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Valence and Rydberg Excitations of 2,4- and 2,6-Difluorotoluene as Studied by Vacuum Ultraviolet Synchrotron Radiation and Ab Initio Calculations Alessandra Souza Barbosa, Filipe Ferreira da Silva, Andre Rebelo, Søren Vrønning Hoffmann, Marcio H. F. Bettega, and Paulo Limao-Vieira J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b07815 • Publication Date (Web): 21 Oct 2016 Downloaded from http://pubs.acs.org on October 21, 2016

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The Journal of Physical Chemistry

Valence and Rydberg Excitations of 2,4- and 2,6-Difluorotoluene as Studied by Vacuum Ultraviolet Synchrotron Radiation and Ab Initio Calculations A. Souza Barbosa 1,2, F. Ferreira da Silva 2, A. Rebelo 2, S. V. Hoffmann 3, M. H. F. Bettega 1, P. Limão-Vieira 2,*

1

Departamento de Física, Universidade Federal do Paraná, Caixa Postal 19044, 81531-990 Curitiba, Paraná,

Brazil 2

Laboratório de Colisões Atómicas e Moleculares, CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal

3

ISA, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000, Aarhus C, Denmark

Abstract Here we report novel comprehensive investigations on the electronic state spectroscopies of isolated 2,4- and 2,6-difluorotoluene in the gas phase by high-resolution vacuum ultraviolet (VUV) photoabsorption measurements in the 4.4–10.8 eV energy-range, with absolute crosssection values derived. We also present the first set of ab initio calculations (vertical energies and oscillator strengths), which we have used in the assignment of valence transitions of the difluorotoluene molecules, together with calculated ionisation energies to obtain the Rydberg transitions for both molecules. The measured absolute photoabsorption cross sections have been used to estimate the photolysis lifetimes of 2,4- and 2,6-difluorotoluene in the Earth’s atmosphere.

*

Corresponding author: Tel: (+351) 21 294 78 59, Fax: (+351) 21 294 85 49

Email address: [email protected] (P. Limão-Vieira)

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1. INTRODUCTION The electronic state spectroscopy of polyatomic molecules is crucial for the study of their structure and dynamics providing key insights into photo-induced dissociation processes with applications in several scientific and technological environments as is the case of modelling biological relevant mechanisms. Difluorotoluenes, C7H6F2 (Figure 1), in particular 2,4-difluorotoluene (2,4-DFT), belongs to the group of molecules that has been used as a nucleoside that mimics thymidine in DNA replication mechanisms.1 Differences in binding or activity measured between natural DNA bases (e.g. thymine) and non-natural analogues (as is the case of 2,4-DFT) have served to suggest that geometric matching, potentially even in the absence of hydrogen bonding between adjacent bases, may be sufficient to direct nucleotide selection during replication.2 From the fundamental point of view, free radical formation produced from precursor difluorotoluene molecules have been the subject of comprehensive investigations to assess the nature of their chemical reactivity,3 although halogen-substituted benzyl radicals have been less studied as other larger aromatic radicals.3-5 2,4-difluorotoluene is isoelectronic with 2,6-difluorotoluene (2,6-DFT) and these substitutions create a number of interesting features that need to be explored from the spectroscopic perspective. In order to facilitate a comparison, we report the results of a comprehensive investigation of the electronic state spectroscopies of 2,4-DFT and 2,6-DFT by high resolution vacuum ultraviolet (VUV) photoabsorption spectroscopy and ab initio theoretical calculations of the vertical excitation energies and oscillator strengths for the neutral electronic transitions. The ionisation energies for the lowest-lying ionic states are obtained using the Outer-Valence Green’s Function (OVGF) method. Due to lack of spectroscopic data related to difluorotoluenes, we make use of the information available on other analogues that include the work of Know et al.6 on the vibrational spectra of p-, m-, and o-difluorobenzene cations in the ground electronic states measured by vacuum ultraviolet mass-analysed threshold ionisation spectroscopy, as well as on difluorobenzyls vibronic emission by Lee and co-workers.7,8 We note spectroscopic studies on 2,6-DFT methyl rotor dependence in the first singlet state S1 and the cation ground state D0.9 In addition, to further our knowledge on the spectroscopy of difluorotolune species and their role as toluene related molecules, we compare our experimental and theoretical results with recent detailed information on the electronic state spectroscopy of toluene by means of high-resolution VUV synchrotron radiation and ab initio calculations.10 In the next section, we present some computational details and provide a brief information on the spectroscopy of both molecules. In addition to identifying the optical 2 ACS Paragon Plus Environment

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electronic transitions, the present work provides reliable photoabsorption cross sections in the range 4.4–10.8 eV. In section 3, we present a brief discussion of the experimental details and section 4 is devoted to the results and discussion. Given the potential atmospheric relevance of these molecules, absolute photoabsorption cross sections are used to estimate photolysis rates in the Earth’s atmosphere, from ground level up to the stratopause (≤50 km altitude). Finally, some conclusions that can be drawn from this study are given in section 5.

2. COMPUTATIONAL DETAILS AND SPECTROSCOPY OF 2,4- AND 2,6DIFLUOROTOLUENE The calculations of the excited electronic states were carried out in the ground state optimized geometry of 2,4-DFT and 2,6-DFT molecules, employing the time-dependent density functional theory (TDDFT),11,12 with the long range corrected version of B3LYP functional using the Coulomb-attenuating method, and Dunning’s augmented correlation consistent valence double zeta basis set (CAM-B3LYP/aug-cc-pVDZ)13 as implemented in the package GAMESS-US.14 In order to optimize the electronic ground state geometries, we employed the density functional theory (DFT) with the same functional and basis set. From the optimization of the ground state structures of difluorotoluenes we found three conformers for 2,4-DFT (belonging to the C1 and Cs symmetry groups) and two for 2,6DFT (both belonging to the Cs symmetry group, staggered and eclipsed conformers), depending on the rotation of the methyl group. The calculated optimized geometries, electron configuration and total energies are presented in the Supporting Information for all conformers. Although the total energy for both conformers are very close (the relative energies are 0.0002 eV for the most stable conformers of 2,4-DFT and 0.002 eV for 2,6-DFT), we will present here only the calculated results for the most stable conformers for each molecule. In the Supporting Information, the calculated data for the other conformer of both molecules is presented. The outermost molecular orbitals of 2,4-DFT are … (24a′)2 (25a′)2 (26a′)2 (27a′)2 (4a″)2 (5a″)2 (6a″)2 and of 2,6-DFT are … (3a″)2 (24a′)2 (25a′)2 (26a′)2 (4a″)2 (27a′)2 (5a″)2 (6a″)2. The most active occupied and virtual orbitals in the excited states of both molecules are presented in the Supporting Information data. 2,4-DFT highest occupied molecular orbital (HOMO) and the second highest occupied molecular orbital (HOMO-1) have π character, whereas de lowest unoccupied molecular orbital (LUMO) character goes toward a Rydberg type orbital, and (LUMO+1) and (LUMO+4) are of π* anti-bonding character. Regarding 2,6-DFT molecular orbitals, the (HOMO) and (HOMO-1) are of π



