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35Cl and 37Cl nuclear magnetic resonance of chloroorganic compounds
29 May 1998
Chemical Physics Letters 288 Ž1998. 809–815
35
Cl and 37Cl nuclear magnetic resonance of chloroorganic compounds Vladimir N. Torocheshnikov, Nickolai M. Sergeyev NMR Laboratory, Department of Chemistry, Moscow State UniÕersity, 119899 Moscow, Russia Received 5 March 1998
Abstract It has been shown that 35Cl and 37Cl chemical shifts and linewidths can be obtained using lineshape analysis. For a series of chloroorganic compounds the positions of the lines in the 35Cl and 37Cl NMR spectra were determined to an accuracy of 5–10 ppm, while the accuracy of the linewidths was 10–100 Hz. It has been also found that the ratio of 35Cl and 37Cl linewidths is not constant as expected from the assumption that the spin–spin relaxation of 35Cl and 37Cl is completely determined by the quadrupole mechanism. From 35Cl NMR data the DrH induced isotope shift for hydrochloric acid varies from 3.4 to 5.5 ppm, depending on its concentration in water. 13C– 35Cl, 13C– 37Cl, 19 F– 35Cl, 19 F– 37Cl coupling constants are reported. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction 35
Interest in Cl NMR is limited due to the large linewidths which are usually of the order of 1000– 10000 Hz w1x in most chloroorganic compounds. The broadening of 35 Cl NMR is due to a fast relaxation of the 35 Cl nuclei, which is mainly caused by the large quadrupole moment of the 35 Cl nucleus and rather large electric field gradients along the C–Cl bonds in chloroorganic compounds. Substantial line narrowing is observed for Cly anions due to the almost symmetrical electron distribution in such anions and in its complexes with counter anions w2x. This is applicable also to the case of 37Cl NMR, for which the quadrupole moment is only ; 20% less, but it has only half the sensitivity and it is almost three times less in natural abundance. Recent results on 35 Clr 37 Cl induced 13 C isotope shifts in chloromethanes w3x and on estimates of the indirect coupling constants 13 C– 35 Cl and 13 C– 37 Cl persuaded us to return to the 35 Cl and 37Cl NMR of
chloroorganic compounds in a search for possible ways of more accurate measurements of NMR parameters. Indeed, in spite of the very broad lines in 35 Clr 37 Cl NMR spectra, modern NMR spectrometers can give a rather high sensitivity and applying lineshape analysis can give an accuracy in line position much better than the linewidth itself w4,5x At the final stage of our study a Letter by Fedotov et al. w6x on 35 Clr 37 Cl NMR of small organic compounds was published. The authors w6x also used quantum mechanical density matrix functional calculations w7x to demonstrate how to predict 35 Cl chemical shifts in chloroorganic compounds. The results of the Letter w6x will be discussed.
2. Experimental NMR measurements were performed on a Varian VXR-400 spectrometer at resonance frequencies of
0009-2614r98r$19.00 q 1998 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 3 6 2 - 5
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V.N. TorocheshnikoÕ, N.M. SergeyeÕr Chemical Physics Letters 288 (1998) 809–815
39.193 and 32.623 MHz for 35 Cl and 37Cl, respectively. The measurements were done with acquisition times of 0.3–0.4 s and usually 400–1000 scans were enough to obtain very good signal to noise ratios. Proton decoupling was used in all experiments. Several halomethanes were measured including CH 3 Cl, CH 2 Cl 2 , CHCl 3 , CCl 4 , CHF2 Cl and CF2 Cl 2 . All chloroorganic compounds were studied as neat liquids with small amounts of C 6 D6 added for 2 D locking. The first four halomethanes were measured in conditions similar to those used to study 35 Clr 37 Cl induced 13 C isotope shifts Žsee Ref. w3x.. The organic compounds were standard commercial products: namely, n-propyl chloride, i-propyl chloride, n-butyl chloride, n-decyl chloride, a-chlorotoluene, a-chlorbenzaldehyde. For fluorochloromethanes 19 F NMR spectra were also measured in order to determine the 35 Clr 37 Cl induced isotope shifts on 19 F nuclei. 35 Cl chemical shifts were measured relative to the 35 Cl resonance of a solution of NH 4 Cl as the external reference. In addition, we measured the 35 Cl and 37Cl NMR of aqueous solutions of hydrochloric acid with varying concentrations of both HCl and DCl to look for possible DrH induced isotope shifts on chlorine chemical shifts. The experiments with hydrochloric acid were performed in coaxial sample tubes with inner tubes Žof ; 5 mm o.d.. filled with DCl and an
outer 10 mm o.d. tube filled with HCl. All experiments were made at 258C.
