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Hydrogen bond formation in N-ethyl urethane solutions
European Polymer Journal, 1972, Vol. 8, pp. 339-350. Pergamon Press. Printed in England.
HYDROGEN BOND FORMATION IN N-ETHYL URETHANE SOLUTIONS J. BAILEY and S. M. WALKER Donnan Laboratories, University of Liverpool, P.O. Box 147, Liverpool L69 3BX
(Received 10 August 1971) Abstract--Complementary NMR and ultrasonic techniques have been used to determine the degree and strength of hydrogen bonds formed by N-ethyl urethane. Experiments were made on systems in which the urethane molecules existed in a controlled environment simulating the behaviour in polyurethane elastomers. Pure ethyl urethane is found to exist as a cyclic dimer over a wide concentration range while 1:1 urethane:solvent dimers predominate in ester-rich environments or in ether-rich surroundings. THE HIGH frequency dielectric behaviour o f N-ethyl urethane C 2 H s N H C O O C 2 H s has been studied recently. (~) A relaxation in the G H z region was f o u n d at r o o m temperature and assigned to the re-orientation o f long h y d r o g e n - b o n d e d molecular chains. The high dielectric constant coupled with the similarity o f the activation energies for dielectric relaxation and viscous flow ( ~ 26 kJ m o l e - 2 ) were used to support this hypothesis. The current importance o f polyurethanes as thermoplastic materials and the close relation between their physical properties and the inter-chain hydrogen b o n d energies makes it desirable to obtain some m o r e direct evidence on the energetics o f this type o f bond. It is usually understood that the hydrogen b o n d i n g occurs between the N - - H groups o f the urethane segments and the carbonyl or ether groups o f the c o m o n o m e r used in the polycondensation reaction. Solutions o f N-ethyl urethane (U), in ethyl propionate (R) and 1,2-dimethoxyethane (M) provide models for this type o f interaction. Spectroscopic methods for the study o f hydrogen b o n d i n g are carried out c o m m o n l y using N M R c2) and i.r. c3) techniques, but recent interest has been generated in the use o f ultrasonics to study association equilibria in solution. (4,5) W e report here the results o f a parallel investigation using acoustics and N M R on N-ethyl urethane and its solutions in inert and interactive solvents. EXPERIMENTAL N-Ethyl urethane was obtained 99" 9 ~ pure from several sources--K and K laboratories, Fluka chemicals and Ralph Emmanuel. No differences between the samples could be detected spectroscopically. 1,2-Dimethoxyethane and ethyl propionate were obtained as laboratory grade reagents from Hopkin & Williams, and twice distilled before use. Ultrasonic absorption measurements were made using the pulse-echo tecahnique at frequencies of 5, 10, 15, 25, 30, 35 and 45 MHz. Details of the apparatus have been given elsewhere, re) The measuring cell was sufficiently large that no corrections for diffraction effects were needed even at 5 MHz. The error in measurement of the absorption coefficients is estimated at 4 ~ at 5 MHz, falling to 2 ~ at the highest frequency. Sound velocity measurements are accurate to 1 ~. NMR experiments were carried out at 36° and 70° using a Varian A-60 spectrometer and T.M.S. reference. Liquid density and viscosity measurements were made using calibrated dilatometers and viscometers respectively. Although densities are accurate to 0" 1 ~, the thermal expansion coefficients, obtained from reciprocal density versus temperature graphs, are only accurate to 7~. 339
340
J. BAILEY and S. M. WALKER RESULTS A N D DISCUSSION
N - E t h y l urethane
Any fluid existing in two states differing in energy and/or volume will, at some characteristic sound frequency f,, exhibit ultrasonic relaxation. This condition is reached when the changes in the populations of the states can no longer follow the rapid temperature or pressure changes accompanying the sound wave. The onset of a relaxation is manifest as a change in the quantity a / f 2 which follows the relation a / f 2 = A/[1 q- (f/f,)2] q_ B
(1)
for a single relaxation. Here a is the measured absorption at frequency f, A is a parameter of the relaxation related to the relaxing part of the specific heat 8Cp and B represents contributions from classical (viscothermal) absorption and higher frequency relaxation processes. TABLE 1. ULTRASONIC ABSORFFION DATA FOR PURE N-~rlYL
Temperature (K)
(all 2) experimental × 1017cm-l see2
(all 2) classical x 1017cm-1 see2
243 253 263 276 296 309 321
390 252 184 134 104 96 94
,~370 227 155 100 56 40 31
(a/f2) experimental-(a/f z) classical × 101~cm-1 sec2 with estimated error 20 4- 20 25 -4- 10 29 q- 8 34 4- 5 48 + 2 56 4- 2 63 4- 2
At all temperatures in the region -- 50° to + 50 ° for pure urethane, the value of a / f 2 was constant over the whole frequency range. There is, however, a marked temperature dependence (Table 1) with a rapid, almost exponential increase, in a / f 2 with decreasing temperature. Equation (1) implies that frequency-independent values for a / f 2 can be found only when f , ~ f , or f ~ f i . In view of the very high dielectric relaxation frequency, tl) the former condition is presumed. Consequently, the measured a / f 2 quantities represent a series of values of A q- B. The large increase in a / f 2 is accounted for, in part, by a large increase in the liquid viscosity. The contribution to a l l 2 which arises from this source is given by the Stokes-Navier equation as el.,, = 30 3"
(2)
Using Eqn. (2) and the experimentally determined densities and viscosities, the calculated viscosity contribution is shown in Table 1. Acceptable agreement between these data and experiment is found at low temperatures, but the correspondence becomes progressively poorer as the temperature is raised. The difference between the two is attributed to relaxation processes other than shear viscosity. Although the
Hydrogen Bond Formation in N-Ethyl Urethane Solutions
341
available frequency range is insufficient to characterize the behaviour, the comparatively small differences imply the existence of only one further relaxation. In addition, it seems reasonable that, whatever the relaxation frequency., the B value is given by the classical viscosity term and thus estimates of A can be made. These values were used for a plot of In (ATac/(7 - 1)) against T - i , the slope of which is related to (AH* -AH°)/R. (7) This is shown in Fig. 1 and a linear plot is obtained between --10 ° and + 50 °. At lower temperatures the points, while still in qualitative agreement, are subject to a large absolute experimental error contained in the very large absorption coefficients. The slope of the graph gives the value --18.5 kJ mole-1 for AH* -- AH °.
