High resolution jet-cooled infrared absorption spectra of (HCOOH)2, (HCOOD)2, and HCOOH—HCOOD complexes in 7.2 μm region

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 v 21 (H—C/O—H in-plane bending) fundamental band of (HCOOH)2, the v 15 (H—C/O—D in-plane bending) fundamental band of HCOOH—HCOOD, and the v 20 (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 v 21 fundamental band of (HCOOH)2 and the previously measured v 22 fundamental and v12 + v 14 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 v 21 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 v 15 vibrational mode of HCOOH—HCOOD and 1391.084 39(17) cm−1 for the v 20 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.