Formate Ion: Structure and Spectroscopic Properties M. A. Moreno, O. Ga ´lvez, B. Mate ´, V. J. Herrero, and R. Escribano* Instituto de Estructura de la Materia, CSIC, Serrano 123, 28006 Madrid, Spain ReceiVed: September 1, 2010; ReVised Manuscript ReceiVed: NoVember 22, 2010 The formate anion HCOO - is present in a multitude of systems of relevance, and it is characterized by its plasticity, adopting several different structures. This work provides a theoretical study of the ion focused on two of these structures, a crystal and an isolated species. Crystals of sodium formate and ammonium formate are studied using CASTEP, a solid-oriented computing package. Individual molecules of the same systems and of the formate and ammonium ions are also studied, using the Gaussian code at the MP2/aug-cc-pvTZ level. All theoretical calculations are contrasted by comparison to observed infrared spectra, recorded by using different techniques. In addition, a topological analysis of the bonding properties of the isolated molecules is presented. 1. Introduction The carboxylate (RCOO - ) functional group is omnipresent in biology, forming part of all amino acids. The simplest carboxylate, the formate ion HCOO - , is present also in a multitude of systems and could be found, e.g., in natural waters as a result of photooxidation of humic substances. 1 Perhaps one of its most recent and exciting occurrences would be its possible detection in astrophysical ices (see ref 2), especially in relation with spectroscopic features tentatively assigned to their coun- terion ammonium, NH 4 + , particularly the elusive 6.85 µm band. 3 The astrophysical interest of this species has prompted several recent investigations, of either theoretical 4 or experimental nature, 5 and is also one of the main reasons why we started this research. One of the peculiarities of the formate ion is the different structure that it can adopt depending on its environment and physical state. This was already a focus of research in the 1930s, 6,7 when a special configuration was proposed for the formate ion in solution, while the so-called normal structure was retained for the acid, its esters, and the formate ion in the crystal. 8 The structure may vary depending also on the cation with which it is associated. X-ray diffraction studies showed that crystalline sodium formate and ammonium formate (refs 8 and 10, respectively) present different configurations, with possible resonant structures in the former and H-bonding between O and N atoms in the latter. We are not aware of theoretical calculations of these systems as solid crystals, although other theoretical and spectroscopic studies of species containing the formate ion have been reported over the years (see, for instance, ref 11 for work on calcium formate crystals, ref 12 for lithium formate monohydrate, and ref 13 for solid amino formate); such calculations are very useful for predicting and assigning the corresponding vibrational spectra. Consideration of the formate ion as an isolated species has on the other hand induced several theoretical investigations. To the best of our knowledge, the most recent one is the work cited, 4 in which HCOO - , NH 4 + , HCOOH, and several hydrated clusters of these species were studied using density functional (DFT) methods. The calculations allowed a discussion on the assign- ment of the observed spectra. Molecular dynamics methods have also been employed to study some specific properties of the formate ion. 14 In this investigation, we have pursued a deeper understanding of the structure and spectroscopic properties of the formate ion. We have recorded spectra of liquid solutions of formate salts and also of low-temperature samples obtained after most of the water content had been removed. Using specific theoretical methods for solids, we have modeled the crystals of sodium formate and ammonium formate, and after relaxing the structure, we have predicted their vibrational spectra, which are then compared to the experimental recordings. We have also studied the structure of ionic and neutral species involved in the molecules mentioned above by high-level ab initio methods, followed by the prediction of their anharmonicity-corrected spectra. From the optimized structures, we have conducted a study of the bonding properties of the formate ion, which provides a clear graphical picture of the structural differences found for this ion. 2. Methodology Experimental Section. We have made use of the hyper- quenching experimental technique, 15 which in our case consisted of a rapid expansion of droplets of the appropriate liquid solution into a low-temperature chamber containing a cold substrate, where the droplets immediately freeze, followed by a controlled warming to induce water sublimation. The substrate is then cooled again, leading to the crystalline salts. The whole process is monitored by infrared (IR) spectroscopy, using a Bruker Vertex 70 FTIR spectrometer coupled to the cold chamber. A more thorough description of our experimental setup and procedure is provided in ref 3. Solutions of HCOONa and HCOONH 4 were prepared at room temperature with a 7:100 salt:water ratio. Vacuum expansion and freezing at 14 K, and subsequent warming at 210 K and recooling again at 14 K, were performed as indicated above. Spectra of the corresponding samples at each stage are shown below, with a full description and a subsequent discussion. Theoretical. We have performed theoretical calculations of two types, specific for solid systems on one hand and for isolated individual molecules and ions on the other. Solid crystals of sodium formate and ammonium formate have been treated using CASTEP, 16 a program specifically designed to deal with several properties of solids, not only geometry optimization and prediction of spectra, as used in this investigation, but also J. Phys. Chem. A 2011, 115, 70–75 70 10.1021/jp108326x  2011 American Chemical Society Published on Web 12/13/2010