J. Phys. Chem. 1996, 100, 4385-4389
4385
Resonance Two-Photon Ionization Spectroscopy of the Aniline Dimer J.-H. Yeh, T.-L. Shen, D. G. Nocera, and G. E. Leroi* Department of Chemistry and the LASER Laboratory, Michigan State UniVersity, East Lansing, Michigan 48824
I. Suzuka, H. Ozawa, and Y. Namuta Department of Industrial Chemistry, College of Engineering, Nihon UniVersity, Koriyama 963, Japan ReceiVed: August 21, 1995; In Final Form: December 6, 1995X
The resonance two-photon ionization (R2PI) spectra of the jet-cooled aniline dimers (An-d0)2, (An-d5)2, and (An-d7)2 have been recorded; the first 225 cm-1 region above the (An-d0)2 origin was also reproduced by fluorescence excitation spectroscopy. Relatively sharp structure near the origin of the local excited state is attributed to intermolecular modes, perhaps of multiple conformers. Broader features at higher energies are assigned to symmetric ring modes coupled to an underlying excimer state. The dispersed fluorescence spectrum excited at the dimer electronic origin suggests that excimer formation occurs soon after the local excited state is pumped. Molecular dynamics simulations support the inference from the spectroscopic observations that hydrogen bonding governs the structure of the aniline dimer.
Introduction It has been well recognized that molecular clusters may serve as a means to bridge the gap between the gaseous and condensed phases, enabling interesting aspects of the condensed phase, such as the mechanisms for condensation and solution dynamics, to be studied at the molecular level. Understanding the bonding and the conformation of clusters is a fundamental first step in this process. Van der Waals forces govern the formation of many molecular clusters and adducts with rare gases or other solvents. Specific electrostatic interactions, such as multipole forces and hydrogen bonding, may control the structure of clusters in other cases. Dimers can be studied to probe the intermolecular forces acting within larger clusters. Owing to its relatively stronger attractive forces, H-bonding may determine the primary orientation between monomers within a dimer where H-bonds can be formed, being augmented by van der Waals forces to provide the minimum energy geometry. Aniline (phenylamine, C6H5NH2) is an example of such a system. The molecule is of interest as a participant in both electron and proton transfer processes; therefore, we were interested in studying the interactions that lead to the formation of the homodimer, the simplest cluster. The crystal structure of aniline has been reported; it reveals that both H-bonding and stacking forces are important in the crystalline phase.1 Ito’s analysis of the ultraviolet absorption spectra of phenol and aniline, interacting with hydrogen bond acceptors in hexane solutions, suggested that the aniline hydrogen-bonded complex was more strongly bound than that of phenol.2 Theoretically, a relatively low level calculation favored a head-to-tail conformation for the aniline dimer.3 The phenol dimer has been carefully studied by one- and two-color resonance-enhanced multiphoton ionization (REMPI), rotational coherence spectroscopy (RCS), and zero-kinetic-energy photoelectron (ZEKE) spectroscopies.4-6 Its geometry reflects both H-bonding and van der Waals forces. We wish to begin gaining similar insight into the geometry of the aniline dimer. Recently, we have obtained the one-color REMPI spectra of the following monomeric aniline isotopomers: An-d0, An-d1, An-d2, An-d5, An-d6, and An-d7 and identified several lowX
Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-4385$12.00/0
frequency modes in the lowest excited electronic state on the basis of the isotope shift pattern.7 We are interested in the structure of the dimers, the forces that govern their formation, and the roles played by the low-frequency vibrations in the first electronic excited state. We have therefore measured the onecolor resonance-enhanced two-photon ionization (R2PI) spectra of the following mass-selected isotopomers: (An-d0)2, (An-d5)2, and (An-d7)2, cooled by expansion in a supersonic jet. To investigate the dynamics of energy transfer following electronic excitation, dispersed fluorescence from the nondeuterated dimer has also been recorded. Implications regarding the conformation, stabilization, and dynamics of the aniline