Yuanzineng kexue jishu (Jul 2023)
Theoretical Study on B—Cl Stretching Infrared Spectrum of Boron Trichloride at Low Temperature
Abstract
Commercial high-power and narrow linewidth mid-infrared lasers make it possible to efficiently separate the two stable isotopes of 11B and 10B 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 11BF3 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 228 000 vib-rotational transition frequencies around 10 μm were calculated based on the infrared selected rules of the symmetric planar molecules with Jmax=200. The line intensities can also be calculated with the equations given in this paper including the statistical weights, and Hönl-London coefficients. Then, the infrared spectrum around 10 μm 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 11B and 10B 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 12CO2, the 11BCl3 isotopologues may be better excited by P(6) and P(4) emission lines of carbon dioxide laser within the method of SILARC.
