NMR Spectroscopy MCQ Quiz in தமிழ் - Objective Question with Answer for NMR Spectroscopy - இலவச PDF ஐப் பதிவிறக்கவும்

The Correct Answer is 4.

Concept:-

NMR: 

• NMR spectroscopy relies on the magnetic properties of certain atomic nuclei, particularly those with an odd number of protons or neutrons.
• When placed in a magnetic field, these nuclei can absorb and emit electromagnetic radiation (radio waves) at characteristic frequencies.
• Chemical Shift indicates the electronic environment around the nucleus. Different environment cause different chemical shift.
• Number of peaks in a NMR spectroscopy is 2nI + 1, where I is nuclear spin.

Explanation:-

The ¹H-NMR spectrum of [Ru(η⁴ - C₈H₈)(CO)₃] at low temperature (-140^oC) is different 1H-NMR signals from C₈H₈ ring broadened approximately equally. There is a possibility of fluxional process which result in random shift.

Conclusion:-

The number of signals of complex [Ru(η⁴ - C₈H₈)(CO)₃] observed at low temperature (–140 °C) in its spectrum is Four.

The Correct Answer is Option 1.

Concept:-

Mass Spectroscopy

Mass spectrometry is an analytical technique used to identify and quantify the chemical composition of a sample based on the mass-to-charge ratio of charged particles.

The basic principle of mass spectrometry involves the following steps

Ionization: The sample is ionized to generate charged particles (ions). This can be achieved using various ionization techniques, such as electron impact ionization (EI), electrospray ionization (ESI), matrix-assisted laser desorption/ionization (MALDI), or chemical ionization (CI).

Mass analysis: The ions generated in the ionization step are then separated based on their mass-to-charge ratio (m/z) using a mass analyzer. The most abundant ion formed during ionization gives the tallest peak in mass spectrum. This peak is known as Base Peak.

During the ionization process molecule might absorb some extra energy and this extra energy can be lost by the fragmentation of molecule. One of the important fragmentation pattern is Mc- Lafferty Rearrangement. 

The Mc- Lafferty rearrangement typically occurs in molecules containing a carbonyl group (such as ketones, aldehydes, and esters) adjacent to a carbon-carbon double bond or aromatic ring. The rearrangement involves several steps:

1. Formation of a cyclic transition state: The carbonyl oxygen atom attacks the adjacent carbon-carbon bond, forming a cyclic transition state. This transition state involves the migration of a hydrogen atom from the beta position (adjacent to the carbonyl carbon) to the carbonyl oxygen atom, leading to the formation of a five-membered cyclic transition state.
2. Cleavage of the bond: The cyclic transition state undergoes cleavage of the bond adjacent to the carbonyl group, resulting in the formation of a neutral molecule and a fragment ion. The neutral molecule typically contains a terminal double bond or aromatic ring, while the fragment ion retains the carbonyl group.
3. Fragmentation: The neutral molecule and the fragment ion are detected in the mass spectrum, providing information about the structure of the original molecule.

Mc- Lafferty Rearrangement in aldehydes is as follows-

¹H- NMR Spectroscopy- NMR gives the information about the number of magnetically distinct atoms of the type being studied, e.g. Hydrogen and Carbon nuclei.

Chemical shift is obtained in NMR which expresses the amount by which a proton resonance is shifted from the standard(TMS).

Chemical shift value of various protons are-

• R-CH₃- 0.7-1.3
• R-CH₂-R- 1.2-1.4
• R₃-CH- 1.4-1.7
• C₆H₆- 6.5-8.0

Explanation:- 

¹H NMR: δ,7.9 (d, J = 8 Hz, 2H), 6.6 (d, J = 8 Hz, 2H)- indicates the presence of para- substituted aromatic ring.

Thus, Correct structure of compound is 

Also from Mass: m/z 165, 137, 120, 92

Conclusion:- 

Data from both ¹H- NMR and Mass suggests that Option 1 is the correct structure.

The correct answer is The protons are better shielded 

Concept:-

• Chemical Shift Definition: Chemical shift is the measure of the difference in resonance frequency of a nucleus in a sample compared to a reference standard (TMS).

Explanation:-

• Chemical Shift Standard: TMS (Tetramethylsilane) is widely used as an NMR reference standard because its proton NMR signal is assigned a chemical shift of exactly 0 ppm. This makes it a convenient reference point for comparing and analyzing other NMR spectra.
• High Symmetry: TMS possesses a high degree of symmetry due to the presence of identical methyl groups. This symmetry minimizes magnetic anisotropy effects, contributing to the sharp singlet observed in its NMR spectrum.
• Effective Shielding: The symmetric arrangement of methyl groups around the silicon atom results in effective shielding of the protons, making them less susceptible to external magnetic influences.
• Better Shielding: TMS is chosen as a reference because its protons are highly shielded. Shielding results in a lower effective magnetic field experienced by the protons, leading to a downfield shift in the NMR spectrum.
• TMS is chemically inert, which means it does not readily react with other substances in the sample.
• Its non-polar structure, containing only carbon and hydrogen atoms, makes TMS suitable for use in a wide range of solvents and sample matrices without introducing unwanted interactions.
• TMS is the preferred NMR reference standard because of its well-defined chemical shift, high symmetry, effective shielding of protons, inert nature, and non-polar structure.

