Chlorine-35 Nuclear Quadrupole Resonance Studies of Trichloromethyl Compounds Edward G . Brame, Jr. E . I . du Pont de Nemours and Company, Inc., Elastomer Chemicals Department, Experimental Station, Wilmington, Del. 19898 Chlorine-35 nuclear quadrupole resonance (NQR) spectrometry was applied to the study of trichloromethyl compounds. A correlation of spectra obtained at 77 O K with structure of these trichloromethyl compounds was prepared upon averaging the frequency of the multiplet splitting patterns observed for the trichloromethyl group. A relationship consistent with findings in NMR is seen for the NQR correlation of the trichloromethyl structures.
CHLORINE-35 NUCLEAR QUADRUPOLE RESONANCE (NQR) spectrometry has been used in studying the structure of chlorine-containing compounds to a much greater extent than any of the other isotopes capable of being examined by NQR, because of the frequency range over which aSC1 resonances appear and also because the resonance lines are not easily saturated with high radio-frequency power levels ( I , 2). Even with the various NQR studies that have been performed on chlorine-containing compounds, there have been no correlation studies reported involving the trichloromethyl group except for a specific kind of correlation shown by G. K. Semin (3) on a homologous series of tetrachloroalkanes. The molecular formula for these tetrachloroalkanes is C1(CHS)n--ICC13,where n is the number of methylene groups that separates the trichloromethyl group from the chloromethyl group. In the NQR studies performed by G. K. Semin on these compounds, two interesting phenomena were observed. The first was that a shift to lower frequency was observed for the resonances of both the chloromethyl and the trichloromethyl groups as n was varied from 1 to 13. Second, a change in the splitting pattern for the trichloromethyl group was observed as n was varied from 1 to 13. For n equals 1, a three-line pattern of equal spacing was observed. However, on increasing n to 5, the splitting pattern changed to show one of the three lines had shifted to higher frequencies relative to the other two. Then, on increasing n to 9 and beyond, the pattern reversed itself to show that the line separated from the other two was at the low frequency side of the pattern. G. K. Semin attempts to explain the observations by stating that they “may be due to the fact that intermolecular Van der Waals interactions for such large molecules deform one of the CCI3-group chlorine atoms more than is usually possible.” He further states, “it follows from the symmetry of arrangements of the C-CI and C-H dipoles relative to the chlorine atoms in the trichloromethyl group that the chlorine atom in the symmetry plane of the molecule must differ from the chlorine atoms lying outside this plane. Competition between the contribution of the C-C1 dipole in the chloromethyl group to the electric field gradient and the sum of contributions of the C-H dipoles, which increases with n, apparently results in a large splitting for n equals 2 and n (1) E. A. C. Lucken, “Nuclear Quadrupole Coupling Constants,” Academic Press, New York, N. Y.,1969. (2) E. G. Brame, Jr., ANAL.CHEM., 39,918 (1967). (3) G. K. Semin and V. I. Robas, UDC541.67,lIO (1966).
greater than 9 with a slight degree of splitting (of a different type) for intermediate values of n.” Based on the results reported by Graybeal and Cornwell (4) for trichloroacetonitrile and Allen (5) for chloral hydrate in addition to Semin’s results, it is apparent that the splittings of all trichloromethyl group resonances are sensitive to the crystallographic environment. Because of the appearance of multiplet splittings in the nuclear quadrupole resonance spectra of trichloromethyl groups both Graybeal and Cornwell, and Allen stated that the average frequency should be taken in order to calculate quadrupole coupling constants, eQq, or even to discuss relationships with chemical bonding. As a result of these previously reported findings, it is apparent that spectra-structure correlations of the type we reported previously ( 2 ) for various kinds of chlorine-containing compounds can also be applied to the trichloromethyl group. In this paper we extend our previous correlations to a correlation between average resonance frequencies of trichloromethyl groups and their structures. EXPERIMENTAL
