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Chapter 13

Ch.13 Summary.docx

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Barb Morra

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CHM 247 CH.13 Structure Determination: Nuclear Magnetic Resonance Spectroscopy • Nuclear magnetic resonance (NMR) spectroscopy: A spectroscopic technique that provides information about the hydrogen-carbon framework of a molecule. NMR works by detecting the energy absorptions accompanying the transitions between nuclear spin states that occur when a molecule is placed in a strong magnetic field and irradiated with radiofrequency waves. Nuclear Magnetic Resonance Spectroscopy • External magnetic field denoted B .0Spinning nuclei act like tiny magnets and interact with the external magnetic field because they are positive. Not all nuclei do this, but H 13 and C nuclei do have spins. • Nuclear spins are oriented randomly in the absence of a magnetic field, but have a specific orientation in the presence of a magnetic field. Some are aligned parallel, others antiparallel with the field. Parallel spin state is slightly lower in energy and therefore favoured. • If nuclei are irradiated with electromagnetic radiation of the proper frequency, energy absorption occurs and the lower state ‘spin-flips’to the higher energy state. When this occurs, said to be in resonance with the applied radiation. • The energy differences between the 2 states vary. • Frequency required to flip depends on the strength of the external magnetic field and on the identity of the nuclei. • Strong magnetic field  Large energy difference and a higher frequency is needed. • The energy difference depends on the strength of the magnetic field. • ΔE=hv E=Energy (J/mol) h=Planck’s constant (1.58x10 -34cal·s) v=Frequency (Hz) • Field strengths of 4.7 T to 7.0 T are common. • Radiofrequency energy in the 200MHz range brings protons to resonance and a rf energy 13 of 50MHz brings a C nucleus to resonance. • Only nuclei with BOTH even number of protons and neutrons do not give rise to magnetic phenomena. 13.2 The Nature of NMR Absorptions 1 13 • The absorption frequency is not the same for all H and C nuclei. • All nuclei are surrounded by electrons. When an external magnetic field is applied, the electrons moving around set up tiny local magnetic fields of their own. These local fields act in opposition to the applied field so that the effective field actually felt by the nucleus is a bit weaker than the applied field. • Beffective appliedBlocal • Shielding: An effect observed in NMR that causes a nucleus to absorb toward the right (upfield) side of the chart. Shielding is caused by donation of electron density to the nucleus. • We say the nuclei is shielded from the full effect of the applied field by the surrounding electrons. Each nucleus has different arrangements of electrons, each is shielded differently, and therefore the effective magnetic fields are slightly different. • Each peak on an NMR spectrum corresponds to a chemically distinct H and C nucleus in the molecule. Though since in portion such as –OCH the3hydrogens are equal, there is 1 13 only 1 observable peak. H and C can’t be observed on the same spectrum. • Abasic NMR has an organic sample dissolved in a suitable solvent (usually deuteriochloroform CDCl , 3hich has no hydrogens) and placed in a thing glass tube between the poles of a magnet. The magnetic field causes the molecules to align in 1 of 2 orientations, and the sample is irradiated with rf energy. If the frequency of the rf irradiation is held constant and the magnetic field strength is varied, each nucleus comes into resonance at a slightly different field strength. • IR radiation is a nearly instantaneous process; the NMR spectroscopy is much slower.A blurring effect occurs due to being slower. Due to this effect, NMR spectroscopy can be used to measure the rates and activation energies of very fast processes. • For cyclohexane, only a single NMR is seen as the ring-flip occurs so rapidly (at room temperature). Lowering the temperature to -90C allows for 2 sets of absorption peaks to be seen. Knowing the temperature at which signal blurring begins to occur, the activation energy can be calculated for the ring flip (45 kJ/mol). 13.3 Chemical Shifts • Upfield: The right-hand portion of the NMR chart. • Downfield: Referring to the left-hand portion of the NMR chart. • NMR spectra are displayed on charts that show the applied field strength increasing from left to right. The left part is then the downfield, or the deshielded side, while the right part is upfield, or the shielded side. • Nuclei that absorb on the downfield side of the chart require a lower field strength for resonance. • For reference a small amount of tetramethylsilane [TMS; (CH ) Si] is added to the 3 4 sample so that a reference peak is produced when the spectrum is run. • Chemical shift: The position on the NMR chart where a nucleus absorbs. By convention, the chemical shift of tetramethylsilane (TMS) is set at 0, and all other absorptions usually occur downfield (to the left of the chart). Chemical shifts are expressed in delta units (), where 1  equals 1 ppm of the spectrometer operating frequency. • Delta scale: An arbitrary scale used to calibrate NMR charts. One delta unit ( is equal to 1 part per million (ppm) of the spectrometer operating frequency. 1 • Example, is we were measuring the H NMR spectrum of a sample using an instrument operating at 200 MHz, 1  would be 1 millionth of 200,000,000 Hz, or 200 Hz. • = Observed chemical shift (number of Hz away from TMS) / Spectrometer frequency in MHz • By using a system of measurement in which NMR absorptions are expressed in relative terms (ppm relative to spectrometer frequency) rather than absolute terms (Hz), it’s possible to compare spectra obtained on different instruments. • **The chemical shift of an NMR absorption in  units is constant, regardless of the operating frequency of the spectrometer. 1 • Almost all H NMR absorption occur from 0 to 10  downfield from the proton absorptions of TS. Almost all the C absorptions occur from 1 to 220  downfield from the carbon absorption of TMS. • Higher field strength NMRs allow for more widely separated absorptions. Chance of overlap is less when using a higher field strength. 13.4 C NMR Spectroscopy: SignalAveraging and FT-NMR • Carbon-13 is the only naturally occurring carbon isotope with a nuclear spin, but it isn’t very common. Low abundance issue overcome by use of signal averaging and Fourier- transform NMR (FT-NMR). Signal averaging increases instrument sensitivity, and FT- NMR increases instrument speed. • Averaging helps due to the background noise that shows up being averaged out to 0 while the peaks still stand out over the multitude of trials. This is a long process, 5-10 minutes, so its value is limited. • The rf frequency is held constant while the strength of the magnetic field is varied. This way all signals in the spectrum are recorded sequentially. In the FT-NMR technique, all signals are recorded simultaneously. Asample will be placed in a magnetic field of constant strength and will be irradiated with a short pulse of rf energy that covers the entire range of useful frequencies. • Speed of FT-NMR is a few seconds. The speed of FT-NR with the sensitivity enhanc
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