Magnetic shielding isotropic

The NMR magnetic shielding for atoms like carbon is affected greatly by what it is bonded to and the type of bond to its neighbor. Use the inner carbon atoms of normal butane as the reference atom and calculate the shift in C isotropic shielding for 2-butene and 2-butyne. Can you explain these shifts as a function of the changing molecular environments ... 

When comparing the magnetic shielding tensors in Cgo and it is observed that the six extra electrons in the latter yield an important increase in the anisotropy and a strong shielding effect in the isotropic part of the nuclear shielding. 

Figure 1. Basis set and method dependence of the calculated isotropic aluminum magnetic shielding for A1H. The RHF data are denoted by the filled circles and the MP2 data are denoted by the open squares. The horizontal scale is a qualitative representation of the basis set size.

The mean absolute deviations for isotropic NMR chemical shifts show quite clearly that the atomic functions must be allowed to depend on the magnetic field if close agreement with experiment is to be obtained at the minimal or split valence shell levels. Within the GIAO framework, the results show a marked improvement as the theoretical model is improved. However, the study presented here is by no means exhaustive and further theoretical work is necessary to document more adequately the performance of these computational methods for magnetic shielding parameters. 

Comparison of Calculated C, and Isotropic Nuclear Magnetic Shielding Constants (in ppm) Obtained at HF-SCF, MP2, and LMP2 Levels Using the GIAO Ansatz Together with a qz2p Basis ... 

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