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Dipole moments of the sulfides of Pb, Sn, and Si from Stark effect measurements
JOURNAL
OF
Dipole
MOLI~XULAR
SPECTROSCOPY
30,
102-110 (1969)
Moments of the Sulfides of Pb, Sn, and Si from Stark Effect Measurements’ A. NARASIMHA Chemistry
Department,
MURTY~
AND R.
Rice University,
F.
CURL,
Houston,
JR.
Texas YYOOl
The Stark components of some microwave rotational transitions of the sulfides of lead, tin, and silicon were studied in the frequency region 20-36 GHz at temperatures ranging from 680 to 8OO”C, and the dipole moments were determined (Debye): ppm = 4.02 f 0.10; po+s) = 3.38 f 0.07; p(sis, = 1.74 f 0.07.
With the advent of high temperature microwave spectroscopy in recent years, it was possible to determine the molecular constants for a number of diatomic molecules (1-C). Dipole moment measurements have not been carried out for many of these simple molecules. We have developed a simple Stark modulation high temperature cell, which has all the advantages of a conventional room temperature system and can be conveniently used for the study of Stark effects of rotational transitions. The Stark septum is held in the center of an X-band stainless steel waveguide, parallel to the broad face, by means of stainless steel screws passing through fine holes in the narrow face of the guide at 10 discreet locations (Fig. 1). The screws are insulated by quartz or ceramic washers situated outside the guide on the narrow face. This arrangement eliminates microwave power dielectric loss in the Stark insulators since they are outside the waveguide. This also has the further advantage of reducing the deposition of sample on the Stark insulators with a consequent lowering of resistance and deleterious effect on the Stark waveform. The cell can be conveniently dismantled and reassembled for cleaning purposes. A detailed description of the absorption system is presented elsewhere (7). The microwave spectra of many group IV/VI compounds have been studied by Hoeft (5). A coaxial type cylindrical waveguide absorption cell (8) was used in his investigations. The Stark electric field is inhomogeneous with this arrangement and is not suitable for dipole moment measurements. The present paper presents the results of our studies on the Stark energy levels of the sulfides of lead, tin, and silicon. 1Supported by Grant GP-6305 x of the National Science Foundation. * Present address: Physics Department, Grambling College, Grambling, 71245. 102
Louisiana
DIPOLE
MOMENTS
SECTION
0%’ PbS,
SnS,
AND 8iS
103
OF THE STARK CELL 0.25”HOLE
DIA.
--r S.S. STEP WASHER STAINLESS
QUARTZ
STEEL
OR ALSIMAG 222
WASHER
WALL THICKNESS
SEPTUM THICKNESS TAPPED
0.125”
0.0625”
HOL
2-56
GAP
S.S. SCREW
FIG. 1. Cross section of the Stark cell (not to scale) EXPERIMENTAL
METHODS
With the exception of the absorption cell and the associated heating arrangcment, etc., the rest of the electronic units are all the same as that of a CO~~VCIItional 100 IcHz Stark modulation microwave spectrograph. The basic measurement is the determination of Stark component frequency as a function of voltage applied to the Stark electrodes. The frequencies are measured by mixing the klystron frequency with a harmonic of a FX4 Gertsch frequency generator (500-1000 MHz). The difference frequency is heard as an audio beat note on a National HRO receiver. The frequencies of the receiver and Gertsch were in turn determined b? a HP XZ4511electronic frequency counter. Since accurate voltage measurements of a square \vave are not easily made, only :I very small square wave voltage necessary to modulate the absorption was applied to the guide. The Stark component frequency was then measured as a function of a superimposed dc voltage shifting the square wave base. The dc voltage was derived from a regulated de supply or a bank of batteries and could accurately he measured on a digital voltmeter. The estimated error in frequency measurement is about ~0.3 MHz and that in voltage measurement is fO.2 V. There may be a much larger systematic error in the voltage. CALIBRATION
The frequency tional transition pressed as (9 I :
OF THE WAVEGUIDE
shift AV of a Stark component of a J .+ J + 1, AN = 0 rotaof a diatomic molecule for an impressed voltage V can be ex-
104
MURTY
AND CURL, JR.
