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Molecular dynamics simulations are used to characterize the hydrates of Xe, methane, and CO2, allowing for a systematic comparison of the structural and dynamical properties for these three hydrates. Although the host−guest interaction energy for the T= 0 K structures is most attractive in the case of Xe, other structural and dynamical properties from the simulations indicate that, in fact, host−guest coupling is most important for the CO2hydrate. Specifically, the host lattice of CO2hydrate expands more with increasing temperature than do the lattices of the xenon and methane hydrates, and the translational and rotational dynamics of the water molecules are predicted to be most perturbed in the CO2hydrate. The simulations predict that the CO2and xenon hydrates have lower speed of sound values and lower themal conductivities than methane hydrate or the empty lattice.

We report the vibrational predissociation spectrum of the NO^-·H2O complex in the OH stretching region. The ionic H-bonded OH stretch is much less red-shifted and less structured in the vibrational spectrum of NO^-·H2O than in that of the O2^-·H2O cluster, despite the larger proton affinity of NO^- relative to O2^-. It is argued that the differences between the spectra of these two species are primarily a consequence of the greater charge-transfer character in the O2^-·H2O cluster and that the charge-transfer component is much less important in NO^-·H2O because of its triplet spin character.

We report for the first time the vibrational spectrum of a molecular anion−water complex in which the water molecule adopts a symmetrical binding motif. In the specific complex considered, S%@mt;sys@%O%@sx@%2%@be@%-%@sxx@%%@mx@% ·H2O, each hydrogen atom is attached to one of the oxygen atoms of the anion. The vibrational spectrum is rather simple and is analyzed with the aid of isotopic substitution and ab initio calculations of the complex structure and harmonic vibrational frequencies. The symmetric and asymmetric OH stretching modes in the S%@mt;sys@%O%@sx@%2%@be@%-%@sxx@%%@mx@% ·H2O complex are much closer in energy than in the isolated water molecule, an effect that is traced to the reduction in the HOH angle of the water molecule upon complexation.

Circumtrindene, a C36H12 open geodesic dome with alternating five- and six-membered rings, is of interest as a vesicle for carrying out chemical reactions and also as a model for certain carbon nanotube caps. Here the addition of an oxygen atom to circumtrindene is examined by means of density functional calculations. O-atom addition to a C−C bond shared between the central and adjacent hexagons gives an epoxide structure while O-atom addition to a C−C bond shared by the central hexagon and an adjacent pentagon results in cleavage of the C−C bond and an R−O−R insertion product. Despite their structural dissimilarity, the two oxidation products are predicted to be of comparable stability (with binding energies of −74.9 and −78.7 kcal/mol, respectively), and to have a large (55.7 kcal/mol) barrier for interconversion.

Photoexcitation of I^-(H2O)n clusters can lead to charge transfer states with the excess electron localized on the water cluster, making these ideal systems for studying electron/water cluster dynamics. In the present study, the MP2 method is used to study the structure and infrared spectrum of the ground state of I^-(H2O)4 and the CASPT2 method is used to characterize the low-lying electronically excited states of the complex. The calculated IR spectrum is in good agreement with experimental results, providing support for a crown-like structure of I^-(H2O)4. The 2¹A excited state is predicted to be 29 meV below the ground state of the neutral cluster (at the geometry of the ground-state anion), and the vertical excitation energy for the 1¹A → 2¹A charge-transfer transition is calculated to be 4.60 eV, in good agreement with the experimental value of 4.5 eV. Potential energy curves for the relaxation of the photoexcited I^-(H2O)4 cluster from the crown-like to the planar structure are presented.

Spin-polarized density functional calculations are used to study the adsorption of O atoms and O2 molecules on graphite and on a (8,0) single-walled carbon nanotube. An O atom is found to bind to graphite and to the outside and inside surfaces of the nanotube to give stable epoxide-like structures. Of these, the most stable is the epoxide on the outer surface of the nanotube, with a calculated adsorption energy of 44 kcal/mol. In the case of O2, both physisorbed and chemisorbed species are identified. The O2 molecule is predicted to only weakly physisorb (adsorption energy ≈ 0.9 kcal/mol) to the graphite and the (8,0) nanotube surfaces. However, these adsorption energies are expected to be underestimated due to inadequate treatment of the dispersion interactions. The chemisorbed species are metastable in the sense that they lie energetically above the isolated X ³Σg⁺ O2 and graphite or nanotube systems. In the case of the outer wall of the nanotube, the chemisorbed species is predicted to lie only 10.2 kcal/mol above the reactants. The reaction of O2 with the nanotube surface to give two epoxide groups is predicted to be slightly exothermic.

The complete-active-space SCF and CASMP2 methods are used to characterize the low-lying excited singlet and triplet states of diacetylene. With the exception of the 1 ¹Δu state, all excited states considered are found to have geometrical structures distorted away from linear. Three of the triplet states are predicted to have non-planar C2-symmetry structures. The other states are predicted to be planar, existing as both cis (C2v) and trans (C2h) isomeric forms. Vibrational frequencies are calculated for the low-lying electronically excited states, and vertical and adiabatic excitation energies are reported.

