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Temperature variation of the isotope shift of fluorine nuclei
JOURNAL
OF MAGNETIC
RESONANCE
8,406-409
(1972)
Communication TemperatureVariation of the Isotope Shift of Fluorine Nuclei That nearby isotopic substitution alters the chemical shift of a fluorine atom has been known for some time. Tables showing the shift occurring upon the change from 12 to 13 of the mass number of the carbon atom to which a fluorine atom is attached have been compiled (I, 2) and a similar effect from the carbon atom removed by two bonds from the fluorine atom is known (2). Recently a chlorine two-bond isotope effect has been found for CFCIJ (3) and similar chlorine effects have been observed for some halogenated ethanes and propanes ($5). As for chemical shift changes which are caused in fluorine, as well as in hydrogen, by the substitution of deuterium for hydrogen in a nearby position, the heavier isotope invariably causes a shift of the fluorine resonance to a higher field. The observation that the several resonances for the fluorine nucleus in CFQ, corresponding to the four combinations of chlorine isotopes present as shown in Fig. 1
FIG. 1. Fluorine magnetic resonance spectrum of trichlorofluoromethane at 230 K, showing the four peaks corresponding to various isotopic combinations of chlorine-35 and chlorine-37. Frequency increases toward the left, with the bar corresponding to 1 Hz.
for a temperature of 230 K, were better resolved at low temperatures (3) raised the question as to whether the difference resulted from narrower lines at the lower temperature or from greater chemical shift differences under this condition. We have accordingly made careful observations of the temperature dependence of the isotope shifts produced by chlorine as well as of those produced by the substitution of 13C for 12C, both one and two bonds removed from the resonant fluorine nucleus. We present here a preliminary report of our findings, which demonstrate that all three kinds offluorine isotope shift uniformly increase as the temperature is lowered. Examples of the experimental results, as obtained on a Varian XL-100 spectrometer, are given in Tables 1 and 2. Copyright 0 1972 by Academic Press, Inc. All rights of reproduction in any form reserved.
406
407
COMMUNICATIONS TABLE
1
EFFECT OFSIJB~TITLJTI~NOF CHLORINE-37 FORCHLORINE-~~ONTHE~HEMICAL SHIFTSOFAFLUORINENUCLEUSTWOBONDSREMOVED~ Temperature W) Molecule CF35C1237C1 CF3W3’CI 2 CF3’CI 3 CFz3’CICCl 3 CF35C137C1CF2CFCIBr CF3’CI,CF2CFClBr
193
230
0.8 f- 0.1 -
0.67 zk 0.01 1.35 + 0.05 2.0 zt 0.15
0.83 + 0.02
0.71 f 0.02
-
0.59 + 0.01
-
0.63 z!c 0.03 1.22 i 0.02
0.60 zt 0.02 -
0.61 & 0.02
0.52 + 0.02
0.60 rt 0.01
0.55 f 0.03
CFC12CF2CF3’ClBr
-
0.74 It 0.02 1.35 + 0.06 -
CF237ClCFBrCF2CN
-
0.66 * 0.02
n Shifts
are to lower
frequency
in hertz,
295
305
-
0.53 + 0.02 1.06 zt 0.05 -
at 94.1 megahertz.
