Published: January 25, 2011 r2011 American Chemical Society 1143 dx.doi.org/10.1021/jp111279t | J. Phys. Chem. A 2011, 115, 1143–1151 ARTICLE pubs.acs.org/JPCA Intermolecular Potential Energy Surface of the N 2 -CO Dimer: Ab Initio Investigation and Analytical Representation M. H. Karimi-Jafari,* ,† A. Maghari, ‡ and A. Farjamnia ‡,† † Computational Chemistry Laboratory, NSTRI, Tehran, Iran and ‡ Department of Physical Chemistry, School of Chemistry, University of Tehran, Tehran, Iran b S Supporting Information ABSTRACT: In this work, for the first time, an analytical four-dim- ensional representation for the intermolecular potential of the N 2 -CO dimer is constructed from ab initio calculations. The most stable structure of dimer is found to be a distorted T-shape con- formation with CO forming the top and N 2 the leg of T. Important structures of the dimer are characterized, and surprisingly, it is found that in contrast with general assumptions, the potential energy surface of the N 2 -CO dimer has a single symmetry unique mini- mum. The energy profile of a minimum energy path that connects two T-shaped saddle points to the minimum structure is derived. Important structures are characterized along this path to represent the concerted internal rotation of monomers within the complex. The second virial coefficient is calculated from the fitted PES, and reasonable agreement is found with recent experimental results. ’ INTRODUCTION Nitrogen and carbon monoxide are important atmospheric species and knowledge of their binary interaction is important for modeling and simulation of their spectroscopic and bulk proper- ties. The N 2 -CO complex has been first detected spectro- scopically in the infrared region of the CO stretching vibration in 1996 by Kawashima and Nishizawa 1 and by Xu and McKellar. 2 Using predictions from these infrared studies, rotational spectra of N 2 -CO have then been observed in the microwave and millimeter wave 3-5 regions for both spin modifications and various substituted isotopes. The infrared spectrum of the complex has been studied again with much more detail by Xia, McKellar, and Xu. 6 The most recent spectroscopic work is an investigation of the bending vibration of the complex in the millimeter wave range by Surin and co-workers. 5 Useful informa- tion could be extracted from these longstanding spectroscopic studies about the structure and dynamics of the N 2 -CO complex. In their infrared studies Xu and McKellar 2 noted that the N 2 -CO falls midway between the He-CO and Ne-CO cases, surprisingly close to the free rotor limit. The observed ground state rotational constant implies an effective separation of 4.025 Å between the N 2 and CO centers of mass in the complex. They found this distance consistent with the N 2 having some- what free internal rotation in the complex, so that it spends part of the time with its axis perpendicular to the intermolecular axis, and part of the time parallel to it. Further structural information on the angular location of monomers within the complex emerged from the microwave and millimeter wave investigations of Xu and co-workers. 3 Their examination of the type of possible transitions establishes that the complex has to be planar. As a sensitive indicator of geometry and internal motions, the sign and magnitude of 14 N nuclear quadrupole coupling constant were analyzed in their work at different rotational states. Accordingly, they concluded that the N 2 -CO complex exhibits complicated internal dynamics that depends sensitively upon isotopic com- position and rotational state. Thus one cannot treat the N 2 -CO complex as a semirigid rotor. For the two ortho-N 2 states investigated by them, it was shown that the N 2 -CO complex in its real ground state (with K = 0, where quantum number K refers to the projection of the angular momentum of monomers on to the intermolecular axis) has an approximate T-shaped structure with N 2 forming the top and CO the leg, and that the oxygen atom is on average closer to the N 2 monomer than the carbon atom. In this structure the N 2 monomer undergoes only slightly hindered internal rotation in the complex. The lower K =1 state has a somewhat opposite geometry, with N 2 forming the leg and CO forming the top of T, and the N 2 monomer is somewhat more localized than in the ground state. Microwave measure- ments of Xu and Jager 4 extend the investigations of ref 3 to the para-N 2 species of the complex. They found it unreliable to Received: November 27, 2010 Revised: January 3, 2011