Low-n Rydberg Transitions of Liquid Ketones Studied by Attenuated

ARTICLE pubs.acs.org/JPCA

Low-n Rydberg Transitions of Liquid Ketones Studied by Attenuated Total Reflection Far-Ultraviolet Spectroscopy Yusuke Morisawa,† Akifumi Ikehata,‡ Noboru Higashi,§ and Yukihiro Ozaki*,† †

Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan National Food Research Institute, National Agriculture and Food Research Organization, Tsukuba, Ibaraki 305-8642, Japan § KURABO Industries, LTD., Shimokida-cho, Neyagawa, Osaka 572-0823, Japan ‡

ABSTRACT: Far-ultraviolet (FUV) spectra in the 8.55-6.20 eV (145-200 nm) region were measured for several kinds of ketones in the liquid phase to investigate low-n Rydberg transitions using a uniquely developed technique of attenuated total reflection (ATR) FUV spectrometry. Assignments of the transitions are attempted for absorptions in this region by comparing the spectra for the liquid phase with those for the gas phase and ab initio calculations at the equation-of-motion coupled cluster theory with single and double substitutions at the aug-cc-pVDZ level. The transition from a nonbonding electron (n) to the 3s Rydberg orbital was found at around 6.7 eV for all investigated liquid ketones. Another intense band also appeared in the higher-energy region (ca. 8.5 eV) for all the ketones. A significant shoulder was found at around 7.4 eV for branched ketones. This shoulder band near 7.4 eV was assigned to the n-3p Rydberg transition. Band broadening and higher energy shifts were observed in the spectra of the liquid phase ketones in comparison with those of the gas phase ketones.

1. INTRODUCTION Spectra in the far-ultraviolet (FUV) region have been studied for molecules in the gas phase.1 These studies revealed that various kinds of molecules have strong absorptions due to electronic transitions to low-lying Rydberg states in the FUV region until their vertical ionized energy.1 Recently, we have developed a unique technique of attenuated total reflection (ATR) in the FUV region (ATR-FUV).2 By use of ATR-FUV spectroscopy, spectra in the region from 8.55 to 4.13 eV (145 to 300 nm) can be measured for various kinds of liquids such as liquid water,3 aqueous solutions,2,4,5 and liquid alcohol.2,6 Because of the advantage of online analysis of the ATR method, studies on quantitative analysis of aqueous solutions have been carried out.4,5 Strong absorptions of liquids in the FUV region may be used for novel analytical methods as well as in physicochemical studies. Temperature dependence of the absorption due to liquid H2O and D2O in the region of 8.5-7.0 eV was measured, which includes the peak top region of the absorption.3 The ATR-FUV technique allows for measurement of experimental data, which would elucidate the relationship between hydrogen bonding and the electronic structure of water. The isotope effects of absorptions for liquid methanol were observed.6 This result suggests that the Rydberg states of molecules exist even in the liquid phase. Recently, ATR-FUV spectra of aliphatic and branched hydrocarbons in the liquid phase have been reported.7 In this paper, the molecular structure and electronic absorption in the FUV region in the liquid phase are discussed. These experiments revealed that shapes and transition energies of the absorptions due to the Rydberg r 2011 American Chemical Society

transition in the liquid phase are different from but still correlate with those in the gas phase. However, information about Rydberg transitions in the liquid phase has yet to be reported. This paper reports band assignments of FUV spectra of liquid ketones and their Rydberg transitions. Electronic transitions of acetone in the gas phase have been studied by photoabsorption,1,8-12 energy loss,13,14 and resonanceenhanced multiphoton ionization (REMPI) spectroscopy.15-18 Since acetone is one of the benchmark molecules for quantum chemical calculations of electronic states for molecules containing a carbonyl group, several theoretical studies have been conducted on the vertical transition energy of acetone in the FUV region; examples of methods adopted in such studies include a complete active space self-consistent field (CAS SCF) method in combination with a multiconfigurational second-order perturbation approach (CASPT2),18 equation-of-motion coupled cluster (EOM-CC) method,19 random-phase approximation (RPA), time-dependent density function theory (TD-DFT), and EOM coupled-cluster theory with single and double substitutions (CCSD).20 These studies revealed the assignments for a few absorptions in the 7.45-4.38 eV region of acetone in the gas phase. The first electronic transition, the valence n-π* transition, is a dipoleforbidden transition; nonetheless, it is observed in the UV region (5.4-3.8 eV) as an electrovibronic transition.1 In the FUV region, n-3s Rydberg transitions of acetone in the gas phase Received: September 6, 2010 Revised: December 7, 2010 Published: January 12, 2011 562

