NMRSTUDYOF DIETHYLAMINE HYDROGEN BONDING
481
A Nuclear Magnetic Resonance Study of Diethylamine Hydrogen Bonding’
by Charles S. Springer, Jr., and Devon W. Meek Departnzent of Chemistry, The Ohw State University, Columbus, Ohio 48.210
(Received August $3, 1966)
The concentration dependence of the chemical shift of the diethylamine N H proton was studied both in the hydrogen-bonding solvent acetonitrile and in the nonhydrogen-bonding solvent cyclohexane. I n the cyclohexane solutions the N-H---N bonds in pure diethylamine break upon dilution and the resonance position of the N-H peak changes toward higher magnetic fields, whereas in acetonitrile solutions the N-H resonance changes systematically toward lower fields as the concentration of amine decreases. Thus, the N-H group of diethylamine forms a stronger hydrogen bond with acetonitrile than with itself. Rapid exchange of the diethylamine N-H proton appears to result from trace amounts of water present in acetonitrile and cyclohexane.
Introduction The self-association of liquid amines via hydrogen bonds has been studied by infrared spectroscopy in both the N-H fundamental2 and overtone region^,^ by nmr in carbon t e t r a c h l ~ r i d e and , ~ ~ ~by the enthalpy of mixing of diethylamine with monoethylamine and triethylamine.6 Since the N-H chemical shift of tris(2-N-methylaminoethyl) borate, B(OCH2CH2NHCH&, was strongly dependent on concentration in a~etonitrile,~ an investigation of the proton nuclear magnetic resonance of the “model” compound, (C2H5)2NH, was undertaken in order to determine the behavior of the N-H group in the hydrogen-bonding solvent acetonitrile.
Experimental Section Reagents. Acetonitrile was purified and the water content was determined as reported previously.’ Cyclohexane was refluxed overnight over calcium hydride and then fractionated. The middle portion was stored in a desiccator and opened only in a drybox which was cont,inuously flushed with nitrogen. Eastman White Label diethylamine was distilled from anhydrous BaO and protected from moisture. The middle fraction was stored in a desiccator and opened only in the drybox. Nmr Spectra. A Varian A-60 nmr spectrometer with a 60-Mc probe was operated at 40’. All nmr samples were prepared in a drybox as described prev i o u ~ l y . ~All spectra were calibrated internally with tetramethylsilane. The near-infrared spectra were
obtained with a Cary Model 14 recording spectrophotometer using a set of matched 1-cm quartz cells.
Results and Discussion Diethylamine in Acetonitrile. The significant portions of the nmr spectra of diethylamine in acetonitrile (that of the CH3 and N H groups) are shown in Figure 1, and the data are given in Table I. A plot of the N-H resonance position, in T units, as a function of concentration in acetonitrile is shown in Figure 2. The resonance position of the N-H peak shifts systematically to lower T values as the concentration of diethylamine is decreased in acetonitrile, indicating that the solute-solvent hydrogen bonds are stronger than those between diethylamine molecules, that is, that acetonitrile is a stronger hydrogen-bond acceptor than diethylamine. Since only one sharp N H peak is seen, the equilibrium between (C2H&NH-HN(C2H5)2 and (C2HJ2NH--N=C-CHa must be very rapid, so that the spectrometer records the average resonance. ~
~~
(1) Grateful acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the grant (1518-A1,3)which supported this research. (2) A. G. Morita, Spectrochim. Acta, 17, 365 (1961). (3) K.B. Whetsel and J. H. Lady, J. Phys. Chem., 6 8 , 1010 (1964). (4) V. P. Bystrov and V. P. Leaina, Opt. i Spektroskopiya, 16, 790 (1964); Chem. Abstr., 61,4186a (1964). ( 5 ) J. Feeney and L. H. Sutcliffe, J . Chem. SOC.,1123 (1962). (6) H.J. Bittrich, G. Duering, and G. Linke, Wiss. 2. Tech. Hochsch. Chem. Leuna-Merseburg, 4, 245 (1962); Chem. Abstr., 59, 3371d (1963). (7) D.W.Meek and C. S. Springer, Jr., Inorg. C h m . , in press.
Volume 70, Number 2 February 1966
482
II
CHARLES S. SPRINGER, JR.,AND DEVONW. MEEK
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1
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I ! i o 0 8 8 9 9 '01156 0218
I
1
in CH$N
' 0343 !
