11 January 1999 Ž . Chemical Physics Letters 299 1999 334–344 Density functional theory exploring the HONO potential energy surface Branko S. Jursic ) Department of Chemistry, UniÕersity of New Orleans, New Orleans, LA 70148, USA Received 5 August 1998; in final form 5 October 1998 Abstract The potential energy surface was carefully explored for the HONO molecular system with hybrid, gradient-corrected, and local spin density approximation. Computed geometries, relative energies, activation barriers, enthalpies of formation, and bond dissociation energies for this molecular species, as well as for related molecules, were calculated and compared with experimental and the complete basis set obtained values. The harmonic frequencies with the moment of inertia for all stationary points on the potential energy surface were calculated with both gradient-corrected and hybrid density functional theory methods. The reliability of the density functional theory method for exploring the small polar nitrogen system was discussed. q 1999 Elsevier Science B.V. All rights reserved. 1. Introduction There is considerable interest in exploring the structural and energetic properties of nitrous acid due to its presence in combustion as well as atmospheric chemistry. This compound is a well-known source of q wx NO electrophiles in organic synthetic chemistry 1 . Nitrous acid is very unstable in its pure form, but it is a well-known and important reagent in aqueous solutions and has been extensively investigated in wx the gas phase 2 . In the gas phase, equilibrium of the HONO decomposition with the formation of equimo- lar amounts of water, nitric oxide, and nitrogen ) Corresponding author. Fax: q1 504 280 6860; e-mail: bsjcm@uno.edu dioxide can be reached. It was estimated that the wx enthalpy for this reaction is 38 kJr2 mol HNO 2. 2 From microwave spectroscopy we know that in the gaseous phase nitrous acid is predominantly in the trans form. The structural parameters are r s HO ˚ ˚ ˚ 0.954 A, r s 1.433 A, r s 1.177 A, a s HO – N NO HON wx 102.18, a s 110.78 2 . From the infrared data it ONO was suggested that the cis form is only 0.5 kcalrmol higher in energy than trans-HONO. Although the HO–N bond should have a single-bond character, the rotational barrier is experimentally estimated to be 10.8 kcalrmol. We have previously demonstrated that density Ž . functional theory DFT methods are exceptionally accurate for computing structural parameters of small w x polar molecular systems 3–8 , their bond dissocia- 0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 98 01274-3