Toluene Valence and Rydberg Excitations as Studied by ab initio

Article pubs.acs.org/JPCA

Toluene Valence and Rydberg Excitations as Studied by ab initio Calculations and Vacuum Ultraviolet (VUV) Synchrotron Radiation C. Serralheiro,†,‡ D. Duflot,§ F. Ferreira da Silva,† S. V. Hoffmann,∥ N. C. Jones,∥ N. J. Mason,⊥ B. Mendes,‡ and P. Limaõ -Vieira*,†,⊥

Downloaded by UNIV OF PENNSYLVANIA on August 28, 2015 | http://pubs.acs.org Publication Date (Web): August 13, 2015 | doi: 10.1021/acs.jpca.5b05080

†

Laboratório de Colisões Atómicas e Moleculares, CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal ‡ Centro de Engenharia Mecânica e Sustentabilidade de Recursos (MEtRICs), Departamento de Ciências e Tecnologia da Biomassa, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal § Laboratoire de Physique des Lasers, Atomes et Molécules (PhLAM), UMR CNRS 8523, Université de Lille, F-59655 Villeneuve d’Ascq Cedex, France ∥ ISA, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000, Aarhus C, Denmark ⊥ Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, U.K. S Supporting Information *

ABSTRACT: The electronic spectroscopy of isolated toluene in the gas phase has been investigated using high-resolution photoabsorption spectroscopy in the 4.0−10.8 eV energy range, with absolute crosssection measurements derived. We present the first set of ab initio calculations (vertical energies and oscillator strengths), which we use in the assignment of valence and Rydberg transitions of the toluene molecule. The spectrum reveals several new features not previously reported in the literature, with particular relevance to 7.989 and 8.958 eV, which are here tentatively assigned to the π*(17a′) ← σ(15a′) and 1π*(10a″) ← 1π(14a′) transitions, respectively. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of toluene in the upper stratosphere (20−50 km).

1. INTRODUCTION Toluene, C7H8, is a volatile organic compound (VOC) relevant within urban pollution, where fossil fuel, industrial and biomass burning are the common sources of emission.1,2 It is also of interest due to its carcinogenic and mutagenic effects on living organisms and the human health.3,4 Toluene also takes part in reactions promoting photochemical smog and other local atmospheric effects, which may contribute significantly to ozone formation in the troposphere.5 Trost et al.1 have measured the room-temperature rate coefficient for reactions of the OH radical with toluene (kOH = 5.96 × 10−12 cm3 molecule−1 s−1), showing that this is likely to be the main sink mechanism for these molecules in the troposphere. Night time reactivity of NO3 in urban areas with large concentrations of VOCs, may lead to oxidation of toluene, contributing therefore to the formation of HNO3, which is related to particulate formation and harmful health effects. Studies on NO3-initiated oxidation of toluene, at room temperature, have been reported by Atkinson et al.6 with a rate constant value of (6.75 ± 1.75) × 10−17 cm3 molecules−1 s−1. C7H8 has an estimated global warming potential (GWP) between 2.7−3.3 relative to CO2 on a 100-year time scale.2,7 The present work is part of a wider research program aimed at understanding the spectroscopy of volatile organic © 2015 American Chemical Society

compounds and the role of these trace gases in atmospheric chemistry and physics. Our knowledge of the VUV electronic state spectroscopy of toluene remains poorly quantified in a wide wavelength region. Indeed, experimental information on toluene is mainly restricted to 230 < λ < 290 nm (4.27 < E < 5.39 eV),1,8 with these studies including no assignments of the absorption bands. The data of Shaw et al.9 covers the energy region (8.55−11.81 eV) with a few assignments of Rydberg series converging to the B̃ 2A′ ionization threshold. Bolovinos and co-workers10 have reported lower valence-shell electronic transitions in the 4−7 eV energy region, whereas electron energy-loss spectra have been reported by Ari et al.11 and Yamamoto et al.12 Infrared and Raman spectra on liquid toluene and several deuterated derivatives have been obtained to provide a consistent set of vibrational assignments,13 while Krogh-Jespersen et al.14 reported on the multiphoton ionization spectrum of toluene in the 1B2 state and recently Gardner and co-workers15,16 have performed comprehensive spectroscopic studies employing resonance-enhanced multiphoton ionization (REMPI), zero kinetic energy (ZEKE) spectroscopy and timeReceived: May 28, 2015 Revised: August 3, 2015 Published: August 5, 2015 9059

DOI: 10.1021/acs.jpca.5b05080 J. Phys. Chem. A 2015, 119, 9059−9069

a

9060

fL

− 0.000910 0.002643 0.008436 0.000037 0.012872 0.000238 0.465263 0.030133 0.002299 0.477187 0.324926 0.008863 0.000937 0.008941 0.004756 0.067849 0.000009 0.000965 0.002909 0.037270 0.006234 0.002897 0.000014 0.006580 0.000318 0.003242 0.004029 0.000001 0.000283 0.020755 0.005089 0.000036 0.013783

