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The Aniline—Water Complex
JOURNAL OF MOLECULAR SPECTROSCOPY ARTICLE NO.
190, 278 –289 (1998)
MS987600
The Aniline—Water Complex Rotational Spectrum and Molecular Structure Ute Spoerel and Wolfgang Stahl Institut fu¨r Physikalische Chemie, RWTH Aachen, Templergraben 59, D-52056 Aachen, Germany Received January 6, 1998; in revised form March 26, 1998
The rotational spectra of aniline–water and its 18O isotopomer have been studied in the microwave region between 3 and 26.5 GHz using a pulsed molecular beam FT microwave spectrometer. The spectra were described in terms of a centrifugally distorted asymmetric rotor. Assuming a linear hydrogen N...H–O bond and that the water molecule was located in the symmetry plane of aniline, two structures turned out to be possible. In structure (I) the free water proton is directed toward the aniline ring. In structure (II) the proton is bent away from it. Ab initio calculations indicate that only structure (I) is supported by the experimental results. © 1998 Academic Press INTRODUCTION
Water is the most important inorganic solvent. In the solution process it often undergoes chemical reactions. Among these are acid– base reactions in which water either serves as proton donor or as a proton acceptor. In solution these reactions yield hydrated ions. If no hydration takes place, e.g., in the gas phase, the ion pair does not separate. Instead, a hydrogen-bonded complex is formed. An example for such a complex, in which water serves as a proton acceptor, is phenol– water (1). In the water dimer (2) one water molecule acts as a proton donor, the other one as an acceptor. Trimethylamine– water (3) and ammonia–water (4) are examples where water is a proton donor. Hydrogen-bonded complexes are also studied with REMPI experiments to obtain information on the electronically excited state. For this purpose a chromophore, like an aromatic ring system, should be present in the complex. Therefore we decided to investigate the aniline–water complex as a model system and to provide rotational constants for the ground state. In addition, we were interested in studying the change in the nuclear quadrupole coupling constants of the 14N nucleus when aniline undergoes complexation. These may also be compared to the constants for the liquid phase, as soon as these become available. Thereby information on the molecular environment of aniline in an aqueous solution may be obtained. In aqueous solutions aniline reacts as a base (5) and anilinium ions can be formed. The aniline–water complex may be considered as the first step of this reaction. EXPERIMENT
All spectra were taken in the range from 3 to 26.5 GHz using the molecular beam (MB) Fourier transform microwave
(FTMW) spectrometer (6, 7) at the University of Kiel. Aniline was kept in a stainless steel container upstream from the nozzle and heated to about 35°C. Helium containing 0.35% water was passed over the aniline sample at a pressure of 100 kPa. For the measurements on aniline–water–18O a mixture of about 50% water and 50% water–18O was used. RESULTS
I. Spectral Analysis We started our investigation by predicting rotational constants for aniline–water using the structure of ammonia–water as a model. In the structure of ammonia–water the distance between oxygen and nitrogen is 2.983 Å and the linear N...H–O bond is bent away from the C3-axis of ammonia by 23.1 (2)° (4). We assumed that the structures of aniline, C6H5NH2, and of water remain unchanged upon complexation. The water molecule was assumed to be located in the ac-plane of aniline. Further, a linear N . . . H–O bond with a length of 3 Å was assumed to form an angle of 110° with the ring plane of aniline. Using the aniline structure given by Lister et al. (8) and the water structure of Taft and Dailey (9), we calculated the rotational constants to be A 5 3492 MHz, B 5 1035 MHz, and C 5 969 MHz. Transforming the dipole moments of aniline (8) and water (10) into the inertia axes system of aniline–water, we predicted an a- and c-type spectrum for the complex. While scanning for some predicted lines we found several a-type branches. All lines were split into multiplets due to 14 N-quadrupole coupling. A least-squares fit yielded improved rotational B and C constants, which in turn enabled us to find more a- and also c-type lines. Analysis of the very narrow 14 N-hyperfine structure (hfs) confirmed our rotational assignment. In Fig. 1 a quite well resolved hfs of the 414–313
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THE ANILINE–WATER COMPLEX
large amplitude motion of the amino group. Under molecular beam conditions usually only the lower state can be observed because the upper state is too high to be populated. In the case of aniline–water we did not observe any tunneling splitting. We believe that the inversion is completely quenched in the complex. However, even if inversion could take place, it would be not observable for the same reasons as in the case of the monomer. II. Molecular Structure
FIG. 1. JKaKc 5 414 2 313 transition of aniline–water. Power spectrum, sample interval 100 ns, 8 k Fourier transformation, average of 545 experimental cycles, polarization frequency 8651.61 MHz.
transition is shown as an example. In some cases the hfs could only be partially resolved. All measured lines are complied in Table 1. Some lines were additionally broadened or showed very narrow splittings in the order of 1–2 kHz. The origin of this effect is not quite clear and it was therefore ignored. Instead these splittings were averaged out by decreasing the resolution element of the Fourier transform. As a consequence the standard deviation of the fit increased to about 4 kHz, which is slightly higher than the experimental accuracy. The rotational constants, the quartic centrifugal distortion constants according to the reduction of van Eijck (11), and the diagonal elements of the quadrupole coupling tensor are given in Table 2. The corresponding correlation matrix is given in Table 3. By comparing the quadrupole coupling elements of aniline–water and its 18O isotopomer, we were also able to determined the xac off-diagonal element. Its determination will be described in Part III. The rotational constants were in agreement with two different structures of the complex. In both cases we assumed a linear N . . . H–O hydrogen bond and that the water molecule was located in the symmetry plane of aniline. In structure (I) the free water proton is directed toward the aniline ring; in structure (II) the proton is bent away from it. For these two structures we predicted two sets of rotational constants for the 18O isotopomer. These were A 5 3117.2 MHz, B 5 1057.7 MHz, and C 5 1023.7 MHz for structure (I), and A 5 3111.7 MHz, B 5 1059.4 MHz, and C 5 1025.9 MHz for structure (II). The lines of the 18O isotopomer were found to be very close to the frequencies predicted with the rotational constants of structure (I). They are given in Table 1 and were analyzed in the same way as described before. The spectroscopic constants are also presented in Table 2, the corresponding correlation matrix in Table 3. The aniline monomer shows an inversion splitting due to the
Our spectral analysis provided us with three rotational constants, five quartic distortion constants, and the diagonal elements of the quadrupole coupling tensor for aniline–water and its 18O isotopomer. We used the rotational constants to determine the position of the water molecule with respect to the aniline monomer. In order to obtain the structure of aniline– water we assumed the structures of aniline and water to remain unchanged in the complex. We used the structure for Lister et al. (8) for aniline and the structure of Taft et al. (9) for water. We further assumed a linear N . . . H–O bond and that the water molecule was located within the mirror plane of aniline. The existence of such a mirror plane results from the consideration of the planar moments of aniline and its water complex. The difference of the planar moments DP bb 5 P bb~aniline 2 water! 2P bb~aniline!
