J . Phys. Chem. 1987, 91, 1236-1241
1236
for gelation. The gel dose for the vacuum exposure of PPFPO was simulated by determining the combined effects of scission and cross-linking on an initial Poisson distribution. In addition, molecules with p < 9 were removed from the system as they were formed by the irradiation. The pertinent quantities l/(2fz,o) and t were computed for a range of r / c values defined by the following: r = 0.0028 cmZ/pC and c = r/3, r/4, and r / 5 . The results for this investigation are contained in Figure 22. The line for l/(2fz,o) is independent of the r / c value because it only depends on the scission probability; the line for t , of course, changes with c. For a r l c = 3 and 4, we find a gel dose at -22 and 38 pC/cm2. A gel dose is also found for r / c = 5 at -54 pC/cm2. In contrast to this when the same calculation was repeated for the case where no molecules were removed from the system, the 1/(2f2,,) and t lines did not cross the r / c = 5 and r l c = 4 within the range of Q studied. Thus, the condition for gelation, that is r / c < 4, is relaxed for the vacuum exposure of PPFPO. This is primarily due to the removal of low molecular weight material from the film by the vacuum system, and it is this effect that is responsible for
the rapid etch rate and gelation of the system. Hence, we conclude that removal of molecular fragments from a system under irradiation has a profound effect on the radiation induced physical properties of the system. Concluding Remarks and Summary
The radiation induced mechanisms operative when PPFPO is exposed to a high-energy electron beam within a vacuum are influenced by the pumping action of the vacuum system. Due to main chain scission reactions, lower molecular weight polymers are produced with vapor pressures high enough to be removed by the pumping action of the vacuum system. The net result of the removal of material by the vacuum system is to produce an etch rate that decreases with increasing initial molecular weight. In addition, the removal of lower molecular weight materials from the irradiated PPFPO films inhibits the lowering of the average molecular weight and in effect permits gel formation when the ratio r / c > 4. Registry No. PPFPO, 25038-02-2
35CINQR Study of Molecular Motion of Polycrystalline Insecticide p ,p’-DDT. 1. Modulation Effects in Temperature Dependence of ”CI NQR Spin-Lattice Relaxation Time in p,p’-DDT Boleslaw Nogaj’ Institute of Physics, A. Mickiewicz University, Grunwaldzka 6, 6 0 - 780 Poznafi. Poland (Received: April 28, 1986)
Investigation of 35ClNQR spin-lattice relaxation times (TI) was performed for the polycrystalline insecticide p,p’-DDT (CC1,CH(C6H4C1),) at temperatures ranging from 77 K to the melting point of the compound (380 K). A change of conformationof two phenyl rings (at 165 K) and hindered rotations of phenyl rings and CC13groups were found. The modulation minima in the temperature dependence of T , were ascribed to hindered rotations of two dynamically nonequivalent groups of phenyl rings. The contribution of individual relaxation mechanisms in TI was discussed. The activation energies for individual hindered rotations were determined from temperature dependences of the relaxation time TI as well as from the shift of the modulation minima.
I. Introduction The activity of insecticides has been found to depend on the shape of the molecules and on the charge distribution within it. DDT-type insecticides have molecules shaped like wedges. The top of the wedge is made of the groups -CC13 in DDT, -CHCl2 in DDD, or =CC12 in DDE, while its base is phenyl rings. Charge distribution in a wedgelike molecule is essential. Holan and Spurling’ have shown that with increasing total negative charge at the top of the wedge the lethal dose of the compound (LD,,) decreases. Chlorine atoms that occur at characteristic sites at both the top and the base of the wedge allow NQR investigation on the 3sCl isotope to be performed. Thus, the measurement of 35ClNQR frequency provides information on charge distribution in a molecule of the insecticide. As far as one could find, the only literature data on 3sClNQR studies are the resonance frequencies for p,p’-DDT measured at 77 K given in Bray’s work.z In our paper3 the 3sCl NQR studies of insecticides were developed on p,p’-DDMU, p,p’-DDE, and p,p’-DDD. This paper is a continuation of these studies. We have concentrated our interest on p,p’-DDT, the insecticide that has been known for 11 1 years and has become a standard. This compound has won a distinguished place in the history of world agriculture. Present address: Institut d’Electronique Fondamentale, Labratoire associt au CNRS, BBtiment 220, UniversitC Paris XI, 91405 Orsay Cedex, France.
