case of neutral jet-cooled species, the technique of population labeling has been used. This is known as infraredultraviolet

Transcription

1 JOURNAL OF CHEMICAL PHYSICS VOLUME 110, NUMBER 9 1 MARCH 1999 Autoionization-detected infrared spectroscopy of intramolecular hydrogen bonds in aromatic cations. I. Principle and application to fluorophenol and methoxyphenol Eiji Fujimaki, Asuka Fujii, a) Takayuki Ebata, and Naohiko Mikami a) Department of Chemistry, Graduate School of Science, Tohoku University, Sendai , Japan Received 15 October 1998; accepted 24 November 1998 A new infrared spectroscopic technique for jet-cooled molecular cations is applied to observe intramolecular hydrogen bonds in substituted phenol ions. Vibrational transitions of an ion core of high Rydberg states are measured by detecting molecular ions prepared through vibrational autoionization. The observed infrared spectra practically provide vibrational frequencies of the corresponding bare molecular ion. The OH stretching vibrations of ortho-, meta-, and para-isomers of fluorophenol and methoxyphenol cations are observed. The OH stretching vibrational frequency of the ortho-isomer shows a characteristic redshift due to the intramolecular hydrogen bond. The redshift increases with ionization, indicating a significant enhancement of the intramolecular hydrogen bond strength American Institute of Physics. S I. INTRODUCTION Intramolecular hydrogen bonds, which form between neighboring proton donating and accepting groups, have been the subject of various studies. 1,2 Intramolecular hydrogen bonds are an important factor in some macroscopic properties, such as melting and boiling points, density, and viscosity. They also play a significant role in determination of the geometrical structures of biological molecules, such as proteins. The most powerful probe for intramolecular hydrogen bonds is the observation of OH stretching vibrations, as in the case of intermolecular hydrogen bonds. In many intramolecular hydrogen-bonded molecules, the interaction is subjected to severe steric restrictions associated with the resulting structure of the molecule, and the OH stretch frequency results in much smaller redshifts in comparison with intermolecular hydrogen-bonded OH stretches. Most of the previous infrared studies on intramolecular hydrogen bonds have been performed in condensed phases, in which there are many perturbations, such as polarization effects, dipole dipole interactions, and so on. Therefore, we should be careful to eliminate perturbations from surrounding molecules in order to elucidate the nature of intramolecular hydrogen bonds. From this standpoint, an isolated cold molecule in a supersonic jet is an ideal system for the study of intramolecular hydrogen bonds. However, because of the difficulty in applying infrared IR spectroscopy to the jet-cooled species, electronic spectroscopy has often been used to characterize intramolecular hydrogen-bonded molecules under jetcooled conditions. 3 Recent development of multi photon and multi color laser spectroscopic techniques in combination with the molecular beam method enables us to apply IR spectroscopy even to jet-cooled molecules and molecular clusters. 4,5 In the a Authors to whom correspondence should be addressed. case of neutral jet-cooled species, the technique of population labeling has been used. This is known as infraredultraviolet IR-UV double resonance spectroscopy, and employs UV laser light to detect the population changes caused by IR absorption. 6 This method has been used to observe the OH stretching vibrations of jet-cooled tropolone, hydroxytropolone, 10 and methyl salicylate, 11 and to discuss the proton tunneling and transformation from the intra- to intermolecular hydrogen bond with respect to the stepwise solvation process. And very recently, the technique was used in a systematic study on the intramolecular hydrogen bond in jet-cooled halophenol. 12,13 In contrast to the development of our understanding of intramolecular hydrogen bonds in neutral molecules, information on these bonds in molecular cations is extremely scarce. To our knowledge, there has been no report on the infrared spectra of intramolecular hydrogen-bonded cations in gas phase. A lack of such research is clearly due to technical difficulties in infrared spectroscopy of molecular cations, and especially of those in a jet expansion; generally, the diffuse electronic spectra of molecular ions especially in the case of large molecules prevent application of the population labeling technique to spectroscopy involving molecular beams. Infrared multiphoton dissociation IRMPD 14 used for cluster cations is also ineffective for molecular cations because these cations have larger dissociation energies than cluster ions. Matrix isolation has also been used for IR spectroscopy of cold radical species that include molecular cations. 15 However, perturbation from the matrix is problematic in the study of weak intramolecular hydrogen bonds. Though perturbations on vibrational frequencies from the matrix are estimated to be smaller than a few tens of cm 1, red shifts due to intramolecular hydrogen bonds are expected to be on the same order. Moreover, in the case of large polyatomic molecules, such as aromatic cations, identification of spectral carriers becomes a serious problem, since fragmentation cannot be more or less avoidable in the ionization /99/110(9)/4238/10/$ American Institute of Physics