where K = E2/V2 is the guide constant determined by calibration with a substance of known dipole moment, and C, the Stark effect constant, equals 0.50348 MHz/( Debye) (Volt/cm). A JM
= 6M2(8J2 + 16J + 5) - SJ(J + 1)2(J + 2) . . . J(J + 2)(2J - 1)(2J + 1)(2J + 3)(2J + 5)
(2)
In the present case the uncertainties in the dipole moment value are mainly due to the inaccuracies in the measurement of frequency and voltage both for the known compound in the determination of K, and the new compound under investigation. Normally for cells operating at room temperature, the guide constant K is determined taking OCS as a standard whose dipole moment is accurately known (10) (0.7124 D). Due to the rapid decomposition of the sample beyond 2OO”C, it has not been possible for us to obtain useful measurements around 700°C with OCS. Several experiments were made at room temperature with OCS to assure ourselves that the Stark septum is always fixed at the same position every time it is reassembled after cleaning (7). The noticeable thermal expansion of the stainless steel vacuum chamber containing the absorption cell clearly indicated the necessity of :alibrating the cell at the temperature at which the three sulfides were studied (-700°C). Lide (6) observed that around 6OO”C, there is an increase of about 10 % in the dimensions of his cell (of considerably different construction). The dipole moments of KC1 and NaCl are accurately known from the recent molecular beam resonance experiments of Hebert et al. (11) . The cell constant was determined at 700-800” C by measurement of the Stark effect of these two compounds. The lines of both compounds are broadened by quadrupole structure making very accurate measurement difficult. Each Stark lobe contains several quadrupole components. As these were unresolved the data was treated as though quadrupole structure was absent. The calibration measurements are plotted in Fig. 2. The experimental points were least square fitted with a straight line which was not required to pass through the origin. The resulting values of the cell constant were 6.28 & 0.1 cmL2 from NaCl and 6.58 f 0.2 cmF2 from KCl. The uncertainties listed are estimated standard deviations in all cases. The average K value, 6.43 =t 0.2 cm-‘, was used in the evaluation of the dipole moments of the sulfides. RESULTS
AND DISCUSSION
PBS Reagent grade sample supplied by General Chemical Company, New York, was used without any further purification. Though there was considerable decomposition of the sample into metallic lead, sufficient vapor pressure of the undecomposed PbS molecules was always available and the lines were fairly strong. The u = 0, J = 2 -+ 3 transition of “*Pb 32Swas observed at a temperature of about 75O”C, and the M = 1 and M = 2 Stark component frequencies were
DIPOLE
I
2
MOMENTS
3
4
OF PbS,
5
6
SnS, AND
7
v2 x w4 FIG. 2. Stark effect measurementsof KC1 and NaCl for calibration of the Stark cell.
6
105
SiS
9
10
(volts I2 at a temperature
of about
800°C
measured for various dc voltages. Figure 3 is a plot of Av vs V2 for the M = 1 Stark components of the three sulfides. Similar plots were obtained for M = 2 lobes also. The scatter was appreciably higher for SiS M = 0 lobe.
The commercial sample obtained from K & K labs, Plainview, New York, was used. In this case also the decomposition of SnS to metallic tin did not cause any problem except that large quantities of the sample were needed for each run. Since the entire absorption cell is in the hot zone, the metallic globules condensed on the cold ends of the chamber. The v = 0, J = 2 --+ 3 transition of “‘Sn 32Swas observed at a temperature of about 680°C. Both the M = 1 and M = 2 lobes were studied as a function of Stark voltage. SiS The sample of silicon sulfide was prepared in the manner suggested by Hoeft (la). Equimolar components of silicon and ferrous sulfide were heated in a quartz tube to a temperature of about 1200°C and the reaction was allowed to continue for a period of about 12 hr. The tube was flushed with helium through a bubble chamber to avoid interaction with the outside atmosphere. Dark brown-black substance condensed on the cold ends of the tube. Appreciable amounts of yeho\\
106
MURTY
AND
CURL,
JR.
