J. Phys. Chem. A 2010, 114, 11263–11268
11263
Mass-Analyzed Threshold Ionization Spectroscopy of Ortho Fluorinated 2-Phenylethanol: Identification of an Additional Gauche Conformer† R. Karaminkov, S. Chervenkov,‡ and H. J. Neusser* Physikalische und Theoretische Chemie, Technische UniVersita¨t Mu¨nchen, Lichtenbergstr. 4, 85748 Garching, Germany ReceiVed: April 30, 2010; ReVised Manuscript ReceiVed: August 28, 2010
The cationic ground state of the ortho fluorinated 2-phenylethanol has been investigated by combination of mass-analyzed threshold ionization (MATI) spectroscopy and quantum chemistry ab initio density functional theory (DFT) calculations employing the hybrid functional M05 with cc-pVDZ basis set. The MATI spectra measured via vibronic bands in the S1 intermediate state of the most stable gauche conformer stabilized by an intramolecular OH · · · π hydrogen bond are structureless. The MATI spectrum recorded via a small band blueshifted by 3 cm-1 from the 000 electronic origin of the gauche conformer features well-resolved peaks and is assigned to a cationic gauche structure without an OH · · · π bond. The ab initio calculations are qualitatively consistent with the experimental observations and show that the presumable conformer giving rise to the observed MATI spectrum retains its structure during ionization, whereas the lowest-energy gauche conformer undergoes a significant structural change resulting in a break of the OH · · · π bond, thus leading to unfavorable Franck-Condon factors for the threshold ionization. I. Introduction
SCHEME 1: Atom Labels of the 2-oFPE Monomer
Mass-analyzed pulsed field threshold ionization spectroscopy has established itself over the past two decades as a powerful tool for studying the ionic states of isolated molecules and molecular complexes in the gas phase.1-13 Mass analyzed threshold ionization (MATI) spectroscopy is based on pulsed field ionization of high-lying long-lived Rydberg states and, in addition, to the zero-electron kinetic energy (ZEKE)14 spectroscopy, it features the advantage of mass selectivity. The latter makes possible for the dissociation threshold of the cationic cluster to be detected, which allows for the MATI technique to provide not only the adiabatic ionization energy (AIE) of the studied isolated species but also the binding energies of binary molecular complexes in both neutral ground, S0, electronic state and in the cationic ground, D0, state.2,4,8,15 The versatility of the MATI spectroscopy has been demonstrated also by applying it for identification of molecular conformations,16 in particular, of biologically relevant molecules both in the neutral and cationic state,17,18 thus complementing the information obtained by other spectroscopic methods.19-24 It has been used also as a sensitive probe for the presence of weak intramolecular OH · · · π hydrogen bonds. Pulsed field ionization spectroscopy is particularly suitable for monitoring changes occurring upon ionization. Such changes are expected for flexible molecules stabilized by a nonconventional hydrogen bond between the side chain and the π-electrons of the aromatic ring.24-28 Here, the ionization leads to the ejection of a π-electron from the aromatic ring and therefore to a weakening or even rupture of π-hydrogen bond.17,18 Recently, we have shown that the OH · · · π bonded gauche conformations in 2-para-fluorophenylethanol (2-pFPE)27 and 2-ortho-fluorophenylethanol (2-oFPE)28 are not affected by
the fluorination in the benzene ring even though the fluorination leads to a decreasing of the π-electron density in the ring.17,18 The results in this paper are based on combination of MATI spectroscopy yielding the first threshold ionization spectrum of 2-oFPE and high-level quantum-chemistry ab initio calculations for this species. The new results complement the ones on 2-phenylethanol (2-PE) and 2-pFPE.17,18 The aim of this series of studies is the understanding of the effect of the electronegative fluorine atom substituted at different positions in the aromatic ring on the conformational structures and their stability in the neutral and cationic state. The ortho position of the F atom is expected to lead to a stronger interaction with the side chain than in the para-substituted 2-PE due to the vicinity of the partial positive charge at the terminal hydrogen atom of the hydroxyl group to the electronegative fluorine atom. The structure and the atom labels of 2-oFPE are shown in Scheme 1.
