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PR Separations and relative Q-branch intensities in infrared band countours of mono-substituted benzenes
Volume 6. number 5
PR
CHEMICAL PHYSICS LETTERS
SEPARATIONS
IN INFRARED
AND
BAND
Department
RELATIVE
CONTOURS
dF
Q-BRANCH
I Eeptember 1970
INTENSITIES
MONO-SUBSTITUTED
V. N. SARIN,. M. M. RAI, H. D. BIST Indian Institute Gf Technology, Kanpur-16,
of Physics,
BENZENES
Xndta
and Department
of Physics,
D. P. KHANDELWAL A?B. TechnoEogicaL Institute,
Kanpur, India
Received 14 May 1970 In this communication we report the PR separations (AU& and relative Q-branch intensities /Itotal) in infrared band envelopes of benzaldehyde, chlorobenzene. phenol-&. phenol-4 and phe49 molecules and compare them with values calculated on the basis of semi-empirical formuIae.
For a linear molecule the PR separation, 6, in a vibration rotation vapour phase band contour at T°K is given under well known approximations r11 by 6 = (aBkT/Jzc)1’2, (1) where B is the rotational constant and other symbols have their usual meanlug. For prolate symmetric top molecules Gerhard and DeMison [Z] deduced the following expressions for PR separation (A-R) and the relative intensity of Q branch (IQ) with respect to that of the whole band (Itotal) for the parallel type bands (2) and
-=IQ Itotal
lop&@
+ (l+@]
- [P/(l+s)]1’2
@/(l+s)ll’z
(j)
where B = (A/B) - 1; and S(8) is given by the empirica1 relation log@@)]
= 0.721/(4 +&13
(4)
For asymmetric top molecules, us&g the work of Badger and.ZumwaIt [3], Seth-Paul and coworkers [4,5] have given general empirical formulae for PR separations of both the parallel and perpendicular, type bands. For the near prolate molecules, which are.our Present interest, these formulae are. AbpRA(,,)
,‘S@)g;
:
(5)
hUpRB(Lj = 5,
(6)
AuPRC(I)
(7)
= (3/2)S(BW,
where 8 and fi are given by same formulae as 6 and S respectively, except for replacing B, in effect, by B = B(BC)/(BcC). However, in accordance with general formulae for near asymmetric top molecules [6] we consider it more desirable to use B = (B+C)/2. For computing the intensity ratio (IQ/&& in A-type bands we use Gerhard and Dennison’s [Z] formula repIacing Sbyi?. The rotational constants for the molecules chosen are available from microwave data [?-lo]. Using these we have deduced the branch separations AuPR for A(a), B(I) and C(l) type bands; and also the ~Q&&l for the A(111bands using our modified s value in formula (3). These cakulated values
are summarized
in table
I. Values
of
Aupp. C(l! are not given in the table since they are just 3/2 times the AVPR A(R) under the appraRimationS used. The vapour -phase infrared spectra
of the five molecules were recorded on a Perkin-Elmer 621 spectrophotometer fitted with a frequency marker accessory and a&o equipped with a 10 meter variable path gas cell. They show a large number of bands ‘all of which could hot be examined for their Avp~ and iQ/ltotaI values due either Jo the complex structure resulting from overtones, binarg combiiions, Fernhi resonances; hot bands etc.; or rjue to their very feebIe intensities - es._ -. 473. _. . - .,: _, ,.,
Volume 6, number5
CHEMSCALPWSICS_LET'TERS
lSeptember1970
Table1 Bohtfonalparam~ters and calcuktedPR separations at300°Kfortl~einfraredbandcontours of mono-substituted benzene8 Molecule
(T&&j
&zj.
