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Two-color two-photon REMPI and ZEKE spectroscopy of supersonically cooled o-aminobenzoic acid
Chemical Physics Letters 398 (2004) 351–356 www.elsevier.com/locate/cplett
Two-color two-photon REMPI and ZEKE spectroscopy of supersonically cooled o-aminobenzoic acid Chengyin Wu, Yonggang He, Wei Kong
*
Department of Chemistry, Oregon State University, Gilbert 153, Corvallis, OR 97331-4003, USA Received 21 July 2004; in final form 20 September 2004 Available online 7 October 2004
Abstract We report studies of supersonically cooled o-aminobenzoic acid using two-color resonantly enhanced multiphoton ionization and zero kinetic energy photoelectron spectroscopy. Vibrational modes of the first electronically excited state (S1) of the neutral species and those of the ground state cation (D0) have been assigned, and the ionization threshold has been determined. The results are analyzed in comparison with the other two isomers of aminobenzoic acid, mainly in terms of electron back donation of the two substituents in both the S1 and the D0 states. The change in the intramolecular hydrogen bond upon electronic excitation and ionization will also be discussed. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction o-Aminobenzoic acid (OABA), also named anthranilic acid, is interesting both from a chemistry and a biology point of view. Like p-aminobenzoic acid (PABA) and m-aminobenzoic acid (MABA), OABA is a doubly substituted electron push–pull aromatic system with the donor (–NH2) and the acceptor (–COOH) groups connected by the p-ring [1–4]. Upon electronic excitation, however, both electron rich substituents can back donate to the electron deficient ring through hyper conjugation. On the biological aspect, OABA has largely been used as a convenient fluorescent probe in internally quenched fluorescent peptides because of its high quantum yield, small size, and convenient spectral regions [5–13]. These properties are essential in reducing the potential influence of the probe on the substrate–protease interaction. Interestingly, when OABA is bound to proline and tryptophan [13], its fluorescence quantum *
Corresponding author. Fax: +1 541 737 2062. E-mail addresses: [email protected], wei.kong@oregonstate. edu (W. Kong). 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.09.095
yield decreases by an order of magnitude. The origin of this behavior is still unknown. Spectroscopic studies of OABA in both the gas phase and in solutions are limited [14–17]. The most recent work has been reported by Southern et al. [17] on the vibrational spectroscopy of both the ground state (S0) and the first electronically excited state (S1). The authors have concluded that only rotamer I exists in the molecular beam even though OABA has two rotameric structures differing by a 180° rotation of the carboxyl group. From the comparison of the NH stretching frequency between OABA and aniline, the authors have also concluded that a much stronger intramolecular hydrogen bond is formed in the electronically excited state than in the ground state. More recently, Stearns et al. [18] have studied the water complex of OABA and found that water has essentially no effect on the intramolecular hydrogen bond in both the ground and the excited state. On the theoretical front, Pirowska et al. [19] have calculated the geometry changes of OABA upon S1 S0 electronic excitation. The length of the hydrogen bond ˚ in the excited state, lending decreases by 0.114 A credence to the conclusion of Southern et al. [17].
C. Wu et al. / Chemical Physics Letters 398 (2004) 351–356
Recently, we have investigated the spectroscopic properties of the first excited electronic state and the cationic ground state (D0) of a series of substituted aromatic systems [3,4,20,21]. Our results indicate that upon electronic excitation and ionization, the substituent would back donate electrons to the aromatic ring, ensuring the integrity of the ring structure. Consequently, a strong propensity of Dv = 0 has been observed in the two-color zero kinetic energy (ZEKE) photoelectron spectrum, where v represents the vibrational quantum number of the intermediate state. From the bi-substituted systems such as PABA and MABA, we have also learned that the distance between the substituents affects the dynamics of electron back donation [3,4]. In OABA, the carboxyl group is ortho to the amino group, and a much stronger intramolecular hydrogen bond should exist between the substituents. The effect of this intramolecular hydrogen bond on the electronic structure of the related states is therefore worth investigating. In this report, we present studies of two-color twophoton resonantly enhanced multiphoton ionization (REMPI) and ZEKE spectroscopy of OABA. Complete assignments of the ZEKE and REMPI spectra are achieved with the assistance of ab initio and density functional calculations (DFT). Based on the experimental results and theoretical calculations, changes in the molecular geometry and the intramolecular hydrogen bond upon electronic excitation and ionization will be discussed.
2. Experimental The experimental apparatus has been reported in our previous publications [22,23]. Briefly, it consisted of a pulsed molecular beam source and a time-of-flight mass spectrometer, which could be converted into a pulsed field ionization (PFI) zero kinetic energy photoelectron spectrometer. OABA was purchased from Aldrich Co. and used without further purification. The sample was heated to 145 °C in the nozzle (diameter: 1 mm), and the vapor was seeded into 2 atm of argon. Both the pump and the probe beams were generated by frequency doubling of the output of Nd:YAG (Spectra Physics, GCR 230 and 190) pumped dye laser systems (Laser Analytical Systems, LDL 2051 and LDL 20505). The absolute wavelength of each laser was calibrated using an iron hollow cathode lamp filled with neon. The relative timing between the two laser pulses was controlled by a delay generator (Stanford Research, DG535). In the ZEKE experiment, molecules excited to ZEKE states were allowed to stay for 500–600 ns in the presence of a spoiling field of 1 V/cm, and ionization and extraction was achieved by a pulsed electric field of 16 V/cm.