dimer are presented in this report. Experimental Section The R2PI spectra of supersonically cooled, isotopomeric aniline homodimers have been obtained by employing a QuantaRay DCR-2 YAG laser to pump the dyes R610/R640, R610, R510/R590, and R590. The output from the dye laser was doubled by a Quanta-Ray wavelength extension system (WEX) to cover the spectral region from 303 to 280 nm (33 000-35 700 cm-1). The appropriate wavelength of the laser output is collimated to an estimated 3 mm diameter spot where it crosses a skimmed molecular beam pulsed at 10 Hz. The time-of-flight mass spectrometer (R. M. Jordon Co.), which has a mass resolution of approximately 800 in the M ) 200 Da range, is differentially pumped. The main chamber, which handles the primary gas load, is pumped by a 6 in. diffusion pump which maintains a pressure on the order of 10-6 Torr. The flight tube is pumped by a Balzers TPU 240 turbo pump, which maintains vacuum below 10-7 Torr at the multichannel plate (MCP) detector. The seeded molecular beam is prepared by passing 3 atm of He (99.995% purity) carrier gas through a quartz sample cell placed behind the current loop-type pulsed nozzle (R. M. Jordon Co.). After expansion through a 500 µm orifice, the seeded molecular beam passes through a 750 µm skimmer located ca. 4 cm from the pulsed nozzle and intersects the collimated laser beam about 12 cm downstream. The ions produced in the ionization region are accelerated through the drift tube to the detector, a Galileo 3 stage MCP which is normally biased at 1900 V (∼106 gain). The signal output from © 1996 American Chemical Society
4386 J. Phys. Chem., Vol. 100, No. 11, 1996
Figure 1. R2PI spectra of aniline-d0: (a) monomer and (b) homodimer.
the MCP is monitored by a Tektronix DSA 602A storage oscilloscope with a 11A72 amplifier. The data processing system consists of a model SR250 Stanford Research Systems boxcar integrator, which integrates the signals from the MCP detector and the photodiode that monitors the laser pulses, and a software system based on the National Instruments Labview program. Bad shots are removed prior to data normalization and averaging. To discriminate spectral features arising from hot bands, the (An-d0)2 spectrum was also measured with up to 7 atm of He carrier gas pressure. Fluorescence excitation spectra of (An-d0)2 were obtained under similar pulsed expansion conditions, in a chamber with pressure maintained below 10-5 Torr. Selected excitation radiation from a doubled dye laser (Lambda Physik LPD 3000, rhodamine 6G or rhodamine B dye), pumped by a Lambda Physik Lextra 50 excimer laser, is focused about 4 cm from the nozzle orifice. The fluorescence is collected at 90° by a lens system matched to a Nalumi 75 cm monochromator (1200 gr/mm grating blazed at 5000 Å and used in second order with 500 µm slits) and detected by a Hamamatsu R562 photomultiplier. A boxcar integrator (SR250, Stanford Research) processes the signals from the detector and the photomultiplier that monitors the laser pulses and passes them to a recorder. The conformational stability of the aniline dimer was simulated with the Biograf molecular dynamics program (Molecular Simulations, Inc.), using a 10-12 potential for the hydrogen-bonding interaction in a Dreiding force field. Several different initial conformations were employed, and rigid monomer units were assumed in each dimer conformational optimization. The ground state dimer configuration was also calculated with CAChe molecular mechanics software (CAChe Scientific, Inc.). All aniline isotopomers were purchased from Aldrich. And0 and -d5 were purified by distillation before use; An-d7 was used as received. Some sample contamination occurred during the course of our experiments. However, owing to the mass selection provided by the gated signals, the effects are minimal. Results and Assignments The R2PI spectrum of the aniline-d0 dimer is shown in Figure 1, together with that of the monomer. Essentially identical relative intensities are measured in the fluorescence excitation experiments. This gives us confidence that the spectrum in Figure 1b is not affected by contributions from fragmentation owing to the excess energy deposited in the R2PI process.
Yeh et al.
Figure 2. R2PI spectra within 225 cm-1 of the respective origins for (a) (An-d0)2, (b) (An-d5)2, and (c) (An-d7)2.
