3_B.Tech_NMR_RC_Part 1 & 2
Page 1: Nuclear Magnetic Resonance Spectroscopy Principle & Applications
Introduction to Nuclear Magnetic Resonance (NMR) Spectroscopy and its significance in characterizing organic molecules.
Page 2: Energy Absorption
Formula: DE = hn
Electromagnetic radiation is absorbed when the energy of a photon matches the energy difference between two states.
Page 3: Types of States in NMR
NMR focuses on changes in the direction of spin orientation due to radiofrequency radiation absorption.
Key states: electronic, vibrational, rotational, nuclear spin across UV-Vis, infrared, microwave, and radiofrequency ranges.
Page 4: Applications of NMR
NMR is a vital analytical technique to identify carbon-hydrogen frameworks in organic molecules.
Two common types of NMR spectroscopy:
1H NMR: identifies the type and number of hydrogen atoms.
13C NMR: identifies different carbon atom types.
NMR utilizes low-energy radio waves to interact with nuclei (1H and 13C), influencing nuclear spins.
Page 5: Understanding Nuclear Spin
Nuclei with odd atomic or mass numbers exhibit nuclear spin, generating a magnetic field due to their charge and spin.
Page 6: The Effect of External Magnetic Field
In an external magnetic field, spinning protons behave like bar magnets, with states of lower and higher energy depending on alignment.
Page 7: Energy States of Nuclei
Spinning nuclei can align with or against an external magnetic field, and absorption of a photon can flip the spin state of protons.
Page 8: Useful Nuclei in Organic Chemistry
Protons and 13C are key:
1H: 99% natural abundance.
13C: 1.1% natural abundance.
Only nuclei with odd mass or atomic numbers exhibit NMR signals (e.g., 1H, 13C, 19F, 31P).
Page 9: Spin Distribution in Absence of a Magnetic Field
In the absence of an external magnetic field, nuclear spins are randomly distributed.
Page 10: Nuclear Magnetic Moments Alignment
An external magnetic field causes nuclear magnetic moments to align either parallel or antiparallel to the field direction.
Page 11: Excess Moments and Energy Difference
Slight excess of nuclear moments aligns parallel to the applied field, creating a very small energy difference (<0.1 cal) between states.
Page 12: NMR Characterization Variables
Two key variables define NMR:
Applied magnetic field (B0) measured in tesla (T).
Frequency (n) of radiation for resonance, measured in hertz (Hz) or megahertz (MHz).
Page 13: Energy Differences without Magnetic Field
No energy difference exists without a magnetic field; energy differences are proportional to the external field strength.
Page 14: Relationship Between Energy and Magnetic Field Strength
Leveraging the formula: DE = hn = hB0/2, where gyromagnetic ratio (γ) is specific to each nucleus, indicative of required photon for resonance.
Low-energy radio frequencies are employed.
Page 15: Key Relationships in NMR
The absorbed radiation's frequency correlates with energy difference between nuclear spin states and is proportional to the external magnetic field strength.
Page 16: Different Frequencies for Different Nuclei
In a B0 = 4.7 T field:
1H absorbs at 200 MHz.
13C absorbs at 50.4 MHz.
Page 17: Molecular Environment's Effect on NMR Frequencies
The absorbed frequency for a specific nucleus varies based on its molecular environment, crucial for structural determination.
Page 18: Relationship of Frequency and Magnetic Field
The frequency for resonance and magnetic field are proportionally related, characteristic of the NMR spectrometer's designation (e.g., 300 MHz).
Page 19: Overview of NMR Spectrometry
Sample dissolved in solvent within NMR tube, exposed to magnetic field and RF radiation. Detected energy is used to produce an NMR spectrum.
Modern NMR employs superconducting magnets.
Page 20: NMR Spectrum Visualization
NMR spectrum plots intensity of peaks against chemical shifts (ppm).
