Chlorine Nuclear Quadrupole Relaxation and Cationic Motion
in Trimethylsulfonium Hexachloroselenate(IV): [(CH 3 ) 3 Sl 2 SeCl^
Makoto Kaga, Tetsuo Asaji, Ryuichi Ikeda 3 , and Daiyu Nakamura b
D e p a r t m e n t o f C h e m i s t r y , F a c u l t y o f Science, N a g o y a U n i v e r s i t y ,
N a g o y a 464-01, Japan
Z. N a t u r f o r s c h . 4 7 a , 2 7 4 - 2 7 6 (1992); received July 19, 1991
The 3 5 C1 N Q R spin-lattice a n d spin-spin r e l a x a t i o n times, 7 ^ a n d T2Q, respectively, and the ' H
N M R spin-lattice r e l a x a t i o n time T 1 H at 32 a n d 60 M H z were determined f o r [ ( C H 3 j 3 S ] 2 S e C l 6 as
f u n c t i o n s o f temperature. T h e r a p i d decrease o f
observed above ca. 250 K w i t h increasing
temperature was a t t r i b u t e d to the onset o f r e o r i e n t a t i o n o f the [ S e C l 6 ] 2 " c o m p l e x anion w i t h the
-1
a c t i v a t i o n energy £ a = 42 + 5 kJ m o l . W h e n cooled f r o m ca. 250 K , T 1 Q showed an a n o m a l o u s
decrease. T h i s 7"1Q decrease was explained by electric field gradient m o d u l a t i o n related to some
cationic m o t i o n . Possible o r i g i n s o f the c a t i o n i c m o t i o n are discussed.
Key words:
Spin-lattice r e l a x a t i o n , N Q R , N M R , E F G m o d u l a t i o n , M o l e c u l a r m o t i o n .
1. Introduction
2. Experimental
A new relaxation mechanism of chlorine NQR, unexplainable by reorientation or lattice vibrations of
complex anions containing the resonant nuclei, has
been observed in trimethylsulfonium hexachlorometallates(IV), [(CH 3 ) 3 S] 2 MC1 6 (M = Sn, Pt) [1], This
mechanism, giving rise to relaxation maximum with
decreasing temperature, is expected to be related with
some motion of the counter cation. The C 3 reorientation of CH 3 group about its triad axis seems to be
little important because no such relaxation anomaly
has been observed in trimethylammonium hexachlorostannate(IV), [(CH 3 ) 3 NH] 2 SnCl 6 , and tetramethylphosphonium hexachlorostannate(IV), [(CH3)4P]2SnCl6,
having crystal structures very close to that of the trimethylsulfonium salt.
In order to obtain further information about the
electric field gradient (EFG) modulated by cationic
motions, 35 C1 NQR spin-lattice and spin-spin relaxation times, T1Q and 7 2 Q , respectively, and the *H
NMR spin-lattice relaxation time T1H are measured
for [(CH 3 ) 3 S] 2 SeCl 6 as functions of temperature.
Using a pulsed NQR spectrometer described elsewhere [2, 3], T1Q and T2Q were determined from the
height of echo signals observed after the pulse sequences, n-x-n/2-Te-n
and tz/2-t-tz, respectively,
where the echo height was observed at various interpulse spacing time T, while Tc was set constant (ca.
150 ps). The length of a n pulse was ca. 30 ps. Recovery
and decay of the echo amplitude against T and 2T in
the TLQ and T2Q measurements, respectively, could be
fitted to the simple exponential law usually employed
in pulsed NMR experiments [4], A pulsed NMR spectrometer [5] was employed to determine TLH at Larmor frequencies of 32 and 60 MHz, using the usual
7i—t —7i/2 method. The error of the relaxation times is
expected to be less than ca. 10 and 20% in NMR and
NQR measurements, respectively. The sample temperature was controlled within +0.5 K and estimated
to be accurate within + 1 K. Differential thermal analysis (DTA) was performed employing a homemade
apparatus [6]. The sample, which was used in the previous NQR study [7], was employed in the present
NQR measurements. The sample for the NMR measurements was newly prepared in a similar way as in
* Presented at the X l t h I n t e r n a t i o n a l S y m p o s i u m on
Nuclear Q u a d r u p o l e Resonance Spectroscopy, L o n d o n ,
U . K . , July 1 5 - 1 9 , 1991.
a
Present address: D e p a r t m e n t of C h e m i s t r y , U n i v e r s i t y of
Tsukuba, T s u k u b a 305, Japan.
b
Deceased.
[7].
Reprint requests to D r . Tetsuo Asaji, D e p a r t m e n t o f Chemistry, F a c u l t y of Science, N a g o y a U n i v e r s i t y , N a g o y a 464-01,
Japan.