3. Spectra For the series of chloromethanes in both 35 Cl and Cl NMR we observed only one rather broad line. Assuming a Lorentzian line approximation we used the QUADR program w8x and obtained the linewidths and line positions listed in Table 1. In each case we used lines with a high signal to noise ratio. It was suggested that T2 s T1 s 1rpD n 1r2 . For the series of chloromethanes CH 4y nCl n linewidths from 3498 " 3 Hz in CH 3 Cl to 15501" 9 Hz in CCl 4 were obtained. The relaxation times decrease with the number of chlorines as follows: 91, 40, 23 and 21 ms, respectively, for n s 1, 2, 3 and 4. We also measured 35 Cl NMR spectra of several simple organic compounds ŽTable 2.. In the case of n-decyl-chloride the 35 Cl NMR signal was not observed at all probably due to a very broad line Žthe linewidth is presumably more than 30000 Hz.. The line positions were measured relative to Cly in NH 4 Cl with a rather high accuracy of ; 3–10 ppm. In the case of fluorochloromethanes we also measured the 19 F NMR spectra. For CHF2 Cl we ob37
Table 1 The 35 Clr 37 Cl spectral parameters of some halomethanes Compound CH 3 Cl CH 2 Cl 2 CHCl 3
Linewidth ŽHz.
T1 Žms.
Ratio D n 1r2 Ž 35 Cl.rD n 1r2 Ž 37 Cl.
Cl Cl
3498 " 3 2396 " 4
91 140
1.46
Cl Cl
7961 " 5 4775 " 4
40 66
1.67
Cl Cl
13810 " 7 7885 " 10
23 40
1.75
Cl Cl
15501 " 9 8593 " 56
21 37
1.81
Cl Cl
4214 " 8 2657 " 9
77 120
1.59
Cl Cl
6480 " 10 4051 " 8
49 78
1.56
Nucleus 35 37 35 37 35 37
CCl 4
35
CHFCl 2
35
CF2 Cl 2
35
37
37
37
V.N. TorocheshnikoÕ, N.M. SergeyeÕr Chemical Physics Letters 288 (1998) 809–815
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Table 2 35 Cl NMR spectral data of several organic compounds Compound
n-Pr-Cl i-Pr-Cl n-Bu-Cl n-dezyl-Cl C 6 H 5 CH 2 Cl C 6 H 5 CŽO.Cl
Linewidth ŽHz.
7724 " 9 6126 " 6 10927 " 10 ) 30000 24486" 148 25686 " 190
T1 Žms. 41 52 29 - 10 13 12
Chemical shift Žppm, relative to Cly . our data
lit. data ŽRef. w9x.
q119 " 3 q234 " 3 q126 " 3 n.o. 170 " 10 480 " 12
98 251 125
n.o.s not observed.
served a doublet due to 19 F– 1 H coupling ŽFig. 1. with the 19 F– 1 H geminal coupling constant equal to 62.33 Hz, each line being split into two components with an intensity ratio of 3:1, which can obviously be assigned to the 35 Cl and 37Cl isotopomers, respectively. The individual lineshapes for each isotopomer were processed using the QUADR program. This resulted in the values J Ž 19 F– 35 Cl. s 17.7 " 0.5 Hz and J Ž 19 F– 37 Cl. s 12.2 " 0.9 Hz. In the QUADR calculations we used the measured T1 relaxation times for both 35 Cl and 37Cl nuclei. We also measured the 13 C NMR spectrum of CH 3 Cl Žsee also Ref. w3x. where all components of the 13 C–H 1:3:3:1 quartet are additionally split into two lines due to the 35 Clr 37 Cl induced isotope effect ŽFig. 2.. The lineshapes were used in the QUADR fitting which gave
J Ž 13 C– 35 Cl. s 9.7 " 0.2 and J Ž 13 C– 37 Cl. s 8.8 " 0.5 Hz. We also obtained isotope effects for the 13 C– 19 F coupling constants due to 35 Clr 37 Cl substitution. Fig. 3 gives the same 19 F NMR spectrum as Fig. 2 but with a higher gain. It allows one to see the 13 C-satellites due to H 13 CF2 Cl. From the signals of the isotopomers H 13 CF235 Cl and H 13 CF237Cl we found slightly different coupling constants: 288.25 and 288.16 Hz for the 35 Cl and 37Cl isotopomers, respectively. Finally, we have measured the 35 Cl chemical shifts for Cly for the isotopomeric hydrochloric acids HCl and DCl. Measurements were done for solutions of HCl and DCl in water at different concentrations. HCl and DCl solutions were placed in coaxial outer
Fig. 1. The 19 F NMR spectrum of CHF2 Cl ŽVarian VXR-400, 376.28 MHz.. The splitting a represents 2 J Ž19 F– 1 H. equal to 63.23 Hz. Lines b and c were used to obtain the values of 2 J Ž19 F– 35 Cl. and 2 J Ž19 F– 37 Cl., which equal 17.6 and 12.2 Hz, respectively.
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V.N. TorocheshnikoÕ, N.M. SergeyeÕr Chemical Physics Letters 288 (1998) 809–815
Fig. 2. The proton-coupled 13 C NMR spectrum of CH 3 Cl ŽVarian VXR-400, 100.1 MHz.. The insert corresponds to one of the central lines, split due to the 35 Clr 37 Cl isotope effect into two components a and b from two isotopomers CH 3 35 Cl and CH 337Cl, respectively.
Fig. 3. 13 C satellites in the 19 F NMR spectrum of CHF2 Cl. The splittings a and b represent 1 JŽ13 C– 1 H. in CHF235 Cl and CHF237Cl and equal 288.16 and 288.25 Hz, respectively. The splitting c, equal to 141 ppb, is the 19 F isotope shift in CHF2 Cl due to 12 Cr 13 C substitution.
