Electronic State Spectroscopy of 1,4-Pentadiene ... - ACS Publications

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Electronic State Spectroscopy of 1,4-Pentadiene As Studied by VUV Photoabsorption Spectroscopy and ab Initio Calculations S. Sério,† Y. Nunes,† S. V. Hoffmann,‡ N. J. Mason,§ D. Duflot,∥ and P. Limaõ -Vieira*,†,§ †

Laboratório de Colisões Atómicas e Moleculares, CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal ‡ Institute for Storage Ring Facilities, Aarhus University, Ny Munkegade, DK-8000, Århus C, Denmark § Centre of Earth, Planetary, Space and Astronomy Research, Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes MK7 6AA, U.K. ∥ Laboratoire de Physique des Lasers, Atomes et Molécules (PhLAM), UMR CNRS 8523, Université Lille 1, F-59655 Villeneuve d’Ascq Cedex, France ABSTRACT: We present high resolution VUV photoabsorption spectra of 1,4-pentadiene, C5H8, over the wavelength range 115−247 nm (10.8−5.0 eV). These spectra reveal several new features not previously reported in the literature. These measurements are complemented by the first ab initio calculations for the three most abundant conformational isomers of 1,4-pentadiene, C5H8, which we then use in the assignment of valence and Rydberg transitions. Calculations of the two lowest energy ionic states of 1,4-pentadiene are also presented and compared with the experimental data available in the literature. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of 1,4-pentadiene in the upper stratosphere (20−50 km).

1. INTRODUCTION In this paper we report the results of extensive study of the electronic state spectroscopy of 1,4-pentadiene, CH2 CHCH2CHCH2 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. This work was partially engendered by the need to understand the spectroscopy of larger hydrocarbons used in industrial applications such as organic synthesis and polymer manufacturing.1 Our knowledge of the VUV spectroscopy of larger hydrocarbons remains poorly quantified; indeed as far as we are aware, experimental information on C5Hx isomers is restricted to photoionization mass spectrometry2 and the derivation of valence ionization energies by means of photoelectron spectroscopy.3,4 However, because 1,4-pentadiene, C5H8, is an isomeric form of isoprene (CH2CHC(CH3)CH2) we may also compare the present results with the valence shell electronic spectroscopy of isoprene. In a recent publication, we have reported an extensive study of the electronic spectroscopy of isoprene combining theoretical calculations with electron scattering, photoelectron spectroscopy and absolute photoabsorption measurements.5 Recent DFT calculations and electronic energies obtained from CBS(T,Q)-QCISD(T)/B3LYP/6-311G** electronic structure calculations extrapolated to the complete basis set limit, have shown that the lowest configurations are the (gauche) © XXXX American Chemical Society

and (cis) CH2CHCH2CHCH2 isomers with no symmetry (C1 group).2 However, according to the present work, the three isomeric forms of 1,4-pentadiene, C5H8 (Table 1a)), are almost certainly all populated at room temperature, because the relative energies of the C2, C1, and Cs isomers are less than 0.6 kcal/mol (section 5 and Table 1b)). This paper is therefore arranged in the following order. A discussion of the structure and properties of 1,4-pentadiene, followed by a discussion of our theoretical calculations of the electronic states. We will then describe the experimental methodology before presenting the VUV photoabsorption spectra. Each of the features in the spectra are assigned and comparisons made between the present photoabsorption spectrum and that of isoprene.

2. BRIEF SUMMARY OF THE STRUCTURE AND PROPERTIES OF 1,4-PENTADIENE, C5H8 The geometries and interatomic distances (Table 1a) for the three isomers of 1,4-pentadiene calculated at the CCSD/cc-pVTZ level (frozen core) are shown in Figure 1. The calculated electron configurations of the ground states are as follows. C2 isomer: (a) core orbitals (1a)2 (1b)2 (2a)2 (2b)2 (3a)2; (b) valence orbitals (4a)2 (3b)2 (5a)2 (4b)2 (6a)2 (5b)2 (7a)2 Received: June 15, 2012 Revised: July 17, 2012

A

dx.doi.org/10.1021/jp305905j | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Table 1. Calculated Geometriesa and Calculated Relative Energiesb of 1,4-Pentadiene, C5H8 (a) Calculated Geometries this work (FC-CCSD/cc-pVTZ) C2 CC CC CH

1.5059 1.3319 1.0811 1.0829

CCC C2C1H C1C2H C2C3H CCC C1C2C3C4 C2C3C4C5

C2 C1 Cs

1.0928 124.6 121.5/121.3 119.5 109.8 111.8 −117.8 −117.8

ref 26

Cs

C1

C2

1.5054 1.3314 1.0810 1.0828 1.0854 1.0951/1.0911 124.5 121.5/121.3 119.2 108.8/109.9 112.2 −121.7 121.7

