HW 3 Multinuclear NMR Name:
Points: C6800 Date:
Max. 100 points Spring 2017 Version A
1. (5 pts) Photolysis of HSiCl3 provides a compound with an empirical formula Si5Cl12. The
product displays two resonances in the 29
Si NMR spectra in a 4:1 intensity ratio. Derive
structural formula of this molecule.
2. (12 pts) How would you distinguish these two isomers? Give the symmetry point groups of
the molecules. Calculate chemical shifts of aromatic carbons (use your table of 13
C
increments).
OH
OH
3. The 13
C{1
H} NMR spectrum in the picture belongs to Li[Al(CH3)4] in dimethoxyethane.
a) (10 pts) Explain multiplicity (number of lines) of the signal, intensities of lines in the
multiplet, and mark a coupling constant in the spectrum.
b) (5 pts) Which property of this molecule allows observing the splitting? Give the symmetry
point group.
c) (5 pts) How would this spectrum change after turning off {1
H} decoupling?
d) (5 pts) Designate this spin system by prime notation (consider only H, C, and Al nuclei).
4. Consider two derivatives of the following anion:
A: R = F, B: R = CHMePh
a) (2 pts) Draw clearly all symmetry elements present in the anion A.
b) (6 pts) Give the symmetry point group of anions A and B.
c) (8 pts) Are there any geminal groups in A? If yes, what class do they
belong to? homotopic (H), enantiotopic (E) and diastereotopic (D)
d) (8 pts) How many resonances do you expect in the 19
F NMR spectra of
A and B.
e) (5 pts) How would you explain, that in A there is actually only one
signal at room temperature.
5. (20 pts) How would you explain the number of signals in the 29
Si NMR spectra the
following molecules:
a) R = Ph, 145.9;
b) R = (R,S)-CHMePh, 146.1, 146.4, 146.6;
c) R = (R)-CHMePh, 146.2, 146.8;
6. (9 pts) The anion Tl3Se7
5has
symmetry point group C3v. Consider the following
isotopologue enriched to 100% both in Se = 77
Se, and Tl = 205
Tl:
Designate this spin system by prime and bracket notations: