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Chem. 133 – 4/24 Lecture

Chem. 133 – 4/24 Lecture. Announcements. Lab Reports (Set 2 period 3) due today New Handout: Homework Set 3 – changed Set 3.1 due date and date of Quiz to 5/1 Term Project Progress Report – now due 5/6 Today’s Lecture NMR Interpretation (spin-spin coupling and examples) Instrumentation

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Chem. 133 – 4/24 Lecture

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  1. Chem. 133 – 4/24 Lecture

  2. Announcements • Lab Reports (Set 2 period 3) due today • New Handout: Homework Set 3 – changed Set 3.1 due date and date of Quiz to 5/1 • Term Project Progress Report – now due 5/6 • Today’s Lecture • NMR • Interpretation (spin-spin coupling and examples) • Instrumentation • Mass Spectrometry • Overview • Ionization (if time)

  3. NMR SpectrometryMore on Spin-Spin Coupling Both homonuclear (1H –1H) and heteronuclear (1H –19F) splitting can occur Nuclei must be close enough for magnetic fields to be observable (normally 3 bonds or less for 1H –1H) The number of split peaks = n + 1 for n neighboring equivalent nuclei (for I = ½ nuclei causing splitting; I = 1 causes splitting 2n+1) The distance between split peaks is constant in Hz (not ppm) and is the same for both nuclei (e.g. splitting constant for A proton caused by B proton will be the same for both A and B protons) In the case of one set of equivalent nuclei causing splitting, you should be able to predict the pattern caused If more than one set of nuclei cause splitting, the result is “complex” (although you can predict number of peaks if splitting constants are similar)

  4. NMR SpectrometryInterpretation Examples Predict Spectra (# equivalent peak, relative locations of peaks, relative peak areas, and splitting patterns) for the following compounds: CH3CHBrCH3 (CH3)2CHCOCH3 CH3CH2OCH2F (CH3)2C=CHCH3 CHDClOCH3 CH3CH2CHBr2 ClCH2CHClF What type of groups caused this:

  5. NMR SpectrometryInstrumentation Magnet Needs a) high field strength and b) very homogeneous field Why high field strength? greater sensitivity (N*/N0 lower with higher B0) easier to resolve overlapping peaks (δ const. in ppm, J in Hz) 2.35 T Magnet (100 MHz) TMS overlapping peak of ethyl group J = 7 Hz Δδ = 0.14 ppm (14 Hz) 11.8 T Magnet (500 MHz) no longer overlapping J = 7 Hz Δδ = 0.14 ppm = 70 Hz

  6. NMR SpectrometryInstrumentation Magnet (cont.) Why homogeneous field? needed to obtain high resolution example, to resolve 2 Hz splitting in a 600 MHz instrument, a resolution required is 600,000,000/2 = 3 x 108; so magnetic field (B0) must vary by less than 1 part in 300,000,000 over the region where the sample is detected done by shims (small electromagnets in which current is varied) and spinning sample (to reduce localized inhomongenieties)

  7. NMR SpectrometryInstrumentation Light Source Radio waves produced by RF AC current with antenna Continuous in CW (continuous wave) instruments Pulsed in FT (Fourier Transform) Instruments Sample Typically contains: active nuclei, sample matrix, and deuterated solvents (for proton NMR) Deuterated solvent used to reduce interference and to use “lock” (CW NMR to locate frequency based on D signal) Light Detector same antenna producing light (at least in FT NMR)

  8. NMR SpectrometryInstrumentation Interaction of light with sample in FTNMR Numerous precessing nuclei can be represented by net vector RF pulse causes rotation about x-axis (in y-z plane) During relaxation back to ground state, RF signal is “picked up” (antenna picks up y-axis component) supposed to be spiral path made vector head z y x B0

  9. NMR SpectrometryInstrumentation Electronics for Detection Antenna picks up RF signal pulse RF is difficult to digitize So signal split into RF component and lower frequency component Lower frequency component is digitized (this is observed FID) Digitized signal is then processed (filtered by exponential multiplication and Fourier transformed to to frequency domain) antenna Signal Splitting Removal of RF signal Low frequency signal Fourier Transformed Data Conversion to digital

  10. NMR SpectrometryAdditional Topics 13C NMR Lower sensitivity due to lower frequency and lower abundance Useful for determining # equiv. C atoms, types of functional groups (particularly for C atoms with no protons attached like C-CO-C) Typically done with proton decoupling (removing splitting caused by neighboring protons) to enhance sensitivity Solids Analysis Suffers from wide peak width Peak width made narrow by using “magic angle” spinning Spin Decoupling and 2-Dimensional Methods Used to determine connectivity between protons

  11. NMR SpectrometrySome Questions The use of a more powerful magnet will result in better sensitivity and better resolution (separation of protons from different environments). Explain why. What is magnetic field homogeneity and why is it important in NMR? If it is not good, what is the effect? Why are more repeated scans typically used for 13C NMR?

  12. Mass SpectrometryIntroduction • One of the Major Branches of Analytical Chemistry (along with spectroscopy, chromatography, and electrochemistry) • Roles of Mass Spectrometry • Qualitative analysis (less useful than NMR for true unknowns, but can be applied to very small samples) • Quantitative analysis (often used for quantitative analysis)

  13. Mass SpectrometryIntroduction • Main information given • molecular weight • number of specific elements (based on isotope peaks) • molecular formula (with high resolution MS) • reproducible fragment patterns (to get clues about functional groups and/or arrangement of components or to confirm compound identity)

  14. Mass SpectrometryMain Components to Instruments • Ionization Source (must produce ions in gas phase) • Separation of Ions (Mass Filter) • Detection of Ions • Note: most common instruments run in order 1 → 2 → 3, but additional fragmentation to generate different ions can occur after step 2 (1 → 2 → 1 → 2 → 3)

  15. Mass SpectrometryOverview of Component Types • Ionization Types

  16. Mass SpectrometryOverview of Component Types • Separation Types (Ion Filters) In addition, there are 2D MS, such as quadrupole - quadrupole

  17. Mass SpectrometryOverview of Component Types • Detectors

  18. Mass SpectrometryIon Source • Gas Phase Sources • Electron Impact • M + e-→ M*+ + 2e- • (electrons accelerated from hot filament source) • However, M*+ typically has extra energy and can undergo decomposition: M *+→ X+ + Y· (where X and Y are fragments) • Only the charged fragments are seen, but often if M *+→ X+ + Y·, it also may form X· + Y+.

  19. Mass SpectrometryIon Source • EI Fragmentation Example: + charged fragment m/z = 43 (16 + 15 + 12) charged fragment m/z = 77 (5*13 + 12)

  20. Mass SpectrometryIon Source • Fragmentation Example 2: mass peak at 49 (and 51) - observed CH2Cl2+ CH2Cl+ + Cl· CH2Cl2 ·+ Cl+ mass peak at 35 (and 37) - not observed Presence of ions also depends on their stability

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