2 J. Chem. Phys., Vol. 110, No. 9, 1 March 1999 Fujimaki et al FIG. 1. Schematic representation of the principles of a infrared-ultraviolet IR-UV double resonance spectroscopy and b autoionization-detected infrared ADIR spectroscopy. process. Spectral overlap with neutral bands also causes difficulty in assignments of spectra. In previous letters, 16,17 we proposed a new technique for infrared spectroscopy of jet-cooled molecular cations. This technique was based on the fact that, in very high Rydberg states, the geometric structure of the ion core is the same as that of the corresponding bare ion, because of the very small interaction between the ion core and the Rydberg electron. Such a small interaction enables us to prepare an isolatedcore excitation in which only the ion core of the high Rydberg states is subject to photoabsorption and the Rydberg electron behaves as a spectator during the transition. 18 In the isolated-core excitation, therefore, transitions of the ion core can be practically regarded as being identical to those of the corresponding bare ion. The very high Rydberg states with the cold ion core are prepared by two-color double resonance techniques. Vibrational excitation of the ion core results in vibrational autoionization autoionization accompanied by vibrational deexcitation, 19 and the ion signal is monitored as a function of the frequency of the tunable infrared light. We call this technique autoionization-detected infrared ADIR spectroscopy. ADIR spectroscopy is based on essentially the same concept as the photoinduced Rydberg ionization PIRI spectroscopy developed by Johnson and co-workers, in which electronic transitions of the ion core are measured by detecting the following electronic autoionization The structural similarity between the ion core of the Rydberg states and the bare ion is also utilized in zero kinetic energy photoelectron spectroscopy ZEKE-PES 23 and mass-analyzed threshold ionization MATI spectroscopy. 24 These techniques have been widely applied for vibrational spectroscopy of various molecular and cluster ions; however, there is a difficulty in obtaining high-frequency vibrations, such as OH and CH stretches, by these means. This difficulty arises partly due to poor Franck Condon factors between the ground and Rydberg states, and partly due to band congestion in the electronic transition used in the final step of the measurement. On the other hand, use of IR spectroscopy is more effective for observing the high-frequency vibrational modes, so that ADIR spectroscopy provides us with the information complementary to the results obtained from PIRI spectroscopy and ZEKE-PES. Very recently, Gerhards et al. also proposed a similar infrared spectroscopic technique IR/PIRI spectroscopy for jet-cooled species. 25 This technique employs measurement of the MAT1 signal reduction induced by infrared absorption of the ion core, so that the essential concept between it and ADIR spectroscopy is the same. We have previously applied ADIR spectroscopy to observation of the OH stretching vibrations in phenol and ortho-fluorophenol cations. 16,17 A large redshift of the OH stretch frequency of the ortho-fluorophenol cation was found, resulting in the first observation of intramolecular hydrogen bonds in jet-cooled molecular cations. In the present paper, we present a comprehensive report on the observation of intramolecular hydrogen bonds in fluorophenol and methoxyphenol cations. Both the fluorine atom and methoxyl groups are electron-attracting, and intramolecular hydrogen bond formation with the neighboring hydroxyl group is expected. The infrared spectra of ortho-, meta-, and paraisomers of fluorophenol and methoxyphenol in S 0 the neutral ground state and D 0 the ground state of the cation were measured by using IR-UV and ADIR spectroscopy, respectively. The isomer dependence of the OH stretching vibrational frequencies provides clear evidence of intramolecular hydrogen bond formation in ortho-isomers. Enhancement of the hydrogen bond strength upon ionization is discussed. II. EXPERIMENT A. IR-UV double resonance spectroscopy of the neutral ground state The details of IR-UV double resonance spectroscopy of jet-cooled molecules have been described elsewhere. 5,7,11,26 Only a brief description is given here. The principles of the spectroscopy are shown schematically in Fig. 1 a. The S 1 S 0 transitions of molecules were induced by a pulsed UV laser. The resonance enhanced multiphoton ionization REMPI signal was monitored as a measure of the population in the S 0 vibrational ground state of the molecule. A pulsed IR beam was introduced 50 ns prior to the UV beam, and its wavelength was scanned. When the IR laser frequency was resonant with the vibrational transition, the molecule was pumped to the vibrationally excited level, resulting in the reduction of the ionization signal. Thus, by scanning the IR laser wavelength, an ionization dip spectrum was obtained, which corresponded to the infrared spectrum of the ground state molecule. B. ADIR spectroscopy of the ionic ground state Figure 1 b shows the excitation scheme of ADIR spectroscopy. Jet-cooled molecules are excited to high Rydberg states converging to the vibrationless level (v 0) of the ground state of the ion by using two-color double resonance excitation via the 0 0 band of the S 1 S 0 transition. The IR laser light excites the isolated ion core of the Rydberg states to the vibrationally excited level (v 1). Because of the vi-

3 4240 J. Chem. Phys., Vol. 110, No. 9, 1 March 1999 Fujimaki et al. brational energy of the ion core, the core-excited Rydberg states lie above the first ionization threshold. Energy exchange between the ion core and Rydberg electron results in the spontaneous ejection of the electron, i.e., vibrational autoionization. After an appropriate delay time see Sec. III A, a pulsed electric field is applied to the interaction region, and the ions produced by autoionization are extracted into a time-of-flight mass spectrometer. The ions are massanalyzed and detected by a channel electron multiplier. While monitoring the autoionization signal, the infrared wavelength is scanned. Two UV beams were used to excite molecules to the high Rydberg states. One was a second harmonic of the output of a dye laser pumped by a pulsed YAG laser. Its typical output power was a few hundreds of nj/pulse, and was kept low enough to avoid generating a one-color ion background signal. The other UV beam was a second harmonic of the output of another