Chemical Physics Letters 610–611 (2014) 351–356 Contents lists available at ScienceDirect Chemical Physics Letters jou rn al hom epage: www.elsevier.com/locate/cplett Movement of Ng 2 molecules confined in a C 60 cage: An ab initio molecular dynamics study Munmun Khatua, Sudip Pan, Pratim K. Chattaraj ∗ Department of Chemistry and Centre for Theoretical Studies, Indian Institute of Technology, Kharagpur 721302, India a r t i c l e i n f o Article history: Received 12 May 2014 Received in revised form 25 June 2014 In final form 26 June 2014 Available online 8 July 2014 a b s t r a c t An ab initio molecular dynamics study on Ng 2 @C 60 (Ng = He Kr) systems is performed to analyze the movement of Ng 2 molecules inside a C 60 cage. Within 500 fs time window, the He 2 undergoes preces- sion encompassing translation, vibration and rotation readily whereas other Ng 2 molecules show usual vibration but the degrees of translation and rotation decrease with an increase in size of the Ng atoms. Increase in interaction between the Ng centers and cage carbons and an increased distortion of cage in moving from He to Kr seem to be responsible for this. During the movement, the Ng 2 units behave as single entity. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The long-established chemical principle that noble gases (Ng) are inert and do not take part in chemical bonding was abandoned after the synthesis of Xe + [PtF 6 ] - by Bartlett [1,2]. Prompted by this synthesis within a short duration, a good number of Kr Rn com- pounds [3,4] were synthesized, which further raised the interest of the researchers in this developing arena named Ng chemistry. The contributions of Räsänen and co-workers [3–5] in synthesizing successfully a series of compounds of type H(Ng)Y (Ng = Ar, Kr, Xe; Y = electron-withdrawing group) and that of Feldman et al. [6] in preparing Ng hydrides and other related Ng compounds are consid- ered as significant in enriching this field. The Ng-compounds were also investigated theoretically by different researchers [7–14]. The study of chemical systems under unusual conditions like the endohedrally confined one has been a matter of great inter- est to the scientists. Ever since the discovery of fullerenes in 1985 [15], several methods were developed for producing endohedrally doped fullerenes since the space inside could hold atoms and even small molecules. Methods such as ion bombardment and high pressure/high-temperature techniques [16–22] were used but the yield was low (<1%) and so the ‘molecular surgery’ method was used where a hole is chemically opened on the surface of the fullerene cage, and a gas molecule is inserted reversibly through the orifice [23–30]. Komatsu et al. added groups to C 60 to create a ∗ Corresponding author. E-mail addresses: pkc@chem.iitkgp.ernet.in, pratim.chattaraj@gmail.com (P.K. Chattaraj). 13-membered ring in the carbon cage [27] large enough to permit H 2 and He to enter [28,29]. They then devised a sequence of reac- tions that closed the hole, trapping H 2 inside to produce H 2 @C 60 in high yield [30]. Several theoretical and experimental [16,31–34] studies were also carried out on the endohedrally trapped Ng com- pounds like Ng n @C 60 and Ng n @C 70 . These unusual endohedrally trapped compounds are stable with a van der Waals type of inter- action in most cases in that there is no chemical bond between the trapped atom and the carbon cage. Yet, they are very stable, since several C C bonds must break to allow the atom to escape. In addi- tion to the fullerene cages, inclusion of Ng atoms in much smaller cages like C 10 H 16 , C 20 H 20 and Mo 6 Cl 8 F 6 was also studied [35–38]. From a theoretical perspective, noble gas dimers (Ng 2 ) confined in a C 60 and other cages have revealed how the valence electrons of atoms, which have a fully occupied valence shell, change when they are confined to strong interatomic interactions at equilibrium geometry. The theoretical data suggest of a possible new type of inter-atomic interaction, which would provide material for further stimulating discussions about the nature of a chemical bond. Cer- tain quantum-chemical studies about the endohedrally confined Ng dimers i.e. Ng 2 (Ng = He Xe) in C 60 fullerene cage [39–41] revealed that the Ng Ng distances in Ng 2 @C 60 are considerably shorter than those in free Ng dimers. All these Ng 2 @C 60 systems are thermodynamically unstable toward the loss of Ng atoms. Krapp and Frenking [40] investigated whether the bonds in between two Ng atoms or in between Ng and C atoms in Ng 2 @C 60 systems could be called a chemical bond. With the help of detailed bonding analysis and IUPAC definition of a chemical bond, they termed the Ng Ng and Ng C bonds for Ng = Ar Xe as genuine chemical bonds whereas those in He 2 @C 60 and Ne 2 @C 60 are just weakly bonded http://dx.doi.org/10.1016/j.cplett.2014.06.052 0009-2614/© 2014 Elsevier B.V. All rights reserved.