In order to assign the observed vibronic structures in both the REMPI and ZEKE spectra, we used the GAUSSIAN 03 suite [24] to calculate the geometry and vibrational frequencies. Density functional theory calculations using the Becke 3LYP functional were carried out with the 6-31+G(d) basis set for both the S0 and D0 states. The calculation for the S1 state was performed at the CIS level using the 6-31G(d,p) basis set. Good agreement between experimental and theoretical results was obtained when a scaling factor of 0.95 for the calculated frequencies of the S1 state was used [25], while no scaling factor was used for the D0 state.
3. Results 3.1. Two-color 1+1 0 REMPI spectrum The two-color 1+1 0 REMPI spectrum of OABA is displayed in Fig. 1. The ionization laser was set at 285 nm and was temporally overlapped with the scanning resonant laser. The linewidths of most transitions are on the order of 5 cm1, though the linewidth of the excitation laser is only 0.2 cm1. Unresolved rotational profiles are believed to contribute to the observed width. The spectrum is similar to that reported by Southern et al. [17], and assignment for all the observed features is listed in Table 1, including our own calculation results. Modes that are associated with the motion of the aromatic ring are labeled using the convention of VarsanyiÕs nomenclature [26]. Other modes that mainly involve the bending motion of the –NH2 and –COOH moieties are named with letters from A to D in the order of increasing frequency.
2
1
3
n
Ion intensity (arbitrary units)
352
D
2,1
1,1 m
2,2
1,2
n
B D
000 m
B
1
3
2
4
q
C
3
2
1 1
1 6a 6b
0
200
400
600
800
1000
1200
1400
Excitation Energy (cm-1) - 28594 cm-1 Fig. 1. (1+1 0 ) REMPI spectrum of supersonically cooled OABA. The spectrum is shifted by 28 594 cm1 (the origin of the S1 S0 transition) to emphasize the frequencies of the different vibrational modes of the S1 state.
C. Wu et al. / Chemical Physics Letters 398 (2004) 351–356
353
Table 1 Observed vibrational frequencies and assignments for the S1 state of OABA
220 254 355 379 418 506 544 595 602 632 672 713 762 772 797 839 884 923 961 988 1013 1050 1092 1130 1140 1178 1218 1258 1342
b
Calculation
Assignment
105*2 258 371 208*2 415
A20 ; c ðACOOH torsionÞ B10 ; b ðACOOHÞ C10 ; b ðANH2 Þ 10b20 ; c ðringÞ D10 ; b ðACOOHÞ; b ðANH2 Þ and b ðringÞ B20 6a10 ; b ðringÞ A20 10b20 B10 C10 or 16b10 ; c ðringÞ 6b10 ; b ðringÞ B10 D10 C20 B30 C10 D10 B10 6a10 D20 19a10 ; b ðringÞ B20 D10 B10 C20 C10 6b10 B40 C30 B10 D20 C20 D10 B20 6b10 6a10 6b10 B20 C20 D30 B20 D20
258*2 560 105*2+208*2 (258 + 371) or 603 626 258 + 415 371*2 258*3 371 + 418 258 + 560 415*2 875 258*2 + 415 258 + 371*2 371 + 626 258*4 371*3 258 + 415*2 371*2+415 258*2+626 560 + 626 258*2 + 371*2 415*3 258*2 + 415*2
+
a
The calculation was on the CIS/6-31G(d,p) level, and the values include a scaling factor of 0.95. b b and c represent in-plane bending and out-of-plane bending vibrations.
The spectrum is dominated by progressions and combinations of modes B, C, and D. This rich activity of the in-plane bending modes of the substituents reflects a large geometry change upon electronic excitation. On the other hand, the ring deformation modes, such as modes 6a, 6b, 12, and 1, play only minor roles. Two of the most active ring deformation modes are labeled in Fig. 1 for comparison. Interestingly, in PABA and MABA, the REMPI spectra also showed rich activities of the substituents [3,4]. This common theme among the three isomers of aminobenzoic acid implies active participation of the substituents in this electronic excitation. 3.2. ZEKE spectra Using the origin band and a few strong bands involving modes B and D as intermediate states, we recorded the ZEKE spectra of OABA as shown in Fig. 2. In Table 2, the assignment of the observed features is summarized in comparison with our own calculation results. No scal-
.B
+
D
+2
+2
A D
+2
2+
Ion Intensity
Experimental
a
.B D .D .B
+3
B
+2
B D
+
+
+
B
+2
+
BD
+
+2
.0
A
0
200
+
D E+
0+
400
+
+ 6a 6b
600
800
1000
Ion Internal Energy (cm-1) Fig. 2. Two-color ZEKE spectra of OABA recorded via the following vibrational levels of the S1 state as intermediate states: (a) 00; (b) B1; (c) D1; (d) B2; (e) B1D1. The energy in the abscissa is relative to the ionization threshold at 63 232 cm1. The assignment in the figure refers to the vibrational levels of the cation, and the corresponding vibrational level of the intermediate state is labeled by a black dot in each panel.