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character, whereas (LUMO+2) and (LUMO+3) are of π* anti-bonding character, and (LUMO+6) character goes toward a Rydberg type orbital. Recent studies on the electronic excitation of toluene molecule10 have shown that the lowest Rydberg states may overlap with valence states resulting in a complex intensity distribution in the electronic spectrum. Given the similarities in the photoabsorption spectrum of toluene with the spectra of the difluorotoluenes (see Figure 2), it is therefore necessary to assign in the photoabsorption spectra features that are due to Rydberg and valence states. Rydberg states may be identified through knowledge of the ionisation energies and the application of quantum defect theory. The lowest-lying adiabatic ionisation energy of 2,6DFT has been reported at 9.134 eV9 and to the best of our knowledge there is no other work in the literature reporting experimental ionisation energies for 2,4-DFT, such that the Rydberg states were assigned through the calculated ionisation energies. These were obtained at the outer valence Green’s function (OVGF)15 calculations16 using the Gaussian0917 package and the cc-pVTZ basis set with the optimized ground state geometry.

3. EXPERIMENTAL DETAILS 3.1 Difluorotoluene samples The liquid samples used in the VUV photoabsorption measurements were purchased from Sigma-Aldrich, with a stated purity of ≥ 99%. The samples were degassed by a repeated freeze–pump–thaw cycles.

3.2 VUV photoabsorption High-resolution VUV photoabsorption spectra of 2,4-DFT and 2,6-DFT were recorded at the UV1 beam line of the ASTRID synchrotron facility, Aarhus University, Denmark (Figure 2). The experimental setup has been described in detail elsewhere18 so only a short review will be given here. Briefly, vacuum ultraviolet radiation passes through a static gas chamber fitted with a photomultiplier to measure the transmitted light intensity. The incident wavelength is selected using a toroidal dispersion grating with 2000 lines/mm providing a resolution of 0.075 nm, corresponding to 3 meV at the midpoint of the energy range studied. For wavelengths below 200 nm (energies above 6.20 eV), helium was flushed through the small gap between the photomultiplier and the exit window of the gas cell to prevent any absorption by molecular oxygen in the air contributing to the spectrum. The sample pressure is measured using a capacitance manometer (Baratron). To ensure that the data is free of any saturation effects the absorption cross-sections were measured over the pressure range 0.02– 4 ACS Paragon Plus Environment

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1.00 mbar, with typical attenuations of less than 40%. The synchrotron beam ring current was monitored throughout the collection of each spectrum and background scans were recorded with the cell evacuated. The Schuman–Rünge (6.9–9.5 eV) absorption band of O2 is used to test the absolute cross-section measurements because its broad unstructured nature minimises the effect of differences in energy resolution.19 SO2 is used to calibrate the energy scale as it has absorption bands with clearly defined sets of sharp absorption peaks in the ranges 3.8 to 5.1 eV20 and 5.15 to 7.25 eV,21 and the sharp N2 absorption peaks are used for calibration in the 8.2–9.9 eV range. Absolute photoabsorption cross sections were then obtained using the Beer-Lambert attenuation law: It = I0 exp (-nσx), where It is the radiation intensity transmitted through the gas sample, I0 is that through the evacuated cell, n the molecular number density of the sample gas, σ the absolute photoabsorption cross section, and x the absorption path length (25 cm). In order to accurately determine cross-sections, the VUV spectrum was recorded in small (5 nm or 10 nm) sections, with at least 1 nm overlap to the adjoining sections. The accuracy of the cross section is estimated to be better than ± 5%. Only when absorption by the sample is very weak (I0 ≈ It), does the error increase as a percentage of the measured cross section.

4. RESULTS AND DISCUSSION High-resolution VUV photoabsorption cross sections of 2,4-DFT and 2,6-DFT are shown in Figure 2 and compared with previous data of toluene,10 at energies from 4.4 to 10.8 eV. A close inspection of this figure reveals that 2,4-DFT and 2,6-DFT show identical absorption features (in shape and in energy position), where the lowest band of 2,4-DFT is significantly more intense than in 2,6-DFT. Another interesting aspect in respect to toluene is that the most intense absorption features of 2,4-DFT and 2,6-DFT are slightly blue-shifted due to the presence of fluorine atoms. Figures 3 – 6 show expanded views in the 4.4–10.8 eV energy region of the difluorotoluenes together with some assignments. The most intense absorption bands can be classified as a mixture of Rydberg series and molecular valence transitions of the (π* ← π) character. Tables 1 and 2 compare experimental results with the theoretical calculations for 2,4-DFT and 2,6-DFT, showing very good agreement. In these tables we present only the leading terms of the calculations (with highest oscillator strengths), and the complete tables and similar calculations obtained for the others conformers of both molecules can be found in Supporting Information. In Tables 3–5 we present comprehensive vibrational assignments in the 4.4–10.8 eV absorption bands of 2,4-DFT and 2,6-DFT. 5 ACS Paragon Plus Environment


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The three lowest calculated vertical ionisation energies (IEs) obtained with the OVGF method, are IE1 = 9.022 eV, IE2 = 9.552 eV and IE3 = 12.297 eV for 2,4-DFT, and IE1 = 9.247 eV, IE2 = 9.255 eV and IE3 = 12.333 eV for 2,6- DFT (Table S5 of the Supporting Information data summarizes the calculated IEs for the difluorotoluenes conformers obtained with two different basis sets). The proposed assignments of the Rydberg series for 2,4-DFT and 2,6-DFT are summarized in Tables 6 and 7. We now discuss the valence and Rydberg excitation of both difluorotoluene molecules highlighting the most relevant features assigned with the help of the theoretical calculations as well as from a direct comparison with the previous investigation of toluene.10

