Autoionization-detected infrared (ADIR) spectroscopy of molecular cations

Journal of Electron Spectroscopy and Related Phenomena 108 (2000) 21–30 www.elsevier.nl / locate / elspec

Autoionization-detected infrared (ADIR) spectroscopy of molecular cations Asuka Fujii*, Naohiko Mikami Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980 -8578, Japan

Abstract A newly developed technique for infrared spectroscopy of cold molecular cations in the gas phase is reviewed. Very high Rydberg states converging to the first ionization threshold are prepared by two-color double resonance excitation. Vibrational excitation of the ion core of the Rydberg states induces autoionization, leading to the molecular ion. An infrared spectrum is obtained by monitoring the ion current due to the autoionization as a function of the vibrational excitation laser frequency. The observed spectrum practically provides the vibrational frequency of the bare ion. This new technique is called autoionization-detected infrared (ADIR) spectroscopy, and it is very advantageous to observe high frequency vibrations of molecular cations, such as OH and CH stretches. The following applications of ADIR spectroscopy to the phenol derivative cations are reviewed in this paper; (1) OH stretching vibrations of typical intramolecular hydrogen-bonded cations. (2) Unconventional intramolecular hydrogen bonds between the hydroxyl and alkyl groups. (3) Aromatic and alkyl CH stretching vibrations in aromatic cations.  2000 Elsevier Science B.V. All rights reserved. Keywords: ADIR spectroscopy; Cold molecular cations; Gas phase

1. Introduction For highly excited Rydberg states, it is well known that the geometric structure of their ion core is quite similar to that of the corresponding bare ion, because of very small interaction between the ion core and a Rydberg electron. Zero kinetic energy– photoelectron spectroscopy (ZEKE-PES) and mass analyzed threshold ionization (MATI) spectroscopy have been developed on the basis of this nature of high Rydberg states to provide precise vibrational frequencies of ionic species [1–3]. Both techniques have been widely applied as vibrational spectroscopy of various molecular and cluster ions, however, there is a difficulty in obtaining high frequency vibrations, *Corresponding author. Fax: 181-22-217-6785. E-mail address: [email protected] (A. Fujii)

such as OH and CH stretches. This difficulty comes partly from poor Franck–Condon factors between the intermediate and Rydberg states and also from the band congestion in electronic transitions used as the final step of the measurement. The weak interaction between the ion core and the high Rydberg electron enables us to perform ‘isolated-core excitation’, in which only the ion core is subject to photoabsorption and the Rydberg electron behaves as a spectator during the transition. In the isolated-core excitation, thus, transitions of the ion core are practically regarded as those of the corresponding bare ion. This phenomenon was first applied for the generation of doubly excited atoms [4]. Johnson and co-workers measured electronic transitions of benzene and phenol cations by detecting the electronic autoionization signal following the isolated-core excitation of high Rydberg states [5–7].

0368-2048 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 00 )00142-0

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They called this method photoinduced Rydberg ionization (PIRI) spectroscopy. We apply the isolated-core excitation for vibrational spectroscopy of molecular ions. 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 de-excitation), and the ion signal is monitored as a function of the frequency of tunable infrared light. We call this new infrared (IR) spectroscopic technique autoionization-detected infrared (ADIR) spectroscopy [8–13]. Infrared spectroscopy has an advantage of observing high frequency vibrational modes, so that ADIR spectroscopy provides us with the complementary information to ZEKE and PIRI spectroscopy, which is useful to measure lowfrequency modes such as intermolecular vibrations. Recently Gerhards et al. also proposed a similar IR spectroscopic technique (IR / PIRI spectroscopy) for jet-cooled species [14,15]. This technique employs the measurement of the MATI signal reduction induced by an IR absorption of the ion core, so that its essential concept is very similar to that of ADIR spectroscopy. For polyatomic molecular ions except the cases of small sized ones, conventional IR methods are generally not applicable in respect of their sensitivity as well as their selectivity. On the other hand, because of the autoionization-detection of the IR absorption, the sensitivity of ADIR spectroscopy is high enough to apply to molecular cations in a supersonic jet. At present, ADIR spectroscopy (and its related technique, IR / PIRI) is the unique IR spectroscopic method for jet-cooled molecular cations. Measurements under the supersonic jet condition allow us to observe molecular vibrations of the cations with the complete elimination of perturbations from surrounding molecules. Such a perturbation often interferes with the analysis of IR spectra obtained by the matrix isolation technique. Another advantage of ADIR spectroscopy is its high selectivity. Unambiguous identification of the spectral carrier is inherently performed by the stepwise excitation of the high Rydberg states. Spectral overlap with neutral bands is no longer a problem in ADIR spectroscopy. Moreover, the discrimination of rotational

