J. Phys. Chem. 1996, 100, 17145-17147
17145
Infrared Depletion Spectroscopy of the Aniline Dimer Ko-ichi Sugawara,* Jun Miyawaki, Taisuke Nakanaga, and Harutoshi Takeo National Institute for AdVanced Interdisciplinary Research and National Institute of Materials and Chemical Research, 1-1-4 Higashi, Tsukuba 305, Japan
Gerhard Lembach, Schahla Djafari, Hans-Dieter Barth, and Bernd Brutschy J.W. Goethe UniVersita¨ t Frankfurt, Institut fu¨ r Physikalische und Theoretische Chemie, Marie-Curie-Strasse 11, 60439 Frankfurt am Main, Germany ReceiVed: June 27, 1996; In Final Form: August 7, 1996X
The infrared depletion spectrum of the aniline dimer, formed in a supersonic jet, has been recorded in the N-H stretch region by combining infrared laser excitation and resonant two-photon ionization/time-of-flight mass spectrometry. Only two bands have been found at 3394.0 and 3465.9 cm-1 ((0.5 cm-1) in the region 3130-3530 cm-1. These are red-shifted by 27.8 and 42.3 cm-1 from the symmetric and asymmetric NH stretching vibrations of the aniline monomer, respectively. A configuration with mutual NH2‚‚‚π bonds and phenyl groups stacked in parallel is suggested for the ground-state aniline dimer.
Introduction Infrared depletion spectroscopy (IRDS) combined with resonantly enhanced multiphoton ionization and time-of-flight mass spectrometry (REMPI-TOFMS) has been confirmed to be a powerful method for investigating the structures of clusters in their ground state.1-8 Neutral clusters produced in a supersonic jet are sensitively and selectively detected with TOFMS after selective ionization with a pulsed UV laser. Before exposure to the UV laser radiation, a tunable pulsed IR laser beam is directed into the jet in order to deplete the population of clusters in their vibrational ground state, which causes a decrease in the cluster ion intensities. For comparison, the sensitivity of conventional infrared absorption spectroscopy is not so high to observe the infrared spectra of clusters in a supersonic jet. Even if it is possible to measure these spectra, it will be difficult to distinguish the absorption of one species from the others. The present IRDS is a suitable method to obtain the infrared spectra of clusters and to study their structures and molecular interactions due to its high sensitivity and selectivity. Felker and co-workers combined stimulated Raman spectroscopy with REMPI-TOFMS to record vibrational spectra of weakly bound clusters and discussed their intermolecular interactions and mode-selective dynamics.9 Hydrogen-bonded clusters involving phenol and substituted phenols were considered as prototypes for the hydrogen-bonding interactions of larger species. They found very large vibrational frequency shifts for the OH stretching modes of phenol-water clusters, and the shift was closely related to the gas-phase acidity of the phenols.10 Similar measurements for phenol-(H2O)1-3 were done using the IRDS-REMPI method by Tanabe et al.3 Recently, we observed the infrared spectra of NH2 stretch modes of aniline and aniline-Arn (n ) 1, 2) clusters using IRDS.8 The vibrational frequencies of the symmetric and asymmetric stretching modes of the three species were determined, and their structures were discussed. Hydrogen-bonding interactions of basic molecules with the NH group are also of great interest in view of comparing them with those of acidic molecules such as phenol. In the case of the phenol dimer, the X
Abstract published in AdVance ACS Abstracts, October 1, 1996.