dye laser, and its typical power was about 0.5 mj/pulse. Both laser beams were introduced coaxially into a vacuum chamber after they passed through a lens of f 500 mm. The IR light was prepared by difference frequency generation between the second harmonic of a YAG laser and the output of a YAG pumped dye laser. The output power of the IR light was typically 1 mj/pulse, and was focused by a lens of f 250 mm. The IR beam was counterpropagated with the UV laser beams. Pulse widths of all the laser beams were about 5 ns. No delay time was provided between the UV pulses, while the IR light was introduced 5 ns after the UV excitation. A time-of-flight mass spectrometer of Wiley McLaren type with a 60 cm long flight tube was used. 27 The pulsed electric field of 25 V/cm was used to repel ions and that of 125 V/cm for ion extraction. The pulse duration was 8 s, and the pulse rising rate was 15 V/ns. The field ionization effects of the Rydberg states are discussed in Sec. III A. 28 The sample vapor was seeded in He gas of 3 atm stagnation pressure, and was expanded into the vacuum chamber through a pulsed valve. The typical background pressure of the chamber was Torr. The jet expansion was skimmed by a skimmer of 2 mm diameter, and the resulting molecular beam was introduced into the interaction region. All samples were purchased from the Tokyo Kasei Co. and Aldrich, and were used without further purification. All samples except para-methoxyphenol were kept at the room temperature in a sample room prior to the jet valve. In the case of para-methoxyphenol, the sample was heated up to 370 K. III. RESULTS AND DISCUSSION A. Details of the procedure in ADIR spectroscopy: Phenol as an example In this subsection, we describe the procedure of ADIR spectroscopy in detail, taking the case of phenol as an example. Figure 2 shows two-color multiphoton ionization spectra of phenol near the first ionization threshold (IP 0 ). The first excitation laser was fixed to the origin band of the S 1 S 0 transition of phenol cm 1, and the second UV laser wavelength was scanned. In Fig. 2, the spectra FIG. 2. Two-color multiphoton ionization spectra of phenol around the first ionization threshold. In the spectra, the first excitation laser wavelength is fixed at the origin band of the S 1 S 0 transition of phenol, and the second excitation laser is scanned. The delay time between the laser pulses and rise of the pulsed electronic field is changed from a 100 to e 170 ns. Approximate principle quantum numbers of the Rydberg states are shown at the top of the spectra. The arrow indicates the energy of the Rydberg state for the measurement of the infrared spectrum shown in Fig. 3 see text. a e represent the change of the threshold with respect to the delay time between the laser pulses and the rise of the pulsed electric field. Since the IP 0 of phenol has been found to be cm 1, 23 the ionization signals below the threshold come from the field ionization of high Rydberg states converging to the v 0 level of the ion. 28 The approximate principle quantum numbers n of the Rydberg states are indicated at the top of the figure zero quantum defects of the Rydberg states are assumed. When the delay time is increased, the field ionization signal decreases and the observed threshold shifts to IP 0, because the Rydberg states decay and disappear, presumably due to predissociation. From this delay-time dependence, we roughly estimated the lifetime of the high Rydberg states: about 80 ns for the n 80 state. For ADIR spectroscopy, we fixed the second excitation laser wavelength at cm 1 in the total energy which corresponds to excitation of the n 83 Rydberg state, assuming the zero quantum defect, and the delay time between the laser pulses and ion extraction field was set to 170 ns to avoid background ion signals due to field ionization. The IR laser was introduced and its wavelength was scanned over the 3 m region. By monitoring the ion signal, the infrared spectrum shown in Fig. 3 was obtained; an intense peak appears at cm 1. From the band position, this peak is uniquely assigned to the OH stretching vibration of the ion

4 J. Chem. Phys., Vol. 110, No. 9, 1 March 1999 Fujimaki et al FIG. 3. ADIR spectrum of the phenol cation in the 3 m region. core. Because of the negligible perturbation from the Rydberg electron, the observed frequency can be practically regarded as being identical to that of bare ion; the accuracy of the vibrational frequency is essentially the same as that in ZEKE-PES measurements. In contrast, the present spectroscopy is free from the problem of the absolute frequency uncertainty due to the extraction field that is known to occur for ZEKE-PES. 23 Here the following should be noted in regard to ADIR spectroscopy. i The vibrational selection rule for ADIR spectroscopy is expected to be the same as that for ordinary infrared transitions. This is because the Rydberg electron plays only the role of a spectator during the infrared transition, and the transition is localized in the ion core. This situation is substantially different from the case of ZEKE spectroscopy, in which the symmetry and angular momentum of the Rydberg electron should be taken into account, making the selection rules fairly complicated. 23 ii The band intensity in ADIR spectroscopy does not depend only on the infrared absorption intensity but also on the dynamics of the Rydberg states. The decay dynamics of core-excited Rydberg states is characterized by a complicated competition between the processes of autoionization and neutral decay mainly predissociation. 29 The appearance of the vibrational spectrum suggests that the neutral decay rate, which is roughly estimated as 80 ns at n 80, is not substantially faster than the autoionization rate. iii The autoionization mechanism is not simple when the competitive predissociation is present. 