Fundamental spectroscopy and Organic chemistry problem
(18 + 13 marks)
1. Draw the schematic energy diagram and show ESR hyperfine splitting for an electron
interacting with two non-equivalent protons.
2. Assign the structure to the molecule having molecular formula C8H10. Assign the signals.
IR : 3000-2900, 1600, 1500, 1430, 1400, 750, 690 cm-1.
NMR : δ (in ppm) 7.4 (m, 5H), 2.6 (q, J = 7 Hz, 2H), 1.0 (t, J = 7 Hz, 3H).
Ans: The molecular formula of given organic compound is C8H10.
J = 7 Hz
H
H
2.6 (q)
H
H
3000-2900, 1600,
-1
1500, 750, 690 cm
1.0 (t)
H
H
H
7.4 (m)
H
H
H
3. Assign the structure of the molecule having molecular formula C3H6O. Assign the signals.
IR : 2720, 1720 cm-1.
NMR : δ (in ppm) 0.9 (t, J = 6Hz, 3H), 2.4 (dq, J = 2 & 6 Hz, 2H), 9.7 (t, J = 2Hz, 1H).
Ans: The molecular formula of the compound is C3H6O containing aldehyde group.
H
(0.9, t)
H
O
H
H
6 Hz
H H
2 Hz
2720 and 1720 cm
-1
(9.7, t)
(2.4, dq)
4. The fundamental vibrational frequency of A2 is 4400 cm-1. Assuming that the atomic mass
of B is double that of A, estimate the fundamental vibration frequencies of AB and B2.
5. State with brief reasoning, which of the following molecules can show a pure rotational
spectrum CO2, OCS, N2, ethylene, benzene, water.
Which of the above molecule can show a vibrational spectrum?
Hint: The molecules possesses permanent dipole moment are microwave active while
those molecules have permanent dipole moment or zero dipole moment, but it can be changed or
generate dipole moment are infra red active.
6. Sketch the diagram of allowed transitions between the energy levels and the rotational
spectrum of a non-rigid diatomic molecule. Compaired the spacing between the allowed
rotational energy levels of rigid and non-rigid molecules.
7. Predicts the structure of the two compounds whose NMR data is given. Assign the signals.
i.
C3H3Cl5 : δ (in ppm) 4.52 (t, 1H); 6.07 (d, 2H).
ii.
C3H5Cl3 : δ (in ppm) 2.2 (s, 3H); 4.02 (s, 2H).
Ans: i. Cl2CH-CH2-CCl3
ii. CH3-CCl2-CH2Cl
8. Predicts the structure of the two compounds whose IR data is given. Assign the IR bands.
i.
C3H6O : 1620 cm-1.
ii.
C4H6 : 3300 and 2250 cm-1.
Ans: i. CH2=CH-O-CH3
ii. HC≡C-CH2CH3
9. Predict the structure of the compound C8H10 which shows PMR signals at δ (in ppm) 0.9
(t, 3H); 2.3 (q, 2H), 7.0 (s, 5H). Assign the NMR signals.
Ans: C6H5-CH2-CH3
10. The mass spectra of a compound C5H12O has ions at m/z = 88, 43 and 29. Derive its
structure and suggest the possible structures of ions.
Ans: CH3-CH2-CH2-O-CH2-CH3
11. How will you distinguish between thiophenol (Ph-SH), styrene (Ph-CH=CH2) and phenyl
acetylene by IR spectroscopy.
Ans: Thiophenol shows stretching vibration of –S-H group, styrene shows vibrations of –
C=C and olefinic C-H and phenyl acetylene shows vibrations of C≡C and ≡C-H bond
vibrations.
12. Based on the spectral data given below deduce the structure of the hydrocarbon. Assign
the signals. IR : 3030, 1600, 1500, 700 cm-1 and characteristic strong band at 2210 cm-1.
PMR : δ (in ppm) 3.1 (s, 3H); 7.4 (s, 5H).
Mass m/z = 108 (M+).
Ans: Molecular formula is C7H8O and structure is- C6H5-O-CH3.
13. Assign the structure to the molecule having given data:
Molecular formula : C8H8O2.
IR : 3000-2900, 1745, 1600, 1500, 750, 690 cm-1.
NMR : δ (in ppm) 7.3 (m, 5H); 3.9 (s, 3H).
Ans:
H
O
H
H
H
O
H
H
H
methyl benzoate
750 and 690 cm
-1
H
3.9 (s, 3H) indicates presence of -OCH 3 group.
