Vibrational spectra of formic acid and its dimer: I. Spectroscopic accuracy through matrix-isolation IR spectroscopy and anharmonic computations

Abstract

This work constitutes part I of a series on vibrational spectroscopy of formic acid. As the simplest carboxylic acid, formic acid and its cyclic dimer are well-established models for investigating vibrational signatures of hydrogen-bonded organic acids. This has led to numerous highly specialized studies on formic acid, rendering the field increasingly fragmented. Here we demonstrate how a straightforward combination of experimental and theoretical approaches can provide a complete description of the vibrational structure of formic acid. Using matrix-isolation Fourier-transform infrared (MI-FTIR) spectroscopy with argon and neon as hosts, we record mid-IR spectra of HCOOH, HCOOD, DCOOH, and DCOOD and assign all fundamental bands, as well as numerous combination bands, overtones, and resonances. Matrix-induced frequency shifts in neon average about 2 cm⁻¹, with maximum shifts of 6 cm⁻¹, making neon a close approximation to the gas phase IR spectrum. Anharmonic vibrational calculations based on a high-quality potential energy surface (PES) reproduce gas phase reference data with mean absolute deviations of ∼2 cm⁻¹ for the monomer and ∼4 cm⁻¹ for the dimer. Taken together, the calculations approach spectroscopic accuracy for the matrix-isolation data. We demonstrate this accuracy for the trans-formic acid monomer and its deuterated isotopocules (HCOOH, HCOOD, DCOOH, DCOOD) as well as for the cyclic non-polar dimer in C₂ₕ symmetry ((HCOOH)₂, (DCOOD)₂), using both vibrational perturbation theory (VPT2) and vibrational configuration interaction theory (VCI). Our results reconnect the diverse threads of formic acid IR spectroscopy and establish a framework for interpreting matrix-, isotope-, and cluster-induced frequency shifts.