A G3 Study of the Structure of Carbon-Nitrogen Nanoclusters Muneerah M. Al Mogren, Adel A. El-Azhary,* and Wad Z. Alkiali Chemistry Department, Faculty of Science, King Saud UniVersity, P.O. Box 2455, Riyadh 11451, Kingdom of Saudi Arabia Majdi Hochlaf UniVersite ´ Paris-Est, Laboratoire Modelisation et Simulation Multi Echelle, MSME UMR 8208 CNRS, 5 bd Descartes, 77454 Marne-la-Valle ´e, France ReceiVed: June 11, 2010; ReVised Manuscript ReceiVed: October 5, 2010 Possible structures of the carbon-nitrogen clusters of the form C m N n (m ) 1-4, n ) 1-4, m + n ) 2-5) were predicted for the neutral, anion, and cation species in the singlet, doublet, and triplet states, whenever appropriate. The calculations were performed at the G3, MP2(fc)/6-311+G*, and B3LYP/6-311+G* levels of theory. Several molecular properties related to the experimental datassuch as the electronic energy, equilibrium geometry, binding energy, HOMO-LUMO gap (HLG), and spin contamination 〈S 2 〉swere calculated. In addition the vertical electron attachment, the adiabatic electron affinity, and vertical ionization energy, of the neutral clusters were calculated. Most of the predicted lowest energy structures were linear, whereas bent structures became more stable with the increase of the cluster size and increase of the number of the N atoms. In most of the predicted lowest energy structures, the N atom prefers the terminal position with acetylenic bond. The calculated BE of the predicted clusters increases with the increase of the cluster size for the neutral and cation clusters but decreases with the increase of the cluster size for the anion clusters. The predicted clusters are characterized by high HLG of about 11 eV on the average, with that of the anion clusters is smaller than that for the neutral and cation clusters. It is concluded then that the anion clusters are less stable than the corresponding neutral and cation clusters. Finally, the N 2 loss reaction is treated. 1. Introduction Since clusters have properties distinctly different from their bulk state, the study of the structure and properties of nano- clusters has recently drawn much attention. Cluster properties, such as electron affinities and ionization energies, are functions of the size and the structure of these clusters, and they are converging for large sized clusters to the bulk values. The understanding and the knowledge of such evolutions is manda- tory for their applications in reactivity, for example, as catalysts, and in nanotechnology. One of the most important clusters is the carbon-nitrogen clusters. Carbon-nitrogen clusters are abundant in interstellar space and in planetary atmospheres, for example, Titan, and have unusual reactivity and thus are difficult to study on earth. 1-7 It has also been reported, theoretically, that carbon-nitride films are harder than diamond. 8-18 There are several reports of the structure of carbon-nitrogen clusters 19-61 where the electronic, vibrational, rotational, and structural properties of small, medium, and large sized carbon-nitrogen clusters are investi- gated. Briefly, Wang et al. 25 performed HF calculations on linear C n N anion clusters and showed that odd-n isomers are much more stable than even-n isomers, in good agreement with the mass spectrometry studies for the lowest-lying linear and cyclic structures. Tang et al. 32 produced carbon-nitrogen anion clusters with various composition by laser mass ablation of K 3 [Fe(CN) 6 ]. The authors rationalized the trend of odd/even alternation in stability according to the variation in bonding character, energy differences, electron affinities, and incremental binding energies (BEs). The results of the calculations were in good agreement with the phenomena observed in the experimental mass spec- trometric studies. Pascoli and Lavendy 36 investigated the structure of the C n N anion (n ) 1-7) clusters using the B3LYP method. The calculated results were compared with those calculated at the MP2, MP4, QCISD(T), and CCSD(T) levels. All the methods employed, except the MP2 method, revealed that the C n N anion (n ) 2-7) clusters are linear with 1 Σ + state when n is odd and 3 Σ - state when n is even. Jiang et al. 48 investigated the geometrical and electronic properties of the C m N 2 (m ) 1-14) neutral, anion, and cation clusters using the B3LYP method. It was concluded that the C m N 2 (m ) 2-14) cation clusters and the C m N 2 (m ) 6-14) anion clusters have linear structure with D ∞h symmetry except for the C 11 N 2 cation cluster, whereas the C m N 2 (m ) 1-5) anion clusters have chain- like structures. The two N atoms favor to bond at the ends in the linear and chain-like structures. The calculated HLG and adiabatic ionization potential showed that clusters with even m are more stable than those with odd m. More specifically, it turns out from previous works that several features of the stability and the isomeric forms of the carbon- nitrogen clusters are not well established. For instance, the relative stability of the linear, of astrophysical importance, and the ring and membred forms, which play a priori the role intermediates in the synthesis of large and bulk carbon-nitrogen clusters, is not fixed. For instance, linear C 2n-1 N molecules have two low-lying 2 Σ + and 2 Π states, making the identification of their ground state not obvious either theoretically or experimentally. The present theoretical contribution presents a systematic study of the possible structures of C m N n (m ) 1-4, n ) 1-4, * To whom correspondence should be addressed. E-mail: azhary60@ hotmail.com. Phone: (9661) 467 4367. Fax: (9661) 467 5992. J. Phys. Chem. A 2010, 114, 12258–12268 12258 10.1021/jp105390c  2010 American Chemical Society Published on Web 10/29/2010