J. CHEM. SOC. FARADAY TRANS.. 1994. 94361, 843-844 843 Far-IR Study of the Hydrogen-bond Vibration of Intramolecular Bonds in Substituted 2-Diethylaminomethylphenol N-Oxides, as a Function of the pK, of the Phenolic Group Bogumil Brzezinski Faculty of Chemistry,A, Mickiewicz University, Grunwaldzka 6 PL-60-780Poznari, Poland Arno Rabold and Georg Zundel Institute of Physical Chemistry, University of Munich, Theresienstr. 4 I, 0-80333Munich, Germany The intramolecular hydrogen-bond vibration in R-substituted 2-diethylaminomethylphenol N-oxides in the far- infrared region has been studied. With decreasing pK, value of the phenolic group this vibration shifts first towards higher and then towards lower wavenumbers. The proton is transferred within the OH.* -ON-: 0-0-.H+ON hydrogen bond with increasing acidity of the phenolic group. This bond is strongest for R = 4 -NO,, and the hydrogen bond vibration is very broad in this system. It has been found that the change of the position of the hydrogen bond vibration is independent of the mass of the substituents. All results in this work agree very well with results obtained for corresponding intermolecular hydrogen bonds. Recently we have studied intermolecular hydrogen bonds between aliphatic N-oxides and phenols as a function of t!ie pK, value of the phenols by FTIR'"3as well as 'Hand '.'C NMR ~pectroscopy.~ For this family of systems the OH. .ON 0--..H 'ON equilibrium could be completelq shifted from the left- to the right-hand side by altering the acidity of the phenol.If the acidity is low, only two bands are observed in the region 3200-1700 ern-.'. With increasing acidity a continuum arises which extends increasingly toward smaller wavenumbers. With the 3,4-dini trophenol (pK, 3.42) system an intense continuum is observed, but only in the region 1500-600 cm-', extending with less intensity to 1I)O cm -1.1.3 With further increasing acidity of the phenols the intensity of the continuum in the 1500-600 cni-' regicm decreases and moves back towards higher wavenumber.' .' The changes of the continuum are discussed with regard 10 the shape of the proton potential in ref. 2. Particularly inter- esting is the behaviour of the far-infrared hydrogen-bmd vibration. This is independent of changes in the mass of the phenol as well as the mass of the aliphatic N-oxides (see Fig.2 in ref. 2).2*3The band for the hydrogen-bond vibration shifts with the decreasing pK, value of the phenol first towards higher and then lower wavenumbers. which COrri:- sponds to the changes of the infrared continuum. Analogous results were obtained from 'H and I3C NMR spectros-copy.2*4 We have already studied intramolecular OH. .ON 5; O--..H+ONhydrogen by MIR and 'H and I3C NMR spectroscopies. In this paper the hydrogen bond vibta- tion of corresponding intramolecular hydrogen bonds in t be far-infrared region are studied. Experimental Ten 2-diethylaminomethylphenols and their N-oxides were synthesized following the procedures given in ref.7 and 3. respectively. The N-oxides were dissolved in chloroform -acetonitrile (3 : 1).The solvents were dried with 3 A molecular sieves. The concentration of the solutions was 0.2 mol dm . '. All preparations and transfers of solutions were carried out in a carefully dried glovebox. The spectra were measured with an FTIR Bruker IFS 1 13v spectrometer using a cell with Si windows (mean layer thich- ness 0.26 mm). A set of 400 scans was collected with a resolution of 1 cm-using an He-cooled bolometer. Results and Discussion Ten substituted 2-diethylaminomethylphenolN-oxides were studied in chloroform-acetonitrile (3 : 1) solution in the far-infrared region [Fig. l(a)-(c)]. The systems and all data obtained are summarized in Table 1.The positions of the hydrogen-bond vibrations are shown as a function of the pK, value of the parent phenol in Fig. 2. The hydrogen-bond vibration shifts toward higher wave- numbers with increasing acidity of the phenolic group. The largest shift is observed for R = 4-N02, indicating that this system has the strongest intramolecular hydrogen bond. This is in good agreement with the 'H and I3C NMR data.6 As the acidity increases further, the