Conclusion:-

So, TMS is a universally preferred NMR reference because The protons are better shielded 

Concept:

• Proton nuclear magnetic resonance or 1H NMR is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen nuclei within the molecules of a substance, in order to determine the structure of its molecules.
• Proton NMR spectra are characterized by chemical shifts in the range of +14 to -4 ppm and by spin-spin coupling between protons. The integration curve for each proton reflects the abundance of the individual protons.

Explanation:

• The compound (C5H5)2Fe(CO)2 contains both \(\left( {{{\rm{\eta }}^{\rm{1}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) and \(\left( {{{\rm{\eta }}^{\rm{5}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) ligands.
• The 1H NMR spectrum at room temperature shows two singlets of equal area.
• This is because the five equivalent protons of the \(\left( {{{\rm{\eta }}^{\rm{1}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) ring will show one singlet in 1H NMR spectra.
• While, the \(\left( {{{\rm{\eta }}^{\rm{1}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) ring will also show one singlet in 1H NMR spectra, although the protons are not all equivalent.

• This is because of the “ring Whizzer” mechanism, by which the five ring positions of the monohapto ring of \(\left( {{{\rm{\eta }}^{\rm{1}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) interchange via 1,2-metal shift so rapidly at room temperature that the NMR spectrometer can detect only the average signal for the ring.
• At lower temperature, this process is slower, and the different resonances for the protons of \(\left( {{{\rm{\eta }}^{\rm{1}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) become apparent at lower temperatures, the peak at 4.5 ppm \(\left( {{{\rm{\eta }}^{\rm{5}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) remains constant, but the other peak at 5.7 ppm for \(\left( {{{\rm{\eta }}^{\rm{1}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) spreads and then splits into four peaks of intensity ratio (5 ∶ 2 ∶ 2 ∶ 1).

​Conclusion:

Hence, the hapticities of C5H5- are η5 and η1

Concept:

• In the absence of an external magnetic field, the two components of the proton spin have the same energy, they are degenerate.
• When there is an external magnetic field present, the components have different energy which is expressed by saying that 'the magnetic field removes the degeneracy of the spin components'. This is known as Zeeman Effect.
• The energy difference between the two components is given by:

Δ E = gNβNBz, where g = gyrometric ratio, B = strength of the electric field.

• In order to induce a transition between the two energy states, i.e, taking a proton from an upward direction to a downward spin, we have to apply an oscillating radiofrequency field perpendicular to the direction of Bz.
• When the provided energy hν is the photon is equal to the difference of energy between the two spin states Δ E, the proton 'flips'.

Hence, ΔE = hν = gNβNBz

• This is called the Bohr Frequency condition.
• We know that at the point of resonance,

ΔE = hν = gNβNBz

or, \(ν =\frac{B_N× g×\beta_N }{h}\)

Explanation:-

• The frequency of NMR frequency (in MHz) of the proton in a magnetic field of intensity 1.4092 tesla is (given gn = 5.585 and μN= 5.05× 10-27JT-1 will be

\(\nu _{0}= \frac{g_{N}\mu _{N}B_{0}}{h}\)

or, \(\nu _{0}= \frac{5.585\times 5.05\times 10^{-27}J.T^{-1}\times 1.4092T}{6.64\times 10^{-34}Js}\)

or, \(\nu _{0}= 5.98\times 10^{7}s^{-1}\)

or it is approximately equals to, \(\nu _{0}= 60MHz\)

Conclusion:-

• Hence, the NMR frequency (in MHz) will be 60 MHz.

Concept:

The difference between high and low resolution spectra:

→ low-resolution NMR spectrum

• The number of peaks tells you the number of different environments the hydrogen atoms are in.
• The ratio of the areas under the peaks tells you the ratio of the numbers of hydrogen atoms in each of these environments.
• The chemical shifts give you important information about the sort of environment the hydrogen atoms are in.

→ High-resolution NMR spectra

In a high-resolution spectrum, you find that many of what looked like single peaks in the low-resolution spectrum are split into clusters of peaks.

Explanation:

→ There are 6 equivalent protons in acetone, thus it will give only one signal. There is no further splitting of protons due to which there is only one signal in both low-resolution and high-resolution spectra.