†
Part of the “Klaus Mu¨ller-Dethlefs Festschrift”. * Author to whom correspondence should be addressed. E-mail:
[email protected]. ‡ Present address: Max-Planck-Institut fu¨r Quantenoptik, Hans-Kopfermann-Str. 1, D-85748 Garching, Germany.
II. Experimental Details The experimental setup has been described in detail elsewhere29,30 and will be discussed only briefly here. 2-oFPE
10.1021/jp103941d 2010 American Chemical Society Published on Web 09/15/2010
11264
J. Phys. Chem. A, Vol. 114, No. 42, 2010
Figure 1. One-color S1 r S0 REMPI spectrum of 2-oFPE, recorded at the monomer mass channel m/z ) 140. Conformer assignments from our recent work28 and relative peak positions are included in the spectrum. Bands originating from the most stable neutral gauche conformer 1 are designated by “g”. A more precise value for the position of the 000 band is given in our recent high-resolution work on 2-oFPE.28
(Sigma Aldrich, 99% purity) was heated to 100 °C in an internal sample holder located behind the valve (General Valve series 9, diameter of the orifice: 0.5 mm). The sample vapor was expanded in a supersonic jet using argon as a backing gas at a stagnation pressure of 2-3 bar. The molecular beam is intersected by the counterpropagating, frequency-doubled outputs of two dye lasers (FL 3002 and LPD 3000, Lambda Physik) operated with Coumarin 153, which are synchronously pumped by a XeCl excimer laser (EMG 1003i, Lambda Physik). The MATI technique was used to obtain the threshold ionization spectra of 2-oFPE. The excited Rydberg molecules were separated from the directly produced photoions in the first stage of the acceleration region of a linear reflecting time-offlight mass spectrometer (RETOF)31 by a weak electric field (ca. 0.18-0.22 V/cm) and ionized with a delay of ca. 20 µs by a strong electric field of 1000 V/cm in the second stage. The resulting threshold ions were accelerated and injected by the same electric field into the drift region of the linear reflecting RETOF mass spectrometer. The mass-resolved signals were detected at different mass channels and recorded with gated integrators, then digitized, and finally processed in a personal computer under LabVIEW environment. III. Results A. REMPI Spectrum. First, we recorded the one-color resonance enhanced multiphoton ionization (REMPI) spectrum of the excited, S1, electronic state of 2-oFPE, shown in Figure 1. In our recent publication, the pronounced peak at 37 589.2 cm-1 was identified by a rotational analysis of the highly resolved vibronic band as the origin of the gauche conformer, which is stabilized by an OH · · · π hydrogen bond.28 Due to its differing rotational structure, the closely lying vibronic band at +3 cm-1 was assumed to originate from another conformer, in particular, from one of the adjacent gauche conformers. The recently assigned vibronic bands in the low-frequency region in the REMPI spectrum near the origin of the gauche conformer are designated in Figure 1. Most of these bands correspond to vibrations of the most stable gauche structure 1 (see Figure 3). The position of the +3 cm-1 band is shown on an enlarged scale in the inset of Figure 1. To examine the behavior of the
Karaminkov et al.
