Probing the Lowest-Lying Electronic States of Acrylic Acid by

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Probing the Lowest-Lying Electronic States of Acrylic Acid by Experimental and Theoretical Methods Mónica Mendes, Alessandra Souza Barbosa, Filipe Ferreira da Silva, Nykola C. Jones, Søren Vrønning Hoffmann, Gustavo Garcia, Marcio H. F. Bettega, and Paulo Limao-Vieira J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b06626 • Publication Date (Web): 19 Sep 2018 Downloaded from http://pubs.acs.org on September 20, 2018

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The Journal of Physical Chemistry

Probing the Lowest-Lying Electronic States of Acrylic Acid by Experimental and Theoretical Methods M. Mendes,1,2 A. S. Barbosa,3 F. Ferreira da Silva,1 N. C. Jones,4 S. V. Hoffmann,4 G. García,2 M. H. F. Bettega3 and P. Limão-Vieira1,* 1

Atomic and Molecular Collisions Laboratory, CEFITEC, Department of Physics, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal

2

Instituto de Física Fundamental, Consejo Superior de Investigaciones Científicas (CSIC), Serrano 113-bis, 28006 Madrid, Spain

3

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

4

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

ABSTRACT We report a combined experimental and theoretical study of the electronic state spectroscopy of acrylic acid (C3H4O2) in the gas phase, by high-resolution vacuum ultraviolet (VUV) photoabsorption measurements in the 4.0-10.8 eV energy range, together with ab initio calculations (vertical energies and oscillator strengths), which were used in the assignment of the valence transitions. We also discuss the Rydberg transitions for this molecular target, obtained using the experimental ionisation energies available in the literature. The experimental spectrum presented in this paper represents the highest resolution data yet reported for acrylic acid and reveals new features not previously reported in the literature. The dominant transitions have been assigned to (π*(4a″) ← π(3a″)) and (π*(4a″) ← π(2a″)), the latter exhibiting excitation of the ߥହᇱ ሺܽᇱ ሻ C=O stretching mode with mean energy of 0.155 eV. The measured absolute photoabsorption cross-sections have been used to calculate the photolysis lifetime of acrylic acid in the upper stratosphere (20–50 km).

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1. INTRODUCTION Acrylic acid (2-propenoic acid, CH2=CHCOOH) (Figure 1), which is the simplest unsaturated carboxylic acid, shows interesting spectroscopic and photochemical properties related to its role in atmospheric and astronomical environments, although only a few publications are found in the literature. In the last few years, the international scientific community has been devoting relevant attention to the role of complex organic molecules in the interstellar medium (IM) and in circumstellar envelopes. This has attracted considerable interest in the field of astrochemistry, because of their possible implications in the origin of life.1 Acrylic compounds (such as acrylonitrile2) have been observed in the Orion KL nebula3 and towards the Sagittarius B2(N) molecular nebula.3,4 Thus, acrylonitrile and other acrylic compounds may serve as prototype molecules for chain elongation reactions leading to larger molecular targets that have been detected in the IM and other astronomic environments.5 Recently, Calabrese et al.5 reported that although acrylic acid has not been observed in the IM, “given the abundance of oxygencontaining compounds in different interstellar sources, a search for this molecule seems promising”. Additionally, these authors have pointed out that such molecules could be formed through reactions involving methane and carbonic acid present in interstellar ices, similar to the formation of acetic acid.5,6 Therefore, the investigation of acrylic acid appears to be extremely relevant for its detection in astronomical observations,7 as well as other derivatives such as methyl acrylate. Katrib and Rabalais8 He(I) photoelectron spectroscopy and ab initio calculations have assigned the vibrational structure the first, second and third ionic bands of acrylic acid. Resonances and inductive effects were observed between the C=C π molecular orbital and the n and/or π orbitals of the COOH group. In a very recent work Shemesh and Gerber9 present a description of the reaction pathways for photochemistry dynamics of acrylic acid through two different theoretical methods – OM2/MRCI and the ab initio ADC(2) – to describe the first ππ* excited state. In this work the relevance of the photo-induced decomposition mechanisms of acrylic acid, especially in the production of OH radicals which are highly reactive and contribute to the formation of ozone and photochemical smog, is reported.9,10 However, we note that a comprehensive description of the low-lying electronic and ionic states is limited to very old publications from 195111 and 1976,12 so detailed investigations combining high-resolution vacuum ultraviolet (VUV) spectroscopy with full support of theoretical calculations are needed for acrylic acid. This information is relevant to assess the role of certain electronic states participating in key reaction mechanisms. In the present paper, 2 ACS Paragon Plus Environment

