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Some results of TDPAD measurements in fluorine compounds
Journal ofMolecular Structure, 192 (1989) 383-386 Elsevier Science Publishers B.V., Amsterdam - Printed
383 in The Netherlands
SOME RESULTS OF TDPAD MEASUREMENTS COMPOUNDS
IN FLUORINE
E. BERTHOLDT, M. FRANK, F. GUBITZ, W. KREISCHE, B. LijSCH, CH. OTT, B. RGSELER, M. SCHNEIDER, F. SCHWAB, K. STAMMLER and G. WEESKE Physikalische Institut, Uniuersittit Erlangen-Niirnberg (Received
12 February
(F.R.G.)
1988)
ABSTRACT In recent years a great variety of compounds containing i9F have been investigated. By plotting the data in a suitable manner general trends in the behaviour of the coupling constants can be recognized. These trends enable us to predict the coupling constants of as yet unexamined simple fluorine containing compounds. One of these is OF, for which, according to the behaviour of the compounds on which our predictions are based, a 19Fcoupling constant of approximately 100 MHz is expected. In analogy to the work done on trifluoroaminoboranes the electron distribution of the C-F bond in different organic molecules is determined. Also some data gained from experiments performed with fluorinated aromatic compounds are given.
METHOD
The angular distribution of the gamma radiation can be expanded in terms of Legendre polynomials IV(&)
after a nuclear
reaction
=AklAk2Pk(cosO)
The expansion coefficients depend on the spin of the decaying level, the multipolarity of the radiation and the nuclear alignment. An interaction of the excited level with its environment results in a perturbation of the angular distribution. The type of perturbation is characteristic for the kind of interaction. In the case of electric quadrupole interaction of a 5/2 + level the time dependent angular distribution is given by w(O)
=A,,G,,(t)P,(cosW
For static equals
randomly
G22(t) =azO(r)+s~,(r)
0022-2860/89/$03.50
oriented exp(--n
interactions OLD)
the perturbation cos(nol(~)t)
0 1989 Elsevier Science Publishers
B.V.
function
G,,(t)
384
where the asymmetry
parameter
v is defined as
with 1V,, ( 2 1V,, ( 2 1V,, I. The exponential term takes into account a possible Lorentzian spread of the interactions. The 197 keV 5/2+ level is the second excited level of “F. It was populated via a lgF (p,p’ ) “F* reaction with a pulsed 5 MeV proton beam from the Erlangen EN Tandem accelerator. Defining after background correction a ratio R (t) from the coincidence counting rates N( 180” ,t) and N(90”,t) in a suitable manner, this ratio equals the perturbation function times an effective amplitude
R(t) :=2
N(l80”,t) -N(90”,t)
=AeffG
(t)
22 22
N(180",t)+2N(90°,t)
Thus, by fitting the theoretical function GZ2(t) to the experimentally gained data R (t ) information about coupling constants, effective amplitudes, asymmetry parameters and relative frequency distributions can be obtained. RESULTS
Prediction of the O-F-coupling constant In the framework of the Townes and Dailey theory the molecular by: eqmol= [ (l-s+d)(l-i)-n]
efg is given
eq,,
where s and dare the portions of s and d hybridisation; i is the degree of ionicity of the intramolecular bond and 7~represents eventually occurring double bonds. From the Pauling formula, that relates the ionicity and the electronegativity difference X, - Xn of an A-B molecule via: i=exp(
= (X,-X,)‘/4)
one gets i= 0.09.
125
I
V.Grp
VI.Grp
VlI.Grp
IlI.Grp
IV.Grp
lLGrp
LGrp
--1---I
100
9 50
25
I,,,
F Cl Br
,,,,,,,,,,,l,,lll~r~w~rl)
III,
I
At
0
S Se Te W
N
P As Sb 81
Fig. 1. Trends in the coupling constants
C Si Ge Sn Pb
‘I B
of several fluorides.