isomers which are often co-existing in the jet expansion is also easily done because of the significant difference of the origins of the electronic transition among the isomers. In the case of neutral jet-cooled species, the technique of population labeling has been used to observe their IR spectra [16]. This is known as infrared–ultraviolet (IR-UV) double resonance spectroscopy, and employs electronic transitions to detect the population changes caused by IR absorption. The combination of the IR-UV and ADIR spectroscopy enables us to compare IR spectra in the neutral and cationic ground states, providing important information on the structural changes upon ionization. We have applied ADIR spectroscopy to observation of the OH and CH stretching vibrations in substituted phenol cations [8–13]. In the present paper, we review representative results of the ADIR spectroscopic studies and show the high potential of the ADIR technique to elucidate structures of molecular cations.

2. Experiment

2.1. IR-UV double resonance spectroscopy of the neutral ground state ( S0 ) The details of IR-UV double resonance spectroscopy of jet-cooled molecules have been described elsewhere [16]. Only a brief description is given here. The principles of the spectroscopy are shown schematically in Fig. 1a. The S 1 –S 0 transitions of molecules are induced by a pulsed UV laser. The resonance enhanced multiphoton ionization (REMPI) signal is monitored as a measure of the population in the S 0 vibrational ground state of the molecule. A pulsed IR beam is introduced 50 ns prior to the UV beam, and its wavelength is scanned. When the IR laser frequency is resonant with the vibrational transition, the molecule is 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 is obtained, which corresponds to the IR spectrum of the ground state molecule.

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Fig. 1. Schematic representation of the principles of (a) infrared–ultraviolet (IR-UV) double resonance spectroscopy and (b) autoionizationdetected infrared (ADIR) spectroscopy.

2.2. ADIR spectroscopy of the ionic ground state ( D0 ) Fig. 1b shows the excitation scheme of ADIR spectroscopy. Jet-cooled molecules are excited to high Rydberg states converging to the vibrationless level (v 5 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 5 1). Because of the vibrational energy of the ion core, the coreexcited 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 [17]. After an appropriate delay time, a pulsed electric field is applied to the interaction region, and the ions produced by autoionization are extracted into a timeof-flight mass spectrometer. The ions are mass-analyzed and detected by a channel electron multiplier. While monitoring the autoionization signal, the IR

laser wavelength is scanned. The IR absorption of the ion core is detected as an enhancement of the ionization signal. More detailed experimental conditions of ADIR spectroscopy are available in Refs. [8,11].

3. Applications of ADIR spectroscopy

3.1. Intramolecular hydrogen bonds in aromatic cations [9,11] Intramolecular hydrogen bonds, which form between neighboring proton donating and accepting groups, have been the subject of various studies [18,19]. The most powerful probe for intramolecular hydrogen bonds is the observation of OH stretching vibrations, as in the case of familiar intermolecular hydrogen bonds. Most of the previous IR studies on intramolecular hydrogen bonds, however, have been performed in condensed phases, in which there are many perturbations, such as polarization effects,

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dipole–dipole interactions, and so on. In many intramolecular hydrogen-bonded molecules, the hydrogen bond is subjected to severe steric restrictions associated with the structure of the molecule, and the OH stretch frequency results in much smaller redshifts in comparison with intermolecular hydrogenbonded OH stretches. Therefore, elimination of any perturbations from the surrounding molecules is required to elucidate the nature of intramolecular hydrogen bonds. From this standpoint, an isolated cold molecule in a supersonic jet is an ideal system for study of intramolecular hydrogen bonds. Phenol derivatives in which an electron attracting group, such as fluorine and methoxyl, is substituted are typical examples of intramolecular hydrogenbonded molecules. The hydroxyl group donates the proton, and the electron attracting group, which is negatively charged, accepts the proton. Among three structural isomers of mono-substituted 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. 2. The intramolecular hydrogen bond is expected only in the cis rotational isomer of the o-isomer because of the steric restriction. 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 [20,21]. A great advantage of IR-UV spectroscopy for neutrals and of ADIR spectroscopy for cations is that discrimination of the rotational isomers can easily be done by the selection of the laser frequency of the first excitation step of the high Rydberg states. The OH stretching vibrations of jet-cooled fluorophenols in the neutral ground state were recently studied by Fujii and co-workers [22,23]. They employed IR-UV double resonance spectroscopy and obtained isomer-separated IR spectra in the 3 mm region. As is shown in Table 1, the OH stretching frequencies of m- and p-isomers are quite similar to that of phenol (3657 cm 21 ), 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 21 ) shows a substantial low frequency shift of 26 cm 21 compared with the average frequency (3660 cm 21 ) among the other

