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

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.