TRB PG Assistant - Infrared spectroscopy: selection rules, vibrational modes and characteristic IR regions Online Test (MCQ Quiz)

1. What fundamental principle governs the absorption of infrared radiation by molecules?

A) Changes in electronic configuration B) Changes in nuclear spin states C) Changes in vibrational and rotational energy levels D) Changes in translational energy levels

2. Which property must a molecule possess for a vibration to be IR active?

A) A permanent dipole moment B) A change in dipole moment during the vibration C) A change in polarizability during the vibration D) Symmetry in its equilibrium structure

3. What type of molecular motion is directly probed by Infrared (IR) spectroscopy?

A) Electronic transitions B) Rotational transitions C) Vibrational transitions D) Translational transitions

4. For a molecule with N atoms, how many fundamental vibrational modes are expected?

A) 3N B) 3N - 3 C) 3N - 6 D) 3N - 5

5. How many fundamental vibrational modes does a linear molecule with N atoms have?

A) 3N B) 3N - 3 C) 3N - 5 D) 3N - 6

6. What is the number of degrees of freedom for translation for a molecule in 3D space?

A) 1 B) 2 C) 3 D) 6

7. What is the number of degrees of freedom for rotation for a non-linear molecule in 3D space?

A) 1 B) 2 C) 3 D) 6

8. What is the number of degrees of freedom for rotation for a linear molecule in 3D space?

A) 1 B) 2 C) 3 D) 6

9. Which selection rule states that a vibrational transition is allowed if it involves a change in the molecule's dipole moment?

A) Laporte rule B) Spin multiplicity rule C) Electric dipole selection rule D) Magnetic dipole selection rule

10. Which of the following molecules is IR inactive?

A) H2O B) CO2 C) N2 D) HCl

11. Why is N2 IR inactive?

A) It is a linear molecule. B) It has no permanent dipole moment and its vibration does not change the dipole moment. C) It has a very strong bond. D) It only undergoes stretching vibrations.

12. What type of vibration in CO2 is IR active?

A) Symmetric stretching B) Asymmetric stretching C) Bending vibration D) All stretching vibrations

13. Which vibrational mode of CO2 is IR inactive?

A) Asymmetric stretch B) Symmetric stretch C) Bending D) Rocking

14. What is the approximate wavenumber range for the 'fingerprint region' in an IR spectrum?

A) 4000-2500 cm⁻¹ B) 2500-1500 cm⁻¹ C) 1500-400 cm⁻¹ D) 400-100 cm⁻¹

15. What does the 'fingerprint region' of an IR spectrum typically contain?

A) Characteristic functional group absorptions B) Unique absorption bands for complex molecular structures C) Overtone and combination bands D) Simple stretching vibrations of common bonds

16. Which functional group typically absorbs in the 3300-2500 cm⁻¹ region?

A) C=O stretch B) O-H stretch (carboxylic acid) C) C-H stretch D) N-H stretch

17. The C=O stretching vibration of a ketone is usually observed in which region?

A) 3600-3200 cm⁻¹ B) 3000-2850 cm⁻¹ C) 1725-1705 cm⁻¹ D) 1680-1640 cm⁻¹

18. What type of vibration is responsible for absorption around 3300 cm⁻¹ for alcohols and phenols?

A) C-H stretch B) O-H stretch C) N-H stretch D) C=C stretch

19. A sharp, strong absorption band around 2250 cm⁻¹ typically indicates the presence of which functional group?

A) Aldehyde B) Nitrile (C≡N) C) Alkyne (C≡C) D) Amine

20. The C-H stretching vibrations for sp³ hybridized carbon atoms are usually found in which range?

A) 3100-3000 cm⁻¹ B) 3000-2850 cm⁻¹ C) 2260-2100 cm⁻¹ D) 1680-1640 cm⁻¹

21. Which functional group exhibits a strong IR absorption band around 1650 cm⁻¹?

A) Alkane C-C single bond B) Alkene C=C double bond C) Alkyne C≡C triple bond D) Aromatic C=C bonds

22. The C-H stretching vibrations for sp² hybridized carbon atoms (alkenes, aromatics) are typically observed above:

A) 3100 cm⁻¹ B) 3000 cm⁻¹ C) 2900 cm⁻¹ D) 2850 cm⁻¹

23. What type of vibration gives rise to a broad absorption band in the 3600-3200 cm⁻¹ region?

A) O-H stretch (alcohol/phenol, hydrogen-bonded) B) N-H stretch (primary amine) C) C-H stretch (aromatic) D) C=O stretch (ketone)

24. Which of the following is a bending vibration?

A) Symmetric stretch B) Asymmetric stretch C) Scissoring D) Twisting

25. The number of vibrational modes for a non-linear molecule with 4 atoms is:

A) 9 B) 10 C) 11 D) 12

26. The number of vibrational modes for a linear molecule with 3 atoms is:

A) 6 B) 7 C) 8 D) 9

27. What is the primary reason for the characteristic absorption frequencies of different functional groups?

A) Mass of the atoms involved and bond strength B) Molecular geometry C) Electronic configuration of the molecule D) Polarity of the molecule

28. A strong absorption band around 1000-1200 cm⁻¹ often indicates the presence of which type of bond?

A) C-H B) C=C C) C-O D) C-N

29. Which vibration would typically occur at a higher wavenumber: C=O stretch or C=C stretch?

A) C=O stretch B) C=C stretch C) They occur at similar wavenumbers. D) It depends on the molecular structure.

