3765
NOTES
Calculation of Inter-Ring Proton Couplings
by George F. Adams Propellants Laboratory, Picatinny Arsenal, Dozer, N e w Jersey (Received M a y 27, 1971)
07801
Publication costs assisted by the Propellants Laboratory, Picatinny Arsenal
have adapted finite-field perturbation Pople, et theory to the INDO approximate molecular orbital method2 in order to calculate nmr spin-coupling constants. This note will discuss an application of this procedure. Inter-ring proton couplings, especially those over more than four bonds, are of great interest in organic c h e m i ~ t r y . ~Much of the experimental work in this area has been done on substituted compounds. This limits the ability to correlate the information on interring couplings with other molecular parameters. Jarvis and Moritz4 have reported an attempt to determine experimentally the six inter-ring coupling constants in naphthalene. They were not able, however, to determine both the magnitude and sign of all these parameters. Since previous INDO calculations of nmr spin-coupling constants have given good agreement with the experimentally determined values, we have used the INDO method to calculate the inter-ring proton couplings. The IKDO and experimental results for naphthalene are given in Table I; the labeling of the protons is indicated in Figure 1.
Table I : Inter-Ring Proton Couplings in Xaphthalene
Coupling
Calcd coupling constant
Obsd coupling constant
+1.80 -0.44
SO.63 -0.16 $0.21
$0.54 -0.58 +0.40
+0.02 +1.95
...
between protons separated by an odd number of bonds and negative between protons separated by an even number of bonds. Of interest in this respect is the fact that the INDO calculation predicts J 1 , 3 = f1.95. These protons are separated by an even number of bonds, and one would expect a negative coupling constant. The positive sign is an indication that there is a significant contribution to the coupling from the cr-electron framework. The calculated J1,3 has the same sign as the experimentally determined J 1 , 3 . The signs of the other proton-proton coupling constants are in agreement with the experimental signs, except for JZJ. However, the magnitude of this calculated coupling infers that the suggestion of Jarvis and Moritz that Jz,, is zero may be correct. 8
1
Figure 1. Labeling of the naphthalene protons. 1.40; TO-H = 1.08; LCCC = L C C H = 120".
TC-0
=
There has been a question of the sensitivity of the calculated coupling constant to small changes in the molecular geometry. The naphthalene geometry in the calculation uses the bond lengths and angles suggested by Pople and Gordonq6 Their suggested carbon-hydrogen bond length is 1.08 A. Calculations were done for two other carbon-hydrogen distances, and the results are summarized in Table 11. The variations in J are quite small, especially when one considers the semiempirical nature of the molecular orbital calculation. More significant is the fact that the calculated signs of the coupling constants are unaffected by these variations.
10.281 .
I
.
$1.25
In their paper, Jarvis and Moritz speculate that J z , 8 is negative, but the IXDO calculations do not support this contention. This result is in agreement with Barfield's5 suggestion that coupling should be positive
(1) J. A . Pople, J. W. MoGiver, Jr., and N. 8. Ostlund, J . Chem. Phys., 49, 2965 (1968). (2) J. A. Pople, D. L.Beveridge, and P. A . Dobosh, {bid., 46, 2026 (1967). (3) K. D. Bartle, D. W. Jones, and R. S. Mathews, Rev.P u r e App2. Chem., 19, 191 (1969). (4) M. W. Jarvis and A. G. Moritz, Aust. J . Chem., 24, 89 (1971). (5) M. Barfield, J . Chem. Phys., 48, 4463 (1968). (6) J. A. Pople and M. S. Gordon, J . A m e r . Chem. Sac., 89, 4253 (1967).
T h e Journal of Physical Chemistry, Val. 76, N o . 24, 1971
Nms
3766 Table 11: Variation in J with C-H Bond Length IO-".
&.