S0022-3654(96)01901-6 CCC: $12.00
frequency shift of the hydrogen-bonded OH stretching band was 126.4 cm-1 to the red and that of the hydrogen-accepting OH stretching band was only 2.0 cm-1.10 Therefore, it may be expected for the aniline dimer that only one vibrational mode among the four NH stretching modes significantly shifts while the others remain basically at monomer positions if the aniline dimer has a structure similar to that of the phenol dimer. Very recently Yeh et al. reported the REMPI spectra of the jet-cooled aniline dimer and its deuterated species.11 They also calculated the conformational stability of the ground-state dimer using molecular dynamics and molecular mechanics calculations. The molecular dynamics simulation showed that the most stable configuration was a head-to-head structure with the hydrogen bond of the N-H‚‚‚N and the phenyl groups stacked roughly parallel. On the other hand, the lowest energy configuration calculated by the molecular mechanics method was a head-totail structure with mutual hydrogen bonds of the NH2‚‚‚π type and the phenyl groups stacked parallel. One would expect band origins for both the proton donor and the acceptor moiety of the head-to-head dimer configuration as reported for the phenol dimer. For the latter, the origin of the donor is shifted by 305 cm-1 to the red relative to the origin of the monomer and that of the acceptor is shifted to the blue.10,12-14 Even though they could find only one origin which might be suitable for the headto-tail configuration, the observed REMPI spectrum of the dimer was assigned to the head-to-head configuration because of, first, the large red-shift of the band origin, and second, the high frequency of the hydrogen-bond stretching vibration. It is still very ambiguous as to which aniline dimer is formed in a supersonic jet, the head-to-head or head-to-tail. In the present study, the infrared spectrum of the ground-state dimer in the N-H stretch region has been recorded using IRDS combined with REMPI-TOFMS to obtain direct information on the hydrogen-bonding interaction of the dimer. Experimental Section The experimental apparatus and procedures have been previously reported.8 Aniline clusters were formed in the supersonic expansion of a mixture of aniline vapor and helium at room temperature with a stagnation pressure of 1.5 bar. The mixture was injected into a vacuum chamber through a pulsed valve © 1996 American Chemical Society
17146 J. Phys. Chem., Vol. 100, No. 43, 1996
Sugawara et al.
Figure 1. REMPI spectrum of the aniline dimer in the region 33 100-33 800 cm-1. The sharp strong band with the lowest frequency at 33 357 cm-1 is assigned to the 000 band of the aniline dimer.
(General Valve, 0.8 mm diameter), which was operated at 10 Hz with a pulse length of 250 µs, and then introduced into the ionization region of a TOFMS through a skimmer with a 1 mm diameter. Aniline clusters in their ground state, formed in the supersonic jet, were detected by TOFMS after resonant two-photon ionization (R2PI) via the S1 state. The intermediate S1 state was excited with a UV laser at 299 nm, generated by the frequency doubling of the output of a dye laser (Continuum, ND6000). The pulse and bandwidths of the laser were 5 ns and 0.1 cm-1, respectively. The energy of the UV laser beam was reduced to a few hundreds microjoules per pulse in order to minimize the fragmentation of the aniline clusters by UV multiphoton absorption. The infrared absorption bands of the aniline clusters were recorded as the attenuated ion signal induced by the depletion of the clusters in the ground state. The infrared light source was generated by difference frequency mixing of a dye laser with the fundamental of a Nd:YAG laser (Quanta Ray, Wex1). The IR output was about 1 mJ/pulse at 3 µm, and the pulse duration was 6 ns. The bandwidth of the IR laser was less than 1 cm-1, which is mainly determined by the bandwidth of the Nd:YAG laser. The infrared frequency was calibrated by monitoring the wavelength of the dye laser using a wavemeter (Burley, WA4500). The cluster beam was first irradiated with the IR laser and then probed with the UV laser pulse with a delay of 50 ns. The IR and UV beams were focused at the center of the ionization region of the TOFMS using concave mirrors with a focal length of 70 cm. The frequency of the UV laser was fixed on a resonance line of the aniline dimer, while the IR laser was scanned around the N-H stretching region 3130-3530 cm-1. Results and Discussion Infrared Depletion Spectrum. Figure l shows the REMPI spectrum of the aniline dimer in the region 33 100-33 800 cm-1. This spectrum is in good agreement with that very recently observed by Yeh et al.11 They assigned the sharp strong peak at 33 357 cm-1 to the 000 transition and the other bands at higher energy to the vibrational transitions from the ground state to the van der Waals vibrational modes of the S1 state of the aniline dimer. The 000 transition of the dimer is red-shifted from that of the monomer by 672 cm-1. Beside the sharp peaks one can see a broad background signal in Figure 1. The TOF mass spectrum of the cluster beam was composed of (aniline)n+, n ) 1-4, with intensities of 0.4:1.0:0.08:0.01 for n ) 1-4,
Figure 2. Infrared depletion spectra of (a) the aniline dimer and (b) the aniline monomer in the NH2 stretch region. The UV laser frequency was fixed to the respective 000 bands, and the resulting ion intensity was measured as a function of the IR laser frequency.
