ELSEVIER THEO CHEM Journal of Molecular Structure (Theochem) 315 (1994) 173-178 Valence bond studies of N204* Yirong MO*, Wei Wu’, Qianer Zhang Department of Chemistry, Xiamen University, Xiamen 361005, People’s Republic of China Received 2 November 1993;revised 5 January 1994;accepted 17 March 1994 Abstract A non-empirical valence bond method is used to investigate the conformation of N204. Calculations of the planar conformer shows that the 7~ resonance energy between the two NO1 fragments is only 3.60 kJ mol-‘, so that the planarity cannot result primarily from the presence of N-N r bonding in accordance with previous results. We also present a technique for studying the origin of the rotation barrier for NzO,,. Our results indicate that the variations of the one- electron orbital energies of the N2P, and 02P, atomic orbitals as well as their electron populations are responsible for the stability of the planar conformer compared with the skew conformer. 1. Introduction As a weakly bonded dimer of N02, dinitrogen tetroxide (Nz04) has an experimental dissociative energy of 53.1 kJmo1 [l], while the value for a typical N-N bond is as high as 297 kJmo1 [2]. N204 has several other prominent features: (a) the bond length between the two nitrogen atoms [2] is exceptionally long (1.782A) compared to the bond length in N,H, (1.47A [3]), (b) the barrier to rotation around the N-N bond is com- paratively high (previous values are about 8-12 kJmol_’ [1,4], but the most recent experi- ment gives rise to a value of 28.0 kJmol_’ [5]) and the planar structure is the most stable con- * Dedicated to Professor W.J. Orville-Thomas for his numerous contributions to our knowledge of molecular structure as well as the Journal of Molecular Structure which has accumulated 300 volumes under his editing. * Corresponding author. ’ Present address: Department of Chemistry, University of Pisa, 35-56100 Pisa, Italy. former; (c) the angle O-N-O is quite large. The experimental conformation for N204 [6] is shown in Fig. 1. Theoretically, whether the 7r bond between the nitrogen atoms exists and why N204 prefers a planar conformer are two intriguing questions. Regarding the bonding pattern in N204, most theoreticians [7-91 conclude that there is r bonding as well as c bonding in the N-N bond, while others [lo] believe that the long N-N distance can be explained by a “n-only” bond. The above works are exclusively based on qualita- tive arguments or semiempirical calculations. Bauschlicher et al. [l l] performed a series of CAS-SCF calculations using a double-c plus polarization quality basis set. Their results show that the correlation of the x space allows the formation of some N-N 7r bonding which shortens the N-N bond and presumably is responsible for the observed planar conformer. However, other authors [ 12-191 hold different opinions. Ahlrichs and Keil [ 121 performed a 0166-1280/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDZ 0166-1280(94)03778-J