  We have conducted an extensive search for nitrogen-, oxygen- and
sulfur-bearing heterocycles toward Taurus Molecular Cloud 1 (TMC-1) using the
deep, broadband centimeter-wavelength spectral line survey of the region from
the GOTHAM large project on the Green Bank Telescope. Despite their ubiquity in
terrestrial chemistry, and the confirmed presence of a number of cyclic and
polycyclic hydrocarbon species in the source, we find no evidence for the
presence of any heterocyclic species. Here, we report the derived upper limits
on the column densities of these molecules obtained by Markov Chain Monte Carlo
(MCMC) analysis and compare this approach to traditional single-line upper
limit measurements. We further hypothesize why these molecules are absent in
our data, how they might form in interstellar space, and the nature of
observations that would be needed to secure their detection.
