Photoelectron spectroscopy of organic pollutants: Chlorophenols

Photoelectron spectroscopy of organic pollutants: Chlorophenols

Journal of Electron Spectroscopy and Related Phenomena 239 (2020) 146919 Contents lists available at ScienceDirect Journal of Electron Spectroscopy ...

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Journal of Electron Spectroscopy and Related Phenomena 239 (2020) 146919

Contents lists available at ScienceDirect

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Photoelectron spectroscopy of organic pollutants: Chlorophenols


Igor Novak Charles Sturt University, POB 883, Orange NSW 2800, Australia



Keywords: Chlorophenols Photoelectron spectroscopy

The electronic structures of poly-chlorophenols (CP) have been studied in the gas phase by UV photoelectron spectroscopy and high-level quantum chemical calculations. We relate their electronic structures to chemical reactions involved in CP removal during water treatment processes.

1. Introduction

2. Theoretical and experimental methodology

Chlorophenols (CP) are derivatives of phenol containing one or more chlorine substituents. Because both the hydroxyl group and chlorines are considered ring-activating substituents CP are more reactive than the parent phenol. This reactivity is expressed in the fact that CP are more readily oxidized and can undergo electrophilic or nucleophilic substitution reactions. CP have antimicrobial properties and act as wood preservatives, herbicides, insecticides and fungicides. CP are used in manufacturing paper, textiles, wood products. Their presence has been detected in wastewater released during oil refining and coking processes. They are also by-products of industrial processes in manufacturing of various polymers, dyes, textiles, resins and explosives. They have also been detected in landfill leachates. CP are widespread and persistent in the environment [1]. They are mutagenic, carcinogenic and immunogenic for humans and other organisms. CP are volatile and may react with OH radicals in the atmosphere, but their main polluting effects are tied to water phases which we shall consider in this work. The electronic structure of these compounds can be expected to play an important role in determining their chemical reactivity which in turn governs their chemical behavior in the environment. One of the best methods to study valence electronic structure is UV photoelectron spectroscopy (UPS). The electronic structure of parent phenol and some mono-chlorophenols have been studied by UPS previously [2,3], but the main focus of our investigations is on CP with more than one chlorine substituent since these are the ones of greatest environmental concern. We present a discussion of spectra and electronic structure of four poly-chlorophenols and the relationship between electronic structure and environmentally important chemical reactivity.

Sample compounds were obtained from Sigma-Aldrich and their purity was checked by mass spectrometry. The spectra were recorded on Vacuum Generators UV-G3 photoelectron spectrometer with sample probe at temperatures of 150°, 30°, 60° and 50 °C for pentachlorophenol (PCP), 2,3-chlorophenol (23CP), 2,3,4,5-chlorophenol (2345CP) and 2,4,5-chlorophenol (245CP) samples, respectively. The different temperatures used are due to different volatilities and melting points of individual compounds. The probe temperatures were set so that sufficiently high vapour pressures could be generated in the ionization chamber. There were no signs of sample decomposition in the spectra and all the spectra were of the same quality irrespective of different measurement temperatures (above). The spectral resolution was 25 and 70 meV for HeI and HeII spectra, respectively when measured as FWHM of the 3p−1 2P3/2 Ar+ ⟵Ar(1S0) line. Vertical ionization energies were taken as band maxima except for bands whose Franck-Condon envelope indicates that 0-0 vibronic transition is the strongest. In these cases the adiabatic energy was measured as energy of 0-0 transition as is the established practice [4]. The geometry optimizations and thermodynamic calculations were performed with the Gaussian 16 software package [5]. Vertical valence ionization energies were calculated with EOM-IP-CCSD method implemented in ORCA software [6]. The vibrational analysis confirmed that the optimized molecular geometries were true minima (no imaginary frequencies) on the potential energy surface. The molecular geometries were fully optimized at B3LYP/6-311+ +(3df,3pd) level prior to EOM-IP-CCSD calculations which used augCC-pvDZ basis set. The optimized geometries and total energies are given in Supplementary data.

E-mail address: [email protected] Received 14 November 2019; Received in revised form 5 December 2019; Accepted 17 December 2019 Available online 23 December 2019 0368-2048/ © 2019 Elsevier B.V. All rights reserved.

Journal of Electron Spectroscopy and Related Phenomena 239 (2020) 146919

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Fig. 1. HeI photoelectron spectrum of 2,3-dichlorophenol (23CP).

