecular layers.

Abstract: We present measurements of the thickness as a function of time of liquid films as they are squeezed between molecularly smooth mica surfaces. Three Newtonian, nonpolar liquids have been studied: octamethylcyclotetrasiloxane, n‐tetradecane, and n‐hexadecane. The film thicknesses are determined with an accuracy of 0.2 nm as they drain from ∼1 μm to a few molecular layers. Results are in excellent agreement with the Reynolds theory of lubrication for film thicknesses above 50 nm. For thinner films the drainage is slower than the theoretical prediction, which can be accounted for by assuming that the liquid within about two molecular layers of each solid surface does not undergo shear. In very thin films the continuum Reynolds theory breaks down, as drainage occurs in a series of abrupt steps whose size matches the thickness of molecular layers in the liquid. The presence of trace amounts of water has a dramatic effect on the drainage of a nonpolar liquid between hydrophilic surfaces, causing film rupture whi...

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TL;DR: In this paper, the free energy of hydration of methanol and ethane in dilute soluton was calculated using Monte Carlo simulations using double-wide sampling, and it was shown that only two or three Monte-Carlo simulations are necessary to obtain results with high precision.

Abstract: Perturbation theory has been applied to calculate the relative free energies of hydration of methanol and ethane in dilute soluton. It is demonstrated that only two or three Monte Carlo simulations using double‐wide sampling are necessary to obtain results with high precision. The small statistical uncertainty in the computed change in free energy of hydration and the good accord with experimental thermodynamic data are most encouraging for application of the procedure to a wide range of problems. Structural effects accompanying the mutation of methanol to ethane in water are also discussed; hydrogen bonding to the solute is essentialy eliminated by only a 25% reduction in the atomic charges of methanol.

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Stevens Institute of Technology

^{1}, Ohio State University^{2}, Drexel University^{3}, Clarkson University^{4}TL;DR: In this article, the shape consistent effective potential procedure was used to compute averaged relativistic effective potentials (AREP) and spin-orbit operators for the atoms Li through Ar.

Abstract: A refined version of the ‘‘shape consistent’’ effective potential procedure of Christiansen, Lee, and Pitzer was used to compute averaged relativistic effective potentials (AREP) and spin‐orbit operators for the atoms Li through Ar. These are tabulated in analytic form. Small optimized Gaussian basis sets with expansion coefficients for the lowest energy state for each atom are given and the reliability of the potentials relative to all electric calculations is discussed. Finally a procedure for computing molecular moments and Breit corrections is suggested.

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TL;DR: In this article, the interaction energy of two HF molecules at the single configuration Hartree-Fock level was calculated using 34 basis sets in an effort to assess the reliability and usefulness of the counterpoise correction to account for basis set incompleteness.

Abstract: We have calculated the interaction energy of two HF molecules at the single‐configuration Hartree–Fock level using 34 different basis sets in an effort to assess the reliability and usefulness of the counterpoise correction to account for basis set incompleteness. We find large counterpoise corrections for all configurations studied, and we show that using a large enough basis set so that the counterpoise correction is small does not guarantee accurate results. Furthermore even for smaller basis sets the inclusion of counterpoise corrections does not systematically improve the accuracy of the calculations.

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TL;DR: Very small ZnS and CdS crystallites were made and stabilized in aqueous and methanolic media without organic surfactants in this paper, and they were characterized by transmission electron microscopy and in situ optical spectroscopy (λ≳200 nm).

Abstract: Very small ZnS and CdS crystallites are made and stabilized in aqueous and methanolic media without organic surfactants. Low temperature (−77 °C) synthesis in methanol produces the smallest crystallites, ≈30 A diameter cubic CdS and <20 A diameter cubic ZnS. The crystallites are characterized by transmission electron microscopy and in situ optical spectroscopy (λ≳200 nm). The crystallites are too small to exhibit bulk band gaps in their optical spectra. In the band gap region, the small crystallites show a higher energy absorption threshold with a resolved spectral feature (quantum size exciton peak), not present in the spectra of larger crystals. The far ultraviolet spectra are unaffected by size at present resolution. These results can be understood in terms of the crystallite molecular orbitals, and an elementary confined electron and hole model.

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TL;DR: In this article, it was shown that "impulsive" stimulated Raman scattering (ISRS) should occur, with no laser intensity threshold, when a sufficiently short laser pulse passes through many types of matter.