E (eV)

− 5.078 6.275 6.319 6.519 6.792 6.858 7.017 7.037 7.158 7.174 7.232 7.410 7.486 7.550 7.636 7.680 7.759 7.872 8.044 8.095 8.120 8.159 8.261 8.270 8.291 8.338 8.365 8.410 8.419 8.657 8.692 8.884 8.964 92 94 137 99 139 161 158 110 157 163 119 126 172 176 135 156 135 177 154 148 151 178 105 95 152 96 93 150 165 194 165 170 186 99

⟨ r2⟩a

(a′) (a′) (a″) (a′)

4pσ (a′)

4sσ (a′)

4pσ (a″)

3dπ (a′) 3dπ (a″) σ*(CH) (a′)c

3pπ 3dσ 3dσ 3dσ

3pσ/σ*(CH) (a′) 3pσ/σ*(CH) (a″)

3sσ/σ*(CH) (a′)

HOMO 3π (16a′)

(a′) (a′) (a″) (a″)

4pσ (a′)

σ*(CH) (a′)c

3dπ (a″)

3dπ (a′)

3dσ 3pπ 3dσ 3dπ

3pσ/σ*(CH) (a″) 3pσ/σ*(CH) (a′)

3sσ/σ*(CH) (a′)

HOMO−1 2π (9a″)

HOMO−4 → LUMO

HOMO−2 → LUMO+1 HOMO−3 → LUMO+1

HOMO−2 → LUMO HOMO−3 → LUMO

other

HOMO−1 → LUMO+HOMO → 3pπ (a″)

HOMO → LUMO+HOMO−1 → 3pπ (a″)

HOMO−1 → LUMO+HOMO → LUMO+1

HOMO−1 → LUMO+HOMO → LUMO+1

mixed character

8.958

7.96(3) 8.07(7)

44.11

23.61 18.24

24.83

23.65

7.60(2)

7.989

23.65

223.15

223.15

22.18

1.02

cross section (Mb)

7.60(2)

6.786

6.786

6.05(7)

4.766

expt (eV)b

Mean value of r2 (electronic radial spatial extents). bthe last decimal of the energy value is given in brackets for these less-resolved features. cthis MO has some Rydberg character.

1 1A″ 2 1A′ 3 1A′ 2 1A″ 4 1A′ 3 1A″ 4 1A″ 5 1A′ 5 1A″ 6 1A′ 7 1A′ 8 1A′ 6 1A″ 9 1A′ 7 1A″ 8 1A″ 10 1A′ 9 1A″ 11 1A′ 10 1A″ 12 1A′ 13 1A′ 11 1A″ 12 1A″ 13 1A″ 14 1A′ 15 1A′ 14 1A″ 15 1A″ 16 1A′ 17 1A′ 16 1A″ 17 1A″

state X̃ 1A′

Table 1. Calculated Vertical Excitation Energies (EOM-CCSD Level at the aug-cc-pVTZ+R Basis Set) (eV) and Oscillator Strengths (Singlet States) of Toluene (C7H8) Staggered Conformation Compared with Experimental Data and Other Work (Energies in eV) (Details in Text)


The Journal of Physical Chemistry A Article


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The Journal of Physical Chemistry A


Table 2. Calculated Vertical Ionisation Energies (in eV) and Intensities (CCSD/(T)-cc-pVTZ geometry) of Toluene, C7H8 Compared with Previous Work 16a′−1

9a″−1

8a″−1

15a′−1

Koopmans theorema OVGFa pole strengtha P3a pole strengtha OVGF/ANOb pole strengthb OVGF/cc-pVTZc pole strengthc ΔSCF/TD-DFTd OVGFd SAC−CId

8.762 8.808 0.895 8.996 0.885 8.81 0.90 8.845 0.89 8.851 8.705 8.541

9.042 9.073 0.893 9.241 0.884 9.11 0.89 9.095 0.89 9.232 8.973 8.824

13.177 11.876 0.900 12.034 0.890 11.91 0.90 11.792 0.90 11.766 11.775 11.749

13.197 11.832 0.891 12.038 0.881 12.03 0.90 11.798 0.90 11.769 11.742 11.726

photoelectronb Penning ionizationc

8.845 8.89

9.38 9.28

11.240 11.42

− 11.83

14a′−1

7a″−1

Theoretical 13.216 14.645 12.017 13.559 0.825 0.899 12.033 13.499 0.815 0.892 12.14 13.59 0.83 0.90 12.063 13.387 0.83 0.90 11.948 13.071 12.006 13.494 12.036 13.388 Experimental 11.45 − 11.83 (13.2)

13a′−1

12a′−1

6a″−1

5a″−1

15.351 13.838 0.882 13.925 0.872 13.96 0.88 13.775 0.88 13.673 13.801 13.737

15.648 14.569 0.875 14.440 0.869 14.43 0.88 14.166 0.88 − − −

15.810 14.256 0.879 14.291 0.869 14.69 0.88 14.422 0.87 − − −

17.136 15.180 0.870 15.405 0.859 15.36 0.87 15.213 0.87 − − −

− 13.80

− 14.05

− 14.05

− 14.97

11a′−1 17.156 15.459 0.864 15.434 0.856 15.42 0.87 15.334 0.87 − − − − (15.5, 15.6)

10a′−1 18.774 16.877 0.849 16.872 0.840 16.93 0.85 16.828 0.85 − − − 16.420 16.40

a

This work. bReference 9, eclipsed, B3LYP geometry. cReference 31, eclipsed. dReference 32, 6-311G(d,p) basis set, C2v symmetry, CAM-BL3YP/6311G(d,p) geometry.