[1]
with P bb 5
1 ~I 1 I cc 2 I bb! 2 aa
[2]
is only 0.69 uÅ2 for aniline–water and 0.72 uÅ2 for aniline– water–18O, respectively. This strongly indicates that the equilibrium structure of the complex has an ac-symmetry plane with the water molecule located within this plane. This is also supported by the fact that no mb-type lines could be observed. The ‚Pbb of 0.7 uÅ2 may be attributed to an out-of-plane vibration of the free water proton as well as of the whole water molecule. So far we have no experimental evidence to support the assumption of a linear hydrogen bond. To gain experimental insight in this problem, it would be necessary to measure various isotopomers of aniline–water. Unfortunately this was not possible. We will refer to this point in the following parts of this article. Fitting the hydrogen bond length and the angle between the hydrogen bond and the ring plane of aniline to the rotational constants of aniline–water and aniline–water–18O, we determined two possible structures for the complex. In structure (I) the hydrogen bond length between N and O is 3.028 Å, and the angle between the hydrogen bond and the aniline ring is 101.31°. The free water proton is directed toward the aniline ring. In structure (II) the hydrogen bond length is 3.035 Å, and
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TABLE 1 Measured Transitions of Aniline–Water and Aniline–Water–18O
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THE ANILINE–WATER COMPLEX
TABLE 1—Continued
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TABLE 1—Continued
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TABLE 1—Continued
the angle between the hydrogen bond and the aniline ring is 99.12°. In this case the free water proton is directed away from the aniline ring. Structure (I) reproduces the rotational constants of the measured aniline–water isotopomers within 0.5 MHz for aniline-water and 0.3 MHz for aniline–water–18O. On the other hand, structure (II) reproduces the rotational constants for aniline–water only within 2.8 MHz and for aniline–water–18O only within 3.6 MHz. Therefore we favor TABLE 2 Rotational, Centrifugal Distortion (van Eijck), and Quadrupole Coupling Constants of Aniline–Water and Aniline–Water–18O, Ir Representation
structure (I), although the isotopic shifts cannot definitely rule out structure (II). The atomic coordinates of both structures in their principal axes systems and the calculated rotational constants for aniline–water and aniline–water–18O are given in Table 4. Figures 2 and 3 show projections of structure (I) and (II) onto their acand ab-planes. We were now also able to calculate the rs-coordinates of oxygen in the principal axes system of aniline–water. Using Kraitchman’s equations (13) we calculated the values given in Table 4. With this method it is only possible to determine the TABLE 3 Correlation Matrices of Aniline–Water and Aniline–Water–18O
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TABLE 4 Structure (I) and (II) of Aniline–Water
absolute value of the coordinates. Comparing the rs-coordinates of oxygen to the fitted coordinates of structure (I) or (II) in the principal axes system of aniline–water, one can see that the rs-coordinates resemble the coordinates of structure (I). Since the deviation between the ro- and the rs-coordinates for structure (I) are 0.005 Å for the a- and 0.001 Å for the c-coordinate, this gives a hint towards the quality of the fitted ro-structure. Measurements on deuterated isotopomers would help much in deciding whether the free water proton is directed away or toward the aniline ring. Unfortunately aniline undergoes a fast exchange of its amino protons with D2O. Measurements on deuterated aniline and D2O were also not successful because an exchange between the deuteriums and the protons of water traces present in the spectrometer and in the carrier gas
evolved. Spectra of aniline–water with 13C–aniline in natural abundance could not be observed because of their very low intensity. In principle it should also be possible to distinguish between structure (I) and (II) by their different dipole moments. In structure (I) the dipole moment of the water molecule is directed almost along the a-axis, in structure (II) almost along the c-axis. The dipole moment of aniline does not change significantly between the two structures. Despite small induced dipole moments the dipole moments of structure (I) or (II) should be very different. Therefore we tried Stark measurements. Unfortunately the Stark shifted lines which show an additional splitting due to the hyperfine structure turned out to be very weak. One reason for this finding is probably the inhomogeneity of the Stark field. Even with very low Stark fields we were not able to measure all expected Stark shifted
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TABLE 4—Continued
lines. Therefore we suspended these measurements and performed ab initio calculations instead. The ab initio calculations indeed turned out to be in line with our experimental results so far. They will be discussed in Part IV.
FIG. 2. Structure (I) of aniline–water with CS symmetry. Projection onto ac-plane and ab-plane in the principal axes system of the complex.
