Chapter 19, Problem 19.42QAP

Interpretation Introduction

(a)

Interpretation:

The major advantage and disadvantages of two dimensional NMR methods over conventional one-dimensional techniques are to be stated.

Concept introduction:

Nuclear magnetic resonance spectroscopy is a method used to determine the physical and chemical nature of atoms in different compounds. In two-dimensional nuclear magnetic resonance spectroscopy information about atoms is plotted in two frequency axes.

Answer to Problem 19.42QAP

The major advantage of two dimensional NMR method is that chemical shift and coupling constants are obtained and disadvantage is the complex analysis of two dimensional NMR method.

Explanation of Solution

The advantages and disadvantages of two dimensional NMR methods over conventional one-dimensional techniques are as mentioned below.

S. No.	Advantages	Disadvantages
1	One dimensional NMR gives information about simple organic compounds while two dimensional NMR gives information about complex organic compounds.	The two dimensional NMR consists of different phases of cross and diagonal peak multi-pet.
2	Two dimensional NMR gives information about the chemical shift, coupling constant and signals of compounds.	The analysis of two dimensional NMR is complex as compared to one dimensional NMR.
3	The spectrum of complex compounds like proteins, heterocyclic compounds can only found by two dimensional NMR.	The intense peak may get obscured by cross-peaks in two dimensional NMR.

Interpretation Introduction

(b)

Interpretation:

The pulse sequence used in COSY and HETCOR experiments and behavior of magnetization vector $M$ as a result of applied pulses is to be stated.

Concept introduction:

The structure of heterocyclic complex compounds is determined by two dimensional NMR. The two dimensional NMR involves the techniques COSY and HETCOR. In the HETCOR experiment, the behavior of a single bond between two different nucleoli is observed. The COSY experiment interprets the resonance of correlating protons when there is spin-spin bond coupling in protons.

Answer to Problem 19.42QAP

The pulse sequence used in COSY and HETCOR experiments explains the single bond coupling between two different nucleoli and resonance in the compound. The magnetizing behavior of vector $M$ is explained on the basis of the evolution period and relaxation time in the detection coil.

Explanation of Solution

The COSY and HETCOR are two pulse sequence experiments. There is an evolution period between two pulses. The evolution period is the difference between time periods of pulses. In the chemical shift, coupling constant arises between this evolution period. The COSY and HETCOR experiments give the spectrum of the complex compound. The two pulses are applied with some time duration on a sample of the compound. This process is applied again after the sample of the compound achieves equilibrium. The spectrum of a compound is obtained by repeating this process several times. The chemical shift, coupling constant and signals are obtained by analyzing the spectrum.

The behavior of the magnetization, vector $M$ is found by applying an oscillatory voltage in the detection coil. The sample of a compound is kept in the detection coil. The magnetization vector is nonzero when the evolution period is restricted by relaxation time. Then the signal of applied pulses gets damped.

Interpretation Introduction

(c)

Interpretation:

The identification of resonances in ${\text{H}}_1$ and ${\text{H}}_{11}$ as a pair is to be stated.

Concept introduction:

The resonances in the compound are found by the spectrum of the compound.

Answer to Problem 19.42QAP

The resonances in atom ${\text{H}}_1$ and ${\text{H}}_{11}$ is explained on the basis of chemical shift between different protons present between ${\text{H}}_1$ and ${\text{H}}_{11}$ atoms.

Explanation of Solution

The resonance in the compound is found by the environment of ${\text{H}}_1$ and ${\text{H}}_{11}$.

The below figure represents the proton NMR of ${\text{H}}_1$.

The ${\text{H}}_1$ consist of two double bonds, first one between ${\text{H}}_1$ and ${\text{H}}_2$, second between ${\text{H}}_1$ and spatially near ${\text{H}}_{11}$. The chemical shift due to atom ${\text{H}}_1$ is $8.8\text{ppm}$. The intensity of ${\text{H}}_1$ atom increases due to the disappearance of peaks in the spectrum of ${\text{H}}_{11}$ atom.

The double bond is also present between atoms ${\text{H}}_{11}$ and ${\text{H}}_{10}$. The chemical shift due to the atom ${\text{H}}_{11}$ is $7.8\text{ppm}$.

Interpretation Introduction

(d)

Interpretation:

The differences in the resonance of atoms ${\text{H}}_1$ and ${\text{H}}_{11}$ is to be stated.

Concept introduction:

The resonance in different atoms can be distinguished by chemical shift, coupling constant and signal of pulses.

Answer to Problem 19.42QAP

The resonance of atoms ${\text{H}}_1$ and ${\text{H}}_{11}$ is distinguished by the chemical shift.

Explanation of Solution

The atom ${\text{H}}_1$ is attached in the thiophene ring. The sulfur is an electrophile so it has ann electron-withdrawing nature. The ${\text{H}}_1$ atom is de-shielded and shows the peak at a higher chemical shift. The ${\text{H}}_{11}$ .atom is attached with a benzene ring and shielded. The ${\text{H}}_{11}$ .atom shows a peak at low chemical shift.

On the basis of chemical shift resonance of atoms ${\text{H}}_1$ and ${\text{H}}_{11}$ is distinguished because both the atoms have same signal.

Interpretation Introduction

(e)

Interpretation:

The resonances in ${\text{H}}_2$, ${\text{H}}_8$, ${\text{H}}_9$, and ${\text{H}}_{10}$ is to be stated.

Concept introduction:

The resonance of different atoms in a compound is explained on the basis of chemical shift. The chemical shift is the resonate frequency of the nucleus in the magnetic field applied. On the basis of the position and chemical shift of nucleus resonance in the compound can be explained.

Answer to Problem 19.42QAP

The atoms ${\text{H}}_2$, ${\text{H}}_8$, ${\text{H}}_9$, and ${\text{H}}_{10}$ are assigned on the basis of chemical shift and signals.

Explanation of Solution

The peaks of ${\text{H}}_2$ in the spectrum is spilt due to the double bond between ${\text{H}}_1$ and ${\text{H}}_2$. The chemical shift of ${\text{H}}_2$ atom is due to the resonance due to the double bond between ${\text{H}}_1$ and ${\text{H}}_2$. The ${\text{H}}_2$ gives the signal of $9.2\text{ppm}$ due to the chemical shift. The double bond ${\text{H}}_8$ atom is split due to the adjacent atom ${\text{H}}_9$. The chemical shift of ${\text{H}}_8$ atom is $8\text{ppm}$. The ${\text{H}}_9$ gives multiplet due to adjacent atoms ${\text{H}}_8$ and ${\text{H}}_{10}$. The ${\text{H}}_{10}$ gives multiplet due to adjacent atoms ${\text{H}}_9$ and ${\text{H}}_{11}$. The environment of ${\text{H}}_9$ and ${\text{H}}_{10}$ atom is the same and shows the signal at the chemical shift $7.6\text{ppm}$.