Structure Determination: Nuclear Magnetic Resonance Spectroscopy

The amount of energy required to spin-flip a nucleus depends both on the strength of the external magnetic field and on the nucleus. At a field strength of 4.7 T, rf energy of 200 MHz is required to bring a ${}^{\mathbf{1}}\mathbf{H}$ nucleus into resonance, but energy of only 187 MHz will bring a ${}^{\mathbf{19}}\mathbf{F}$ nucleus into resonance. Calculate the amount of energy required to spin-flip a ${}^{\mathbf{19}}\mathbf{F}$ nucleus. Is this amount greater or less than that required to spin-flip a ${}^{\mathbf{1}}\mathbf{H}$ nucleus?

The integrated 1H NMR spectrum of a compound of the formula ${\mathbf{C}}_4{\mathbf{H}}_{10}\mathbf{O}$is shown in Figure. Propose a structure.

Identify the indicated sets of protons as unrelated, homotopic, enantiotopic, or diastereotopic:

a. 

(b)

(c) 

(d) 

(e) 

(f) 

How many kinds of electronically non-equivalent protons are present in each of the following compounds, and thus how many NMR absorptions might you expect in each?

$$\begin{gathered} \left(\mathrm{a}\right){\mathbf{CH}}_3{\mathbf{CH}}_2\mathbf{Br}\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{\left(}\mathbf{b}\mathbf{\right)}\mathbf{ }{\mathbf{CH}}_3{\mathbf{OCH}}_2\mathbf{CH}{\mathbf{\left(}{\mathbf{CH}}_3\mathbf{\right)}}_2 \\ \mathbf{\left(}\mathbf{c}\mathbf{\right)}\mathbf{ }{\mathbf{CH}}_3{\mathbf{CH}}_2{\mathbf{CH}}_2{\mathbf{NO}}_2\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{\left(}\mathbf{d}\mathbf{\right)}\mathbf{ }\mathbf{Methylbenzene} \\ \mathbf{\left(}\mathbf{e}\mathbf{\right)}\mathbf{ }\mathbf{2}\mathbf{-}\mathbf{Methyl}\mathbf{-}\mathbf{1}\mathbf{-}\mathbf{butene}\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{\left(}\mathbf{f}\mathbf{\right)}\mathbf{ }\mathbf{cis}\mathbf{-}\mathbf{3}\mathbf{-}\mathbf{Hexene} \end{gathered}$$

How many absorptions would you expect (S)-malate, an intermediate in carbohydrate metabolism, to have in its ${}^1\mathbf{H}$  spectrum? Explain.

Propose structures for compounds that fit the following descriptions: 

(a) A hydrocarbon with seven lines in its ¹³C NMR spectrum 

(b) A six-carbon compound with only five lines in its ¹³C NMR spectrum 

(c) A four-carbon compound with three lines in its ¹³C NMR spectrum

Question: 3-Bromo-1-phenyl-1-propene shows a complex NMR spectrum in which the vinylic proton at C2 is coupled with both the C1 vinylic proton (J = 16 Hz) and the C3 methylene protons (J = 8 Hz). Draw a tree diagram for the C2 proton signal, and account for the fact that a five-line multiplet is observed .

Calculate the amount of energy required to spin-flip a proton in a spectrometer operating at 300 MHz. Does increasing the spectrometer frequency from 200 to 300 MHz increase or decrease the amount of energy necessary for

resonance?

2-Chloropropene shows signals for three kinds of protons in its 1H NMR spectrum. Explain.

The following NMR peaks were recorded on a spectrometer operating at

200 MHz. Convert each into O¹ units.

(a) CH₃CI; 1454Hz  

(b)  CHCI₃; 610Hz  

(c)  CH₃OH; 693Hz

(d)   CH₂CI₂; 1060Hz  

When the ¹H NMR spectrum of acetone, CH₃COCH₃, is recorded on an instrument operating at 200MHz , a single sharp resonance at 2.1 σis seen.

(a) How many hertz downfield from TMS does the acetone resonance correspond to?

(b) If the  ¹H  NMR spectrum of acetone were recorded at 500NHz , what would the position of the absorption be in σunits?

(c) How many hertz downfield from TMS does this 500MHz resonance correspond to?

