Nuclear magnetic resonance, Cheat Sheet of Chemistry

Download Nuclear magnetic resonance and more Cheat Sheet Chemistry in PDF only on Docsity! Nuclear Magnetic Resonance (NMR) Spectroscopy • The nuclei of many atoms have a property called nuclear spin (I > 0). • We can picture this as the nucleus ‘rotating’ - like the earth . • Any spinning object has angular momentum. For sub-atomic particles (electrons, nuclei), the angular momentum is quantized, i.e. angular momentum can only have certain fixed values or more simply we can say that only certain rotation speeds are possible. • The nuclear spin is characterized by a quantum number, I (spin quantum number). • The nuclear spin is also associated with a magnetic Larmor precession • The spin axis will rotate around the direction of the field. The angle is always the same. • Precession of the +1/2 component around the field direction. This is called Larmor precession. • The frequency of precession is given by ωo = ϒBo . Bo is the field strength (in Tesla) and ϒ is the magnetogyric ratio (in T -1s -1). It is the ratio of magnetic moment to angular momentum. This has a different value for every nucleus. Nuclear spins- energy levels ¢ With no external field the energy of the @ and £ state is the same. Once we apply a field they become different in energy. 2 B-spin state ae. . y magnetic Se A field is applied a-spin state no applied magnetic field * The energy gap is proportional to the magnetic field strength. Applied magnetic field - 1H 1.4 (T)_2.3 (T) 7-046(T) 14.096 (T) T T @ AE=y/B,h/2x _|_.600 MHz. Energy ——» a state Magnetic Field Strength, B,/T Shielding of external magnetic field * Moving electrons generate a magnetic field. * Electrons moving around a nucleus change the total field experienced by the nucleus. * This electron circulation causes a small magnetic field, B,, =o Bo at the nucleus which opposes the external field Bp. This is known as diamagnetic shielding. * The effective field at the nucleus is: Bure = By - By, = By C1 -O) Where o is a shielding constant. B sh By By B eff Chemical shift and the delta scale, 6 * The actual absorption frequencies depend on the magnet used in the apparatus, making it difficult to compare spectra from different machines. So the delta scale, is used instead. vey § =— 2! x 10° ppm Operating The 6 scale uses a reference compound and records the absorption of a peak (v) as a shift from the reference (Vj <). By dividing via Voperating 1t becomes independent of the actual field — and hence universal for all NMR machines. The scale is normally given in parts per million, ppm. 6 scale reference - tetramethylsilane, TMS *Because of molecular symmetry all 12 protons of TMS absorb at the same frequency giving a strong single peak. CH, H,c— 4 —CH, *The silicon in TMS shifts the absorption peak far upfield. bey * The vast majority of compounds have shifts downfield of TMS. * The TMS peak ts given v= 0, 6 = 0 on the delta scale. e@ Drew lTT §& crnlare ce orn faerew fi IfV Usefulness of tetramethylsilane, TMS CH . *It is chemically inert and miscible with a large range of solvents H,C—Si—CH, | *Its twelve protons are magnetically equivalent. CH, * It is highly volatile and can be easily removed from the sample. 5 y-V ref x 6 0 V Openstin Ppa *Values of 5 for a given proton is always the same irrespective of whether the measurement is made at 60 (1.4 T), 100 (2.3T), 300 (7 T) or 600 (14 T) MHz NMR spectrophotometer. *e.g. at 60 MHz NMR spectrophotometer, the proton shift in CH3Br is 162 Hz from TMS, 5 = (162/60 x 106) x 10°= 2.7 ppm *On the other hand, at 100 MHz NMR spectrophotometer, the proton shift in CH3Br is 270 Hz from TMS, 5 = (270/100 x 10°) x 10°= 2.7 ppm Factors influencing Chemical shift values * Local diamagnetic shielding (1) -I effect CH3F = 4.27 CHCl, 7.27 CHCl 3.06 CH,Cl, 5.30 CH;Br 2.69 CH;Cl «3.05 CHs| 2.16 2) Hydrogen bondin (2) Hydrog g oe % & b+ & Ha —_ll ees H——O ---€----H-}—O--------- H——O-*=* CH, CH, CH; CH; Free (dilute) alcohol 5=0.5-1 ppm Inter molecular hydrogen bonding (Concentrated), 5=4-5 ppm Pulsed Fourier Transform (FT) spectrometers: *In CW type NMR spectrometer, each type of proton is excited individually and its resonance peak is observed and recorded independently of all the others. It is very lengthy process as each distinct type of proton need to be scanned separately. *In the modern FT spectrometers, a pulse is applied for 5 to 30 u sec having a whole range of frequencies, which excites all the nuclei simultaneously and all the signals are collected at the same time. The initial curve is known as FID curve. Then we apply FT to get the normal mode of NMR spectrum. 5 values of protons | | J | | | | | | | ; Ethers Sulfides Sat. alkanes pili Aromatics = -RC=CH) OCH RSCH «RH Alcohols RCH=CHR-—+* HO-CH——_—s > + RCBCH PhO-CH-——+ Ar-CHt—4- 4 R,C=CR-CH FCH Cl-CH__I-CH Br-CH-———+ arden Esters RCO;CH ++ +——4NC-CH O,N-CH-— -——R,N-CH RCO,H Amide RCONH ROH POH R,NH T T T T T T T T T T T 120 WO 00 909 80 70 60 50 40 30 20 10 00 ppm(s)  H3G £ CHy mers H-CSe—e-0-H Hc OH CHy Ts Ts | iz TT T T T T T TT T T T T T T T T io 9 8 7 6 5 43 2 1 04109 8 7 6 3 2 04 gL H-9 tl Hyo—@ Hye—¢—6" ‘CHy O-CHy i Ag Hac | Ts r— ts T T T T T T T T T T T T T T T T T 10 9 6 7 6 5 4 3 2 14 08 109 €$ 7 6 3 2 04 HH es Hye CH Cl-cHy-C—cl ey CHy TMS | TMs! Sy 7 —— T 109 86 7 6 5 43 2 1 085 109 8 7 6 a @ 04 Some example spectra Integration or integral curve: The area under the curve is integrated. The value is proportional to the number of protons in each peak