c
4'2
4"0
32
34
[ITx
36
;38
40
10 4 K -l
Fio. 1. Graph of log (ACT-a/y_ 1) vs. 1/T for N-ethyl urethan¢. Further meaning to these results is given by a N M R study since a very satisfactory method exists for investigating hydrogen bonding by this technique. (2'a) The method may be applied to solutions in which the various association equilibria are sufficiently fast, compared to the time scale of the experiment, that only a single proton peak is observed. For urethane solutions the frequency of this peak is the weighted mean of the frequencies of the N - - H species taking part (from monomer U, dimer U2, etc.). Since the mean degree of aggregation is concentration dependent, then the N - - H resonance frequency, or chemical shift, 8Nn, also varies with concentration. For solutions of N-ethyl urethane in inert solvents the relevant equations are,
8Nn = 8=[U] -k 28==K2[U]2 -t- . . . . . . nS,, K,[u]" C C ---- [u] -+- 2K2[u] 2 -+- . . . . . nK,[u~'
(3) (4)
342
J. BAILEY and S. M. WALKER
where 8,, is the chemical shift of the N - - H proton in the aggregate U,, [u] is the monomer concentration, C the total solute concentration and K~ the equilibrium constant. Equations (3) and (4) can be made tractable by the important assumption that only one of the several equilibrium constants K~ is significant, i.e. one species (in addition to monomer) is dominant. Thus, 8N, = 8,[u]
+ nS,,, K,,[u]"
(5)
C C = [u] q-
nKn[u] ~.
(6)
The characteristic sigmoid shape of the experimental 8N, vs. log C plot can be curve fitted for various separate values of n. 5, is found by extrapolation to infinite dilution and the quantity (~, -- ~,,) is related to the straight portion of the graph at intermediate concentrations. Figure 2 shows this together with the calculated fit for a monomer/dimer equilibrium. Excellent agreement between the two is obtained up to a
240
28o-
260
300
~\
320 60
340
~~
360
\
380
~o
400
I -I-6
r -0-8 0r log c
l 0.8
FIG. 2. Plot of chemical shift of the N--H proton versus concentration of N-ethyl urethane in CC14. The circles are the experimental results at 360 and the triangles at 70°. The full line represents the theoretical behaviour at 36° expected for a U2 dimer. concentration of about 1 M, above which higher aggregates become significant. The parameters used in the fit are, n = 2, 8u = 260 Hz, 8,2 = 445 Hz and Kz = 0.40 1.mole -1 at 36 °. Figure 2 also shows the behaviour at 70 ° and simply sliding the abscissa enables the new equilibrium constant to be calculated, c2) At 70°, K2 assumes the value of 0.15 1.mole -~ implying that the enthalpy change accompanying the
Hydrogen Bond Formation in N-Ethyl Urethane Solutions
343
formation of the dimer is 26.0 kJ mole- 1. Since the activation energy for hydrogen bonding is usually very lOW, tg) this answer compares very favourably with the ultrasonic calculation of -- 18.5 kJ mole- 1 for AH* -- AH ° (implying that AH* = 7.5 kJ mole-1). This is excellent evidence that the NMR and acoustic experiments are looking at the same equilibrium. The detection of predominantly dimeric species by these two techniques must be contrasted with the long hydrogen-bonded chains postulated from dielectric measurements. ") This relatively high enthalpy value for N - - H . . . . O bonds (9) suggests that more than one hydrogen bond is taking part in the equilibrium. Consequently it seems reasonable that N-ethyl urethane exists as a predominantly cyclic dimer, Et
H
--
\/
-- -- O
OEt
\/
N
C
I
I
C
N
/\
/\
Et O
O
--
--
--
Et
H
with each hydrogen bond having an associated enthalpy change of 13 kJ mole-1 with an activation energy barrier to scission of 3" 8 kJ mole- 1.
N-Ethyl urethane in 1,2-dimethoxyethane Although neither pure urethane nor 1,2-dimethoxyethane show any evidence of ultrasonic relaxation over the available frequency and temperature range, marked anomalous absorption occurs in all mixtures of the two liquids. The data can be fitted satisfactorily to the single relaxation equation (Eqn. 1) using a least squares procedure to evaluate A, B and f,. This information is shown in Table 2. The value of A passes through a maximum for solutions containing approximately 33 ~. by volume of urethane. It is reasonable to assume that this behaviour can be ascribed to urethane (U)-dimethoxyethane (M) aggregation. Indeed the peak in the absorption coefficient at intermediate concentrations is characteristic of such equilibria. (1°) TABLE 2. RELAXATIONPARAMETERSFOR URETHANE--DIMETHOXYETHANESOLUTIONS Temperature (K)
Percentage urethane A × 10tTcm-t sec2
B × 10tTcm-t sec2
f, MHz
~Cp x 10" J c m -a K - t
296
5 15 33 50
99 239 310 160
51 56 45 51
3-4 4"2 5"2 6"0
12"2 35"7 72"4 56"9
321
5 15 33 50
79 208 299 174
60 58 35 47
4"2 4"9 5"7 6"2
12"8 37"4 77"9 70.4
344
J. BAILEY and S. M. WALKER
In a discrete volume of the mixture, the magnitude of the relaxing specific heat 8Ce will be proportional to the number o f hydrogen bonds within it. Specifically, since there is no relaxation contribution from the individual components separately, 8Ce is
3¢
T o E 3"
25
~ 2c x
I
0
I0
0
20
~
30
Urethane,
40
50
%
FIG. 3. Graph of 8C,/[u] vs. urethaae concentration in 1,2-dimethoxyethane. proportional to the number of U--M bonds. For dilute urethane solutions it is reasonable to assume that all urethane molecules are bonded to solvent and the number of bonds of the type U--M will equal the number o f urethane molecules. Consequently ~Cp per unit volume should be proportional to the urethane concentration. Figure 3, which shows a plot of 8Ce/[u] vs. [u] for two different temperatures, indicates that 8Ce/[u] does become constant in dilute solution ( < 15 ~o). This quantity takes the values 2.924 × l0 -a at 23 ° and 3"201 × 10 -a J 1. cm -a K -1 mole -1 at 48 ° when extrapolated to zero urethane concentration. Small deviations from the constancy of 8Ce appear even in quite dilute solution and become very pronounced above 15 per cent. The concentrations of larger aggregates than UM will become significant under these conditions and the effects of species like U2M, UaM must be considered. It is not difficult to show that, for species such as U,M, then n is given by, n ~
Original urethane concentration Concentration of bonded dimethoxyethane"
The denominator in this equation can be obtained from the value of 8Ce/[u] at infinite dilution since U--U bonds do not contribute to 8Ce and at this concentration
Hydrogen Bond Formation in N-Ethyl Urethane Solutions
345
TABLE 3. R e s t ~ r s FOR URETItAN~--D~OXYE'THANE SOLUTIONS Percentage urethane
296K
321K
5 15 33 50
1 "00 1"04 1"12 2"1
I "00 1 "04 1"11 1"9
n
Bond energy kJ m o l e - ~
U~M
7" 1 5"9 6"7 7"1
K3 296K
321K
3"25 3"36
3"56 3"65
all urethane is bonded to dimethoxyethane. Table 3 lists the result of this calculation and shows that UM species are dominant up to about 33 per cent above which U2M aggregates become more significant. Also, the number of U - - M bonds in any given solution is almost temperature independent. It is permissible therefore to obtain an estimate of the U - - M bond energy by plotting log \ - ~ - ~
]-)]
against T - I . (7) The values obtained and shown in Table 3 agree quite well with other estimates o f N H . . . . O bond energies and with the U--U bond energy. For 15 and 33 per cent solutions, the value of n is much less than two and the analysis can be extended to include UM and U2M species only.
