Dependence of NMR isotropic shift averages and nuclear shielding tensors on the internal rotation of the functional group X about the C-X bond in seven simple vinylic derivatives H2C=CH-X

The 'Gauge Including Atomic Orbitals' (GIAO) approach is used to investigat e the question of intramolecular rotation. Ab initio GIAO calculations of N MR chemical shielding tensors carried out with GAUSSIAN 94 within the SCF-H artree-Fock approximation are described. In order to compare the calculated chemical shifts with experimental ones, it is important to use consistent nuclear shieldings for NMR reference compounds like TMS. The influence of r otating functional groups X=CH3, CHO, NO2, NH2, CONH2, COOH or C6H5 on the shielding tensors in seven vinylic derivatives H2C=CH-X is studied; the mol ecules are propene, acrolein, nitroethylene, ethyleneamine, acrylamide, acr ylic acid and styrene. We observe a marked dependence of nuclear shielding and chemical shift on the torsional movement. Different Boltzmann averages over the conformational states are considered and compared for gas phase, l iquid and solid state NMR. Their applicability to model cases for rigid or freely rotating molecules and for fixed molecules (e.g. polymers or protein s) with rapidly rotating groups is discussed and simple calculation models are presented. On the basis of this work it can be concluded that intramole cular rotation clearly affects the observed averages. Effects of up to 2 pp m have been observed for isotropic chemical shifts, and up to 17 ppm differ ence have been observed for individual tensor components, for example, of t he carboxylic C-13 atom in acrylic acid. The variation of the shielding ten sor on a nucleus in a fixed molecular backbone resulting from an attached r otating group furthermore leads to a new relaxation mechanism by chemical s hift anisotropy.