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The van der Waals vibrational frequencies of the aniline-carbon monoxide complex in its S1 state
Chemical Physics ELSEVIER
Chemical Physics 215 (1997) 291 - 298
The van der Waals vibrational frequencies of the aniline-carbon monoxide complex in its state Johann-Georg J ickel a, Reiner Schmid a, Harold Jones a,*, T. Nakanaga c, Harutoshi Takeo b a Abteilung Chemische Physik, Universit~t Ulm, Albert Einstein-Allee 11, 89069 Ulm, Germany b National Institute for Advanced Interdisciplinary Research, 1-1-4 Higashi, Tsukuba, Ibaraki 305, Japan ¢ National Institute of Materials and Chemical Research, 1-1-4 Higashi, Tsukuba, lbaraki 305, Japan
Received 31 July 1996; in final form 13 November 1996
Abstract A portion of the S1-S o band of the aniline-CO van der Waals (vdW) complex has been observed near 297 nm, using REMPI/TOF spectroscopy. Compared to the same band in aniline itself, this represents a shift to longer wavelengths of over 300 c m - J, which is one of the largest red shifts so far observed for this type of vdW complex. The spectrum observed consisted of five almost equally intense, relatively sharp sub-bands and we have assigned these as the 0 ° band plus four signals arising from transitions to vibrational levels in the S 1 state which are associated with the van der Waals bond. In order to account for these four sub-bands, it appears that transitions to a minimum of three different vdW vibrational modes are required. Two signals appear to involve the first and second level of the same vibration, the other two seem to arise from at least a further two different vibrational levels. The large values of both the red shift of the S1-S o band and the frequencies of the vdW modes indicate that aniline-CO is a relatively strongly bonded complex. It was concluded that the CO molecule probably lies in the symmetry plane above the aromatic ring of the aniline molecule. 1. Introduction Van der W a a l s ( v d W ) complexes of aniline with various atoms or molecules produced in a supersonic jet expansion have been subjected to spectroscopic investigation for well over a decade. The techniques so far successfully employed include laser induced fluorescence (LIF), resonance enhanced multiphoton ionisation coupled with time-of-flight mass spectrometry ( R E M P I / T O F ) , and more recently, thresh-
* Corresponding author.
old photoelectron spectroscopy. The studies have been almost invariably carried out on the S t - S 0 band ( A 1B2-X IAi) o f aniline near 294 nm, which is a c o n v e n i e n t r e g i o n for i n v e s t i g a t i o n with frequency-doubled dye lasers. The most extensive work has been carried out on a n i l i n e - r a r e gas complexes and the complex with argon has been particularly well characterised [1]. It was assumed that the complex had C S symmetry with the rare gas atom located above the aromatic ring in the plane of symmetry. With such weakly-bound complexes, the spectra observed usually resemble that of aniline itself, simply shifted slightly to longer wavelengths. In the case
0301-0104/97/$17.00 Copyright © 1997 Published by Elsevier Science B.V. All rights reserved. PII S0301-0104(96)00359-X
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of the rare gases, the red shift of the complex lies between 5.3 c m - t for Ne and 82 cm-~ for Kr [1]. Using laser induced fluorescence spectroscopy, Yamanouchi et al. [1] were able to resolve the rotational structure of the 0 ° vibronic band of both the aniline-Ar and aniline-Ne complexes. This work was later extended to the complexes with N 2, H 2 and CH 4 [2]. Under the assumptions that the complex had C s symmetry and that the complexing particle could be considered as a point mass, several structural parameters were estimated from band-contour analysis. A prominent feature of the spectra of the complexes which is not observed with aniline alone, is the appearance of a number of lines which arise from the low-frequency vibrational modes of the weak vdW bond. In the case of argon, the lines have been assigned to specific vibrational modes. Bieske et al. [3] were able to convincingly demonstrate that four lines observed in a number of complexes of argon with substituted benzenes arise from the stretching and bending motions of the vdW bond. The four lines, which lay within 50 c m - l of the 0 ° origin band of the complex, were assigned as being associated with the following vibrational levels in the S t state: the first excited level of a stretching motion within the plane of symmetry of the molecule (Sz~), the first and second excited levels of an in-plane bending motion (bz~ and bx 2) and the second excited level of an out-of-plane bend (byg), where the complex is assumed to have C s symmetry with the plane of symmetry in the z - x plane. Since only transitions to totally symmetric levels are allowed, the transition associated with the first excited level of the out-of-plane bend, bye, was not observed [3]. Similar assignments have been made in the cases of argon complexes with other compounds e.g., Ref. [4]. An alternative assignment for the vdW structure of aniline-Ar has been given by Hermine et al. [5] who, as a result of calculations and using intensity 1 arguments, exchanged the signals assigned as sz0 and by ~. However, the theoretical results were somewhat inconclusive [5] and the earlier assignment [3] was considered by Hermine et al. [5] to be also possible. After considering all the data available, we still favor the original assignment of Bieske et al. [3] since this seems to be more consistent with the shifts
in the origin of the S I - S 0 bands and the spectral patterns observed for the argon complexes of other aromatic compounds studied in the same work [3]. In the case of the aniline complexes with molecules such as N 2 and methane, signals were observed in a region 50 c m - i to 100 c m - I below the 0 ° band of aniline itself [2,6,7] which almost certainly arise from vdW motions. However, no easily identifiable spectral pattern appeared to be present. We have recently carried out ultraviolet-infrared double resonance measurements [8] on the complexes of aniline with the molecules N 2, CH 4, CHF 3 and CO and were able to show that in the ground electronic state of the neutral complex and in its cation, the frequencies of the two vibrational stretching modes of the amino group of aniline were only slightly affected by the formation of a complex. This has led us to the conclusion that bonding in these complexes takes place predominantly with the -rr-system of the aromatic ring, as is the case with the argon complex, rather than with the amino group. In this paper we report R E M P I / T O F measurements carried out on the aniline-carbon monoxide complex. These appear to be the first measurements on this complex and in this case we have observed a pattern of lines which we interpret as being qualitatively very similar to that observed with the anilineAr complex. If it is assumed that the argon atom, which is situated above the aromatic ring, is simply replaced with a diatomic molecule, the vdW vibrational structure would be expected to remain basically the same, except that two extra modes are required to describe the torsional motion of the diatomic molecule: one motion about an axis parallel to the plane of the benzene ring (ty) and another about the axis perpendicular to this plane (tz). Transitions to excited levels of these torsional motions may give rise to additional signals over and above those observed for the argon complex. The situation is more complicated if the torsional barrier for the mode t z is very low. In the extreme lower limit of a zero barrier, the motion results in a free internal rotation about the z-axis. Such a situation has been considered in detail by Nowak et al. [9] in their work on complexes of N 2, CO 2, and CO with benzene. The complicated spectra they observed were interpreted as being a mixture of transitions
