The ^ { 13 } C substitutions of molecule HC _ { 7 } N were observed in TMC-1 using the J = 12 - 11 , J = 13 - 12 rotational transitions in the frequency range 12.4 to 13.6 GHz .
We present the first detection the ^ { 13 } C isotopic species of HC _ { 7 } N in the interstellar medium , based on the average of a number of weak rotational transitions .

This paper describes the calibration and data averaging process that is also used in a search for large cyanopolyyne molecules in TMC-1 using the 100m Robert C. Byrd Green Bank Telescope ( GBT ) .
The capabilities of the GBT 11 to 15 GHz observing system are described along with a discussion of numerical methods for averaging observations of a number of weak spectral lines to detect new interstellar molecules .