[Babcock_Hall]

We made a vinyl derivative of a 5-hydroxypyridine, and we could not find a signal corresponding to the phenolic carbon in the C-13 NMR spectrum.  One factor is that it is quaternary, but I wonder whether that is the whole story or not.  Could tautomerism lead to exchange broadening?
EDT
I remembered that we had made an isomer of the compound above, one with a hydroxyl group at the 3-position of the pyridine ring.  This compound displayed a broad peak near 155.0 ppm with low S/N.  Therefore, I am tempted to say that for some reason the C-OH carbon is expected to be broad, and this may be why we did not observe it in the compound discussed in the previous paragraph. 

[rolnor]

It could be completely in its carbonyl-form? Can you run the stm and compare?

[Babcock_Hall]

The signal to noise ratio in the C-13 spectrum is low, which could be a factor.  I do not see anything in the carbonyl region except the acetone NMR solvent peak.  What is stm?  Mr. Google suggested that it was scanning tunneling microscope.