Aniline trimer formation is negligible under our experimental conditions. When the trimer concentration is intentionally enhanced, no sharp features are observed when the R2PI spectrum is scanned in the dimer resonance range. The dimer electronic transition origin is assigned to the first sharp peak with strong intensity, at 33 352 cm-1. We expect that the strong S1 r S0 transition origin of the aniline monomers7,8 will also be strong in the local excited state of the dimer. Following some well-resolved structure near the origin, the dimer spectrum is marked by broad bands displaced by ca. 510, 980, 1460, and 1930 cm-1. In general, the intermolecular interaction is a sufficiently weak perturbation that the intrinsic vibronic transitions of a monomer should be preserved, albeit slightly shifted. Thus the four broad bands are probably due to the 6a01, 1201, and/or 6a02, 6a011201, and 6a021201 transitions, which involve symmetric ring modes and are among the highest intensity bands in the An-d0 monomer R2PI spectrum.7 (The 101 vibration in the dimer may be responsible for the weak peak near 824 cm-1 in Figure 1b.) Their frequencies are slightly higher than the ground state values, as would be expected in a hydrogen-bonded environment. Similar peaks are observed for (An-d5)2 and (And7)2. The spectral breadth might arise from coupling with the intermolecular modes, the existence of multiple conformations for the supersonically cooled dimers, or the mixing between the first excited state of the dimer and a lower lying excimer state. The rising background shown in Figure 1b suggests the existence of the excimer state. The R2PI spectra in the well-resolved region, the first 225 cm-1 above the respective origins, are reproduced in Figure 2 for the aniline-d0, -d5, and -d7 dimers; the fluorescence excitation spectrum of (An-d0)2 in this range matches the photoionization results peak for peak, with little difference in the relative intensities. The dimer transition origins are displaced 678, 691, and 668 cm-1 to lower energy with respect to those of the And0, -d5, and -d7 monomers; the stabilization energy in the dimer excited state is thus significantly larger than that in the ground state. In comparison to other clusters and dimers,4,9,10 these are large red shifts, suggesting that hydrogen bonding is important in stabilizing the aniline dimer. An upper bound to the stablization energy resulting from van der Waals forces plus H-bonding to a neighboring aromatic ring is given by the aniline: benzene cluster, for which the red shift upon electronic excitation is 450 cm-1.10 This adduct was reported to have an almost parallel, but displaced, (sandwich) conformation between the two aromatic moieties of the heterodimer. Thus we would
R2PI Spectroscopy of the Aniline Dimer
J. Phys. Chem., Vol. 100, No. 11, 1996 4387
TABLE 1: Peak Positions (Wavenumbers, (2 cm-1) Relative to the Respective Origins and Tentative Assignments of Low-Frequency Modes in the R2PI Spectra of (An-d0)2, (An-d5)2, and (An-d7)2 assignment
(An-d0)2
(An-d5)2
V1 V2 V3 V4 V5 V6 V1 + V2 2V1 + V4 V2 + V4 2V3 V3 + V4 V1 + V5 V2 + V5 V3 + V5 V4 + V5 V6 + V1 2V5 V6 + V2 V6 + V3 V6 + V4 V6 + V5 V6 + V2 + V5 2V6; 10b?
6 15 25 34 58 103 18 44 48 52 60 67 74 85 94 108 114 119 126 137 158 178 210 (or 16a?)