Figure 2. MATI spectra of 2-oFPE measured (a) via the000 band at 37589.2 cm-1 and (b) the closely lying band at +3 cm-1 of the REMPI spectrum in Figure 1. Verticals bars and bold values indicate some of the theoretically predicted vibrational frequencies of conformer 2.
gauche conformer after ionization, we performed MATI scans after two-photon excitation via various intermediate vibronic S1 states. B. MATI Spectra. We have recorded MATI spectra via the prominent S1 vibronic bands 000, +43g, and +85g in the REMPI spectrum (see Figure 1). However, the MATI spectra via these bands are unstructured and do not display clear vibrational peaks. For demonstration, the MATI spectrum measured via the origin band 000 is shown in Figure 2a; it is noisy with unclear onset and increasing background for higher excitation energies. The scanned spectral region covers more than 450 cm-1, that is, the region where the adiabatic ionization energy is expected to appear. The MATI spectra via the other two vibronic bands in the S1 r S0 spectrum of the gauche conformer 1 (+43g and +85g) are similar and therefore, not shown. By contrast, the MATI spectrum via the band at +3 cm-1 (Figure 2b) manifests a well-resolved vibronic structure with a strong peak as its onset on the red side, which indicates the AIE. The spectrum is extended to higher excitation energies. The signal-to-noise ratio was improved by averaging over five scans. The value for the AIE is 72 159 ( 5 cm-1, where the ionization energy is given without field correction.32 There is a second strong vibronic band at +22 cm-1, whereas the spectrum at higher energies has an increasing background with some structure on it. C. Computational Results. For theoretical investigation of the conformeric structure, vibrational modes, and energetics of the 2-oFPE cation, we employed the Gaussian 03 suite of programs33 using density functional theory (DFT) with M0534,35 functional and the cc-pVDZ basis set. As a starting point for the optimizations in the cation, we have used the energetically most stable geometries of the neutral conformers in the ground, S0, electronic state.28 The results show that the initial number of nine neutral structures reduces to six in the cation, where none of the cationic conformers exhibits a nonclassical OH · · · π hydrogen bond between the terminal OH group of the side chain and the π electrons of the benzene ring. All cationic structures are depicted in Figure 3. The neutral gauche conformers 1 and 2 produce identical cationic structures, having the same values for their normal-mode vibrational frequencies and their energies (see Figure 3). These identical structures correspond to con-
Gauche Conformer of Ortho Fluorinated 2-Phenylethanol
J. Phys. Chem. A, Vol. 114, No. 42, 2010 11265
Figure 3. Conformations of the 2-oFPE in the ground, S0, electronic state and their corresponding cationic structures obtained from them by ab initio quantum chemistry full structural optimization at the DFT M05/cc-pVDZ level of theory.
former I in the cation (see Figure 3). The same situation exists for the anti structures 5 and 6, which converge to cationic structure II, and for the adjacent gauche conformers 8 and 9, which produce cationic structure III. In the cation the energy ordering is different compared to that in the neutral ground state.28 In the cation, the most stable conformer is conformer VI (0 cm-1), followed by conformers IV (36 cm-1), III (60 cm-1), I (295 cm-1). The highest-energy conformers are the
anti structures II (791 cm-1) and V (860 cm-1). The calculated normal-mode frequencies up to 520 cm-1 in ascending order as well as the frequencies of two ring-specific normal modes for all conformers are listed in Table 1. IV. Discussion As mentioned above, the MATI spectra measured via the origin band of the neutral gauche conformer and the closely
11266
J. Phys. Chem. A, Vol. 114, No. 42, 2010
Karaminkov et al.
TABLE 1: Theoretically Predicted Normal-Mode Vibrational Frequencies up to 520 cm-1 in Ascending Order, and Two Ring-Specific Modes for the 6 Stable Conformations of the 2-oFPE Cation Calculated at the DFT M05/cc-pVDZ Level of Theorya
a
No scale factors have been applied to correct the normal-mode frequencies.
