bluuebunny/crossref_metadata_2025 huggingface dataset:
Ipsilateral Actions of Feline Corticospinal Tract Neurons on Limb Motoneurons
Authors: Edgley S. A. , Jankowska E. , Hammar I.
Published: September 2004

Contralateral pyramidal tract (PT) neurons arising in the primary motor cortex are the major route through which volitional limb movements are controlled. However, the contralateral hemiparesis that follows PT neuron injury on one side may be counteracted by ipsilateral of actions of PT neurons from the undamaged side. To investigate the spinal relays through which PT neurons may influence ipsilateral motoneurons, we analyzed the synaptic actions evoked by stimulation of the ipsilateral pyramid on hindlimb motoneurons after transecting the descending fibers of the contralateral PT at a low thoracic level. The results show that ipsilateral PT neurons can affect limb motoneurons trisynaptically by activating contralaterally descending reticulospinal neurons, which in turn activate spinal commissural interneurons that project back across to motoneurons ipsilateral to the stimulated pyramidal tract. Stimulation of the pyramids alone did not evoke synaptic actions in motoneurons but potently facilitated disynaptic EPSPs and IPSPs evoked by stimulation of reticulospinal tract fibers in the medial longitudinal fascicle. In parallel with this double-crossed pathway, corticospinal neurons could also evoke ipsilateral actions via ipsilateral descending reticulospinal tract fibers, acting through ipsilaterally located spinal interneurons. Because the actions mediated by commissural interneurons were found to be stronger than those of ipsilateral premotor interneurons, the study leads to the conclusion that ipsilateral actions of corticospinal neurons via commissural interneurons may provide a better opportunity for recovery of function in hemiparesis produced by corticospinal tract injury.
Functional Properties of Motoneurons Derived from Mouse Embryonic Stem Cells
Authors: Miles Gareth B. , Yohn Damien C. , Wichterle Hynek , Jessell Thomas M. , Rafuse Victor F. , Brownstone Robert M.
Published: September 2004

The capacity of embryonic stem (ES) cells to form functional motoneurons (MNs) and appropriate connections with muscle was investigatedin vitro. ES cells were obtained from a transgenic mouse line in which the gene for enhanced green fluorescent protein (eGFP) is expressed under the control of the promotor of the MN specific homeobox geneHb9. ES cells were exposed to retinoic acid (RA) and sonic hedgehog agonist (Hh-Ag1.3) to stimulate differentiation into MNs marked by expression of eGFP and the cholinergic transmitter synthetic enzyme choline acetyltransferase. Whole-cell patch-clamp recordings were made from eGFP-labeled cells to investigate the development of functional characteristics of MNs. In voltage-clamp mode, currents, including EPSCs, were recorded in response to exogenous applications of GABA, glycine, and glutamate. EGFP-labeled neurons also express voltage-activated ion channels including fast-inactivating Na+channels, delayed rectifier andIA-type K+channels, and Ca2+channels. Current-clamp recordings demonstrated that eGFP-positive neurons generate repetitive trains of action potentials and thatl-type Ca2+channels mediate sustained depolarizations. When cocultured with a muscle cell line, clustering of acetylcholine receptors on muscle fibers adjacent to developing axons was seen. Intracellular recordings of muscle fibers adjacent to eGFP-positive axons revealed endplate potentials that increased in amplitude and frequency after glutamate application and were sensitive to TTX and curare. In summary, our findings demonstrate that MNs derived from ES cells develop appropriate transmitter receptors, intrinsic properties necessary for appropriate patterns of action potential firing and functional synapses with muscle fibers.
L1.1 Is Involved in Spinal Cord Regeneration in Adult Zebrafish
Authors: Becker Catherina G. , Lieberoth Bettina C. , Morellini Fabio , Feldner Julia , Becker Thomas , Schachner Melitta
Published: September 2004

Adult zebrafish, in contrast to mammals, regrow axons descending from the brainstem after spinal cord transection. L1.1, a homolog of the mammalian recognition molecule L1, is upregulated by brainstem neurons during axon regrowth. However, its functional relevance for regeneration is unclear. Here, we show with a novel morpholino-based approach that reducing L1.1 protein expression leads to impaired locomotor recovery as well as reduced regrowth and synapse formation of axons of supraspinal origin after spinal cord transection. This indicates that L1.1 contributes to successful regrowth of axons from the brainstem and locomotor recovery after spinal cord transection in adult zebrafish.
Prevalence of Palatal and Alveolar Cysts in Babies with Cleft Lip and Palate
Authors: Alves Karina Mirela Ribeiro Pinto , Peixoto Virginia , Gomide Márcia Ribeiro , Carrara Cleide Felíciode Carvalho , Costa Beatriz
Published: September 2004

Objective

To evaluate the prevalence of palatal and alveolar cysts in babies with cleft lip and/or palate.

Design

Cross-sectional.

Setting

Hospital for Rehabilitation of Craniofacial Anomalies, University of São Paulo (HRAC-USP), Bauru, São Paulo, Brazil.

Participants

Two hundred ninety-one Caucasian babies divided into four groups according to the type of cleft: cleft lip with or without cleft alveolus (70), complete unilateral cleft lip and palate (112), complete bilateral cleft lip and palate (56), and cleft palate (53).