3. Results and discussion 3.1. Photoelectron spectra The experimental spectra and the results of EOM-IP-CCSD calculations are given in Figs. 1–3 and Table 1. The summarized results obtained from photoelectron spectra in the form of valence orbital energy correlation diagrams are shown in Figs. 4 and 5. The photoelectron spectrum of PCP was measured at HeI and HeII photon energies. It is well established that ionizations of molecular orbitals with C2p character show increase in relative band intensity and those with Cl3p character show a decrease in relative band intensity on going from HeI to HeII radiation [4]. This intensity variation was used as an aid in the assignment of π-ionizations and halogen lone pair ionizations. We have also compared UPS of our compounds with UPS of chlorobenzenes [7,8] and with the Penning ionization spectrum of phenol [9] as an aid in the assignment. The final assignments based on all these considerations are summarized in Figs. 4 and 5 and in Table 1. The UPS of

Fig. 3. HeI and HeII photoelectron spectra of pentachlorophenol (PCP). Table 1 Vertical ionization energies (Ei/eV) from UPS spectra of CP, EOM-IP-CCSD calculated energies and MO assignmentsa. Molecule

UPS band






8.9, 9.23 11.06 11.77 12.28


12.60 12.85 13.4 13.86 9.0, 9.30 11.38, 11.56

8.84, 9.22 11.02, 11.16 11.72 12.06, 12.19, 12.24 12.58, 12.59 12.88 13.58 13.85, 14.27 8.76, 9.30 11.04, 11.65, 11.66


12.15 12.65 13.40

12.12 12.75 13.35, 13.41


14.3 8.95 9.35 11.05,11.25


11.90 12.10 12.35 12.80 13.50 14.23 8.85 (9.56) 11.15 11.30 11.5 11.96 12.30 12.50 13.10 13.95

14.34 8.75 9.28 10.99, 11.59 12.04 12.09 12.11 12.52 13.37, 14.14, 8.65 9.49 11.20 11.37 11.58 11.95 12.41 12.88 13.15 13.88,

π3(a”), π2(a”) nCl (a’), πCl (a”) nCl (a’) nCl (a’), πCl (a”) nCl (a’) πCl (a”), nCl (a’) πCl (a”) πCl (a”) σ (a’), σ (a’) π3(a”), π2(a”) nCl− (a’), π1 (a”) nCl+ (a’) πCl (a”) σ(a’) πCl (a”) σ(a’) σ(a’) π3(a”) π2(a”) nCl(a’), πCl(a”), nCl(a’) πCl(a”) nCl(a’) nCl(a’) πCl(a”) πCl(a”), σ(a’) σ(a’), π1(a”) π3 (a”) π2 (a”) πCl (a”) nCl(a’) nCl(a’) nCl(a’) πCl (a”) πCl (a”) σ(a’) σ(a’), π1(a”)






13.39 14.28


values in brackets correspond to adiabatic ionization energies.

Fig. 2. HeI photoelectron spectra of 2,4,5-trichlorophenol (245CP) and 2,3,4,5tetrachlorophenol (2345CP). 2

Journal of Electron Spectroscopy and Related Phenomena 239 (2020) 146919

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Fig. 4. Correlation diagram for 23CP and 245CP: energy levels and assignments based on measured ionization energies of 23CP, 245CP and related molecules. The energy levels for chlorobenzenes are from [7,8] and phenol from [9].

The consequence is that, depending on the positions of chloro substituents, one π-orbital may be destabilized more than the other due to the presence/absence of resonance interactions between substituent(s) and that π-orbital. For example, chloro substitution at 4-position in phenol exerts negligible resonance effect on π2 orbital, but on the other hand the same substituent will destabilize π3 orbital. As a result the corresponding Δπ value will differ from reference Δπ value of 0.72 eV in phenol.

polychlorobenzenes reported earlier [7,8] helped us to assign and compare regions of chlorine lone pair ionizations and of HOMO and HOMO-1 ionizations in CP and chlorobenzenes. The ionization energies of well resolved spectral bands, which correspond to π3 (HOMO) and π2 (HOMO-1) ionizations depend on the topology of substitution. The π3 and π2 ionization energies increase (vs. parent phenol) as can be seen from Δπ3 and Δπ2 values (Table 2). This can be expected due large electronegativity of chlorine substituent (inductive effect). This inductive shift towards higher ionization energies is especially pronounced for π1 ionization as indicated in Figs. 4 and 5. HOMO and HOMO-1 ionizations are shifted by different amounts (vs. phenol) and this can be quantified by defining Δπ= π3-π2. We interpret the results by reference to the well-known inductive and resonance effects. Inductive effect (I) of chloro substituents can be expected to stabilize (increase ionization energies) of both π3 and π2 orbitals to a similar degree. On the other hand, the resonance effect (R) can be expected to destabilize these orbitals (reduce their ionization energies) albeit not to the same extent. The two effects are cumulative and we can estimate their relative importance in individual molecules by the following considerations. The electron localization and nodal properties of π3 and π2 orbitals are different (see MO diagram below).