1.5084/1.5006 1.3321/1.3317 1.0809/1.0816 1.0811/1.0829 1.0847/1.0850 1.0948/1.0936 125.7/124.5 120.9/121.8

ref 27

Cs

C1

C2

1.508 (2) 1.334 (2) 1.090 (2)

Cs

C1

1.505 (1) 1.336 (1) 1.074 (2)

125.5 (6) 123.2 (9) 117.3 (3) 109.7 (6) 108.9 (19) 108.9 (19) −122.2 (78) −128.6 (84) −122.2 (78) 128.6 (84) (b) Calculated Relative Energies

113.1 (11) −116.9 (7) −4.3 (69)

113 (1) −120 −120

125 (1) 126 (2) 114 (2) 110 (2) 113 (1) −120 120

MP2c

MP2+ZPEc

CCSDd

CCSD(T)d

MM2e

0.00 0.23 0.56

0.00 0.03 0.50

0.00 0.31 0.51

0.00 0.25 0.51

0.00 0.35 ∼0.00

113 (1) −115 −5

a Compared with previous works. Bond lengths in Å and angles in deg. bIn kcal/mol. cUsing MP2 geometry and harmonic frequencies. dUsing CCSD geometry. eReference 26.

(6b)2 (8a)2 (7b)2 (9a)2 (8b)2 (10a)2 (9b)2. The highest occupied molecular orbital (HOMO) is 9b, πb (Figure 1a) and the second highest occupied orbital (HOMO−1) is 10a, πa in the neutral ground state; both are localized in the carbon− carbon double bonds with π(CC) character. The lowest unoccupied molecular orbital (LUMO) is 10b, πb* (Figure 1a) and the LUMO+1 orbital (11a, πa*) is mainly of π* antibonding character. C1 isomer: (a) core orbitals (1a)2 (2a)2 (3a)2 (4a)2 (5a)2 (6a)2; (b) valence orbitals (7a)2 (8a)2 (9a)2 (10a)2 (11a)2 (12a)2 (13a)2 (14a)2 (15a)2 (16a)2 (17a)2 (18a)2 (19a)2. The HOMO (19a) (Figure 1b) and the HOMO−1 (18a) are π (CC) in character. They are labeled π+ and π− because they exhibit a symmetric and antisymmetric combination of the individual π(CC) bonds. However, their antibonding π* counterparts, LUMO (20a) and LUMO+1 (21a), are localized and are designated as π* and π′*, respectively. Cs isomer: (a) core orbitals (1a′)2 (1a″)2 (2a′)2 (2a″)2 (3a′)2; (b) valence orbitals (4a′)2 (3a″)2 (5a′)2 (4a″)2 (6a′)2 (7a′)2 (5a″)2 (8a′)2 (6a″)2 (9a′)2 (10a′)2 (7a″)2 (11a′)2 (8a″)2. The highest occupied molecular orbital (HOMO, 8a″, π″) (Figure 1c) and the second highest occupied orbital (HOMO−1, 11a′, π′) are localized in the carbon−carbon double bonds with π (CC) character, the latter showing a small σ(CCC) contribution. The lowest unoccupied molecular orbital (LUMO, 12a′, π′*) as well as the LUMO+1 orbital (9a″, π″*) are mainly of π* antibonding character. Previous experimental studies on the valence ionization energies of hydrocarbons reported a vertical ionization energy (IE) of 1,4-pentadiene of 9.62 eV.3 In the present analysis of the VUV spectra, this vertical ionization energy has been used to calculate quantum defects to identify the Rydberg character of several excited states (section 5.2).

synchrotron facility at the University of Aarhus, Denmark (Figures 2−4). The experimental apparatus has been described in detail elsewhere6 so only a brief 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 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 are free of any saturation effects, the absorption cross sections were measured over the pressure range 0.02− 1.00 Torr, with typical attenuations of less than 10%. 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 is the molecular number density of the sample gas, σ is the absolute photoabsorption cross section, and x is the absorption path length (25 cm). The accuracy of the cross section is estimated to be ±5%. Only when absorption by the sample is very weak (I0 ≈ It) does the error increase as a percentage of the measured cross section. The liquid sample of C5H8 was purchased from Fluka with a quoted purity of ≥99%. The sample was degassed by repeated freeze−pump−thaw cycles prior to use.

4. COMPUTATIONAL METHODS Ab initio calculations were performed to determine the excitation energies of the neutral (Tables 2 −4), and the ionization energies of the ground neutral state (Table 5) using the

3. EXPERIMENTAL PROCEDURE High-resolution VUV photoabsorption spectra of 1,4-pentadiene were recorded using the UV1 beamline of the ASTRID B



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Figure 1. (a) C2, (b) C1, and (c) Cs isomer molecular orbitals of 1,4-pentadiene, C5H8.

