NMRSTUDYOF METHYLN-VINYLCARBAMATE SELF-ASSOCIATION Pure second-order decay kinetics of a transient is somewhat unique and may represent a simple bimolecular triplet annihilation process. 2o When one observes an isosbestic point during the steady illumination of bianthrones, as in the case of 2,2’-dibromobianthrone, the permanent photochemical reaction is the conversion to the corresponding helianthrone with essentially no naphthodianthrone being formed. In other cases such as bianthrone and 2,2’dimethylbianthrone, significant amounts of helianthrone are photochemically produced prior to formation of the corresponding naphthodianthrone. To verify this observat,ion, an authentic sample of helianthrone was synthesized.21 The spectrum of this sample (A, 450 mp) is identical with the helianthrone produced photochemically from bianthrone. Further, on irradiation (A 366 mp), helianthrone is converted to
3485
naphthodianthrone (4 = 0.09) ; the concurrent disappearance of the helianthrone absorption band and the appearance of the 418 mp naphthodianthrone band corresponds precisely t o the spectral changes observed in the conversion of bianthrone to naphthodianthrone except that an isosbestic point appears a t approximately 450 mp. Therefore it appears that helianthrone is the only permanent photoproduct of bianthrone in benzene. Acknowledgment. This work was supported by the Army Research Office (Durham) under Contract No. DA-31-124-ARO-D-475 and is gratefully acknowledged. (20) C. A. Parker, “Advances in Photochemistry,” Vol. 11, W. A. Noyes, Jr., G. S. Hammond, and J. N. Pitts, Jr., Ed., Interscience Publishers, New York, N. Y., 1964,p 305. (21) R.Scholland J. Mansfield, Chem. Bey., 43, 1734 (1910).
Nuclear Magnetic Resonance Study of Methyl N-Vinyl Carbamate Self-Association by W. C. Meyer and J. T. K. Woo Physical Research Laboratory, The Dow Chemical Company, Midland, Michigan 48640
(Received April g3,1565)
The dependence of the WH frequency of methyl N-vinyl carbamate as a function of temperature and concentration in carbon tetrachloride solution follows best theoretical curves derived from a monomer-dimer equilibrium. Thermodynamic constants of association were AH = -3.2 i: 0.3 kcal mol-’ and A S O a o z = -11 & 4 cal mol-’ deg-I. The existence of a mixture of cyclic cis and open chain trans dimers is postulated to account for chemical shift differences of dilute carbamate monomer, pure monomer-DMSO complexes, and the frequency of “pure” carbamate dimer.
Introduction Hydrogen bonding of N-substituted amides is better described when equilibria among various self-associated species are taken into account.’ On the other hand, carbamates, which contain the urethane linkage -NHCOO-, may be confined to a single hydrogen-bonded n-mer. The dielectric constant of ethyl S-methyl carbamate, for example, supports a cyclic cis dimer for this molecule in solution.2 If a specific hydrogenbonded species predominates, then thermodynamic properties of association are amenable to study. It has already been shown that methyl N-vinyl carbamate forms a 1: 1 hydrogen-bonded complex with dimethyl sulfoxide The data implied monomeric carbamate participated in the equilibrium ; however, its concentration was kept purposely low to
avoid complications of self-association. The present work investigates the nature of methyl N-vinyl carbamate self-association using nmr spectroscopy. The type of association is established and thermodynamic constants are derived from the equilibrium. The values compare favorably with hydrogen-bond strengths and entropies of the N H . . - O bond type of other systems. The possibility of distinguishing trends of NH. -0 bond strengths with K-substitution or with various carbonyl-containing complexing agents (as opposed t o self-association) is raised. (1) L. A. LaPlanche, H. B. Thompson, and M. T. Rogers, J . Phys, Chem., 69,1482 (1965). (2) G. R. Leader and J. F. Gormley, J . Amer. Chem. Soc., 73, 6731 (1951). (3) W. C.Meyer and J. T. K. Woo, J . Phys. Chem., 73, 2989 (1969).