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are observed at 6.35 eV as strong absorptions.1,9-12,18-20 Weak absorptions at around 7.4 eV are assigned to the n-3p Rydberg transitions.12,13,17-20 In the region higher than 7.5 eV, successive absorptions occur. The assignments of these absorptions generally include the 3d Rydberg transitions.12,18-20 Nobre et al. suggested that the n-4s transition also exists in this region in their Rydberg series assignments.12 Merch
an et al. also suggested the existence of the n-4s transition but did not perform the ab initio calculation.18 Several research groups discussed the contribution of the valence π-π* transition in this region.12,18,19 Nobre et al. tentatively assigned a spectral feature at around 7.85 eV to the valence π-π* transition on the basis of the absence of the Rydberg series assignment.12 With respect to other ketones in the gas phase, the absorption spectra in the region from 7.5 to 6.2 eV were measured by several research groups.1,8 Strong absorption is observed at around 6.3 eV, as in the case of acetone. Absorptions in the region of 7.5-7.0 eV of other ketones are stronger than that of acetone. These absorptions are generally attributed to the n-3s and n-3p transitions due to the similarity among the ketones in terms of excitation energy and intensity.13 The intensity of the n-3p transition should differ between acetone and other ketones because the selection rule is different between C2v and Cs.13 Theoretical studies on ketones based on ab initio calculations have not yet been carried out, except for the case of acetone. Spectra in the FUV region of ketones in the condensed phase have not been well explored, except for the solvatochromism of the n-π* transition of acetone.21 The n-π* transition of acetone, however, is forbidden, and it has been discussed as to how the vertical energy of acetone can be estimated experimentally.21,22 Absorption spectra of aliphatic ketones were obtained using paraffin as a solvent1 in the region from 7.0 to 6.0 eV (177 to 206 nm); the spectra showed a broad band at around 6.7 eV. Differences in the electronic states of molecules between the gas and liquid phases are still not well understood. For example, the 1B1-1A1 absorption of water appears at 7.4, 8.3, and 8.6 eV (167, 149, and 144 nm) for the vapor,23 liquid,3,24,25 and solid25,26 phases, respectively. These large blue shifts that occur upon the transition from the gas phase to the condensed phases should come from changes in the ground and excited states caused by molecular interactions and exchanging repulsive interactions between excited-state electrons and ground-state electrons of neighboring molecules. The Rydbergization of valence-type antibonding orbitals of water26 and NO27 was discussed in the condensed phase. In the rare gas matrices of Ar, Kr, and Xe, shifts of transition energy from the gas phase increase continuously by the lattice constant in the solid.26 In this paper, ATR-FUV absorption spectra of several kinds of liquid ketones, including branched ketones, are reported for the region of 8.55-6.20 eV (145-200 nm). This is the first report of the FUV spectrum of a ketone in neat liquid. Two or three characteristic absorption bands were observed in these spectra. The absorptions were assigned by comparing the obtained spectra with those for the gas phase. The results of this study suggest that the Rydberg states of molecules can be observed for liquids and the states are related to molecular structure features such as length or branching of alkyl chain among ketones.