MOLPRO program.7 First, the ground state geometries of the three isomers were optimized at the frozen core (FC) MP2 and CCSD levels8 using Dunning’s aug-cc-pVTZ basis set.9 When the UV spectra are calculated, it should be noted that obtaining accurate vertical excitation energies and oscillator strengths remain a difficult task even for small organic

molecules such as C5H8 (see, for example, ref 10 for a recent review). Although the computational cost of TDDFT is low, these methods are known to give poorly description of Rydberg or charge transfer states11 whereas multireference methods such as CASPT212 or CASSCF/MRCI techniques become prohibitive when a large number of Rydberg orbitals have to be C



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Figure 2. VUV photoabsorption cross section (Megabarn = 10−18 cm2) of 1,4-pentadiene, C5H8.

Figure 3. Vibrational progressions and Rydberg series assignment in the 5.0−8.0 eV absorption band of 1,4-pentadiene, C5H8.

mixture of Rydberg series and molecular valence transitions of (π* ← π) character. It is interesting to note that, generally speaking, isoprene and 1,4-pentadiene VUV spectrum show some similarities regarding their overall shape, but whereas for the former the lowest lying excited state has a threshold around 5.2 eV, the latter appears at 1 eV higher. Although the fine structure observed in isoprene5 is absent or appears diffuse in 1,4-pentadiene (Figure 2), the mixed Rydberg-valence character in the low-energy range is observed for both molecules. In fact, the calculations reported in Tables 2 and 4 indicate that the electronic transitions for 1,4-pentadiene have mixed valenceRydberg character for C2 and Cs symmetries, respectively. The only difference seems to lie with the C1 symmetry where this character appears at higher energies. The results of theoretical calculations obtained for all three conformers are presented in Tables 2−4 and are compared with experimental results. The data obtained from experiments are in a reasonably good agreement with theoretical predictions. 5.1. Valence State Spectroscopy of 1,4-Pentadiene. According to the calculations presented in Tables 2−4, the

included in the active space. However, coupled cluster methods have recently proven to give reliable results and appear to provide a good compromise between computational cost and accuracy.13 The electronic spectrum of C5H8 was therefore computed at the EOM-CCSD level.14 To this end, a set of diffuse functions (6s, 6p, 5d), taken from Kaufmann et al.15 and localized on the central carbon atom, were added to the original basis set for a better description of the Rydberg states (aug-ccpVTZ+R basis set). The corresponding oscillator strengths were calculated in the length gauge (Tables 2−4). Finally, the lowest vertical ionization energies of C5H8 were also obtained at the RCCSD16 and RCCSD(T) levels17 and the molecular orbitals shown in Figure 1 have been plotted using ChemCraft18 with isovalues of 0.07.

5. ELECTRONIC STATE SPECTROSCOPY: RESULTS AND DISCUSSION The absolute VUV photoabsorption cross section of 1,4pentadiene is shown in Figure 2, extending from 5.0 to 10.8 eV. The major absorption bands can be classified mainly as a D



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Figure 4. Vibrational progressions and Rydberg series assignment in the 8.0−11.0 eV absorption band of 1,4-pentadiene, C5H8.

Table 2. C2 Isomer Calculated Vertical Excitation Energies (EOM-CCSD/cc-pVTZ + Rydberg Level at the FC-CCSD/cc-pVTZ Geometry) (eV) and Oscillator Strengths Compared with the Present Experimental VUV Absorption Cross Sections of 1,4-Pentadiene, C5H8a sym

E (eV)

fL

⟨r2⟩

0.0518 0.0390 0.3954 0.0029 0.1500 0.0129 0.0051 0.0000 0.0187 0.0014 0.0541 0.0009 0.0025 0.0083 0.0430 0.1580 0.0277 0.0169 0.0000 0.0007 0.0006 0.0028 0.0109 0.0279 0.0000 0.0021 0.0005 0.0005 0.0091 0.0013 0.0005 0.0306 0.0111

72 121 127 98 124 117 112 133 144 150 115 199 181 182 186 205 205 212 290 353 385 343 210 204 211 242 225 503 490 524 528 525 620 620

1

X A 1 B 1 A 1 B 1 A 1 B 1 A 1 B 1 A 1 B 1 A 1 B 1 A 1 B 1 A 1 B 1 B 1 A 1 B 1 A 1 B 1 A 1 A 1 B 1 B 1 A 1 A 1 B 1 A 1 B 1 A 1 A 1 B 1 B

7.008 7.176 7.342 7.458 7.637 7.729 7.795 7.949 8.036 8.038 8.368 8.395 8.458 8.469 8.496 8.529 8.537 8.633 8.657 8.698 8.730 8.758 8.785 8.804 8.840 8.872 8.921 8.970 8.984 9.002 9.012 9.026 9.030

(9b, πb)

(10a, πa)

mixed character

exp (eV)

cross section (Mb)

