Exam (elaborations)
Introduction to Spectroscopy January exam 2019/2020
- Module
- CHEM 40003
- Institution
- Imperial College London (ICL)
Introduction to Spectroscopy January exam 2019/2020
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Introduction to Spectroscopy January 2020 answersIntroduction to Spectroscopy
Outline answers:
1)
a)
Use E (J) = hv (Hz) = hc/ (nm) = hcv (cm-1) 1 mark for the correct formulae
a) 4545.5 nm and 4.38x10-20 Joules 1 mark
b) 1.11x109 nm and 1.79x10-25 Joules 1 mark
0.5 marks each for the 4 correctly calculated values
(3 marks)
b)
Ni/Nj = exp (-E/kT) 1 mark
Need to convert 650 nm to J and use the J values from a): 3.06 x 10-19 J
Relative population at room temperature, 298K, Ni/Nj = 2.39e-5 for 2200 cm-1 (IR) 1mark
Ni/Nj = 4.84e-33 for 650 nm (UV/vis) 1 mark
Ni/Nj = 0.99996 for 270 MHz (NMR) 1 mark
0.5 marks for each correct value and 0.5 for correctly naming the technique
(4 marks)
c)
NMR<IR<UV/vis (1 mark)
The least sensitive is NMR, to increase its sensitivity we need to reduce the temperature
and this will reduce the ratio (2 marks) other valid answers may also be considered
(3 marks)
d) A = AMX6 B = AMX C = AMX2 (1 mark each correct answer)
Problem solving
Course content: identifying inequivalent nuclei and stating spin systems
(3 marks)
1
, Introduction to Spectroscopy January 2020 answers
e)
• correct number of resonances in spectrum; four distinct environments (1 marks)
• correct chemical shifts within a reasonable range (error max. 2 ppm) (1 mark)
• correct coupling patterns, 2 x doublet of doublets; 1 x triplet; 1 x quartet (2 marks)
• appropriate annotation on spectrum indicating coupling constants (1 mark)
• correct integration of signals (1 mark)
• maximum of 6 marks
Problem solving
Course content: annotating NMR spectra, labelling inequivalent nuclei and relating the
molecular structure to spectral data.
f)
Compound E
• comment on identification from mass spectrum data molecular ion peak at 120
m/z in relation to the saturation (1 mark)
• 19F NMR spectrum interpretation: triplet of triplets pattern can only be achieved
with the saturation of the double bond with hydrogen. Appropriate description of
this coupling pattern and its relation to the structure above must be given. (2
marks)
• Alternatively, an appropriate coupling tree could be used to explain the spectrum
in relation to the structure
• Comment on 2J and 3J value in discription (1 mark)
• No marks are given for just providing the structure of the molecule without
explanation
Problem solving
Course content: predicting the structure of molecules from the 19F NMR spectrum and
MS data
2
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