III. The Quadrupole Coupling Tensor of Aniline–Water After determining two possible structures of aniline–water and measuring the diagonal elements of the quadrupole coupling tensors (qct) of two isotopomers, we were able to determine one further off-diagonal element, xac, of the qct of aniline–water. Due to the Cs symmetry of the complex, the other two off-diagonal elements, xab and xbc, are zero. If one assumes that the bond characteristic around N is not affected on the substitution of 16O by its isotope 18O, then the changes in the diagonal elements of the qct of aniline–water–18O can only be due to the rotation of the principal axes system around the b-axes. Rudolph (14) calculated a rotation matrix that depends only on the rotational constants of the measured isotopomeres without regarding to structural details. Using this rotation matrix, the ambiguity of the structure determination that results in two possible structures for the complex can be avoided. So we could determine xac without regarding to structural details. The result is given in Table 2. The error of xac results from the errors of the rotational constants. After determining the qct of aniline–water we were interested in the effect of the hydrogen bond on the original qct of aniline. Therefore we rotated the qct of aniline–water into the principal inertia axes system of aniline and compared the obtained qct to the experimental values. The diagonal elements of the qct of
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FIG. 4.
MP2-fit parameters a, r, b of the hydrogen bond of aniline–water.
axes systems of the two structures into the principal axes system of aniline is not possible. The principal axes systems of the two structures are too similar. IV. Ab Initio Calculations
FIG. 3. Structure (II) of aniline–water with CS symmetry. Projection onto ac-plane and ab-plane in the principal axes system of the complex.
aniline have been determined by Kleibo¨hmer and Sutter (15). The qcts are given in Table 5. One can see that the qct of aniline is reproduced quite well. Deviations between experimental and calculated quadrupole coupling elements range between 0.13 and 0.33 MHz or 5.2% and 10.8%. The largest deviation is along the c axis. This seems sensible since the free electron pair of nitrogen lies mostly along the c axis. This electron pair is involved in the hydrogen bond of aniline–water. Therefore changes in the electronic distribution should have their maximum in this region. The calculated values are smaller than the experimental ones. Nevertheless the differences are not as big as one could expect. A comparison of aniline–water with other hydrogen bonded aniline complexes, e.g., aniline–methanol (16), will be presented in subsequent papers. Distinguishing between structure (I) and (II) of aniline– water by rotating the qct of aniline–water from the principal TABLE 5 Calculated and Measured Quadrupole Coupling Tensors of Aniline and Aniline–Water
We performed ab initio calculations on aniline–water in order to get more information about the potential energy surface of the complex. Therefore we carried out single-point calculations on different structures of aniline–water. We again assumed a linear hydrogen bond and that the structures of the monomers were unchanged on complexation. Calculations carried out on other complexes, like tetrahydropyran–water (17) or equatorial piperidine–water (18), showed these assumptions to be in agreement with theoretical data. Starting from the two experimental structures of aniline– water, we altered the bond angle between the hydrogen bond and the aniline ring, a, the hydrogen bond length, r, and also the angle between the mirror plane of the complex in the experimental structures and the free water proton, b, as depicted in Fig. 4. For each of these structures we calculated the stabilization energies and optimized the bonding parameters for structure (I) and (II) by least-squares fits of a two-dimensional Taylor expansion taking into account terms up to second order. Stabilization energies, EStab, were calculated using the full counterpoise procedure in order to reduce basis set superposition errors (BSSE) (19). Schu¨tz et al. (20) performed calculations on phenol–water using 6-31G(d,p) and 6-31111G(d,p) basis sets. They found that for phenol–water and the 6-31G(d,p) or 6-311G11(d,p) basis sets the contributions of BSSE and MP2 correlation corrections canceled fortuitously rather closely so that HFcalculations yielded quite reasonable results for stabilization energies. For aniline–water we decided to make calculations on HF- and MP2-level. To estimate the influence of diffuse functions, we also carried out calculations using a basis set of Spackman (21) which is still of a moderate size but includes diffuse functions. This basis set was used very successfully in calculating stabilization energies of argon-van der Waals complexes (22, 23). It yielded on MP2-level a stabilization energy for structure (I) which was only 7.3% less than with 6-31G(d,p). Since this calculation needed quite more computer time than calculations with 6-31G(d,p) we finally decided to use the 6-31G(d,p) basis set. All calculations were done with Gaussian 92 (24).
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TABLE 6 MP2/6-31G(d,p)-Optimized and Experimental Structures of Aniline and Water
The stabilization energies in dependence to a, b, r are given in Table 8. Within 5° deviation between the free water proton and the mirror plane the stabilization energy does not change within the given precision for structure (I) at MP2-level, at HF-level it changes by 0.006 kcal mol21. This indicates that the structure is rather floppy and large amplitude motions are probably present. However, except rather large centrifugal distortion constants, no detailed information could be extracted from the experimental data. For structure (II) the optimization was done the same way. The least-squares fits yielded the optimized parameters a 5 104.46(46)° and r 5 3.175(75) Å on MP2-level, see also Table 7. The stabilization energies in dependence of a, b, r are given in Table 8. b shows a flat maximum in the potential energy from b 5 150° to b 5 210°. b reaches its minimum in stabilization energy at 0(18)°. So optimizing structure (II) results in structure (I). The same result was obtained at HFlevel. We yielded the optimized parameters a 5 107.46(96)°, r 5 3.225(65) Å, b 5 0(18)°. Our ab initio calculations are also confirmed by calculations done by Gerhards (25) on HF-level with the basis set 6-31G**. Also in this case the optimized structure of aniline–water corresponds to structure (I) even if a structure corresponding to structure (II) was used as a starting point. DISCUSSION
We optimized the structures of aniline and water at the MP2/6-31G(d,p) level before calculating structures of the complex using CS symmetry for aniline and C2v symmetry for water. The parameters of the optimized and experimental structures are given in Table 6. Numbering of atoms follows Fig. 2 and 3. One can see that for aniline the deviation between experimental and optimized parameters is at worst 0.3%. Only the bond length and bond angle of the amino protons differ far more, by 8.3% or 28.1%, respectively. This deviation might be due to the experimental data since Lister et al. report that these values were only poorly determined (8). For water the deviation between experimental and optimized values is only 0.8% which is also very good. In order to optimize the structures (I) and (II), we started from structures consisting of the optimized structures of aniline and water connected by the bond parameters, a, b, r. First we optimized a, then r always assuming CS symmetry for the complex. Then the energy dependence for deviation from CS symmetry was examined by varying b. The least-squares fits yielded the optimized parameters a 5 110.77(1)°, r 5 3.155(65) Å, b 5 0.0(40)° on MP2-level, see also Tale 7. At HF-level we derived the optimized parameters a 5 113.05(80)°, r 5 3.206(64) Å, b 5 0.0(42)°. The resulting stabilization energies show a very shallow potential for the torsion of the free water proton around the hydrogen-bond axis.