ing factor for the theoretical vibrational frequency is used, and the agreement seems reasonable. The same labeling scheme as that of the S1 state is used for the D0 state, but a superscript Ô+Õ is used to distinguish the modes of the D0 state. The identity of the intermediate level for each ZEKE spectrum is marked on the figure by a black dot. The spectra in Fig. 2 are remarkably simple, which can be partially attributed to state selection achieved in typical REMPI experiments. Each spectrum is dominated by a single strong transition, and in most cases, it can be assigned as the same vibrational level as that of the corresponding S1 state. This strong correlation between the intermediate level and the final vibrational level of the ion has been reported in our previous studies of p-aminobenzoic acid [3], m-aminobenzoic acid [4], 4aminopyridine [20], and 2-chloropyrimidine [21]. We have also derived a propensity rule of Dv = 0 to describe this effect. Assignment of Fig. 2 is straightforward. The first feature in the spectrum obtained via the origin band of the S1 state is the origin band of the cation 0+0. The resulting adiabatic ionization energy, including a correction due to field ionization [27], is 63232 ± 6 cm1 (7.8398 ± 0.0007 eV). In contrast, Meeks et al. [16] determined a higher value of 8.0 eV using He(I) photoelectron spectroscopy. This difference might be a result of the low resolution of typical PES experiments or small Franck–Condon factors at the threshold during single photon ionization at 21.2 eV. Other features are mostly high overtone bands and combination bands of the bending modes of the substituents.
354
C. Wu et al. / Chemical Physics Letters 398 (2004) 351–356
Table 2 Observed vibrational frequencies and assignments in the ZEKE spectra of OABA Intermediate level in the S1 state 000
B10
D10
B20
0 190 258 403 489
400 515
516
548 633 662
662 774 805 919 992
a
Calculation
Assignmenta
78*2 259 409 510 259*2 559 640 259 + 409 259*3 409*2 259*2 + 409 78*2 + 409*2
0+ A+2 B+ D+ E+ B+2 6a+ 6b+ B+D+ B+3 D+2 B+2D+ A+2D+2
B10 D10
The Ô+Õ in the superscript represents the cationic state.
4. Discussion Based on the work of Southern et al., only rotamer I exists in a supersonic molecular beam [17]. The agreement between our experimental and theoretical results, as shown in Table 1 and Fig. 1, further supports this conclusion. The spectral congestion is thus not a result of different rotamers; rather, it reflects the rich Franck–Condon activity. The calculated geometry parameters are listed in Table 3 and the numbering scheme is shown in Fig. 3. OABA is nonplanar in the ground state and planar in the S1 and D0 states. During the S1 S0 transition, the bond lengths for C1AN7 and C2AC8 decrease, which indicates participation of both polar groups in the conjugation with the aromatic ring. The resulting chemical bonds between the adjacent moieties take on
Table 3 Molecular geometry parameters of OABA in the S0, S1, and D0 states S0
S1
D0
Bond length (A˚) C1AC2 C2AC3 C3AC4 C4AC5 C5AC6 C6AC1 C1AN7 N7AH1 N7AH2 C2AC8 C8AO1 C8AO2 O2AH H1AO1
1.425 1.409 1.382 1.403 1.381 1.413 1.363 1.010 1.005 1.466 1.222 1.359 0.968 1.950
1.483 1.399 1.404 1.401 1.400 1.402 1.324 1.004 0.994 1.432 1.223 1.340 0.947 1.863
1.449 1.377 1.413 1.407 1.373 1.436 1.327 1.027 1.014 1.496 1.214 1.330 0.972 1.840
Dihedral angle (°) H1AN7AC1AC2 H2AN7AC1AC6
7.125 9.745
0 0
0 0
Fig. 3. Structure and numbering scheme of OABA.