Abstract: It is shown that ‘‘impulsive’’ stimulated Raman scattering (ISRS) should occur, with no laser intensity threshold, when a sufficiently short laser pulse passes through many types of matter. ISRS excitation of coherent optic phonons, molecular vibrations, and other excitations (including rotational, electronic, and spin) may play important roles in femtosecond pulse interactions with molecules, crystals, glasses (including optical fibers), semiconductors, and metals. Spectroscopic applications of ISRS, including time‐resolved spectroscopy of vibrationally distorted molecules and crystals, are discussed.

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TL;DR: In this article, the viscosities of cyclohexane suspensions of sterically stabilized (hard) silica spheres are reported as a function of shear rate (γ) and volume fraction.

Abstract: The viscosities of suspensions of sterically stabilized (hard) silica spheres in cyclohexane are reported as a function of shear rate (γ) and volume fraction (6×10−4<φ<0.6). The shear thinning scales according to (ηr−η∞)/(η0−η∞) =1/(1+1.31ηγa3/kT) with limiting low and high shear viscosities described up to φ∼0.35 by η0=1+5/2φ+(4±2)φ2+(42±10)φ3 , η∞=1+5/2φ+(4±2)φ2+(25±7)φ3 . At higher volume fractions the viscosity becomes more sensitive to φ and diverges at φm=0.63±0.02 (γ→0) , φm=0.70±0.02 (γ→∞) . The experimental results compare well with existing hard sphere theories and the data of Krieger (1972) for aqueous lattices. Even at the highest volume fraction neither yield stresses nor shear thickening are observed.

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TL;DR: In this paper, a modification of the CI(SD) energy functional is proposed which leads to size consistency through the use of partial normalization denominators, derived from simple principles: correct description of separated two-electron systems and certain invariance requirements.

Abstract: A modification of the CI(SD) energy functional is proposed which leads to size consistency through the use of partial normalization denominators. The method is derived from simple principles: correct description of separated two‐electron systems and certain invariance requirements. This approach is connected to CEPA‐1. The theoretical framework allows for a simple rationalization of connections between CI(SD), CEPA‐1, and the linear version of CP–MET. As demonstrative applications we report comparisons with full CI calculations for BH, NH3, H2O, HF, and Re, De for F2, N2, O2, Cl2, NO, and CO obtained for very large basis sets.

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TL;DR: In this paper, a theory of NMR shielding tensors is derived from Ramsey's expressions, using the framework of the random phase approximation (RPA) and localized molecular orbitals, which allow an analysis of the shielding into intrinsic bond and bond-bond coupling contributions.

Abstract: A theory of NMR shielding tensors is derived from Ramsey’s expressions, using the framework of the random phase approximation (RPA) and localized molecular orbitals. By expanding angular momentum terms relative to a local origin for each orbital and using properties of the RPA solutions, we arrive at shielding expressions that contain no reference to an overall gauge origin and that lead to appropriate damping of basis set errors in contributions from distant groups. The expressions allow an analysis of the shielding into intrinsic bond and bond–bond coupling contributions. The resulting method is a variant of the coupled‐Hartree–Fock approach. Ab initio results are presented for 13C isotropic shieldings and shielding tensors in a number of organic molecules ranging in size up to benzene. These results agree very well with experiment, for both isotropic shieldings and principal tensor components, even in double‐zeta basis sets. For some low symmetry molecules we study the asymmetry of the shielding tensor...

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TL;DR: In this paper, a series of extensive simulations with the MCY (Matsuoka-Clementi-Yoshimine) potential for water have been performed in reaction field geometry, and it is shown that consistent and accurate results may be obtained for the full frequency-dependent dielectric constant, from the static up to the submillimeter range.