We analyzed the rotational spectra of aniline–water and aniline–water–18O. We assumed that the aniline–water complex is formed by aniline acting as a proton acceptor and water acting as a proton donor. This assumption is based on the chemical behavior of aniline and water. We then derived two different structures for the complex. In structure (I) the free water proton is directed toward the aniline ring, in structure (II)
TABLE 7 BSSE-Corrected HF- and MP2-Stabilization Energies for MP2-Optimized Structures (I) and (II), Experimental, HF- and MP2-Optimized Bonding Parameters a, r, b
a): stabilization energies in kcal mol21 b): r(N-O) in Å, a and b in degrees c): b 5 0° after optimization, this means that optimization of b of structure (II) results in a structure analogue to structure (I). Stabilization energies of structure (II) with b 5 180° are E(HF) 5 23.514 kcal mol21 and E(MP2) 5 24.267 kcal mol21. d): calculated on MP2/6-31G** level
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TABLE 8 MP2/6-31G(d,p)-Stabilization Energies of Aniline–Water (I) and (II) in Dependence of Parameters a, r, b
Note. Stabilization energy is in kcal mol21, a, b in degrees, r in Å
it is bent away from it. CS symmetry is assumed for this complex due to experimental findings. The planar moments of aniline and aniline–water indicate CS symmetry. Furthermore b-type transitions which would be possible without CS symmetry could not be observed. We could not distinguish between structure (I) and (II) by measuring the rotational spectra of different isotopomers. Measurements on 13C isotopomers of aniline in natural abundance
were not possible due to the low intensities of the transitions. Measurements on deuterated aniline and deuterated water were not successful because of fast isotopic exchange with normal water. Stark measurements which would be sensible due to the different dipole moments of the two proposed structures were not successful. The Stark shifted lines became very weak due to a splitting into a large number of M components and to an inhomogenous Stark field. It was also not possible to distinguish between structure (I) and (II) by rotation of the qct since the principal axes systems of the two structures are quite similar. We further tried to obtain information on the complex by ab initio calculations. We calculated different structures in order to get some points on the potential energy surface. When varying the angle between the hydrogen bond and the CS plane of aniline structure (II) with optimized parameters r and a the free water proton is finally found at a position corresponding to structure (I). Also these results let us prefer structure (I). All experimental and theoretical results favor structure (I). This seems sensible since the free water proton which is positively charged can form a very weak bond with the negatively charged aromatic system of aniline. Supposedly several isomers of the complex will be possible. Due to low temperatures in the molecular beam we could only observe the lowest energy form. When comparing the derived structural data of aniline–water to other N-containing water complexes, like trimethylamine– water (3) or ammonia–water (4), the results are in agreement with the chemical behavior of the N containing subunits. For example, the NO bond length, which can be considered to be a function of the bond energy, changes from 2.881 Å (trimethylamine–water) to 2.983 Å (ammonia–water) and to 3.082 Å (aniline–water). In trimethylamine–water the electron density of N is increased if compared to ammonia–water due to the positive inductive effect of the methyl groups. As a consequence the hydrogen bond is tighter and shorter than in ammonia–water. In aniline–water the electron density of N is decreased due to a positive mesomeric effect of the amino group. Electrons are pushed into the phenyl ring system. As a consequence the hydrogen bond is weaker and longer than in ammonia–water. The aniline–water complex has been treated as a rigid rotor within this paper. No indications for large amplitude motions like tunneling splittings were found. However, it is clear from the ab initio calculations that large amplitude motions should be present. Therefore the accuracy of our results may be overestimated in some cases.
ACKNOWLEDGMENTS We thank the members of the Kiel microwave group for help and discussion, the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie, and the Land Schleswig-Holstein for funds.
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THE ANILINE–WATER COMPLEX
REFERENCES 1. M. Gerhards, M. Schmitt, K. Kleinermanns, and W. Stahl, J. Chem. Phys. 104, 967 (1996). 2. E. Zwart, J. J. ter Meulen, W. L. Meerts, and L. H. Coudert, J. Mol. Spectrosc. 147, 27 (1991). 3. M. J. Tubergen and R. L. Kuczkowski, J. Am. Chem. Soc. 115, 9263 (1993). 4. T. Herbine and T. R. Dyke, J. Chem. Phys. 83, 3768 (1985). 5. W. J. Perllmann (Ed.), ‘‘Taschenbuch der Chemie,’’ 2. ed., VEB Deutscher Verlag der Wissenschaften, Berlin, 1959. 6. U. Andresen, H. Dreizler, J. -U. Grabow, and W. Stahl, Rev. Sci. Instrum. 61, 3694 (1990). 7. J. -U. Grabow and W. Stahl, Z. Naturforsch. 45a, 1043 (1990). 8. D. G. Lister, J. K. Tyler, J. H. Høg, and N. W. Larsen, J. Mol. Struct. 23, 253 (1974). 9. H. Taft and B. P. Dailey, J. Chem. Phys. 51, 1002 (1969). 10. M. Lichtenstein, V. E. Derr, and J. J. Gallagher, J. Mol. Spectrosc. 20, 391 (1966). 11. B. P. van Eijck, J. Mol. Spectrosc. 53, 246 (1974). 12. J.-U. Grabow, N. Heineking, and W. Stahl, J. Mol. Spectrosc. 152, 168 (1992).
