ELSEVIER 27 October 1995 Chemical Physics Letters 245 (1995) 143-149 CHEMICAL PHYSICS LETTERS Ab initio study of protonated nitrosamide: a possible intermediate in the deNOx process Sudhir A. Kulkarni, Savita S. Pundlik Department of Chemistry, University~Pune, Pune 411007, India Received 21 June 1995;in final form l0 August 1995 Abstract Ab initio calculations on protonated nitrosamide, a possible intermediate in the deNOx process, have been performed. A mechanism involving the formation of an intermediate NH3NO + was proposed by Egsgaard, Carlsen and Madsen and tested by performing flame experiments with mass spectrometry. In the present work, the absence of NH3NO + in the flame has been supported on the basis of its structure. This cation is observed to be a loosely bound adduct of NH 3 and NO + at and beyond the HF level of theory. On the other hand, NH2NOH + is found to be a covalently bonded compound at all theoretical levels. Topographical analysis of electron density and electrostatic potential has been utilized to gain insight into bonding properties and to predict the possible sites of protonation. The vibrational spectra of both protonated nitrosamide cations are reported. The feasibility of proton transfer reactions of NH3NO + with ammonia and water are investigated using heats of reactions. These values indicate that such a reaction is possible with NH 3 but unfavorable with H20. I. Introduction The reaction between NH 2 and NO radicals has received considerable attention [1-10] since it pro- vides a means of eliminating NO, a major atmo- spheric pollutant, NH 2 + NO --, [NHzNO ] --* N 2 + .... The products of this deNOx process have been identified to be N 2 and H20 [1-3] and it has been proposed [4,5] that NHzNO is a crucial intermediate. Nitrosamide was earlier believed to have a planar structure [6,7]. However, it is shown [8] that the two hydrogens in NHzNO are out of plane by about 10°. Various pathways for the above reaction have been studied and the kinetic data [4] suggests that ni- trosamide actually exists in a vibrationally excited state. It was proposed that energetically excited NH2NO would either isomerize to hydroxydiimide or go back to reactants, rather than getting deexcited [7]. This particular intermediate isomer, however, has not been identified in the thermal deNOx pro- cess. Also, the lifetime for [NH2NO*] has been reported to be quite small in the microsecond regime near the ground state [9] and even smaller for the excited state [4]. In view of these lhcts, a search for other alternative mechanisms is warranted. An ionic mechanism has been suggested by Egs- gaard et al. [10] involving the formation of N-proto- nated nitrosamide NH3NO ÷. The experiments were performed on a methane/ammonia flame with nitric oxide as a reagent gas [10]. The mass spectra of various regions of the experimental chamber were found to have mainly NH~- and NO ÷. However, the 0009-2614/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0009-261 4(95)00999-X