πb → 3pb + πa → 3sa

6.49(4) 6.49(4) 6.981

22.1 22.1 56.9

πb → 3pa + πb → πa*

7.26(3)

50.2

8.157

37.4

8.157

37.4

8.509

36.0

8.92(0)

33.1

8.71(9) 8.768

33.5 34.5

πb → 3sa πb → πa* πb → 3pb πa → πa* πb πb πb πb πb πb πb πb πb

→ → → → →

3da 3db 3da 3db 3da

πb πb πb πb πb

→ → → → →

πb* 4sa 4pb 4pa 4pb

→ → → →

3pa + πa → 3pb 3pb + πa → 3sa 3da + πa → 3pb 3pb + πa → πa*

πb → 3da + πa → πb*

πb → πb* + πa → 3da πa πa πa πa

→ → → →

3db 3db 3da 3da

πb → 4da πb → 5pa + πa → 3db πb → 4da πb → 4db + πa → 4sa πb → 4db + πa → 4sa πb → 4da πb → 5sa +πa → 4pb E



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Table 2. continued sym 1

A 1 B 1 A 1 B 1 A 1 B

E (eV)

fL

⟨r2⟩

9.067 9.096 9.119 9.139 9.141 9.163

0.0009 0.0042 0.0104 0.0020 0.0005 0.0093

382 648 614 1002 891 707

(9b, πb)

(10a, πa)

mixed character

exp (eV)

cross section (Mb)

8.92(0)

33.2

πb → 4db + πa → 4sa πb → 4da + πa → 4pb πb → 5pb + πa → 4pa πb → 5pa πb → 5pb πb → 4da + πa → 4pb

⟨r2⟩ is the mean value of r2 (electronic radial spatial extents); the last decimal of the energy value is given in parentheses for these less-resolved features. a

Table 3. C1 Isomer Calculated Vertical Excitation Energies (EOM-CCSD/cc-pVTZ + Rydberg level at the FC-CCSD/ cc-pVTZ Geometry) (eV) and Oscillator Strengths Compared with the Present Experimental VUV Absorption Cross Sections of 1,4-Pentadiene, C5H8a sym E (eV) X 1A A 1 A 1 A 1 A 1 A 1 A 1 A 1 A 1 A 1 A 1 A 1 A 1 A 1 A 1

6.951 7.118 7.448 7.543 7.668 7.732 7.814 7.908 8.111 8.189 8.269 8.356 8.417 8.476

fL

⟨r2⟩

0.0316 0.0156 0.0626 0.0288 0.0694 0.0865 0.0362 0.0846 0.1376 0.0964 0.0871 0.0322 0.0136 0.0135

71 121 122 123 126 125 122 136 136 114 136 123 115 200 201

exp (eV)

cross section (Mb)

6.49(4)

22.1

π− → 3p π− → 3p 7.26(3)

50.2

(19a, π+) (18a, π−) π+ → 3s

reminds of the sudden polarization effect found in excited states of ethene and other polyenes (see, e.g., ref 19). Because of this behavior, the C1 (π* ← π) transitions could have a doubly excited character, which cannot be described by EOM-CCSD methods. Therefore, multireference calculations would be necessary to clarify this point. The vibrational series observed throughout the spectrum are tentatively assigned by comparison with the vibrational modes in isoprene.5 The structure between 5.0 and 8.0 eV (Figure 3 and Table 6) is proposed to be due to CC stretching (with an average value of 0.164 eV), as in isoprene (ν8).4 The feature at 5.61(0) eV is tentatively assigned as the origin of the band. Further overlap of vibronic structures associated with both valence and Rydberg transitions is proposed to account for the features above 8.0 eV (Figure 4, Table 6 and 7). In the energy range 8.0−10.8 eV, the observed fine structure is also tentatively assigned to CC symmetric stretching with a mean value of 0.176 eV, with the (0−0) transition lying at 8.157 eV, although the broad nature of the features suggests that further modes and combinations may also contribute to the observed structure. In particular, the normal mode configuration may lead to Fermi resonances, notably around the feature at 8.23(4) eV, 0.09(1) eV from ν00, which is assigned to νA (CH rocking) and/or νB(CH wagging) modes, with values in the ground state of 0.118 and 0.123 eV, respectively.20 However, the complexity of the structure makes it difficult to assign transitions unambiguously. Indeed, the present calculations (Tables 2−4) predict a number of low-intensity transitions (Rydberg) that have not been identified in the VUV spectrum. The vibronic structure beginning at 8.157 eV (Table 6) is proposed to be due to the Rydberg transitions (3da/4sa ← πb) for the C2 isomer, (3da′/4sa′ ← π″) for the Cs isomer, and (3d ← π+) for the C1 isomer, combined with several quanta of CC stretching mode. 5.2. Rydberg Transitions. The VUV spectrum above 6.0 eV consists of a few structures superimposed on a diffuse absorption feature extending to the lowest ionization energy (IE). The proposed Rydberg structures are labeled in Figures 3 and 4 and are presented in Table 7. The peak positions, En, have been tested using the Rydberg formula: En = Ei − R/ (n − δ)2, where Ei is the ionization energy (IE1) with a vertical value of 9.62 eV, 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. Quantum defects in the range 0.9−1.0, ∼0.5, and 0.02−0.16 are expected for ns, np, and nd transitions, respectively.21 The experimental values for the lowest terms of the ns, np, and nd (n = 3) Rydberg series (Table 7) are in good agreement with the calculations in Tables 2−4.

