MAXT. ROGERS AND JAMESC. WOODBREY
540
Vol. 66
A PROTON MAGNETIC RESONANCE STUDY OF HINDERED INTERNAL ROTATION I N SOME SUBSTITUTED N,N-DIMETHYLAMIDES' BYM A X T. ROGERS AXD ,JAMESC. WOODBREY~ Kedzie Chemical Laboratory, Michigan State University, East Lansing, Michigan Received November 80,1901
Several studies of hindered internal rotation about the C-N bond in amides have been made by high-resolution proton magnetic resonance (p.m.r.) spectroscopy. Various methods have been reported for obtaining the activation energies E , and frequency factors A for internal rotation from the temperature dependence of the p.m.r. spectrum. Recent methods make it possible to reduce the rather large experimental errors associated with these parameters in the earlier studies. We therefore have made a redetermination of the energy barrier and frequency factor for internal rotation about the central C-N. bond of N,N-dimethylacetamide (DATA). The same experimental techniques and methods of calculation then were applied for the measurements of the barrier heights for internal rotation in a series of substituted N,N-dimethylamides, none of which had been studied previously. A detailed treatment of the errors in the present method leads t o the conclusion that the probable error in E, has been reduced to less than dz0.8 kcal./mole. The effect of structure on E, therefore may be discussed with some assurance that differences among different substituted amides are significant.
Introduction The p.m.r. spectrum of DMA a t vo = 60 Mc. and room temperature shows a single line for the C-methyl protons but a chemical shift doublet for the protons of the N-methyl groups (A, B of I). Gutowsky and HolmS observed that the separation
..
:O \&N/
/
CHI
\
(A) . .
..
-:O:
\
CHa
C=N
/
CHz (B) HS I I1 between the lines of the doublet decreased upon heating the sample until a t a sufficiently high temperature, termed the coulescence temperature, a single line remained. They found an energy barrier E, = 12 * . 2 kcaI./moIe and a frequency factor A = lo7 to 1O1O sec.-l for DMA and E a = 7 + 3 kcal./mole and A = lo3 to lo7 see.-' for N,N-dimethylformamide (DMF) from a series of measurements of apparent chemical shift at a spectrometer frequency vo = 17.735 Me. Several other studies have been made on formamide, DMF, and DMA4J The earlier work and theory have been reviewedas Piette and Anderson' have shown that the mean lifetime T of exchangeable nuclei a t chemical sites (as A or B of I) may be related to the changes in the resonance line widths which accompany the exchange averaging. Grunwald, Loewenstein, and Meiboom8 and Loewenstein and Meiboomg noted that the mean lifetime T of exchangeable nuclei at a chemical site may be
HaC
(1) Presented before the Division of Physical Chemistry a t the 138th National Meeting of the American Chemical Society, New York, N. Y., Sept., 1960. Abstracted in part from a thesis submitted by J. C.Woodbrey in partial fulfillment of the requirements for the Ph.D. degree, June, 1960. Supported by grants from the National Science Foundation and from the Atomic Energy Commission. (2) Union Carbide Research Fellow, 1958-1959. (3) H. 9. Gutowsky and C. H. Holm, J . Chem. Phys., 25,1228 (1956). (4) B. Sunners, L. H. Piette, and W. G. Schneider, Can. J . Chem., 88, 681 (1960). (5) C. Franconi and G. Fraenkel, J . Am. Chem. Soc., 8 2 , 4478 (1960). (6) J. A. Pople, W.G. Schneider, and H. J. Bernstein, "High-resolution Nuclear Magnetic Resonance," McGraw-Hill Book Co., Inc., New York, N. Y.,1959. (7) L. H. Piette and W. A. Anderson, J . Chem. P h y s . , 80, 899 (1959). (8) E,Grunwald, A. Loewenstein, and 8. Meiboom, J . Chem. Phya., 27, 630 (1957). (9) A. Loewenstein and S.Meiboom, ibid., 27, 1067 (1957).
