Freitag vormittags Vorträge - Bunsentagung - Technische Universität ...

Freitag 21.05.2004, 17:50 Uhr
Vortrag - Theoretische Chemie
Calculation of detailed NMR spectra from
molecular dynamics simulations
Thomas Heine
C
29
Institut für Physikalische Chemie und Elektrochemie, TU
Dresden, D-01062 Dresden
The simulation of NMR spectra of optimised gas-phase molecules
has become an established field of quantum chemistry.
However, this type of simulation does not reflect experimental
conditions of NMR spectroscopy, where the time a spectrum is
recorded takes usually several micro or even milli seconds. As
the spectrum is usually taken at finite temperatures (300K) it
corresponds rather to an average of various conformations than
to one frozen geometry. In addition, solvents may influence the
spectrum considerably, as they act both on the geometry of the
solvated molecule and directly on the shielding constant.
In this presentation, the influence of these effects on computed
chemical shifts is discussed. Trajectories of gas phase and solvated
molecules are calculated using density-functional based
Born-Oppenheimer molecular dynamics. The NMR chemical
shifts for series of snapshots of these trajectories is then computed
using DFT-NMR computations. In some cases, for example
in solvated zeolite precursors, the NMR chemical shielding can
vary by a large number, like 50 ppm for 29 Si. On the first glance,
this disagrees with the very sharp signals which are observed in
experiment, where the line width is typically a small fraction of
a ppm.
Computation of NMR line widths, using the auto correlation
function of the chemical shielding, show that the NMR signals
are a subject of motional narrowing. However, this straightforward
type of line width simulation is computationally very
demanding, as it involves very frequent calculation of the shielding
constant, typically for snapshots taken all ≈3 fs for 1 H, or
≈10 fs for 13 C, for quite long trajectories (at least some 10 ps).
Using small benchmark molecules (small carbon hydrates and
TMS), an approximation for the simulation of the line width
for larger molecules is proposed. The method is applied for endohedral
fullerene Sc3N@C80, for C60, and series of zeolite
precursors.