Infrared Spectroscopy of the Microhydrated Nitrate Ions NO 3 − (H 2 O) 1−6 †

Infrared Spectroscopy of the Microhydrated Nitrate Ions NO 3 − (H 2 O) 1−6 †,10.1021/jp9017103,Journal of Physical Chemistry A,Daniel J. Goebbert,Etie

Infrared Spectroscopy of the Microhydrated Nitrate Ions NO 3 − (H 2 O) 1−6 †   (Citations: 1)
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We present infrared photodissociation spectra of the microhydrated nitrate ions NO3 - (H2O)1-6, measured from 600 to 1800 cm -1 . The assignment of the spectra is aided by comparison with calculated B3LYP/aug- cc-pVDZ harmonic frequencies, as well as with higher-level calculations. The IR spectra are dominated by the antisymmetric stretching mode of NO3 - , which is doubly degenerate in the bare ion but splits into its two components for most microhydrated ions studied here due to asymmetric solvation of the nitrate core. However, for NO3 - (H2O)3, the spectrum reveals no lifting of this degeneracy, indicating an ion with a highly symmetric solvation shell. The first three water molecules bind in a bidentate fashion to the terminal oxygen atoms of the nitrate ion, keeping the planar symmetry. The onset of extensive water-water hydrogen bonding is observed starting with four water molecules and persists in the larger clusters. I. Introduction Nitrate ions, NO3 - , are ubiquitous in aqueous-phase chem- istry. They are of importance in diverse fields like atmospheric chemistry 1 and nuclear waste treatment 2 and as a source of nitrogen for the production of nucleic acids and proteins. 3 NO3 - is one of the most abundant ionic species in the troposphere and can be formed by rapid acid dissolution of nitric acid in aerosol particles. 1 The heterogeneous chemistry of aerosols containing such strong acids is of importance for many atmospheric cycles and is primarily governed by the nature of the aerosol surface, which determines if the adsorbed acid molecule dissociates and how the conjugate base anion is hydrated. 4 It is difficult to obtain such detailed microscopic information directly from bulk measurements. Spectroscopic experiments on isolated anion-water clusters can aid in characterizing the solvent-solute (and solvent-solvent) interac- tion at the molecular level one water molecule at a time. 5,6 In this work we investigate the microhydration of NO3 - through the infrared spectroscopy of NO3 - (H2O)n clusters, n ) 1-6. The gas-phase vibrational spectroscopy of NO3 - ·Ar was studied recently by Johnson and co-workers. 7 NO3 - is planar and has three equivalent N-O bonds (D3h symmetry). The six vibrational degrees of freedom lead to two bending and two stretching modes, of which two are doubly degenerate. The two bending modes (eand a2'') are infrared-active but have low intensity. The symmetric stretching vibration ν1 (a1' )i s not infrared-active, while the degenerate ν3 (e') antisymmetric stretching vibration is infrared-active. The vibrational predis- sociation spectrum of NO3 - ·Ar reveals only a single band at 1349 cm -1 , which was assigned to the ν3 mode, suggesting that ‡ Fritz-Haber-Institut der Max-Planck-Gesellschaft. §
Journal: Journal of Physical Chemistry A - J PHYS CHEM A , vol. 113, no. 26, pp. 7584-7592, 2009
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