132 CHEMICAL SENSORS: FUNDAMENTALS. VOLUME 3: POLYMERS & OTHER MATERIALS relatively better understanding of their conformational behavior in comparison to that of calix[6]arenes and calix[8]arenes 9. CONFORMATIONAL CHARACTERIZATION OF CALIX[n]ARENES X-ray crystallography is considered the best method for ascertaining the conformation of calixarenes and thiacalixarenes in the solid state; NMR spectroscopy has been known to be useful for conformational analysis in solution. The shape and multiplicity of methylene bridge (ArCH 2 Ar) protons provide a characteristic pattern for each conformer (Gutsche et al. 1983). The following pattern for the methy- lene proton have been observed in proton NMR: The cone conformer shows one pair of doublets (J = 12 Hz), while the partial cone conformer shows two pairs of doublets (ratio 1:1) or one pair of one doublet and one singlet (ratio 1:1). The 1,2-alternate conformer exhibits one singlet and two doublets (J = 12 Hz) in the ratio 1:1, and the 1,3-alternate conformer shows only a singlet. 13 C NMR spectroscopy indicates that the cone and 1,3-alternate conformers show only one signal, while the partial cone and 1,2-alternate conformers show two signals for the methylene bridge carbons. Generally, a combination of 1 H NMR and 13 C NMR is considered suffi cient to assign a particular structural conformation to calixarenes. Detailed spectral patterns observed for the different conformations of p-tert-butylcalix[4] arene have been reported (Gutsche et al. 1983).