J.-G. Ji~ckel et al./ Chemical Physics 215 (1997) 291-298
arising from free rotation and some of the vdW vibrational modes mentioned above. The analysis of the spectra relied heavily on the results of theoretical calculations which were used to determine a number of basic properties of the complex. There are however, a few examples of complexes formed between carbon monoxide and various aromatic compounds for which exact structural information has been determined. For example, the microwave work of Bander and coworkers on the complexes of CO with benzene [10], pyridine [11] and pyrrole [12]. In the case of the CO-benzene complex, the parameters determined from the experimental data [10] differ considerably from those obtained by Nowak et al. [9] from their theoretical calculations. To what extent these discrepancies lead to a reduction in the confidence of the correctness of the assignments made for the UV spectrum of the benzene-CO complex [9] will not be discussed here. For our purposes, it suffices to point out, that this complex is certainly relatively weakly bound [10] and, no
293
matter how the UV spectrum is assigned in detail, the experimental observations [9] show that only a relatively small shift in the origin of the UV band was produced on forming the complex. 2. E x p e r i m e n t a l
The R E M P I / T O F apparatus consisted of a single vacuum chamber (ca. 50 liter volume) with the 50 cm long drift tube of a linear TOF mass spectrometer aligned with the axis of the free expansion jet. The chamber and the TOF were connected over an opening 5 mm in diameter. Pressures in the region of 10 -3 mbar were maintained in the main chamber using a 700 l / s diffusion pump, whereas the mass spectrometer was kept at 10 -6 to 10 -7 mbar with the aid of a 300 l / s turbo-molecular pump. The aniline-CO complex was produced by passing carbon monoxide gas over liquid aniline at room temperature. The gas mixture was admitted at a stagnation pressure of 1 bar into the vacuum cham-
Aniline/CO 1000
by~
complex
bx
sxol 800
Origin 600
,400
200
vdW S t r u c t u r e I
-100
-200 2c-.~. OOO
2S6. 000
297.000
rim
=~
Fig. 1. The REMPI spectrum of the S~-S 0 band of the aniline-CO van der Waals complex in the region 294 to 298 nm. As can be seen, five sub-bands of similar intensity were observed. That at the longest wavelength was assigned as the 0o° origin band of the complex. The four other spectral features appear to arise from at least three different vibrational modes of the vdW bond. Tentative individual assignments are as shown in the figure (see text). The spectrum appears to display no indication of the effects of large amplitude motions in the complex. The positions of the spectral features scale relative to the 0o° origin band of aniline are indicated by the approximate wavenumber scale in the figure.
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ber through a pulsed nozzle (General Valve) with a 0.5 mm orifice. Tunable ultraviolet radiation in the 295 nm region was produced by pumping a Lumonics HD-300 dye laser with the second harmonic of a Lumonics HY1200 Nd:YAG laser and then doubling the output in a KDP crystal (Lumonics HT1000). Pulse energies up to 10 mJ with pulse lengths of the order of 5 ns and a nominal line width of approximately 0.12 c m - l were produced. The spectrum shown in Fig. 1 was recorded with a pulse energy of less than 1 mJ with the laser beam focused into the vacuum chamber. Frequency calibration of all the spectra reported in this paper was carried out using a Burleigh WA 4500 wavemeter. The ion signal was displayed on a digital oscilloscope (Tektronix TDS 320) and mass selected spectra were obtained by digitally recording the output of a box-car integrator (Stanford Research).
3. Observations and discussion The spectrum of the aniline-CO complex observed in the region 295 to 297 nm is shown in Fig. 1. As can be seen, five sharp features of almost equal intensity were observed in this region. The wavenumbers obtained are given in Table 1. In this figure the central signal appears to be most intense, but this is a instrumental factor which depends largely on the calibration curve of the frequency doubler. In reality, all five signals appear to have similar intensity.
Table 1 Observed wavenumbers o f the transitions of the a n i l i n e - C O complex
Assignment a
Wavenumber
0o°
33686
bb;i ib
33756 33795
bx2ol tY° b ty o I Sz0
33821 33863
a Vibrational levels designated as in the a n i l i n e - A r complex [3], see text. b ty o1 is assumed to be coincident with either one of these t w o transitions.
In order to be certain that none of the signals of Fig. 1 arose from higher clusters via fragmentation, the spectrum of aniline/(CO), complexes, with n = 2 and 3, were recorded. In both cases, a broad, featureless signal was observed to the long-wavelength side of the range shown in Fig. 1. As can be seen from the data of Table 1, the closest spacing between any two of the signals is 25 c m - 1 and the other spacings are >/40 c m - i. These signals appear to be far too widely spaced to have their origin with an internal rotation of the CO molecule relative to aniline. Under these circumstances, in the free-rotor limit, we would expect an energy level structure of Erot = m2B, were B is the rotational constant for the CO molecule. Assuming B = 1.93 cm -] for CO in its ground state [13], to account for even the smallest observed spacing we would require a transition involving large values of the quantum number m, or large changes in this quantum number, in order to produce the required frequency interval. While this possibility cannot be totally excluded, it seems far more likely that the pattern of signals we have observed (Fig. 1) is vdW vibrational structure. Indeed, as will be seen, based on the experimental data, we arrive at the conclusion that the aniline-CO complex is far more strongly bonded that the benzene-CO equivalent. Since the aniline molecule does not possess the high symmetry of benzene, strong bonding in itself would tend to preclude the possibility of free rotation. The spectrum observed (Fig. 1) consists of five almost equally intense, relatively sharp sub-bands. The only internally consistent assignment we have found, can be made by assigning these as the 0 ° band plus four signals arising from transitions to vdW vibrational levels in the S~ state. In order to account for these four sub-bands, it appears that transitions to a minimum of three different vdW vibrational modes are required. Two signals appear to involve the first and second levels of the same vibration, the other two seem to arise from at least a further two different vibrational levels This situation is very reminiscent of that observed for the aniline-argon complex [3] and in order to make a comparison we make the following tentative assignment. If we assign the longest wavelength signal close to 297 nm as the 0 ° band, the remaining four signals may be assigned as in the aniline-Ar
J.-G. J~ckel et al. / Chemical Physics 215 (1997) 291-298 Table 2 Vibration e n e r g y o f the van der W a a l s levels in the S I state (cm- t ) Aniline-CO
Aniline-Ar
Ratio
70 135 177 109 c or 135 c
22 b 41 b 49 b
3.2 3.3 3.6
109
39 b
2.8
351
53.18 a
6.6
in plane d bend b x stretch s . torsion ty
v v v v
= = = =
l 2 I 1
out o f plane a bend by v = 2 red shift o f 0 ° b a n d
a b c d
F r o m reference [1 ]. F r o m reference [3]. See text. The s y m m e t r y plane p e r p e n d i c u l a r to the a r o m a t i c ring.