(b) valence orbitals (6a′)2 (7a′)2 (3a″)2 (8a′)2 (4a″)2 (9a′)2 (10a′)2 (11a′)2 (5a″)2 (6a″)2 (12a′)2 (13a′)2 (7a″)2 (14a′)2 (15a′)2 (8a″)2 (9a″)2 (16a′)2. The highest occupied molecular orbital (HOMO) 16a′ and the second highest occupied molecular orbital (HOMO−1) 9a″ have π character and may be labeled 3π and 2π, respectively, whereas the HOMO−2 (8a″) and HOMO−3 (15a′) have σ character. The lowest unoccupied molecular orbitals (LUMO, 10a″), (LUMO+1, 17a′), and the (LUMO+2, 18a′), are mainly of π* antibonding character (1π*, 2π* and 3π*, respectively). These are shown in the Supporting Information. The earlier studies of Shaw and co-workers9 have shown that the lowest Rydberg states may overlap with valence states resulting in a complex intensity distribution in the electronic spectrum. It is therefore necessary to separate features in the photoabsorption spectra due to Rydberg states from those arising from valence states. The former states may be identified through knowledge of the ionization states energies and the application of quantum defect theory. The three lowest ionization energies, which are needed to calculate the quantum defects associated with transitions to Rydberg orbitals, have been identified experimentally to be at 8.845, 9.38, and 11.24 eV.9

resolved slow-electron velocity-map imaging (tr-SEVI) techniques to probe the low energy states of toluene (X̃ 1A1 and Ã 1 B2)15,16 and the toluene cation (X̃ 2B1).15 Finally, we note comprehensive spectroscopic studies on toluene methyl rotor dependence of a three state Fermi resonance in S1 at ∼460 cm−117 and of the lowest 345 cm−1 of S0 toluene by laserinduced fluorescence,18 whereas Borst and Pratt19 reported on toluene structure, dynamics, and barrier to methyl group rotation in its electronically excited state. In this paper, we report the results of an extensive study of the electronic state spectroscopy of toluene by high resolution VUV photoabsorption spectroscopy and ab initio theoretical calculations of the vertical excitation energies and oscillator strengths for the neutral electronic transitions. The ionization energies for the lowest ionic states are also estimated using different levels of theory. In the next section, we provide a brief summary of the structure and properties of toluene. In addition to identifying the optical electronic transitions of toluene, the present work provides reliable photoabsorption cross sections in the range 4.0−10.8 eV. In section 3, we present the computational methods and in section 4 a brief discussion of the experimental details. Section 5 is devoted to the results and discussion with a comparison with other absolute photoabsorption cross sections. The absolute photoabsorption cross sections are used to calculate photolysis rates at 20−50 km altitude in the Earth’s atmosphere. Finally some conclusions that can be drawn from this study are given in section 6.

3. COMPUTATIONAL DETAILS Given that the calculations cannot deal with the methyl group rotation in the Born−Oppenheimer approximation, the results show that the staggered configuration is a minimum while the eclipsed is a saddle point. As so, ab initio calculations were performed for the staggered conformation only (for the eclipsed see Table S3 in Supporting Information) to determine the excitation energies of the neutral molecules (Table 1) and the ionization energies (Table 2) using the MOLPRO program.20 First, the ground state geometry was optimized at the MP2 and CCSD(T) levels using the Dunning’s cc-pVTZ basis set.21 It is well-known that the calculation of electronic spectra, even for small organic molecules, is a difficult task (see ref 22 for a recent review). The very fast TDDFT method is known to have problems in describing Rydberg states.23 Multireference methods such as CASPT2 or CASSCF/MRCI

2. BRIEF SUMMARY OF THE STRUCTURE AND PROPERTIES OF TOLUENE Geometry optimization revealed two almost degenerate stationary points of Cs symmetry, eclipsed and staggered, the latter 5 cm−1 more stable than the former (see Supporting Information). Harmonic frequencies calculations revealed that only the staggered configuration is a minimum, whereas the eclipsed a saddle point for the (free) methyl rotation. Toluene has symmetry Cs in the electronic ground state and the symmetry species available are A′ and A″. The calculated electron configuration of the X̃ 1A′ ground state is as follows: (a) core orbitals (1a′)2 (2a′)2 (1a″)2 (3a′)2 (2a″)2 (4a′)2 (5a′)2; 9061


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Figure 1. High resolution VUV photoabsorption spectrum of toluene, C7H8 in the 4.0−10.8 eV photon energy range.

