COMMUNICATIONS TO THE EDITOR
Aug. 5, 1963
should be those where I varies from 180" (IIa) to 125" (Ilb). We could test further the above hypothesis by measuring JH~H. for cis (111) and trans (V) isomers provided J H ~were H a~function of the dihedral angle, as J is
B V
VI
2327
-
VI1
of the =NNHX structure indicates that the conformation of aliphatic oximes in benzene is VI, not VII. Microwave spectroscopy has shown4that the conformation of formaldoxime in the gas phase is VI. For =NNHX compounds conformation IV is preferred over the one which is analogous to VI, because of unfavorable interactions between X and R1in VI. Acknowledgment.-We thank the United States Atomic Energy Commission for financial support, Grant A T ( 11- 1)- 1189.
Hx
Hx
-N z' CHu I11 (cis)
rCHa
L N / Z
IV (trans)
in ethane derivatives3 The data summarized in Table I are consonant with the advanced explanation. As expected from a dihedral angle of about 180", JCis for isopropyl compounds is large (7.6 c.P.s.). For ethyl compounds 11, predicts JCiS larger than J C i sfor methyl compounds, while I I b predicts the reverse. The data indicate that both IIa and IIa are important contributors, but do not allow assign( 4 ) I N Levine, J M o l Spectvy, 8,276 (1962) J . KARABATSOS ment of relative contributions. J ' s , especially Jlrans of KEDZIECHEMICAL LABORATORY GERASIMOS ROBERTA. TALLER oximes, are solvent dependent. In addition, Jlrans STATE UNIVERSITY MICHIGAN of FLOIE M . VANE EASTLASSING,MICHIGAN oximes depends on concentration, e.g., J t r a n s of isoRECEIVED M A Y25, 1963 butyraldoxime = 6.55, 6.35, 6.30 C.P.S. in carbon tetrachloride solutions of 20%, lo%, and 5% concentration, Structural Studies by Nuclear Magnetic Resonance. respectively. Preliminary temperature studies also show that J,,, of isobutyraldoxime (neat) decreases by IV. Conformations of syn-anti Isomers from Chemical 0.10 C.P.S. and Jtyan, by about 0.20 C.P.S. as the temShifts and Spin-Spin Coupling Constants perature is raised from 38" to about 70". These reSir : sults are easily understood in terms of self-association We reported1 in our n.m.r. studies on structural asand hydrogen bonding between solute and solvent. signments to s y n and anti isomers that a-methyl hyTABLE I drogens when cis to Z resonate a t higher fields (shielded) Solvent than when trans; for a-methylene hydrogens the chemiJ E ~ ~ H JHQH. ~ (Concd., R Compound 5-10 %I (cis) (trans) >N-Z ca
cal shift difference between cis and trans hydrogens is smaller than it is for a-methyl hydrogens, and in many cases this difference is zero; a-methine hydrogens when cis to Z resonate a t appreciably Lower fields (deshielded) than when trans. In this respect a-methine hydrogens behave as hydrogens directly bonded to the imino carbon.2 This puzzling behavior of a-hydrogens is explicable from conformational considerations. Because of steric interactions between Z and isopropyl when cis to each other, the preferred conformation of the isopropyl group might be I (I 180' or slightly smaller). Conformation I places the methine hydrogen in or near the C=N-Z
-
H
H,
Me