0022-365418712091-1236$01 .50/0
The application of DDT in the period 1955-1972 in 117 countries resulted in complete suppression of malaria and averted the danger of this disease from more than 1 billion p e ~ p l e . ~Despite the fact that this compound has been replaced by other insecticides, it still remains a standard for a large group of insecticides, especially DDT-like. Though many attempts have been undertaken to explain the insecticidal activity of p,p’-DDT, no definite answer has been obtained. According to some of the t h e ~ r i e s the , ~ insecticidal activity of this compound is associated with the probable rotations of phenyl rings. The aim of this paper was to study the possible reorientations of atomic groups. A molecule of p,p’-DDT alone is interesting for a physicist who can expect the possibility of reorientations of phenyl rings as well as CC13groups on the basis of its structure. Studies of nuclear quadrupole spin-lattice relaxation times measured on chlorines at phenyl rings as well as on chlorines of CCl, groups should prove interesting, and it is the subject of this paper. (1) Holan, G.; Spurling, T.H. Experienrin 1974, 30, 480. (2) Bray, P. J. J. Chem. Phys. 1955, 23, 703. (3) Nogaj, B.; Pietrzak, J.; Wielopolska, E.; Schroeder, G.; Jarczewski, A. J . Mol. Strucr. 1982, 83, 265. (4) Metcalf, R. L. J. Agric. Food Chem. 1973, 21, 511. (5) Riemschneider, R.; Otto, H. D. 2. Naturforsch., B: Anorg. Chem., Org. Chem., Biochem., Biophys., Biol. 1954, 9, 95.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987 1237
Molecular Motion of p,p‘-DDT
H
11. The Influence of Molecular Librations and Reorientations on Nuclear Quadrupole Relaxation Nuclear quadrupole spin-lattice relaxation time ( T I )is de-
termined by the interaction between the system of the quadrupole nuclei and phonons in the crystal. Thus, TI studies provide information about the dynamics of the crystalline lattice. Fluctuations of the electric field gradient may result from different kinds of molecular motions, so a few relaxation mechanisms can be distinguished. According to Bayer6 as well as Woessner and Gutowsky,’ the fluctuations of electric field gradient caused by librational motions lead to the following relation between the relaxation time TI and temperature
(n
Tl-l =
(1)
U P
where a is a constant and n = 2. According to Alexander and Tzalmona theory: abrupt changes in the electric field gradient due to hindered rotation of molecular groups containing quadrupole nuclei (e.g., CC13 groups) lead to the exponential temperature dependence of the relaxation time TI-‘ = 6 exp(
-2)
(9,)CI,
AI
--@ -@- CI,
bJ
/?\
CI,
CI‘
(9.J
(?‘I
CIS
Figure 1. Structural formula of p,p’-DDT and 35ClNQR frequencies assigned to each chlorine atom (see Table I). TABLE I: 35CINQR Frequencies ( Y ~ ) ,Line Widths (6wQ), and Spin-Lattice Relaxation Times ( T I )in p,p’-DDT at 77 K no. of NOR line Y n . MHz by,.‘ kHz TI. ms Y1 34.8670 4.80 348 Y2 34.9732 4.30 330 38.4845 38.8115 39.0362
y3
u4
Y5
5.44 4.80 4.88
440 460 450
Full width at half-intensity.
