Intermolecular Transfer Processes Research Articles

Collective Proton Transfers in Cyclic Water–Ammonia Tetramers: A Path Integral Machine-Learning Study

We present results from machine-learning-based path integral molecular dynamics simulations that describe isomerization paths articulated via collective proton transfers along mixed, cyclic tetramers combining water and ammonia at cryogenic conditions. The net result of such isomerizations is a reverse of the chirality of the global hydrogen-bonding architecture along the different cyclic moieties. In monocomponent tetramers, the classical free energy profiles associated with these isomerizations present the usual symmetric double-well characteristics whereas the reactive paths exhibit full concertedness among the different intermolecular transfer processes. Contrastingly, in mixed water/ammonia tetramers, the incorporation of a second component introduces imbalances in the strengths of the different hydrogen bonds leading to a partial loss of concertedness, most notably at the vicinity of the transition state. As such, the highest and lowest degrees of progression are registered along OH···N and O···HN coordinations, respectively. These characteristics lead to polarized transition state scenarios akin to solvent-separated ion-pair configurations. The explicit incorporation of nuclear quantum effects promotes drastic depletions in the activation free energies and modifications in the overall shape of the profiles which include central plateau-like stages, indicating the prevalence of deep tunneling regimes. On the other hand, the quantum treatment of the nuclei partially restores the degree of concertedness among the evolutions of the individual transfers.

The effect of hydrogen bonds on the ultrafast relaxation dynamics of a BODIPY dimer.

The influence of hydrogen bonds (H-bonds) in the structure, dynamics, and functionality of biological and artificial complex systems is the subject of intense investigation. In this broad context, particular attention has recently been focused on the ultrafast H-bond dependent dynamical properties in the electronic excited state because of their potentially dramatic consequences on the mechanism, dynamics, and efficiency of photochemical reactions and photophysical processes of crucial importance for life and technology. Excited-state H-bond dynamics generally occur on ultrafast time scales of hundreds of femtoseconds or less, making the characterization of associated mechanisms particularly challenging with conventional time-resolved techniques. Here, 2D electronic spectroscopy is exploited to shed light on this still largely unexplored dynamic mechanism. An H-bonded molecular dimer prepared by self-assembly of two boron-dipyrromethene dyes has been specifically designed and synthesized for this aim. The obtained results confirm that upon formation of H-bonds and the dimer, a new ultrafast relaxation channel is activated in the ultrafast dynamics, mediated by the vibrational motions of the hydrogen donor and acceptor groups. This relaxation channel also involves, beyond intra-molecular relaxations, an inter-molecular transfer process. This is particularly significant considering the long distance between the centers of mass of the two molecules. These findings suggest that the design of H-bonded structures is a particularly powerful tool to drive the ultrafast dynamics in complex materials.

A theoretical study of 3,5-diazido-1,2,4-triazole: the role of the hydrogen bonding interaction in stabilizing the molecular system

3,5-Diazido-1, 2, 4-triazole (DATZ) is a compound that has a good thermal stability and can be used to produce high energetic ionic salts. The conformations of DATZ were searched by the molecular dynamics simulations and optimized by the molecular mechanics and dispersion-corrected density functional theory methods. The dimer and trimer of DATZ were constructed from the most stable monomer. The hydrogen bonding interactions, which were found to be critically important in increasing the stability of the dimer and trimer, were investigated with the help of the natural bond orbital and the quantum theory of atoms in molecules analyses. The changes in thermodynamic functions, stabilization interaction energies, and hydrogen-bonding energies show that the trimer is most likely the existing form of DATZ. The intramolecular, intermolecular, and water catalytic proton transfer processes were simulated to investigate the proton transfer mechanism. The intermolecular transfer process requires the lowest activation energy (42.56 kJ mol−1) and is the most likely process of proton transfer. DATZ is not only a proton acceptor but also a proton donor. Its weak acidity was quantified as pKa = 10.16. The solvation energy estimated using the conductor-like polarizable continuum model in water is the largest (−99.96 kJ mol−1), revealing that DATZ is more stable in water than in another seven solvents.