Figure 2. High resolution VUV photoabsorption spectrum of toluene, C7H8, in the 4.4−5.5 eV photon energy range. See text for details on the assignments.

rev. D.01 package, using the cc-pVTZ basis sets and geometry.29

techniques become prohibitive when a large number of Rydberg orbitals have to be included in the active space. However, coupled cluster methods have recently been proven to produce reliable excitation spectra of benzene complexes, which is very similar to toluene.24 Thus, the toluene VUV spectrum was calculated at the EOM-CCSD (equation-ofmotion coupled cluster method restricted to single and double excitations) level.25 To this end, the basis set was extended to aug-cc-pVTZ for all atoms and set of diffuse functions (5s, 5p, 2d), taken from Kaufmann et al.26 and localized at the center of the carbon ring, was added for a better description of Rydberg states (aug-cc-pVTZ + R basis set). The oscillator strengths were obtained using the length gauge. The vertical ionization energies were also obtained at partial third order (P3) propagation and OVGF27 calculations28 using the Gaussian09

4. EXPERIMENTAL DETAILS 4.1. Toluene Sample. The liquid sample used in the VUV measurements was purchased from Sigma-Aldrich, with a stated purity of ≥99.5%. The sample was degassed by a repeated freeze−pump−thaw cycles. 4.2. VUV Photoabsorption. High-resolution VUV photoabsorption spectra of toluene were recorded using the UV1 beamline of the ASTRID synchrotron facility at Aarhus University, Denmark (Figure 1). The experimental apparatus has been described in detail elsewhere30 so only a short review will be given here. Briefly, synchrotron radiation passes through a static gas sample and a photomultiplier is used to measure the transmitted light intensity. The incident wavelength is selected 9062


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The Journal of Physical Chemistry A Table 3. Proposed Vibrational Assignments in the 4.4−5.5 and 5.5−6.5 eV Absorption Bands of Toluene, C7H8a this workb energy (eV)


4.537 4.552 4.572 4.584 4.615 4.628 4.645 4.650 4.673 4.689 − 4.697 4.707 4.716 4.73(8) (s) 4.744 4.759 4.766 4.769 4.798 4.806 4.821 − 4.833 − 4.860 4.88(3) (s) 4.886 4.916 − 4.938 − − 4.976 5.027 5.055 5.09(8) (b) 5.116 5.142 5.18(1)(w) 5.21(2) (b) 5.23(6) (b) 5.26(3) (b) 5.29(2) (b) 5.32(3) (w) 5.35(1) (w) 5.38(4) (w)

Hickman et al.33 assignment

1402 1001 14012001 1101 3001 1411 ? 000 1410 3010 − 14012010 1110/19102010 2910 1010/18101910 1010/18101910 29103010 810 910 610 2910(1010/18101910) 810(1110/19102010) − 9102910 − 6102910 810910 920820 9202910 − 620 − − 930 6201010 9302910 940 820920 9402910 9401010 950 ? 9502910 9501010 960 830930 9602910

energy (eV)

Bolovinos et al.10 assignment

1602 1201 16012501 1301 3801

4.547 4.550 4.572 4.582 4.606 − 4.644 4.647 − 4.688 − 4.693 4.704 4.713 4.738 4.740 4.754 4.763 4.767 4.795 4.804 4.819 − 4.832 − 4.861 4.883 4.886 − − − − − − − − − − − − − − − − − − −

− 000(0a1′ → 3a1″) 000(0a1′ → 0a1′ ) − 3810 − 16102510 1310/24102510 3710 1210/23102410 1210/23102410 37103810 1010 1110 810 3710(1210/23102410) 1010(1310/24102510) − 11103710 − 8103710 10101110 1120 − − − − − − − − − − − − − − − − − − −

energy (eV)

assignment

− − − − − − − 4.647 − − − − 4.704 4.712 4.740 − − 4.763/4767 − 4.794 4.806 4.819 4.828 4.833 4.851 4.860 4.882 − 4.914 4.921 4.935 4.944 4.948 4.998 5.030 − − − − − − − − − − − −

− − − − − − − 000 − − − − 1110 2910 1010 − − 910/810 − 610 29101010 2910910 − 1020 1110610 2910610/1010810 910810 − 810610/910610 29101010910 1110920 2910920 2910910810 920810 910810610 − − − − − − − − − − − −

a

Key: (s) shoulder feature; (w) weak feature;(b) broad structure (the last decimal of the energy value is given in brackets for these less-resolved features). ? feature remains unassigned. bVibrational modes notation adopted from ref 15.