6
2
0
-I N I ;
-3
-5
-71
I 4
I 8
I
I 12
I 20
16 v2
x 10-4,
(volts
I
’
24
I
J
28
)2
FIG. 3. Stark effect measurements of the sulfides of lead, tin, and silicon
and white polymer also were produced and condensed on the colder ends. The reaction was not very efficient, and the size of the furnace imposed limitations on the amount of reactants that could be used at a time. About 25 g of the sample could be prepared in two experiments. The v = 0, J = 14 2 transition of *‘Si 32S was studied at a temperature of about 770” C. All the results are summarized in Table I. More precise measurements could not be made in the case of SiS because of lack of sufficient microwave power in that frequency region and enough sample to make a large number of measurements. In the case of NaCl and ICC1 the increased breadth (-3 MHz) of the lines reduced the precision. All the reported values of p and K are mean values of two sets of measurements performed under similar conditions. The values of Av/V’ and error estimates were obtained by the method of least squares, except that the estimates of error in final p values were obtained by considering the consistency betneen two lobes. Figures 4 and 5 present the nature of variation of the dipole moment with respect to the internuclear distance T, and the row of the periodic table. For comparison, similar plots for group V trichlorides are also shown. The dipole moment of CS of 1.97 D (13) seems relatively large compared to the value of 1.74 D of SiS, especially since the electronegativity difference of C and S is very small. However, there exists a puzzling question concerning the sign of the
DIPOLE
MOMENTS
OF PbS, SnS, AND
TABLE
Transition
I
Gradient [(Av/p X 10’) MHz/(Volt)2]
Stark component Lobe
0.102
-0.125
NaCl
750
Dipole moment (Debye units)
f
0.002
+
0.003
0.7124 (assumed) K = 7.79 h 0.16 cmd2 mean iY = 7.77 & 0.1 crnm2 k’ = 7.76 =t 0.20 crxd2
0.24
9.002 (assumed) k’ = 6.28 + 0.10 cm-%
12.267 +
we2
107
SiS
=26051.52 mean K = 6.43 + 0.20 cme2
KC1
770
“24
10.2688 (assumed K = 6.58 f 0.20 cme2
-1.634
f
0.05
-0.261
f
0.004
1.869 f
0.080
/J = 3.95 f 0.06 r(PbS) = 4.02 xt 0.10 cc = 4.09 f 0.10
f
0.002
p
1.104 *
0.030
)
=23067.32 PbS
750
Y2_3
=20883.75 SIlS
-0.159
680
= 24572.53
Estimated from Fig. 4 r(GeS) = 2.25
GeS
SiS
= 3.34f 0.05 p(SnS) = 3.38 f 0.07 &J = 3.11 f 0.07
770
VI-t?
=36309.62
M
=
1
JI = 0
0.350
-0.387
+
0.0”
h
0.04
/.I = 1.77 f 0.05 r(SiS) = 1.74 f 0.07 ,U = 1.68 f 0.10
dipole moment of CS. Bird and Mockler (13) appear to have felt that the most reasonable picture of the bonding in CS gives a dipole moment with S positive (-C = S+) . On this basis the plot of p vs 1’ (Fig. 4)) shows a more regular behavior than with C+S-. Similarly in the case of trichlorides also, if this sign change is incorporated for NC13 the variation becomes smoother. There appears a fairl? linear behavior in both the groups from the third row. Hence a reasonable estimate of the dipole moment of GeS could be made from this plot by interpolation and is found to be 2.25 D. Figure 5 shows the variation of both r and b with respect to the row of the
108
MURTY AND CURL, JR.
