Oct., 1959
NUCLEAR MAGNETIC SHIELDING OF FIBIN~CHLOROFLUOROCARBONS 1701
NUCLEAR MAGNETIC SHIELDING OF Fl9 I N SOME CHLOROFLUOROCARBOXS’ BY T. S. SMITH* AND E. A. SMITH Gooduear Atomic Corporation, Portsmouth, Ohio Received December 1.2, 1968
The ositions of the nuclear magnetic resonance lines of the various F’Qgroups in dichlorotetrafluoroethane (CClzFCFS and C&F&C1F2) trichlorotrifluoroethane (CC12FCClFz), trichlorofluoromethane (CChF), perfluorodimethylcyclohexane (CsFla), and trichioroheptafluorobutane (CFsCC12CClFCF3) have been measured with respect to the FI9 line of trifluoroethanoic acid (CFaCOOH). The magnetic shielding constant for the F19groups increases with an increase of the total electronegativity of the atoms associated with the nearest carbon atom.
Introduction Since Knight3 first reported that the external conditions for nuclear magnetic resonance of a particular nucleus depended upon the chemical compound in which the nucleus resided, many investigator^^-'^ have made experimental and theoretical studies of the effect. The chemical shift is caused by a magnetic shielding of the nucleus due to the electron distribution about the nucleus of interest. Ramseyl’ has developed a formal procedure for calculating the shielding as a function of the externally applied magnetic field. It consists of two terms: (a) Lamb’s” diamagnetic correction and (b) a paramagnetic term arising from a spherical asymmetry of the electrical potential a t the nucleus due to the electrons. Unfortunately, the results of Ramsey’s study can only be used to calculate the shielding for linear molecules for which spin-rotational interaction data are known. A series of binary covalent compounds was examined by Gutowksy and HoffmanL2in an attempt to determine the effect of the electronic structure associated with bond formation and of the electronic configuration of the other atoms in the molecule. The results of their examination showed a decrease of the nuclear magnetic shielding of the F19 nucleus with increased electronegativity18 of the adjacent atom. Investigations by Gutowsky and others13 on t.he substituent effects in some fluorobenzene derivatives led to an apparent direct (1) This work was performed under Contract AT-(33-2)-1 with the United States Atomic Energy Commission, and was presented at the Annual Meeting of the American Physical Society, New York, January 30, 1958. (2) Department of Physics, Ohio University, Athens, Ohio. (3) W. D. Knight, Phys. Rev., 7 6 , 1259 (1949). (4) H. S. Gutowsky and B. R. McGarvey, J. Chem. Phys., 20, 1472 (1952). ( 5 ) N. Bloetnbergen and T. J. Rowland, Acta Metalluwia, 1, 731 (1953). (6) W. C. Dickenson, Phys. Rev., 7 7 , 736 (1950). (7) H. S. Gutowsky and C. J. Hoffman, ibid., 80, 110 (19501. (8) W. G. Proctor and F. C. Yu, ibid.. 7 7 , 717 (1950). (9) W. C. Dickenson, %bid.,81,717 (1951). (10) J. E. Welts, Chem. Reus., 66, 829 (1955). (11) N. F. Ramsey, Phys. Rev.. 78, G99 (1950). (12) H. S. Gutowsky and C. J. Hoffman, J. Chem. Phys., 19, 1259 (1951). (13) H. 8. Gutowsky, e t al., J. A m . Chsm. Soc., 74, 4809 (1952). (14) L. H. Meyer and H. S. Gutowsky, THIEJOURNAL, 67, 481 (1953). (15) J. N. Shoolery, J . Chsm. Phys., 21, 1899 (1953). (16) A. Saika and C. P. Slichter, ibid., 2 2 , 26 (1954). (17) W . E. Lamb, Jr., Phys. Rev.. 60, 817 (1941). (18) L. N. Ferguson, “Electron Structures of Organic hIolecules,” Prentioe-Hall, Inc.,New York, N. Y., 1952.
