Complex between Formic Acid and Nitrous Oxide: A Matrix-Isolation and Computational Study

The complex of formic acid (FA, HCOOH) with nitrous oxide (N₂O) was studied experimentally and computationally. Eight structures of the trans-FA···N₂O complex and nine structures of the cis-FA···N₂O complex were found at the density functional theory (M06-2X and wB97XD), MP2(full), and CCSD(T)-F12a levels. Two structures of the trans-FA···N₂O complex (1t and 3t) and two structures of the cis-FA···N₂O complex (1c and 3c) were identified by infrared spectroscopy in an argon matrix. Structure 1t with the bonded OH group appears in the matrices after deposition. Structures 3t and 3c with the free OH groups were prepared by vibrational excitation of the trans-FA conformer in structures 1t and 3t, respectively. Structure 3t is thermally unstable and relaxes to structure 1t at ∼15 K. Structure 1c with the bonded OH group is made by vibrational excitation of trans-FA monomer combined with thermal annealing. The lifetimes of the cis-FA···N₂O complex structures 1c (81.5 min) and 3c (18.7 min) in an argon matrix at 4.3 K are longer than that of cis-FA monomer (6.3 min). This difference agrees with the calculated stabilization barriers of these species.