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we report an extensive study of VUV photoabsorption measurements combined with ab initio theoretical calculations of the vertical excitation energies and oscillator strengths to interpret the electronic state spectroscopy of C3H4O2 below the lowest ionisation energy. The outline of this paper is as follows, in the next section we discuss the experimental method, while in section 3 we present some computational details of the calculations. Section 4 is devoted to the presentation and discussion of the valence and Rydberg transitions and Section 5 compares the present data with other absolute photoabsorption cross-sections and the photolysis rates of C3H4O2 are calculated from 20–50 km altitude in the Earth’s atmosphere. Finally in Section 6 there is a brief summary and conclusions.

2. EXPERIMENTAL METHODS 2.1. Acrylic Acid sample The liquid sample used in the VUV photoabsorption measurements was purchased from Sigma-Aldrich, with a stated purity of ≥ 99%. The sample was degassed by a repeated freeze– pump–thaw cycles and used without any further purification or treatment.

2.2. VUV photoabsorption The high-resolution VUV photoabsorption spectrum of acrylic acid (Figure 2) was recorded at the AU-UV beam line of the ASTRID2 synchrotron facility at Aarhus University, Denmark. The experimental apparatus has been described in detail previously,13 with some recent modifications reported in detail in Ref.14 Briefly, synchrotron radiation passes through a static gas sample and the transmitted light intensity is detected by a photomultiplier tube (PMT). 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), the small gap between the PMT and the exit window (MgF2) of the gas cell is evacuated using a scroll pump to prevent any absorption by O2 in the air contributing to the spectrum. Measurement at higher wavelengths are carried out with this section at atmosphere so that O2 absorbs any higher order light produced by the monochromator. The acrylic acid sample absolute pressure in the absorption cell is measured by a capacitance manometer (Chell CDG100D). In order to guarantee the absence of any saturation effects in the data recorded, the absorption crosssections were measured over the pressure range 0.02–0.15 mbar, with typical attenuations of less than 50%. The synchrotron beam ring current was monitored throughout the collection of each spectrum and background scans were recorded with the cell evacuated. 3 ACS Paragon Plus Environment


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Absolute photoabsorption cross-sections, σ, (in units of megabarn, 1 Mb ≡ 10-18 cm2) were then obtained using the Beer-Lambert attenuation law: It = I0 exp (-Nσl), where It is the light intensity transmitted through the gas sample, I0 is that transmitted through the evacuated cell, N the molecular number density of the sample gas, and l the absorption path length (15.5 cm). In order to accurately determine cross-sections, the VUV spectrum was recorded in small (5 or 10 nm) sections, with at least 10 points overlap of 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.

3. THEORETICAL METHODS The calculations of the excited electronic states were carried out in the ground state optimized geometry of the acrylic acid molecule, employing time-dependent density functional theory (TD-DFT)15,16 with the PBE017 functional and Dunning’s augmented correlation consistent valence double zeta basis set (PBE0/aug-cc-pVDZ)17 as implemented in the package GAMESS-US.18 The combination of the PBE0 functional with the aug-cc-pVDZ basis set provides accurate low-lying valence and Rydberg excitations, as reported previously in the literature.19 We also performed calculations of the excited electronic states employing two other functionals. These results are tabulated in the Supporting Information (Tables S3 and S4). In order to optimize the electronic ground state geometries, we employed density functional theory (DFT) with the same functional and basis set employed in the excited electronic states calculations. From the optimization of the ground state structure we found four conformers from which the two lowest energy conformers are both belonging to the Cs symmetry group (s-cis and s-trans, depending on the rotation of the carboxyl group). Both conformers are schematically represented in Figure 1. The relative energy of the conformers is 0.013 eV (1.25 kJ mol−1), where s-cis is the most stable conformer and at room temperature the calculated population for the monomers is 62% for s-cis and 38% for s-trans, in agreement with previous results.20 For further explanation about the two high energy conformers, s-cis/anti and s-trans/anti predicted to be 54.9 and 50.6 kJ mol−1 (49.6 and 45.8 kJ mol−1 including the zero-point correction) from the most stable forms, see a comprehensive description in Ref. [21]. The calculated optimized geometries, electron configuration and total energies are presented in the Supporting Information (SI) for both conformers. Although in the gas phase carboxylic acids may be present as a mixture of monomers and dimers, it has been shown that the dimer concentration is very low in the pressure range studied (< 4%).21 4 ACS Paragon Plus Environment

