Structural Anomaly in Glasses: Molecular Dynamics Study of Organic Radical in Dibutylphthalate at Different Temperatures.

Understanding the heterogeneous nano/microscopic structures of various organic glasses is fundamental and necessary for many applications. Recently, unusual structural phenomena have been observed experimentally in various organic glasses near their glass transition temperatures (Tg), including dibutyl phthalate (DBP). In particular, the librational motion of radical probe in the glass is progressively suppressed upon temperature increase. In this work, we report in-depth molecular dynamics studies of structural anomalies in DBP glass, that revealed insights into the general mechanism of these phenomena. In particular, we have evidenced that the two types of solvation within alkyl chains coexist, allowing only small-angle wobbling of the solute molecule (TEMPO radical), and another favouring large-angle rotations. The former solvation assumes constrained location of the solute near carboxyl groups of DBP, while the latter is coupled to the concerted movement of butyl chains. Remarkably, excellent qualitative and quantitative agreement with previous experimental results were obtained. As such, we are certain that the above-mentioned dynamic phenomena explain the intriguing structural anomalies observed in DBP and some other glasses in the vicinity of Tg.

Phosphine oxides as NMR and IR spectroscopic probes for the estimation of the geometry and energy of PO⋯H-A hydrogen bonds.

In this work we evaluate the possibility of using the NMR and IR spectral properties of the PO group to estimate the geometry and strength of hydrogen bonds which it forms with OH-, NH- and CH-acids. The results of the DFT study of 70 hydrogen-bonded 1 : 1 complexes of a model trimethylphosphine oxide, Me3PO, with various proton donors in the gas phase and in aprotic medium (modelled as a polarizable continuum) are presented. Four types of hydrogen bonds with the general formula Me3PO⋯H-A were considered, where the A atom is O, C, and N (neutral or cationic acids). Within the selected set of complexes the hydrogen bond energy varies over a wide range (ca. 0-85 kJ mol-1). We show that it is possible to use simple correlations to estimate the energy and geometry of OHO, NHO and CHO hydrogen bonds from the changes of isotropic 31P NMR chemical shifts and harmonic PO stretching vibration frequencies upon complexation. Such correlations also could be used to estimate the proton-donating ability (and Brønsted acidity; pKa) of OH acids.

Mixed-Valence Compounds as Polarizing Agents for Overhauser Dynamic Nuclear Polarization in Solids*.

Herein, we investigate a novel set of polarizing agents-mixed-valence compounds-by theoretical and experimental methods and demonstrate their performance in high-field dynamic nuclear polarization (DNP) NMR experiments in the solid state. Mixed-valence compounds constitute a group of molecules in which molecular mobility persists even in solids. Consequently, such polarizing agents can be used to perform Overhauser-DNP experiments in the solid state, with favorable conditions for dynamic nuclear polarization formation at ultra-high magnetic fields.

Organic Mixed-Valence Compounds and the Overhauser Effect in Insulating Solids.

Recent experiments have shown that the organic free radical 1,3-bisdiphenylene-2-phenylallyl (BDPA) can induce an Overhauser effect dynamic nuclear polarization in insulating solids, a feat previously considered not to be possible. Here, we establish that this peculiar ability of the BDPA radical stems from its mixed-valence nature and the ensuing intramolecular charge transfer. Using state-of-the-art DMRGSCF calculations, we confirm the class II mixed-valence nature of BDPA with the characteristic double-well potential energy surface, and we investigate the mechanism of the consequent electron hopping. A two-component vibronic Hamiltonian is then employed to compute the rate of electron hopping from a quantum dynamical time-propagation of the density matrix. The predicted hyperfine coupling oscillations indeed fall within the frequency range required for an Overhauser effect. The paradigm of mixed-valence compounds as a mining source opens many possibilities for the development and fine tuning of novel polarizing agents.

Inherent heterogeneities and nanostructural anomalies in organic glasses revealed by EPR.

Intriguing heterogeneities and nanostructural reorganizations of glassy ionic liquids (ILs) have recently been found using electron paramagnetic resonance (EPR) spectroscopy. Alkyl chains of IL cations play the key role in such phenomena and govern the anomalous temperature dependence of local density and molecular mobility. In this paper we evidence and study similar manifestations in a variety of common non-IL glasses, which also contain molecules with alkyl chains. A series of phthalates clearly demonstrates very similar behavior to imidazolium-based ILs with the same length of alkyl chain. Glasses of alkyl alcohols and alkyl benzenes show only some similarities to the corresponding ILs, mainly due to a lower glass transition temperature hindering the development of the anomaly. Therefore, we demonstrate the general nature and broad scope of nanoscale structural anomalies in organic glasses based on alkyl-chain compounds. The 'roadmap' for their occurrence is provided, which aids in understanding and future applications of these anomalous nanoheterogeneities.

Nanocage formation and structural anomalies in imidazolium ionic liquid glasses governed by alkyl chains of cations.

Intriguing nanostructuring anomalies have been recently observed in imidazolium ionic liquids (ILs) near their glass transition points, where local density around a nanocaged solute progressively grows up with temperature. Herewith, we for the first time demonstrate experimentally and theoretically, that these anomalies are governed by alkyl chains of cations and crucially depend on their length. Electron Paramagnetic Resonance (EPR) spectroscopy on a series of ILs [Cnmim]BF4 (n = 0-12) shows that only the chains with n = 3-10 favor anomaly. Moreover, remarkable even vs. odd n peculiarities were systematically observed. Finally, similar anomaly was for the first time observed for a non-IL glass of dibutyl phthalate, which structurally mimics cations of imidazolium ILs. Therefore, such anomalous density behavior in a glassy state nanocage goes far beyond ILs and proves to be a more general phenomenon, which can be structurally tuned and rationally adjusted for various potential applications in nanoscale materials.

