Intercalation of Halogen Molecules in Alkali Fluoride Lattices: A

J. Am. Chem. SOC. 1995. 117, 2877-2883

2877

Intercalation of Halogen Molecules in Alkali Fluoride Lattices: A Theoretical Study Eliseo Ruiz and Santiago Alvarez* Contribution from the Depanameni de Quimica Inorghnica, Universiiat de Barcelona, Diagonal 647. 08028 Barcelona, Spain Received July 28, 1994@

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Abstract: A theoretical study of the host guest interactions in the intercalation compounds MF XY and 2MF * XY (M = Li, K. Cs; X, Y = C1, Br, I) is presented. A qualitative description of the bonding is obtained in the form of a band interaction diagram by using a Crystal Orbital Difference (COD) analysis of extended Hiickel tight-binding band calculations. Quantitative estimates of the host. .fguest interaction energies in these compounds are obtained from periodic Hartree-Fock (HF) calculations on the solids and molecular orbital calculations on a cluster model, including electron correlation through a second-order Moeller-Plesset (MP2) treatment. Periodic HF calculations correctly predict the trends in the structural and vibrational variations resulting from host * * * guest interactions. with formation energies of ca. 2 kcallmol for the CsF. Br2 and CsF. 12 intercalates. General trends in the MF-XY family indicate that the intercalation compounds are not stable for M = K or Li and XY = Bn, and that the host.. guest interaction energy depends on the energy of the HOMO of the guest molecule XY and on its dipole moment.

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h i m g the wicr) oi intcralation mnpounJ\ k n w n . those tiinned h) i n i l u w n ., halogen i mdeiulc\ in a l a t t i i e oi C \ F

unique in that the guest ntiilcculcs .ire neither pl:iccd in the r m Jer Wads p i p o i il two-Jimeninmal Idtiie. a i in graphite or in metal chakogcnidc dcriutives. ndr c c u p y n g a g e s or c h m d i n Ihrce-dimen*imal mucturw. a, in 7 c ~ i l i t einlerch i e 5 Inhtedd. the halogen niolecules trandorni the cubic 1atti;e o i C\F i n t o ;I ki)creJ muirure and inicrc:ilate betaeen the ionicdl! b i m J In)cr> of C+. The nnl! cnmpomd pret ioL\I) reponed i n which Brl mlilciule\ :ire interc;ilaled i n a twoJinicnsiond hmt 15 t h d of graphite.' Several i.,mpwnd~ u ith I\\ o difierent btoichiomctries h i v e hccii reported. C4:. X. and ?CsF.X. I X= Br. IJ,hut the mil! * t ~ ~ t ~ riharacteriied ~ l l y one \o i3r I. C\F. Br:. "hi5 yclloaorang.' product retain\ B r ~ninlciulcs even \vIieii hc3ted at 101l 'C. The inlcrs\ting ~tru;~urcui CsF. Rr? rfigurc I J