We compared Δπ value for phenol (0.72 eV) with Δπ values for individual CP in Table 2. We note that Δπ value in case of 245CP is very similar to phenol which indicates that R affects both π orbitals equally. In 4CP (ref. [3]) Δπ values are larger than in phenol. This can be 3

Journal of Electron Spectroscopy and Related Phenomena 239 (2020) 146919

I. Novak

Fig. 5. Correlation diagram for 2345CP and pentachlorophenol (PCP): energy levels and assignments based on experimental ionization energies of 2345CP, PCP and related molecules. The energy levels for chlorobenzenes are from [7,8] and for phenol from [9]. Table 2 1st vertical ionization energies (Ei) and shifts in π ionization energies (Δπ) for CPa−b. Molecule


Δπ /eV

Δπ2 /eV

Δπ3 /eV

Phenol (P) 2CP 3CP 4CP 23CP 245CP 2345CP 23456CP

8.61 8.90 8.86 8.70 9.0 8.85 8.95 9.0

0.72 0.58 0.49 1.1 0.30 0.71 0.40 0.33

0.17 0.02 0.47 −0.03 0.23 0.02 −0.10

0.29 0.25 0.09 0.39 0.24 0.34 0.29

a Experimental ionization energies for phenol and di-chlorophenols were taken from refs. [2,3]. b Δπ3= π3- π3(phenol); Δπ2= π2- π2(phenol); Δπ= π3 - π2.

Fig. 6. Mechanism of reaction of OH radical with PCP.

interpreted as larger resonance destabilization of π3 orbital vs. π2. In the remaining molecules, Δπ values are smaller than in phenol which suggests that opposite is the case: π3 is destabilized less than π2 by the resonance effect of chlorine substituents. We would like to mention at this point that a more quantitative analysis of the complex interplay of effects of different substituents in benzene derivatives is possible by measuring core ionization energies of C1s electrons [10]. This is especially true if different types of substituents are present in phenol molecule unlike here where all substituents are of the same type.

3.2. Electronic structure and chemical properties Removal of chlorophenols from water occurs via different chemical pathways and in a wide pH range (2–10). At low pH (pH≈2) direct oxidation of CP takes place and proceeds via electrophilic, nucleophilic or dipolar addition reactions. At pH > 9 the formation of hydroxyl radicals is faster which speeds up the CP oxidation process [1]. At intermediate pH both mechanisms are active so that the knowledge of CP valence ionization energies is relevant. Other observations are that OH radicals react faster with less chlorinated CP while ozone is more reactive towards more chlorinated CP. How can we relate this observation to the electronic structures of CP? 4

Journal of Electron Spectroscopy and Related Phenomena 239 (2020) 146919

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Fig. 7. Formation of pre-reaction complex and its rearrangement into OH-adduct.

to be virtually identical [14]. 4. Conclusions In our work we have determined the electronic structures of polychlorophenols using photoelectron spectroscopy. We have then related the structures to observed chemical reactivities of CP in environmentally important oxidation reactions with OH radicals and ozone.

Fig. 8. Calculated structures of pre-reaction complexes between OH radical and chlorobenzene (left) and hexachlorobenzene HCB (right) [12].

OH radical reacts with CP via OH addition producing OH-adduct first which subsequently rearranges into final product o-chloranil as shown in Fig. 6 [11]. However, recent computational study on chlorobenzenes (chlorobenzene; CB and hexachlorobenzene; HCB) has shown that there is another step before OH-adduct is reached [12]. This is the formation of pre-reaction complex (PRC) which subsequently rearranges into OHadduct (Fig. 7). The structures of two PRCs (for cases of CB and HCB) are given in Fig. 8. We assume that analogy between reaction of OH radical with chlorobenzenes and with CP applies at least qualitatively. We also note that π1 orbital energy in chlorobenzenes and in CP decreases significantly with the increasing degree of chlorination. π1 orbital is important because it is delocalized throughout aromatic ring and thus provides the strongest interaction with OH radical in PRC (Fig. 8). The optimized geometries shown in Fig. 8 were obtained at MP2/6311 G(d,p) level while the relative energies of the two complexes were obtained using G3 composite method [12]. The net result is that lower π1 orbital energy makes OH-HCB pre-reaction complex15 kJ/mol more stable than OH-monochlorobenzene pre-reaction complex (Fig. 8). Expressed in the language of quantum chemistry, strong electrostatic interaction between negatively charged oxygen in OH radical and large positive electrostatic potential in HCB aromatic ring are established due to strong electron-withdrawing effects of five chlorine substituents [12]. For pre-reaction complexes between OH radical and monochlorobenzene the same interaction is much weaker (Fig. 8), because partially positively charged hydrogen atom now interacts with aromatic ring whose electrostatic potential is only slightly positive due to the presence of single electron withdrawing chlorine substituent. The fact that PRC of OH-HCB is more stable (than OH-monochlorobenzene PRC) increases the reaction barrier (by 10.9 kJ/mol) towards rearrangement into OH-adduct and in turn leads to slower reactions of more chlorinated chlorobenzenes with OH radicals. We can apply analogous reasoning to reactions and PRC stability for e.g. OH-CP and OH-PCP complexes. This provides a possible rationalization of the fact that OH radicals react faster with less chlorinated CP. While the computational study of PRC complexes is beyond the scope of this study we mention that the complex between neutral benzene molecule and OH radical was isolated and studied by experimental and theoretical methods [13]. Two lowest vertical valence ionization energies (VIE) of phenol and phenolate ion in water solution have been measured and calculated [14]. The results are somewhat ambiguous because the calculations give virtually the same first vertical ionization energies for the two species, while experimental results show VIE difference of 0.7 eV between the two species. The bands in the experimental spectra are broad (FWHM =1 eV) due to condensed phase broadening so the actual difference may be < 0.7 eV. The second vertical ionization energies in phenol and phenolate in solution were found

