Solvent Effect on Characteristic Vibration of IR Spectrum of 4,4'-Dibromodiphenyl Ether

The optimal geometry, IR spectrum and vibration assignment of 4,4'-dibromodiphenyl ether(BDE-15) in gas phase were calculated via the density functional theory(DFT) at the level of B3LYP/6-31+G(d). Based on the vibration assignment, the calculation of vibration frequencies and intensities of 5 main vibration types of BDE-15 in 25 kinds of solvents was carried out by means of a self-consistent reaction field(SCRF) theoretical model at the level of B3LYP/6-31+G(d) to analyze the solvent effect on the vibration of IR spectrum of BDE-15. To study the solvent effect further, C―O asymmetric stretching vibration fluctuating, which is relatively acute in both vibration frequency and intensity, was selected as the characteristic vibration to establish different linear solvation energy relation(LSER) models for solvent categoring. Solvent parameters(α, β, π^*), acceptable number(AN) and quantitative structure- activity relationship(QSAR) models were established via chemical quantum parameters of solvent moleculer, which were first been introduced to investigate different solvent-solute interaction mechanisms in alcoholic and non-alcoholic solvents on molecular level. At last, a single solvent molecule was embedded in the framework of polarizable continuum model(PCM) to validate the effect of hydrogen bonding on solvent-solute interaction in alcohol solvents. The obtained results show that 5 main vibration types of BDE-15 in different solvents have small variation range in frequency and intensity and all the vibration frequencies in solvents are lower than those in gas phase, decreasing along with the increasing of the dielectric constant(ε) of solvents exponentially. In contrast, all the vibration intensities in solvents are greater than those in gas phase and present positive exponential trend. Twenty-five solvents were divided into two categories(non-alcoholic solvents and alcoholic solvents) by LSER. The C―O asymmetric stretching vibration was mainly regulated by non-specific interaction(dipole-dipole and dipole-induced dipole inter- action) and the combination of non-specific interaction and electronic effect in non-alcoholic solvents and alcoholic solvents, respectively. R² values of the two QSAR models established for different categories are 0.940 and 0.997, both showing good fitness and the decisive parameters for the two models are μ, α_zz and α_yy, E_HOMO separately, which reflect that the interaction mechanism is coincide with the conclusion obtained by LSER model. The present of hydrogen bonding in alcoholic solvents improves the differentiation degree of vibration frequency of the two models with the variance increasing from 0.281 to 0.513 and the intensity from 5.263 to 10.382, which is conducive to the identification of polybrominated diphenyl ether(PBDE) homologues, indicating this research would provide theoretical gist for linking the experimental IR spectrum data with the identification of PBDE homologues.