1.08 Calculated coupling
1.10
Coupling
f1.95
JC.7
f2.19 +1.98 -0.47 +O.h8
JL.8
-0.62
-0.58
J1.S
JX.6
J,,,
1.m
constant
f1.80 -0.44 f0.54
+1.74 f1.63
-0.40 f0.499 -0.54
Aclnowledgment. This work was completed while the author was a National Research Council Resident Research Associate at Picatinny Arsenal. The author thanks the MUCOM DPSO for use of computer facilities. He also thanks Dr. Lee Pedersen of the University of North Carolina and Dr. Ted Vladimiroff of Picatinny Arsenal for encouragement and assistance.
~ . ~ .,,~ ...... . :. . . . ...
.II.,.
b
C
Carbon-13 Nuclear Magnetic Resonance Spectrosopy. IV.
Bromo-Substituted
E t h a n e s and Ethylenes' by Goh Miyajima* Naka W w b , Hitachi. Ltd.. Ichise 886,Kotsuta-shi, Iharaki, Japan
and Kensuke Takahashi Department of Synthetic Chemistry, Nogoya Institute o/ Technology, (Rcceiued Jonuwy 11, 1871) Gokiso-cho. Showa-ku. N a ~ o y aJapan ,
Figure 1. Typical carhon-13 nmr spectra: (a) a full spectrum of a mixture of cis- and transdibromoethylenes, with the signal mersged for 20 scans; (b) an expanded spectrum of CBn in CHeBrCBs with 130 scans; (c) an expanded spectrum of half of CHBrlCHBrawith 2.5 scans. Applied frequency increases from right to left at a constant magnetic field. The sweep rate for one scan was 50 ppm/64 see, 51 Hz/128 sec, and 34 Hs/128 sec for (a), (b), and ( e ) , respectively.
Publication m 8 t 8 assisted by S d a h u . Hitochi. Ltd.,
Carbon-13 nuclear magnetic resonance spectra have been investigated by many authors, but the data are not fully accumulated. In a previous report, chloro-substituted ethanes and ethylenes were studied.* As a comparison, other halogeno-substituted compounds with rather simple skeletons such as the above are interesting in many respects. This note presents the nmr data of bromo-substituted ethanes and ethylenes. Most experimental procedures were similar to those described in the previous reports.*,3 Chemical shifts of the samples were compared with a sample of 55% 13CC-enrichedmethyl iodide diluted with one-third volume of benzene, and then the values were recalculated to refer to CS,. The signal of the methyl iodide was at 214.5 ppm from CS,. Most compounds were measured in neat liquid. Ethyl bromide and dibromoand tetrabromoethanes were obtained from commercial sources. Other samples were prepared by the usual methods of successive dehydrobromination, bromination, and debromination of dibromo- and tetrabromoethanes. CHaCBr3 could not be obtained in spite of The J o u m l of Phypienl Chemislry, Vol. 76. No. 64,1871
much effort. Some of the compounds seem to be nnstable under the measuring conditions, as the 13C nmr signals sometimes changed with time. This feature was especially apparent in the ethylene derivatives; the reason is not clear at the present time. Some typical spectra are shown in Figure 1. The spectra with 800-Hz widths can be shown in full on an nmr chart, but further details of the signals must be observed on the expanded spectra taken with a sweep rate of 1-2.5 He/ sec, as shown in Figures l b and c. 13C nmr data for 14 bromo-substituted ethanes and ethylenes except CHaCBr3are given in Table I. The data for vinyl bromide (no. 11) are only approximately given in Table I. It seems that the exact data for this compound must be obtained from the exact analysis. The case is the same for the corresponding data of vinyl
(1) Presented in brief at the 7th Symposium on Nuclear Magnetic Resonance Speetmscopy, Meijo University, Nagoya, Japan. Nov 1968, Abstract p 31. (2) G. Miyaiiriia and K. Takahashi, J . Phys. Chem.. 15, 331 (1971). (3) G. Miyajima. Y. Utsurni, and K . Takahashi. ibid., 73, 1378 (1969).