V.N. TorocheshnikoÕ, N.M. SergeyeÕr Chemical Physics Letters 288 (1998) 809–815
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Fig. 4. The 35 Cl NMR spectrum of HCl and DCl solutions in H 2 O and D 2 O, respectively ŽVarian VXR-400, spectra were run for the coaxial tubes arrangement.. The high-field line assigned to DCl is markedly broadened.
Ž10 mm o.d.. and inner Ž5 mm o.d.. sample tubes, respectively. A typical spectrum is given in Fig. 4, where a high-field signal is attributed to the DCl solution. Spectra were always a superimposition of two Lorentzian lines and their positions and linewidths were obtained from the QUADR calculations. In each case the DCl line was observed to the high field. Pairs of HCl and DCl solutions were prepared with equal concentrations Žaccuracy "0.1 molrl. and the corresponding 35 Cl chemical shifts
Fig. 5. 35 Cl chemical shifts in ppm in the HClrDCl experiments vs. concentration. 35 Cl chemical shifts were measured relative to external NH 4 Cl.
are given in Fig. 5. Concentrations were in the range 0.1–10 molrl.
4. Discussion 4.1. Cl chemical shifts 35
Cl chemical shifts in several organic compounds have been measured previously several times Žsee reviews w1,9x and Ref. w10x.. It is worth noting that the 35 Cl chemical shifts in the literature are usually given without error estimates and so does not allow one to discuss the possible use of 35 Cl NMR in structural investigations. Another factor needed to be taken into account while comparing data from different sources is the solvent effect Žsee, e.g., data in Ref. w6x.. The accuracy obtained in the present study seems to be quite realistic and allows us to compare the present data with literature data and to estimate the accuracy of the literature values as ca. "20 ppm. Our data show that 35 Cl chemical shifts are extremely sensitive to the immediate environment. An increase in the number of carbon atoms attached to the C–Cl group leads to a substantial deshielding effect. Taking into account the large range of 35 Cl shielding constants Ž; 1000 ppm. an accuracy of 3–5 ppm may indeed be decisive in solving many structural problems in organochlorine chemistry. It is supported by the recent progress in calculations of 35 Cl chemical shifts w6x where the accuracy of calcu-
V.N. TorocheshnikoÕ, N.M. SergeyeÕr Chemical Physics Letters 288 (1998) 809–815
814
lations is comparable with the experimental error limits. 4.2. Linewidths of
35
Cl and
37
Cl NMR
The systematic line broadening of 35 Cl NMR with the number of chlorine atoms at the carbon in the halomethane series CH nCl 4yn was first observed many years ago of Saito w11x. He obtained the following data for the T1 relaxation times: 100, 40, 30 and 20 ms for n s 1, 2, 3 and 4, respectively. It seems that only in the case of chloroform are the deviations between the data by Saito and the data of the present study beyond the limits of error Žcf. 30 ms in Ref. w11x with 23 ms in the present study.. It is likely that our data are more accurate as the possible errors in the linewidth are less than 1% in our experiments. This is also supported by our measurements of the linewidth in 37Cl NMR where the T1 values for CHCl 3 and CCl 4 are also close to each other. We also paid attention to the data on the ratio of relaxation times T1Ž 37 Cl.rT1Ž 35 Cl. Žor the linewidths D n 1r 2 Ž 35 Cl.rD n 1r2 Ž 37 Cl.. in the series of chloromethanes CH 4y nCl n . An assumption can be made that the 35 Cl and 37Cl relaxation rates are fully determined by the quadrupole contributions. Thus the ratio D n 1r2 Ž 35 Cl.rD n 1r2Ž 37 Cl.. should be equal to the ratio of the squares of the corresponding quadrupole moments Ži.e. 1.61, neglecting any possible isotope effects on electric field gradients.. Our e x p e rim e n ta l d a ta s h o w th e ra tio D n 1r2 Ž 35 Cl.rD n 1r2Ž 37 Cl.. is not a constant equal to 1.61 but instead it varies from 1.46 in CH 3 Cl to 1.81 in CCl 4 ŽTable 1.. The variations we observed are supported by the changes in the ratio of D n 1r2 Ž 35 Cl.rD n 1r2 Ž 37 Cl. were observed previously by Bastow et al. w12x in a study of 35 Cl and 37Cl relaxation in NH 4 ClO4y and ND4 ClO4y and by Lambert and Schiff w13x while studying 35 Cl and 37Cl spectra of some perchlorates. According to Hertz w14x the ratio of D n 1r2 Ž 35 Cl.rD n 1r2 Ž 37 Cl. can vary from 1.61 to 1 in cases of exchange of chlorine ions. As any chlorine exchange is excluded in halomethanes and as we observed deviations in the h ig h e r v a lu e s of th e ra tio D n 1r2 Ž 35 Cl.rD n 1r2Ž 37 Cl.. Žup to 1.81., the explanation for these observations remains unclear.
