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 investigate the question of intramolecular rotation. Ab initio GIAO calculations of NMR chemical shielding tensors carried out with GAUSSIAN 94 within the SCF-Hartree-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 rotating 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 molecules are propene, acrolein, nitroethylene, ethyleneamine, acrylamide, acrylic 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, liquid and solid state NMR. Their applicability to model cases for rigid or freely rotating molecules and for fixed molecules (e.g. polymers or proteins) with rapidly rotating groups is discussed and simple calculation models are presented. On the basis of this work it can be concluded that intramolecular rotation clearly affects the observed averages. Effects of up to 2 ppm have been observed for isotropic chemical shifts, and up to 17 ppm difference have been observed for individual tensor components, for example, of the carboxylic ¹³C atom in acrylic acid. The variation of the shielding tensor on a nucleus in a fixed molecular backbone resulting from an attached rotating group furthermore leads to a new relaxation mechanism by chemical shift anisotropy.