T H E
J O U R N A L
OF
PHYSICAL CHEMISTRY
Registered in U . S. Patent Ofice @ Copyright, 1070, by the American Chemical Society
VOLUME 74, NUMBER 6 MARCH 19, 1970
Rotational Barriers in 0-(N,N-Dimethylcarbamoyl) Oximes. Experimental Results and Nonempirical Molecular Orbital (NEMO) Calculations by C. Hackett Bushweller, Philip E. Stevenson, John Golini,' and James W. O'Neil Department Of ChenZistTy, WOTCesteT Polytechnic Institute, WOTCesteT, Massachusetts Of 609 (Received OCtObeT $7, 1969)
A total line shape analysis of the temperature dependence of the proton magnetic resonance spectrum of the N(CH& group in 0-(N,N-dimethylcarbamoy1)-acetoneoxime gives AH* = 15.4 kcal/mol, AS* = $2.7 eu AG* = 14.7 kcal/mol at -loo, and E. = 16.0 kcal/mol for rotation about the carbonyl carbon-nitrogen bond. Similar behavior is observed in 0-(N,N-dimethylcarbamoy1)cyclohexanone oxime and 0-(N,N-dimethylcarbamoy1)fluorenone oxime. Nonempirical molecular orbital (NEMO) calculations have been performed indicating substantial T bonding involving both oxygen atoms and the nitrogen atom of the carbamoyl group.
Resonance interactions coupled with inductive and complete nmr line shape analysis was performed on the steric effects are important in determining the magnitemperature dependence of the N(CH& resonance tude of barriers about formally single or double bonds. Examples of increased barriers about single bonds due (1) National Science Foundation Undergraduate Research Particito resonance effects include amides,2 f ~ r m a m i d e s , ~ pant, Summer 1969. enamines,* amidines,6 l-acylpyrroleslB triazenes,' hy(2) R. C.Neuman, Jr., and V. Jonas, J. Amer. Chem. Soc., 90, 1970 (1968),and references therein. d r a z o n e ~ ,N-nitrosoamines,* ~ and N,N-dimethylmeth(3) M. Rabinovitz and A. Pines, ibid., 91, 1585 (1969),and references anesulfinamides. * Indeed, resonance and steric therein. effects appear to be major factors in the drastic reduc(4) A. Mannschreck and V. Koelle, Tetrahedron Lett., 863 (1967); tion of the barrier to rotation about formal carbonA. P. Downing, W. D. Ollis, and J. 0. Sutherland, Chem. Commun., 143 (1967). carbon double bonds in ketene mercaptals, lo ketene (5) D. J. Bertelli and J. T.Gerig, Tetrahedron Lett., 2481 (1967). amina1s,l0vinylamines," and vinyl ethers.12 (6) T.Natsuo and H. Shosenji, Chem. Commun., 501 (1969). There have been few reports concerning the detection (7) N. P. Marullo, C. B. Mayfield, and E. H. Wagener, J. Amer. of a barrier to rotation about the carbonyl carbonChem. Soc., 90,510 (1968). nitrogen bond in carbamates13* or ureaslaband no data (8) C . E. Looney, W. D. Phillips, and E. L. Reilly, ibid., 79, 6136 (1957); 8. Andreades, J . Org. Chem., 27, 4163 (1962); A. Mannsconcerning 0-(N,N-dimethylcarbamoyl) oximes.14s chreck, H. Munsch, and A. Matteus, Angew. Chem., 5 , 728 (1966); This paper concerns the measurement using variable P.K. Korver, P. J. Van Der Haak, and T. J. DeBoer, Tetrahedron,22, 3157 (1966). temperature nmr spectroscopy of the rotational barrier (9) H. J. Jakobsen and A. Senning, Chem. Commun., 617 (1967). about the carbonyl carbon-nitrogen bond in 0-(N,N(10) A. Isaksson, J. Sandstrom, and I. Wennerbeck, Tetrahedron dimethylcarbamoy1)acetone oxime (I), 0-(N,N-diLett., 2233 (1967). methy1carbamoyl)cyclohexanone oxime (11), and 0(11) Y.Shvo, E.C. Taylor, and J. Bartulin, ibid., 3259 (1967). (N ,N-dimethylcarbamoy1)fluorenone oxime (111). A (12) Y.Shvo, ibid., 5923 (1968). 1155