where T~ is the correlation time at infinitely high temperature, the following general formula describing the relaxation time T I under the modulation mechanism is obtained:
where 6 is a constant, E , is the activation energy for hindered rotation, and R is the gas constant. If atomic groups in the neighborhood of the resonating nuclei undergo a rotation with a frequency close to the observed N Q R frequency, then in the temperature dependence of TI a so-called modulation minimum is observed. The appearance of such minima has rarely been reported in the literature, and for the first time their explanation was given by Woessner and Gutowsky.’ In the above-described circumstances the quadrupole spin-lattice relaxation time can be given as
=
[
d exp(
$) +
g exp(
-%)]-I
(7)
If the electric field gradient undergoes modulation, the parameters d and g from eq 7 take the following form:
d = 3r0(
g) (9)
(3) where wQ is the resonance angular frequency, T , is the correlation time, and C is a constant. The parameter C depends on the character of the modulating effect. If the rotating atomic groups modulate the electric field gradient on the quadrupolar nucleus (ref 7 ) , the C parameter can be expressed as
wd(
c=
g) 2
(4)
3
where qzd is the contribution of motions of adjacent groups or molecules to the whole electric field gradient qzz. When the rotating groups include e.g. hydrogen atoms, then the magnetic dipole-dipole interaction between the quadrupolar nucleus and rotating protons undergoes m o d u l a t i ~ n . ~In. ~this ~ case the C parameter is a function of a few variables (5)
where yI and ys are gyromagnetic ratios for two magnetically interacting different nuclei and Oi, vi,ri are the polar coordinates of the radius, i.e. 3 vector connecting the quadrupolar nucleus of spin I with the rotating nucleus of spin S. Taking the correlation function T , as r, =
To
exp(
g)
( 6 ) Bayer, H. Z . Phys. 1951, 130, 227. (7) Woessner, D. E.; Gutowsky, H. S . J . Chem. Phys. 1963, 39, 440. (8) Alexander, S.; Tzalmona, A. Phys. Rev. 1965, 138, 845. (9) Zussman, A.; Alexander, S . J. Chem. Phys. 1968, 48, 3534 (10) Tzalmona, A.; Kaplan, A. J . Chem. Phys. 1974, 61, 1912.
When the magnetic dipole-dipole interactions are modulated, the same parameters can be described as
d = wQ*TO/C g = l/Cro
(10) (1 1)
In most actual cases it is extremely hard to distinguish between the two kinds of interactions in the modulation mechanism. 111. Experimental Section
35ClN Q R spin-lattice relaxation times T I have been investigated in p,p’-DDT (CCl3CH(C6H4C1),,Aldrich) at temperatures from 77 K to the melting point of the compound (380 K). Measurements were performed at the laboratory of the University of Florida, Gainesville, by means of a pulse spectrometer with quadrature detection and Fourier transform (FT) capability achieved with a Nicolet 1180 data processor system. The standard 1/2r--T--1/2r sequence was applied. The sample was contained in a copper cell in a temperature-controlled cryostat which provided temperature stability better than 0.1 K and an absolute accuracy of 0.2 K.
IV. Results and Discussion The structural formula of p,p’-DDT and 35ClNQR frequencies ascribed to individual chlorine atoms, line widths, and spin-lattice relaxation times at 77 K are given in F i g u r e 1 and Table 1. Detailed crystallographic data are given in ref 11. The structure of the p,p’-DDT molecule implie. hat chlorine atoms at phenyl rings should be characterized by s.milar dynamics. It is also expected that the dynamics of the three chlorine atoms of the CC13 group should be the same. This conclusion has been (1 1) De Lacy, T. P.; Kennard, C. H. L. J . Chem. Soc., Perkin Trans. 2 1972, 2148.
1238 The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
Nogaj
TABLE II: Computer Fit of Measured 3%1 NQR Spin-Lattice Relaxation Times in p,p'-DDT to the Appropriate Formulas-Theories (Eq 1, 2, and 12y temp range, K eq no. TI-+"), s-I chlorines at phenyl rings 77-165 I Tl-'(ul) = 1.075846 X 10-3T'8395 I1 T1-I(v2)= 0.964806 X 10-3Tl8741
253-325 332.5-336.6 335.2-358.2 77-120 77-165 200-293 341-378 200-378b
CClp group chlorines
" R is expressed in J mol-l
TI-'(vl,v2) = 1.166830 X 10-17"98" TI-'(uI,v2) = 9.356582 X l O I 4 exp(-84.452 X 103/RT) TI-I(uI,v2) = 16.058692 X 1014 exp(-91.001 X 103/RT) TI-I(v3,u4,vS) = 1.423498 X 10-2T11598 Tl-1(up,v4,u5)= 1.423509 X 10-2T11598 + 2.572825 X lo7 exp(-19.308 X 103/RT) Tl-'(vp,v4,us) = 6.558933 X 10-7T3.1822 Tl-1(v3,v4,~5) = 6.437749 X l O I 4 exp(-79.971 X 103/RT) T1-'(u3,v4,u5) = 6.558933 X 10-7T)'822+ 5.185533 X l O I 5 exp(-86.586 X 103/R3")
111 IV V VI VI1 VI11
IX X
bMinima are disregarded.