MeHC=N-NHCsHs(NOdz MeHC=N-NHCnHs( NOdi EtHC=X-NHCIHa(NOz)i r-PrHC=N-P\'HCeHs(NOz)z MeHC=N-NHm MeHC=P\'-NH@ EtHC=N-NH@ r-PrHC=N-KH@ MeHC=N-NCdi4NOz(o) MeHC=N-NHMe MeHC=N-NHMe MeHC=N-NHCONHz MeHC=N-P\'HCSNHa MeHC=N-OH MeHC=N-OH EtHC=N-OH EtHC=N-OH EtHC=N-OH I PrHC=N-OH I PrHC=Pi-OH I - PrHC=N-OH I-PIHC=N-OH
b,
C,
d, e , f
h b b g,
i, k , 1
m
a a , i,k, n b a, i k, 1 b
b a , b , i,k e, E, 1 a b, k,1 B
a g
k 1
5.60 i 0.05 5 50 f 0.05 5.4 i 0.1 7.6 i 0.1 5.75 i 0.05 5.80 f 0.05 5.2 i 0.1 7.6 i 0.1 5.50 f 0.05 5.80 i 0.05 5.80 i 0.05 5.80 i 0.05 5.80 i 0.05 5.80 i 0.05 5.75 i 0.05 5.75 f 0.05 5.75 i 0.05 5.75 + 0.05 7.65 i 0.05 7.60 2s 0.05 7.55 f 0.05 7.50 f 0.05
5.40 i 0.05 5.30 =t 0.05 5.0 f 0.1 4.9 i 0.1 5.55 j l 0.05 5 60 i 0.05 5 2 f 0.1 5.0 =t 0 . 1 5.30 & 0.05 5.50 f 0 . 0 5 5 60 =t 0.05 5.50 j. 0.05 5.50 i 0.05 6.30 j. 0.05 6.10 i 0.05 6 10 zk 0 05 5 90 =t0 05 6.00 r!= 0 05 6 65 f 0 05 6.15 f 0 05 6.30 f 0.05 6.0 j l 0 . 1
Neat. * Methylene bromide Kitrobenzene Dioxane. Pyridine. Dimethylformamide. Acetone, Quinoline. Benzene Carbon tetrachloride. Dimethyl sulfoxide. Methanol Isodurene a
I
IIa
IIb
plane and explains the similarity in behavior between niethine and hydrogens directly bonded to the imino carbon (deshielded). The angle I will depend on the size of R , ie., as R increases in size I will increase and approach 180". As I increases, the methine hydrogen will become more deshielded. Our data support this hypothesis, e.g., in oximes (2 = OH), when R = H, T(trans) = 7.55 and cis) = 6.87; when R = Me, r(trans) = 7.52 and cis) = 6.55. From similar arguments the important conformations of the ethyl group ( I ) G. J . Karabatsos, R. A. Taller, and F. M . Vane, J . A m . Chem. SOL., 86, 2320 (1963). ( 2 ) We also observed t h e same behavior in nitrosamines,
I n cases such as oximes where all a-hydrogens are deshielded (cis lower t h a n trans), we find t h e same t r e n d ; ;.e. a-methine hydrogens are more deshielded (0.7-1.0 p . p . m . in carbon tetrachloride) t h a n a-methyl hydrogens (0.034.05p.p,m, in carbon tetrachloride).
Exceptionally high Jtvansvalues for oximes4 result in > J c i s for acetaldoxime and propionaldoxime. We find JCjnHA (V) = 162 =k 2 C.P.S. and J c ~(VI) ~ H= ~ 174 =k 2 C.P.S. From the dependence of J C 1 a - H on the yos-character of the carbon atomic orbitals we conclude Jtya,,
that the imino carbon orbital in C-HB has more s (3) M . Karplus, J . Chem. Phys., 30, 11 (1959); J . Phys. Chem., 64, 1793 (1860). (4) Our studies confirm t h e correctness of oxime assignments by W. P . Phillips, A n n . N . .'l Acad. Sci., TO, 817 (1958); E.Lustig, J . Phys. Chem ,
86, 481 (1961).
COMMUNICATIONS TO THE EDITOR
2328
character than in C-HA (the C-HB bond is shorter than the C-HA). Analogous results were obtained with formamide5(VII), J N I ~=H92 ~ c.P.s., J N ~ =~ 88 H C.P.S. ~ Part of the difference, therefore, between JCisand J t r a n s (Table I) could be due to changes in the hybridization of the imino carbon. Evaluation of conformational and hybridization contributions must await temperature studies and C13labeling. Acknowledgment.-We thank the United States Atomic Energy Commission for financial support, Grant AT(l1-1)-1189.