C. H. BUSHWELLER, P. E. STEVENSON, J. GOLINI,AND J. W. O'NEIL
1156
(111)
in I. A significantly lower barrier in I as compared to normal amides is qualitatively consistent with nonempirical molecular orbital (NEXO) 14b calculations reported here indicating substantial electronic delocalization over the oxime oxygen, the carbonyl group, and the carbamoyl nitrogen.14"
Experimental Section The nmr spectra were recorded using a Varian HRBOA spectrometer equipped with a custom-built variable temperature probe. Spectral calibrations were performed by the audiomodulation technique using a Hewlett-Packard 651-A audiooscillator and a HewlettPackard 5221B electronic counter. Temperature measurements were performed using a calibrated copper-constantan thermocouple placed about 1 in. below the sample and were taken simultaneously with the recording of the spectrum. I n the probe used, the temperature gradient over the sample length is no more than 0.1O and temperature measurements at the sample are accurate to *0.1". 0-(N, N-Dinzethylcarbamoyl) acetone Oxime. Acetone oxime (5.3 g, 0.079 mol) , SIN-dimethylcarbamoyl chloride (8.5 g, 0.079 mol), and 30 ml of anhydrous pyridine were mixed at room temperature and allowed to stand at 0" for 9 days during which time pyridinium chloride precipitated. The reaction mixture was poured into a mixture of 50 ml of concentrated hydrochloric acid, 100 g of sodium chloride, ice, and water totaling 500 ml. This mixture was continuously extracted (ether) for 3 days, the ether solution was dried (Na2S04),and the ether was removed under vacuum. The residue was fractionated to give 8.5 g (75%) of 0-(N,N-dime thylcarb amoyl) acetone oxime : bp 8 6-87 O (3 mm); nmr peaks (60 MHz; CSZ) at 6 1.91 (6H) and 6 2.88 (6H). Anat. Calcd for CBHIZNZOZ: C, 49.98; H, 8.39; E, 19.43; 0, 22.19. Found: C, 49.92; H, 8.40; N, 19.34; 0,22.14. 0-(N,N-Dimethylcarbamoy1)cyclohexanone Oxime. Cyclohexanone oxime (25.0 g, 0.22 mol), N,N-dimethylcarbamoyl chloride (23.8 g, 0.22 mol), and 50 ml of pyridine were treated identically with the immediately The Journal of Phgsical Chemistry
previous preparation. The ether was removed from the solution resulting from the continuous extraction, and the residue was recrystallized from petroleum ether to give 22.0 g (55%) of 0-(N,N-dimethylcarbamoy1)cyclohexanone oxime: mp 56.5-57.5'; nmr peaks (60 MHz, CS2) at 6 3.03 (6H), 6 2.50 (4H), and 6 1.75 (6H). Anal. Calcd for C9Hl0N2O2. C, 58.67; H, 8.76; N, 15.19; 0, 17.38. Found: C, 58.74; H, 8.80; N, 15.19; 0, 17.35. 0-(N,N-Dimethylcarbamoy1)JEuorenone Oxime. Fluorenone oxime (4.1 g, 0.021 mol), N,X-dimethylcarbamoyl chloride (2.3 g, 0.021 mol), and 30 ml of pyridine were treated as in the two previous preparations. Upon addition of the hydrochloric acid, sodium chloride, ice, and water mixture, a yellow solid formed slowly. The yellow solid was separated by filtration and recrystallized from petroleum ether to give 4.2 g (75%) of 0-(NIX-dimethylcarbamoy1)fluorenone oxime : mp 110-11lo; nmr peaks at 6 8.1-7.1 (8H) and 6 3.06 (6H).