Results

A low prevalence of palatal and alveolar cysts was observed among patients with the four different types of clefts, with no statistically significant difference between genders. The maxilla and the anterior area of the mouth were more affected than the mandible and the posterior area.

Conclusions

The low prevalence of palatal and alveolar cysts in the four groups of babies with clefts included in this study may have been due to the high mean age of the sample.
Dentoalveolar Growth of Patients with Complete Unilateral Cleft Lip and Palate by Early Two-Stage Furlow and Push-Back Method: Preliminary Results
Authors: Kitagawa Taiji , Kohara Hiroshi , Sohmura Taiji , Takahashi Junzo , Tachimura Takashi , Wada Takeshi , Kogo Mikihiko
Published: September 2004

Objective

This study examined dentoalveolar growth changes prior to the time of palatoplasty up to 3 years of age by the early two-stage Furlow and push-back methods.

Subjects

Thirty-four Japanese patients with complete unilateral cleft lip and palate (UCLP) treated with either a two-stage Furlow procedure (Furlow group: seven boys, eight girls) from 1998 to 2002 or a push-back procedure (push-back group; 12 boys, 7 girls) from 1993 to 1997.

Method

Consecutive plaster models were measured by three-dimensional laser scanner, before primary palatoplasty, before hard palate closure (Furlow group only), and at 3 years of age. Bite measures were taken at 3 years of age.

Results

In the Furlow group, arch length, canine width, first and second deciduous molar width and cross-sectional area, and depth and volume at midpoint showed greater growth than in the push-back group. In the Furlow group, the crossbite score was also better than in the push-back group at 3 years of age. In comparison with the push-back group, inhibition of growth impediment in the anterior region was observed in the horizontal direction in the Furlow group. In the midregion, it was observed in the horizontal and vertical directions, and in the posterior region it was observed in the horizontal direction.

Conclusion

The results demonstrate that the early two-stage Furlow method showed progressive alveolar growth. Therefore, the early two-stage Furlow method is a more beneficial procedure than the push-back method.
Management of the Impaired Adult Cleft Patient: The Last Chance
Authors: Felemovicius J. , Ortiz-Monasterio F.
Published: September 2004

Objective

Unfortunately, not all patients receive the benefits of a well-trained, experienced, multidisciplinary cleft team, and a significant number of patients reach adulthood with severe neglected cleft related problems. The vast majority of this group have made their way through several surgical procedures, usually performed by different surgeons in different centers, and still carry the original cleft stigmata aggravated by variable degrees of maxillary hypoplasia, fistulae, scarring, muscle pull, soft tissue tension, lost teeth, dental adaptation, and instability of bony fragments. Adult patients who reach this stage with their original deformities have, in most instances, lost their best opportunity to be properly treated. With this assumption in mind, an aggressive surgical plan focused mainly on the deformities that can objectively achieve functional and aesthetical improvement should be individualized for each patient. In this article, experience with 38 consecutive adult patients with cleft managed following a two-stage protocol used at our cleft clinic for the last 4 years is presented.

Conclusion

An algorithm to help definitively solve these deformities is suggested.
Demographic and Reproductive Factors Associated with Hemifacial Microsomia
Authors: Werler Martha M. , Sheehan Jane E. , Hayes Catherine , Padwa Bonnie L. , Mitchell Allen A. , Mulliken John B.
Published: September 2004

Objective

To identify demographic and reproductive risk factors for hemifacial microsomia in offspring.

Design

In a case-control study, maternal interviews were conducted within 3 years after delivery. Cases with hemifacial microsomia were ascertained from craniofacial centers in 26 cities in the United States and Canada. Controls were patients of the cases’ pediatricians. Two hundred thirty-nine cases were compared with 854 controls. Odds ratios for various infant and maternal factors were estimated.

Results

Cases had lower birth weights, were more often male or a twin, and had more relatives with craniofacial malformations or hearing loss than controls. Case mothers had lower family incomes, had a lower body mass index, had more vaginal bleeding in the second trimester, and were more likely to have had a spontaneous abortion in a previous pregnancy.

Conclusions

Nonmodifiable factors (age and parity) were not associated with hemifacial microsomia risk. Factors that are related to poverty (low family income, late recognition of pregnancy, and low body mass index) are associated with an increase in risk. High risk estimates for multiple pregnancies and second-trimester vaginal bleeding suggest a vascular etiology.
Approaches of South African Traditional Healers regarding the Treatment of Cleft Lip and Palate
Authors: Dagher Danielle , Ross Eleanor
Published: September 2004

Objective

Due to the large number of Black South Africans known to consult with traditional healers and the influential role they play in the developing world, this study investigated the approaches of a group of 15 traditional healers toward the treatment of cleft lip and palate.

Design

An exploratory-descriptive, qualitative research design was employed, which involved the use of standardized, semistructured interviews, conducted with the assistance of an interpreter who was fluent in several African languages.

Setting

Interviews were conducted outdoors in places in which traditional healers usually consult with their patients. These areas were in the fields close to the traditional healers’ homes or under a highway bridge in the Johannesburg inner city.

Participants

Individual interviews were conducted with 15 African traditional healers who had been practicing in their callings for at least 1 year.

