It turns out that the decoupler 90 deg. pulse width was not set properly.
After doing a pulse width calibration, and running a series of experiments
where the decoupler offset is set in 1 ppm steps on either side of the
acetone 1H resonance (I used acetone in d-methanol as the test case since
it most closely resembled the problematic sample). With the decoupler
pulse width set right, a range of 20 ppm was covered with all 13C peaks
singlets of about equal height.
Thanks to ammrl'ers (in general), and Neil E. Jacobsen (in particular) for
the helpful advice.
------------------------------------------------------------------------------
Edward T. Chainani Nuclear Magnetic Resonance Laboratory Manager
National Chemistry Instrumentation Center (NCIC)
Ateneo de Manila University phone: (632) 924-4601 loc. 2309
Loyola Heights, Quezon City e-mail: etch@ginto.chem.admu.edu.ph
Philippines
------------------------------------------------------------------------------