Table Salt and Other Alkali Metal Chloride Oligomers: Structure, Stability, and Bonding F. Matthias Bickelhaupt,* ,† Miquel Sola `,* ,‡ and Ce ´ lia Fonseca Guerra † Afdeling Theoretische Chemie, Scheikundig Laboratorium der Vrije UniVersiteit, De Boelelaan 1083, NL-1081 HV Amsterdam, The Netherlands, and Institut de Quı ´mica Computacional, UniVersitat de Girona, Campus MontiliVi, E-17071 Girona, Catalonia, Spain Received February 20, 2007 We have investigated table salt and other alkali metal chloride monomers, ClM, and (distorted) cubic tetramers, (ClM) 4 , with M ) Li, Na, K, and Rb, using density functional theory (DFT) at the BP86/TZ2P level. Our objectives are to determine how the structure and thermochemistry (e.g., Cl-M bond lengths and strengths, oligomerization energies, etc.) of alkali metal chlorides depend on the metal atom and to understand the emerging trends in terms of quantitative Kohn-Sham molecular orbital (KS-MO) theory. The analyses confirm the high polarity of the Cl-M bond (dipole moment, VDD, and Hirshfeld atomic charges). They also reveal that bond overlap derived stabilization (approximately -26, -20, and -8 kcal/mol), although clearly larger than in the corresponding F-M bonds, contributes relatively little to the (trend in) bond strengths (-105, -90, and -94 kcal/mol) along M ) Li, Na, and K. Thus, the Cl-M bonding mechanism resembles more closely that of the even more ionic F-M bond than that of the more covalent C-M or H-M bonds. Tetramerization causes the Cl-M bond to expand, and it reduces its polarity. 1. Introduction Clusters are intermediates in the transition between gaseous and condensed phases, and their investigation provides thus valuable insight into how physicochemical properties evolve going from molecular systems to the solid state. 1 Here, we focus on table salt and other alkali metal chloride molecules and clusters (ClM) n (M ) alkali metal), which occur in hot vapors of these materials. 2 Structural and thermochemical data about these species are still incomplete in spite of various pioneering experimental 2,3 and theoretical 4-9 inves- tigations. It is known that particularly compact and stable clusters (XM) n arise for the so-called magic numbers: n ) 4, 6, 9, 12, 15, and 18. 2a These clusters exist in various isomeric forms that may be in thermal equilibrium with each other. The tetramer (XM) 4 , for example, occurs among others as a cube (1), ladder (2), and ring (3); 2a the cubic isomer (1) being typically (but not always) the most stable form. 4-7 Earlier theoretical studies have shown that the cube (1) is the lowest-energy structure for (ClNa) 4 and (ClK) 4 followed by the ring (3), which is higher in energy by 5-15 kcal/ mol, depending on the level of theory. 4d,6a,d,e,7b,c For the less * To whom correspondence should be addressed. E-mail: FM.Bickelhaupt@few.vu.nl (F.M.B.), miquel.sola@udg.es (M.S.). Fax: +31-20-59 87 629 (F.M.B.), +34-972-41 83 56 (M.S.). † Scheikundig Laboratorium der Vrije Universiteit. ‡ Universitat de Girona. (1) (a) Alonso, J. A. Chem. ReV. 2000, 100, 637. (b) Berry, R. S. Chem. ReV. 1993, 93, 2379. (2) (a) Hargittai, M. Chem. ReV. 2000, 100, 2233. (b) Hargittai, M.; Hargittai, I. The Molecular Geometries of Coordination Compounds in the Vapor Phase; Elsevier: Amsterdam, 1997. (3) (a) Campana, J. E.; Barlak, T. M.; Colton, R. J.; DeCorpo, J. J.; Wyatt, J. R.; Dunlap, B. I. Phys. ReV. Lett. 1981, 47, 1046. (b) Pflaum, R.; Pfau, P.; Sattler, K.; Recknagel, E. Surf. Sci. 1985, 156, 165. (c) Conover, C. W. S.; Yang, Y. A.; Bloomfield, L. A. Phys. ReV.B 1988, 38, 3517. (d) Twu, Y. T.; Conover, C. W. S.; Yang, Y. A.; Bloomfield, Phys. ReV.B 1990, 42, 5306. (e) Hebert, A. J.; Lovas, F. J.; Melendres, C. A.; Hollowell, C. D.; Story, T. L., Jr.; Street, K., Jr. J. Chem. Phys. 1968, 48, 2824. (f) Lide, D. R., Jr.; Cahill, P.; Gold, L. P. J. Chem. Phys. 1964, 40, 156. (g) Honig, A.; Mandel, M.; Stitch, M. L.; Townes, C. H. Phys. ReV. 1954, 96, 629. (h) Clouser, P.; Gordy, W. Bull. Am. Phys. Soc. 1963, 8, 326. (i) Lee, C. A.; Fabricand, B. P.; Carlson, R. O.; Rabi, I. I. Phys. ReV. 1953, 91, 1395. (j) Trischka, J. W.; Braunstein, R. Phys. ReV. 1954, 96, 968. (k) Bulewicz, E. M.; Phillips, L. F.; Sugden, T. M. Trans. Faraday Soc. 1961, 57, 921. (l) Brewer, L.; Brackett, E. Chem. ReV. 1961, 61, 425. (m) Martin, T. P. Phys. Rep. 1983, 95, 167. Inorg. Chem. 2007, 46, 5411-5418 10.1021/ic070328u CCC: $37.00 © 2007 American Chemical Society Inorganic Chemistry, Vol. 46, No. 13, 2007 5411 Published on Web 06/01/2007