Results

Traditional healers interviewed had treated one to six persons with a cleft lip, cleft palate, or both. Most informants believed that clefts were caused by the ancestors, spirits, and witchcraft. A variety of plant and animal products were used to treat these conditions and were augmented by spiritual resources from the ancestors. All except one healer had undergone formal training, although they had received no specific training relative to cleft lip and palate. Most participants reported referring patients to Western health care practitioners who were referred to as modern doctors but did not receive reciprocal referrals from these professionals. Patients generally consulted with traditional healers because this approach was part of their culture.

Conclusions

These findings have important implications for health care professionals as well as traditional healers in terms of cross-cultural consultation, collaboration, and information sharing with regard to cleft lip and palate; the potential use of traditional healers in primary health care and education; and further research.
Heterochromatin structure and function
Authors: Dillon Niall
Published: September 2004

Abstract Although heterochromatin has long been used as a model for studying chromatin condensation and heritable gene silencing, it is only relatively recently that detailed information has become available on the mechanisms that underlie its structure. Current evidence suggests that these operate on at least three different levels. A regular nucleosome array may facilitate packaging of the chromatin into a highly condensed configuration. Methylation of histone H3 lysine 9 and lysine 27 generates heterochromatin marks that are recognised through binding of heterochromatin proteins such as HP1. Finally, very recent studies using genetic and biochemical approaches have indicated that the RNAi machinery plays an important role in the formation of heterochromatin.
ALS2CL, the novel protein highly homologous to the carboxy‐terminal half of ALS2, binds to Rab5 and modulates endosome dynamics
Authors: Hadano Shinji , Otomo Asako , Suzuki-Utsunomiya Kyoko , Kunita Ryota , Yanagisawa Yoshiko , Showguchi-Miyata Junko , Mizumura Hikaru , Ikeda Joh-E
Published: September 2004

ALS2, the causative gene product for juvenile recessive amyotrophic lateral sclerosis (ALS2), is a guanine‐nucleotide exchange factor for the small GTPase Rab5. Here, we report a novel ALS2 homologous gene, ALS2 C‐terminal like (ALS2CL), which encodes a 108‐kD ALS2CL protein. ALS2CL exhibited a specific but a relatively weak Rab5‐GEF activity with accompanying rather strong Rab5‐binding properties. In HeLa cells, co‐expression of ALS2CL and Rab5A resulted in a unique tubulation phenotype of endosome compartments with significant colocalization of ALS2CL and Rab5A. These results suggest that ALS2CL is a novel factor modulating the Rab5‐mediated endosome dynamics in the cells.
Is the cytoplasmic loop of MerT, the mercuric ion transport protein, involved in mercury transfer to the mercuric reductase?
Authors: Rossy Emmanuel , Sénèque Olivier , Lascoux David , Lemaire David , Crouzy Serge , Delangle Pascale , Covès Jacques
Published: September 2004

In MerT, the mercury transporter, a first cysteine pair, located in the first trans‐membrane helix, receives mercury from the periplasm. Then, a second cysteine pair, housed in a cytoplasmic loop connecting the second and the third trans‐membrane helices, is thought to transfer the metal to another cysteine pair located in the N‐terminal extension of the mercuric reductase. We found that a 23‐amino acid synthetic peptide corresponding to the cytoplasmic loop can bind one mercury atom per molecule and that this mercury atom can be transferred specifically to MerAa. The solution structure of Hg‐bound ppMerT has been solved by 1H NMR spectroscopy.
Functional characterization of Ih‐channel splice variants from Apis mellifera
Authors: Gisselmann Günter , Wetzel Christian H. , Warnstedt Maike , Hatt Hanns
Published: September 2004

We isolated splice variants of the AMIH cDNA by means of polymerase chain reaction and homology screening. Splicing at one site generates at least four different channel transcripts (AMIH, AMIHL, AMIHM and AMIHT), which code for ion‐channel proteins that vary in the interloop regions between the membrane‐spanning domains S4 and S5. HEK293 cells in which the AMIHL splice variants were functionally expressed generated currents that were activated by hyperpolarizing voltage steps. Compared to AMIH, AMIHL cells showed pronounced differences in the voltage dependency of activation: the incorporation of 32 extra amino acids between S4 and S5 shifts the activation curve by +25 mV. Intracellular cAMP made the current‐activation potential still less negative and accelerated the activation more effectively than it does in AMIH cells. In vertebrates, functional diversity of I
h‐channels is generated by four different genes. In Apis mellifera, splice variants coded by the single gene AMIH could generate a similar diversity.
Predictability of Brain-Computer Communication
Authors: Kübler Andrea , Neumann Nicola , Wilhelm Barbara , Hinterberger Thilo , Birbaumer Niels
Published: September 2004

