Nmr inorganic chemistry 2019 compact

Spectroscopic Methods in
Inorganic Chemistry
Part 3: NMR
Dr. Chris, August 2019
Rotating Charge produces a
magnetic moment μ
For ELECTRONS that means
that molecules become dia- or
paramagnetic.
For the NUCLEUS it means that
it gets a magnetic moment
that can be adjusted to an
outer magnetic field.
The spin I of particles is QUANTIZISED, means it can
only have certain values
For the nucleus of elements, the spin depends on the
number of protons and neutrons:
Most important for NMR are elements with I = ½
For bigger I there is a quadrupole moment which affects
the broadness of the peaks
NMR active elements
http://www.fhi-
berlin.mpg.de/acnew/department/pages/teaching/pages/teaching__wintersemester__2007_2
008/pelzer_nmr_301107.pdf
Printed with FinePrint trial version - purchase at www.fineprint.com

In a strong magnetic Field B0 the spins can have the
values + or – ½.
The energy difference between these states is: The E is expressed as
frequency  in MHz:
In a magn. Field of 2.35 T
this energy difference for 1H
would be 100 MHz.
(We call the instrument
“100 MHz spectrometer”)
The induced magnetization is
measured (not the absorption of
radio waves)
= h * 
NMR Spectrometer
see: https://www.youtube.com/watch?v=sfiQFQYgJuQ
https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/spectrpy/nmr/nmr1.htm
Energies involved
Energy difference between two spin states is less
than 0.1 cal/mole
Compare:
Vibrational transitions (IR) 1,000 cal/mole
Electronic transitions (UV) 1,000,000 cal/mole
∆E proportional to B0
→ Strong magne c ﬁelds (1-20 T ≈ 20-900 MHz for 1H)
Earth's magnetic field ≈10-4 T

RF Pulse Experiment
The sum of all
magn.moments is a vector
that rotates around B0
B1 as magn.field
perpendicular to B0
causes the
magn.moments to move
towards the y-axis
Fourier Transformation
All possible resonance frequencies are sent together
instead of going from slowly from low to high -
e.g. in the case of 4 different proton signals:
https://www2.chemaistry.msu.edu/faculty/reu
sch/virttxtjml/spectrpy/nmr/nmr2.htm#pulse

Chemical Shift δ
The magn.Field B0 also
affects the electrons moving
around the nucleus.
They will align with the B0
field, creating a local field
opposing B0 for the nucleus.
The nucleus is shielded by
the electrons.
Spin – Spin coupling
Spin Coupling
http://orgchem.colorado.edu/Spectroscopy/nmrthe
ory/splitting.html
Example

Calculate J (in Hz)
The coupling constant J must be independent of the
instrument (the used magnetic field):
Convert δ to Hz
Δδ = 0.07 ppm
The measuring frequency was 200 MHz
J = 0.07 1/106 x 200 MHz = 14 Hz
13C NMR
13C has I = ½; its natural abundance is 1.1%
• 13C sensitivity is only 1/5700 that of 1H
• 13C experiments require higher
concentrations and more scans/time
• S/N increases with square root of # of scans
1H coupled and de-coupled
How would the 13C NMR spectrum of:
CH3 – CH2 – CH - CH3
|
Br
look like with and without 1H-13C coupling ?
Step 1: identify how many different C atoms ?
Step 2: estimate chemical shift
Step 3: estimate coupling to adjacent protons.

Exercise
How would the spectrum look like for
1H NMR
19F NMR
31P NMR
(all have I = ½)

NMR OF NUCLEI WITH SPIN > 1/2

Example 1: Compare CH4 and BH4
-
How would the 1H NMR spectra look like ?
 Consider that C has an NMR-active isotope 13C
 11B has a nuclear spin of 3/2
https://www.youtube.com/watch?v=x6lRDdR4Dj4
1H NMR of pure 13CH4
The protons are
equivalent but they
couple with the 13C
-> doublet
CH4 contains 1.1% of 13C -> coupling between the H
would result in a doublet
13C – 1H coupling
And a main peak in the middle from 1H alone
1H NMR of CH4
1.1% of the molecules have 13C -> doublet,
But 99% have no coupling

B has I = 3/2 => possible ml are 3/2, ½, -½ and -3/2
E
B0
l
4 possible spin orientations
 3 possible transitions
(in a 11B NMR spectrum !)
 Δml = +/- 1
-3/2
+3/2
+1/2
-1/2
The 4 protons can interact with 4
different B spin states => quadruple
signal (4 peaks in 1H NMR)
https://www.youtube.com/watch?v=ApCTiSjKg5s
1H NMR of BH4
-
1H NMR of BH4
-
The 4 protons are equivalent – they couple with the 11B
Because 11B has 4 different energy states, we get a quartet
How about 11B NMR of BH4
- ?
The interactions of the B-spin with the 4 H-spins
The 4 protons can have 5 different total spins:
½ ½ ½ ½ Spin 2 -> 1 way
½ ½ ½ -½ Spin 1 -> 4 ways
½ ½ -½ -½ Spin 0 -> 6 ways
½ -½ -½ -½ Spin -1 -> 4 ways
-½ -½ -½ -½ Spin -2 -> 1 way
Splitting pattern by spin ½ nuclei
1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
Number of peaks: 2 x I x n + 1
( = 2 x ½ x 4 +1 = 5 )
“Pascal triangle”

Example 2: Diborane
Consider only the 2 H in the bridge
 What is the 1H NMR spectrum
 What is the 11B NMR spectrum ?
11B spectrum
2 equivalent B atoms couple with
2 equivalent protons
 2 (1/2) 2 + 1 = 3 signals
Coupling by spin ½ nuclei ->
use the Pascal triangle
1H NMR spectrum
Number of peaks: 2*I*n + 1 = 2*(3/2)*2 + 1 = 7
The proton interacts with 2 B spins -> no Pascal triangle !
which can have the following orientations:
3/2 3/2 I = 3 1 way
3/2 ½ and ½ 3/2 I = 2 2 ways
½ ½ and 3/2 -½ and -½ 3/2 I = 1 3 ways
½ -½ and -½ ½ I = 0 4 ways
and 3/2 -3/2 and -3/2 3/2
…… and backwards: 3 / 2 / 1 -> in total 7 peaks
Splitting by spin-3/2 (11B)
NO Pascal triangle !!
We have to find out all
possible spin
combinations of the
two 11B !

How would the spectra change if
we had 3 bridging Hydrogens ?
19F NMR example (spin ½)
To calculate the coupling
constant J, we have to
convert the difference in
δ to Hz
Δδ = 0.07
The measuring
frequency was 282 MHz
J = 0.07 1/106 x 282 MHz
= 20 Hz

We refer this simply to
be a multiplet
Read the spectrum from left to right
Solid State NMR (SSNMR)
http://www.fhi-
berlin.mpg.de/acnew/department/pages/teaching/pages/teaching__wintersemester__2007_2
008/pelzer_nmr_301107.pdf
DYNAMIC NMR

Principle
How would the spectrum look like if the groups
A and B and “frozen” and if they can quickly
interchange ?
Example: Methanol

Example: Re Complex
Fast change in coordination of the
bidentate ligand at high T
-> only 2 groups of 1H atoms
At low T (< 0 deg) the exchange is
slow and 4 groups of 1H atoms can
be detected.

Nmr inorganic chemistry 2019 compact

More Related Content

What's hot

Similar to Nmr inorganic chemistry 2019 compact

More from Chris Sonntag

Recently uploaded

Nmr inorganic chemistry 2019 compact