partial double bond characters. The ring structure, on the other hand, remains largely unchanged, except for an elongation of the C1AC2 bond. This stability of the aromatic ring explains the inactivity of the ring breathing mode 1. In comparison, mode 1 is the most active in the REMPI spectrum of PABA [3], while it is only a weak feature in MABA [4]. This progressive change in activity for the ring breathing mode from OABA to PABA reflects the effect of the intramolecular hydrogen bond. The ZEKE spectra in Fig. 2 follow the propensity rule, and this observation is consistent with our previous results on both mono- and bi-substituted compounds [3,4,20,21]. Table 4 lists the Mulliken charge [28] distribution of OABA based on our calculation. The large negative charge on the amino group in the S1 state re-
Table 4 Mulliken charge distributions of OABA in the S0, S1, and D0 states –NH2 –COOH Benzene ring C1 C2
S0
S1
D0
+0.224 0.235 +0.013 0.956 1.119
0.121 0.066 +0.187 +0.403 0.253
+0.486 +0.013 +0.501 1.625 +1.522
C. Wu et al. / Chemical Physics Letters 398 (2004) 351–356
flects the substantial contribution of the atomic orbitals of the amino nitrogen to the p* orbital, similar to the case of MABA [4]. Further ionization removes this p* electron, resulting in a large positive charge on the amino group. We thus believe that in the S1 state, both polar groups donate electrons to the ring in this bi-substituted compound, while in the D0 state, it is the amino group that dominates the electron back donation. As a result, the electron deficiency of the ring is alleviated, and the rigidity of the ring structure is preserved during both electronic excitation and further ionization. It is interesting to compare the three isomers of aminobenzoic acid in terms of changes in geometry and charge distribution upon electronic excitation and ionization. From Table 3, the geometric changes in OABA are quite similar to those in MABA, including the change in the bond length C1AN7 and the planarity of the S1 and D0 states. In Table 4, the charge distributions in the S1 state and the D0 state are also similar for OABA and MABA. In contrast, the amino group and the carboxyl group have almost equal share of the positive charge in the D0 state of PABA, and the S1 state of PABA is non-planar [3]. The bond length for C1AN7 in PABA becomes progressively shorter with successive excitation. These results imply that in the S1 and D0 states, the ortho and the meta compounds are similar, while they are considerably different from the para compound. This conclusion is similar to that of Jain et al. [15], and it is different from the general belief in organic chemistry [29]. However, one should notice that all of the above discussions are applicable to the S1 and D0 states, while the resonance and the inductive effect discussed in organic chemistry are for the ground state. A unique feature of OABA compared with the other two isomers of aminobenzoic acid is the intramolecular hydrogen bond. Based on the large red shift of the NH stretching frequency compared with aniline [30], Southern et al. [17] concluded on a much stronger intramolecular hydrogen bond in the S1 state than that in the S0 state. Unfortunately, the range of our measurement was insufficient to observe this mode either for the S1 state or for the D0 state. Our theoretical calculation for the S1 state, on the other hand, is consistent with the above conclusion. For the D0 state, our result indicates a moderate red shift, and it is therefore likely that the strength of the intramolecular hydrogen bond in the D0 state falls in between that of the S0 and the S1 state.
5. Conclusion Spectroscopic properties of the excited and the ground cationic state of OABA have been studied using two-color REMPI and ZEKE techniques. With the aid of ab initio calculation, we have achieved complete
355
assignment of the progressions and combinations of in-plane bending modes of the substituents in the REMPI spectra. We also report a propensity of Dv = 0 in the ZEKE spectra and discuss the effect of electron back donation from the substituents to the aromatic ring. The adiabatic ionization energy has been determined to be 63232 ± 6 cm1. Based on our calculation, the strength of the intramolecular hydrogen bond of the D0 state should be in between those of the S0 and the S1 state. Different from the general belief in organic chemistry, OABA and MABA demonstrate similar electronic and geometric properties upon excitation and ionization, while they are different from PABA. Acknowledgements This work is supported by the National Science Foundation, Division of Chemistry. Acknowledgment is made to the Donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Wei Kong is an Alfred P. Sloan research fellow.
References [1] M. Tang, J. Isbell, B. Hodges, J. Brodbelt, J. Mass Spectrom. 30 (1995) 977. [2] K.B. Wiberg, J. Org. Chem. 67 (2002) 4787. [3] Y. He, C. Wu, W. Kong, J. Chem. Phys. 121 (2004) 3533. [4] Y. He, C. Wu, W. Kong, J. Chem. Phys. 2004 (in press). [5] A. Carmel, A. Yaron, Eur. J. Biochem. 87 (1978) 265. [6] L.B. Hersh, J.T. Gafford, J.C. Powers, T. Tanaka, E.G. Erdos, Biochem. Biophys. Res. Commun. 110 (1983) 654. [7] E.K. Bratovanova, D.D. Petkov, Anal. Biochem. 162 (1987) 213. [8] J.R. Chagas, L. Juliano, E.S. Prado, Anal. Biochem. 192 (1991) 419. [9] M. Meldal, K. Breddam, Anal. Biochem. 195 (1991) 141. [10] N. Nishino, Y. Makinose, T. Fugimoto, Chem. Lett. 77 (1992) 77. [11] C.G. Knight, Method. Enzymol. 248 (1995) 18. [12] R.F. Turchilleo, M.T. Lamy-Freund, I.Y. Hirata, L. Juliano, A.S. Ito, Biophys. Chem. 73 (1998) 217. [13] A.S. Ito, R.F. Turchilleo, I.Y. Hirata, M.H.S. Cezari, M. Meldal, L. Juliano, Biospectrosopy 4 (1998) 395. [14] A. Tramer, J. Phys. Chem. 74 (1970) 887. [15] D.V.H.S. Jain, F.S. Nandel, P. Singla, D.J. Kaur, Indian J. Chem. 25A (1986) 15. [16] J. Meeks, A. Wahlborg, S.P. McGlynn, J. Electron Spectrosc. Relat. Phenom. 22 (1981) 43. [17] C.A. Southern, D.H. Levy, G.M. Florio, A. Longarte, T.S. Zwier, J. Phys. Chem. A 107 (2003) 4032. [18] J.A. Stearns, A. Das, T.S. Zwier, Phys. Chem. Chem. Phys. 6 (2004) 2605. [19] K. Pirowska, P. Kolek, J. Goclon, J. Najbar, Chem. Phys. Lett. 387 (2004) 165. [20] Y. He, C. Wu, W. Kong, J. Chem. Phys. 120 (2004) 7497. [21] Y. He, C. Wu, W. Kong, Chem. Phys. Lett. 391 (2004) 38. [22] Y. He, C. Wu, W. Kong, J. Phys. Chem. A 107 (2003) 5145. [23] Y. He, C. Wu, W. Kong, J. Phys. Chem. A 108 (2004) 943. [24] M.J. Frisch et al., GAUSSIAN 03: Revision A 7, Gaussian Inc., Pittsburgh, Pennsylvania, 2003.
