TRB PG Assistant - Infrared spectroscopy: selection rules, vibrational modes and characteristic IR regions One Line Questions & Answers

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

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

3. What is the number of degrees of freedom for rotation for a linear molecule in 3D space? — 2

4. What is the approximate wavenumber for the C≡C stretching vibration of an alkyne? — 2150 cm⁻¹

5. What is the approximate wavenumber for the C≡N stretching vibration of a nitrile? — 2150 cm⁻¹

6. The C-N stretching vibration typically appears in the wavenumber range of: — 1350-1000 cm⁻¹

7. What is the approximate frequency range for the C-H stretching vibrations in aldehydes? — 2850-2650 cm⁻¹

8. The bending vibrations of CH2 groups typically appear in the region: — 1470-1430 cm⁻¹

9. The C-H stretching vibrations for sp² hybridized carbon atoms (alkenes, aromatics) are typically observed above: — 3000 cm⁻¹

10. The C-H stretching vibrations for sp³ hybridized carbon atoms are usually found in which range? — 3000-2850 cm⁻¹

11. What is the characteristic IR absorption range for C-H stretching in aromatic rings? — 3100-3000 cm⁻¹

12. The characteristic absorption of aromatic C=C stretching vibrations is observed in the region: — 1600-1450 cm⁻¹

13. The C=O stretching vibration of a ketone is usually observed in which region? — 1725-1705 cm⁻¹

14. For a molecule with N atoms, how many fundamental vibrational modes are expected? — 3N - 6

15. How many fundamental vibrational modes does a linear molecule with N atoms have? — 3N - 5

16. What is the approximate wavenumber range for the 'fingerprint region' in an IR spectrum? — 1500-400 cm⁻¹

17. The number of vibrational modes for a linear molecule with 3 atoms is: — 6

18. The number of vibrational modes for a non-linear molecule with 4 atoms is: — 9

19. Which property must a molecule possess for a vibration to be IR active? — A change in dipole moment during the vibration

20. A sharp, strong absorption band around 2250 cm⁻¹ typically indicates the presence of which functional group? — Nitrile (C≡N)

21. An IR absorption band that is very sharp and strong around 3000 cm⁻¹ is likely due to: — O-H stretch (dilute solution)

22. Which functional group exhibits a strong IR absorption band around 1650 cm⁻¹? — Alkene C=C double bond

23. Which vibrational mode of CO2 is IR inactive? — Symmetric stretch

24. Which of the following vibrations is typically IR inactive in methane (CH4)? — Symmetric stretch

25. Which of the following is NOT a factor influencing the frequency of a stretching vibration? — Number of rotational energy levels

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

27. A broad and strong absorption band between 1300 and 1000 cm⁻¹ is characteristic of: — C-O bond

28. What type of vibration is responsible for absorption around 3300 cm⁻¹ for alcohols and phenols? — O-H stretch

29. Which functional group typically absorbs in the 3300-2500 cm⁻¹ region? — O-H stretch (carboxylic acid)

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

31. What fundamental principle governs the absorption of infrared radiation by molecules? — Changes in vibrational and rotational energy levels

32. What does the 'fingerprint region' of an IR spectrum typically contain? — Unique absorption bands for complex molecular structures

33. What type of molecular motion is directly probed by Infrared (IR) spectroscopy? — Vibrational transitions

34. Which of the following molecules is IR inactive? — N2

35. Which of the following statements about the vibrational modes of water (H2O) is correct? — It has 3N-6 = 3 vibrational modes, all IR active.

36. What is the effect of hydrogen bonding on the O-H stretching frequency? — It decreases the frequency and broadens the band.

37. Why is N2 IR inactive? — It has no permanent dipole moment and its vibration does not change the dipole moment.

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

39. What is the primary reason for the characteristic absorption frequencies of different functional groups? — Mass of the atoms involved and bond strength

40. What type of vibration gives rise to a broad absorption band in the 3600-3200 cm⁻¹ region? — O-H stretch (alcohol/phenol, hydrogen-bonded)

41. 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: — Molecules without a permanent dipole moment can still be IR active if their vibrations induce a temporary dipole moment.

42. Which vibrational mode of a diatomic molecule like HCl is IR active? — Only stretching

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

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

45. Which of the following is a bending vibration? — Scissoring

46. What type of vibration in CO2 is IR active? — Asymmetric stretching

47. Which factor influences the exact position of a functional group absorption in an IR spectrum? — The mass of the atoms and the force constant of the bond

48. The intensity of an IR absorption band is primarily related to: — The change in dipole moment during the vibration

49. Why are overtone and combination bands generally weaker than fundamental bands in IR spectroscopy? — They violate the selection rules more significantly.

50. In IR spectroscopy, the term 'fundamental vibration' refers to: — Transitions from the ground state to the first excited vibrational state.

Infrared spectroscopy: selection rules, vibrational modes and characteristic IR regions - One Line Questions