^
I
1 J
L
r,lNTERNUCLEAR
DISTANCE,
d
FIG. 4. Variations of dipole moment as a function of internuclear distance in group IV subsulfides and group V trichlorides; all the r values of the sulfides except for CS are taken from Ref. (6); p and r of CS from Ref. (IS) ; GeS p value is interpolated from the known r value from Ref. (14); the /J and T values of PC13 and AsCla were obtained from Ref. (17) ; that for SbCla from Ref. (f8); r of NC18estimated from the bond distances in other nitrogen chlorine compounds Ref. (16); p of NC18 from Ref. (16). periodic table. No experimental data is availab!e on the internuclear distance of NC&. It is expected to be about 1.85 f 0.07 A from the bond lengths in other nitrogen chlorine compounds. If this value is assumed, the bond lengths show a smooth increase in the case of group V trichlorides while there is an abrupt change at the second row in the case of group IV subsulfides. Here also it may be noted that there is a close similarity in the nature of variation of both r and p from the third row onwards. However, if a sign change in dipole moment occurs for the second row elements (C and N) then this similarity continues throughout in both the groups. RECEIVED: August 20, 1968
DIPOLE
^-
GROUP
d
-
MOMENTS
OF PbS,
P
SnS, AND
SiS
109
TRICHLORIDES
N-Cl;
2
4
3 ROW OF
PERIODIC
FIG. 5. Variation of p and r with respect for BiCh was obtained from Ref. (19).
5
6
TABLF
to the row of the periodic
table. The T value
REFERENCES 1. A. HONIG, M. MANDEL, M. L. STITCH, AND C. H. TOWNES, Phys. Rev. 96, 629 (1954). d. A. H. BARRETT AND M. MANDEL, Phys. Rev. 109, 1572 (1958). 5. J. R. RUSK AND W. GORDY, Phys. Rev. 137, 817 (1962); 136, 1303 (1965). 4. P. A. TATE AND M. W. P. STRANDBERG, 1. Chem. Phys. 22, 1389 (1954). 5. J. HOEFT, 2. Naturforsch. 19a, 1134 (1964); 2Oa, 313, 826, 1122, 1327 (1965); 21a, 437, 1122, 1240, 1884 (1966). 6. D. R. LIDE, JR., J. Chem. Phys. 42, 1013 (1965). 7. A. NARASIMHA MURTY AND R. F. CURL, JR., Rev. Sci. Instr., 39,1885 (1968). 8. J. HOEFT, 2. Naturforsch. 2Oa, 313 (1965). 9. C. H. TOWNES AND A. L. SCHAWLOW, “Microwave Spectroscopy.” McGraw-Hill New York. 1955.
110
MURTY AND CURL, JR.
10. S. A. MARSHALL AND J. WEBER, Phys. Rev. 106, 1502 (1957). 11. A. J. HEBERT, F. J. LOVAS, C. A. MELENDRES, C. D. HOLLOWELL, T. L. STORY, AND K. STREET, J. ChenL. Phys. 48,2824 (1968). 12. J. HOEFT, Z. Naturjorsch. 2Oa, 1327 (1965). 1s. It. C. MOCI~LER AND G. R. BIRD, Phys. Rev. 98, 1837 (1955). 14.J. HOEFT, Z. Naturjorsch. 20a, 826 (1965). 15. “Tables of Interatomic Distances and Configuration in Molecules and Ions and Supplement (1956-1959) Special Publication No. 18,” (L. E. SUTTON, ed.), Chem. Sot., London, 1958. 16. E. ALLENSTEIN, Z. Anorg. Al/gem. Chern. 308, 3 (1961). 17. P. KISLUIK AND C. H. TOWNES,Phys. Rev. 78, 347a (1959); J. Chem. Phys. 18, 1109 (1950). 18. P. KISLUX. J. Chem. Phys. 22, 86 (1954). 19. H. A. SKINNER AND 1,. E. SUTTON, Z’rans. Faraday Sot. 36, 681 (1940).
OF
Dipole
MOLI~XULAR
SPECTROSCOPY
30,
102-110 (1969)
Moments of the Sulfides of Pb, Sn, and Si from Stark Effect Measurements’ A. NARASIMHA Chemistry
Department,
MURTY~
AND R.
Rice University,
F.
CURL,
Houston,
JR.
Texas YYOOl
The Stark components of some microwave rotational transitions of the sulfides of lead, tin, and silicon were studied in the frequency region 20-36 GHz at temperatures ranging from 680 to 8OO”C, and the dipole moments were determined (Debye): ppm = 4.02 f 0.10; po+s) = 3.38 f 0.07; p(sis, = 1.74 f 0.07.