Acknowledgements Author thanks Charles Sturt University for research grant (CSU OPA 4068) and acknowledges Dr B. Kovač for performing spectral measurements. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi: References [1] P. van Aken, N. Lambert, R. van den Broek, J. Degreve, R. Dewil, Advances in ozonation and biodegradation processes to enhance chlorophenol abatement in multisubstrate wastewaters: a review, Environ. Sci. Water Res. Technol. 5 (2019) 444–481. [2] L. Klasinc, B. Kovač, H. Güsten, Photoelectron spectra of acenes. Electronic structure and substituent effects, Pure Appl. Chem. 55 (1983) 289–298. [3] E.E. Tseplin, S.N. Tseplina, G.M. Tuimedov, O.G. Khvostenko, Photoelectron and UV absorption spectroscopy for determination of electronic configurations of negative molecular ions: Chlorophenols, J. Electron Spectrosc. Relat. Phenom. 171 (2009) 37–46. [4] J.H.D. Eland, Photoelectron Spectroscopy: An Introduction to Ultraviolet Photoelectron Spectroscopy in the Gas Phase, 2nd ed., Butterworths-Heinemann, London, 2013. [5] M.J. Frisch, et al., Gaussian16, rev. A-03, Gaussian, Inc., Wallingford, CT, 2016. [6] F. Neese, Software update: the ORCA program system, version 4.0. WIREs, Comput. Mol. Sci. 8 (2018) e1327. [7] B. Ruščić, L. Klasinc, A. Wolf, J.V. Knop, Photoelectron spectra of and ab initio calculations on chlorobenzenes. 1. Chlorobenzene and dichlorobenzenes, J. Phys. Chem. A 85 (1981) 1486–1489. [8] B. Ruščić, L. Klasinc, A. Wolf, J.V. Knop, Photoelectron Spectra of and Ab Initio Calculations on Chlorobenzenes. 2. Trichlorobenzenes, Tetrachlorobenzenes, and Pentachlorobenzene, J. Phys. Chem. A 85 (1981) 1490–1495. [9] N. Kishimoto, M. Furuhashi, K. Ohno, Penning ionization of substituted benzenes (aniline, phenol and 3 thiophenol) by collision with He*(2 S) metastable atoms, J. Electron Spectrosc. Relat. Phenom. 113 (2000) 35–48. [10] F. Rondino, D. Catone, G. Mattioli, A.A. Bonapasta, P. Bolognesi, A.R. Casavola, M. Coreno, P. O’Keeffe, L. Avaldi, Competition between electron-donor and electron-acceptor substituents in nitrotoluene isomers: a photoelectron spectroscopy and ab initio investigation, RSC Adv. 4 (2014) 5272–5282. [11] L.K. Weavers, N. Malmstadt, M.R. Hoffmann, Kinetics and Mechanism of Pentachlorophenol Degradation by Sonication, Ozonation, and Sonolytic Ozonation, Environ. Sci. Technol. 34 (2000) 1280–1285. [12] G. Kovačević, A. Sabljić, Atmospheric oxidation of halogenated aromatics: comparative analysis of reaction mechanisms and reaction kinetics, Environ. Sci. Processes Impacts 19 (2017) 357–369. [13] A. Mardyukov, E. Sanchez-Garcia, R. Crespo-Otero, W. Sander, Interaction and reaction of the phenyl radical with water: a source of OH radicals, Angew. Chem. Int. Ed. 48 (2009) 4804–4807. [14] D. Ghosh, A. Roy, R. Seidel, B. Winter, S. Bradforth, A.I. Krylov, First- Principle Protocol for Calculating Ionization Energies and Redox Potentials of Solvated Molecules and Ions: Theory and Application to Aqueous Phenol and Phenolate, J. Phys. Chem. B 116 (2012) 7269–7280.