[urn] = tul t,,,----q
[u2m] K2 -
f,,ml tu------I
Kx K3 =
Under these conditions, values for K3 may be calculated from the data and reasonable constancy is maintained as shown in Table 3. In fact, the derivation of Ka is subject to very large procedural errors so that the agreement is all the more gratifying. A rough estimate of the U--U bond energy can be derived from these data since, K1 = e x p ( - - A G 1 / R T )
where
K2 = exp (-- AGe/RT) then and
AG, = G m - - G ~ - - G , , AG2 =
G,2m -- G,m -- G~
Ka -----exp (-- [(AG~ -- AG2)/RT]) dlnKa d(1/r)
AH1 -- AH2 R
where
AH, -- AH2 = 2H~m -- Hu2m -- Hm
or
AH, -- AH2 = (H,m -- H , -- H,,) -- (H,, 2. -- H , -- Hm).
The first term on the R.H.S. of this last equation is simply the U - - M hydrogen bond energy as in the reaction U + M ~ - UM, values for which have been given in Table 3. Thus, the temperature dependence of K~ yields information on the second term and this is the U- - U bond energy for the reaction U + UM.~- U2M. In this case the value derived is 9.2 kJ m o l e - ' .
346
J. BAILEY and S. M. WALKER
390
380
370 == ~o
A 360
350
:340
I o.l
Urethone,
I 0-2 mole
I 0-3
r 0.4
fraction
1~o. 4. Plot of the chemical shift of the NmH proton as a function of mole fraction of urethane in (A) ethyl propionate and (B) 1,2-dimethoxyethane. The analysis of N M R data for the bchaviour of 8N, is much more complex than in the simple case of urethane only, since two equilibria need to be analysed simultaneously. Huggins et al. have shown that, provided the solute is bonded to solvent in a dimeric aggregate only, then a plot of 8Nn against the mole fraction of urethane should be a straight line. Figure 4 shows that this is so up to a concentration of about 30 per cent, above which complex aggregates become more significant. The deviation closely parallels the acoustic behaviour shown in Fig. 3 and provides further evidence that the N M R and ultrasonic techniques are giving complementary data on this system. More quantitative information can be obtained only by extending the treatment based on Eqns. (3) and (4). Including the species UnM gives, 8u[u] -t- 28,2K2[u] 2 -t- (n -t-- 1) 8,,mKm[u]"[m] [u] + 2K2[u] 2 + (n + I) K=[u]"[m]
8NH
C,, = [u] + 2K2[u] 2 + nK=[u]"[m] Cm
=
[m] + K,,,[u]"[m]
where Cu and C= are the concentrations of solute and solvent respectively, 8~,,, is the chemical shift of the unto aggregate and Km the equilibrium constant for its formation. We know that in dilute solution only monomers and dimers will need to be considered, ~NH
8. + 28.,K2[u] + 28,,d~'~,[m] 1 + 2K2[u] + 2Kin[m]
Hydrogen Bond Formation in N-Ethyl Urethane Solutions
347
390
380
/
370
360
350
340 -8
-10
I -12
-14
l -16
I -18
l -20
~ -22
t -24
-26
(~N,-~u) Hz I mole -I C FIG. 5. G r a p h
o f u r e t h a n e c h e m i c a l s h i f t versus (~Nn - -
~,,)/C
for (A) ethyl propionate
solutions and (B) 1,2-dirnethoxyethane solutions.
If, furthermore, the basic assumption made in the ultrasound analysis is also made here, namely that in very dilute solution all the urethane is bonded to solvent, then putting [u] --~ 0, 8Nn =
3, + 2~,,K,[ml 1 + 2K=[m]
and
Cm = [m].
Thus a plot of (SNn -- 8=)/Cm vs. 8sa should be a straight line with slope --2/(,. Figure 5 illustrates this graph where again linearity is observed for the more dilute solutions with deviations beginning to occur at about 30 % urethane. Quite eleady then, the N M R analysis confirms the validity of assumptions made in the acoustic deviations. The value of K,~ is 1.48 mole I.- 1 at 36°.
