This paper describes the theoretical study on borazine and substituted borazines by the means of density functional theory. Amino group was used as the common donor group while four acceptor groups namely –NO2, –C≡N, –COCl and –NMe2 were chosen as the acceptor groups, for substituted borazines. Structural variations, vibrational properties, non-linear optical properties and electronic absorption spectra were calculated. Different combinations of basis sets and functionals were tested for calculation of non-linear optical properties.
Geometry optimizations for all the molecules were done at B3LYP/6-311++G** level of theory. The optimized parameters were compared to the data available for such systems.
Vibrational spectra of all the optimized structures were carried out using the same parameters of basis set and functional.
The authors calculated the nonlinear optical properties based on static hyperpolarizability components as described in section 3.3.
“The static hyperpolarizability components (β and γ) have been calculated by using energy based equations in finite field methods. First we applied different field strengths to calculate β and γ and to avoid numerical instability. Since the finite field equations are sensitive to the precision in the energy calculations we have also obtained β and γ for various methods and basis sets. The optimized structures at B3LYP/6-311++G** level of theory are used for β and γ calculations since at this level of theory, the geometrical parameters and vibrational frequencies for borazine are in excellent agreement with the available experimental determinations.”
Electronic transitions for the geometry optimized structures were calculated at B3LYP/6-311++G** level of theory.
The geometry optimizations at B3LYP/6-311++G** correspond very well to the available experimental data, as pointed out by the authors. In this regard, the procedure is reproducible to high degree of accuracy. Interestingly, I found that the two acceptor groups –CF3 and –SO3H were not considered for calculations and tried to replicate the methods with these. The optimization results were similar to the reported ones. The elongation of B-N bond length by 0.01 and shortening of B-H bond by 0.008 follows the pattern. The N-H bond did not show any variation.
The vibrational spectra were well reproduced in every case.
The static hyperpolarizability components (β and γ) for all the structures show excellent reproducibility. The structures with acceptor groups –CF3 and –SO3H also confirm the findings.
The electronic structural studies are somewhat inadequate in my opinion. A correlation of theoretical and experimental transitions would be better. In general, the calculations are reproducible.
I would recommend including the acceptor groups –CF3 and –SO3H for a more thorough study.
A correlation of theoretical electronic transitions with experimental findings will complete the information presented in this paper.