6 16 22 27 59 102
66 73 83
(An-d7)2 6 13 22 30 56 103 18
54 62
108 116 121 125 131 162 211
117 132 161 170 (or 16a?) 203
expect the red shift in the absence of N-H‚‚‚N hydrogen bonding to be in the range 450-40 cm-1, the value for the T-shaped benzene dimer.11-14 Clearly H-bonding between the amino groups helps govern the formation of the aniline dimer. In the local excited state, where only one aniline molecule of the dimer is excited, the overall symmetry is quite low, probably C1. In principle, this can lead to site splitting, where two origins for each aniline dimer conformer could be observed; one corresponds to excitation of the proton donor, the other to a transition in the proton acceptor. We assign the origins marked in Figure 2 to excitation in the proton donor. If transitions corresponding to excitation of the proton acceptor are present, they are lost within the broad structure at higher energies. The wavenumbers of the sharper peaks near the R2PI origins of the isotopomeric aniline dimers are listed in Table 1. On the basis of the R2PI spectra of the monomers,7 the peaks observed in the first 225 cm-1 above the origins are not intrinsic normal modes of the monomers themselves. (Exceptions might be the 10b and 16a fundamentals, symmetry inactive in the monomer, which should lie below 225 cm-1.) We did not observe a long progression built on the dimer electronic origin, which would indicate a large geometry change upon electronic excitation.15 The low-frequency transitions could arise from strong mixing between the local excited state and a lower lying excimer state. Deuteration would greatly perturb this coupling and change the structure of the spectra.16 The similarity between the well-defined spectral features of (An-d0)2 and the isotopomers in the first 225 cm-1 indicates that this coupling is rather weak. Therefore, the most likely assignments for the peaks in Figure 2 are intermolecular vibrations, which only weakly interact with the background excimer state. Intermolecular stretching within the hydrogen-bonded dimer is expected in the 100 cm-1 range;4-6 bending and wagging motions should lie below 60 cm-1.10,17-19 Moreover, it is unlikely that the aniline dimer exists in only a single conformation; the presence of multiple conformers will compound the interpretation of the dimer spectra. Thus the assignments given in Table 1, most complete for (An-d0)2 where the peaks are most distinct, must be considered tentative. We denote the intermolecular vibrations
Figure 3. Dispersed fluorescence from (An-d0)2 excited under 000 excitation at 33352 cm-1.
from lowest frequency to highest as V1-V6. The intermolecular stretch along the hydrogen bond is V6. It is the most prominent feature in the low-frequency region (Figure 2) and is also discernible as combination mode structure on the broader intramolecular bands shown in Figure 1. To study the energy transfer dynamics after the local excited state of the aniline dimer is formed, the dispersed fluorescence spectrum of the undeuterated dimer was obtained under 000 excitation at 33 352 cm-1; it is displayed in Figure 3. The dispersed fluorescence consists primarily of broad emission between 32 200 and 27 000 cm-1, with maximum intensity near 30 000 cm-1. Judging from the ca. 3000 cm-1 red shift of the dispersed fluorescence maximum from the excitation energy, there is apparently another electronic state, likely an excimer state, coupled to the local excited electronic state of the dimer. Following Franck-Condon excitation of the dimer, energy is transferred to this state, whereupon it can relax by radiative decay. Because the fluorescence is observed upon 000 excitation of the local excited state, the barrier for energy transfer to the excimer state must be very low. The breadth of the luminescence envelope suggests that the intermolecular distance between the monomers in the excimer state is shorter than that in the ground state of the dimer. Hence, when the excitation energy relaxes from the excimer state it reaches the repulsive portion of the ground state potential, leading to broad emission and dissociation. The coupling between the local excited state and excimer-like state tends to be weak; thus the vibronic levels are not strongly mixed and sharp peaks are preserved in the first few hundred wavenumbers of the R2PI spectra. The overall excitation, coupling, and relaxation process is sketched in Figure 4; this model has been applied to other molecular clusters.16,20 Discussion The most stable conformation determined by the Biograf optimization for the ground state aniline dimer has a head-tohead orientation, with the phenyl rings stacked roughly parallel, but with slightly displaced geometry. The dihedral angle between the two aromatic rings is about 24°, and the distance between the ring centers is ca. 4 Å. The geometry of the lowest energy conformation is thus consistent with hydrogen bonding between an amino proton on one aniline monomer (the donor) and the nitrogen atom on the acceptor. As noted above, the experimental stabilization energy suggests that H-bonding is important in the formation of the aniline dimer, augmented by
4388 J. Phys. Chem., Vol. 100, No. 11, 1996
Figure 4. Schematic energy level diagram for the aniline dimer excimer formation and relaxation process.
