Infrared and density functional theory studies of formic acid hydrate clusters in noble gas matrices

•An extensive spectroscopic observation of formic acid hydrate clusters in noble gas matrices.•Anharmonic DFT calculations of candidate structures.•The observed clusters are assignable to moderately sized ones (HCOOH)n(H2O)m (n = 1–2, m = 1–4).

Infrared absorption spectra of formic acid hydrate clusters (HCOOH)m(H2O)n have been measured in noble gas matrices (Ar and Kr). The concentration dependence of the spectra and the comparison with a previous experimental study on HCOOH(H2O) and HCOOH(H2O)2 [Geoge et al., Spectrochim. Acta, Part A 60 (2004) 3225] led to the identification of large clusters. Density functional theory calculations at the B3LYP-DCP/6-31+G(2d,2p) level were carried out to determine the anharmonic vibrational properties of the clusters, enabling a consistent assignment of the observed vibrational peaks to specific clusters.