Unraveling the Anomalous Solvatochromic Response of the Formate Ion Vibrational Spectrum: An Infrared, Ar-Tagging Study of the HCO 2 ¯ , DCO 2 ¯ , and HCO 2 ¯ ·H 2 O Ions (original) (raw)

2011, The Journal of Physical Chemistry Letters

b S Supporting Information T he formate ion, HCO 2 h , is ubiquitous in aqueous chemistry and biology, and its structural properties have been extensively catalogued over many decades using a wide variety of spectroscopic techniques. 1À5 There is renewed interest, however, in this ion's intrinsic behavior due to its increasingly important role in the chain of events leading to activation of CO 2 to transportable fuels, for example, by reductive addition of hydride. 6,7 The electronic and vibrational level structures of the isolated ion were accurately calculated in 2007 by Botschwina and co-workers, 8 and two aspects of the potential curve describing CH dissociation are nonintuitive. First, the ∼18700 cm À1 bond dissociation energy 5,9,10 is much lower than is typical for CH bonds of closed-shell singlets (e.g., 43370 cm À1 for HCN) 11 and second, the shape of the CH potential does not conform to a Morse form, instead exhibiting a sharp change in slope around a bond length of 2 Å where it flattens toward the low dissociation asymptote. Both these features are consistent with the 2003 experimental observation of the CH stretching (ν CH) fundamental of formate in a Ne matrix 12 at 2456 cm À1 , which is far below the typical 2950 cm À1 value found for aliphatic hydrocarbons. 13 More interestingly, this value is also much lower than that long known for HCO 2 h in aqueous media (2803 cm À1) 4 and in crystals (2830 cm À1). 14 In this paper, we explore the mechanism underlying the unusual shape of the CH potential, the low intrinsic ν CH frequency, and its anomalously large solvent dependence. This is accomplished by analyzing the vibrational