Showing papers in "Molecular Physics in 2009"
TL;DR: In this article, it was shown that the densities of C,H,N,O molecular crystals are, in many instances, given quite well by the formula M/Vm, in which M is the molecular mass and Vm is the volume.
Abstract: Recent work by others has shown that the densities of C,H,N,O molecular crystals are, in many instances, given quite well by the formula M/Vm, in which M is the molecular mass and Vm is the volume ...
363 citations
TL;DR: In this article, it was shown that the presence of several electron-withdrawing NO2 groups and/or aza nitrogens, which is the reason for the anomalously positive surface potentials, also weakens the C-NO2 and or N−NO2 bonds, thus, insofar as these are trigger linkages, the rupture of which is a key step in detonation initiation, an approximate correlation between the features of the surface potential and sensitivity may be expected.
Abstract: Earlier work has shown a link—not necessarily a causal relationship—between the impact sensitivities of energetic compounds and the electrostatic potentials on their molecular surfaces. The latter are anomalous in that the positive regions are strikingly dominant, contrary to what is typical of organic molecules. In this work, we show that the presence of several electron-withdrawing NO2 groups and/or aza nitrogens (common features of energetic systems), which is the reason for the anomalously positive surface potentials, also weakens the C–NO2 and/or N–NO2 bonds. Thus, insofar as these are trigger linkages, the rupture of which is a key step in detonation initiation, an approximate correlation between the features of the surface potentials and sensitivity may be expected. A group of eight nitramines is used to demonstrate this. Work is in progress to elucidate the basis for the surface potential–sensitivity link when a non-trigger-linkage mechanism is operative.
266 citations
TL;DR: In this paper, a review article summarizes some of the principal theoretical approaches to understand quantum effects in barrierless collisional processes in these barrierless processes, where the de Broglie wavelength is long compared with molecular dimensions.
Abstract: The recent development of a range of new methods for producing samples of gas-phase molecules that are translationally cold ( K) or ultracold ( mK) is driving efforts to study reactive and inelastic collisional processes in these temperature regimes. In this review article the new methods for cold/ultracold molecule production are reviewed in the context of their potential or current use in collisional studies and progress in the application of these methods is highlighted. In these sub-Kelvin temperature ranges, where the de Broglie wavelength is long compared with molecular dimensions, quantum effects may play a crucial role in the collision dynamics. Reactions with no potential energy barrier are of greatest importance, and this review article summarizes some of the principal theoretical approaches to understanding quantum effects in these barrierless processes.
259 citations
TL;DR: In this article, the TIP4P/2005 model was used to investigate the maximum and minimum in density and the isothermal compressibility along a number of isobars, and the model correctly describes the decrease in the temperature of the density maximum with increasing pressure.
Abstract: The so-called thermodynamic anomalies of water form an integral part of the peculiar behaviour of this both important and ubiquitous molecule. In this paper our aim is to establish whether the recently proposed TIP4P/2005 model is capable of reproducing a number of these anomalies. Using molecular dynamics simulations we investigate both the maximum in density and the minimum in the isothermal compressibility along a number of isobars. It is shown that the model correctly describes the decrease in the temperature of the density maximum with increasing pressure. At atmospheric pressure the model exhibits an additional minimum in density at a temperature of about 200K, in good agreement with recent experimental work on super-cooled confined water. The model also presents a minimum in the isothermal compressibility close to 310K. We have also investigated the atmospheric pressure isobar for three other water models; the SPC/E and TIP4P models also present a minimum in the isothermal compressibility, although...
170 citations
TL;DR: In this article, 4-cyanobenzylideneamino antipyrine (CBAP) has been synthesized and characterized by elemental analysis, FT-IR, UV-VIS and X-ray single crystal diffraction techniques.
Abstract: 4-(4-cyanobenzylideneamino)antipyrine (CBAP) has been synthesized and characterized by elemental analysis, FT-IR, UV-VIS and X-ray single crystal diffraction techniques. Crystallographic study reveals that the compound adopts trans configuration about the Schiff base imine double bond. The substituted p-cyanophenyl ring indirectly linked to the pyrazoline ring by the C=N double bond is almost coplanar with the pyrazole ring, whereas the phenyl ring directly attached to the pyrazoline ring forms an effective dihedral angle. Density functional calculations have been carried out to optimize and to characterize the title compound by using B3LYP method at 6-31G(d) basis set. The calculated results show that the optimized geometry can well reproduce the crystal structural parameters and the theoretical vibrational frequencies show good agreement with experimental values. On the basis of theoretical vibrational analyses, the thermodynamic properties (standard heat capacities, standard entropies, and standard ent...