K-I.
T IKI 80
651
1w
T [KI
-
~
150
200-
300
r
80
100
m
150
300
- Loo
6.0-
-300
-m
-
5.0
-100 LO -
2 0 , ,
,
,
,
35
LC
15 "
l
1L
!
l
12
,
,
,
,
,
,
, mt , 2 I5
,
lo3/ T IK-'] 3 0 8
10
8
i
i
P
,
8
l
6
-50
'5
8
%
8
L
m 2 P
io3/ T i K-' I Figure 2. Temperature dependence of 35ClNQR spin-lattice relaxation time in p,p'-DDT (lines vI and v2). confirmed by T1measurements a t only one temperature (77 K, Table I). However, the TI investigqtion in a wide temperature range (Figures 2 and 3) revealed a series of new interesting phenomena. The earliest results of quadrupole relaxation studies in p,p'-DDT were published in the conference materials.I2-I4 Table I1 presents the numerically obtained fits of the measured T , relaxation times in individual characteristic temperature ranges to the proper theories-formulas (eq 1, 2, and 12). At low temperatures the influence of small-amplitude librations of phenyl and CC13 group chlorine atoms leads to almost a square temperature dependence of TI (Table 11, eq I, 11, and VI). In this temperature range the behavior of relaxation times is typical and consistent with the results of Bayer, Woessner, and Gqtowsky and with eq 1. Starting from about 125 K the dependence T l ( T ) for Cl,, Cl,, and C15 becomes exponential, which indicates the appearance of the hindered rotation of CCl, groups. On the basis of the computer fitting of T , data in the temperature range from 77 to 165 K to the formula that takes into account the influence of both librations and hindered rotations of CC13 groups on TI relaxation times
the activation energy for the hindered rotation of the CC13group (12) Nogaj, B.; Brookeman, J. R. In Proceedings of rhe XVl Seminar on N M R and Its Applications, Cracow, 1983; Institute of Nuclear Physics: Cracow, 1984; Report No. 1237/PL, pp 138, 145. (13) Nogaj, B. In Proceedings of the XVZZ Seminar on N M R and Its Applications, Cracow, 1984; Institute of Nuclear Physics: Cracow, 1985; Report No. 1287/PL, p 191. (14) Nogaj, B. In Proceedings of the RAMIS-85 Conference, Pornah, 1985; Institute of Molecular Physics of Polish Academy of Sciences: PoznaA, 1985; p 581.
P -a5 B A
t l
t
U
lo
6
1 3 / T IK41
L'
m2
P.
Figure 3. Temperature dependence of 35ClNQR spin-lattice relaxation time in p,p'-DDT (lines u3, u4, and u s ) .
