Last time: Chemical shift (1H and 13C)
We can make a reasonable prediction of chemical shifts for different types of protons and 13C nuclei. We can also arrive at fairly decent estimates of these shifts from empirical calculations. In simple cases, we wil
First problem (cont.):
Second problem (cont.):
top: CH only; middle CH and CH3 up, CH2 down.
Problem 5 (cont.):
Problem 6 (cont.):
Chemistry 439 Quiz I
March 26, 2002
Name Student Number
This quiz consists of 25 points. Show all work to receive lll (or partial) credit.
1. I have just measured a vicinal lH-lH coupling constant at 300 MHz to be 7 Hz. What
is the value of this couplin
Chemistry 439 Quiz II
April 27, 2004
This quiz consists of 50 points.
1. Consider the following pulses sequence:
Magnetization is left on the z-axis during τm.
(3 pts) What is the name of this experiment?
(3 pts) What happens during τm?
(3 pts) What
Beyond infinity: 3D and 4D spectroscopy When the size of your molecule becomes large, spectral overlap is more common and analysis is problematic if not impossible. Adding a dimension or two helps (other methods, like adding some deuterium to remove dipol
• We have seen through the COSY experiment that we
can correlate spins that are J-coupled. That is, cross
peaks are present for all coupled spins. These can be
observed over 2, 3, and sometimes 4 bonds (see any
ring structure for the long range coup
Last time: UV spectroscopy and degrees of unsaturation
Cubane has only 5: C8H 8 2x8=16; 16+2= 18 that is, a saturated hydrocarbon would have C8H 18 as a molec.form. 18-8=10 10/2=5 DU's Halogens take the place of H; O do nothing to m.f., and N subtract 1 f
Other shielding effects
H C C H CH C H
a) Effective for acetylene b) not as effective for acetylene Therefore, shifts for alkyne protons are only ~2.9-1.9.
Hax is shielded by 2-3 and 5-6 c-c bonds; upfield by ~0.5 ppm
Compare methyl, methylene, and methin
Tree diagrams on more complicated systems
With diastereotopic groups:
From here to infinity: living with strong coupling
Where are those J's in these spectra? With strong coupling, measurement may be impossible
When the chemical shift difference (i
Infrared Spectroscopy (IR)
1 2 c
The two atoms that are bonded are treated like a harmonic oscillator (two masses joined by a spring)
M 1M 2 M1+ M2
M1 and M2 are atomic weights
and K is the force constant
C=O is at 1710
for the dimer, but variable (1760, monomer)
OH stretch is one broad peak but can be weak in nujol mulls
O-H oop at ~900 is for the hydrogen bonded OH group
More carboxylic acids
Sodium salts: RCOONa
We wish to learn about the molecular
weight, or at least, molecular fragments
Electron Impact MS:
•Cause ionization (loss of an electron)
•Accelerate in an electric field
•Send through a magnetic field and
determine the mass/charge ratio
Mass spectroscopy (continued) Ionization methods Electron impact (ionization) Chemical ionization Desorption ionization (MALDI) Spray ionization (ESI)
1. 2. 3. 4.
Electron Impact (Ionization)
Good for mw< 1000 Da Good for less polar molecules 70 eV or gr
Practical aspects of NMR data collection Shimming
The problem with the magnetic field is that it is not homogeneous over the entire sample. We can correct this by "shimming the magnet" There are two types of "shims" available: cryoshims (set up when the m
Practical aspects of NMR data collection
Read chapter 2 and section 5.1
The sample is subjected to irradiation by one or more radio-frequency (rf) fields (B1, B2, etc.). Under the effect of B1 applied along x, M is rotated into the xy plane perpendicular
More practical stuff
More basics of data collection
• Why bother with FT NMR?
Signal averaging. Important for low concentrations, for
low γ nuclei, and for nuclei with low natural abundance.
13C has a 1.1% natural abundance and is ¼ the γ of 1H.
The Nuclear Overhauser Effect
Read section 5.4
The nuclear Overhauser effect (NOE) The NOE is the fractional change in intensity of one NMR line when another resonance is irradiated. The NOE is due to dipolar interactions between different nuclei and is c
In these experiments, we are trying to transfer magnetization from our sensitive, high , spins (1H) to our insensitive, low , spins (13C). Later, we will use this "polarization transfer" to correlate the 1H spins with their 13C partn