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Experimental and theoretical studies on the photodegradation of 2-ethylhexyl 4-methoxycinnamate in the presence of reactive oxygen and chlorine species

2-Ethylhexyl 4-methoxycinnamate (EHMC) is one of the most commonly used sunscreen ingredient. In this study we investigated photodegradation of EHMC in the presence of such common oxidizing and chlorinating systems as H2O2, H2O2/HCl, H2O2/UV, and H2O2/HCl/UV. Reaction products were detected by gas chromatography with a mass spectrometric detector (GC-MS). As a result of experimental studies chloro-substituted 4-methoxycinnamic acid (4-MCA), 4-methoxybenzaldehyde (4-MBA) and 4-methoxyphenol (4-MP) were identified. Experimental studies were enriched with DFT and MP2 calculations. We found that reactions of 4-MCA, 4-MBA and 4-MP with Cl2 and HOCl were in all cases thermodynamically favorable. However, reactivity indices provide a better explanation of the formation of particular chloroorganic compounds. Generally, those isomeric forms of mono- and dichlorinated compounds which exhibits the highest hardness were identified. Nucleophilicity of the chloroorganic compounds precursors were examined by means of the Fukui function.


Introduction
Substances which absorb UV light and convert it into heat, fluorescence or phosphorescence, are often used as cosmetics ingredients and in photostabilization of polymeric materials and paints [1]. Because cinnamic acid derivatives convert absorbed light energy into heat with excellent efficiency [2,3] they are among the most popular UV sunscreen ingredients. Recently there has been a growing interest in the research of sunscreens [4][5][6][7][8][9][10].
There are many studies dealing with photodegradation of different compounds used as polymer additives [11], personal care products [10,12], dyes [13], and pharmaceuticals [14,15]. From the usability point of view, the UV absorbers should exhibit high photostability. Nevertheless there are studies which show that these compounds are relatively light-sensitive in environments of different polarity such as water [4,12], methanol [16], and petroleum jelly [17].  found that 2-ethylhexyl 4-methoxycinnamate (EHMC) undergoes photolysis which results in formation of 4-methoxybenzaldehyde and 2-ethylhexyl alcohol. From the theoretical point of view, stability of organic compounds can be evaluated using thermochemical calculations. In many cases however the thermodynamic

Central European Journal of Chemistry
Experimental and theoretical studies on the photodegradation of 2-ethylhexyl 4-methoxycinnamate in the presence of reactive oxygen and chlorine species approach does not provide a complete description and hence reactivity indices should be taken into account. It is well known that reactive oxygen and chlorine species (ROCS) are formed in many natural and industrial processes. Hydrogen peroxide and its mixtures with HCl are the sources of many various chlorinating and oxidizing agents such as OH • , HO 2 • , O 2 , Cl • , Cl -, Cl + , Cl 2 , HOCl, H 2 OCl + , ClO - [18]. According to Nakajima et al. [19] chlorinated EHMC derivatives can be formed during EHMC photodegradation in the presence of HOCl. As we reported in our previous paper, chlorinating and oxidizing agents significantly enhance photodegradation of UV absorbers [20]. The main objective of this study has been to research EHMC photodegradation in the presence of common oxidizing and chlorinating model systems such as H 2 O 2 , H 2 O 2 /HCl, H 2 O 2 /UV and H 2 O 2 / HCl/UV [14,18,[21][22][23]. In order to achieve this goal the experimental methods, such as GC/MS chromatography, spectrophotometric measurements as well as the theoretical tools (thermodynamic calculations, reactivity indices) were applied.

Reaction conditions
First, methanolic solutions of EHMC, H 2 O 2 and HCl were prepared. These solutions were then mixed in molar proportions according to Table 1. 900 mL of each mixture was subjected to the UV light using a photoreactor equipped with a medium pressure mercury lamp (Heraeus, TQ 150W), which emits radiation in the 200-600 nm range. The mixtures in the photoreactor were stirred using a magnetic stirrer (200 rpm). The temperature of the system was kept at 20°C.