complex [3] i.e. in ascending wave number we would have bx0, beg, bx 2 and sx0. In this way one may provisionally label the signals as shown in Table 1. This assignment gives the energies of the vdW vibrational levels in the S 1 state of the aniline-CO complex shown in Table 2 together with the red shift of the 0 ° band. The results obtained for the complex with argon [3] are included in this table for comparison. As can be seen, with this assignment the values obtained are unusually large. The vibrational energies of the vdW modes of the CO-complex are three times as large as those measured in the Ar-complex, slightly more for the in-plane motions (b~ and s z) and slightly less for the out-of-plane b e n d (by). One of the main strengths of this assignment is that, in accordance with expectation, the second excited level of bx lies slightly below twice the energy of the v=l.
The next problem in this simple model is to account for the two additional torsional modes which are to be expected in this complex. In the case of the pyrrole-CO complex, Bettens et al. [ 12] carried out a normal mode analysis of the vdW motions based on the vibrational information contained in the centrifugal distortion parameters they determined. They showed that, as already noted by other authors e.g. [3], the vdW vibrational modes are substantially mixed. In particular the two bending modes, b x and by were shown to be strongly mixed and the higher
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frequency torsional mode, ty, contained a considerable contribution from t z. Only t:, the low lying torsional mode, and s=, the stretching mode, were relatively pure. The fundamental frequency of t z in the pyrrole-CO complex was calculated to be slightly lower than the out-of-plane bend, by. However, since t= may actually be a large amplitude motion, the frequency will be critically dependent on the barrier to this motion and hence difficult to predict for aniline-CO. However, the data so far available from other complexes would lead one to believe that, the transition tz0 would probably lie between the signal assigned as bx~ in Fig. 1 and the 0 ° origin band. (i.e. t z = 1 lies below 70 c m - l ) . As can be seen from Fig. 1 there is no prominent signal in this region, 1 which would either mean that tz0 is coincident with bx 0, or that the transition is not allowed. This latter circumstance would be expected if the CO molecule lay in the C s symmetry plane of the complex. In the CO complexes with both benzene [9] and pyrrole [12], the frequency of the torsional mode t y was calculated to be somewhat lower in frequency than the stretching mode. This would mean that t y 01 is either coincident with the signal we have provisionally assigned a s by 2 or with that assigned as b x 02 in Fig. 1, i.e the first excited level of t y is either 109 c m - l or 135 c m - l (Tables 1 and 2). It must be stressed that although this tentative assignment seems reasonably internally consistent, it is not claimed that this simple model is an accurate description of the vibrational modes involved. The vibrational modes are almost certainly strongly mixed and reliable structural and force field information will be required to be able to do this. A feature of the spectrum shown in Fig. 1 which is rather unusual, is the fact that all five signals have similar intensities. Normally one would expect considerable variation in the intensities of the individual signals and in most cases so far reported, the 0 ° band was by far the most intense feature. As mentioned above, the vdW vibrational modes are considerably mixed and this probably explains why the transitions to different vibrational levels tend to have similar probabilities. From the calculations of Bettens et al. [12], it would seem that this tendency would be particularly pronounced for the second excited levels of the bending modes, where very strong mixing would be expected. The signal as-
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signed as the 0 ° band here (Fig. 1) is relatively much weaker than that observed in other similar vdW complexes. However, all that would be necessary for this situation to arise is that the Franck-Condon factor shifts somewhat in favor of the vdW vibrational levels and this is quite conceivable. The assignments we have made for the signals in Table 1, lead to vibrational frequencies which are three times as large as those observed for the aniline-Ar complex. This requires that the vdW bond formed with CO is considerably stronger than that formed with the argon atom. This assignment is supported by the observed change in red shift of the 0 ° band from going from argon to carbon monoxide. In the aniline-Ar complex the shift is 53.18 cm[1 ], compared to the 351 cm-J observed here (Table 2) for the aniline-CO complex. Indeed, as can be shown by a simple calculation, these two pieces of information, i.e. an increase in the red shift of almost a factor of 7 times and an increase in the vibrational frequency by a factor of three, correlate reasonably well. Assuming the following: (1) a linear relationship between the red shift and the vibrational force constants of the vdW bond, (2) the molecules involved behave as point masses and (3) a purely harmonic stretching vibration. From the observed red shift of the 0 ° band of the argon complex [1] and that of the CO complex (Table 2) the ratio of the vibrational frequencies VCO/VAr is calculated to be approximately 2.93. This value is somewhat smaller than the 3.6 actually observed for the stretching mode (Table 2), but is midway between those observed for the two bending modes (Table 2). What, of course, has been assumed here is that the observed red shift of the 0 ° band provides information on the strength of the vdW bond. This has been proposed by other authors when dealing with vdW complexes formed between aromatic compounds and rare gases e.g. [14]. This relationship can only be expected to hold,for weak interactions in which any resonance effects between the electronic states of the molecules forming the complex have only very small effects. 3.1. The structure of the aniline-CO complex We have no direct information over the structure of the complex from this work. However, as men-
