Date: Tue, 27 Jun 2017 18:18:05 +0000
From: Knudsen, Jens
Sent: Tuesday, June 27, 2017 12:31 PM
To: 'ammrl_at_ammrl.org'
Subject: Mysteriously missing methyl carbon
Hello all,
I have a question for this very knowledgeable group of NMR people. A couple of times now, I have encountered small molecule structures with methyl groups with its 13C signal missing.
In one instance it was an N-methyl group (as a part of an amide group), several other (multi-substituted) rings nearby containing eg. -CF(CF3)2. I do see most of the double heptet lines from that substituent (around 92-94 ppm, in 13C APT) but the N-methyl group signal (an expected singlet) is only observed indirectly in an HSQC at ~34 ppm.
Another curious molecule has the methyl group as a (carbon-)substituent of an isoxazole ring The ring carbons are observed in both 13C and APT, but the methyl-substituent carbon is again only observed indirectly via the HSQC near ~36.75 ppm.
Help - what gives? These samples were both run in DMSO-d6, but what could possible obscure the carbon signal from my methyl groups? Relaxation issues?
Jens Knudsen
Research Scientist/NMR Spectroscopist,
Analytical Chemistry
Olon Ricerca Biosciences LLC
Phone: 440.357.3242
Fax: 440.357.3654
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