We have searched for the 2 mm transitions of H 2 C O H + ( 2 02 − 1 01 ) and H 2 13 C O ( 2 02 − 1 01 , 2 12 − 1 11 , a n d 2 11 − 1 10 ) toward the dense interstellar molecular clouds Orion A, TMC-1 and L134N using the FCRAO 14 m telescope. None of the transitions have been detected except the H 2 13 C O transitions toward Orion-KL. We set upper limits for the abundances of the protonated formaldehyde ion ( H 2 C O H + ) , which are close to the abundances expected from ion-molecule chemistry.

We have mapped the C 3 H 2 2 12 − 1 01 transition line toward the Sgr A molecular cloud on a 1' grid spacing and derived C 3 H 2 column densities of 3 \~ 7 × 10 14 c m − 2 for molecular clouds of Sgr A. The fractional abundances of C 3 H 2 relative to H 2 are obtained to be 3 \~ 6 × 10 − 9 , which are slightly lower than that for the cold dark cloud TMC-1 but are enhanced by factors of 5-60 compared to those for Sgr B2 and the Orion extended ridge. We also estimate from the C 3 H 2 column densities total masses of \~ 10 6 M ⨀ for two clouds (M - 0.13 - 0.08 and M - 0.02 - 0.07), which are thought to be close to the virial equilibrium. We suggest that the large abundance of C 3 H 2 in Sgr A may be partly due to the activities of the Galactic center.