tioned in the Introduction, the results of our infrared-UV double resonance measurements on various aniline complexes [8] were interpreted as showing that bonding took place with the aromatic ring rather than with the amino group. Since a similar situation was found for both the ground electronic state of the neutral complex and that of the cation of the complex [8], it would seem likely that things are not very different in the S~ state of the neutral complex. Consequently, we favor a structure with the CO molecule above the ring, as in the pyrrole-CO complex [12] and not one with the CO bound to the nitrogen of the heterocyclic ring as in the pyridineCO complex [11]. The simplicity of the spectral pattern observed (Fig. 1) and its similarity to that observed for the aniline-Ar complex suggests a structure in which the CO molecule lies in a symmetry plane of the complex. 3.2. The bonding in the complex The assignments we have made here (Table 1) result in large values for the red shift of the origin of the UV band and in comparatively high frequencies for the vdW vibrational modes. These two parameters would be expected to be correlated and consequently the large values of both would seem to be mutually supportive. The results of our double resonance work on complexes of aniline with argon, nitrogen, methane, CHF 3 and CO [8] have shown that a linear relationship exists between the red shift of the origin band and the change in the vibrational frequency of the two stretching modes of the amino group of the aniline molecule. The aniline-CO complex displayed both the largest red shift and the largest changes in vibrational frequencies. This observation would tend to support the view that, at least for the complexes mentioned above, the red shift is a reasonably reliable indicator for the relative strength of the binding interaction between aniline and the particles listed. It would appear that the interaction between aniline and the particles mentioned above is considerably stronger than is the case for the same substance with benzene. For example, the red shift for benzene-Ar and aniline-Ar are 21 cm-~ and 53 cm-l respectively e.g., Ref. [3], and the corresponding
J.-G. Ji~ckel et al. / Chemical Physics 215 (1997) 291-298
vdW stretching frequencies (s z) are 40 c m - t and 49 cm- t. In the case of benzene with N 2 and CO [9], as mentioned in the Introduction, the shifts in the UV band origins were assigned [9] as being very small and the vdW stretching frequencies lie in the region 60 to 70 c m - t [9]. The red shift for the aniline-N 2 complex is on the other hand roughly 130 c m - t [2,6,8] which compares much more favorably with the value of 351 c m - t (Table 2) produced by the assignment given here than would be the case if only the data of the benzene complexes [9] were considered. The conclusion we draw from these data is that the vdW bonding between aniline and all the complexing particles mention above, is considerably stronger than is the case in the equivalent benzene complex. The reason for this almost certainly has its origin with the same factors which make aniline far more reactive than benzene. This is generally considered to be caused by a delocalisation of the electron lone pair on the nitrogen atom of the amino group, thus increasing and extending the electron density over the aromatic ring. Indeed, the aniline molecule in the S l state is planar and the structural change is accompanied with large change in dipole moment [5]. This probably means that this delocalisation process is more pronounced than in the ground state and that the interactions which bind the vdW complexes are stronger in the S t state than in the ground state. This effect is observed experimentally as a large red shift in the origin of the UV band concerned, a point which has been discussed by several authors [5,14]. The question now arises, what factors contribute to the strength of the vdW bond in the aniline-CO complex compared to, say, aniline-N2? Some of the factors usually considered to be of importance for the formation of vdW bonds would not appear to be relevant in this case. Carbon monoxide has only a very small dipole moment and is certainly not easily polarised. Since carbon monoxide possesses a considerable quadrupole moment [15], one possibility is that multipole interactions are important. Another lies with the chemical nature of carbon monoxide, which is well known for its ability to form carbonyls with various metal atoms. The stability of these complexes is usually explained over the ability of carbon
297
monoxide to act as an acceptor of Tr-electrons e.g. [16]. While the situation here is completely different from that present in the metal carbonyls, this effect may provide some basis for the postulation of a relatively strong interaction between carbon monoxide and an electron-rich "rr-system such as that present in aniline. The spectrum shown in Fig. 1, appears to be one of the few cases in which such a clean vdW vibrational structure, similar to that observed in the complexes of argon with various aromatic compounds [3], has been observed. The aniline-N 2 complex, for example, displays no easily identifiable structure other than the 0 ° origin band [2,6,8]. Measurements have been recently carried out in these laboratories on the phenol and carbon monoxide complex [17]. An initial analysis of the complicated spectrum indicates a red shift in the region of 190 c m - l and a structure which appears to contain contributions from vdW vibrations and large amplitude internal motions.
4. Conclusions The aniline-CO vdW complex gives rise to five relatively sharp spectral features within a region 170 to 350 c m - t below the 0 ° origin band of aniline itself. We have assigned the signal at longest wavelength as being the 0 ° origin band of the complex and the other four signals as arising from the vibrational modes associated with the vdW bond. In making this assignment we arrive at unusually large values for the shift of the 0 ° origin band (351 cm- l ) and correspondingly large values for the vibrational frequencies. These values indicated that the CO molecule is relatively strongly bonded to aniline. The spectrum indicates that at least three different vdW vibrational modes are involved in the production of the spectrum and these have been provisionally labeled in a manner analogous to that given for the aniline-At complex [3]. The presence of strong bonding would appear to explain the simplicity of the observed spectrum and the absence of complicating features which would indicate the presence of large amplitude motions in the complex.
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The similarity of the spectral pattern to that of the aniline-Ar complex [3] seems to indicate that the structure of the aniline-CO complex may be similar to its argon counterpart and to the structure of the pyrrole-CO complex [11]. This conclusion is supported by the results of infrared-ultraviolet double resonance measurements on a number of aniline complexes [8], which indicated that the complexing particle interacts with the aromatic ring rather than the amino group of aniline. Although this proposed assignment appears to be internally consistent, since the amount of the experimental data is somewhat limited, further measurements will be required to confirm it in detail.
Acknowledgements This work is supported by the DFG and in part by the Fonds der Chemischen Industrie.
References [1] K. Yamanouchi, S. lsogai and S. Tsuchiya, Chem. Phys. 116 (1987) 123; K. Yamanouchi, H. Watanabe, S. Koda and S. Tsuchiya, Chem. Phys. Lett. 107 (1984) 290.