π− → 3s π+ → 3p π+ → 3p

π+ π+ π+ π+

→ → → →

3p 3d π* π′*

π+ → 3d π+ → 3d

8.157 8.157 π− → π * 8.157 π− → π′* 8.157

37.4 37.4 37.4 37.4

⟨r2⟩ is the mean value of r2 (electronic radial spatial extents); the last decimal of the energy value is given in parentheses for the less-resolved features.

a

absorption bands centered at 6.981 and 8.157 eV have been assigned to (πa*(11a) ← πb(9b)) and (πb*(10b) ← πa(10a)/ (3da ← πb(9b)) for the C2 isomer whereas for Cs to (3pa′/ π′*(12a′) ← π″(8a″)) and (π″*(9a″) ← π′(11a′)) transitions, respectively (Figures 2−4). The first band is reported with a maximum absolute cross section of 56.9 Mb whereas the second with a value of 37.4 Mb (Tables 2−4). Although the calculations indicate a mixed valence/Rydberg character (3da) in the C2 isomer for the 8.157 eV band, the rather high calculated intensity is primarily due to the important valence π* character of the MO, so the transition should be labeled as (πb*(10b)(CC) ← πa(10a)). It should be noted that although in the C1 symmetry (Table 3), the calculations predict the (π*(20a) ← π+(19a)) transition at 8.111 eV. Using the calculated oscillator strengths in Tables 2−4, the C2, C1, and Cs isomers contribute to the 8.157 eV band with 0.158, 0.138, and 0.099, respectively. Pure Rydberg transitions with high oscillator strengths in this energy range are discussed in section 5.2. The prediction of the C1 isomer (π* ← π) transitions well above the Cs and C2 corresponding transitions maybe be related to the localization of the π* MO’s in C1, as shown in Figure 1b. This relocalization effect in a symmetry broken geometry F



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Table 4. Cs Isomer Calculated Vertical Excitation Energies (EOM-CCSD/cc-pVTZ + Rydberg level at the FC-CCSD/cc-pVTZ Geometry) (eV) and Oscillator Strengths Compared with the Present Experimental VUV Absorption Cross Sections of 1,4-Pentadiene, C5H8a sym

E (eV)

fL

⟨r2⟩

X A′ A″ A′ A″ A″ A′ A′ A″ A′ A″ A′ A″ A′ A″ A″ A′ A″ A′ A′ A″ A″ A′ A″ A″ A′ A′ A′ A″ A′ A′ A″ A′ A″ A″ A″ A′ A′ A″ A″ A′

7.017 7.085 7.403 7.416 7.518 7.649 7.848 7.894 8.038 8.101 8.296 8.392 8.448 8.497 8.540 8.553 8.557 8.599 8.612 8.650 8.651 8.660 8.686 8.715 8.768 8.812 8.815 8.897 8.932 8.940 9.012 9.036 9.053 9.089 9.097 9.118 9.127 9.170 9.184

0.0338 0.0344 0.1309 0.1333 0.0002 0.0899 0.0580 0.0151 0.0289 0.0509 0.0985 0.0654 0.0077 0.0258 0.0139 0.0523 0.0026 0.0338 0.0202 0.0097 0.0179 0.0033 0.0099 0.0068 0.0515 0.0130 0.0145 0.0014 0.0050 0.0014 0.0194 0.0014 0.0003 0.0287 0.0078 0.0052 0.0020 0.0005 0.0020

72 72 120 115 109 122 122 128 136 148 124 102 157 187 232 203 221 210 216 217 295 201 224 338 285 255 338 397 404 403 456 184 413 573 673 609 670 632 830 653

(8a″, π″)

(11a′, π′)

mixed character

π″ → 3sa′

exp (eV)

cross section (Mb)

6.49(4)

22.1

6.981 6.981

56.9 56.9

7.26(3)

50.2

8.157 8.157

37.4 37.4

8.157

37.4

8.157

37.4

8.92(0)

33.1

8.509

36.0

8.71(9)