Page 21: Characteristics of NMR Absorptions
NMR absorptions manifest as sharp peaks; chemical shifts range typically from 0 to 10 ppm; reference peak at 0 ppm due to TMS.
Page 22: Chemical Shift in NMR Spectrum
Chemical shift measured in parts per million (ppm), relative to TMS signal.
Page 23: Chemical Shift Calculation
Chemical shift = (Shift downfield from TMS in Hz) / Total spectrometer frequency (Hz).
Delta scale utilized for consistency across spectrometer types.
Page 24: Role of TMS in NMR
TMS is an inert reference compound yielding a single peak; organic protons typically reside downfield of TMS peak.
Page 26: NMR Spectrum Features
Four key features of a 1H NMR spectrum inform structure:
Number of signals
Position of signals
Intensity of signals
Spin-spin splitting of signals.
Page 27: Varied Proton Environments in NMR
Distinct proton environments lead to varied absorption frequencies, crucial for structure determination.
Page 28: Shielding and Deshielding in Protons
Protons' magnetic environments influence their absorption frequencies: electron density impacts shielding.
Page 29: Shielding Effects in Magnetic Field
The external magnetic field interacts with electron-generated local magnetic fields, affecting perceived field strength for protons.
Page 30: Shielding from Triple Bonds
Electrons in carbon-carbon triple bonds cause induced magnetic fields that further weaken applied fields, leading to proton shielding.
Page 31: Varying Shielding Among Protons
Protons in different chemical environments are subject to heterogeneous shielding influences, creating distinct absorption characteristics.
Page 32: Example of NMR Spectrum for Shielded Protons
Representation showing differences in absorption fields among shielded and deshielded protons.
Page 33: Consequences of Deshielding
Deshielded protons require higher frequencies for resonance and show downfield shifts in absorption.
Page 34-35: Benzene Ring NMR Signals
In benzene, carbon electrons generate magnetic fields that reinforce applied fields, moving protons downfield due to deshielding.
Page 36: Aldehyde Proton in NMR
The aldehyde proton absorption location illustrates deshielding effects near electronegative atoms.
Page 37: Summary of Shielding and Deshielding
Concepts summarizing effects of electron density on NMR signals, elucidating upfield versus downfield shifts in resonance.
Page 38: Signals Correspond to Proton Environments
Count of NMR signals indicates distinct proton types, aiding in structural identification of compounds.
Page 39-40: Case Studies of NMR Signals Count
Examples with different organic compounds detailing NMR signal counts based on proton types and environments.
Page 41: Analyzing NMR Peak Patterns
Understanding different splitting types: triplets, doublets, quartet formations due to adjacent protons.
Page 42: Chemical Shift Values Overview
Summary of typical chemical shifts for various proton types in organic compounds.
Page 43-45: Electronegativity Effects on Proton Shifts
Influence of electronegativity on chemical shifts, noting deshielding effects alongside electron cloud interactions.
Page 46: Integrating NMR Signals
Key principles on interpreting area under NMR peaks, correlating with number of absorbing protons.
Page 47-48: NMR Integration Illustration
Example provides insights into determining proton types and amounts via integration in an NMR spectrum.
Page 49-55: Spin-Spin Splitting Mechanics
Spin-spin interactions detailed with rules defining splitting patterns observable in NMR spectra.
Page 56: Rules of Splitting in NMR
Overview of conditions influencing splitting patterns based on proton equivalency and proximity across bonds.
Page 57-61: Example Spectra and Analyzing Aromatic Signals
Visual illustrations of 1H NMR spectra from specific compounds; analysis of benzene-related peaks.
Page 62: NMR Structure Determination Example
Steps detailing how to interpret NMR data to deduce structures from given molecular formulas, showcasing practical applications.
Page 63-66: Final Structure Analysis Using NMR Data
Logical steps and calculations leading to the resolution of structures from NMR spectral information along with decision-making based on chemical shifts.
Page 67-68: Practice Problems Overview
Problem statements posed for practice in identifying structures and molecular formulas using spectral data.