Volume 78, Number 10 October 1989
3486
W. C. MEYERA N D J. T. K. Woo
Experimental Section The preparation and characterization of methyl Nvinyl carbamate has been discussed previo~sly.~ Freshly distilled CCld was used as solvent. 460 A Varian Associates A-60 spectrometer followed the N H chemical-shift dependence as a function of concentration and temperature. The broadness of the NH peak reduced experimental accuracy to f l 440 Hz for all solutions except those of low concentration, I where the N H signal enters the region of the complex b pattern of the a-vinyl proton. Observing the changing 420 ratio of peak heights to valley depressions within this region with varying temperature, in conjunction with scans run at temperatures sufficiently low (-20") to shift the NH signal beyond the low-field border of the 400 pattern, enabled the N H peak position to be estimated to =!=3-Hzaccuracy at the lower concentrations. All spectra were calibrated by the side-band technique using tetramethylsilane as internal reference. Chem380 ical shifts were recorded on the precalibrated 50.0 f - 1.0 -0.5 0 0.5 LOG C 0.2-Hz chart scale. Lower temperatures also reduced the rate of carFigure 1. Chemical shift (Hz downfield from internal bamate degradation. Apparently thermal polymerizatetramethylsilane) of methyl N-vinyl carbamate Ic" tion occurs since the vinyl proton pattern disappears proton for best fit monomer-n-mer model a t 28.5': solid curve, n = 2, dimer association; dashed curve, n = 3, and insoluble material is created upon storage of solutrimer association; dotted curve, n = 4, tetramer association. tions a t 0" over a period of days. Another complication enters from storage of solutions in that the N H peak shifts upfield upon standing. Moisture pickup computer program was devised to solve eq 1 for M I is one possible explanation. In any case only freshly using assumed values for K,. VI was established from prepared solutions were used. A given chemical shift extrapolation to infinite dilution of frequency-conis an average value of three separate solutions. Reprocentration curves. v, was adjusted until the theoducibility was =!= 1Hz. retical linear region of the plot matched the slope of the experiment'al points in their respective linear region. Results and Discussion A Kn could be chosen to give the best theoretical fit Many methods of relating chemical shifts of hydrofrom comparing experimental and calculated fregen-bonded protons to chemical equilibria have been devised to encompass a variety of situati~ns.l~~-'The approach of Saunders and Hyne4 is most appropriate to Table I: Methyl N-Vinyl Carbamate NH Frequency as a the present problem and it has been adopted in this Function of Concentration and Temperature in CCla work. Other than assuming a time-average peak is ____l_l_pNHa C, seen of the frequencies of all species contributing to mol 1. - 1 10 100 20.5O 28.5" 45.5O the equilibria, the only simplification required for 0.087 393 ... ... 384 382 analysis of the model is that only one equilibrium 0.174 410 403 399 395 393 constant is significant, i.e., a particular hydrogen0.347 434 425 420 414 405 bonded species prevails. 0 695 453 445 438 431 420 Under these conditions the total YH concentration 1.04 460 456 450 441 430 1.39 466 460 456 449 437 is given by N
_______-
C = M1
+ nK,Ml"
1.74 2.08 2.75
(11
and the weighted average frequency is v = (VIM1
+ nv,K,Mi")/C
a
...
466 471
...
46 1 466
...
454 460 46 8
444 451 456
13s down field from internal tetramethylsilane.