33.5

8.768

34.5

π′ → 3sa′ π″ → 3pa′/π′* π″ → 3pa′/π′* π′ → 3pa′ π′ → 3pa′ π″ → 3sa′/π′* π′ → 3da′ + π″ → 3pa″ π′ → 3pa″ + π″ → 3sa′ π′ → 3da′ π′ → π″* π′ → 3da′ π″ π″ π″ π″

→ → → →

3da′ 3da′ 3da″ 3da′ π′ → 3da′ + π″ → 3da″ π′ → 3da′ π″ → 3da′ + π′ → 3da″

π″ → 4sa′ π″ → 3da′ + π′ → 3da′ π′ → 3da″ π″ → 4pa′ π″ → 3da′ + π′ → 3da′ π′ → 3da′ π′ → 4pa′ π″ → 4pa′ π″ → 4pa″ + π′ → 4sa′ π′ → 4pa′ π″ → 4da′ + π′ → 4pa″ π″ → π″* + σ a′(CC) → π′* π″ → 4da′+ π′ → 4da″ π″ π″ π″ π″ π″ π″

→ → → → → →

4da′ 4da′ 4da″ 4da″ 4da′ 5sa′ π′ → 4da′

⟨r2⟩ is the mean value of r2 (electronic radial spatial extents); the last decimal of the energy value is given in parentheses for these less-resolved features. a

Table 5. Calculated Vertical Ionization Energies (MP2/aug-cc-pVTZ Geometry) of 1,4-Pentadiene (eV) C2

C2

C1

2

2

2

9.624 9.792 9.770

10.021 10.194 10.147

9.62 ± 0.02

10.12 ± 0.0.2

B

RMP2 RCCSD RCCSD(T) MP2-CBS//B3LYP/6-311G++** 2 exp4

A

The lowest transition energy calculated for the three isomers (Tables 2−4) is also tentatively assigned to the Rydberg transitions (3sa ← πb, 9b), (3s ← π+, 19a), and (3sa′ ← π″, 8a″) for C2, C1, and Cs, respectively, with a quantum defect δ = 0.91 (Table 7). The n = 4 member may be accompanied by vibronic

A

9.957 9.730 9.864 9.17

Cs 2

A″

9.700 9.852 9.836

Cs 2

A′

9.850 10.026 9.980

structure, as previously reported for the valence transition (π* ← π), which is proposed to be mainly due to excitation of the CC stretching mode (Figure 4 and Table 6). The higher members of this Rydberg series are proposed to extend to n = 8. G



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Table 6. Proposed Vibrational Assignments in the 5.0−10.8 eV Absorption Bands of 1,4-Pentadiene, C5H8a energy (eV)

ΔE(ν′CC) (eV)

assignment

Table 7. Energies (eV), Quantum Defects, and Assignments of the ns, np, and nd Rydberg Series Converging to the X̃ 2 A (9b−1) Ionic Electronic Ground State of 1,4-Pentadiene, C5H8

ΔE(ν′A/ν′B) (eV)

First Band 5.61(0) 5.77(8)

ν00 1νCC

6.49(4) 6.662 6.808 6.981

1νCC 2νCC 3νCC 4νCC

8.157 8.23(4) 8.332 8.42(3) 8.509 8.59(2) 8.695 8.768 8.86(9) 8.94(5) 9.043 9.13(0) 9.225 9.31(5) 9.385 9.49(3) 9.567

ν00 1νA/1νB 1νCC 1νA/1νB 2νCC 1νA/1νB 3νCC 1νA/1νB 4νCC 1νA/1νB 5νCC 1νA/1νB 6νCC 1νA/1νB 7νCC 1νA/1νB 8νCC

(w) (s)

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

0.16(8)

0.168 0.146 0.173 Second Band 0.09(1) + 1νCC + 2νCC + 3νCC + 4νCC + 5νCC + 6νCC + 7νCC

0.175 0.189 0.177 0.169 0.186 0.176 0.174 0.177 0.174 0.185 0.182 0.185 0.160 0.178 0.182

9.73(9) (b) 9.91(1) (b) 10.08(8) (s)

ν00 1νCC 2νCC

0.172 0.177

10.436 10.615

1νCC

0.179

vertical energy

quantum defect (δ)

assignment

IE1 = 9.62 eVa 6.49(4) (s) 8.157 8.768 9.043 (s) 9.225 (s) 9.336 (w) 7.26(3) (s) 8.509 8.92(0) (w) 9.16(4) (w) 9.29(4) (s) 9.37(8) (s) 8.157 8.71(9) (s) 9.04(3) (s) 9.225 9.336

0.91 0.95 1.00 1.14 1.13 1.07 0.59 0.50 0.59 0.54 0.54 0.50 −0.05 0.11 0.14 0.13 0.08

3s 4s 5s 6s 7s 8s 3p 4p 5p 6p 7p 8p 3d 4d 5d 6d 7d

a

Vertical value. (s) Indicates a shoulder; (w) weak structure. The last decimal of the energy value is given in parentheses for these lessresolved features.

