Lecture Organic chemistry - Chapter 10: Nuclear magnetic resonance (NMR) spectroscopy. In this chapter, the following content will be discussed: What is spectroscopy? General spectrometer, the chemical shift δ, complex splitting patterns,.and other contents. | Chapter 10: Nuclear Magnetic Resonance (NMR) Spectroscopy What is spectroscopy? Molecular response to radiative stimulus is quantized (“geared”). (Molecule = nuclei + electrons). Excitation: 1. Electronic (UV-visible spectra) 2. Vibrational (IR) 3. Rotation (microwave) 4. Nuclear spin orientation in magnet (NMR) Spectrometer scans ν to find ΔE : The spectrum ΔE = hν ν = c/λ Excitation causes an “absorption”, a peak in a graph A General Spectrometer kcal mol-1 What is ∆E in NMR? Nuclei behave as tiny magnets (random orientation). In a magnetic field H0, they organize with (α, lower energy) or against (β, higher energy) the field. ∆E = hν0 resonance frequency: The frequency that matches exactly the energy difference. Protons as Tiny Magnets Line Up With and Against an External Magnetic Filed Ratio α:β ~ 1:1 Absorption of Light, Spin Flip, and Resonance Spectral Line ν0 is proportional to H0. (ν0 is specific for element/isotope) For 1H: at H0 = 21,150 gauss, ν0 = 90 MHz. ∆E (300 MHz) ~ | Chapter 10: Nuclear Magnetic Resonance (NMR) Spectroscopy What is spectroscopy? Molecular response to radiative stimulus is quantized (“geared”). (Molecule = nuclei + electrons). Excitation: 1. Electronic (UV-visible spectra) 2. Vibrational (IR) 3. Rotation (microwave) 4. Nuclear spin orientation in magnet (NMR) Spectrometer scans ν to find ΔE : The spectrum ΔE = hν ν = c/λ Excitation causes an “absorption”, a peak in a graph A General Spectrometer kcal mol-1 What is ∆E in NMR? Nuclei behave as tiny magnets (random orientation). In a magnetic field H0, they organize with (α, lower energy) or against (β, higher energy) the field. ∆E = hν0 resonance frequency: The frequency that matches exactly the energy difference. Protons as Tiny Magnets Line Up With and Against an External Magnetic Filed Ratio α:β ~ 1:1 Absorption of Light, Spin Flip, and Resonance Spectral Line ν0 is proportional to H0. (ν0 is specific for element/isotope) For 1H: at H0 = 21,150 gauss, ν0 = 90 MHz. ∆E (300 MHz) ~ 10-5 kcal mol-1. Nα/Nβ = NMR “active” nuclei: 1H and 13C (not 12C) At H0 = 70500 gauss, ν0 = 300 MHz. H0 (earth): gauss! A Hypothetical NMR Spectrum: Active Elements Absorb at Differing Frequencies The NMR Spectrometer Solvents: CDCl3, CD2Cl2, THF-d8, etc. High Resolution NMR! Why are there two peaks? The Chemical Shift Causes shielding, ., Peak moves to the right Electrons in Vicinity of Nucleus Affect ν Consider H+: No e, no shielding, peak furthest to the left. But: When we add an e-withdrawing group: ., CH3 Cl, causes deshielding (to left). Chemical shift provides a finely tuned picture of electronic environment around each H. C H H H H + - Electrons cause shielding: Peak moves upfield (to right). If we substitute with e-negative groups, shielding of observed nucleus is diminished; or nucleus is “deshielded” (relatively). One e-poor neighbor Two e-poor neighbors The Chemical Shift δ The position of peak relative to an internal standard. Most common is .