[2] K. Yamanouchi, S. Isogai and S. Tsuchiya, J. Mol. Struct. 146 (1986) 349. [3] E.J. Bieske, M.W. Rainbird, I.M. Atkinson and A.E.W. Knight, J. Chem. Phys. 91 (1989) 752. [4] P.M. Weber, J.T. Buontempo, F. Novak and S.A. Rice, J. Chem. Phys. 88 (1988) 6082. [5] P. Hermine, P. Pameix, B. Coutant, F.G. Amar and Ph. Br~chignac, Z. Phys. D. At. Mol. Clusters 22 (1992) 529. [6] M.F. Hineman, S.K. Kim, E.R. Bernstein and D.F. Kelly, J. Chem. Phys. 96 (1992) 4904. [7] M.R. Nimlos, M.A. Young, E.R. Bernstein and D.F. Kelly, J. Chem. Phys. 91 (1989) 5268. [8] R. Schmid, K.P. Chowdhury, J. Miyawaki, K. Sugawara, T. Nakanaga, H. Takeo and H. Jones, J. Chem. Phys. submitted. [9] R. Nowak, J.A. Manapace and E.R. Bernstein, J. Chem. Phys. 89 (1988) 1309. [10] Th. Brupbacher and A. Bauder, J. Chem. Phys. 99 (1993) 9394. [11] R.P.A. Bettens and A. Bauder, J. Chem. Phys. 102 (1995) 1501. [12] R.P.A. Bettens, S.R. Huber and A. Bauder, J. Phys. Chem. 98 (1994) 4551. [13] K.P. Huber and G. Herzberg, Constants of diatomic molecules (Van Nostrand, New York, 1979). [14] M. Mons, J. Le Calv6, F. Piuzzi and !. Dimicoli, J. Chem. Phys. 92 (1990) 2155. [15] S. Gustavson and W. Gordy, J. Chem. Phys. 52 (1970) 579. [16] F.A. Cotton and G. Wilkinson, Anorganische chemie (Verlag Chemie, Weinheim, 1982). [17] Th. Engst, J.-G. J~ickel, H. Takeo and H. Jones, unpublished results.
Chemical Physics 215 (1997) 291 - 298
The van der Waals vibrational frequencies of the aniline-carbon monoxide complex in its state Johann-Georg J ickel a, Reiner Schmid a, Harold Jones a,*, T. Nakanaga c, Harutoshi Takeo b a Abteilung Chemische Physik, Universit~t Ulm, Albert Einstein-Allee 11, 89069 Ulm, Germany b National Institute for Advanced Interdisciplinary Research, 1-1-4 Higashi, Tsukuba, Ibaraki 305, Japan ¢ National Institute of Materials and Chemical Research, 1-1-4 Higashi, Tsukuba, lbaraki 305, Japan
Received 31 July 1996; in final form 13 November 1996
Abstract A portion of the S1-S o band of the aniline-CO van der Waals (vdW) complex has been observed near 297 nm, using REMPI/TOF spectroscopy. Compared to the same band in aniline itself, this represents a shift to longer wavelengths of over 300 c m - J, which is one of the largest red shifts so far observed for this type of vdW complex. The spectrum observed consisted of five almost equally intense, relatively sharp sub-bands and we have assigned these as the 0 ° band plus four signals arising from transitions to vibrational levels in the S 1 state which are associated with the van der Waals bond. In order to account for these four sub-bands, it appears that transitions to a minimum of three different vdW vibrational modes are required. Two signals appear to involve the first and second level of the same vibration, the other two seem to arise from at least a further two different vibrational levels. The large values of both the red shift of the S1-S o band and the frequencies of the vdW modes indicate that aniline-CO is a relatively strongly bonded complex. It was concluded that the CO molecule probably lies in the symmetry plane above the aromatic ring of the aniline molecule. 1. Introduction Van der W a a l s ( v d W ) complexes of aniline with various atoms or molecules produced in a supersonic jet expansion have been subjected to spectroscopic investigation for well over a decade. The techniques so far successfully employed include laser induced fluorescence (LIF), resonance enhanced multiphoton ionisation coupled with time-of-flight mass spectrometry ( R E M P I / T O F ) , and more recently, thresh-
* Corresponding author.
old photoelectron spectroscopy. The studies have been almost invariably carried out on the S t - S 0 band ( A 1B2-X IAi) o f aniline near 294 nm, which is a c o n v e n i e n t r e g i o n for i n v e s t i g a t i o n with frequency-doubled dye lasers. The most extensive work has been carried out on a n i l i n e - r a r e gas complexes and the complex with argon has been particularly well characterised [1]. It was assumed that the complex had C S symmetry with the rare gas atom located above the aromatic ring in the plane of symmetry. With such weakly-bound complexes, the spectra observed usually resemble that of aniline itself, simply shifted slightly to longer wavelengths. In the case
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of the rare gases, the red shift of the complex lies between 5.3 c m - t for Ne and 82 cm-~ for Kr [1]. Using laser induced fluorescence spectroscopy, Yamanouchi et al. [1] were able to resolve the rotational structure of the 0 ° vibronic band of both the aniline-Ar and aniline-Ne complexes. This work was later extended to the complexes with N 2, H 2 and CH 4 [2]. Under the assumptions that the complex had C s symmetry and that the complexing particle could be considered as a point mass, several structural parameters were estimated from band-contour analysis. A prominent feature of the spectra of the complexes which is not observed with aniline alone, is the appearance of a number of lines which arise from the low-frequency vibrational modes of the weak vdW bond. In the case of argon, the lines have been assigned to specific vibrational modes. Bieske et al. [3] were able to convincingly demonstrate that four lines observed in a number of complexes of argon with substituted benzenes arise from the stretching and bending motions of the vdW bond. The four lines, which lay within 50 c m - l of the 0 ° origin band of the complex, were assigned as being associated with the following vibrational levels in the S t state: the first excited level of a stretching motion within the plane of symmetry of the molecule (Sz~), the first and second excited levels of an in-plane bending motion (bz~ and bx 2) and the second excited level of an out-of-plane bend (byg), where the complex is assumed to have C s symmetry with the plane of symmetry in the z - x plane. Since only transitions to totally symmetric levels are allowed, the transition associated with the first excited level of the out-of-plane bend, bye, was not observed [3]. Similar assignments have been made in the cases of argon complexes with other compounds e.g., Ref. [4]. An alternative assignment for the vdW structure of aniline-Ar has been given by Hermine et al. [5] who, as a result of calculations and using intensity 1 arguments, exchanged the signals assigned as sz0 and by ~. However, the theoretical results were somewhat inconclusive [5] and the earlier assignment [3] was considered by Hermine et al. [5] to be also possible. After considering all the data available, we still favor the original assignment of Bieske et al. [3] since this seems to be more consistent with the shifts