29 Internal conversion from the Rydberg states to a dissociative valence state above IP 0 causes not only predissociation but also electronic autoionization transfer of the electronic energy to the ejected electron. The electronic autoionization process is generally much faster than the vibrational autoionization; however, its effective rate is lowered because the internal conversion process creates a bottleneck. FIG. 4. Possible structural and rotational isomers of fluorophenol. the steric restriction. Discrimination of the rotational isomers has been the subject of many spectroscopic studies. 30,31 It has been established for aromatic molecules that rotational isomers of a structural isomer have almost the same vibrational structure, whereas the origins of their electronic transitions are known to be significantly different. A great advantage of IR-UV spectroscopy for neutrals and of ADIR spectroscopy for cations is that the difference in the electronic transition energy enables discrimination of the rotational isomers. Figure 5 shows the REMPI spectrum of the origin region for the S 1 S 0 transition of jet-cooled o-fluorophenol. A strong band appears at cm 1, and a much weaker one is observed at cm 1. On the basis of the dispersed fluorescence study, it was tentatively concluded that the former band at cm 1 is the 0-0 band of the cis- B. Fluorophenol 1. Structural and rotational isomers Among three structural isomers of monosubstituted phenols i.e., ortho-, meta-, and para-isomers, there are cis and trans rotational isomers for the o- and m-isomers, as shown in Fig. 4. The intramolecular hydrogen bond is expected only in the cis isomer of o-fluorophenol because of FIG. 5. Resonance enhanced multiphoton ionization REMPI spectrum of the S 1 S 0 transition of o-fluorophenol around the origin. The strong band at cm 1 is the origin band of the cis-isomer see text.

5 4242 J. Chem. Phys., Vol. 110, No. 9, 1 March 1999 Fujimaki et al. TABLE I. The OH stretching vibrational frequencies of fluorophenol in cm 1. ortho meta para OH average a v OH b phenol c S 0 d D 0 cis cis trans a Average of the OH frequencies of all isomers other than the o-isomer. b v OH v OH average v OH o-isomer. c From Refs. 12 and 13. d This work. FIG. 6. REMPI spectrum of the S 1 S 0 transition of m-fluorophenol around the origin. The bands at and cm 1 are the origin bands of the cis- and trans-isomers, respectively. isomer, and the latter is due to the trans-isomer. 31 Recently, IR-UV double resonance spectroscopy was applied to these two rotational isomers, and demonstrated the OH stretching vibrations in their neutral ground states. 12,13 The results, however, revealed that both bands must be associated with one of the isomers, because both exhibit the same OH stretching frequency. Therefore, it has been concluded that the previous assignment was incorrect, and both bands have been reassigned to the cis-isomer, which should be more stable than the trans-isomer because of the intramolecular hydrogen bond. The REMPI spectrum at the origin region of the S 1 S 0 transition of m-fluorophenol is reproduced in Fig. 6, showing two bands having almost the same intensity. It was reported by Oikawa et al. that both bands show similar dispersed fluorescence spectra, and it was concluded that the band at cm 1 is due to the cis- and the band at cm 1 to the trans-isomer. 30 The intensities of the origin bands suggest roughly equal populations of the two rotational isomers. The REMPI spectrum of p-fluorophenol is given in Fig. 7, which well reproduces the spectrum reported by Tembreull et al. 32 Since no rotational isomer is expected for p-fluorophenol, the lowest energy band at cm 1 is the 0 0 band of p-fluorophenol. FIG. 7. REMPI spectrum of the S 1 S 0 transition of p-fluorophenol around the origin. 2. OH stretching vibrations in the neutral ground state The OH stretching vibrations of jet-cooled fluorophenols in the neutral ground state were recently studied by M. Fujii and co-workers. 12,13 They employed IR-UV double resonance spectroscopy and obtained isomer-separated IR spectra in the 3 m region. The reported vibrational frequencies of the OH stretches are tabulated in Table I. We repeated this experiment and confirmed their results. As was described in Sec. III B I, all the vibronic bands of o-fluorophenol were reassigned to the cis-rotational isomer, and no band attributed to the trans-isomer was found. As seen in Table I, the OH stretching frequencies of m- and p-isomers are quite similar to that of phenol, indicating that the fluorine substitution induces a minor change in the force field of the hydroxyl group through the aromatic ring. On the other hand, only the cis-isomer of o-fluorophenol 3634 cm 1 shows a substantial low-frequency shift of 26 cm 1 compared with the average frequency 3660 cm 1 among the other isomers. This shift constitutes clear evidence for the presence of an intramolecular hydrogen bond between the fluorine atom and the hydroxyl group. The OH stretch frequency of the cis-isomer of m-fluorophenol is shifted by only 1 cm 1 from that of the trans-isomer. Moreover, these frequencies are very close to that of the p-isomer. These facts mean that the intramolecular hydrogen bond does not form in m-fluorophenol because of the steric restriction. 3. OH stretching vibrations in the ionic ground state The ADIR spectra in the OH stretching region of the fluorophenol cations are shown in Fig. 8. In each spectrum, the high Rydberg states were pumped by the two-color double resonance excitation via the 0 0 band of the S 1 S 0 transition, as was shown in Sec. III A, and the autoionization signal due to the IR absorption of the ion core was recorded as a function of the IR wavelength. The peak positions of the spectra are also tabulated in Table I. Upon the ionization, the OH stretching vibrational frequency of every isomer cation is reduced by over 100 cm 1 compared to those of its neutral form. This ionizationassociated shift is roughly as large as that of phenol. As in the case of the neutral isomers, only the cis-isomer of the o-fluorophenol cation shows a remarkable lowfrequency shift with respect to other structural and rotational isomers. From the viewpoint of molecular structure, it is clear that this substantial low-frequency shift of OH stretch-