(diethyl ketone, DEK), 3-methyl-2-butanone (methyl i-propyl ketone, MIPK), and 4-methyl-2-pentanone (i-butyl methyl ketone, MIBK) were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). All the regents were dried using 4-Å molecular sieves (Wako, Osaka, Japan). 2.2. Spectral Measurements. The details of the ATR-FUV spectrometer used are reported elsewhere.2,3,6 We adopted an internal reflection element (IRE) made of sapphire with an incident angle of 60°. First, we measured a reflection spectrum through the prism, which served as the background intensity I0. Then, we placed a liquid sample on the prism and measured its ATR intensity I. The ATR absorbance A is defined by the following formula: A = -log (I/I0). All the spectra were collected at room temperature (23-24 °C). 2.3. Spectral Analysis. The absorption coefficient ε is expressed as ε ¼ 4πk=λc where κ is an absorption index derived from ATR absorbance using the Kramers-Kronig transformation (KKT) for ATRFUV.3 The investigated ketones have several absorption bands in the 8.55-6.20 eV region. To obtain spectral parameters (center energy, bandwidth, and band area), the spectra are reproduced by two or three Gaussian functions. It should be noted that for the KKT of ATR-FUV spectra of ketones, the higher-energy side of the spectra is cut off by the instrumental limits. It is unknown which spectra the liquid spectra of these ketones should be connected with at 8.55 eV. In the present study, we connect the observed ATR spectra to the fourth or third polynomial smoothly to the flat end of the highenergy region. We use the spectra averaged by these treatments. Because of the variations of polynomials, peak positions in the higher-energy region diverge by approximately 0.04 eV. On the other hand, peak positions in the lower energy band do not change with the variations in connecting functions. From the present argument, we can say that the peak positions in the higher-energy region include an error of approximately 0.1 eV, whereas for a band in the lower-energy region, the result is satisfactory because the band is observed in whole. This result should also be confirmed by the independence of the variations in connecting polynomial functions. 2.4. Theoretical Study. To discuss the relationship between molecular structures and spectra, we carried out ab initio quantum chemical calculations for acetone, MEK, MPK, DEK, and MIPK. As described above, we carried out the calculations using the GAUSSIAN 09 program28 at the EOM-CCSD/aug-ccpVDZ level of approximation for the vertical energy of the electronic transitions. Geometric optimizations were calculated at the MP2/6-311þþG** level of approximation. To discuss a solvent shift, we used a polarlizable continuum model (PCM).29 To optimize the geometry of acetone in the neat liquid, a DFT calculation with coulomb-attenuating method (CAM)-B3LYP30 was used.

3. RESULTS 3.1. Observed Spectra. Figure 1 shows an ATR-FUV spectrum of liquid acetone (solid line) and the corresponding spectrum of the molar absorption coefficient ε derived by KKT (dashed line). As described above, an ATR-FUV spectrum is pulled to the lower energy side by the contribution of the complex refractive index.3 For example, the spectrum of liquid

2. EXPERIMENTAL SECTION 2.1. Materials. Acetone, 2-butanone (methyl ethyl ketone, MEK), 2-pentanone (methyl n-propyl ketone, MPK), 3-pentanone 563

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Figure 1. ATR-FUV spectrum of liquid acetone (solid line) and corresponding spectrum of molar absorption coefficient ε derived by KKT (dashed line).

Figure 3. Second derivatives of FUV-ε spectra of (a) aliphatic ketones terminated by CH3 (MEK, MPK, and DEK) and (b) branched ketones (MIBK and MIPK) in the liquid phase.

Table 1. Fitting Parameters of Band A and Band C for Four Kinds of Aliphatic Ketones;Acetone, MEK, MPK, and DEKa

Figure 2. FUV-ε spectra of (a) aliphatic ketones terminated by CH3 (MEK, MPK, and DEK) and (b) branched ketones (MIBK and MIPK) in the liquid phase.

acetone

MEK

MPK

DEK

6.682(4) 909(10)

6.689(4) 786(22)

6.610(4) 416(12)

Band A

xc A

6.758(4) 2044(11)

w

0.429(4)

0.354(4)

0.308(8)

0.335(10)

Band C

xc

8.47(1)

8.50(1)

8.65(1)

8.49(1)

A

36580(110)

41850(140)

56300(500)

43340(160)

w

1.661(4)

1.855(5)

2.090(12)

1.774(5)

Aobs

20000(110)

22040(140)

256600(500)

23140(160)