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[25] M.E. Casida, C. Jamorski, K.C. Casida, D.R. Sulahub, J. Chem. Phys. 108 (1998) 4439. [26] G. Varsanyi, Assignment of Vibrational Spectra of Seven Hundred Benzene Derivatives, Wiley, New York, 1974. [27] E.W. Schlag, ZEKE Spectroscopy, Cambridge University Press, Cambridge, 1998.
[28] R.S. Mulliken, J. Chem. Phys. 23 (1955) 1833. [29] A. Streitwieser Jr., C.H. Heathcock, Introduction to Organic Chemistry, third edn., Macmillan Publishing Company, New York, 1989. [30] T. Nakanaga, F. Ito, J. Miyawaki, K. Sugawara, H. Takeo, Chem. Phys. Lett. 261 (1996) 414.
Two-color two-photon REMPI and ZEKE spectroscopy of supersonically cooled o-aminobenzoic acid Chengyin Wu, Yonggang He, Wei Kong
*
Department of Chemistry, Oregon State University, Gilbert 153, Corvallis, OR 97331-4003, USA Received 21 July 2004; in final form 20 September 2004 Available online 7 October 2004
Abstract We report studies of supersonically cooled o-aminobenzoic acid using two-color resonantly enhanced multiphoton ionization and zero kinetic energy photoelectron spectroscopy. Vibrational modes of the first electronically excited state (S1) of the neutral species and those of the ground state cation (D0) have been assigned, and the ionization threshold has been determined. The results are analyzed in comparison with the other two isomers of aminobenzoic acid, mainly in terms of electron back donation of the two substituents in both the S1 and the D0 states. The change in the intramolecular hydrogen bond upon electronic excitation and ionization will also be discussed. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction o-Aminobenzoic acid (OABA), also named anthranilic acid, is interesting both from a chemistry and a biology point of view. Like p-aminobenzoic acid (PABA) and m-aminobenzoic acid (MABA), OABA is a doubly substituted electron push–pull aromatic system with the donor (–NH2) and the acceptor (–COOH) groups connected by the p-ring [1–4]. Upon electronic excitation, however, both electron rich substituents can back donate to the electron deficient ring through hyper conjugation. On the biological aspect, OABA has largely been used as a convenient fluorescent probe in internally quenched fluorescent peptides because of its high quantum yield, small size, and convenient spectral regions [5–13]. These properties are essential in reducing the potential influence of the probe on the substrate–protease interaction. Interestingly, when OABA is bound to proline and tryptophan [13], its fluorescence quantum *
Corresponding author. Fax: +1 541 737 2062. E-mail addresses: [email protected], wei.kong@oregonstate. edu (W. Kong). 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.09.095
yield decreases by an order of magnitude. The origin of this behavior is still unknown. Spectroscopic studies of OABA in both the gas phase and in solutions are limited [14–17]. The most recent work has been reported by Southern et al. [17] on the vibrational spectroscopy of both the ground state (S0) and the first electronically excited state (S1). The authors have concluded that only rotamer I exists in the molecular beam even though OABA has two rotameric structures differing by a 180° rotation of the carboxyl group. From the comparison of the NH stretching frequency between OABA and aniline, the authors have also concluded that a much stronger intramolecular hydrogen bond is formed in the electronically excited state than in the ground state. More recently, Stearns et al. [18] have studied the water complex of OABA and found that water has essentially no effect on the intramolecular hydrogen bond in both the ground and the excited state. On the theoretical front, Pirowska et al. [19] have calculated the geometry changes of OABA upon S1 S0 electronic excitation. The length of the hydrogen bond ˚ in the excited state, lending decreases by 0.114 A credence to the conclusion of Southern et al. [17].