relation between the Hammett substituent constantlg and the magnetic shielding constant for the Fl9 nucleus in the number one position on the benzene ring. Fluorine magnetic resonance shifts also have been measured in the ha10methanes.l~ The relation between the shielding constant b and the sum of the electronegativities of the atoms present exhibits a simple relation which is shown in Fig. 1. The shielding constant b is defined as (HR- Hc)/ H R X 106 where H R is the externally applied magnetic field for which resonance occurs at a fixed frequency for a reference nucleus, and H , is the externally applied field for which resonance occurs a t the same frequency for the nucleus of interest. This allows a direct comparison to be made of shielding effects without regard to the magnitude of the magnetic field. Shoolery’s studied the effect of substituting various atomic groups for one of the hydrogen atoms of C2H6. He found a direct relationship between (&, - ~ c H ,and ) the electronegativity of the substituent; where ~ C isHthe~ shielding constant for the protons in the CH2-plus-substituent group and ~ C H is , the shielding constant for the protons in the CH3 group. Saika and Slichter’e simplified the procedure devised by Ramsey l l for calculating the second-order paramagnetic term and were able to calculate the shift between Fz and HF. Their calculations agree well with the experimental results. In view of the evidence presented above, an examination was undertaken to investigate the FI9 shielding constants for some of the more complicated molecules. Apparatus and Materials Measurements were made a t 30 Me. by means of a Varian Associates High Resolution Nuclear Magnetic Resonance Spectrometer. For calibration purposes side-bands20-25 were created by subjecting the steady magnetic field to a 600 cycle sine wave modulation from a Hewlett-Packard Function generator Model 202A. The 600 cycles per second frequency was checked by means of a Hewlett-Packard Electronic timer Model 524B whose accuracy had been checked against the frequency signal from WWV radio. This permitted a frequency regulation of the side-bands accurate to five significant figures. The spectra were recorded on a Minneapolis-Honeywell Model 153 variable span, strip chart recorder running a t 2.75 inches per minute. The materials investigated include: 1. A mixture of 93y0 I ,2-dichloro-l, 1,2,a-tetrafluoroethane (CCIF2CClFZ) and (19) L. P. Hammett, “Physical Organic Chemistry,” RIcGraw-Hill Book Co., New York, N. Y., 1940. (20) C. H. Townes and E’. R. Merritt, Phgs. Rev., 7 2 , 1266 (1947). (21) R. Karplus, ibid., 73, 1027 (1948). (22) J. H. Burgess and R. M. Brown. Rev. Soi. Instr., 23,334 (1952). (23) J. N. Shoolery and E. J. Alder, J . Chem. Phys., 25, 805 (1955).
1702
T.S. SMITHAND E. A. SMITH
Vol. 63
I-
c:
r
\
I X
0
0
0
c
CO“
00‘
SUM OF ELECTRONEGATIVITIES OF THE ATOMS.
Fig. 1.-Relation of 8 to total electronegativities of adjacent atoms (data from Meyer and Gutowsky14).
\ ‘ -i, \,
-8
Fig. 3.-Influence
I
CC\F,CCIF,
- spc
\
CCl2FQ
0
, ,
I
, ,
, ,
6 IO 12 14 SUM OF ELECTRONEGATIVITIES OF NEAREST NEIGHBOR GROUP.
6
of total electronegativities of neighboring groups on 6 for FIB.
