solid state nmr section 2.3-

What is the difference between the direct and indirect dipole-dipole interactions?

Indirect is the J coupling and is through bonds (electron-coupled)

Direct dipole-dipole is through space and directly between two nuclei

What are the types of direct dipole-dipole interactions?

Homonuclear - both nuclei are the same type

Heteronuclear - the two nuclei are different types

What is an isolated spin pair?

A heteronuclear direct dipole-dipole interaction between two spin a half nuclei that only interact with each other

What is the tensor that describes the isolated spin pair? What will the spectrum look like?

Dipolar coupling tensor

The a axis of this tensor corresponds to the direction of the internuclear vector

The spectrum will have two lines with frequencies:

Viso ± ½ D (3cos2(theta) - 1)

What is the dipolar coupling tensor defined as?

GammaA x GammaA (h/2pi) (muo/4pi) 1/r(cubed)

Gamma are the gyromagnetic ratios of the two types of nuclei involved in the interaction

The brackets are constants, with muo meaning the permeability constant

R is the internuclear distance

For a heteronuclear direct dipole-dipole interaction between 2 spin ½ nuclei that interact just with each other, what does the spectrum look like and how can the splitting be calculated?

For a specific orientation of the internuclear vector r relative to the applied magnetic field

Get two peaks separated by dipolar splitting

D (3cos2(theta) - 1)

This is the distance between the peaks

What can be learned from heteronuclear direct dipole-dipole interactions?

The dipolar splitting is independent of the magnitude of the applied magnetic field

It is a short range interaction, the magnitude of the dipolar coupling is significantly higher when the internuclear distance is short

R can be determined by measuring D from the spectrum, which is most straightforward when one dipolar interaction dominates the spectrum (isolated spin pair)

Analogous pair of lines observed with the same dipolar splitting

For a heteronuclear direct dipole-dipole isolated pair of spin ½ nuclei, how does the spectrum change with orientation of the crystal for a single crystal measurement?

Theta = 0, distance between peaks is 2D, 3cos2(theta) - 1 = 2

Theta = 90, distance between peaks is D, 3cos2(theta) - 1 = -1

Theta = 54.7, 3cos2(theta) - 1 = 0, only one peak

For a heteronuclear direct dipole-dipole interaction between two spin ½ nuclei, what does the powder nmr spectrum look like?

The spectrum has two broad peaks that join in the middle at the isotropic frequency (theta = 54.7)

The largest peaks are where theta = 90, at viso ± ½ D

The smaller peaks are where theta = 0, at viso ± D

This is called the dipolar powder pattern

What can be said about direct dipole-dipole interactions between heteronuclei?

Is one nucleus experiences these interactions with more than one nucleus, the spectrum becomes complicated

As it depends on the internuclear distance r, it is dominated by interactions with nuclei in close proximity to the measured nucleus

They are important when the observed nucleus has another type of nmr active nucleus with high isotopic abundance in close vicinity

When are homonuclear direct dipole-dipole interactions important?

When the isotope is abundant and densely distributed in the solid

it is more likely to have two nuclei of a given type closer to each other in this case