Topic 11: Measurement & Data Processing

Question 1

function of IR spectroscopy

Answer 1

• identifies the bonds in a molecule

- since different bonds absorb different frequencies

Question 2

how mass spectrometry works

Answer 2

• the gaseous molecules are shot with e-s to form cation fragments
• the peak in the mass spectrum with the greatest mass comprises of ions that passed through unscathed
• but other peaks appear as well (the cation fragments)
• the ‘fragmentation patterns’ are then used to analyze the composition of the molecule
• REMEMBER! generally the fragment that gives the most stable ion is formed!

Question 3

index of hydrogen deficiency

Answer 3

• tells us how unsaturated the molecule is

- the number tells us how many H2 molecules can be added

Question 4

how IR spectroscopy works

Answer 4

• IR is absorbed by polar bonds and will cause stretching/bending
• the change in energy produces a change in the molecule’s dipole moment
• the intensity of the absorption depends on the bond polarity (more polar = more stretch/bend observed)

Question 5

how IR spectroscopy works

Answer 5

• IR is absorbed by polar bonds and will cause stretching/bending
• the change in energy produces a change in the molecule’s dipole moment
• the intensity of the absorption depends on the bond polarity (more polar = more stretch/bend observed)

Question 6

why is it that only polar bonds can interact w electromagnetic radiation?

Answer 6

• polar bonds have separate areas of partial positive and negative charges
• this allows the electric field component to excite vibrational energy

Question 7

trends in wavelength required to induce stretch/bend

Answer 7

• symmetric stretch/bend takes less energy than asymmetric
• multiple bonds need higher frequencies
  e. g. ≡ 𝐶 − 𝐻 has higher wavenumber (≈3300) than = 𝐶 − 𝐻 (≈3100); and = 𝐶 − 𝐻 has higher wavenumber than −𝐶 − 𝐻 (≈2900)
• in terms of hybridization, sp hybridization has 50% s-orbital character, sp2 has 33% s-orbital character and sp3 has 25% s-orbital character

Question 8

how nuclear magnetic resonance (NMR) spectroscopy works

Answer 8

• uses radio waves to see how much energy is required to stimulate H+ in an element
• using radio waves to see how much energy it is required to stimulate the
  protons (H+
  ) in an element. Hydrogen atoms in different position in the molecule will require
  different amount of energy

Question 9

principle behind NMR spectroscopy

Answer 9

• a proton has a spin that creates a magnetic field, just like a magnet bar
• proton in its low energy state spins parallel to an external magnetic field (its N gets attracted to the southern magnetic pole, which is geographically the North Pole, and its S gets attracted to the northern magnetic pole, which is geographically the South Pole)
• in NMR, energy is applied in the form of radio waves to flip the direction of magnetic field of hydrogen atoms
• when these flipped protons flip back again to its low energy state, each one gives out energy that is recorded in the NMR spectrometer

Question 10

NMR spectroscopy: what number of peaks tells you

Answer 10

tells you the number of possible “chemical environments” a hydrogen can be in

Question 11

NMR spectroscopy: what the areas under the peaks tell you

Answer 11

i. e. integration trace
- tells you how many protons are sending the signal
- essentially reveals the ratio of H in the molecule

Question 12

NMR spectroscopy: chemical shift

Answer 12

• the energy required to flip the proton in relation to energy required to flip TMS (tetramethylsilane, Si(CH3)4)
• value depends on the environment

Question 13

NMR spectroscopy: splitting pattern

Answer 13

AKA spin-spin splitting/coupling

• the signals/peaks are actually split, not straight
• this is because a proton’s magnetic field is influenced by neighboring protons
• neighboring protons may spin in the same direction (thus intensifying the magnetic field) or in the opposite direction (thus decreasing the magnetic field)
• this is because each proton can spin either up or down with equal probability
• number of splitting = number of neighbor protons + 1

Question 14

how x-ray crystallography works

Answer 14

• it sends X-ray radiation on a sample at an angle θ
• we can deduce the bond length, bond angle, and the structure from the reflected radiation (either constructive or destructive)

Question 15

why is TMS used as the standard for NMR?

Answer 15

• it has one peak; all hydrogens are in identical chemical environment
• it is inert and has low boiling point (easily separated from other compounds)
• silicon absorbs different wavelengths from carbon-hydrogen bonds (so it doesn’t interfere)

Question 16

calculating chemical shift

Answer 16

chemical shift = (shift in relation to TMS / frequency of spectrometer) * 10^6