lecture 13 - Molecular Spectroscopy Nuclear magnetic...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
1 Nuclear Magnetic Nuclear Magnetic Resonance Resonance Chapter 13 Molecular Spectroscopy Molecular Spectroscopy ± Nuclear magnetic resonance (NMR) spectroscopy : a spectroscopic technique that gives us information about the number and types of atoms in a molecule, for example, about the number and types of hydrogen atoms using 1 H-NMR spectroscopy carbon atoms using 13 C-NMR spectroscopy phosphorus atoms using 31 P-NMR spectroscopy Nuclear Spin States ± An electron has a spin quantum number of 1/2 with allowed values of +1/2 and -1/2 this spinning charge creates an associated magnetic field in effect, an electron behaves as if it is a tiny bar magnet and has what is called a magnetic moment Nuclear Spin States Nuclear Spin States ± The same effect holds for certain atomic nuclei any atomic nucleus that has an odd mass number, an odd atomic number, or both also has a spin and a resulting nuclear magnetic moment the allowed nuclear spin states are determined by the spin quantum number, I , of the nucleus Nuclear Spin States a nucleus with spin quantum number I has 2 I + 1 + 1 spin states; if I = 1/2, there are two allowed spin states The table below gives the spin quantum numbers and allowed nuclear spin states for selected isotopes of elements common to organic compounds 1 H 2 H 12 C 13 C 14 N 16 O 31 P 32 S Element Nuclear spin quantum number ( I ) Number of spin states 1/2 1 0 0 0 1/2 1 2312 31 1/2 21 Nuclear Spins in B Nuclear Spins in B 0 within a collection of 1 H and 13 C atoms, nuclear spins are completely random in orientation when placed in a strong external magnetic field of strength B 0 , however, interaction between nuclear spins and the applied magnetic field is quantized, with the result that only certain orientations of nuclear magnetic moments are allowed
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
2 Nuclear Spins in B Nuclear Spins in B 0 for 1 H and 13 C, only two orientations are allowed Nuclear Spins in B Nuclear Spins in B 0 ± In an applied field strength of 7.05T, which is readily available with present-day superconducting electromagnets, the difference in energy between nuclear spin states for 1 H is approximately 0.120 J (0.0286 cal)/mol, which corresponds to electromagnetic radiation of 300 MHz (300,000,000 Hz) 13 C is approximately 0.030 J (0.00715 cal)/mol, which corresponds to electromagnetic radiation of 75MHz (75,000,000 Hz) Nuclear Spin in B 0 the energy difference between allowed spin states increases linearly with applied field strength Nuclear Spin in B Nuclear Spin in B 0 values shown here are for 1 H nuclei Nuclear Magnetic Resonance when nuclei with a spin quantum number of 1/2 are placed in an applied field, a small majority of nuclear spins are aligned with the applied field in the lower energy state Nuclear Magnetic Resonance Nuclear Magnetic Resonance the nucleus begins to precess and traces out a cone-shaped surface, in much the same way a spinning top or gyroscope traces out a cone- shaped surface as it precesses in the earth’s gravitational field we express the rate of precession as a frequency in hertz
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 16

lecture 13 - Molecular Spectroscopy Nuclear magnetic...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online