Binding of an Electron by a Finite Fixed Dipole.

In this work, it is demonstrated that a simple analytical expression, (A/R2) exp(-B/µ-µcr), where µ is the dipole moment, µcr is the critical dipole moment, and A and B are constants, accurately describes the binding energy of an electron in the field of a finite fixed dipole over a wide range of dipole moments. It is also demonstrated that this expression provides an accurate fit to the experimental electron binding energies for the dipole-bound anions of a series of phenoxy radicals. A simple extension of this expression is found to be applicable when the dipole model is extended to include short-range repulsion and polarization interactions.

Assessment of the performance of common density functional methods for describing the interaction energies of (H2O)6 clusters.

Localized molecular orbital energy decomposition analysis and symmetry-adapted perturbation theory (SAPT) calculations are used to analyze the two- and three-body interaction energies of four low-energy isomers of (H(2)O)(6) in order to gain insight into the performance of several popular density functionals for describing the electrostatic, exchange-repulsion, induction, and short-range dispersion interactions between water molecules. The energy decomposition analyses indicate that all density functionals considered significantly overestimate the contributions of charge transfer to the interaction energies. Moreover, in contrast to some studies that state that density functional theory (DFT) does not include dispersion interactions, we adopt a broader definition and conclude that for (H(2)O)(6) the short-range dispersion interactions recovered in the DFT calculations account about 75% or more of the net (short-range plus long-range) dispersion energies obtained from the SAPT calculations.

Infrared spectroscopy of small protonated water clusters, H(+)(H2O)n (n = 2-5): isomers, argon tagging, and deuteration.

Infrared photodissociation spectroscopy is reported for mass-selected H(+)(H(2)O)(n) complexes and their deuterated analogues with and without argon "tagging." H(+)(H(2)O)(n)Ar(m) and D(+)(D(2)O)(n)Ar(m) complexes are studied in the O-H (O-D) stretching region for clusters in the small size range (n = 2-5). Upon infrared excitation, these clusters fragment by the loss of either argon atoms or one or more intact water molecules. Their excitation spectra show distinct bands in the region of the symmetric and asymmetric stretches of water and in the hydrogen bonding region. Experimental studies are complemented by computational work that explores the isomeric structures, their energetics and vibrational spectra. The addition of an argon atom is essential to obtain photodissociation for the n = 2-3 complexes, and specific inclusion of the argon in calculations is necessary to reproduce the measured spectra. For n = 3-5, spectra are obtained both with and without argon. The added argon atom allows selection of a subset of colder clusters and it increases the photodissociation yield. Although most of these clusters have more than one possible isomeric structure, the spectra measured correspond to a single isomer that is computed to be the most stable. Deuteration in these small cluster sizes leads to expected lowering of frequencies, but the spectra indicate the presence of the same single most-stable isomer for each cluster size.

Application of the diffusion Monte Carlo method to the binding of excess electrons to water clusters.

The diffusion Monte Carlo (DMC) method is used to calculate the electron binding energies of two forms of (H(2)O)(6)(-). It is found that the DMC method, when using either Hartree-Fock or density functional theory trial wave functions, gives electron binding energies in excellent agreement with the results of large basis set CCSD(T) calculations. This demonstrates that the DMC method will be a viable method for characterizing larger (H(2)O)(n)(-) ions for which CCSD(T) calculations are not feasible.

A modified MSEVB force field for protonated water clusters.

A multistate empirical valence bond (MSEVB) model for protonated water clusters, which incorporates the TIP4P water model, is presented. This model which is designated MSEVB4P represents a significant improvement over the original model of Voth et al. (J. Phys. Chem. B 1998, 102, 5547) which was based on the TIP3P water model and a smaller improvement over the recently introduced MSEVB3 model (J. Phys. Chem. B 2008, 112, 467) which is based on the SPC/Fw (J. Chem. Phys. 2006, 124, 024503) water model.