4.3. Cl r 37 Cl induced isotope shift for
19
F
In the case of fluorinated compounds we found small 19 F isotope shifts due to 35 Clr 37 Cl substitution. For CHF2 Cl the average splitting between the two components Žsee Fig. 1. was 6 ppb and the three components in the 19 F spectrum of CF2 Cl 2 were split by 11.4 ppb. Similar values for the isotope shifts over two bonds were recently observed for several fluorochloroorganic compounds w15x. The present results show that this isotope shift depends mainly on the number of fluorine atoms at the carbon atom. 4.4. Cl–F coupling constants Limited data on 19 F–Cl geminal coupling constants can be found in two reviews w1,9x where they cover the range from 11 to 28 Hz. Unfortunately, all the previous studies used indirect relaxation measurements and the accuracy reported seems to be overestimated. For CHF2 Cl the averaged value of 28 Hz was reported by Sears w16x without any error limits. Our data Ž J Ž 19 F– 35 Cl. s 17.7 " 1.0 and J Ž 37 Cl– 19 F. s 12.2 " 1.0 Hz. differ substantially from the literature value. It is also worth noting that the ratio J Ž 19 F– 35 Cl.rJ Ž 37 Cl– 19 F. is equal 1.45 " 0.2 which differs from the theoretical ratio of 1.20. The data obtained in this Letter are obtained with a realistic estimate of accuracy. 4.5. Isotope effects on 13C– 19 F coupling constants due to 35Cl r 37 Cl substitution We also found small secondary isotope effects on the 13 C– 19 F coupling constants due to 35 Clr 37 Cl substitution. If defined as follows Žfor the definition, see Ref. w5x. s
D J Ž 13 C, 19 F . s J Ž 13 C, 19 F . Ž in CHF237Cl . yJ Ž 13 C, 19 F . Ž in CHF235 Cl . ,
it equals 0.09 Hz. This isotope effect is positive when the negative sign of 1 J Ž 13 C, 19 F. is taken into account. 4.6. C, Cl coupling constants It was one of the aims of the present study to obtain more accurate estimates of the 13 C– 35 Cl and 13 C– 37 Cl coupling constants. They were previously estimated from the 13 C line broadenings w3x using a simplified approach Ži.e., neglecting, for example,
V.N. TorocheshnikoÕ, N.M. SergeyeÕr Chemical Physics Letters 288 (1998) 809–815
any difference in 13 C– 35 Cl and 13 C– 37Cl couplings.. As a result a mean value for J Ž 13 C, Cl. of 11 " 2 Hz was obtained for CH 3 Cl w3x. Using independent data on the chlorine relaxation time T1 and also applying the QUADR program we have now obtained two different coupling constants: J Ž 13 C– 35 Cl. s 9.7 " 0.2 and J Ž 13 C– 37Cl. s 8.8 " 0.5 Hz. Their ratio is equal to 1.1Ž"0.1.. This is in accord with the ratio of gyromagnetic ratios but it is obvious that the accuracy obtained does not allow one to search for any isotope effects due to 35 Clr 37 Cl substitution. The values we obtained are more accurate that those reported before. 4.7. D r H induced acid
35
C isotope shifts in hydrochloric
For solutions of HCl and DCl in water we obtained the results presented in Fig. 5. It is clearly seen that for both solutions the high-field shift Ži.e., an increase of shielding. is observed for 35 Cl nuclei with a decrease of concentration of HCl in water. A similar trend for 19 F chemical shifts was observed for an aqueous solution of HF w17x. In all cases we observed the high-field displacement of the 35 Cl NMR of DCl relative to HCl Žfrom about 3.4 to 5.2 ppm.. Though a detailed investigation seems to need the equilibrium processes for HCl in water to be taken into account, one can state that the high-field displacement of the order of 4 ppm is in agreement with a high-field shift observed in the 19 F NMR spectrum of HFrDF in the gaseous phase Žand equal to ; 2.5 ppm w18x.. It is also worth noting that in all cases the DCl signal was substantially more broadened than that of HCl. This fact can be used when the equilibria Cly Ž H 2 O . n | HCl Ž H 2 O . ny1 q OHy and Cly Ž D 2 O . n | DCl Ž D 2 O . ny1 q ODy are taken into account.
815
Acknowledgements The authors are grateful to Dr. W.T. Raynes for useful comments. This work was supported by the Russian Foundation for Basic Research Žgrant 97-0333574a..