in this temperature range was determined to be 19.3 kJ mol-' (Table 11, eq VII). At about 165 K the exponential decrease in T l ( T ) for CC13 group chlorine nuclei is stopped, and the rapid jumps in TI for phenyl chlorines Cll and Clz are observed. At this temperature the change in conformation of phenyl rings is supposed to occur, which may disturb the hindered rotation of CCl, groups. Indeed, after this change, i.e. since a temperature of about 167.5 K, only a slight and still diminishing difference in T 1values for v1 and v2 lines is observed. This supports the conclusion as to the occurrence of new more uniform conformations of two phenyl rings in the p,p'-DDT crystal. No sooner than above 188 K the CCI, groups can perform the librations of a greater amplitude which is marked by a decrease in T I (for v,, u4, and v 5 ) above this temperature. The ordinary hindered rotations of CC1, groups can start only at about room temperature (-295 K). This is indicated by the change in the character of the dependence T l (T ) from the
Molecular Motion of p,p'-DDT TABLE III: Temperatures at Which the Modulation Minima in T , ( T ) Occur and the Correlation Times 7, at the Minima Obtained from the Condition 7L1= cog minima in T,(7') measd for the freq Tmin, K 7cfi", 10-9 s VI, y2 337.55 ( T A ) 4.645 4.654 359.92 ( F ) y3, Y4r y5 338.98 (79) 4.178 4.186 361.45 ( p )
square to the exponential one (Figure 3). Above 305 K the change in the character of the temperature dependence of TI time for v 1 and v2 lines (Figure 2) indicates the onset of hindered rotation of phenyl rings about the axis passing through the central carbon atom of the pp'-DDT molecule and the chlorine atom at the ring. As a result of the rotations, two clearly visible minima are observed to appear in the temperature dependence of T I both for chlorine atoms at phenyl rings and for chlorine atoms of CC13 groups (Figures 2 and 3). Precisely at the minimum in the relaxation time T I W Q T ~= 1 , which means that a t this temperature the frequency of the hindered rotation of phenyl rings has reached the N Q R frequency at which T I is measured. The presence of the two minima proves a certain (in this case small) dynamical nonequivalence of the two phenyl rings in the p,p'-DDT molecule. Precise determination of the minima in T I (T ) (Table 111) proves that the minima in T I measured for CC13 group chlorines appear at a temperature 1.5 K higher than the T I minima measured on chlorines at phenyl rings. It is quite understandable when we regard the fact that the resonance frequencies for phenyl ring chlorines occur at about 34 MHz while for CCI, group chlorines they occur at about 38 MHz. From the condition 7C-l = wQ we could find the correlation times for hindered rotations of phenyl rings at individual minima (Table 111). A more thorough analysis of the modulation minima will be given later. Coming back to the T I ( dependence above the temperature range in which the modulation minima are found, no new phenomena have been observed. We can only say that CC13 groups rotate still faster and faster which is evidenced by broadening of the v3, v4, and v 5 lines and their disappearance a t 378 K. As to the lines v l and v2, we can observe them up to the melting point of the compound (380 K). This independently confirms the character of phenyl rings motion that we have accepted and which does not produce significant averaging of the electric field gradient a t the chlorine atoms at phenyl rings. Let us consider again the range from 200 to 380 K in which few relaxation mechanisms are met. As far as the phenyl rings are concerned, we can only talk about the Bayer libration mechanism and the modulation mechanism. In the case of chlorine atoms of CCl, groups within 200-378 K, one more mechanism should be considered which is associated with the hindered rotation of CC13 groups. This may be expressed as the sum (.T1-1 ) obsd =
Replacing (A) by eq 1 and (B) by eq 2 and writing (C) and (D) with the help of eq (7), we arrive at the following expression (for the CC13 group):
The most important parameters in the above equation are activation energies for hindered rotation of individual atomic groups: E, for the CC13 group, E,' and E," for the two dynamically nonequivalent phenyl rings. The computer fit of experimental results to eq 14 was performed adding step by step the influence of particular relaxation mech-
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
1239
T [KI
m
q 0
-21
'
4.0
3.0
3.5
2.5
10yT[K-'l Figure 4. Temperature dependence of 35ClNQR spin-lattice relaxation time in p,p'-DDT (lines u3. u4, and us): 0, experimental points; A, fit function that takes into account only librations (eq 1 and Table 11, eq VIII); B, fit function that takes into account only hindered rotations of CCI, groups (eq 2 and Table 11, eq IX). T [KI
"
3.5
3.0
\
25
lO?T [K-'] Figure 5. Temperature dependence of 35ClNQR spin-lattice relaxation 0, experimental points; solid line, time in p,p'-DDT (lines Y,, u4, and 4: fit function that takes into account both librations and hindered rotations of CCI, groups (eq 12 and Table 11, eq X).
