JACKD,GRAYBEAL AND PAULJ. GREEN
2948 The cross-combination ratio for CFZH and CF2C1 radicals is given by
which accounts for all the collisional events between the radicals, omitting the trace products, and assuming that all the C2F4 results from CFzH and CFzCl interaction. The data in Table I1 yield an average value of 1.99 for $) which is the collision theory prediction. Bellas, et u Z . , * ~ give P ! = 1.9 at 25 Torr and 2.4 at 200-313 Torr; both values are uncorrected for disproportionations, Correcting by the factor (1.17 X 1.19)-”2 gives 1.6 and 2.0, respectively.
Since this work was originally submitted a report on the homogeneous gas-phase decomposition of ethyl fluoride has been given2gand a retraction of the previously published5&low value for the activation energy has been made.29n30A study has also appeared of the pyrolysis of a series of fluoro- and fluorochloroethanes. The pyrolysis of CH&Fd2l yields CH2=CF2 HCI; the elimination of H F is very minor in contradistinction to the chemical activation report.5~
+
(29) M. Day and A. F. Trotman-Dickenson, J . Chem. Soc., A’, 233 (1969). (30) A. W. Kirk, A. F. Trotman-Dickenson, and B. L. Trus, ibid., 3058 (1968). (31) D. Sianesi, G. Nelli, and R . Fontanelli, Chim. Ind. (Milan), 50, 619 (1963).
Nuclear Quadrupole Resonance Zeeman Study of Polycrystalline Group IVa Tetrachlorides
by Jack D. Graybeall and Paul J. Green Department of Chemietry, West Virginia University, Morgantown, West Virginia 96606
(Received February 10, 1969)
The asymmetry parameters of the electric field gradients at the chlorine nuclei in four group IVa tetrachlorides have been determined by employing a polycrystalline Zeeman analysis of their nuclear quadrupole resonance transitions. The parameters found were: CCla (-
717,
where Av = line width at zero magnetic field and y = gyromagnetic ratio for nuclei being observed. The shape of the unbroadened absorption line which was observed in the present work was either a first or second derivative type depending on the particular sample in the spectrometer and the side band being observed. Due to the necessity of selecting an absorption side band which was experimentally most amenable, the treatment of envelope shapes arising from both types of line shapes must be considered. Morino and Toyama'O have presented an expression for the first derivative envelope shape when the resonance has a finite line width
vv vo IC1
a #" '
-v,
%
vn
2%
i0)
where t(1
f e) =
1 Ahs(2n)
Figure 2. Theoretical line shapes for (A) first derivative of an absorption with H = 0; (B) envelope of a first derivative curve with H # 0, H parallel to H,f and 7 = 0; ( C ) second derivative of an absorpt.ion with H = 0; (D) envelope of a second derivative curve with H # 0, H parallel to H,r and q = 0.
I
Av = line width of resonance at zero magnetic field and N = normalization constant. Taking the derivative of eq 3 with respect to E gives the second derivative envelope -24-
II I I
(8'
Figure 3. Theoretical envelope shapes to illustrate the effect of increased asymmetry.
Inspection of both eq 3 and 7 shows that the envelope shape is highly dependent on both the asymmetry parameter, 7, and the line width, A v . Using a FORTRAK Iv program and an IBN-7040 computer the envelope shapes, dl(e)/dr and d21(t)/de2, as functions of frequency, at intervals of 0.05 kHz and up to a maximum frequency of 4vX, were computed for several values of 7, Ae, and H . Sample plots of these computations are shown in Figures 2-5. I n Figures 3-5 only the upper sides of the curves are shown. The plots shown are selected to illustrate the effects of both asymmetry and line width. The Journal of Physical Chemistry
Experimental Section Since the ultimate S/N for the residual envelope diminishes with increasing static field, it was necessary to have original unsplit S/N values of 50 or better to perform an analysis. Because of this requirement it was necessary to use a superregenerative oscillator even though the analysis was complicated by the presence of side-band absorptions, A self-quenched oscillatorlB with manual coherence control was used. For 36Cl, where v H = 0.4 kHz/H (in gauss), operation up to H = (16) C.Dean, Rep. Sci. Instrum., 29, 1947 (1958).
