J . Phys. Chem. 1987, 91, 3974-3977
3974
occasions, are gratefully acknowledged. We thank Stephen E. Anderson for completing the last few experimental measurements. T.D.T. thanks the University of Liverpool for providing its hospitality and the Royal Society for its support while this article
was being written. Registry No. CH3CH2F,353-36-6; CH3CH2CI,75-00-3;CH3CH,Br, 74-96-4; CH3CH21,75-03-6; fluoroethene, 75-02-5; chloroethene, 7501-4; bromoethene, 593-60-2;iodoethene, 593-66-8.
Solvent Effect on the Intramolecular Hydrogen Bond Strength and on the Isotopic Ratio v,,+/v,,+ in a Trisubstituted Mannich Base Maria Rospenkt and Th. Zeegers-Huyskens* Department of Chemistry, University of Leuven. Celestijnenlaan 200F, B- 3030 Heverlee, Belgium (Received: January 20, 1987)
The electronic (300-440 nm) and the infrared spectra (3000-700 cm-') of 2-(N,N-dodecylaminomethyl)-3,6-dichloro-4nitrophenol and its deuteriated OD analogue have been studied in 10 different organic solvents. The strength of the intramolecular NH+-O- hydrogen bond decreases with the Onsager parameter of the solvent. The v ~ ~ + . values . . ~ - ranging from 2800 to 2700 cm-' suggest that the hydrogen bond is of medium strength. The isotopic ratio VNH+/VND+ values varying from 1.322 (CC14) to 1.350(C,H4Cl,) suggest a double-minimum curve with a relatively high barrier for the proton motion. The experimental - ~ - are mprkedly higher for the intra- than for the intermolecular results show that, for the same isotopic ratio, the v ~ ~ +values hydrogen bonds. From literature data, the same trend can be found for intra- and intermolecular OH-0 bonds. This effect can be accounted for, at least qualitatively, by the nonlinearity of the intramolecular hydrogen bonds.
Introduction As shown by recent infrared data,'-4 dipole moment measurements, and X-ray ortho Mannich bases are classes of compounds characterized by stable intramolecular hydrogen bonds. By introducing
is of the proton-transfer type. The isotopic ratio is intimitely related to the strength of the hydrogen bond, to the intermolecular distances, and to the shape of the potential curve for the proton motion. It seemed therefore interesting to investigate the solvent effect on the isotopic ratio vNH+/VND+ and to compare the results with those previously reported for normal OH-N bonds; for these, the isotopic ratio vOH/VOD has been shown to decrease with the polarity of the m e d i ~ m . ~
Experimental Section The ultraviolet spectra were taken with a Cary 219 spectrophotometer using I-cm cells and concentrations between 2 X low3 various X groups into the phenyl ring and changing the amines and 8 X M. The infrared spectra were recorded with a condensed with the appropriate phenols, it is possible to modulate Perkin-Elmer 580B spectrophotometer using cells with KBr within a broad scope the acidic and basic properties of the inwindows of 0.05-0.2 cm. In the weakly polar solvents, the solteracting centers. According to ultraviolet data, the intramolecular ubility of DCNMBHp, is very low and maximal concentrations proton-transfer constant KpTdepends on the substituents implanted on the phenolic ring, on the solvent, and on the t e m p e r a t ~ r e . ~ - ~ (about 0.015 M) were used in order to obtain representative spectra. When three or four chlorine atoms are implanted on the aromatic DCNMBH was synthesized recently in the laboratory of Proring, the hydrogen bond is predominantly of the OH-N type fessor L. Sobczyk by a described procedure." The deuteriated whatever the polarity of the ~ o l v e n t . ~In , ~ 2-(N,N-dimethylanalogue was obtained by dissolving the base in C H 3 0 D under aminomethyl)-4-nitrophenol,two NH+.-O- bonds related through nitrogen atmosphere. This operation was repeated several times an inversion center form a cyclic dimer in the solid state while and about 7 0 4 0 % deuteriation was achieved. Owing to the rapid in weakly polar solvents, intramolecular OH-N hydrogen bonding exchange with the hydrogen atoms, the infrared spectra of is predominating.* The purpose of this work is to investigate the solvent effect on the strength of the hydrogen bond and in 2-(N,N-dodecyl(1) Sucharda-Sobczyk,A.; Sobcyzyk, L. Bull. Acad. Pol. Sci., Ser. Sci. aminomethyl)-3,6-dichloro-4-nitrophenoland its O D deuteriated Chim. 1978, 26, 549. analogue: ( 2 ) Koll, A.; Glowiak, T. J . Crystallogr. Spectrosc. Res. 1985, IS, 411. (3) Rospenk, M.; Zeegers-Huyskens, Th. Bull. SOC.Chim. Belg. 1985, 94, 469
