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.