Abstract Since 1996 we have been teaching more than 18 severely or totally paralyzed patients to successfully control the movements of a cursor on a computer screen by means of systematic changes in the amplitudes of their slow cortical potentials (SCPs; Birbaumer, Ghanayim, Hinterberger, Iversen, Kotchoubey et al., 1999 ). Patients learned regulation of their SCP amplitudes by means of a brain-computer interface (BCI) and on-line feedback about the time course of SCP amplitude shifts, represented by cursor movements on a computer screen. When patients were able to successfully regulate their SCP amplitude, they were trained to use this ability to communicate with friends and caregivers by means of a Language Support Program ( Perelmouter, Kotchoubey, Kübler, Taub, & Birbaumer, 1999 ). Having a reliable predictor of progress in training would be particularly helpful because training patients at their homes requires substantial effort and a positive outcome is desirable given limited personal and financial resources. In this study we present data from healthy participants (n = 10) and a sample of patients (n = 10), diagnosed with amyotrophic lateral sclerosis, who participated in six BCI training sessions; six patients continued training for another six sessions. All participants except one achieved stable significant cursor control. The number of sessions needed to achieve significant cursor control (initial training phase) correlated moderately with the number of sessions needed to achieve a correct response rate of 70% (advanced training phase). Individual differences in performance remained stable within the six training sessions. After six sessions both groups had achieved significant cursor control, but patients' performance was poorer than that of healthy participants. The patients, however, were trained once a week only, and for some patients longer breaks in training occurred. We conclude that learning during the initial training phase indicates the duration of training that will be necessary to achieve 70% correct responses. A higher frequency of training sessions per week seems necessary to achieve faster progress.
Seismic attenuation in a nonvolcanic swarm region beneath Wakayama, southwest Japan
Authors: Matsunami Koji , Nakamura Masao
Published: September 2004

Seismic attenuation in the Wakayama swarm area of southwest Japan is estimated by optimally fitting the theoretical S coda model to an average S coda envelope in the frequency range of 1–48 Hz. The average S coda envelope is confirmed to have a common decay curve independent of the source‐station distance. Intrinsic attenuation is found to peak at around 4 Hz, suggesting thermal diffusion in grain size or crack length domains with dimensions of the order of 0.7–1 mm, which is reasonable for crustal rock. Scattering attenuation is well approximated by the relation 0.054f−1 in this frequency range, with an inferred peak at 0.5 Hz, suggesting that the statistical model is characterized by an exponential autocorrelation function with a correlation length of about 1 km and a fractional velocity fluctuation of 21%. Scattering is the dominant cause of attenuation at frequencies below 2 Hz, while intrinsic attenuation becomes predominant above 4 Hz. Although values of coda attenuation (Qc−1) generally lie between the total and intrinsic attenuation, the differences between these three values becomes very small above 4 Hz, indicating that the coda attenuation measurements provide a reasonably good estimate of the total attenuation. The total attenuation was found to coincide well with the total apparent attenuation estimated by a linear inversion using direct S waves from swarm earthquakes, confirming the validity of the present results.
Upper crustal structure of southwestern British Columbia from the 1998 Seismic Hazards Investigation in Puget Sound
Authors: Ramachandran K. , Dosso S. E. , Zelt C. A. , Spence G. D. , Hyndman R. D. , Brocher T. M.
Published: September 2004

This paper applies nonlinear three‐dimensional travel time tomography to refraction data recorded during the 1998 Seismic Hazards Investigation in Puget Sound (SHIPS) to derive the first large‐scale, high‐resolution upper crustal velocity model for southwestern British Columbia. A minimum structure P wave velocity model is constructed using 175,000 first arrival travel times picked from data recorded by 58 temporary onshore stations. The model details forearc crustal structures related to terrane accretion and subsequent basin formation to a depth of about 10 km. The Metchosin igneous complex (correlative with the Eocene Crescent‐Siletz Terrane in Washington) is imaged as a laterally extensive WNW trending high‐velocity anomaly underlying southernmost Vancouver Island and much of the Strait of Juan du Fuca. Northeast of the Strait of Georgia, the southwesterly dip of the contact between the Wrangellia terrane rocks of Vancouver Island and the Coast Plutonic Complex suggests Wrangellia rocks are down‐faulted against the plutonic complex. At the southwestern end of the Strait of Juan de Fuca, the 50 km long WNW trending Clallam basin has a maximum thickness of 5–6 km. Near the eastern end of the Strait of Juan de Fuca, Port Townsend basin has an inferred thickness of approximately 4–5 km. The southern end of the 9 km thick Georgia basin is bounded by a high‐velocity basement ridge. Beneath the Strait of Georgia, clusters of well‐located earthquakes have a prominent NW trend and coincide spatially with rapid lateral velocity changes. Clusters of microearthquakes there are associated with the intersection of several east trending structural highs within this NW trend.
Improving 104‐year surge level estimates using data of the ECMWF seasonal prediction system
Authors: van den Brink H. W. , Können G. P. , Opsteegh J. D. , van Oldenborgh G. J. , Burgers G.
Published: September 2004

The vulnerability of society on extreme weather has resulted in extensive research on the statistics of extremes. Although the theoretical framework of extreme value statistics is well developed, meteorological applications are often limited by the relative shortness of the available datasets. In order to overcome this problem, we use archived data from all past seasonal forecast ensemble runs of the European Centre for Medium‐Range Weather Forecasts (ECMWF). For regions where the forecasts have very little seasonal skill the archived seasonal forecast ensembles provide independent sets that cumulate to over 1500 years. We illustrate this approach by estimating 104‐year sea‐surge levels at high‐tide along the Dutch coast. No physical mechanisms occur in the ECMWF model that make the distribution of very extreme surges different from what is inferred from a direct analysis of the observations. In comparison with the observational sets, the ECMWF set shows a decrease in the statistical uncertainty of the estimated 104‐year return value by a factor four.
Large‐scale elongated gas blowouts along the U.S. Atlantic margin
Authors: Hill Jenna C. , Driscoll Neal W. , Weissel Jeffrey K. , Goff John A.
Published: September 2004