108 citations
TL;DR: The correlation consistent composite approach (ccCA) has been made more robust by modifying the basis set used in computing B3LYP equilibrium geometries and harmonic vibrational frequencies so that the correlation consistent basis sets are used throughout ccCA as mentioned in this paper.
Abstract: The correlation consistent Composite Approach (ccCA) has been made more robust by (a) modifying the basis set used in computing B3LYP equilibrium geometries and harmonic vibrational frequencies so that the correlation consistent basis sets are used throughout ccCA; (b) separately extrapolating the MP2 and Hartree–Fock complete basis set limit energies; (c) uniformly treating unrestricted open shell wave functions; (d) utilizing newly recommended enthalpies of formation for C, B, Al, and Si atoms; and (e) using theoretically derived vibrational scale factors. This modified ccCA formulation has been used to compute the 454 energetic properties (enthalpies of formation, dissociation energies, ionization potentials, electron affinities, and proton affinities) in the G3/05 test set. This new formulation, which does not contain any optimized parameters, has a small systematic statistical bias (mean signed deviation of −0.20 kcal mol−1), and has a mean absolute deviation of 1.01 kcal mol−1 with the incorporation...
95 citations
TL;DR: In this article, a model for the scatte is proposed to evaluate the effect of nuclear recoil on the self and distinct structure factors extracted from time-of-flight neutron diffraction data.
Abstract: Time-of-flight neutron diffraction methods are widely used to study the structure of liquids and glasses. The scattering nuclei in these experiments suffer nuclear recoil in the course of the neutron scattering process, which gives rise to distortions to both the self and distinct structure factors extracted from the data. These distortions are in general difficult to evaluate quantitatively, especially when the mass of the nucleus is similar to that of the neutron, such as when hydrogen is present in the material being studied. Traditional treatments of this effect generally assume the neutron energy is lower than the excitation energy of an atom, but for time-of-flight diffraction this is never the case, and the experiments typically sample a wide range of energy transfers from sub-meV to tens of eV. In addition, by attempting to produce an analytical correction, such methods invariably make a long list of approximations which can be difficult to justify in particular cases. Here, a model for the scatte...
94 citations
TL;DR: In this paper, several damping functions for Coulomb, induction, and dispersion interactions within the framework of the general effective fragment potential (EFP) method were implemented and analyzed.
Abstract: This work presents the implementation and analysis of several damping functions for Coulomb, induction, and dispersion interactions within the framework of the general effective fragment potential (EFP) method. Damping is necessary to obtain the correct asymptotic short-range behavior of these interactions. Correctly chosen damping functions allow a balanced description of different parts of intermolecular potential energy surfaces and improve the accuracy of predicted intermolecular distances and binding energies. The performance of different damping functions is tested by comparing the EFP energy terms with the symmetry adapted perturbation theory (SAPT) energy terms in a range of intermolecular separations for ten molecular dimers. The total EFP binding energies in these dimers were compared with the binding energies obtained from SAPT and coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)]. A formula for electrostatic damping that was derived from first principles...
92 citations
TL;DR: In this paper, a density functional theory study is presented on changes in band gap effects of nanodiamonds (hydrogen terminated diamond-like molecules, diamondoids) depending on size, shape, and the incorporation of heteroatom functionalities.
Abstract: A density functional theory study is presented on changes in band gap effects of nanodiamonds (hydrogen terminated diamond-like molecules, diamondoids) depending on size, shape, and the incorporation of heteroatom functionalities. Strong quantum confinement effects were identified at particle sizes from 0.5 to at least 2 nm, when the band gaps of these nanodiamonds are reduced to 6.7 eV. Octahedral and tetrahedral nanodiamonds show the same trends in band gap narrowing, and it is the dimension rather than the shape/morphology of the nanodiamonds that alters the band gaps. Band gap tuning through external (by C–H bond substitution) or internal (by replacing CH or CH2 moieties) doping is non-additive for the same dopant. Push-pull doping, with electron donating and electron withdrawing groups is most effective and reduces the band gaps of diamondoids to that of bulk diamond. Further reductions down to 1–2 eV are conceivable with charged external substituents. The combination of increasing the size of the na...
