NMR

• The molecular ion peak in mass spectrometry gives the Mr
• Atoms with an odd mass number have nuclear spin and behave like tiny magnets.
• The process by which nuclei absorb energy and flip between two spin states is called resonance
• Nuclei in different chemical environments cause peaks in different positions in an NMR spectrum.
• In NMR spectroscopy, deuterated solvents such as CDCl₃ are commonly used because they do not contain protons
• The TMS standard in NMR spectroscopy appears at 0 ppm.
• TMS produces a peak upfield from other signals in the NMR spectrum.
• The units of the chemical shift scale in NMR spectroscopy are ppm
• The number of peaks in a ¹³C NMR spectrum corresponds to the number of different carbon environments
• Match the type of carbon with its chemical shift range in ppm:

  -C-C- ↔️ 5-40

  R-C-CI ↔️ 10-70

  R-C=O ↔️ 190-220
• Propane in a ¹³C NMR spectrum shows two peaks, both in the 5-40 ppm range.
• Benzene in a ¹³C NMR spectrum shows only one peak in the range of 110-160
• How many peaks does the ¹³C NMR spectrum of propan-2-ol show?

  3
• How many peaks does the ¹³C NMR spectrum of propanone show?

  2
• The ¹³C NMR spectrum of butanal shows 4 peaks.
• ¹³C NMR spectroscopy can distinguish between butanal and butanone.

  False
• How many possible mono-nitro products are formed in the nitration of methylbenzene?

  3
• ¹³C NMR spectroscopy can distinguish isomer 1 of nitrated methylbenzene but not isomers 2 and 3 because they have the same number of peaks.
• A tri-nitro product of methylbenzene with 5 peaks in its ¹³C NMR spectrum has some symmetry.
• What does the number of peaks in a ¹H NMR spectrum indicate?

  Hydrogen environments
• The chemical shift range for RCH₃ protons in ¹H NMR is 0.7-1.2 ppm.
• The chemical shift range for O-H protons in ¹H NMR is 3.1-4.2 ppm.
• What is the chemical shift range for protons in RCH₂CI or Br in ¹H NMR?

  3.1-3.9 ppm
• The relative area of a peak in a ¹H NMR spectrum is related to the number of protons in that environment.
• What is the n+1 rule used for in ¹H NMR spectroscopy?

  Peak splitting
• What is related to a peak in a ¹H NMR spectrum?

  Number of protons
• The relative heights of integration traces are proportional to the peak area and therefore to the number of H atoms in each environment.
• Arrange the following integration ratios from smallest to largest: 3:1, 6:4, 3:2

  1️⃣ 3:1

  2️⃣ 3:2

  3️⃣ 6:4
• The n+1 rule is used to determine the number of protons bonded to adjacent carbon atoms.
• Match the number of adjacent protons with the appearance of the signal in a ¹H NMR spectrum:

  0 ↔️ Singlet

  1 ↔️ Doublet

  2 ↔️ Triplet

  3 ↔️ Quartet
• O-H and N-H protons are not usually split because O and N are highly electronegative atoms.
• What splitting pattern is observed for an ethyl group in a ¹H NMR spectrum?

  Triplet-quartet
• An IR spectrum showing a broad peak in the region 3230-3550 cm⁻¹ indicates the presence of an O-H bond in an alcohol.
• What splitting pattern and integration ratio are observed for adjacent CH₂ groups in a ¹H NMR spectrum?

  Triplet, 2:2
• ¹H NMR spectra are obtained using samples dissolved in deuterated solvents or CCl₄.
• 3 Hydrogen, Triplet with 2 Hydrogen, Quartet

  Ethyl group
• 9 Hydrogen, Singlet

  $R-CH_3$ x3
• 3 Hydrogen, singlet

  isolated methyl group
• 2x 2 Hydrogen Triplet

  $-CH_2CH_2-$
• 6 Hydrogen, singlet with 1 hydrogen, multiplet (7)

  $CH_3CHCH_3$