In May 2000 we surveyed a series of en echelon, asymmetric depressions along the outer shelf off Virginia and North Carolina using high‐resolution chirp and side‐scan sonar. The features, which are elongated parallel to the shelf edge and have steep landward walls, are ∼4 km long, 1 km wide, and up to 50 m deep. On the basis of internal stratal geometry interpreted from chirp profiles, the depressions do not appear to result from simple, down‐to‐the‐east, normal displacement along deep‐seated faults or structure. Rather, the depressions seem to have been excavated primarily by gas expulsion, creating large‐scale asymmetric gas escape structures that have been termed “gas blowouts.” Gas appears to have been trapped beneath a shelf edge delta that is a few tens of meters thick and exhibits internal soft sediment deformation suggestive of progressive downslope (seaward) creep. These new data suggest the blowouts occurred when thin‐skinned deformation and creep of the surficial deltaic sediment layers combined with updip/upslope gas migration, ultimately leading to gas pressure in excess of the overburden. The location of the expulsion craters along the shelf edge and their elongated, asymmetric shapes strongly suggests a causal relation between the downslope creep of the delta and the expulsion event. We suggest a positive feedback between upward migration of gas‐rich fluids through the low‐stand delta and the downslope creep processes. While the complex interplay between differential permeability, overpressure, and upslope fluid migration remains poorly understood, we suggest such interactions may play an important role in controlling slope stability.
An extremum principle of evaporation
Authors: Wang Jingfeng , Salvucci Guido D. , Bras Rafael L.
Published: September 2004

It is proposed, on the basis of an argument of thermodynamic equilibrium, that land‐atmosphere interactive processes lead to thermal and hydrologic states of the land surface that maximize evaporation in a given meteorological environment. The extremum principle leads to general equations linking surface energy fluxes to surface temperature and soil moisture. The hypothesis of maximum evaporation has been tested with data from three field experiments. We found strong evidence suggesting that evaporation is maximized and furthermore that it is determined by the state variables (temperature, soil moisture, and sensible heat flux into the atmosphere) and relatively insensitive to water vapor pressure deficit. The theory allows an independent estimate of the coefficient in the Priestley‐Taylor formula for potential evaporation, which is consistent with the widely accepted value of 1.26.
Soil moisture controls on canopy‐scale water and carbon fluxes in an African savanna
Authors: Williams Christopher A. , Albertson John D.
Published: September 2004

Tower‐based measurements of mass and energy exchanges at the end of the growing season in central Botswana were used to evaluate functional relationships commonly applied to predict water and carbon fluxes between savanna landscapes and the atmosphere. Following a large rainfall event, daily evapotranspiration (ETdaily) exhibited an exponential decay consistent with a derived analytical expression based on critical and wilting‐point soil moisture limits for savanna vegetation native to the study region. A piecewise linear soil moisture limitation function provided good estimates of ETdaily as a function of potential evapotranspiration and soil moisture (R2 = 0.92). Comparison of a soil moisture mass balance with measured ETdaily indicated deeper root water uptake at a site with more woody vegetation compared with a grass‐dominated site. Linear correlation (R2 = 0.90) of daytime CO2 flux and evapotranspiration supported a constant water use efficiency to estimate carbon fluxes from water fluxes. Daytime and nighttime CO2 fluxes responded similarly to soil drying, enabling estimation of total daily CO2 flux from ETdaily. These experimental results support a simple model of savanna land‐atmosphere exchange over interstorm periods.
Resonance spectrum and dissociation dynamics of ozone in the B23 electronically excited state: Experiment and theory
Authors: Deppe Sabine F. , Wachsmuth Uwe , Abel Bernd , Bittererová Martina , Grebenshchikov Sergy Yu. , Siebert Rüdiger , Schinke Reinhard
Published: September 2004

The rovibrational spectrum assigned to the low-lying B23 electronic state of ozone is measured with intracavity laser absorption spectroscopy. The experimental results are interpreted by means of quantum dynamical calculations on a global ab initio potential energy surface. The observed spectrum is shown to originate from the vibrational ground state in the local minimum of the B23 potential. The spectrum of short-lived resonance states in this local minimum is analyzed. Additionally, the global minimum of the surface is shown to lie in the dissociation channel in the van der Waals region. This region supports a short sequence of weakly bound vibrational states.
Dissipative curve crossing problem. I. High-barrier crossing
Authors: Rips Ilya
Published: September 2004