4.1. Valence state spectroscopy of 2,4- and 2,6-difluorotoluene The theoretical calculations for 2,4-DFT and 2,6-DFT presented in Tables 1 and 2, show that the absorption bands centred at 4.702, 6.14(7) and 6.892 eV, and 4.789, 6.15(3) and 6.888 eV, respectively, have been assigned mainly to (π* ← π) transitions. The calculated wave functions show a mixing between excitations from the (HOMO) (π) and (HOMO-1) (π) to the (LUMO+1) (π*) and (LUMO+4) (π*) MO’s, and also a 3s Rydberg character for the (LUMO) in the case of 2,4-DFT, whereas for 2,6-DFT excitations are mainly assigned from (HOMO) (π) and (HOMO-1) (π) to the (LUMO+2) (π*) and (LUMO+3) (π*) MO’s. An identical behaviour, although with slightly different MOs ordering, has been observed for toluene.10 The calculated transition energies at the TDDFT level (Tables 1 and 2) are in very good agreement with the experimental data, with the exception for the lowest absorption band where overestimation is by about 0.6 eV when compared to experiment. The assignments in the lowest absorption band of 2,6-DFT are in reasonable agreement with Walker et al.9 reporting the origin of the band at 4.705 eV (263.520 nm). The first absorption bands are reported here with a maximum absolute cross-section value of 4.8 Mb (at 4.702 eV) and 1.2 Mb (at 4.789 eV), whereas the second have a maximum of 19.8 Mb (at 6.14(7) eV) and 22.1 Mb (at 6.15(3) eV), and the third at 165.0 Mb (at 6.892 eV) and 189.4 Mb (at 6.888 eV), for 2,4-DFT and 2,6-DFT (see Tables 1 and 2), respectively. The feature at 6.14(7) eV (for 2,4-DFT) with a rather weak calculated oscillator strength, f0 ≈ 0.0021 at 6.291 eV (Table 1), is here assigned to a 3s(28 a') ← π(6a"), i.e. an n = 3s member of a Rydberg series converging to the ionic electronic ground state (see Section 4.2). However, the calculations predict at 6.102 eV another transition with a slightly high intensity f0 ≈ 0.0061 (Table 1) which is mainly due to the pure valence π* character of the 6 ACS Paragon Plus Environment

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molecular orbital. Pure Rydberg transitions (Table 6) with considerable values of oscillator strengths in this energy range are discussed in Sec. 4.2. In the next sections we present a detailed assignment of the absorption features based on the data provided by the vibronic bands of Lee and Lee,8 (in CS symmetry) for 2,4-DFT, whereas Lee and Baek,7 Know et al.6 and Walker et al.9 (in C2v symmetry) will be used for 2,6-DFT. Of relevance that the normal mode description may lead to Fermi resonances, making assignments particularly challenging.

Valence excitation in the range 4.4−5.5 eV The lowest-lying excited state of 2,4-DFT has been assigned according to the calculation results in Table 1 to the π*(7a") ← π(6a")/π*(8a") ← π(5a") transition, with a maximum at 4.614 eV, a local cross-section value of 4.9 Mb (Figure 3 (a)) and showing extensive fine structure. The vertical transition is observed at 4.702 eV (4.8 Mb) against the 5.319 eV predicted from the TDDFT calculations, where such ~ 0.5 eV difference is reasonable given the current level of accuracy of the method employed. The (0–0) transition is tentatively assigned at 4.614 eV, and similarly to 2,6-DFT (see below), this absorption band shows resolvable vibronic structure,9 with assignments presented in Table 3. Note that identical behaviour was reported for toluene.10 The present high-resolution spectrum reveals that the fine structure is due to several modes (see Table 3 for the proposed detailed assignments based on the data of 2,4-difluorobenzyl radical8), with the main contributions from the in-plane ring deformation mode, ν′6a (a′) and ring breathing mode, ν′1 (a′). These modes also appear coupled with other modes. Due to the complexity of such fine structure in the absorption band in Figure 3(a), and in order to avoid congestion, we have represented a few modes only. It is noteworthy that the normal vibrational description may lead to some Fermi resonances, making assignments particularly difficult. The features below the 0 transition (see Table 3) were identified as possible hot-bands based on the previous work of Lee and Lee,8 although we have not been able to perform any assignment. The lowest-lying excited state of 2,6-DFT peaking at 4.789 eV with a local crosssection value of 1.2 Mb, has been assigned to the π*(7a") ← π(6a")/π*(8a") ← π(5a") transition, and shows extensive fine structure. Assignments for this energy band have been revisited and compared with detailed data of Walker et al.9 (Table 3). The origin of the band has been identified at 4.705 eV (37947.6 cm-1)9 in very good agreement with the present value at 4.708 eV. The high-resolution VUV spectrum of 2,6-DFT reveals that the fine structure (Figure 3(b) and Table 3) is due to several modes, with major contributions from in-plane CH bending ν′9a (a1), ring deformation, ν′6a (a1) and C–C stretching, ν′14 (b2). Note that here and 7 ACS Paragon Plus Environment


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below, we adopt Lee and Baek4 description of the vibrational modes in the C2v symmetry. These modes also appear coupled with other modes. We also note that the feature at 5.075 eV, 0.392 eV from the (0–0) transition can be also assigned to υCH stretching mode characterized by 0.395 eV in the ground state.22 In order to avoid congestion, only a few modes have been represented in Figure 3(b). The features below the 0 transition are tentatively assigned to hot-bands (Table 3) based on previous work.7 Interesting to note that in the case of toluene the origin of the lowest-lying absorption band was proposed at 4.650 eV,10 close to the value reported here for 2,4-DFT, although the difluorotoluenes fine structure relative intensities do not vary appreciably and show less resolvable features when compared with toluene.

Valence excitation above 5.5 eV The feature at 6.14(7) eV (for 2,4-DFT), with a calculated oscillator strength, f0 ≈ 0.0061 (at 6.102 eV), is here assigned to π*(7a") ← π(5a")/π*(8a") ← π(6a") with the aid of theoretical calculations (see Table 1). The (0–0) transition is assigned at 5.87(1) eV (Figure 4(a), Table 4) and the (0.137±0.003) eV average value is assigned to excitation of mode ν′14 (a′) and/or ν′7a (a′). According to the calculations, the 3s(28a') ← π(6a") Rydberg transition has a very close energy value (6.291 eV) to the valence transition, with an oscillator strength value (0.0021) that may contribute to the feature observed in this energy range. The next absorption band (6.4-7.6 eV, Figure 5(a)) with the highest oscillator strength (f0 ≈ 0.60), peaking at 6.892 eV, has been assigned to (π*(8a") ← π(5a")/π*(7a") ← π(6a") at 7.042 eV), although it may also be assigned to (π*(7a") ← π(5a")/π*(8a") ← π(6a") at 6.885 eV) with an oscillator strength f0 ≈ 0.51. According to the calculations, at 7.401 eV a 3p(9a") ← π(6a") Rydberg transition converging to IE1=9.022 eV with an oscillator strength of 0.027 can contribute to the experimental feature assigned at 6.832 eV (see Table 6), although we note an ~ 0.5 eV difference between theoretical and experimental values. A few quanta of in-plane CH bending mode, ν′9b (a′), are excited (Table 5). Another two absorption regions have been assigned at 8.077 and 8.744 eV (Table 1) mainly with Rydberg character. These energy regions exhibit evidence for transitions to Rydberg states (section 5.2 and Table 6), converging to the ionic electronic ground and first excited states. The rather low-lying background signal contributing to these bands may be related to the close-lying ionisation continua, which becomes more noticeable above ~ 9.0 eV. As far as 2,6-DFT is concerned, the lowest valence excitation band in this energy range (Figure 4(b)), and peaking at 6.15(3) eV, is assigned to the (π*(7a") ← π(5a")/π*(8a") 8 ACS Paragon Plus Environment