Molecular dynamics simulations of methane hydrate using polarizable force fields.

Molecular dynamics simulations of methane hydrate have been carried out using the polarizable AMOEBA and COS/G2 force fields. Properties calculated include the temperature dependence of the lattice constant, the OC and OO radial distribution functions, and the vibrational spectra. Both the AMOEBA and COS/G2 force fields are found to successfully account for the available experimental data, with overall somewhat better agreement with experiment being found for the AMOEBA model. Comparison is made with previous results obtained using TIP4P and SPC/E effective two-body force fields and the polarizable TIP4P-FQ force field, which allows for in-plane polarization only. Significant differences are found between the properties calculated using the TIP4P-FQ model and those obtained using the other models, indicating an inadequacy of restricting explicit polarization to in-plane only.

The chemisorption of dibenzo[a,j]coronene on Si(001)-2 x 1.

Adsorption structures of the dibenzo[a,j]coronene (C(32)H(16)) molecule on the clean Si(001)-2 X 1 surface were investigated by scanning tunneling microscopy (STM) in conjunction with electronic structure calculations. The dibenzo[a,j]coronene molecules were found to adsorb on three different sites: one major adsorption site and two minor adsorption sites. The formation of four to eight Si-C covalent bonds is responsible for the different surface bonding structures observed. Bond strain effects due to out-of-plane bending of the molecule play a significant role in governing the surface bond energies. The geometries of the three adsorption sites were established by comparison of the experimental and simulated STM images. By applying an electrical pulse, the molecule can be made to hop from one site to another site without breaking the dibenzo[a,j]coronene molecular structure.

Infrared spectrum and structural assignment of the water trimer anion.

The bending vibrational spectrum of the perdeutero isotopomer of the water trimer anion has been measured and compared with spectra calculated using the MP2, CCSD, and Becke3LYP electronic structure methods. Due to its low electron binding energy (approximately 150 meV), only the OD bending region of the IR spectrum of (D2O)3(-) is accessible experimentally, with electron ejection dominating at higher photon energies. The calculated spectrum of the isomer having three water molecules arranged in a chain agrees best with the experimental spectrum. In the chain isomer, the excess electron is bound to the terminal water monomer with two dangling OH groups. This is consistent with the electron binding mechanism established previously for the (H2O)n(-) (n = 2, 4-6) anions.

Negative ions of ethylene sulfite.

The formation of negative ions in molecular beams of ethylene sulfite (ES, alternately called glycol sulfite or ethylene glycol, C(2)H(4)SO(3)) molecules has been studied using both Rydberg electron transfer (RET) and free electron attachment methods. RET experiments with jet-cooled ES show an unexpected broad profile of anion formation as a function of the effective quantum number (n(*)) of the excited rubidium atoms, with peaks at n(max)(*) approximately 13.5 and 16.8. The peak at n(max)(*) approximately 16.8 corresponds to an expected dipole-bound anion with an electron binding energy of 8.5 meV. It is speculated that the peak at n(max)(*) approximately 13.5 derives from the formation of a distorted C(2)H(4)SO(3)(-) ion. We suggest that quasifree electron attachment promotes the breaking of one ring bond giving a long-lived acyclic anion and term this process incomplete dissociative electron attachment. Theoretical calculations of plausible ionic structures are presented and discussed. Electron beam studies of ES reveal the presence of multiple dissociative attachment channels, with the dominant fragment, SO(2)(-), peaking at 1.3 eV and much weaker signals due to SO(3)(-), SO(-), and (ES-H)(-) peaking at 1.5, 1.7, and 0.9 eV, respectively. All of these products appear to originate from a broad temporary negative ion resonance centered at approximately 1.4 eV.

Infrared photodissociation spectroscopy of Mg(+)(H2O)Ar(n) complexes: isomers in progressive microsolvation.