Results and Discussion Examination of the nmr spectrum (60 JIHz) of the N(CH& protons (6 2.88) of 0-(N,N-dimethylcarbamoy1)acetone oxime (I; 0.5 M in CSZ)at 35" revealed a sharp singlet resonance characteristic of rapid exchange on the nmr time scale. Upon lowering the temperature, the N(CH& resonance broadens in a manner characteristic of a decreasing rate of conformational exchange and then separates into two resonances of equal intensity in typical fashion. A total line shape analysis of this spectral behavior was performed using computer-generated spectra based on the treatment of Gutowsliy and Holm.15 Equal populations (or intensities) of the two resonances were assumed and the relaxation time (Tz)mas calculated from the width at half-height of either NCH3 resonance under conditions of slow exchange.15 This spectral behavior is entirely consistent with a kinetic effect on the nmr spectrum due to a decreasing rate of rotation about the carbonyl carbon-nitrogen bond and not to a temperature dependence of the chemical shift between the two nonequivalent NCH, resonance^.^ Indeed, in the sample studied, a negligi-
(13) (a) T. H. Siddall, 111,and W. E. Stewart, J . Ow.Chem., 34, 3261 (1967); (b) T. M. Valega, ibid., 31, 1150 (1966); E. H. White, M . C. Chen, and L. A. Dolak, ibid., 31, 3038 (1966); M. T. Rodgers and J. C. Woodbrey, J . Phys. Chem., 6 6 , 540 (1962); S. van der Werf, T. Olijnsma, and J. B. F. N. Engberts, Tetrahedron Lett., 689 (1967); E. Lustig, W. R. Benson, and N. Duy, J . Org. Chem., 32, 851 (1967). (14) (a) C. H. Bushweller and M . A. Tobias, Tetrahedron Lett., 595 (1968) ; (b) M. D. Newton, F. P. Boer, and W. N . Lipscomb, J . Amer. Chem. Soc., 88, 2353 (1966); (c) For another recent report concerning experimental and theoretical barriers in thioamides, see J. Sandstrom, J . Phys. Chem., 71, 2318 (1967). (15) H. S. Gutowsky and C. H. Holm, J . Chem. Phys., 25, 1228 (1956).
ROTATIONAL BARRIERS TO
O-(N,N-DIMETHYLCARBAMOYL) OXIMES
1157
Similar spectral behavior to that found in I was observed in I1 and 111. The free energies of activation (AG*) for rotation about the central C-N bond in I, 11, and I11 are compiled in Table 11. The rate conTable I1 : The Free Energies of Activation (AG *) for Rotation about the Carbonyl Carbon-Nitrogen Bond of Various 0-(N,N-Dimethylcarbamoyl) Oximes a t - 10" Compound
'
Ib IIC IIId 1
I
3.70 3.74
I
I
I
3.78
?IT
3.86
3.82 x
~
~
I
I
3.90
3.94
I
3.98
-1
3
Figure 1. The Arrhenius plot for rotation about the carbonyl carbon-nitrogen bond in 0-(N,N-dimethylcarbamoy1)acetone oxime (I). E, = 16.0 f 0.5 kcal/mol, log Y O = 12.0, AH = 15.4 i 0.5 kcal/mol, A S * = 2.7 i 1.5 eu, and AG*-loo = 14.7 i 0.1 kcal/mol.
*
ble temperature dependence of the chemical shift between NCH3 resonances w'as observed from -50 to -20". Thus, this source of error is apparently not important in our study but has been shown to be important in other compound^.^ For all spectra used in subsequent calculations, it was assured that the internal TMS line shape was Lorentzian in order to avoid the errors associated with asymmetry of line shapes. A mathematical comparison3 of the experimental and calculated nmr line shapes at various temperatures gave a series of first-order rate constants ( k ) for rotation about the carbonyl carbon-nitrogen bond in I (Table I). An Arrhenius plot of these data (Figure 1) gave
Av, Hea
3.2 2.7 8.9
k, 880-1
AQ*, kcal/mol
3.9 4.2 12
14.7 j = 0 . 1 14.6 f 0 . 1 14.1 i 0 . 3
Chemical shift between NCHI groups. 0 . 4 M in CSZ. 0.1M in CSg.
'0 . 5
M in CSg.
stants for I1 and I11 were derived from a complete nmr line shape analysis at one temperature. Measurements on I11 were complicated by sparing solubility. The substantially enhanced chemical shift between NCH3 resonances in 111as compared to I and I1 (Table 11)is interesting and suggests a not unexpected diamagnetic anisotropic effect of the aromatic nucleus. The entropy of activation (Ah'*) for C-N bond rotation in I is not atypical of such a kinetic process (Table 111). The substantially lower barrier ( A H * ) to rotation about the central C-N bond in 0-(N,N-dimethylcarbamoyl) oximes and carbamates13aas compared to normal amides2 and formamides* is consistent with the possibility of cross conjugation in these systems. Perusal of Table I11 illustrates the trends. In seeking a rationalization for the observed trend, postulation of resonance interactions in ureas13* and carbarnateslab with "competition" between canonical forms (e.g., IV and V) seems reasonable to account for the lowered barriers.