Variational approach based on the diabatic variational transition state theory optimization is extended and applied to the calculation of the radiationless transition rate for asymmetric high-barrier normal and inverted crossings. Scaling argument is employed to derive an analytic expression for the renormalized frequency as a function of the asymmetry of the crossing and of the dissipation strength for the particular case of Ohmic dissipation (Debye relaxation). The effect of the asymmetry of the crossing on the physical parameters and on the radiationless transition rate is explored. The effective adiabaticity parameter increases with the increasing asymmetry of the crossing in the normal region and decreases with the increasing asymmetry in the inverted region. The physical behavior of the radiationless transition rate in the normal and inverted regions is qualitatively different in the strong dissipation (Smoluchowski) limit. In the inverted region the rate exhibits stretched exponential decay as a function of the dissipation strength reflecting adiabatic suppression in the strong dissipation limit. The accuracy of the approach decreases with the increasing (decreasing) asymmetry of the crossing in the normal (inverted) region and breaks down in the activationless case.
Irreversible versus reversible aggregation: Mean field theory and experiments
Authors: Odriozola G. , Leone R. , Schmitt A. , Callejas-Fernández J. , Martı́nez-Garcı́a R. , Hidalgo-Álvarez R.
Published: September 2004

Colloidal aggregation processes arising at different electrolyte concentrations were studied by means of experiments and confronted with theoretical predictions of different kinetic aggregation models. For this purpose, aqueous dispersions of relatively large polystyrene microspheres were chosen as experimental systems. Aggregation was induced by adding KBr electrolyte to the initially stable particle dispersions. During the aggregation processes, the cluster-size distribution was monitored by means of single cluster light scattering. Analyzing the time evolution of the monomer concentration, we found that the processes arising even at moderate electrolyte concentrations cannot be described by pure time-independent irreversible aggregation models. Hence, alternative models such as time-dependent irreversible aggregation and several reversible aggregation models were also tested. The model that considers a time-dependent sticking probability was found to fit the data quite satisfactorily. Nevertheless, the fitted was so slow that it seems not very likely to find such a behavior in real systems. The aggregation-fragmentation models reported in the literature were unable to reproduce the experimental observations. Hence, a more realistic reversible aggregation model was developed. This model accounts also for reenforced or double bonds between the constituent particles. The corresponding fit improved significantly and reached the same quality as the time-dependent model. Moreover, the obtained fitting parameters were in qualitative agreement with the DLVO predictions and so, reversible aggregation seems to be a more reasonable explanation for the experimental data than time-dependent irreversible aggregation. However, no definite statement on the possible secondary bond fragmentation mechanism may be made since both the applied shear stress in the measuring cell and thermal fluctuations can cause weaker bonds to break.
Many-body interaction analysis: Algorithm development and application to large molecular clusters
Authors: Kulkarni Anant D. , Ganesh V. , Gadre Shridhar R.
Published: September 2004

A completely automated algorithm for performing many-body interaction energy analysis of clusters (MBAC) [M. J. Elrodt and R. J. Saykally, Chem. Rev. 94, 1975 (1994); S. S. Xantheas, J. Chem. Phys. 104, 8821 (1996)] at restricted Hartree-Fock (RHF)/MÃ Plesset 2nd order perturbation theory (MP2)/density functional theory (DFT) level of theory is reported. Use of superior guess density matrices (DM’s) for smaller fragments generated from DM of the parent system and elimination of energetically insignificant higher-body combinations, leads to a more efficient performance (speed-up up to 2) compared to the conventional procedure. MBAC approach has been tested out on several large-sized weakly bound molecular clusters such as (H2O)n, n=8, 12, 16, 20 and hydrated clusters of amides and aldehydes. The MBAC results indicate that the amides interact more strongly with water than aldehydes in these clusters. It also reconfirms minimization of the basis set superposition error for large cluster on using superior quality basis set. In case of larger weakly bound clusters, the contributions higher than four body are found to be repulsive in nature and smaller in magnitude. The reason for this may be attributed to the increased random orientations of the interacting molecules separated from each other by large distances.
Rovibrational and dynamical properties of the hydrogen bonded complex (CH2)2S-HF: A combined free jet, cell, and neon matrix-Fourier transform infrared study
Authors: Asselin P. , Goubet M. , Lewerenz M. , Soulard P. , Perchard J. P.
Published: September 2004

Fourier transform infrared spectra of the νs (HF stretching) band of the (CH2)2S-HF complex have been recorded at 0.1–0.5 cm−1 resolution in a cooled cell, in a supersonic jet expansion seeded with argon and in a neon matrix at 4.5 K. The combination of controlled temperature effects over a range of 40–250 K and a sophisticated band contour simulation program allows the separation of homogeneous and inhomogeneous contributions and reveals significant anharmonic couplings between intramolecular and intermolecular vibrational modes similar to our previous work on (CH2)2S-DF. The sign of the coupling constants is consistent with the expected strengthening of the hydrogen bond upon vibrational excitation of HF which also explains the observed small variations of the geometrical parameters in the excited state. The analysis of sum and difference combination bands involving νs provides accurate values of intermolecular harmonic frequencies and anharmonicities and a good estimate of the dissociation energy of the complex. Frequencies and coupling parameters derived from gas phase spectra compare well with results from neon matrix experiments. The effective linewidth provides a lower bound for the predissociation lifetime of 10 ps. The comparison between effective linewidths and vibrational densities of states for (CH2)2S-HF and -DF complexes highlights the important role of intramolecular vibrational redistribution in the vibrational dynamics of medium strength hydrogen bonds.
Comparison of lattice and continuum treatments of n-alkanes with experimental data
Authors: Porter J. A. , Lipson J. E. G.
Published: September 2004