(2)
The monomer concentration M1 has associated with it a peak situated a t VI. The pure n-mer frequency is v, with an equilibrium constant K, for hydrogen bonding. The data in Table I were plotted in the suggested" v vs. log C manner for n = 2, 3, and 4. A The Journal of Physical Chemistry
474 478
7
(4) M. Saunders and J. B. Hyne, J. Chem. Phws., 2 9 , 1319 (1958). J. Phys. Chem., 71, 3779 (1967). (6) R. A. Murphy and J. C. Davis, Jr., ibid., 72, 3111 (1968). (7) T. A. Wittstruck and J. F. Cronan, ibid., 72, 4243 (1968). (5) D. M. Porter and W. S. Brey, Jr.,
NMRSTUDY OF METHYL N-VINYLCARBAMATE SELF-ASSOCIATION 480
Table 11: Equilibrium Constants and NH Carbamate Frequencies Giving Best Fit for Monomer-n-mer Models 10
460 Vl
KZ Yl
440
YI
K)
2
3487
Y3
420
100
Kh
400
V4
380
360 0
1.0
2.0
G , MOL
3.0
B L - '
Figure 2. Linear scale carbamate NH frequency-concentration plot using best fit monomer-dimer equilibrium constants for theoretical solid curves.
quencies for a given C, a task aided by directing attention again to the linear portion of the plot and having the computer program tabulate frequencies for each selected K,. Figure 1 illustrates final curves for monomer-dimer, monomer-trimer, and monomertetramer models for the data of 28.5'. The best approximation is without question the monomer-dimer equilibrium. Trimer and tetramer models require unrealistic values of YI, when viewed from the extrapolated experimental intercept, for these best-fit curves. Unfortunately lower concentrations, where distinction among various equilibria is most pronounced, could not be examined because of the a-vinyl proton pattern masking the N H resonance in this region. To better judge the agreement and also appease the criticism of such plots with respect to error in the best fit,' a linear v us. C dependence is presented in Figure 2, where the solid theoretical curves were generated using the best Kz for each temperature. Solubility limitations prevented concentrations greater than 3 M from being used. Evidently dimer association i s the major hydrogen-bonded species for all concentrations, since no obvious deviation of the best-fit curves from the data occurs.s Table I1 summarizes the best fit results. Like most extrapolations to infinite dilution in hydrogen-bonding studies, the value of u1 in this work is uncertain. The
28.5'
45.50
370 1.15 541
370 1.02 534
n = 2 370 0.90 532
3 70 0.70 530
370 0.50 525
380 2.74 515
387 1.78 507
n = 3 387 1.30 507
387 0.87 505
387 0.65 494
386 25 49 1
385 9.4 492
n = 4 387 4.8 493
384 2.2 491
383 0.7 49 1
b Y1
20.5'
continually changing slope at low concentrations requires measurements of very dilute solutions before an extrapolation can be confidently accepted. Wevertheless the value of yl can be bracketed using timeaveraging techniques on the signal and temperature variation on dilute solutions. In no instance could an KH peak be detected upfield of 370 Hz, the high-field frequency edge of the a-vinyl proton pattern. At infinite dilution the value of V I = 370 Hz appears to be a reasonable intercept from the extrapolation, and t,his value was used in the monomer-dimer computations. PI was considered temperature invariant because as lower carbamate concentrations are approached, the chemical-shift span becomes less for a given concentration as the temperature range is traversed, with indications that a common intercept is possible. However, the above conclusion is tentative. (A Y I independent of temperature would of course rule out any appreciable carbamate-CC14 interaction.) The temperature variation of K z yields thermodynamic constants of AH = -3.2 f 0.3 kcal mol-' and ASoaoz= -11 A 4 cal mol-' deg-l for dimer association. These constants are typical for the N H - s . 0 type of hydrogen bonding. The enthalpy is somewhat more negative than that found for methyl N-vinyl carbamate association with DMSO ( A H = -2.4 f 0.3 kcal and there is more molecular ordering implicated in the dimer equilibrium. CarbamateDMSO complexes must necessarily be open-chain species. Open chain carbamate dimers should have virtually identical thermodynamic constants as the DMSO complexes, whereas an equilibrium of cis cyclic dimers should have more entropy loss and negative enthalpy from the above. Thus the presence of cis dimer species is suggested from the thermodynamic data, albeit from the two studies there is some overlap (8) A reviewer has emphasined, rightly so, that no claim is made that no other species are present or that dimer association is proven. The data merely affirm such is the major operational equilibrium. Volume 79, Number 10 October 1060