13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
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J. Kraitchman, Am. J. Phys. 21, 17 (1953). H. D. Rudolph, J. Mol. Spectrosc. 89, 430 (1981). B. Kleibo¨hmer and D. H. Sutter, Z. Naturforsch. 43a, 561 (1988). M. Ha¨ckel and W. Stahl, submitted for publication. U. Spoerel, W. Stahl, W. Caminati, P. Favero, submitted for publication. U. Spoerel and W. Stahl, submitted for publication. F. Boys and F. Bernardi, Mol. Phys, 19, 553 (1970). M. Schu¨tz, T. Bu¨rgi, S. Leutwyler, and T. Fischer, J. Chem. Phys. 98, 3763 (1993). M. A. Spackman, J. Phys. Chem. 93, 7594 (1989). E. Kraka, D. Cremer, U. Spoerel, I. Merke, W. Stahl, and H. Dreizler, J. Phys. Chem. 99, 12466 (1995). U. Spoerel, H. Dreizler, W. Stahl, E. Kraka, and D. Cremer, J. Phys. Chem. 100, 14298 (1996). M. J. Frisch, G. W. Trucks, M. Head-Gordon, P. M. W. Gill, M. W. Wong, J. B. Foresman, B. G. Johnson, H. B. Schlegel, M. A. Robb, E. S. Replogle, R. Gomperts, J. L. Andres, K. Raghavachari, J. S. Binkley, C. Gonzalez, R. L. Martin, D. J. Fox, D. J. Defrees, J. Baker, J. J. P. Stewart, and J. A. ‘‘Gaussian 92, Revision C,’’ Gaussian, Inc., Pittsburgh PA, 1992. M. Gerhards, private communication (1996).
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MS987600
The Aniline—Water Complex Rotational Spectrum and Molecular Structure Ute Spoerel and Wolfgang Stahl Institut fu¨r Physikalische Chemie, RWTH Aachen, Templergraben 59, D-52056 Aachen, Germany Received January 6, 1998; in revised form March 26, 1998
The rotational spectra of aniline–water and its 18O isotopomer have been studied in the microwave region between 3 and 26.5 GHz using a pulsed molecular beam FT microwave spectrometer. The spectra were described in terms of a centrifugally distorted asymmetric rotor. Assuming a linear hydrogen N...H–O bond and that the water molecule was located in the symmetry plane of aniline, two structures turned out to be possible. In structure (I) the free water proton is directed toward the aniline ring. In structure (II) the proton is bent away from it. Ab initio calculations indicate that only structure (I) is supported by the experimental results. © 1998 Academic Press INTRODUCTION
Water is the most important inorganic solvent. In the solution process it often undergoes chemical reactions. Among these are acid– base reactions in which water either serves as proton donor or as a proton acceptor. In solution these reactions yield hydrated ions. If no hydration takes place, e.g., in the gas phase, the ion pair does not separate. Instead, a hydrogen-bonded complex is formed. An example for such a complex, in which water serves as a proton acceptor, is phenol– water (1). In the water dimer (2) one water molecule acts as a proton donor, the other one as an acceptor. Trimethylamine– water (3) and ammonia–water (4) are examples where water is a proton donor. Hydrogen-bonded complexes are also studied with REMPI experiments to obtain information on the electronically excited state. For this purpose a chromophore, like an aromatic ring system, should be present in the complex. Therefore we decided to investigate the aniline–water complex as a model system and to provide rotational constants for the ground state. In addition, we were interested in studying the change in the nuclear quadrupole coupling constants of the 14N nucleus when aniline undergoes complexation. These may also be compared to the constants for the liquid phase, as soon as these become available. Thereby information on the molecular environment of aniline in an aqueous solution may be obtained. In aqueous solutions aniline reacts as a base (5) and anilinium ions can be formed. The aniline–water complex may be considered as the first step of this reaction. EXPERIMENT
All spectra were taken in the range from 3 to 26.5 GHz using the molecular beam (MB) Fourier transform microwave
(FTMW) spectrometer (6, 7) at the University of Kiel. Aniline was kept in a stainless steel container upstream from the nozzle and heated to about 35°C. Helium containing 0.35% water was passed over the aniline sample at a pressure of 100 kPa. For the measurements on aniline–water–18O a mixture of about 50% water and 50% water–18O was used. RESULTS
I. Spectral Analysis We started our investigation by predicting rotational constants for aniline–water using the structure of ammonia–water as a model. In the structure of ammonia–water the distance between oxygen and nitrogen is 2.983 Å and the linear N...H–O bond is bent away from the C3-axis of ammonia by 23.1 (2)° (4). We assumed that the structures of aniline, C6H5NH2, and of water remain unchanged upon complexation. The water molecule was assumed to be located in the ac-plane of aniline. Further, a linear N . . . H–O bond with a length of 3 Å was assumed to form an angle of 110° with the ring plane of aniline. Using the aniline structure given by Lister et al. (8) and the water structure of Taft and Dailey (9), we calculated the rotational constants to be A 5 3492 MHz, B 5 1035 MHz, and C 5 969 MHz. Transforming the dipole moments of aniline (8) and water (10) into the inertia axes system of aniline–water, we predicted an a- and c-type spectrum for the complex. While scanning for some predicted lines we found several a-type branches. All lines were split into multiplets due to 14 N-quadrupole coupling. A least-squares fit yielded improved rotational B and C constants, which in turn enabled us to find more a- and also c-type lines. Analysis of the very narrow 14 N-hyperfine structure (hfs) confirmed our rotational assignment. In Fig. 1 a quite well resolved hfs of the 414–313
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large amplitude motion of the amino group. Under molecular beam conditions usually only the lower state can be observed because the upper state is too high to be populated. In the case of aniline–water we did not observe any tunneling splitting. We believe that the inversion is completely quenched in the complex. However, even if inversion could take place, it would be not observable for the same reasons as in the case of the monomer. II. Molecular Structure
FIG. 1. JKaKc 5 414 2 313 transition of aniline–water. Power spectrum, sample interval 100 ns, 8 k Fourier transformation, average of 545 experimental cycles, polarization frequency 8651.61 MHz.
