NMR
of Oriented Molecules, Fields of Research
Molecular Structure Determination
Basic Theory
NMR of Oriented Molecules,
Fields of Research, Molecular Structure Determination, Basic Theory
Accuracy Limiting Factors
NMR of Oriented Molecules,
Fields of Research, Molecular Structure Determination, Accuracy Limiting
Factors
Nonrigid Molecules
NMR of Oriented Molecules,
Fields of Research, Molecular Structure Determination, Nonrigid Molecules
NMR of Oriented Molecules,
Fields of Research, Molecular Structure Determination, Possible Accuracy
of Structure Determination
Possible Accuracy of Structure Determination
Most observed direct couplings are in the range of
102-103Hz. It is usually possible to obtain
linewidths of the order of 1 Hz, so that 0.1 Hz can definitely be determined
(sometimes even 0.01 Hz). So that the relatively accuracy of couplings is
usually 0.0001.
Since the internuclear distances are inversely proportional to cube
of direct couplings, the relatively accuracy of the distances is alsu further
increased to 0.00003.
Consequently, distances should, in principle, be measurable with an
accuracy of at least 0.01 pm. Unfortunately, this accuracy is not meaningful,
because several approximation are made in the necessary corrections to
the measured direct couplings. These are neglect of pseudodipolar coupling
contributions to the measured couplings, limited accuracy in the vibration
corrections, and assumptions made in the deformation corrections (see Factors
Limiting the Accuracy of Structure Determination)
Taking these facts into account, it is possible to state that an accuracy
of distance determination by NMR of oriented molecules exceeding 0.1 pm
in bond lengths and 0.1° in bond angles is definitely unrealistic.
However 1 pm may measured reliably.
Return to "NMR of Oriented Molecules"
Last Update: April 22th, 1997
By: Alexan Shahatuni (msrc@moon.yerphi.am)