| β-carotene is the polyenes linear Biological molecule which have nine C=C, and it have special optocal property. Our study indicate thatβ-carotene have largely Raman active, and its Raman scattering cross section (RSCS) can reached 10-23-10-20 cm2 molecule-1Sr-1 by 514.5 laser excitation. This result mainly from molecular structural order besides resonance Raman. When p-carotene have large molecular structural order, it can produce strong RSCS in the effect of laser due to coherent weakly damped CC vibrations. We found in the solution 1,2-dichloroethane ofβ-carotene and iodine, the UV spectra of theβ-carotene in complex shift from 460nm to 1000nm with complex formation, and the resonance Raman ofβ-carotene in complex disappeared with complex formation. RSCS ofβ-carotene that didn't in the complex reduced. We think this is due to P-carotene and iodine form complex in solution, and the complex is unstable. So the molecular structural order of the (3-carotene which didn't in the complex reduced with complex increased, and lead to RSCS reduced. This article is very useful for the study of the structure, property of the polyenes linear Biological molecule in solution.There are three parts in this paper, as follow:一.β-carotene and iodine form charge transfer complex in 1,2-dichloroethane Solution Fig.1 is the UV-visible spectra ofβ-carotene with different concentration of iodine in 1,2-dichloroethane. Fig.l The UV-visible Spectra ofβ-carotene and its complex in different concentration of I2 (The concentration of I2:a:0,b:1×10-5M,c:2×10-5M,d:3×10-5M,e:4×10-5M,f: 5×10-5M, g:6×10-5M, h:8×10-5M)We found the UV-visible spectra ofβ-carotene changed a lot after adding iodine, because P-carotene and iodine can form charge transfer complex in polar solvent 1,2-dichloroethane, and its form constant is about 2-2.5. The equation of form complex is: If a solvent with a higher dielectric constant was chosen, C40H56·nI2 is best represented by [C4oH56I]+I3-, so the equation can represented byIn the course of time, isomerization, as well as iodination of theβ-carotene would take place. At the same time the UV-Vis spectra at 460nm ofβ-carotene disappeared in polar solvent 1,2-dichloroethane. And a new, intense absorption band of complex which is formed by P-carotene and iodine was found at 1000nm.二. Effect of complex formation on Raman scattering cross section ofβ-caroteneFig.2 is the Raman spectra ofβ-carotene with different concentration of iodine in 1,2-dichloroethane. Fig.2 The Raman Spectra ofβ-carotene and its complex in different concentration of I2 (The concentration of I2:a:0, b:2×10-5M, c:4×10-5M, d:6×10-5M, e:8×10-5M)We measured the Raman spectra ofβ-carotene at different concentration of iodine by 514.5nm excitation laser, and used n-hexane as an internal standard(RSCS is 1.01×10-30cm2 moleculele-1 Sr-1).We calculated the RSCS ofβ-carotene at different concentration of iodine. We found that the RSCS ofβ-carotene reduced with the concentration of iodine increased, that is the RSCS ofβ-carotene reduced with the complex increased, and in other words is the concentration of P-carotene decreased. The result is in Table 1, and the RSCS(σR) is calculated by the following expression,whereσR is RSCS,the subscripts X is 1520 and 1155 cm-1 Raman band ofβ-carotene, C1452 and Cx are the molar concentrations of n-hexane andβ-carotene, respectively. (IX/I1452) is the Raman intensity ratio of the 1452cm-1 band of n-hexane relative to the 1520cm-1 and 1155cm-1 band ofβ-carotene. Table.l The value of Raman scattering cross section ofβ-carotene in different concentration三. The structural order and Raman cross section ofβ-caroteneThe study proofed that the polyenes linear Biological molecule have series optical properties under the conditions of have large structural order, and including have large RSCS. Though the molecule chain of theβ-carotene is shorter than others, it has the same properties. We studied the RSCS of theβ-caroten in different solvents, and found its value was very large, for example, the RSCS of C=C is larger than 10-22 cm2molecule-1 Sr-1.The RSCS different with the concentration and solution difference, and increased with the solvent concentration decreased. The reason is molecule structural order increased more than Resonance effect. In the solution of P-carotene and iodine, the process ofβ-carotene and iodine formation complex was reversible, so all the physical parameters was unstable, even some thermodynamic parameters couldn't have stable value. The instability of solution increased with the complex increased. The structural order ofβ-carotene that didn't in the complex weaked, which lead to the weak of coherent weakly damped CC vibrations, and made the RSCS reduced. Besides the number of 3 increased with the concentration of iodine increased in polar solvent of 1,2-dichloroethane, and made the intermolecular interaction more complex, also made the molecule structural order ofβ-carotene weaked, then the RSCS reduced.
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