Multiple-time scale accelerated molecular dynamics: addressing the small-barrier problem

We present a method for accelerated molecular-dynamics simulation in systems with rare-event dynamics that span a wide range of time scales. Using a variant of hyperdynamics, we detect, on the fly, groups of recurrent states connected by small energy barriers and we modify the potential-energy surface locally to consolidate them into large, coarse states. In this way, fast motion between recurrent states is treated within an equilibrium formalism and dynamics can be simulated over the longer time scale of the slow events. We apply the method to simulate cluster diffusion and the initial growth of Co on Cu(001),where time scales spanning more than 6 orders of magnitude are present, and show that the method correctly follows the slow events, so that much larger times can be simulated than with accelerated molecular dynamics alone.     

Thermal desorption of large molecules from solid surfaces

We use molecular-dynamics simulations and importance sampling to obtain transition-state-theory rate constants for thermal desorption of an n-alkane series from Au(111). We find that the binding of a large molecule to a solid surface involves different types of local minima. The preexponential factors increase with increasing chain length and can be substantially larger than typical estimates for small molecules. Our results match recent experimental studies and indicate that a proper treatment of conformational isomerism and entropy, heretofore not found in coarse-grained models, is essential to quantitatively describe the thermal desorption of large molecules from solid surfaces.     

Enhancing Teaching Assistants' (TAs') Inquiry Teaching by Means of Teaching Observations and Reflective Discourse

Recent education reform efforts advocate teaching the process of science (inquiry) in undergraduate lecture and laboratory classes. To meet this challenge, professional development for the graduate student instructors (teaching assistants, or TAs) often assigned to teach these classes is needed. This study explored the implementation of an observation protocol designed to support peer observation and reflection among TAs teaching inquiry‐based undergraduate biology laboratories. The researchers of this study and TA‐peers who are experienced in teaching inquiry used the protocol to observe novice TAs at the beginning, mid‐point, and end of a semester‐long teaching assignment. Novice TAs used a modified version of this protocol to observe an experienced and a novice TA. Analysis of post‐semester interview data indicated engaging in both sets of observations and post‐observation discussions facilitated by the protocol gave novice TAs new ways to teach content, guidance on implementing pedagogical theory, and means to improve communications with students and classroom management skills. Researchers also used quantitative data collected from the observations to document frequency of teaching and student behaviors associated with teaching inquiry and how these frequencies changed during the semester. Considerations for how to use both sets of data to inform changes in TA professional development are discussed.     

Are You Convinced? Middle‐Grade Students' Evaluations of Mathematical Arguments

Students learn norms of proving by observing teachers generating proofs, engaging in proving, and generalizing features of proofs deemed convincing by an authority, such as a textbook. Students at all grade levels have difficulties generating valid proof; however, little research exists on students' understandings about what makes a mathematical argument convincing prior to more formal instruction in methods of proof. This study investigated middle‐school students' (ages 12–14) evaluations of arguments for a statement in number theory. Students evaluated both an empirical and a general argument in an interview setting. The results show that students tend to prefer empirical arguments because examples enhance an argument's power to show that the statement is true. However, interview responses also reveal that a significant number of students find arguments to be most convincing when examples are supported with an explanation that “tells why” the statement is true. The analysis also examined the alignment of students' reasons for choosing arguments as more convincing along with the strategies they employ to make arguments more convincing. Overall, the findings show middle‐school students' conceptions about what makes arguments convincing are more sophisticated than their performance in generating arguments suggests.     

Photoelectron spectroscopy of HC4N-

We report the 364-nm photoelectron spectrum of HC(4)N(-). We observe electron photodetachment from the bent X(2)A" state of HC(4)N(-) to both the near-linear X(3)A" and the bent ? (1)A' states of neutral HC(4)N. We observe an extended, unresolved vibrational progression corresponding to X(3)A" ← X(2)A" photodetachment, and we measure the electron affinity (EA) of the X(3)A" state of HC(4)N to be 2.05(8) eV. Photodetachment to the bent ? (1)A' state results in a single intense origin peak at a binding energy of 2.809(4) eV, from which we determine the singlet-triplet splitting (ΔE(ST)) of HC(4)N: 0.76(8) eV. For comparison and to aid in the interpretation of the HC(4)N(-) spectrum, we also report the 364-nm photoelectron spectra of HCCN(-) and DCCN(-). Improved signal-to-noise over the previous HCCN(-) and DCCN(-) photoelectron spectra allows for a more precise determination of the EAs and ΔE(ST)s of HCCN and DCCN. The EAs of HCCN and DCCN are measured to be 2.001(15) eV and 1.998(15) eV, respectively; ΔE(ST)(HCCN) is 0.510(15) eV and ΔE(ST)(DCCN) is 0.508(15) eV. These results are discussed in the context of other organic carbene chains.     