related to T , the ratio of maximum to central minimum v-mode intensities, and to the coupling constant when 7 is very large, for symmetrical spinspin multiplets. The mean lifetime 27 of a methyl group at one of the chemical sites (A or B of I) is similarly related to r, the ratio of maximum to central minimum v-mode intensities, and to 6u = v4 - v g , the chemical-shift difference under conditions where there is no rotationaI averaging. We have found for the compounds studied here that the use of these latter line-shape parameters leads to much more precise values of 7 than does either of the earlier methodssJ which also were tried. The errors in the barrier heights, which were obtained from the temperature dependence of T , are =!= 0.3 to =!= 0.8 kcal./mole for the compounds studied if the limits of 90% confidence are computed statistically. The standard deviations are smaller but we have preferred the more severe criterion of error. Since comparisons of barrier heights in different compounds become more significant as errors are reduced, we have used a ratio method similar to that of Loewenstein and Meiboomgfor the analyses of the p.m.r. spectra of a series of substituted N,N-dimethylamides over a range of temperature. The effects of substituents on the barrier heights and frequency factors have been discussed. Experimental Spectrometer.-The spectra were obtained by use of a Varian Associates high-resolution nuclear magnetic resonance (n.m.r.) spectrometer with Model V-4311 R F probe for operation at YO = 60.000 Mc. and Model V-4310 R F probe for operation at vu = 40.000 Mc. A Model V-4320 s p ~ ndecoupler allowed fluorine-proton spin-spin interactions t o be decoupled while observing proton spectra at Y O = 60.000 Mc. Ambient room temperature was regulated to rt2'. All accessory components and the spectrometer console were supplied from an a x . line regulated by a Sorensen Model 1000 S voltage regulator. Radiofrequencies were measured with a calibrated Collins 51J-M4 communications receiver. A vacuum-jacketed variable-temperature receiver-coil insert' was eFployed to provide sample temperatures from - 100 to 220 The design was somewhat similar to earlier models.lOJ1 Sample temperature could be controlled t o better than =k0.loin the region 0 to 100' and to better than rt0.5' through the extreme high and low regions. Electrical shim coils constructed in this Laboratory were employed to provide high-resolution fluorine spectra a t a spectrometer frequency = 60.000 Mc. Sample tubes were precision drawn,la thin-wall Pyrex tubes, 0.192 rt 0.002 in.
.
(IO) J. N. Shoolery and J. D. Roberts, Rea. Sci. Instr., 28, 61, (1957).
(11) C. Franconi and G. Fraenkel, dbid., 81, 657 (lQ60).
HINDERED INTERNAL ROTATION IN SUBSTITUTED N,N-DIMETHYLAMIDES
March, 1962
541
TABLE I CHEMICAL ~ H I F T EIN ~ THE N.M.R. SPECTRAOF SOMEN,N-DISUBSTITUTED AMIDESOR THE TYPER‘CONRn (YO
Amides
t,
c.
Proton spectxa HCON(CHa)Z CHsCO N(CHa) z CHaC€IzCON(CHa)r CClaCON(CHa)n CF&ON(CHa)n CHe=CHCON(CHa)z CeHsCON(CHs)z‘: ClCON (CHa)i CHaCHzOCON(CHa)n CHaOCON(CFs)a
YB
uc
of coupling constants,b o.p.8.
Magnitudes
R‘ Group
UA
ID
-N( CH& Group
232.3d 205.7 225.2 132.9 -26.8 217.3/ 2 :i 4.3 215.3 202.9 -26.6 209.8 -24.2 229.5 25 28
26 -27.6 -27.5
60.000 Mc.)
-NRe Group
241.7d 216.3 234.4 150.5 224. 7f 224.8 212.1 216.3
-71.4’
261.5
261.7(CHz)’ 341.1(CH1)~ “vinyl” spectrum* -60.7 (CaHa) 168.2(CH2)’ 332, ^(CHa)g 170.9
-X( CH& Group
Fluorine spectra CHsCO N(CFa)i CHaOCON(CFa)t CHsCON(CHa)a
25 2B 25
-315 . 2/
701.8‘
-315.6 462.73
JAC JBC= 5 . 6 J A C = JBO = 0.0 2JAC P J B O
1.4
Chemical shifts in C.P.S. (increasing with the applied field) for protons are relative to the ring protons of toluene as external reference and are relative to 1,2.-dibromo-1,1,2,2-tetrafluoroethane as external reference for fluorine. Reference tubes ’ amide solution in diA 36.34 mole % All JAB = 0.0 C.P.S. were 1-mm. Pyrex capillaries concentric to sample tubes. Center of a poorly resolved 1:3:6:10:12:12:10:6:3:1 multiplet. bromomethane. d Center of a resolved 1 : l doublet. Thirteen of the 15 theoretical lines of the a Center of a resolved 1:2: 1 triplet. f Center of a resolved 1:3:3: 1 quartet, ABC “vinyl” spectrum were observed; the intensities of two of the three combination lines were apparently too small for observation. Centerof aresolved 1:6:15:20:15:6:1 septet. f Center of aresolved 1:3:6:10:12:12:10:6:3:1 multiplet. o.d., straight to within 0.003 in. over an 8 in. length, and with hemispherical bottoms, Samples were thoroughly degassed and sealed in vacuo. Materials ,-The materials employed were purified by fractional distillation in vacuo or by repeated recrystallization. Some were commercial products and some were prepared in this Laboratory. Methyl-N,N-bis-(trifluoromethyl)-carbamate and perfluoro-N,N-dimethylacetamide were the gifl, of Profs. R. Dresdner and J. A. Young, University of Florida. Experimental and literature values for physical constants agreed for all the materials used except N,N-dimethyltrichloroacetamide, for which no literature values were found. ThiEi material. nrenared in this Laboratory, had b. 85.2’ (5 mm.). ‘dlalid. for C4H6O N C k C, 25.22; 3.18; N,7.36; C1, 55.85. Found: C, 25.43; H, 3.25; N,7.52; C1, 55.66. Experimental Method.-High-resolution n.m.r. spectra for the amides were obtained at a series of temperatures. A linear sweep rate of -0.012 p.p.m./sec. was used. Internal frequency separations were measured by the audiosideband technique and counted with a calibrated electronic frequency counter (Hewlett-Packard Model 521 A). The chemical-shift difference 6v between lines of the resolved -N( C H S ) ~doublet was taken as the largest measureable value which was the limiting maximum value a t the lower temperatures. The ratio of the average of the intensity maxima of the two components of the doublet to the central minimum was taken from the spectrum as recorded on a Varian Associates modified Model G-10 recorder. The average of thle base lines on either side of the doublet was the zero of intensity. All spectra were recorded a t the same value of the applied R F field H I which was always much below the lowest value giving noticeable R F saturation and a t the maximum time-constant setting of the frequbncy response control of the spectrometer. From eight to twelve spectra were obtained a t each temperature, half being recorded with increasing and half with decreasing linear sweep fields. The probe was moved slightly between each pair of observations to search for increasing field homogeneity. Field homogeneity was repeal edly checked by observing the beating and/or decay of “wiggles” on rapidpassage for standard samples of acetaldehyde and/or pentachloroethane, respectively.
