Chemical shift basic concepts

However, one of the most successfiil approaches to systematically encoding substructures for NMR spectrum prediction was introduced quite some time ago by Bremser [9]. He used the so-called HOSE (Hierarchical Organization of Spherical Environments) code to describe structures. As mentioned above, the chemical shift value of a carbon atom is basically influenced by the chemical environment of the atom. The HOSE code describes the environment of an atom in several virtual spheres - see Figure 10.2-1. It uses spherical layers (or levels) around the atom to define the chemical environment. The first layer is defined by all the atoms that are one bond away from the central atom, the second layer includes the atoms within the two-bond distance, and so on. This idea can be described as an atom center fragment (ACF) concept, which has been addressed by several other authors in different approaches [19-21]. 

I assume that you are conversant with basic principles of XH or proton NMR spectroscopy as applied to small molecules. In particular, I assume that you understand the concepts of chemical shift (8) and spin-spin coupling, classical continuous-wave methods of obtaining NMR spectra, and decoupling experiments to determine pairs of coupled nuclei. If these ideas are unfamiliar to you, you may wish to review NMR spectroscopy in an introductory organic chemistry textbook before reading further. 

The basic idea behind the DOSY concept is similar to the one behind multidimensional NMR. In 2-D NMR, a modulation in the phase or signal intensity with respect to a known time increment is recovered by inverse FT. In a DOSY experiment, the diffusion coefficient is recovered from the signal decay as a function of a diffusion increment by an ILT. In fact, the approximate ILT of the signal amplitude with respect to q, where q is defined as ygSf (t), yields the second dimension of the spectrum which correlates the chemical shift with the diffusion coefficient. Therefore, it was termed diffusion ordered spectroscopy (DOSY). However, unlike the FT of the time domain signal that yields a unique solution, ILT does not yield a unique solution. Therefore, several software packages were developed to overcome this problem. Readers interested in more details concerning the DOSY techniques can consult a recent extensive review on the subject [17]. 

Basic Concepts of Nuclear Shieldings and Chemical Shifts 56... 

BASIC CONCEPTS OF NUCLEAR SHIELDINGS AND CHEMICAL SHIFTS... 

The basic concept of utilizing anisotropic chemical shifts in macroscopically aligned membranes, pioneered by Opella and Cross, was initially developed using gramicidin A. The 15 amino acid peptide is readily synthesized with selective N or C-labels in the backbone, " or with N or F-labels in the side chains (as well as H for quadrupolar couplings). Numerous individual orientational constraints were thus used to determine the detailed p-helical structure of the membrane-bound ion channel, which was refined... 

Two-dimensional NMR resulted naturally from pulse programming concepts [20]. The basic idea is that a second dimension could be simulated by acquiring a series of spectra in which a delay period in the pulse sequence was incrementally increased. In what seemed at first like pure magic, it became possible, for example, to correlate a carbon s chemical shift in one dimension with the proton chemical shift of its attached proton in a second dimension. Again, the ingenuity for designing pulse programs to tease out the data of interest was boundless, and development of new experiments has continued unabated until the present. 

The 3-cyclopropenyl anion is more basic than the allyl anion and cyclopropyl anion, its acyclic and saturated counterparts. This can be accounted for by the small central C-C—C bond angle and the resulting electrostatic repulsion in the constrained anion. No additional parameter is needed to account for the weak acidity of cyclopropene at the aUyhc position. Consequently, on the basis of the thermodynamic definition of antiaromaticity, this concept is not needed to describe the 3-cyclopropenyl anion. Magnetic criteria such as nuclear independent chemical shifts (NICSs) lead to a different conclusion, but in this instance there is no energetic basis for this view. Consequently, the 3-cyclopropenyl anion is best described as non-aromatic despite 50 years of thought to the contrary. 

The basic concept for using the distance- and angle-dependent dipole-dipole coupling for structural studies is displayed in Figure 6, details are described in recent reviews. In a 2D experiment, DQ coherence is created, for example, between two with like or different chemical shifts. During excitation of the DQ coherence, the dipole-dipole coupling between the... 

When a molecule takes part in a reaction, it is properties at the molecular level which determine its chemical behaviour. Such intrinsic properties cannot be measured directly, however. What can be measured are macroscopic molecular properties which are likely to be manifestations of the intrinsic properties. It is therefore reasonable to assume that we can use macroscopic properties as probes on intrinsic properties. Through physical chemical models it is sometimes possible to relate macroscopic properties to intrinsic properties. For instance 13C NMR shifts can be used to estimate electron densities on different carbon atoms in a molecule. It is reasonable to expect that macroscopic observable properties which depend on the same intrinsic property will be more or less correlated to each other. It is also likely that observed properties which depend on different intrinsic properties will not be strongly correlated. A few examples illustrate this In a homologous series of compounds, the melting points and the boiling points are correlated. They depend on the strengths of intermolecular forces. To some extent such forces are due to van der Waals interactions, and hence, it is reasonable to assume a correlation also to the molar mass. Another example is furnished by the rather fuzzy concept nucleophilicity . What is usually meant by this term is the ability to donate electron density to an electron-deficient site. A number of measurable properties are related to this intrinsic property, e.g. refractive index, basicity as measured by pK, ionization potential, HOMO-LUMO energies, n — n ... 

Lavoisier s vision of the mathematization of chemistry placed him in the Newtonian tradition of ethers and imponderable subtle fluids. In shifting the focus of the Chemical Revolution away from the originality of Lavoisier s experiments on combustion and towards the power, fruitfulness and intellectual context of his basic chemical concepts and categories, postpositivist scholars undermined the positivist-Whig notion of Lavoisier s crucial year and any clear sense of its cruciality . ... 

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