Multiple state transition interface sampling of alanine dipeptide in explicit solvent

We have applied the recently developed multiple state transition interface sampling approach to alanine dipeptide in explicit water. We extract the rate constant matrix for configurational changes between each pair of metastable states. The results are comparable with values from previous literature and show that the method is applicable to biomolecular systems.     

Synthesis and Characterization of Fumarate Copolyesters for Use in Bioresorbable Bone Cement Compositions

Unsaturated amorphous copolyesters of varied composition were prepared by transesterification copolymerization of diethyl fumarate, and two diols, 1,2‐propanediol and 2‐methyl‐1,3‐propanediol. The copolyesters were characterized by IR, ¹H‐ and ¹³C‐NMR, GPC, DSC, and TGA. The glass transition is changing with composition from 0°C to 19°C as the content of 1,2‐propanediol residue in the copolyester increases. The copolyester structure and composition have an impact on the compressive strength and hydrolytic stability of the composites prepared by crosslinking the fumarate double bonds with N‐vinyl pyrrolidone in the presence of inorganic filler, calcium sulfate dihydrate, with the addition of a radical initiator, benzoyl peroxide, at ambient temperatures.     

Oxidative modification of native protein residues using cerium(IV) ammonium nitrate

A new protein modification strategy has been developed that is based on an oxidative coupling reaction that targets electron-rich amino acids. This strategy relies on cerium(IV) ammonium nitrate (CAN) as an oxidation reagent and results in the coupling of tyrosine and tryptophan residues to phenylene diamine and anisidine derivatives. The methodology was first identified and characterized on peptides and small molecules, and was subsequently adapted for protein modification by determining appropriate buffer conditions. Using the optimized procedure, native and introduced solvent-accessible residues on proteins were selectively modified with polyethylene glycol (PEG) and small peptides. This unprecedented bioconjugation strategy targets these under-utilized amino acids with excellent chemoselectivity and affords good-to-high yields using low concentrations of the oxidant and coupling partners, short reaction times, and mild conditions.     

Chirality and Polarity in the f‐Block Borates M4[B16O26(OH)4(H2O)3Cl4] (M=Sm,Eu, Gd,Pu, Am,Cm, and Cf)

The reactions of trivalent lanthanides and actinides with molten boric acid in high chloride concentrations result in the formation of M₄[B₁₆O₂₆(OH)₄(H₂O)₃Cl₄] (M=Sm, Eu, Gd, Pu, Am, Cm, Cf). This cubic structure type is remarkably complex and displays both chirality and polarity. The polymeric borate network forms helical features that are linked via two different types of nine‐coordinate f‐element environments. The f–f transitions are unusually intense and result in dark coloration of these compounds with actinides.     

In‐house and synchrotron X‐ray diffraction studies of 2‐phenyl‐1,10‐phenanthroline,protonated salts,complexes with gold(III) and copper(II), and an orthometallation product with palladium(II)

Different salts of the 2‐phenyl‐1,10‐phenanthrolin‐1‐ium cation, (pnpH)⁺, are obtained by reacting 2‐phenyl‐1,10‐phenanthroline (pnp), C₁₈H₁₂N₂, (I), with a variety of anions, such as hexafluoridophosphate, C₁₈H₁₃N₂⁺·PF₆⁻, (II), trifluoromethanesulfonate, C₁₈H₁₃N₂⁺·CF₃SO₃⁻, (III), tetrachloridoaurate, (C₁₈H₁₃N₂)[AuCl₄], (IV), and bromide (as the dihydrate), C₁₈H₁₃N₂⁺·Br⁻·2H₂O, (V). Compound (I) crystallizes with Z′ = 2, with both independent molecules adopting a coplanar conformation. In (II)–(IV), a hydrogen bond exists between the cation and anion, while one of the lattice water molecules serves as a hydrogen‐bonded bridge between the cation and anion in (V). Reaction of (I) with HAuCl₄ gives the salt complex (IV); however, reaction with KAuCl₄ produces the monodentate complex trichlorido(2‐phenyl‐1,10‐phenanthroline‐κN¹⁰)gold(III), [AuCl₃(C₁₈H₁₂N₂)], (VI). Dichlorido(2‐phenyl‐1,10‐phenanthroline‐κ²N,N′)copper(II), [CuCl₂(C₁₈H₁₂N₂)], (VII), results from the reaction of CuCl₂·2H₂O and (I), in which the Cu^II center adopts a tetrahedrally distorted square‐planar geometry. The pendent phenyl ring twists to a bisecting position relative to the phenanthroline plane. The square‐planar Pd^II complex, bromido[2‐(phenanthrolin‐2‐yl)phenyl‐κ³C¹,N,N′]palladium(II), [PdBr(C₁₈H₁₁N₂)], (VIII), is obtained from the reaction of (I) with [PdCl₂(cycloocta‐1,5‐diene)], followed by addition of bromine. A coplanar geometry for the pendent ring is adopted as a result of the tridentate bonding motif.