2951
NQR STUDY OF GROUP IVA TETRACHLORIDES
3 a
--lit DERIVATIVE --2nd DERIVATIVE
IAl
-I5
1
t
-8
Figure 4 . Theoretical envelope shapes to illustrate the effect of increased line width.
I
-2Yn
(CI H 8 8 g0055 V,,: 3.3KH7. 7) :0.45
A€= 1.1
N
\
-s
t
\
---2nd 1st DERIVATIVE DERIVATIVE :1000
IV"
Figure 5 . Theoretical envelope shape for parameters similar to those observed for SEI4, GeC14, and SnCll (for example, compare to Figure 70).
25 G was possible before there was appreciable sideband interference. The static magnetic field was provided by a pair of 15-in. diameter Helmholz coils mounted horizontally. Since the method of analysis involved an extrapolation to infinite field strength and did not require absolute field strength measurements, the magnet current was calibrated using an Alpha 100-G rotating coil gauss meter and the magnitude of the field was measured by using an ammeter in series with the magnet. Samples of 10 or 100 g could be used by placing them in either of two nipple dewars with the oscillator coil external to the dewar. The samples were frozen rapidly to ensure polycrystallinity. All measurements on the group IVa tetrachlorides were made at 77°K. Prior to investigating the group IVa tetrachlorides the resonances of p-C12C6H4 and KCIOa were analyzed. The former produced results in complete agreement with the experimental work of Morino and Toyama.lo
Figure 6. Observed envelope shape of the *5Cl resonance in KCIOa at 25" with H parallel to Hrfa t static fields of (A) 0 G, (B) 15 G, and (C) 25 G.
The residual envelope shape for KClOa, shown in Figure 6 for two field strengths, was analogous to that expected for r] = 0 up to a field of 50 G. Beyond this point the signal is lost into the noise. This experiment places an upper limit on r] of 0.02 which is in agreement with r] = 0.006 as reported in the 1iterat~re.I~ All of the tetrachlorides except CCl, have a four-line Nqr pattern with one line lying several quench frequencies below a closely spaced triplet. The lowest line of the CCI4fifteen-line pattern was investigated. The more isolated line of each set was chosen in order to minimize the interference of sidebands at higher magnetic fields. A second line for SnCl4 was analyzed and gave the same results as the lowest one. The experimental selection of the investigated resonance is believed to be justified. Typical observed envelope shapes at several different static fields for the compounds studied are shown in Figures 7-10. The method devised by R40rino,l' whereby a set of envelope shape parameters, 6i/2VH, are plotted vs. 1/H and extrapolated to infinite field strength was used to obtain the asymmetry parameters. For a transition with a very small line width the measurements of the separations between the deflections occurring at f(1 i= v)vH are given by 2 v v H . For transitions having large line widths, as shown for example by Figure 5, the deflections are more diffuse and shifted from the theoretical positions. The problem of precisely locating the deflection points is increased in difficulty due to the low S/N values for the observed envelopes. In order to best analyze the data one employs a set of three experimentally measurable parameters, 6i/2VH, all of which approach 7 as H + 03. The measured parameters are (1) 61, the horizontal spacing between the positions of the (17) H. Zeldes and R. Livingston, J. Chem. Phys., 26, 1102 (1957). Volume 78, Number 9
September 1069
JACKD. GRAYBEAL AND PAUL J. GREEN
2952
n
Figure 8. Observed envelope shape of the V I a6C1resonance in GeC14at 77°K with H parallel to H,f for static fields of (A) 0 G, (B) 6 G, (C) 10 G, and (D) 15 G.