where the UV laser was fixed to the 000 transition of the dimer. When the cluster beam was cooled by using argon instead of helium, the broad background became much stronger than the sharp peaks and the relative intensity of the dimer mass signal was also decreased. These results indicate that the broad background band is attributed to the larger clusters, and their contribution to the dimer signal is small at 33 357 cm-1 as shown in Figure 1. In Figure 2a, the observed IR depletion spectrum of the aniline dimer is shown in the region 3130-3530 cm-1, where the UV laser is fixed to the 000 transition. Here, two strong bands are observed at 3394.0 and 3465.9 cm-1 ((0.5 cm-1). The fluctuation of the baseline (≈5%) was mainly due to the power instability of the UV laser. For the sake of comparison, the IR depletion spectrum of the aniline monomer measured under the same conditions with the UV laser exciting the 000 transition of the monomer is shown in Figure 2b. One observes the symmetric and asymmetric stretching modes of the NH2 group at 3421.8 and 3508.2 cm-1, respectively.8 The IR laser easily causes saturation of the depletion of the monomer signal at 50%,
Infrared Depletion Spectroscopy of Aniline Dimer while the depletion of the dimer signal is almost 80%. This may be attributed to the vibrational predissociation of the dimer, suggesting that the binding energy of the aniline dimer is smaller than one IR photon energy. The widths of both peaks in the dimer spectrum are about 4 cm-1 (fwhm), which are comparable to those of the monomer. Structure of Aniline Dimer. Yeh et al. calculated the structure of the aniline dimer by using two different computational methods in order to discuss their experimental results of REMPI spectroscopy.11 It is interesting that the two calculations gave different results, as previously mentioned. One conformer is a head-to-head structure, where the phenyl rings are stacked roughly parallel and the NH2 groups of the two anilines form a normal hydrogen bond with each other. The other is a headto-tail structure with the phenyl rings stacked in parallel but displaced in such a manner that the amino hydrogens on each aniline point toward the phenyl ring of the other. They adopted the former, based on the large red-shift of the electronic band origin and the high frequency of the intermolecular hydrogenbond stretching. Now, given the results of our IR spectrum together with the result of their calculations, we will discuss the structure of the aniline dimer. Generally, in a hydrogen bond of type X-H‚‚‚Y-H, the Y-H stretching vibration of the acceptor is only slightly affected while that of the donor is very sensitive to hydrogen-bond formation. For example, the OH stretching of the phenol dimer is redshifted from that of the monomer by 2 and 126 cm-1 for the acceptor and donor, respectively.10 Very recently, Iwasaki et al. obtained the IR depletion spectra of phenol-amine complexes. The OH donor stretch of the phenol-ammonia cluster is red-shifted by 364 cm-1, while that of the NH acceptor stretch is shifted by only 4 cm-1.15 If the aniline dimer forms a hydrogen bond between the NH2 groups as suggested by Yeh et al.,11 one should observe NH donor and NH acceptor bands. Thus, the head-to-head conformer may be ruled out. Our observation is consistent with the other conformation suggested by the calculations in the study of Yeh et al. In this structure, both NH2 groups are equivalent which reasonably explains the observed IR spectrum in which