transition is shown as an example. In some cases the hfs could only be partially resolved. All measured lines are complied in Table 1. Some lines were additionally broadened or showed very narrow splittings in the order of 1–2 kHz. The origin of this effect is not quite clear and it was therefore ignored. Instead these splittings were averaged out by decreasing the resolution element of the Fourier transform. As a consequence the standard deviation of the fit increased to about 4 kHz, which is slightly higher than the experimental accuracy. The rotational constants, the quartic centrifugal distortion constants according to the reduction of van Eijck (11), and the diagonal elements of the quadrupole coupling tensor are given in Table 2. The corresponding correlation matrix is given in Table 3. By comparing the quadrupole coupling elements of aniline–water and its 18O isotopomer, we were also able to determined the xac off-diagonal element. Its determination will be described in Part III. The rotational constants were in agreement with two different structures of the complex. In both cases we assumed a linear N . . . H–O hydrogen bond and that the water molecule was located in the symmetry plane of aniline. In structure (I) the free water proton is directed toward the aniline ring; in structure (II) the proton is bent away from it. For these two structures we predicted two sets of rotational constants for the 18O isotopomer. These were A 5 3117.2 MHz, B 5 1057.7 MHz, and C 5 1023.7 MHz for structure (I), and A 5 3111.7 MHz, B 5 1059.4 MHz, and C 5 1025.9 MHz for structure (II). The lines of the 18O isotopomer were found to be very close to the frequencies predicted with the rotational constants of structure (I). They are given in Table 1 and were analyzed in the same way as described before. The spectroscopic constants are also presented in Table 2, the corresponding correlation matrix in Table 3. The aniline monomer shows an inversion splitting due to the
Our spectral analysis provided us with three rotational constants, five quartic distortion constants, and the diagonal elements of the quadrupole coupling tensor for aniline–water and its 18O isotopomer. We used the rotational constants to determine the position of the water molecule with respect to the aniline monomer. In order to obtain the structure of aniline– water we assumed the structures of aniline and water to remain unchanged in the complex. We used the structure for Lister et al. (8) for aniline and the structure of Taft et al. (9) for water. We further assumed a linear N . . . H–O bond and that the water molecule was located within the mirror plane of aniline. The existence of such a mirror plane results from the consideration of the planar moments of aniline and its water complex. The difference of the planar moments DP bb 5 P bb~aniline 2 water! 2P bb~aniline!
[1]
with P bb 5
1 ~I 1 I cc 2 I bb! 2 aa
[2]
is only 0.69 uÅ2 for aniline–water and 0.72 uÅ2 for aniline– water–18O, respectively. This strongly indicates that the equilibrium structure of the complex has an ac-symmetry plane with the water molecule located within this plane. This is also supported by the fact that no mb-type lines could be observed. The ‚Pbb of 0.7 uÅ2 may be attributed to an out-of-plane vibration of the free water proton as well as of the whole water molecule. So far we have no experimental evidence to support the assumption of a linear hydrogen bond. To gain experimental insight in this problem, it would be necessary to measure various isotopomers of aniline–water. Unfortunately this was not possible. We will refer to this point in the following parts of this article. Fitting the hydrogen bond length and the angle between the hydrogen bond and the ring plane of aniline to the rotational constants of aniline–water and aniline–water–18O, we determined two possible structures for the complex. In structure (I) the hydrogen bond length between N and O is 3.028 Å, and the angle between the hydrogen bond and the aniline ring is 101.31°. The free water proton is directed toward the aniline ring. In structure (II) the hydrogen bond length is 3.035 Å, and
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TABLE 1 Measured Transitions of Aniline–Water and Aniline–Water–18O
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TABLE 1—Continued
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TABLE 1—Continued
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TABLE 1—Continued
the angle between the hydrogen bond and the aniline ring is 99.12°. In this case the free water proton is directed away from the aniline ring. Structure (I) reproduces the rotational constants of the measured aniline–water isotopomers within 0.5 MHz for aniline-water and 0.3 MHz for aniline–water–18O. On the other hand, structure (II) reproduces the rotational constants for aniline–water only within 2.8 MHz and for aniline–water–18O only within 3.6 MHz. Therefore we favor TABLE 2 Rotational, Centrifugal Distortion (van Eijck), and Quadrupole Coupling Constants of Aniline–Water and Aniline–Water–18O, Ir Representation
structure (I), although the isotopic shifts cannot definitely rule out structure (II). The atomic coordinates of both structures in their principal axes systems and the calculated rotational constants for aniline–water and aniline–water–18O are given in Table 4. Figures 2 and 3 show projections of structure (I) and (II) onto their acand ab-planes. We were now also able to calculate the rs-coordinates of oxygen in the principal axes system of aniline–water. Using Kraitchman’s equations (13) we calculated the values given in Table 4. With this method it is only possible to determine the TABLE 3 Correlation Matrices of Aniline–Water and Aniline–Water–18O
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TABLE 4 Structure (I) and (II) of Aniline–Water
absolute value of the coordinates. Comparing the rs-coordinates of oxygen to the fitted coordinates of structure (I) or (II) in the principal axes system of aniline–water, one can see that the rs-coordinates resemble the coordinates of structure (I). Since the deviation between the ro- and the rs-coordinates for structure (I) are 0.005 Å for the a- and 0.001 Å for the c-coordinate, this gives a hint towards the quality of the fitted ro-structure. Measurements on deuterated isotopomers would help much in deciding whether the free water proton is directed away or toward the aniline ring. Unfortunately aniline undergoes a fast exchange of its amino protons with D2O. Measurements on deuterated aniline and D2O were also not successful because an exchange between the deuteriums and the protons of water traces present in the spectrometer and in the carrier gas
evolved. Spectra of aniline–water with 13C–aniline in natural abundance could not be observed because of their very low intensity. In principle it should also be possible to distinguish between structure (I) and (II) by their different dipole moments. In structure (I) the dipole moment of the water molecule is directed almost along the a-axis, in structure (II) almost along the c-axis. The dipole moment of aniline does not change significantly between the two structures. Despite small induced dipole moments the dipole moments of structure (I) or (II) should be very different. Therefore we tried Stark measurements. Unfortunately the Stark shifted lines which show an additional splitting due to the hyperfine structure turned out to be very weak. One reason for this finding is probably the inhomogeneity of the Stark field. Even with very low Stark fields we were not able to measure all expected Stark shifted
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TABLE 4—Continued
lines. Therefore we suspended these measurements and performed ab initio calculations instead. The ab initio calculations indeed turned out to be in line with our experimental results so far. They will be discussed in Part IV.