Analytical methods
Changes in EHMC concentration were monitored using Agilent 8452A spectrophotometer. The absorbance changes of the EHMC solution were measured at λ max = 310 nm. Samples of 0.1 ml were taken from the reactor at different time intervals and diluted 60 times with methanol before measuring the spectra. All spectrophotometric measurements were performed in standard quartz cuvettes of 1 cm optical path. Methanol was used as a blank sample. Each photodegradation experiment was performed three times and the mean values of absorbance were used for kinetic analysis.
Qualitative analyses of reaction mixtures were performed according to the following procedure. After UV radiation, solutions (ca. 900 mL) were concentrated to a volume of 1 mL using rotary vacuum evaporator (p=122 mbar, t=40°C). In the next step, 0.1 mL of concentrate was diluted with 1 mL of ethyl acetate. So prepared samples were analyzed by HEWLETT PACKARD 5890 gas chromatographer equipped with a mass spectrometric detector and the ZB-5MS column (0.25 mm × 30 m × 0.25 µm). The operating conditions were as follows: 100°C-10°C min -1 -200°C/2 min-10°C min -1 -300°C 2 min -1 . The volume of the sample was 1 μL. Helium was used as a carrier gas. The schematic diagram of the experimental procedures is illustrated in Fig. 1.

Calculation details
All geometry optimizations and thermodynamic analysis, were performed applying the Becke threeparameter (B3) hybrid method [24][25][26] with the Lee-Yang-Parr (LYP) functional [27] and 6-31+G(d,p) basis set (B3LYP/6-31+G (d,p)). For open-shell systems, the spin-unrestricted calculations were applied (UB3LYP/6-31+G(d,p)). In order to get more accurate results, a large integration grid and tight convergence criteria were used. Vibrational analysis was performed for all compounds and no imaginary frequencies were found. Changes of thermodynamic functions, enthalpy (∆H) and Gibbs free energy (ΔG), were calculated for standard conditions (T=298.15 K and P=101325 Pa). For each compound only the most stable conformation of a molecule was taken into account (global minima on the potential energy hypersurface). In order to include the solvent effect the self-consistent reaction field Energy of the lowest unoccupied molecular orbital, E LUMO and the highest occupied molecular orbital, E HOMO were calculated for the optimized at B3LYP/6-31+G(d,p) structures using the Møller-Plesset perturbation method [33-38] and 6-31+G(d,p) basis set (MP2/6-31+G(d,p)). In order to characterize aromaticity of the studied compounds, a harmonic oscillator model of aromaticity (HOMA) index [39,40] was calculated according to the following equation: where N is the number of bonds in the aromatic ring while α and R opt are constants depending on the type of bond (for C−C bond α is 257.7, and R opt is 1.388 Å).
The condensed Fukui function f j was calculated from the Eq. 5 according to the procedure proposed by Yang and Mortier [41]: where q is NBO charge population calculated at the UB3LYP/6-311++G(2d,2p) level while N denotes number of electrons in the optimized structure.