hydrogen-bond vibration shifts towards lower wavenumbers. For the most symmetrical system, R = 4-NO2, i.e. the system in which the fluctuation of the proton is fa~test,~ the hydrogen-bond vibration is very broad. and the continuous absorption extends down to 100 em-'. The most interesting result is that the shift of the h>.drogen-bond vibration is independent of the mass of the phenol.i.e. for all compounds the same 'reduced mass' deter- mines the position of the hydrogen-bond vibration. This demonstrates that the shift is only determined by the force constant. An analogous result was obtained for the intermo- lecular hydrogen-bond vibration and explained by the fact that the centre of gravity of the bridged atoms is far from the hydrogen-bond axk2 Therefore. a large part of the mass of the molecules is only slightly involved in the vibration. and basically only the bridged atoms are involved in the hj drogen-bond vibrations. In the case of the intramolecular bonds it is immediately clear that the mass of the substituents does not essentially influence the position of the hydrogen- bond vibration.The position of the b(N0) vibration for each compound is also given in Table 1. This band is found in the region 460- 470 cm-'. With increasing acidity it shifts first towards higher wavenumbers and after the most symmetrical system (R = NO,) it is no longer observed. 0.8:I0.71 350 300 250 200 150 100 wavenumber/cm-’ (b) 0.7‘ 350 300 250 200 150 100 wavenum ber/cm -0.7 350 300 250 200 150 100 wavenumber/cm-Fig. 1 FTIR spectra of solutions of R-substituted 2-diethylaminomethylphenol N-oxides: (a) (-) 4-But, (---) 4-F, (* * *) 4-C6H,, (-. -) 4C1; (b) (-) 4-C02C2H5, (---) 4-NO2, (a(-. -) 4-CN, (. . -) CCO,CH,, and (c) (-) 3,4,6-c1,, .) 3,4-(No212 300 250 c 1 E -3 200 I I I150 ‘ 4 6 8 10 12 PK, Fig.2 Position of the hydrogen-bond vibration (v,) as a function of the pK, of the parent phenols J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Hydrogen-bond (v,) and 4NO) vibrations (an-’)of R-substituted 2-diethylaminomethylphenol N-oxides and the pK, values of the parent phenols compound R PK,” V, WO) 1 4-But 10.20 172 461.O 2 4-F 9.81 189 462.5 3 4-C6H5 9.55 194 463.0 4 4-c1 9.37 212 464.0 5 4-C0,C2H5 8.50 221 465.5 6 7 4-C02CH, 4-CN 8.47 7.95 237 256 465.5 466.0 8 4-N02 7.15 289 468.0 9 3,4,6-C13 6.72 255 - 10 3,WNO2)2 5.42 170 - @ From ref. 9. Conclusions The hydrogen-bond vibration of substituted 2-di-ethylaminomethylphenol N-oxides observed in the far-infrared region shifts with increasing acidity of the phenolic group first towards higher and then lower wavenumbers.Thus, with increasing transfer of the proton in the OH. .ON 0-.* .H +ON hydrogen bonds, the bonds ini- tially become stronger and then again weaker. The bond is strongest in the most symmetrical system and the hydrogen bond vibration is broadened. These results are analogous to results obtained for corresponding intermolecular hydrogen bonds. The only difference is that intramolecular hydrogen bonding is strongest for the R = 4-N02 system (pK, of the parent phenol is 7.15) whereas the intermolecular bond is strongest for the 3,4-dinitrophenol system (pK, = 5.42).Our thanks are due to the Polish Ministry of National Edu-cation, the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie for providing facilities for this work. References 1 B. Brzezinski, B. Brycki, G. Zundel and Th. Keil, J. Phys. Chem., 1991,%, 8598. 2 Th.Keil, B. Brzezinski and G. Zundel, J. Phys. Chem., 1992,96, 4421. 3 B. Brzezinski, G. Schroeder, G. Zundel and Th. Keil, J. Chem. SOC.,Perkin Trans. 2, 1992,819. 4 B. Brycki, B. Brzezinski, G. Zundel and Th. Keil, Magn. Reson. Chem., 1992,30,507. 5 B. Brzezinski, J. Olejnik, G. Zundel and R. Kramer, J. Mol. Struct., 1989,212, 247. 6 B. Brzezinski, B. Brycki, H. Maciejewska-Urjasz and G. Zundel, Magn. Reson. Chem., 1993,31,642. 7 B. Brycki, B. Brzezinski, H. Maciejewska and G. Zundel, J. Mol. Struct., 1991,246, 61. 8 B. Brzezinski and G. Zundel, J. Mol. Struct., 1984,118,311. 9 G.Kortiim, W.Vogel and K. Anchussov, Dissociation Constants of Organic Acids in Aqueous Solutions, Butterworth, London, 1961. Paper 3/05929D; Received 4th October, 1993