van der Waals attractions. Indeed, rehybridization of the nonbonding electrons on the nitrogen atom upon electronic excitation of the proton donor in the calculated conformation should provide both resonance stabilization of the donor’s π-electron system and stronger intermolecular hydrogen bonding. Moreover, subsequent realignment of the aromatic moieties into the more parallel alignment required for excimer formation is facilitated. A second dimer configuration in which the aromatic rings are stacked parallel, but displaced such that the amino hydrogens on each aniline monomer point toward the π-cloud of the neighboring phenyl group, in a head-to-tail orientation is also relatively stable. The lowest energy configuration calculated by the CAChe program had this geometry, with a ring centerto-center distance of 3.99 Å. For this conformation, the two local origin wave functions should mix and the energy levels split; however, for one state the transition dipoles will cancel, and thus only a single transition origin should be observed.13b The orientation of the monomers is also well suited to excimer formation. This conformation is thus consistent with the spectroscopic observations, also. In a hydrogen-bonded heterodimer, site splitting is reflected in the following way: the absorption spectrum of the proton donor moiety is shifted to lower energy with respect to that of the free molecule, whereas, the corresponding transition for the proton acceptor is blue shifted.21,22 The trend also applies to the head-to-head H-bonded homodimer. A good example is the phenol dimer, where the excitation origin for the proton donor moiety is red shifted by 303 cm-1 and that of the proton acceptor is blue shifted by 353 cm-1 with respect to the monomer.4,6 As mentioned above, absorption measurements in solution suggest that when aniline acts as a proton donor it experienced a larger red shift than that of phenol.2 Therefore the assignment of the peak at 33 352 cm-1 as the S1 r S0 excitation origin for the proton donor in (An-d0)2 seems quite reasonable. Similarly, we might expect a blue shift of the same magnitude for the proton acceptor moiety; however, no salient peak is observed in the 1000-1400 cm-1 range from the donor origin. Perhaps the acceptor origin intensity is too low to be distinguished within the broad structure. The single origin observed for the dimer transition is also consistent with a displaced sandwich configuration involving π-type hydrogen bonding, although the red shift is larger than one would expect. The frequency for the intermolecular H-bond stretching vibration is also much larger than this conformation would suggest, and we therefore attribute the major spectro-
Yeh et al. scopic features observed experimentally to a head-to-head dimer configuration. The appropriate molecular symmetry group for the aniline monomer is G8. Under conditions where the NH2 torsional barrier is sufficiently high that level splitting arising from tunneling is not observed, as is the case in both the ground and first excited electronic states, the C2V molecular point group can be employed.23 However, when the dimer is formed the potential for the inversion motion will no longer be symmetric with respect to the aromatic plane. (In the S1 state, aniline is quasi-planar, with an inversion barrier of about 20 cm-1;7 stabilization due to hydrogen bonding may be as high as 1500 cm-1.6,24) Thus the effective point group for the aniline monomer in the dimer environment should be lowered from C2V to C1, and in principle all normal modes could be vibronically active. In practice, additional work is needed before definitive assignments can be made for the spectra displayed in Figures 1 and 2. These include hole-burning experiments, to identify how many conformers contribute to the spectra; REMPI measurements on other isotopomeric aniline homo- and heterodimers, to locate the local origin for the proton acceptor excitation and to solidify the assignments of the broad structure in the region of the intramolecular vibrations in the excited electronic state; and quantum chemistry