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As such, we show only the calculated results for the acrylic acid monomers, while the results for the dimers are presented in the SI. The outermost molecular orbitals for acrylic acid ground state (ܺ෨ 1A′) conformer s-cis is … (1a″)2 (13a′)2 (14a′)2 (15a′)2 (2a″)2 (3a″)2 (16a′)2 and for s-trans is … (13a′)2 (1a″)2 (14a′)2 (15a′)2 (2a″)2 (3a″)2 (16a′)2. The most active occupied and virtual orbitals in the excited states of acrylic acid for both monomer conformers are also presented in the SI.

4. RESULTS AND DISCUSSION The high-resolution VUV photoabsorption spectrum of acrylic acid, measured at room temperature in the photon energy range 4.0–10.8 eV, is shown in Figure 2, where the main absorption bands (I–V) can be classified as members of Rydberg series and molecular valence transitions of the type (π* ← nO) and (π* ← π). The assigned character of these transitions for the monomeric conformers of acrylic acid is further supported by TD-DFT calculations, as summarised in Table 1, and discussed in the next sections. Note that the calculations are in good agreement with the experimental findings within 0.4 eV, which is certainly expected for the level of accuracy used. Similar results for the dimeric conformers of acrylic acid are shown in the SI (Table S2). In Table 2 we present a comprehensive vibrational assignment of the fine structure in the absorption bands of acrylic acid, while in Table 3 we propose assignments of the Rydberg series converging to the ionic electronic ground, first and second excited states, principal quantum numbers (n) and quantum defects (δ). The three lowest experimental vertical ionisation energies (IEs) of 10.78, 10.95 and 12.00 eV [21] were used to estimate the Rydberg series using the well-known Rydberg formula, En = IE − R/(n − δ)2, with R the Rydberg constant (13.61 eV). Assignments in the VUV spectrum for higher members of the Rydberg series, where n ≥ 4 members are expected to lie, is rather complex due to the presence of other valence and Rydberg transitions, and no special attempts have been made for the series converging to IE2 and IE3. Therefore, with quantum defect calculations as our only guide, we cannot propose assignments for these bands with confidence, so values for higher n Rydberg states in Table 3 are tentative assignments. The identification of Rydberg states was based more firmly on the symmetry and shape of the mono-occupied orbitals and the values of the oscillator strengths for the lowest values of the principal quantum number, n.

Valence and Rydberg transitions A. The energy range 4.0-5.6 eV (band I)



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The VUV photoabsorption experimental results show a first band at 5.22(7) eV (~240 nm) with a local maximum cross-section of 0.3 Mb. The TD-DFT calculations identified this band as a transition from the oxygen non-bonding nO lone pair electrons of the carboxyl group to the first π antibonding MO, (π* ← nO) (4a" ← 16a′), at 4.779 eV and 4.728 eV for the s-cis and s-trans monomers, respectively (Table 1), while for the dimers an estimated energy value of 5.105 and 5.067 eV, for cis-cis and trans-trans, respectively have been obtained (Table S2). Previous VUV absorption data from Morita et al.12 report this transition at ~5.0 eV for the monomer, and at ~5.1 eV for the dimer, in good agreement with the present results.

