Abstract
Static and frequency-dependent electronic (hyper)polarizabilities of the dimethylnaphthalene (DMN) isomers were computed in vacuum using the Coulomb-attenuating Density Functional Theory method. The nonlinear optical Second Harmonic Generation (SHG) and Electro-Optical Pockels Effect (EOPE) were investigated at the characteristic Nd:YAG laser wavelength of 1064 nm. The response electric properties especially the longitudinal polarizability, polarizability anisotropy, and first-order hyperpolarizability are significantly affected by the position of the methyl groups. The SHG and EOPE techniques can be potentially useful to discriminate the α,α-DMN isomers (2,6-DMN < 2,7-DMN < 2,3-DMN) as well as the β,β-DMN isomers (1,5-DMN < 1,4-DMN < 1,8-DMN). The (hyper)polarizability differences among the investigated DMNs were elucidated through density analysis calculations. The predicted polarizabilities exhibit good linear relationships with the experimental first-order biomass-normalized rate coefficient, a physicochemical property connected to the rates of biodegradation processes of polycyclic aromatic hydrocarbons.
Highlights
Polycyclic aromatic hydrocarbons (PAHs) are very stable organic lipophilic pollutants principally produced during incomplete combustion processes, often exhibiting high levels of toxicity, mutagenicity, and carcinogenicity to humans and/or other living creatures [1,2,3,4,5]
DMNs can exist as ten isomeric forms (Figure 1): 1,2-DMN, 1,3-DMN, 1,4-DMN, 1,5DMN, 1,6-DMN, 1,7-DMN, 1,8-DMN, 2,3-DMN, 2,6-DMN, and 2,7-DMN
The average polarizabilities (⟨α⟩) of DMNs regularly increase in the order ⟨α⟩(α,α-DMNs) < ⟨α⟩(α,β-DMNs) < ⟨α⟩(β,β-DMNs), they slightly vary among the ten isomers
Summary
Polycyclic aromatic hydrocarbons (PAHs) are very stable organic lipophilic pollutants principally produced during incomplete combustion processes, often exhibiting high levels of toxicity, mutagenicity, and carcinogenicity to humans and/or other living creatures [1,2,3,4,5]. On the basis of experimental and theoretical investigations, the active site of the enzyme which controls the biodegradative mechanism is mainly characterized by hydrophobic residues [12, 13], involving contributions from dispersive and/or inductive forces in enzyme-substrate complex formation This result has been corroborated by recent computational studies on the electronic polarizabilities (α) [14, 16] and Raman spectra [15] of alkylated-naphthalenes, the average polarizabilities [14], and summation of the Raman activities [15] of DMNs being found to be linearly related to the experimental kb values. The main focus of this study is to determine the static and frequency-dependent electronic α and β values of the series of the DMN isomers, aiming to explore the effects of the position of the CH3 groups on these electric properties, potentially helpful for the isomeric discrimination. In the present study we used the Coulomb-attenuating hybrid exchange-correlation functional (CAM-B3LYP) [36], which has been recently employed with success for computing electronic (hyper)polarizabilities of organic compounds [37,38,39,40,41,42,43,44,45,46,47,48]
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