N-Ethyl urethane in ethyl propionate The important difference between this system and the previous one in acoustic terms is the presence of a relaxation in pure ethyl propionate due to cis/trans conformational interconversion, c~.12) This behaviour introduces an additional complication into the analysis of the urethane mixtures. Table 4 shows that, once again, a maximum A is found at intermediate urethane concentrations indicating the presence of specific solute/solvent interactions. The relaxation behaviour closely parallels the dimethoxyethane system and a similar analysis should be feasible. Note that the relaxation frequency in the mixtures is essentially identical to that of the pure propionate and that only one relaxation is detected. The analytical plocedure, if it is to be analogous to the previous system, implies
J. BAILEY and S. M. W A L K E R
348
TABLE 4. RZLAXATIONPARAMETE~ FOR ~ A N E - - E I ' H Y L PROPIONATE SOLUTIONS Temperature (K)
Percentage urethane
A x 10tTcm-tsec 2
B × 1017cm-XsecZ
f, MHz
~Cp × 104 J c m -3 K -1
276
5 10 15 33 50
42 54 80 167 200
44 42 37 29 57
10"2 10"0 9"8 8"9 7"9
13"2 21"1 30"2 54"4 55"0
296
5 10 15 33 50 75
50 60 79 157 129 42
42 34 37 28 42 77
12"0 11"9 12-0 11"1 10"0 12"0
18"5 27"3 37"0 63"0 46"0 17"5
321
5 10 15 33 50 75
61 65 76 108 115 62
39 52 26 35 22 60
15-0 15"0 15"9 13"2 11-0 12"0
29"5 39"9 YJ'0 53"1 44.6 23"8
that the relaxation process in the solvent/solute system is acoustically independent of the propionate equilibrium whether these be molecularly dependent or not. The first step in the treatment must involve the correction of the results to allow for contributions from the ester conformers. The 8Ce values can be adjusted assuming only TABLE5
Temperature
Percentage
Correction to be applied to 8C~. for contributions from ester conformers
(K)
urethane
x 10 t J crn -3 K -I
276
5 I0 15 33 50
3"81 3"39 2-97 1"38 0
22"7 20"7 21.4 18"7 13"I
296
5 I0 15 33 50
7"50 6"61 5"80 2"76 0
26"3 25"8 25-1 21"9 10.9
75
0
321
5 10 15 33 50 75
16"7 14"8 13"0 6" 12 0 0
6Cpoor~ [u] x
104 Jl. c m -3 K -I mole -~
N
2"5
31 "9 30"7 30"6 17" 5 10" 7 ,,, 3"8
Hydrogen Bond Formation in N-Ethyl Urethane Solutions
349
dimer aggregation (UR) using the data in reference (6). The magnitude of this correction is shown in Table 5. Again, an essentially constant value for 8C~,¢orr./[u] is found with the extrapolated values ([u] ~ 0) of 2.24 × 10 -a at 3 °, 2.66 × 10 -3 at 23 ° and 3.22 x 10 -3 J1. cm -a K -1 mole -1 a t 4 8 °. Departure from the predicted behaviour occurs above about 15 % concentration due to urethane self-association and the appearance of UnR aggregates; Table 6 shows the estimates of the mean urethane chain length in these higher aggregates together with the U - - R hydrogen bond energy and K3 for the equilibrium U + UR U2R. The temperature dependence of Ks yields a value of 8.0 kJ mole -1 for the enthalpy of formation of a U - - U hydrogen bond in the equilibrium U + UR ~ U2R. TABLE 6. RESULTSFOR URETHANE--ETHYL PROPIONATE SYSTEM
Percentage urethane
276K
n 296K
321K
U--R Bond energy kJ mole -1
276K
Ka 296K
321K
5 10 15 33 50
1"01 1"07 1"05 1"18 1"87
1"01 1"03 1"06 1-22 2"42
1"01 1"05 1"06 1"70 3
10"9 11"3 10"9 10"5
3-62 1.20 3"50 2"43
3"55 3"54 2"60 1"75
3"46 1"70 2-90 0"60
The N M R analysis is identical with that given previously and the results are expressed in Figs. 4 and 5. Deviations from linearity in the graphs occur at about the same concentration as the acoustic results. The value of K, is 2.7 mole 1.- 1 at 36 °. Qualitative studies on solutions of urethane in methyl acetate and ethyl acetate show that the acoustic behaviour is very similar to the propionate system with single relaxations being observed at similar frequencies to the pure ester. Conversely no unusual behaviour can be detected for solutions of urethane in materials such as acetone or diethyl ether, both of which possess potentially strong hydrogen bonding sites. These facts lead to the inescapable conclusion that the two equilibria are molecularly interdependent. It seems as if the internal rotation of the ester acts as a ratedetermining step for the breaking of the hydrogen bond between urethane and ester. In the absence of the conformational equilibrium (e.g. acetone) even the hydrogen bonding equilibrium cannot be detected. Although 1,2-dimethoxyethane itself exhibits no ultrasonic relaxation, it does at least possess the potential for conformational change (7) and this could explain the presence of a hydrogen bonding relaxation in the mixtures. The failure to observe a conformational equilibrium may be due to a fortuitously very low or zero enthalpy difference between the conformers. This suggested mechanism would automatically result in identical relaxation frequencies and the lower AH values and enhanced A values in the dimers have their origin in the difference in hydrogen bonding potential between the conformers. Acknowledgements--We are indebted to Professor A. M. North for much valuable advice. We thank the S.R.C. for the award of a maintenance grant to one of us (J.B.) and for an equipment grant.
350
J. BAILEY and S. M. W A L K E R REFERENCES
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)
A. T. Bullock, A. M. North and J. B. ShortaU, Europ. Polym. J. 4, 587 (1968). J. Feeney and S. M. Walker, J. chem. Soc. (A) 1148 (1966). S. Bratoz, Adv. Quantum Chem. 3, 209 (1967). J. H. Andreae, P. D. Edmonds and J. F. McKellar, ,4custica 15, "/4 (1965). G. R. Hammes and A. C. Park,,/. ,4m. chem. Soc. 90, 4151 (1968). J. Bailey and A. M. North, Trans. Faraday Soc. 64, 1499 (1968). J. Lamb, in Physical Acoustics (edited by W. P. Mason), Vol. IIA, chapter 4. Academic Press (1965). M. Satmders and J. B. Hyne, J. chem. Phys. 29, 1319 (1958). G. C. Pimentel and A. L. McClellan, The Hydrogen Bond. Freeman (1960). J. M. M. Pinkerton, Proc. phys. Soc. Lond. 6213, 129 (1949). C. M. Huggins, G. C. Pimentel and J. M. Shoolery, J. phys. Chem. 60, 1311 (1956). J. Bailey, S. M. Walker and A. M. North, J. molec. Structure 6, 53 (1970).
R~sum6----4)n a utilis6 des techniques compl6mentaires de R M N et d'ultrasons pour d6terminer le degr6 et la force des liaisons H form6es darts le N-~thyl m6thane. On a fait ces experiences sur des syst~mes o/~ les mol6cules d'ur6thane sont darts un environnement contr616, simulant le comportemerit dans les 61astom~res du polyur6thane. On a trouv6 que l'ur6thane d'6thyle pur existait sous la forme de dim~res cycliques dans un grand domaine de concentrations tandis que darts les milieux riches en ester ou en 6ther les dim~res ur6thane-solvant en proportion 1 : 1 pr6dominent. Sommario---Per determinare il grado e la resistenza dei legami dell'idrogeno formati dall'N-etil uretano, si sono impiegate tecniche agli ultrasuoni e NMR. Si sono effettuati esperimenti su sistemi nei quali le molecote di uretano si trovano in un ambiente controllato che simula il comportamento da esse avuto negli elastomeri di poliuretano. Si ~ trovato che dell'etil uretano pure ~ presente come dimero ciclico per una laxga gamma di concentrazioni, mentre i dimeri uretano:solvente 1:1 predominano in ambienti ricchi di esteri o eteri. Zusanunenfassung--Komplementfire N M R und Ultraschall Methoden wurden verwendet, um den Grad und die Stfirke yon durch N-.~thylurethan gebildeten Wasserstoffbindungen zu bestimmen. Die Versuche wurden an Systemen durchgeffihrt, in denen die Urethanmolektile in einer kontrollierten Umgebung filmlich dem Verhalten in Polyurethan Elastomeren vorlagen. Es wird festgestellt, dab reines .~thylurethan als ein cyclisches Dimer fiber einen weiten Konzentrationsbereich vorliegt, wrthrend 1 : 1 Urethan-Lfsungsmittel Dimere in einer Ester-reichen oder ,~ther-reichen Umgebung fiberwiegen.
