The Hydrogen Atom Is a Magnet.

Transcription

1 The Hydrogen Atom Is a Magnet Nuclear Magnetic Resonance Spectroscopy (NMR) Proton NMR A hydrogen nucleus can be viewed as a proton, which can be viewed as a spinning charge. As with any spinning charge, an intrinsic magnetic field is set up On the left, a toy top precesses about its vertical axis. The hydrogen, atom on the right, precesses about a magnetic field. Because it has only one proton, a single mass with a positive charge, it has a large magnetic moment (red arrow). A spinning proton produces a magnetic field similar to a bar magnet.

2 Resonance If H H is aligned with H o, this will be a lower energy arrangement than when H H is opposed to H o. This creates an energy gap (ΔE) and if energy equal to ΔE is applied to the proton, it "flips spin states." This means that when the proton absorbs the energy, the proton magnetic field changes from aligned to opposed (low energy to high energy) - it flips its spin state. In order to absorb ΔE, a particular magnetic field must be applied; when this occurs they are said to be in resonance. If the magnetic field changes, ΔE changes. The sample is immersed in a very strong magnetic field and this aligns the nuclei that have spin, like a compass needle aligning with the Earth's field. The alignment takes a number of patterns depending on the total spin. Each alignment has a different energy. The nucleus is rattled with a pulsed radio wave. When the correct frequency is applied during this pulse, the nucleus jumps from one alignment (energy ) to another alignment. This is a "resonance" similar to when a person pushes a swing a the same rate as the swing's period - the swing gets higher.

3 ΔE the nucleus precesses around its axis with a precessional frequency,! prec H H aligned opposite H o HIGHER ENERGY small "E large "E H H aligned with H o LOWER ENERGY requires smaller H o requires larger H o External Magnetic Field H o The proton spins "off-axis," and it precesses around the axis at a certain frequency, the precessional frequency. This information becomes important if we place the proton, which is now a small magnet, into the field of a large external magnet represented by H o. As with any two magnets, the small magnet will be influenced by the large magnet and the field of the smaller magnet will orient itself relative to the larger magnet (H o ). There are two possible orientations, the proton magnetic field H H can be aligned with H o or opposed to H o This is an energy gap, represented by ΔE.

4 Spin Quantum Number: I Moving charge creates magnetic fields, spinning as moving. Neutrons are made of 3 quarks and so the charge associated with the neutron, while totaling zero, is not symmetric.. One side can be thought of as slightly positive while the other slightly negative. Spinning generates the magnetic field. Spin" is strictly a "property" of the subatomic particles, a quantum number. Protons and neutrons each have a spin of size "1/2". To get the total spin of a nucleus, we must add the spins of all the nuclear members of an isotope vectorially. This in turn gives the nuclear magnetic moment of the nucleus. If the total spin of a nucleus is 0 then NMR cannot detect the nucleus. O-16 and C-12 are examples of spin 0 so cannot be detected. The spin quantum number is I. Here, if I = 0, there is no NMR. Nuclei with spin of 1/2 include H-1, the proton and C-13. Commonly used nuclei with Spin of 1 include H-2 and N-14.

5 Nuclei and Spin Common nuclei and their spin quantum numbers. Nucleus Number of protons Number of neutrons Spin (I) 1 H 1 0 1/2 2 H C C 6 7 1/2 16 O O F /2 15 N 7 8 1/2 Proton NMR is common since the proton is a common nuclei 12 C C H H N N O O O 0.20 All the other isotopes are low abundance, so Fourier Transform techniques are required, and Longer acquisition times, to obtain good NMR

6 Spin and Orientation: Signals per Nucleus 1 H I = 1 2 2I+1 = 2 2 orientations 1 signal 2 H I = 1 2I+1 = 3 3 orientations 3 signals There are 2 orientations for spin = 1/2, so there is one transition (one signal) upon absorption. For spin = 1, there are 3 orientations, and 3 possible transitions (3 signals) per nucleus. Transition = Signal Therefore: For spin = 1/2, one signal per nucleus ( 1 H, 13 C) For spin = 1, 3 signals per nucleus ( 2 H)

7 The instrument: NMR Instrument

8 FT-NMR Time and Frequency Domains: Fourier Transform ww.chemie.uni-erlangen.de/ bauer/music3.htm Fourier transform a short blast of radio waves is delivered to the sample and then re-emitted radiation by the sample is monitored over time. Because frequency and time are related by the Heisenberg Uncertainty Principle (like energy and position are), if we know the duration of the radiation pulse precisely we will have many different frequencies present at the same time. All of the nuclei are excited by the pulse, but then they began to "relax" and emit radio waves of an energy that matches their ΔE. The result is a free induction decay (FID). The complex FID pattern contains information on all the nuclei that were excited by our radiation pulse, and we can convert theses oscillations in time back to each nucleus' frequency by using a mathematical process called a Fourier transform.

9 Low Field, High Field and Zero Low Field (downfield) (upfield) High Field Intensity Typical proton NMR spectrum TMS The change will be measured in Hertz (Hz) while ΔE is measured in megahertz (MHz). This means that the signal in Hz is in millionths relative to ΔE. ppm As ΔE is changed incrementally, each proton will resonate (absorb ΔE) for its particular value of H o and we generate a series of absorption peaks for each different proton. This means that each peak will represent a signal for a different type of proton. We must establish a zero point. We use tetramethylsilane [(Me 3 ) 4 Si; TMS] as an internal standard. This molecule gives rise to one peak for the methyl groups and we measure everything relative to it.