The parameters were derived by fitting each band with two Gaussian functions. The units of band centers (xc), areas (A), and bandwidths (w) are eV, M-1 cm-1 eV, and eV, respectively. Aobs denotes the area of Band C in the observed region. The numbers in parentheses show errors estimated with a fitting or experimental error of 1σ. a

acetone contains two broad bands in the region of 8.55-6.20 eV. Their peaks appear at 7.99 and 6.71 eV in the ATR spectrum (solid line). The spectrum of the absorption cross section (dashed line) can be fitted by two Gaussian functions whose centers are located at 8.47 and 6.76 eV. Figure 2a shows FUV-ε spectra of aliphatic ketones terminated by CH3 (MEK, MPK, and DEK). Figure 2b shows those of branched ketones (MIPK and MIBK). All the ketones have an absorption band at around 6.7 eV. However, the higher-energy regions of all ketones are significantly different from each other in their spectral shapes. The branched ketones;MIPK and MIBK;have a clear shoulder near 7.3 eV. Parts a and b of Figure 3 show the second derivative spectra of parts a and b of Figure 2, respectively. The second derivative spectra of aliphatic ketones (Figure 3a) show two negative peaks near 8.0 and 6.7 eV, whereas branched ketones (Figure 3b) have another negative peak between them (7.3 eV). These results strongly suggest that the branched ketones have a third component near 7.3 eV. In this paper, the absorption peaks at around 6.7 and 8.5 eV are referred to as Band A and Band C, respectively. The shoulder seen for the branched ketones is referred to as Band B.

We collected the fitting parameters for Band A and Band C for the four aliphatic ketones (acetone, MEK, MPK, and DEK) and for Band A, Band B, and Band C for the branched ketones (MIPK and MIBK); these parameters are summarized in Tables 1 and 2, respectively. With respect to the aliphatic ketones, two Gaussian functions were used to obtain the parameters; whereas for the branched ketones, three Gaussian functions were used to obtain the parameters. 3.2. Results of Ab Initio Quantum Chemical Calculations. Table 3 summarizes the transition energies of acetone calculated in the present study using EOM-CCSD/aug-cc-pVDZ and in other theoretical studies using CASPT2,18 EOM-CC/POL1,19 and EOM-CCSD/6-311(2þ,2þ)G**.20 These results are compared with experimental results for acetone in the gas phase studied by photoabsorption12 and REMPI.17,18 The present calculated results using EOM-CCSD/aug-cc-pVDZ give satisfactory transition energies for the n-π*, n-3s, and n-3p 564

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transitions, as in the case of other studies. However, the assignment of the n-3d transition has still not been understood clearly in the present study because of confusion of the intensity and the excitation energy as can be seen in a previous work.18 The calculated values of oscillator strengths using EOM-CCSD/augcc-pVDZ are quite similar to those obtained using EOM-CC/ POL1.19 Figure 4 shows the calculated transition energies, oscillator strengths, and spectral simulations (dashed line) for (a) acetone, (b) MEK, (c) MPK, (d) DEK, and (e) MIPK using the EOMCCSD/aug-cc-pVDZ level approximation together with their observed spectra (solid line). To reproduce the observed spectra in the liquid phase, transition widths of 0.4, 1.1, and 1.8 eV were employed for transitions around 6.4, 7.5, and 8.0 eV and higherenergy transitions, respectively. According to the theoretical

results, the following assignments can be considered for these ketones: (1) There is an isolated transition from n to the excitation orbital, whose representation is A1 for C2v species (acetone and DEK) or A0 for Cs species (MEK, MPK, and MIPK) in the region of 6.4-6.3 eV. This transition is assigned to the n-3s transition. (2) There is a group of transitions containing three transitions from n to the orbitals, whose representations are A1, A2, and B2 for the C2v species or two of A0 and A00 for the Cs species in the region of 7.60-7.13 eV. These transitions are ascribed to the n-3p transition. (3) There is a group of transitions containing 5-7 transitions from n in the region of 8.9-7.8 eV. For acetone, the upper orbital of the transitions is 5 orbitals of the 3d Rydberg transition. With regard to the other ketones, because of their similarity in strength and transition energy, they include the same kinds of transitions. Additional transitions should contain 4s or higher components. (4) There is a transition that has the strongest oscillator strength in this region, which is assigned to the π -π* transition.