Fig. 2. Structural and rotational isomers of phenol derivatives.

isomers. This shift constitutes clear evidence for the presence of an intramolecular hydrogen bond between the fluorine atom and the hydroxyl group. The trans isomer of o-fluorophenol was not found in the jet expansion, because all molecules populate in the more stable cis conformation. The ADIR spectra in the OH stretching region of the fluorophenol cations are shown in Fig. 3. The observed OH frequencies are also tabulated in Table 1. Upon the ionization, the OH stretching vibrational frequency of every isomer cation is reduced by over 100 cm 21 compared to those of its neutral form. This ionization-associated 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 low-frequency shift of the OH frequency with respect to the other structural and rotational isomers. From the viewpoint of molecular structure, it is clear that this substantial low-frequency shift of OH stretching vibrations is due to intramolecular

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Table 1 The OH stretching vibrational frequencies of fluorophenol in cm 21 Ortho Cis S0 c D0 d

3634 3495

Meta Cis

Trans

3659 3542

3658 3544

Para

nOH (average)a

DnOH b

Phenol

3664 3546

3660 3544

26 49

3657 3534

a

Average of the OH frequencies of all isomers other than the o-isomer. DnOH 5 nOH (average)2 nOH (o-isomer). c From Refs. [22,23]. d This work. b

Fig. 3. ADIR spectra of fluorophenol and phenol cations in the OH stretching vibrational 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.

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 21 , which is about twice as large as that of the neutral, indicating an enhancement of the intramolecular hydrogen bond upon ionization. Similar enhancement of the intramolecular hydrogen bond strength was found for methoxyphenol. Methoxyphenol also has three structural isomers, i.e. o-, m-, and p-isomers, and there exist rotational isomers due to the relative conformation between the methoxyl and hydroxyl groups. The OH stretching frequencies of neutral methoxyphenol are tabulated in Table 2. The OH stretch frequency of the cis isomer of the o-isomer (3599 cm 21 ) shows a remarkable red-shift from those of the m- and pisomers (3654–3662 cm 21 ). The red-shift is 60 cm 21 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 in the cis isomer of o-methoxyphenol. It was also found that o-methoxyphenol in a jet populates only in the cis conformation stabilized by the intramolecular hydrogen bond, while two rotational isomers were found for each m- and p-isomer. The ADIR spectra of the OH stretching vibration of the rotational isomers of the o-, m-, and pmethoxyphenol cations are shown in Fig. 4, and their OH frequencies are tabulated in Table 2 with those of the neutrals. The OH stretching frequencies of all the isomer cations were substantially reduced over 100 cm 21 upon the ionization. Among them, the o-isomer cation exhibited a much larger red-shift

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Table 2 The OH stretching vibrational frequencies of methoxyphenol in cm 21

S0 D0 a b

Ortho Cis

Meta

Para

Isomer A

Isomer B

Isomer a

Isomer b

3599 3487

3657 3584

3654 3589

3662 3576

3661 3573

nOH (average)a

DnOH b

Phenol

3659 3581

60 94

3657 3534

Average of the OH frequencies of all isomers other than the o-isomer. DnOH 5 nOH (average)2 nOH (o-isomer).

than the others; this difference was found to be 94 cm 21 . This remarkable frequency reduction manifests a significant enhancement of the intramolecular hydrogen bond strength associated with the ionization of the o-isomer. Such enhancement of the intramolecular hydrogen bonding upon the ionization can be qualitatively understood by examining the character of the highest occupied molecular orbital (HOMO) of the molecules, if Koopman’s theory is applicable for the ion. Fig. 5 shows the HOMO wavefunction of the cis isomer of o-fluorophenol obtained in the ab initio calculation with the HF-SCF / 6-31G1(d, p) level

Fig. 4. ADIR spectra of the OH stretching vibrations of methoxyphenol cations in the ground state. (a) Cis rotational isomer of o-methoxyphenol. (b, c) Rotational isomers of m-methoxyphenol. (d, e) Rotational isomers of p-methoxyphenol.