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← π(6a")) transition in agreement with previous reports.9 The 0 transition is here proposed at 5.92(6) eV. The mean energy spacing of 0.077 eV (Figure 4(b) and Table 4) has been assigned to ring breathing, ν′1 (a1) with 699 cm-1 (0.087 eV) in the neutral ground state.7 The next band with the highest oscillator strength (f0 ≈ 0.68), and peaking at 6.888 eV (Figure 5 (b)), has been assigned to (π*(8a") ← π(6a")/π*(7a") ← π(5a")) in Table 2, whereas in the case of toluene it was reported at 6.786 eV and assigned to a mixed Rydberg/valence (1π* ← 3π) and (3pπ (a") ← 2π) character.6 We note that the calculated vertical excitation energy predicts at ~ 7.69 eV a Rydberg transition associated to (HOMO) (6a") → (LUMO+6) (9a"), (HOMO) (6a") → (LUMO+9) (10a") with an oscillator strength f0=0.0133. In the energy region above 7.8 eV (Figures 5(b) and 6(b)), two transitions to Rydberg states (section 5.2 and Table 7), converging to the ionic electronic ground and first excited states are assigned. This energy region also shows some quanta of vibrational excitation of ring deformation mode, ν′6a (a1), in-plane C-H bending mode, ν′9a (a1) and ring breathing mode, ν′1 (a1) (see Table 5). Finally, we note in Figure 6(b) a rather broad background contribution that can be attributed to the low-lying ionisation continua. Finally, from the comparative point of view of 2,4-DFT and 2,6-DFT most intense absorption feature, we note somehow that the former molecule vibrational excitation pattern resembles that observed in toluene.10

4.2 Rydberg transitions The VUV photoabsorption spectrum above 6.0 eV shows structures superimposed on a diffuse absorption feature extending to the lowest ionisation energies, which are here reported for the first time. The calculated vertical ionisation energies values mentioned above are used to estimate the Rydberg series. The proposed Rydberg structures are listed in Tables 6 and 7, for 2,4-DFT and 2,6-DFT molecules according to the ionisation energies (Ei), principal quantum numbers (n), and quantum defects (δ). Features in the photoabsorption spectra related to Rydberg states, En, are tested by the Rydberg formula: En = Ei – R / (n - δ)2, where R is the Rydberg constant (13.61 eV). The lowest-lying transition energies are tentatively assigned to 3s at 6.14(7) and 5.92(6) eV (Tables 6 and 7) for the Rydberg series converging to the ionic electronic ground states of 2,4-DFT and 2,6-DFT, with average quantum defects δ=(0.83±0.03) and δ=(0.94±0.07), respectively. These are accompanied by vibrational structure, which for the former molecule is proposed to be due to excitation of the in-plane ring stretching mode υ′14 (a′) (or even in-plane C-H stretching mode υ′13 (a′)), with an average value of 0.137 eV 9 ACS Paragon Plus Environment


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(Figure 3(a) and Table 4), against 0.153 eV for the electronic ground state;6 as far as the latter molecule is concerned, excitation is proposed to be due to ring breathing υ′1 (a1), with an average value of (0.077±0.005) eV (Figure 3(b) and Table 4), against 0.087 eV for the electronic ground state.7 These assignments are suggested following the assumptions of Lee and Lee8 and Lee and Baek7 reporting vibrational frequencies at Cs and C2v symmetries for 2,4-DFT and 2,6-DFT, respectively. The higher members of these Rydberg series are proposed to extend up to n = 8 (Tables 6 and 7). The first members of the np and nd series of 2,4-DFT and 2,6-DFT are associated with structures at 6.832 eV (n = 3, δ = 0.51) and 8.077 eV (n = 4, δ = 0.20) (Table 6), and 6.888 eV (n = 3, δ = 0.54) and 7.667 eV (n = 3, δ = -0.04) (Table 7), respectively. The nd series (for 2,4-DFT) shows vibrational excitation of several quanta for the n = 4 of in-plane ring deformation mode υ′6 (a′) (or υ′6a (a′) according to Ref. 9). Close to the lowest ionic limit, weak features tentatively assigned to in-plane ring deformation mode (υ′6) appear at 8.73(1) and 8.90(7) eV (Table 5) for 2,4-DFT, whereas ring deformation mode (υ′6) appear at 8.95(8) eV for 2,6-DFT (Table 5). The first members of the ns, np and nd series converging to the ionic electronic first excited state of 2,4-DFT are associated with peaks at 6.88(8) eV (n = 3, δ = 0.74), 6.982 eV (n = 3, δ = 0.70) and 7.845 eV (n = 3, δ = 0.17), respectively (Table 6). Rydberg transitions (Figure 5(a)) are accompanied by vibronic structure which has been tentatively assigned to excitation of the υ′6 (a′) and υ′8 (a′) modes (Table 5). Regarding 2,6-DFT, the first members are observed at 5.926 eV (n = 3, δ = 0.98), 7.029(1) eV (n = 3, δ = 0.54) and 7.739 eV (n = 3, δ = 0.02), respectively (Table 7). Concerning the Rydberg series converging to the ionic electronic second excited state of 2,4-DFT, the lowest members for ns, np and nd (Table 6) are suggested at 10.27(2) eV (n = 4, δ = 1.38), 9.801 eV (n = 3, δ = 0.65) and 10.422 eV (n = 3, δ = 0.29), respectively. The feature at 10.27(2) eV has a considerable large quantum defect for an ns series but that is probably due to the contribution of the vibrational modes. Some of the fine structure has been assigned to vibrational excitation involving in-plane C–F stretching mode υ′7b (a′) and also ring deformation mode υ′8 (a′). Finally, as far as 2,6-DFT is concerned, n = 3 for ns, np and nd are proposed at 9.085, 10.188 and 10.498 eV, respectively, (Table 7) with quantum defects 0.95, 0.48 and 0.28, respectively. The fine structure has been mainly assigned to vibrational excitation involving ring deformation mode υ′6a (a1), in-plane C-H bending mode, ν′9a (a1) and also ring breathing mode υ′1 (a1).