B. The energy range 5.6-7.3 eV (band II) The most intense absorption band is centred at 6.637 eV, with its maximum absolute crosssection value of 43.4 Mb assigned to the (π* ← π) (4a" ← 3a") transition. The electronic transition is calculated at 6.259 and 6.472 eV, for the s-cis and s-trans conformers, with oscillator strengths of 0.2368 and 0.3923, respectively (Table 1) and at 6.297 and 6.412 eV for the cis-cis and trans-trans conformers (Table S2). This absorption band has been previously reported12 at 6.71 eV for the monomer and at 6.51 eV for the dimer. The calculated HOMO-1 (3a″) is predominantly located in the C=C bond and promotion to the LUMO (4a″) is highly favourable (Figure 3), resulting in the highest calculated oscillator strength. Moreover, the HOMO-1 also shows some π(C=O) character consistent with the photodissociation studies of Kitchen and co-workers,22 where due to the large electron spin density of the C=C bond, intramolecular charge transfer may occur from the double bond to the COOH group. Another relevant aspect of this absorption band pertains to the role of the underlying contribution that may lead to dissociation. The studies of Kitchen and coworkers,22 are not conclusive enough to confirm the correlation between the initially prepared singlet state (ππ*, S2) and the low-lying excited singlet and triplet states as well as the ground electronic state. Electron energy loss spectroscopy data under dipolar allowed and dipolar forbidden transitions would certainly help clarifying the role of such intermediate states.

C. The energy range 7.3-8.2 eV (band III) The absorption in this energy range has a maximum at 7.672 eV with a cross-section value of 28.9 Mb. The 0଴଴ origin band is observed at 7.514 eV (28.8 Mb) and in reasonable agreement with the value of 7.46 eV reported by Morita et al.12 (see Table 2). TD-DFT calculations assign this band to a transition of mixed Rydberg (3s ← nO) (17a′ ←16a′) and valence (π∗ ← π) (4a″ ← 2a″) character at 7.200 eV and 7.201 eV with oscillator strengths 6 ACS Paragon Plus Environment

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0.1111 and 0.0879 for s-cis and s-trans monomers, respectively (see Table 1), while for the dimers estimated energy values of 6.954 and 6.907 eV, for cis-cis and trans-trans, respectively have been obtained (Table S2). This absorption band also exhibits some fine structure which is tentatively assigned to the contribution of the C=O stretching mode, ߥହᇱ ሺܽᇱ ሻ, with an average value of (0.155 ± 0.004) eV (1250 cm-1). This assignment is in agreement with the Katrib and Rabalais8 photoelectron spectrum showing a vibrational frequency of 0.164 eV (1320 cm-1) in the lowest ionic state (10.77 eV band) and the infrared spectrum of Charles et al.23 which reports a value of 0.216 eV (1740 cm-1) for s-cis conformer (0.218 eV, 1752 cm-1 for s-trans) in the ground state. The feature at 7.514 eV is also proposed to belong to the lowest Rydberg transition (3s) converging to the ionic electronic ground state (IE1 = 10.78 eV) and to the ionic electronic first excited state (IE2 = 10.95 eV), with quantum defects δ = 0.96 and δ = 1.00, respectively (see Table 3).

D. The energy range above 8.2 eV (bands IV and V) The energy region above 8.2 eV shows a quite rich fine structure due to vibrational excitation (Table 2) and a close inspection of the spectrum reveals the presence of three electronic transitions clearly visible from the different slopes of the photoabsorption features in the 8.2 – 9.0 eV and 9.0 – 10.8 eV photon energy range. From the calculations in Table 1 these electronic transitions are Rydberg in character. The feature at 8.557 eV with a crosssection of 16.3 Mb, and calculated at 8.945 eV and 9.004 eV with oscillator strengths of 0.0465 and 0.0256 for the s-cis and s-trans conformers, respectively, is here assigned to (3p ← π) (5a″ ← 3a″) and to (π* ← π) (6a″ ← 3a″) (see Table 1). The calculated energy values for the dimers are 8.901 and 8.903 eV, for cis-cis and trans-trans, respectively have been obtained (Table S2). The 0଴଴ transition assigned at 8.21(6) eV is accompanied by a weak vibrational progression up to three quanta of the C=O stretching mode, ߥହᇱ ሺܽᇱ ሻ , with an average value of (0.164 ± 0.003) eV (≈ 1320 cm-1). This feature is also associated with the first member of an np Rydberg series converging to the ionic electronic ground state (IE1) (n = 3, δ = 0.70) and the ionic electronic first excited state (IE2) (n = 3, δ = 0.77) (Table 3). The feature at 8.545 eV is assigned to 5ଷ଴ and to the lowest member of the ns Rydberg series converging to the ionic electronic second excited state (IE3 = 12.00 eV) (n = 3, δ = 1.01). Additionally, this transition may also be assigned to a combination of C=C stretching mode, ᇱ ሺ ᇱሻ ߥ଺ᇱ ሺܽᇱ ሻ, and C‒C‒O bending mode, ߥଵଷ ܽ , (Table 2), with values in the ground state for s-

