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Quadrupole hyperfine structure in the rotational spectrum of bromo- and chlorobenzene
CHEMICAL PHYSICSLETTERS
Volume 12, number 1
QUADRUPOLE
I
T-ZYPERFINE STRUCTURE IN THE ROTATIONAL OF BROMO- AND CHLOROBENZENE
December
1971
SPECTRUM
W. CAMINATI and A.M. MlRRI Laboratorio di Spettroscopia Molecolare, C.N.R. and Istituto Chimico “G. Ciamician”, Universiti di Bobgna, Bologna. Italy
Received 7 September 1971
The hyperfine quadrupole couplingstructure in the rotitional spectrum of brcmoanalyzed and the symmetry parameter n was evaluated in each case.
No data are available in the literature concerning the percentage of double bond character of the C-X bonds in the series chloro-, bromo-, and iodobenzene. Accurate measurements of the quadrupole coupling constants and of the asymmetry parameter q= oLbb-xo)&?~ which is the quantity directly bound to the asymmetry of the electronic distribution around the halogen nucleus, were carried out very recently only for iodobenzene [ 11. A comparison between the double bond character of the C-X bond in this molecule and in the other benzene derivatives appears interesting therefore. Rosenthal and Dailey [2] measured the rotational spectrum of bromobenzene but only a very rough value of the percentage of double bond character could be evaluated from the hyperfme structure, due to the high values of J involved in the transitions they measured. The rotational spectrum of chlorobenzene was studied by Poynter [3], but the asymmetry parameter 77could not be obtained at all. The evaluation of double bond character from the length of the C-Cl bond and from the value of the electric dipole moment appears to be unreliabIe since in the first case 15% and in the second case 4% of double bond character was obtined [3]. We therefore measured new lines corresponding to the, lowest possible J.values, depending~on the intensity of the spectrum, for both bromobenzkre and chloro‘benzene.
The Lines with k_L = 2 and 0 were found the most dependent on the asymmetry parameter q_ Some lines
already measured by Rosenthal and Dailey [2] and by Poynter [3] were also remeasured. We found out that our measurements coincide within 0.2 MHz with the previous ones [2,3]. In the case of bromobenzene, the frequency of each line was corrected for the second order quadrupole coupling effect, which was caIcuIated in the same manner as for iodobenzene [I]. This correction is only a little larger than the experimental error which
was eval-aated
lines and
to be LO.05
ML+
for the strongest
40.1 MB2 for the weakest ones, but the
agreement between the doub!e bond character evaluated for the two different isotopic species, definitely improved when this correction was made. In tables I and 2 ail the line frequencies and second order corrections for C6H5 7gBr and C6HSS1Br are listed. No second order quadrupoIe couphng correction was necessary in the case of chlorobenzene since the maximum contribution was considerably smaller than tile experimental accuracy in the frequency measurements. In table 3 &I the rotational
transitions
wticii
were.
foi C6H,C!1are Listed. ln table 4 alI the moIecuIar donstants which were determined in this work and in the previous one [l] for the three compounds and the values obtained by Rosenthal and Dailey [2] aid by Poynter [3] for
measured
.’ .,
and chlorobenzene has been
-I’
.’
127
Volume 12, number
1
CHEMICAL PHYSICS LECERS
1 December
Table t CgHsBr(79)
hnsitions
&l&hted
Table 2 values
determined
&H&(81)
with
A = 5663.50 MHzB = 994.666 MHz. C= 846.82 MHz,& = = 292.5 MHz. The trantitions are AF= +I the lower F vaIue is only given. All frequencies given are in MHz
Transitions
F lower
624 + 725
707 ?8OB
15/z 9/2 i312 II/2
13 044.94 iJ45.82 055.09 056.42
0.12 -0.20 -0.09 0.03
15/2 1712
14 449.36
449.34
0.04
449.90 452.71 453.28
449.96 452.70 453.21
0.06
-0.10 -0.05
1712. 14 960.77 1112 962.01. 15/2 967.69 13/2 969.25
960.79 962.03 967.66 969.17
-0.16 -0.06 c.01
805 * 909
17/2
16 182.76
182.76
0.05
IV/2
183.26
183.27
0.03
Caicuhted values determhed with
= 464.1 MHz, q,fj = -242.7 MHz. Th$&xitions tl transitions; the ldwer F value is onlj)@n. given are in MHz
C&Ill&d seand order contributions
044.93 045.‘71 055. i0 056.45
1312 1112 725 -* 825
Frequencies c&c. expt.