These data are obviously consistent with the hypothesis of Gutowsky (6) that isotope effects are related to the nature of molecular vibrations. In the vibrational model, the difference upon isotopic substitution is attributed to the fact that the zero-point energy of the system containing the heavier mass is lower, corresponding to smaller vibrational excursions within the potential well. Furthermore, if the potential well is asymmetric, in the absence of excitation to higher vibrational levels the two isotopomers have different molecular geometries. At the limit of high temperature, when vibration is extensively excited so that higher levels are populated, the fine-grain structure of the vibrational ladder tends to be obliterated and the isotope effect disappears, as is found in our experimental results. The question must next be asked: Is the variation of vibrational excitation over the range of temperature in question sufficient to explain the observed change in the isotope effect with temperature? It is possible to attempt only a qualitative answer to this question, in part because the detailed nature of the atomic motions involved in particular vibrational modes is not known for most of the molecules studied. If we examine the vibrational analysis of CFCl, (7), there is a frequency at 244 cm-’ which corresponds to a motion involving primarily bond bending, and another at 349 cm-l which is primarily bond stretching. The first is relevant to the two-bond chlorine isotope effect and corresponds to a population of the first and second excited states of about 30 % at room temperature. We may presume that the frequency of the second gives a reasonable estimate for some of those vibrations in fluorocarbon derivatives which include changes in C-F distances. For this frequency approximately 12% of the molecules are excited at 233 K and 21% at 303 K, a difference more than enough to account for the typical 4 % change in isotope effect between these temperatures. Results in Table 2 indicate that the one-bond effect is generally smaller for sp2 carbons than for sp3 carbons, which is consistent with the higher vibrational frequencies observed for olefins, although the temperature dependence does not show the expected
408
COMMUNICATIONS TABLE
2
EFFECT 0F S~JL~STITLJTION 0~ CARBON-13 FOR C.~IIB~N-~~ ON THE CHEMICAL FLUORINE NUCLEI ONE AND Two BONDS REMOVED“ One-bond Molecule
P-GJWz CF2=CC12 cis CFCI=CFCI CFCI=CC12 GWX CF&X13 CF#3CF2CI CF,CF,CF,COCl C&CF#ZF2COCl a Shifts
are to lower
shift
233 K
303 K
8.10 9.50 10.38 10.62 12.40 12.10 13.26 11.95 12.21
7.80 9.20 10.08 10.28 12.10 11.70 13.05 11.60 11.96
frequency
in hertz,
SHIFTS OF
Two-bond Difference 0.30 0.30 0.30 0.34 0.30 0.40 0.21 0.35 0.25
233
K
shift
303 K
2.90
Difference
-
-
4.42 3.17 0.74
2.50 3.88 2.61 0.64
0.40 0.54 0.56
1.40
1.20
1.58 I .90 1.64
1.46 1.80 1.56
0.20 0.12 0.10 0.08
0.10
at 94.1 megahertz.
difference between the groups. It is also significant that the relative change with temperature (for fluorines attached to sp2 carbons, even the absolute change) for the two-bond 13C effect is larger than the change of the one-bond effect, because one would expect the two-bond effect to be associated with bending vibrations, and these are of frequencies lower than those of vibrations involving bond stretching. In summary, our observations lend strong support to the vibrational model of isotope effects, although lack of complete correlation with vibrational frequencies makes it probable that other factors, possibly including changes in hybridization of the carbon atom, also play a role in determining the magnitude of the effect. WALLACES.BREY,JR. KENNETH H. LADNER
Department of Chemistry, University of Florida, Gainesville, Florida 32601 RONALD E. BLOCK
Papanicolaou Cancer Research Institute, Miami, Florida 33136 WILLIAM
A. TALLON
Columbia State Community College, Columbia, Tennessee 38401 Received August IO,1972
REFERENCES 1. H.
BATIZ-HERNANDEZ AND R. A. BERNHEIM, “Progress troscopy,” (J. W. Emsley, J. Feeney, and L. H. SutcliITe, Oxford, 1967.
in Nuclear Eds.), Vol.
Magnetic 3, Chap.
Resonance 2. Pergamon
SpecPress,
COMMUNICATIONS
409
2. J. W. EMSLEY, J. FEENEY, AND L. H. SUTCLIFFE, “High Resolution Nuclear Magnetic Resonance Spectroscopy,” Vol. 2, pp. 875,962-3,1022. Pergamon Press, Oxford, 1966. 3. P. R. CAREY, H. W. KROTO, AND M. A. TURPIN, Chem. Commun. 188 (1969). 4. A. DEMARCO AND G. GA-IT, J. Magn. Resonance 6,200 (1972). 5. L. CAVALLI, J. Magn. Resonance 6,298 (1972). 6. H. S. GUTOWSKY, J. Chem. Phys. 31,1683 (1959). 7. T. Y. Wu, J. Chem. Phys. 10,116 (1942).
