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Lecture 14

CHEM 213 Lecture Notes - Lecture 14: Hydrogen Bond, Geminal, Chemical Shift


Department
Chemistry
Course Code
CHEM 213
Professor
Dr.Dave Berg
Lecture
14

Page:
of 2
Chem 213!
Lecture **!
Oct. 14/2016!
!
Aldehydes:!
O=C-H!
-double bond is shielded to lower ppm!
-protons de shielded to higher ppm!
!
Karplus showed relationship between J and alpha!
-depends on how hydrogens are aligned with each other!
-strong coupling if protons elapsed!
-staggered --> coupling constant drops o rapidly!
-at 90 degrees falls o to zero!
-transmission is poor if there is no orbital overlap on the two centres!
-dihedral angle!
!
The size of the coupling constant depends on the nuclei and their distance relationship and the karplus
relationship...!
-constant magnitude-->certain amount of hertz--> energy is fixed-->NOT dependent on field!
-chemical shift does depend on field-->absolute frequency in hertz!
!
-Trans coupling is largest ~16Hz!
-Cis coupling middle ~ 8Hz!
-Geminal baby coupling ~2 Hz!
########!
####!
#########Geminal^!
-mono-substituted alkene gets 12 lines!
#-easily identify which is which from the coupling constants!
!
-ethylene standard for ppm ~5.25ppm!
!
-coupling in saturated system = 0 if four bonds away!
#-not true for pi system!
!
Exchangeable protons move on and o very quickly about 10^5 per second!
-nmr relaxation time is relatively slow, ms-many ms even to seconds!
#-NMR time scale is 10^-2-10^-3 fastest an NMR can measure!
#-sees where the Hydrogens are on average!
#-therefore you lose coupling because coupling requires you to have a fixed relationship!
-Sharp signal is from really fast or really slow average!
!
-the more acidic the more delta positive, therefore, higher in chemical shift!
-saturated alcohols between 1-5 ppm because not very acidic!
-chemical shifts are extremely dependent upon hydrogen bonding-->solvent eects are huge!
#-bond polarizability factor!
-can make exchangeable portends disappear by adding D2O!
-deuteriums swap with H!
!
!
-cooling temperature broadens peak!
-somewhere between zero and minus 5-->coalescent temperature!
#-rate of exchange exactly measures nmr rate --> no peak disappears!
-peak reappears at minus 5!
-rate at which protons exchanging is slower than the nmr therefore can see the protons and coupling is resolved!
-reach the "stopped exchange region", you get simple coupled spectrum you would expect!
-can measure how fast the process is occurring from numbers (times etc) from NMR --> dynamic NMR
spectroscopy!
find more resources at oneclass.com
find more resources at oneclass.com
!
-take advantage of the exchangeable proton!
#-amide, amine, alcohol, acid!
!
Amides!
-two lumpy peaks!
-due to hydrogens being inequivalent!
#-amides have a barrier to rotation due to pseudo db character, thats why the protons are inequivalent!
!
END OF MIDTERM ! MATERIAL
find more resources at oneclass.com
find more resources at oneclass.com