were carried out in the pressure range 0.02−1.00 Torr 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. Furthermore, the agreement of previous cross sections measured at the ASTRID beamline with the most precise data available in the literature (see Eden et al.22 and references therein), suggest that the present 1,4-pentadiene cross sections can be relied upon across the energy range studied up to 10.8 eV (Figure 2). As far as we are aware, no previous absolute VUV photoabsorption cross sections of 1,4-pentadiene are available. The present absolute cross sections can be used in combination with solar actinic flux23 measurements from the literature to estimate the photolysis rate of 1,4-pentadiene in the atmosphere from an altitude close to the ground to the stratopause at 50 km. Details of the calculation program are presented in a previous publication by Limão-Vieira et al.24 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 72 sunlit hours were calculated at altitudes above 24 km. This indicates that 1,4-pentadiene molecules can be broken up quite efficiently by VUV absorption at these altitudes. At ground level the photolysis lifetimes increases to 180 sunlit hours. Recently, relative-rate and absolute techniques have been used to obtain rate coefficients for the reactions between the NO3 radical and 1,4-pentadiene at (2.3 ± 0.5) × 10−14 cm3 molecule−1 s−1.25 Bale et al. have observed that the absence of conjugation in the case of 1,4-pentadiene means that its consumption through reaction with NO3 during the night is comparable with that by ozone, but the dominant loss process is the day-time reaction with the hydroxyl radical, with a lifetime of 3 h;25 this may provide a main reactive sink mechanism in the Earth’s atmosphere. Therefore, compared with radical reactions,

a

(w) means weak structure; (s) indicates a shoulder; (b) is a broad feature. The last decimal of the energy value is given in parentheses for these less-resolved features.

The first members of the np and nd series are associated with the peaks at 7.26(3) eV (δ = 0.59) and 8.157 eV (δ = −0.05), respectively (Table 7). The calculated values for the np series are 7.637, 7.732, and 7.649 eV for C2, C1, and Cs isomers, respectively. Regarding the nd Rydberg series, the calculated values are reported at 8.529, 8.417, and 8.392 eV for C2, C1, and Cs isomers, respectively. Both sets of theoretical values are in reasonably good agreement with the experiment, taking into account that these transitions for C2 and Cs isomers also have valence character. The nd series shows a n = 3 vibrational excitation with several quanta of CC stretching mode coupled with CH rocking and wagging modes. 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. The clear increase in the absorption with energy in the range above ∼9.0 eV may be related to low-lying predissociative or dissociative excited neutral states. Above the lowest ionic limit, weak features tentatively assigned to CC stretching excitation appear at 9.73(9) eV and extend up to 10.61(5) eV. 5.3. Absolute Photoabsorption Cross Sections and Atmospheric Photolysis. The present optical measurements H



