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CHEM 2331H
T.Andrew Taton

Chapter 13 Book Notes Nuclear Magnetic Resonance Spectroscopy 13.1) Introduction • Nuclear Magnetic Resonance spectroscopy (NMR): most powerful tool for organic structure determination o like infrared spectroscopy: uses small sample, no harm to sample o gives info on compound structure; many structures can only be determined by NMR spectrum o usually used w/ other spectroscopy forms and chemical analysis complicated organic molecule structures 1 13 • Ochemists mostly use proton ( H) and carbon-13 ( C)NMR b/c H and C = major components of organic compounds o but can also use N, F, and P 31 o history: NMR 1 used to study protons (H nuclei) most commonly used H NMR 1 o NMR usually = proton magnetic resonance unless specified differently 13.2) Theory of Nuclear Magnetic Resonance • Nucleus w/ odd atomic/mass # has nuclear spin, observed by NMR spectrometer (ex: proton: simplest nucleus, atomic # = 1) o kind of like rotating +charged sphere, charge moves like current in wire loop o generates magnetic field (magnetic movement) B (like bar magnet field, straight lines up/down) • When proton placed in external magnetic field, twists to align self w/ field (like how bar magnet would act) o quantum mechanics requires proton be aligned w/ or against external field o alpha (α)-spin state: w/ field, lower energy o beta (β)-spin state: against field, higher energy • When no magnetic field  random proton magnetic movement orientations; Field applied each proton assumes alpha/beta state o alpha is lower energy, so more alpha than beta spins • Magnetic field strength ↑ energy difference btwn 2 spin states ↑ h o Eqn: ∆E=γ 2π B 0 , where ΔE = energy difference btwn 2 states, h = Planck’s constant, o B = 0xternal magnetic field strength; measured in gauss, 1 Tesla (T) = 10000 gauss o γ = gyromagnetic ratio, 26,753 sec gauss for a proton; depends on magnetic movement of whatever nucleus we’re studying • Small energy difference btwn proton’s spin states, but can still be detected by NMR o when proton interacts w/ photon w/ right amt electromagnetic energy, spin can flip from alpha to beta/vice versa • Nucleus = in resonance when subjected to right combo of magnetic field and electromagnetic radiation to flip spin; NMR spectrometer detects this energy absorption h • E = hν(frequency)  ∆ E=hν=γ 2π B0 o  ν=γ 1 B 0 2π • For currently available magnetic fields, proton resonance frequencies occur in radio- frequency (RF) spectrum region o NMR spectrometers usually designed for most powerful magnet that’s practical for spectrometer price range o more powerful magnet ΔE ↑, more easily detected, frequency difference btwn signals ↑ more clearly resolved, easy to interpret spectra o past: most common operating frequency for student spectrometers = 60 MHz, corresponding magnetic field = 14092 gauss o higher resolution instruments 200-600 MHz or higher, corresponding fields = 46972-140918 gauss 13.3) Magnetic Shielding by Electrons • Real protons aren’t naked surrounded by electrons shield from external magnetic field o induced magnetic field caused by electrons circulating; opposes external magnetic field • same effect w/ wire in magnetic field, • here, electron cloud = like wire, rotates in response to external field circular current w/ opposing magnetic field to external field o magnetic field @ nucleus weaker than external magnetic field (nucleus = shielded) o B effectiveexternalshielding need to have greater external field if want resonance @ certain frequency) • Protons in different chemical environments shielded by different amts not in resonance @ same combo of frequency and magnetic field o ex: methanol: electronegative O withdraws some electron density from hydroxyl proton hydroxyl proton not as shielded as methyl protons absorbs @ lower field than methyl protons o AKAhydroxyl proton = deshielded b/c of O o absorb radiation @ different magnetic field strengths • Organic molecule structures = complex different electron shielding effects @ different positions; measuring required field strengths for resonance @ various protons learn 2 things o 1) # different absorptions (aka signals/peaks) tells how many different proton types present o 2) amt shielding implies electronic structure of molecule close to each type of proton o 3) intensities of signals imply # each type of proton present o 4) splitting of the signals: info about other nearby protons • Nuclear magnetic resonance spectrum: graph of energy of absorption as f(magnetic field strength) 13.4) The NMR Spectrometer • Original, simplest NMR spectrometer type has 4 parts o 1) Stable magnet w/ sensitive controller produce precise magnetic field o 2) Radio-frequency (RF) transmitter, emitting precise frequency (continuous wave (CW)) o 3) Detector: measures sample’s absorption of RF energy o 4) Recorder: plots detector output vs. applied magnetic field • Printer records graph: y axis= absorption, x axis = applied magnetic field o upfield: higher magnetic field values, more shielded protons o downfield: lower magnetic field values, more deshielded protons 13.5) The Chemical Shift • 13.5A) Measurement of Chemical Shifts o Chemical shift: difference (in ppm) btwn resonance frequency of proton being observed and of tetramethylsilane (TMS); variations of positions b/c of electronic shielding/deshielding o Hard to measure absolute field, where proton absorbs w/ enough accuracy to distinguish individual protons b/c signals have very small differences  determine value relative to reference compound added to sample  magnetic field strength difference btwn sample proton resonances and reference proton resonances can be measured very accurately o Most common reference compound = tetramethylsilane (CH ) Si (3 4)  Silicon = less electronegative than C TMS methyl groups = relatively electron richwell-shielded protons  TMS protons absorb @ higher field strength than most H’s bonded to C/other elements  most NMR signals = downfield (to left, deshielded) of TMS  all 12 TMS protons absorb @ exact same applied magnetic field 1 strong absorption o Add small amt TMS to sample instrument measures magnetic field difference btwn where sample protons absorb and where TMS protons absorb  distance downfield of TMS = chemical shift for each proton type  newer spectrometers operate @ constant magnetic field, measure chem. shifts as frequency difference btwn sample proton resonances and TMS proton