solid state nmr section 4 part 1

What are some nmr active nuclei and their spin states?

Spin I = ½ 1H, 13C, 15N, 19F, 29Si, 31P

Spin I = 1 2H, 14N

Spin I = 5/2, 17O, 27Al

What are some high and low abundance isotopes?

High = 1H, 12C, 14N, 16O, 19F, 31P, 27Al

Low = 2H, 13C, 15N, 17O, 29Si

What are some important anisotropic nmr interactions in solids?

Shielding aniosotropy (chemical shift)

Direct dipole-dipole interaction

Quadrupolar interactions (I = 1)

Indirect dipole-dipole interaction (j coupling)

why are powder nmr spectra usually very broad?

they are the summation of the spectra for all crystal orientations

which aniosotrpic nmr interactions depend on the orientation of the crystal relative to Bo?

shielding (chemical shift) and indirect electron coupled dipole-dipole interaction (j coupling) and quadrupolar interactions

indirect can normally be ignored in solids as it is much weaker than the direct interactions

which anisotropic nmr interactions depend on the orientation of the internuclear vector relative to Bo?

direct through space dipole-dipole interaction

also: high res solid state 1H nmr challenging due to strong 1H 1H direct interactions

how can anisotropic nmr interactions be averaged for 13C nmr?

high power heteronuclear decoupling - remove direct 13C - 1H dipole-dipole interactions

magic angle spinning - remove 13C chemical shift anisotropy, not fast enough to remove 13C - 1H interactions

direct 13C-13C not significant bc low abundance

no quadrupolar interaction bc 13C has I = ½

why is high res 13C nmr used?

characterise polymorphs

determine the number of independent molecules in a crystal structure

determine the symmetry of the site occupied by a molecule in a crystal

detect disorder

identify dpecific intermolecular interactions

determine aspects of molecular conformation

what does the number of peaks in high res 13C nmr depend on?

details of the crystal structure (including symmetry) and not the symmetry of the isolated molecule

what is a polymorph?

set of materials containing the same molecule in different crystal structures

what is an asymmetric unit?

smallest part of the unit cell that can be used to make the full crystal structure using the crystallographic symmetry operations

use translation operations to replicate the unit cell in 3D space

how can you use solid state nmr to determine if an OH group is engaged in hydrogen bonding?

prepare the material with a deuterated OH group

record the solid state 2H nmr spectrum

the value of the 2H nmr quadrupole coupling constant is sensitive to the O-D -O distance in the hydrogen bond

chi is bigger from a non-hydrogen bonded group