三氯化硼分子在10μm波段低温光谱的理论研究

Theoretical Study on B—Cl Stretching Infrared Spectrumof Boron Trichloride at Low Temperature

商业化的高功率、窄线宽中红外激光器使得采用激光辅助抑制冷凝方法(SILARC)高效分离11B和10B两种稳定同位素成为可能。然而,中红外激光器的可调谐范围受限以及价格高昂的问题成为制约该应用的一大难题。准确确认高效率激发气相硼化物的激光频率成为解决该难题的办法之一。本文采用高精度从头算量子化学理论对三氯化硼分子的电子基态进行了深入研究。在二阶微扰MP2理论和6-311++G(3df, 2pd)基组下,优化了三氯化硼的分子结构,基于PT2理论计算获得了振动基态和基频(硼-氯伸缩振动模)转动参数、振转耦合等光谱参数。基于理论模拟谱与实验光谱相对满意的匹配,确定了三氯化硼同位素分子在10μm波段基频振动带的可信赖的非谐振频率。以上结果表明,本文实现了三氯化硼分子变温光谱(30~300 K)的模拟,确定了10μm波段的P(4)和P(6)二氧化碳激光器更适用于激光辅助抑制冷凝方法(SILARC)的激发11B同位素分子。

Commercial high-power and narrow linewidth mid-infrared lasers make it possible to efficiently separate the two stable isotopes of 11 B and 10 B using the method of selective laser assisted retarded condensation(SILARC).However,a significant issue that limits this application is the restricted tunable range and expensive cost of mid-infrared lasers.One of the solutions to this issue is to accurately determine the laser frequency,which is located around the gaseous boride vib-rotational transition with efficient excitation.In this research,the electronic ground state of boron trichloride was investigated using high-level ab initio quantum chemistry theory.The second-order Moller-Plesset perturbation(MP2)theory and the 6-311++G(3df,2pd)basis set are found to provide the most accurate calculated molecular and spectroscopic parameters compared to the experimental values for 11 BF 3 isotopologues.This combination was then used to optimize the geometries of eight boron trichloride isotoplogues.The anharmonic vibrational frequencies were derived with the PT2 theory for forty fundamental bending and stretching bands of all stable isotopologues.Comparisons of the calculated vibrational frequencies with the experimental values in matrix spectrum show better agreement than previous ab initio calculations in literature.And smaller difference of vibrational frequencies can be found for the bending bands than the stretching modes.A set of eighty spectroscopic parameters is also obtained for the ground states and B—Cl stretching fundamental states.In total,more than 228000 vib-rotational transition frequencies around 10μm were calculated based on the infrared selected rules of the symmetric planar molecules with J max=200.The line intensities can also be calculated with the equations given in this paper including the statistical weights,and H nl-London coefficients.Then,the infrared spectrum around 10μm can be simulated at different temperatures with the considerations of the populations at different lower rotational states.The simulated room-temperature spectra with the matrix experimental vibrational frequencies present significant difference from absorption and transmission experimental spectra in previous work.The gaseous anharmonic frequencies have 10.5 cm-1 and 11.9 cm-1 shifts from the matrix values for 11 B and 10 B isotopologues,respectively.The experimental transmittance is also well reproduced with the corrected frequencies and the ab initio calculated vib-rotational constants.Then the absorption spectrum with natural abundance can be accurately calculated at variable temperatures(30-300 K).The simulations show that the stronger absorption in R branch is around 957.36 cm-1 below 200 K,and less frequency shift along with the temperature.The above results indicate that from a comparison between the line intensities of 00011←10001 band for 12 CO 2,the 11 BCl 3 isotopologues may be better excited by P(6)and P(4)emission lines of carbon dioxide laser within the method of SILARC.