OF MAGNETIC
RESONANCE
8,406-409
(1972)
Communication TemperatureVariation of the Isotope Shift of Fluorine Nuclei That nearby isotopic substitution alters the chemical shift of a fluorine atom has been known for some time. Tables showing the shift occurring upon the change from 12 to 13 of the mass number of the carbon atom to which a fluorine atom is attached have been compiled (I, 2) and a similar effect from the carbon atom removed by two bonds from the fluorine atom is known (2). Recently a chlorine two-bond isotope effect has been found for CFCIJ (3) and similar chlorine effects have been observed for some halogenated ethanes and propanes ($5). As for chemical shift changes which are caused in fluorine, as well as in hydrogen, by the substitution of deuterium for hydrogen in a nearby position, the heavier isotope invariably causes a shift of the fluorine resonance to a higher field. The observation that the several resonances for the fluorine nucleus in CFQ, corresponding to the four combinations of chlorine isotopes present as shown in Fig. 1
FIG. 1. Fluorine magnetic resonance spectrum of trichlorofluoromethane at 230 K, showing the four peaks corresponding to various isotopic combinations of chlorine-35 and chlorine-37. Frequency increases toward the left, with the bar corresponding to 1 Hz.
for a temperature of 230 K, were better resolved at low temperatures (3) raised the question as to whether the difference resulted from narrower lines at the lower temperature or from greater chemical shift differences under this condition. We have accordingly made careful observations of the temperature dependence of the isotope shifts produced by chlorine as well as of those produced by the substitution of 13C for 12C, both one and two bonds removed from the resonant fluorine nucleus. We present here a preliminary report of our findings, which demonstrate that all three kinds offluorine isotope shift uniformly increase as the temperature is lowered. Examples of the experimental results, as obtained on a Varian XL-100 spectrometer, are given in Tables 1 and 2. Copyright 0 1972 by Academic Press, Inc. All rights of reproduction in any form reserved.
406
407
COMMUNICATIONS TABLE
1
EFFECT OFSIJB~TITLJTI~NOF CHLORINE-37 FORCHLORINE-~~ONTHE~HEMICAL SHIFTSOFAFLUORINENUCLEUSTWOBONDSREMOVED~ Temperature W) Molecule CF35C1237C1 CF3W3’CI 2 CF3’CI 3 CFz3’CICCl 3 CF35C137C1CF2CFCIBr CF3’CI,CF2CFClBr
193
230
0.8 f- 0.1 -
0.67 zk 0.01 1.35 + 0.05 2.0 zt 0.15
0.83 + 0.02
0.71 f 0.02
-
0.59 + 0.01
-
0.63 z!c 0.03 1.22 i 0.02
0.60 zt 0.02 -
0.61 & 0.02
0.52 + 0.02
0.60 rt 0.01
0.55 f 0.03
CFC12CF2CF3’ClBr
-
0.74 It 0.02 1.35 + 0.06 -
CF237ClCFBrCF2CN
-
0.66 * 0.02
n Shifts
are to lower
frequency
in hertz,
295
305
-
0.53 + 0.02 1.06 zt 0.05 -
at 94.1 megahertz.