cis/s-trans of (0.203 eV (1637 cm-1)/0.202 eV (1625 cm-1)) and (0.079 eV (637 cm-1)/0.072 7 ACS Paragon Plus Environment


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eV (584 cm-1)), respectively.8,23,24 The data of Katrib and Rabalais8 reports a 580 cm-1 (0.072 ᇱ ሺ ᇱሻ eV) progression in the third photoelectron band related to ߥଵଷ ܽ . Another interesting aspect

related to the vibrational assignment pertains to the difficult task of clearly identifying closely ᇱ ሺ ᇳሻ spaced vibronic interactions as C–O torsion mode, ߥଵଽ ܽ with a value of 0.077 eV (617 cm1

) and 0.071 eV (576 cm-1) for s-cis/s-trans ground-state acrylic acid, associated with singlet

excited states and to the presence of strong Fermi resonances. The next absorption band at 9.421 eV with a cross-section value of 31.4 Mb (Figure 1) is predicted at 9.139 eV and 9.618 eV for s-cis and s-trans conformers with a calculated oscillator strengths of 0.066 and 0.0479, respectively. Calculated energies of 9.065 and 9.047 eV, for cis-cis and trans-trans dimers, respectively have been obtained (Table S2). This absorption band is assigned to a transition of mixed Rydberg (3d ← nO) (21a′ ← 16a′) and valence (π* ← π) (6a″ ← 3a″) character (Table 1) and is composed of vibrational structure which is tentatively attributed to excitation of the C=O stretching mode, ߥହᇱ ሺܽᇱ ሻ, and C=C stretching mode, ߥ଺ᇱ ሺܽᇱ ሻ, (Table 2). The 0଴଴ origin band lying at 8.991 eV shows a progression of three quanta of C=O stretching mode, ߥହᇱ ሺܽᇱ ሻ, with an average value of (0.159 ± 0.011 eV) (≈ 1280 cm-1). The feature at 9.414 eV is assigned to a combination of the C=C stretching ᇱ ሺܽᇱ ሻ. mode, ߥ଺ᇱ and the C‒C‒O bending mode, ߥଵଷ Excitation of these modes is also observed

in the higher photon energy region. The features at 9.32(2) eV and 9.626 eV are related to n = 4 members of Rydberg series converging to the ionic electronic ground state at 10.78 eV (IE1), and assigned to 4s with δ = 0.94 and 4p with δ = 0.57, respectively. The feature at 9.414 eV is assigned to 4s with a quantum defect δ = 1.02, converging to the ionic electronic first excited state (IE2). A shoulder at 9.47(9) eV is tentatively assigned to a 3p term of a series converging to the ionic electronic second excited state, with a quantum defect δ = 0.68 (Table 3). The feature at 9.801 eV is tentatively assigned to the 0଴଴ origin and lies on an absorption band with a different slope from the previous band. At least five quanta of C=O stretching mode ߥହᇱ ሺܽᇱ ሻ are visible although combination with the C=C stretching mode, ߥ଺ᇱ ሺܽᇱ ሻ and the C‒C‒O ᇱ ሺ ᇱሻ bending mode, ߥଵଷ ܽ are also possible (Table 2). Moreover, the absorption band also ᇱ ሺ ᇱሻ exhibits progressions of C=O stretching mode, ߥହᇱ ሺܽᇱ ሻ together with ߥ଺ᇱ ሺܽᇱ ሻ andߥଵଷ ܽ . Some

of the peaks are associated with Rydberg series converging to IE1, IE2 and IE3 (see Table 3). The first transition lying at 9.801 eV is also associated with n = 4 term of a nd Rydberg series converging to the ionic electronic ground state (10.78 eV) with a quantum defect δ = 0.27. The feature at 9.164 eV is tentatively assigned to a 3d Rydberg member converging to ionic electronic first excited state with a quantum defect of 0.24. The peak at 10.146 eV 8 ACS Paragon Plus Environment