positioned band at +3 cm-1 in the S1 r S0 spectrum show a completely different behavior upon ionization. The lack of structure in the MATI spectrum via the strong gauche origin (see Figure 2a) can be attributed to an unfavorable FranckCondon factor resulting from a significant structural change upon ionization. On the other hand, the presence of vibrational structure in the MATI spectrum via the band at +3 cm-1 band (see Figure 2b) points to minor changes after ionization of this conformer.21,36 This is confirmed by our quantum chemistry ab initio calculations. The most stable and experimentally confirmed gauche structure 1 in the neutral ground state is geometrically distorted upon ionization, which results from breaking of the stabilizing OH · · · π bond (see Figure 3). The produced cationic geometry I has a different orientation of the side chain, which is identical to the one of the neutral gauche conformer 2. The same occurs for the neutral adjacent gauche structure 9, which in the cation transforms to conformer III, where the OH · · · π bond, which is present in the neutral state, does not exist in the cation. The neutral anti conformer 5 gives rise to conformer II in the cation. On the contrary, the structures of the neutral conformers 2-4 and 6-8, which are not stabilized by a nonclassical OH · · · π bond, are preserved or only slightly changed upon ionization. This result makes conformers 2-4 and 6-8 in the neutral ground state candidates for the origin of the vibronic band at +3 cm-1, which produces a resolved MATI
TABLE 2: Assignment of the Observed Vibronic Bands in the MATI Spectra Shown in Figure 2b on the Basis of the Theoretically Predicted Frequencies (cm-1) for the Observed Gauche Conformer of the 2-pFPE Cation (Conformer 2g in Figure 3) Calculated at the DFT M05/cc-pVTZ Level of Theory exp. freq.
theor. freq.
assignment
22 101 197 265 382 737 760
27 104 205 266 382 716 757
τ1 side-chain torsion about C2C7 β1 bending side chain vs ring side chain wagging mode butterfly mode β2 puckering mode υ6 benzene breathing out-of-plane ring bending
spectrum in the cationic ground state. Recently, we have tentatively assigned the band at +3 cm-1 as originating from the adjacent gauche conformer 9.28 Now, by comparison of the measured vibrational frequencies of the cation (see Figure 2a and Table 2) with the theoretical findings (see Table 1) we can refine this assignment. The MATI spectrum via the vibronic band at +3 cm-1 in the intermediate electronic, S1, state shown in Figure 2b has a specific vibrational low-frequency mode located only 22 cm-1 above the AIE. By inspection of Table 1 containing the
Gauche Conformer of Ortho Fluorinated 2-Phenylethanol theoretical normal-mode frequencies of the cationic structures calculated at the DFT M05/cc-pVDZ level of theory, one finds that only conformer I has a low-frequency mode in that region. Conformer I is produced either from the neutral OH · · · π bonded conformer 1 by distortion of its side-chain, or from the neutral gauche conformer 2. In the latter case, the structure is completely preserved during the ionization process, which presumably leads to a favorable Franck-Condon factor for the MATI transitions. The MATI spectrum in Figure 2b becomes noisy with increasing excitation energy. That is why it is difficult to assign the other calculated vibrational frequencies to peaks in the MATI spectrum. We have marked the theoretical positions of the vibronic peaks of conformer I by vertical bars in Figure 2b. Only the theoretical modes which are in the range of visible vibronic peaks in the MATI spectrum are shown, for example, the vibrational mode at 382 cm-1, which