Crash course in interpretting NMR spectra for lab. NMR = the workhorse of characterization tools reveals connectivity & alkyl chains

Transcription

1 EM 222 secion 01 rash course in inerpreing NM specra for lab NM = he workhorse of characerizaion ools reveals conneciviy & chains For a basic overview of NM (o help inerpre specra): - rea hese summary slies - rea h.13 secions 1 (superficially) + 7 & 10 (eails) (eails & explanaions laer in class) (1) elpful websie for specroscopy opics: Michigan Sae Universiy hp:// Nuclear magneic resonance (NM) specroscopy - he basic principles Many nuclei spin abou heir axis, like e - s P 19 F generae own iny magneic fiel If place sample in magneic fiel, B nuclei s fiel can (α) line up wih fiel O (β ) align opposie B aio waves (low freq. raiaion) = sufficien energy o excie nuclei o higher E spin sae (β ) NM = anoher ype of absorpion specroscopy 1 NM specrum: x-axis = relaive freq. of raio wave absorbe: chemical shif δ y-axis = absorpion inensiy, which scales wih # s absorbing (2) δ = relaive o wha! More in class laer

2 Informaion provie by a 1 NM specrum 1) hemical shif (δ) = peak posiion: e - -richness of s environmen neighbouring groups elecrons shiel nearby nuclei (make E α-β lower) near EWG eshiele s peak appears a higher 2) Mulipliciy = peak shape: inicaes # of s on ajacen aom(s) s feel presence of s on nex-oor aom: --- n Effec of coupling : sees 0 peak no spli = single 1 s sees 1 peak spli ino 2 = ouble 1:1 sees 2 peak spli ino 3 = riple 1:2:1 sees 3 peak spli ino 4 = quare 1:3:3:1 q 3) Inensiy = peak area: scales wih # s causing he peak eighs of inegraion race relaive peak heighs # s/peak relaive inensiy (3) A QUATET A DOUBLET chemical shif, eference compoun Si( 3 ) 4 TMS δ = 0 ppm A SINGLET haracerisic chemical shifs for 1 NM specra EWG & sp 2 e-neg. of sp 2 >sp 3 EWGs... = e - -rich If know poenial srucure: look for key feaures in NM ( 3, Ph, ec) Alkyl region (δ = 0-3 ppm): mos cluere, bu yiels mos informaion omplex peaks = muliples: can be ue o overlapping, or q peaks STATEGY FO INTEPETTING SPETA: seps 1&2 ogeher 1) hemical shifs: 1 s ienify well-resolve peaks, hen overlapping peaks 2) Mulipliciy: ienify isolae s (singles) use mulipliciy o ienify neighbours on chain 3) Inegraion: eermine #s per peak & correlae wih propose IDs 4) ombine everyhing: propose a srucure woul i give observe specrum (4) Don forge oher ools: anyhing learne from UV/Vis, I, MS

3 Example: For maximum learning, follow/work alongsie WITOUT skipping ahea o see answer Eluciae he srucure of his molecule Imagine also have I specrum complex specrum bu srong broa peak a ~3300 cm -1 TMS eaing specrum from righ o lef Ieas (colour coe ) nearer EWG -- very near EWG -O! (suggese by I!!) On ring, seeing 1 On Ph ring, on each sie On Ph ring, on each sie On ring, seeing mul 2.95 mul = O 4.9 s 6.7 (5) egion Mul. couple o 2 couple o 1 couple o MANY couple o many isolae, 4 o -X- couple o couple o couple o couple o 1 # s Space for you o work (6)

4 Try o piece his ino a srucure: sar wih Ph ring (arbirary ) 2 ND 1 ST 1 is besie 1 & no oher s follow conneciviy suggese by mulipliciies 1 besie his + 1 on oher sie Mul. # s D 4 T Ieas (colour coe ) On ring, besie 1 On Ph ring, on each sie On Ph ring, on each sie On ring, besie 1 anoher 1 shoul have 1 on each sie (7) O conclue Wha o we know abou 6 T subsiuens vs look a 2 s besie s more eshiele han = a heeroaom O ' 5 T our las 1 shoul have an on only one sie Try o piece his ino a srucure: coninue wih group on Ph ring Follow conneciviy suggese by mul. & posiions WE KNOW: 1 ST = chain on Ph ring PLAE TO STAT: Fin aom aache o Ph 2 ND Ph aachmen = sp 2 like an EWG (δ 2.95) is on Ph! Mul. 3 - mul mul s sees 2 sees 1 sees many sees many isolae #s Ieas (colour coe ) near EWG -- NEA EWG -O O 3 D O 3 is ' couple o his = or O 3 3 ' 4 T Sill missing 2 & 3 Bu (8) T 3 is couple o a 2 an he only one no accoune for ye is 2

5 Answer: correc! I akes pracice: sar wih simple examples in ex (simpler han his) be organize be logical raw possibiliies es hypoheses ge righ peaks be paien. TMS egion Iea nearer EWG -- very near EWG Mul. mul mul couple o 2 couple o 1 couple o MANY couple o many # s (9) = O s isolae, 4 o -X- couple o 1 couple o 2 couple o 2 couple o O On ring, besie = On Ph ring, on each sie On Ph ring, on each sie On ring, besie =