Article

(4) Bünzli, J. C.; Burak, A. J.; Frost, D. C. Tetrahedron 1973, 29, 3735. (5) Martins, G.; Ferreira-Rodrigues, A. M.; Rodrigues, F. N.; de Souza, G. G. B.; Mason, N. J.; Eden, S.; Duflot, D.; Flament, J.-P.; Hoffmann, S. V.; Delwiche, J.; Hubin-Franskin, M.-J.; Limão-Vieira, P. Phys. Chem. Chem. Phys. 2009, 11, 11219. (6) Eden, S.; Limão-Vieira, P.; Hoffmann, S. V.; Mason, N. J. Chem. Phys. 2006, 323, 313. (7) MOLPRO, version 2010.1, a package of ab initio programs, H.-J. Werner, P. J. Knowles, F. R. Manby, M. Schütz, P. Celani, G. Knizia, T. Korona, R. Lindh, A. Mitrushenkov, G. Rauhut, T. B. Adler, R. D. Amos, A. Bernhardsson, A. Berning, D. L. Cooper, M. J. O. Deegan, A. J. Dobbyn, F. Eckert, Goll, E.; C. Hampel, A. Hesselmann, G. Hetzer, T. Hrenar, G. Jansen, C. Köppl, Y. Liu, A. W. Lloyd, R. A. Mata, A. J. May, S. J. McNicholas, W. Meyer, M. E. Mura, A. Nicklass, P. Palmieri, K. Pflüger, R. Pitzer, M. Reiher, T. Shiozaki, H. Stoll, A. J. Stone, R. Tarroni, T. Thorsteinsson, M. Wang, A. Wolf, see http://www.molpro. net. (8) Hampel, C.; Peterson, K.; Werner, H.-J. Chem. Phys. Lett. 1992, 190, 1. (9) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007. (10) Serrano-Andres, L.; Merchan, M. J. Mol Struct. (THEOCHEM) 2005, 729, 99. (11) Dreuw, A.; Head-Gordon, M. Chem. Rev. 2005, 105, 4009. (12) Roos, B. O. In Theory and Applications of Computational Chemistry: The First Forty Years; Dykstra, C., et al., Eds.; Elsevier: Amsterdam, 2005; Chapter 27. (13) Bartlett, R. J.; Musila, M. Rev. Mod. Phys. 2007, 79, 291. (14) Hampel, C.; Peterson, K.; Werner, H.-J. Chem. Phys. Lett. 1992, 190, 1. (15) Kaufmann, K.; Baumeister, W.; Jungen, M. J. Phys. B 1989, 22, 2223. (16) Knowles, P. J.; Hampel, C.; Werner, H.-J. J. Chem. Phys. 1993, 99, 5219. (17) Watts, J. D.; Gauss, J.; Bartlett, R. J. J. Chem. Phys. 1993, 98, 8718. (18) See http://www.chemcraftprog.com/index.html. (19) Viel, A.; Krawczyk, R. P.; Manthe, U.; Domcke, W. Ang. Chem. Int. Ed. 2003, 32, 3434. (20) Pachenko, Y.; Bock, C. W.; Larkin, J. D.; Abramenkov, A. V.; Kühnemann, F. Struct. Chem. 2008, 19, 421. (21) Sandorfy, C., Ed. The Role of Rydberg States in Spectroscopy and Photochemistry; Kluwer Academic Publishers; Amsterdam, 1999. (22) Eden, S.; Limão-Vieira, P.; Hoffmann, S. V.; Mason, N. J Chem. Phys. 2007, 331, 232. (23) Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling, Evaluation number 12, NASA, Jet Propulsion Laboratory, JPL Publication 97-4, January 15, 1997. (24) Limão-Vieira, P.; Eden, S.; Kendall, P. A.; Mason, N. J.; Hoffmann, S. V. Chem. Phys. Lett. 2002, 364, 535. (25) Bale, C. S. E.; Canosa-Mas, C. E.; Flugge, M. L.; Wayne, R. P. Phys. Chem. Chem. Phys. 2002, 4, 5821. (26) Persuad, L.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.; Webber, S. E.; White, J. M. J. Am. Chem. Soc. 1987, 109, 7309. (27) Terbrake, J. H. M. J. Mol. Struct. 1984, 118, 73.

UV photolysis is not expected to play a significant role in the tropospheric removal of these molecules.

6. IONIZATION ENERGIES The calculated vertical IEs, using several methods, are presented in Table 5. Generally speaking, all methods agree with each other. The measured lowest vertical IE of 1,4pentadiene (9.62 eV)4 agrees reasonably well with the RCCSD and RCCSD(T) theoretical predictions for the three isomers. As far as the second calculated IE for the C2 and Cs isomer is concerned, these values are close to 10.12 eV obtained by the experimental work of Bünzli et al.4 The calculated IEs of the three isomers are too close to be able to be discriminated in the photoelectron spectrum. 7. CONCLUSIONS The present work provides the first complete study of the VUV electronic spectra of 1,4-pentadiene and provides the most reliable set of absolute photoabsorption cross sections available between 5.0 and 10.8 eV. The observed 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 for the three isomers. Fine structure has been assigned to vibrational series, dominantly involving excitation of the CC stretching mode. The theoretical results are in good agreement with the experiments and predict significant mixing of Rydberg and π* states. The photolysis lifetimes of 1,4-pentadiene 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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AUTHOR INFORMATION

Corresponding Author

*Tel.: + 351 − 21 294 78 59. Fax: + 351 − 21 294 85 49. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.L.V. acknowledges the visiting fellow position in the Molecular Physics group, Open University, U.K. P.L.V. and N.J.M. acknowledge the support from the British Council for the Portuguese-English joint collaboration. S.S., Y.N., and P.L.V. acknowledge the Portuguese grant PEst-OE/FIS/ UI0068/2011. We acknowledge the beam time at the ISA synchrotron at University of Aarhus, Denmark. We also acknowledge the financial support provided by the European Commission through the Access to Research Infrastructure action of the Improving Human Potential Programme, FP6Transnational Access Programme IA-5F5:R113-CT-2004506008. This work forms part of the EU COST Actions CM0601 and CM0805 programmes “ECCL” and “The Chemical Cosmos”, respectively.

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REFERENCES

(1) Salari, D.; Jodaei, A. Iran Polym. J. 2006, 15, 55. (2) Hansen, N.; Klippenstein, S. J.; Miller, J. A.; Wang, J.; Cool, T. A.; Law, M. E.; Westmoreland, P. R.; Kasper, T.; Kohse-Hö1inghaus, K. J. Phys. Chem. A 2006, 110, 4376. (3) Bieri, G.; Burger, F.; Heilbronner, E.; Maier, J. P. Helv. Chim. Acta 1977, 60, 2213. I


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