  Ethyl phosphaethyne, C2H5CP, has been characterized spectroscopically in the
gas phase for the first time, employing millimeter- and submillimeter-wave
spectroscopy in the frequency regime from 75 to 760 GHz. Spectroscopic
detection and analysis was guided by high-level quantum-chemical calculations
of molecular structures and force fields performed at the coupled-cluster
singles and doubles level extended by a perturbative correction for the
contribution from triple excitations, CCSD(T), in combination with large basis
sets. Besides the parent isotopologue, the singly substituted 13C species were
observed in natural abundance up to frequencies as high as 500 GHz. Despite the
comparably low astronomical abundance of phosphorus, phosphaalkynes, R--CP,
such as C2H5CP are promising candidates for future radio astronomical
detection.