em in TI me Mg Co or Ea Li Nn
K Rb 13
FI
Using the values eqat= 125 MHz, s = 0.15 and assuming that no double bonds occur, results in an O-F coupling constant of about 97 MHz. A value of the same size can be found by evaluation of the trend for the known coupling constants of the B-F, C-F, N-F and the F-F bonds. An interpolation yields about 100 MHz for the O-F bond. This is shown by the guideline in Fig. 1. ELECTRON DISTRIBUTIONS AND NQI-DATA IN SOME AROMATIC COMPOUNDS
We have measured the quadrupole coupling data in some fluorobenzenes. These measurements were performed at various temperatures with a minimum temperature of about 10 K. The observed temperature dependence of the coupling constants was a very weak one. In Table 1 the data for the lowest temperatures are given. Again the Townes and Dailey theory was used to analyze the lgF C-F coupling constant to obtain information about the electron distribution of the C-F-bond. Within the mentioned model the main contributions to the EFG come from the lowest unfilledp shell. TABLE 1 Experimental data of some fluorobenzenes Compound Hexafluorobenzene 1-Chloro-3-fluorobenzene 1-Chloro-4-fluorobenzene Cyanuric-fluoride
VQ
(MHz)
61.42 56.18 38.95 58.95 41.48 57.65
(17) (39) (38) (13) (17) (17)
4,
(%)
6.94 8.47 2.02 6.27 7.05 6.95
(42) (99) (50) (51) (59) (37)
7
T
0.09 0.19 0 0.15 0 0.25
10 10
W)
10 11
In fluorine compounds lgF uses an sp hybrid with an amount s2 of s character for the covalent bonds. Neglecting possible changes in the radial dependence of the wave functions a simple additive relation for the EFG holds EFG= 1 afbiefgi i
where efgi is the contribution of a “pure” orbital, Uiis the hybridization coefficient of the respective orbital and bi is the corresponding orbital population factor. This method has been successfully applied in the literature for boron and nitrogen containing compounds [ 11. Since in the case of fluorine only 2p orbitals are relevant only two expectation integrals have to be evaluated: (a) the contribution of the 2p, orbital to qzz
(b) the contribution (VL
13co;:-1~
of a 2pxy orbital to qzz
%J=-;qo
All the other contributions can be obtained from these by cyclic changes of indices. It is assumed that the antibonding o orbital is filled with two electrons, the bonding one being filled with 2 -ai, electrons due to the covalent character of the C-F bond. In order to obtain a nonvanishing asymmetry parameter the px and py orbitals have to be populated unequally. So the pr orbital is assigned to be full whereas the px orbital is only filled with 2-p, electrons. Using these plausible assumptions and the ai coefficients for sp-hybridization one obtains
V,Z ShZ-----
1+;
q() l-s2
In Table 2 the numerical data obtained are listed for the compounds from Table 1 with some additional organic molecules. So a charge transfer from the fluorinep, orbital to the nonlocalized n-system and a lower apopulation in the bonding sp hybrid can be observed.
TABLE 2 Numerical data for some fluorinated
agents
Compound
VQ
Tetrafluoromethane Teflon Chloro-trifluoroethylene Hexafluorobenzene 1-Chloro-4-fluorobenzene Cyanuric-fluoride
58.9 58.7 59.8 61.4 58.9 57.9
(MHz)
0 0 0 0.09 0.15 0.22
2-s))
2 -Ph
1.454 1.456 1.446 1.414 1.428 1.424
2.00 2.00 2.00 1.97 1.95 1.93
ACKNOWLEDGEMENT
This work was supported Technologie.
by the Bundesministerium
fur Forschung
und
REFERENCE 1 J. Olliges, A. Lijtz, J. Voitljinder, H. Barfuss, G. Bijhnlein, F. Gubitz, W. Ittner, dorfer, W. Kreische and B. Riiseler, J. Mag. Reson., 69 (1986) 302.
G. Lanzen-
383 in The Netherlands
SOME RESULTS OF TDPAD MEASUREMENTS COMPOUNDS
IN FLUORINE
E. BERTHOLDT, M. FRANK, F. GUBITZ, W. KREISCHE, B. LijSCH, CH. OTT, B. RGSELER, M. SCHNEIDER, F. SCHWAB, K. STAMMLER and G. WEESKE Physikalische Institut, Uniuersittit Erlangen-Niirnberg (Received
12 February
(F.R.G.)
1988)
ABSTRACT In recent years a great variety of compounds containing i9F have been investigated. By plotting the data in a suitable manner general trends in the behaviour of the coupling constants can be recognized. These trends enable us to predict the coupling constants of as yet unexamined simple fluorine containing compounds. One of these is OF, for which, according to the behaviour of the compounds on which our predictions are based, a 19Fcoupling constant of approximately 100 MHz is expected. In analogy to the work done on trifluoroaminoboranes the electron distribution of the C-F bond in different organic molecules is determined. Also some data gained from experiments performed with fluorinated aromatic compounds are given.
METHOD
The angular distribution of the gamma radiation can be expanded in terms of Legendre polynomials IV(&)
after a nuclear
reaction
=AklAk2Pk(cosO)
The expansion coefficients depend on the spin of the decaying level, the multipolarity of the radiation and the nuclear alignment. An interaction of the excited level with its environment results in a perturbation of the angular distribution. The type of perturbation is characteristic for the kind of interaction. In the case of electric quadrupole interaction of a 5/2 + level the time dependent angular distribution is given by w(O)
=A,,G,,(t)P,(cosW
For static equals
randomly
G22(t) =azO(r)+s~,(r)
0022-2860/89/$03.50
oriented exp(--n
interactions OLD)
the perturbation cos(nol(~)t)
0 1989 Elsevier Science Publishers
B.V.