HYDROGEN BOND FORMATION IN N-ETHYL URETHANE SOLUTIONS J. BAILEY and S. M. WALKER Donnan Laboratories, University of Liverpool, P.O. Box 147, Liverpool L69 3BX
(Received 10 August 1971) Abstract--Complementary NMR and ultrasonic techniques have been used to determine the degree and strength of hydrogen bonds formed by N-ethyl urethane. Experiments were made on systems in which the urethane molecules existed in a controlled environment simulating the behaviour in polyurethane elastomers. Pure ethyl urethane is found to exist as a cyclic dimer over a wide concentration range while 1:1 urethane:solvent dimers predominate in ester-rich environments or in ether-rich surroundings. THE HIGH frequency dielectric behaviour o f N-ethyl urethane C 2 H s N H C O O C 2 H s has been studied recently. (~) A relaxation in the G H z region was f o u n d at r o o m temperature and assigned to the re-orientation o f long h y d r o g e n - b o n d e d molecular chains. The high dielectric constant coupled with the similarity o f the activation energies for dielectric relaxation and viscous flow ( ~ 26 kJ m o l e - 2 ) were used to support this hypothesis. The current importance o f polyurethanes as thermoplastic materials and the close relation between their physical properties and the inter-chain hydrogen b o n d energies makes it desirable to obtain some m o r e direct evidence on the energetics o f this type o f bond. It is usually understood that the hydrogen b o n d i n g occurs between the N - - H groups o f the urethane segments and the carbonyl or ether groups o f the c o m o n o m e r used in the polycondensation reaction. Solutions o f N-ethyl urethane (U), in ethyl propionate (R) and 1,2-dimethoxyethane (M) provide models for this type o f interaction. Spectroscopic methods for the study o f hydrogen b o n d i n g are carried out c o m m o n l y using N M R c2) and i.r. c3) techniques, but recent interest has been generated in the use o f ultrasonics to study association equilibria in solution. (4,5) W e report here the results o f a parallel investigation using acoustics and N M R on N-ethyl urethane and its solutions in inert and interactive solvents. EXPERIMENTAL N-Ethyl urethane was obtained 99" 9 ~ pure from several sources--K and K laboratories, Fluka chemicals and Ralph Emmanuel. No differences between the samples could be detected spectroscopically. 1,2-Dimethoxyethane and ethyl propionate were obtained as laboratory grade reagents from Hopkin & Williams, and twice distilled before use. Ultrasonic absorption measurements were made using the pulse-echo tecahnique at frequencies of 5, 10, 15, 25, 30, 35 and 45 MHz. Details of the apparatus have been given elsewhere, re) The measuring cell was sufficiently large that no corrections for diffraction effects were needed even at 5 MHz. The error in measurement of the absorption coefficients is estimated at 4 ~ at 5 MHz, falling to 2 ~ at the highest frequency. Sound velocity measurements are accurate to 1 ~. NMR experiments were carried out at 36° and 70° using a Varian A-60 spectrometer and T.M.S. reference. Liquid density and viscosity measurements were made using calibrated dilatometers and viscometers respectively. Although densities are accurate to 0" 1 ~, the thermal expansion coefficients, obtained from reciprocal density versus temperature graphs, are only accurate to 7~. 339
340
J. BAILEY and S. M. WALKER RESULTS A N D DISCUSSION
N - E t h y l urethane
Any fluid existing in two states differing in energy and/or volume will, at some characteristic sound frequency f,, exhibit ultrasonic relaxation. This condition is reached when the changes in the populations of the states can no longer follow the rapid temperature or pressure changes accompanying the sound wave. The onset of a relaxation is manifest as a change in the quantity a / f 2 which follows the relation a / f 2 = A/[1 q- (f/f,)2] q_ B
(1)
for a single relaxation. Here a is the measured absorption at frequency f, A is a parameter of the relaxation related to the relaxing part of the specific heat 8Cp and B represents contributions from classical (viscothermal) absorption and higher frequency relaxation processes. TABLE 1. ULTRASONIC ABSORFFION DATA FOR PURE N-~rlYL
Temperature (K)
(all 2) experimental × 1017cm-l see2
(all 2) classical x 1017cm-1 see2
243 253 263 276 296 309 321
390 252 184 134 104 96 94
,~370 227 155 100 56 40 31
(a/f2) experimental-(a/f z) classical × 101~cm-1 sec2 with estimated error 20 4- 20 25 -4- 10 29 q- 8 34 4- 5 48 + 2 56 4- 2 63 4- 2
At all temperatures in the region -- 50° to + 50 ° for pure urethane, the value of a / f 2 was constant over the whole frequency range. There is, however, a marked temperature dependence (Table 1) with a rapid, almost exponential increase, in a / f 2 with decreasing temperature. Equation (1) implies that frequency-independent values for a / f 2 can be found only when f , ~ f , or f ~ f i . In view of the very high dielectric relaxation frequency, tl) the former condition is presumed. Consequently, the measured a / f 2 quantities represent a series of values of A q- B. The large increase in a / f 2 is accounted for, in part, by a large increase in the liquid viscosity. The contribution to a l l 2 which arises from this source is given by the Stokes-Navier equation as el.,, = 30 3"
(2)
Using Eqn. (2) and the experimentally determined densities and viscosities, the calculated viscosity contribution is shown in Table 1. Acceptable agreement between these data and experiment is found at low temperatures, but the correspondence becomes progressively poorer as the temperature is raised. The difference between the two is attributed to relaxation processes other than shear viscosity. Although the
Hydrogen Bond Formation in N-Ethyl Urethane Solutions
341
available frequency range is insufficient to characterize the behaviour, the comparatively small differences imply the existence of only one further relaxation. In addition, it seems reasonable that, whatever the relaxation frequency., the B value is given by the classical viscosity term and thus estimates of A can be made. These values were used for a plot of In (ATac/(7 - 1)) against T - i , the slope of which is related to (AH* -AH°)/R. (7) This is shown in Fig. 1 and a linear plot is obtained between --10 ° and + 50 °. At lower temperatures the points, while still in qualitative agreement, are subject to a large absolute experimental error contained in the very large absorption coefficients. The slope of the graph gives the value --18.5 kJ mole-1 for AH* -- AH °.