8,.
Results Chemical shifts (v-vref), in c.P.s., for the primcipal lines in the n.m.r. spectra of the substances studied are listed in Table I. All lines were measured a t a spectrometer frequency vo = 60.000 Me. Proton chemical shifts are relative to the ring (12) Wilmad Glass Co., Vineland, N.
J.
Fig. 1.-The 60.000 Mc. n.m.r. spectra of N,N-dimethylacetamide (DMA) and N,N-dimethyltrifluoroacetamide (DMTFA); (a) proton resonance of DMA, (b) proton resonance of DMTFA, and (c) fluorine resonance of DMTFA.
protons of toluene as an external reference and fluorine chemical shifts are relative to 1,Z-dibromo1,1,2,2-tetrafluoroethaneas an external reference, Where two values are quoted for an -NR2 group, they represent the limiting maximum separation observed €or the doublet a t the temperature shown. The p.m.r. spectrum of DMA is shown in Fig. l a , The high-field line a t 261.5 C.P.S. arises from the C-methyl protons and the doublet a t 205.7 and 216.3 C.P.S. is due to the protons of the two nonequivalent N-methyl groups. Very weak nonequivalent spin-spin coupling of the C-methyl protons with the protons of each N-methyl group causes the effective natural line width for the highfield N-methyl group to be broader than that for the low-field N-methyl group. These spin-spin couplings could not be resolved. A similar, but much smaller, difference in line width is observed
542
MAXT. ROGERS AND JAMESC.WOODBREX
Eig. 2 -The p.m.r. spectrum of N,N-dimethyltrifluoromebarnide at various temperatures under conditions of double irradiation; Y O = 60.000 Me., v1 = 56.452 Mc. T h e linear sweep rate and recorder gain were not exactly the same for all the doublets shown.
for the two X-methyl resonances in N,X-dimethylpropionamide (DMP) but was not noticeable for N,N-dimethylacrylamide and K,N-dimethylbenz.amide (DMB). The p.m.r . spectrum of ?J, N-dimet hyltrifluoroacetamide is shown in Fig. l b and the fluorine resonance spectrum is shown in Fig. IC. Koaequivalent spin-spin coupling of the protons of the two non-equivalent S-methyl groups with the fluorine nuclei of the C-trifluoromethyl group produces the two p.m.r. quartets. These two quartets are simplified to a single doublet by decoupling the proton-fluorine spin-spin interactions by irradiating the sample with a strong 56.452 Mc. fluorine-resonance RF field in addition to the weak 60.000 Ilk. proton-resonance RF field. The doublet resulting from the double irradiation of N,X-dimethyltrifluoroacetamide at various temperatures is shomn in Fig. 2. The similarity of the line widths for the components of each doublet is a good indication of the completeiiess of the fluorine-proton decoupling. These doublets are representative of those observed for many of the other amides studied without double irradiation. The mean lifetime of protons at sites A and B (as in I), T A and T B , respectively, must be equal and the quantity T = [ T ~ T B / T * PB] may be related t o the ratio T of maximum to central minimum v-mode intensities for the -N(CHJ2 doublet by the equation
Vol. 66
doublet is negligible [l/rz.k < < 6 v > > I/T~B,where 2 / T u and 2 / T 2 ~are the line widths at one-half maximum intensity of the components A and B, respectively, in radians per second (r.p.s.), and in the absence of rotational averaging]. It also assumes that the fraction of protons is the same at each site (Pa = Pg, T A = T B = 2 7 ) , that RB saturation is negligible, and that “slow-passage’’ conditions are maintained. Equation 1 is easily derived from equation A1 in the Appendix by imposing the first and second restrictions just mentioned and by defining r = (vmax,/vmin.) = [v(2Au = &~,)/r(Aw = O)], where 6w, is given by equation 6 of ref. 3. Segligible RF saturation and “slow-passage” conditions