On the Electronic Structure of H-Ng-Ng-F (Ng ) Ar, Kr, Xe) and the Nonlinear Optical Properties of HXe 2 F A. Avramopoulos,* ,†,| L. Serrano-Andre ´s,* ,‡ J. Li, § and M. G. Papadopoulos* ,† Institute of Organic and Pharmaceutical Chemistry, National Hellenic Research Foundation, 48 Vas. Constantinou AVe., Athens 116 35, Greece, Molecular Sciences Institute, UniVersitat de Vale `ncia, Apartado 22085, Vale `ncia ES-46071, Spain, Accelrys Inc., Telesis Court, San Diego, California 92121, United States, and Department of Informatics and Computer Technology, Lamia Institute of Technology, Third Km Old National Road, 35100, Lamia, Greece Received August 23, 2010 Abstract: The electronic ground state of H-Ng-Ng-F (Ng ) Ar, Kr, Xe) has been studied theoretically by employing the ab initio complete active space valence bond (CASVB) and multi- state complete active space perturbation theory (MS-CASPT2) methods. Both levels of theory confirm the diradicaloid character (DC) of the HNg 2 F ground state, increasing in the order Ar > Kr > Xe. The very significant effect of the first and, even more, the second Xe atom on the (hyper)polarizabilities has been shown and interpreted. Thus, the present results demonstrate a mechanism for producing very large (hyper)polarizabilities. I. Introduction There is currently a great need for novel materials for photonic applications (e.g., optical communications, data storage, and signal processing). Such materials must have a series of properties, prominently among which is the large magnitude of their nonlinear optical (NLO) properties. Various mechanisms and phenomena have been investigated, which lead to such large NLO properties, for instance, conjugation or charge transfer. 1-3 Some time ago, we reported that insertion of a noble gas atom into a chemical bond, for example, Ar into HF leading to HArF 4 or Xe into HC 2 H leading to HXeC 2 H, 5 dramatically enhances the NLO properties. Pauling predicted that stable bonds could be formed by heavy noble gas atoms, due to the reduced stability of the outer electrons, caused by the strong screening of the inner electrons. 7 The first Xe derivative, XePtF 6 , was reported by Bartlett in 1962, 8 and soon after that, Claassen et al. reported that xenon and fluorine can react to form XeF 4 (solid), which is stable at room temperature. 9 After Bartlett’s work, a large number of Xe derivatives were synthesized, and as Christie noted, this discovery “triggered a world-wide frenzy in this area.” 10 Various groups have inserted noble gas atoms in the bonds A-B (e.g., H-C, H-O, H-S, and C-C), leading to A-Ng-B and thus producing a large variety of compounds. 11-14 Among those, one notes HNgY, where Y is an electronegative element or group (e.g., OH, CN, SH), which has been studied both experimentally and theoretically. 15-17 These derivatives are metastable, with a global minimum HY + Ng. The bonding in HNgY involves the structure HNg + Y - , where HNg + is covalently bound and linked to Y - by a Coulombic attraction. 18 It has been found that the inserted noble gas atom has a considerable positive charge. 5,19 In general, there is a large energy barrier which prevents these molecules from dissociation. 20 A notable derivative in this series is HArF, which was synthesized by Khriachtchev et al. 21 from the photolysis of hydrogen fluoride in a solid argon matrix. This is the first experimentally observed covalent neutral con- densed phase argon derivative. 22 Among the most remarkable properties of the derivatives resulting by insertion of a noble gas atom are the linear and nonlinear optical (L&NLO) properties. It has been found that, * Corresponding authors. E-mail: aavram@eie.gr (A.A.), Luis.Serrano@uv.es (L.S.-A.), mpapad@eie.gr (M.G.P.). † National Hellenic Research Foundation. | Lamia Institute of Technology. ‡ Universitat de Vale `ncia. § Accelrys Inc. J. Chem. Theory Comput. 2010, 6, 3365–3372 3365 10.1021/ct100471k  2010 American Chemical Society Published on Web 10/14/2010