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 0.01−1.08 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. Absolute photoabsorption cross sections are 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 9063


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Figure 3. High resolution VUV photoabsorption spectrum of toluene, C7H8, in the 5.5−6.5 eV photon energy range.

of the sample gas, σ the absolute photoabsorption cross section, and x the absorption path length (25 cm). 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.

bands centered at 4.766, 6.05(7) and 6.786 eV have been assigned to (π* ← π) transitions. The calculated wave functions show a mixing between excitations from the HOMO (3π) and HOMO−1 (2π*) to the 1π* and 2π* MO’s, and 3pπ (a″) Rydberg character for the latter. It seems that the calculated transition energies are overestimated by about 0.2 eV when compared to experiment. These assignments are in reasonable agreement with Bolovinos et al.10 reporting these bands at 4.763, 6.04, and 6.78 eV, respectively while Yamamoto et al.’s12 electron energy loss data place the first two bands at 4.8 and 6.7 eV, respectively. The first band is reported with a maximum absolute cross section of 1.02 Mb, whereas the second has a maximum of 22.18 Mb (Table 1) and the third at 223.15 Mb. The feature at 7.989 eV, with a rather weak calculated oscillator strength, fL ≈ 0.003, is here tentatively assigned for the first time to 2π*(17a′) ← σ(15a′) with the help of the theoretical calculations (Table 1). Finally, the VUV spectrum reveals another broad band at 8.958 eV which is also tentatively assigned for the first time to 1π*(10a″) ← 1π(14a′), (HOMO−4) (Table 1). Although we have identified an n = 3p member of a Rydberg series converging to the ionic electronic second excited state of toluene (see section 5.2), the rather high calculated intensity (fL ≈ 0.014) is primarily due to the pure valence π* character of the MO, as indicated by the typical low ⟨r2⟩ value of 99 au.2 Pure Rydberg transitions (Table 5 and 6) with high oscillator strengths in this energy range are discussed in section 5.2. Valence and Vibronic Excitation in the Range 4.4−5.5 eV. The lowest-lying excited state of toluene peaking at 4.766 eV with a local cross section value of 1.02 Mb, has been assigned to the (π* ← π) (1A″ ← 1A′) transition, and shows an extensive fine structure which is much better resolved than in previous absolute cross section measurements.1,10,12,33,34 Assignments for this energy band have been revisited and compared with the detailed data of Bolovinos et al.,10 and Hickman et al.,33 with several new features reported here for the first time (Table 3). The origin of the band has been identified at 4.647 eV from fluorescence excitation experiments on toluene (Table 3) from Hickman et al.33 Moreover, we note that the Bolovinos et al.10

5. ELECTRONIC STATE SPECTROSCOPY: RESULTS AND DISCUSSION The absolute high resolution VUV photoabsorption cross section of toluene is shown in Figure 1, extending from 4.0 to 10.8 eV and Figures 2−5 show expanded views in the 4.4−10.8 eV energy region. The major absorption bands can be classified as a mixture of Rydberg series and molecular valence transitions of (π* ← π) character. Table 1 compares the experimental results with the theoretical calculations and demonstrates reasonably good agreement. The calculations indicate that the electronic transitions for toluene have mixed valence-Rydberg character. The calculated vertical ionization energies (IEs) are presented in Table 2 and compared against Kishimoto et al.31 and Chrostowska et al.32 Generally speaking the P3, OVGF and Koopman’s theorem theoretical methods agree with each other with the exception of Koopman’s theorem above the second ionization energy. The measured lowest vertical IE of toluene (8.845 eV)9 agrees reasonably well with the three theoretical predictions. The lack of agreement of the results from Koopman’s theorem with the other methods is not surprising due to the improper electron correlation treatment. The P3 and OVGF results are very close to each other and previous studies, the differences coming from different geometry and basis sets. They reproduce quite well the experimental data from Shaw et al.9 and Kishimoto and co-workers.31 Because of several numbering schemes of the vibrational modes of toluene in the literature, we have followed the notation adopted in the studies of Gardner et al.15 and Gascooke et al.17,18 in the following sections. 5.1. Valence State Spectroscopy of Toluene. According to the calculations in Table 1 (for the eclipsed conformation see Table 3 in the Supporting Information), the absorption 9064