References w1x J.W. Akitt, in: J. Mason ŽEd.., Multinuclear NMR, Plenum, New York, 1987, p.447. w2x J. Roberts, J. Schnitker, J. Phys. Chem. 97 Ž1993. 5410. w3x N.M. Sergeyev, P. Sandor, N.D. Sergeyeva, W.T. Raynes, J. Magn. Reson. A 115 Ž1995. 174. w4x G.H. Weiss, J.A. Ferreti, J.E. Kiefer, J. Magn. Reson. 46 Ž1982. 49. w5x N.M. Sergeyev, in: P. Diehl, R. Kosfeld, H. Gunther ŽEds.., ¨ NMR Basic Principle and Progress, vol. 22, Springer, Berlin, 1990, p.31. w6x M. Fedotov, O.L. Malkina, V.G. Malkin, Chem. Phys. Lett. 258 Ž1996. 330. w7x V.G. Malkin, O.L. Malkina, M.E. Casida, D.R. Salahub, J. Am. Chem. Soc. 116 Ž1994. 5898. w8x I.F. Leshcheva, V.N. Torocheshnikov, V.A. Chertkov, N.M. Sergeyev, J. Magn. Reson. 94 Ž1991. 1. w9x T. Drakenberg, S. Forsen, in: The Multinuclear Approach to NMR Spectroscopy, D. Reidel, Dordrecht, 1982, p.405. w10x K. Barlos, J. Kroner, H. Noth, B. Wrackmeyer, Chem. Ber. 111 Ž1978. 1833–1838. w11x Y. Saito, Can. J. Chem. 43 Ž1965. 2530. w12x T.J. Bastow, R.J.C. Brown, S.L. Segal, J. Chem. Phys. 89 Ž1988. 1203. w13x J.B. Lambert, W. Schiff, J. Am. Chem. Soc. 110 Ž1988. 6364. w14x H.G. Hertz, Z. Electrochem. 65 Ž1961. 36. w15x M. Tordeuax, I. Baime, C. Wakselmon, New. J. Chem. 19 Ž1995. 287. w16x R.E. Sears, J. Chem. Phys. 56 Ž1972. 983. w17x K. Schaumburg, C. Deverell, J. Am. Chem. Soc. 90 Ž1968. 2495. w18x D.K. Hindermann, C.D. Cornwell, J. Chem. Phys. 48 Ž1968. 2017.
Chemical Physics Letters 288 Ž1998. 809–815
35
Cl and 37Cl nuclear magnetic resonance of chloroorganic compounds Vladimir N. Torocheshnikov, Nickolai M. Sergeyev NMR Laboratory, Department of Chemistry, Moscow State UniÕersity, 119899 Moscow, Russia Received 5 March 1998
Abstract It has been shown that 35Cl and 37Cl chemical shifts and linewidths can be obtained using lineshape analysis. For a series of chloroorganic compounds the positions of the lines in the 35Cl and 37Cl NMR spectra were determined to an accuracy of 5–10 ppm, while the accuracy of the linewidths was 10–100 Hz. It has been also found that the ratio of 35Cl and 37Cl linewidths is not constant as expected from the assumption that the spin–spin relaxation of 35Cl and 37Cl is completely determined by the quadrupole mechanism. From 35Cl NMR data the DrH induced isotope shift for hydrochloric acid varies from 3.4 to 5.5 ppm, depending on its concentration in water. 13C– 35Cl, 13C– 37Cl, 19 F– 35Cl, 19 F– 37Cl coupling constants are reported. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction 35
Interest in Cl NMR is limited due to the large linewidths which are usually of the order of 1000– 10000 Hz w1x in most chloroorganic compounds. The broadening of 35 Cl NMR is due to a fast relaxation of the 35 Cl nuclei, which is mainly caused by the large quadrupole moment of the 35 Cl nucleus and rather large electric field gradients along the C–Cl bonds in chloroorganic compounds. Substantial line narrowing is observed for Cly anions due to the almost symmetrical electron distribution in such anions and in its complexes with counter anions w2x. This is applicable also to the case of 37Cl NMR, for which the quadrupole moment is only ; 20% less, but it has only half the sensitivity and it is almost three times less in natural abundance. Recent results on 35 Clr 37 Cl induced 13 C isotope shifts in chloromethanes w3x and on estimates of the indirect coupling constants 13 C– 35 Cl and 13 C– 37 Cl persuaded us to return to the 35 Cl and 37Cl NMR of
chloroorganic compounds in a search for possible ways of more accurate measurements of NMR parameters. Indeed, in spite of the very broad lines in 35 Clr 37 Cl NMR spectra, modern NMR spectrometers can give a rather high sensitivity and applying lineshape analysis can give an accuracy in line position much better than the linewidth itself w4,5x At the final stage of our study a Letter by Fedotov et al. w6x on 35 Clr 37 Cl NMR of small organic compounds was published. The authors w6x also used quantum mechanical density matrix functional calculations w7x to demonstrate how to predict 35 Cl chemical shifts in chloroorganic compounds. The results of the Letter w6x will be discussed.
2. Experimental NMR measurements were performed on a Varian VXR-400 spectrometer at resonance frequencies of
0009-2614r98r$19.00 q 1998 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 3 6 2 - 5
810
V.N. TorocheshnikoÕ, N.M. SergeyeÕr Chemical Physics Letters 288 (1998) 809–815
39.193 and 32.623 MHz for 35 Cl and 37Cl, respectively. The measurements were done with acquisition times of 0.3–0.4 s and usually 400–1000 scans were enough to obtain very good signal to noise ratios. Proton decoupling was used in all experiments. Several halomethanes were measured including CH 3 Cl, CH 2 Cl 2 , CHCl 3 , CCl 4 , CHF2 Cl and CF2 Cl 2 . All chloroorganic compounds were studied as neat liquids with small amounts of C 6 D6 added for 2 D locking. The first four halomethanes were measured in conditions similar to those used to study 35 Clr 37 Cl induced 13 C isotope shifts Žsee Ref. w3x.. The organic compounds were standard commercial products: namely, n-propyl chloride, i-propyl chloride, n-butyl chloride, n-decyl chloride, a-chlorotoluene, a-chlorbenzaldehyde. For fluorochloromethanes 19 F NMR spectra were also measured in order to determine the 35 Clr 37 Cl induced isotope shifts on 19 F nuclei. 35 Cl chemical shifts were measured relative to the 35 Cl resonance of a solution of NH 4 Cl as the external reference. In addition, we measured the 35 Cl and 37Cl NMR of aqueous solutions of hydrochloric acid with varying concentrations of both HCl and DCl to look for possible DrH induced isotope shifts on chlorine chemical shifts. The experiments with hydrochloric acid were performed in coaxial sample tubes with inner tubes Žof ; 5 mm o.d.. filled with DCl and an
outer 10 mm o.d. tube filled with HCl. All experiments were made at 258C.
