Department of Chemistry, Korea University, Seoul 136-701, South Korea.
J Chem Phys. 2010 Feb 21;132(7):074506. doi: 10.1063/1.3322893.
Determining absolute configuration of chiral molecule can be achieved by using vibrational optical activity (VOA) measurement methods, such as vibrational circular dichroism and Raman optical activity (ROA). Recently, vibrationally resonant sum frequency generation and difference frequency generation utilizing circularly polarized beams were theoretically studied and shown that they can be alternative and complementary VOA measurement techniques [S. Cheon and M. Cho, Phys. Rev. A 71, 013808 (2005); J. Phys. Chem. A 113, 2438 (2009)]. Even for randomly oriented chiral molecules in solutions, the sum- and difference-frequency-generation signals induced by linearly polarized incident beams with mutually perpendicular polarization directions can be nonzero and are determined by the optical activity hyperpolarizability given by a product of antisymmetric Raman tensor and vibrational transition dipole. If one of the beams involved in the three-wave-mixing processes is circularly polarized and if the difference signal is measured, not only the same optical activity hyperpolarizability but also that including electric quadrupole-ROA tensor are required to determine the signals. Here, we carried out quantum chemistry calculations to obtain these quantities for a representative chiral molecule, (S)-methyl lactate, and numerically simulated the corresponding spectra. It is shown that the circular polarization three-wave-mixing signal intensities are quantitatively similar to those of the linear polarization three-wave-mixing signals, respectively, and that they are sensitive to the absolute configuration of chiral molecule. The calculation results thus suggest that these two novel techniques will be of use in studying molecular chirality even in time domain, once polarization-modulated ultrashort pulses are used to carry out circular polarization three-wave-mixing experiments.
确定手性分子的绝对构型可以通过使用振动圆二色性（VCD）和拉曼圆二色性（ROA）等振动旋光活性（VOA）测量方法来实现。最近，利用圆偏振光束的振动共振和和频差频产生在理论上进行了研究，并表明它们可以作为替代和互补的 VOA 测量技术[S. Cheon 和 M. Cho，Phys. Rev. A 71, 013808（2005）；J. Phys. Chem. A 113, 2438（2009）]。即使在溶液中随机取向的手性分子中，由相互垂直偏振方向的线偏振入射光束诱导的和频与差频产生信号也可以不为零，并且由不对称拉曼张量和振动跃迁偶极子的乘积给出的旋光超高极化率确定。如果三波混合过程中涉及的光束之一是圆偏振的，如果测量差信号，则不仅需要相同的旋光超高极化率，还需要包括电四极 ROA 张量，才能确定信号。在这里，我们进行了量子化学计算，以获得代表手性分子（S）-乳酸甲酯的这些量，并对相应的光谱进行了数值模拟。结果表明，圆偏振三波混合信号强度与线偏振三波混合信号强度分别定量相似，并且它们对手性分子的绝对构型敏感。因此，这些计算结果表明，一旦使用偏振调制超短脉冲进行圆偏振三波混合实验，这两种新的技术将有助于研究分子手性，甚至在时域内。
