Monday, May 11, 2009

J. Phys. Chem. B. , Vol. 113, Issues 16-19

Solid State NMR Study and Density Functional Theory (DFT) Calculations of Structure and Dynamics of Poly(p-xylylenes)

A. Sroka-Bartnicka, S. Olejniczak, W. Ciesielski, A. Nosal, H. Szymanowski, M. Gazicki-Lipman and M. J. Potrzebowski*

J. Phys. Chem. B, 2009, 113 (16), pp 5464–5472
DOI: 10.1021/jp900788m

Abstract:High resolution solid state 13C nuclear magnetic resonance (SS NMR) measurements were carried out on poly(p-xylylene) (PPX). The samples comprised vapor-deposited specimens as well as pure α and β polymorphs of this polymer. The measurements were performed using cross-polarization and magic angle spinning (CP/MAS) techniques. Density functional theory gauge-including-atomic-orbital (DFT GIAO) calculations of NMR shielding parameters 13C σii were performed for the optimized geometry and structure of a xylylene trimer, acquired from the X-ray data, including intermolecular interactions. Two-dimensional phase adjusted spinning sideband (2D PASS) correlation was employed for the assignment of the values of the principal elements 13C δii of the chemical shift tensor (CST). A comparative analysis of shielding (σii) versus chemical shift (δii) parameters showed substantial differences between the molecular dynamics of α and β polymorphs. This observation was further supported by the measurements of 13C T1 relaxation times and the analysis of cross-polarization kinetics. Frequency switched Lee−Goldburg heteronuclear correlation (FSLG HETCOR) for the 1H−13C system was used in order to analyze molecular packing in both polymorphs. As a result of all of the above measurements, new insight into the mechanism of thermal phase transition from the α to the β polymorph of poly(p-xylylene) is presented.

Approximate Reconstruction of Continuous Spatially Complex Domain Motions by Multialignment NMR Residual Dipolar Couplings

Charles K. Fisher and Hashim M. Al-Hashimi*
J. Phys. Chem. B, 2009, 113 (18), pp 6173–6176
DOI: 10.1021/jp900411z

Abstract:NMR spectroscopy is one of the most powerful techniques for studying the internal dynamics of biomolecules. Current formalisms approximate the dynamics using simple continuous motional models or models involving discrete jumps between a small number of states. However, no approach currently exists for interpreting NMR data in terms of continuous spatially complex motional paths that may feature more than one distinct maneuver. Here, we present an approach for approximately reconstructing spatially complex continuous motions of chiral domains using NMR anisotropic interactions. The key is to express Wigner matrix elements, which can be determined experimentally using residual dipolar couplings, as a line integral over a curve in configuration space containing an ensemble of conformations and to approximate the curve using a series of geodesic segments. Using this approach and five sets of synthetic residual dipolar couplings computed for five linearly independent alignment conditions, we show that it is theoretically possible to reconstruct salient features of a multisegment interhelical motional trajectory obtained from a 65 ns molecular dynamics simulation of a stem−loop RNA. Our study shows that the 3-D atomic reconstruction of complex motions in biomolecules is within experimental reach.

A Dynamic Magic Angle Spinning NMR Study of the Local Mobility of Alanine in an Aqueous Environment at the Inner Surface of Mesoporous Materials

Tal Amitay-Rosen, Shifi Kababya and Shimon Vega*
J. Phys. Chem. B, 2009, 113 (18), pp 6267–6282
DOI: 10.1021/jp810572r
Abstract: Dynamic deuterium magic angle spinning NMR has been applied to study the slow motion of small molecules close to a silica surface. In particular, alanine-d3 molecules dissolved in an aqueous solution were loaded into the pores of the mesoporous materials SBA-15 and MCM-41. Deuterium spectra were measured as a function of the water content of these materials and the temperature. From the analysis of these spectra and the corresponding proton spectra, using a simple molecular exchange model, relatively slow desorption rates of the binding of alanine to the inner pore surface were obtained and were correlated with the low proton concentrations at the pore surfaces.

Proton Mobilities in Phosphonic Acid-Based Proton Exchange Membranes Probed by 1H and 2H Solid-State NMR Spectroscopy

Gunther Brunklaus*, Siri Schauff, Dilyana Markova, Markus Klapper, Klaus Mllen and Hans-Wolfgang Spiess
Max-Planck-Institut fr Polymerforschung, Postfach 3148, D-55021 Mainz, Germany
J. Phys. Chem. B, 2009, 113 (19), pp 6674–6681
Abstract:Two novel phosphonic acid-based “dry” proton exchange membrane materials that may allow for fuel cell operation above 100 °C have been prepared and characterized via solid-state 1H and 2H MAS NMR spectroscopy. We obtained information on both the nature of hydrogen bonding and local proton mobilities among phosphonic acid moieties. In particular, 2H MAS NMR line shape analysis yielded apparent activation energies of the underlying motional processes. Using this approach, we have investigated both a model compound and a novel PEM system. It was found that the relation of estimated hydrogen-bond strength and local proton mobility accessible by solid-state NMR and bulk proton conductivity is complex. Improvements through admixture of a second component with protogenic groups are suggested.

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