Structure Determination

Question 1

How do you find the M_r of a compound from mass spectroscopy?

Answer 1

• Locate the molecular ion peak; M⁺
• This is located just before the M+1 peak
• Electrons in the spectrometer bombard the sample molecules and break electrons off, forming ions.
• Only the charged fragment, X⁺, is detected; uncharged fragments (free radicals) are not deflected.

M^+• → X⁺ + Y^•

E.g. for pentane
C₅H₁₀ → [C₅H₁₀]⁺ + e^-

• The mass/charge value of the molecular ion peak s the molecular mass.

Question 2

What speices give higher peaks?

Answer 2

• More stable X⁺ ion fragments give higher peaks; more abudnant, therefore higher peaks.
• Carbocations and acylium ions are two very stable fragment ions.

Question 3

How do you idenitfy structures and functional groups from mass spectroscopy?

Work out the stucture of a molecule.

Answer 3

1. Identify the fragments; read the mass/charge ratio and identify the fragments; e.g. a peak at 15 m/z is likely to have a CH₃ group, a peak at 17 m/z means it’s likely to have an OH group.
   The fragments add up to give the molecular ion, the second furthest one to the right.
2. Piece together to form a molcule with the correct M_r: ethanol has all the fragments on the spectrum (check if proposed molecule has all fragments), if molecular mass of the molecule you think it is matches to the m/z value of the M peak, it is it.

• Fragments all contain an ion and a free radical; each peak would add up to give the m/z of the molecular ion if the free radical were present.

Question 4

How do you use the M+1 peak to work out the number of carbon atoms in a molecule?

E.g. If a molecule has a molecular peak with a relative abundance of 44.13%, and a M+1 peak with a relative abundance of 1.41%, how many carbon atoms does it contain?

Answer 4

height of M+1 peak x 100 = no. of carbon atoms
height of M peak in a compound

1.41 x 100 = 3.195; so the molecule contains 3
44.13 carbon atoms.

Question 5

What information does nuclear magnetic resonance give you?

Answer 5

NMR spectroscopy tells you about the position of ¹³C or ¹H atoms in a molecule.

Question 6

How does ¹³C spec compare with ¹H spec?

Answer 6

• ¹³C n.m.r gives a far simpler spectrum than ¹H n.m.r.
• Carbon usually has 6 protons and 6 neutrons, thus it doesn’t have spin.
• A ¹³C spectrum gives you one peak corresponding to each different carbon environment in the molecule.
• Nuclei in different environments absorb different amounts of energy; a nucleus is partly shielded from the effects of an external magnetic field by its surrounding electrons. And other atoms and groups of atoms around a nucleus will also affect the amount of electron shielding.

Question 7

Why is chemical shift measured to tetramethylsilane (TMS) as a standard?

Answer 7

• It has 12 hydrogen atoms in all identical environments, so it produces a single absorption peak, well away from most other absorption peaks.
• It is inert; doesn’t react with the sample, is non-toxic, and volatile; easy to remove from the sample.
• Single peak produced by TMS is given a chemical shift value of 0.

Question 8

How does proton NMR work?

Answer 8

1. Count the number of peaks; no. of peaks = no. of environments
2. Look at the integration trace; tells you the area ratio, e.g. 1:3 = for every 1 H at δ= 11.5ppm, there are 3 H atoms in the other environment.
3. Use the table to tidentify the H atom causing the chemical shift
4. Use splitting patterns to find out more about structure; peaks split into the no. of hydrogens on the neighbouring carbon, +1.
   * *n + 1 rule**.

Question 9

What are the 1H n.m.r. samples dissolved in?

Answer 9

• Samples are dissolved in hydrogen-free solvents; otherwise would turn up on the spectrum.
• Deuterated solvents are often used; H atoms have been replaced by deuterium (D or ²H). Isotope of hydrogen with two nucleons; a proton and a neutron; doesn’t have a spin due to even no. of nucleons. (thus doesn’t create magnetic field)
• CCl₄ also used/