架設快速低振動頻率的拉曼光譜儀並應用於即時監控結晶態1,1'-binaphthyl的熔化過程

Abstract

研究凝態物質的分子間作用力和晶格振動可以藉由低頻率(小於200波數)拉曼光譜學，但此技術的困難在於雷利散射比所要的拉曼訊號強上幾個級數。市面上典型的notch濾片無法有效的過濾雷利散射，因此要得到200波數以下的拉曼訊號相當困難。雖然單通道偵測器結合三分光儀(triple monochromator) 可以有效的分離雷利散射和拉曼散射並得到低頻率(小於 50 cm−1)且高訊雜比的拉曼光譜圖，但如果研究相變化這類型的動力學過程，多通道的偵測器才有辦法以較短的曝光時間(小於1秒)完成測量。在本論文中我們使用充滿碘蒸氣的玻璃槽做為消除雷利散射的濾片並結合多通道的拉曼光譜儀，我們能夠以小於1秒的測量時間記錄10波數以下的史托克斯和反史托克斯的拉曼訊號。接著將此低振動頻率拉曼系統應用到結晶態1,1'-binaphthyl。這個分子在結晶態有兩個晶型: 類順式與類反式，並有著不同的熔點。我們發現兩個晶型在低頻率範圍(−200–+200 cm−1)呈現截然不同的拉曼光譜。為了瞭解低振動頻率訊號的來源，快速的加熱樣品並以0.2秒的測量時間記錄每一張光譜圖，由此觀察兩個晶型的拉曼光譜隨溫度的變化。除此之外，藉由史托克斯和反史托克斯譜帶強度的比率，每個數據點的樣品溫度可以高精準度的被估計。將這兩組溫度變化的拉曼光譜數據搭配理論計算的結果，我們推測類反式中的26 cm−1峰和類順式中的100 cm−1峰分別是碳碳單鍵的扭曲振動和平面外變形振動(分子內振動)。由於分子間的作用力與熱膨脹造成低振動頻率波帶往低頻率位移有關，實驗得知類反式的拉曼波帶位移的程度較類順式少，因此我們認為類反式的分子間作用力較類順式強，而文獻中由單晶繞射實驗所得到的結果與我們的推測吻合。

Low-frequency (<200 cm−1) Raman spectroscopy enables us to investigate intermolecular vibrations and lattice vibrations in condensed phase materials. However, low-frequency Raman signals can be almost completely buried under vast Rayleigh scattering. Typical commercial notch filters are not effective enough to eliminate Rayleigh scattering and do not allow us to reach the frequency region below 200 cm−1. Moreover, in order to keep track of dynamic processes such as phase transitions, multichannel detection rather than single-channel detection with a double/triple monochromator is required. In this work, by combining an I2 vapor-containing cell as a Rayleigh rejection filter with a multichannel Raman spectrometer, we have successfully recorded Raman spectra down to ~10 cm−1 in both Stokes and anti-Stokes sides simultaneously. The constructed Raman spectrometer has then been applied to crystalline 1,1'-binaphthyl, which has two different crystal polymorphs, that is, the cisoid and transoid forms. They show quite distinct spectral features in the low-frequency region (−200–+200 cm−1). Real-time tracing of the melting process of two crystalline forms have also been conducted with rapid heating. A series of Raman spectra have been recorded every 0.2 sec, in which spectral changes of low-frequency bands are clearly observed. In addition, the sample temperature has been accurately estimated from the Stokes/anti-Stokes intensity ratio of the Raman bands. Base on the temperature-dependent Raman spectra and theoretical calculations, the 26 cm−1 band of the transoid form and the 100 cm−1 band of the cisoid form are assigned to the torsional vibration and out-of-plane deformation of the C–C single bond, respectively. Moreover, the peak position of each low-frequency band has been determined from the fitting. We found that peak shifts due to thermal expansion for the transoid form are smaller than those for the cisoid form. Thus, intermolecular interactions in the transoid form are stronger compared with those in the cisoid form. This finding is consistent with their X-ray crystal structures.