  It has been more than 30 years since the enigmatic 21 {\mu}m emission feature
was first discovered in protoplanetary nebulae (PPNs). Although dozens of
different dust carrier candidates have been proposed, there is as yet no widely
accepted one. We present the results of molecular observations toward 21{\mu}m
objects using the 10m Submillimeter Telescope of Arizona Radio Observatory at
the 1.3 mm band and the 13.7 m telescope of Purple Mountain Observatory at the
3mm band, aiming to investigate whether the gas-phase environments of these
unusual sources have some peculiarities compared to normal PPNs. We detect 31
emission lines belonging to seven different molecular species, most of which
are the first detection in 21 {\mu}m PPNs. The observations provide clues on
the identification of the 21 {\mu}m feature. We report a correlation study
between the fractional abundance of gas-phase molecules and the strengths of
the 21 {\mu}m emission. Our study shows that given the small sample size, the
21 {\mu}m feature has weak or no correlations with the gas-phase molecules.
Future radio observations of high spatial and spectral resolution toward a
large sample are desirable to elucidate the 21 {\mu}m emission phenomena.
