Torsional energy pattern of HCOOCH₃, HCOOCH₂D, and HCOOCD₂H. The energy pattern consists of two sublevels, 405 MHz apart, for the lighter symmetrical species (Ilyushin et al. 2009) which belong to the A₁ and E symmetry species of C_3v. For the mono-deuterated species, three nondegenerate sublevels arise. Two close-lying A′ and A′′ levels, only 80 MHz apart, can be seen about 10 cm^-1 above an A′ isolated sublevel (Margulès et al. 2009). For the doubly deuterated species, the pair of close-lying levels is about 10 cm^-1 below the isolated sublevel. For this species, the tunneling splitting is 10.8 MHz as obtained in this work.