&?.j
1564.6
(Id&
1205.6"a)
-’ s
MJP~(~,
S@)
Aycmfmj
_
IQ&otal
C6H5CHO
-5224.6
1385
2.7:'2 1.211
.8.8
10.6
0.180
C6H5C1
5672.950 1576.774 1233.672b) 1405
3.037 1.201
8.8
10.6
0.173
C6D50H .C6H50D
4602.677 2422.815 1596.93Oc) 2010 5603.25 2528.43 1743.15d) 2136
1.330 '1.285 1.626 1.266
10.6 10.9
13.6 13.8
0.232 0.216
C6H50B
5650.450 2619.190 17W.843c)
1.563 1.270
11.1
14.1
0.222
2205
* The rotational constants A, Band Ctabulatedhere are from microwavedata:a)ref.[7];b) ref.[8);c) rsf.PI and d) ref.[lo]. Table2 TheQbandpeaks. observedandcalculat.edPR separations andl /&,talforA(H).B(L)and C(1)bandsoifewprolateasymmetrictopmoecties B (cmY-1) c&&HO
450.3,c
Iii 10.7 10.5
15.9
0.31
-
10.6 10.6
0.18 0.22
0.18 Q.18
10.8 (l071.O),B 8.4 1167.0.A 10.3
10.6 8.8 10.6
0.17 0.18
0.18 0.18
825.3,A 1003.0.A 1024.8,A
C6H5Cl
1313.1,B* 1386.6.B*
10.0 9.8
8.8 8.8
0.18 0.18
-
(1459.4),B
8.8
8.8
-
-
466.9
706.0,A 740.6,C 806.0,A 1025.4,A 1091.6.A 1175.5.A
15 11 15 10.6 10.6 10.5 9.8
1208.0.A 10.0 1232.0,A 11.0 (1270.5),B*-10.0 (129&3).B* 11.0 (1448.0).B* 10.5 1483.5,A 10.8 *indicates thehyhridband:
15.9
0.25
-
10.6 15.9
0.16 0.23
0.17 -
10.6 0.17 10.6 0.17. 10.6 0.18 10.6 0.16
0.17 0.17 0.17 o.i7
10.6 0.17 0.17 10.6 0.19 0.17 8.8 8.8 8.8 10.6 0.18 0.17
C&.OH
C6H5OD
C6H50H
Avp~(crn-1) @&otal obs. talc. obs. cak.
307.4,c
19
336.4,B* 551.3,c 753.7,A
13.8 19
20.4 13.5
20.4 0.31 10.8 0.24 20.4 0.31 13.6 0.22 0.23 10.6 0.20 13.6 0.20 0.23 10.6 0.19 10.6 0.20 20.7 0.26 13.8 0.19
13.2 12.5 21
13.8 13.8 21.2
13.6 19.7 20.3
11.1 21.2 21.2
0.25 0.22 0.24 0.25
20.4 13.0 11.4 13.5 13.3 14.5 22.1
21.2 14.1
0.25 0.19
13.2
1020.8,B 1178.6.A
13.4 12.3
1236.7.B 1372.5,B* 686.7,C
10.2 13.1
917.3.A* 1499.6,A 2699.8.A* 309.6.C 402.9.B* 686.0.c 751.2,C 881.2.C 1025.6,A* 1069.5.B 1343.5.B* 1471.5,,B* 1501.2,A 3655.5,A+
0.20 0.18
0.22 11.1 0.15 11.1 0.15 11.; 0.20 14.1 0.22 0.22 14.1 0.25 0.22
Volume 6, number 5
CHEMICAL
PHYSlCS
pecially in isotopically Iabelled molecules [U-15]. For our present purpose we have chosen only those band&of medium intensity which either exhibit no overlapping with other bands or which could easily be analysed into components. In the latter case we were guided by the typical contours of A, B and C-type bands. The Q positions given in table 2 are usually accurate within f 0.2 cm-l. The P and R maxima are rather broad and hence measurement of PR separation has larger uncertainty. However, in the distinct bands chosen by us the AvpR values were reproducible within f 0.8 cm-l in A and B-type bands and within k 1.5 cm-l in C-type bands. For deducing ZQ/Ztotd, linear absorbance plots were used and Q branch contribution was deduced by a method analogous to that of Franks and Innes [16]. These intensity ratios ar’c reproducible within f 20% The letters A, B and C in front of the Q positions indicate the type of band based on earlier assignments both in phenol and its isotopic species [ll, 121 and a!so in chlorobenzene [13-151. The benzaldehyde assignments will be discussed in detail shortly [l?]