With the advent of high temperature microwave spectroscopy in recent years, it was possible to determine the molecular constants for a number of diatomic molecules (1-C). Dipole moment measurements have not been carried out for many of these simple molecules. We have developed a simple Stark modulation high temperature cell, which has all the advantages of a conventional room temperature system and can be conveniently used for the study of Stark effects of rotational transitions. The Stark septum is held in the center of an X-band stainless steel waveguide, parallel to the broad face, by means of stainless steel screws passing through fine holes in the narrow face of the guide at 10 discreet locations (Fig. 1). The screws are insulated by quartz or ceramic washers situated outside the guide on the narrow face. This arrangement eliminates microwave power dielectric loss in the Stark insulators since they are outside the waveguide. This also has the further advantage of reducing the deposition of sample on the Stark insulators with a consequent lowering of resistance and deleterious effect on the Stark waveform. The cell can be conveniently dismantled and reassembled for cleaning purposes. A detailed description of the absorption system is presented elsewhere (7). The microwave spectra of many group IV/VI compounds have been studied by Hoeft (5). A coaxial type cylindrical waveguide absorption cell (8) was used in his investigations. The Stark electric field is inhomogeneous with this arrangement and is not suitable for dipole moment measurements. The present paper presents the results of our studies on the Stark energy levels of the sulfides of lead, tin, and silicon. 1Supported by Grant GP-6305 x of the National Science Foundation. * Present address: Physics Department, Grambling College, Grambling, 71245. 102
Louisiana
DIPOLE
MOMENTS
SECTION
0%’ PbS,
SnS,
AND 8iS
103
OF THE STARK CELL 0.25”HOLE
DIA.
--r S.S. STEP WASHER STAINLESS
QUARTZ
STEEL
OR ALSIMAG 222
WASHER
WALL THICKNESS
SEPTUM THICKNESS TAPPED
0.125”
0.0625”
HOL
2-56
GAP
S.S. SCREW
FIG. 1. Cross section of the Stark cell (not to scale) EXPERIMENTAL
METHODS
With the exception of the absorption cell and the associated heating arrangcment, etc., the rest of the electronic units are all the same as that of a CO~~VCIItional 100 IcHz Stark modulation microwave spectrograph. The basic measurement is the determination of Stark component frequency as a function of voltage applied to the Stark electrodes. The frequencies are measured by mixing the klystron frequency with a harmonic of a FX4 Gertsch frequency generator (500-1000 MHz). The difference frequency is heard as an audio beat note on a National HRO receiver. The frequencies of the receiver and Gertsch were in turn determined b? a HP XZ4511electronic frequency counter. Since accurate voltage measurements of a square \vave are not easily made, only :I very small square wave voltage necessary to modulate the absorption was applied to the guide. The Stark component frequency was then measured as a function of a superimposed dc voltage shifting the square wave base. The dc voltage was derived from a regulated de supply or a bank of batteries and could accurately he measured on a digital voltmeter. The estimated error in frequency measurement is about ~0.3 MHz and that in voltage measurement is fO.2 V. There may be a much larger systematic error in the voltage. CALIBRATION
The frequency tional transition pressed as (9 I :
OF THE WAVEGUIDE
shift AV of a Stark component of a J .+ J + 1, AN = 0 rotaof a diatomic molecule for an impressed voltage V can be ex-
104
MURTY
AND CURL, JR.