6 J. Chem. Phys., Vol. 110, No. 9, 1 March 1999 Fujimaki et al FIG. 8. ADIR spectra of fluorophenol and phenol cations in the OH stretching vibration region. a cis-rotational isomer of o-fluorophenol cation. b cis- and c trans-rotational isomers of m-fluorophenol cation, respectively. d p-fluorophenol cation. e phenol cation. ing vibrations is due to intramolecular hydrogen bond formation. To our knowledge, this is the first observation of the intramolecular hydrogen bond in molecular cations in the gas phase. The low-frequency shift from the average of the other isomers was found to be 49 cm 1, which is about twice as large as that of the neutral, indicating an enhancement of the intramolecular hydrogen bond upon ionization. The OH frequencies of the m- and p-isomers show highfrequency shifts from that of phenol. This tendency is also seen in the neutrals, but it is much more significant in the cations, as shown in Table I. This shift is clearly due to the electronic effect of the fluorine substitution. The difference between the OH stretch frequencies of the cis- and transisomers of m-fluorophenol also increases up to 2 cm 1 upon the ionization. However, the OH frequency of the cis-isomer is lower than that of the trans-isomer in the cation, while the cis-isomer has a higher frequency in the neutral ground state. Moreover, the difference is still very small, and both the frequencies are close to that of the p-isomer. This fact indicates that local interactions between the fluorine atom and hydroxyl group are negligible in the m-isomer, as was observed in S 0. In Fig. 8, it can be seen that the bandwidths of the OH stretches are rather broad. Though such bandwidths must carry more information about the dynamical processes of the vibrationally excited level, the careful elimination of saturation effects required for discussion of the widths has not yet been achieved. Since our focus here is on the frequency FIG. 9. Possible rotational and structural isomers of methoxyphenol. shifts of the OH stretches, we will reserve discussion of the bandwidth for a future study. C. Methoxyphenol 1. Structural and rotational isomers Methoxyphenol also has three structural isomers, i.e., o-, m-, and p-isomers. Intramolecular rotations of the methoxyl and hydroxyl groups are responsible for four rotational isomers in the o- and m-isomers, and for two in the p-isomer. These structural and rotational isomers are schematically illustrated in Fig. 9. Figure 10 shows the REMPI spectrum of the S 1 S 0 transition of o-methoxyphenol. The strong band appearing at cm 1 is assigned to the origin. As will be shown later, IR hole-burning and IR-UV spectroscopy to discriminate rotational isomers reveal that all vibronic bands in the REMPI spectrum are attributed to a single rotational isomer which involves an intramolecular hydrogen bond. From the viewpoint of the steric restriction, it is reasonable to suppose that the intramolecular hydrogen bond is formed only in the isomer I in Fig. 9, and therefore it is concluded that o-methoxyphenol has only the isomer I form in a jet. To confirm the above, we calculated the electronic energies and harmonic vibrational frequencies of the rotational isomers by means of ab initio calculations at the Hartree Fock-self-consistent field HF-SCF level using the 6-31G

7 4244 J. Chem. Phys., Vol. 110, No. 9, 1 March 1999 Fujimaki et al. FIG. 10. a REMPI spectra of the S 1 S 0 transition of o-methoxyphenol around the origin. b Infrared hole-burning spectrum in the same region. The infrared wavelength is fixed at the OH stretching band of the isomer l of o-methoxyphenol see text. basis set of the GAUSSIAN94 program package. 33 The potential minima corresponding to the isomers I and III were obtained, and their relative total energies and OH stretching frequencies are tabulated in Table II. We could not find the minima corresponding to the isomers II and IV. In these two structures, the repulsion between the hydroxyl and methyl group is so large that the corresponding local minima might not be present or might be too shallow for the isomer to be stable. The total energy of the isomer I is more stable by ev than that of the isomer III, and its OH stretch frequency is 35 cm 1 lower than that of the isomer III. These results are a clear indication of the intramolecular hydrogen bond in the isomer I. And although the calculation level may not be high enough to accurately obtain the energy difference between the isomers, the large energy difference qualitatively supports the exclusive population of the isomer I in a jet. Experimental evidence of the exclusive population is given in the next section. The S 1 S 0 electronic transition of m-methoxyphenol, reproduced in Fig. 11, shows two bands designated as A and B, respectively at and cm 1, which are assigned as the origin bands of two rotational isomers. Starting from either band, similar progressions occur and result in rich vibronic structures. Four energy minima corresponding to the isomers I IV in Fig. 9 were found for TABLE II. Calculated energies and OH stretching vibrational frequencies of structural and rotational isomers of methoxyphenol. The calculated level is HF-SCF/6-31G. FIG. 11. REMPI spectrum of the S 1 S 0 transition of m-methoxyphenol around the origin. The bands A and B are the origin bands of two rotational isomers. m-methoxyphenol by means of the ab initio calculations. The calculated total energy differences among these four structures were much smaller than that of the o-isomers, and their OH stretch frequencies are also degenerated, as shown in Table II. The calculations, however, suggest that the isomers I and IV have slightly lower energies than the others, and thus these two isomers are the most probable candidates for the spectral carriers of the electronic