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1
1
1
I I
I
1
0.492
I
%
Table I: Proton Nmr Data"Sb of Diethylamine in Acetonitrile as a Function of Concentration Conon, mole fraction of (C2Ha)zNH
CHa triplet, 7, ppm
CHz quartet, r , ppm
NH singlet, r , ppm
8.98 8.99 8.99 8.99 8.98 9.00 9.00 9.00 9.00 9.02 8.97
7.44 7.43 7.42 7.45 7.43 7.44 7.43 7.43 7.45 7.43 7.41
8.83 8.95 8.99 9.03 9.07 9.14 9.18 9.20 9.23 9.27 9.29
0 . 0166" 0.0268 0.0330 0.0665 0.0889 0.1156 0.2180 0.3430 0.4920 0.7440
Figure 1. The CHs and N-H regions from eight different proton nmr spectra of diethylamine in acetonitrile a t 40'. Concentration of diethylamine (given in mole fraction) increases from left to right. Assignments of the resonance position? are given by the numbers a t the bottom of each spectrum. Resonance of the CHa group is a t I 8.97-9.00, whereas the N-H resonance varies systematically from T 8.83 to 9.29.
1.OOOO*
a At the mole fraction 0.0166 the total area under the peaks assigned as CHs f NH is 7.11 relative to 4.00 for the area of the methylene peaks (theory = 7.0:4.0). * In pure diethylamine the areas of the CHa and CH2 peaks are 6.14 and 4.12 relative to the N-H = 1.00 (theory = 6.0 and 4.0:1.0, respectively).
the amine molecule, can be represented by the equations
R
R
\
N-Hf
H
e
\a/
N
0 - H z
/ \
R'
fO-H
H R
R
E
0
I
I
I
I
I
Mole f r a c t i o n of
I
I
I
I
0
Ell"
Figure 2. Concentration dependence of the NH resonance position ( T N H ) in the proton nmr spectrum of diethylamine in both acetonitrile and cyclohexane at 40".
The sharp peaks can be attributed to rapid exchange of the amino proton caused by a trace of water in the hygroscopic solvent, acetonitrile. The rapid exchange of the labile amine proton eliminates the quadrupole broadening effect of NI4, thus resulting in a sharp resonance peak. Oggs reports that only 0.1 ppm of water is necessary to give a sharp NH resonance peak for liquid ammonia. If the forms of the (CzH&NH molecule in which it is hydrogen bonded, either to itself or to the solvent, are in rapid equilibrium with the free amine species, an exchange scheme, in which water acts as an acid toward The Journal of Physical Chemistry
(1)
R
H
N R
R
e/
\@/
+N
/\
H
\
R
R
\ NH /
R
R
+
/ HN \
(3)
R
Most of the exchange which involves an ionic intermediate probably occurs by route 1. Huyskens and Huyskensg~lo observed a rapid proton exchange between primary and secondary amines with alcohols. They discounted the possibility that the exchange occurred via ionic intermediates and proposed a simultaneous exchange of two hydrogens through a (8) R. A. Ogg, Jr., J. Chem. Phys., 2 2 , 560 (1954). (9) P. Huyskens and T. 2. Huyskens, Bull. SOC.Chim. Belges, 69, 267 (1960). (10) T. Z.Huyskens, P. Huyskens, and P. Van Tiggelen, ibid., 70, 386 (1961).
NMRSTUDYOF DIETHYLAMINE HYDROGEN BONDING
cyclic transition state. The rapid exchange of a secondary amine was envisaged to occur as R
-3
JH-,+
R>K.
,H--4O) -R R>&'*-O
'*O--R
and the transition state for a primary amine can be either
H.. R , /**% 'O-R or R-X&-H+O-R E/"-'..H/ \H;;:* Since water is more acidic than aliphatic alcohols, ionic intermediates become even more probable in aqueous exchange processes. Muney and Coetzee" observed that extensive interaction occurs between a series of amines and water in acetonitrile, but that only a moderate increase in conductivity results. Ionization of the nitrogen base may still result from a trace of water by a series of reactions such as Kz +
Ki
B:
+ H2OJB:H-0
__
\
f-
H
K8
13H+OH-
BH+soivated
OH-solvated
provided the product K2K3 is small. Walden and Birr12 found that incompletely substituted alkylammonium chlorides are very weak electrolytes in acetonitrile. The corresponding hydroxides will possibly be even weaker electrolytes since the hydroxide ion is very small. Thus, the stability of the BH+OH- ion pairs and the undoubtedly small values of Kaare major causes of the extreme weakness of nitrogen bases as electrolytes in acetonitrile. From a consideration of the specific conductance of electrolytes dissolved in l-butanol and from the sharp N-H resonance of diethylamine in l-butanol, Huyskens, et u Z . , ~ ~ ' O ruled out the ionic species (C2H&NH2+ in the rapid hydrogen exchange between diethylamine and l-butanol. However, from the above discussion of the electrolytic properties of alkylammonium hydroxides in acetonitrile, it is apparent that conductance measurements do not accurately measure the number of ionic species present. The purified acetonitrile contained less than 2 X mole fraction of water even after all of the spectral measurements. The highest concentration of (C2H&NH in CHPCN studied was a mole fraction of 0.7440. This solution, of course, contained the smallest ratio of water (from the solvent) to diethyl-
483
amine of all solutions investigated. This ratio, calculated from the concentrations of water and diethylamine, is
