Dedicated to Professor Shigeru Nagase on the occasion of his 65th birthday With metal atoms or clusters entrapped in their inner spaces, endohedral metallofullerenes (EMFs) are viewed as unique compounds with a variety of potential applications. [1][2][3][4][5][6][7][8][9][10] These EMFs are commonly divided into several forms by virtue of the encapsulated moieties.[8] The tri-metallic nitride templated endohedral metallofullerenes (TNT EMFs) often attract much attention due to their high stability and yield resulting from strong metal-cage interactions. [5,6,[10][11][12] Particularly, many efforts have been devoted to gadolinium nitride cluster fullerenes, because they can be used as excellent contrast agents for magnetic resonance imaging (MRI). [13,14] To date, six such EMFs, Gd 3 N@C 2n (2n = 78~88), have been successfully synthesized. [15][16][17][18][19][20] Although the gadolinium nitride cluster fullerenes have been discussed with several experimental methods, [15][16][17][18][19][20][21][22] only a few theoretical studies have been carried out on these significant chemical compounds. [23À 26] As the smallest Gd 3 Ncontaining EMFs, Gd 3 N@C 78 was revealed by X-ray diffraction to have the C 2 (22010)ÀC 78 cage OK which violates the isolatedpentagon rule (IPR) and has two pairs of pentagon adjacencies.[15] (The fullerene isomers are coded by the spiral number, which was proposed by Fowler and Manolopoulous. [27] ) Because of its relatively small carbon cage and large metal cluster, Gd 3 N@C 2 (22010)ÀC 78 is a good candidate for revealing the structures, electronic properties and metal-cage interactions in gadolinium nitride cluster fullerenes.Many other C 78 -based EMFs have been extracted successfully and most of them were determined to possess the IPR carbon cage D 3h (24109)ÀC 78 , such as La 2 @C 78 , [28,29] Ce 2 @C 78 , [30] Ti 2 C 2 @C 78 , [31][32][33][34] and Sc 3 N@C 78 . [35,36] In contrast, the major isomers of Dy 3 N@C 78 and Tm 3 N@C 78 were proposed to bear the same non-IPR carbon cage with Gd 3 N@C 78 . [37][38][39] Thus, as the only isolated and characterized non-IPR carbon cage, C 2 (22010)-C 78 is an exceptional structure among 24109 classical C 78 isomers.[27] Herein, we report-for the first time-a thorough quantum chemical investigation on the electronic structures, electron affinity, ionization potential, thermodynamic stability, and IR vibrational frequencies of Gd 3 N@C 78 endohedral metallofullerene systems including both IPR and non-IPR cages.Several electronic states with different spin multiplicities were calculated to search for the ground state of Gd 3 N@C 2 (22010)-C 78 . The results revealed that a high-spin state with S = 21/2 has the lowest relative energy, indicating that seven 4f electrons of each Gd 3 + ion are ferromagnetically coupled in the ground state. Interestingly, the relative energy of the low-spin state with S = 7/2, which represents seven 4f electrons in one Gd 3 + ion that are non-ferromagnetically coupled with fourteen 4f electrons on other two Gd 3 + ion...