already are implied by equation Al. The assumption that the overlap effect is negligible is not valid for all the amides studied. The method used for correcting for the effect of overlap for these amides is discussed in the Appendix. The experimental activation energy E,, here identified with the barrier height restricting internal rotation about the central C-K bond of the amide, is obtained from the measurements of the rate of internal rotation (1/2T) at a series of temperatures by fitting the data to the Arrhenius equation log ( 1 / 2 r ) = log A
- EJ2.3026RT
(2)
T.-alues of E, and the frequency factor A were derived from linear plots of log ( 1 / 2 ~us. ) 103/2.3026RT by the method of least squares. To illustrate the precision of the present measurements the Arrhenius plots for the compounds studied are shomn in Fig. 3. The activation energies and frequency factors obtained from these plots are shown in Table I1 along with the free energies of activation AF” based on the absolute reaction rate theory. Coalescence temperatures T , also are given. The free energies of activation are computed from the relationship AFT” = 2.3026RTlog
(*y) -
(3)
where T is taken from the Arrhenius plot as the least-squared value at the temperature T . Transmission coeficients K are assumed to be unity. Each coalescence temperature is taken from the least-squared Arrhenius plot as the lowest temperature for which r is unity. The precision obtained by use of this method is satisfactory for most of the substituted X,N-dimethylamides studied because values of 6v were large enough that overlap of the doublet components could be neglected (ie., 1 / T 2 ~> l / T z ~ )the , ratio r could be measured quite precisely, and the changes in r are more pronounced than changes in line widths or changes in apparent chemical-shift differences. The chief disadvantage was the limited temperature range (about 25’) 1 = =t----..\/T f ( r 2 -- r)’/a which could be used. The data of Table 111, from *d2 which the Arrhenius plot for N,N-dimethyltrifor ?r d’2rav > 1 (1) fluoroacetamide in Fig. 3 was drawn, illustrate where BY is the chemical-shift difference between the magnitude of the average deviations in r and ) a typical case. The limits of E, the two N-methyl sesonances A and B in the ab- in log ( 1 / 2 ~ in sence of rotational averaging. Equation 1 implies and log A were computed for each compound for that the effect of overlap of the components of the 9O(r, confidence using convenltional statistical
+
March, 1962
HINDERED INTERNAL ROTATION IN SUBSTfTuTED N,N-bIMETHYLAMIDES
543
TABLE I1 VALUESOF E,, I,OG A, AF*2ss.z A N D T , FOR HINDERED INTERNAL ROTATION ABOUT THE CENTRALC-N BOND OF SOMESUBSTITUTED W,N-DIMETHYLAMIDES AB DETEEMINED BY PROTON MAGNETICRESONANCE SPECTROBCOPY"~~ ( y o = 60.000 Mc.) Ea
Amide N,N-D1tneth:ilformamide N,N-Dimeth:ilacetsmide N,N-Dimethylpropionamide N,N-Dimethyltrifluoroacetamide N,N-Dimethyltrichloroaoetamide N,N-Dimethylacrylamide N,N-Dimethylbenzamidea N,N-Dimethj,lcarbamyl chloride
koal./hole log A 18 3 f 0.7 10 8 f 0 . 4 10 6 i . 4 7.8 .2 9 . 2 zt . 7 7.3 i .5 9 . 3 f . 6 6.8 f . 4
*
AF*zss.z, koal./ mole 21.0 17.4 16.7 17.6
OK. 421 6 360.3 534.4
Tc,
367.9
9 . Q i: . 3
9.1 f . 2
14.9
287.1
6.8 i .7
6 . 0 f . 5 16 1 7.2 i .4 15.3 6.1 f . 3 1 6 . 5
284.9 326.0
7 . 7 j. . 5 7.3 i ,5
These results are all derived from equations 1, 2, and 3 without corrections for the effect of overlap of the components of the chemical-shift doublets. * Thevalues and errors given include the limits of 90% confidence. A 36.34 mole % amide solution in CHZBr2. a
methods. For N,N-dimethyltrifluoroacetamide these limits (Table 11) are 9.3 It 0.6 kcal./mole in Ea and 6.8 i 0.4 in log A.
IOs
/ 2.ZOP6
RT,(MOLES/KCALl.
