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The rotationally resolved infrared spectra of (HCOOH), (HCOOD), 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 (H-C/O-H in-plane bending) fundamental band of (HCOOH), the v (H-C/O-D in-plane bending) fundamental band of HCOOH-HCOOD, and the v (H-C-O in-plane bending) fundamental band of (HCOOD). Strong local perturbations caused by the rotation-tunneling coupling between two tunneling components are observed in (HCOOH). The v fundamental band of (HCOOH) and the previously measured v fundamental and v + v 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. The band-origin of the v band of (HCOOH) is 1371.776 74(8) cm, and the tunneling splitting decreases to 0.000 38(18) cm upon the vibrational excitation. The vibrational energy is 1386.755 49(16) cm for the v vibrational mode of HCOOH-HCOOD and 1391.084 39(17) cm for the v vibrational mode of (HCOOD). No apparent spectral splittings are resolved for HCOOH-HCOOD and (HCOOD) under our experimental conditions. The tunneling splitting in the ground state of HCOOH-HCOOD is estimated to be 0.001 13 cm from its average linewidth.

The vibration-rotation-tunneling absorption spectra of the formic acid dimer (HCOOH)2 have been measured in the C-O stretch region at 1215-1240 cm(-1) using a rapid-scan tunable diode laser spectrometer in conjunction with a slit supersonic expansion. The ν5 fundamental band of the HCOOH monomer is identified and the perturbed band-center is 1220.83329(10) cm(-1). Three vibrational bands centered at 1219.71, 1225.35, and 1233.95 cm(-1) are assigned to the two combination bands and the ν22 fundamental band of (HCOOH)2 unambiguously. The transition frequencies of these three vibrational bands are fitted together using a standard Watson A-reduced Hamiltonian, yielding precise rotational and centrifugal distortion constants for each tunneling level in the ground and excited vibrational states. The fitting results of the vibrational band centered at 1225.35 cm(-1) are in good agreement with a previous high resolution study [M. Ortlieb and M. Havenith, J. Phys. Chem. A. 111, 7355 (2007)]. The tunneling splittings in the vibrationally excited states are -0.00304(16), -0.01023(11), and -0.00318(12) cm(-1), respectively, where the minus indicates that the upper tunneling component lies energetically below the lower tunneling component. A three-state deperturbation analysis using the Fermi coupling constants obtained from a previous vibrational analysis [F. Ito, Chem. Phys. Lett. 447, 202 (2007)] fails to get the normal order of the tunneling levels for all the three excited vibrational states simultaneously.

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