30c: Determining molecular structure from H-NMR spectra

Roxi Hulet

9 Feb 202118:40

EducationalLearning

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TLDRThis educational video script guides viewers through basic NMR analysis, using molecular formulas and proton NMR spectra to deduce molecular structures. It covers calculating HDI, interpreting integrations, and drawing molecular fragments based on splitting patterns, ultimately assembling the complete molecule.

Takeaways

🔍 Start with the molecular formula and calculate the Hydrogen Deficiency Index (HDI) to get initial information about the presence of double bonds, triple bonds, or rings.
📊 Check the integration of the NMR spectrum to ensure it matches the number of hydrogen atoms in the molecular formula.
🧩 Draw fragments of the molecule that are consistent with the splitting pattern and integration for each peak.
🔗 Combine the fragments to form the complete molecular structure.
🌐 An HDI of zero indicates no double bonds, triple bonds, or rings are present in the molecule.
🔬 The integration ratio can be calculated by dividing the peak areas in the NMR spectrum to determine the relative number of hydrogen atoms.
🔍 Splitting patterns in the NMR spectrum (e.g., septet, doublet, quartet) provide information about the number of neighboring hydrogen atoms within three bonds.
🔎 The presence of a double bond or ring can be inferred from an HDI greater than zero.
🌀 Benzene rings typically show peaks around seven parts per million in the NMR spectrum and have an HDI of 4.
🏷️ Halogens like iodine or bromine can be placed in the molecule based on their lack of splitting influence on neighboring hydrogen atoms.

Q & A

What is the first step in the basic NMR analysis described in the video?
-The first step is to start with the molecular formula and calculate the Hydrogen Deficiency Index (HDI) to gather initial information about the presence of double bonds or rings.
How is the HDI calculated, and what does an HDI of zero indicate?
-HDI is calculated by taking 3 times the number of carbon atoms, adding 2, and then subtracting the number of hydrogen atoms and halogens. An HDI of zero indicates no double bonds, triple bonds, or rings are present in the molecule.
What do the integration values in an NMR spectrum represent?
-The integration values represent the ratio of the number of hydrogen atoms (protons) that contribute to each peak in the spectrum.
How is the ratio of protons determined from the integration values?
-The integration values are divided by the smallest integration value to get the ratio of protons.
What does a splitting pattern indicate in an NMR spectrum?
-The splitting pattern indicates the number of neighboring hydrogen atoms within three bonds of a given hydrogen atom, following the n+1 rule where n is the number of neighbors.
In the first example, what does the septet splitting pattern tell us about the hydrogen atom?
-The septet splitting pattern indicates that the hydrogen atom has six neighboring hydrogen atoms, suggesting the presence of two methyl groups (each with three hydrogen atoms) nearby.
What does a doublet splitting pattern indicate about the number of neighboring hydrogen atoms?
-A doublet splitting pattern indicates that there is one neighboring hydrogen atom (n+1 = 2, so n = 1).
Why are certain chemical shifts ignored initially in the analysis process?
-Chemical shifts are initially ignored because the analysis focuses on the integration and splitting patterns to build molecular fragments. The shifts may be considered later if necessary.
How do you handle complicated splitting patterns, such as a quintet or sextet?
-Complicated splitting patterns, like a quintet or sextet, are initially set aside if they seem tricky. The analysis proceeds with simpler patterns, and the complex ones are revisited later to see how they fit with other fragments.
What is the significance of a peak around 7 ppm in an NMR spectrum?
-A peak around 7 ppm typically corresponds to hydrogen atoms on a benzene ring, indicating the presence of an aromatic ring in the molecule.