Table 2. Fitting parameters of Band A, Band B, and Band C for Two Kinds of Branched Ketones;MIPK and MIBKa MIPK Band A

Band B

Band C

MIBK

xc

6.606(4)

6.606(4)

A

482(22)

557(25)

w

0.310(9)

0.304(9)

xc

7.56(2)

7.54(2)

A

8400(2000)

8900(1900)

w

1.12(5)

1.10(4)

Aobs xc

8300(2000) 8.75(2)

8700(1900) 8.77(2)

A

42000(4000)

53000(4000)

w

1.79(14)

1.82(12)

Aobs

17000(4000)

21000(4000)

4. DISCUSSION 4.1. Assignments of FUV Spectra of Acetone. To assign the bands observed in the 8.55-6.2 eV region of acetone in the liquid, the liquid phase spectra should be compared with those of acetone in the gas phase. As mentioned in section 1, the lowest allowed transition found at 6.35 eV for the gas phase was assigned to the n-3s Rydberg transition.1,10-12,15 Successive absorptions in the region of 8.5-7.3 eV correspond to the n-3p and n-3d components of acetone in the gas phase.12,13,17-20 Band A near 6.75 eV of the liquid phase acetone is observed at a slightly higher energy than the n-3s transition of the gas phase acetone (6.35 eV). As will be discussed in section 4.2, we determined the correlations between the n-3s transition and

a The parameters were derived by fitting each band with two Gaussian functions. The units of band centers (xc), areas (A), and bandwidths (w) are eV, M-1 cm-1 eV, and eV, respectively. Aobs denotes the area of Band C in the observed region. The numbers in parentheses show errors estimated with a fitting or experimental error of 1σ.

Table 3. Transition Energies of Acetone Calculated in the Present Study (EOM-CCSD/aug-cc-pVDZ) and in Previous Studies by Other Groups EOM-CCSD/aug-cc-pVDZa

Previous results EOM-CCSD/6-311

excited state

a

exptl (gas phase)

energy

oscillator strength

EOM-CC/POL1b

CASPT2c

(2þ,2þ)G**d

11A2

(n-π*)

4.49e

4.49

0.0000

4.48

4.18

4.47

11B2

(n-3s)

6.36e

6.40

0.0314

6.39

6.58

6.42

21A2

(n-3p)

7.45e

7.43

0.0000

7.41

7.34

7.31

21A1

(n-3p)

7.36f 7.41f

7.47

0.0001

7.45

7.26

7.41

21B2

(n-3p)

7.45f

7.60

0.0071

7.51

7.48

7.39

31B2

(n-3d)

8.09c

8.04

0.0414

7.95

8.04

7.82

31A1

(n-3d)

7.8c

8.24

0.0702

8.23

7.91

8.02

11B1

(n-3d)

8.17c

8.49

0.0160

8.43

8.20

8.11

41B2

(n-3d)

8.76

0.0000

8.48

8.18

8.10

31A2

(n-3d)

8.79

0.0000

8.44

8.09

8.08

41A1 21B1

(π-π*) (σ-π*)

9.21 9.29

0.2743 0.0016

9.15 9.30

9.16 9.10

8.59 9.31

41A2

(σ-π*)

9.47

0.0000

9.45

8.60

Present study. b Reference 19. c Reference 18. d Reference 20. e Reference 12. f Reference 17. 565

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Figure 4. Transition energies, oscillator strengths, and spectral simulations for (a) acetone, (b) MEK, (c) MPK, (d) DEK, and (e) MIPK calculated by EOM-CCSD/aug-cc-pvDZ. Stick diagram: Calculated transition energy and oscillator strength. Black line, simulated ε spectra; red line, observed spectra in the liquid phase.