Each of the following compounds has a single NMR peak. Approximately where would you expect each compound to absorb?

Identify the different types of protons in the following molecule, and tell

where you would expect each to absorb:

How many peaks would you expect in the ¹H NMR spectrum of 1,4-dimethylbenzene(para-xylene, or p-xylene)? What ratio of peak areas would you expect on integration of the spectrum? Refer to Table 13-3 for approximate chemical shifts, and sketch what the spectrum would look like. (Remember from Section 2-4 that aromatic rings have two resonance forms.)

Predict the splitting patterns you would expect for each proton in the following molecules:

Draw structures for compounds that meet the following descriptions:

(a) C₂H₆O ; one singlet

(b)  C₃H₇CI ; one doublet and one septet

(c)  C₄H₈CI₂O ; two triplets

(d)  C₂H₈O  ; one singlet, one triplet, and one quartet

How could you use 1H NMR to determine the regiochemistry of electrophilic addition to alkenes? For example, does addition of HCl to 1-methylcyclohexene yield 1-chloro-1-methylcyclohexane or 1-chloro-2-methylcyclohexane?

Question: Predict the number of carbon resonance lines you would expect in the C- 13 NMR spectra of the following compounds:

(a) Methylcyclopentane

(b) 1- Methylcyclohexane

(c) 1,2- Dimethylbenzene

(d) 2- Methyl-2-butene

(e)

(f)

Classify the resonances in the ¹³C NMR spectrum of methyl propanoate,${\mathbf{CH}}_3{\mathbf{CH}}_2{\mathbf{CO}}_2{\mathbf{CH}}_3$ .

Assign a chemical shift to each carbon in 6-methyl-5-hepten-2-ol.

Estimate the chemical shift of each carbon in the following molecule. Predict which carbons will appear in the DEPT-90 spectrum, which will give positive peaks in the DEPT-135 spectrum, and which will give negative peaks in the DEPT-135 spectrum.

Propose a structure for an aromatic hydrocarbon, C₁₁H₁₆, that has the following ¹³C NMR spectral data:

Broadband decoupled: 29.5, 31.8, 50.2, 125.5, 127.5, 130.3, 139.8

DEPT-90: 125.5, 127.5, 130.3 

DEPT-135: positive peaks at 29.5, 125.5, 127.5, 130.3 d; negative peak at

50.2 

We saw in Section 9-3 that addition of H-Br to a terminal alkyne leads to the Markovnikov addition product, with the Br bonding to the more highly substituted carbon. How could you use ¹³C NMR to identify the product of the addition of 1 equivalent of H-Br to 1-hexyne?

Into how many peaks would you expect the ¹H NMR signals of the indicated protons to be split? (Green =Cl.)

a) 

b) 

The following 1H NMR absorptions were obtained on a spectrometer

operating at 300 MHz. Convert the chemical shifts from $\delta$units to hertz

downfield from TMS.

(a) 2.1 (b) 3.45 (c) 6.30 (d) 7.70 $\delta$

Is a nucleus that absorbs at 6.50 $\mathrm{\delta }$ more shielded or less shielded than a nucleus that absorbs at 3.20 $\delta$? Does the nucleus that absorbs at 6.5 $\delta$ require a stronger applied field or a weaker applied field to come into resonance than the one that absorbs at 3.20 $\delta$?

How many types of non-equivalent protons are present in each of the following molecules?

The following compounds all show a single line in their ${}^{\mathbf{1}}\mathbf{H}$ NMR spectra. List them in order of expected increasing chemical shift:

${\mathbf{CH}}_{\mathbf{4}\mathbf{,}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{Cl}}_{\mathbf{2}}$,cyclohexane ,${\mathbf{CH}}_{\mathbf{3}}{\mathbf{COCH}}_{\mathbf{3}}\mathbf{,}{\mathbf{H}}_{\mathbf{2}}\mathbf{C}\mathbf{=}{\mathbf{H}}_{\mathbf{2}}\mathbf{C}$,benzene

How many signals would you expect each of the following molecules to have in its ${}^1\mathrm{H}$and  ${}^{13}C$spectra? 

a. 

b.

c. 

d.

e.

f.