Characterization of the reaction mixture
In order to examine the effect of oxidizing and chlorinating agents on EHMC photodegradation, the following model systems were prepared: EHMC; EHMC/H 2 O 2 ; EHMC/H 2 O 2 /HCl. Spectrophotometric measurements showed that in non-irradiated solutions a decrease in absorbance at λ max = 310 nm was very small. In case of the EHMC/H 2 O 2 system the absorbance diminished by about 10% after 28 days while the presence of HCl slightly accelerated the EHMC degradation. After 24 hours of irradiation of the EHMC solution without HCl and H 2 O 2 the absorbance was reduced by about 24% (Fig. 2a). Addition of H 2 O 2 significantly influenced the rate of EHMC photodegradation. After 10 hours of irradiation the absorbance decreased by half. As one can see from Fig. 2a, the highest rate of EHMC conversion was achieved in case of the EHMC/H 2 O 2 /HCl/UV system. When the molar ratio of the EHMC/H 2 O 2 /HCl system was 1:10:10 photodegradation was considerably faster than in other cases (Fig. 2b). The GC/MS analysis showed that the main reactions occurring in the EHMC/H 2 O 2 /UV system were trans-cis photoisomerisation and photolysis, which confirmed the presence of 2-ethylhexyl alcohol (EHA) and Z-EHMC in the reaction system (Fig. 3). Noteworthy, this is consistent with the results reported in earlier studies [12,42,43]. The EHMC conversion significantly increased with the addition of HCl to the system (Fig. 2a). As one can see from Fig. 3, much more of photodegradation products were formed during EHMC UV irradiation in the presence of chlorinating agents than in case of the EHMC/H 2 O 2 /UV system. The following chloroorganic compounds, 3-chloro-4-methoxybenzaldehyde (3-Cl-4-MBA), 3,5-dichloro-4-methoxybenzaldehyde   Experimental and theoretical studies on the photodegradation of 2-ethylhexyl 4-methoxycinnamate in the presence of reactive oxygen and chlorine species

Thermodynamic insight into formation of detected products
Because the reaction proceeds very slowly without UV irradiation, the EHMC degradation is probably mainly photochemical and radical in nature.  In order to evaluate which compounds are more favorable to form, the standard ∆H and ∆G of mono-and dichlorination reactions (Fig. 6) and negative logarithm of the equilibrium constant, pK, were analyzed ( Table 3). Based on the approach presented by Li et al. [47], all of the available ring positions in 4-MCA, 4-MBA, and 4-MP were examined. It is well known that ΔG is a much better criterion of thermodynamic stability because it includes the entropy factor. Generally, the entropy contribution to the ∆G (-T∆S) is the most significant when R=OH or Me and almost negligible when R=Ph (Table 3). Nevertheless, a good correlation coefficient between the calculated values of ∆G and ∆H (r=0.99) indicates that both functions are equivalent in evaluating the thermodynamic stability of chloroorganic compounds.
H 2 O 2 /HCl system is mainly a source of HOCl and Cl 2 . From the thermodynamic point of view HOCl is a more efficient chlorinating agent than Cl 2 ( Table 3). This is a general rule since the gas phase enthalpies of monochlorination of benzene with Cl 2 and HOCl, calculated from the experimental data [48][49][50][51], are -28.85 kcal mol -1 and -46.79 kcal mol -1 respectively. As one can see from Table 3, reactions of 4-MCA, 4-MBA, and 4-MP with Cl 2 and HOCl are in all cases exothermic (∆H<0) and favorable (∆G<0). However, the ∆H and ∆G values calculated for the isodesmic reactions ( Fig. 6; R=Me, Ph) indicate that chlorination of 4-MCA, 4-MBA, and 4-MP is less favorable than chlorination of methane or benzene.
In our experiments the vicinal dichlorinated compounds were not detected in the reaction mixture. Obviously chlorination of 4-MCA and 4-MBA occurs according to the electron-donating resonance effect of the MeO substituent (+R) and the electron-withdrawing resonance effects of the CH=CHCOOH and CHO groups (-R). From the thermodynamic point of view formation of vicinal dichlorinated isomers, 2,3-diCl-4-MCA, 2,3-diCl-4-MBA and 2,3-diCl-4-MP is also less favorable than formation of 2,5-diCl-4-MP, 3,5-diCl-4-MBA and 3,5-diCl-4-MCA detected in the reaction mixture ( Table 3). It is worth mentioning that DFT thermochemical calculations presented by Li et al. [47] demonstrated that the repulsion of the nearest chlorine atoms in a molecule is a destabilizing factor.

