One-Dimensional Zigzag Chains of Cs<sup>-</sup>: The Structures and Properties of
Li<sup>+</sup>(Cryptand[2.1.1])Cs<sup>-</sup> and Cs<sup>+</sup>(Cryptand[2.2.2])Cs<sup>-</sup>
IchimuraAndrew S.
HuangRui H.
XieQingshan
MorganelliPhilip
BurnsAmy
DyeJames L.
2006
The crystal structure and properties of lithium (cryptand[2.1.1]) ceside, Li<sup>+</sup>(C211)Cs<sup>-</sup>, are reported.
Li<sup>+</sup>(C211)Cs<sup>-</sup> is the second ceside and third alkalide with a one-dimensional (1D) zigzag chain of alkali
metal anions. The distance between adjacent Cs<sup>-</sup> anions, 6 Å, is shorter than the sum of the van der Waals
radii, 7 Å. Optical, magic angle spinning NMR, two-probe alternating and direct current conductivity, and
electron paramagnetic resonance measurements reveal unique physical properties that result from the overlap
of adjacent Cs<sup>-</sup> wave functions in the chain structure. The properties of cesium (cryptand[2.2.2]) ceside,
Cs<sup>+</sup>(C222)Cs<sup>-</sup>, were also studied to compare the effects of the subtle geometric changes between the two 1D
zigzag chain structures. Li<sup>+</sup>(C211)Cs<sup>-</sup> and Cs<sup>+</sup>(C222)Cs<sup>-</sup> are both low-band-gap semiconductors with
anisotropic reflectivities and large paramagnetic <sup>133</sup>Cs NMR chemical shifts relative to Cs<sup>-</sup>(g). An electronic
structure model consistent with the experimental data has sp<sup>2</sup>-hybridized Cs<sup>-</sup> within the chain and sp-hybridized
chain ends. Ab initio multiconfiguration self-consistent field calculations on the ceside trimer, Cs<sub>3</sub><sup>3-</sup>, support
this model and indicate a net bonding interaction between nearest neighbors. The buildup of electron density
between adjacent Cs<sup>-</sup> anions is visualized through an electron density difference map constructed by subtracting
the density of three cesium atoms from the short Cs<sub>3</sub><sup>3-</sup> fragment.