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absorption spectrum of toluene places the optically allowed transition at 4.647 eV in very good agreement with the present value at 4.650 eV. Of relevance the recent contribution from Borst and Pratt on the structure, dynamics, and barrier to methyl group rotation in its electronically excited state19 placing the first excitation band origin at 37476.56(2) cm−1 (4.6465 eV). The present high-resolution spectrum reveals that the fine structure is due to several modes (see Table 3 for the proposed detailed assignments), with the main contributions from C−C symmetric stretching mode, ν′9 (a′) and ring breathing mode, ν′10 (a′), respectively. These modes also appear coupled with other modes. Because of the complexity of such fine structure in the absorption band in Figure 2, 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. Of relevance that the features below the 000 transition are tentatively assigned to hot-bands based on the previous work of Hickman et al.33 The feature at 4.628 eV is tentatively assigned a sequence band, 1411, further to KrogJespersen et al.14 and Hickman et al.33 assignments. Finally, we note that Morrison and Laposa35 have identified from phosphorescence and fluorescence studies, in the 4.4−5.4 eV energy region, contribution from singlet to triplet excitation. Such band assignments would benefit from extensive electron energy loss spectroscopy studies regarding the nature of the underlying triplet states and the assignments. This is, however, beyond the scope of this contribution. Valence and Vibronic Excitation in the Range 5.5−7.0 eV. The lowest valence excitation band in this energy range (Figures 3 and 4) has been assigned to the (1π* ← 2π + 2π* ← 3π) transition in agreement with references.10,12 The (0−0) transition has been identified by Bolovinos et al.,10 at 5.83 eV, which is in very good agreement with the present value of 5.83(7) eV (see Table 4). The 900 cm−1 (0.112 eV) spacing reported by Bolovinos et al.10 is in very good agreement with the mean value of 0.116 eV (Figure 3 and Table 4) which has been assigned to excitation of ring deformation mode, ν′9 (a′). The next band with the highest oscillator strength ( f L ≈ 0.48),

Table 4. Proposed Vibrational Assignments in the 5.5−6.4 eV Absorption Band of Toluene, C7H8 energy (eV) 5.83(7) 5.95(5) 6.05(7) 6.17(8) 6.29(0)

(b)a (b) (b) (b) (b)

assignment

ΔE (eV)

000 910 − 910 920

− 0.118 − 0.119 0.112

a

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

peaking at 6.786 eV, has been assigned to (1π* ← 3π + 3pπ (a″) ← 2π). According to the calculations, the 3pπ (a″) ← 3π Rydberg transition has a very close energy and a large oscillator strength (0.32) and contributes to the feature observed in this energy range. A few quanta of C−C−C in-plane ring deformation mode, ν′29 (a′), are excited. This energy region exhibits evidence for transitions to Rydberg states at 6.29(0) and 6.786 eV (see section 5.2 and Table 6), converging to the ionic electronic ground and first excited states, respectively, and also shows some quanta excitation number of C−C−C in-plane ring deformation mode, ν′29 (a′) (Table 5) with mean excitation energy of 0.062 eV. 5.2. Rydberg Transitions. The VUV photoabsorption spectrum above 6.0 eV consists of a few structures superimposed on a diffuse absorption feature extending to the lowest IEs, which are here reported for the first time. The experimental ionization energy values of Shaw et al.9 in Table 6 are used to estimate the Rydberg series. The proposed Rydberg structures are labeled in Figures 3−5 and listed in Table 5. The peak positions have been tested using the Rydberg formula: En = Ei − R/(n − δ)2, where Ei is the ionization energy, n is the principal quantum number of the Rydberg orbital of energy En, R is the Rydberg constant (13.61 eV), and δ the quantum defect resulting from the penetration of the Rydberg orbital into the core. The experimental values for the lowest terms of the ns, np, and nd (n = 3) Rydberg series (Table 6) are in good agreement with the calculations in Table 1. 9065


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The Journal of Physical Chemistry A Table 5. Proposed Vibrational Assignments in the 6.4−8.8 eV Absorption Band of Toluene, C7H8a ΔE (eV)

energy (eV)

6.67(6) (s) 6.73(8) (s) 6.786 6.83(9) (s)

− 2910 2920 2930

− 0.062 0.048 0.053

7.16(7) (s) 7.20(8) (s)

3d 3d + 2910

− 0.041

8.958 9.014 9.090 9.164 9.211 9.287 9.37(1) (b)

3p 3p 3p 3p 3p 3p 3p

7.41(1) 7.45(5) 7.52(3) 7.58(3)

4s 4s 4s 4s

2910 2920 2930 2940

− 0.044 0.068 0.060

9.47(9) (b) 9.54(5) (b)

3d 3d + 2910

− 0.066

7.96(3) (b) 8.020

4s 4s + 2910

− 0.057

9.84(8) (b,s) 9.92(7) (b) 9.99(8) (w) 10.08(0) (s)

4s 4s + 2910 4s + 2920 4s + 2930

− 0.079 0.071 0.082

8.287 8.33(8) (b)

5d 5d + 2910

− 0.051

8.440 8.49(8) (b)

6d 6d + 2910

− 0.058

10.129 10.196 10.27(2) (b) 10.332

4p 4p + 2910 4p + 2920/4d 4p + 2930/4d + 2910

− 0.067 0.076 0.060

8.54(5) (b) 8.604 8.65(8) (b)

5s 5s + 2910 5s + 2920/5p + 2910

− 0.059 0.054

10.39(3) (b) 10.45(4) (b)

5s 5s + 2910

− 0.061

10.59(7) (s) 10.67(9) (b)

6s 6s + 2910

− 0.082

8.750 8.818 8.88(1) (s)

2910 2920 2930

− 0.068 0.063


energy (eV)

(b) (b) (w) (w)

assignment

+ + + +

ΔE (eV)

assignment + + + + + +

2910/7s 2920/7s + 2910 2930/ 7s + 2920 2940/7s + 2930 2950/7s + 2940 2960/7s + 2950

− 0.056 0.076 0.074 0.047 0.076 0.084

a

Key: (s) shoulder structure; (w) weak structure; (b) broad structure (the last decimal of the energy value is given in brackets for these less-resolved features).

