Conclusions With large basis sets and high-level theoretical methods, the structure and physical properties of the exocyclic ring isomer of diacetylene, cyclopropenylidenecarbene (: C= CC2H2), are predicted. Several of the vibrational frequencies appear to have IR intensities strong enough for detection. Moreover, cyclopropenylidenearbene has a large dipole moment, raising the hope that experimental and astronomical investigations will ...