Carbon-13 chemical shift anisotropy

Nuclear magnetic shielding tensors computed by the gauge including atomic orbital (GIAO) method in the Hartree–Fock self-consistent-field (HF-SCF) framework are partitioned into magnetic contributions from chemical bonds and lone pairs by means of natural chemical shielding (NCS) analysis, an extension of natural bond orbital (NBO) analysis. NCS analysis complements the description provided by alternative localized orbital methods by directly calculating chemical shieldings due to delocalized features in the electronic structure, such as bond conjugation and hyperconjugation. Examples of NCS tensor decomposition are reported for CH4, CO, and H2CO, for which a graphical mnemonic due to Cornwell is used to illustrate the effect of hyperconjugative delocalization on the carbon shielding.

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Natural chemical shielding analysis of nuclear magnetic resonance shielding tensors from gauge-including atomic orbital calculations

Methods are described for determining the mutual orientation of molecular interaction tensors based on two‐dimensional NMR powder spectra. Necessary conditions are derived under which this determination can be made, procedures for the computation and analysis of 2D powder spectra are indicated and a technique for the construction of ridge plots is worked out. Chemical shift resolved dipolar powder spectra provide an easily interpreted representation of the mutual orientation of chemical shielding and dipolar coupling tensors. The various features and practical applications of such spectra are demonstrated with 2D powder spectra of benzene, lead formate, and methyl formate.

Orientation of tensorial interactions determined from two‐dimensional NMR powder spectra

Magnetic shielding constants are calculated for the nuclei in H2O and (H2O)2 using a slightly extended$set of atomic functions modified by gauge factors. Calculated shielding results for the theoretical equilibrium structures of H2O and (H2O)2 lead to a value for the hydrogen bond shift of −2.95 ppm, which is in reasonable agreement with the available experimental data. The anisotropy of the magnetic shielding tensor associated with the hydrogen bonded proton is found to increase by 12 ppm on dimer fromation. This suggestes that proton magnetic shielding anisotropies may be more sensitive to features of hydrogen bonding than are isotropic values. An analysis of the theoretical results reveals that the downfield hydrogen bond shift arises (a) from a decrease of the electronic charge on the hydrogen bonded proton, and (b) from deshielding effects of the currents induced on the oxygen atom of the porton acceptor by the external magnetic field. For (H2O)2 these two types of contribution are of equal importance. The latter type of contribution (b) is found to be almost tottally responsible for the anisotropy changes produced on dimer formation. The sensitivity of the shielding tensor associated with the hydrogen bonded proton to changes in the intermolecular geometrical parameters of (H2O)2 is examined. The calculatted isotropic and anisotropic values of thius tensor show large changes of the O⋅⋅⋅O distance%is reduced below 3.5 A. In contrast, the sensitivity of this shielding tensor to the relative orientation of the water monomers in (H2O)2 is quite small. Finally, the results of relaxing the geometry of the proton donor are reported. The isotropic value of the shielding at the hydrogen bonding proton is found to decrease markedly as the O−H distance of O−H⋅⋅⋅O is increased slightly from its equilibrium value.

Theoretical studies of magnetic shielding in H2O and (H2O)2

Nuclear spin relaxation has been developed as a standard method for studying molecular motions in liquids, solids, polymers, and—to a lesser extent—gases, staring with the pioneering work of Bloembergen, Purcell, and Pound [1]. Of the great variety of molecular motions possible (e.g., translations, rotations, vibrations) rotations are particularly important for nuclear spin relaxation. Conversely, nuclear spin relaxation can be especially successful if information about rotational motions is desired. In this case nuclear spin relaxation can yield quantitative information over an extraordinary wide range of characteristic frequencies, from about 1 Hz to 1014Hz. It shoud be noted that, typically, the nuclear spin relaxation times actually observed are much longer than the characteristic times of rotation of molecules. Therefore, a theory is needed which links the time constants of nuclear spin relaxation to the correlation functions describing the rotational motion, and so a considerable part of this volume is devoted to this goal. For rapid motions in liquids of low viscosity nuclear spin relaxation thus in principle, is inferior to scattering experiments (e.g., light and neutron scattering) which allow the determination of correlation functions.

J. Kempf

Papers

13C anisotropic chemical shift in organic solids: Benzoic acid and derivatives, benzophenone, and thiobenzophenone

13C Anisotropic chemical shift in a single crystal of benzophenone

Proton magnetic shielding in malonic acid by multiple pulse techniques