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ᇱ ሺ ᇱሻ ܽ vibrational excitation modes and may be accommodates a combination of ߥ଺ᇱ ሺܽᇱ ሻ + ߥଵଷ

also present in other Rydberg series (5d, δ = 0.37, IE1; 5s, δ = 0.89, IE2; 3d, δ = 0.29, IE3). The peak at 10.03(1) eV is assigned to 5p, with a calculated quantum defect δ = 0.74, converging to the ionic electronic ground state. We note that this quantum defect is slightly higher than for the other members of the same series and that can be related to the broadness and relative low intensity of this feature which may also accommodate the 4d term (δ = 0.15) converging to the ionic electronic first excited state (Table 3).

5. ABSOLUTE PHOTOABSORPTION CROSS-SECTIONS AND ATMOSPHERIC PHOTOLYSIS Previous absolute VUV photoabsorption cross-sections of acrylic acid are only available in the wavelength range 159–295 nm (4.2–7.8 eV),10 with 91.4 Mb at 185 nm (6.71 eV), higher than the present value of 43.4 Mb. High-resolution VUV absolute photoabsorption crosssections in combination with solar actinic flux measurements from the literature25 can be used to determine the photolysis rates of acrylic acid in the Earth’s atmosphere (0–50 km altitude). Since 2002 we have employed a simple methodology to calculate photolysis lifetimes which are the reciprocal of the photolysis rate at a given altitude.26 The quantum yield for dissociation is assumed to be unity. Photolysis lifetimes of less than one sunlit day were calculated at altitudes above 12 km. This indicates that acrylic acid molecules are efficiently broken up by UV absorption at these altitudes. At lower altitudes the photolysis lifetimes increase to 1.5 sunlit days. The room temperature work of Teruel et al.27 reports a comprehensive study on the reactions of acrylic acid with OH radicals and Cl atoms, with rate constants of (1.75 ± 0.47) × 10–11 cm3 molecule–1 s–1 and of (3.99 ± 0.84) × 10–10 cm3 molecule–1 s–1, respectively, which may provide a main reactive sink mechanism in the Earth’s atmosphere. Additionally, Teruel et al.27 have also calculated the tropospheric reaction lifetimes of acrylic acid with OH, Cl and O3 yielding 8 hours, 3 days and 25 days, respectively. These authors point out that due to the typical atmospheric 12 h average concentration of OH (2×106 molecule cm-3), the reaction with this radical is the major mechanism for loss of acrylic acid molecules. However, in coastal areas and in the marine boundary layer, where Cl concentrations can reach significant levels (1×105 atom cm-3), reactions initiated by chlorine may become dominant in acrylic acid degradation.27 Therefore, compared with radical reactions, UV photolysis is not expected to play a significant role in the tropospheric removal of these molecules.



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6. CONCLUSION The present study provides the first complete VUV photoabsorption spectrum of acrylic acid and most reliable set of cross-section values in the energy range 4.0 to 10.8 eV. The observed photoabsorption bands can be classified as members of Rydberg series and molecular valence transitions of the type (π* ← nO) and (π* ← π). Ab initio calculations on the vertical excitation energies and oscillator strengths were performed to support the assignments of the transition bands predicting significant mixing of Rydberg and π* states and are in good agreement with the experimental findings. Above 8.2 eV the photoabsorption spectrum shows a quite rich vibronic structure which has been mainly assigned to the C=O ᇱ ሺ ᇱሻ stretching, ߥହᇱ ሺܽᇱ ሻ, C=C stretching, ߥ଺ᇱ ሺܽᇱ ሻ, and C‒C‒O bending modes, ߥଵଷ ܽ . Photolysis

lifetimes of acrylic acid have been derived for the Earth’s troposphere and stratosphere.