Using a homologous series of n-alkanes as a model system, we compare the predictions of a lattice Born-Green-Yvon (BGY) theory and a continuum BGY theory with experimental results. We find that both theories are capable of describing the fluid properties and critical points of alkanes ranging from heptamers (n-C7) to nonadecamers (n-C19). We probe the connection between the lattice and continuum BGY models and extend our discussion to include a sampling of other lattice and continuum treatments.
Cycloaddition reactions of cyanogen (C2N2) on the Si(100)-2×1 surface
Authors: Lim Chultack , Choi Cheol Ho
Published: September 2004

Multireference as well as density functional theories in combination with the surface integrated molecular orbital molecular mechanics were adopted to study the surface reactions of cyanogens on Si(100)-2×1 surface. Three different products were identified as minima in the initial surface reaction. Among these, the [2+2] product is both kinetically easily accessible and thermodynamically the most stable. Therefore, it can be considered as the experimentally found strongly bound surface species. Unlike other conjugated systems, the [4+2] product is less stable than the [2+2] product. Subsequent surface isomerization studies revealed that kinetically favorable channels exist between the initially formed low-temperature species and the high-temperature species, indicating that surface morphology changes gradually as a function of surface temperature. Theses two channels eventually lead to the same final surface products, which is consistent with experiment. Current study shows that the subsequent surface isomerizations are the key reactions to better understand the complex surface structures and their properties.
The behavior of mixed-metal oxides: Physical and chemical properties of bulk Ce1−xTbxO2 and nanoparticles of Ce1−xTbxOy
Authors: Wang Xianqin , Hanson Jonathan C. , Liu Gang , Rodriguez José A. , Iglesias-Juez Ana , Fernández-Garcı́a Marcos
Published: September 2004

The physical and chemical properties of bulk Ce1−xTbxO2 and Ce1−xTbxOy nanoparticles (x⩽0.5) were investigated using synchrotron-based x-ray diffraction (XRD), x-ray adsorption near edge spectroscopy (XANES), Raman spectroscopy (RS), and first-principles density-functional (DF) calculations. DF results and Raman spectra point to a small tetragonal distortion after introducing terbium in ceria. The results of XRD show a small contraction (⩽ 0.08 Å) in the cell dimensions. The presence of Tb generates strain in the lattice through the variation of the ionic radii and creation of crystal imperfections and O vacancies. The strain increases with the content of Tb and affects the chemical reactivity of the Ce1−xTbxOy nanoparticles towards hydrogen, SO2, and NO2. DF calculations for bulk Ce1−xTbxO2 and Ce8−nTbnO16 (n=0, 1, 2, or 4) clusters show oxide systems that are not fully ionic. The theoretical results and XANES spectra indicate that neither a Ce↔Tb exchange nor the introduction of oxygen vacancies in Ce1−xTbxOy significantly affect the charge on the Ce cations. In contrast, the O K-edge and Tb LIII-edge XANES spectra for Ce1−xTbxOy nanoparticles show substantial changes with respect to the corresponding spectra of Ce and Tb single oxide references. The Ce0.5Tb0.5Oy compounds exhibit a much larger Tb3+/Tb4+ ratio than TbO1.7. A comparison with the properties of Ce1−xZrxOy and Ce1−xCaxOy shows important differences in the charge distribution, the magnitude of the dopant induced strain in the oxide lattice, and a superior behavior in the case of the Ce1−xTbxOy systems. The Tb-containing oxides combine stability at high temperature against phase segregation and a reasonable concentration of O vacancies, making them attractive for chemical and catalytic applications.
Spin relaxation in isolated molecules and clusters: The interpretation of Stern-Gerlach experiments
Authors: Knickelbein Mark B.
Published: September 2004

Intramolecular spin relaxation may occur in isolated molecules or clusters provided that the density of rovibrational eigenstates is sufficiently high to serve as an energy bath and angular momentum is conserved. In the coupled, zero-field limit, total angular momentum (J) is the sum of spin (S) and rotational (N) momenta such that J and MJ are good angular momentum quantum numbers. In the coupled limit, transitions between Zeeman levels (ΔMJ≠0) cannot occur in the absence of an external torque. However, in the high-field limit, J and MJ are no longer good quantum numbers, as N and S are decoupled and only their projections on the z axis defined by the external field are invariant. In this case MN and MS remain as good quantum numbers so that angular momentum conserving transitions can occur subject to the selection rule ΔMN=−ΔMS. Determination of the magnetic moments of isolated molecules and clusters via a thermodynamics-based analysis requires that their magnetizations are measured at sufficiently large fields that spin-rotation effects become negligible and the Zeeman level structure approaches the free-spin case.
Positronium lifetime in polymers
Authors: Camacho Abel
Published: September 2004

A model describing the relationship between the orthopositronium lifetime and the volume of a void, located in a synthetic zeolite, is analyzed. Our idea, which allows us to take into account the effects of temperature, comprises the introduction of a non-Hermitian term in the Hamiltonian, which accounts for the annihilation of the orthopositronium. The predictions of the present model are also confronted against an already known experimental result.
A novel method for calculating rate constants of diffusion-influenced reactions
Authors: Ivanov Konstantin L. , Lukzen Nikita N.
Published: September 2004

