致力於發展低振動頻率拉曼光學活性光譜儀

Abstract

拉曼光學活性為旋光光譜之一，提供一項用來決定光學活性分子的絕對組態和研究生物分子的分子構向的利器．雖然在拉曼光學活性中，不論圓偏極入射或圓偏極散射的架構皆沿用多年且已生產大量的光譜，但是這些所觀測到的光譜至今仍然被限制於指紋區（200–1800振動波數）之內．然而，未開發的低振動頻率區（0–200振動波數）可提供有關分子間作用力和晶體震動豐富的資訊，有助於我們了解固體晶格中的旋光性． 本研究中，我們的目標是開發低振動頻率拉曼光學活性光譜．以後向散射圓偏極入射為基礎架設光譜儀，其中可使拉曼光學活性觀測低於200振動波數以下的反射體積式布拉格光柵用來作為改向入射光同時收集散射光的裝置．為了減少系統雜訊，其容易覆蓋掉微弱的拉曼光學活性訊號，我們並履行虛擬鏡像異構物的概念．α-蒎烯為拉曼光學活性中常見的標準樣品，其成對的鏡像異構物的初步光譜可測得於70–1900振動波數範圍．經由虛擬異構物的方法校正系統雜訊後，其中許多拉曼訊號彼此可呈現相當勻稱的鏡像．

Raman optical activity (ROA), which is one of chiroptical spectroscopies, has provided a powerful tool to determine the absolute configuration of chiral molecules and to investigate the molecular conformation of biomolecules. Although ROA, with either incident circularly polarized (ICP) or scattering circularly polarized (SCP) configuration, has been employed for many years and yielded a large number of ROA spectra, ROA observation has so far been limited to the fingerprint region (200－1800 cm−1). However, the unexplored low-frequency region (0－200 cm−1) is abundant in information on intermolecular interactions and lattice vibrations and hence would help us to understand the chirality in crystalline solids. In this work, we aim to develop low-frequency Raman optical activity (LF-ROA) spectroscopy. In the backscattering ICP-based spectrometer that we have constructed, a reflective volume Bragg grating that makes ROA measurements below 200 cm−1 feasible was installed to direct the incident light and collect the scattered light simultaneously. We also implemented a concept of virtual enantiomers in order to minimize systematic artifact signals that could easily overwhelm extremely small ROA signals. Preliminary ICP-ROA spectra of the two enantiomers of α-pinene, which is a standard sample for ROA, were measured in the 70–1900 cm−1. They showed reasonably good mirror images for several Raman bands thanks to artifact correction using a virtual enantiomer method.