78 citations
TL;DR: A review of peptide and carbohydrate systems in the gas phase can be found in this article, where the authors attempt to move towards aspects of their biological relevance by reviewing some recent and current investigations, chosen not just because they are "do-able" but because of the biochemical questions they address.
Abstract: Spectroscopic investigations of biomolecular structure and interactions in the gas phase, free from environmental ‘disturbance’, have become a growth industry over the last decade. The great majority however, have taken their ‘bio-pedigrees’ as a given rather than an issue to be addressed. This review attempts to move towards aspects of their biological, or perhaps more accurately, biochemical relevance, by reviewing some recent and current investigations of peptide and carbohydrate systems in the gas phase, chosen not just because they are ‘do-able’ but because of the biochemical questions they address. Hopefully, this will help to foster better communication between the molecular physics, biochemistry and molecular biology communities.
78 citations
TL;DR: For both edge-ring and face-ring-substituted dimers, interaction energies correlate with σm for the substituents, indicating that substituent effects can be understood qualitatively in terms of simple electrostatic effects, although in the latter case dispersion results in some scatter in the data.
Abstract: Substituent effects in the edge-to-face configuration of the benzene dimer have been studied using modern density functional theory. An accurate interaction potential energy curve has been computed for the unsubstituted dimer using ab initio methods with large basis sets. The recommended binding energy for the edge-to-face benzene dimer is 2.31 kcal mol−1, estimated at the counterpoise-corrected CCSD(T)/aug-cc-pVTZ level of theory. For both edge-ring and face-ring-substituted dimers, interaction energies correlate with σm for the substituents, indicating that substituent effects can be understood qualitatively in terms of simple electrostatic effects, although in the latter case dispersion results in some scatter in the data. In contrast to prevailing models of substituent effects in benzene dimers, polarization of the π-system of the substituted ring does not induce substituent effects. For edge-ring-substituted dimers, substituent effects arise from differential electrostatic interactions between the hy...
TL;DR: In this paper, the accuracy of one-electron basis sets for F12 explicitly-correlated molecular electronic-structure methods is assessed by analyzing the Hartree-Fock and second-order correlation contributions to the atomisation energies of the molecules.
Abstract: One-electron basis sets for F12 explicitly-correlated molecular electronic-structure methods are assessed by analysing the accuracy of Hartree–Fock energies and valence-only second-order correlation energies of a test set of 106 small molecules containing the atoms H, C, N, O and F. For these molecules, near Hartree–Fock-limit energies and benchmark second-order correlation energies (accurate to within 99.95% of the basis-set limit) are provided. Absolute energies are analysed as well as the Hartree–Fock and second-order correlation contributions to the atomisation energies of the molecules. Standard basis sets such as the Karlsruhe def2-TZVPP and def2-QZVPP sets and the augmented correlation-consistent polarised valence X-tuple zeta (aug-cc-p VXZ, X = D, T, Q, 5) sets are compared with the specialised cc-pVXZ-F12 (X = D, T, Q) sets that were recently optimised by Peterson and co-workers [J. Chem. Phys. 128, 084102 (2008)] for use in F12 theory. The results obtained from F12 explicitly-correlated molecula...
TL;DR: In this article, the experimental and theoretical UV, NMR and vibrational spectra of 2-chloro-6methylaniline (2-Cl-6-MA, C7H8NCl) were studied.
Abstract: In this work, the experimental and theoretical UV, NMR and vibrational spectra of 2-chloro-6-methylaniline (2-Cl-6-MA, C7H8NCl) were studied. The ultraviolet absorption spectra of compound that dissolved in ethanol were examined in the range of 200–400 nm. The 1H, 13C and DEPT NMR spectra of the compound were recorded. FT-IR and FT-Raman spectra of 2-Cl-6-MA in the liquid phase were recorded in the region 4000–400 cm−1 and 3500–50 cm−1, respectively. The structural and spectroscopic data of the molecule in the ground state were calculated using density functional theory (DFT) employing B3LYP exchange correlation and the 6-311++G(d,p) basis set. The vibrational frequencies were calculated and scaled values were compared with experimental FT-IR and FT-Raman spectra. The observed and calculated frequencies were found to be in good agreement. The complete assignments were performed on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) meth...