c
4'2
4"0
32
34
[ITx
36
;38
40
10 4 K -l
Fio. 1. Graph of log (ACT-a/y_ 1) vs. 1/T for N-ethyl urethan¢. Further meaning to these results is given by a N M R study since a very satisfactory method exists for investigating hydrogen bonding by this technique. (2'a) The method may be applied to solutions in which the various association equilibria are sufficiently fast, compared to the time scale of the experiment, that only a single proton peak is observed. For urethane solutions the frequency of this peak is the weighted mean of the frequencies of the N - - H species taking part (from monomer U, dimer U2, etc.). Since the mean degree of aggregation is concentration dependent, then the N - - H resonance frequency, or chemical shift, 8Nn, also varies with concentration. For solutions of N-ethyl urethane in inert solvents the relevant equations are,
8Nn = 8=[U] -k 28==K2[U]2 -t- . . . . . . nS,, K,[u]" C C ---- [u] -+- 2K2[u] 2 -+- . . . . . nK,[u~'
(3) (4)
342
J. BAILEY and S. M. WALKER
where 8,, is the chemical shift of the N - - H proton in the aggregate U,, [u] is the monomer concentration, C the total solute concentration and K~ the equilibrium constant. Equations (3) and (4) can be made tractable by the important assumption that only one of the several equilibrium constants K~ is significant, i.e. one species (in addition to monomer) is dominant. Thus, 8N, = 8,[u]
+ nS,,, K,,[u]"
(5)
C C = [u] q-
nKn[u] ~.
(6)
The characteristic sigmoid shape of the experimental 8N, vs. log C plot can be curve fitted for various separate values of n. 5, is found by extrapolation to infinite dilution and the quantity (~, -- ~,,) is related to the straight portion of the graph at intermediate concentrations. Figure 2 shows this together with the calculated fit for a monomer/dimer equilibrium. Excellent agreement between the two is obtained up to a
240
28o-
260
300
~\
320 60
340
~~
360
\
380
~o
400
I -I-6
r -0-8 0r log c
l 0.8
FIG. 2. Plot of chemical shift of the N--H proton versus concentration of N-ethyl urethane in CC14. The circles are the experimental results at 360 and the triangles at 70°. The full line represents the theoretical behaviour at 36° expected for a U2 dimer. concentration of about 1 M, above which higher aggregates become significant. The parameters used in the fit are, n = 2, 8u = 260 Hz, 8,2 = 445 Hz and Kz = 0.40 1.mole -1 at 36 °. Figure 2 also shows the behaviour at 70 ° and simply sliding the abscissa enables the new equilibrium constant to be calculated, c2) At 70°, K2 assumes the value of 0.15 1.mole -~ implying that the enthalpy change accompanying the
Hydrogen Bond Formation in N-Ethyl Urethane Solutions
343
formation of the dimer is 26.0 kJ mole- 1. Since the activation energy for hydrogen bonding is usually very lOW, tg) this answer compares very favourably with the ultrasonic calculation of -- 18.5 kJ mole- 1 for AH* -- AH ° (implying that AH* = 7.5 kJ mole-1). This is excellent evidence that the NMR and acoustic experiments are looking at the same equilibrium. The detection of predominantly dimeric species by these two techniques must be contrasted with the long hydrogen-bonded chains postulated from dielectric measurements. ") This relatively high enthalpy value for N - - H . . . . O bonds (9) suggests that more than one hydrogen bond is taking part in the equilibrium. Consequently it seems reasonable that N-ethyl urethane exists as a predominantly cyclic dimer, Et
H
--
\/
-- -- O
OEt
\/
N
C
I
I
C
N
/\
/\
Et O
O
--
--
--
Et
H
with each hydrogen bond having an associated enthalpy change of 13 kJ mole-1 with an activation energy barrier to scission of 3" 8 kJ mole- 1.
N-Ethyl urethane in 1,2-dimethoxyethane Although neither pure urethane nor 1,2-dimethoxyethane show any evidence of ultrasonic relaxation over the available frequency and temperature range, marked anomalous absorption occurs in all mixtures of the two liquids. The data can be fitted satisfactorily to the single relaxation equation (Eqn. 1) using a least squares procedure to evaluate A, B and f,. This information is shown in Table 2. The value of A passes through a maximum for solutions containing approximately 33 ~. by volume of urethane. It is reasonable to assume that this behaviour can be ascribed to urethane (U)-dimethoxyethane (M) aggregation. Indeed the peak in the absorption coefficient at intermediate concentrations is characteristic of such equilibria. (1°) TABLE 2. RELAXATIONPARAMETERSFOR URETHANE--DIMETHOXYETHANESOLUTIONS Temperature (K)
Percentage urethane A × 10tTcm-t sec2
B × 10tTcm-t sec2
f, MHz
~Cp x 10" J c m -a K - t
296
5 15 33 50
99 239 310 160
51 56 45 51
3-4 4"2 5"2 6"0
12"2 35"7 72"4 56"9
321
5 15 33 50
79 208 299 174
60 58 35 47
4"2 4"9 5"7 6"2
12"8 37"4 77"9 70.4
344
J. BAILEY and S. M. WALKER
In a discrete volume of the mixture, the magnitude of the relaxing specific heat 8Ce will be proportional to the number o f hydrogen bonds within it. Specifically, since there is no relaxation contribution from the individual components separately, 8Ce is
3¢
T o E 3"
25
~ 2c x
I
0
I0
0
20
~
30
Urethane,
40
50
%
FIG. 3. Graph of 8C,/[u] vs. urethaae concentration in 1,2-dimethoxyethane. proportional to the number of U--M bonds. For dilute urethane solutions it is reasonable to assume that all urethane molecules are bonded to solvent and the number of bonds of the type U--M will equal the number o f urethane molecules. Consequently ~Cp per unit volume should be proportional to the urethane concentration. Figure 3, which shows a plot of 8Ce/[u] vs. [u] for two different temperatures, indicates that 8Ce/[u] does become constant in dilute solution ( < 15 ~o). This quantity takes the values 2.924 × l0 -a at 23 ° and 3"201 × 10 -a J 1. cm -a K -1 mole -1 at 48 ° when extrapolated to zero urethane concentration. Small deviations from the constancy of 8Ce appear even in quite dilute solution and become very pronounced above 15 per cent. The concentrations of larger aggregates than UM will become significant under these conditions and the effects of species like U2M, UaM must be considered. It is not difficult to show that, for species such as U,M, then n is given by, n ~
Original urethane concentration Concentration of bonded dimethoxyethane"
The denominator in this equation can be obtained from the value of 8Ce/[u] at infinite dilution since U--U bonds do not contribute to 8Ce and at this concentration
Hydrogen Bond Formation in N-Ethyl Urethane Solutions
345
TABLE 3. R e s t ~ r s FOR URETItAN~--D~OXYE'THANE SOLUTIONS Percentage urethane
296K
321K
5 15 33 50
1 "00 1"04 1"12 2"1
I "00 1 "04 1"11 1"9
n
Bond energy kJ m o l e - ~
U~M
7" 1 5"9 6"7 7"1
K3 296K
321K
3"25 3"36
3"56 3"65
all urethane is bonded to dimethoxyethane. Table 3 lists the result of this calculation and shows that UM species are dominant up to about 33 per cent above which U2M aggregates become more significant. Also, the number of U - - M bonds in any given solution is almost temperature independent. It is permissible therefore to obtain an estimate of the U - - M bond energy by plotting log \ - ~ - ~
]-)]
against T - I . (7) The values obtained and shown in Table 3 agree quite well with other estimates o f N H . . . . O bond energies and with the U--U bond energy. For 15 and 33 per cent solutions, the value of n is much less than two and the analysis can be extended to include UM and U2M species only.
