1. Chem. 133 – 4/21 Lecture

2. Announcements HW Set 3: 3.1 and 3.2 posted, but 3.2 was revised 4/19 or 4/20; first assignment + quiz on 4/23 Today’s Lecture NMR Effect of Environment on Magnetic Field at Nucleus Instrumentation Mass Spectrometry Introduction and Components

3. NMR Spectrometry Magnetic Anisotropy Besides effects from electron withdrawing (or electron supplying), electron currents outside of the σ bonds can affect H 0 This can occur from the induction of larger scale electron circulations Example: benzene ring (δ ~ 7 to 8 ppm – much greater than expected based on local electron density) H H HH H H H Applied p-orbitals π electrons circulate This induces magnetic field in same direction as H applied e-e- Effect is the same as deshielding and similar electron currents can originate in alkenes and alkynes

4. NMR Spectrometry Other Effects on Spectra Number of peaks (equal to number of equivalent nuclei) Peak position (discussed already) Peak area –Proportional to number of nuclei of given type/environment (for 1 H, not for 13 C) –Given by integration Peak width (affected by relaxation) Multiplets (coming)

5. NMR Spectrometry Spin-Spin Coupling We have seen that both H σ bond electrons and neighboring π bond electrons affect H 0. In addition, neighboring NMR active nuclei affect H 0. Example: CHCl 2 CH 2 Br –CH 2 Br protons are affected by spin of CHCl 2 proton (so split into two peaks from spin up and spin down CHCl 2 proton) –CHCl 2 proton is affected by two CH 2 Br protons (three possibilites: two spins up, spins up and down, two spins down) –Spin up + spin down twice as likely because either nuclei can be spin up Low Resolution B0B0 -CHCl 2 -CH 2 Br B0B0 Opposing spins

6. NMR Spectrometry More Spin-Spin Coupling Example: CHCl 2 CH 2 Br –Energy view of spin-spin splitting –CH 2 Br nuclei can be aligned with or against the magnet –Each CH 2 Br state is slightly higher or lower depending on state of CHCl 2 –When all spins are “up”, the energy is the lowest –The unaligned CH 2 Br ( 1 H) is similarly affected –Transitions only involve the CH 2 Br nuclei (see plot) – the CHCl 2 nuclei can’t flip –Splitting of CHCl 2 by CH 2 Br is similar HoHo CH 2 BrCHCl 2 E Down- field coupling Up-field coupling

7. NMR Spectrometry More on Spin-Spin Coupling Both homonuclear ( 1 H – 1 H) and heteronuclear ( 1 H – 19 F) splitting can occur (although homonuclear splitting is more common) Nuclei must be close enough for magnetic fields to be observable (normally 3 bonds or less for 1 H – 1 H) The number of split peaks = n + 1 for n neighboring equivalent nuclei (for I = ½ nuclei causing splitting) 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)

8. NMR Spectrometry Interpretation Examples Predict Spectra (# equivalent peak, relative locations of peaks, relative peak areas, and splitting patterns) for the following compounds: –CH 3 CHBrCH 3 –(CH 3 ) 2 CHCOCH 3 –CH 3 CH 2 OCH 2 F –(CH 3 ) 2 C=CHCH 3 –CHDClOCH 3 –CH 3 CH 2 CHBr 2 –ClCH 2 CHClF What type of groups caused this:

9. NMR Spectrometry Instrumentation Magnet –Needs a) high field strength and b) very homogeneous field –Why high field strength? greater sensitivity (N*/N 0 lower with higher B 0 ) easier to resolve overlapping peaks (δ const. in ppm, J in Hz) TMS overlapping peak of ethyl group J = 7 Hz 2.35 T Magnet (100 MHz) Δδ = 0.14 ppm (14 Hz) J = 7 Hz Δδ = 0.14 ppm = 70 Hz 11.8 T Magnet (500 MHz) no longer overlapping

10. NMR Spectrometry Instrumentation 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 10 8 ; so magnetic field (B 0 ) 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)

11. NMR Spectrometry Instrumentation 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)

12. NMR Spectrometry Instrumentation 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) B0B0 z x y supposed to be spiral path made vector head

13. NMR Spectrometry Instrumentation 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 Removal of RF signal Low frequency signal Conversion to digital Fourier Transformed Data Signal Splitting

14. NMR Spectrometry Additional Topics 13 C 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

15. NMR Spectrometry Some Questions 1.The use of a more powerful magnet will result in better sensitivity and better resolution (separation of protons from different environments). Explain why. 2.What is magnetic field homogeneity and why is it important in NMR? If it is not good, what is the effect? 3.Why are more repeated scans typically used for 13 C NMR?

16. Mass Spectrometry Introduction 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)

17. Mass Spectrometry Introduction 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)

18. Mass Spectrometry Main Components to Instruments 1.Ionization Source (must produce ions in gas phase) 2.Separation of Ions (Mass Filter) 3.Detection of Ions 4.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) 5. Common as chromatographic detector

19. Mass Spectrometry Overview of Component Types Ionization Types TypePhaseFragmentation ICPLiquid feedGives elements Electron Impact (EI)gaslots Chemical Ionization (CI) gassome Electrospray (ESI)liquidvery little APCIliquidsome MALDIsolidsome DESIPortableVery little

20. Mass Spectrometry Overview of Component Types Separation Types (Ion Filters) TypeSpeedBasisCost Magnetic SectorslowAcceleration in magnetic fieldmoderate Double FocusingslowMagnetic plus electric fieldhigh QuadrupolefastPassage through ac electric fieldmoderate Ion trapfastOrbit in quadrupolemoderate Time-of-Flightvery fastTime to travel through tubemoderate Newer High Resolution variesVarious, usually involving orbitshigh In addition, there are 2D MS, such as quadrupole - quadrupole

21. Mass Spectrometry Overview of Component Types Detectors TypeInternal Amplifications? Uses Faraday CupNoIsotope Ratio MS Electron MultiplierYesFairly Common Microchannel plateYesHigher end instruments InductionNoUsed in FT-ICR