In the present work we suggest a method for calculating rate constants of chemical processes affected by mobility of reactants. The method is based on the encounter theory. Unlike the widely accepted model of collision complexes it provides a general formal solution for practically arbitrary reaction scheme.
Thermodynamics of attractive hard rods: A test of mean field density functional theory
Authors: Khakshouri Sascha , Ford Ian J.
Published: September 2004

Mean field density functional theory (MFDFT) has been employed to calculate the free energy of a pair of attractive hard rods on a ring. The results for homogeneous and optimal inhomogeneous density profiles have been compared with the exact free energy as a test of the approach. We discuss the problems in applying MFDFT to small systems and suggest modifications which allow a reasonably accurate treatment of this particular, rather extreme, case.
Charge renormalization of charged spheres based on thermodynamic properties
Authors: Hsin Wei Lun , Wang Tzu-Yu , Sheng Yu-Jane , Tsao Heng-Kwong
Published: September 2004

At strong electrostatic coupling, counterions are accumulated in the vicinity of the surface of the charged particle with intrinsic charge Z. In order to explain the behavior of highly charged particles, effective charge Z* is therefore invoked in the models based on Debye-Hückel approximation, such as the Derjaguin-Landau-Verwey-Overbeek potential. For a salt-free colloidal suspension, we perform Monte Carlo simulations to obtain various thermodynamic properties ω in a spherical Wigner-Seitz cell. The effect of dielectric discontinuity is examined. We show that at the same particle volume fraction, counterions around a highly charged sphere with Z may display the same value of ω as those around a weakly charged sphere with Z*, i.e., ω(Z)=ω(Z*). There exists a maximally attainable value of ω at which Z=Z*. Defining Z* as the effective charge, we find that the effective charge passes through a maximum and declines again due to ion-ion correlation as the number of counterions is increased. The effective charge is even smaller if one adopts the Debye-Hückel expression ωDH. Our results suggest that charge renormalization can be performed by chemical potential, which may be observed in osmotic pressure measurements.
Rovibrational distributions of HF in the photodissociation of vinyl fluoride at 193 nm: A direct MP2 quasiclassical trajectory study
Authors: Martı́nez-Núñez Emilio , Vázquez Saulo
Published: September 2004

Quasiclassical trajectory calculations were performed to calculate rovibrational distributions of the nascent HF fragment in the photodissociation of vinyl fluoride at 193 nm. The trajectories were initiated at the transition states of the four-center (4C) and three-center (3C) HF elimination channels, using a microcanonical, quasiclassical normal-mode sampling. In general, the calculated distributions are in reasonably good agreement with experiment. In particular, the trajectory distributions show bimodal character, although not as pronounced as that observed experimentally. The calculations predict that the 3C and 4C distributions are rather similar to each other, which suggests that the low-J and high-J components of the rotational distributions cannot be specifically assigned to each of these channels.
Electronic excitation and charge transfer processes in collisions between Mg(3 1S) atoms and Rb+(1S) ions in the 0.07–4.00 keV energy range
Authors: Sabidó M. , de Andrés J. , Sogas J. , Lucas J. M. , Albertı́ M. , Bofill J. M. , Aguilar A.
Published: September 2004

Inelastic collision processes between neutral Mg atoms and Rb+ ions, both in their ground states, have been studied by means of a crossed molecular beam technique measuring the decay fluorescence of the excited species formed. Emissions corresponding to Mg(3 1P1), Mg(3 3D3,2,1), and Mg(4 3S1), formed by direct target excitation, Rb(5 2P3/2,1/2), Rb(6 2P3/2,1/2) produced by electron capture and also the phosphorescent emission due to decay of Mg(3 3P1), have been detected and the corresponding absolute cross-section values measured both as total values and resolved into their J states. No polarization measurements could be made. Ab initio calculations using pseudopotentials have been performed and from these a manifold of adiabatic energy curves correlating with the different entry and exit channels have been obtained, allowing to propose a qualitative interpretation of the results, such as the shape of the cross section vs energy for different transitions and the oscillating nature of the branching ratios due to interference effects.
Inelastic hard rods in a periodic potential
Authors: Cecconi Fabio , Marconi Umberto Marini Bettolo , Diotallevi Fabiana , Puglisi Andrea
Published: September 2004

A simple model of inelastic hard rods subject to a one-dimensional array of identical wells is introduced. The energy loss due to inelastic collisions is balanced by the work supplied by an external stochastic heat bath. We explore the effect of the spatial nonuniformity on the steady states of the system. The spatial variations of the density, granular temperature, and pressure induced by the gradient of the external potential are investigated and compared with the analogous variations in an elastic system. Finally, we study the clustering process by considering the relaxation of the system starting from a uniform homogeneous state.
The integral encounter theory of multistage reactions containing association-dissociation reaction stages. III. Taking account of quantum states of reactants
Authors: Ivanov Konstantin L. , Lukzen Nikita N. , Doktorov Alexander B.
Published: September 2004

The formalism developed in Part I [K. L. Ivanov, N. N. Lukzen, A. A. Kipriyanov, and A. B. Doktorov, Phys. Chem. Chem. Phys. 6, 1706 (2004)] of the present contribution is extended to treat the reacting particles with internal quantum states. Initial spatial correlations of reactants are considered in the framework of this formalism as well.