These data are obviously consistent with the hypothesis of Gutowsky (6) that isotope effects are related to the nature of molecular vibrations. In the vibrational model, the difference upon isotopic substitution is attributed to the fact that the zero-point energy of the system containing the heavier mass is lower, corresponding to smaller vibrational excursions within the potential well. Furthermore, if the potential well is asymmetric, in the absence of excitation to higher vibrational levels the two isotopomers have different molecular geometries. At the limit of high temperature, when vibration is extensively excited so that higher levels are populated, the fine-grain structure of the vibrational ladder tends to be obliterated and the isotope effect disappears, as is found in our experimental results. The question must next be asked: Is the variation of vibrational excitation over the range of temperature in question sufficient to explain the observed change in the isotope effect with temperature? It is possible to attempt only a qualitative answer to this question, in part because the detailed nature of the atomic motions involved in particular vibrational modes is not known for most of the molecules studied. If we examine the vibrational analysis of CFCl, (7), there is a frequency at 244 cm-’ which corresponds to a motion involving primarily bond bending, and another at 349 cm-l which is primarily bond stretching. The first is relevant to the two-bond chlorine isotope effect and corresponds to a population of the first and second excited states of about 30 % at room temperature. We may presume that the frequency of the second gives a reasonable estimate for some of those vibrations in fluorocarbon derivatives which include changes in C-F distances. For this frequency approximately 12% of the molecules are excited at 233 K and 21% at 303 K, a difference more than enough to account for the typical 4 % change in isotope effect between these temperatures. Results in Table 2 indicate that the one-bond effect is generally smaller for sp2 carbons than for sp3 carbons, which is consistent with the higher vibrational frequencies observed for olefins, although the temperature dependence does not show the expected
408
COMMUNICATIONS TABLE
2
EFFECT 0F S~JL~STITLJTION 0~ CARBON-13 FOR C.~IIB~N-~~ ON THE CHEMICAL FLUORINE NUCLEI ONE AND Two BONDS REMOVED“ One-bond Molecule
P-GJWz CF2=CC12 cis CFCI=CFCI CFCI=CC12 GWX CF&X13 CF#3CF2CI CF,CF,CF,COCl C&CF#ZF2COCl a Shifts
are to lower
shift
233 K
303 K
8.10 9.50 10.38 10.62 12.40 12.10 13.26 11.95 12.21
7.80 9.20 10.08 10.28 12.10 11.70 13.05 11.60 11.96
frequency
in hertz,
SHIFTS OF
Two-bond Difference 0.30 0.30 0.30 0.34 0.30 0.40 0.21 0.35 0.25
233
K
shift
303 K
2.90
Difference
-
-
4.42 3.17 0.74
2.50 3.88 2.61 0.64
0.40 0.54 0.56
1.40
1.20
1.58 I .90 1.64
1.46 1.80 1.56
0.20 0.12 0.10 0.08
0.10
at 94.1 megahertz.
difference between the groups. It is also significant that the relative change with temperature (for fluorines attached to sp2 carbons, even the absolute change) for the two-bond 13C effect is larger than the change of the one-bond effect, because one would expect the two-bond effect to be associated with bending vibrations, and these are of frequencies lower than those of vibrations involving bond stretching. In summary, our observations lend strong support to the vibrational model of isotope effects, although lack of complete correlation with vibrational frequencies makes it probable that other factors, possibly including changes in hybridization of the carbon atom, also play a role in determining the magnitude of the effect. WALLACES.BREY,JR. KENNETH H. LADNER
Department of Chemistry, University of Florida, Gainesville, Florida 32601 RONALD E. BLOCK
Papanicolaou Cancer Research Institute, Miami, Florida 33136 WILLIAM
A. TALLON
Columbia State Community College, Columbia, Tennessee 38401 Received August IO,1972
REFERENCES 1. H.
BATIZ-HERNANDEZ AND R. A. BERNHEIM, “Progress troscopy,” (J. W. Emsley, J. Feeney, and L. H. SutcliITe, Oxford, 1967.
in Nuclear Eds.), Vol.
Magnetic 3, Chap.
Resonance 2. Pergamon
SpecPress,
COMMUNICATIONS
409
2. J. W. EMSLEY, J. FEENEY, AND L. H. SUTCLIFFE, “High Resolution Nuclear Magnetic Resonance Spectroscopy,” Vol. 2, pp. 875,962-3,1022. Pergamon Press, Oxford, 1966. 3. P. R. CAREY, H. W. KROTO, AND M. A. TURPIN, Chem. Commun. 188 (1969). 4. A. DEMARCO AND G. GA-IT, J. Magn. Resonance 6,200 (1972). 5. L. CAVALLI, J. Magn. Resonance 6,298 (1972). 6. H. S. GUTOWSKY, J. Chem. Phys. 31,1683 (1959). 7. T. Y. Wu, J. Chem. Phys. 10,116 (1942).