Table I: The Rate Constants for Rotation about the Carbonyl Carbon-Nitrogen Bond in
0-(N,N-Dimethylcarbamoy1)acetoneOxime ( I ) As a Function of Temperature T,OC
k, 800-1
-1.5 -2.6 -6.2 -9.5 -10.0 -11.2 -14.4 -20.2 -24.1
10.00 9.10 6.25 4.17 3.85 3.34 2.38 1.20 0.71
E, = 16.0 f 0.5 kcal/mol, log vo = 12.0, AH* = 15.4 f 0.5 kcal/mol, and A S * = 2.7 f 1.5 eu. The free energy of activation (AG') a t -10" is 14.7 f 0.1 kcal/mol as calculated from the Eyring equation assuming a transmission coefficient of 1.
0-
1
(V)
Using the NEMO method,14bwe have calculated the energy difference between the equilibrium (planar) geometry (VI) and the transition state (twisted) geometry X
X
Volume 74, Number 6 March 10, 1970
1158
C. H. BUSHWELLER, P. E. STEVENSON, J. GOLINI,AND J. W. O'NEIL
Table 111: Pertinent Rotational Barriers about the C-N(CH& Bond of Various Compounds Compound
AS*, eu
AH*, kcal/mol
AU*, kcal/mol
Referenae
19.0
f2.7
18.2 (25')
2
20.2
-1.7
21 .O (128')
3
..
10.7 (-39')
13a
11
-14
13b
14.7 (-10')
This work
Table IV: Overlap Populations and Computed Barriers to Rotation about the C-N Bond of Various Compounds Total Overlap populations CO,CS,CSe CN
n-Overlap populations CO,CS,CSe CN
2%
Compound
atomic units
Barrier, kcal/mol
0
I1
H-C-NHz
planar" twisted"
- 53.80202 - 53.73804
0.2288 0.3201
0 2235 0.0721
0.9499 1 0195
1,0507 0.9467
40.8
planar twisted
-152.70024 - 152.61606
0.1535 0.3006
0.2683 0.0794
0.8066 0.9219
1.1296 0.9920
52.8
I
I
S
/I
H-C-NHz Se
11
H-C-NH,
calculation l b planar twisted calculation 2b planar twisted
-750.07300 - 749.98779
0.1469 0.2775
0.2747 0.0811
0.8488 0.9630
1.1470 0.9990
53.4
- 750.10474 - 750.01343
0.1211 0.2741
0.2933 0.0869
0.7503 0.8805
1.1566 1 0142
57.2
planar
-76.55418
0,1458' 0. 1973d 0,1792' 0. 2327d
0.1638
0. 7943c
0.9382
0.0528
0. 8522d 0. 824gC 0. 870Sd
0.8378
0.1666 0.0535' 0.2084g 0.0650
I
0
I1
HO-C-NHz
twisted
- 76.50322
31.9
0
I/
HzN-C-NHz
planar one twist'
-71.19315 - 71.14532
0.1901 0.2259
two twists%
- 71
0.3021
I
07872
0.8460 0.8675
0.9641 0. 8740' 0.9870' 0.8677
0 9299 I
30. Oh 41.7j
" Planar = VI; twisted = 1711. b These calculations were performed using two different sets of available parameters. To OH To nitrogen on twisted NH2 group. To nitrogen on planar NHz group. oxygen. To C=O oxygen, e Only one NHz group twisted. Barrier to rotation about one NH2group if the other remains planar. Both NH2 groups twisted with respect to NCO plane. j Barrier to rotation of one NHZ group if other NHz group is twisted.
'
'
(VII) of formamide, thioformamide, selenoformamide, urea, and carbamic acid (H2N-COzH). The calculated energy differences, Le., barriers to rotation, are based on an approximate formula for total energy proposed and Lipscomb'6 (eq ')' The first by term on the right side of eq 1 The Journal of Physical Chemistry
@
E =
i
EtA
+
> .
elrn
(1)
3
is a summation of one electron energies over occupied orbitals in the sepawtecl atoms; hence it is independent of (16) M. D. Newton, F. P. Boer, and W. N. Lipscomb, J. Amer. Chem.
Soc., 88, 2367 (1966).