Ion-molecule complexes of the form Mg(H2O)Ar(n)+ (n = 1-8) are produced by laser vaporization in a pulsed-nozzle cluster source. These complexes are mass-selected and studied with infrared photodissociation spectroscopy in the O-H stretch region. The spectra are interpreted with the aid of ab initio calculations on the n = 1-5 complexes, including examination of various isomeric structures. The combined spectroscopic and theoretical studies reveal the presence of multiple isomeric structures at each cluster size, as the argon atoms assemble around the Mg(+)(H2O) unit. Distinct infrared resonances are measured for argon-on-metal, argon-on-OH and argon-on-two-OH isomers.

Calculation of the photodetachment cross sections of the HCN- and HNC- dipole-bound anions as described by a one-electron Drude model.

The Drude model for treating the interaction of excess electrons with polar molecules is extended to calculate continuum functions and to evaluate photodetachment cross sections. The approach is applied to calculate the cross sections for photodetachment of dipole-bound electrons from HCN(-) and HNC(-). In addition, an adiabatic model separating the angular and radial degrees of freedom of the excess electron is introduced and shown to account in a qualitative manner for the cross sections.

Infrared signature of structures associated with the H+(H2O)n (n = 6 to 27) clusters.

We report the OH stretching vibrational spectra of size-selected H+(H2O)n clusters through the region of the pronounced "magic number" at n = 21 in the cluster distribution. Sharp features are observed in the spectra and assigned to excitation of the dangling OH groups throughout the size range 6

Should extended disability be an exclusion criterion for tertiary rehabilitation? Socioeconomic outcomes of early versus late functional restoration in compensation spinal disorders.

STUDY DESIGN: A prospective cohort design with two groups of patients representing short-term or long-term disability (n = 497) who were selected from a larger cohort (n = 938) of consecutively treated spinal disorder patients with chronic compensation injuries. OBJECTIVES: To prospectively evaluate the impact of length of spinal disability on socioeconomic outcomes of medically directed rehabilitation. SUMMARY OF BACKGROUND DATA: Despite an increasing tendency of managed care organizations to limit rehabilitation services for disabled workers with chronic spinal disorders, there has been a surprising lack of prospective research evaluating the impact of length of disability on objective socioeconomic treatment outcomes. Although only approximately 10% of all patients with spinal disorders are disabled beyond 4 months, they account for nearly 80% of all workers' compensation expenditures. Little is known about whether relatively early intervention improves outcomes after chronicity has been established or whether any predictors distinguish between these groups. METHODS: Two comparison groups of functional restoration tertiary treatment graduates were identified from the same community referral pool. The "long-term disabled" group involved a minimum of 18 months of disability (n = 252). This group was compared with a "short-term disabled" group (n = 245), no more than 8 months since injury, but chronic based on a minimum of 4 months after injury. The long-term disabled group showed significantly higher rates of pretreatment surgery than the short-term disabled group (P < 0.001). All patients were evaluated prospectively with specific physical, psychological, and occupational measurements. They also underwent a structured interview 1 year after treatment evaluating work status, health care use, and recurrent injury. RESULTS: The short-term disabled group showed statistically higher return to work (P < 0.001) and work retention (P < 0.05) relative to the long-term disabled group. However, health care use and recurrent lost time injury claims were low in both groups and did not differ significantly. No predictors of outcome were found among the prospectively collected physical performance or psychosocial variables. CONCLUSIONS: This study suggests that early tertiary nonoperative care, once patients with chronic spinal disorders are identified as having potentially high-cost chronic pain and disability, is efficacious in achieving goals of better work return and work retention. Such early rehabilitation may also prevent significant indemnity expense, as well as some late surgical interventions sought by progressively more desperate patients. However, individuals with long-term disability achieve respectable work return and retention rates, while faring no worse on other socioeconomic outcomes that represent major "cost drivers" to the workers' compensation system. Early intervention is not a panacea or a necessary condition for the successful rehabilitation of workers with disabling chronic spinal disorders.