The thermal stability and thermal degradation of blends of syndiotactic polystyrene and polyphenylene ether

Blends of syndiotactic polystyrene (sPS) and polyphenylene ether (PPE) have been prepared by co-precipitation from solution, followed by compression moulding. Three different molecular weight polystyrenes were used. Differential scanning calorimetry shows that partial crystallization of the sPS occurs, but a single glass transition indicates that the blend components are miscible in the amorphous phase. Thermogravimetry has been used to compare the thermal stability of the blends and the pure polymers, and IR spectroscopy used to give information on the structural changes consequent upon heating. The three different molecular weight polystyrenes degraded in an identical manner, suggesting that initiation is by random chain scission. The onset of sPS degradation in the blend occurs at higher temperatures than in the pure polymer indicating stabilisaton of that component, whilst degradation of PPE in the blends is accelerated relative to the pure polymer. IR spectroscopy shows that PPE undergoes structural rearrangement in which the ether link is broken and chain continuity is re-established through the methyl group before any mass loss is apparent. The sPS degradation temperatures are coincident with the temperature of this rearrangement and it is proposed that the PPE acts as a radical acceptor, interrupting the intermolecular transfer process in the degrading polystyrene. This cross-termination process limits the PPE rearrangement and consequently reduces the ultimate stability of that polymer.

Synthesis of alpha- and beta-glycosyl donors with a disaccharide beta-D-Gal-(1-->3)-D-GalNAc backbone

The synthesis of thioglycosyl donors with a disaccharide beta-D-Gal-(1-->3)-D-GalNAc backbone was studied using the glycosylation of a series of suitably protected 3-monohydroxy- and 3,4-dihydroxyderivatives of phenyl 2-azido-2-deoxy-1-thio-alpha- and 1-thio-beta-D-galactopyranosides by galactosyl bromide, fluoride, and trichloroacetimidate. In the reaction with the monohydroxylated glycosyl acceptor, the process of intermolecular transfer of thiophenyl group from the glycosyl acceptor onto the cation formed from the molecule of glycosyl donor dominated. When glycosylating 3,4-diol under the same conditions, the product of the thiophenyl group transfer dominated or the undesired (1-->4), rather than (1-->3)-linked, disaccharide product formed. The aglycone transfer was excluded when 4-nitrophenylthio group was substituted for phenylthio group in the galactosyl acceptor molecule. This led to the target disaccharide, 4-nitrophenyl 2-azido-4,5-O-benzylidene-2-deoxy-3-O-(2,3,4,6-tetra-O-acetyl-beta-D- galactopyranosyl)-1-thio-beta-D-galactopyranoside, in 57% yield. This disaccharide product bears nonparticipating azide group in position 2 of galactosamine and can hence be used to form alpha-glycoside bond. 2-Azide group and the aglycone nitro group were simultaneously reduced in this product and then trichloroacetylated, which led to the beta-glycosyl donor, 4-trichloroacetamidophenyl 4,6-O-diacetyl-2-deoxy-3-O-(2,3,4,6-tetra- O-acetyl-beta-D-galactopyranosyl)-1-thio-2-trichloroacetamido-beta-D- galactopyranoside, in 62% yield. The resulting glycosyl donor was used in the synthesis of tetrasaccharide asialo-GM1.

Thermal Degradation of Polystyrene

AbstractThe thermal degradation of polystyrene in the absence of oxygen is examined, at temperatures above or below 300°C. The decrease in degree of polymerisation occurring either with negligible volatilisation at lower temperatures, or with evolution of low molecular weight products at higher temperatures, is simulated by Monte‐Carlo procedures. The results suggest that below 300°C, scission of the polymer chains is random, and involves also a minor fraction of some abnormal, as yet unidentified, structures. Above 300°C, an extensive intermolecular transfer process seems to be the main cause of the decrease in degree of polymerisation, whereas an overall depolymerisation of 100 structural units at each chain scission is the source of volatilisation.