Propose structures for compounds with the following formulas that show only one peak in their ${}^1\mathrm{H}$NMR spectra:

a. ${\mathrm{C}}_5{\mathrm{H}}_{12}$

b. ${\mathrm{C}}_5{\mathrm{H}}_{10}$

c. ${\mathrm{C}}_4{\mathrm{H}}_8{\mathrm{O}}_2$

Predict the splitting pattern for each kind of hydrogen in the following molecules:

(a) $\left({\mathrm{CH}}_3\right){}_3\mathrm{CH}$ 

(b) ${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CO}}_2{\mathrm{CH}}_3$

(c) trans-2-Butene

Predict the splitting pattern for each kind of hydrogen in isopropyl propanoate,${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CO}}_{\mathbf{2}}\mathbf{CH}{\mathbf{\left(}{\mathbf{CH}}_{\mathbf{3}}\mathbf{\right)}}_{\mathbf{2}}$.

Identify the indicated sets of protons as unrelated, homotopic, enantiotopic, or diastereotopic:

a. 

b.

c. 

Identify the indicated sets of protons as unrelated, homotopic, enantiotopic, or diastereotopic:

The acid-catalyzed dehydration of 1-methylcyclohexanol yields a mixture of two alkenes. How could you use ${}^1\mathrm{H}$NMR to help you decide which was which?

How could you use ${}^{\mathbf{1}}\mathbf{H}$NMR to distinguish between the following pairs of isomers?

Propose structures for compounds that fit the following NMR data:

(a)  C₅H₁₀O

0.95$\mathrm{\delta }$(6H,double, J = 7Hz)

2.10$\mathrm{\delta }$ (3H,singlet)

2.43$\mathrm{\delta }$ (1H, mutiplet)

(b)  C₃H₅Br

2.32$\mathrm{\delta }$(3H,singlet)

5.35 $\mathrm{\delta }$ (3H,singlet)

 5.45$\mathrm{\delta }$(1H, broad signet )

Propose structures for the two compounds whose ¹H NMR spectra are

shown.

(a)  C₄H₉Br

(b) C₄H₈CI₂

How many ₁₃C NMR absorptions would you expect for cis-1,3-dimethylcyclohexane?For trans-1,3-dimethylcyclohexane? Explain.

How many absorptions would you expect to observe in the NMR

spectra of the following compounds?

(a) 1,1-Dimethylcyclohexane

 (b)  CH₃CH₂OCH₃

(c) tert-Butylcyclohexane

 (d) 3-Methyl-1-pentyne

(e) cis-1,2-Dimethylcyclohexane 

(f) Cyclohexanone

Suppose you ran a DEPT-135 spectrum for each substance in Problem

13-47. Which carbon atoms in each molecule would show positive

peaks, and which would show negative peaks?

How could you use  and ¹H ¹³C NMR to help distinguish the following

isomeric compounds of the formula C₄H₈?

How could you use NMR, NMR, and IR spectroscopy to help you

distinguish between the following structures?

The compound whose ¹H NMR spectrum is shown has the molecular

formula C₃H₆Br₂. Propose a structure.

Question: When measured on a spectrometer operating at 200 MHz, chloroform $\left(CHC{l}_3\right)$shows a single sharp absorption at 7.3$\delta$ .

(a) How many parts per million downfield from TMS does chloroform absorb?

(b) How many hertz downfield from TMS would chloroform absorb if the measurement were carried out on a spectrometer operating at 360 MHz?

(c) What would be the position of the chloroform absorption in $\delta$ units when measured on a 360 MHz spectrometer?

Question: Identify the indicated protons in the following molecules as unrelated,

homotopic, enantiotopic, or diastereotopic.

a)

b)

Question: How many electronically non-equivalent kinds of protons and how

many kinds of carbons are present in the following compound? Don’t

forget that cyclohexane rings can ring-flip.

How many absorptions would you expect the following compound to have in its ¹H and ¹³C NMR spectra?

Question: How many signals would you expect each of the following molecules to have in its  ${}^{1}H$sand ${}^{13}C$spectra?

a)

b)

c)

d)

e)

f)

Question: The following ¹H NMR absorptions were obtained on a spectrometer

operating at 200 MHz and are given in hertz downfield from the TMS

standard. Convert the absorptions to $\delta$ units.

(a) 436 Hz (b) 956 Hz (c) 1504 Hz