Fig. 5. The HOMO wavefunction of the cis-isomer of o-fluorophenol obtained by the ab initio calculation with the HF-SCF / 631G1(d, p) level.

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[24,25]. The orbital has mixed characters of the non-bonding electron on the oxygen atom and the p 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 wavefunction 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.

3.2. Unconventional intramolecular hydrogen bond in the cresol cation [10,13] The examples of intramolecular hydrogen bond described in the previous section are of the typical hydrogen bond. The fluorine and methoxyl groups are known to be electron-attracting from the aromatic ring, and the high proton affinities of these groups are reasonably expected. On the other hand, alkyl groups are generally considered to have no protondonating nor -accepting abilities, and are known to be a non-participating group in the hydrogen bond [19]. Fig. 6 shows IR spectra of rotational and structural isomers of jet-cooled neutral cresol (methylphenol) observed by the IR-UV double resonance technique. Both the o- and m-cresol have the cis and trans rotational isomers due to the conformation of the hydroxyl group relative to the methyl group [26]. Sharp dips in the spectra correspond to absorption due to the OH stretching vibration. In contrast to fluorophenol and methoxyphenol, the OH stretch band of every isomer of cresol appears in the same region, and the frequency difference among these five isomers is extremely small (within 3 cm 21 ). This result directly indicates that the intramolecular hydrogen bond is negligible in neutral o-cresol, as is reasonably expected. ADIR spectra of the OH stretches of the cresol cations are shown in Fig. 7. Upon ionization, the OH stretch frequencies shift to red over 100 cm 21 , as usual. However, a significant feature in the spectra of the cations is seen for the substantial red-shift of the OH band in the cis isomer of the o-cresol cation in comparison with all of the other isomers. The trans isomer of the o-cresol cation should give the same

Fig. 6. Infrared spectra of isomer-separated neutral cresol in the OH stretching vibrational region. The IR-UV double resonance technique is utilized to measure the spectra of jet-cooled molecules.

electronic substitution effects as the cis isomer to the hydroxyl group through the phenyl ring, however, its OH frequency is almost the same as those of m- and p-isomers. Therefore, the origin of the characteristic red-shift of the cis-isomer of the o-cresol cation is concluded to be due to local interactions between the hydroxyl and methyl group, i.e. intramolecular hydrogen bond. The steric configuration of the cis isomer and the red-shift of the OH frequency indicate that the methyl group should be the proton acceptor in this hydrogen bond while the hydroxyl group the donor. Such an intramolecular hydrogen bond has not been known, and this is the first observation of the new interaction. The OH frequency red-shift of the cis isomer from

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phenyl ring, and the partial charge increases upon the ionization. Such increase of the charge distribution might induce mixing of the non-classical character which contributes to the intramolecular hydrogen bond. It should be worthwhile to note that knowledge of the hydrogen bond has been accumulated for over 50 years, however the topic of the hydrogen bond is still a rapidly growing field. New concepts of hydrogen bonds, such as X–H----p and X–H----H–Y type hydrogen bonds, have been established in the last decade [19,28]. Much of our present knowledge on the hydrogen bond is concerned with neutral molecules, while studies in cations are still scarce because of experimental and theoretical difficulties. Development of the ADIR spectroscopic technique enables us to find a new type of intramolecular hydrogen bond for the first time, because both the rotational isomer separation and complete elimination of solvation effects are required for the observation and characterization of this unconventional hydrogen bond between the hydroxyl and methyl groups.