Fig. 3.-Arrheniusplots for the process of internal rotation about the central C-N bond of amides of the type RCON(CH& Each curve is designated with the appropriate functional group R. A 36.34 mole 7' amide solution in CH2Br2. (I
son7 did not yield a reliable value for E, in DMP since the range of line widths that could be used was small (0.3 to 3.Q c.P.s.) and the errors in TABLEI11 measurement of the widths large ( f 0.3 c.P.s.). The method of Gutowsky and Holm3 was applied TEMPERATURE DEPENDENCE OF THH, RATE OF IXTERNAL to D M F but the limits of 90% confidence in E,. ROTATION ABOUT THE CENTRAL C-N BOND were 14.3 i 3 kcal./mole. In the region of slow O F N,N-DIMETHYLTRIFLUOROACETAMIDEa rotational rates 6Yobsd can be obtained precisely Y O = 60.000 Mc., 6v = 7.48 i 0.30 c.p.s, a t 21.4' but it is very insensitive to 7. Near coalescence, 103/2.3026R T , r t, oc. mole/kcak. 6Yobad becomes quite sensitive to 7 but the former 89 87 =k 0 11 0 . 6 0 2 0 f 0 0002 1 066 zt 0 008 1 1584 zt 0,0040 cannot be measured precisely because of the lines . 6 0 7 7 i .0001 1 2 2 1 i : 014 1 IO02 i 0041 86 48 i .06 broadening which accompanies the averaging, . 6 1 4 4 z t ,0001 1 5 2 7 f 023 1 0 2 8 3 i 0044 82 54 i 07 .6195 f .0001 1 843 zt 011 0 9756 =t 0016 process. TQ 62 f 07 ,6251 i ,0001 2 324 f 012 0 Ql.53f 0013 76 47 f .04 Keglecting the effect of overlap of the corn-, 75 40 i 04 6 2 7 0 ~ 0001 2 426 i: 031 o 9645 =t 0042 ponents of the N-methyl doublet we find E, = 70 21 i: .15 .6366 * ,0003 3 513 i .069 0 8146 zt 0004 10.6 f 0.4 kcal,/mole and log A = 7.8 f 0.2' in 62 07 f . i i 6519 I 0002 5 822 =t 189 o 6975 i 0701 DMA (Table 11). When the overlap correctians a The errors given are the average deviations from the average of five or more measurements. b These values were are made, as discussed in the Appendix, the values calculated from equation 1 . E, = 11.6 i 0.8 kcal./mole and log A = 8.4 i 0.6 The slope b = - Ea and the iiitercept a = log A are obtained. For DMF, each component of the of the linear plot of y = log (1/27) us. x = 103/ N-methyl chemicaLshift doublet consists of a re2.3026 RT mere found by the method of least solvable spin-spin doublet (Table I). Keglecting squares. 'The limits of 90% confidence in E, are the effect of overlap of these components we find E, = 18.3 i 0.7 kcal./mole and log A = 10.8 A= given by13 0.4 in DMF. The appropriate corrections6 for the effect of overlap would inerease the uncor(4) rected values we find for E, and A in DMF. where 2 is the average of the n values of xiand Overlap corrections for all the other amides studied are estimated to be smaller than the normal experimental errors given in Table 11. z t depends on the number of degrees of freedom (n Discussion 2) and the confidence limits and converts the standThe energy barrier internal rotation ard error to the confidence limits desired. The about the central C-N hindering bond of DMA is 11.6 * limits of 90% confidence in log A were computed 0.8 kcal./mole, in agreement with the value 12 f from 2 kcal./mole reported earlier by Gutowsky and Holm. Without making the appropriate cor2. J r e c t i o n ~for ~ the spin-spin splitting and the overlap Attempts to measure E, by other methods did of the chemical-shift doublet we find E , = 18.3 f not yield satisfactory results for the compounds 0.7 kcal./mde for the barrier hindering rotation in studied here. The method of Piette and Ander- DMF. This value is at variance with two values, 7 f 3 kcal./mole3 and 9.6 f 1.5 kcal./molej re(13) J. F. Kenney and E. S. Keeping, "hlathematios of Statistics," ported earlier. We cannot offer an explanation fop part two, 2nd ed., D. Van Nostrand Co., Ino., New York, N. Y . , 1951, pp. 207-211,416-417. these large discrepancies. Our data for DMF,,
MAXT. ROGERS AND JAMESC. WOODBREY
544
\\
7-
'
5-
LS 3-
I-
, Lcg,,
mn.
Fig. 4.-Plots of rRV. US. log ( l / g ) for PA = Pn = l / z l 6w = 66.288 r.p.s., and (a) l / T z ~= 2.20 r.p.s., l/Tzn = 1.90 r.p.s., (b) 1 / T z ~= 4.30 r.p.s., 1 / T z ~= 3.80 r.p.s., (c) 1/T2A = 6.46 r.p.s., 1/T2n = 5.70 r.p.s.