EOM-CC/POL1,19 and the present calculation (EOM-CCSD/ aug-cc-pVDZ), the ratios of the total oscillator strength of n-3p and n-3d to n-3s of acetone were estimated to be 3.5, 4.5, and 4.3, respectively. The oscillator strength of the π-π* transition was calculated to be 21.5, 8.1, and 8.7 times that of n-3s by CASPT2,18 EOM-CC/POL1,19 and EOM-CCSD/aug-ccpVDZ, respectively. Thus, the large intensity of Band C may be explained by the contribution of the π-π* transition directly as an overlapped absorption caused by a red shift from the gas phase as seen in the π-π* transition of amides31,32 or indirectly by intensity borrowing, for example, which results in the higher energy shift of Rydberg states in the liquid phase. 4.2. Assignments of FUV Spectra of Ketones. FUV spectra in the gas phase were studied by photoabsorption spectroscopy in the region of 7.5-6.2 eV1,9 for all the investigated ketones in the present study and by energy loss spectroscopy in the region of 9.0-6.0 eV for MEK.13 To the best of our knowledge, theoretical studies on the relationship between FUV spectra and the molecular structure of a ketone have not been conducted, the exception being acetone. Thus, the present calculation may be the first such research in this area. In the gas spectra of ketones investigated in the present study, a peak is found in the region of 6.20-6.35 eV.9 We observed the corresponding peak for the liquid phase in the region of 6.75-6.60 eV as Band A. The peak shift between the gas and liquid phases is very similar for all the investigated ketones. Thus, the excitation energy in the liquid phase is correlated with that in the gas phase,9 as shown in Figure 5. A similar correlation can be seen for the oscillator strength in Figure 6. The correlations of both excitation energy and intensity strongly suggest that the lowest energy band observed for the liquid phase originates from the n-3s Rydberg transition, as is the case in the gas phase. The spectral simulation based on the result of EOM-CCSD/aug-cc-pVDZ reproduces Band A in the lower-energy region as shown in Figure 4.

Band A in terms of the 0-0 transition and the oscillator strength among all the investigated ketones. The correlations strongly suggest that Band A of the liquid phase originates from the n-3s transition, as in the case of the gas phase. The spectral simulation based on the result of EOM-CCSD/aug-cc-pVDZ reproduces Band A in the lower-energy region as shown in Figure 4. We observed Band C as a large and broad absorption at 8.47 eV for liquid acetone. As for the spectra of acetone in the gas phase, a strong, broad, and complicated structured band was observed in the region of 8.5-7.5 eV, and the most intense absorption peak was found at 8.10 eV.12 The assignment of this absorption has not yet been elucidated. The theoretical studies conducted using CASPT2,18 EOM-CC,19 and EOM-CCSD20 predicted that there are two components of the n-3d Rydberg state and that the intensities of these transitions strongly depend on the methods of calculations, as discussed in ref 18. Mercian et al. measured a (3 þ 1) REMPI spectrum for acetone in the gas phase. In the REMPI spectrum, two intense peaks were observed at 8.09 and 8.20 eV.18 They suggested that the absorption at 8.09 is due to the n-3dx2-y2 Rydberg transition on the basis of consideration of the selection rule for the (3 þ 1) REMPI, although the calculated intensity of the band did not correspond to their result. They concluded that the Rydberg states in this region are strongly perturbed by the valence antibonding π-π* state and that the difference between the experimental and theoretical calculation is a result of the perturbation. Since the FUV spectra of liquid phase ketones show no structured absorption and they heavily overlap each other, their detailed assignment would not be easy. In this paper, the intensity ratio between Band A and others for acetone is discussed. The area of Band B and Band C (ABandBþBandC = Atotal - ABandA) is 9.0 times that of Band A for acetone in the liquid phase. According to theoretical calculations based on CASPT2,18 566

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Table 4. Comparison of a Gas-to-Liquid Shift between Experiments and Ab Initio Calculation Using PCM for the n-π* and the n-3s Rydberg Transition of Acetone experiments gas

a

neat liq shift

1 A2 (n-π*) 4.43

b

4.51

11B2 (n-3s) 6.36a

6.75c

1

a

TD-CAM-B3LYP/aug-cc-pVTZ gas

neat liq

shift

0.08

4.43

4.60

0.17

0.39

6.39

6.62

0.22

Reference 12. b Reference 22. c Present result.

