Recent
astronomical observations have shown that interstellar complex
organic molecules (COMs) exist even in cold environments (∼10
K), while various interstellar COMs have conventionally been detected
in hot gas (≳100 K) in the vicinity of high-mass and low-mass
protostars. However, the formation pathway of each interstellar COM
remains largely unclear. In this work, we demonstrate that an automated
reaction path search based on transition state theory, which does
not require predetermined pathways, is helpful for investigating the
formation pathways of interstellar COMs in the gas phase. The exhaustive
search within the electronic ground states helps elucidate the complex
chemical formation pathways of COMs at low temperatures. Here we examine
the formation pathways of dimethyl ether (CH3OCH3) and methyl formate (HCOOCH3), which are often detected
in the cold and hot gas of star-forming regions. We have identified
a barrierless and exothermic formation path of CH3OCH3 by reaction between neutral species; CH3O + CH3 → H2CO···CH4 →
CH3OCH3 is the most efficient path in the large
chemical network constructed by our automated reaction path search
and is comparable to those of previous studies. For HCOOCH3, we obtain complex pathways initiated from reactions between neutral
species; HCOO and CH3 generate HCOOCH3 and its
isomers without external energy. However, we also identified competing
reaction branches producing CO2 + CH4 and CH3COOH, which would be more efficient than the formation of
HCOOCH3. Thus, the gas-phase formation of HCOOCH3 through reactions between neutral species would not be efficient
compared to CH3OCH3 formation.