The general procedure in obtaining scans of the Fl9 spectra was one of a continuous, slow sweep of the magnetic field while the reference sample and the test sample were alternately placed in and removed from the field. Figure 2 shows SB a typical spectrum thus obtained. The zero of the recorder is on the far left of the scale; therefore, the scan was obSB tained with the recorder paper moving from left to right. The direction of increasing field is indicated by an arrow, H INCREASING Figure 2 is a acan of CCbF and F-114. The position Fig. 2.-N.m.r. spectrum of CC12FCF5, CClF2CClF2, and marked x indicates where the CCLF sample was removed and the F-114 sample inserted in the probe as the field was CCbF. being swept. The two small signals upfield from the 7% 1,1-dichloro-1,2,2,2-tetrafluoroethane (CC12FCFa) (de- CClFzCClF2 line aye the lines of CClzFCFs which are present termined by infrared analysis) supplied by E. I. du in F-114. (A series of runs on various concentrations of CClFCFJ in CClFzCClFz showed no effect on the positions Pont de Nemours and Co. under the trade name “Freon114.” In this paper the mixture is referred to as F-114. of their respective lines.) The side-bands (SB) of each line 2. A mixture of 85.5% l,l-dichloro-1,2,2,2-tetrafluoro- are separated by 1200 cycles per second. I n actual practice ethane (CC12FCFa) and 14.5 701,2-dichloro-l,1,2,2-te~ra- this distance on a tracing is about 300 mm. This measurefluoroethane (CClF2CClF2) supplied by General Chemical ment provided a direct calibration on every scan. No Company under the trade name “Genetron.” The isomer attempt was made to eliminate drift in the magnetic field concentrations were also determined by infrared analysis. during this experiment because uniform drift would not 3. Trichloromonofluoromethane (CChF) supplied by Olin affect the results. I n order to check the uniformity of Mathieson Company. Infrared analysis of this material drift it wag necessary to measure the separations between showed 98% CC13F and 2% CClzFz. 4. l,l,2-Trichloro- different pairs of side-bands on a single scan such aa those 1,2,2-trifluoroethane (CC12FCClF2) supplied by E. I. du shown in Fig. 2. Since the distance between the sidePont de Nemours and Company under the trade name bands around the line of CClF?CClF2 was found to be equal “Freon-113.’’ Mass spectroscopic analysis indicated this to the distance between the side-bands around the line of compound to be 99% .pure. The only impurity detected CClaF, the total magnetic field sweep rate was the same wm NZ. 5 . 2,2,3-Tr1chloroheptafluorobutane (CF&C12- during the recording of both pairs of side-bands. UniCC1FCF3) supplied by the Hooker Electrochemical Com- formity of drift was all that was necessary because of the pany. A mass spectroscopic analysis of this material in- method of calibration used. A direct measurement of the dicated less than one per cent. Nz aa the only impurity. 6 . distance between line- in Fig. 2 determined their separation Perfluorodimethylcyclohexane ( C8Fla) supplied by E . I. in cycles per second. By use of the rate of precession of du Pont de Nemours and Company. This compound con- F19, 30 megacycles per second in a field of 7500 gauss or tained only traces of Ng, A and 0 2 as indicated by mass l / d milligauss per cycle per second, the shift in cycles per spectroscopic analysis. 7. Trifluoroethanoic acid ( CFs- second can be expressed in milligauss. The spectra of the other compounds were obtained in the COOH) supplied by the Minnesota Mining and Manufactursame manner as the spectrum shown in Fig. 2. In cases ing Company as chemically pure. Each sample was prepared by condensing the material where the reference line fell upon, or very near, a line from the vapor phase into a 5 mm. 0.d. Pyrex glass sample in the spectrum of the sample, only the side-bands of the tube. The spectra were obtained while each sample was in reference line were recorded by substituting and removing the reference Sam le at the proper times. Thus very small an atmosphere of its own vapor and a t room temperature. field shifts could {e measured. The various lines in a parProcedure ticular spectrum were identified easily by means of their All observed F19resonance lines were measured relative to multiplets caused by spin-spin interactions through the valence bond electron spins.24*26The spectrum of CsFle the line of CClF2CClF2. The position of the CClF?CClF’, line was then measured relative to the h e of trrfluoroethanoic acid. These data were used to compare the results (24) H. 8. Gutowsky, D. W. McCall and C. P. Slichter, J . Chem. of this work with previour efforts in which trifluoroethanoic Phys., 21, 270(1953). acid had been used as a reference.14 (25) J. T.Arnold, Phys. Rev., 102, 136 (1956).
