NOTES
The Nuclear Magnetic Resonance Spectra of Three Unsymmetrical o-Dihalobenzenes
by William B. Smith and G. Mattney Cole Department of Chemistry, Tlsxas Christian University, Fort Worth, Texas (Received August 9, 1.966)
Recently, Martin and Dailey have reported the chemical shifts and coupling constants for a large series of disubstituted benzenes.1 While the interplay of the several factors which determine the proton chemical shifts in such substances could not be completely delineated, they did note empirically a simple substituent additivity rule which allowed the calculation of the proton chemicd shift in the para-disubstituted compounds with excellent accuracy.2 While not quite so accurate, the application of these same substituent constants was found to apply to the metu-disubstituted compounds and to the two protons opposjte to the substituents in the ortho-disubstituted examples. I n the latter compounds, no correlation was found with the shifts of the protons adjacent to the substituents.2 We have now determined the n.m.r. parameters for three unsymmetrically substituted ortho dihalobenzenes. I n part, our interest in these compounds stemmed from the observations of Martin and Dailey. The complexities of determining the parameters for an unsymmetrical four-spin system also presented an intriguing challenge. The n.m.r. spectra of l-chloro-2-bromo-, l-chloro-2iodo-, and 1-bromo-2-iodobenzene were determined in dilute solutions of carbon tetrachloride and are shown in Figures 1 and 2. Though the parameters for the symmetrical ortho dihalobenzenes have been reported before,lJ these compounds were run again under the conditions used in this study, and the spectra were analyzed following the considerations of Grant, Hirst, and Gutowsky.8 The results of this study are given in Table I. The spectra of an unsymmetrical four-spin system is determined by ten individual parameters (six coupling constants and four chemical shifts). The basis functions and matrices used ia describing such a system have been given by Reilly and S ~ a l e n . I~n general, it is not possible to write down explicit formulas for the line frequencies and intensities for this case though under certain special conditions the matrices may be simplified to tractable expres~ions.~t6 As will be noted in Table I, there is comparatively little variation among the six coupling constants for the three symmetrical dihalobenzenes, and it was
4413
assumed as a first approximation that this would hold true for the unsymmetrical cases as well. Trial calculations using the Freqint IV A 1620 computer program and averaged values for the coupling constants and various trial values of the chemical shifts suggested that the spectral line frequencies of the 1-chloro-2-iodo- and 1-bromo-2-iodobenzene spectra could be well approximated as ABMX cases in which all off-diagonal matrix are neglected. elements other than those involving JAB The energy levels for the ABMX case have been given by Reilly and S ~ a l e n and , ~ explicit equations for the line frequencies are readily derived therefrom. Transitions were assigned according to the frequency sum rule given by Reilly and S ~ a l e n . Once ~ the experimental line frequencies were assigned, the spectral parameters for these two cases were readily derived from the frequency and energy level equations. These values were checked and the intensities were calculated with the computer program. The parameters are given in Table I and the calculated spectra are shown in Figure 1. The average deviation was zkO.1 C.P.S. for all 32 lines in both cases. As will be noted in Figure 1,the intensity fit is not exact in this approximation. When all off-diagonal elements were included in the calculation (ABCD case), the intensity pattern matched the experimental spectrum but the line positions shifted slightly. This indicated that a slight adjustment of the parameters was required. Since we did not have access to an iterative type program, no further improvement was attempted. Examination of the 1-chloro-2-bromoben~enespectrum (Figure 2) clearly indicated that the system could only be treated as a true ABCD case. From our knowledge of the order of the lines in the two simpler cases, it was possible to construct an energy level diagram for this case again using the Reilly-Swalen sum rules. Several trial calculations indicated how variations in each parameter affected the energy levels. With this information, an exact ABCD solution was possible. The energy level table is shown in Figure 3. The line assignments correspond to those in Figure 2. The average deviation was zk0.1 c.p.s. The parameters are given in Table I. I n keeping with the observations of o t h e r ~ , ~ - ~ ~ ~ ~
~~
(1) J. S. Martin and €3. P. Dailey, J. Chem. Phys., 37, 2694 (1962). (2) J. 8. Martin and €3. P. Dailey, ibid., 39, 1722 (1963). (3) D. M. Grant, R. C. Hirst, and H. 5. Gutowsky, ibid., 38, 470 (1963). (4) C. A. Reilly and J. D. Swalen, ibdd., 34,980 (1961). (6) See, for instance: (a) R. J. Abraham and H. J. Bernstein, Can. J . Chem., 39, 216 (1960); (b) N. V. Riggs, Australian J. Chem., 16, 621 (1963); ( 0 ) V. J. Kowalewski and D. G . deKowalewski, J. Chem. Phys., 36, 266 (1961); 37, 2603 (1962). (6) H. Spiesecke and W. G . Schneider, ibid., 35, 731 (1961).
Volume 69, Number 18 December 1966
4414
NOTES
4 60
400
Figure 1. The n.m.r. spectra of 1-chloro-2-iodobenzene (upper) and I-bromo-2-iodobenzene (lower). The calculated spectra are ABMX approximations.