and will exhibit a minimum. (3) The parameter 83/2VH will approach the 1/H = 0 axis along the curve, 7 K . ( A V / ~ V H ) ’ (4) . The parameter 62/2VH will lie between Figure 7. Observed envelope shape of the VI W 1 resonance in the latter two. Sic14 at 77°K with H parallel t o H,f for static field of (A) The experimental values of the 6i were obtained using 0 G, (B) 4 G, (C) 8 G, and (D) 14 G. frequency measurements which were made by interpolation between successive side band absorptions and by maximum and minimum of the first derivative of the endetermining this spacing by measurement of the quench velope; ( 2 ) &, the horizontal position between the halffrequency with a Tektronix 546 oscilloscope. The height points of adjacent deflections of the first derivaerror involved in this method is much less than that associated with the visual selection of the envelope peak tive of the envelope; (3) &, the horizontal distance beand half-height) positions. The measured values of tween the points where adjacent deflections of the first &/2vH are accurate to f 0.04. The experimental and derivative of the envelope intersect the zero intensity extrapolated values of the envelope shape parameters line. These parameters as related to both first and secfor SiCL, GeCL, and SnCld are listed in Table I. The ond derivatives of the envelopes are shown in Figure 5 . uncertainties listed with each parameter reflect the reFor the case of zero line width all three of these paramesults of several determinations of the & values at a given ters would be equivalent. For the case of a large line field plus the errors associated with both static field and width they all approach a common value as the magquench frequency measurements. The asymmetry netic field strength becomes very large and the condition parameters are the averages of the three extrapolated indicated by eq 2 is well satisfied. The parameters, envelope shape parameters for each compound. The 6i/2vH, are plotted us. 1,” and extrapolated to infinite extrapolation procedure is shown in Figure 11. They field to obtain 7 . The extrapolation procedure is guided are listed in Table 11, along with (1) the average nuclear by the following points.1° (1) In the low-field limit all quadrupole coupling constants as determined using eq 1 three curves will be asymptotic to a linear function of 1/H. The asymptote of 81/2VH will be Av/2vH and can and assuming 7 = 0, and as determined by using the measured values of the 7’s) and ( 2 ) the number of unbalbe obtained by measuring the line width, Av, of the zero Rt, calculated anced p electrons, f = le2&qZZ~mo~//e2QqZzl field resonance. ( 2 ) The parameter 61/2VH will approach the 1/H = 0 axis along the curve 7 - A V / ~ V H using the latter values for the coupling constants.
+
The Journal of Physical Chemistry
2953
NQRSTUDYOF GROUPIVA TETRACHLORIDES Table I : Results of the Envelope Shape Analysis of the Lowest Nqr Line of the Group IVa Tetrachlorides at 77°K and Various Strengths of the Zeeman Fields
-----------------
Magnetic field, G
m
Compound
SiC1,
Parameter
6
8
10
14
16
(Extrapolated)
81/2VH &/2VH 88/2VH
0.42 0.56 0.75
0.39 0.50 0.70
0.39 0.52 0.68
0.36 0.48 0.64
0.38 0.50 0.61
0.39 f 0.05 0.45 =k 0.05 0.49 =k 0.05 m
GeCl,
W2VH 82/2VH
6
10
15
20
(Extrapolated)
0.39 0.56
0.36 0.50 0.64
0.34 0.45 0.58
0.32 0.41 0.51
0.31 =k 0.05 0.35 & 0.05 0.39 =k 0.05
&/~VH
_------------_
Magnetic field, G---------------m
SnCl,
81/2 VH 62/2VX
10
16
20
40
(Extrapolated)
0.35
0.25 0.45 0.55
0.22 0.38 0.45
0.19 0.25 0.31
0.23 i 0.05 0.25 Z!C 0.05 0.27 i 0.05
&/~VH
Figure 9. Observed envelope shape of the VI W 1 resonance in SnClr at 77°K with H parallel to H,f for static fields of ( A ) 0 G, (B) 10 G, (C) 15 G, and (D) 40 G.
Figure 10. Observed envelope shape of the VI W l resonance in CCL at 77°K with H parallel to H,r for static fields, of (A) 2 G, (B) 6 G, (C) 10 G, (D) 15 G, (E) 20 G.
These results confirm the validity of prior qualitative discussions based on coupling constants determined by assuming r] = 0. Carbon tetrachloride presents a somewhat different situation from the other tetrachlo-
rides. As is shown in Figure 10, up to a field of 20 G no discernable dips in the envelope could be detected. At higher magnetic field strengths the low intensity of the resonance and the interference of side bands precluded volume 79, Number 9 September 1969
JACK D. GRAYBEAL AND PAUL J. GREEN
2954 ~
~~
~
Table I1 : Asymmetry Parameters and Corrected Coupling Constants of Group IVa Tetrachlorides Compound
aG a5
2hv,MHn
9