anisms. Let us consider the results for the CCl, group. Regarding only the librations which hold for the temperatures from 200 to 293 K we come to eq VI11 of Table I1 according to eq 1 . A graphical illustration is curve A in Figure 4. Having assumed that only reorientations of CCl, groups take place (no librations) between 341 and 378 K, we come to eq IX of Table I1 shown graphically as line B in Figure 4. The activation energy of a CC13 group determined in this way is equal to 80.0 kJ mol-'. However, it is beyond question that in the above-discussed temperature range the librations of chlorine atoms of CCl, groups also take place. Taking into account this mechanism of relaxation as well, we obtained a slightly higher value of the activation energy for the CC13 group equal to 86.6 kJ mol-' (Table 11, eq X; Figure 5). Considering all relaxation mechanisms-librations, hindered rotation of CCI, groups, and hindered rotation of two phenyl rings-we could fit all experimental data, including the modulation minima, to eq 14 (Figure 6). From this fitting we expected to obtain the value of the activation energy for hindered rotation of phenyl rings, E,' and E,". However despite the mathematically good fit, we did not obtain reasonable values of E,' and E,". The values that we obtained were too high. This results from the fact that we could not distinguish the two different interactions in the modulation mechanism (modulation of electric field gradient and modulation of magnetic dipole-dipole interactions) and we did not take into account in eq 14 the factor describing the intensities of each mechanism of relaxation in the respective temperature ranges. Equation 14 is just a mathematical sum of the contributions of particular mechanisms. Both imperfections of eq 14 are hard to overcome. Now, let us analyze the first of them. Both interactions in the modulation mechanism (pure quadrupolar and magnetic) lead to the same mathematical description of the temperature dependence of T I ,namely eq 7. The phenyl rings rotation (with hydrogen
1240 The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
Nogaj
T [KI 2
w)
360 iL
2 1
’I
I=
Figure 6. Temperature dependence of 35C1NQR spin-lattice relaxation time in p,p’-DDT (lines v3, v4, and v5): 0,experimental points; solid line, fit function that takes into account librations, hindered rotations of CCI, groups, and the influence of two rotating nonequivalent phenyl groups (two modulation minima) (eq 14).
atoms rotating) alone can suggest that magnetic dipole-dipole interaction between the rotating H atoms and C1 atoms studied should contribute to quadrupole spin-lattice relaxation in p,p’DDT. Ainbinder et al.I5 proved that the contribution of the magnetic mechanism of relaxation to the quadrupole T I value is small when compared with that of the pure quadrupolar mechanism. However, the final conclusion is that the separation of the two interactions in the modulation mechanism (describable by the same mathematical formula) is very hard. Moreover, the modulation mechanism is very weak when compared with the effect of rotations of resonating nuclei. Let us assume that the modulation minima considered result only from the modulation of the electric field gradient on the chlorine nuclei studied due to rotation of phenyl rings; we neglect the magnetic interactions. Now apply eq 3 and 4 but on the condition that W ~ =T 1, ~which means that we are interested only in the minimum in T , ( T ) . In this case we obtain
From this formula we can find the contribution of the modulated (due to neighboring phenyl rings rotation) q z i gradient to the total electric field gradient qzz. The ratio qZz//qzzisabout 0.2%for the relaxation observed on chlorines at phenyl rings and about 0.8% for the relaxation observed on chlorine atoms of the CCl, group. The low values of qz;/qzz prove that the modulation mechanism is very weak in this case, but simultaneously they show a high sensitivity of the method of relaxation time measurements in molecular dynamics studies. Now, we shall analyze the second drawback of eq 14, which is a mathematical sum of contributions of all relaxation mechanisms. As follows from the analysis of intensities of individual relaxation mechanisms for temperatures between 341 and 378 K and for relaxation observed on chlorine atoms of CCl, groups, the mechanism associated with hindered rotation of CCl, groups dominates. This is evidenced by the exponential character of the T 1(7+) dependence. The modulation mechanism associated with rotation of neighboring phenyl rings gains significance only within the narrow range of temperatures when we are close to the situation when the condition TLI = wQ is fulfilled. That is why we observe very sharp minima in T l ( T )and obtain too high values of E,’ and E,’’ from eq 14. The aforementioned incorrectness in the determination of E,’ and E,” can be avoided if the number of overlapping relaxation mechanisms could be limited. Fortunately, this is the case when we study relaxation on chlorines at phenyl rings in p,p’-DDT (Figure 2). Then we observe small changes in T l relaxation time caused by a weak libration mechanism (librations of the chlorine atoms studied) and a stronger modulation mechanism (hindered (15) Ainbinder, N. E.; Volgina, G. A.; Kyuntsel, I. A.; Mokeeva, V. A,; Osipenko, A. N.; Soifer, G. B. Zh. Eksp. Teor. Fir. 1979, 77, 692.