[urn] = tul t,,,----q
[u2m] K2 -
f,,ml tu------I
Kx K3 =
Under these conditions, values for K3 may be calculated from the data and reasonable constancy is maintained as shown in Table 3. In fact, the derivation of Ka is subject to very large procedural errors so that the agreement is all the more gratifying. A rough estimate of the U--U bond energy can be derived from these data since, K1 = e x p ( - - A G 1 / R T )
where
K2 = exp (-- AGe/RT) then and
AG, = G m - - G ~ - - G , , AG2 =
G,2m -- G,m -- G~
Ka -----exp (-- [(AG~ -- AG2)/RT]) dlnKa d(1/r)
AH1 -- AH2 R
where
AH, -- AH2 = 2H~m -- Hu2m -- Hm
or
AH, -- AH2 = (H,m -- H , -- H,,) -- (H,, 2. -- H , -- Hm).
The first term on the R.H.S. of this last equation is simply the U - - M hydrogen bond energy as in the reaction U + M ~ - UM, values for which have been given in Table 3. Thus, the temperature dependence of K~ yields information on the second term and this is the U- - U bond energy for the reaction U + UM.~- U2M. In this case the value derived is 9.2 kJ m o l e - ' .
346
J. BAILEY and S. M. WALKER
390
380
370 == ~o
A 360
350
:340
I o.l
Urethone,
I 0-2 mole
I 0-3
r 0.4
fraction
1~o. 4. Plot of the chemical shift of the NmH proton as a function of mole fraction of urethane in (A) ethyl propionate and (B) 1,2-dimethoxyethane. The analysis of N M R data for the bchaviour of 8N, is much more complex than in the simple case of urethane only, since two equilibria need to be analysed simultaneously. Huggins et al. have shown that, provided the solute is bonded to solvent in a dimeric aggregate only, then a plot of 8Nn against the mole fraction of urethane should be a straight line. Figure 4 shows that this is so up to a concentration of about 30 per cent, above which complex aggregates become more significant. The deviation closely parallels the acoustic behaviour shown in Fig. 3 and provides further evidence that the N M R and ultrasonic techniques are giving complementary data on this system. More quantitative information can be obtained only by extending the treatment based on Eqns. (3) and (4). Including the species UnM gives, 8u[u] -t- 28,2K2[u] 2 -t- (n -t-- 1) 8,,mKm[u]"[m] [u] + 2K2[u] 2 + (n + I) K=[u]"[m]
8NH
C,, = [u] + 2K2[u] 2 + nK=[u]"[m] Cm
=
[m] + K,,,[u]"[m]
where Cu and C= are the concentrations of solute and solvent respectively, 8~,,, is the chemical shift of the unto aggregate and Km the equilibrium constant for its formation. We know that in dilute solution only monomers and dimers will need to be considered, ~NH
8. + 28.,K2[u] + 28,,d~'~,[m] 1 + 2K2[u] + 2Kin[m]
Hydrogen Bond Formation in N-Ethyl Urethane Solutions
347
390
380
/
370
360
350
340 -8
-10
I -12
-14
l -16
I -18
l -20
~ -22
t -24
-26
(~N,-~u) Hz I mole -I C FIG. 5. G r a p h
o f u r e t h a n e c h e m i c a l s h i f t versus (~Nn - -
~,,)/C
for (A) ethyl propionate
solutions and (B) 1,2-dirnethoxyethane solutions.
If, furthermore, the basic assumption made in the ultrasound analysis is also made here, namely that in very dilute solution all the urethane is bonded to solvent, then putting [u] --~ 0, 8Nn =
3, + 2~,,K,[ml 1 + 2K=[m]
and
Cm = [m].
Thus a plot of (SNn -- 8=)/Cm vs. 8sa should be a straight line with slope --2/(,. Figure 5 illustrates this graph where again linearity is observed for the more dilute solutions with deviations beginning to occur at about 30 % urethane. Quite eleady then, the N M R analysis confirms the validity of assumptions made in the acoustic deviations. The value of K,~ is 1.48 mole I.- 1 at 36°.
N-Ethyl urethane in ethyl propionate The important difference between this system and the previous one in acoustic terms is the presence of a relaxation in pure ethyl propionate due to cis/trans conformational interconversion, c~.12) This behaviour introduces an additional complication into the analysis of the urethane mixtures. Table 4 shows that, once again, a maximum A is found at intermediate urethane concentrations indicating the presence of specific solute/solvent interactions. The relaxation behaviour closely parallels the dimethoxyethane system and a similar analysis should be feasible. Note that the relaxation frequency in the mixtures is essentially identical to that of the pure propionate and that only one relaxation is detected. The analytical plocedure, if it is to be analogous to the previous system, implies
J. BAILEY and S. M. W A L K E R
348
TABLE 4. RZLAXATIONPARAMETE~ FOR ~ A N E - - E I ' H Y L PROPIONATE SOLUTIONS Temperature (K)
Percentage urethane
A x 10tTcm-tsec 2
B × 1017cm-XsecZ
f, MHz
~Cp × 104 J c m -3 K -1
276
5 10 15 33 50
42 54 80 167 200
44 42 37 29 57
10"2 10"0 9"8 8"9 7"9
13"2 21"1 30"2 54"4 55"0
296
5 10 15 33 50 75
50 60 79 157 129 42
42 34 37 28 42 77
12"0 11"9 12-0 11"1 10"0 12"0
18"5 27"3 37"0 63"0 46"0 17"5
321
5 10 15 33 50 75
61 65 76 108 115 62
39 52 26 35 22 60
15-0 15"0 15"9 13"2 11-0 12"0
29"5 39"9 YJ'0 53"1 44.6 23"8
that the relaxation process in the solvent/solute system is acoustically independent of the propionate equilibrium whether these be molecularly dependent or not. The first step in the treatment must involve the correction of the results to allow for contributions from the ester conformers. The 8Ce values can be adjusted assuming only TABLE5
Temperature
Percentage
Correction to be applied to 8C~. for contributions from ester conformers
(K)
urethane
x 10 t J crn -3 K -I
276
5 I0 15 33 50
3"81 3"39 2-97 1"38 0
22"7 20"7 21.4 18"7 13"I
296
5 I0 15 33 50
7"50 6"61 5"80 2"76 0
26"3 25"8 25-1 21"9 10.9
75
0
321
5 10 15 33 50 75
16"7 14"8 13"0 6" 12 0 0
6Cpoor~ [u] x
104 Jl. c m -3 K -I mole -~
N
2"5
31 "9 30"7 30"6 17" 5 10" 7 ,,, 3"8