trtiti&s~
p.= 5668,&V MHz,B = 984.704 MHz, C =,838.858MHz,%a
558.9MHz. xbb
trkitions;
‘Transitions
726 +
826
803 -+ 909
0.10
Frequencies C&2. expt.
F lower
707 -+ %I8
826 + 927
AU
YC AI= = frequencies
Calculated second order contributions
15/2 17/2
14 319.37 319.79
319.34 319.76
1312 1112
322.17 322.61
322.18 322.55
-0.07 -0.04
1712 11/2 lS/2 13/2
14 812.07 813.12 817.90 819.11
812.15 813.12 817.85 819.10
0.07 -0.12 0.04 0.01
17/Z 1912 1512 1312
16 038.76 039.19 040.95 041.40
038.73 039.17 040.9s 041.33
0.02 0.03 -0.06 -0.03 -0.08 0.00 -0.02 0.02 0.03 -0.04 -0.03
1912
16 721.25
1a12
722.34
1712 15/z
725.39 726.60
721.23 722.35 725.39 726.67
0.03 0.04
0.06
1512
185.36
185.36
-0.08
1312
135.92
185.93
-0.04
&a-’ 927
19/2 1312 17,‘2 1512
16 890.53 891.82 895.48 896.98
890.54 891.77 895.46 896.94
0.08 -0.11 -0.03 -0.01
909 +
10010
19/2 21/2 1712 1512
17 740.99 741.41 742.75 743.17
740.92 741.38 742.71 743.10
90’9 -r
1912 21/2 1712 IS/2
17 898.65 599.17 900.?6 901.25
898.59 899.20 900.82 901.24
0.03 0.04 -0.06 -oil4
927 +
a3
21/2 15/2 1912
18 642.14 643.17 645.20
642.13a) 0.05 643.14@ -0.06 645.21 a) -0.01
21i2
18 832.00
832.03@.
lOOlO
9 27 + 1028
102s +‘ll59
lSi2 1912 1712
833.24 835.69 837.07
2312
20 781.72
1712 2112 1912 llzs --* 1lZI0
by Roskthal
0.06
735.38a)
736.40 a) -0.05
737.61
737.58 a)
738.71
‘--’
j.
a) Identified
-738.70a) -0.02
..,:
0.00
20 571.47 572.41
571.39”) 0.04 572.37 a) -0.05
2112 ‘, 1912
573.81 574.84
573.7ga) 0.00 574.82 a) -0.01
25/2 22 505.19 1912 So&OS 2312 507.00
505.18a) 0.03 506.09 a) -0.04 s07.02a) 0.00
2112
508.02a)
507.92
-0.01
by R&enthl
and Dailey [ 2j.
0.00
The cor:ection of each f?eqi&ncy for .order e.ffect in the case of-bromobenzene
and Dailey [21.
‘.
646.35 n)
0.04
736.45
the second has the gf-
feet of changing the valuesof xapoutsidethe range ‘. ’ of the experjxnental Sxuracy ijveti by RoSenthaI incj
-..&iley‘.i?)
compirisgn are, listed_.The dquble bond chaiacier 6s sugg&ted by Goldste$ [4]. ... ,: ; -.