ASSOCIATED CONTENT Supporting Information Additional figures, tables and Cartesian coordinates of all optimized structures (PDF)

AUTHOR INFORMATION Corresponding Author *(P.L.-V.) E-mail: [email protected]. ORCID P. Limão-Vieira: 0000-0003-2696-1152 N. C. Jones: 0000-0002-4081-6405 S. V. Hoffmann: 0000-0002-8018-5433 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS MM and FFS acknowledge the Portuguese National Funding Agency FCT-MCTES through grants PD/BD/106038/2015 and researcher position IF-FCT IF/00380/2014, respectively, and 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). ASB and MHFB acknowledge the Brazilian Agency Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), under CAPES/FCT Programme (CAPES Process No. 23038.002465/2014-87, FCT Process No. 2267). MHFB 10 ACS Paragon Plus Environment

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also acknowledges support from CNPq and CENAPAD-SP for computational support. ASB and MHFB acknowledge Prof. Carlos de Carvalho for computational support at LFTC-DFisUFPR and at LCPAD-UFPR. GG acknowledges partial ﬁnancial support from the Spanish Ministerio de Economia, Industria y Competitividad (Project No. FIS2016-80440). The authors wish to acknowledge the beam time at ISA synchrotron, Aarhus University, Denmark. Some of this work forms part of the EU COST Action Our Astro-Chemical History CM1401.



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Figure captions Figure 1. Molecular structure of acrylic acid for the s-cis- (left) and s-trans conformers (right). Figure 2. High-resolution VUV photoabsorption spectrum of acrylic acid in the 4.0–10.8 eV photon energy range together with some assignments of the main absorption features. See text for details.

Figure 3. Calculated highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) for acrylic acid cis conformer.

Table captions Table 1. Calculated Vertical Excitation Energies (TD-DFT/PBE0/aug-cc-pVDZ) and Oscillator Strengths (Singlet States) of Monomeric s-cis and s-trans Conformers of Acrylic Acid, C3H4O2, Compared with Experimental Data. Energies in eV. Table 2. Proposed Vibrational Assignments in the 7.5−10.8 eV Absorption Bands of Acrylic Acid.

Table 3. Energy Value (eV), Quantum Defect (δ) and Tentative Assignment of the Rydberg Series Converging to the Ionic Electronic Ground, First and Second Excited States of Acrylic Acid, C3H4O2.



Page 16 of 22

Figure 1. Molecular structure of acrylic acid for the s-cis (left) and s-trans conformers (right).


Page 17 of 22

Figure 2. High-resolution VUV photoabsorption spectrum of acrylic acid in the 4.0–10.8 eV photon energy range together with some assignments of the main absorption features. See text for details.

50 ν6 + ν13

(π* ← π)

45

nν5 (π* ← π)

40

Cross-section [Mb]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nν5

ν6 + ν13

35

nν5 nν5

30 3p

3s

25

nd

20

(π* ← nO)

4 5

3

3

4

× 50 nd 3

10

4

np 3

5

ns

IE3 IE2 IE2

np 3 ns

15

3d

4

3

4

5

IE2

5

IE1

5 6 5

6

0 4

5

6

7 8 Photon energy [eV]

9

10

11



Page 18 of 22

Figure 3. Calculated highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) for acrylic acid s-cis conformer. HOMO (16a′)

HOMO-1 (3a″)

HOMO-2 (2a″)

HOMO-3 (15a′)

LUMO (4a'')

LUMO+1 (17a')

LUMO+5 (5a'')

LUMO+6 (6a'')

LUMO+7 (21a')


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Table 1. Calculated Vertical Excitation Energies (TD-DFT/PBE0/aug-cc-pVDZ) and Oscillator Strengths (Singlet States) of Monomeric s-cis and s-trans Conformers of Acrylic Acid, C3H4O2, Compared with Experimental Dataa. Energies in eV.

experimental Cross-section

TD-DFT/PBE0/aug-cc-pVDZ

TD-DFT/PBE0/aug-cc-pVDZ

(s-cis conformer)

(s-trans conformer)

calculated

dominant

energy

excitation(s)

A''

4.779

43.4

A'

6.259

III

28.8

A'

7.200

8.557

IV

16.3

A'

8.945

9.421

V

31.4

A'

