Comments on: You’re best to know complex NMR

By: NMR Slave

NMR Slave — Fri, 24 Oct 2008 02:55:52 +0000

ACD labs software is great, but maybe better for pharma or high throughput applications due to the cost. For a moderate organic lab, an old mac and iNMR can be better and only cost the same as some new packets of tubes. The new version is also available at a reduced price for October.

By: Darksyde

Darksyde — Thu, 23 Oct 2008 22:56:32 +0000

crap, next time I’ll read all the comments first before chiming in.

By: Darksyde

Darksyde — Thu, 23 Oct 2008 22:55:48 +0000

This has to do with molecular tumbling, correlation times, and the like. The confusing thing is that the ROESY pulse sequence and the TOCSY pulse sequence are idemtical, and I never understood why.

I’ve only ever done peptides (14-37 aa) and NOESY was par for the course.

By: Kyle Finchsigmate

Kyle Finchsigmate — Thu, 23 Oct 2008 15:30:51 +0000

So it would seem!

By: ChemE

ChemE — Thu, 23 Oct 2008 06:21:42 +0000

Given your complaints related to a lack of discussion about chemistry articles, it appears you should write about characterization techniques if you’re jonesing for some threads.

By: Eugene

Eugene — Wed, 22 Oct 2008 16:14:53 +0000

DQF-COSY is really for determining H,H coupling constants. It’s fine if you have a decent amount of sample (it’s less sensitive than COSY by a factor of 2) and quite a lot of time. You need a lot of time because you need a lot of increments to get enough resolution in T1 to avoid the cancellation of adjacent signals. There are published procedures on how to narrow the spectral window and computationally remove the aliased signals. In principle, that would let you “zoom in” on a particular multiplet to get its couplings, but it’s complicateed in practice. For most applications, it is unnecessary. It’s way easier to use some other technique, like 1D-TOCSY, to get the couplings. However, one case where it might come in handy is where the spectrum has a lot of singlets in it arising from methyl groups. They don’t show up in DQF-COSY, so they won’t swamp the spectrum.

By: Rhenium

Rhenium — Wed, 22 Oct 2008 15:09:55 +0000

Yay for crystal structures, that’s the gold standard in my (inorganic) book.

By: fussboy labmonkey

fussboy labmonkey — Wed, 22 Oct 2008 04:52:15 +0000

also – dqf cosy has a significant advantage over cosy in that sign of the signal is conserved, so identifying multiplets becomes a lot easier b/c the alternating colors… it takes more time to run the sample, but saves so much time in interpreting

By: Eugene

Eugene — Wed, 22 Oct 2008 02:30:51 +0000

I would suggest it is perfectly acceptable to quote C13 chemical shifts from 2D correlation spectra. If anything, I should think a 2D spectrum would give more detailed information than a 1D one, so it should be considered better, assuming it’s of good quality.

By: Kyle Finchsigmate

Kyle Finchsigmate — Wed, 22 Oct 2008 00:27:38 +0000

All good points. I run COSY-45, not the old fashioned kind, and I always obtain a C13 because it’s something that must go into my thesis for every novel compound I create per PI policy but I have yet to find it necessary for structural determination when I have all the 2D data handy.

I can’t comment on OEM NMR software since there are very few data stations to work on that aren’t connected to an instrument. Our department has a site license for ACD labs, so everyone has a copy of version 10 or 11 installed on their laptops or desktops which means NMR processing is always done away from the instruments and out of the NMR lab. So… I really don’t know how to manipulate the spectra much on the OEM software other than putting peaks in and integrating and phasing…