RMN - Blindagem e deslocamento químico

KiFacil

8 Jun 202106:51

Summary

TLDRThis educational video delves into nuclear magnetic resonance (NMR), explaining the chemical shift concept and how it varies with the electronic environment around protons. Protons in different chemical environments absorb energy at different frequencies due to shielding by surrounding electrons. The video uses analogies like electrical currents to explain these concepts and introduces the use of tetramethylsilane (TMS) as a reference compound. It also touches on how chemical shifts are measured in parts per million (ppm) and ends with a teaser for the next topic: signal splitting.

Takeaways

🧲 The video continues the study of Nuclear Magnetic Resonance (NMR) and explains the interaction of radiofrequency waves with matter.
🌐 The concept of chemical shift and shielding is introduced, explaining how the environment of a proton affects its energy absorption in a magnetic field.
🔗 Protons absorb energy differently depending on their chemical environment and neighboring atoms, which influences the electron density around them.
💡 Electrons around a proton create an induced magnetic field that opposes an externally applied magnetic field, leading to shielding effects.
📉 The greater the electron density around a nucleus, the greater the induced field that opposes the external field, resulting in a lower effective magnetic field experienced by the nucleus.
🔢 The frequency at which a nucleus absorbs energy is directly proportional to the magnetic field it processes, with higher electron densities leading to lower absorption frequencies.
📊 Chemical shifts are measured relative to a reference compound, typically tetramethylsilane (TMS), which has equivalent protons and minimal interference.
🔄 The concept of 'parts per million' (ppm) is introduced as a standardized scale for chemical shifts, with TMS protons set at 0 ppm.
🧪 The video uses examples to illustrate how different protons in a molecule can have different chemical shifts based on their electron shielding.
📈 The NMR spectrum is explained to start from higher delta values and go to lower ones, with peak positions depending on proton shielding.

Q & A

What is the main topic of the video script?
-The main topic of the video script is Nuclear Magnetic Resonance (NMR), specifically focusing on the concepts of chemical shift and shielding.
How does the chemical environment affect the absorption of energy by protons in NMR?
-The chemical environment affects the absorption of energy by protons because it influences the electron density around the nucleus. Protons surrounded by more electron density experience a stronger induced magnetic field that opposes the external magnetic field, leading to a reduced effective magnetic field and absorption at lower frequencies.
What is meant by 'shielding' in the context of NMR?
-In NMR, 'shielding' refers to the effect where electrons around a nucleus create an induced magnetic field that opposes an externally applied magnetic field, thus reducing the effective magnetic field experienced by the nucleus.
How does the proximity to an electronegative atom like oxygen affect the shielding of protons?
-Proximity to an electronegative atom like oxygen leads to a decrease in electron density around the protons, resulting in less shielding. This makes the protons more sensitive to the external magnetic field and they absorb at higher frequencies.
What is the significance of the chemical shift in NMR spectroscopy?
-The chemical shift in NMR spectroscopy is significant because it allows for the identification of different chemical environments of protons within a molecule, which is crucial for determining molecular structures.
Why is tetramethylsilane (TMS) used as a reference compound in NMR?
-Tetramethylsilane (TMS) is used as a reference compound in NMR because it has 12 equivalent protons, leading to a single, intense signal that is far from the region where most hydrogen atoms absorb, minimizing interference.
How is the chemical shift measured in NMR?
-The chemical shift in NMR is measured in parts per million (ppm), which is calculated by dividing the difference in resonance frequency (in hertz) of a proton by the operating frequency of the spectrometer (in megahertz).
What is the significance of the term 'ppm' in NMR spectroscopy?
-In NMR spectroscopy, 'ppm' (parts per million) is a dimensionless unit used to express the chemical shift, providing a standardized scale that is independent of the magnetic field strength of the spectrometer.
How does the script describe the difference between protons A and B in the molecule methoxyethane?
-The script describes protons A in methoxyethane as being less shielded than protons B due to their proximity to the electronegative oxygen atom, resulting in protons A having a lower chemical shift (absorbing at lower frequencies) compared to protons B.
What is the purpose of the 'splitting' concept mentioned at the end of the script?
-The 'splitting' concept mentioned at the end of the script refers to the appearance of multiple peaks for a single proton due to the interaction with neighboring protons, which is important for determining the connectivity of atoms within a molecule.
Why is it important to understand the concept of chemical shift and shielding for NMR spectroscopy?
-Understanding chemical shift and shielding is important for NMR spectroscopy as it allows for the identification of different types of protons in a molecule and their relative positions, which is crucial for structural determination and analysis of organic compounds.

Outlines

00:00

🧲 Introduction to Nuclear Magnetic Resonance (NMR) and Chemical Shift

This paragraph introduces the concepts of nuclear magnetic resonance (NMR), focusing on how radiofrequency waves interact with matter. It explains the chemical shift and shielding effect, which influence the absorption of energy by protons in different chemical environments. Protons are shielded by electrons, and the density of electrons around the nucleus affects the effective magnetic field experienced by the nucleus. The paragraph uses an analogy of an electric current in a conductor to illustrate how electrons create an induced magnetic field that opposes an externally applied magnetic field. The higher the electron density, the greater the induced field, leading to a lower effective field and absorption at lower frequencies. The script also discusses the use of tetramethylsilane (TMS) as a reference compound in NMR spectroscopy, with its protons being well shielded and thus appearing at a standard position in the spectrum.