with the appropriate corrections for the spin-spin couplings and the effect of overlap of the chemicalshift doublet, would give a value for Ea greater than 18.3 kcal./mole. These corrections are estimated to be of the order of fl to +2 kcal./mole. Our uncorrected value is in agreement with an earlier value of about 18 kcal./mole14 for the barrier height in DMF. These high barriers are the same as the value 18 rt 3 kcal./mole reported4 for the barrier height restricting internal rotation about the C-N bond in formamide. The values of Eafor DMP (9.2 0.8 kcal./mole), N,N-dimethyltrichloroacetamide(9.9 f 0.3 kcal./ mole) and N,N-dimethyltrifluoroacetamide (9.3 =k 0.6 kcal./mole) are smaller than for DMA but the differences are not very large compared to the errors involved. Since the -CF3, -CCb, -C~HS,and -CH3 groups differ greatly in ability to withdraw electrons, in ability to participate in hyperconjugation, and in size it does not appear that these factors influence the barrier heights very strongly. The barrier height is significantly smaller in N,Ndimethylacrylamide, E, = 6.8 i 0.7 kcal./mole, and in N,N-dimethylcarbamyl chloride (DMCC), E, = 7.3 i 0.3 kcal./mole. I n these compounds cross-conjugation as represented by structure I11 (for example) may tend to reduce the double bond character of the central C-N by competing with the resonance form to which the major portion of the energy barrier is attributed (structures analogous to 11). The lower barrier in DMB (7.7 -:O:
I11 could similarly result from crossconjugation. The latter value was measured in a 36.3 mole yo solution of DMB in dibromomethane so it is not directly comparable with the other f 0.5kcal./mole)
(14) Technical Information Bulletin from the Radio-frequency Spectroscopy Laboratory of Varian Associates, Instruments Division, "01.2 of Series A , No. 28, Palo Alto, California, 1967.
Vol. 66
values. However, it should be comparable with ' solution in the value for DMP in a 36.3 mole % dibromomethane and with the value for DMCC in a 36.3 mole % solution in dibromomethane, which are estimated to be 9.7 f 0.7 and 8.5 =t0.6 kcal./ mole, respectively. 15pi8 A number of N,N-disubstituted amides failed to show the expected doublet for the -NRz resonance, even at the lowest temperatures a t which we were able to make observations. Thus ethyl N,N-dimethylcarbarnate, R = CH3 and R' = C2H6 in VI, and methyl N,N-bis-(trifluoromethy1)-carbamate, R = CF3 and R' = CHBin VI, show a single sharp resonance line for the -NRz group down to their respective freezing points. The spectrum of perfluoro-N,K-dimethylacetamide (VII) consists of a single 1:3 :3 :1 spin-spin quartet for the N(CFa)a group at temperatures down to the freezing point. One cannot say whether in these cases the chemicalshift differences between the -NR2 groups are zero or whether the exchange rates ( 1 / 2 r ) between the
VI
VI1
sites remain rapid down to the freezing points. For the carbamates cross-conjugation involving important structures such as VI would tend to make the two C-0 bonds more similar. Since the chemical-shift difference vA - VB depends" extensively on the magnetic anisotropies of these bonds, it seems likely that the similarity of the two C-0 bonds results in a small chemical-shift difference. To the extent that structures such as VI contribute to the ground state the barrier restricting internal rotation about the central C-N bond would, of course, be correspondingly lower. Both effects may operate simultaneously. The failure to detect a chemical-shift difference between the N-trifluoromethyl groups in VI1 is quite unexpected since the barrier height might be expected to be about the s&meas that for N,Ndimethyltrifluoroacetamide (9.3 i= 0.6 kcal./mole). Possibly the chemical-shift difference between the N-trifluoromethyl groups ol perfluoro-N,N-dimethylacetamide is very small even under conditions of no rotational averaging about the central C-N bond. The strongly electron-wi thdrawing trifluorometliyl groups bonded directly to nitrogen may, however, suppress the double-bond character of the central C-N bond. This latter effect would suppress the energy barrier and, to the extent it is important in VII, it also mould be operative in methyl-N,K-bis- (trifluoromethyl) -carbamate. (15) J. C. Woodbrey and M. T. Rogers, J . Am. Chem. SOC.,84, 13 (1962). (16) It has been pointed out by a referee that the differences among barrier heights in the different pure materials may be, a t least in part, solvent effects, since each is measured in a different solvent-namely itself. One might expect that these disubstituted amides would be rather similar as solvents so that at least the large differenoes among the barriers should be significant. However, until measurements of a variety of these compounds are made in a single solvent this point will not be settled. (17) P. T. Narasirnhan and M. T. Rogers, J . Phys. Chsm., 6.3, 1388 (1959).