Data used in the preparation of the spectra-structure correlation chart for trichloromethyl groups were obtained from several literature sources (3-8) as well as from studies performed in this laboratory on a variety of trichloromethyl containing compounds. Only data reported at 77 OK were used in the correlations. All samples examined in this laboratory were analyzed at 77 OK except for a and P-parachloral which were examined at ambient temperatures. The instrument used in our examination of the trichloromethyl containing samples was the Wilks Scientific Corp., Model NQR-1 A, spectrometer operating over a frequency range from 30-60 MHz. The quench frequency varied automatically from 30 to 45 KHz during scanning over this frequency range. Time constant used in the detection of the resonance signals was 10 seconds. Chart speed used was 8 inches/hour and the sweep rate used was about 3 MHz/hour. As discussed previously (2) we decided that any data included in correlation charts should be expressed in terms of resonance frequency rather than in terms of the quadrupole coupling constants. This same procedure was used for the correlation chart described herein. For the frequency measurements made on the samples examined in our laboratory we used a standard grid dip meter. The accuracy of the measurement using the grid dip meter was about +0.1 MHz. Whenever a frequency measurement was made, the marker switch on the console was moved to the right and to the left in order to throw the oscillator out-of-balance. This condition caused the pen to move vertically on the chart. At that precise time the reading was taken on the grid dip meter for the frequency measurement. Interpolations were then required to determine the resonance frequency for each observed peak (4) J. Graybeal and D. Cornwell, J . Phys. Chem., 62,483 (1958). (5) H. Allen, ibid.,57, 501 (1953). (6) S. L. Segel and R. G. Barnes, USAEC, IS-1222, September 1965. (7) H. 0. Hopper and P. J. Bray,J. Chem. Phys. 33, 334 (1960). (8) E. I. Fedin and G. K. Semin, Zh. Srrukt. Khim., 1,464 (1960).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 1, JANUARY 1971
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39.5 MHz
MHz
2 iz
CCL.5 cc13
77°K
Figure 3. W l NQR spectrum of trichloro-Nmethylacetamide at 77 O K
Figure 1. WI NQR spectrum of hexachloroethane at 77 O K
39 0
MHz
,
O-CHZ
CCL3-CH I ‘O-CHz
77°K
Figure 2. WI NQR spectrum of chloralide at 77 O K
in the spectrum. For the NQR spectra of CC13 groups, this meant that several interpolations were required. RESULTS AND DISCUSSION
In order to prepare a meaningful and useful spectrastructure correlation for use in structure determinations, we followed the work of Graybeal and Cornwell, and Allen in averaging the frequency of the multiplet lines observed for the trichloromethyl group prior to formulating the correlation chart. Three basically different types of multiplet line patterns were observed in our investigation of different kinds of trichloromethyl groups. These different patterns observed are illustrated in Figures 1-3. Figure 1 shows the 35Clres36
e
onance pattern obtained on hexachloroethane. Two lines are seen surrounded by fine structure that is dependent upon the quench frequency (4). One line appears a t 40.3 MHz and the other line appears at 40.5 MHz. The area ratio is about 1 :2 between the lower frequency line and the higher frequency line. The fact that only two lines are seen suggests that only one of the three chlorines in each C c l 3group is crystallographically different from the other two chlorines. Figure 2 shows the pattern obtained on chloralide. Instead of two lines being observed we find three lines for the CC13 group in this compound. One appears at 38.1 MHz, another at 38.4 MHz, and the third at 38.8 MHz. Again fine structure is also observed. Here we see that the lines are not only equivalent in area but they are also evenly spaced. Thus, each chlorine is not only different in a crystallographic sense but there is also a regularity in the degree of difference among the three chlorines. The third example as seen in Figure 3 is a spectrum of trichloro-N-methylacetamide. Again three lines are observed for the CC13 group. They appear at 39.2 MHz, 39.8 MHz, and 40.1 MHz. In this spectrum the three lines are not evenly spaced. The two higher frequency lines are spaced closer together than the two lower frequency lines. In fact there is a factor of 2 difference in the relative difference of the line spacings between the upper two lines and the lower two lines. Since the line intensities are nearly the same for all three lines, each line is assigned to one of the three chlorines in the CC13 group. Only through obtaining X-ray data on this material will it be known exactly how the crystal structure can be related to the spacing differences observed in the NQR spectrum. A correlation between X-ray data and NQR data appears to be essentially possible because of the illustration provided by Allen (5) on chloral hydrate and because of the kind of difference both we and Semin (3) noted in the investigation of trichloromethyl compounds. The spectra-structure correlation chart which we prepared from our work and from data reported in the literature (3-8) is shown in Figure 4. Of the 25 different compounds included in this correlation chart, 10 were analyzed by us for use in
ANALYTICAL CHEMISTRY, VOL. 43, NO. 1, JANUARY 1971
a 3
Z I
Table I. Comparison of WI NQR Data with lH NMR Data on Cyclic Trimers of Trichloroacetaldehyde NQR of CC13groups NMR of CH (ambient temp.) groups ____ Line Average Line frequency, Frequency, position Samples MHz MHz (PPm) a-Parachloral 39.7 6.15
FI -
-c-Cl (1)
-c
0 -c-0
n W
0
V
-c1
(3)
13)
I1
2 +a
/3-Parachloral W LL 3
‘ O -CH,CHZ, CH, (4)
c
0
3
i(L
(3)