C. Wu et al. / Chemical Physics Letters 398 (2004) 351–356
Recently, we have investigated the spectroscopic properties of the first excited electronic state and the cationic ground state (D0) of a series of substituted aromatic systems [3,4,20,21]. Our results indicate that upon electronic excitation and ionization, the substituent would back donate electrons to the aromatic ring, ensuring the integrity of the ring structure. Consequently, a strong propensity of Dv = 0 has been observed in the two-color zero kinetic energy (ZEKE) photoelectron spectrum, where v represents the vibrational quantum number of the intermediate state. From the bi-substituted systems such as PABA and MABA, we have also learned that the distance between the substituents affects the dynamics of electron back donation [3,4]. In OABA, the carboxyl group is ortho to the amino group, and a much stronger intramolecular hydrogen bond should exist between the substituents. The effect of this intramolecular hydrogen bond on the electronic structure of the related states is therefore worth investigating. In this report, we present studies of two-color twophoton resonantly enhanced multiphoton ionization (REMPI) and ZEKE spectroscopy of OABA. Complete assignments of the ZEKE and REMPI spectra are achieved with the assistance of ab initio and density functional calculations (DFT). Based on the experimental results and theoretical calculations, changes in the molecular geometry and the intramolecular hydrogen bond upon electronic excitation and ionization will be discussed.
2. Experimental The experimental apparatus has been reported in our previous publications [22,23]. Briefly, it consisted of a pulsed molecular beam source and a time-of-flight mass spectrometer, which could be converted into a pulsed field ionization (PFI) zero kinetic energy photoelectron spectrometer. OABA was purchased from Aldrich Co. and used without further purification. The sample was heated to 145 °C in the nozzle (diameter: 1 mm), and the vapor was seeded into 2 atm of argon. Both the pump and the probe beams were generated by frequency doubling of the output of Nd:YAG (Spectra Physics, GCR 230 and 190) pumped dye laser systems (Laser Analytical Systems, LDL 2051 and LDL 20505). The absolute wavelength of each laser was calibrated using an iron hollow cathode lamp filled with neon. The relative timing between the two laser pulses was controlled by a delay generator (Stanford Research, DG535). In the ZEKE experiment, molecules excited to ZEKE states were allowed to stay for 500–600 ns in the presence of a spoiling field of 1 V/cm, and ionization and extraction was achieved by a pulsed electric field of 16 V/cm.
In order to assign the observed vibronic structures in both the REMPI and ZEKE spectra, we used the GAUSSIAN 03 suite [24] to calculate the geometry and vibrational frequencies. Density functional theory calculations using the Becke 3LYP functional were carried out with the 6-31+G(d) basis set for both the S0 and D0 states. The calculation for the S1 state was performed at the CIS level using the 6-31G(d,p) basis set. Good agreement between experimental and theoretical results was obtained when a scaling factor of 0.95 for the calculated frequencies of the S1 state was used [25], while no scaling factor was used for the D0 state.
3. Results 3.1. Two-color 1+1 0 REMPI spectrum The two-color 1+1 0 REMPI spectrum of OABA is displayed in Fig. 1. The ionization laser was set at 285 nm and was temporally overlapped with the scanning resonant laser. The linewidths of most transitions are on the order of 5 cm1, though the linewidth of the excitation laser is only 0.2 cm1. Unresolved rotational profiles are believed to contribute to the observed width. The spectrum is similar to that reported by Southern et al. [17], and assignment for all the observed features is listed in Table 1, including our own calculation results. Modes that are associated with the motion of the aromatic ring are labeled using the convention of VarsanyiÕs nomenclature [26]. Other modes that mainly involve the bending motion of the –NH2 and –COOH moieties are named with letters from A to D in the order of increasing frequency.
2
1
3
n
Ion intensity (arbitrary units)
352
D
2,1
1,1 m
2,2
1,2
n
B D
000 m
B
1
3
2
4
q
C
3
2
1 1
1 6a 6b
0
200
400
600
800
1000
1200
1400
Excitation Energy (cm-1) - 28594 cm-1 Fig. 1. (1+1 0 ) REMPI spectrum of supersonically cooled OABA. The spectrum is shifted by 28 594 cm1 (the origin of the S1 S0 transition) to emphasize the frequencies of the different vibrational modes of the S1 state.
C. Wu et al. / Chemical Physics Letters 398 (2004) 351–356
353
Table 1 Observed vibrational frequencies and assignments for the S1 state of OABA
220 254 355 379 418 506 544 595 602 632 672 713 762 772 797 839 884 923 961 988 1013 1050 1092 1130 1140 1178 1218 1258 1342
b
Calculation
Assignment
105*2 258 371 208*2 415
A20 ; c ðACOOH torsionÞ B10 ; b ðACOOHÞ C10 ; b ðANH2 Þ 10b20 ; c ðringÞ D10 ; b ðACOOHÞ; b ðANH2 Þ and b ðringÞ B20 6a10 ; b ðringÞ A20 10b20 B10 C10 or 16b10 ; c ðringÞ 6b10 ; b ðringÞ B10 D10 C20 B30 C10 D10 B10 6a10 D20 19a10 ; b ðringÞ B20 D10 B10 C20 C10 6b10 B40 C30 B10 D20 C20 D10 B20 6b10 6a10 6b10 B20 C20 D30 B20 D20
258*2 560 105*2+208*2 (258 + 371) or 603 626 258 + 415 371*2 258*3 371 + 418 258 + 560 415*2 875 258*2 + 415 258 + 371*2 371 + 626 258*4 371*3 258 + 415*2 371*2+415 258*2+626 560 + 626 258*2 + 371*2 415*3 258*2 + 415*2
+
a
The calculation was on the CIS/6-31G(d,p) level, and the values include a scaling factor of 0.95. b b and c represent in-plane bending and out-of-plane bending vibrations.
