RESUMEN
It has been recently shown that in the presence of an applied voltage, hydrogen and deuterium nuclei can be separated from one another using graphene membranes as a nuclear sieve, resulting in a 10-fold enhancement in the concentration of the lighter isotope. While previous studies, both experimental and theoretical, have attributed this effect mostly to differences in vibrational zero point energy (ZPE) of the various isotopes near the membrane surface, we propose that multi-dimensional quantum mechanical tunneling of nuclei through the graphene membrane influences this proton permeation process in a fundamental way. We perform ring polymer molecular dynamics calculations in which we include both ZPE and tunneling effects of various hydrogen isotopes as they permeate the graphene membrane and compute rate constants across a range of temperatures near 300 K. While capturing the experimentally observed separation factor, our calculations indicate that the transverse motion of the various isotopes across the surface of the graphene membrane is an essential part of this sieving mechanism. An understanding of the multi-dimensional quantum mechanical nature of this process could serve to guide the design of other such isotopic enrichment processes for a variety of atomic and molecular species of interest.
RESUMEN
Biological systems have been shown to shuttle excess protons long distances by taking advantage of tightly organized hydrogen-bonded water bridges in hydrophobic protein cavities, and similar effects have been observed in carbon nanotubes. In this theoretical study we investigate how quantum effects of proton motion impact the rate constants for charge transfer in a model system consisting of a donor and acceptor molecule separated by a single-molecule water bridge. We calculate quantum and classical rate constants for the transfer of an excess proton over two possible paths, one with an H3O+ intermediate, and one with an OH- intermediate. Quantum effects are included through ring polymer molecular dynamics (RPMD) calculations. We observe a 4-fold enhancement of reaction rate constants due to proton tunneling at temperatures between 280 and 320 K, as shown by transmission coefficient calculations. Deuteration of the donor and acceptor proton are shown to decrease the reaction rate constant by a factor of 50, and this is another indicator that tunneling plays an important role in this proton transfer mechanism.
RESUMEN
BACKGROUND: Derangements of the plantar plate and joint capsule are an underrecognized cause of lesser metatarsalgia. Fluoroscopic arthrography and magnetic resonance (MR) arthrography are both used for diagnosis. Currently there are no studies comparing the effectiveness of these two modalities. METHODS: Patients suspected of having plantar plate or capsular tears underwent both fluoroscopic arthrography and MR arthrography; the imaging findings were then compared and correlated with intraoperative findings, when available, to evaluate the effectiveness of the different imaging modalities. Forty consecutive patients underwent both fluoroscopic and MR arthrography. RESULTS: Thirty-two of 40 patients (80%) were found to have tears of the plantar plate, joint capsule, or both. MR arthrography identified all 32 tears. Four cases in the first 29 patients, 13.8%, demonstrated discrepancy where a tear was identified only on the MR arthrogram. A midpoint review of the data was performed. Of the 4 missed tears they were all noted to be plantar lateral in location. Four other patients in this group had plantar lateral tears that were not missed. These patients had an additional steep lateral oblique image on fluoroscopic arthrography, which showed the plantar lateral tear. Therefore an additional steep lateral oblique image was performed routinely capturing these small tears in the last 11 patients. CONCLUSION: MR arthrography was more accurate in identifying tears of the plantar plate and capsule than fluoroscopic arthrography. Fluoroscopic arthrography with additional views, like a steep lateral oblique view, was found to be as reliable, and more cost-effective, than MR arthrography. LEVEL OF EVIDENCE: Level II, prospective comparative study.