05:01

📊 Understanding Chemical Shifts and Predicting Spectra

This paragraph delves into the concept of chemical shifts, which are measured in parts per million (PPM) relative to a reference compound, typically TMS. It explains that the resonance of protons from TMS is set at 0 PPM by convention. The script guides through a qualitative prediction of the NMR spectrum for methoxy-ethane, discussing the classification of equivalent protons and their chemical shifts based on their electronic environment. Protons in different groups (A and B) are compared, with group B protons being more shielded due to their proximity to the electronegative oxygen atom in the nitrile group, resulting in a higher chemical shift relative to TMS. The paragraph concludes with a teaser for the next video, which will cover signal splitting in NMR spectra.

Mindmap

Keywords

💡Nuclear Magnetic Resonance (NMR)

Nuclear Magnetic Resonance (NMR) is a physical phenomenon in which nuclei in a magnetic field absorb and re-emit electromagnetic radiation, and it is the basis for NMR spectroscopy. In the context of the video, NMR is used to study the interaction of radiofrequency waves with matter, specifically how the absorption of energy by protons depends on their chemical environment. The video aims to further the understanding of NMR by explaining concepts like chemical shift and shielding.

💡Chemical Shift

Chemical shift is a phenomenon in NMR spectroscopy where the resonant frequency of a nucleus depends on the magnetic field it experiences. This shift is influenced by the electron density around the nucleus, which is affected by the chemical environment. The video explains that not all protons absorb energy at the same magnetic field strength due to differing chemical environments, which leads to the concept of chemical shift.

💡Shielding

Shielding in NMR refers to the effect where electrons around a nucleus create a secondary magnetic field that opposes the applied magnetic field. This reduces the effective magnetic field experienced by the nucleus. The video uses the analogy of an electric current in a circular conductor to explain how electrons shield the nucleus from the external magnetic field, leading to different absorption frequencies for protons in different chemical environments.

💡Electronegativity

Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. In the script, it is mentioned that oxygen, being highly electronegative, tends to draw electron density towards itself, which affects the shielding of protons near it. This concept is crucial for understanding how the chemical environment impacts the chemical shift in NMR.

💡Tetramethylsilane (TMS)

Tetramethylsilane (TMS) is a common reference compound used in NMR spectroscopy. It has 12 equivalent protons, which results in a single, intense signal that is far upfield (at low chemical shift values). The video explains that TMS is used as a reference because its protons are well shielded due to the electronegativity of silicon, which helps to standardize chemical shifts in NMR spectra.

💡Parts Per Million (PPM)

Parts per million (PPM) is a dimensionless concentration unit used to express the chemical shift in NMR spectroscopy. It is calculated by dividing the difference in resonance frequency by the operating frequency of the spectrometer. The video mentions that TMS is assigned a chemical shift of 0 PPM, and other protons' shifts are measured relative to this reference.

💡Upfield and Downfield

In NMR spectroscopy, upfield refers to signals that occur at lower frequencies (lower chemical shifts), while downfield refers to signals at higher frequencies (higher chemical shifts). The video discusses how the chemical environment affects the protons' shielding, leading to different chemical shifts that appear upfield or downfield relative to the TMS reference.

💡Equivalent Protons

Equivalent protons are protons that are in the same chemical environment and thus have the same chemical shift. The video describes how to group equivalent protons in a molecule, such as methoxyethane, to predict their NMR signals. This concept is important for interpreting NMR spectra and understanding the structure of molecules.

💡Signal Splitting

Signal splitting, or multiplicity, refers to the appearance of an NMR signal as a multiplet due to the coupling between neighboring nuclei. Although not explicitly mentioned in the script, the预告 of the next video suggests that this concept will be covered, which is important for understanding the complexity of NMR spectra and the interactions between protons.

💡Methoxyethane

Methoxyethane is a molecule used as an example in the video to illustrate the concept of chemical shift and the grouping of equivalent protons. The video explains how the protons in methoxyethane can be classified into equivalent groups and how their chemical shifts will differ based on their proximity to electronegative atoms like oxygen.

Highlights

Introduction to Nuclear Magnetic Resonance (NMR) studies.

Understanding the interaction of radiofrequency waves with matter.

The concept of chemical shift and how it depends on the chemical environment of protons.

The idea that not all protons absorb energy at the same magnetic field strength.

The influence of neighboring atoms on the electron density around a proton.

Proton shielding by surrounding electrons is compared to an electric current in a coil.

The induced magnetic field opposes the applied magnetic field, affecting the effective field experienced by the nucleus.

Frequency of absorption is directly proportional to the magnetic field experienced by the nucleus.

Example of protons in different chemical environments absorbing at different frequencies.

The importance of the chemical shift in understanding proton absorption.

Explanation of how a carbon atom's connection to hydrogen affects electron density.

The concept of chemical shift is crucial for distinguishing between different protons.

The use of tetramethylsilane (TMS) as a reference compound in NMR spectroscopy.

TMS has 12 equivalent protons, leading to a single, intense signal in the spectrum.

The chemical shift is measured in parts per million (ppm) relative to TMS.

The NMR spectrometer starts scanning from higher to lower delta values.

The positions of peaks in the spectrum depend on the proton shielding.

Qualitative prediction of the spectrum for methoxyethane (Trio).

Separation of equivalent protons into groups with the same chemical shift.

The difference in chemical shift between protons A and B in methoxyethane due to their proximity to electronegative atoms.

Anticipation of the concept of signal splitting in the next video.