VOl. 85
ORTHO
a
META
(5) B. Sunners, L. H . Piette, and W. G. Schneider, Can. J. Chem., 38, 681 (1960). T h e N - H A = 1.002 k 0.005A. and t h e N-HB = 1.014 i 0.005A. according t o C . C. Costain and J , M . Dowling, J. Chem. Phys., 82, 158 (1960)
KEDZIECHEMICAL LABORATORY GERASIMOS J. KARABATSOS MICHIGAN STATEUNIVERSITY ROBERTA. TALLER EASTLANSING, MICHIGAN FLOIEM . VANE RECEIVED MAY25, 1963
?r-Electron Densities in Arylmethyl Carbanions by Nuclear Magnetic Resonance Spectroscopy
Sir : Nuclear magnetic resonance spectroscopy offers a unique approach toward the explication of s-electron densities in aromatic molecules. I t has been amply demonstratedl-10 that, in general, the chemical shift (6) of an aromatic proton is approximately proportional
i I . " * " ' . . 1 . . 3 .O
Fig. 2.-Proton
I
I 8P
1 . . . . 1 . * . . 1 . . . . 1 . . .
Fig. 1.-Proton
3.0 4 -0 n.m.r. spectrum of triphenylmethyllithium in THF.
(1) P. L . Corio and B. P. Dailey, J . A m . Chem. Soc.. 78, 3043 (1956). R B.Moodie, T . M Connor, and R . Stewart, C a n . J . Chem., 87, 1402 (1959) (3) D. E. O'Reilly and H . P. Leftin, J. Phys Chem., 64, 1555 (1960), according t o Berry, Dehl, and Vaughn (see ref. 18b) t h e chemical shifts assigned t o the orlho and mefa protons of t h e triphenylrnethyl carbonium ion are reversed (4) G Fraenkel, et al , J . A m Chem. Soc., 82, 5846 (1960). ( 5 ) I. C . Smith and W. G . Schneider, C a n . J. Chem., 89, 1158 (1961). (6) H Spiesecke and W . G Schneider, Tetrahedron Letters, 468 (1961). (7) H . Spiesecke and W . G Schneider, J . Chem. Phys., 86, 731 (1961). (8) W. SeifTert, H . Zimmermann, and G Scheibe, Angew. Chem., Intern. E d . E n p i . , 1, 265 (1962). (9) J. C . Schug and J C Deck, J. Chem. P h y s . , 37, 2618 (1962). (10) T Schaefer and W . G Schneider, C a n . J . Chem., 41,966 (1963). (2)
4 .O
f i .
1
*
. I S.0
n.m.r. spectrum of diphenylmethyllithium in THF.
to the s-electron density on the carbon to which it is attached. Using this principle, the electronic structures of various aromatics, ]-lo including arylmethyl carbonium ion^,^,^, lo have been investigated. We now have been able to show this technique to be particularly useful for arylmethyl carbanions, with triphenylmethyllithium exhibiting the first spectrum of a monosubstituted benzene readily interpretable by first-order analysis. The ?r-electron densities of such carbanions have not been experimentally available previously. Our results are contrary to the predictions of classical resonance theory or Hiickel LCAO molecular orbital calculations, and strongly support the self-consistent field (s.c.f .) calculations of Brickstock and Pople." The 60 Mc.p.s. proton n.m.r. spectra of triphenylmethyllithium, diphenylmethyllithium, and benzyllithium in tetrahydrofuran (THF) are shown in Fig. 1 , 2 ,and 3. The spectrum of triphenylmethyllithium is surprisingly uncomplicated, consisting of three discrete multiplets with integrated intensities of 2 : 2 : 1. Firstorder analysis gives chemical shifts of 2.69 r , 3.48 r , and 4.04 r for the ortho, meta, and para protons, respectively, with coupling constants of J o - m = 7.9 C.P.S. and Jm-* = 6.6 C.P.S. Further, the spectrum is neither solvent nor cation sensitive; only minor shifts were observed between spectra run in THF, hexamethylphosphoramide, and dimethyl sulfoxide, and between the spectra of triphenylmethylsodium and triphenylmethyllithium in THF. Thus, the alkali triphenylmethyls must be essentially completely ionized, l 2 in accord with the conclusion of Ziegler and Wollschitt13 derived from conductance data. (11) A. Brickstock and J . A. Pople, Trans. Faraday Sac., 6 0 , 901 (1954) (12) This conclusion cannot be extrapolated t o diphenylmethyllithium and benzyllithium until t h e proper experiments have been completed. (13) K . Ziegler and H . Wollschitt, A n n . , 479, 123 (1930).