7.3 (m, 5H) indicates presence of monosubstituted benzene ring
3000 - 2900 cm
-1
is due to olefinic C - H streching vibrations
-1
1745 cm is due to C=O streching vibrations of ester
-1
1600 - 1500 cm is due to aromatic C=C streching vibtration.
is bending vibrations indicates presence of monosubstituted aromatic ring
14. Deduce the structure of molecule having molecular formula C4H8O.
IR : 1710 cm-1.
NMR : δ (in ppm) 0.9 (t, J = 6 Hz, 3H); 2.2 (s, 3H); 2.6 (q, J = 6Hz, 2H).
Ans:
(q, 2H)
H3C
(t, 3H)
O
1710 cm
-1
(s, 3H)
CH3
butan-2-one
15. The IR spectrum of the hypothetical gas AB shows an abosorption band at 3000 cm-1.
Calculate the force constant of the A-B bond. (Take reduced mass = 1 x 10-27 Kg).
16. The IR spectra of water molecule shows three bands with maxima at 3756 cm-1, 3652 cm1
, and 1595 cm-1. Explain whether the molecule will be linear or bend.
17. Assign the structure to the molecule having given data:
Molecular formula: C9H10O2.
IR : 1730, 1600, 1250, 750, 700 cm-1.
NMR : δ (in ppm) 1.3 (t, J = 7 Hz, 3H); 4.4 (q, J = 7 Hz, 2H); 7.4 (m, 3H); 8.1 (m, 2H).
18. Deduce the structure of molecule having molecular formula C5H10O.
UV : No λ max above 200 nm.
IR : No bands above 3000 cm-1 and 2000-1600 cm-1 region.
NMR : δ (in ppm) 1.22 (d, J = 6.5 Hz, 3H); 2.0 (m, 4H); 3.65 (t, J = 6 Hz, 2H);
3.8 (m, 1H).
19. State the rule of mutual exclusion in connection with the IR and Raman spectra of
molecules. Illustrate the use of this in understanding the molecular structure by giving one
example.
20. Sketch schematically the potential energy curves of a diatomic molecule for ground and
excited states. Represents the transition corresponding to υ’-progression originating from
the lowest state.
21. Assign the structure of the molecule containing nitrogen.
Molecular weight : 71 (m/z = 71 M+).
IR (neat) : 2941 – 2857, 2247, 1460 cm-1.
UV : No λ max above 200 nm.
PMR : δ (in ppm) 4.22 (s, 2H); 3.49 (s, 3H).
22. Deduce the structure of molecule having molecular formula C6H10O.
IR (neat) : 1715 cm-1.
NMR : δ (in ppm) 1.6 (quintet, 2H); 1.7 (quintet, 4H); 2.25 (t, 4H).
23. Sketch schematically the normal modes of vibration of CO2 molecule. Indicate which are
active in infrared and which are Raman active.
24. With the help of a schematic diagram show ‘vibrational coarse structure’ of an electronic
absorption band for diatomic molecule.
25. The electronic absorption spectrum of gaseous O2 at room temperature exhibits vibrational
structure having convergence limit at 56876 cm-1. The oxygen molecule in the
corresponding excited state dissociates into one atom in the ground state and the other
atom in an excited state with energy 15875 cm-1. Estimate the dissociation energy (in
kJ/mole) of molecular O2 in the ground electronic state.
26. Assign the structure to the molecule having given data:
Molecular formula : C9H10O.
IR : No bands above 3100 cm-1 and no band in 2000-1650 cm-1 region.
NMR : δ (in ppm) 1.15 (t, J = 7.5 Hz, 3H); 3.5 (q, J = 7.5 Hz, 2H); 4.4 (s, 2H); 7.2 (s, 5H)
Mass : m/z = 91 (base peak).
27. Deduce the structure of molecule having molecular formula C10H11O2Cl.
UV : λmax = 245 nm (ε 18,000).
IR (neat) : 3000 - 2920, 1745, 1600, 1580, 820 cm-1.
PMR : δ (in ppm) 2.0 (s, 3H); 2.8 (t, J = 6 Hz, 2H); 4.1 (t, J = 6.0 Hz, 2H);
7.1 (d, J = 8 Hz, 2H); 7.3 (d, J = 8 Hz, 2H).
28. Draw rotational energy level diagram for a heteronuclear diatomic molecule behaving as
non-rigid rotator. Draw the resulting microwave spectrum showing intensities and
separations quantitatively.
29. Explain the concept of group frequencies of the vibrations of the molecule. State the
factors causing shift in group frequencies.
30. Deduce the structure of the compound C7H8O having following spectral data. Justify your
answer.
IR : 3350 (broad), 1600, 1100, 760 cm-1.
PMR : δ (in ppm) 3.8 (1H); 4.3 (2H); 7.2 (5H).
31. Assign the structure of the compound which shows following spectral properties.
MS : m/z = 134 (M+).