transitions. The nearly degenerated electronic energies of the isomers I and IV are consistent with the comparable intensities of the two progressions in the electronic spectrum. There is, unfortunately, no definite evidence for the identification, so that this assignment is still tentative. Figure 12 shows the origin region of the REMPI spectrum of the S 1 S 0 transition of p-methoxyphenol; two intense bands appear at and cm 1, denoted as band and, respectively. The two-color ionization thresholds via the excitation of these two bands represent the different ionization energies, as shown in Fig. 13. Therefore, these two bands are attributed to two rotational isomers of p-methoxyphenol. The HF-SCF/6-31G level ab initio calculations predict two potential minima having similar energies and OH stretch frequencies, as shown in Table II. They correspond to the rotational isomers I and II in Fig. 9, though it is difficult to predict which structure corresponds to which band in the electronic spectrum. 2. OH stretching vibrations in the neutral ground state The IR-UV double resonance spectra of o-, m-, and p-methoxyphenol are shown in Fig. 14. In each spectrum, a Rotational isomer a Relative energy/ ev b OH frequency /cm 1 ortho- isomer I lowest 4011 isomer III meta- isomer I isomer II isomer III isomer IV lowest 4049 para- isomer I isomer II lowest 4050 a Corresponding to the schematic representations in Fig. 9. b The energy of the lowest rotational isomer is set to zero in each structural isomer. FIG. 12. REMPI spectrum of the S 1 S 0 transition of p-methoxyphenol around the origin. The bands and are the band origin bands of two rotational isomers.

8 J. Chem. Phys., Vol. 110, No. 9, 1 March 1999 Fujimaki et al FIG. 13. Two-color multiphoton ionization spectra of p-methoxyphenol around the first ionization threshold. The first excitation laser wavelength is fixed at the a band and b band, respectively, of the S 1 S 0 transition shown in Fig. 12. The second laser wavelength is scanned. single band due to the OH stretching vibration is shown the splitting of the bands is due to the absorption of the IR light caused by atmospheric water in the optical path. Asdescribed above, m- and p-methoxyphenol exhibit two rotational isomers in a jet, while o-methoxyphenol shows only the single isomer having the intramolecular hydrogen bond. In each IR-UV spectrum, the UV laser wavelength was fixed at the origin band of each rotational isomer. FIG. 14. IR spectra of the OH stretching vibrations of methoxyphenol in the neutral ground state measured by using IR-UV double resonance. a isomer I of o-methoxyphenol. b A and c B rotational isomers of m-methoxyphenol, respectively. d and e rotational isomers of p-methoxyphenol, respectively. The splitting of the bands b, c, and e is due to the infrared light absorption of the atmospheric water in the optical path. The OH stretch frequency of the o-isomer shows a remarkable redshift from those of the m- and p-isomers. The redshift is 60 cm 1 from the average of the others, and is much larger than that in fluorophenol. This constitutes clear evidence for the presence of an intramolecular hydrogen bond between the hydroxyl and methoxyl groups, supporting the assignment of the rotational isomer described in the above section. The OH stretch frequencies of the rotational isomers of m- and p-methoxyl phenol are very close in value, and are also very close to that of phenol. This means that the methoxyl substitution effect through the aromatic ring is very small for the hydroxyl group. In addition, the hydroxyl group is spatially apart from the methoxyl group in these isomers, so that the interaction through space i.e., the intramolecular hydrogen bond might be negligible. This is also supported by the fact that the difference between the OH stretch frequencies of the two rotational isomers of m-methoxyphenol is only 3 cm 1, and that the difference between the rotational isomers of p-methoxyphenol is also the same value. On the basis of the results of the IR-UV spectroscopy, the discrimination of the isomers in the electronic spectrum can be performed by the IR hole-burning technique. 7 We applied this technique to o-methoxyphenol. The IR wavelength was fixed at the OH stretching frequency of the isomer I, 3599 cm 1, and then the UV wavelength was scanned around the spectral region of the S 1 S 0 transition by monitoring the ionization signal. With this method, all the vibronic bands attributed to the isomer I should disappear, while those of other isomers having different OH frequencies should show no change in intensity. The resulting holeburning spectrum of o-methoxyphenol is shown in Fig. 10 b. All the vibronic bands disappeared with the excitation by IR light, of which the wavelength was resonant with the OH stretching vibration of the isomer I. This result indicates that the isomer I is the only species existing in the jet expansion. 3. OH stretch vibration in the ground state of the cation The ADIR spectra of the OH stretching vibrations of the rotational isomers of o-, m-, and p-methoxyphenol are shown in Fig. 15. The OH stretching frequencies of all the isomer cations were substantially reduced over 100 cm 1 upon the ionization. Among them, o-methoxyphenol exhibited a much larger redshift than the others; this difference was found to be 94 cm 1, as indicated in Fig. 15. This remarkable frequency reduction manifests a significant enhancement of the intramolecular hydrogen bond strength associated with the ionization of the o-isomer. For the m- and p-isomers, the behaviors of the OH frequencies upon the ionization were very similar to those of the fluorophenol cations; the blue shift from phenol became larger, and a slight difference in the OH frequency shifts was found between the two rotational isomers of the m-isomers. No evidence was found for the intramolecular hydrogen bond in the m- and p-isomers, and it is clear that the steric restriction prohibits the intramolecular hydrogen bond.