Salt Bridge in Aqueous Solution: Strong Structural Motifs but Weak Enthalpic Effect.

Salt bridges are elementary motifs of protein secondary and tertiary structure and are commonly associated with structural driving force that increases stability. Often found on the interface to the solvent, they are highly susceptible to solvent-solute interactions, primarily with water but also with other cosolvents (especially ions). We have investigated the interplay of an Arginine-Aspartic acid salt bridge with simple salt ions in aqueous solution by means of molecular dynamics simulations. Besides structural and dynamical features at equilibrium, we have computed the mean force along the dissociation pathway of the salt bridge. We demonstrate that solvated ions influence the behavior of the salt bridge in a very specific and local way, namely the formation of tight ionic pairs Li+/Na+-Asp-. Moreover, our findings show that the enthalpic relevance of the salt bridge is minor, regardless of the presence of solvated ions.

The Conformational Ensemble of Polyglutamine-14 Chains: Specific Influences of Solubility Tail and Chromophores.

We address polyglutamine-14 in aqueous solution with specific chromophores and a solubility chain by means of a multiscale simulation approach, combining atomistic molecular dynamics simulations and coarse-grained Monte-Carlo conformational sampling. Despite the intrinsically disordered nature of the amyloidogenic polyglutamine, we observe transient characteristic structural motifs which exhibit a specific hydrogen bonding pattern. We illustrate the relationship between structure pattern and the distance distribution of a pair of chromophores attached to the peptide termini, in light of specific influence of a short solubility tail and the chromophores themselves on the conformational ensemble.

Molecular Mechanism of Overhauser Dynamic Nuclear Polarization in Insulating Solids.

Dynamic nuclear polarization (DNP), a technique that significantly enhances NMR signals, is experiencing a renaissance owing to enormous methodological developments. In the heart of DNP is a polarization transfer mechanism that endows nuclei with much larger electronic spin polarization. Polarization transfer via the Overhauser effect (OE) is traditionally known to be operative only in liquids and conducting solids. Very recently, surprisingly strong OE-DNP in insulating solids has been reported, with a DNP efficiency that increases with the magnetic field strength. Here we offer an explanation for these perplexing observations using a combination of molecular dynamics and spin dynamics simulations. Our approach elucidates the underlying molecular stochastic motion, provides cross-relaxation rates, explains the observed sign of the NMR enhancement, and estimates the role of nuclear spin diffusion. The presented theoretical description opens the door for rational design of novel polarizing agents for OE-DNP in insulating solids.

Hydrogen Bonding in Bis(6-amino-1,3-dimethyluracil-5-yl)-methane Derivatives: Dynamic NMR and DFT Evaluation.

Three bis(6-amino-1,3-dimethyluracil-5-yl)-methane derivatives were studied experimentally by variable-temperature (1)H NMR in polar aprotic solutions (CD2Cl2, C5D5N, C2D2Cl4) and computationally by DFT. The unusual for diarylmethanes coplanar conformation of dimethyluracil rings of each molecule is held by a pair of unequal intramolecular N-H···O hydrogen bonds. We show the presence of two dynamic processes involving breakage/formation of these bonds. First, it is two independent NH2 group rotations, each coupled to nitrogen inversion. Second, it is uracil ring rotations (ring flips). The thermodynamic parameters (ΔH(‡), ΔS(‡), and ΔG(‡)) of both processes were estimated by the full line shape analysis of NMR signals and also by DFT calculations. We demonstrate that, though the ring flips exchange pairs of NH protons, the two processes are not coupled: during the ring flip NH2 groups do not rotate, and during the NH2 rotation the rings do not necessarily rotate. Unlike in many other diarylmethanes, the ring flips in the studied compounds are happening stepwise; i.e., the configuration when both rings are "in flight" at the same time is energetically unfavorable (small degree of "cog wheel effect"). The signs of the ΔS(‡) values indicate that the molecular flexibility increases during the NH2 rotations, but decreases during the ring flips.

Proton transfer in a short hydrogen bond caused by solvation shell fluctuations: an ab initio MD and NMR/UV study of an (OHO)(-) bonded system.

We present a joint experimental and quantum chemical study on the influence of solvent dynamics on the protonation equilibrium in a strongly hydrogen bonded phenol-acetate complex in CD2Cl2. Particular attention is given to the correlation of the proton position distribution with the internal conformation of the complex itself and with fluctuations of the aprotic solvent. Specifically, we have focused on a complex formed by 4-nitrophenol and tetraalkylammonium-acetate in CD2Cl2. Experimentally we have used combined low-temperature (1)H and (13)C NMR and UV-vis spectroscopy and showed that a very strong OHO hydrogen bond is formed with proton tautomerism (PhOH···(-)OAc and PhO(-)···HOAc forms, both strongly hydrogen bonded). Computationally, we have employed ab initio molecular dynamics (70 and 71 solvent molecules, with and without the presence of a counter-cation, respectively). We demonstrate that the relative motion of the counter-cation and the "free" carbonyl group of the acid plays the major role in the OHO bond geometry and causes proton "jumps", i.e. interconversion of PhOH···(-)OAc and PhO(-)···HOAc tautomers. Weak H-bonds between CH(CD) groups of the solvent and the oxygen atom of carbonyl stabilize the PhOH···(-)OAc type of structures. Breaking of CH···O bonds shifts the equilibrium towards PhO(-)···HOAc form.