2312 17/2
11hr9-+ 1&,o
0.05
_.,
.:;
, Ah&h&
the reiatioh &&ested
.stein is only an’approximate
evaluated as :
128
646.35
1o;g 3 1129
833.25 a) -0.08 835.68 a) -0.02 837.12a) -0.01 751.77 2)
1712
782.86 a) -0.06 784.58 a) -0.01 785.81 a),. -0.02
-2512 22 735.42’
1912 2312 2112
a) Identified
782.86 784.53 785.77
1971
;
’
;
,,
;,;
. .
‘;
by cold-
‘one 143 , ,the percent-
.,-, _’ .’ ,.Y,, ; ..:
.,’
..
..
..
Volume 12, number 1
CHEMICAL PHYSICS LETTJZRS
1 December
1971
:, Table 3 _ CeHsCl(35) transitions. Calculated values determined with A = 5672.95 MHz, B = 1576.787 MHz. C = 1233.674 MHz, =a = -71,OY MHz;, xbb = 38,18 MHz. l?x trYlsitions ye ti = +l transitions; the lo&r F value is only given < frequencies xc in
MHz Transitions
F
Frequencies
IO%%
202 + 303
220
+
321
!/2 312
8 345.57 345.7 11
345:60
512 712
350.16 350.01 I
350.06
8 500.76 513.49 518.67 531.41
s/2
10 514.95 515.81 516.69 517.54
514.91 515.752) 516.73 5t7.573)
s/2 912 312
11 423.38 425.90 430.65 433.17
423.35 2) 425.90a) 430.64 a) 433.18
912
14 396.48
;;;,$a’
712
312 92
712
%I-+ 422
712 11/2 512 a) Identified
Quadrupole
coupling constants
by Poynter
and percentage
@5&robeozene
I*
S/2 312 7/2 l/2
313’414
42 - h.3
~calc.
500.86 513.61 531.49
397.01 400.20 400.73
. 400.19a) 400.69
[ 3 1.
Table 4 double bond cberacter of carbon-halogen benzene. ~4 and xbb are in MHz
bond for chloro-,
Br”bromcbenzene
Jh%OflIO-
benzene
This work
&la xbb ;
XaJ xbb .;
and iodo;
iodo-
berucne Mini and CZlmiIlati [l]
-71.09 38.18 -0.074 (3.26
f t f f
Poynter
[4]
-71.10
bromo-
0.10 0.49 0.015 0.67)s
f 0:50.
558.9 -292.5 -0.046 (2.26
* 1.3 (k 0.5) f 0.004 * 0.19%
Rosenthal
464.1 -242.7 -0.046 (231
f r * *
1.8 0.7 0.007 0.33)%
and Dailey [3]
567-c4 -0.049 f o.ozo (2.5 * IS)%
+00*3 -0.029 * 0.03 1 (1.5 f 1.5EG
-L892.1 c 2.2 976.2 i 1.5 -0.03 1 ?z0.0025 (1.75 *o-L+%
VoJume 12. tiumber 1
CHEMICAL PHYSICS LEtiRS
b&d character is clearly decreasing froin chIoro-‘to bromo- and iodobenzene, as is to be expected. Despite the greater polar&ability of the
.age pf double
Jarger atoms,
the atomic
orbital
a-erhpping
is in fact
rmallergoing’fromch!orine to iodine in view of the C-X bond length w&h is rapidly increasing in the sanie order and in view of the more diffuse nature of the p brbitals in going from chlorine to iodine. The value of 4% double bond character obtained from electric dipole moment 131 .in the case of cblorobenzene is in good agreement with the results obtained in this work from the asymmetry parameter q. Since ti error of about 10% is l~s$ly involved in
1 December
1971
the approximate relation between v and percexitage of double bond charact&, more precise data on the: qtiadrupole c&pLng constants are not nec+$ at this stage: ‘- .a k References [I] A.M.Mirri and W.Caminati, Chem. Phys. Letters 8 (1971) ,409. [2] E.Rqsenthal and +P.Dailey, J. Chem. Phys. 43 (1965) 2093. 133 R.L.Poynter, J. Chem. Phys. 39 (1963) 1962. [4] J.H.Goldstein, J. Chem. Phys. 24 (1956) 106.
