Question 1

Which of the following nuclei is NOT NMR active and why?

Accepted Answer

$$^{12}\mathrm{C}$$ because it has an even number of protons and neutrons.

Question 2

What is the reference compound used in both $$^{1}\mathrm{H}$$ and $$^{13}\mathrm{C}$$ NMR spectroscopy, and at what chemical shift does it appear?

Accepted Answer

Tetramethylsilane (TMS), $$0$$ ppm

Question 3

A proton NMR spectrum of ethylbenzene shows overlapping signals for some aromatic protons. What is the most likely reason for this overlap?

Accepted Answer

The electronic effect of the ethyl group is similar to that of a hydrogen.

Question 4

Which of the following statements about chemical shift ($$\delta) in$$ $$^{1}\mathrm{H}$$ NMR is correct?

Accepted Answer

Protons near electronegative atoms appear at higher $$\delta$$ values (downfield).

Question 5

A compound has the formula $$\mathrm{CH_3CH_2CH_2NO_2}. $$The $$^{1}\mathrm{H}$$ NMR spectrum shows signals at $$1.04$$ ppm, $$2.07$$ ppm, and $$4.37$$ ppm. Which set of protons is most deshielded?

Accepted Answer

The protons at $$4.37$$ ppm (adjacent to $$\mathrm{NO_2$})$$

Question 6

Which of the following best explains why the O–H signal in ethanol is broad and sometimes disappears upon addition of $$\mathrm{D_2O}$$?

Accepted Answer

Rapid hydrogen exchange with $$\mathrm{D_2O}$$ replaces O–H with O–D, removing the signal.

Question 7

In $$^{1}\mathrm{H}$$ NMR, what is the expected multiplicity for the methylene protons in $$\mathrm{CH_3CH_2Cl}$$?

Accepted Answer

Quartet

Question 8

Which of the following best describes the (N+1) rule for $$^{1}\mathrm{H}$$ NMR splitting?

Accepted Answer

A proton with $$N$$ equivalent neighboring protons will be split into $$N+1$$ peaks.

Question 9

Which of the following compounds will show only one signal in its $$^{1}\mathrm{H}$$ NMR spectrum?

Accepted Answer

$$\mathrm{C_6H_6} ($$benzene)

Question 10

A quartet in $$^{1}\mathrm{H}$$ NMR has a 1:3:3:1 intensity ratio. What is the most likely cause?

Accepted Answer

Splitting by three equivalent neighboring protons.

Question 11

Which of the following statements about $$^{13}\mathrm{C}$$ NMR is FALSE?

Accepted Answer

Integration of $$^{13}\mathrm{C}$$ NMR signals gives the number of hydrogens attached to each carbon.

Question 12

Which of the following best describes the effect of increasing the applied magnetic field ($$B_0) on $$the energy difference between $$\alpha$$ and $$\beta$$ spin states?

Accepted Answer

The energy difference increases.

Question 13

Which of the following protons would appear furthest downfield (highest $$\delta$$ value) in a $$^{1}\mathrm{H}$$ NMR spectrum?

Accepted Answer

Aldehyde proton ($$\mathrm{RCHO})$$

Question 14

Which of the following is a real-world application of NMR technology?

Accepted Answer

Magnetic Resonance Imaging (MRI) for medical diagnostics

Question 15

A $$^{1}\mathrm{H}$$ NMR spectrum of a compound shows a singlet at $$2.1$$ ppm integrating for $$3$$ protons. Which group is most likely present?

Accepted Answer

Methyl group adjacent to a carbonyl ($$\mathrm{$$COCH_3$$})$$

Question 16

Which of the following statements about enantiotopic hydrogens is correct?

Accepted Answer

They are chemically equivalent and indistinguishable by NMR.

Question 17

Which of the following best explains why the $$^{1}\mathrm{H}$$ NMR spectrum of cyclohexane-d$$_{11}$$ shows one sharp signal at room temperature but two signals at low temperature?

Accepted Answer

Chair–chair interconversion is fast at room temperature but slows at low temperature.

Question 18

Which of the following coupling constants ($$J) $$would you expect for trans-vinylic protons?

Accepted Answer

$$15 Hz$$

Question 19

Which of the following statements about the splitting of $$^{13}\mathrm{C}$$ NMR signals is correct?

Accepted Answer

The number of peaks for a carbon is $$N+1$$, where $$N is $$the number of hydrogens attached to that carbon.

NMR Spectroscopy: Principles and Applications in Organic Chemistry

Guida di studio - Domande di pratica

Study guide - Flashcard