3. Spectra For the series of chloromethanes in both 35 Cl and Cl NMR we observed only one rather broad line. Assuming a Lorentzian line approximation we used the QUADR program w8x and obtained the linewidths and line positions listed in Table 1. In each case we used lines with a high signal to noise ratio. It was suggested that T2 s T1 s 1rpD n 1r2 . For the series of chloromethanes CH 4y nCl n linewidths from 3498 " 3 Hz in CH 3 Cl to 15501" 9 Hz in CCl 4 were obtained. The relaxation times decrease with the number of chlorines as follows: 91, 40, 23 and 21 ms, respectively, for n s 1, 2, 3 and 4. We also measured 35 Cl NMR spectra of several simple organic compounds ŽTable 2.. In the case of n-decyl-chloride the 35 Cl NMR signal was not observed at all probably due to a very broad line Žthe linewidth is presumably more than 30000 Hz.. The line positions were measured relative to Cly in NH 4 Cl with a rather high accuracy of ; 3–10 ppm. In the case of fluorochloromethanes we also measured the 19 F NMR spectra. For CHF2 Cl we ob37
Table 1 The 35 Clr 37 Cl spectral parameters of some halomethanes Compound CH 3 Cl CH 2 Cl 2 CHCl 3
Linewidth ŽHz.
T1 Žms.
Ratio D n 1r2 Ž 35 Cl.rD n 1r2 Ž 37 Cl.
Cl Cl
3498 " 3 2396 " 4
91 140
1.46
Cl Cl
7961 " 5 4775 " 4
40 66
1.67
Cl Cl
13810 " 7 7885 " 10
23 40
1.75
Cl Cl
15501 " 9 8593 " 56
21 37
1.81
Cl Cl
4214 " 8 2657 " 9
77 120
1.59
Cl Cl
6480 " 10 4051 " 8
49 78
1.56
Nucleus 35 37 35 37 35 37
CCl 4
35
CHFCl 2
35
CF2 Cl 2
35
37
37
37
V.N. TorocheshnikoÕ, N.M. SergeyeÕr Chemical Physics Letters 288 (1998) 809–815
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Table 2 35 Cl NMR spectral data of several organic compounds Compound
n-Pr-Cl i-Pr-Cl n-Bu-Cl n-dezyl-Cl C 6 H 5 CH 2 Cl C 6 H 5 CŽO.Cl
Linewidth ŽHz.
7724 " 9 6126 " 6 10927 " 10 ) 30000 24486" 148 25686 " 190
T1 Žms. 41 52 29 - 10 13 12
Chemical shift Žppm, relative to Cly . our data
lit. data ŽRef. w9x.
q119 " 3 q234 " 3 q126 " 3 n.o. 170 " 10 480 " 12
98 251 125
n.o.s not observed.
served a doublet due to 19 F– 1 H coupling ŽFig. 1. with the 19 F– 1 H geminal coupling constant equal to 62.33 Hz, each line being split into two components with an intensity ratio of 3:1, which can obviously be assigned to the 35 Cl and 37Cl isotopomers, respectively. The individual lineshapes for each isotopomer were processed using the QUADR program. This resulted in the values J Ž 19 F– 35 Cl. s 17.7 " 0.5 Hz and J Ž 19 F– 37 Cl. s 12.2 " 0.9 Hz. In the QUADR calculations we used the measured T1 relaxation times for both 35 Cl and 37Cl nuclei. We also measured the 13 C NMR spectrum of CH 3 Cl Žsee also Ref. w3x. where all components of the 13 C–H 1:3:3:1 quartet are additionally split into two lines due to the 35 Clr 37 Cl induced isotope effect ŽFig. 2.. The lineshapes were used in the QUADR fitting which gave
J Ž 13 C– 35 Cl. s 9.7 " 0.2 and J Ž 13 C– 37 Cl. s 8.8 " 0.5 Hz. We also obtained isotope effects for the 13 C– 19 F coupling constants due to 35 Clr 37 Cl substitution. Fig. 3 gives the same 19 F NMR spectrum as Fig. 2 but with a higher gain. It allows one to see the 13 C-satellites due to H 13 CF2 Cl. From the signals of the isotopomers H 13 CF235 Cl and H 13 CF237Cl we found slightly different coupling constants: 288.25 and 288.16 Hz for the 35 Cl and 37Cl isotopomers, respectively. Finally, we have measured the 35 Cl chemical shifts for Cly for the isotopomeric hydrochloric acids HCl and DCl. Measurements were done for solutions of HCl and DCl in water at different concentrations. HCl and DCl solutions were placed in coaxial outer
Fig. 1. The 19 F NMR spectrum of CHF2 Cl ŽVarian VXR-400, 376.28 MHz.. The splitting a represents 2 J Ž19 F– 1 H. equal to 63.23 Hz. Lines b and c were used to obtain the values of 2 J Ž19 F– 35 Cl. and 2 J Ž19 F– 37 Cl., which equal 17.6 and 12.2 Hz, respectively.