TL;DR: In this article, it was shown that MP2 can violate N-representability by having natural occupation numbers greater than 2 or less than 0, even though the energy appears well behaved, due to the fact that the MP2 effective one-particle density matrix is dependent on the inverse of the orbital hessian matrix.
Abstract: By examining the natural orbital occupation numbers, it was discovered that unrestricted Moller–Plesset perturbation theory (MP2) can violate N-representability by having natural occupation numbers greater than 2 or less than 0, even though the energy appears well behaved. Analytically, this problem stems from the fact that the MP2 effective one-particle density matrix is dependent on the inverse of the orbital hessian matrix (A′). When a molecular system goes through the point where spin-restricted orbitals become unstable to unrestriction, one of the eigenvalues of the A′ matrix will become 0, making it a singular matrix, thereby causing problems in the relaxed one-particle density matrix and discontinuities in the first order properties such as nuclear forces and dipole moments. Since the new ‘double-hybrid’ density functionals also involve a second-order perturbation expression for correlation using Kohn–Sham orbitals, these functionals also exhibit similar issues near the unrestriction point. The unp...
TL;DR: In this paper, a simple quantum chemical model is proposed to simulate the effect of external forces acting on a single molecule, which involves optimising the geometry of a molecule with an external force applied to selected nuclei.
Abstract: A very simple quantum chemical model is proposed to simulate the effect of external forces acting on a single molecule. It involves optimising the geometry of a molecule with an external force applied to selected nuclei. This approach is used to investigate conformational transitions of the pyranose ring, which have been the subject of several atomic force microscopy (AFM) experiments, and to generate a number of previously unknown isomers of azobenzene.
TL;DR: In this paper, the authors reported values of the virial coefficients B n of the Lennard-Jones (LJ) model, as computed by the Mayer sampling Monte Carlo method.
Abstract: We report values of the virial coefficients B n of the Lennard-Jones (LJ) model, as computed by the Mayer Sampling Monte Carlo method. For n = 4 and 5, values are reported for 103 temperatures T = 0.62 to 40.0 (in LJ units); for n = 6, 31 values are reported for T = 0.625 to 20.0; for n = 7, 15 values are reported from T = 0.625 to 10; and for n = 8, four values are reported from T = 0.75 to 10. Data are used to estimate the location of the LJ critical point, and the critical temperature estimated this way is given to within 0.8% of the established value, while the critical density is too low by 10%. Data derived from the virial equation of state (VEOS) are compared to pressures and internal energies calculated by Monte Carlo simulation. Simulations of systems ranging from 125 to 30,000 particles are extrapolated to infinite system size, and it is shown that the VEOS–when applied at densities where the series has reached convergence–provides results closer to the infinite-system values than obtained by an...
TL;DR: The infrared spectrum of the nitrate radical (NO3) was studied on the basis of ab initio calculations that use the equation-of-motion coupled-cluster method known as EOMIP-CCSD as discussed by the authors.
Abstract: The infrared spectrum of the nitrate radical (NO3) is studied on the basis of ab initio calculations that use the equation-of-motion coupled-cluster method known as EOMIP-CCSD. While the spectrum associated with vibrational levels is simple, with only the ν2 fundamental having appreciable intensity, that for the e′ vibrational states is extremely rich. The first 20 levels of e′ symmetry (not including those that feature two quanta of excitation in the out-of-plane ν2 mode) are calculated. The major features are: the ν4 fundamental has a large (positive) anharmonicity, several combination bands involving ν4 and its overtones are prominent in the spectrum, and the ν3 fundamental is calculated to lie at 1067 cm−1 and have a small intensity. In fact, of the 20 levels studied, the predicted dipole strength associated with the transition to ν3 ranks nineteenth. The results are broadly consistent with an earlier study that was based on a diabatic representation that also indicated that ν3 is well below the 1492 ...