transition than the n-π* transition means that the 3s orbital should be more delocalized and nondirectional than the π* orbital. Quantum chemical calculations with a PCM are widely used to estimate transition energies in solvents. Recently, Aidas et al.35 reported the calculated result of the valence n-π* transition for acetone in neat liquid and its aqueous solution by using a combined quantum mechanics/molecular mechanics (QM/MM) model coupled to classical molecular dynamics (MD) simulations and the PCM. Since we focused on the n-3s Rydberg transition, we used TD-CAM-B3LYP to reproduce the transition energy of the Rydberg orbitals. Table 4 shows results of the calculation of TD-CAM-B3LYP/aug-cc-pVTZ for acetone in the gas phase and in the neat liquid. The calculated results for the gas phase correspond well to the experimental results. The calculated gas-to-liquid energy shift of the n-3s Rydberg transition is estimated to be 0.22 eV. It reproduces the larger shift of the n-3s transition than n-π* transition but it is a little smaller than the experimental results as shown in Table 4. The discrepancy between the shift observed in the FUV absorption spectra and that calculated with PCM should result from the neglect of intermolecular interactions, as Aidas et al. pointed out.35 The higher energy shift of the Rydberg transition was observed for the n-3s Rydberg transition of water.3 The shift of the band of the n-3s transition in acetone (0.4 eV) is smaller than that in water (approximately 1 eV).3 It is known that the shift in the n-3s transition of water depends on its temperature.3 This dependence means that the liquid structure is related to the spectral shift and is attributed to hydrogen bonding. From the analogy to water, the smaller shift in the liquid acetone indicates that the interaction between acetones is weaker than hydrogen bondings of water. More detail discussion should be carried out after more data on solvatochromism of n-3s Rydberg transition will be obtained. The bandwidth of the n-3s transition is 0.38 eV for acetone. The origin of the bandwidth is unclear, as in the cases of other liquids3,6 and solids.36 Since the shape of Band A can be fitted well using a Gaussian function, the origin should be statistical, i.e., environmental perturbation.

Figure 5. Correlation between excitation energy of n-3s transition in the liquid phase and that in the gas phase.

Figure 6. Correlation between oscillator strength of the n-3s transition in the liquid phase and that in the gas phase.

Band B is observed at 7.40 and 7.46 eV for MIPK and MIBK, respectively (Figure 2b and Figure 3b). Ito et al. reported that MIPK and MIBK in the gas phase show absorption at around 7.10 and 7.11 eV (174.6 and 174.2 nm), respectively.9 This band is observed especially for branched ketones.9 The difference in the molecular symmetry between C2v (acetone and DEK) and Cs (MEK, MPK, MIPK, and MIBK) may change the intensity of the n-3p transition.13 As can be seen in Figure 3a, a weak peak is observed at around 7.2 eV for MEK and DEK. The peak of MEK is stronger than that of DEK. From this result, the second band at around 7.4 eV of the branched ketones is assigned to the n-3p Rydberg transition. The theoretical calculations using EOMCCSD/aug-cc-pVDZ support this conclusion, as shown in Figure 4. FUV spectra of ketones in the gas phase in the region above 7.5 eV have not yet been reported. Thus, only theoretical calculations in the present study are available for the analysis of Band C of the ketones in the liquid phase. As discussed in the previous section for acetone, the intensity relation between Band A and Band C of the investigated ketones cannot be explained by the assignment of Band C to the n-3d Rydberg transition, and thus, the π-π* transition should be added to the assignment of Band C. 4.3. Peak Shift and Broadening of n-3s Transition for Acetone in Liquid Phase. The n-3s Rydberg transition of acetone was observed at 6.76 eV in the spectrum of the liquid phase. The transition energy shifts to a higher energy side by 0.39 eV from the corresponding position of in the gas phase. The shift of 0.39 eV is significantly larger than that of the n-π* transition (0.05 eV).21 In the case of the n-π* transition the shift originates from a difference in the dipole-dipole interaction in the liquid phase between the ground and excited states of acetone.33,34 According to this theory, the larger blue shift in the n-3s

