APPLICABILITY OF A REPORTED SIMPLE METHOD OF CALCULATING
INDUCED FIELDS TO CALCULATE CONTOUR-MAPS OF RING-CURRENT-EFFECT SHIELDING VALUES FOR MACRO- (BIO-) MOLECULAR STRUCTURE DETERMINATIONS
S.Aravamudhan, Department of Chemistry, North Eastern Hill University,Shillong 793022 MEGHALAYA
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In the literature attempts to make use of NMR methods
for the geometrical, structural determination have been abundantly reported. Initial efforts were to make use of the dipolar
interaction parameters to determine the internuclear distances in solid state single crystal materials1 and after the advent
of the Nuclear Overhauser Effect (NOE) the dipolar-interaction dependent relaxation mechanisms and the related equations could
be used2 with the experimentally determined enhancement factors to quantitatively infer on the internuclear distances in small
molecules even in the liquid state studies which made the utility of High Resolution NMR Spectroscopy extend into the realms
of structural determinations comparable2 with the X-ray methods. Further adapating the NOE consequences with the more recent
multiple-pulse FT NMR techniques with the capabilities of the Highfield Spectrometers NMR could be potentially used with several
combinations of theoretcal and experimental parameters fitting into optimisation computational techniques for the macromolecular
structural determinations - the most preferred interaction being the dipole-dipole interactions manifesting in NMR, for such
The more commonly used chemicalshift parameter which is used for electronic geometrical structural detrminations
and the spinspin coupling constants in the liquid state samples become tractably and quantitatively distance-dependent mainly
due to the ringcurrent effects when they are as large as in the case of aromatic rings present in the molecule and efforts
to calculate these effects have been reported4 with tables and contour diagrams, in the literature for the reading-out of
distance values when the corresponding Shielding parametrs (measured experimentally as chemicalshifts) values can be obtained
from NMR experiments.
It has been possible to enunciate5 a simple, programmable summing method to calculate the induced
fields due to electron-circulations in molecular specimens and in sample specimens of macroscopic size used in NMR studies
by considering the magnetic susceptibilty as the available input parameter. But till now the considerations have been with
isotropic susceptibility values in homogeneously magnetized specimens.
In the case of an aromatic ring the corresponding
susceptibility parameter is an anisotropic second rank tensor property and it is intended in this contribution to put forward
the practicabilities and limitations of the mathematical method mentioned as above5 while applying it with the available susceptibility
tensor values with an outlook projected for the possibility of using these calculations effectively for the macromolecular
structural determinations4 .
These projections have not yet reached a level to be discussed with quantitative exapmles.
But the confidence obtained till now with these methods5 it should be possible to expose a possibilty for the macromolecular
case and from the comparison of the available literature reports it should be possible to obtain from the users an overview
even at the outset for the actual contexts where this can be of added advantage and whether there could be any at all.
the light of an earlier-literature coverage the present method of calculations will be described and summarised with comments
even without actual quantitative calculated values would be depicted in the poster presentation.
1. Introduction to Magnetic Resonance with
applications to Chemistry & Chemical Physics, A.Carrington and A.D.McLachlan, Harper International Edition 1967, Harper
& Row, Chapter 3 - Nuclear Resonance in Solids, Sections3.3-3.5
2. (a) The review article on Double Resonance Techniques
in NMR ,Angewandte Chemie Vol.10 by von Philipsborn (b) The book on Nuclear Overhauser Effect by J.H.Noggle et.al ,Academic
3. The several applications of NOESY technique to Macromolecular Structural determinations and its variations.
Applications of Ring-current Calculations to the Proton NMR of Proteins and Transfer RNA , Stephen J.Perkins in BIOLOGICAL
MAGNETIC RESONANCE, Vol.4, Edited by Lawrence J.Berliner and Jaques Reuben, Chapter 4, pages 193-336. The Y , f energy contour
diagrams(the Ramachandran diagrams) and the similar corresponding Y, f spin-spin coupling contour digrams(Karplus equations)
are well known.
5. (a) A Report on the Demagnetization Factor and Induced Field Calculations for Standard Sample Shapes,
S.Aravamudhan , Symposium on Spacially Resolved Magnetic Resonance & 7th NMRS Symposium , Feb.2001 , C.L.R.I / I.I.T.,Madras,
Chennai (b) Induced Fields Due to a Magnetized Specimen S.Aravamudhan , 2nd Alpine Conference on Solid State NMR , Chamonix
Mont-Blanc, France , POSTER 2 , Sept. 2001. (c) Implications of Calculating the Full Shielding Tensor of Benzene Molecule
Using a Magnetic Dipole Approximation , S.Aravamudhan et.al., 4th NMRS Symposium , Feb.1998, I.I.Sc., Bangalore. (d) Investigating
the Feasibility of Calculating Intramolecular Shielding Tensors Using a Magnetic Dipole Model , S.Aravamudhan , Proceedings
of the Joint 29th Ampere-International Conference on Magnetic Resonance & Related Phenomena, Technische Universitt Berlin,
Berlin page 501,August 1998.