in the origin of the S I - S 0 bands and the spectral patterns observed for the argon complexes of other aromatic compounds studied in the same work [3]. In the case of the aniline complexes with molecules such as N 2 and methane, signals were observed in a region 50 c m - i to 100 c m - I below the 0 ° band of aniline itself [2,6,7] which almost certainly arise from vdW motions. However, no easily identifiable spectral pattern appeared to be present. We have recently carried out ultraviolet-infrared double resonance measurements [8] on the complexes of aniline with the molecules N 2, CH 4, CHF 3 and CO and were able to show that in the ground electronic state of the neutral complex and in its cation, the frequencies of the two vibrational stretching modes of the amino group of aniline were only slightly affected by the formation of a complex. This has led us to the conclusion that bonding in these complexes takes place predominantly with the -rr-system of the aromatic ring, as is the case with the argon complex, rather than with the amino group. In this paper we report R E M P I / T O F measurements carried out on the aniline-carbon monoxide complex. These appear to be the first measurements on this complex and in this case we have observed a pattern of lines which we interpret as being qualitatively very similar to that observed with the anilineAr complex. If it is assumed that the argon atom, which is situated above the aromatic ring, is simply replaced with a diatomic molecule, the vdW vibrational structure would be expected to remain basically the same, except that two extra modes are required to describe the torsional motion of the diatomic molecule: one motion about an axis parallel to the plane of the benzene ring (ty) and another about the axis perpendicular to this plane (tz). Transitions to excited levels of these torsional motions may give rise to additional signals over and above those observed for the argon complex. The situation is more complicated if the torsional barrier for the mode t z is very low. In the extreme lower limit of a zero barrier, the motion results in a free internal rotation about the z-axis. Such a situation has been considered in detail by Nowak et al. [9] in their work on complexes of N 2, CO 2, and CO with benzene. The complicated spectra they observed were interpreted as being a mixture of transitions
J.-G. Ji~ckel et al./ Chemical Physics 215 (1997) 291-298
arising from free rotation and some of the vdW vibrational modes mentioned above. The analysis of the spectra relied heavily on the results of theoretical calculations which were used to determine a number of basic properties of the complex. There are however, a few examples of complexes formed between carbon monoxide and various aromatic compounds for which exact structural information has been determined. For example, the microwave work of Bander and coworkers on the complexes of CO with benzene [10], pyridine [11] and pyrrole [12]. In the case of the CO-benzene complex, the parameters determined from the experimental data [10] differ considerably from those obtained by Nowak et al. [9] from their theoretical calculations. To what extent these discrepancies lead to a reduction in the confidence of the correctness of the assignments made for the UV spectrum of the benzene-CO complex [9] will not be discussed here. For our purposes, it suffices to point out, that this complex is certainly relatively weakly bound [10] and, no
293
matter how the UV spectrum is assigned in detail, the experimental observations [9] show that only a relatively small shift in the origin of the UV band was produced on forming the complex. 2. E x p e r i m e n t a l
The R E M P I / T O F apparatus consisted of a single vacuum chamber (ca. 50 liter volume) with the 50 cm long drift tube of a linear TOF mass spectrometer aligned with the axis of the free expansion jet. The chamber and the TOF were connected over an opening 5 mm in diameter. Pressures in the region of 10 -3 mbar were maintained in the main chamber using a 700 l / s diffusion pump, whereas the mass spectrometer was kept at 10 -6 to 10 -7 mbar with the aid of a 300 l / s turbo-molecular pump. The aniline-CO complex was produced by passing carbon monoxide gas over liquid aniline at room temperature. The gas mixture was admitted at a stagnation pressure of 1 bar into the vacuum cham-
Aniline/CO 1000
by~
complex
bx
sxol 800
Origin 600
,400
200
vdW S t r u c t u r e I
-100
-200 2c-.~. OOO
2S6. 000
297.000
rim
=~
Fig. 1. The REMPI spectrum of the S~-S 0 band of the aniline-CO van der Waals complex in the region 294 to 298 nm. As can be seen, five sub-bands of similar intensity were observed. That at the longest wavelength was assigned as the 0o° origin band of the complex. The four other spectral features appear to arise from at least three different vibrational modes of the vdW bond. Tentative individual assignments are as shown in the figure (see text). The spectrum appears to display no indication of the effects of large amplitude motions in the complex. The positions of the spectral features scale relative to the 0o° origin band of aniline are indicated by the approximate wavenumber scale in the figure.
J.-G. Ji~ckel et al. / Chemical Physics 215 (1997) 291-298
294
ber through a pulsed nozzle (General Valve) with a 0.5 mm orifice. Tunable ultraviolet radiation in the 295 nm region was produced by pumping a Lumonics HD-300 dye laser with the second harmonic of a Lumonics HY1200 Nd:YAG laser and then doubling the output in a KDP crystal (Lumonics HT1000). Pulse energies up to 10 mJ with pulse lengths of the order of 5 ns and a nominal line width of approximately 0.12 c m - l were produced. The spectrum shown in Fig. 1 was recorded with a pulse energy of less than 1 mJ with the laser beam focused into the vacuum chamber. Frequency calibration of all the spectra reported in this paper was carried out using a Burleigh WA 4500 wavemeter. The ion signal was displayed on a digital oscilloscope (Tektronix TDS 320) and mass selected spectra were obtained by digitally recording the output of a box-car integrator (Stanford Research).
3. Observations and discussion The spectrum of the aniline-CO complex observed in the region 295 to 297 nm is shown in Fig. 1. As can be seen, five sharp features of almost equal intensity were observed in this region. The wavenumbers obtained are given in Table 1. In this figure the central signal appears to be most intense, but this is a instrumental factor which depends largely on the calibration curve of the frequency doubler. In reality, all five signals appear to have similar intensity.
Table 1 Observed wavenumbers o f the transitions of the a n i l i n e - C O complex
Assignment a
Wavenumber
0o°
33686
bb;i ib
33756 33795
bx2ol tY° b ty o I Sz0
33821 33863
a Vibrational levels designated as in the a n i l i n e - A r complex [3], see text. b ty o1 is assumed to be coincident with either one of these t w o transitions.