agrees nicely with a peak in the MATI spectrum. We would like to note that the vibrational frequencies above 100 cm-1 up to 400 cm-1 do not differ very much for the different conformations. Therefore, they are not so suitable for identification of the conformeric structure as is the pronounced low-frequency mode at +22 cm-1, which is theoretically predicted only for conformer I. In the higherfrequency range between 700 and 800 cm-1, the experimental MATI spectrum in Figure 2b features two relatively wellresolved peaks at 737 and 760 cm-1, respectively. The two bands in question presumably originate from benzene-ring modes, which typically appear in that frequency range. Indeed, looking up at Table 1, we see that the theoretically predicted specific ring mode υ6a,b and the out-of-plane benzene ring bending mode for conformer I appear at 716 and 757 cm-1, respectively. The good agreement between the experimental and the theoretical frequencies for these modes is a further confirmation that the experimental MATI spectrum originates from conformer I. There are two other conformers, IV and VI, for which the theoretically predicted frequencies for the above vibrational modes are very similar to the discussed experimental frequencies. Those conformers, however, can be ruled out as structures producing the experimental MATI spectrum since their theoretical lowest-frequency modes lie away from the experimental band at +22 cm-1. The assignments of the observed experimental vibronic modes are summarized in Table 2. The lowfrequency mode at 27 cm-1 is the one that becomes active during the ionization step and induces some slight bending of the side chain. For a more detailed comparison, calculation of the Franck-Condon factors for the ionization process would be necessary, which is beyond the scope of this work.37 On the basis of Franck-Condon arguments, one would favor conformer 2 as giving rise to the structured MATI spectrum of cationic conformer I (see Figure 2b). On the other hand, our previous ab initio calculations28 show that conformer 2 in the neutral ground state lies at 487 cm-1 above the most stable conformer 1 (see Figure 3), and the potential barrier between them is very low. This suggests relaxation of the less stable conformer 2 to the most stable conformer 1. This would preclude direct spectroscopic observation of the neutral conformer 2 in the cold molecular beam, thus a direct production of the MATI spectrum in Figure 2b from excitation of conformer 2 in the S0 state is not likely. A tentative explanation would be to assume conformational changes in the intermediate first excited, S1, electronic state leading to conformer 2 after excitation of the lowest energy conformers 7 and 9. To distinguish between both possibilities improved high resolution spectra of the band at +3 cm-1 could provide additional information.
J. Phys. Chem. A, Vol. 114, No. 42, 2010 11267 V. Conclusions Employing MATI spectroscopy combined with high-level ab initio quantum-chemistry calculations, we have identified a new conformation of the 2-oFPE cation. We have shown that the nonconventional π-hydrogen bond stabilizing the gauche conformers in the neutral ground state breaks upon ionization as a result of the electron-density depletion in the benzene ring after the ejection of a π electron from there, thus leading to conformers in the cation without an OH · · · π bond. Compared to 2-PE and 2-pFPE, the fluorination at ortho position leads to the presence of a non-OH · · · π bonded gauche conformer; whereas in the former cases anti structures were observed. Acknowledgment. Financial support from the Deutsche Forschungsgemeinschaft is gratefully