User:Benjah-bmm27/degree/3/NCN

NMR

General

Common spin-½ nuclei

Nucleus	Abundance
1H	100%
19F	100%
31P	100%
103Rh	100%
183W	15%
195Pt	33%

Quadrupolar nuclei

Quadrupolar nuclei (those with a spin I > ½) relax quickly as a direct result of the Heisenberg uncertainty principle (see Spectral line#Broadening due to local effects). This results in broad lines, limiting the usefulness of NMR for studying quadrupolar nuclei.

Chemical shift anisotropy

Even spin-½ nuclei sometimes show broad lines, due to chemical shift anisotropy. In non-spherical molecules, the same nuclei have different chemical shifts in different directions in the solid state. In solution, tumbling averages these differences if the difference between chemical shifts in parallel and perpendicular directions, Δδ < 5000 Hz. This is the case for protons but not for ¹⁹⁵Pt, where Δδ can be as much as 100,000 Hz.

High-field NMR is not suitable for nuclei with large Δδ, as Δδ in Hz increases with the strength of the magnetic field used. 60 MHz spectrometers are therefore better suited to ¹⁹⁵Pt NMR, as much less line broadening occurs.

Chemical shift

Nuclei other than ¹H exhibit large ranges of chemical shifts, δ.

Effect of screening

Chemical shift is determined by the magnetic field at the nucleus.

$B=B_{0}\left(1-\sigma \right)\,\!$

• B: magnetic field at the nucleus
• B₀: applied magnetic field
• σ: screening constant

Screening constant

$\sigma =\sigma ^{D}+\sigma ^{P}\,\!$

• σ^D: diamagnetic contribution to the screening constant, due to electron density
• σ^P: paramagnetic contribution to the screening constant

Paramagnetic screening

$\sigma ^{P}\propto {\frac {1}{^{3}\Delta E}}\,\!$

• ³ΔE is the difference in energy between the ground state and low energy excited states
• A smaller ³ΔE results in more mixing of the ground and excited states and so a larger value for σ^P
• ¹H in C-H bonds has no low energy excited states, so has a small δ range of 0-10 ppm
• ³¹P has some low energy excited states, so Δδ ~ 300 ppm
• ¹⁹⁵Pt has many low energy excited states, so Δδ ~ 1000 ppm
• σ^P is difficult to predict, which makes chemical shifts difficult to predict

Coupling constants

Square planar and octahedral complexes

For example, [(Ph₃P)₂PdCl₂]. Can have cis or trans phosphines.

²J_PP(cis) ~ 10-20 Hz

²J_PP(trans) ~ 200-400 Hz

• Cause: the two phosphines bond to the metal via the same orbital when trans, but via different orbitals when cis.
• Consequence: easier magnetization transfer and thus much greater coupling constants for trans phosphines.

Fermi contact interaction

Coupling is transmitted through electron density. A and B are two nuclei, and can be the same element or different.

$J_{AB}\propto \gamma _{A}\;\gamma _{B}\;S_{A}^{2}(0)\;S_{B}^{2}(0)\;P_{AB}^{2}\,\!$

• γ: magnetogyric ratio
• S²(0): s electron density at nucleus
• P²(AB): s character in A-B bond

Also important in ESR, see Fermi contact interaction.

ESR

Comparison with NMR

Technique	Line origin	Coupling constant
NMR	chemical shift, δ	J-coupling, J
ESR	Landé g-factor, g	Hyperfine coupling, A