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The higher members of the Rydberg series, for which the relative intensities decrease, may result from overlap with other transitions and possible vibronic structure contributions, and so no attempt has been made to assign these structures. Yet, a close inspection of Figure 2 above 9 eV shows a clear increase in the absorption background with energy, which may be related to contribution of the underlying ionisation continua. Vibrational excitation in the Rydberg series of 2,4-DFT and 2,6-DFT converging to the different ionisation energies, appears somehow identical from the point of view of similar modes being active (with prominence to ring deformation mode), however not as rich as it appears in toluene.10

4.3 Absolute photoabsorption cross sections and atmospheric photolysis A careful set of measurements have been performed on 2,4-DFT and 2,6-DFT in the pressure range 0.02–1.00 mbar and revealed no changes in absolute cross sections or peak energies as a function of pressure. From our previous experience in obtaining photoabsorption cross sections measured on the UV1 beamline at the ASTRID facility compared with the most accurate data in the literature (e.g. Ref. 23), the present spectra are free of any saturation effects, indicating therefore that the cross sectional values can be relied upon across the energy range studies, 4.4–10.8 eV. As far as authors are aware there are no cross section values available in the literature for 2,4-DFT and 2,6-DFT to compare with. The present cross sections of 2,4-DFT and 2,6-DFT above 180 nm (below 6.89 eV) can be used in combination with the solar actinic flux24 to estimate the photolysis rates of these molecules in the Earth’s atmosphere from the ground level up to the limit of the stratopause (50 km). Comprehensive details on the calculation can be found in Ref. 25 in which the quantum yield for dissociation following absorption is assumed to be unity. Photolysis lifetimes of less than a day were estimated at ground level for 2,4-DFT, whereas for 2,6-DFT photolysis lifetimes of less than a day were estimated at altitudes above 10 km (less than two days at 0 km altitude). These results show that these difluorotoluenes can be broken up very efficiently by UV absorption radiation even at low altitudes. However, we are not aware of any detailed study on the rate coefficients obtained for reactions between these molecules and OH radical (and even other atmospheric relevant species as Cl, O3, NO3), which in the case of toluene were identified to provide the main sink mechanism in the troposphere in detriment to photolysis.10

5. CONCLUSIONS 11 ACS Paragon Plus Environment


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Here we report for the first time high-resolution VUV photoabsorption cross sections and a comprehensive study on the electronic state spectroscopy of 2,4-DFT and 2,6-DFT in the 4.4–10.8 eV energy-range. The absorption features have been assigned to valence and Rydberg excitations with the aid of ab initio calculations of vertical excitation energies and oscillator strengths for these molecules. The ionisation energies for the lowest ionic states were obtained using the Outer-Valence Green’s Function (OVGF) method. The theoretical results have been found to be in very good agreement with the experimental findings, where a relevant Rydberg and π* anti-bonding mixing character is observed. The detailed analyses of the fine structure, especially in the lowest-lying absorption band (4.4–5.5 eV), revealed vibronic features which have been assigned in generally consistent agreement with the 2,6DFT data of Walker and co-workers.6 The photolysis lifetimes of 2,4-DFT and 2,6-DFT have been derived in the Earth’s atmosphere, showing that solar photolysis is estimated to be a weak sink mechanism as was observed in the case of the toluene molecule.7

Supporting Information 2,4-Difluorotoluene and 2,6-Difluorortoluene Optimized Geometries (CAMB3LYP/aug-ccpVDZ), Total Energy and Calculated Electron Configuration; Molecular Orbitals; Calculated Vertical Excitation Energies and Oscillator Strengths (Singlet States) of the C1 conformer of 2,4-DFT; Calculated Vertical Excitation Energies and Oscillator Strengths (Singlet States) of the Cs conformer of 2,4-DFT; Calculated Vertical Excitation Energies and Oscillator Strengths (Singlet States) of the Eclipsed Conformer of 2,6-DFT; Calculated Vertical Excitation Energies and Oscillator Strengths (Singlet States) of the Staggered Conformer of 2,6-DFT; OVGF Calculated Ionisation Energies (IE) for 2,4-DFT and 2,6-DFT Most Stable Conformers. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGEMENTS

ASB and MHFB acknowledge the Brazilian Agency Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), under CAPES/FCT Programme (process number 23038.002465/2014-87). MHFB acknowledges support from the Brazilian agency Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), and from FINEP (under project CT-Infra). ASB and MHFB acknowledge computational support from Professor Carlos M. de Carvalho at LFTC-DFis-UFPR, and together with FFS the “convénio FCT CAPES nº 2267”. AR and FFS acknowledge the Portuguese National Funding Agency FCTMCTES through PD/BD/114449/2016 and researcher position IF-FCT IF/00380/2014, and 12 ACS Paragon Plus Environment

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together with PLV the research grant UID/FIS/00068/2013. This work was also supported by Radiation Biology and Biophysics Doctoral Training Programme (RaBBiT, PD/00193/2010); UID/Multi/ 04378/2013 (UCIBIO). Authors wish to acknowledge the beam time at the ISA synchrotron facility at Aarhus University, Denmark. We also acknowledge the financial support provided by the European Commission through I3 Integrated Activity on Synchrotron and Free Electron Laser Science (IA-SFS), contract number RII3-CT-2004-506008, under the Research Infrastructure Action of the FP6 EC programme Structuring the European Research Area.

Author Information Corresponding Author *Telephone: (+351) 21 294 78 59. Fax: (+351) 21 294 85 49. E-mail: [email protected] (P L.-V.).

Notes The authors declare no competing financial interest.



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References (1) Kool, E. T.; Sintim, H. O. The Difluorotoluene Debate—A Decade Later. Chem. Commun. 2006, 35, 3665–3675. (2) Guckian, K. M.; Krugh, T. R.; Kool, E. T. Solution Structure of a DNA Duplex Containing a Replicable Difluorotoluene–Adenine Pair. Nature Struct. Biol. 1998, 5, 954–959. (3) Tan, X.Q.; Wright, T.G.; Miller, T.A. Electronic Spectroscopy of Free Radicals in Supersonic Jets: Jet Spectroscopy and Molecular Dynamics; Hollas, J.M.; Phillip, D.; Eds.; Blackie Academic & Professional, London, 1994. (4) Langkilde, F.W.; Bajdor, K.; Wibrandt, R.; Negri, F.; Zerbetto, F.; Orlandi, G. Resonance Raman-Spectra and Quantum-Chemical Vibrational Analysis of The C7H7⋅ and C7D7⋅ Benzyl Radicals. J. Chem. Phys. 1994, 100, 3503–3513. (5) Cossart-Magos, C.; Cossart, D. Rotational Band Contours of The First Electronic Transition Origin Bands of p-Xylyl And p-Fluorobenzyl. Mol. Phys. 1988, 65, 627–647. (6) Kwon, C. H.; Kim, H. L.; Kim, M. S. Vacuum Ultraviolet Mass-Analyzed Threshold Ionization Spectroscopy of p-, m-, and o-Difluorobenzenes. Ionization Energies and Vibrational Frequencies and Structures of the Cations. J. Chem. Phys. 2003, 118, 6327–6335. (7) Lee, S. K.; Baek, D. Y. Observation of Vibronic Emission Spectrum of the Jet-Cooled 2,6Difluorobenzyl Radical. J. Phys. Chem. A 2000, 104, 5219–5221. (8) Lee, G. W.; Lee, S. K. Observation of Vibronic Emission Spectrum of Jet-Cooled 2,4Difluorobenzyl Radical in a Corona Excitation. Chem. Phys. Lett. 2006, 430, 8–12. (9) Walker, R. A.; Richard, E. C.; Lu, K.-T.; Weisshaar, J. C. Methyl Group Internal Rotation in 2,6-Difluorotoluene (S1) and 2,6-Difluorotoluene+ (D0). J. Phys. Chem. 1995, 99, 12422– 12433. (10) Serralheiro, C.; Duflot, D.; Ferreira da Silva, F.; Hoffmann, S. V.; Jones, N. C.; Mason, N. J.; Mendes, B.; Limão-Vieira, P. Toluene Valence and Rydberg Excitations as Studied by ab initio Calculations and Vacuum Ultraviolet (VUV) Synchrotron Radiation. J. Phys. Chem. A 2015, 119, 9059–9069. (11) Bauernschmitt, R.; Ahlrichs, R. Treatment of Electronic Excitations Within the Adiabatic Approximation of Time Dependent Density Functional Theory. Chem. Phys. Lett. 1996, 256, 454–464. (12) Casida, M. E. Time-Dependent Density-Functional Theory for Molecules and Molecular Solids. J. Mol. Struct. Theochem. 2009, 914, 3–18. (13) Yanai, T.; Tew, D. P.; Handy, N. C. A New Hybrid Exchange-Correlation Functional Using the Coulomb-Attenuating Method (CAM-B3LYP). Chem. Phys. Lett. 2004, 393, 51–57. 14 ACS Paragon Plus Environment