ROTATIONAL BARRIERS TO 0-(N,N-DIMETHYLCARBAMOYL) OXIMES
1159
the various structures. A consideration of the Mulliken molecular geometry. The second term is the convenoverlap populationsz1 (Table IV) for pertinent bonds in tional sum over the eigenvalues of the occupied molecthe various structures suggests strongly the widely ular orbitals. Equation 1 allows energy differences held bonding mechanism for the barrier to C-N bond between different geometries to be equated to the difrotation. This, of course, is consistent with the simple ferences between the eigenvalue sums. resonance picture presented previously. Indeed, The parameters employed in these calculations have been taken from the l i t e r a t ~ r e . ’ For ~ ~ ~the ~ ~first ~ ~ ~ the 7r-overlap populations (Table IV) for carbamic acid indicate substantial 7~ bonding not only between row atoms, they differ in one important respect from the carbonyl carbon and nitrogen but also between the those originally recommended in that all p- and d-orbital carbonyl carbon and hydroxyl oxygen leading to a anisotropy in the parameters is averaged out. This lowering of the barrier compared to amides. The was done to preserve a uniform set of parameters for same considerations apply to urea in the case where both equilibrium (VI) and transition state (VII) geomone NH2 group rotates while the other remains planar etries. I n addition, not enough is known about the dif(Table IV). However, for simultaneous rotation of ferences among components of p and d orbitals in sulfur both NH2 groups, all ?r overlap is minimized and the and selenium to make any distinctions. Therefore, avercalculated barrier approaches that in normal amides aged parameters for 3p, 3d, and 4p orbitals were used (Table IV). It seems clear that the mechanism for for these atoms. rotation about the C-N bond in urea is rotation of one Molecular geometries have been taken from the NH2 group at a time. literature when possible.: The C=Se bond length was estimated to be 1.82 A by comparison of hydrogen Acknowledgment. We gratefully acknowledge supsulfide and sulfur dioxide with hydrogen selenide and port of this work by Research Corporation (Cottrell selenium d i o ~ i d e . ’ ~The structure of the yet to be Grant) and the National Science Foundation (COSIP isolated carbamic acid was derived from formamide Grant) as well as assistance in the syntheses from Dr. with a hydroxyl group replacing the aldehydic proton. M. A. Tobias of Mobil Chemical Company. The NCO bond angle in carbamic acid was assumed to be the same as the NCH bond angle in formamide. (17) W. E.Palke and W. N. Lipscomb, J. Amer. Chem. Soc., 88, 2384 (1966); F. P. Boer and W. N. Lipscomb, J. Chem. Phys., 50, 989 The C-0 bond length and O-H bond len8th in carbamic (1969). acid were assumed to be 1.31 and 0.95 A, respectively, (18) J. E.Worsham, H. A. Levy, and S. W. Peterson, Acta Cryst., 10, 319 (1957); R.J. Kurland and E. B. Wilson, Jr., J. Chem. Phys., 27, being taken from formic acid.Ig 585 (1957);M. R.Truter, J. Chem. Soc., 997 (1960). I n all of these calculations, the equilibrium geometries (19) “Tables of Interatomic Distances and Configurations in Molewere planar (VI). The transition state (VII) geometry cules and Ions,” Special Publication No. 11, The Chemical Society, London, 1958. was constructed from the equilibrium geometry (VI) (20).Similar results were obtained in test calculations of the ethane by rotating the NH, group as a unit about the C-N barrier and of both barriers in propylene. These discrepancies are bond until the NH2plane was perpendicular to the plane due partly to the inherent approximations in the theory but also partly to a failure to use the optimum geometry for the transition state. of the remaining atoms. In the case of NHzCHO, we sought an optimum geometry through The computed barriers ( A H * ) are compiled in Table variation of the HNH angle and variation of the angle between the HNH plane and CN bond. These variations preserve all bond disIV. Although the calculated barriers are approxitances, as well as the mirror plane of the molecule. The results mately a factor of two higher than the measured values indicate a transition geometry in which the bonds about N are a t angles typical of NHs. The discrepancy between calculated and (Table 111),20 the sequence of calculated barriers clearly experimental values is reduced, but only by about one-third. indicates the experimentally observed trend, and the (21) R. S. Mulliken, J. Chem. Phys., 23, 1833, 1841, 2238, 2343 origin of the variation in the rotational barriers for (1955).
Volume 7.4, Number 6 March 19, 1070