30. Why are overtone and combination bands generally weaker than fundamental bands in IR spectroscopy?

A) They violate the selection rules more significantly. B) They involve transitions between higher energy levels. C) They are less common vibrational modes. D) They are often degenerate.

31. An IR absorption band that is very sharp and strong around 3000 cm⁻¹ is likely due to:

A) Aliphatic C-H stretch B) Aromatic C-H stretch C) O-H stretch (dilute solution) D) N-H stretch (primary amine)

32. What is the effect of hydrogen bonding on the O-H stretching frequency?

A) It increases the frequency and broadens the band. B) It decreases the frequency and broadens the band. C) It increases the frequency and sharpens the band. D) It decreases the frequency and sharpens the band.

33. Which type of vibration involves the change in the angle between two bonds connected to a common atom?

A) Stretching B) Bending C) Torsional D) Rocking

34. What is the approximate wavenumber for the C≡C stretching vibration of an alkyne?

A) 1650 cm⁻¹ B) 2150 cm⁻¹ C) 2800 cm⁻¹ D) 3300 cm⁻¹

35. Which factor influences the exact position of a functional group absorption in an IR spectrum?

A) The atomic number of the elements in the bond B) The mass of the atoms and the force constant of the bond C) The number of lone pairs on the atoms D) The wavelength of the incident IR radiation

36. The intensity of an IR absorption band is primarily related to:

A) The energy of the transition B) The change in dipole moment during the vibration C) The frequency of the vibration D) The mass of the vibrating atoms

37. What is the characteristic IR absorption range for C-H stretching in aromatic rings?

A) 3100-3000 cm⁻¹ B) 3000-2850 cm⁻¹ C) 2260-2100 cm⁻¹ D) 1600-1450 cm⁻¹

38. A broad and strong absorption band between 1300 and 1000 cm⁻¹ is characteristic of:

A) C-H bond B) C=O bond C) C-O bond D) C≡N bond

39. Which of the following vibrations is typically IR inactive in methane (CH4)?

A) Asymmetric stretch B) Symmetric stretch C) Bending D) Rocking

40. The selection rule for vibrational spectroscopy states that for a transition to be allowed, the dipole moment must change during the vibration. This implies that:

A) Only molecules with a permanent dipole moment can show IR absorptions. B) Molecules without a permanent dipole moment can still be IR active if their vibrations induce a temporary dipole moment. C) All molecular vibrations are IR active. D) Only stretching vibrations are IR active.

41. In IR spectroscopy, the term 'fundamental vibration' refers to:

A) Transitions from the ground state to the first excited vibrational state. B) Transitions involving higher energy vibrational levels. C) Transitions that are most intense. D) Transitions that are least affected by anharmonicity.

42. Which type of bending vibration involves the two hydrogen atoms moving in opposite directions, perpendicular to the bond axis?

A) Scissoring B) Rocking C) Wagging D) Twisting

43. What is the approximate frequency range for the C-H stretching vibrations in aldehydes?

A) 3000-2850 cm⁻¹ B) 2850-2650 cm⁻¹ C) 1740-1720 cm⁻¹ D) 1680-1640 cm⁻¹

44. The C-N stretching vibration typically appears in the wavenumber range of:

A) 1700-1600 cm⁻¹ B) 1600-1450 cm⁻¹ C) 1350-1000 cm⁻¹ D) 1000-600 cm⁻¹

45. Which of the following statements about the vibrational modes of water (H2O) is correct?

A) It has 3N-6 = 3 vibrational modes, all IR active. B) It has 3N-5 = 4 vibrational modes, all IR active. C) It has 3N-6 = 3 vibrational modes, two IR active and one inactive. D) It has 3N-6 = 3 vibrational modes, one IR active and two inactive.

46. The characteristic absorption of aromatic C=C stretching vibrations is observed in the region:

A) 3100-3000 cm⁻¹ B) 1680-1640 cm⁻¹ C) 1600-1450 cm⁻¹ D) 900-650 cm⁻¹

47. Which vibrational mode of a diatomic molecule like HCl is IR active?

A) Only stretching B) Only bending C) All vibrations D) Diatomic molecules do not have IR active vibrations.

48. What is the approximate wavenumber for the C≡N stretching vibration of a nitrile?

A) 1700 cm⁻¹ B) 2150 cm⁻¹ C) 2800 cm⁻¹ D) 3300 cm⁻¹

49. The bending vibrations of CH2 groups typically appear in the region:

A) 3000-2850 cm⁻¹ B) 1470-1430 cm⁻¹ C) 720 cm⁻¹ D) 1000-1200 cm⁻¹

50. Which of the following is NOT a factor influencing the frequency of a stretching vibration?

A) Bond strength (force constant) B) Masses of the bonded atoms C) Hybridization of the atoms D) Number of rotational energy levels

Infrared spectroscopy: selection rules, vibrational modes and characteristic IR regions - Online Test

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