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(14) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; SU, S. J.; et al. General Atomic and Molecular Electronic-Structure System. J. Comput. Chem. 1993, 14, 1347–1363. (15) Ortiz, J. V.; Zakrzewski, V. G.; Dolgounircheva, O. Conceptual Perspectives in Quantum Chemistry; Calais, J.-L., Kryachko, E., Eds.; Kluwer Academic: Boston, 1997. (16) Ferreira, A. M.; Seabra, G.; Dolgounitcheva, O.; Zakrzewski, V. G.; Ortiz, J. V. Application and Testing of Diagonal, Partial Third-Order Electron Propagator Approximation. In Quantum Mechanical Prediction of Thermochemical Data; Cioslowski, J., Ed.; Kluwer: Dordrecht, The Netherlands, 2001; p 131. (17) Frisch, M. J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G. ; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09, revision C.01; Gaussian, Inc.: Wallingford, CT, 2010. (18) Eden, S.; Limão-Vieira, P.; Hoffmann, S. V.; Mason, N. J. VUV Photoabsorption in CF3X (X = Cl, Br, I) Fluoro-Alkanes. Chem. Phys. 2006, 323, 313–333. (19) Watanabe, K. Ultraviolet Absorption Processes in the Upper Atmosphere. Adv. Geophys. 1958, 5, 153–221. (20) Vandaele, A.C.; Simon, P.C.; Guilmot, J.M.; Carleer, M.; Colin, R. SO2 Absorption Cross-Section Measurement in the UV Using a Fourier-Transform Spectrometer. J. Geo. Res., 1994, 99, 25599–25605. (21) Freeman, D.E.; Yoshino, K.; Esmond, J.R. Parkinson, W.H. High-Resolution Absorption Cross-Section Measurements of SO2 at 213-K in the Wavelength Region 172-240 nm. Planet. Space Sci., 1984, 32, 1125–1134. (22) Rong, Z.; Zhu, C.; Henry, B.R. CH Stretching Overtone Spectra of Fluorine Substituted Toluenes. J. Phys. Chem. A 2003, 107, 10771–10780. (23) Eden, S.; Limão-Vieira, P.; Hoffmann, S. V.; Mason, N. J. Spectroscopy of CH3Cl and CH3I. Chem. Phys. 2007, 331, 232–244. (24) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina, M. J. Chemical Kinetics and Photochemical Data for Use in Stratospheric Modelling, Evaluation No. 12, 15 January 1997. (25) Limão-Vieira, P.; Eden, S.; Kendall, P.; Mason, N. J.; Hoffmann, S. V. VUV PhotoAbsorption Cross-Section for CCl2F2. Chem. Phys. Lett. 2002, 364, 535–541.



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Figure captions Figure 1 – Molecular Structure of 2,4-DFT and 2,6-DFT Figure 2 – High Resolution VUV Photoabsorption Spectra of 2,4-DFT and 2,6-DFT Compared With Available Data for Toluene (Ref. 10) in the 4.4 – 10.8 eV Photon Energy Range Figure 3 – High Resolution VUV Photoabsorption Spectrum of (a) 2,4-DFT and (b) 2,6-DFT, in the 4.4 – 5.5 eV Photon Energy Range. See Text For Details on The Assignments Figure 4 – High Resolution VUV Photoabsorption Spectrum of (a) 2,4-DFT and (b) 2,6-DFT, in The 5.5 – 6.5 eV Photon Energy Range Figure 5 – High Resolution VUV Photoabsorption Spectrum of (a) 2,4-DFT and (b) 2,6-DFT, in The 6.4 – 8.8 eV Photon Energy Range Figure 6 – High Resolution VUV Photoabsorption Spectrum of (a) 2,4-DFT and (b) 2,6-DFT, in The 8.6 – 10.8 eV Photon Energy Range

Table captions Table 1 – Calculated Vertical Excitation Energies (TD-DFT/CAM-B3LYP/aug-cc-pVDZ) (eV) and Oscillator Strengths (Singlet States) of 2,4-DFT (C7H6F2) Compared With Experimental Data Table 2 – Calculated Vertical Excitation Energies (TD-DFT/CAM-B3LYP/aug-cc-pVDZ) (eV) and Oscillator Strengths (Singlet States) of 2,6-DFT (C7H6F2) Compared With Experimental Data Table 3 – Proposed Vibrational Assignments in The 4.4−5.5 eV Absorption Bands of 2,4DFT and 2,6-DFT Table 4 – Proposed Vibrational Assignments in The 5.5−6.4 eV Absorption Band of 2,4-DFT and 2,6-DFT Table 5 – Proposed Vibrational Assignments in The 6.4−10.8 eV Absorption Band of 2,4DFT and 2,6-DFT Table 6 – Energy Value (eV), Quantum Defect (δ) and Assignment of The Rydberg Series Converging to the Ionic Electronic Ground, First and Second Excited States of 2,4-DFT, C7H6F2 Table 7 – Energy Value (eV), Quantum Defect (δ) and Assignment of The Rydberg Series Converging to the Ionic Electronic Ground, First and Second Excited States of 2,6-DFT, C7H6F2


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Figure 1 – Molecular Structure of 2,4-DFT and 2,6-DFT



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Figure 2 – High Resolution VUV Photoabsorption Spectra of 2,4-DFT and 2,6-DFT Compared With Available Data for Toluene (Ref. 10) in the 4.4 – 10.8 eV Photon Energy Range


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Figure 3 – High Resolution VUV Photoabsorption Spectrum of (a) 2,4-DFT and (b) 2,6-DFT, in the 4.4 – 5.5 eV Photon Energy Range. See Text For Details on The Assignments



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Figure 4 – High Resolution VUV Photoabsorption Spectrum of (a) 2,4-DFT and (b) 2,6-DFT, in The 5.5 – 6.5 eV Photon Energy Range


