{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

NMR - CH 19 NMR Spectroscopy HW 19-7 19-11 19-31 Proton NMR...

Info icon This preview shows pages 1–12. Sign up to view the full content.

View Full Document Right Arrow Icon
CH 19: NMR Spectroscopy HW: 19-7, 19-11, 19-31 Proton NMR Spectrum
Image of page 1

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

View Full Document Right Arrow Icon
NMR Spectroscopy-The Physical Principles N S Based upon nuclear spin Spinning charges produce magnetic fields S N 1 H, 13 C, 19 F, and 31 P have two possible quantum mechanical spin states, +1/2 and -1/2 *Key Concepts
Image of page 2
NMR Spectroscopy-The Physical Principles In the absence of an applied magnetic field (B 0 ) the spin states have the same E. When a magnetic field is applied the states split in E. This opens up transitions (absorptions) that can be induced with radiofrequencies. These are detected during an NMR experiment and are called resonances .
Image of page 3

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

View Full Document Right Arrow Icon
General Instrument Schematic Spins are aligned with the large magnets. • The magnetic component of the RF input at the right frequency causes some spins to flip. Spin flip is detected by the RF output coils.
Image of page 4
NMR Spectroscopy-The Physical Principles The energy gap is very small, about 0.1 cal/mol for an applied field of 20 Tesla. Compare this to a typical IR transition (1-10 kcal/mol) and electronic transition (1000 kcal/mol).
Image of page 5

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

View Full Document Right Arrow Icon
Magnetic Fields Magnetic field is usually given in units of gauss or Tesla. These are units of energy. 1 T = = N/qv The magnetic field is the force acting on a charged particle (q) moving with velocity (v) through a magnetic field. 1 G = 10 -4 T Newton (coulomb)(meter/sec)
Image of page 6
NMR Spectroscopy-The Physical Principles The frequency of the transition is ν 0 = γ B 0 2 π where B 0 is the applied magnetic field and γ is the magnetogyric ratio. γ has a different value for each type of nucleus. Example 19-1 Many proton NMR instruments employ a magnet that provides a field strength of 4.69 T. At what frequency would the hydrogen nucleus absorb in such a field? ν 0 = γ B 0 2 π = (2.68 x 10 8 T -1 s -1 )(4.69 T)/2 π = 2.00 x 10 8 s -1 = 200 MHz Resonance or Larmor frequency
Image of page 7

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

View Full Document Right Arrow Icon
NMR Spectroscopy-The Physical Principles The dependence of the splitting energy on magnetic field presents a small problem that we will address later: If two researchers want to compare their data using magnets of different strengths, they have to adjust for that difference.
Image of page 8
NMR Spectroscopy The magnetic field strength can be varied at constant RF frequency or vice versa.
Image of page 9

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

View Full Document Right Arrow Icon
NMR Spectroscopy-The Physical Principles Relaxation Processes in NMR If the nuclei that become aligned against the magnetic field do not “relax” back, then the signal will decrease or “saturate”. Relaxation processes should be very fast to avoid saturation. Unfortunately, the faster the relaxation, the more NMR lines are broadened.
Image of page 10
NMR Spectroscopy-The Physical Principles Line Broadening We examined several mechanisms of line broadening in CH 8 (pp. 220): Doppler and Pressure Broadening. *Key Concept: The Heisenberg Uncertainty Principle is another source of line broadening. Δ t > 1/ Δν Δ E = h/ Δ t Δ x Δ p = h/2 The width of a line resulting from a transition between two states would approach zero only if the lifetime of the excited state approached infinity.
Image of page 11

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

View Full Document Right Arrow Icon
Image of page 12
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern