High resolution jet-cooled infrared absorption spectra of (HCOOH)2, (HCOOD)2, and HCOOH—HCOOD complexes in 7.2 <i>μ</i>m region

Abstract

The rotationally resolved infrared spectra of (HCOOH)₂, (HCOOD)₂, and HCOOH-HCOOD complexes have been measured in 7.2 μm region by using a segmented rapid-scan distributed-feedback quantum cascade laser absorption spectrometer to probe a slit supersonic jet expansion. The observed spectra are assigned to the v₂₁ (H-C/O-H in-plane bending) fundamental band of (HCOOH)₂, the v₁₅ (H-C/O-D in-plane bending) fundamental band of HCOOH-HCOOD, and the v₂₀ (H-C-O in-plane bending) fundamental band of (HCOOD)₂. Strong local perturbations caused by the rotation-tunneling coupling between two tunneling components are observed in (HCOOH)₂. The v₂₁ fundamental band of (HCOOH)₂ and the previously measured v₂₂ fundamental and v₁₂ + v₁₄ combination bands [K. G. Goroya et al., J. Chem. Phys. 140, 164311 (2014)] are analyzed together, yielding a more precise tunneling splitting in the ground state, 0.011 367(92) cm^-1. The band-origin of the v₂₁ band of (HCOOH)₂ is 1371.776 74(8) cm^-1, and the tunneling splitting decreases to 0.000 38(18) cm^-1 upon the vibrational excitation. The vibrational energy is 1386.755 49(16) cm^-1 for the v₁₅ vibrational mode of HCOOH-HCOOD and 1391.084 39(17) cm^-1 for the v₂₀ vibrational mode of (HCOOD)₂. No apparent spectral splittings are resolved for HCOOH-HCOOD and (HCOOD)₂ under our experimental conditions. The tunneling splitting in the ground state of HCOOH-HCOOD is estimated to be 0.001 13 cm^-1 from its average linewidth.