Table 6. Energy Value (eV), Quantum Defect (δ) and Assignment of the Rydberg Series Converging to the Ionic Electronic Ground (4a″−1), First (3a″−1) and Second (14a′−1) Excited States of Toluene, C7H8a En IE1 = 8.845 eV − 7.41(1) (b) 8.01(4) (b) 8.33(2) (w) 8.49(8) (w)

δ

En

assignment

− 0.92 0.95 0.85 0.74

3s 4s 5s 6s 7s

6.29(0) (b) 7.60(2) (w) 8.07(7) (b) 8.37(7) (w) 7.16(7) (s) 7.91(2) (b) 8.287 8.440 IE2 = 9.38 eV

0.69 0.69 0.79 0.61 0.15 0.18 0.06 0.20

3p 4p 5p 6p 3d 4d 5d 6d

− 7.96(3) (b) 8.54(5) (b) − 9.014

− 0.90 0.96 − 0.90

3s 4s 5s 6s 7s

IE2 = 9.38 eV 6.786 8.07(7) (w) 8.62(2) (b)

δ

assignment

0.71 0.77 0.76

3p 4p 5p

0.23 0.19 0.15 −0.10

3d 4d 5d 6d

IE3 = 11.24 eV 7.912 9.84(8) (b,s) 10.39(3) (b) 10.67(9) (b) 8.958 10.129 10.57(9) (s)

0.98 0.87 0.99 1.07 0.56 0.50 0.46

3s 4s 5s 6s 3p 4p 5p

9.47(9) (b) 10.27(2) (b) 10.66(0) (s)

0.22 0.25 0.16

3d 4d 5d

7.60(2) (w) 8.44(0) 8.801 9.014

a

Key: (b) broad structure. (w) weak structure. (s) shoulder structure (the last decimal of the energy value is given in brackets for these less-resolved features). 9066


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∼7.0 eV, may be related to low-lying predissociative or dissociative excited neutral states. 5.3. Absolute Photoabsorption Cross Sections and Atmospheric Photolysis. The present optical measurements were carried out in the pressure range 0.01−1.08 mbar and reveal no evidence for changes in absolute cross sections or peak energies as a function of pressure, thus we believe the present spectra are free of any saturation effects. Previous absolute VUV photoabsorption cross sections of toluene are available in the wavelength ranges 290−230 nm (4.27−5.39 eV).1,8 Trost et al.1 and Etzkorn et al.8 reported absolute cross sections of magnitude 1.4 Mb (267 nm, 4.64 eV) and 1.28 Mb (266.6 nm, 4.65 eV), respectively, compared to the present value of 1.59 Mb at 4.650 eV. Such differences are mainly attributable to experimental resolution, where Trost et al.1 and Etzkorn et al.8 claim 0.11 and 0.15 nm, against the present value of 0.075 nm. Fally et al.’s34 UV Fourier transform absorption cross sections of toluene at room temperature, are in good agreement with the magnitude of the values obtained in the 4.4−5.1 eV energy region. The agreement of previous cross sections measured on the UV1 beamline on ASTRID with the most precise data available in the literature (see Eden et al.36 and references therein) indicate that the present toluene cross sections can be relied upon across the energy range studied (4.0−10.8 eV). The present absolute cross sections below 6.89 eV (above 180 nm) can be used in combination with solar actinic flux37 measurements from the literature to estimate the photolysis rate of toluene in the atmosphere from an altitude close to the ground to the stratopause at 50 km. Details of the calculation program are presented in ref 38 in which the quantum yield for dissociation following absorption is assumed to be unity. The reciprocal of the photolysis rate at a given altitude corresponds to the local photolysis lifetime. Photolysis lifetimes of less than 100 sunlit days were calculated at altitudes above 15 km with less than a day above 24 km. This shows that toluene molecules can be broken up quite efficiently by UV absorption at these altitudes (>24 km). From the ground level up to the tropopause the lifetimes are considerably high, such that photolysis is an unlikely sink mechanism. However, the rate

