Computational note on the structure and vibrational spectra of organic micropollutants: 1-Ethylnaphthalene and 2-ethylnaphthalene

Abstract

2011 Elsevier B.V. All rights reserved. Polycyclic aromatic hydrocarbons (PAHs) constitute a class of widespread and persistent carcinogenic and mutagenic hazards [1]. However, it is well-known that in many different environments PAHs are prevalently formed by alkylated mixtures [2]. Microbial biodegradation pathways of alkylated PAHs are significantly affected by position of substituents [3]. For instance, in aqueous systems the biodegradation rate of 2-ethylnaphthalene (2-EN) is predicted to be ca. four times higher than that of 1-ethylnaphthalene (1-EN) [3]. In addition, the rate constant for the gasphase reaction of hydroxyl radical with 2-EN was estimated to be greater than that with 1-EN [4]. Therefore the ability to discriminate these isomers is of great environmental interest. Previous studies revealed that vibrational spectroscopies can be useful to distinguish alkyl-naphthalene isomers [5–7]. In this work, we report the structure, energetics, IR and Raman spectra of ethylnaphthalene isomers obtained in the gas-phase by using DFT and ab initio methods. All calculations were carried out by Gaussian 03 program [8]. Molecular structures of possible rotamers (Fig. S1, Supplementary data) were optimized using B3LYP functional with 6-31G⁄ basis set. Table S1 in the Supplementary data reports the relative stability of the rotamers obtained by HF, MP2 and B3LYP methods. At all levels of calculation 1-EN(a) and 2-EN(a) (Fig. 1) are predicted to be the most stable aand b-substiAll rights reserved. alysis, Monitoring and Miniria 8, Catania 95125, Italy. tuted rotamers, respectively. Interestingly, 1-EN(a) is 1–2 kcal/mol less stable than 2-EN(a) owing to stronger alkyl-hydrogen steric repulsions (Fig. 1). Selected geometrical parameters of 1-EN(a), 2-EN(a) and naphthalene are collected in Table S2. The results for naphthalene show that the calculated values are in good agreement with the experimental ones. The largest geometrical differences between 1-EN(a) and 2-EN(a) are found for C1–C9 and C2–C3 bond lengths (0.01 A). Vibrational wavenumbers, IR and Raman spectra of 1-EN(a) and 2-EN(a) were calculated under the harmonic approximation at the B3LYP/6-31G⁄ level, confirming that both these rotamers are equilibrium structures. Wavenumber values were corrected by a single scaling factor of 0.9594 [9]. In agreement with the experimental UV Raman spectra excited at 233 nm [10], the high and middle spectral zones (Figs. S2–S5 of the Supplementary data) are not much informative to discriminate the investigated isomers. Differently, the low-energy IR spectral zone (Fig. 2) is dominated by a strong absorption band located near 800 cm 1 and assigned to out-of-plane C–H bending deformation (Fig. S6). This transition is red-shifted by 33 cm 1 on passing from 2-EN(a) to 1-EN(a), and concomitantly it increases in intensity by a factor of two. Additionally, the low-wavenumber Raman spectral region (Fig. 3) is characterized by an isolated peak near 700 cm 1 attributed to rings breathing vibration (Fig. S6), which on going from 2-EN(a) to 1-EN(a) is shifted downward by 70 cm . Thus the low-energy IR and Raman spectral regions can be useful to identify ethylnaphthalene isomers. Note also that, calculated depolarization ratios for the natural and plane-polarized lights (Table S3) increase by ca. 20% on passing from 1-EN(a) to 2-EN(a). Fig. 1. Lowest-energy structure of 1-ethylnaphthalene and 2-ethylnaphthalene. Fig. 2. Simulated B3LYP/6-31G⁄ IR spectra (Lorentzian lineshapes, half-height of 10 cm ) of ethylnaphthalene isomers. Fig. 3. Simulated B3LYP/6-31G⁄ Raman spectra (Lorentzian lineshapes, half-height of 10 cm ) of ethylnaphthalene isomers. A. Alparone, V. Librando / Computational and Theoretical Chemistry 965 (2011) 244–245 245