FIG. 2. Structure (I) of aniline–water with CS symmetry. Projection onto ac-plane and ab-plane in the principal axes system of the complex.
III. The Quadrupole Coupling Tensor of Aniline–Water After determining two possible structures of aniline–water and measuring the diagonal elements of the quadrupole coupling tensors (qct) of two isotopomers, we were able to determine one further off-diagonal element, xac, of the qct of aniline–water. Due to the Cs symmetry of the complex, the other two off-diagonal elements, xab and xbc, are zero. If one assumes that the bond characteristic around N is not affected on the substitution of 16O by its isotope 18O, then the changes in the diagonal elements of the qct of aniline–water–18O can only be due to the rotation of the principal axes system around the b-axes. Rudolph (14) calculated a rotation matrix that depends only on the rotational constants of the measured isotopomeres without regarding to structural details. Using this rotation matrix, the ambiguity of the structure determination that results in two possible structures for the complex can be avoided. So we could determine xac without regarding to structural details. The result is given in Table 2. The error of xac results from the errors of the rotational constants. After determining the qct of aniline–water we were interested in the effect of the hydrogen bond on the original qct of aniline. Therefore we rotated the qct of aniline–water into the principal inertia axes system of aniline and compared the obtained qct to the experimental values. The diagonal elements of the qct of
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FIG. 4.
MP2-fit parameters a, r, b of the hydrogen bond of aniline–water.
axes systems of the two structures into the principal axes system of aniline is not possible. The principal axes systems of the two structures are too similar. IV. Ab Initio Calculations
FIG. 3. Structure (II) of aniline–water with CS symmetry. Projection onto ac-plane and ab-plane in the principal axes system of the complex.
aniline have been determined by Kleibo¨hmer and Sutter (15). The qcts are given in Table 5. One can see that the qct of aniline is reproduced quite well. Deviations between experimental and calculated quadrupole coupling elements range between 0.13 and 0.33 MHz or 5.2% and 10.8%. The largest deviation is along the c axis. This seems sensible since the free electron pair of nitrogen lies mostly along the c axis. This electron pair is involved in the hydrogen bond of aniline–water. Therefore changes in the electronic distribution should have their maximum in this region. The calculated values are smaller than the experimental ones. Nevertheless the differences are not as big as one could expect. A comparison of aniline–water with other hydrogen bonded aniline complexes, e.g., aniline–methanol (16), will be presented in subsequent papers. Distinguishing between structure (I) and (II) of aniline– water by rotating the qct of aniline–water from the principal TABLE 5 Calculated and Measured Quadrupole Coupling Tensors of Aniline and Aniline–Water
We performed ab initio calculations on aniline–water in order to get more information about the potential energy surface of the complex. Therefore we carried out single-point calculations on different structures of aniline–water. We again assumed a linear hydrogen bond and that the structures of the monomers were unchanged on complexation. Calculations carried out on other complexes, like tetrahydropyran–water (17) or equatorial piperidine–water (18), showed these assumptions to be in agreement with theoretical data. Starting from the two experimental structures of aniline– water, we altered the bond angle between the hydrogen bond and the aniline ring, a, the hydrogen bond length, r, and also the angle between the mirror plane of the complex in the experimental structures and the free water proton, b, as depicted in Fig. 4. For each of these structures we calculated the stabilization energies and optimized the bonding parameters for structure (I) and (II) by least-squares fits of a two-dimensional Taylor expansion taking into account terms up to second order. Stabilization energies, EStab, were calculated using the full counterpoise procedure in order to reduce basis set superposition errors (BSSE) (19). Schu¨tz et al. (20) performed calculations on phenol–water using 6-31G(d,p) and 6-31111G(d,p) basis sets. They found that for phenol–water and the 6-31G(d,p) or 6-311G11(d,p) basis sets the contributions of BSSE and MP2 correlation corrections canceled fortuitously rather closely so that HFcalculations yielded quite reasonable results for stabilization energies. For aniline–water we decided to make calculations on HF- and MP2-level. To estimate the influence of diffuse functions, we also carried out calculations using a basis set of Spackman (21) which is still of a moderate size but includes diffuse functions. This basis set was used very successfully in calculating stabilization energies of argon-van der Waals complexes (22, 23). It yielded on MP2-level a stabilization energy for structure (I) which was only 7.3% less than with 6-31G(d,p). Since this calculation needed quite more computer time than calculations with 6-31G(d,p) we finally decided to use the 6-31G(d,p) basis set. All calculations were done with Gaussian 92 (24).
