THe NMR - NMR = Nuclear Magnetic Resonance Physical...

Info iconThis preview shows pages 1–4. Sign up to view the full content.

View Full Document Right Arrow Icon
NMR = N uclear M agnetic R esonance Some (but not all) nuclei, such as 1 H, 13 C, 19 F, 31 P have nuclear spin. A spinning charge creates a magnetic moment, so these nuclei can be thought of as tiny magnets. If we place these nuclei in a magnetic field, they can line up with or against the field by spinning clockwise or counter clockwise. Alignment with the magnetic field (called α ) is lower energy than against the magnetic field (called β ). How much lower it is depends on the strength of the magnetic field Physical Principles: Note that for nuclei that don’t have spin, such as 12 C, there is no difference in energy between alignments in a magnetic field since they are not magnets. As such, we can’t do NMR spectroscopy on 12 C. S A spinning nucleus with it's magnetic field aligned with the magnetic field of a magnet α-σπιν στατε, φαωοραβλε, λοϖερενεργψ Ν Σ Ν N S β-σπιν στατε, υνφαωοραβλε, ηιγηερενεργψ Α σπιννινγ νυχλευσϖιτηιτ220dσμ αγνετιχφιελδ αλιγνεδ αγαινσττηε μ αγνετιχφιελδ οφαμ αγνετ Σ Ν
Background image of page 1

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

View Full DocumentRight Arrow Icon
NMR: Basic Experimental Principles Imagine placing a molecule, for example, CH 4 , in a magnetic field. We can probe the energy difference of the α - and β - state of the protons by irradiating them with EM radiation of just the right energy. In a magnet of 7.05 Tesla, it takes EM radiation of about 300 MHz (radio waves). So, if we bombard the molecule with 300 MHz radio waves, the protons will absorb that energy and we can measure that absorbance. In a magnet of 11.75 Tesla, it takes EM radiation of about 500 MHz (stronger magnet means greater energy difference between the α - and β - state of the protons) But there’s a problem. If two researchers want to compare their data using magnets of different strengths, they have to adjust for that difference. That’s a pain, so, data is instead reported using the “chemical shift” scale as described on the next slide. E B o ∆Ε=ηξ300ΜΗζ ∆Ε=ηξ500ΜΗζ 7.05Τ 11.75Τ α προτονσπινστατε (λοϖερενεργψ29 β προτονσπινστατε (ηιγηερενεργψ29 Graphical relationship between magnetic field (B o) and frequency (n) for 1 H NMR absorptions at no magnetic field, there is no difference beteen a - and b- states. 0 T
Background image of page 2
The Chemical Shift (Also Called δ ) Scale Here’s how it works. We decide on a sample we’ll use to standardize our instruments. We take an NMR of that standard and measure its absorbance frequency. We then measure the frequency of our sample and subtract its frequency from that of the standard. We then then divide by the frequency of the standard. This gives a number called the “chemical shift,” also called δ , which does not depend on the magnetic field strength. Why not? Let’s look at two examples. Of course, we don’t do any of this, it’s all done automatically by the NMR
Background image of page 3

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

View Full DocumentRight Arrow Icon
Image of page 4
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 10/14/2010 for the course CHEM Chemistry taught by Professor Nitche during the Fall '08 term at Berkeley.

Page1 / 17

THe NMR - NMR = Nuclear Magnetic Resonance Physical...

This preview shows document pages 1 - 4. Sign up to view the full document.

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