# NMR _ IR Spectroscopy Problems I

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```					                                 Spectroscopy Problems I
Eight structure elucidation problems based on 1H nmr, 13C nmr and some infrared spectroscopic
data are presented below. In each case, from the nineteen formulas shown at the bottom of the
page (A through S), select that one which best fits the evidence. Enter the appropriate letter in the
answer box to the right of the question. Do not enter punctuation (eg. commas or semicolons) or
spaces.

1. A C5H12O2 compound has strong                     2. A C4H8O2 compound has a strong
infrared absorption at 3300 to 3400 cm-1             infrared absorption at 1150 cm-1, but no
The 1H NMR spectrum has three singlets               absorption at 3300 to 3400 cm-1. It's 1H
at δ 0.9 , δ 3.45 and δ3.2 ppm; relative             NMR spectrum shows a singlet at δ
areas 3:2:1. The 13C NMR spectrum shows              3.55 ppm. The 13C NMR spectrum
three signals all at higher field than δ100          shows one signal at δ 66.5 ppm.
ppm. Suggest a structure for this                    Suggest a structure for this compound.
compound.

3. A C9H12O compound has strong infrared             4. A C10H14 compound. The 1H NMR
absorption at 3300 to 3400 cm-1. The 13C             spectrum has two singlets at δ 2.45 and
NMR spectrum of this compound has six                7.0 ppm (ratio = 6:1). The 13C NMR
discrete signals. It's 1H NMR spectrum has           spectrum shows three signals at
three sets of lines: singlets at δ1.1 (6H),          δ132.9, 130.5 and 18.9 ppm. Suggest a
1.9 (1H) and 7.3 (5H) ppm. Suggest a                 structure for this compound.
structure for this compound.

5. A C8H4N2 compound shows a sharp                   6. A C14H22 compound. The 1 H NMR
infrared absorption at 2230 cm-1. It's 1H            spectrum has two singlets at δ 1.1 and
NMR spectrum has a singlet at δ 7.6 ppm.             7.25 ppm (ratio = 9:2). The 13C NMR
The 13C NMR spectrum shows three                     spectrum shows four signals at δ 147,
signals at δ132, 119 and 117 ppm. Suggest            125, 39.3 and 30.8 ppm. Suggest a
a structure for this compound.                       structure for this compound.

7. A C9H12O3 compound has strong                     8. A C9H18O compound has a strong
infrared absorption near 1100 cm-1. Its 1H           infrared absorption at 1710 cm-1. Its 1H
NMR spectrum has sharp singlet peaks at              NMR spectrum has a single sharp peak
δ 3.6 and 6.6 ppm (intensity ratio 3:1). Its         (a singlet) at δ 1.2 ppm. Its 13C NMR
13
C NMR spectrum shows three lines at δ             spectrum shows three lines at δ210, 45
165, 115 and 55 ppm. Suggest a structure             and 25 ppm. Suggest a structure for
for this compound.                                   this compound.

1
2
Interpreting a NMR Spectrum
The following 1Hnmr spectrum of a C10H12O2 compound was obtained on a 90 MHz
spectrometer.
A series of questions are presented below the spectrum. To answer each question enter a number
or letter in the designated answer box.
Do not enter punctuation (eg. commas or semicolons) or spaces.

1. Ignoring the TMS signal at δ = 0, how many discrete groups of proton signals are present in
this spectrum? ...
2. What is the multiplicity ( s, d, t, q ) of the highest field signal from this sample? ...
3. The sample has a singlet at δ = 3.8 ppm. In units of Hertz (Hz) how far is this signal from the
TMS signal? ...
4. What structural feature is suggested by the singlet at δ = 3.8 ppm? ...
A CH3–C=O B –CH2– C –O–H D –O–CH3 E C–CH3 F C=C–H
5. From multiplet line separations (Js), which of the other signals is coupled to the quartet at δ =
2.9 ppm? ...
A δ = 1.2 ppm         B δ = 3.8 ppm    C δ = 6.9 ppm   D δ = 7.9 ppm

6. Using the integrator trace and the formula of the sample, assign a whole number ratio to the
sample signals as follows:
7.9 ppm signal       ;    6.9 ppm signal    ;   3.8 ppm signal      ;   2.9 ppm signal
;     1.2 ppm signal

3
Interpreting a NMR Spectrum

The 1Hnmr spectrum of the unknown C10H12O2 compound is shown above. Seven structural
formulas of some possible candidates for this compound are presented below. After reviewing the
chemical shifts, coupling patterns and integration values shown in this spectrum, select the
structure most likely to give this spectrum, and enter the corresponding letter ( A through G ) in
Do not enter punctuation (eg. commas or semicolons) or spaces.

4
1. Ignoring the TMS signal at δ = 0, how many discrete groups of proton signals are
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present          in              this             spectrum?              ...
2. What is the multiplicity ( s, d, t, q ) of the highest field signal from this sample? ...
t

3. The sample has a singlet at δ = 3.8 ppm. In units of Hertz (Hz) how far is this signal
342
from                 the             TMS              signal?            ...
d
4. What structural feature is suggested by the singlet at δ = 3.8 ppm? ...
A CH3–C=O        B –CH2–      C –O–H      D –O–CH3       E C–CH3     F C=C–H
5. From multiplet line separations (Js), which of the other signals is coupled to the
a
quartet         at       δ               =        2.9       ppm?               ...
A δ = 1.2 ppm            B δ = 3.8 ppm     C δ = 6.9 ppm            D δ = 7.9 ppm

6. Using the integrator trace and the formula of the sample, assign a whole number ratio
to            the            sample             signals            as          follows:
2                         2                         3
7.9 ppm signal        ;    6.9 ppm signal       ;   3.8 ppm signal        ;   2.9 ppm
2                         3
signal         ;   1.2 ppm signal

5
Spin-Spin Splitting in 1H NMR
1.What splitting pattern in the 1H nmr spectrum would you expect for the hydrogen atom(s)
colored red in the compounds shown below?
Your choices are: s singlet d doublet t triplet q quartet m multiplet. Enter the appropriate
letter in the answer box to the right of each formula.
Do not enter punctuation (eg. commas or semicolons) or spaces.

A                                B                                C

D                                E                                F

2. Which of the following compounds would be expected to show spin-spin splitting in their 1H
NMR spectra? (Check the appropriate button).

No                                           No
Compound        Splitting                  Compound          Splitting
Splitting                                    Splitting

(CH3)3C-O-CH3                             (CH3)2CH-O-CH3

Br-CH2CH2CH2-Br                              Br-CH2CH2-Br

6
Structurally Equivalent Atoms & Groups
For each of the compounds A through F indicate the number of structurally-distinct groups of
carbon atoms, and also the number of distinct groups of equivalent hydrogens. Enter a number
from 1 to 9 in each answer box.

Number of distinct carbon atoms: ...
A
Number of distinct hydrogen groups:

Number of distinct carbon atoms: ...
B
Number of distinct hydrogen groups:

Number of distinct carbon atoms: ...
C
Number of distinct hydrogen groups:

Number of distinct carbon atoms: ...
D
Number of distinct hydrogen groups:

Number of distinct carbon atoms: ...
E
Number of distinct hydrogen groups:

Number of distinct carbon atoms: ...
F
Number of distinct hydrogen groups:

Number of distinct carbon atoms: ...
A
Number of distinct hydrogen groups:

7
Number of distinct carbon atoms: ...
B
Number of distinct hydrogen groups:

Number of distinct carbon atoms: ...
C
Number of distinct hydrogen groups:

Number of distinct carbon atoms: ...
D
Number of distinct hydrogen groups:

Number of distinct carbon atoms: ...
E
Number of distinct hydrogen groups:

Number of distinct carbon atoms: ...
F
Number of distinct hydrogen groups:

Spectroscopy Problems II

8
Eight structure elucidation problems based on 1H nmr, 13C nmr and infrared spectroscopic data are
presented below. In each case, from the nineteen formulas shown at the bottom of the page (A
through S), select that one which best fits the evidence. Enter the appropriate letter in the answer
box to the right of the question. Do not enter punctuation (eg. commas or semicolons) or spaces.

1. A C9H10O compound has a strong             R     2. A C5H10O2 compound shows a strong         C
infrared absorption at 1720 cm-1. Its 1H            infrared absorption at 1100 cm-1, but no
NMR spectrum has signals at δ 2.8                   absorption at 3300 to 3400 cm-1. Its 1H
(mult., 4H), 7.3 (s, 5H) and 9.8 (t, 1H)            NMR spectrum has sharp singlet peaks
ppm. Its 13C NMR spectrum shows seven               at δ1.3 and 4.0ppm (intensity ratio 3:2).
lines at δ200, 138, 129, 128, 125, 35 and           Its 13C NMR spectrum shows three lines
30 ppm. Suggest a structure for this                at δ 98, 68 and 20 ppm. Suggest a
compound.                                           structure for this compound.

3. A C9H10O2 compound has strong              D     4. A C9H10O2 compound has strong             L
infrared absorption at 1690 and 1100 cm-1           infrared absorption at 1695 cm-1. The 1H
. Its 1H NMR spectrum has sharp singlet             NMR spectrum has five sets of lines: a
peaks at δ 2.8 and 3.8 ppm (3H each) and            triplet at δ1.3(3H), a quartet at
two doublets at δ 6.9 and 7.8 ppm (2H               δ>4.1(2H), a doublet at δ7.0(2H), a
each). Its 13C NMR spectrum shows                   doublet at δ7.8(2H) and a singlet at
seven lines Suggest a structure for this            δ9.8(1H) ppm. Suggest a structure for
compound.                                           this compound.