Volume 12, number 1
QUADRUPOLE
I
T-ZYPERFINE STRUCTURE IN THE ROTATIONAL OF BROMO- AND CHLOROBENZENE
December
1971
SPECTRUM
W. CAMINATI and A.M. MlRRI Laboratorio di Spettroscopia Molecolare, C.N.R. and Istituto Chimico “G. Ciamician”, Universiti di Bobgna, Bologna. Italy
Received 7 September 1971
The hyperfine quadrupole couplingstructure in the rotitional spectrum of brcmoanalyzed and the symmetry parameter n was evaluated in each case.
No data are available in the literature concerning the percentage of double bond character of the C-X bonds in the series chloro-, bromo-, and iodobenzene. Accurate measurements of the quadrupole coupling constants and of the asymmetry parameter q= oLbb-xo)&?~ which is the quantity directly bound to the asymmetry of the electronic distribution around the halogen nucleus, were carried out very recently only for iodobenzene [ 11. A comparison between the double bond character of the C-X bond in this molecule and in the other benzene derivatives appears interesting therefore. Rosenthal and Dailey [2] measured the rotational spectrum of bromobenzene but only a very rough value of the percentage of double bond character could be evaluated from the hyperfme structure, due to the high values of J involved in the transitions they measured. The rotational spectrum of chlorobenzene was studied by Poynter [3], but the asymmetry parameter 77could not be obtained at all. The evaluation of double bond character from the length of the C-Cl bond and from the value of the electric dipole moment appears to be unreliabIe since in the first case 15% and in the second case 4% of double bond character was obtined [3]. We therefore measured new lines corresponding to the, lowest possible J.values, depending~on the intensity of the spectrum, for both bromobenzkre and chloro‘benzene.
The Lines with k_L = 2 and 0 were found the most dependent on the asymmetry parameter q_ Some lines
already measured by Rosenthal and Dailey [2] and by Poynter [3] were also remeasured. We found out that our measurements coincide within 0.2 MHz with the previous ones [2,3]. In the case of bromobenzene, the frequency of each line was corrected for the second order quadrupole coupling effect, which was caIcuIated in the same manner as for iodobenzene [I]. This correction is only a little larger than the experimental error which
was eval-aated
lines and
to be LO.05
ML+
for the strongest
40.1 MB2 for the weakest ones, but the
agreement between the doub!e bond character evaluated for the two different isotopic species, definitely improved when this correction was made. In tables I and 2 ail the line frequencies and second order corrections for C6H5 7gBr and C6HSS1Br are listed. No second order quadrupoIe couphng correction was necessary in the case of chlorobenzene since the maximum contribution was considerably smaller than tile experimental accuracy in the frequency measurements. In table 3 &I the rotational
transitions
wticii
were.
foi C6H,C!1are Listed. ln table 4 alI the moIecuIar donstants which were determined in this work and in the previous one [l] for the three compounds and the values obtained by Rosenthal and Dailey [2] aid by Poynter [3] for
measured
.’ .,
and chlorobenzene has been
-I’
.’
127
Volume 12, number
1
CHEMICAL PHYSICS LECERS
1 December
Table t CgHsBr(79)
hnsitions
&l&hted
Table 2 values
determined
&H&(81)
with
A = 5663.50 MHzB = 994.666 MHz. C= 846.82 MHz,& = = 292.5 MHz. The trantitions are AF= +I the lower F vaIue is only given. All frequencies given are in MHz
Transitions
F lower
624 + 725
707 ?8OB
15/z 9/2 i312 II/2
13 044.94 iJ45.82 055.09 056.42
0.12 -0.20 -0.09 0.03
15/2 1712
14 449.36
449.34
0.04
449.90 452.71 453.28
449.96 452.70 453.21
0.06
-0.10 -0.05
1712. 14 960.77 1112 962.01. 15/2 967.69 13/2 969.25
960.79 962.03 967.66 969.17
-0.16 -0.06 c.01
805 * 909
17/2
16 182.76
182.76
0.05
IV/2
183.26
183.27
0.03
Caicuhted values determhed with
= 464.1 MHz, q,fj = -242.7 MHz. Th$&xitions tl transitions; the ldwer F value is onlj)@n. given are in MHz
C&Ill&d seand order contributions
044.93 045.‘71 055. i0 056.45
1312 1112 725 -* 825
Frequencies c&c. expt.