In order to be certain that none of the signals of Fig. 1 arose from higher clusters via fragmentation, the spectrum of aniline/(CO), complexes, with n = 2 and 3, were recorded. In both cases, a broad, featureless signal was observed to the long-wavelength side of the range shown in Fig. 1. As can be seen from the data of Table 1, the closest spacing between any two of the signals is 25 c m - 1 and the other spacings are >/40 c m - i. These signals appear to be far too widely spaced to have their origin with an internal rotation of the CO molecule relative to aniline. Under these circumstances, in the free-rotor limit, we would expect an energy level structure of Erot = m2B, were B is the rotational constant for the CO molecule. Assuming B = 1.93 cm -] for CO in its ground state [13], to account for even the smallest observed spacing we would require a transition involving large values of the quantum number m, or large changes in this quantum number, in order to produce the required frequency interval. While this possibility cannot be totally excluded, it seems far more likely that the pattern of signals we have observed (Fig. 1) is vdW vibrational structure. Indeed, as will be seen, based on the experimental data, we arrive at the conclusion that the aniline-CO complex is far more strongly bonded that the benzene-CO equivalent. Since the aniline molecule does not possess the high symmetry of benzene, strong bonding in itself would tend to preclude the possibility of free rotation. The spectrum observed (Fig. 1) consists of five almost equally intense, relatively sharp sub-bands. The only internally consistent assignment we have found, can be made by assigning these as the 0 ° band plus four signals arising from transitions to vdW vibrational levels in the S~ state. In order to account for these four sub-bands, it appears that transitions to a minimum of three different vdW vibrational modes are required. Two signals appear to involve the first and second levels of the same vibration, the other two seem to arise from at least a further two different vibrational levels This situation is very reminiscent of that observed for the aniline-argon complex [3] and in order to make a comparison we make the following tentative assignment. If we assign the longest wavelength signal close to 297 nm as the 0 ° band, the remaining four signals may be assigned as in the aniline-Ar
J.-G. J~ckel et al. / Chemical Physics 215 (1997) 291-298 Table 2 Vibration e n e r g y o f the van der W a a l s levels in the S I state (cm- t ) Aniline-CO
Aniline-Ar
Ratio
70 135 177 109 c or 135 c
22 b 41 b 49 b
3.2 3.3 3.6
109
39 b
2.8
351
53.18 a
6.6
in plane d bend b x stretch s . torsion ty
v v v v
= = = =
l 2 I 1
out o f plane a bend by v = 2 red shift o f 0 ° b a n d
a b c d
F r o m reference [1 ]. F r o m reference [3]. See text. The s y m m e t r y plane p e r p e n d i c u l a r to the a r o m a t i c ring.
complex [3] i.e. in ascending wave number we would have bx0, beg, bx 2 and sx0. In this way one may provisionally label the signals as shown in Table 1. This assignment gives the energies of the vdW vibrational levels in the S 1 state of the aniline-CO complex shown in Table 2 together with the red shift of the 0 ° band. The results obtained for the complex with argon [3] are included in this table for comparison. As can be seen, with this assignment the values obtained are unusually large. The vibrational energies of the vdW modes of the CO-complex are three times as large as those measured in the Ar-complex, slightly more for the in-plane motions (b~ and s z) and slightly less for the out-of-plane b e n d (by). One of the main strengths of this assignment is that, in accordance with expectation, the second excited level of bx lies slightly below twice the energy of the v=l.
The next problem in this simple model is to account for the two additional torsional modes which are to be expected in this complex. In the case of the pyrrole-CO complex, Bettens et al. [ 12] carried out a normal mode analysis of the vdW motions based on the vibrational information contained in the centrifugal distortion parameters they determined. They showed that, as already noted by other authors e.g. [3], the vdW vibrational modes are substantially mixed. In particular the two bending modes, b x and by were shown to be strongly mixed and the higher
295
frequency torsional mode, ty, contained a considerable contribution from t z. Only t:, the low lying torsional mode, and s=, the stretching mode, were relatively pure. The fundamental frequency of t z in the pyrrole-CO complex was calculated to be slightly lower than the out-of-plane bend, by. However, since t= may actually be a large amplitude motion, the frequency will be critically dependent on the barrier to this motion and hence difficult to predict for aniline-CO. However, the data so far available from other complexes would lead one to believe that, the transition tz0 would probably lie between the signal assigned as bx~ in Fig. 1 and the 0 ° origin band. (i.e. t z = 1 lies below 70 c m - l ) . As can be seen from Fig. 1 there is no prominent signal in this region, 1 which would either mean that tz0 is coincident with bx 0, or that the transition is not allowed. This latter circumstance would be expected if the CO molecule lay in the C s symmetry plane of the complex. In the CO complexes with both benzene [9] and pyrrole [12], the frequency of the torsional mode t y was calculated to be somewhat lower in frequency than the stretching mode. This would mean that t y 01 is either coincident with the signal we have provisionally assigned a s by 2 or with that assigned as b x 02 in Fig. 1, i.e the first excited level of t y is either 109 c m - l or 135 c m - l (Tables 1 and 2). It must be stressed that although this tentative assignment seems reasonably internally consistent, it is not claimed that this simple model is an accurate description of the vibrational modes involved. The vibrational modes are almost certainly strongly mixed and reliable structural and force field information will be required to be able to do this. A feature of the spectrum shown in Fig. 1 which is rather unusual, is the fact that all five signals have similar intensities. Normally one would expect considerable variation in the intensities of the individual signals and in most cases so far reported, the 0 ° band was by far the most intense feature. As mentioned above, the vdW vibrational modes are considerably mixed and this probably explains why the transitions to different vibrational levels tend to have similar probabilities. From the calculations of Bettens et al. [12], it would seem that this tendency would be particularly pronounced for the second excited levels of the bending modes, where very strong mixing would be expected. The signal as-
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signed as the 0 ° band here (Fig. 1) is relatively much weaker than that observed in other similar vdW complexes. However, all that would be necessary for this situation to arise is that the Franck-Condon factor shifts somewhat in favor of the vdW vibrational levels and this is quite conceivable. The assignments we have made for the signals in Table 1, lead to vibrational frequencies which are three times as large as those observed for the aniline-Ar complex. This requires that the vdW bond formed with CO is considerably stronger than that formed with the argon atom. This assignment is supported by the observed change in red shift of the 0 ° band from going from argon to carbon monoxide. In the aniline-Ar complex the shift is 53.18 cm[1 ], compared to the 351 cm-J observed here (Table 2) for the aniline-CO complex. Indeed, as can be shown by a simple calculation, these two pieces of information, i.e. an increase in the red shift of almost a factor of 7 times and an increase in the vibrational frequency by a factor of three, correlate reasonably well. Assuming the following: (1) a linear relationship between the red shift and the vibrational force constants of the vdW bond, (2) the molecules involved behave as point masses and (3) a purely harmonic stretching vibration. From the observed red shift of the 0 ° band of the argon complex [1] and that of the CO complex (Table 2) the ratio of the vibrational frequencies VCO/VAr is calculated to be approximately 2.93. This value is somewhat smaller than the 3.6 actually observed for the stretching mode (Table 2), but is midway between those observed for the two bending modes (Table 2). What, of course, has been assumed here is that the observed red shift of the 0 ° band provides information on the strength of the vdW bond. This has been proposed by other authors when dealing with vdW complexes formed between aromatic compounds and rare gases e.g. [14]. This relationship can only be expected to hold,for weak interactions in which any resonance effects between the electronic states of the molecules forming the complex have only very small effects. 3.1. The structure of the aniline-CO complex We have no direct information over the structure of the complex from this work. However, as men-