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Table 1 – Calculated Vertical Excitation Energies (TD-DFT/CAM-B3LYP/aug-cc-pVDZ) (eV) and Oscillator Strengths (Singlet States) of 2,4-DFT (C7H6F2) Compared With Experimental Data Experimental Energy Cross section (eV) (Mb) 4.702 4.8 6.14(7) 19.8 6.892 165.0 6.892 165.0

8.077 8.077

16.5 16.5

8.744

13.9

State 11A' 21A' 61A' 81A' 91A'' 111A' 141A'' 161A' 171A' 221A' 271A''

Energy (eV) 5.319 6.102 6.885 7.042 7.233 7.401 7.726 8.090 8.112 8.459 8.758

TD-DFT-B3LYP/aug-cc-pVDZ Dominant excitation(s) 6a"→ 7a", 5a"→ 8a" 6a"→ 8a", 5a"→ 7a" 5a"→ 7a", 6a"→ 8a" 5a"→ 8a", 6a"→ 7a" 5a"→ 30 a', 5a"→ 29 a' 6a"→ 9a" 6a"→ 33 a', 6a"→ 32 a' 5a"→ 9a" 6a"→ 10 a" 6a"→ 11 a" 6a"→ 36 a', 26 a'→ 7a"

f0 0.0215 0.0061 0.5167 0.6011 0.0210 0.0279 0.0062 0.0241 0.0200 0.0053 0.0088

(the last decimal of the energy value is given in brackets for these less-resolved features)



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Table 2 – Calculated Vertical Excitation Energies (TD-DFT/CAM-B3LYP/aug-cc-pVDZ) (eV) and Oscillator Strengths (Singlet States) of 2,6-DFT (C7H6F2) Compared With Experimental Data Experimental Energy Cross section (eV) (Mb) 4.789 1.2 6.15(3) 22.1 6.888 6.888

189.4 189.4

7.739 8.282

19.0 15.9

9.085

18.1

State 11A' 21A' 61A'' 71A' 81A' 91A'' 111A' 121A' 191A' 211A' 291A' 301A'' 311A'' 331A' 361A''

Energy (eV) 5.413 6.103 6.785 6.914 6.956 7.165 7.621 7.691 8.327 8.439 8.903 9.004 9.054 9.103 9.314

TD-DFT-B3LYP/aug-cc-pVDZ Dominant excitation(s) 6 a"→ 7 a", 5 a"→ 8 a" 5 a"→ 7 a", 6 a" → 8 a" 5 a"→ 29 a', 5 a"→ 28 a' 6 a"→ 8 a", 5 a" → 7 a" 5 a"→ 8 a", 6 a"→ 7 a" 6 a"→ 30 a' 5 a"→ 9 a" 6 a"→ 9 a", 6 a"→ 10 a" 6 a"→ 10 a", 6 a"→ 9 a" 5 a"→ 10 a" 5 a"→ 11 a" 5 a"→ 37 a' 26 a'→ 8 a" 4 a"→ 7 a" 5 a"→ 36 a', 6 a"→ 38 a'

f0 0.0030 0.0133 0.0123 0.6774 0.4975 0.0151 0.0065 0.0133 0.0161 0.0083 0.0064 0.0174 0.0081 0.0087 0.0198

(the last decimal of the energy value is given in brackets for these less-resolved features)


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Table 3 – Proposed Vibrational Assignments in The 4.4−5.5 eV Absorption Bands of 2,4DFT and 2,6-DFT 2,4-DFT

2,6-DFT

Energy (eV)

Assignment

∆E (eV)

4.595 4.601 4.606 4.614 4.682(s) 4.691 4.702 4.711 4.723 4.737 4.750(s) 4.763(s) 4.770 4.779 4.787 4.802(s) 4.809 4.851 4.856 4.866 4.873 4.936 5.015 5.098

n/a n/a n/a 0 6 9 +10 /16 1 12 7 18 6 /13 6 /14 6 +1 12 6 1 18 + 6 12 6 + 1 18 / 6 +1 6 + 1 + 12 1 6 +1 6 +1 6 +1

-0.019 -0.013 -0.008 – 0.068 0.077 0.088 0.097 0.109 0.123 0.136 0.149 0.156 0.165 0.173 0.188 0.195 0.237 0.242 0.252 0.259 0.322 0.401 0.484

4.625 4.725 4.822

– 12 12

Energy (eV)

4.675 4.680 4.689 4.698 4.708 4.747(s) 4.756(s) 4.759 4.763 4.772 4.779 4.789 4.802 4.813 4.824 4.828 4.845 4.853 4.864 4.869 4.872 4.883 4.899 4.910 4.941 – 4.951 0.100 4.974 0.097 4.987 4.997 5.024 5.024 5.075 5.100

Assignment

9 n/a n/a n/a 0 9 9 n/a 6 6 16 1 11 9 + 6 n/a 12 18 14 n/a 13 1 18 + 9 11 13 + 9 12 1 18 11 14 13 1 11 / υCH 1

∆E (eV)

-0.033 -0.028 -0.019 -0.010 – 0.039 0.048 0.051 0.056 0.065 0.072 0.081 0.094 0.106 0.117 0.120 0.137 0.145 0.157 0.162 0.164 0.176 0.191 0.203 0.233 0.243 0.266 0.279 0.289 0.316 0.316 0.367 0.392

(n/a) feature remains unassigned. (s) shoulder structure (the last decimal of the energy value is given in brackets for these less-resolved features)



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Table 4 – Proposed Vibrational Assignments in The 5.5−6.4 eV Absorption Band of 2,4-DFT and 2,6-DFT 2,4-DFT Energy (eV)

Assignment

2,6-DFT ∆E

Energy

(eV)

(eV)

Assignment

∆E (eV)

5.92(6) (b) 0 /3s

–

6.01(2) (b) 14 /7

0.141 6.00(4) (b) 1 / 3s + 1

0.078

6.14(7) (b) 14 /7 /3s

0.135 6.07(7) (b) 1 / 3s + 1

0.073

6.28(4) (b) 14 /7 /3s + 14 /7 0.137 6.15(3) (b) 1 / 3s + 1

0.076

6.42(1) (b) 14 /7 /3s + 14 /7 0.137 6.22(4) (b) 1 / 3s + 1

0.071

6.309

1 /3s + 1

0.085

6.387

1 / 3s + 1

0.078

5.87(1) (b) 0

–

(b) broad structure (the last decimal of the energy value is given in brackets for these less-resolved features)


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Table 5 – Proposed Vibrational Assignments in The 6.4−10.8 eV Absorption Band of 2,4DFT and 2,6-DFT 2,4-DFT

2,6-DFT

6.83(4) 6.88(8) 6.93(4) 6.98(2)

– 9 / 3s(IE2) 9 / 3s+9 9 / 3s + 9 / 3p

∆E (eV) – 0.054 0.046 0.048

7.65(6) 7.84(0) 8.04(0) 8.23(3)