Abstract: Despite the sizable literature on computer simulations of water and aqueous solutions, very little is known about one of the most interesting properties of water, namely its dielectric constant. In this paper it is demonstrated that the methodological as well as technological problems that have hitherto impeded the calculation of dielectric properties of realistic model systems have now been overcome. Using a small dedicated array processor, a series of extensive simulations with the MCY (Matsuoka–Clementi–Yoshimine) potential for water have been performed in reaction field geometry, and it is shown that consistent (i.e., free from boundary effects) and accurate results may be obtained for the full frequency‐dependent dielectric constant, from the static up to the submillimeter range. At the same time it is found that the rather popular MCY model is not able to satisfactorily reproduce the dielectric properties of real water: Both the static dielectric constant and the principal dielectric relaxation time are much lower than the experimental values, and the temperature dependence of the Kirkwood g factor has the wrong sign. It is concluded that in the future the calculation of the dielectric constant, being a measure of orientational correlations between molecules, will play an important part in assessing the reliability of model potentials for water.

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TL;DR: In this article, the dynamics of the F+H/sub 2/ reaction have been investigated in a high resolution crossed molecular beam study, and the results strongly suggest that dynamical resonances play a significant role in the reaction dynamics.

Abstract: The dynamics of the F+H/sub 2/ reaction have been investigated in a high resolution crossed molecular beam study. Differential cross sections and kinetic energy distributions were obtained for each HF vibrational state. The v = 1 and v = 2 states were predominantly backward scattered, but substantial forward scattering was observed for HF (v = 3) over the range of collision energies accessible in our apparatus, from 0.7 to 3.4 kcal/mol. The results strongly suggest that dynamical resonances play a significant role in the reaction dynamics of F+H/sub 2/ and that resonance effects are most prominent in the v = 3 product channel. Quantal reactive scattering calculations on F+H/sub 2/ predict that the v = 2 channel should be most strongly affected by resonances. This discrepancy is attributed to inadequacies in the potential energy surface used in the calculations, and several modifications to the surface are proposed based on the experimental results. Other features of the reaction are also discussed, including the integrated partial cross sections, the effect of H/sub 2/ rotation, and the reactivity of F(/sup 2/P/sub 1/2/).

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IBM

^{1}TL;DR: Vibrational frequencies of neutral molecules computed at three levels of theory were compared with experiment and the effect of scaling was investigated to determine how accurately vibrational frequencies could be predicted.

Abstract: The initial detections of IR vibration-rotation bands in polyatomic molecular ions by recent spectroscopic advances were guided by ab initio prediction of vibrational frequencies. The present calculations predict the vibrational frequencies of additional ions which are candidates for laboratory analysis. Neutral molecule vibrational frequencies were computed at three levels of theory and then compared with experimental data; the effect of scaling was also investigated, in order to determine how accurately vibrational frequencies could be predicted. For 92 percent of the frequencies examined, the relatively simple HF/6-31G theory's vibrational frequencies were within 100/cm of experimental values, with a mean absolute error of 49/cm. On this basis, the frequencies of 30 molecular ions (many possessing astrophysical significance) were computed.

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TL;DR: In this paper, Hartree-Fock and valence effective Hamiltonian (VEH) calculations on polyparaphenylene, polypyrrole, and polythiophene dimers and polymer chains are presented.

Abstract: We present ab initio Hartree–Fock and valence effective Hamiltonian (VEH) calculations on polyparaphenylene, polypyrrole, and polythiophene dimers and polymer chains. These polymeric materials are among the most studied compounds in the field of conducting polymers. We examine, as a function of the torsion angle between consecutive rings, the evolution of electronic properties such as ionization potential, bandgap and width of the highest occupied bands and of the carbon–carbon bond length between rings. This investigation is motivated by the fact that many derivatives of these compounds have substituents that lead to an increase of the torsion angle between adjacent rings, as a result of steric interactions. As expected, on going from a coplanar to a perpendicular conformation, the ionization potential and bandgap values increase and the width of the highest occupied bands decreases. This makes it more difficult to ionize or reduce the polymer chains and can result in achieving lower maximum conductivities on doping. However, since the evolution of the electronic properties is found to follow a cosine law (related to the decrease of the overlap between the π orbitals on adjacent rings), the modifications up to a ∼40° torsion angle are not very large. For instance, in all three polymers, the ionization potential value for a 40° torsion angle is about 0.4 eV larger than the coplanar conformation value. Therefore, substituents that lead to torsion angles between consecutive rings smaller than 40° are quite acceptable. Finally we discuss the importance, for the substituted compounds, of the possibility of achieving a coplanar conformation upon doping, in order to permit high intrachain mobilities of charge carriers such as bipolarons.