TL;DR: In this paper, the photodynamics of pyrrole were performed at multireference configuration interaction level involving five electronic states with a simulation time of 200 fs and the analysis of the time dependence of the average state occupations showed that the deactivation of Pyrrole to the electronic ground state takes place in about 140 fs.
Abstract: Non-adiabatic on-the-fly-dynamics simulations of the photodynamics of pyrrole were performed at multireference configuration interaction level involving five electronic states with a simulation time of 200 fs. The analysis of the time dependence of the average state occupations shows that the deactivation of pyrrole to the electronic ground state takes place in about 140 fs. This deactivation time agrees very well with the experimentally measured time constant of 110 fs for the formation of fast hydrogen atoms. After excitation into the S4 state, 80% of the trajectories followed the NH-stretching mechanism giving rise to a population of fast H atoms. The computed average kinetic energy is in good accord with the experimentally observed average kinetic energy of the fast hydrogen atoms. It is found that 10% of trajectories followed the ring-puckering mechanism and 3% followed the ring-opening mechanism. This latter mechanism was characterized in pyrrole for the first time and involves the conical intersect...
TL;DR: In this article, the authors constructed the complete liquid crystal phase diagram of hard plate-like cylinders for variable aspect ratio using Onsager's second virial theory and employing the Parsons-Lee decoupling approximation to account for higher-body interactions in the isotropic and nematic fluid phases.
Abstract: We construct the complete liquid crystal phase diagram of hard plate-like cylinders for variable aspect ratio using Onsager's second virial theory and employing the Parsons–Lee decoupling approximation to account for higher-body interactions in the isotropic and nematic fluid phases. The stability of the solid (columnar) state at high packing fraction is included by invoking a simple equation of state based on a Lennard–Jones–Devonshire cell model which has proven to be quantitatively reliable over a large range of packing fractions. By employing an asymptotic analysis based on the Gaussian approximation we are able to show that the nematic–columnar transition is universal and independent of particle shape. The predicted phase diagram is in qualitative agreement with simulation results.
TL;DR: In this article, the influence of strong directional or bonding interactions on the phase diagram of complex fluids, and in particular on the liquid-vapour critical point, was investigated, and results were interpreted in terms of the equilibrium cluster structures of the coexisting phases.
Abstract: We investigate the influence of strong directional, or bonding, interactions on the phase diagram of complex fluids, and in particular on the liquid–vapour critical point. To this end we revisit a simple model and theory for associating fluids which consist of spherical particles having a hard-core repulsion, complemented by three short-ranged attractive sites on the surface (sticky spots). Two of the spots are of type A and one is of type B; the interactions between each pair of spots have strengths , and . The theory is applied over the whole range of bonding strengths and results are interpreted in terms of the equilibrium cluster structures of the coexisting phases. In systems where unlike sites do not interact (i.e. where ), the critical point exists all the way to . By contrast, when , there is no critical point below a certain finite value of . These somewhat surprising results are rationalised in terms of the different network structures of the two systems: two long AA chains are linked by one BB ...
TL;DR: In this paper, the starting spatial disposition of components was assumed to be random, and the calculated structural parameters of reverse micelles (such as their shape, size, and distribution of components) were found to agree with the available experimental data.
Abstract: Formation of reverse micelles in the water–hexane–AOT system has been investigated by molecular dynamics simulations. In order to optimize calculation strategy, the starting spatial disposition of components was assumed to be random. Such an approach was found to provide a lower dependence of final results on initial conditions as compared to the calculations using pre-assembled reverse micelles as a starting geometry. The calculated structural parameters of reverse micelles (such as their shape, size, and distribution of components) were found to agree with the available experimental data.
TL;DR: In this paper, a coarse grained force field model has been developed to study aqueous solutions of the room temperature ionic liquid 1-n-decyl-3-methylimidazolium bromide using molecular dynamics (MD) simulations.
Abstract: A coarse grained force field model has been developed to study aqueous solutions of the room temperature ionic liquid 1-n-decyl-3-methylimidazolium bromide using molecular dynamics (MD) simulations. Spontaneous self-aggregation of amphiphilic cations to form quasi-spherical micelles has been observed in dilute solutions. Two types of growth mechanism have been observed during the MD simulations, including the fusion of small micelles to form larger ones. Aggregates are found to be poly-disperse and the aggregation number, as determined from the MD simulations, compares well with results from small angle neutron scattering. The dynamics of micelles is discussed. Formation of the hexagonal columnar phase at a concentration of 37% (w/w) of water has been observed with a spacing of the columns in good agreement with the experimentally determined value. The vapor–liquid interface of an aqueous ionic liquid solution has been investigated and the structure at the interface has been characterized. The surface are...