5. A C4H8O3 compound has strong               K     6. A C9H9N compound shows infrared           Q
infrared absorption at 2500 to 3300 cm-1            absorption at 2250 cm-1. It's 1H NMR
and 1710 cm-1 . The 1H NMR spectrum                 spectrum has three signals: sharp
has four signals: a doublet at δ1.2(3H), a          singlets at δ 2.4 (3H), 3.8 (2H) and a
quartet at δ4.5(1H), a singlet at δ3.6(3H)          broader one at 7.2 (4H) ppm. The 13C
and a singlet at 12.5 ppm. The 13C NMR              NMR spectrum has seven signals, five
spectrum has four signals at δ177, 70, 54           at fields lower than δ100 ppm and two
and18 ppm. Suggest a structure for this             at higher fields. Suggest a structure for
compound.                                           this compound.

7. A C4H11N compound shows two                M     8. A C5H11N compound shows infrared          N
infrared absorption bands at 3200 to 3400           absorption at 3300 cm-1. It's 1H NMR
cm-1 and a strong band at 2900 cm-1. It's           spectrum has three signals: singlets at
1
H NMR spectrum shows two singlets at              δ1.0 (6H), 1.4 (1H) and 2.7 (4H) ppm.
δ 1.0 (9H) and 1.3 (2H) ppm. The 13C                The 13C NMR spectrum has three
NMR spectrum has two signals, both at               signals, all at fields higher than δ100
fields higher than δ100 ppm. Suggest a              ppm. Suggest a structure for this
structure for this compound.                        compound.

9
A Structural Formula Puzzle

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Two isomeric C8H10 hydrocarbons (A & B) both give the
same saturated hydrocarbon product, C, on exhaustive          A        excess H2
catalytic hydrogenation. Both A & B, but not C, show
strong absorption of UV light, λmax = 245 nm. Addition of     or
B
} Pt catalyst C
=
one equivalent of HBr to either A or B produces the same
mixture of C8H13Br isomers (D & E).

1. How many double bonds must be present in compound A?                (enter a number from 0 to 9

In compound B?           (enter a number from 0 to 9

(enter a
2. What must be the relationship of these functional groups? word or a short phrase in the answer
space)

Spectroscopic evidence concerning the structures of these compounds:

Compound A: five 13C nmr signals, two from sp2 carbons & three from sp3 carbons. A
complex 1H nmr signal at δ 5.6 ppm, 1:4 ratio with respect to higher-field signals.
Compound B: four 13C nmr signals, two from sp2 carbons & two from sp3 carbons. A
complex 1H nmr signal at δ 5.6 ppm, 1:4 ratio with respect to higher-field signals.
Compound D: A complex 1H nmr signal at δ 5.5 ppm, 1:9 ratio with respect to higher-
field signals.
Compound E: No 1H nmr signals in the region below (lower field than) δ 5.0 ppm.

3. From the information given above you should be able to write structural formulas for
compounds A, B, D & E. When you have done so continue the problem by clicking here.

From the information given at the beginning of this problem you should have concluded that
compounds A & B have two double bonds each. The UV absorption indicates that these double
bonds must be conjugated. Given the common bicyclic skeleton of these isomers (hydrogenation
to C), only five conjugated diene structures can be written: 1 through 5 below.

We can now examine these isomeric dienes and predict how their different structures would be
reflected in differences in their 13C and 1H nmr spectra.
You must enter numbers for both carbon and proton nmr before checking your answer.

11
Number of expected            Number of H atoms giving
Compound        13
C nmr signals       1
H nmr signals at δ > 5.0 ppm

1

2

3

4

5

With this analysis accomplished, the spectroscopic data from A & B can be used to make
structural assignments:

Five 13C nmr signals, two from sp2 carbons & three from sp3
4. Which compound (1 through       carbons.
5) is A?                         { A complex 1H nmr signal at δ 5.6 ppm, 1:4 ratio with respect
to higher-field signals.
Four 13C nmr signals, two from sp2 carbons & two from sp3
5. Which compound (1 through       carbons.
5) is B?                         { A complex 1H nmr signal at δ 5.6 ppm, 1:4 ratio with respect
to higher-field signals.

6. Finally, you should be able to write structural formulas for the HBr addition products,
compounds D & E.
Remember, D shows a complex 1H nmr signal at δ 5.5 ppm, 1:9 ratio with respect to higher-field
signals; whereas, E has no 1H nmr signals in the region below (lower field than) δ 5.0 ppm. To
draw a structural formula, use the drawing window opened by the Draw Formulas button. When
you are finished, Submit your formula and then check it by pressing the appropriate Check
Compound button. A help screen is available for the drawing window.

12

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