tioned in the Introduction, the results of our infrared-UV double resonance measurements on various aniline complexes [8] were interpreted as showing that bonding took place with the aromatic ring rather than with the amino group. Since a similar situation was found for both the ground electronic state of the neutral complex and that of the cation of the complex [8], it would seem likely that things are not very different in the S~ state of the neutral complex. Consequently, we favor a structure with the CO molecule above the ring, as in the pyrrole-CO complex [12] and not one with the CO bound to the nitrogen of the heterocyclic ring as in the pyridineCO complex [11]. The simplicity of the spectral pattern observed (Fig. 1) and its similarity to that observed for the aniline-Ar complex suggests a structure in which the CO molecule lies in a symmetry plane of the complex. 3.2. The bonding in the complex The assignments we have made here (Table 1) result in large values for the red shift of the origin of the UV band and in comparatively high frequencies for the vdW vibrational modes. These two parameters would be expected to be correlated and consequently the large values of both would seem to be mutually supportive. The results of our double resonance work on complexes of aniline with argon, nitrogen, methane, CHF 3 and CO [8] have shown that a linear relationship exists between the red shift of the origin band and the change in the vibrational frequency of the two stretching modes of the amino group of the aniline molecule. The aniline-CO complex displayed both the largest red shift and the largest changes in vibrational frequencies. This observation would tend to support the view that, at least for the complexes mentioned above, the red shift is a reasonably reliable indicator for the relative strength of the binding interaction between aniline and the particles listed. It would appear that the interaction between aniline and the particles mentioned above is considerably stronger than is the case for the same substance with benzene. For example, the red shift for benzene-Ar and aniline-Ar are 21 cm-~ and 53 cm-l respectively e.g., Ref. [3], and the corresponding
J.-G. Ji~ckel et al. / Chemical Physics 215 (1997) 291-298
vdW stretching frequencies (s z) are 40 c m - t and 49 cm- t. In the case of benzene with N 2 and CO [9], as mentioned in the Introduction, the shifts in the UV band origins were assigned [9] as being very small and the vdW stretching frequencies lie in the region 60 to 70 c m - t [9]. The red shift for the aniline-N 2 complex is on the other hand roughly 130 c m - t [2,6,8] which compares much more favorably with the value of 351 c m - t (Table 2) produced by the assignment given here than would be the case if only the data of the benzene complexes [9] were considered. The conclusion we draw from these data is that the vdW bonding between aniline and all the complexing particles mention above, is considerably stronger than is the case in the equivalent benzene complex. The reason for this almost certainly has its origin with the same factors which make aniline far more reactive than benzene. This is generally considered to be caused by a delocalisation of the electron lone pair on the nitrogen atom of the amino group, thus increasing and extending the electron density over the aromatic ring. Indeed, the aniline molecule in the S l state is planar and the structural change is accompanied with large change in dipole moment [5]. This probably means that this delocalisation process is more pronounced than in the ground state and that the interactions which bind the vdW complexes are stronger in the S t state than in the ground state. This effect is observed experimentally as a large red shift in the origin of the UV band concerned, a point which has been discussed by several authors [5,14]. The question now arises, what factors contribute to the strength of the vdW bond in the aniline-CO complex compared to, say, aniline-N2? Some of the factors usually considered to be of importance for the formation of vdW bonds would not appear to be relevant in this case. Carbon monoxide has only a very small dipole moment and is certainly not easily polarised. Since carbon monoxide possesses a considerable quadrupole moment [15], one possibility is that multipole interactions are important. Another lies with the chemical nature of carbon monoxide, which is well known for its ability to form carbonyls with various metal atoms. The stability of these complexes is usually explained over the ability of carbon
297
monoxide to act as an acceptor of Tr-electrons e.g. [16]. While the situation here is completely different from that present in the metal carbonyls, this effect may provide some basis for the postulation of a relatively strong interaction between carbon monoxide and an electron-rich "rr-system such as that present in aniline. The spectrum shown in Fig. 1, appears to be one of the few cases in which such a clean vdW vibrational structure, similar to that observed in the complexes of argon with various aromatic compounds [3], has been observed. The aniline-N 2 complex, for example, displays no easily identifiable structure other than the 0 ° origin band [2,6,8]. Measurements have been recently carried out in these laboratories on the phenol and carbon monoxide complex [17]. An initial analysis of the complicated spectrum indicates a red shift in the region of 190 c m - l and a structure which appears to contain contributions from vdW vibrations and large amplitude internal motions.
4. Conclusions The aniline-CO vdW complex gives rise to five relatively sharp spectral features within a region 170 to 350 c m - t below the 0 ° origin band of aniline itself. We have assigned the signal at longest wavelength as being the 0 ° origin band of the complex and the other four signals as arising from the vibrational modes associated with the vdW bond. In making this assignment we arrive at unusually large values for the shift of the 0 ° origin band (351 cm- l ) and correspondingly large values for the vibrational frequencies. These values indicated that the CO molecule is relatively strongly bonded to aniline. The spectrum indicates that at least three different vdW vibrational modes are involved in the production of the spectrum and these have been provisionally labeled in a manner analogous to that given for the aniline-At complex [3]. The presence of strong bonding would appear to explain the simplicity of the observed spectrum and the absence of complicating features which would indicate the presence of large amplitude motions in the complex.
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The similarity of the spectral pattern to that of the aniline-Ar complex [3] seems to indicate that the structure of the aniline-CO complex may be similar to its argon counterpart and to the structure of the pyrrole-CO complex [11]. This conclusion is supported by the results of infrared-ultraviolet double resonance measurements on a number of aniline complexes [8], which indicated that the complexing particle interacts with the aromatic ring rather than the amino group of aniline. Although this proposed assignment appears to be internally consistent, since the amount of the experimental data is somewhat limited, further measurements will be required to confirm it in detail.
Acknowledgements This work is supported by the DFG and in part by the Fonds der Chemischen Industrie.
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