3. Results
The temperature dependence of the 35 C1 NQR frequencies observed for [(CH 3 ) 3 S] 2 SeCl 6 agreed well
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275
M . K a g a et al. • C h l o r i n e Nuclear Q u a d r u p o l e Relaxation in [ ( C H 3 ) 3 S ] 2 S e C l 6
—i—i—r
30 -
—i—i—I—r
i
i
I/Q/MHZ
[(CH3)3S]2SeCl6
i
[(CHj)jS]2SeCl6
• •
•
T= 2 4 5 K
f
21.6
20-
S
•
•
•
10 -
0
21.45
_l
I
I L.
21.50
-I
21.55
I
1 L.
21.60
21.65
UQ/MHZ
Fig. 1. The frequency dependence of the spin-echo-height
observed for [(CH 3 ) 3 S] 2 SeCl 6 at T = 245 K .
with those reported earlier [7]. The signal could be
observed only in the restricted temperature range of
ca. 150-350 K. However, no phase transition was observed in DTA measurements. Since NQR signals with
a very broad line-width (50-100 kHz) were observed,
the resonance frequency was determined by plotting
echo-heights against the irradiated rf frequencies at
each temperature. One such plot, bearing a marked
asymmetry, is shown in Figure 1. The frequencies at
the maximum echo-heights are plotted as functions of
temperature in Figure 2. The bar shows the halfheight width of the echo-envelope.
The temperature dependences of 35 C1 T1Q and T 2Q ,
determined at the frequencies with the maximum
echo-heights, are shown in Fig. 3 together with that of
T 1H ..The shortening of T2Q is responsible for the disappearance of the NQR signal both at low and high
temperatures. The rapid decrease of T1Q observed
above ca. 250 K with increasing temperature is
attributable to the onset of reorientation of the
[SeCl 6 ] 2 - complex anions with the activation energy
£ a = 42 + 5 k J m o l " 1 [8]. When cooled from ca. 250 K,
T1Q decreased to 3 ms at ca. 170 K. This T1Q decrease
is much steeper than those in the corresponding Pt
and Sn complexes [1].
The 32 MHz T, H showed two minima of 27 ms (at
ca. 130 K) and 48 ms (at ca. 370 K) describable by the
C 3 reorientations of the C H 3 groups and the whole
cation about their triad axes, respectively, by referring
to the reported assignments of ' H NMR observed in
the analogous complexes [(CH 3 ) 3 S] 2 MC1 6 (M = Pt,
Sn, Te) having the isomorphous structure with the
present complex [7]. The activation energy £ a = 6 ± 0.5
kJ m o l - 1 evaluated for the C H 3 reorientation in the
temperature range of ca. 150-250 K disagrees with
£ a = 9 + l kJ m o l - 1 derived from the slope of log T1Q
vs. T _ 1 plot below ca. 250 K.
4*
•
•J
21.5
'
250
150
J 1 •
1
77 K
350
Fig. 2. The temperature dependence of 35 C1 N Q R frequency
of [(CH 3 ) 3 S] 2 SeCl 6 . The bar shows the half-height width of
the echo-envelope.
500 300
200
7VK
100
9
11
10
3
13
K/T
35
Fig. 3. The temperature dependences of C1 N Q R spinlattice and spin-spin relaxation times, T 1Q (•) and T2Q (•),
respectively, compared with that of 'H N M R spin-lattice
relaxation times T 1 H at L a r m o r frequencies of 32 ( x ) and 60
(A) M H Z .
4. Discussion
The above mentioned new relaxation mechanism of
chlorine NQR observed in the low-temperature range
for [(CH 3 ) 3 S] 2 MC1 6 (M = Pt, Sn, Se) is explainable by
the E F G modulation due to some cationic motion.
This motion was expected in the previous report [1] to
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276
M. K a g a et al. • C h l o r i n e Nuclear Q u a d r u p o l e Relaxation in [ ( C H 3 ) 3 S ] 2 S e C l 6 276
be characteristic for the [(CH 3 ) 3 S] + cation having
lone-pair electrons and not attributable to a simple
CH 3 reorientation by 120° about the S - C bond. This
is because the chlorine relaxations were quite normal
in [(CH 3 ) 3 NH] 2 SnCl 6 and [(CH 3 ) 4 P] 2 SnCl 6 , and
moreover, for [(CH 3 ) 3 S] 2 PtCl 6 , the correlation time
of the motion responsible for the E F G modulation
was elucidated to be much shorter than that of the
CH 3 reorientation [1], It has recently been reported
that a similar EFG modulations due to cationic motions is operative also in [(CH 3 ) 4 N] 2 MC1 6 (M = Pb,
Sn, Te), in which a detailed analysis of the chlorine
relaxation is prevented by the existence of a structural
phase transition [9]. Now, our question is what is the
origin of the cationic motion which causes the E F G
modulation. To answer this question, the cationic
orientations in the crystal lattice are compared for the
series of complexes so far studied in Table 1. In the
compounds in which chlorine E F G modulation was
observed, at least some cations [(CH 3 ) 3 YL] + , where L
is C H 3 , H or lone-pair electrons and Y is N or S, are
located at a high symmetry site and the Y - L vector
points at an empty site of the complex ions in the cubic
structure. This cationic orientation may be related to
a possible disorder in the cationic position, which is
suggested by the very broad NQR linewidth observed.