5. CONCLUSION FUV spectra were measured for acetone, MEK, MPK, MBK, DEK, MIPK, and MIBK using the ATR-FUV technique. The n = 3 Rydberg transitions were investigated for these compounds in the liquid phase. Three regions of absorption bands were found in these spectra. A comparison of the spectra of the liquid phase with those of the gas phase revealed that Band A and Band B are strongly correlated with the molecular structure of the investigated ketones and are assigned to the n-3s and n-3p Rydberg transitions, respectively. The ab initio calculations of the 567

dx.doi.org/10.1021/jp108510c |J. Phys. Chem. A 2011, 115, 562–568

The Journal of Physical Chemistry A

ARTICLE

investigated ketones were carried out using EOM-CCSD/aug-ccpVDZ. These calculations supported the above assignments, which confirm that the Rydberg transitions exist even in the liquid phase. Although the assignment of Band C is still unclear similar to the case of the gas phase, the contribution of the π-π* transition may be involved in Band C. Band broadening and the higher energy shift from the gas phase to the liquid phase were discussed for acetone. As to the gas-to-liquid shift for acetone, the PCM calculation at the TDCAM-B3LYP/aug-cc-pVTZ level was carried out. The gas-to-liquid shift estimated by the PCM was smaller than the observed shift. This discrepancy is expected to originate from perturbation by neighboring molecules.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This study was supported by the System Development Program for Advanced Measurement and Analysis (Program-S) of the Japan Science and Technology Agency (JST). This study was partially supported by a Grant-in-Aid for Young Scientists (B) from the Japan Society for the Promotion of Science (JSPS). ’ REFERENCES (1) Robin, M. B. Higher Excited States of Polyatomic Molecules; Academic Press: New York, 1974, Vol II. (2) Higashi, N.; Ikehata, A.; Ozaki, Y. Rev. Sci. Instrum. 2007, 78, 103107. (3) Ikehata, A.; Higashi, N.; Ozaki, Y. J. Chem. Phys. 2008, 129, 234510. (4) Higashi, N.; Ozaki, Y. Appl. Spectrosc. 2004, 58, 910–916. (5) Higashi, N.; Yokota, H.; Hiraki, S.; Ozaki, Y. Anal. Chem. 2005, 77, 2272–2277. (6) Morisawa, Y.; Ikehata, A.; Higashi, N.; Ozaki, Y. Chem. Phys. Lett. 2009, 476, 205–208. (7) Tachibana, S; Morisawa, Y.; Ikehata, A.; Sato, H.; Higashi, N.; Ozaki, Y. Appl. Spectrosc. 2011in press. (8) Barnes, E. E.; Simpson, W. T. J. Chem. Phys. 1963, 39, 670–675. (9) Ito, H.; Nogata, Y.; Matsuzaki, S.; Kuboyama, A. Bull. Chem. Soc. Jpn. 1969, 42, 2453–2458. (10) Robin, M. B.; Kuebler, N. A. J. Mol. Spectrosc. 1970, 33, 274– 291. (11) McDiarmid, R. J. Chem. Phys. 1991, 95, 1530–1536. (12) Nobre, M.; Fernandes, A.; Ferreiru da Silva, A.; Antunes, R.; Almeida, D.; Kokhan, V.; Hoffmann, S. V.; Mason, N. J.; Eden, S.; Limao-Vieira, P. Phys. Chem. Chem. Phys. 2008, 10, 550–560. (13) Doering, J. P.; McDiarmid, R. J. Chem. Phys. 1982, 76, 1838– 1844. (14) Walzl, K. N.; Koerting, C. F.; Kuppermann, A. J. Chem. Phys. 1987, 87, 3796–3803. (15) Gaines, G. A.; Donaldson, D. J.; Strickler, S. J.; Vaida, V. J. Phys. Chem. 1988, 92, 2762–2796. (16) Philis, J. G.; Goodman, L. J. Chem. Phys. 1993, 98, 3795–3802. (17) Thakur, S. N.; Guo, D.; Kundu, T.; Goodman, L. Chem. Phys. Lett. 1992, 199, 335–340. (18) Merch
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