Outlines

00:00

🔍 Basic NMR Analysis

This paragraph introduces the process of basic NMR (Nuclear Magnetic Resonance) analysis. The focus is on proton NMR spectroscopy, where a molecular formula is used alongside an NMR spectrum to determine the structure of a molecule. The steps outlined include calculating the Hydrogen Deficiency Index (HDI) to infer the presence of double bonds, triple bonds, or rings. Next, the integration of the NMR peaks is discussed to ensure it matches the number of hydrogen atoms in the molecular formula. The process then involves drawing fragments of the molecule that match the splitting patterns and integration, eventually assembling these fragments into the complete molecular structure. The example given involves a molecule with a molecular formula that results in an HDI of zero, indicating no double or triple bonds or rings, and the integration confirms the presence of one hydrogen atom generating a peak and six hydrogen atoms generating another peak.

05:00

🧩 Fragment Assembly in NMR Analysis

This paragraph delves deeper into the process of assembling molecular fragments based on NMR data. It starts by calculating the HDI for a molecule with the formula C2H4Cl2, which results in an HDI of zero, suggesting no complex structures like double bonds or rings. The integration ratios are then analyzed, confirming the presence of one hydrogen atom and three hydrogen atoms in different parts of the molecule. The speaker illustrates how to draw fragments that match the integration and splitting patterns, such as a quartet indicating three neighboring hydrogens and a doublet indicating one. The paragraph also discusses the placement of chlorine atoms in the molecule. The process is demonstrated with additional examples, including a molecule with C4H9Br, where the HDI is zero, and the integration confirms the presence of various hydrogen atoms in different environments.

10:02

🔬 Advanced Fragment Assembly and Double Bond Consideration

This paragraph continues the discussion on assembling molecular fragments using NMR data, with a focus on more complex scenarios involving double bonds. The HDI for a molecule with the formula CHC3H4Br is calculated, resulting in an HDI of one, indicating the presence of a double bond or a ring. The integration is analyzed, confirming the presence of one, two, and three hydrogen atoms in different environments. The speaker then attempts to draw fragments that match the integration and splitting patterns, such as a doublet and a singlet. The paragraph highlights the importance of considering the chemical shift in determining the placement of hydrogen atoms, especially when they are part of a carbon-carbon double bond. The example concludes with the successful assembly of the molecular structure based on the NMR data.

15:02

🌐 Benzene Ring Identification in NMR Spectroscopy

This paragraph focuses on identifying a benzene ring in an NMR spectrum. The HDI for a molecule with the formula C8H10 is calculated, resulting in an HDI of four, which is a strong indicator of a benzene ring. The integration ratios are analyzed, confirming the presence of five hydrogen atoms in the benzene ring and additional hydrogen atoms in other parts of the molecule. The speaker discusses the chemical shift of the benzene hydrogens and how they typically appear around seven parts per million in the NMR spectrum. The process involves drawing fragments that match the integration and splitting patterns, such as a quartet and a triplet, and assembling these fragments to form the complete molecular structure, which includes a benzene ring and additional carbon chains.

Mindmap

Keywords

💡NMR Analysis

NMR Analysis, or Nuclear Magnetic Resonance Analysis, is a technique used to determine the structure of molecules. In the video, it is used to analyze proton NMR spectra along with molecular formulas to deduce the structure of the molecule. The process involves calculating the Hydrogen Deficiency Index (HDI), examining integration to match the number of hydrogen atoms, and drawing fragments based on splitting patterns and integration.

💡Proton NMR

Proton NMR, or Proton Nuclear Magnetic Resonance, is a type of NMR spectroscopy that focuses on the behavior of hydrogen atoms (protons) in a molecule. The video script discusses analyzing proton NMR spectra to determine the structure of molecules, highlighting how the integration and splitting patterns in the spectra provide insights into the molecular structure.

💡Molecular Formula

A molecular formula is a representation of the numbers and types of atoms in a molecule. In the context of the video, the molecular formula is used in conjunction with NMR data to determine the structure of the molecule. The script mentions calculating the Hydrogen Deficiency Index from the molecular formula to gain initial insights into the presence of double bonds, triple bonds, or rings.

💡Hydrogen Deficiency Index (HDI)

The Hydrogen Deficiency Index (HDI) is a calculation used in NMR analysis to determine the presence of unsaturation in a molecule, such as double bonds or rings. In the video, HDI is calculated by taking three times the number of carbon atoms plus two, subtracting the number of hydrogens and halogens, to provide initial information about the molecule's structure.