9.139

energy

band

5.22(7)

I

0.3

6.637

II

7.514

a

(Mb)

state

calculated

dominant

energy

excitation(s)

A''

4.728

16a′ → 4a″

0.0001

0.2368

A'

6.472

3a″ → 4a″

0.3923

0.1111

A'

7.201

0.0465

A'

9.004

3a″ → 6a″

0.0256

0.0660

A''

9.618

3a″ → 22a′

0.0479

f0

state

16a′ → 4a″

0.0000

3a″ → 4a″ 16a′ → 17a′ 2a″ → 4a″ 3a″ → 5a″ 3a″ → 6a″ 16a′ → 21a′ 3a″ → 6a″

16a′ → 17a′ 2a″ →4a″

f0

0.0879

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

19

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Table 2. Proposed Vibrational Assignments in the 7.5−10.8 eV Absorption Bands of Acrylic Acida,b.

Acrylic Acid, C3H4O2 energy (eV)

assignment

∆E(eV)

energy (eV)

assignment

∆E(eV)

7.514

0଴଴

–

9.801

0଴଴

–

7.672

5ଵ଴

0.158

9.967

5ଵ଴

0.166

7.832

5ଶ଴

0.150

10.146

5ଶ଴

0.169

7.989

5ଷ଴

0.157

10.32(3) (s)

5ଷ଴

0.177

10.48(9) (s)

5ସ଴

0.166

10.64(2) (b)

5ହ଴

0.163

8.21(6) (w)

0଴଴

–

8.282

–

–

8.38(3) (s)

5ଵ଴

0.167

9.879

5ଵ଴

–

8.446

5ଶ଴

0.164

10.03(1) (w,b)

5ଶ଴

0.152

8.545

5ଷ଴ /6ଵ଴ + 13ଵ଴

0.162/0.263

10.146

6ଵ଴ + 13ଵ଴

0.267

10.213

5ଷ଴

0.182

8.991

0଴଴

–

10.39(3) (w)

5ସ଴ /6ଶ଴ + 13ଶ଴

0.180/0.247

9.164

5ଵ଴

0.173

10.66(1) (b)

6ଷ଴ + 13ଷ଴

0.272

9.32(2) (s)

5ଶ଴

0.158

6ଵ଴ + 13ଵ଴

0.250

9.967

6ଵ଴ + 13ଵ଴

–

9.47(9) (s)

5ଷ଴

0.157

10.213

6ଶ଴ + 13ଵ଴

0.246

9.626

5ସ଴

0.147

10.489

6ଷ଴ + 13ଶ଴

0.276

9.414

a

(w) weak feature. (s) shoulder structure. (b) broad structure (the last decimal of the energy

value is given in brackets for these less-resolved features). b

the minimum vertical excitation energy observed for the excited state as the band origin,

may be different from the adiabatic band origin.


Page 21 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Energy Value (eV), Quantum Defect (δ) and Tentative Assignment of the Rydberg Series Converging to the Ionic Electronic Ground, First and Second Excited States of Acrylic Acid, C3H4O2a.

En

δ

assignment

IE1 = 10.78 eV

En

δ

assignment

8.21(6) (w)

0.73

3p

IE2 = 10.95 eV

7.514

0.96

3s

9.32(2) (s)

0.94

4s

9.967

0.91

5s

9.164

0.24

3d

10.213

1.10

6s

10.03(1) (w,b)

0.15

4d

10.32(3) (s)

0.34

5d

8.21(6) (w)

0.70

3p

9.626

0.57

4p

IE3 = 12.00 eV

10.03(1) (w,b)

0.74

5p

8.545

1.01

3s

10.32(3) (s)

0.54

6p 9.47(9) (s)

0.68

3p

10.146

0.29

3d

8.991

0.24

3d

9.801

0.27

4d

10.146

0.37

5d

7.514

1.00

3s

9.414

1.02

4s

10.146

0.89

5s

IE2 = 10.95 eV

a

(s) shoulder structure. (w) weak feature. (b) broad structure (the last decimal of the energy

value is given in brackets for these less-resolved features).



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TOC


Probing the Lowest-Lying Electronic States of Acrylic Acid by

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