HIGH-RESOLUTION SOLID-STATE NMR
An Introductory Description and Leading Literature Indications
for Beginners and Aspirants to Start-up to Reach the Front-line
The line-width of the NMR lines in a High-Resolution liquid-state NMR spectrum is of the order of 0.2 Hz which is the
common-practice observed linewidth value. This is in accordance with the specifications of the spectrometer-manufacturers
and is the limiting value determined by the highest homgeneity of the magnetic field, which the spectrometer assembly is capable
of reaching. The inherent linewidths of the samples(solutes in solvents),usually dilute solutions, can be less than this field-inhomogeneity
value. These very small linewidths (in a proton 300 MHz system, this line-width is of the order of one part in thousand millions)
occurs due to the following sample characteristic: the molecules tumble (rotate & translate) at a very fast rate compared
to the frequencies(the energies of interactions, of the spin systems in the sample surroundings, expressed in units of frequencies)
and the motion averages-out all such interactions which can cause a broadening of the NMR lines. It is also a characteristic
feature of the spins in solutions that the spin-lattice and the spin-spin relaxation times are equal. When the relaxation
times are long, the NMR line-widths are small and vice-versa.In solid samples it can happen and it is usually so that the
spin lattice relaxation times are very much larger than the spin-spin relaxation times:the spin-spin relaxation times occur
in the timesacles of few tens of micro second only.On the contrary, in solid-samples (single crystal and powder specimens)
the molecules are fixed in space and do not undergo fast motions. And the spin interactions can be of the order of tens of
KHz, and hence the line widths occuring in solid-state NMR are of the aorde of KHz. It is the interactions of this order of
magnitude which are averaged in solutions to result in line widths of such small values which can be less than the 0.2 Hz.It
is also well known feature in a liquid state HR NMR spectrometer the sample is spinning to achieve the narrow linewidth. And
this spinning is about the direction of the Externally Applied, Steady, High, Magnetic Field. It was pointed out by E.R.Andrew
(E.R.Andrew, NUCLEAR MAGNETIC RESONANCE IN RAPIDLY ROTATING SOLIDS, in "Magnetic Resonance", Eds.,C.K.Coogan,N.S.Ham,S.N.Stuart,J.R.Pilbrow
and G.H.H.Wilson, page 163 Plenum, New York,1970) who pointed out the favourable effects of spinning a sample when the spinning
axis is oriented at an angle of 54 deg. 44 mins. with respect to the direction of the external magnetic field. It can produce
spectra qualitatively as narrow as the corresponding solution spectra of the sample.Besides the linewidth aspect of the solid
state NMR, the following essential structural aspects can be studied by measuring the NMR parametrs manifest in solid state.
1) In single crystals the main linewidth contributing interaction is the dipole-dipole interaction between the spins in their
fixed coordinates. When the interacting spins form isolated pairs of closely located spins, then splittings of the order of
few KHz can be the dominant feature in the spectrum from which it would be possible to measure internuclear distances with
accuracies comparable to X-ray techniques particularly from proton NMR (PMR). (A.Carrington and A.D.McLachlan, Introduction
to Magnetic Resonance, Chapter 3 entitled "Nuclear Resonance in Solids"- Chapter 1 of this book describes the NMR experiment
in sec. 1.2). These dipole-dipole interactions are dependent on the angle of orientation of the specimen,the internuclear
vectors in particular,and hence reflect the anisotropy of the interaction. 2) The Shielding of nuclei (mesured as chemical
shift parameters) are also anisotropic interactions and the angular dependences are averaged out in soultion state in which
only the isotropic values can be measured. The anisotropy of these interactions are characterized by principal-directions
and principal-values in the principal axis system (PAS). The anisotropic interaction parametrs have tensor forms with nine
components (3 x 3 matrix) in the orthogonal axis system of cartesian coordinates. In their principal axis system these matrices
are diagonal.The trace of the matrix is the isotropic value for the interaction. The anisotropy parameters and the assymmetry
of the interactions can be revealing rich structural informations. The magic angle spinning experiment (MASS) yields the isotropic
value for the interaction in solid state as much as what is obtainable in liquid state. Besides by the variation of this magic
angle spinning experiments, called the OFF-Magic Angle Spinning Experiments, and by processing the data on side-bands and
main-bands of the spinning sample spectra, it is possible to get the anisotropic parameters as well in favouarable cases.
Hence MASS technique and its variations are advantageous in cases when the soulte is insoluble in solvents,and information
on anisotropy parametrs is an important criterion for favouring such experimental measurements. (B.C.Gerstein,R.G.Pembleton,R.C.Wilson
and L.M.Ryan,J.Chem.Phys.,Vol.66,page 362(1977); J.Herzfeld and A.E.Berger, J.Chem.Phys.,Vol. 73, page 6021,(1980); M.M.Maricq
and J.S.waugh,J.Chem.Phys., Vol 70, page 3300 (1979) ).
A
combination of High Power decoupling and and MASS was demonstrated for rare spins (J.Schefer,E.O.Stejskal and R.Buchdahl,
Macromolecules Vol 10,page 384 (1977) )
In
the MASS experiment , the time - dependence rendered to the coordinte dependent interactions causes the averaging effect and
hence this is a technique based on Coordinate Space Averaging of the Spin Interactions in NMR.
This
must be clearly distinguished from the coherent averaging in spin space as elucidated in the Solid-State NMR monographs/books
by 1) U.Haeberlen, 2)M.Mehring, and 3) B.C.Gerstein et.al., which are cited in the Bibliogrphy listed out in one of the webpage
of this website.
