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NSC9 - CRSI: Abstract Feb. 1-4, 2007 University of Delhi
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HERE for PROGRAMME of NSC-9
HERE for index to POSTERS at NSC-9
Aravamudhan S---Poster PP-8
A Perspective for Recognizing Discreteness distinctly from Continuum:
Validity of Point Dipole Approximation to Set the Distance Scale.
Department of Chemistry, North Eastern Hill University, Shillong 793022 Meghalaya
The materials in any state of matter are made up of discrete units and depending upon when and how it becomes relevant and possible to count the units as individual entities is when the term discreteness is applicable. However, mostly when ensemble properties are considered and the material as a bulk is what can be tangible for consideration, the term continuum is applicable. But there are instances when the given specimen has to have within itself demarcations as discrete region and continuum. Such are the contexts when it becomes necessary to consider critically and quantitatively a criterion to draw a boundary line within the medium which is otherwise homogeneous and does not have any distinctions characteristically associated with the constituents or the way the constituent units are organized within the material medium which in this context can be special arrangements and the inter unit distances.
The example of a single crystal of organic molecule would present the disposition of the molecules as described above; and, the applicability of magnetic point dipole model for induced field values at specified sites within the single crystal specimen would be clarifying the extent to which the material has to be considered as discrete or as a continuum. This subject of the point dipole approximation and its validity in the contexts of chemist s use in interpreting spectroscopic results has been pointed out in the references 1-4 cited below.
It would be further elucidated that the magnitude of the susceptibility of the molecular unit, the intermolecular distance, and the distance of the specified site from the contributing units determines where the demarcation can be located between discrete region and the continuum . With typical values for the quantities the trends would be illustrated as graphical results, and diagrammatic illustration of the relative distances within the specimen and the related properties to together with the graphical results would be presented to comprehend the terms and defining boundary lines when it is necessary. A typical diagram which would be part of the illustration is displayed below.
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HERE PROGRAMME & Index to posters NMRS2007 ARAVAMUDHAN S---Poster: P-30
Day-4 Thursday 8-2-2007 16:00 to 18:00 hrs Session VIII Biomedical NMR Chairman: S ARAVAMUDHAN
BULK SUSCEPTIBILITY EFFECTS AND NMR CHEMICAL SHIFTS:
THE PERSPECTIVE FOR MOLECULES TO MATERIALS
Department of Chemistry, North Eastern Hill University, Shillong, 793022 Meghalaya India
It is a well established practice to properly account for Bulk Susceptibility effects in the measurements of NMR Chemical shifts (proton chemical shifts in particular) accurately. While accounting for this bulk medium effect, it becomes necessary to consider an Inner Volume Element (I.V.E) described closer to the molecular site surrounding it which is excluded from contributions accounted for by the Bulk Medium, but the contributions within must be accounted for separately and added to the remaining contributions so that no part of the material medium gets an undue exemption from contributing to the chemical shifts. Once the above two are taken into account then the molecular contribution is determined from the experimentally measured values. The contribution from the neighbor molecules in the I.V.E. can be termed intermolecular in nature and to some extent is dependent upon the specific orientation dispositions of the neighbors, and the neighbor-molecule electronic structure. This is simply a discrete summation from individual neighbors in the case of the static orientation in Solid state single crystals. However what happens in such an I.V.E. in liquids is yet to be ascertained since even within this I.V.E., the molecules are under constant motion.
Recent results on these aspects of effect of the bulk medium in the case of diamagnetic specimen seem to provide a much clear perspective on the significance of the chemical shift determinations getting the purview from molecular to material properties of the specimen. A summary of results which highlights this significance would be given along with the specific queries to which those results stand out as clarifications.
IBS2007 Abstract Feb.13-15, 2007 AIIMS, New Delhi
Aromatic Ring Current Shifts in Biological Macromolecules:
Generating Weight Factors for the Instantaneous Shift Values for an Effective Motional Averaging
Department of Chemistry
North Eastern Hill University
Shillong 793022 Meghalaya India
By using the Full Tensor Form of the equation for the aromatic ring current shifts in the NMR spectra of Biological Macromolecules, it has been possible to show (1) that there are definite advantages of calculating the full shielding tensor for such cases of molecules in motion, rather than merely using the equations for the isotropic shift values. However, in the model calculation presented (2) equal weights were given for the shift values sampled at the several instantaneous dispositions during the motion while arriving at a average value. Based on the experiences till now on this type of approach, it is possible now to further investigate for the possible improvements that may result if one uses weight factors generated by specifying the characteristics of the motion and the nature of the shift dependences along the motional paths.
Conventionally, an oscillatory motion (torsion) is satisfactorily taken into account on the basis of the characteristics of Simple Harmonic Motion. In such a case it is also known that depending on the frequency of oscillation and the swing amplitudes, the resident time for the molecule at the equilibrium (mean) position is small compared to the times at the maxima for the amplitude. Hence the shift values at the extremes would be effective more than the shift values corresponding to the mean positions. Hence the weight factor must be more for the values corresponding to the extremes of oscillations than to that at the mean position. Such criteria if it gets built in to the averaging procedure, then the results could have better confidence indices. An approach of this type would be illustrated and to what ever extent possible a comparison with the approaches as in Reference (3) would be presented.
Poster Sheet_11: A specific hypothetical case of how the Tensor Form can be useful when the isotropic values cannot be indicative of the motional state of the system. And further results as captions for graphical plots in the subsequent sheets. This sheet contains the main conclusions which resulted from the Calculations made at this instance of IBS2006.
(2). http://www.geocities.com/sankarampadi/ForIbs2006.html Poster Sheets 10 & 11
(3). Fluctuations and Averaging of Proton Chemical Shifts in the Bovine Pancreatic Trypsin Inhibitor , Jeffrey C. Hoch, Christopher M. Dobson, and Martin Karplus, Biochemistry 21, 1118-1125 (1982)
Sheet-1 : Sample shape criteria for Homogeneous and Inhomogeneous magnetization
Sheet-2 : Division of specimen while calculating induced fields.
Sheet-3 : Further illustrations for sample divisions of magnetized specimen
Sheet-4 : Validity of point dipole approximation and distance scales
Sheet-5 : Recapitulation of topics in Solid State
Sheet-6 : Consequences of Homogeneous and Inhomogeneous magnetization:Illustartion
Sheet-7 : Calculated results for Inhomogeneous magnetization
Sheet-8 : Illustration of the trends on the basis of calculations
Sheet-9 : The necessity of discrete summing at off-centre points within IVE
Sheet-10 : The contents of Sheet-9 further illustarted
Sheet-11 : A hypothetical lattice with a rigorous octahedral symmetry
Sheet-12 : Results of discrete summation at the (centre) origin of the lattice
Sheet-13 : Induced fields at points other than the origin within the lattice as above
Sheet-14 : Graphical plots of the values in the table of Sheet-13
Sheet-15 : Further illustration of trends noticed in Sheet-14
Sheet-16 : Conclusions