The spectrum is dominated by progressions and combinations of modes B, C, and D. This rich activity of the in-plane bending modes of the substituents reflects a large geometry change upon electronic excitation. On the other hand, the ring deformation modes, such as modes 6a, 6b, 12, and 1, play only minor roles. Two of the most active ring deformation modes are labeled in Fig. 1 for comparison. Interestingly, in PABA and MABA, the REMPI spectra also showed rich activities of the substituents [3,4]. This common theme among the three isomers of aminobenzoic acid implies active participation of the substituents in this electronic excitation. 3.2. ZEKE spectra Using the origin band and a few strong bands involving modes B and D as intermediate states, we recorded the ZEKE spectra of OABA as shown in Fig. 2. In Table 2, the assignment of the observed features is summarized in comparison with our own calculation results. No scal-
.B
+
D
+2
+2
A D
+2
2+
Ion Intensity
Experimental
a
.B D .D .B
+3
B
+2
B D
+
+
+
B
+2
+
BD
+
+2
.0
A
0
200
+
D E+
0+
400
+
+ 6a 6b
600
800
1000
Ion Internal Energy (cm-1) Fig. 2. Two-color ZEKE spectra of OABA recorded via the following vibrational levels of the S1 state as intermediate states: (a) 00; (b) B1; (c) D1; (d) B2; (e) B1D1. The energy in the abscissa is relative to the ionization threshold at 63 232 cm1. The assignment in the figure refers to the vibrational levels of the cation, and the corresponding vibrational level of the intermediate state is labeled by a black dot in each panel.
ing factor for the theoretical vibrational frequency is used, and the agreement seems reasonable. The same labeling scheme as that of the S1 state is used for the D0 state, but a superscript Ô+Õ is used to distinguish the modes of the D0 state. The identity of the intermediate level for each ZEKE spectrum is marked on the figure by a black dot. The spectra in Fig. 2 are remarkably simple, which can be partially attributed to state selection achieved in typical REMPI experiments. Each spectrum is dominated by a single strong transition, and in most cases, it can be assigned as the same vibrational level as that of the corresponding S1 state. This strong correlation between the intermediate level and the final vibrational level of the ion has been reported in our previous studies of p-aminobenzoic acid [3], m-aminobenzoic acid [4], 4aminopyridine [20], and 2-chloropyrimidine [21]. We have also derived a propensity rule of Dv = 0 to describe this effect. Assignment of Fig. 2 is straightforward. The first feature in the spectrum obtained via the origin band of the S1 state is the origin band of the cation 0+0. The resulting adiabatic ionization energy, including a correction due to field ionization [27], is 63232 ± 6 cm1 (7.8398 ± 0.0007 eV). In contrast, Meeks et al. [16] determined a higher value of 8.0 eV using He(I) photoelectron spectroscopy. This difference might be a result of the low resolution of typical PES experiments or small Franck–Condon factors at the threshold during single photon ionization at 21.2 eV. Other features are mostly high overtone bands and combination bands of the bending modes of the substituents.
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Table 2 Observed vibrational frequencies and assignments in the ZEKE spectra of OABA Intermediate level in the S1 state 000
B10
D10
B20
0 190 258 403 489
400 515
516
548 633 662
662 774 805 919 992
a
Calculation
Assignmenta
78*2 259 409 510 259*2 559 640 259 + 409 259*3 409*2 259*2 + 409 78*2 + 409*2
0+ A+2 B+ D+ E+ B+2 6a+ 6b+ B+D+ B+3 D+2 B+2D+ A+2D+2
B10 D10
The Ô+Õ in the superscript represents the cationic state.
