Jan. 20, 1962
COMMUNTCATIO NS TO THE
EDITOR
307
Determination of the rate and order of the re- note is to report that by taking advantage of the action from broadening of the ring and butyl lines latter aspect, and by making certain other reasonis straightforward. The reaction is second order able approximations, we have developed a simple with k = 300 M-l set.-' a t 30'; the activation interpretation of the additivity relation for JCH. Malinowski's results4 for the substituted methenergy is 1.0 i= 0.5 kcal. per mole. anes CHXYZ, may be summarized as The principal point of interest is the possibility of detecting properties of an intermediate through JCH ( C H x Y z ) = fx CY f l z (1) the magnetic behavior of the hydroxyl proton. The initial and final magnetic environments of an hydroxyl proton undergoing transfer are identical. where {X is a numerical constant for substituent Broadening of its resonance would be produced by a X, determined from experiment as sufficiently strong magnetic pulse in transit. Applifx JcH(CHBX)- ( ~ / ~ ) J C H ( C & ) ( 2 ) cation of the theory of McConnell and Berger, or in the limit of weak impulses of a random walk model,3 or the appropriate modification of BPP theory4lead (Our interpretation is based on the correlation to the same result. For pulses of duration T each which has been found6-8 between JCHand the s accompanied by a frequency shift 6 the contribu- character of the carbon orbital used in the C-H tion to l / T c is k c ( 2 ~ 6 ~ ) ~ [ 1( 2 ~ 6 ~ ) where ~ ] - ~K is bond. For methane, a valence bond adaptation6 of the rate constant and c the concentration of radical. Under the previously mentioned assumption, values Ramsey's general formulationg has shown that the of 2T6T ranging from 0.4 + 0.1 a t 60' to 0.7 f 0.1 Fermi contact term governs J C H . Moreover, the a t 27" were obtained. If 6 corresponds to a split- calculation made of the CI3-H coupling6 gives JCH ting of twenty gauss in the intermediate, the dura- t o be a function primarily of X, a parameter related sec. a t t o the C-H bond polarity. Upon extension of this tion of the intermediate is T = l X approach to the substituted methanes we find that 60' and T = 2 X sec. a t 27".
+
+
(3) D. Pines and C. P. Slichter, Phys. Rev., 100, 1074 (1956) (4) N . Bloembergen, J. Chem. P k y s . , 2 7 , 572 (1957). (5) Shell Fellow for 1961-1962.
JcH(CHXYZ) = A V ~ ~ ~ H ~ , / A E (3)
where A is a collection of constants, AE is the aver-
DEPARTMENT OF CHEMISTRY age excitation energy,g and 7 is the normalization ~ ; A S H I N G T O N UNIVERSITY ROBERTu'.KREILICK5 S. I. WEISSMAN constant of the perfect pairing ground state wave SAINTLOUIS,MISSOURI function RECEIVED DECEMBER 1, 1961 VALENCE BOND INTERPRETATION OF THE ADDITIVITY RELATION FOR C'3-PROTON COUPLING IN SUBSTITUTED METHANES1
Sir: Several additivity relations have been pointed out r e ~ e n t l y ~for - ~ the internuclear coupling constants observed in high resolution n.m.r. spectra. One involves the constancy of the three H-H coupling constants in monosubstituted ethylenes. Another concerns the additivity of substituent effects upon the Cl3--H coupling in substituted methanes. Such observations are of considerable potential value to valence theory, but thus far detailed interpretations of the results have not appeared. The case of the monosubstituted ethylenes is the least tractable because i t includes three H-H coupling constants, which originate in small deviations from the perfect pairing electronic structure. The coupling between directly bonded nuclei, such as CI3-H, is simpler to treat inasmuch as i t depends almost entirely upon the perfect pairing ~ t r u c t u r e . ~The . ~ main purpose of this (1) Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. The work also was supported by the Office of Naval Research. ( 2 ) G . S. Reddy and J . H. Goldstein, J . Chem. P h y s . , 3.7, 380 (1961). Recently J. Waugh (Mellon N. M . R . Letters, No. 36, 13 (1961)) has pointed out that the sum of J H H in CHn=CH?X is a linear function of the electronegativity of X . (3) C. N. Banwell and N. Sheppard, M o l . P h y s . , 3, 351 (1960). (4) E. R . Malinowski, J. Am. Chcm. Soc., 8 8 , 4479 (1961). (5) H. S. Gutowsky, D. a'. McCall. and C. P. Slichter, J. Chcm. Phys., 21, 279 (1953). (6) M . Karplus and D. Grant, Proc. N a f l . Acod. Sci. U.S., 46, 1269 as well as CH,. (1959); this paper treats BH4- and " I +
4 i i ) = *I[+di)&dj)
+ d d i ) b c ( j ) + X+c(i)ic(j)l
(4)
The quantity C Y His ~ the fractional 2s character of the carbon hybrid orbital
used in the C-H bond. Eq. (3) reduces to JcH(CHXYZ)
To a good approximation,
J&a2 = ~JCH(CH~)CUH' = 5 0 0 a ~C .~P . S . ( 6 )
because AE is not very sensitive to substituents and we have found that q, which depends upon CXHand A, is nearly constant for the ranges of CXHand X involved. Two additional assumptions are required. The first is that all of the carbon 2s orbital is used in forming the C-H, C-X, C-Y and C-2 bonds, or axe
+
ay2
+
ffz*
+ rynz
= 1
(71
The second concerns the manner in which a substituent perturbs the distribution of the carbon 2s orbital among the four bonds. As a model, consider the latter to be four interconnected potential wells for the 2s electron. In CH4 the wells are of the same shape and depth and thus contain the same fraction of the 2s electron, CYH'(CH~) = 1/4. Introduction of a substituent X changes the "depth" of the corresponding well by an effective, (7) J. N. Shoolery, J . Chem. P h y s . , 31, 1427 (1959) ( 8 ) N. Muller and D. E. Pritchard, ibid., 31, 768, 1471 (19691: (9) N. F. Ramsey, P h r s . Rev., 91, 303 (1953).
COMMUNICATIONS TO THE EDITOR
308
characteristic (+ or -) amount Axl which we assume to be independent of the other substituents. In CHXYZ, the resulting difference in the 2s electron content a t the bottom of the wells is Ax Ay Az, which must be accommodated by a change in the common level a t the top of each of the four wells. This, by means of Eq. (7), leads to
1701. 83
THE REACTIONS OF ATOMIC CARBON WITH ETHYLENE
Sir: The reactions of atomic carbon with ethylene have been studied. Recoil Cll from a nuclear reaction was introduced into the gaseous reagent, and analysis made for the Cll-labeled radioactive products by techniques outlined previously. Kesults are summarized in Table I. Column I shows AX Ay + A9 = 4 [ a ~ ' ( C H x y Z )- (1/4)] (8) products obtained with pure ethylene, column I1 For monosubstituted derivatives, Eq. (8) re- with oxygen added as a radical scavenger, and duces to column 111 is representative of runs containing a large excess of neon to thermalize (;.e., remove AX 4[aH2(CH&) - (1/4)] (9) excess kinetic energy from) the carbon atom before which enables us to eliminate Axl Ay, and Az from reaction. Eq. (8) and obtain As with the alkanes previously reported,? the CXH'(CHXYZ) = [aa2(CHaX) - ( ~ / ~ ) ( Y H * ( C H -k' ) ] main products are highly unsaturated hydro[ ~ H * ( C & Y) (2/3)a~~((CHd)] carbons. The high yield of acetylene, in particular, probably results largely from the C-H [ ~ H Y C H J Z-) (2/3)a~*(CHd1 (10) And, finally, the relation between JCHand C Y H ~ , bond insertion process as postulated for the alkanes. in Eq. (6)) converts this to H&=CHz .-,CHZ + HCl1=CH
+
+
+
3
+
JcH(CHXYZ) = Tx
+ by + Sz
+
(1)
(11)