where K = E2/V2 is the guide constant determined by calibration with a substance of known dipole moment, and C, the Stark effect constant, equals 0.50348 MHz/( Debye) (Volt/cm). A JM
= 6M2(8J2 + 16J + 5) - SJ(J + 1)2(J + 2) . . . J(J + 2)(2J - 1)(2J + 1)(2J + 3)(2J + 5)
(2)
In the present case the uncertainties in the dipole moment value are mainly due to the inaccuracies in the measurement of frequency and voltage both for the known compound in the determination of K, and the new compound under investigation. Normally for cells operating at room temperature, the guide constant K is determined taking OCS as a standard whose dipole moment is accurately known (10) (0.7124 D). Due to the rapid decomposition of the sample beyond 2OO”C, it has not been possible for us to obtain useful measurements around 700°C with OCS. Several experiments were made at room temperature with OCS to assure ourselves that the Stark septum is always fixed at the same position every time it is reassembled after cleaning (7). The noticeable thermal expansion of the stainless steel vacuum chamber containing the absorption cell clearly indicated the necessity of :alibrating the cell at the temperature at which the three sulfides were studied (-700°C). Lide (6) observed that around 6OO”C, there is an increase of about 10 % in the dimensions of his cell (of considerably different construction). The dipole moments of KC1 and NaCl are accurately known from the recent molecular beam resonance experiments of Hebert et al. (11) . The cell constant was determined at 700-800” C by measurement of the Stark effect of these two compounds. The lines of both compounds are broadened by quadrupole structure making very accurate measurement difficult. Each Stark lobe contains several quadrupole components. As these were unresolved the data was treated as though quadrupole structure was absent. The calibration measurements are plotted in Fig. 2. The experimental points were least square fitted with a straight line which was not required to pass through the origin. The resulting values of the cell constant were 6.28 & 0.1 cmL2 from NaCl and 6.58 f 0.2 cmF2 from KCl. The uncertainties listed are estimated standard deviations in all cases. The average K value, 6.43 =t 0.2 cm-‘, was used in the evaluation of the dipole moments of the sulfides. RESULTS
AND DISCUSSION
PBS Reagent grade sample supplied by General Chemical Company, New York, was used without any further purification. Though there was considerable decomposition of the sample into metallic lead, sufficient vapor pressure of the undecomposed PbS molecules was always available and the lines were fairly strong. The u = 0, J = 2 -+ 3 transition of “*Pb 32Swas observed at a temperature of about 75O”C, and the M = 1 and M = 2 Stark component frequencies were
DIPOLE
I
2
MOMENTS
3
4
OF PbS,
5
6
SnS, AND
7
v2 x w4 FIG. 2. Stark effect measurementsof KC1 and NaCl for calibration of the Stark cell.
6
105
SiS
9
10
(volts I2 at a temperature
of about
800°C
measured for various dc voltages. Figure 3 is a plot of Av vs V2 for the M = 1 Stark components of the three sulfides. Similar plots were obtained for M = 2 lobes also. The scatter was appreciably higher for SiS M = 0 lobe.
The commercial sample obtained from K & K labs, Plainview, New York, was used. In this case also the decomposition of SnS to metallic tin did not cause any problem except that large quantities of the sample were needed for each run. Since the entire absorption cell is in the hot zone, the metallic globules condensed on the cold ends of the chamber. The v = 0, J = 2 --+ 3 transition of “‘Sn 32Swas observed at a temperature of about 680°C. Both the M = 1 and M = 2 lobes were studied as a function of Stark voltage. SiS The sample of silicon sulfide was prepared in the manner suggested by Hoeft (la). Equimolar components of silicon and ferrous sulfide were heated in a quartz tube to a temperature of about 1200°C and the reaction was allowed to continue for a period of about 12 hr. The tube was flushed with helium through a bubble chamber to avoid interaction with the outside atmosphere. Dark brown-black substance condensed on the cold ends of the tube. Appreciable amounts of yeho\\
106
MURTY
AND
CURL,
JR.