4. Discussion Based on the work of Southern et al., only rotamer I exists in a supersonic molecular beam [17]. The agreement between our experimental and theoretical results, as shown in Table 1 and Fig. 1, further supports this conclusion. The spectral congestion is thus not a result of different rotamers; rather, it reflects the rich Franck–Condon activity. The calculated geometry parameters are listed in Table 3 and the numbering scheme is shown in Fig. 3. OABA is nonplanar in the ground state and planar in the S1 and D0 states. During the S1 S0 transition, the bond lengths for C1AN7 and C2AC8 decrease, which indicates participation of both polar groups in the conjugation with the aromatic ring. The resulting chemical bonds between the adjacent moieties take on
Table 3 Molecular geometry parameters of OABA in the S0, S1, and D0 states S0
S1
D0
Bond length (A˚) C1AC2 C2AC3 C3AC4 C4AC5 C5AC6 C6AC1 C1AN7 N7AH1 N7AH2 C2AC8 C8AO1 C8AO2 O2AH H1AO1
1.425 1.409 1.382 1.403 1.381 1.413 1.363 1.010 1.005 1.466 1.222 1.359 0.968 1.950
1.483 1.399 1.404 1.401 1.400 1.402 1.324 1.004 0.994 1.432 1.223 1.340 0.947 1.863
1.449 1.377 1.413 1.407 1.373 1.436 1.327 1.027 1.014 1.496 1.214 1.330 0.972 1.840
Dihedral angle (°) H1AN7AC1AC2 H2AN7AC1AC6
7.125 9.745
0 0
0 0
Fig. 3. Structure and numbering scheme of OABA.
partial double bond characters. The ring structure, on the other hand, remains largely unchanged, except for an elongation of the C1AC2 bond. This stability of the aromatic ring explains the inactivity of the ring breathing mode 1. In comparison, mode 1 is the most active in the REMPI spectrum of PABA [3], while it is only a weak feature in MABA [4]. This progressive change in activity for the ring breathing mode from OABA to PABA reflects the effect of the intramolecular hydrogen bond. The ZEKE spectra in Fig. 2 follow the propensity rule, and this observation is consistent with our previous results on both mono- and bi-substituted compounds [3,4,20,21]. Table 4 lists the Mulliken charge [28] distribution of OABA based on our calculation. The large negative charge on the amino group in the S1 state re-
Table 4 Mulliken charge distributions of OABA in the S0, S1, and D0 states –NH2 –COOH Benzene ring C1 C2
S0
S1
D0
+0.224 0.235 +0.013 0.956 1.119
0.121 0.066 +0.187 +0.403 0.253
+0.486 +0.013 +0.501 1.625 +1.522
C. Wu et al. / Chemical Physics Letters 398 (2004) 351–356
flects the substantial contribution of the atomic orbitals of the amino nitrogen to the p* orbital, similar to the case of MABA [4]. Further ionization removes this p* electron, resulting in a large positive charge on the amino group. We thus believe that in the S1 state, both polar groups donate electrons to the ring in this bi-substituted compound, while in the D0 state, it is the amino group that dominates the electron back donation. As a result, the electron deficiency of the ring is alleviated, and the rigidity of the ring structure is preserved during both electronic excitation and further ionization. It is interesting to compare the three isomers of aminobenzoic acid in terms of changes in geometry and charge distribution upon electronic excitation and ionization. From Table 3, the geometric changes in OABA are quite similar to those in MABA, including the change in the bond length C1AN7 and the planarity of the S1 and D0 states. In Table 4, the charge distributions in the S1 state and the D0 state are also similar for OABA and MABA. In contrast, the amino group and the carboxyl group have almost equal share of the positive charge in the D0 state of PABA, and the S1 state of PABA is non-planar [3]. The bond length for C1AN7 in PABA becomes progressively shorter with successive excitation. These results imply that in the S1 and D0 states, the ortho and the meta compounds are similar, while they are considerably different from the para compound. This conclusion is similar to that of Jain et al. [15], and it is different from the general belief in organic chemistry [29]. However, one should notice that all of the above discussions are applicable to the S1 and D0 states, while the resonance and the inductive effect discussed in organic chemistry are for the ground state. A unique feature of OABA compared with the other two isomers of aminobenzoic acid is the intramolecular hydrogen bond. Based on the large red shift of the NH stretching frequency compared with aniline [30], Southern et al. [17] concluded on a much stronger intramolecular hydrogen bond in the S1 state than that in the S0 state. Unfortunately, the range of our measurement was insufficient to observe this mode either for the S1 state or for the D0 state. Our theoretical calculation for the S1 state, on the other hand, is consistent with the above conclusion. For the D0 state, our result indicates a moderate red shift, and it is therefore likely that the strength of the intramolecular hydrogen bond in the D0 state falls in between that of the S0 and the S1 state.
5. Conclusion Spectroscopic properties of the excited and the ground cationic state of OABA have been studied using two-color REMPI and ZEKE techniques. With the aid of ab initio calculation, we have achieved complete
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assignment of the progressions and combinations of in-plane bending modes of the substituents in the REMPI spectra. We also report a propensity of Dv = 0 in the ZEKE spectra and discuss the effect of electron back donation from the substituents to the aromatic ring. The adiabatic ionization energy has been determined to be 63232 ± 6 cm1. Based on our calculation, the strength of the intramolecular hydrogen bond of the D0 state should be in between those of the S0 and the S1 state. Different from the general belief in organic chemistry, OABA and MABA demonstrate similar electronic and geometric properties upon excitation and ionization, while they are different from PABA. Acknowledgements This work is supported by the National Science Foundation, Division of Chemistry. Acknowledgment is made to the Donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Wei Kong is an Alfred P. Sloan research fellow.
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