3.3. CH stretching vibrations in the aromatic cations

Fig. 7. Infrared spectra of isomer-separated cresol cations in the OH stretching vibrational region. The ADIR spectroscopic technique is utilized to measure the spectra of the cold and isolated cations.

that of the trans is 25 cm 21 in the o-cresol cation, and this shift is as large as the shift due to the intramolecular hydrogen bond in neutral o-fluorophenol, suggesting the same order of hydrogen bond strength. At present, it is difficult to give an unequivocal explanation for the appearance of the hydrogen bond upon ionization, however, it is worthwhile to note that the carbon cation can be a five coordinate site. The presence of these types of nonclassical cations, such as CH 51 , has been confirmed [27]. The methyl group in cresol is positively charged because of its electron donating ability to the

CH stretching vibrations generally have much weaker IR intensities than OH stretching vibrations. Moreover, several theoretical calculations have predicted decrease of the IR intensity of aromatic CH stretches upon ionization [29]. In fact, there has been no experimental information on CH stretches in aromatic cations, except somewhat equivocal assignments in photoelectron spectra [30]. However, the sensitivity of ADIR spectroscopy is high enough to observe CH stretching vibrations. We succeeded in observing both aromatic and alkyl CH stretches of the jet-cooled p-ethylphenol ( p-EP) cation. This is the first unequivocal data on the CH stretches in aromatic cations. p-EP has been studied by electronic spectroscopy, and it has been confirmed that only one conformer, in which the ethyl group is oriented in a plane perpendicular to the plane of the phenyl ring, exists in the jet expansion [31–33]. Fig. 8a shows the IR spectrum of jet-cooled p-EP in S 0 obtained with IR-UV double resonance spectroscopy. Many absorption bands occur in the 2850 to 3100 cm 21 region. Characteristic vibrational frequencies

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shown in Fig. 8b. Based on the similarity with the neutral IR spectrum, the bands in 2830–3010 cm 21 are easily assigned to the alkyl CH stretches, and those in 3050–3090 cm 21 are attributed to the aromatic CH. An interesting feature of the spectrum of the cation is the distinct high-frequency shifts of the aromatic CH stretches upon ionization. This is rather a surprising result because low frequency shifts of the aromatic CH stretches are expected from the view point of chemical intuition; the extraction of a bonding p electron with ionization might cause a relaxation of the electron density from the C–H bond to the C–C bond, resulting in a reduction of the C–H bond strength. At present, it is hard to rationalize these unexpected high-frequency shifts. High level ab initio quantum mechanical calculations of the open shell system are required to explain these shifts. It is also worthwhile to note that our measurements on benzene and other alkylbenzene cations also show similar features, and it seems to be a general tendency for aromatic cations [34]. In this respect, the simulation of the high frequency shifts of CH stretches provides us with a good measure for the feasibility of ab initio quantum chemical calculations of open shell systems, such as aromatic cations.

4. Concluding remarks Fig. 8. The CH stretch region of the IR spectra of jet-cooled p-ethylphenol in the (a) neutral (S 0 ) and (b) cationic (D 0 ) ground states. The S 0 spectrum was obtained by using the IR-UV double resonance technique, while the D 0 spectrum was measured by the ADIR spectroscopic technique.

of CH stretches are well-known. The absorption bands above 3000 cm 21 are uniquely assigned to CH stretches of the aromatic ring, and those below 3000 cm 21 are due to alkyl CH stretches. Because of the low symmetry of the molecule, all of the CH stretches are IR active, and totally nine (four for the aromatic and five for the alkyl) CH stretch bands are expected to be observed. The observed spectrum shows (at least) five aromatic and six alkyl CH stretch bands, indicating the presence of the Fermi resonance with combination bands. The CH stretch region of the IR spectrum of the p-EP cation measured by ADIR spectroscopy is

We reviewed applications of the new technique of IR spectroscopy for jet-cooled molecular ions (ADIR spectroscopy), in which the IR 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 (¯10 212 mol / l) or lower. ADIR (and its related technique, IR / PIRI) spectroscopy is the unique IR spectroscopic technique applicable to cations of such low concentration. It has already been demonstrated that not only OH stretches but also aromatic and alkyl CH stretches of cations can be measured by ADIR spectroscopy. Another important advantage of ADIR spectroscopy is that it

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allows clear identification of the spectral carrier. The multi-step 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.

Acknowledgements We are grateful to E. Fujimaki for his essential contribution to this work. We also thank A. Iwasaki for assistance in the early stage of this work. The authors express their thanks to Prof. T. Ebata, Dr. H. Ishikawa, and Dr. T. Maeyama for helpful discussions.

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