only two peaks are observed. If the environment of both NH2 groups in the dimer is appreciably different, a splitting of the peak should be observed. However, the widths of both peaks in the dimer spectrum are comparable to those of the monomer and no significant broadening is observed. This means that the splitting of the peaks is smaller than a few inverse centimeters, if it exists. The present observation gives evidence that both of the NH2 groups in the aniline dimer must be nearly equivalent. Thus, we suggest mutual H-π bonds for the structure of the aniline dimer and assign the peak at 3394.0 cm-1 to the symmetric NH2 stretching and that at 3465.9 cm-1 to the asymmetric
J. Phys. Chem., Vol. 100, No. 43, 1996 17147 stretching. Both are red-shifted from the corresponding bands of the monomer by 27.8 and 42.3 cm-1, respectively. A similar type of H-π interaction has been reported for the benzene-water16 and benzene-ammonia17 complexes based on microwave spectroscopy. The moderate shift in the NH stretching frequencies of the aniline dimer relative to those of the monomer supports this structure assignment, considering that a H-π bond is weaker than a typical hydrogen bond. Conclusions The infrared depletion spectroscopy, by combining infrared laser excitation with resonant two-photon ionization/time-offlight mass spectrometry, was applied to investigate the NH stretching vibrations of the aniline dimer produced in a supersonic jet. The main goal was to clarify the type of hydrogen bonds in the dimer containing NH groups. Two strong peaks were observed at 3394.0 and 3465.9 cm-1, which were moderately red-shifted relative to those of the monomer. The present result strongly indicates that both of the NH2 groups in the aniline dimer are almost equivalent. We therefore suggest a sandwich-type structure with mutual H-π bonds and assign the former peak to the symmetric NH2 stretching and the latter peak to the asymmetric stretching. References and Notes (1) Riehn, Ch.; Lahmann, Ch.; Wassermann, B.; Brutschy, B. Ber. Bunsen-Ges. Phys. Chem. 1992, 96, 1161. (2) Brutschy, B. Chem. ReV. 1992, 92, 1567. (3) Tanabe, S.; Ebata, T.; Fujii, M.; Mikami, N. Chem. Phys. Lett. 1993, 215, 347. (4) Pribble, R. N.; Zwier, T. S. Science 1994, 265, 75. (5) Pribble, R. N.; Zwier, T. S. Faraday Discuss. 1994, 97, 229. (6) Pribble, R. N.; Garrett, A. W.; Haber, K.; Zwier, T. S. J. Chem. Phys. 1995, 103, 531. (7) Djafari, S.; Lembach, G.; Barth, H.-D.; Brutschy, B. Z. Phys. Chem., in press. (8) Nakanaga, T.; Ito, F.; Miyawaki, J.; Sugawara K.; Takeo, H. Chem. Phys. Lett., in press. (9) Henson, B. F.; Hartland, G. V.; Venturo, V. A.; Felker, P. M. J. Chem. Phys. 1989, 91, 2751. (10) Hartland, G. V.; Henson, B. F.; Venturo, V. A.; Felker, P. M. J. Chem. Phys. 1992, 96, 1164. (11) Yeh, J.-H.; Shen, T.-L.; Nocera, D. G.; Leroi, G. E.; Suzuka, I.; Ozawa H.; Namuta, Y. J. Phys. Chem. 1996, 100, 4385. (12) Fuke, K.; Kaya, K. Chem. Phys. Lett. 1982, 91, 311. (13) Fuke, K.; Kaya, K. Chem. Phys. Lett. 1983, 94, 97. (14) Dopfer, O.; Lembach, G.; Wright, T. G.; Mu¨ller-Dethlefs, K. J. Chem. Phys. 1993, 98, 1933. (15) Iwasaki, A.; Fujii, A.; Sato, S.; Ebata, T.; Mikami, N. J. Phys. Chem., in press. (16) Suzuki, S.; Green, P. G.; Bumgarner, R. E.; Dasgupta, S.; Goddard, W. A., III; Blake, G. A. Science 1992, 257, 942. (17) Rodham, D. A.; Suzuki, S.; Suenram, R. D.; Lovas, F. J.; Dasgupta, S.; Goddard, W. A., III; Blake, G. A. Nature 1993, 362, 735.
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