However, there are two significant differences between ethylenic hydrocarbons in general and saturated systems: (1) in the presence of 0 2 the CO yield is appreciably less with alkenes than with alkanes. This indicates the alkenes are more reactive. (2) The yields of products containing yields essentially the same equation as Eq. (6)) one more carbon atom than the reacting hydroafter which the analysis is identical. The formalism introduced above enables one to carbon are considerably higher with alkenes than alkanes. Thus about twice as much allene use experimental values of JCHto obtain not only with and methylacetylene form from ethylene, as do C Y Hbut ~ also a x 2 . The relation of these quantities propane and propylene from ethane. to parameters such as bund angles and lengths is The most obvious and satisfactory explanation under investigation and will be reported laterlo of these observations is that C atom can react along with details of the present work. I n addi- directly with the double bondthe to form a n-bonded tion, i t appears that substituent parameters obtained from the monosubstituted methanes can species. This intermediate then can rearrange to allenea be applied to J C H in substituted ethylenes. The Si2"H coupling has been observed in SiH4 and in a number of derivatives." However, our :C" 3. HzC=CHz +HzC-CHz +HzC=C"=CHz inspection of the results shows that the substituent y 1 1 (11) effects are nQt additive. Nonetheless, the deviations from additivity are systematic in that for the (In the case of higher alkenes, the corresponding halosilanes, the deviations follow the sequences : substituted allene largely isomerizes to a conSiHXa > SiHzXz; and F > C1 > Br > I. Several jugated diene.) We are presently attempting to factors can contribute to these effects, of which the confirm this mechanism by showing that C" most likely appears to be the use by Si of 3d appears in the central position of allene.4 orbitals, the importance of which is suggested also The intermediate formed by this double bond by the dependence of JHHupon the H S i - H addition also has sufficient energy to decompose angle.11912The inclusion of 3d orbitals in Eq. (5) with bond rupture. It may thus contribute to the may prevent Eq. (6) from applying to J S i H , yield of acetylene-Cl1 from ethylene. and also it could affect Eq. (7). This presents The cyclopropane observed is consistent with the the attractive possibility of learning something previous hypothesis that a few per cent of the C about the d hybridization from the deviations, atoms react by H atom pick-up from hydrocarbons2 to compensate for the more difficult nature of to form C11H2. This will react in a characteristic the analysis. (1) Contribution No. 1685 from Sterling Chemistry Laboratory, which is exactly the form of additivity observed by M a l i n ~ w s k i . ~A two-center molecular orbital of the form fi cl(lsH) f ce(2sC) f C'd2p0,) (12)
hTOYES CHEMICAL LABORATORY UNIVERSITY OF ILLINOIS
Yale University.
We are indebted to the operating staff of the Yale
H. S. GUTOWSKY University Heavy Ion Accelerator for their assistance. Invaluable URBANA,ILLISOIS CYNTHIA S. JUAN discussions with Professor William Doering of Yale University, are gratefully acknowledged. This work was sponsored by the Atomic 15, 1961 RECEIVED DECEMBER (IO) C. S. Juan and H. S. Gutowsky, to be submitted to J . Chcm.
Phys. (11) E. A. V. Ebsworth and J. J. Turner, J . Chem. Phrs.. in press; we are indebted to the authors for making their results available t o us before publication. (12) J. C. &hug and H . S. Gutowsky, unpublished results.
Energy Commisssion. (2) C. MacKay and R. Wolfgang, J . Am. Chcm. Soc., 83, 2399
(1961). (3) W.R. M o a e and H. R . Ward, J . Org. Chem., 26, 2073 (1960). (4) Note that a certain amount of allene with terminal C11, resulting from the C-H bond insertion mechanism (see ref. Z ) , also may be expected.
