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Písemná zkouška Multinukleární NMR
spektroskpie
Jméno:
Body: C6800 Datum:
Max. 100 bodů Jaro 2009 Varianta A
1. (10 pts) The reaction of Ph3SnLi with H3Si-O-SO3C4F9 provided a salt Li(O-SO3C4F9) and
a compound A, that displayed in the 1
H NMR spectrum a multiplet in the aromatic region
(7.0-7.5 ppm) and another signal at 3.66 ppm.
a) Draw the structure of the product A and assign the symmetry point group label.
b) Describe in detail the appearance of the signal at 3.66 ppm.
c) Describe the expected signals of other NMR active nuclei in the molecule A (not for
C6H5).
HINT: A typical value of 1
JSiH for silyl-germyl hydrides, such as (GeH3)xSiH4-x, is 200 Hz. Coupling constants
2
JSnH with values of around 60 Hz were found in HSi(SnMe3)3 and HSiR2’SnR3 compounds.
2. (10 pts) Consider the following molecule:
a) Find all symmetry elements and sketch them in the picture.
b) Assign the symmetry point group label to this molecule.
c) Draw schematically the 1
H NMR spectrum of this
compound. Important is the number of signals, number of
lines in the multiplets, intensities of the lines. The actual
order of signals on the ppm scale is not important, CH2 are
at a lower field than CH3.
N
Ga
N
Ga
H
H
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3. (10 pts) Consider the following molecule:
a) Assign the symmetry point group label.
b) Label the spin systems of the methine hydrogens using the spin system notation.
c) Give their approximate values of the chemical shift.
d) Explain the values of the chemical shift −4.25 and 21.0 ppm of the methyl groups.
CH3
CH3
CH3
CH3
2-4.25
ppm 21.0 ppm
4. (10 pts) Assign the signals in the 1
H NMR spectrum of compound 4a to the particular CH3
group and explain your assignment – chemical shift and coupling multiplicity.
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5. (10 pts) Label hydrogens in geminal groups in the molecules 4a, b, c as Homotopic,
Enantiotopic, Diastereotopic.
Based on this knowledge, propose a method to study the kinetics of the following
interconversion of two enantiomeric helical conformers of 4a. How would you measure the
rate constant of this process.
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6. (10 pts) The 60 MHz 1
H NMR spectrum of an AB system gives the following four peaks
(in Hz from TMS): 423, 418.5, 416, 411.5. Calculate:
a) δA and δB in ppm, and JAB in Hz
b) The positions of the four peaks if the spectrum is acquired at 300 MHz (in both Hz and
ppm).
7. (10 pts) Draw the 31
P{1
H} NMR spectrum of the Nb complex (R = NMe2, no P-N coupling
observed):
a) Mark clearly with an arrow the position of the chemical shift δ = 17.9 ppm.
b) Label clearly 2
JNbP = 279 Hz.
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8. (10 pts) Consider magnetic and chemical equivalence: Label the spin systems using the
spin system notation. Only take into account high abundance spin-1/2 nuclei (e.g. no C).
9. (10 pts) The P2Se8
2anion
is present in the solution as chair and twist conformers.
Interconversion between the conformers at ambient temperature is slow on the NMR time
scale. Consider for now only the chair conformer.
a) Give the symmetry point group of the anion.
b) Explain the satellite signals in the 31
P{1
H} NMR spectrum.
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10. (10 pts) How many resonances will you observe in a 14
N NMR spectrum of NH4NO3?
Which resonance(s) will be sharp and which will be broad? Explain why.