=
0
H
I I
Jll.lcl*~*c,lIIJz
-1
/F\
-
CI C I C I J3
-2
4
+
lo2
30
3.5
L5
25
103/T [K’] Figure 7. Modulation minima in the temperature dependence of 35CI NQR spin-lattice relaxation time in p,p’-DDT.
E, =
%
1
-n.5~ k-15K
I
AB
/
\
CD
1
At minimum :
127wc=1
/ -C-CI
1-1.LK
~
TB- TA
JQ=38MHz
C ‘I 1/T
*,el ITB,Q,$1
IT--) Figure 8. Illustration of the method of determination of activation energies for hindered rotations from the shift in the modulation minima in temperature dependence of quadrupole spin-lattice relaxation time T , observed on two different groups of quadrupole nuclei (”CI NQR in p,p’-DDT). ITA.
rotations of the neighboring phenyl rings). The strong relaxation mechanism that takes place in the case of the resonating and rotating Cl nuclei of CCl, groups is absent. That is why the observed modulation minima are not as sharp as those observed at the frequencies v3, v4, and v g (Figure 7). For the case considered, the values of E,‘ (84.5 kJ mol-’) and E,” (91.0 kJ mol-’) obtained even directly from the slope of the modulation curves are quite reasonable (see Table 11, eq IV and V). We should, however, also remember that in this case two kinds of interactions, quadrupolar and magnetic, overlap and are responsible for the modulation effect in T’(7). Thus, we may expect that the real values of E,’ and E,“ are slightly lower than the above values. Attempts have been made to distinctly separate the relaxation mechanisms in order to obtain the values of activation energies for hindered rotations of the two dynamically nonequivalent groups of phenyl rings. Let us try to obtain E,’ and E,” without separation of the two effects (quadrupolar and magnetic) in the modulation mechanism of quadrupole relaxation in p,p’-DDT. As mentioned above, the two effects are described by the same mathematical relation (eq 7). Thus, despite the kind of effect,
J. Phys. Chem. 1987, 91, 1241-1246 TABLE I V Activation Energies (in kJ mol-’) for Hindered Rotation of CClj Group before (E,,) and after (E,*) the Conformation Change and for Hindered Rotation of Phenyl Rings (E,’ and E,”) in p,p’-DDT Determined from Nuclear Quadrupole Relaxation (T,) Studies
method Ea1 Ea2 temp dependence of T , 19.3 86.6 shift of modulation minima in TI(?“)
E,‘
E,”
84.5 91.0 70.4 75.1
the T l minimum associated with a given effect always appears a t the same temperature. The minimum in Tl(Z‘) depends only on the condition wQrc = 1, Le. on the kind of the motion responsible for the relaxation and on the 3sCl N Q R frequency. We can take advantage of the fact that the modulation minima for p,p’-DDT can be observed on two different groups of nuclei resonating at two different 35ClN Q R frequencies, about 34 and 38 MHz. Let us compare Figure 7, presenting all the minima observed, and Figure 8, which illustrates the way the E,’ and E,” values are determined from the shift in the modulation minima. Consider the first two minima (A and B in Figure 8) associated with hindered rotation of the same group of phenyl rings. The correlation times 7 , of this group of rings may be expressed separately for the A and B minima as
7,B
K)
= To exp( R P
Taking into account the condition which for the two minima takes the form 2*vQA7cA= 1
(18)
2,vQsrcB = 1
(19)
1241
we obtain the following expression:
E,‘ =
T ~ PInR(vQB/vQA) P - TA
