温度对β胡萝卜素结构有序的影响

Temperature effects on structural order of all-trans-β-carotene

本文测量了全反式β胡萝卜素在二甲基亚砜中81—25℃范围的紫外—可见吸收和拉曼光谱.结果表明,随温度降低,紫外—可见吸收光谱、拉曼光谱都发生红移,拉曼光谱线型变窄,散射截面增加这些现象的发生是由于随温度降低,β胡萝卜素分子的热无序降低、分子结构有序性增加、π电子离域扩展,有效共轭长度增加,分子的电子能隙变窄.另外,随着温度的降低,溶剂密度增加,由Lorentz-Lorenz关系得知相伴的折射率增加,从而引起吸收光谱的红移.CC键键长增加,使CC键拉曼光谱红移;振动弛豫时间变长,各CC键之间的键长差减小,线宽变窄;但由于声子,π电子耦合加强使CC键拉曼线型不对称程度增加,低频端"肩"扩展,CC键的弱阻尼相干振动增加,使拉曼散射截面增加.

The effects of temperature on the visible absorption and Raman spectra of all-trans-β-carotene dissolved in dimethyl sulfoxide at temperatures ranging from 81 ℃ to 18 ℃ were determined. The bands of the visible absorption and Raman spectra of all-trans-β-carotene showed red blue shifts. The bandwidth of the Raman spectra becomes narrow. Raman scattering cross-section increases as the temperature decreases. The red shift of the absorption spectrum is attributed to the thermal conformational change-induced decrease in the effective conjugation length in all-trans-β-carotene chains. The molecular structural order increases and the π-electron delocalization range is extended as the temperature decreases. The red shift in all-trans-β-carotene can be also attributed to the decrease in the liquid density, and the concomitant decrease in the refractive index is shown by the Lorentz-Lorenz relation. The apparent behavior of the temperature-induced band broadening of CC bonds can be associated with the decrease of difference in C-C and C=C bond lengths, and the shorter vibrational relaxation time. The shoulder observed below 1520 cm-1 shows a red shift. The enhancement of coherent weakly-damped CC stretching vibrations may increase the Raman scattering cross-section.