Dissociation of Strong Acid Revisited: X-ray Photoelectron Spectroscopy and Molecular Dynamics Simulations of HNO3 in Water

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• 作者：Tanza Lewis ; Bernd Winter ; Abraham C. Stern ; Marcel D. Baer ; Christopher J. Mundy ; Douglas J. Tobias ; John C. Hemminger
• 刊名：Journal of Physical Chemistry B
• 出版年：2011
• 出版时间：August 4, 2011
• 年：2011
• 卷：115
• 期：30
• 页码：9445-9451
• 全文大小：948K
• 年卷期：v.115,no.30(August 4, 2011)
• ISSN：1520-5207

文摘

Molecular-level insight into the dissociation of nitric acid in water is obtained from X-ray photoelectron spectroscopy and first-principles molecular dynamics (MD) simulations. Our combined studies reveal surprisingly abrupt changes in solvation configurations of undissociated nitric acid at approximately 4 M concentration. Experimentally, this is inferred from shifts of the N1s binding energy of HNO₃(aq) as a function of concentration and is associated with variations in the local electronic structure of the nitrogen atom. It also shows up as a discontinuity in the degree of dissociation as a function of concentration, determined here from the N1s photoelectron signal intensity, which can be separately quantified for undissociated HNO₃(aq) and dissociated NO₃^{鈥?/sup>(aq). Intermolecular interactions within the nitric acid solution are discussed on the basis of MD simulations, which reveal that molecular HNO₃ interacts remarkably weakly with solvating water molecules at low concentration; around 4 M there is a turnover to a more structured solvation shell, accompanied by an increase in hydrogen bonding between HNO₃ and water. We suggest that the driving force behind the more structured solvent configuration of HNO₃ is the overlap of nitric acid solvent shells that sets in around 4 M concentration.}