The lowest transition energy is tentatively assigned to 4s at 7.41(1) eV for the Rydberg series converging to the ionic electronic ground state, with a quantum defect δ = 0.92 (Table 6), accompanied by vibronic structure, which is proposed to be mainly due to excitation of the C−C−C in-plane ring deformation mode, ν′29 (a′), with an average value of 0.057 eV (Figure 4 and Table 5), in very good agreement with 0.060 eV for the X̃ 2A″ state from Shaw et al.9 The higher members of this Rydberg series are proposed to extend to n = 7. The first members of the np and nd series are associated with the peaks at 6.29(0) eV (δ = 0.69) and 7.16(7) eV (δ = 0.15), respectively (Table 6). The nd series shows an n = 3 vibrational excitation with one quanta of C−C−C in-plane ring deformation mode, ν′29 (a′). The higher members of these Rydberg series, for which the relative intensity decreases, are difficult to assign due to overlap with other transitions and possible vibronic structure. Close to the lowest ionic limit, weak features tentatively assigned to C−C−C in-plane ring deformation mode appear at 8.750, 8.818 and 8.88(1) eV. The first members of the ns, np, and nd series converging to the ionic electronic first excited state of toluene (3a″−1) are associated with the peaks at 7.96(3) (δ = 0.90), 6.786 (δ = 0.71), and 7.60(2) eV (δ = 0.23), respectively (Table 6). Rydberg transitions (Figures 4 and 5) are accompanied by vibronic structure which is tentatively attributed to excitation of the ν′29 (a′) mode (Table 5), however, the normal mode configuration may lead to Fermi resonances. As far as members of the Rydberg series converging to the ionic electronic second excited state are concerned, n = 3 for ns, np and nd are proposed at 7.912, 8.958, and 9.47(9) eV, respectively, (Table 6) with quantum defects 0.98, 0.56, and 0.22, respectively. Some of the fine structure has been assigned to vibrational excitation involving C−C−C in-plane ring deformation mode, ν′29 (a′), in good agreement with the previous assignments of Shaw et al.,9 although we were able to assign new features (see Tables 5 and 6 and Figure 5). We have not made any attempt to identify higher members (n > 7) of the Rydberg series, due to the broad and structureless nature of the absorption bands. Note that the clear increase in the absorption with energy, in the range above 9067


Article


1832/2012 and UID/FIS/00068/2013. P.L.-V. acknowledges his visiting Professor position at The Open University, U.K., and together with N.J.M. the support from the British Council for the Portuguese-English joint collaboration. The authors wish to acknowledge the beam time at the ISA synchrotron 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-CT2004-506008, under the Research Infrastructure Action of the FP6 EC programme Structuring the European Research Area. D.D. acknowledges support from the CaPPA project (Chemical and Physical Properties of the Atmosphere), funded by the French National Research Agency (ANR) through the PIA (Programme d’Investissement d’Avenir) under Contract ANR10-LABX-005. This work was performed using HPC resources from GENCI-CINES (Grant 2015-088620). The Centre de Ressources Informatiques (CRI) of the Université of Lille also provided computing time.


coefficients obtained for reactions between the OH radical and toluene1 have shown that this may provide a main reactive sink mechanism in the troposphere. Moreover, secondary organic aerosol formation from the oxidation of toluene by chlorine atoms has been reported by Cai et al.,39 with a rate constant of 6.2 × 10−11 cm3 molecules−1 s−1, where such process can be as important as hydroxyl-radical-initiated oxidation in the early morning in certain coastal or industrialized areas. Compared with radical reactions, UV photolysis is not expected to play a significant role in the tropospheric removal of these molecules. This is in good agreement with Collins et al.6 reporting that oxidation by hydroxyl radicals is the main removal process for organic compounds in the troposphere.

6. CONCLUSIONS The present work provides the first complete study to date of the VUV electronic spectroscopy of toluene and provides the most reliable set of absolute photoabsorption cross sections between 4.0 to 10.8 eV. The observed low energy structure has been assigned to valence and Rydberg transitions on the basis of comparisons with the ab initio calculations of vertical excitation energies and oscillator strengths for this molecule. The theoretical results are in good agreement with the experiments, predicting significant mixing of Rydberg and π* states, and allowed the assignment of new features not previously reported in the literature. The states at 7.989 and 8.958 eV, which are tentatively assigned for the first time to 2π*(17a′) ← σ(15a′) and 1π*(10a″) ← 1π(14a′) transitions, respectively. The analysis of the observed vibronic structure in the photoabsorption spectra is generally consistent with earlier data, although the higher resolution of the present experiments has enabled us to propose assignments for Rydberg series strongly overlapping with several progressions within the absorption bands. The photolysis lifetimes of toluene have also been carefully derived for the Earth’s troposphere and stratosphere and show that solar photolysis is expected to be a weak sink in the terrestrial atmosphere.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b05080. Toluene eclipsed and staggered geometries at the MP2 and CCSD(T)/cc-pVTZ levels of calculation; molecular orbitals calculated vertical excitation energies (EOMCCSD level at the aug-cc-pVTZ+R basis set) (eV), and oscillator strengths (singlet states) of toluene (C7H8) eclipsed conformation compared with experimental data (energies in eV) (PDF)

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REFERENCES

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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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ACKNOWLEDGMENTS F.F.d.S. and P.L.-V. acknowledge the Portuguese National Funding Agency FCT-MEC through researcher and sabbatical grants, IF-FCT IF/00380/2014 and SFRH/BSAB/105792/ 2014, as well as from the research grants PTDC/FIS-ATO/ 9068


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Toluene Valence and Rydberg Excitations as Studied by ab initio

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