Composition and Structure of Hydrates Formed in Aqueous Solutions of Formic Acid

IR spectroscopy and quantum chemical methods are used to study the features of hydration and determine concentration structural areas in the HCOOH-H2O system. Data on the composition and structure of stable heteroassociates (HA) formed in HCOOH aqueous solutions are obtained from the joint analysis of the IR spectra of these solutions and the results of quantum chemical calculations of (HCOOH)m·(H2O)n (m = 1−4, 6, n = 1−10, 14) and HCO2−·(H2O)n (n = 2, 6) complexes. The sequence is established in which these complexes form when pure acid is diluted with water. For HCOOH concentrations from 100% to the ratio HCOOH:H2O = 1:1, all water molecules are involved in the most stable cyclic (HCOOH)2· (H2O)2 HAs with the C2 symmetry. The components of the equimolar solution are almost completely bound into such hydrates. For the HCOOH:H2O ratio ranging from 1:1 to 1:2, less stable (by ∼10%) cyclic HCOOH · (H2O)2 HAs form along with the (HCOOH)2 · (H2O)2 complexes. The solution with a twofold water excess consists of these hydrates. A further increase in the water concentration results in further hydration of HCOOH · (H2O)2 HAs.