As follows from the above formula, the accurate determination of temperatures and 3sCl N Q R frequency at which the minima appear allows one to calculate the activation energies for the hindered rotations studied. Applying the accurate values of T A and P (Table III), the activation energy for hindered rotation of the first group of phenyl rings, E,’ = 70.4 kJ mol-’, was obtained from eq 20 while that for the second group of phenyl rings, E / = 75.1 kJ mol-’, was determined from the analysis of the shift in C and D minima (Figure 8). These values may also be subject to some error. However, they are lower than those obtained from temperature dependences of relaxation time T l and subject to the error due to nonseparation of quadrupolar and magnetic effects. Thus, we may expect that E,‘ and E,” values obtained from the shift of the modulation minima are closer to the real values. Unfortunately, we cannot draw any comparison or make any verification as there are no data reported to have been obtained by other methods. Table IV gives all values of activation energies calculated for hindered rotations of CC1, groups and phenyl rings. A detailed investigation of the frequency and line width of all five 3sCl NQR lines for p,p’-DDT over a wide temperature range should support the conclusions and confirm the activation energy values obtained from nuclear quadrupole relaxation time measurements for this compound. This will be the subject of the following paper.
Acknowledgment. Thanks are due to Professor J . R. Brookeman for making it possible for me to carry out the measurements, the results of which are presented in this paper, during my 1-year (1981-1982) stay at the University of Florida, Gainesville. Registry No. DDT,50-29-3; chlorine-35, 13981-72-1.
35CINQR Study of Molecular Motion of Polycrystalline Insecticide p,p’-DDT, 2. The NQR Frequency and Influence of Molecular Vibrations and Reorientations on the Line Width In p,p’-DDT Boleslaw Nogaj’ Institute of Physics, A . Mickiewicz University, Grunwaldzka 6, 60- 780 Poznaii, Poland (Received: April 28, 1986)
35ClNQR frequencies ( v a ) and line widths (Sv,) of polycrystalline p,p’-DDT (CC13CH(C6H4C1)2) have been studied for temperatures ( T ) from 17 K to the melting point of the compound (380 K). The experimental results of vo(T) have been fitted to the proper theories-formulas (Bayer; Kushida, Benedek, Bloembergen; Brown), and the quality of the fits was analyzed. The fits allowed determination of a few physical parameters. Temperature dependences of the 35Cl NQR line width, in particular temperature ranges, were fitted to the most suitable formulas with an attempt to separate the mechanism of line broadening (hindered rotation of CCI3groups) from the mechanism of line narrowing (hindered rotation of phenyl rings) and from so-called static line width. Hindered rotation of CC13groups and phenyl rings do not affect essentially the 35ClNQR frequencies, but their influence is observed in the line widths. Activation energies for the hindered rotations determined from temperature dependences of vQ and SvQ were compared with their values obtained from quadrupole spin-lattice relaxation studies.
1. Introduction This paper is a continuation of 35c1NQR studies of insecticides (DDMU, DDE, DDD, and DDT).’” The presence of numerous chlorine atoms,in the molecules of the insecticides permits 35Cl
NQR study of their electronic structure, which is essential in attempting to explain their insecticidal In this respect (1) Noeai. B.: Pietrzak. J.: Wielomlska. E.: Schroeder. G.: Jarczewski. A.
J.
Present address: Institut d’Electronique Fondamentale, Laboratoire associe au CNRS, BPtiment 220, UniversitZ: Paris XI, 91405 Orsay Cedex,
France.
0022-3654/87/2091-1241$01..50/0
.id.S X t . i m , 83,’265.
(2) Nogaj, B.; Brookeman, J. R. In Proceedings of the XVI Seminar on N M R and Its Applications, Cracow, 1983; Institute of Nuclear Physics: Cracow, 1984; Report No. 1237/PL, pp 138, 145.
0 1987 American Chemical Society