6
2
0
-I N I ;
-3
-5
-71
I 4
I 8
I
I 12
I 20
16 v2
x 10-4,
(volts
I
’
24
I
J
28
)2
FIG. 3. Stark effect measurements of the sulfides of lead, tin, and silicon
and white polymer also were produced and condensed on the colder ends. The reaction was not very efficient, and the size of the furnace imposed limitations on the amount of reactants that could be used at a time. About 25 g of the sample could be prepared in two experiments. The v = 0, J = 14 2 transition of *‘Si 32S was studied at a temperature of about 770” C. All the results are summarized in Table I. More precise measurements could not be made in the case of SiS because of lack of sufficient microwave power in that frequency region and enough sample to make a large number of measurements. In the case of NaCl and ICC1 the increased breadth (-3 MHz) of the lines reduced the precision. All the reported values of p and K are mean values of two sets of measurements performed under similar conditions. The values of Av/V’ and error estimates were obtained by the method of least squares, except that the estimates of error in final p values were obtained by considering the consistency betneen two lobes. Figures 4 and 5 present the nature of variation of the dipole moment with respect to the internuclear distance T, and the row of the periodic table. For comparison, similar plots for group V trichlorides are also shown. The dipole moment of CS of 1.97 D (13) seems relatively large compared to the value of 1.74 D of SiS, especially since the electronegativity difference of C and S is very small. However, there exists a puzzling question concerning the sign of the
DIPOLE
MOMENTS
OF PbS, SnS, AND
TABLE
Transition
I
Gradient [(Av/p X 10’) MHz/(Volt)2]
Stark component Lobe
0.102
-0.125
NaCl
750
Dipole moment (Debye units)
f
0.002
+
0.003
0.7124 (assumed) K = 7.79 h 0.16 cmd2 mean iY = 7.77 & 0.1 crnm2 k’ = 7.76 =t 0.20 crxd2
0.24
9.002 (assumed) k’ = 6.28 + 0.10 cm-%
12.267 +
we2
107
SiS
=26051.52 mean K = 6.43 + 0.20 cme2
KC1
770
“24
10.2688 (assumed K = 6.58 f 0.20 cme2
-1.634
f
0.05
-0.261
f
0.004
1.869 f
0.080
/J = 3.95 f 0.06 r(PbS) = 4.02 xt 0.10 cc = 4.09 f 0.10
f
0.002
p
1.104 *
0.030
)
=23067.32 PbS
750
Y2_3
=20883.75 SIlS
-0.159
680
= 24572.53
Estimated from Fig. 4 r(GeS) = 2.25
GeS
SiS
= 3.34f 0.05 p(SnS) = 3.38 f 0.07 &J = 3.11 f 0.07
770
VI-t?
=36309.62
M
=
1
JI = 0
0.350
-0.387
+
0.0”
h
0.04
/.I = 1.77 f 0.05 r(SiS) = 1.74 f 0.07 ,U = 1.68 f 0.10
dipole moment of CS. Bird and Mockler (13) appear to have felt that the most reasonable picture of the bonding in CS gives a dipole moment with S positive (-C = S+) . On this basis the plot of p vs 1’ (Fig. 4)) shows a more regular behavior than with C+S-. Similarly in the case of trichlorides also, if this sign change is incorporated for NC13 the variation becomes smoother. There appears a fairl? linear behavior in both the groups from the third row. Hence a reasonable estimate of the dipole moment of GeS could be made from this plot by interpolation and is found to be 2.25 D. Figure 5 shows the variation of both r and b with respect to the row of the
108
MURTY AND CURL, JR.
^
I
1 J
L
r,lNTERNUCLEAR
DISTANCE,
d
FIG. 4. Variations of dipole moment as a function of internuclear distance in group IV subsulfides and group V trichlorides; all the r values of the sulfides except for CS are taken from Ref. (6); p and r of CS from Ref. (IS) ; GeS p value is interpolated from the known r value from Ref. (14); the /J and T values of PC13 and AsCla were obtained from Ref. (17) ; that for SbCla from Ref. (f8); r of NC18estimated from the bond distances in other nitrogen chlorine compounds Ref. (16); p of NC18 from Ref. (16). periodic table. No experimental data is availab!e on the internuclear distance of NC&. It is expected to be about 1.85 f 0.07 A from the bond lengths in other nitrogen chlorine compounds. If this value is assumed, the bond lengths show a smooth increase in the case of group V trichlorides while there is an abrupt change at the second row in the case of group IV subsulfides. Here also it may be noted that there is a close similarity in the nature of variation of both r and p from the third row onwards. However, if a sign change in dipole moment occurs for the second row elements (C and N) then this similarity continues throughout in both the groups. RECEIVED: August 20, 1968
DIPOLE
^-
GROUP
d
-
MOMENTS
OF PbS,
P
SnS, AND
SiS
109
TRICHLORIDES
N-Cl;
2
4
3 ROW OF
PERIODIC
FIG. 5. Variation of p and r with respect for BiCh was obtained from Ref. (19).
5
6
TABLF
to the row of the periodic
table. The T value
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