Computational Studies of Protonated β-d-Galactose and Its Hydrated Complex: Structures, Interactions, Proton Transfer Dynamics, and Spectroscopy

We present an exploration of proton transfer dynamics in a monosaccharide, based upon ab initio molecular dynamic (AIMD) simulations, conducted “on-the-fly”, in β-d-galactose-H⁺ (βGal-H⁺) and its singly hydrated complex, βGal-H⁺-H₂O. Prior structural calculations identify O6 as the preferred protonation site for O-methyl α-d-galactopyranoside, but the β-anomeric configuration favors the inversion of the pyranose ring from the ⁴C₁ chair configuration, to ¹C₄, and the formation of proton bridges to the (axial) O1 and O3 sites. In the hydrated complex, however, the proton bonds to the water molecule inserted between the O6 and Ow sites, and the ring retains its original ⁴C₁ conformation, supported by a circular network of co-operatively linked hydrogen bonds. Two distinct proton transfer processes, operating over a time scale of 10 ps, have been identified in βGal-H⁺ at 500 K. One of them leads to chemical reaction and the formation of an oxacarbenium ion (accompanied by the loss of an H₂O molecule). In the hydrated complex, βGal-H⁺-H₂O, this reaction is suppressed, and the proton transfer, which involves multiple jumps between the sugar and the H₂O, creates an H₃O⁺ ion, relevant, perhaps, to the reactivity of protonated sugars both in the gas and condensed phases. Anticipating future spectroscopic investigations, the vibrational spectra of βGal-H⁺ and βGal-H⁺-H₂O have also been computed through AIMD simulations conducted at average temperatures of 300 and 40 K and also through vibrational self-consistent field (VSCF) calculations at 0 K

Unexpected Cation Dynamics in the Low-Temperature Phase of Methylammonium Lead Iodide: The Need for Improved Models

High-resolution inelastic neutron scattering and extensive first-principles calculations have been used to explore the low-temperature phase of the hybrid solar-cell material methylammonium lead iodide up to the well-known phase transition to the tetragonal phase at ca. 160 K. Contrary to original expectation, we find that the <i>Pnma</i> structure for this phase can only provide a qualitative description of the geometry and underlying motions of the organic cation. A substantial lowering of the local symmetry inside the perovskite cage leads to an improved atomistic model that can account for all available spectroscopic and thermodynamic data, both at low temperatures and in the vicinity of the aforementioned phase transition. Further and detailed analysis of the first-principles calculations reveals that large-amplitude distortions of the inorganic framework are driven by both zero-point-energy fluctuations and thermally activated cation motions. These effects are significant down to liquid-helium temperatures. For this important class of technological materials, this work brings to the fore the pressing need to bridge the gap between the long-range order seen by crystallographic methods and the local environment around the organic cation probed by neutron spectroscopy

Dynamics and Structure of Poly(ethylene oxide) Intercalated in the Nanopores of Resorcinol–Formaldehyde Resin Nanoparticles

The incorporation of high-molecular-weight poly­(ethylene oxide) (PEO) in the nanopores of resorcinol–formaldehyde resin nanoparticles (RNPs) leads to the suppression of polymer crystallization, changes in the chain conformation, and a noticeable slowdown of the two dielectric relaxations that reflect the segmental and local PEO dynamics. Both relaxations are significantly slower than those corresponding to bulk PEO. These results are independent of the pore characteristics of the different RNP materials. The segmental relaxation shows a crossover at ca. 220 K in its temperature dependence from non-Arrhenius to Arrhenius-like behavior on cooling. These results suggest the occurrence of limited cooperativity at low temperatures due to the enhancement of long-living hydrogen bonding between PEO and RNP pore walls