When the C H 3 group or the whole cation reorients
among the disordered sites, the chlorine E F G is very
likely to be modulated. Repulsion between the lonepair electrons of [(CH 3 ) 3 S] + ions and chlorines is expected to be one of the reasons why the lone-pair axis
of the cation prefers to point at an empty site.
As an experimental fact that supports the above
disorder model of the cationic position, in our preliminary measurements we observed a shallow T1H mini-
[1] M . Kaga, P. Koziol, T. Asaji, and D. Nakamura,
Z. Naturforsch. 4 5 a , 1033 (1990).
[2] K . Horiuchi, R. Ikeda, and D. Nakamura, Ber. Bunsenges. Phys. Chem. 91, 1351 (1987).
[3] A. Ishikawa, Ph.D. Thesis, Nagoya University 1990.
[4] T. C. Farrar and E. D. Becker, Pulse and Fourier Transform N M R , Academic Press, New York 1971.
[5] S. Gima, Y. Furukawa, R. Ikeda, and D. Nakamura,
J. M o l . Struct. I l l , 189 (1983).
[6] Y. Kume, R. Ikeda, and D. Nakamura, J. Magn. Reson.
33, 331 (1979).
Table 1. The cationic orientation in the crystal lattice of
[ ( C H 3 ) 3 Y L ] , S n C l 6 and chlorine E F G modulation due
to cationic motions.
Compounds
Crystal
symmetry
Y - L vector
orientation
[(CH 3 ) 3 NH] 2 SnCl 6
cubic
Pa3, Z = 4"
[(CH3)3S]2SnCl6
cubic
Pa3, Z = 4 b
[(CH 3 ) 4 N] 2 SnCl 6
cubic
Fd3c, Z = 32 c
[(CH3)4P]2SnCl6
lower than
cubic d
N - H vector
No
pointed at a
2
[SnCl6] ~ site
lone-pair axis
Yes
pointed at an
empty site
cation at 23 site : Yes
N - C H , vectors
pointed at empty
sites; cation at
4 site: N - C H 3 vector
pointed at one particular halogen
unknown
No
a
b
c
d
Chlorine EFG
modulation
O. K n o p , T. S. Cameron, M . A. James, and M . Falk,
Canad. J. Chem. 61, 1620 (1983).
O. K n o p , A. Linden, B. R. Vincent, S. C. Choi, T. S.
Cameron, and R. J. Boyd, Canad. J. Chem. 67,1984 (1989).
K . Nielsen and R. W. Berg, Acta Chem. Scand. A 34, 153
(1980).
M . Kaga, P. Koziol, T. Asaji, and D. Nakamura, Z. N a t u r forsch. 4 5 a , 1033 (1990).
mum of XH NMR in [(CH 3 ) 3 S] 2 PtCl 6 at a temperature close to that for the minimum of Cl NQR TlQ
reported previously [1]. This result suggests that a
small-angle reorientation of the C H 3 group or the
whole cations, which makes EFG modulation at the
chlorines, takes place in the [(CH 3 ) 3 S] + cation. However, the X-ray diffraction analysis of [(CH 3 ) 3 S] 2 SnCl 6
[10] does not show any remarkable sign of disorder of
the cationic position. Therefore, a possible disorder
will be of hydrogen atoms in the C H 3 groups.
[7] R. Ikeda, D. Nakamura, R. Kadel, and Al. Weiss, Ber.
Bunsenges. Phys. Chem. 87, 570 (1983).
[8] K . R. Jeffrey and R. L. Armstrong, Phys. Rev. 174, 359
(1968).
[9] Y. Furukawa, Y. Baba, S. Gima, M . Kaga, T. Asaji, R.
Ikeda, and D. Nakamura, Z. Naturforsch. 4 6 a , 809
(1991).
[10] O. Knop, A. Linden, B. R. Vincent, S. C. Choi, T. S.
Cameron, and R. J. Boyd, Canad. J. Chem. 67, 1984
(1989).
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