2-烷基TCNQ的红外光谱和结构研究——CH_2剪式振动的双峰与晶体场分裂的关系(英文)

Infrared Band Assignments and Structure of Even-Numbered 2-Alkyl-7,7,8,8-Tetracyanoquinodimethane in Cast Films——Two Components of the CH_2 Scissoring Vibrations not Related to Crystal Field Splitting

测量了2-烷基TCNQ(C8TCNQ,C12TCNQ,C18TCNQ)浇铸膜的红外光谱,发现这类化合物的红外CH2剪式振动在1 471和1 462 cm-1处显示2个峰。这2个峰的强度比(I1 471/I1 462)随着烷基链的长度增加而明显减少。另外,来自CH2的1 471 cm-1吸收峰与来自TCNQ环的1 529 cm-1吸收峰的相对强度随着碳链的长度增加而不发生变化。CnTCNQ系列化合物的红外光谱CH2的剪式振动行为与长链脂肪酸中CH2行为很不相同。在长链脂肪酸中,晶体场效应导致了剪式振动分裂。考虑C12TCNQ的晶体结构,而将1 471cm-1峰归属为非叉指对接的碳链,将1 462 cm-1峰归于叉指对接的碳链,双峰的出现与晶体场分裂无关。并且得到烷基TCNQ中,非叉指对接的碳链的长度与碳链总长度无关的结论。

Infrared （IR） spectra were measured for 2-octyl-, 2-dodecyl-, and 2-octadecyl-7, 7, 8, 8-tetracyanoquinodimethane （C8TCNQ, C12TCNQ and C18TCNQ） in cast films, and it was found that each spectrum shows two components for the CH2 scissoring band at 1 471 and 1 462 cm^-1 Polarized IR measurements showed that the micro-crystallites in the cast films take a random orientation in the plane of the plate. The intensity ratio of the two bands at 1 471 and 1 462 cm^-1 （I1471/I1462） decreases observably with the increase in the length of the alkyl chain. Moreover, the relative intensity of the 1 471 cm-1 CH2 band to a band at 1 529 cm^-1 （ C=C stretching mode of the TCNQ chromophore ring） does not change significantly for the three kinds of C.TCNQ while the relative intensty of the 1 462 cm-1 CH2 band to the hand at 1 529 em 1 increases markedly with the length of the alkyl chain. The above variations of the CH2 scissoring doublet of CnTCNQ are quite different from those of long-chain fatty acids （stearic acid and lignoceric acid） where the splitting of the CH2 scissoring vibration occurs due to a crystal field splitting. Considering the crystal structure of C12TCNQ and the above spectral variations, the authors assign the two components of the CH2 scissoring bands at 1 462 and 1 471 cm^-1 of the C,,TCNQ cast films to the interdigitated and non-interdigitated parts of the alkyl chains, respectively. Furthermore, the conclusion that the length of the non-interdigitated part of the alkyl chain is almost unchanged in the three kinds of even-numbered C.TCNQ could also be reached.