January 30, 2018 Chemistry 328N

Transcription

1 Lecture 4 Some More nmr January 30, 2018

2 Tricks for solving unknowns Review. Empirical formula is lowest common denominator ratio of atomic composition From Homework: unknown has an empirical formula of C 4 H 9.a single high field peak in the 1 H nmr and a molecular ion at M/Z = 114..propose a structure..and predict the M/Z of the most intense fragment you expect to see in the mass spectrum

3 Energy Hz Resonance can be achieved by changing the magnetic field strength at constant frequency or by changing frequency at constant magnetic field strength Field Strength T

4 Nuclear Magnetic Resonance The circulation of electrons around a nucleus in an applied field is called diamagnetic current. This current generates a field that opposes the applied field...diamagnetic nuclear shielding results. Lenz s Law?? The difference in resonance frequencies between the various hydrogen nuclei within a molecule is due to shielding/deshielding is very small but very important

5 Conditions for Resonance It is the frequency of the radiation and the NET field at the nucleus that matters. The NET field is the sum of all incident magnetic fields including those from: The Giant Magnet (applied field) Diamagnetic Shielding field (electrons) Coupling (spin fields of adjacent nuclei) credit card strips, earth s field, etc..

6 Chemical Shift The difference in resonance frequencies for hydrogens in CH 3 Cl compared to CH 3 Br under an applied field of 2.34T is only 35.5Hz, which is 0.35 parts per million (ppm) compared with the irradiating frequency 35 Hz = 0.35 = 0.35 ppm 100 x 106Hz 10 6 Here, 35Hz is the difference in resonance frequency and 100MHz is the 1 H resonance frequency for B=2.34T and = MHz / Tesla

7 Calculating Chemical Shift At 7.05 T, a resonance is at 715 Hz (from TMS) 1. What is the spectrometer frequency? 2. What is the chemical shift in ppm???

8 Chemical Shift Depends on several things (1) electronegativity of nearby atoms, (2) the hybridization of adjacent atoms, and (3) magnetic induction within an adjacent pi bond (1) Electronegativity CH 3 -X CH 3 F CH 3 OH CH 3 Cl CH 3 Br CH 3 I (CH 3 ) 4 C (C H 3 ) 4 Si Electronegativity of X d of H (by definition

9 Chemical Shift (2) The Hybridization of C Effects 1 H chemical Shift Hybrid sp 3 sp sp 2 Type of H Name d RCH 3 RC CH R 2 C=CH 2 alkyl acetylenic vinylic This seems like a Mystery!!

10 Chemical Shift Magnetic induction in pi bonds of a. a carbon-carbon triple bond shields an acetylenic hydrogen and shifts its signal upfield (to the right) to a smaller d value carbon-carbon double bond deshields vinylic a hydrogens and shifts their signal downfield (to the left) to a larger d value

11 Chemical Shift - 1 H-NMR Type of H (C H 3 ) 4 Si RCH 3 d Type of H ROH RCH 2 OR d RCH 2 R R 3 CH R 2 NH O R 2 C=CRC HR 2 RC CH ArC H RCCH 3 O RCCH 2 R

12 Chemical Shift - 1 H-NMR Type of H d Type of H d O RCOC H O RCOC H 2 R RCH 2 I RCH 2 Br RCH 2 Cl RCH 2 F R 2 C=C H 2 R 2 C=C HR ArH O RCH O RCOH

13 is this arrow correct for electron flow??? Induced field opposes big magnet inside the ring

14 Some Confusing Definitions The Right-Hand Rule #2 determines the direction of the magnetic field aroud a currentcarrying wire and vice-versa BUT. When using the Right-Hand Rules, it is important to remember that the rules assume charges move in a conventional current (the hypthetical flow of positive charges). In order to apply the Right-Hand Rule to a moving negative charge, the direction of charge flow must be reversed--.or use your LEFT hand.

15 Outside the ring the induced field helps big magnet

16 1 H-nmr spectrum of benzene

17 Integration H 3 C O H 3 C CH 3 O CH 3 9 linear units linear units up field

18 Equivalent Hydrogens Have exactly the same chemical environment Cl Cl CH 3 CH 3 CH Cl O C C 1,1-Dichloroethane Cyclopentanone H H (Z)-1-Chloropropene Cyclohexene

19 How many equivalent hydrogens?? Cl Cl H Cl This looks easy but it is not!!

20 Changing Spectrometers If the resonance of the peak on a 7.05 T instrument is 715 Hz, what will the resonance frequency be at 23.5 T What is the resonance in ppm at 7.05 T? What is the resonance in ppm at 23.5 T?

21 Chemical shift in ppm (d) is independent of the size of the magnet.! d Down Field Frequency Field Up Field

22 Ethyl Acetate H 3 C O CH 3 4 O

23 Signal Splitting Huh?? Why??? What is going on here?? What a mess!! Signal splitting: splitting of an NMR signal into a set of peaks by the influence of neighboring nonequivalent hydrogens This splitting business is actually rich in information it is a wonderful thing!

24 Origins of Signal Splitting When the chemical shift of one nucleus is influenced by the spin of another, the two are said to be coupled Consider nonequivalent hydrogens H a and H b on adjacent carbons the chemical shift of H a is influenced by whether the spin of H b is aligned with or against the applied field Y Ha Hb C C X

25 Origins of Signal Splitting B 0 H b Magnetic field of H b subtracts from the applied field; H b signal appears at a higher applied field H a H b Magnetic field of H b adds to the applied field; H a signal appears at a lower applied field Remember it is the NET field that counts

26 The signal of Ha is split into two peaks of equal area (a doublet) J ab J ab = coupling constant

27 Origins of signal splitting no neighbors one spin two spins three spins

28 Relative Intensity of Peaks singlet double triplet quartet quintete sextete Pascal s triangle The binomial coefficients

29 The N+1 Rule The 1 H-NMR signal of a hydrogen or set of equivalent hydrogens is split into (N + 1) peaks by a set of N equivalent neighboring hydrogens All neighboring hydrogens in the analysis must have the same chemical shift (magnetically equivalent) If this condition is not met, a graphical tree or second order analysis must be used to predict the splitting pattern. We will explore this condition later

30 Who needs DNA??