IR : 1685, 1600, 820 cm-1.
PMR : δ (in ppm) 2.2 (s, 3H); 2.3 (s, 3H); 7.2 (d, J = 8.0 Hz, 2H); 7.8 (d, J = 8.0 Hz, 2H)
Assign the signals.
What base peak would you expect in the mass spectrum?
32. The Mossbauer spectrum of Na4[Fe(CN)6] consist of single line, while that of a
Na21[Fe(CN)5NO] shows a doublet. Explain the above observation.
33. The Mossbauer spectrum of FeSO4.7H2O shows a quadrupole doublet, while that of
K4[Fe(CN)6] shows a single line. Explain.
34. Isomer shift (δ) for Na4[Fe(CN)6].3H2O relative to stainless steel is 0.083 mm/sec while
that for Na3[Fe(CN)6].3H2O is 0.084 mm/sec. Explain the above observation.
35. A compound C5H8O2 shows characteristic IR peak at 1810 cm-1. Its H-NMR has singlet at
1.1 and 2.2 ppm with peak ratios of 3:1. Propose a structure for the compound. Indicates
briefly your reasons.
36. The Raman spectra in the region of the C≡O and M-C stretching vibrations of the
isostructural series [Ni(CO)4], [Co(CO)4]- and [Fe(CO)4]2- exhibits the following spectral
data.
C≡O region in cm-1
M-C region in cm-1
2[Fe(CO)4]
1788
550, 464
[Co(CO)4]1988, 1883
532, 439
[Ni(CO)4]
2121, 2039
422, 381
Give brief explanation of the above data.
37. State the Boltzmann distribution law (BDL). Use BDL to calculate the ratio of population
at 300 K for energy levels separated by
i.
1000 cm-1.
ii.
10 kJ/mol.
38. A Homonuclear diatomic molecule does not exhibit rotational spectra, whereas
heteronuclear diatomic do. Explain.
39. Write down expression for moment of inertia (I) for diatomic molecule and also for
frequency separation (2B) of the rotation lines.
40. The rotational spectra of CO and HF show series of lines separated by 3,842 cm -1 and 419
cm-1 respectively. Account for the difference.
41. The rotational spectra of CO and HF show equidistance lines spaced 3.8 cm-1 and 42 cm-1
respectively. What is the ratio of moment of inertia ICO/IHF?
42. Iron compounds A, B and C exhibit the following Mossbauer data.
A
B
C
Isomer shift (Fe) mm sec-1
0.2
1.2
1.4
Quadrupole splitting mm sec-1
0.4
2.5
0
Comment on the valency spin state and distortion from cubic structure for the three cases.
43. The compound FeSO4.7H2O shows quadrapole split Mossbauer spectrum while FeCl3
shows a single line. Explain the above behavior.
44. Consider a molecular sample containing an unpaired electron interacting with a nucleus
with spin ½ placed in a strong magnetic field. Sketch a schematic energy level diagram for
the electron. Show the allowed e.s.r. transitions and resulting e.s.r. spectrum. Indicate the
hyperfine splitting in the diagram.
45. With the brief explanation, schematically draw the high resolution 1H n.m.r. spectrum of
acetaldehyde.
46. Metallic iron at room temperature shows six fingered hyperfine structure, while
FeSO4.7H2O shows quadrupole doublet. Explain the above behavior.
47. The EPR spectrum of bis-salicylaldimine Cu(II) with isotopically pure Cu63 shows eleven
lines as again fifteen lines expected. Comment on this observation.
48. The EPR spectrum of the methyl radical shows four lines. Explain this behavior.
49. The infrared absorption spectrum of gaseous NO exhibit the fundamental and the first
overtone transitions centered around 1876.06 and 3724.20 cm-1 respectively. Estimate the
vibrational and anharmonicity of NO.
50. The frequencies of the J = 0 → J = 1 rotational transitions in the microwave absorption
spectra of 12C – 16O and 13C – 16O molecules are observed to be 3.84235 and 3.67337 cm-1
respectively. Assuming atomic weights of 12C = 12.00 and 16O = 16.00, estimate the
atomic weight of 13C.
51. The Infrared spectrum of [Fe2(CO)9] shows bands at 2100cm-1, 2000 cm-1 and 1800 cm-1.
Explain the observed spectrum and write molecular structure.
52. What is Born-Oppenhermer approximation?
53. For the hydrogen atom (I = ½), draw the energy level diagram [in term of applied
magnetic field, H and hyperfine field, A]. Indicates the ESR transitions. State the selection
rule used.
54. Draw the schematic energy diagram and show ESR hyperfine splitting for an electron
interacting with two non-equivalent protons.