function
G,,(t)
384
where the asymmetry
parameter
v is defined as
with 1V,, ( 2 1V,, ( 2 1V,, I. The exponential term takes into account a possible Lorentzian spread of the interactions. The 197 keV 5/2+ level is the second excited level of “F. It was populated via a lgF (p,p’ ) “F* reaction with a pulsed 5 MeV proton beam from the Erlangen EN Tandem accelerator. Defining after background correction a ratio R (t) from the coincidence counting rates N( 180” ,t) and N(90”,t) in a suitable manner, this ratio equals the perturbation function times an effective amplitude
R(t) :=2
N(l80”,t) -N(90”,t)
=AeffG
(t)
22 22
N(180",t)+2N(90°,t)
Thus, by fitting the theoretical function GZ2(t) to the experimentally gained data R (t ) information about coupling constants, effective amplitudes, asymmetry parameters and relative frequency distributions can be obtained. RESULTS
Prediction of the O-F-coupling constant In the framework of the Townes and Dailey theory the molecular by: eqmol= [ (l-s+d)(l-i)-n]
efg is given
eq,,
where s and dare the portions of s and d hybridisation; i is the degree of ionicity of the intramolecular bond and 7~represents eventually occurring double bonds. From the Pauling formula, that relates the ionicity and the electronegativity difference X, - Xn of an A-B molecule via: i=exp(
= (X,-X,)‘/4)
one gets i= 0.09.
125
I
V.Grp
VI.Grp
VlI.Grp
IlI.Grp
IV.Grp
lLGrp
LGrp
--1---I
100
9 50
25
I,,,
F Cl Br
,,,,,,,,,,,l,,lll~r~w~rl)
III,
I
At
0
S Se Te W
N
P As Sb 81
Fig. 1. Trends in the coupling constants
C Si Ge Sn Pb
‘I B
of several fluorides.
em in TI me Mg Co or Ea Li Nn
K Rb 13
FI
Using the values eqat= 125 MHz, s = 0.15 and assuming that no double bonds occur, results in an O-F coupling constant of about 97 MHz. A value of the same size can be found by evaluation of the trend for the known coupling constants of the B-F, C-F, N-F and the F-F bonds. An interpolation yields about 100 MHz for the O-F bond. This is shown by the guideline in Fig. 1. ELECTRON DISTRIBUTIONS AND NQI-DATA IN SOME AROMATIC COMPOUNDS
We have measured the quadrupole coupling data in some fluorobenzenes. These measurements were performed at various temperatures with a minimum temperature of about 10 K. The observed temperature dependence of the coupling constants was a very weak one. In Table 1 the data for the lowest temperatures are given. Again the Townes and Dailey theory was used to analyze the lgF C-F coupling constant to obtain information about the electron distribution of the C-F-bond. Within the mentioned model the main contributions to the EFG come from the lowest unfilledp shell. TABLE 1 Experimental data of some fluorobenzenes Compound Hexafluorobenzene 1-Chloro-3-fluorobenzene 1-Chloro-4-fluorobenzene Cyanuric-fluoride
VQ
(MHz)
61.42 56.18 38.95 58.95 41.48 57.65
(17) (39) (38) (13) (17) (17)
4,
(%)
6.94 8.47 2.02 6.27 7.05 6.95
(42) (99) (50) (51) (59) (37)
7
T
0.09 0.19 0 0.15 0 0.25
10 10
W)
10 11
In fluorine compounds lgF uses an sp hybrid with an amount s2 of s character for the covalent bonds. Neglecting possible changes in the radial dependence of the wave functions a simple additive relation for the EFG holds EFG= 1 afbiefgi i
where efgi is the contribution of a “pure” orbital, Uiis the hybridization coefficient of the respective orbital and bi is the corresponding orbital population factor. This method has been successfully applied in the literature for boron and nitrogen containing compounds [ 11. Since in the case of fluorine only 2p orbitals are relevant only two expectation integrals have to be evaluated: (a) the contribution of the 2p, orbital to qzz
(b) the contribution (VL
13co;:-1~
of a 2pxy orbital to qzz
%J=-;qo
All the other contributions can be obtained from these by cyclic changes of indices. It is assumed that the antibonding o orbital is filled with two electrons, the bonding one being filled with 2 -ai, electrons due to the covalent character of the C-F bond. In order to obtain a nonvanishing asymmetry parameter the px and py orbitals have to be populated unequally. So the pr orbital is assigned to be full whereas the px orbital is only filled with 2-p, electrons. Using these plausible assumptions and the ai coefficients for sp-hybridization one obtains
V,Z ShZ-----
1+;
q() l-s2
In Table 2 the numerical data obtained are listed for the compounds from Table 1 with some additional organic molecules. So a charge transfer from the fluorinep, orbital to the nonlocalized n-system and a lower apopulation in the bonding sp hybrid can be observed.
TABLE 2 Numerical data for some fluorinated
agents
Compound
VQ
Tetrafluoromethane Teflon Chloro-trifluoroethylene Hexafluorobenzene 1-Chloro-4-fluorobenzene Cyanuric-fluoride
58.9 58.7 59.8 61.4 58.9 57.9
(MHz)
0 0 0 0.09 0.15 0.22
2-s))
2 -Ph
1.454 1.456 1.446 1.414 1.428 1.424
2.00 2.00 2.00 1.97 1.95 1.93
ACKNOWLEDGEMENT
This work was supported Technologie.
by the Bundesministerium
fur Forschung
und
REFERENCE 1 J. Olliges, A. Lijtz, J. Voitljinder, H. Barfuss, G. Bijhnlein, F. Gubitz, W. Ittner, dorfer, W. Kreische and B. Riiseler, J. Mag. Reson., 69 (1986) 302.
G. Lanzen-