TL;DR: In this paper, the subject molecules were trapped in solid neon and identified by isotopic shifts, comparison with earlier argon and nitrogen matrix results, and DFT frequency calculations, and observed neon-argon matrix shifts follow the molecular polarities.
Abstract: Laser-ablated lithium atoms react with oxygen molecules, as do thermal lithium atoms, to form the LiO2 and LiO2Li ionic molecules. In addition excess energy associated with the laser ablation process fosters the endothermic reaction to give LiO and ultimately Li2O, which form in higher yield from the analogous N2O reactions. The subject molecules were trapped in solid neon and identified by isotopic shifts, comparison with earlier argon and nitrogen matrix results, and DFT frequency calculations. The observed neon–argon matrix shifts follow the molecular polarities. Molecular orbitals from theoretical analysis show the highly ionic nature of the electron distribution in the LiO2 and LiO2Li molecules.
TL;DR: In this paper, the performance of some commonly used quantum-chemical methods in accurately and reliably describing the influence of applying an external mechanical force has been investigated for a set of small molecules.
Abstract: The performance of some commonly used quantum-chemical methods in accurately and reliably describing the influence of applying an external mechanical force has been investigated for a set of small molecules. By applying coupled-cluster CCSD(T) theory in an extended basis set as benchmark, all methods tested provide a good qualitative description of the physical process, although the quantitative agreement varies considerably. Hartree–Fock (HF) theory overestimates both the values of the bond-breaking point and the rupture force, typically by 20–30%. The same applies to density-functional theory (DFT) based on the local density approximation (LDA). By introducing the generalized gradient approximation (GGA) in the form of the BLYP and PBE functionals, only a slight overestimation is observed. Moreover, these pure DFT methods perform better than the hybrid B3LYP and CAM-B3LYP methods. The excellent agreement observed between the CCSD(T) method and multiconfigurational methods for bond distances significantl...
TL;DR: In this paper, a segmented triple zeta (TZ) basis set for atoms H-Ar was constructed from fully-optimized Gaussian basis sets generated in this work, which were optimized at the second-order Mφller-Plesset level.
Abstract: A contracted basis set of triple zeta (TZ) valence quality for the atoms from K to Kr was constructed from fully-optimized Gaussian basis sets generated in this work. Gaussian polarization functions (d, f, and g symmetries), which were optimized at the second-order Mφller–Plesset level, were added to the TZ set. This extends earlier work on segmented contracted TZ basis set for atoms H-Ar. This set along with the BP86 non-hybrid and B3LYP hybrid functionals were used to calculate geometric parameters, dissociation energy, harmonic vibrational frequency, and electric dipole moment of a sample of molecules and, then, comparison with results obtained with other basis sets and with experimental data reported in the literature is done. CCSD(T) atomic excitation energies and bond lengths, dissociation energies, and harmonic vibrational frequencies of some diatomics were also evaluated. Using density functional theory and gauge-including atomic orbitals, 57Fe and 77Se nuclear magnetic resonance chemical shifts i...
TL;DR: In this paper, the authors revisited the thermochemistry of the carbon clusters C n (n = 2−10) by means of W4 theory and W3.2lite theory.
Abstract: The thermochemistry of the carbon clusters C n (n = 2−10) has been revisited by means of W4 theory and W3.2lite theory. Particularly, the larger clusters exhibit very pronounced post-CCSD(T) correlation effects. Despite this, our best calculated total atomization energies agree surprisingly well with the 1991 estimates obtained from scaled CCD(ST)/6-31G* data. Accurately reproducing the small singlet–triplet splitting in C2 requires inclusion of connected quintuple and sextuple excitations. Post-CCSD(T) correlation effects in C4 stabilize the linear form. Linear/cyclic equilibria in C6, C8, and C10 are not strongly affected by connected quadruples, but they are affected by higher-order triples, which favor polyacetylenic rings but disfavor cumulenic rings. Near the CCSD(T) basis set limit, C10 does undergo bond angle alternation in the bottom-of-the-well structure, although it is expected to be absent in the vibrationally averaged structure. The thermochemistry of these systems, and particularly the longe...
