06
The Journal of Physical Chemistry, Vol. 82, No. 1, 1978
Sat0 et al.
Infrared and Ultraviolet Studies of Aromatic Amines Adsorbed on Aluminum Halides. The Presence of Two Types of Adsorbed Species Hlroyasu Sato, * Yoshihuml Kusumoto, Shin-ichl Arase, Masakaru Suenaga, and Shoko Kammura College of Liberal Arts, Kagoshima University, Korimoto, Kagoshima 890, Japan (Received April 25, 1977) Publication costs assisted by Kagoshima University
IR and UV studies of p-toluidine, p-chloroaniline, 1-naphthylamine,and 2-naphthylamineadsorbed on aluminum halides (chloride or bromide) revealed that the adsorption interaction occurred on the amino group. IR spectra of these systems were mutually alike (type ii) and similar to that of aniline adsorbed on aluminum bromide. For p-toluidine and p-chloroaniline adsorbed on aluminum chloride, the presence of excess amine changed the total adsorption pattern as indicated by the change of the IR spectrum into another type (type i), which was similar to that of aniline adsorbed on aluminum chloride. Two types of IR spectra were distinguishable by positions and shapes of the bands ascribable to N-H stretching, NH2 bending, and C-N stretching modes. The positions of these bands (cm-l) for type i were 3230-3290 (two sharp bands of similar intensity), 1575-1580, and around 1140 (a broad band). Those for type ii were 3170-3205 (a broad band with a shoulder), 1557-1565, and 1175-1200 (a sharp band). The two types of IR spectra indicate the presence of two types of adsorbed species, for which models of adsorption are presented. The IR spectrum of type ii is associated with the stronger adsorption, since it was observed for pairs with a strong donor-strong acceptor. A larger UV band shift was found for the system with an IR spectrum of type ii than the system with that of type i, when both types of IR spectra were found for the same amine-aluminum halide pair. The IR spectrum of type ii has a larger frequency shift than type i except for the C-N stretching mode. The IR spectrum of type i is to be associated with the weaker adsorption. The larger frequency shift of the C-N stretching mode observed in this case can be interpreted by assuming the formation of a polarized C-N bond on adsorption.
Introduction Application of UV and IR spectroscopies in the study of molecules adsorbed on a solid surface is extensive.l Anhydrous aluminum halide has been an object of much interest because of its Lewis-acid nature. Its interaction with aromatic hydrocarbons was studied by UV and IR spectroscopies, showing the formation of u c ~ m p l e x e and s~~~ cation radicals4 on the surface. UV study of aniline (I) and N-methylaniline adsorbed on evaporated films of aluminum chloride (A) showed that the bands shifted to positions closer to those of toluene and ethylbenzene, indicating electron donation from the lone pair on the nitrogen atom in these molecules to the electron-accepting center on the solid.5p6 IR study of I adsorbed on evaporated films of A, aluminum bromide (B), iodide (C), and on gallium chloride (D)7revealed that only the bands related to amino group showed large shifts on adsorption. IR spectra of IB (I adsorbed on B), IC, and ID were mutually alike, and the 1272-cm-l band of I, assigned to the C-N stretching mode (coupled with the NH2 deformation mode)F9 shifted to around 1200 cm-l. IA gave a spectrum with a distinctly different nature, the corresponding band appearing at 1140 cm-l as a very intense and broad band. In this paper, IR and UV studies were made for ptoluidine (11), p-chloroaniline (111), 1-naphthylamine (IV), and 2-naphthylamine (V) adsorbed on evaporated films of A and B in order to obtain information on the structure of adsorbed molecules in relation to the electron-donating ability of these aromatic amines. Experimental Section A (or B) was prepared from aluminum and silver chloride (or bromide) in vacuo, and stored in ampoules with break seals. 11, 111, IV, and V were purified by vacuum sublimation or recrystallization. The cell assembly was provided with a cell with NaCl or KBr window plates, to which were connected two ampoules with break seals containing A (or B) and amine, respectively. After the cell 0022-365417812082-0066$01.OOlO
assembly was degassed thoroughly (more than 2 h at Torr), it was sealed off. Then A (or B) was sublimed onto window plates by heating the ampoule containing A (or B) over an open flame. Amine was then introduced by the same technique. These two procedures were sometimes repeated to sublime amine or aluminum halide in small portions onto window plates, IR and UV measurements being made each time. Window plates were kept at room temperature during adsorption and measurement. IR and UV measurements were made by a JASCO IRA-1 and a Shimadzu MPS-50L recording spectrophotometer, respectively.
Results I R and UV Spectra in the Absence of Excess Amine. Observed IR and UV spectra are shown in Figures 1and 2 with those of free amines. IR bands of free amines were classified into those associated with (a) the aromatic ring (and the substituent other than the amino group) and (b) the amino group (C-N stretching inclusive) by referring to the assignments of the bands of I8igfor I1 and 111, and by referring to the IR spectra of 1-and 2-methylnaphthalene for IV and V. The positions of bands belonging to the groups a and b are shown by vertical solid and dotted lines, respectively. It will be noted that the bands of group a suffered little change on adsorption. Especially, C-H out-of-plane bending modes (shown by asterisks) remained essentially unchanged. The bands of group b showed large shifts. IR spectra of eight adsorbed systems are mutually alike. They are characterized by an intense and broad band a t 3170-3205 cm-l, a band at 1557-1565 cm-l, and a sharp band at 1175-1200 cm-l. Such characteristics are in common with the spectrum of IB.7 The first of the bands mentioned above is undoubtedly due to the N-H stretching mode. The second is assigned to the NH2 bending (scissors) m ~ d e . ~The J ~last one can be assigned to the C-N stretching mode by the following consideration. 0 1978 American Chemical Society
The Journal of Physical Chemistty, Vol. 82, No. 1, 1978 67
Aromatic Amines Adsorbed on Aluminum Halides
? 250
300
Wavelengthlnm
250
300
34@03200
Wavelength/nm
1600
1L00 1200 1000 Wavenurnb?r/crn-l
800
Figure 1. UV and IR spectra for (11) p-toluidine (UV, evaporated film; IR, KBr disk); (IIA) p-toluidine-aluminum chloride; (IIB) p-toluidinealuminum bromide; (111) p-chloroaniline (UV, evaporated film; IR, KBr disk); (IIIA) p-chloroaniline-aluminum chloride; (IIIB) p-chloroaniline-aluminum bromide. Ordinate: Absorbance (UV) and transmittance (IR). Enlarged UV spectra are also given to show structure.
34033200
1600
1400 1200 1030 Wavenunberl cm-l
800
Figure 3. UV and IR spectra of the p-toluidine (11)-aluminum chloride (A) system. Ordinate: Absorbance (UV) and transmittance (IR). UV spectra (280-310 nm) are displaced upward relative to those (A