Inside PEF: Chain Conformation and Dynamics in Crystalline and Amorphous Domains

A thorough vibrational spectroscopy and molecular modeling study on poly­(ethylene 2,5-furandicarboxylate) (PEF) explores its conformational preferences, in the amorphous and crystalline regions, while clarifying structure–property correlations. Despite the increasing relevance of PEF as a sustainable polymer, some of its unique characteristics are not yet fully understood and benefit from a deeper comprehension of its microstructure and intermolecular bonding. Results show that in the amorphous domains, where intermolecular interactions are weak, PEF chains favor a helical conformation. Prior to crystallization, polymeric chains undergo internal rotations extending their shape in a zigzag patternan energetically unfavorable geometry which is stabilized by C–H···O bonds among adjacent chain segments. The zigzag conformation is the crystalline motif present in the α and β PEF polymorphs. The energy difference among the amorphous and crystalline chains of PEF is higher than in PET poly­(ethylene terephthalate) and contributes to PEF’s higher crystallization temperature. The 3D arrangement of PEF chains was probed using inelastic neutron scattering (INS) spectroscopy and periodic DFT calculations. Comparing the INS spectra of PEF with that of poly­(ethylene terephthalate) (PET) revealed structure–property correlations. Several low-frequency vibrational modes support the current view that PEF chains are less flexible than those of PET, posing greater resistance to gas penetration and resulting in enhanced barrier properties. The vibrational assignment of PEF’s INS spectrum is a useful guide for future studies on advanced materials based on PEF

Inside PEF: Chain Conformation and Dynamics in Crystalline and Amorphous Domains

A thorough vibrational spectroscopy and molecular modeling study on poly­(ethylene 2,5-furandicarboxylate) (PEF) explores its conformational preferences, in the amorphous and crystalline regions, while clarifying structure–property correlations. Despite the increasing relevance of PEF as a sustainable polymer, some of its unique characteristics are not yet fully understood and benefit from a deeper comprehension of its microstructure and intermolecular bonding. Results show that in the amorphous domains, where intermolecular interactions are weak, PEF chains favor a helical conformation. Prior to crystallization, polymeric chains undergo internal rotations extending their shape in a zigzag patternan energetically unfavorable geometry which is stabilized by C–H···O bonds among adjacent chain segments. The zigzag conformation is the crystalline motif present in the α and β PEF polymorphs. The energy difference among the amorphous and crystalline chains of PEF is higher than in PET poly­(ethylene terephthalate) and contributes to PEF’s higher crystallization temperature. The 3D arrangement of PEF chains was probed using inelastic neutron scattering (INS) spectroscopy and periodic DFT calculations. Comparing the INS spectra of PEF with that of poly­(ethylene terephthalate) (PET) revealed structure–property correlations. Several low-frequency vibrational modes support the current view that PEF chains are less flexible than those of PET, posing greater resistance to gas penetration and resulting in enhanced barrier properties. The vibrational assignment of PEF’s INS spectrum is a useful guide for future studies on advanced materials based on PEF

Reactivity of Hydrogen on and in Nanostructured Molybdenum Nitride: Crotonaldehyde Hydrogenation

Early-transition-metal nitrides, including γ-Mo₂N, are active and selective for a variety of reactions, including the hydrogenation of organics (e.g., hydrodeoxygenation), CO (e.g., Fischer–Tropsch synthesis), and CO₂. In addition to adsorbing hydrogen onto the surface, some of these materials can incorporate hydrogen into subsurface, interstitial sites. Research described in this paper examined, experimentally and computationally, the nature of hydrogen on and in γ-Mo₂N, with a particular focus on characterizing the interactions of these hydrogens with crotonaldehyde. Hydrogen was added to γ-Mo₂N via exposure to gaseous hydrogen at elevated temperatures, forming γ-Mo₂N‑H_<i>x</i>, where 0.061< <i>x</i> < 0.082. Temperature-programmed desorption (TPD) experiments indicate that γ-Mo₂N‑H_<i>x</i> has at least two distinct hydrogen binding sites and that these sites can be selectively populated. Inelastic neutron scattering and density functional theory calculations indicate the presence of surface nitrogen-bound (κ¹-NH_surf), surface Mo-bound (κ¹-MoH_surf), and interstitial Mo-bound (μ₆-Mo₆H_sub) hydrogens. Selectivities for the hydrogenation of crotonaldehyde, a model of species in biomass-derived liquids, correlated with the populations at these sites. Importantly, materials with high densities of interstitial, hydridic hydrogen were selective for CO hydrogenation (i.e., formation of crotyl alcohol). Collectively the results provide mechanistic insights regarding the desorption and reactivity of hydrogen on and in γ-Mo₂N. Hydrogen adsorption/desorption to γ-Mo₂N is heterolytic; in particular, H₂ adds across a Mo–N bond. Because the surface Mo–H site is energetically unfavorable in comparison to the interstitial site, hydrogen migrates into interstitial sites once the surface NH sites are saturated. Crotonaldehyde adsorption facilitates migration of this interstitial hydrogen back to the surface, forming surface Mo–H that is selective for hydrogenation of the CO bond. These insights will facilitate the design of γ-Mo₂N and other early-transition-metal nitrides for catalytic applications