55. An organic compound A having molecular formula C10H12O2 gives following spectral
data:
IR:- (frequency in cm-1)- 1730(s), 1602(m), 1581(m), cm-1.
1
HNMR (neat): δ 2.0[3H, s]; 3.93[2H, t(J= 7Hz)]; 4.30[2H, t(J= 7Hz)]; 7.30[5H, bs].
Determine the structure of A.
CH3
O
Ans:
1602 & 1581
7.30 (bs, 5H)
O
1730
56. Deduce the structure of the compound based on following data:Molecular formula:C8H9NO2.
U.V.
:- 211 (ε 5550); 274 nm (ε 2450)
I.R.
:- 1725, 1600, 830 cm-1.
M.S. (m/z)
:- 151 (M+), 123, 106, 78, 29.
P.M.R. (δ)
:- 1.35 (t, J=7Hz, 3H), 4.2 (q, J=7Hz, 2H), 7.75 (d, J=8Hz, 2H),
8.7 (d, J=8Hz, 2H).
57. Deduce the structure of the compound based on following data:Molecular formula:C10H12O2.
U.V.
:- 211 (ε 1200).
I.R.
:- 3250-2700 (broad), 1710, 1603, 758, 688 cm-1.
M.S. (m/z)
:- 164 (M+), 105, 77, 60, 45.
P.M.R. (δ)
:- 1.3 (d, J=7Hz, 3H), 2.6 (d, J=7Hz, 2H), 3.24 (sextet, J=7Hz, 1H),
7.20 (m, 5H), 10.8 (bs, exchange with D2O, 1H).
58. Deduce the structure of the compound based on following data:Molecular formula:C15H20O2.
U.V.
:- 275 nm (ε 21000).
I.R.
:- 1720, 1626, 1605, 1150, 850, 820 cm-1.
M.S. (m/z)
:- 232 (M+), 176.
P.M.R. (δ)
:- 1.0 (d, J=7Hz, 6H), 2.0 (m, 1H), 2.38 (s, 3H), 3.95 (d, J=7Hz, 2H),
6.16 (d, J=16Hz, 1H), 7.20 (d, J=8Hz, 2H), 7.41 (d, J=8Hz, 2H),
7.75 (d, J=18Hz, 1H).
59. Deduce the structure based on the following carbon-13 N.M.R. data.
Molecular formula:- C8H10O.
C-13 NMR (δ) :- 38(t), 63(t), 126(d), 128(d), 129(d), 139(s).
60. Deduce the structure of the organic compound having the following analytical and spectral
data.
Analysis: C, 74.98; H, 6.86.
Mass:
176(M+), 131, 103, 77.
IR:
1714, 1639 cm-1.
P.M.R. (δ): 1.31 (t, J=7.1Hz, 3H), 4.2 (q, J=7.1Hz, 2H), 6.43 (d, J=15.8Hz, 1H),
7.24-7.57 (m, 5H), 7.67 (d, J=15.8Hz, 1H).
C.M.R. (δ): 14.3, 60.4, 118.4, 128.1, 128.9, 130.2, 134.5, 144.5, 166.8.
61. The bond distance in 79Br2 is 228 pm. Calculate the principal moment of inertia (IB) of the
molecule.
62. Photolysis of a solution of H2O2 in isopropyl alcohol at 110 K leads to a free radical
formation, the esr spectrum of which consist of seven lines with a hyperfine splitting of
20G, a line width of 10G and intensity ratio of 1 : 6 : 15 : 20 : 15 : 6 : 1.
Which radical is responsible for the esr spectrum? Explain.
Given : i) I(12C) = 0. ii) I(16O) = 0.
iii) I(1H) = ½.
63. Show that in the rotational spectrum the level having maximum population for a diatomic
molecule is given by Jmax = {KT/2Bhc}1/2 – ½ with the symbols having their usual
meanings.
64. The moment of inertia of the diatomic molecule is 1.44 x 10-47 kg m2. Its reduced mass is
10-27 kg. What is its bond length in picometer?
65. On the basis of MO diagram, how will you account for the following force constants
(values in Nm-1)?
C2 (930); N2 (2260); O2 (1140).
66. Explain in brief, how one determines the spectroscopic dissociation energy (D0) using the
Birge-Sponer extrapolation. How D0 differs from the equilibrium dissociation energy.
67. The equilibrium bond length (Re) and dipole moment (μ) for the HBr molecule are 141
pm and 2.6 x 10-30 Cm respectively. Determine the values of charges on H and Br. Reset
the exercise for HCl molecule with Re = 127 pm and μ = 3.4 x 10-30 Cm. Comment upon
the relative values of charges in HCl and HBr.
(Given: │e│ = 1.6 x 10-19 C)