9 4246 J. Chem. Phys., Vol. 110, No. 9, 1 March 1999 Fujimaki et al. FIG. 16. The HOMO wave function of the cis-isomer of o-fluorophenol obtained by the ab initio calculation with the HF-SCF 6-31G (d,p) level. FIG. 15. ADIR spectra of the OH stretching vibrations of methoxyphenol cations in the ground state. a isomer I of o-methoxyphenol. b A and c B rotational isomers of m-methoxyphenol, respectively. d and e rotational isomers of p-methoxyphenol, respectively. D. Intramolecular hydrogen bonds in the aromatic cations The observed OH stretch frequencies of the rotational and structural isomers of fluorophenol and methoxyphenol are tabulated in Tables I and III, respectively. In both cases, only the cis-isomers of the o-isomers show large redshifts compared with the other isomers. This is an unequivocal indication of the presence of the intramolecular hydrogen bond between the hydroxyl group and fluorine or methoxyl groups in both the neutral and cationic states. Ionization significantly enhances the redshift due to the intramolecular hydrogen bond. This shows the increase of the intramolecular hydrogen bond strength with ionization, though the correlation between the redshift and the bond strength is not straightforward, as it was in the case of the intermolecular hydrogen bond. 34,35 The enhancement of the intramolecular hydrogen bonding upon the ionization can be qualitatively understood by TABLE III. The OH stretching vibrational frequencies of methoxyphenol in cm 1. ortho meta para average a v b phenol isomer I isomer A isomer B isomer isomer S D a Average of the OH frequencies of all isomers other than the o-isomer. b v v OH average v OH o-isomer. examining the character of the highest occupied molecular orbital HOMO of the molecules, if Koopman s theory is applicable for the ion. Figure 16 shows the HOMO wave function of the cis isomer of o-fluorophenol obtained in the ab initio calculation with the HF-SCF/6-31G (d,p) level. 36 The orbital has mixed characters of the nonbonding electron on the oxygen atom and the electron in the phenyl ring. The ionization extracts an electron from the HOMO, and it largely reduces the electron density at the oxygen atom, resulting in the enhancement of the acidity of the hydroxyl group. On the other hand, the HOMO wave function has no distribution on the fluorine atom, and the elimination of the electron from the HOMO does not strongly affect the proton affinity of the fluorine atom. The OH frequency redshift of methoxyphenol is about twice that of fluorophenol, both in S 0 and D 0. This reflects the higher proton acceptability of the methoxyl group relative to that of the fluorine atom. Although fluorine is usually one of the strongest proton acceptors, when bound to carbon, it is well known that fluorine loses its high proton acceptability. 2 The weak intramolecular hydrogen bond in fluorophenol is regarded as a typical example of the specialty of carbonated fluorine. IV. CONCLUDING REMARKS We developed a new technique of infrared spectroscopy for jet-cooled molecular ions ADIR spectroscopy, in which the infrared absorption of the ion core of the high Rydberg states is measured by detecting autoionization. The absorption of the ion core practically represents that of the corresponding bare ion. The most significant advantage of ADIR spectroscopy is its high sensitivity. The density of molecular ions in a supersonic jet is roughly estimated as 10 9 /cm 3 ( mol/l ) or lower. ADIR and its related technique, IR/PIRI 25 spectroscopy is a unique infrared spectroscopic technique applicable to cations of such low concentration. Another important advantage of ADIR spectroscopy is that it allows clear