TL;DR: In this article, the analytical partial derivatives of the anharmonic vibrational correction with respect to the various harmonic frequencies are used as a tool to quantitatively assess the existence and strength of first-order, or Fermi, resonances.
Abstract: Vibrational perturbation theory has proven to be a highly accurate and efficient method for extending the harmonic approximation in the treatment of polyatomic molecular vibrations. Unfortunately, accidental near-degeneracies of the harmonic vibrational levels can lead to resonance and a breakdown of the perturbation approximation. These resonances can be resolved by the diagonalization of a small effective Hamiltonian derived from either of the usual Rayleigh–Schrodinger or van Vleck perturbation theories. However, the proper choice of states for inclusion in the effective Hamiltonian is crucial to the accuracy of the results, and is not often clearly evident. It is proposed that the analytical partial derivatives of the anharmonic vibrational correction with respect to the various harmonic frequencies, called ‘Harmonic Derivatives’ in this work, can be used as a tool to quantitatively assess the existence and strength of first-order, or Fermi, resonances. These derivatives are shown to concisely and cle...
TL;DR: In this article, the Free Energies from Adaptive Reaction Coordinate Forces (FEARCF) method was used to calculate the free energy of conformation, configuration and chemical reactions.
Abstract: Previously an adaptive reaction coordinate force biasing method based on probability distributions has been used to calculate the free energy of conformation, configuration and chemical reactions. This method has recently been generalised to perform calculations on multidimensional reaction coordinates. This paper presents details of this method, termed Free Energies from Adaptive Reaction Coordinate Forces (FEARCF). The efficiency of sampling of this method is demonstrated by applying it to the problem of sampling the many characteristic pucker conformations of a β-D-glucose pyranose using a semi-empirical PM3 Hamiltonian. The sampling ratio of the global minimum conformer (4C1) to the highest energy conformer (a planar hexapyranose ring) is 1.7:1. Pucker free energy surfaces such as the one presented here can be a useful tool in the analysis of enzymatic reactions involving molecular ring puckers in the Michaelis complex.
TL;DR: In this paper, a reliable potential for GeO2 from first principles is described, which includes dipole polarization effects, is able to reproduce all the studied properties (structural, dynamical and vibrational) to a high degree of precision with a single set of parameters.
Abstract: The construction of a reliable potential for GeO2 from first principles is described. The obtained potential, which includes dipole polarization effects, is able to reproduce all the studied properties (structural, dynamical and vibrational) to a high degree of precision with a single set of parameters. In particular, the infrared spectrum was obtained using the expression proposed for the dielectric function of polarizable ionic solutions reported by Weis et al. [J. Chem. Phys. 91, 5544 (1989)]. The agreement with the experimental spectrum is very good, with three main bands that are associated with tetrahedral modes of the GeO2 network. Finally, we give a comparison with a simpler pair-additive potential.
TL;DR: In this article, the 2Π ground states and low-lying 4Σ excited states of nine diatomic species formed by combining O, S, or Se with F, Cl, or Br were characterized with multireference configuration interaction calculations in order to compare and contrast the behaviour of the bonding in these two states of the chalcogen halides.
Abstract: The 2Π ground states and low-lying 4Σ– excited states of the nine diatomic species formed by combining O, S, or Se with F, Cl, or Br were characterized with multireference configuration interaction calculations in order to compare and contrast the behaviour of the bonding in these two states of the chalcogen halides. For each of these species, the 2Π ground state is polar covalently bound via simple singlet coupling of unpaired electrons on each atom. But for each compound there is also a bound 4Σ– state where bond formation requires recoupling the valence p 2 pair of electrons on the chalcogen atom. This mode of bonding makes more electrons available for additional bond formation and is the basis of hypercoordination. The behaviour of S and Se differs significantly from that of O. Although the hypervalent 4Σ– states of the diatomic chalcogen halides are bound for all three elements, the O species are only weakly bound and exhibit minimal recoupling compared with the corresponding S and Se compounds.