March, 1962
HINDERED INTERNAL ROTATION IN SUBSTITUTED N,N-DIMETHYLAMIDES
The experimental frequency factors A (Table 11) obtained from the Arrhenius equation vary between lo6 and 1Olo sec.-l. These are much lower than k T / h and suggest that the transmission coefficients K of the Eyring rate expression are low for these internal rotations if Laidler'c; suggestion18 that internal rotations have low entropies of activation is true. As a consequence of the low frequency factors the free energies of activation, as calculated from equation 3 with K = 1, are much larger than the energy values E, determined from the Arrhenius equation. It should be noted that in 1)MF the N-methyl protons resonating a t higher field are coupled stronger to the aldehydic proton than are the Smethyl protons resonating a t lower field; see Fig. 1 and Table I. A similar but much less pronounced effect is observed in DMA and DMP, These latter interactions are rare examples of H-H coupling through five bonds. I[n these amides the trans coupling presumably exceeds the cis coupling as in the case of ethylenic-type fragments.lg Rere the interpretation requires that the N-methyl protons cis to oxygen resonate at higher field than those trans to oxygen, a reasonable assumption in view of the high magnetic anisotropy of the C=O bond1? and the high electron density surrounding the oxygen atom. I n contrast, in N,N-dimethyltrifluoroacetamide the protons resonating a t higher field are coupled more weakly to the fluorine nuclei than are the protons resonating a t lower field. Providing the protons cis to oxygen in this amide resonate a t the higher field, so that lJ,,, H- F I = 2 IJt,,, H- F I , then the cis H-F' coupling observed here must involve a mechanism strikingly different from that for H-H coupling. The above interpretation of the cis-trans relationships suggests that a through-space electron-coupling mechanism, rather than a through-bond mechanism, may dominate for H-F coupling in cases where the proton and fluorine nuclei are very proximate but not directly bonded to each other. It has been suggestedZ0that such a through-space mechanism may be responsible for a rare H-F coupling of nuclei separated by five bonds. The interactions in N,Ndimethyltrifluoroacetamide are other rare examples of H-F coupling of nuclei separated by five bonds. I n particular, the trans coupling in this amide, probably with IJHFI= 0.7 c.P.s., is a very rare case of H-F coupling through five bonds; it is unlikely that this trans coupling could involve any throughspace mechanism.
545
in DMF6 where the formyl-methyl proton-proton couplings can be resolved; see Table I. The non-equivalent couplings of the protons of each N-methyl group of DMA with the C-methyl protons are the cause of the different apparent natural line widths for the two I"-methyl resonances. Each N-methyl proton resonance line actually consists of an envelope resulting from an unresolved 1:3 :3 :1 spin-spin quartet. Because the widths of these envelopes are different, the resulting -N(CH3)2 doublet has considerable asymmetry. The numerical solutionsz1for the exchange averaging of a symmetrical spin-spin doublet therefore are not valid for the -N(CH3)2 doublet of DMA. To account for the asymmetry we have solved the appropriate equations numerically. Following Gutowsky and Holm,8 for the more general case of a completely asymmetrical doublet, their equation 3 may be rewritten as
where
s - raw2
[
REAW 1
+
A ;(
--+~
AB)]'
Similarly, for the more general case, their equation 5 becomes 272AAw5 + 372BAw4 + 4 7 2 c A w 3 f (BD - AE)AwZ 2(CD
- A F ) A w + (CE - B F ) =
+
0 (A 2 )
where
D
3
1
[
+
-- f
T
A ;(
--)] 1
2
- 27s
TZB
Appendix To correct for the effect of overlap of the -IT(CHJ2 chemical-shift doublet the lines arising from the spin-spin couplings of the N-methyl protons with otheir nuclei are not treated individually. Rather, only the effect of such interactions on thc apparent natural line width of each N-methyl resonance is considered. This restriction leads to no serious limitations in the amides studied, except
The effect of overlap on the apparent separation 6w, of the two A and B resonance lines in the absence of exchange is obtained by letting Aw + A w , as 7 4 m in equation A2. The resulting equation, fifth order in 6w, may he used to correct for the effect of overlap on the apparent separation of Co , any two Lorentzian line shapes.
(18) K. J. Iaidler, "Chemical Kinetics," MoGraw-Hill Book Ino., New York, N.Y., 1950, pp. 105-108, 382-387. (19) M. Karplus, J. Chem. Phye., S O , 11 (1959). (20) D. R. Davis, R. I'. Luts, and J. D. Roberts, J . Am. Chsm. SOC., 88, 246 (1960).
(21) "Tables of Exchange Broadened Multiplets," Technical Note No. 2, Contract AF 61 (052)-03, The Weizmann Institute of Science, Rehovot, Israel, 1958.
NOTES
546
Equations A1 and A2 were programmed to a Bendix G-15 digital computer. This program allows one to calculate from equation A2 the set of positions of the maxima and of the central minimum in the shape function v corresponding to a set of values for the rate of internal rotation (I / 2 T). Then, in the same program, the set of inteiisities of urnax, and vmin. corresponding to the same set (1/2r) are computed from equation Al. These programs22allow one to obtain by direct computations the apparent line separations and the relative values of the maxima and central minimum intensities for any Lorentzian doublet as a function of the rate of averaging between the two sites. In addition, the programs may be used for the direct computation of the line position and of relative intensities vrnax. for the coulesced doubletz3 as a function of the rate of averaging. For each amide studied the fraction of protons P A and P B at each site A and B, respectively, must always be the same; Le., PA = PB = l / 2 . The apparent natural line widths, l / n T z ~and l / ~ T z g (22) Copies of the programs may be obtained from J. C. W. (23) &I. Takeda and E. 0. Stejskal, J. Am. Chem. Soc., 81, 62 (1959).