NOTES
3488 of values within the experimental error. The existence of a mixture of dimers is a more logical choice from the viewpoint discussed next. The chemical shift of the pure 1: 1carbamate-DMSO complex (572 Hz at 27”) must correspond to a single hydrogen-bonded K H proton. If secondary effects of near neighbors and shielding effects of the orientation of these groups are ignored, then to a first approximation the position of the K H - - 0 signal of cis cyclic dimers, where both NH protons hydrogen bond, should lie close to the DMSO complex chemical shift. Although all N-substitution is identical, the O= bonding component does not remain carbonyl in character. However, the bond strengths and equilibrium constants of the two systems are very comparable, and since the contribution to a hydrogen bond chemical shift is generally partitioned into a polarization effect and a magnetic anisotropy contribution, differences of NH shifts in the pure complexes should be small if anisotropic effects are small. This, then, contrasts with results of acetic acid hydrogen bonding to various typee). For the open chain components (-OH. * dimer, the N H frequency is the weighted average of the hydrogen-bonded N H proton (-572 Hz using the above arguments) and the unassociated proton on the adjacent molecule. The chemical shift of the latter proton is also presumed to be little affected by orientation factors or the presence of trans hydrogen bonding 9
a
0
to its carbonyl group. It should thus be located near the monomer NH signal (370 Hz). The open chain dimer signal should occur in the region of 470 Hz. At 28.5” the frequency of the “pure” dimer (Table 11) is 532 Ha, a figure removed from either extreme of all cis or all trans-configuration. The discrepancy is reconciled if self-association embodies a mixture of 60% cyclic and 40% open-chain dimers. The values, to be sure, are highly approximate but do serve to indicate a mixture of isomers is likely. The temperature dependence of v 2 (Table 11) agrees with this conclusion, since open-chain dimers are favored a t higher temperatures. However, an alternative explanation for such a trend has been proposed by Muller and Reiter-lo The attempt to quantify hydrogen-bond competition between like carbamate molecules and complexing with DMSO solvent molecules through thermodynamic constants derived from careful nmr measurements is encouraging. Although the error in these small bond strengths remains an obstacle, variation of NH. - 0 systems might reveal trends or effects of N-substitution and of oxygen-containing complexing agents on the strength of this bond class.
(9) N. Muller and I?. I. Rose, J. Phys. Chem., 69,2564 (1965). (10) N. Muller and R. C. Reiter, J. Chem. Phys., 42,3265 (1965).
NOTES
On the Density Distribution of Segments of a Terminally Adsorbed Macromolecule by F. Th. Hesselink van’t H o f Laboratory, State University of Utrecht, Utrecht, The Netherlands (Received February 11, 1969)
The segment density distribution of an adsorbed macromolecule is of interest for the adsorption behavior of macromolecules and of crucial importance for a theory of the stability of dispersions stabilized by adsorbed macromolecules. In this note we will derive density distributions for the segments of a macromolecule terminally adsorbed on a flat impermeable interface with one end group (a “tail”) and with both end groups (a “loop”). With The Journal
of
Physical Chemistry
these results it is then possible to specify the segment distribution for any given mode of attachment of the macromolecule to the interface. The density distribution of segments of a tail of i segments is found by summing the probability PI (i,IC,z) to find the kth segment at a distance x from the interface over all values of IC, where PI (i,k,x) is the probability of finding the terminal segment of a chain of IC segments at x times the probability of finding the remaining i - IC segments a t x > 0. Subscript 1 refers to a tail. Recently, Meier’ derived a density distribution for a tail adsorbed on interface A, a second impermeable interface B being at a distance x = d parallel to A. First he formulated the probability W,(x,d) for the terminal (ith) segment of the tail to have a distance (1) D. J. Meier, J. Phys. Chem., 71, 1861 (1967).