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V.N. TorocheshnikoÕ, N.M. SergeyeÕr Chemical Physics Letters 288 (1998) 809–815
Fig. 2. The proton-coupled 13 C NMR spectrum of CH 3 Cl ŽVarian VXR-400, 100.1 MHz.. The insert corresponds to one of the central lines, split due to the 35 Clr 37 Cl isotope effect into two components a and b from two isotopomers CH 3 35 Cl and CH 337Cl, respectively.
Fig. 3. 13 C satellites in the 19 F NMR spectrum of CHF2 Cl. The splittings a and b represent 1 JŽ13 C– 1 H. in CHF235 Cl and CHF237Cl and equal 288.16 and 288.25 Hz, respectively. The splitting c, equal to 141 ppb, is the 19 F isotope shift in CHF2 Cl due to 12 Cr 13 C substitution.
V.N. TorocheshnikoÕ, N.M. SergeyeÕr Chemical Physics Letters 288 (1998) 809–815
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Fig. 4. The 35 Cl NMR spectrum of HCl and DCl solutions in H 2 O and D 2 O, respectively ŽVarian VXR-400, spectra were run for the coaxial tubes arrangement.. The high-field line assigned to DCl is markedly broadened.
Ž10 mm o.d.. and inner Ž5 mm o.d.. sample tubes, respectively. A typical spectrum is given in Fig. 4, where a high-field signal is attributed to the DCl solution. Spectra were always a superimposition of two Lorentzian lines and their positions and linewidths were obtained from the QUADR calculations. In each case the DCl line was observed to the high field. Pairs of HCl and DCl solutions were prepared with equal concentrations Žaccuracy "0.1 molrl. and the corresponding 35 Cl chemical shifts
Fig. 5. 35 Cl chemical shifts in ppm in the HClrDCl experiments vs. concentration. 35 Cl chemical shifts were measured relative to external NH 4 Cl.
are given in Fig. 5. Concentrations were in the range 0.1–10 molrl.
4. Discussion 4.1. Cl chemical shifts 35
Cl chemical shifts in several organic compounds have been measured previously several times Žsee reviews w1,9x and Ref. w10x.. It is worth noting that the 35 Cl chemical shifts in the literature are usually given without error estimates and so does not allow one to discuss the possible use of 35 Cl NMR in structural investigations. Another factor needed to be taken into account while comparing data from different sources is the solvent effect Žsee, e.g., data in Ref. w6x.. The accuracy obtained in the present study seems to be quite realistic and allows us to compare the present data with literature data and to estimate the accuracy of the literature values as ca. "20 ppm. Our data show that 35 Cl chemical shifts are extremely sensitive to the immediate environment. An increase in the number of carbon atoms attached to the C–Cl group leads to a substantial deshielding effect. Taking into account the large range of 35 Cl shielding constants Ž; 1000 ppm. an accuracy of 3–5 ppm may indeed be decisive in solving many structural problems in organochlorine chemistry. It is supported by the recent progress in calculations of 35 Cl chemical shifts w6x where the accuracy of calcu-
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lations is comparable with the experimental error limits. 4.2. Linewidths of
35
Cl and
37
Cl NMR
The systematic line broadening of 35 Cl NMR with the number of chlorine atoms at the carbon in the halomethane series CH nCl 4yn was first observed many years ago of Saito w11x. He obtained the following data for the T1 relaxation times: 100, 40, 30 and 20 ms for n s 1, 2, 3 and 4, respectively. It seems that only in the case of chloroform are the deviations between the data by Saito and the data of the present study beyond the limits of error Žcf. 30 ms in Ref. w11x with 23 ms in the present study.. It is likely that our data are more accurate as the possible errors in the linewidth are less than 1% in our experiments. This is also supported by our measurements of the linewidth in 37Cl NMR where the T1 values for CHCl 3 and CCl 4 are also close to each other. We also paid attention to the data on the ratio of relaxation times T1Ž 37 Cl.rT1Ž 35 Cl. Žor the linewidths D n 1r 2 Ž 35 Cl.rD n 1r2 Ž 37 Cl.. in the series of chloromethanes CH 4y nCl n . An assumption can be made that the 35 Cl and 37Cl relaxation rates are fully determined by the quadrupole contributions. Thus the ratio D n 1r2 Ž 35 Cl.rD n 1r2Ž 37 Cl.. should be equal to the ratio of the squares of the corresponding quadrupole moments Ži.e. 1.61, neglecting any possible isotope effects on electric field gradients.. Our e x p e rim e n ta l d a ta s h o w th e ra tio D n 1r2 Ž 35 Cl.rD n 1r2Ž 37 Cl.. is not a constant equal to 1.61 but instead it varies from 1.46 in CH 3 Cl to 1.81 in CCl 4 ŽTable 1.. The variations we observed are supported by the changes in the ratio of D n 1r2 Ž 35 Cl.rD n 1r2 Ž 37 Cl. were observed previously by Bastow et al. w12x in a study of 35 Cl and 37Cl relaxation in NH 4 ClO4y and ND4 ClO4y and by Lambert and Schiff w13x while studying 35 Cl and 37Cl spectra of some perchlorates. According to Hertz w14x the ratio of D n 1r2 Ž 35 Cl.rD n 1r2 Ž 37 Cl. can vary from 1.61 to 1 in cases of exchange of chlorine ions. As any chlorine exchange is excluded in halomethanes and as we observed deviations in the h ig h e r v a lu e s of th e ra tio D n 1r2 Ž 35 Cl.rD n 1r2Ž 37 Cl.. Žup to 1.81., the explanation for these observations remains unclear.