Vol. 60
in c.p.s. were the limiting minimum values measured directly from the p.m.r. spectra at the lower temperatures. The effect of overlap on the apparent separation of the two -N(CH3)2 resonance lines was negligible in the absence of internal rotation for each amide studied; i e . , 6w N 60,. When obtaining the rates of internal rotation from the ratios of intensities (except for DMF) the only one of the amides studied for which the apparent natural line widths could not be neglected was DMA. Changes in the apparent natural line widths for DMA due to changes in the applied field homogeneity were determined from the width of the C-methyl resonance line a t each temperature. The ratio of the average of the maxima to central minimum intensities rav. was computed from equations A1 and A2 for the appropriate values of 1/TZ* and 1/Tzg and for different values of T. Typical plots of rBY. vs. log (1/27-) for representative values of l / T z ~and 1/Tzg are shown in Fig. 4. From such plots, the value of log (1/27) for DMA was obtained from the observed values of the natural line widths and of rav. at each temperatme.
NOTES TI-IERMODYXAMICS O F H-BOSDING PYRROLE-PYRIDISES BY
H.4ROLD
J. ~ ~ I M E T TAND E ~ ROBERT H. LINSCLL~
Department o f Chemzstry, C'nzversztu of Veimont, Burlangton, T't. Recewed June 15, 1061
Introduction H-Bonding equilibrium in the system pyrrolepyridine has been investigated by several workers. Vinogradov and Linnellz reported a 1: 1 H-bonded complex with pyrrole-pyridine in CC14 and K = 2.66 1. mole-1 (from infrared)3 at room temperature; the heat of formation was determined calorimetically using pure pyrrole and pyridine (no CC14 solvent) to be 3.8 kcal. /mole. Fusoii and co-workers have reported a number of investigations on pyrrole : an infrared study of solvent effects on the N-H of p y r r ~ l eand , ~ later work5 which yielded K = 2.7 5 0.3 1. mole-l for the pyrrole-pyridine equilibrium at room temperature in CC14 Halleux6 has used infrared to study H-bonding equilibria between various pyridines or anilines and phenol in CCl,. ?;one of these workers has made a thermodynamic study. When the preseiit work was almost finished, Happe's? double resonance nuclear magnetic res(1) Scott Research Laboratories Inc , Perkasie, Pa (2) S N Vinogradov and R. H Linnell, J Chem P h y s , 23, 93 (1955) (3) Recalculated fiom ref 2 Original i a l u e in mole fraction units (4) M L Josien and N Fuson J Chem P h y s , 22, 1169 (1954). ( 5 ) N Fuson, P Pineau and ;\I I. Joeien J . cham p h y s , 5 6 , 454 (1958). (6) A. Halleux, Bull soc chzm Belges 68, 381 (1959) (7) J A. Happe J . P h y s Chem , 65, 72 (1961)
onance study of pyrrole-pyridine association was published, including thermodynamic work. We were interested in studying the pyrrolepyridine association with several pyridines and at several temperatures so as to obtain therniodynamic data useful in understanding basicity and steric and solvent clustering eff'ects in acid-base equilibrium. Experimental All infrared measurements were made on a Perkin-Elmer ;\lode1 112 instrument equipped with a LiF prism. The entire light path was continuously swept with dry air. The source and sample compartment was equipped with a thermostat air-bath consisting of electric heaters, refrigerator coils, air circulating fan, and aothermoregulator-relay, maintaining temperatures to 1 0 . 2 . A 5-mm. XaC1 cavity cell was used for all measurements. Three thermocouples werc inserted into holes bored in the cavity cell and temperatures were read on a L. and S . type K-2 potentiometer. A special clamp arrangement held the Teflon plug in the cavity cell and prevented solvent evaporation. Spectra were run after all three thermocouples came to within 0.5" of the same temperature; this thermal equilibrium requircd from 30-45 min. for each sample. Spectral slit widths of 5-8 cin.-l were used. Eastman sulfur-free CCla, dried over silica gel, was used as a solvent. The pyridines, obtained from Reilly Tar and Chemical Company, were dried over NaOH pellets and purified by distillation in an all-Pyrex Todd column at high reflux ratio, then stored over NaOH pellets under pre-purified Nz in the dark. The F l i n g points of the cuts were found to be: pyridine, 115 ; 2-methylpyridine, 128'; 2,6-dimethglpyridine, 142'. Pyrrole v,-as given to us by E. I. du Pont de Nemours & Company and was purified by distillation under prepurified i%2 at atmospheric pressure in the Todd column at high reflux. The pyrrole sample had a b.p. of 128', and was stored under r\'2 at -10". PyrroleCCId solutions in contact with air, at room temperature,