Reactivity indices and charge density analysis
It is well known that chemical species which are highly reactive are usually very unstable. Because of this fact, stability of compounds can be measured in terms of reactivity indices [52]. It is important to note that aromatic systems are generally more stable than non-aromatic ones and hence aromaticity indices are often used in Table 3. Thermodynamic data calculated for monochlorination and dichlorination reactions shown in Fig. 6. Experimental and theoretical studies on the photodegradation of 2-ethylhexyl 4-methoxycinnamate in the presence of reactive oxygen and chlorine species evaluation of reactivity and stability [40,53]. Generally, chlorinated 4-MCA, 4-MBA and 4-MP isomers which are more aromatic and which exhibit higher hardness than others were formed under the experimental conditions (Table 4). However, in case of monochlorinated 4-MP derivatives, 3-Cl-4-MP, not 2-Cl-4-MP, was detected ( Fig. 4) although 2-Cl-4-MP is slightly more aromatic. As might be expected, the introduction of a chlorine atom into the molecule results in an increase of electronegativity. It means that monochlorinated 4-MCA, 4-MBA and 4-MP are probably less nucleophilic than non-chlorinated ones. Generally speaking the second chlorination step is often kinetically less favorable than the first step. For instance, electrophilic chlorination of benzene using the FeCl 3 /PhNO 2 system is several times faster than in the case of chlorobenzene [54]. According to the Hammond postulate [55] reaction intermediates are energetically close to the transition states, which means that the formation of stable σ-complexes is kinetically advantageous. Obviously, the charge density on carbon atoms determines the stability of the σ-complexes with electrophiles. One of the useful methods for evaluating susceptibility of certain atoms in a molecule to either nucleophilic or electrophilic attack is   (Table 5). This is consistent with the resonance effects of the substituents attached to the benzene ring. In case of 4-MCA and 4-MBA the electron-donating methoxyl group (MeO) is conjugated with the electron-withdrawing substituent, CH=CHCOOH and CHO respectively. The substituent constants σ p of MeO, CH=CHCOOH, and CHO groups are -0.26, 0.62, and 1.03 respectively [61]. This shows that C-2 and C-6 atoms (numeration according to Table 5) in 4-MCA and 4-MBA are the most susceptible to substitution by the chlorinating agent. From the local reactivity point of view nucleophilicity of particular atoms in the molecule can be evaluated by means of the Fukui function f j -.

Product
As we found, chlorination reactions occur at the atoms with the highest values of f j among all sites available for substitution. As it can be expected, the electrophilic attack of a chlorinating agent is, in case of 3-Cl-4-MP, the most probable at C-5 atom. This is of course determined by the electron-donating resonance effects (+R) of the OH and Cl groups attached to the benzene ring (σ R of OH and Cl substituents are -0.43 and -0.16 respectively [61]). Besides, the calculated f j values also indicate that C-5 atom is more nucleophilic than C-3 or C-6.

Conclusions
In this study we examined the effect of common oxidizing and chlorinating agents on EHMC photodegradation. Experimental data demonstrated that EHMC photostability decreases in the presence of ROCS. Spectrophotometric measurements indicated that the reaction proceeds very slowly without UV irradiation. This shows that EHMC is relatively stable in the presence of ROCS when photolysis efficiency is reduced.
The mechanism of 4-MCA, 4-MBA, and 4-MP chlorination is probably complex and consists of radical and ionic steps, including photoinduced C-O bond cleavage, oxidation, and chlorination. DFT thermochemical calculations demonstrated that reactions of 4-MCA, 4-MBA, and 4-MP with Cl 2 and HOCl are in all cases thermodynamically probable (∆G<0) and exothermic (∆H<0). However, the thermodynamic analysis does not provide a sufficient explanation of chloroorganic compounds formation. In most cases chlorinated 4-MCA, 4-MBA, and 4-MP isomers, which are "harder" than others, were identified in the reaction mixture. This is consistent with the "Maximum Hardness Principle", according to which stable chemical species are less polarizable than unstable ones [52]. In order to better understand the mechanism of formation of chloroorganic compounds the local nucleophilicity was described by means of the Fukui function.