💡Integration

Integration in NMR spectroscopy refers to the relative area under the peaks in a spectrum, which corresponds to the number of protons contributing to that peak. The video script emphasizes the importance of matching the calculated integration with the number of hydrogen atoms in the molecular formula to ensure the accuracy of the molecular structure determination.

💡Splitting Pattern

The splitting pattern in NMR spectroscopy is the result of protons being influenced by neighboring protons, leading to multiple peaks for a single chemical environment. In the video, the splitting pattern is used to deduce the number of neighboring protons (n) for each hydrogen atom, which helps in drawing fragments of the molecule.

💡Septet

A septet in NMR spectroscopy is a peak that is split into seven components due to the influence of neighboring protons. The video script uses the example of a septet to illustrate how a single hydrogen atom with six neighboring protons (n=6) would result in such a splitting pattern, helping to deduce the molecular structure.

💡Doublet

A doublet in NMR spectroscopy is a peak that is split into two components, indicating that the proton is influenced by one neighboring proton (n=1). The script discusses using the doublet to infer the presence of a single hydrogen atom with one neighbor, which is crucial for reconstructing the molecular structure.

💡Quartet

A quartet in NMR spectroscopy is a peak that is split into four components, suggesting that the proton is influenced by three neighboring protons (n=3). The video script uses the quartet to deduce the presence of a hydrogen atom with three neighbors, contributing to the molecular structure determination.

💡Benzene Ring

A benzene ring is a type of aromatic ring structure with alternating single and double bonds, consisting of six carbon atoms. In the video, the presence of a benzene ring is inferred from the HDI calculation and the chemical shift of the peaks, which is consistent with the typical NMR characteristics of benzene.

💡Halogens

Halogens, such as chlorine and bromine, are non-metal elements that can be part of a molecular structure. In the video, halogens are considered in the HDI calculation and are used to fill in the last bonds in the molecular structure, indicating their presence and position in the molecule.

Highlights

Introduction to basic NMR analysis using proton NMR spectrum, molecular formula, and integration to determine molecular structure.

Explanation of HDI (Hydrogen Deficiency Index) calculation to identify presence of double bonds, triple bonds, or rings.

Method of using integration to match the number of hydrogen atoms in the molecular formula.

Technique of drawing molecular fragments based on splitting patterns and integration.

Example of HDI calculation for a molecule with no double bonds, triple bonds, or rings (HDI = 0).

Integration analysis showing a ratio of hydrogen atoms and consistency with the molecular formula.

Drawing fragments for a septet peak indicating one hydrogen atom with six neighbors.

Interpretation of a doublet peak with six hydrogen atoms and its corresponding molecular fragment.

HDI calculation for C2H4Cl2 indicating no double bonds, triple bonds, or rings (HDI = 0).

Integration analysis for C2H4Cl2 confirming the ratio of hydrogen atoms matches the molecular formula.

Fragment drawing for a quartet peak indicating three hydrogen atoms with three neighbors.

HDI calculation for C4H9Br showing no double bonds, triple bonds, or rings (HDI = 0).

Integration analysis for C4H9Br with a complex ratio of hydrogen atoms and corresponding molecular fragments.

Drawing fragments for a sextet peak indicating one hydrogen atom with five neighbors.

HDI calculation for CHC with one double bond or ring (HDI = 1).

Integration analysis for CHC with a singlet peak indicating two hydrogen atoms with no neighbors.

Fragment drawing for a doublet peak indicating one hydrogen atom with one neighbor in a carbon-carbon double bond.

HDI calculation for C8H10 indicating a benzene ring (HDI = 4).

Integration analysis for C8H10 with a peak at 7 ppm corresponding to benzene hydrogens.

Drawing fragments for a quartet peak indicating two equivalent hydrogen atoms with three neighbors.

Final assembly of molecular fragments to form a complete structure for C8H10.

Transcripts

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