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TL;DR: In this article, the evolution of the band gap as a function of geometry in conjugated polymers based on aromatic rings was analyzed and it was shown that the gap decreases with increasing quinoid character of the backbone.

Abstract: We describe calculations of the evolution of the band gap as a function of geometry in conjugated polymers based on aromatic rings: polyparaphenylene, polypyrrole, polythiophene. Our results demonstrate that the gap decreases as a function of increasing quinoid character of the backbone and is thus not minimal in the case of zero bond length alternation, in contrast to the situation found in polyacetylene‐like compounds. We stress the consequences of these results for the understanding of the effects of doping and for the design of new organic polymers with small gaps.

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TL;DR: In this paper, the distribution of coherence in a multiple-quantum spectrum was modeled as a Gaussian distribution and the variance of the distribution was associated with the number of coupled spins effectively interacting, and its steady growth with time reflected the continual expansion of the system under the action of dipolar interactions.

Abstract: Recently developed solid state multiple‐quantum NMR methods are applied to extended coupling networks, where direct dipole–dipole interactions can be used to create coherences of very high order (∼100). The progressive development of multiple‐quantum coherence over time depends upon the formation of multiple‐spin correlations, a phenomenon which also accompanies the normal decay to equilibrium of the free induction signal in a solid. Both the time development and the observed distributions of coherence can be approached statistically, with the spin system described by a time‐dependent density operator whose elements are completely uncorrelated at sufficiently long times. With this point of view, we treat the distribution of coherence in a multiple‐quantum spectrum as Gaussian, and characterize a spectrum obtained for a given preparation time by its variance. The variance of the distribution is associated roughly with the number of coupled spins effectively interacting, and its steady growth with time reflects the continual expansion of the system under the action of the dipolar interactions. The increase in effective system ‘‘size’’ is accounted for by a random walk model for the time development of the density operator. Experimental results are presented for hexamethylbenzene, adamantane, and squaric acid. The formation of coherence in systems containing physically isolated clusters is also investigated, and a simple method for estimating the number of spins involved is demonstrated.

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TL;DR: The physical and electronic properties of poly(isothianaphthene) are reported in this paper, including initial characterization, electrochemical cyclic voltammetry, spectroscopy, and transport properties.

Abstract: The physical and electronic properties of poly(isothianaphthene) PITN, are reported, including initial characterization, electrochemical cyclic voltammetry, spectroscopy, and transport properties. PITN has the smallest energy gap of any known conjugated organic polymer, Eg≂1 eV. This novel conjugated polymer exhibits reversible chemical and electrochemical p‐type doping with an associated high contrast color change. After doping, thin films of PITN have very low optical density in the visible portion of the spectrum. Thus, PITN is the first example of a transparent highly conducting polymer (σ∼50 Ω−1 cm−1).

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Rice University

^{1}TL;DR: In this article, a survey study of the reactivity of transition metal clusters in contact with low concentrations of D2, N2 and CO is presented. But the detailed pattern of reactivity differing markedly for each metal.

Abstract: Reactions on the surface of a variety of transition metal clusters have been studied in the gas phase at near room temperature using a newly developed fast-flow reaction device. Initial examples of the use of this device are provided by survey studies of the reactivity of iron, cobalt, nickel, copper, and niobium clusters in contact with low concentrations of D2, N2 and CO. Dissociative chemisorption of D2 is found to occur with dramatic sensitivity to cluster size in the cases of iron, cobalt, and niobium clusters, the detailed pattern of reactivity differing markedly for each metal. The corresponding reaction is also observed with nickel clusters, but here the reactivity shows only a slow, steady increase with cluster size. Copper clusters are found to be completely unreactive to H2 chemisorption under these conditions. Molecular nitrogen is found to chemisorb readily to clusters of cobalt and niobium, with a reactivity pattern very similar to that observed with D2. Iron clusters are found to show slight reactivity with N2; only a small amount of chemisorption is observed on the most reactive clusters at high N2 concentration, but the pattern of this reactivity with cluster size is consistent with that observed in D2 chemisorption. In contrast to these highly structured reactivity patterns of D2 and N2, carbon monoxide is found to show only a slow, monotonic increase in reactivity with cluster size. It is suggested that these dramatic reactivity patterns for chemisorption on metal clusters provide stringent tests for future theories as to the nature of chemisorption on metal surfaces at a detailed, molecular level.