acknowledged. References and Notes (1) Zhu, L.; Johnson, P. M. J. Chem. Phys. 1991, 94, 5769. (2) Krause, H.; Neusser, H. J. J. Chem. Phys. 1992, 97, 5923. (3) Willey, K. F.; Yeh, C. S.; Duncan, M. A. Chem. Phys. Lett. 1993, 211, 156. (4) Braun, J. E.; Grebner, T. L.; Neusser, H. J. J. Phys. Chem. 1998, 102, 3273. (5) Haines, S. R.; Dessent, C. E. H.; Mu¨ller-Dethlefs, K. J. Chem. Phys. 1999, 111, 1947. (6) Kong, W.; Peng, X. S.; Abbot, J. Abstr. Pap. Am. Chem. S. 2000, 219, 317. (7) Braun, J. E.; Neusser, H. J. Mass Spectrom. ReV. 2002, 21, 16. (8) Georgiev, S.; Chakraborty, T.; Neusser, H. J. J. Phys. Chem. A 2004, 108, 3304. (9) Zhang, B.; Li, C.; Su, H.; Lin, J. L.; Tzeng, W. B. Chem. Phys. Lett. 2004, 390, 65. (10) Choi, S.; Choi, K. W.; Kim, S. K.; Chung, S.; Lee, S. J. Phys. Chem. A 2006, 110, 13183. (11) Gaber, A.; Riese, M.; Grotemeyer, J. J. Phys. Chem. A 2008, 112, 425. (12) Tong, X.; Cerny, J.; Mu¨ller-Dethlefs, K.; Dessent, C. E. H. J. Phys. Chem. A 2008, 112, 5866. (13) Gu, Q.; Knee, J. L. J. Chem. Phys. 2008, 128, 064311. (14) Mu¨ller-Dethlefs, K.; Sander, M.; Schlag, E. W. Chem. Phys. Lett. 1984, 112, 291. (15) Ernstberger, B.; Krause, H.; Neusser, H. J. Ber. Bunsenges. Phys. Chem. 1993, 97, 885. (16) Tong, X.; Ford, M. S.; Dessent, C. E. H.; Mu¨ller-Dethlefs, K. J. Chem. Phys. 2003, 119, 12908. (17) Karaminkov, R.; Chervenkov, S.; Neusser, H. J. Phys. Chem. Chem. Phys. 2009, 11, 2249. (18) Georgiev, S.; Karaminkov, R.; Chervenkov, S.; Delchev, V.; Neusser, H. J. J. Phys. Chem. A 2009, 113, 12328. (19) Douberly, G. E.; Ricks, A. M.; Schleyer, P. v. R.; Duncan, M. A. J. Phys. Chem. A 2008, 112, 4869. (20) Gu, Q.; Basu, S.; Knee, J. L. J. Phys. Chem. A 2007, 111, 1808. (21) Dessent, C. E. H.; Geppert, W. D.; Ulrich, S.; Mu¨ller-Dethlefs, K. Chem. Phys. Lett. 2000, 319, 375. (22) Krasnokutski, S. A.; Lei, Y.; Lee, J. S.; Yang, D.-S. J. Chem. Phys. 2008, 129, 124309. (23) He, Y.; Kong, W. J. Chem. Phys. 2006, 124, 204306. (24) Karaminkov, R.; Chervenkov, S.; Neusser, H. J. J. Phys. Chem. A 2008, 112, 839. (25) Hockridge, M. R.; Knight, S. M.; Robertson, E. G.; Simons, J. P.; McCombie, J.; Walker, M. Phys. Chem. Chem. Phys. 1999, 1, 407. (26) Zwier, T. S. J. Phys. Chem. A 2006, 110, 4133. (27) Karaminkov, R.; Chervenkov, S.; Neusser, H. J. Phys. Chem. Chem. Phys. 2008, 10, 2852. (28) Karaminkov, R.; Chervenkov, S.; Neusser, H. J.; Ramanathan, V.; Chakraborty, T. J. Chem. Phys. 2009, 130, 034301. (29) Neuhauser, R. G.; Siglow, K.; Neusser, H. J. J. Chem. Phys. 1997, 106, 896. (30) Georgiev, S.; Neusser, H. J. J. Electr. Spectrosc. Rel. Phenom. 2005, 142, 207213. (31) Ernstberger, B.; Krause, H.; Kiermeier, A.; Neusser, H. J. J. Chem. Phys. 1990, 92, 5285. (32) Chupka, W. A. J. Chem. Phys. 1993, 98, 4520. (33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
11268
J. Phys. Chem. A, Vol. 114, No. 42, 2010
Nakatsuji, H.; Haga, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Comperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, J. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Kamaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzales, C.; Pople, J. A. Gaussian 03, ReVision B.04; Gaussian, Inc.: Pittsburgh, PA, 2003.
Karaminkov et al. (34) Zhao, Y.; Truhlar, D. G. J. Phys. Chem. A 2005, 109, 5656. (35) Zhao, Y.; Schultz, N. E.; Truhlar, D. G. J. Chem. Phys. 2005, 123, 161103. (36) Tong, X.; Ford, M. S.; Dessent, C. E. H.; Mu¨ller-Dethlefs, K. J. Chem. Phys. 2003, 119, 12908. (37) Wang Y. H.; Mineo H.; Chao S. D.; Selzle H. L.; Neusser H. J.; Schlag E. W.; Teranishi Y.; Lin S. H. Theoretical Treatments of the Dynamics of ZEKE States and ZEKE Spectroscopy. J. Phys. Chem. A, in press.
JP103941D
