Detection of HC5N and HC7N Isotopologues in TMC-1 with the Green Bank Telescope

We report the first interstellar detection of DC₇N and six ¹³C-bearing isotopologues of HC₇N toward the dark cloud TMC-1 through observations with the Green Bank Telescope, and confirm the recent detection of HC₅¹⁵$N. For the average of the $¹³$C isotopomers, DC$_7$N, and HC$_5¹⁵N, we derive column densities of 1.9(2)×10¹¹, 2.5(9)×10¹¹, and 1.5(4)×10¹¹ cm⁻², respectively. The resulting isotopic ratios are consistent with previous values derived from similar species in the source, and we discuss the implications for the formation chemistry of the observed cyanopolyynes. Within our uncertainties, no significant ¹³C isotopomer variation is found for HC₇N, limiting the significance CN could have in its production. The results further show that, for all observed isotopes, HC₅N may be isotopically depleted relative to HC₃N and HC₇N, suggesting that reactions starting from smaller cyanopolyynes may not be efficient to form HCₙN. This leads to the conclusion that the dominant production route may be the reaction between hydrocarbon ions and nitrogen atoms.