trtiti&s~
p.= 5668,&V MHz,B = 984.704 MHz, C =,838.858MHz,%a
558.9MHz. xbb
trkitions;
‘Transitions
726 +
826
803 -+ 909
0.10
Frequencies C&2. expt.
F lower
707 -+ %I8
826 + 927
AU
YC AI= = frequencies
Calculated second order contributions
15/2 17/2
14 319.37 319.79
319.34 319.76
1312 1112
322.17 322.61
322.18 322.55
-0.07 -0.04
1712 11/2 lS/2 13/2
14 812.07 813.12 817.90 819.11
812.15 813.12 817.85 819.10
0.07 -0.12 0.04 0.01
17/Z 1912 1512 1312
16 038.76 039.19 040.95 041.40
038.73 039.17 040.9s 041.33
0.02 0.03 -0.06 -0.03 -0.08 0.00 -0.02 0.02 0.03 -0.04 -0.03
1912
16 721.25
1a12
722.34
1712 15/z
725.39 726.60
721.23 722.35 725.39 726.67
0.03 0.04
0.06
1512
185.36
185.36
-0.08
1312
135.92
185.93
-0.04
&a-’ 927
19/2 1312 17,‘2 1512
16 890.53 891.82 895.48 896.98
890.54 891.77 895.46 896.94
0.08 -0.11 -0.03 -0.01
909 +
10010
19/2 21/2 1712 1512
17 740.99 741.41 742.75 743.17
740.92 741.38 742.71 743.10
90’9 -r
1912 21/2 1712 IS/2
17 898.65 599.17 900.?6 901.25
898.59 899.20 900.82 901.24
0.03 0.04 -0.06 -oil4
927 +
a3
21/2 15/2 1912
18 642.14 643.17 645.20
642.13a) 0.05 643.14@ -0.06 645.21 a) -0.01
21i2
18 832.00
832.03@.
lOOlO
9 27 + 1028
102s +‘ll59
lSi2 1912 1712
833.24 835.69 837.07
2312
20 781.72
1712 2112 1912 llzs --* 1lZI0
by Roskthal
0.06
735.38a)
736.40 a) -0.05
737.61
737.58 a)
738.71
‘--’
j.
a) Identified
-738.70a) -0.02
..,:
0.00
20 571.47 572.41
571.39”) 0.04 572.37 a) -0.05
2112 ‘, 1912
573.81 574.84
573.7ga) 0.00 574.82 a) -0.01
25/2 22 505.19 1912 So&OS 2312 507.00
505.18a) 0.03 506.09 a) -0.04 s07.02a) 0.00
2112
508.02a)
507.92
-0.01
by R&enthl
and Dailey [ 2j.
0.00
The cor:ection of each f?eqi&ncy for .order e.ffect in the case of-bromobenzene
and Dailey [21.
‘.
646.35 n)
0.04
736.45
the second has the gf-
feet of changing the valuesof xapoutsidethe range ‘. ’ of the experjxnental Sxuracy ijveti by RoSenthaI incj
-..&iley‘.i?)
compirisgn are, listed_.The dquble bond chaiacier 6s sugg&ted by Goldste$ [4]. ... ,: ; -.
2312 17/2
11hr9-+ 1&,o
0.05
_.,
.:;
, Ah&h&
the reiatioh &&ested
.stein is only an’approximate
evaluated as :
128
646.35
1o;g 3 1129
833.25 a) -0.08 835.68 a) -0.02 837.12a) -0.01 751.77 2)
1712
782.86 a) -0.06 784.58 a) -0.01 785.81 a),. -0.02
-2512 22 735.42’
1912 2312 2112
a) Identified
782.86 784.53 785.77
1971
;
’
;
,,
;,;
. .