38 5
375
40 5
39 5
Figure 4. Spectra-structure correlations of a6Cl quadrupole resonances of trichloromethyl compounds at 77 O K
preparing the chart. They included a number of different
tures. The remaining structures and corresponding data in the chart are from the literature. The numbers in parentheses located to the right of each correlation range indicate the number of compounds used in the correlation. By preparing this spectra-structure correlation chart, it is seen not only more clearly how the different indicated structures affect the 3jCl nuclear quadrupole resonances of trichloromethyl groups but also how the indicated resonance frequencies can be used in determining the kind of groups to which the trichloromethyl groups are attached. This correlation is an extension of the spectra-structure correlation we reported earlier ( 2 )for 35C1resonances in aliphatic compounds. Upon inspection of the chart in Figure 4 we see that the frequency range over which the 35Cl resonances of CC13 groups appear varies from about 38 MHz to about 41.5 MHz. Eight different correlations are included in the chart. None exceeds 1 MHz in its range although only two of the 0 (
a, II
-C--N)
39.3
5.67
38.7
5.46
41 5
AVERAGE FREQUENCY ( M H z )
structures
39.3 38.8 38.9 38.1 38.4
show the relatively large
variation of resonance frequency of about 1 MHz. The range for the other structures does not exceed about 0.5 MHz. From this correlation chart, it is seen that there is a shift to higher frequency as groups with greater electronegativity are attached to CC13 groups. This relationship which was first indicated by Allen ( 5 ) for monochloro compounds is consistent with that known for proton NMR; i.e., there is a shift to higher frequency (lower field) as groups with increasing electronegativity are attached to proton groups such as methyl or methylene groups. Since the correlations in this chart show the specific relations between the kind of group to which CCl, groups are attached and the average frequency of the NQR resonances for those groups, this chart should find a high degree of usefulness for structure studies of trichloromethyl compounds and it should be an aid in identifying the group to which cc13groups are attached. It is sDectra-structure correlations like these that will increase the usefulness of NQR as a structure tool. Up to now NQR has seen only limited use as a means of elucidating structure. In order to illustrate the use of NQR in determining the
structure of isomeric species containing CCI, groups, the results obtained on a- and p-parachloral at ambient temperature are given in Table I. These results are compared in the Table with NMR results ( 9 ) that were reported on dilute solutions of these materials. Even though these two materials are both cyclic trimers of trichloroacetaldehyde the difference between them is in the conformation of the CClp groups. p-Parachloral is the isomer that has all the C c l 3 groups cis to each other and a-parachloral is the isomer that has these CCIB groups cis-trans to each other. Three distinct and separate NQR lines were observed for each of these isomers. The difference in the NQR data between these isomers is clearly seen in the average frequency of the three lines observed. It is seen to be 38.7 MHz for p-parachloral and 39.3 MHz for a-parachloral. This difference of 0.6 MHz is not only significant but also shows the substantial difference that can be observed in NQR data for relative subtle changes in structure. The NMR data ( 9 ) show that p-parachloral, the all cis isomer, has only one line appearing at 5.46 pprn, and that aparachloral, the cis-trans isomer, has two lines appearing at lower fields (higher frequencies) from the single line of pparachloral. These lines are at 6.15 ppm and 5.67 ppm relative to the internal standard of TMS. Upon comparing the NQR data with the NMR data for these two isomeric structures, we find that there is a direct correspondence in the shift to higher frequency between the all cis structure and the cistrans structure. Thus, again we find with this illustration that the frequency (field) shifts observed in the high resolution proton magnetic resonance spectrum not only can be observed in the 35CI NQR spectrum but also are related directly to each other. ACKNOWLEDGMENT
The author wishes to thank Wilks Scientific Company for the use of its Model NQR-1A instrument in this work. Also, the author wishes to thank Drs. 0. Vogl, F. M. Sonnenberg, and F. M. Armbrecht for the various chlorine-containing samples supplied for this study on trichloromethyl compounds. Thanks should also be expressed to V. A. Brown who did the measurements on the instrument. RECEIVED for review May 14, 1970. Accepted September 21, 1970. This work was presented at the 159th National Meeting, American Chemical Society, Houston, Texas, February 1970 and at the 5th Middle Atlantic Regional Meeting, Newark, Del., April 1970. (9) Varian NMR Spectra Catalog, No. 449 and 450, Varian
Associates, Palo Alto, Calif.
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