resonances o Chemical shift units = ppm (parts per million); = dimensionless fraction of either total applied field or total radio frequency  custom: chem. shift btwn NMR signal of proton and TMS = on horizontal x axis of NMR spectrum calibrated in frequency units (hertz, Hz)  to get in ppm  chemical shift (ppm) = shift downfield from TMS (Hz)/total spectrometer frequency (MHz)  chem shift (in ppm) of given proton = same regardless of operating field and frequency of spectrometer b/c dimensionless standardized for all NMR spectrometers o Most common chem. shift scale = δ (delta) scale  TMS signal = defined as 0.00ppm on the scale  most protons = more deshielded than TMS so delta scale increases to left  spectrum calibrated in both frequency and ppm o you say that a certain proton absorbs δ# where # = the ppm chem. shift o chemical shift of methyl protons depends on electronegativity of substituents (the more electronegative, the more deshielding, the larger the chemical shifts) o effect of electronegative substituents decreases over distance/# bonds away (usually 4+ bonds away, effect = negligible)  ex: w/ bromobutane o If more than 1 substituent, deshielding effects = almost additive; chem shift = 2-3 ppm per Cl added (but each one is a bit less than one before) • 13.5B) Characteristic Values of Chemical Shifts o can use table for chemical shifts of different types of compounds (b/c chem shift determined by environment) o Vinyl andAromatic Protons (double bonds and benzene ring)  same type of electron circulation that normally shields nuclei from magnetic field • For benzene ring: ring of pi bonds = like conductor, external magnetic field induces ring current • center of ring: induce field opposes external field • edge of ring: induced field adds to external field • aromatic protons = strongly deshielded large chemical shift  But benzene protons aren’t always in same position, goes through many different ones, so chem. shift = avg for all possible orientations • If hold at a position, chem shift will be lower than actual one  alkene pi electrons also deshield vinyl protons, but to less extent than benzene ring b/c no large effective ring of electrons • pi electrons motion generates induced magnetic field that opposes applied field @ middle of double bond and adds to field @ edge, where the protons are deshielding large chem shift o Acetylenic Hydrogens (terminal alkyne)  acetylenic H’s actually absorb more than vinyl protons even if has more electrons in triple bond so should be more deshielded • this is because proton lies in axis of cylinder of electrons of triple bond • also avg out different orientations for chemical shifts here o Aldehyde Protons  absorb @ even lower fields (more deshielded)  deshielded by circulating electrons in double bond and inductive electron- withdrawing effect from carbonyl O o Hydrogen-Bonded Protons  Depends on concentration: the more concentrated, they absorb at relatively low field b/c more of the H bonds are prevalent…but when in dilute sol’n (diluted by non-H bonding substance), H bonding’s less important  broadening of peak b/c protons exchange from 1 molecule to another during NMR resonance…proton goes through variety of environments absorbs over wider frequency range and field strengths o CarboxylicAcid Protons  strongly deshielded large chemical shifts b/c H bonded to O and next to carbonyl group  molecules often exist as H-bonded dimers proton exchange broaden absorption of acid proton  **reread this section 13.6) The Number of Signals • # NMR signals = # different kinds of protons present in molecule • Chemically equivalent: when protons in identical chemical environments have same shielding same chemical shift • But sometimes there might be fewer signals in NMR spectrum than different types of protons in the molecule o could be because whatever makes them different (substituents) don’t influence electron density/shielding o aka accidentally equivalent 13.7) Areas of the Peaks • Area under peak = proportional to # H’s contributing to it; can’t just compare heights, it’s the area • NMR spectrometers have integrators that compute relative peak areas nd o draws 2 trace (integral trace)rises when goes over peak o amt trace rises = proportional to peak area o can measure integrals using millimeter ruler or newer digital instruments print area of peak corresponding to integral trace height rise • But this all only tells us relative ratio of certain types of H’s, not absolute # we have to find that out ourselves • If know molecular formula, can divide total integrated area by total # H’s to find area per H  then look at individual peak areas to see how many H’s must be there 13.8) Spin-Spin Splitting • 13.8A) Theory of Spin-Spin Splitting o Proton in NMR spectrometer subject to external B, self-induced B, and B of other nearby protons too  spin-spin splitting: signals split into multiplets b/c 2 different types of protons close enough to each other that their magnetic fields influence each other (protons = magnetically coupled) o Splitting happens b/c certain type of proton = under influence of another type of H, but the other type’s not oriented (with/against field) same for every sample molecule o When other H type aligned w/ field, original H feels more field more deshielded; vice versa for when other H aligned against field;  one peak @ lower frequency and one at little higher  peaks split about half and half o Spin-Spin splitting = reciprocal property. If 1 proton splits another, the 2 must split the 1st o Can have more than 2 peaks if more permutations possible…the ones that are more probable have larger peaks o Only chemically inequivalent H’s can split each other b/c @ different resonance • 13.8B) The N + 1 Rule o N + 1 rule: If signal split by N neighboring equivalent protons, it will be split into N + 1 peaks  Relative areas (ratios) of N + 1 multiplets given by Pascal’s triangle o Aromatic protons split each other in complicated manner, can see on high- resolution, but looks like broadened peak at low resolution • 13.8C) The Range of Magnetic Coupling o Spin-spin coupling doesn’t happen (or not as much) if H’s aren’t on adjacent C’s  b/c magnetic coupling happens mainly through molecule bonds (usually protons separated by 3 bonds bonded to adjacent C’s (vicinal protons))  Geminal protons (bonded to same C) split each other only if = nonequivalent (usually = equivalent) o Usually
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