‘;
by cold-
‘one 143 , ,the percent-
.,-, _’ .’ ,.Y,, ; ..:
.,’
..
..
..
Volume 12, number 1
CHEMICAL PHYSICS LETTJZRS
1 December
1971
:, Table 3 _ CeHsCl(35) transitions. Calculated values determined with A = 5672.95 MHz, B = 1576.787 MHz. C = 1233.674 MHz, =a = -71,OY MHz;, xbb = 38,18 MHz. l?x trYlsitions ye ti = +l transitions; the lo&r F value is only given < frequencies xc in
MHz Transitions
F
Frequencies
IO%%
202 + 303
220
+
321
!/2 312
8 345.57 345.7 11
345:60
512 712
350.16 350.01 I
350.06
8 500.76 513.49 518.67 531.41
s/2
10 514.95 515.81 516.69 517.54
514.91 515.752) 516.73 5t7.573)
s/2 912 312
11 423.38 425.90 430.65 433.17
423.35 2) 425.90a) 430.64 a) 433.18
912
14 396.48
;;;,$a’
712
312 92
712
%I-+ 422
712 11/2 512 a) Identified
Quadrupole
coupling constants
by Poynter
and percentage
@5&robeozene
I*
S/2 312 7/2 l/2
313’414
42 - h.3
~calc.
500.86 513.61 531.49
397.01 400.20 400.73
. 400.19a) 400.69
[ 3 1.
Table 4 double bond cberacter of carbon-halogen benzene. ~4 and xbb are in MHz
bond for chloro-,
Br”bromcbenzene
Jh%OflIO-
benzene
This work
&la xbb ;
XaJ xbb .;
and iodo;
iodo-
berucne Mini and CZlmiIlati [l]
-71.09 38.18 -0.074 (3.26
f t f f
Poynter
[4]
-71.10
bromo-
0.10 0.49 0.015 0.67)s
f 0:50.
558.9 -292.5 -0.046 (2.26
* 1.3 (k 0.5) f 0.004 * 0.19%
Rosenthal
464.1 -242.7 -0.046 (231
f r * *
1.8 0.7 0.007 0.33)%
and Dailey [3]
567-c4 -0.049 f o.ozo (2.5 * IS)%
+00*3 -0.029 * 0.03 1 (1.5 f 1.5EG
-L892.1 c 2.2 976.2 i 1.5 -0.03 1 ?z0.0025 (1.75 *o-L+%
VoJume 12. tiumber 1
CHEMICAL PHYSICS LEtiRS
b&d character is clearly decreasing froin chIoro-‘to bromo- and iodobenzene, as is to be expected. Despite the greater polar&ability of the
.age pf double
Jarger atoms,
the atomic
orbital
a-erhpping
is in fact
rmallergoing’fromch!orine to iodine in view of the C-X bond length w&h is rapidly increasing in the sanie order and in view of the more diffuse nature of the p brbitals in going from chlorine to iodine. The value of 4% double bond character obtained from electric dipole moment 131 .in the case of cblorobenzene is in good agreement with the results obtained in this work from the asymmetry parameter q. Since ti error of about 10% is l~s$ly involved in
1 December
1971
the approximate relation between v and percexitage of double bond charact&, more precise data on the: qtiadrupole c&pLng constants are not nec+$ at this stage: ‘- .a k References [I] A.M.Mirri and W.Caminati, Chem. Phys. Letters 8 (1971) ,409. [2] E.Rqsenthal and +P.Dailey, J. Chem. Phys. 43 (1965) 2093. 133 R.L.Poynter, J. Chem. Phys. 39 (1963) 1962. [4] J.H.Goldstein, J. Chem. Phys. 24 (1956) 106.