10 J. Chem. Phys., Vol. 110, No. 9, 1 March 1999 Fujimaki et al identification of the spectral carrier. The multistep excitation of the Rydberg states provides unambiguous identification of the spectral carrier. It also enables us to separate isomers coexisting in a supersonic jet. These are very powerful features in terms of overcoming the problems encountered in conventional matrix isolation techniques. We applied ADIR spectroscopy to the intramolecular hydrogen-bonded species, fluorophenol and methoxyphenol cations, in jet expansions. The OH stretching vibrations of the rotational isomers of the o-, m-, and p-structural isomers were observed, and only the cis-isomer of the o-structural isomer showed the remarkable redshift of the OH stretch frequency from those of the others. This shift provides clear evidence of the intramolecular hydrogen bond, and constitutes the first direct observation of molecular cations. The redshift due to the intramolecular hydrogen bond significantly increases with ionization, indicating strong enhancement of the hydrogen bond strength. In this paper, we presented our results on the conventional intramolecular hydrogen bond of the aromatic cations. Our observation of an unconventional intramolecular hydrogen bond, a hydrogen bond between the hydroxyl and alkyl groups, is reported elsewhere. 37,38 ACKNOWLEDGMENTS We thank Atsushi Iwasaki for his contribution in the early stage of this work. We are grateful to Drs. H. Ishikawa and T. Maeyama for helpful discussion. This work is partly supported by a Grant-in-Aid from the Ministry of Education No and the Sumitomo Foundation. 1 G. C. Pimentel and A. L. McClellan, The Hydrogen Bond W. H. Freeman, San Francisco, G. A. Jeffey, An Introduction to Hydrogen Bonding Oxford University Press, Oxford, 1997, and references therein. 3 For example, Y. Tomioka, M. Ito, and N. Mikami, J. Phys. Chem. 87, T. S. Zwier, Annu. Rev. Phys. Chem. 47, , and references therein. 5 T. Ebata, A. Fujii, and N. Mikami, Int. Rev. Phys. Chem. 17, R. H. Page, Y. R. Shen, and Y. T. Lee, J. Chem. Phys. 88, A. Mitsuzuka, A. Fujii, T. Ebata, and N. Mikami, J. Chem. Phys. 105, R. K. Frost, F. C. Hagemeister, C. A. Arrington, and T. S. Zwier, J. Chem. Phys. 105, R. K. Frost, F. C. Hagemeister, C. A. Arrington, D. Schleppenbach, T. S. Zwier, and K. D. Jordan, J. Chem. Phys. 105, R. K. Frost, F. Hagemeister, D. Schleppenbach, G. Laurence, and T. S. Zwier, J. Phys. Chem. 100, 16, A. Mitsuzuka, A. Fujii, T. Ebata, and N. Mikami, J. Phys. Chem. A 102, T. Omi, H. Shitomi, N. Sekiya, K. Takazawa, and M. Fujii, Chem. Phys. Lett. 252, H. Shitomi, S. Ishiuchi, and M. Fujii, Abstract of the Conference on Molecular Structure, Fukuoka, L. I. Yeh, M. Okumura, J. D. Myers, J. M. Price, and Y. T. Lee, J. Chem. Phys. 91, M. E. Jacox, J. Phys. Chem. Ref. Data 19, , and references therein. 16 A. Fujii, A. Iwasaki, T. Ebata, and N. Mikami, J. Phys. Chem. A 101, A. Fujii, A. Iwasaki, and N. Mikami, Chem. Lett., W. E. Cooke, T. F. Gallager, S. A. Edelstein, and R. M. Hill, Phys. Rev. Lett. 40, H. Lefebvre-Brion and R. W. Field, Perturbations in the Spectra of Diatomic Molecules Academic, Orlando, 1986, Chap D. P. Taylor, J. G. Goode, J. E. LeClaire, and P. M. Johnson, J. Chem. Phys. 103, J. G. Goode, J. E. LeClaire, and P. M. Johnson, Int. J. Mass Spectrom. Ion Processes 159, J. E. LeClaire, R. Anand, and P. M. Johnson, J. Chem. Phys. 106, K. Müller-Dethlefs and E. Schlag, Annu. Rev. Phys. Chem. 42, L. Zhu and P. M. Johnson, J. Chem. Phys. 94, M. Gerhards, M. Schiwek, C. Unterberg, and K. Kleinermanns, Chem. Phys. Lett. 297, T. Watanabe, T. Ebata, S. Tanabe, and N. Mikami, J. Chem. Phys. 105, W. C. Wiley and I. H. McLaren, Rev. Sci. Instrum. 26, W. A. Chupka, J. Chem. Phys. 98, H. Nakamura, Int. Rev. Phys. Chem. 10, , and references therein. 30 A. Oikawa, H. Abe, N. Mikami, and M. Ito, J. Phys. Chem. 88, A. Oikawa, H. Abe, N. Mikami, and M. Ito, Chem. Phys. Lett. 116, R. Tembreull, T. M. Dunn, and D. M. Lubman, Spectrochim. Acta A 42, M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Petersson, J. A. Montgomery, K. Raghavachari, M. A. Al-Laham, V. G. Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C. Gonzalez, and J. A. Pople, GAUSSIAN 94, Revision D.4 Gaussian, Inc., Pittsburgh, A. Iwasaki, A. Fujii, T. Watanabe, T. Ebata, and N. Mikami, J. Phys. Chem. 100, 16, K. Kawamata, P. K. Chowdhury, F. Ito, K. Sugawara, and T. Nakanaga, J. Phys. Chem. A 102, The visualization of the wave function is performed by using the program, G. Schaftenaar, MOLDEN Nijmegen, A. Fujii, E. Fujimaki, T. Ebata, and N. Mikami, J. Am. Chem. Soc. 120, E. Fujimaki, A. Fujii, T. Ebata, and N. Mikami in preparation.