E i f we substi tute wi th e negati ve gr oups shi el

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Unformatted text preview: envi r onment ar ound each H . e + - I f we substi tute wi th e-negati ve gr oups, shi el di ng of e-negati obser ved nucl eus i s dii mi ni shed ; or nucl eus i s d or “ deshi el ded ” (r el ati vel y). T wo e-poor Two nei ghbor s n ei One e-poor nei ghbor n ei The Chemical Shift δ T he posi ti on of peak r el ati ve to an The i nter nal standar d. M ost common i s Tetr amethyl si l ane: (CH 3)4Si or “TM S” , t he “ TM the “zer o” poi nt i n the spectr um. “ zer δ= ν (fr om TM S) H z RF (e.g., 300 M H z) ppm , i s i ndependent of machi ne (90, 300 M H z, etc.) D i sta nce fr om T M S i n H z/300 M H z i n ppm 266 Hz 0.89 ppm 978 Hz 3.26 ppm 541 Hz 1.80 ppm Application of δ 2 si gnal s CH 3 CH 3 Br 2 hυ CH 3CH 2Br M ost deshi el ded + CH 3CH Br 2 + of al l of 2 si gnal s Br CH CH Br Br 1 2 2 sii gnal s EPM of 1-br omopr opane 1-2 1-2 2+ 3-4 Var i abl e C hemi ca l l y equi va l ent H s: sa me δs. U se equi symmetr y oper ati ons: r otati on, mi r r or pl anes. “ F ast” pr ocesses, such as methyl Fast” r otati on, equi l i br ate H s: NM R ti me scal e < 1 sec. sec Ea k For cal i br ati on: A B E a ~ 20 kcal mol -1, k ~ 10-2 sec-1, t 1/ 2 = 1 m i n 20 10 E a ~ 25 kcal mol -1, k ~ 10-6 sec-1, t 1/ 2 = 66 h 25 Recal l : E a = 10.8 kcal mol -1. T o “fr eeze” r i ng fl i p on the NM R ti me scal e: we To “ fr n eed to cool to -90ºC (J ensen) cool Integration Integration Ar ea under peak = r el ati ve # of associ ated H s. Computer -automated . associ 33 e.g. (CH ) CCH OH e.g. 9:2:1 H el ps i n peak assi gnments. Spin-Spin Splitting Spin-Spin (N+1) Rule (N+1) N equi val ent adjacent hydr ogens equi (i .e. on nei ghbor i ng car bons) gi ve r i se to (N+1) peaks. e.g., doubl et (d), tr i pl et (t), e.g., quar tet (q), qui ntet (qui n), et...
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This note was uploaded on 11/11/2009 for the course CHEM 368537 taught by Professor Voldhart during the Spring '09 term at Berkeley.

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