4.3. Cl r 37 Cl induced isotope shift for
19
F
In the case of fluorinated compounds we found small 19 F isotope shifts due to 35 Clr 37 Cl substitution. For CHF2 Cl the average splitting between the two components Žsee Fig. 1. was 6 ppb and the three components in the 19 F spectrum of CF2 Cl 2 were split by 11.4 ppb. Similar values for the isotope shifts over two bonds were recently observed for several fluorochloroorganic compounds w15x. The present results show that this isotope shift depends mainly on the number of fluorine atoms at the carbon atom. 4.4. Cl–F coupling constants Limited data on 19 F–Cl geminal coupling constants can be found in two reviews w1,9x where they cover the range from 11 to 28 Hz. Unfortunately, all the previous studies used indirect relaxation measurements and the accuracy reported seems to be overestimated. For CHF2 Cl the averaged value of 28 Hz was reported by Sears w16x without any error limits. Our data Ž J Ž 19 F– 35 Cl. s 17.7 " 1.0 and J Ž 37 Cl– 19 F. s 12.2 " 1.0 Hz. differ substantially from the literature value. It is also worth noting that the ratio J Ž 19 F– 35 Cl.rJ Ž 37 Cl– 19 F. is equal 1.45 " 0.2 which differs from the theoretical ratio of 1.20. The data obtained in this Letter are obtained with a realistic estimate of accuracy. 4.5. Isotope effects on 13C– 19 F coupling constants due to 35Cl r 37 Cl substitution We also found small secondary isotope effects on the 13 C– 19 F coupling constants due to 35 Clr 37 Cl substitution. If defined as follows Žfor the definition, see Ref. w5x. s
D J Ž 13 C, 19 F . s J Ž 13 C, 19 F . Ž in CHF237Cl . yJ Ž 13 C, 19 F . Ž in CHF235 Cl . ,
it equals 0.09 Hz. This isotope effect is positive when the negative sign of 1 J Ž 13 C, 19 F. is taken into account. 4.6. C, Cl coupling constants It was one of the aims of the present study to obtain more accurate estimates of the 13 C– 35 Cl and 13 C– 37 Cl coupling constants. They were previously estimated from the 13 C line broadenings w3x using a simplified approach Ži.e., neglecting, for example,
V.N. TorocheshnikoÕ, N.M. SergeyeÕr Chemical Physics Letters 288 (1998) 809–815
any difference in 13 C– 35 Cl and 13 C– 37Cl couplings.. As a result a mean value for J Ž 13 C, Cl. of 11 " 2 Hz was obtained for CH 3 Cl w3x. Using independent data on the chlorine relaxation time T1 and also applying the QUADR program we have now obtained two different coupling constants: J Ž 13 C– 35 Cl. s 9.7 " 0.2 and J Ž 13 C– 37Cl. s 8.8 " 0.5 Hz. Their ratio is equal to 1.1Ž"0.1.. This is in accord with the ratio of gyromagnetic ratios but it is obvious that the accuracy obtained does not allow one to search for any isotope effects due to 35 Clr 37 Cl substitution. The values we obtained are more accurate that those reported before. 4.7. D r H induced acid
35
C isotope shifts in hydrochloric
For solutions of HCl and DCl in water we obtained the results presented in Fig. 5. It is clearly seen that for both solutions the high-field shift Ži.e., an increase of shielding. is observed for 35 Cl nuclei with a decrease of concentration of HCl in water. A similar trend for 19 F chemical shifts was observed for an aqueous solution of HF w17x. In all cases we observed the high-field displacement of the 35 Cl NMR of DCl relative to HCl Žfrom about 3.4 to 5.2 ppm.. Though a detailed investigation seems to need the equilibrium processes for HCl in water to be taken into account, one can state that the high-field displacement of the order of 4 ppm is in agreement with a high-field shift observed in the 19 F NMR spectrum of HFrDF in the gaseous phase Žand equal to ; 2.5 ppm w18x.. It is also worth noting that in all cases the DCl signal was substantially more broadened than that of HCl. This fact can be used when the equilibria Cly Ž H 2 O . n | HCl Ž H 2 O . ny1 q OHy and Cly Ž D 2 O . n | DCl Ž D 2 O . ny1 q ODy are taken into account.
815
Acknowledgements The authors are grateful to Dr. W.T. Raynes for useful comments. This work was supported by the Russian Foundation for Basic Research Žgrant 97-0333574a..
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