Hydrogen Bond Formation in N-Ethyl Urethane Solutions
349
dimer aggregation (UR) using the data in reference (6). The magnitude of this correction is shown in Table 5. Again, an essentially constant value for 8C~,¢orr./[u] is found with the extrapolated values ([u] ~ 0) of 2.24 × 10 -a at 3 °, 2.66 × 10 -3 at 23 ° and 3.22 x 10 -3 J1. cm -a K -1 mole -1 a t 4 8 °. Departure from the predicted behaviour occurs above about 15 % concentration due to urethane self-association and the appearance of UnR aggregates; Table 6 shows the estimates of the mean urethane chain length in these higher aggregates together with the U - - R hydrogen bond energy and K3 for the equilibrium U + UR U2R. The temperature dependence of Ks yields a value of 8.0 kJ mole -1 for the enthalpy of formation of a U - - U hydrogen bond in the equilibrium U + UR ~ U2R. TABLE 6. RESULTSFOR URETHANE--ETHYL PROPIONATE SYSTEM
Percentage urethane
276K
n 296K
321K
U--R Bond energy kJ mole -1
276K
Ka 296K
321K
5 10 15 33 50
1"01 1"07 1"05 1"18 1"87
1"01 1"03 1"06 1-22 2"42
1"01 1"05 1"06 1"70 3
10"9 11"3 10"9 10"5
3-62 1.20 3"50 2"43
3"55 3"54 2"60 1"75
3"46 1"70 2-90 0"60
The N M R analysis is identical with that given previously and the results are expressed in Figs. 4 and 5. Deviations from linearity in the graphs occur at about the same concentration as the acoustic results. The value of K, is 2.7 mole 1.- 1 at 36 °. Qualitative studies on solutions of urethane in methyl acetate and ethyl acetate show that the acoustic behaviour is very similar to the propionate system with single relaxations being observed at similar frequencies to the pure ester. Conversely no unusual behaviour can be detected for solutions of urethane in materials such as acetone or diethyl ether, both of which possess potentially strong hydrogen bonding sites. These facts lead to the inescapable conclusion that the two equilibria are molecularly interdependent. It seems as if the internal rotation of the ester acts as a ratedetermining step for the breaking of the hydrogen bond between urethane and ester. In the absence of the conformational equilibrium (e.g. acetone) even the hydrogen bonding equilibrium cannot be detected. Although 1,2-dimethoxyethane itself exhibits no ultrasonic relaxation, it does at least possess the potential for conformational change (7) and this could explain the presence of a hydrogen bonding relaxation in the mixtures. The failure to observe a conformational equilibrium may be due to a fortuitously very low or zero enthalpy difference between the conformers. This suggested mechanism would automatically result in identical relaxation frequencies and the lower AH values and enhanced A values in the dimers have their origin in the difference in hydrogen bonding potential between the conformers. Acknowledgements--We are indebted to Professor A. M. North for much valuable advice. We thank the S.R.C. for the award of a maintenance grant to one of us (J.B.) and for an equipment grant.
350
J. BAILEY and S. M. W A L K E R REFERENCES
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)
A. T. Bullock, A. M. North and J. B. ShortaU, Europ. Polym. J. 4, 587 (1968). J. Feeney and S. M. Walker, J. chem. Soc. (A) 1148 (1966). S. Bratoz, Adv. Quantum Chem. 3, 209 (1967). J. H. Andreae, P. D. Edmonds and J. F. McKellar, ,4custica 15, "/4 (1965). G. R. Hammes and A. C. Park,,/. ,4m. chem. Soc. 90, 4151 (1968). J. Bailey and A. M. North, Trans. Faraday Soc. 64, 1499 (1968). J. Lamb, in Physical Acoustics (edited by W. P. Mason), Vol. IIA, chapter 4. Academic Press (1965). M. Satmders and J. B. Hyne, J. chem. Phys. 29, 1319 (1958). G. C. Pimentel and A. L. McClellan, The Hydrogen Bond. Freeman (1960). J. M. M. Pinkerton, Proc. phys. Soc. Lond. 6213, 129 (1949). C. M. Huggins, G. C. Pimentel and J. M. Shoolery, J. phys. Chem. 60, 1311 (1956). J. Bailey, S. M. Walker and A. M. North, J. molec. Structure 6, 53 (1970).
R~sum6----4)n a utilis6 des techniques compl6mentaires de R M N et d'ultrasons pour d6terminer le degr6 et la force des liaisons H form6es darts le N-~thyl m6thane. On a fait ces experiences sur des syst~mes o/~ les mol6cules d'ur6thane sont darts un environnement contr616, simulant le comportemerit dans les 61astom~res du polyur6thane. On a trouv6 que l'ur6thane d'6thyle pur existait sous la forme de dim~res cycliques dans un grand domaine de concentrations tandis que darts les milieux riches en ester ou en 6ther les dim~res ur6thane-solvant en proportion 1 : 1 pr6dominent. Sommario---Per determinare il grado e la resistenza dei legami dell'idrogeno formati dall'N-etil uretano, si sono impiegate tecniche agli ultrasuoni e NMR. Si sono effettuati esperimenti su sistemi nei quali le molecote di uretano si trovano in un ambiente controllato che simula il comportamento da esse avuto negli elastomeri di poliuretano. Si ~ trovato che dell'etil uretano pure ~ presente come dimero ciclico per una laxga gamma di concentrazioni, mentre i dimeri uretano:solvente 1:1 predominano in ambienti ricchi di esteri o eteri. Zusanunenfassung--Komplementfire N M R und Ultraschall Methoden wurden verwendet, um den Grad und die Stfirke yon durch N-.~thylurethan gebildeten Wasserstoffbindungen zu bestimmen. Die Versuche wurden an Systemen durchgeffihrt, in denen die Urethanmolektile in einer kontrollierten Umgebung filmlich dem Verhalten in Polyurethan Elastomeren vorlagen. Es wird festgestellt, dab reines .~thylurethan als ein cyclisches Dimer fiber einen weiten Konzentrationsbereich vorliegt, wrthrend 1 : 1 Urethan-Lfsungsmittel Dimere in einer Ester-reichen oder ,~ther-reichen Umgebung fiberwiegen.