Recent STM measurements and electronic structure calculations have revealed that the Si(001)/acetylene surface is far more complex than previously assumed, with there being at least three different binding arrangements of an acetylene molecule on the surface. In the present study density functional theory with slab models and periodic boundary conditions are used to study the adsorption of acetylene on the Si(001) surface. Nine different local potential energy minima, including three not previously described, are characterized. The relative stability of the two lowest energy structures is found to reverse with increasing surface coverage. The minimum energy pathways between selected pairs of local minima are calculated.

Excitation functions for the ³B3u π → 3s and ³B1g and ¹Ag π → 3p Rydberg states of ethylene have been measured and found to display structure due to the decay of core-excited anion states. Evidence is also provided that the π^-1(3s)² anion state undergoes significant decay to the π → π* singlet state in highly twisted configurations.

Absolute oscillator strength spectra in the C 1s(280–340 eV) and F 1s(680–740 eV) regions of the perfluoro-n-alkanes from C2to C6and perfluorocycloalkanes from C3to C6have been determined from inner-shell electron energy loss spectra recorded under electric-dipole scattering conditions. The spectral features are interpreted in terms of spatially localized transitions terminating at orbitals of predominantly σ*(C—F) and σ*(C—C) character. When compared to the spectra of the perfluoro-n-alkanes, both the C 1sand F 1sspectra of the perfluorocycloalkanes exhibit additional low-lying bands which are assigned to transitions terminating at σ*(C—C) orbitals which are shifted to low energy by the combination of the strain of cyclization and the inductive effect of the fluorination. The electron transmission spectra of selected perfluorocycloalkanes (which provide information on their anion states) show as well that the electron affinities of the cyclic systems are substantially lower than those of the corresponding perfluoro-n-alkanes, again as a result of a low-lying σ* orbital in the cyclic species. Quantum chemical calculations of the alkane and perfluoroalkane ground-state orbital structures support the experimental results. The localized character of the inner-shell excitations, indicated by the constancy of both term values and oscillator strengths with increasing chain length, contrasts with the more delocalized character of the states accessed in ultraviolet excitation or negative ion formation.

We have performed SCF–CI calculations on the ground states of Li2 and its anion. We predict a value aof 0.45 eV for the adiabatic electron affinity of Li2 and a value of 0.86 eV for the dissociation energy of Li2 −. Thus, the bond strength of the anion is nearly the same as that of the neutral parent. This indicates that the simple MO picture, describing the anion as being formed by the addition of an electron to the antibonding 2σu orbital, is not adequate. The present results are compared to the recent equation‐of‐motion calculation of the Li2 electron affinity by Andersen and Simons.

Electrons may attach to highly polar molecules, giving rise to stable negative ions. In this paper we present a detailed study of the type of diffuse functions that must be included in the basis sets in order to obtain a proper description of such anions. In the case of ionic diatomic molecules the 'extra' electron occupies a non-bonding orbital locating most of its charge density behind the electropositive atom. We demonstrate, with the specific examples of LiH and NaH, that such an orbital may be formed either by hybridizing functions centred on the electropositive atom or by employing functions centred behind the electropositive atom. Our new studies on LiH and NaH demonstrate that the electron affinity is relatively insensitive to the basis set employed, provided that it yields a good estimate of the dipole moment of the neutral molecule and that the 'proper' diffuse functions are included on the electropositive atom. Ab initio calculations are also performed on NaCl and its ground-state negative ion. The anion is predicted to be stable, the calculated electron affinity being 0·65 eV.

Dipole-bound anionic states of HCN, (HF)2, CH3CN, C3H2, C4H2, C5H2, and stretched CH3F are studied using extended one-electron basis sets at the coupled cluster level of theory with single, double, and noniterative triple excitations (CCSD(T)). Orbital relaxation and electron correlation corrections to the Koopmans' theorem prediction of electron binding energy are analyzed, and a physical interpretation of low-order corrections is proposed. It is demonstrated that the second-order dispersion interaction between the loosely bound electron and the electrons of the neutral host should be included into physical models of dipole-bound anions. Higher-order electron correlation corrections are also found to be important, and a slow convergence of the Møller−Plesset series for electron binding energies is documented. Modifications of the potential energy surfaces of the above polar molecules upon electron attachment are studied at the second-order Møller−Plesset level, and Franck−Condon factors for the anion/neutral pairs are calculated. It is predicted that photoelectron spectra of the dipole-bound anions of C4H2 and C5H2 should display vibrational structure.

The interactions of CO2 with small hydrocarbons and fluorocarbons (CH4, C2H6, CF4, and C2F6) are investigated using the Hartree−Fock and second-order many-body perturbation theory (MP2) methods. Hartree−Fock calculations with flexible basis sets fail to give appreciable binding for any of the four dimers. In contrast, MP2 calculations using flexible basis sets and including corrections for basis-set superposition error give binding energies of the CO2−CH4, CO2−C2H6, CO2−CF4, and CO2−C2F6 clusters ranging from −0.79 to −1.17 kcal/mol. The binding energies for the CO2−hydrocarbon complexes are slightly larger in magnitude than the corresponding CO2−perfluorocarbon clusters.