Conclusion: The correct answer is option 3.

Concept:-

• Fluorine-19 nuclear magnetic resonance spectroscopy (fluorine NMR or 19F NMR) is an analytical technique used to detect and identify fluorine-containing compounds.
• 19F has a nuclear spin (I) of 1/2 and a high gyromagnetic ratio (\(\gamma\)). Consequently, this isotope is highly responsive to NMR measurements.
• Furthermore, 19F comprises 100% naturally occurring fluorine.
• The only other highly sensitive spin 1/2 NMR-active nuclei that are monoisotopic (or nearly so) are 1H and 31P spectroscopy.
• The 19F NMR chemical shifts span a range of 800 ppm.
• For organofluorine compounds the range is narrower, being ca. -50 to -70 ppm (for CF3 groups) to -200 to -220 ppm (for CH2F groups).

Explanation:-

• The XeF4 molecule is square planar, as shown below:

• 19F is 100 % abundant and has I = 1/2.
• Xe also has an I = 1/2 nucleus (129Xe) that is about 26 % abundant.
• All of the 19F nuclei are chemically equivalent, so a single chemical shift is observed.
• For the (100-26.4)% = 73.6% of molecules that contain I = 0 Xe nuclei, this gives rise to a single peak in the 19F NMR spectrum.
• However, for the 23.6 % of ions that contain 129Xe, the 19F nuclei are split by the I = 1/2 

• The ¹29Xe nucleus splits into two peaks in the 19F NMR spectrum by using (2nI+1), where I = 1/2 and n=1 for Xe atom.

• The 19F NMR spectrum of XeF4 molecule will be as follows:

• The spectrum can be interpreted in terms of a singlet (the central line) due to 73.6 % of the 19F nuclei, plus an overlapping doublet due to the 26.4 % of the 19F nuclei that couple to 129Xe.
• Statement D is correct as the total number of peaks observed in the 19F spectrum is three
• The center of the doublet coincides with the position of the singlet because all the 19F nuclei resonate at the same frequency. The two side
  peaks in the above are called satellite peaks.
• Hence, two satellite peaks are observed in the 19F NMR spectrum.
• Statement A is correct and statement B is incorrect as the Number of satellite peaks is two.
• Thus, the relative intensities of the spectral peaks are 13.2 %, 76.4 %, and 13.2 %, or 6:1 between the central peak and the two wings.
• Statement C is incorrect as the intensities of each satellite peak are 13.2 %
• Hence, statements A and D are correct.

Conclusion:-

• Hence, the number of satellite peaks is two, and the total number of peaks observed in the 19F NMR spectrum is 3.
• Thus, statements A and D are correct.

Concept:

• Proton nuclear magnetic resonance or ¹H NMR is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen nuclei within the molecules of a substance, in order to determine the structure of its molecules.
• Proton NMR spectra are characterized by chemical shifts in the range of +14 to -4 ppm and by spin-spin coupling between protons. The integration curve for each proton reflects the abundance of the individual protons.

Explanation:

• The compound (C₅H₅)₂Fe(CO)₂ contains both \(\left( {{{\rm{\eta }}^{\rm{1}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) and \(\left( {{{\rm{\eta }}^{\rm{5}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) ligands. The ¹H NMR spectrum at 30ºC shows two singlets of equal area.
• This is because the five equivalent protons of the \(\left( {{{\rm{\eta }}^{\rm{1}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) ring will show one singlet in ¹H NMR spectra. The \(\left( {{{\rm{\eta }}^{\rm{1}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) ring will also show one singlet in ¹H NMR spectra, although the protons are not all equivalent.

• This is because of the “ring Whizzer” mechanism, by which the five ring positions of the monohapto ring of \(\left( {{{\rm{\eta }}^{\rm{1}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) interchange via 1,2-metal shift so rapidly at 30 °C that the NMR spectrometer can detect only the average signal for the ring.
• At lower -80C, this process is slower, and the different resonances for the protons of \(\left( {{{\rm{\eta }}^{\rm{1}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) become apparent at lower temperatures, the peak at 4.5 ppm \(\left( {{{\rm{\eta }}^{\rm{5}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) remains constant, but the other peak at 5.7 ppm for \(\left( {{{\rm{\eta }}^{\rm{1}}}{\rm{ - }}{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{\rm{5}}}} \right)\) spreads and then splits into four peaks of intensity ratio (5 ∶ 2 ∶ 2 ∶ 1).

​Conclusion:

Hence, The number of peaks with relative intensities observed in the ¹H NMR spectra of [(Cp)2Fe(CO)2] at +30°C and –80°C in diethyl ether are, respectively two peaks (1 ∶ 1) and four peaks (5 ∶ 2 ∶ 2 ∶ 1).