4s(IE1) 4s + 8 4s + 8 4s + 8 / 5s

– 0.184 0.200 0.193

8.077 8.13(0) 8.19(5)

4d(IE1) 4d + 6 4d + 6

– 0.053 0.065

8.04(0) 8.10(4) 8.16(8) 8.23(3)

0 6 6 6 / 5s(IE1)

– 0.064 0.064 0.065

8.19(5) 8.38(9)

4d + 6 4d + 6 + 8a

0.194

8.33(2) 8.38(8) 8.45(2) 8.50(4) 8.56(2) 8.62(2)

5p(IE1) 5p + 6 / 4p(IE2) 5p + 6 / 4p + 6 5p + 6 / 4p + 6 / 6s 5p + 6 / 4p + 6 / 6s + 6 / 4d 5p + 6 / 4p + 6 / 6s + 6 / 4d + 6 / 6d(IE1)

– 0.056 0.064 0.052 0.058 0.060

Energy (eV) Assignment

Energy (eV) 6.888 7.02(1)

3p(IE1) 3p + 14 / 3p(IE2)

∆E (eV) – 0.133

7.66(7) 7.73(9)

3d(IE1) 3d + 1

– 0.072

7.73(9) 7.77(8) 7.80(8)

4s (IE1) / 3d(IE2) 4s + 9 /3d + 9 / 4s(IE2) 4s + 9 / 3d + 9 / 4s(IE2) + 9

– 0.039 0.030

8.07(7) 8.12(5) 8.19(5) 8.24(9)

4p(IE1) 4p + 9 / 4p(IE2) 4p + 9 + 1 4p + 8

– 0.048 0.070 0.172

8.28(2) 8.34(9) 8.42(8)

4d(IE1) 4d + 1 / 4d(IE2) 4d + 1 / 5s(IE2) / 4d(IE2) + 1

0.076 0.078 0.079

8.49(2) 8.53(3) 8.58(0) 8.62(2) 8.65(8)

5p(IE1) 5p + 9 / 5p(IE2) 5p + 9 / 5p(IE2) + 9 5p + 9 / 5p(IE2) + 9 5p + 8 / 5d(IE2)

– 0.041 0.047 0.042 0.166

8.70(2) 8.74(4) 8.81(2)

6s(IE1) 6s + 6 / 6s (IE2) 6s + 6 / 6s(IE2) + 6

– 0.042 0.068

8.86(8)

6p + 6 / 8s / 6s(IE2) + 6

0.056

Assignment

8.67(0) 8.73(1) 8.78(7)

7s(IE1) 7s + 6 / 7d(IE1) 7s + 6 / 7d + 6 / 5s(IE2)

0.061 0.056

8.90(2) 8.95(8)

7p / 7s(IE2) 7p + 6 / 7s(IE2) + 6

– 0.056

8.83(7) 8.90(7) 8.96(5)

5p(IE2) 5p + 6 5p + 6

0.070 0.058

9.08(5) 9.24(6) 9.41(4)

3s(IE3) 3s + 8 3s + 8

– 0.161 0.167

9.06(3) 9.24(8) (b) 9.41(4) (b) 9.62(6) (b) 9.80(1) (b) 10.03(1) (b)

6s(IE2) 6s + 8 / 7d(IE2) 6s + 8 / 7d + 8 6s + 8 / 7d + 8 6s + 8 / 7d +8 / 3p(IE2) 6s + 8 / 7d + 8 / 3p + 8

– 0.185 0.166 0.212 0.175 0.230

10.18(8) 10.34(1) 10.49(8) 10.66(9)

3p(IE3) 3p + 8 3p + 8 / 3d 3p + 8 / 3d + 8

– 0.153 0.157 0.171

10.27(2) 10.41(0) 10.55(2)

4s(IE3) 4s + 7 4s + 7

– 0.138 0.142

(b) broad structure (the last decimal of the energy value is given in brackets for these less-resolved features).



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Page 28 of 36

Table 6 – Energy Value (eV), Quantum Defect (δ) and Assignment of The Rydberg Series Converging to the Ionic Electronic Ground, First and Second Excited States of 2,4-DFT, C7H6F2 En

δ

Assignment

IE1 = 9.022 eV

En

δ

Assignment

IE2 = 9.552 eV

6.14(7)

0.83

3s

6.982

0.70

3p

7.65(6)

0.84

4s

8.38(8)

0.58

4p

8.233

0.85

5s

8.83(7)

0.64

5p

8.504

0.87

6s

9.083

0.61

6p

8.67(0)

0.78

7s 7.845

0.17

3d

0.26

4d

6.832

0.51

3p

8.562

7.842

0.60

4p

–

–

5d

8.33(2)

0.56

5p

–

–

6d

8.562

0.56

6p

9.24(8)

0.17

7d

–

3d

IE3 = 12.297 eV

8.077

0.20

4d

8.452

0.12

5d

–

–

3s

8.622

0.17

6d

10.27(2)

1.38

4s

8.731

0.17

7d

–

–

5s

0.65

3p

–

IE2 = 9.552 eV

9.80(1)

6.88(8)

0.74

3s

–

–

4p

8.075

0.96

4s

–

–

5p

8.78(7)

0.78

5s

9.06(3)

0.72

6s

10.422

0.29

3d

–

7s

–

–

4d

–

(the last decimal of the energy value is given in brackets for these less-resolved features).


Page 29 of 36

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Table 7 – Energy Value (eV), Quantum Defect (δ) and Assignment of The Rydberg Series Converging to the Ionic Electronic Ground, First and Second Excited States of 2,6-DFT, C7H6F2 En

δ

Assignment

IE1 = 9.134 eVa

En

δ

Assignment

IE2 = 9.270 eV

5.92(6)

0.94

3s

5.926

0.98

3s

7.739

0.88

4s

7.778

0.98

4s

8.349

0.84

5s

8.428

0.98

5s

8.622

0.90

6s

8.74(4)

0.91

6s

-

7s

8.902

0.91

7s

0.85

8s 7.02(1)

0.54

3p

8.868

6.888

0.54

3p

8.125

0.55

4p

8.07(7)

0.42

4p

8.533

0.70

5p

8.49(2)

0.40

5p

8.812

0.55

6p

8.702

0.39

6p

8.793

0.69

7p

7.739

0.02

3d

8.349

0.15

4d

8.658

0.28

5d

7.66(7)

-0.04

3d

8.28(2)

0.00

4d

3 8.580

0.04

5d

IE3 = 12.294 eV

8.744

0.09

6d

9.08(5)

0.95

3s

10.18(8)

0.48

3p

10.498

0.28

3d

a

experimental value from reference 6. (the last decimal of the energy value is given in brackets for these less-resolved features).



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TOC Graphic


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ACS Paragon Plus Environment


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Valence and Rydberg Excitations of 2,4- and 2,6 ... - ACS Publications

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