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TABLE 6 MP2/6-31G(d,p)-Optimized and Experimental Structures of Aniline and Water
The stabilization energies in dependence to a, b, r are given in Table 8. Within 5° deviation between the free water proton and the mirror plane the stabilization energy does not change within the given precision for structure (I) at MP2-level, at HF-level it changes by 0.006 kcal mol21. This indicates that the structure is rather floppy and large amplitude motions are probably present. However, except rather large centrifugal distortion constants, no detailed information could be extracted from the experimental data. For structure (II) the optimization was done the same way. The least-squares fits yielded the optimized parameters a 5 104.46(46)° and r 5 3.175(75) Å on MP2-level, see also Table 7. The stabilization energies in dependence of a, b, r are given in Table 8. b shows a flat maximum in the potential energy from b 5 150° to b 5 210°. b reaches its minimum in stabilization energy at 0(18)°. So optimizing structure (II) results in structure (I). The same result was obtained at HFlevel. We yielded the optimized parameters a 5 107.46(96)°, r 5 3.225(65) Å, b 5 0(18)°. Our ab initio calculations are also confirmed by calculations done by Gerhards (25) on HF-level with the basis set 6-31G**. Also in this case the optimized structure of aniline–water corresponds to structure (I) even if a structure corresponding to structure (II) was used as a starting point. DISCUSSION
We optimized the structures of aniline and water at the MP2/6-31G(d,p) level before calculating structures of the complex using CS symmetry for aniline and C2v symmetry for water. The parameters of the optimized and experimental structures are given in Table 6. Numbering of atoms follows Fig. 2 and 3. One can see that for aniline the deviation between experimental and optimized parameters is at worst 0.3%. Only the bond length and bond angle of the amino protons differ far more, by 8.3% or 28.1%, respectively. This deviation might be due to the experimental data since Lister et al. report that these values were only poorly determined (8). For water the deviation between experimental and optimized values is only 0.8% which is also very good. In order to optimize the structures (I) and (II), we started from structures consisting of the optimized structures of aniline and water connected by the bond parameters, a, b, r. First we optimized a, then r always assuming CS symmetry for the complex. Then the energy dependence for deviation from CS symmetry was examined by varying b. The least-squares fits yielded the optimized parameters a 5 110.77(1)°, r 5 3.155(65) Å, b 5 0.0(40)° on MP2-level, see also Tale 7. At HF-level we derived the optimized parameters a 5 113.05(80)°, r 5 3.206(64) Å, b 5 0.0(42)°. The resulting stabilization energies show a very shallow potential for the torsion of the free water proton around the hydrogen-bond axis.
We analyzed the rotational spectra of aniline–water and aniline–water–18O. We assumed that the aniline–water complex is formed by aniline acting as a proton acceptor and water acting as a proton donor. This assumption is based on the chemical behavior of aniline and water. We then derived two different structures for the complex. In structure (I) the free water proton is directed toward the aniline ring, in structure (II)
TABLE 7 BSSE-Corrected HF- and MP2-Stabilization Energies for MP2-Optimized Structures (I) and (II), Experimental, HF- and MP2-Optimized Bonding Parameters a, r, b
a): stabilization energies in kcal mol21 b): r(N-O) in Å, a and b in degrees c): b 5 0° after optimization, this means that optimization of b of structure (II) results in a structure analogue to structure (I). Stabilization energies of structure (II) with b 5 180° are E(HF) 5 23.514 kcal mol21 and E(MP2) 5 24.267 kcal mol21. d): calculated on MP2/6-31G** level
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TABLE 8 MP2/6-31G(d,p)-Stabilization Energies of Aniline–Water (I) and (II) in Dependence of Parameters a, r, b
Note. Stabilization energy is in kcal mol21, a, b in degrees, r in Å
it is bent away from it. CS symmetry is assumed for this complex due to experimental findings. The planar moments of aniline and aniline–water indicate CS symmetry. Furthermore b-type transitions which would be possible without CS symmetry could not be observed. We could not distinguish between structure (I) and (II) by measuring the rotational spectra of different isotopomers. Measurements on 13C isotopomers of aniline in natural abundance
were not possible due to the low intensities of the transitions. Measurements on deuterated aniline and deuterated water were not successful because of fast isotopic exchange with normal water. Stark measurements which would be sensible due to the different dipole moments of the two proposed structures were not successful. The Stark shifted lines became very weak due to a splitting into a large number of M components and to an inhomogenous Stark field. It was also not possible to distinguish between structure (I) and (II) by rotation of the qct since the principal axes systems of the two structures are quite similar. We further tried to obtain information on the complex by ab initio calculations. We calculated different structures in order to get some points on the potential energy surface. When varying the angle between the hydrogen bond and the CS plane of aniline structure (II) with optimized parameters r and a the free water proton is finally found at a position corresponding to structure (I). Also these results let us prefer structure (I). All experimental and theoretical results favor structure (I). This seems sensible since the free water proton which is positively charged can form a very weak bond with the negatively charged aromatic system of aniline. Supposedly several isomers of the complex will be possible. Due to low temperatures in the molecular beam we could only observe the lowest energy form. When comparing the derived structural data of aniline–water to other N-containing water complexes, like trimethylamine– water (3) or ammonia–water (4), the results are in agreement with the chemical behavior of the N containing subunits. For example, the NO bond length, which can be considered to be a function of the bond energy, changes from 2.881 Å (trimethylamine–water) to 2.983 Å (ammonia–water) and to 3.082 Å (aniline–water). In trimethylamine–water the electron density of N is increased if compared to ammonia–water due to the positive inductive effect of the methyl groups. As a consequence the hydrogen bond is tighter and shorter than in ammonia–water. In aniline–water the electron density of N is decreased due to a positive mesomeric effect of the amino group. Electrons are pushed into the phenyl ring system. As a consequence the hydrogen bond is weaker and longer than in ammonia–water. The aniline–water complex has been treated as a rigid rotor within this paper. No indications for large amplitude motions like tunneling splittings were found. However, it is clear from the ab initio calculations that large amplitude motions should be present. Therefore the accuracy of our results may be overestimated in some cases.
ACKNOWLEDGMENTS We thank the members of the Kiel microwave group for help and discussion, the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie, and the Land Schleswig-Holstein for funds.
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