Investigation of intra- and intermolecular transfer processes by picosecond fluorescence gating

Fluorescence up-conversion is a promising technique for the investigation of ultrafast processes within and between poly-atomic molecules. Conditions are discussed for up converting broad-band fluorescence efficiently with a time resolution better than 1 ps. Experimental results are presented on vibrational relaxation in the first excited singlet state, internal conversion, and intermolecular energy transfer in liquid solutions.

Intermolecular energy transfer between tris(acetylacetonato)-lanthanoid complexes in n-butanol solution

Intermolecular energy transfer occurs between Tb(aa)33H2O and Ln(aa)33H2O complexes (Ln = Pr, Nd, Sm, Eu, Dy, Ho or Er; aa = acetylacetonate) in n-butanol solution at 293 K. Measurement of the decay time of the Tb3 + 5D4 level indicates that transfer occurs from this level to excited levels of the Ln3+ ions with bimolecular rate constants within the range 0.5–4.9 × 105 dm3 mol s–1. Data from similar measurements on a mixed crystal EuxTb(1–x)(aa)33H2O and other considerations indicate that this is a very short range electron-exchange transfer. Similar measurements of the Tb3+ ion phosphorescence yield indicate the presence of a further intermolecular transfer process between a higher excited state of the Tb3+ complex and the added Ln3+ complexes. The Stern–Volmer quenching constants vary from 11 dm3 mol–1 for Ho and Sm to 110 dm3 mol–1 for Pr. It is concluded that this transfer is unlikely to occur from either the ligand singlet or triplet levels and it is proposed that a higher Tb3+ level such as the 5D3 may be involved in both inter- and intramolecular energy transfer.

Photothermal degradation of copolymers of methyl methacrylate and n-butyl acrylate

The photothermal degradation of copolymers of methyl methacrylate (MMA) and n-butyl acrylate ( n-BuA) covering the whole composition range has been studied at 165°. The gaseous products, which are relatively minor, are hydrogen, carbon monoxide and methane. The liquid products are predominantly MMA, with n-BuA, n-butanol and n-butyraldehyde as minor products. Infra-red spectral changes in the residue were attributed to lactone formation and associated with butanol formation as in the purely thermal reaction The “cold ring” or chain fragment fraction becomes increasingly more abundant as the n-BuA content of the copolymer is increased. All the products and principal features of the reaction are explained in terms of a radical process which is initiated by scission of pendant acrylate units and is propagated by a combination of depropagation and intra- and intermolecular transfer processes, the relative importance of which depends upon copolymer composition. Differences from the thermal reaction and the corresponding reaction in copolymers of methyl methacrylate and methyl acrylate are discussed.

Pyrolysis of α‐ and β‐deuterostyrene polymers

AbstractRecently the depolymerization of addition polymers has been treated in terms of four elementary reactions: initiation, propagation, transfer, and termination; analogous to those reactions which operate in the polymerization process. It is the transfer process which, according to its relative importance, gives rise to differences in polymers in the monomer yield, in the rate vs. conversion behavior, and in the molecular weight vs. conversion behavior. The transfer process is assumed to operate through the abstraction of a hydrogen from the polymer chain by a radical. By substituting a deuterium for the hydrogen at selected points in the polymer chain, the amount of the transfer process should be decreased due to an isotope effect. Two deuterated polymers were made from α‐ and β‐deuterated styrenes and their depolymerization behavior studied. In the case of the α‐deuterostyrene polymer the results are compatible with a ½ decrease in the rate of the transfer process and also an increase in the rate of the propagation. The results with the β‐polymer imply an increase in both the transfer and propagation rates. The data obtained indicate that in polystyrene the intermolecular transfer process is mainly responsible for the fall in molecular weight of the polymer residue during depolymerization.