Table I: N.m.r. Parameters for Dihalobenzenes' Jsa
Dihalobenzenes
1,2-Dichloro
8.06 8-96
1,a-Dibramo 1,2-Diiod0
7.45
7,45 7.91
J66
8.06
8.46 7.96
Jas
J&
J86
7,s
1.52
1.52
0.35
2.63 (2.80)
1.33 1.95
1.33 1.95
0.45 0.05
Y6
Y
(2.95)
2.88 (2.95)
2.63 (2.80)
2.91 (2.93)
2.91
(2.70)
(2.93)
2.45 (2.70)
2.19 (2.63)
3.04 (3.07)
(3.07)
2.19 (2.63)
2.45
^(a
2.88
3.04
1-Chloro->bromo
8.26
7.45
8.26
1.33
1.62
0.50
2.47 (2.64)
2.99 (3.02)
2.86 (2.87)
2.62 (2.86)
1-Chloro-2-iodo
7.70
7.56
7.90
1.75
2.05
0.35
2.21 (2.43)
3.16 (3.16)
2.79 (2.87)
2.63 (2.99)
1-Bromo-2iodo
7.96
7.20
8.20
1.60
1.64
0.40
2.22 (2.50)
3.12 (3.07)
2.90 (2.93)
2.45 (2.84)
Chemical shifts are in -
7.96
J4s
7
units.
~~
The Journal of Physical Chemistry
Values in parentheses are predicted from the values of Martin and Dailey.
NOTES
4415
12
460
400
Figure 2. The n.m.r. spectrum of 1-chloro-2-bromobenzene. The calculated spectrum is a n ABCD case.
the less shielded protons were assigned as being adjacent to the halogen substituents. As will be noted in Table I, there is only a slight and apparently random variation in the coupling constants for all six compounds. The values €or the chemical shifts predicted from the Martin and Dailey relation are indicated in parentheses. In keeping with their observation, the agreement between the predj cted and observed values is quite good for the remote pair of protons (4 and 5 ) , and the predicted value is high by about 0.2-0.5 p.p.m. €or the adjacent hydrogens (3 and 6). Perhaps the most significant observation to be noted in Table I is the fact that the chemical shifts of the protons adjacent to a given halogen seem to be determined solely by the nature of that halogen and appear to be independent of the nature of the other substituent. Numerous discussions of the various factors contributing to the chemical shifts of aromatic protons have now appeared. The operation of “ring currents,” inductive and substituent electric field effects, and the magnetic anisotropy of the substituent
have been considered.2 ~ 6 , 7 While it is generally agreed that substituent effects are at a miminum at a metu position, still the metu-substituent constants of Martin and Daileyz vary by a factor of 4 in going from chlorine to iodine, and the meta proton in iodobenzene is 0.23 7 upfield from that in chlorobenzene.6 The low-field shift of a proton located ortho to a halogen has been noted before, and both Spiesecke and Schneider‘j and Martin and Dailey2have suggested the magnetic anisotropy of the halogen as the most likely explanation. The latter, however, noted that certain aspects of the magnetic anisotropy explanation were unsatisfactory; i.e., the anisotropy was calculated also to affect more remote protons. Recently, Schaefer, Reynolds, and Yonemoto8 have also rejected the magnetic anisotropy argument and have suggested a short (7) J. 5. Martin, Thesis, Columbia University, 1962, and references therein. (8) T. Sohaefer, W. F. Reynolds, and T. Yonemoto, Can. J . Chem., 41, 2969 (1963).
Volume 69, Number 1I December 1966
NOTES
4416
Fz
results from an alteration in the paramagnetic term of the Ramsey shielding equation. A linear relation between the ortho-proton shifts for the aromatic halides was found with an empirical parameter, &. This parameter was defined as P/Irs, where P is the bond polarizability, I is the first ionization potential, and r is the bond length. A plot of the chemical shifts for the various ortho protons in Table I against Q is shown in Figure 4. Benzene is also shown in the plot. As can be seen, the fit to the plot is excellent, and it is presumed that this mode of deshielding of the ortho protons dominates in these compounds. Reasonably, the additivity rule of Martin and Dailey would not be expected to hold in this situation.
12
+I
0
-1
Experimental Section -a
Figure 3. The energy level diagram for l-chloro2-bromobenzene showing the line assignments and frequencies (leveb not to scale; add 400 C.P.S. to each frequency).
2.8
i
2.6
-
The compounds used in this study were all commercially available. Spectra were determined with a Varian A-60 at the ambient temperature of the probe (ca. 40"). Solutions of each compound were made up 5 1 0 % by volume in carbon tetrachloride and outgassed. One drop of tetramethylsilane was added as an internal standard. The instrument was calibrated against chloroform. Each spectrum was recorded several times (4-lo), and the averaged line positions were used in the calculations.
Acknowledgment. We wish to express our gratitude to the Robert A. Welch Foundation for its generous support of this work.
.
w% .
Ff: o
b
-
2.4
.
.Y
a 2
'
(9) P. Hruska, H. M. Hutton, and T. Schaefer, Can. J . Chem., 43, 2392 (1966).
Dissociation Constants for Some Nitrophenols 2.2
and Salicylic Acid in Deuterium Oxide
.
by Paul K. Glasoe L
I 3
2
4
8.
wittenberg University, Springfield, Ohio (Received August 11, 1966)
Figure 4. Plot of the chemical shift data of the o-hydrogens us. the Q values of Hruska, Hutton, and Schaefer.
range van der Waals type interaction between the halogen and the adjacent proton. No quantitative calculations were attempted, and this explanation has been subsequently discarded in favor of a more reasonable explanation. Hruska, Hutton, and Schaefer9 have presented evidence that the low-field shift The J O U T ~of U ~Phvsical Chemistry
I n a report on the dissociation constants of some acids in deuterium oxide, Bell1 noted the poor agreement between his result for 2,4-dinitrophenol and that reported earlier by McDougall and Long.2 He also -~ ~
(1) R. P. Bell and A. T. Kuhn, Trans. Faradall Spc., 59, 1789 (1963). (2) A. C. McDougall and E". A. Long, J . Phys. Chem., 66, 429 (1962).
