Carborane-containing liquid crystals:: Synthesis and structural, conformational, thermal, and spectroscopic characterization of diheptyl and diheptynyl derivatives of p-carboranes

被引:56
作者
Kaszynski, P
Pakhomov, S
Tesh, KF
机构
[1] Vanderbilt Univ, Dept Chem, Organ Mat Res Grp, Nashville, TN 37235 USA
[2] Univ Minnesota, Dept Chem, Xray Crystallog Lab, Minneapolis, MN 55455 USA
关键词
D O I
10.1021/ic010663x
中图分类号
O61 [无机化学];
学科分类号
070301 ; 081704 ;
摘要
Molecular and electronic structures for four p-carborane derivatives were studied in the context of their liquid crystalline properties, Thus molecular and crystal structures of diheptyl and diheptynyl derivatives of 10- and 12-vertex bi-p-carboranes were determined by X-ray crystallography and compared to the results of ab initio calculations at the HF/6-31G* level of theory. Experimentally observed significant positional disorder of one of the substituents in the 10-vertex derivatives, 2[2]a and 2[2]b, was related to conformational properties of the alkyl-carborane bond. Experimental and theoretical studies of the electronic structures were conducted for the four compounds using UV and NMR spectroscopies. The nature of the unique long wavelength absorption band at 232 nm in the diheptynyl derivative 2[2]b was explained using INDO/2//HF/6-31G* analysis. The complete assignment of the C-13 signals was accomplished using a long-range coupling technique and was supported by the calculated (HF/6-31G*) isotropic shielding tensors. Analysis of absorption spectra, NMR substituent effects, and trends in bond lengths shows generally strong cage-acetylene electronic interactions for the 10- vertex p-carborane, while the 12-vertex p-carborane remains largely electronically isolated. Ab initio calculations revealed that 12-vertex p-carborane has significantly larger electronic polarizability and quadrupole moments than the 10-vertex analogues, which are larger than those for bicyclo[2.2.2]octane compounds. All these results on packing, conformational, and electronic properties form the basis for the discussion of thermal behavior of the four carborane compounds, bicyclo[ 2.2.2] octane analogues, and some related compounds.
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页码:6622 / 6631
页数:10
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