PLEASE HELP! Exam next month!! Q on NMR spectroscopy please!!?

Question

Hi pplz!
Im a 3rd year chemistry student at uni but am on a placement therefore ive been sent a bunch of notes thru the post. I was meant to have read the notes nd dun these qs in dec and as its feb, my exams r in march and i STILL dont get the notes (they r RUBBISH nd my lecturer is a bit of a meanie so i dont wanna ring him for help) plzzzz can sum1 help me on here!?!
Basically i am totally not understanding my NMR spectroscopy module - i basically learn best by simple examples and simple instructions on how 2 do things ie for proton nmr last year i had 2 buy a book with simple examples like ethanol for me to get intergrations, peak heights etc. I need similar help now - can some1 plz give me step by step instructions on how to assign peaks for COSY, NOEDIF, PENDANT, HMQC and HMBC and also explain really simply wat they r (or point me to a good website where i can learn) - i will b generous with the points for the best asnwer :D
Thanks!!

Answer 1

so, little theory but a lot of details for resolving bidimensional NMR spectra ?

ok first the links

Theory on protons
http://en.wikipedia.org/wiki/Proton_NMR

General Theory
http://en.wikipedia.org/wiki/Nuclear_magnetic_resonance

Good web site with a nice Introduction on how to resolve NMR spectra:
http://www.org.chemie.tu-muenchen.de/people/rh/h1book/html.txt/english/index.shtml

so for the answers:
A HSQC spectrum is an experiment used frequently in NMR spectroscopy, and is particularly useful in the field of protein NMR spectroscopy. The acronym stands for Heteronuclear Single Quantum Correlation. The spectrum obtained in the experiment has two axis, a proton axis and a heteronuclei axis. In the NMR terminology a heteronucleus is another nucleus other than protons, but most often 13C and 15N. The spectrum contains a peak for each unique proton attached a heteronucleus, though only one kind of heteronucleus can be recorded at the time. Thus, if the chemical shift of either the protons are known, the chemical shift of corresponding heteronuclei can be determined, and vice versa.
So for an heteronuclear 13C-1H NMR HSQC you can see spots that connect each protons with it's carbon, this is great for discovering N-H protons (no spot) or geminal protons (2 different protons attached to the same C)

NOE
n chemistry, the transfer of spin polarization from one spin population to another is generally called Overhauser Effect, after American physicist Albert Overhauser who hypothesized it in the early 1950s. The phenomenon was demonstrated by C. P. Slichter and T. R. Carver in 1953. Overhauser effect occurs under different conditions (e.g., between electrons and atomic nuclei) yet is most commonly observed and used amongst atomic nuclei and then named Nuclear Overhauser Effect (NOE). A very common application is NOESY (Nuclear Overhauser Effect Spectroscopy), a magnetic resonance technique for structure determination of macromolecular motifs. Other techniques include:
* HOESY, Heteronuclear Overhauser Effect Spectroscopy
* ROESY, Rotational Frame Nuclear Overhauser Effect Spectroscopy
* TRNOE, Transferred Nuclear Overhauser Effect
This technique is very useful for proteins and start to be useful only to large molecule, or quasi simmetrical molecules
It works by selectively choose a frequency of a proton and you will se only protons "near" 3-7 angstroms of that selected proton.
NOEDIF means noe difference spectra, no one use it anymore, just do a NOESY, same machine time and it's easier to read.

COSY
COSY is just a bidimensional standard 1H spectrum, where the spots are correlations over 1 single bond.

Total correlation spectrum TOCSY
ou can see spots of correlation for every "chain" of the molecule. This is very useful because think about isopentyl-heptyl-ketone, with a normal proton you can see anything, with a COSY is better, but you don't know if that CH2 is from the pentyl or heptyl part of the molecule, so do a TOCSY and you see 2 groups of spots, and you can easily tell which one is for the pentyl and wich of the heptyl (no correlation throught a ketone)

HMBC or multiple bond heteronuclea correlation:
This is a 2D experiment used to correlate, or connect, 1H and 13C peaks for atoms separated by multiple bonds (usually 2 or 3). The coordinates of each peak seen in the contour plot are the 1H and 13C chemical shifts. This is extremely useful for making assignments and mapping out covalent structure.
The information obtained is an extension of that obtained from an HMQC spectrum, but is more complicated to analyze. Like HMQC, this is an "inverse detection" experiment, and is possible only on newer model spectrometers.
So, if you have acetone, you can see a correlation spot between the two methyls !! (so throught 2 carbons)

PENDANT is a DEPT experiment
DEPT also can distinguish between CH and CH3, unlike APT, although quaternary Cs are not observed in DEPT.
DEPT spectra shown in the figure below are, from top to bottom:
DEPT-135 CH and CH3 peaks up, CH2 peaks inverted
DEPT-90 CH peaks only
DEPT-45 all protonated carbons
An APT spectrum provides nearly equivalent information. Quaternary and CH2 peaks are positive, CH and CH3 peaks are inverted. The disadvantages of APT are its lower sensitivity, the need to wait between scans for 13C relaxation, and no means of distinguishing CH from CH3 peaks.

I can give you another link, it's a NMR manual for technicians, maybe it's useful:
http://www.chem.binghamton.edu/staff/schulte/Manual1.htm

There is only 1 way to lean how to resolve 2D spectra, lot's of exercises !! So go to your lecturer and ask him/her a lot of exercises !!

Have fun