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NUCLEAR MAGNETIC SHIELDING OF F19IN CHLOROFLUOROCARBONS
Oct., 1959
1703
TABLE I POSITIONS OF LINESRELATIVE TO CClF2CClF2 I N CYCLES PER SECOND, POSITIONS OF LINESRELATIVE TO CFJCOOHIN MILLIGAUSS, AND NUCLEAR MAGNETIC SHIELDINQ CONSTANTS WITH RESPECT TO CF3COOH Source of FlO signal
No. of ohs.
10 11
73 16 11 13 13 14
CC1,F CFsCOOH FsC \CC&F CClFr-CClF2 CLFC \CCIFI
14 14 11 11 11
AH(c p s with respect t o dlF,CCIFp
-2172 f 4 181 f 3 400 f 1 171 f 2 0.0
0 1 3 -122 f 2 20 2
*
1455 f 4 68 f 3 0.0 1 3
CF
3472 i 18 1617 f 8
AH(mgauss) with respect to CFsCOOH
-588 0.00 55 -2.5 -45 -45 -76 -40
+1
i0 . 8 & 0.9 i 0.75 1.0 0.9 i0 . 9
** 318 * 6 -28 f 1 -45 * 1 . 0 822 i 18 359 f 8
a 10’
(F)
+ 78.4
+- 07.33 .33 + 6.00 ++ 10.1 6.00 + 5.33 +- 42.4 3.73 +-109.6 6.00 - 47.0
The large number of observations of the line from the CF3 group in CC12FCF3were obtained because CC12FCFa was an impurity in the secondary standard used (CC1F2CCIF2)and this line appeared ._ frequently. was complicated by the presence of a mixture of isomers with Table I1 illustrates the effect on similar F19 the meta and para components predominating,%but the large groups as indicated in Table I could be distinguished. nuclei attached to one carbon by various atoms attached to a nearby carbon in the compound. Results An increase of b indicates a decrease in the shieldThe data derived from all observations are shown ing. In every case an increase in the total elecin Table I. The values in the third column, shifts tronegativity of the atoms in the neighboring in field in cycles per second with respect to CClF2- group results in an increase in the shielding of the CClF2, are expressed with 95% confidence limits. F19of interest as is shown in Fig. 3. These results are the reverG of what might be TABLE I1 expected from a study of the measurements made NUCLEAR MAGNETIC SHIELDINQCONSTANTS OF VARIOUS F’Q binary fluorides.i8 In the case of the binary NUCLEI, “AREST NEIQHBOR ATOMIC GROUP, AND TOTAL fluorides an increase in electronegativity of the ELECTRoNEGATIVITY OF atom bonded to the fluorine produced a decrease Electronegativity in the shielding of the fluorine nucleus. Since the HR - Hc F1° producing Neighbor of neighbor shielding effect for the fluoride ion is small, a de106 x 7 signal group @OUP crease in the shielding indicates an increase in the 10.1 CClF2CClaF 12.5 ionic character of the bond. 6.00 CClFyCClF, 13.5 The shielding effect shown in Fig. 3 is consistent 6.00 CFsCF6.5 with the results obtained for the chlorofluoro5.33 CF3CClZ 8,5 methanes14 (Fig. 1) and with some recent measure3.73 CFsCClF9,5 CClzF 12,5 ments made on solid MnF3.*’ A covalent bond or -7.33 CFIdouble-bond mechanism between the fluorine and 6.00 CClZFCClFt 13.5 carbon atom is essential to explain the change in b. 14.5 0.33 CCI2FCFs The electron removed from the fluorine is one which (26) C. Slesserand 8. R.Schrane (editors), “Preparation, Properties has its spin aligned antiparallel to the applied and Technology of Fluorine and Organio Fluoro Compounds,” Mcmagnetic field. Graw-Hill Book Co.. New York, N. Y., 1951, (National Nuclear I
F,C
Energy Series, Div. vii, Vol. I).
hF
(27) R. G. Shulman and V. Jaccarino, Phys. RGU., 109,1084 (1958).