calculations on the energetics and configurations of the aniline dimer in the ground state, local excited state, and excimer state, respectively. Conclusions Electronic spectra of jet-cooled aniline dimers have been measured by the R2PI method for An-d0, -d5 and -d7. The possibility of contributions from higher aniline clusters is ruled out by mass selection. Additional experiments are required to determine how many conformers contribute to the excitation spectra and which normal modes of the aniline monomer are important in the dimer spectra. Both experimental and theoretical work is needed to reveal the details of the dynamics and geometry in each step of the aniline dimer excitation, coupling, and relaxation process. The present work indicates that hydrogen-bonding interactions, supplemented by van der Waals forces, govern the geometry of the aniline dimer in both the ground and excited electronic states. Dispersed fluorescence spectra suggest energy transfer from the local excited state of the proton donor to an excimer, which appears to have a shorter intermolecular distance, and hence stronger stabilization energy, than the dimer in the ground state. Acknowledgments. We are grateful to Dr. N. A. van Dantzig for helpful discussions. The work at Michigan State University was supported in part by the National Institutes of Health (GM 47274). The Japanese authors gratefully acknowledge a Nihon University research grant. References and Notes (1) Fukuyo, M.; Hirotsu, K.; Higuchi, T. Acta Crystallogr. 1982, B38, 640. (2) Ito, M. J. Mol. Spectrosc. 1960, 4, 106. (3) Cazeau-Dubroca, C. J. Lumin. 1984, 29, 349. (4) (a) Fuke, K.; Kaya, K. Chem. Phys. Lett. 1982, 91, 311; (b) 1983, 94, 97. (5) Connell, L. L.; Ohline, S. M.; Joireman, P. W.; Corcoran, T. C.; Felker, P. M. J. Chem. Phys. 1992, 96, 2585. (6) Dopfer, O.; Lembach, G.; Wright, T. G.; Muller-Dethlefs, K. J. Chem. Phys. 1993, 98, 1933. (7) Yeh, J-H.; Shen, T-L.; Crimmins, T. F.; Nocera, D. G.; Leroi, G. E. J. Chem. Phys., submitted for publication. (8) (a) Brand, J. C. D.; Williams, D. R.; Cook, T. J. J. Mol. Spectrosc. 1966, 20, 359. (b) Quack, M.; Stockburger, M. J. Mol. Spectrosc. 1972,
R2PI Spectroscopy of the Aniline Dimer 43, 87. (c) Mikami, N.; Hiraya, A.; Fujiwara, I.; Ito, M. Chem. Phys. Lett. 1980, 74, 286. (d) Smith, M. A.; Hager, J. W.; Wallace, S. C. J. Chem. Phys. 1984, 80, 3097. (e) Song, X.; Yang, M.; Davidson, E. R.; Reilly, J. P. J. Chem. Phys. 1993, 99, 3224. (9) Bernstein, E. R. In Atomic and Molecular Clusters; Bernstein, E. R., Ed.; Elsevier: New York, 1990. (10) Lahmani, F.; Lardeux-Dedonder, C.; Solgadi, D.; Zehnacker, A. Chem. Phys. 1988, 120, 215. (11) Hopkins, J. B.; Powers, D. E.; Smalley, R. E. J. Phys. Chem. 1981, 85, 3739. (12) Law, K. S.; Schauer, M.; Bernstein, E. R. J. Chem. Phys. 1984, 81, 4871. (13) (a) Kung, K. H.; Selzle, H. L.; Schlag, E. W. J. Phys. Chem. 1983, 87, 5113. (b) Bornsen, K. O.; Selzle, H. L.; Schlag, E. W. J. Chem. Phys. 1986, 85, 1726. (14) Henson, B. F.; Hartland, G. V.; Venturo, V. A.; Felker, P. M. J. Chem. Phys. 1992, 97, 2189. (15) Takayanagi, M.; Hanazaki, I. Chem. Phys. Lett. 1993, 208, 5.
J. Phys. Chem., Vol. 100, No. 11, 1996 4389 (16) Saigusa, H.; Lim, E. C. J. Phys. Chem. 1991, 95, 1194. (17) Abe, H.; Mikami, N.; Ito, M. J. Phys. Chem. 1982, 86, 1768. (18) Bieske, E. J.; Rainbird, M. W.; Knight, A. E. W. J. Chem. Phys. 1991, 94, 7019. (19) Intermolecular modes of stacked dimers where no hydrogen bond to a lone pair is possible are expected to lie below 60 cm-1. For example, the highest intermolecular frequency in the aniline:benzene adduct, the stretch between the two aromatic rings, lies at 56 cm-1 (ref 10). (20) Saigusa, H.; Itoh, M. J. Phys. Chem. 1985, 89, 5436. (21) Pimentel, G. C. J. Am. Chem. Soc. 1957, 79, 3323. (22) Mataga, N.; Kubota, T. Molecular Interactions and Electronic Spectra; Marcel Dekker: New York, 1970. (23) Christoffersen, J.; Hollas, J. M.; Kirby, G. H. Mol. Phys. 1969, 16, 441. (24) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; Freeman: San Francisco, 1960.
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