Angel C. de Dios

Recent progress in understanding chemical shifts

In the past three or four years computer hardware and software developments have reached the stage where the nuclear magnetic resonance (NMR) spectra of many molecular systems can now be accurately evaluated. Detailed analysis of chemical shifts may soon become a routine part of solid (and liquid) state NMR structure prediction in chemistry and biology, and this Article covers the development of the topic from its earliest beginnings.

The NMR Chemical Shift: Insight into Structure and Environment

Publisher Summary This chapter discusses the nuclear shielding surfaces in simple systems: The systems discussed are diatomic molecules to focus on the variation with bond length H 2 O, NH 3 , PH 3 and CH 4 to consider the bond angles and bond lengths. The variations with Φ, Ψ, and x angles are explored in various model peptides, and finally the remote bond stretches. Following the intramolecular shielding surfaces, the intermolecular shielding in atom-atom pairs, atom-ion, and atom-molecule pairs and clusters of three or more, including up to 5 or 16 polar solvent molecules, is discussed in the chapter. The effects of electric fields and electric field gradients on shielding are considered separately. The origin of the temperature dependence, the isotope shifts, the shifts with density and solvent, and the hydrogen-bonding shifts is also shown in this chapter. At present, only small fragments of networks have been treated quantum-mechanically, such as the [SiO 4 ] -4 unit to represent silicates. Approaches to complex systems, such as proteins, have been developed to provide new insight into the interdependence of shielding and structure. However, other systems, such as ionic and covalent solids, have so far been treated only at the empirical level.

NMR chemical shifts and structure refinement in proteins

SummaryComputation of the 13Cα chemical shifts (or shieldings) of glycine, alanine and valine residues in bovine and Drosophila calmodulins and Staphylococcal nuclease, and comparison with experimental values, is reported using a gauge-including atomic orbital quantum-chemical approach. The full ≈24 ppm shielding range is reproduced (overall r.m.s.d.=1.4 ppm) using ‘optimized’ protein structures, corrected for bond-length/bond-angle errors, and rovibrational effects.

129Xe Magic-angle spinning spectra of xenon in zeolite NaA direct observation of mixed clusters of co-adsorbed species

We present the first demonstration that the individual 129Xe resonances corresponding to Xen (n = 1-8) clusters inside the alpha-cages of zeolite NaA can be narrowed under magic-angle spinning (MAS). Under these high-resolution conditions we also observe upon addition of Kr the individual peaks corresponding to mixed clusters, XenKrm, inside the alpha-cages, which will allow the first direct determination of the distribution of co-adsorbates in a microporous solid. Under MAS the chemical shifts of the Xen clusters are shown to be highly sensitive to disorder in the zeolite and provides new, quantitative information about the presence of alpha-cages of several types.

THE NUCLEAR SHIELDING SURFACE: THE SHIELDING AS A FUNCTION OF MOLECULAR GEOMETRY AND INTERMOLECULAR SEPARATION

In making the connection between theoretical shielding values and experiments, it becomes necessary to consider medium effects and rovibrational averaging. The rovibrational averaging which takes the shielding for a molecule at its rigid equilibrium geometry into a thermal average shielding also provides the temperature dependence in the zero-pressure limit and the changes upon isotopic substitution which are experimentally accessible. The averaging of 15N and 31P shielding in the NH3 and PH3 molecules uses ab initio shielding surfaces and intramolecular potential surfaces or their derivatives. General trends are observed in comparing the shielding surfaces in these two molecules with H2O and CH4. A proper account of medium effects requires the knowledge of the intermolecular shielding surfaces, which are explored here for model systems 39Ar in Ar...Ar, Ar...Ne, Ar...Na+, Ar...NaH, and for 21Ne in Ne...Ne and Ne...He, employing ab initio calculations (LORG and SOLO). The shapes of the shielding function σ(R) for two atoms are shown to be similar for intra-and intermolecular shielding.

Chapter 2:Theoretical and physical aspects of nuclear shielding

This chapter is a review of the literature published between June 2012 and May 2013 on physical and theoretical aspects of nuclear magnetic shielding. Theoretical aspects include (a) general theory, for example, newly developed approaches in relativistic theory of nuclear shielding, the relation between the spin-rotation tensor and shielding in relativistic theory, NMR shielding for electronically degenerate states and benchmark studies (b) ab initio and DFT calculations, both relativistic and non-relativistic. Physical aspects include (a) anisotropy of the shielding tensor, (b) shielding surfaces and rovibrational averaging, (c) isotope shifts, (d) intermolecular effects on nuclear shielding, and (e) absolute shielding scales.