. The observed Z /.Ztohl values are given in column 4 for all those B ands which exhibit a Q branch. The calculated values of this ratio are given in column 5 for A-type bands alone, since no formulae are available at present for other bands. For most of the A and B-type bands the calculated and observed PR separations agree within the experimental limitations. In some cases the divergence is, however, large and one must either re-examine the assignment or look for hybridization effects. Those of the A and B-type bands which show large divergence in PR separations have been marked with an asterisk (*) in table 2. On examining their detailed assignments [ll-151 we find that many of these bands pertain to localized modes in which the direction of the change in dipole moment is parallel neither to A nor B axis, and the deviations of Avp~ values would largely be explained by considering the AB hybridization effects [la]. The AYPR deviations in non-localized modes could be used to estimate the direction of the change in effective dipole moment. Present study also indicates that in some cases where the AupR value of band assigned as B-type is close to that expected for an A-type band, the older assignments [12] would need to be re-examined. The band at 1021 cm-l in C8D50H falls in this last category [l2]_ Empirical extension of Dennison and Gerhard formula for ZQ/ZtoM of parallel type bands in symmetrical top tiolecules to the asymmetrical molecules appears to be quite s@isfactory - at
LETTERS
1 September 1970
least in these large molecules with moderate asymmetry. However, the large inherent uncertainty (i 20%) in evaluatiori of ZQ/Ztotd would hardly justify the use of such ratio to deduce the asymmetry parameter K, especially by choosing a solitary band from a complex spectrum [IS]. For C(L) type bands we note that the ratio is larger than for A(s) bands. Fur all Localized modes (e.g. the stretching and bending modes of OH groups in phenols), if one assumes that the change in the magnitude of the dipole moment occurs along the bond for a stretching mode and perpendicular to the bond for a bending mode, all the reported deviations in PR separations are satisfactorily explained. Details will be published elsewhere. Thanks are due to Council Industrial
Research
(India)
of Scientific and for. financial assist-
ance. One of us (H.D.B.) is thankful to Professor 3. C. D. Brand (London, Canada) for useful
discussions.
REFERENCES [I] G. Herzberg. Molecular spectra and molecular
structure. I. Diatomic molecules (Prentice-Hall, New York, 1939). [Z] S. L. Gerhard and D. M. Dennison, Phys. Rev. 43 (l933) 197. [3] R. M-Badger and L. R. Zumwalt. J.Chem. Phys. 6 Q938) 711. [4] W. A. Seth-Paul and G. Dijkstra. Spectrochim. Acta 23A (i967) 3861. [S] W. A-Seth-Paul. J. Mol.Structure 3 (l969) 403. [6] H. C. Allen and P. C. Cross, Molecular vib-rotors (Wiley, New York, 1963). [ 71 T.Kojima. C. R.Quade and C. C. Lin, Bull. Am. Phys.Soc., Ser. II, 7 (1962) 44: L.Bemstein. Numerical data, group 2. vol. 4 (Springer. Berlin, 1967). [E] R. L. Poynter. J. Chem. Phys. 39 (1963) 1962. [9] H. Forest and B. P.Dailey. J. Chem. Phys. 45 (l966) 1736. [lo] T.Kojima. private communication. [ll] H. D. Bisr. J. C. D-Brand and D.R. Williams. J. Mol.Spectry. 21 (1966) 76. [12J EL D. Bist. J. C. D-Brand and D. R.WiLKams, J. Mol.Spectry. 24 (1967) 413. [13] Ii. D. Bisr, V. N. Sarin, A.Ojha and Y. S.Jain. Spectrocbim. Acta. to be published. 1141V. N. Sarin, H. D. Bist and D. P. Khandelwal, Spectrochim.Acta. to be published. (151 Y.S. Jain arid H. D:Bist. Spectrochim.Acta, to be published. [16] L. A. Franks and K. K. tines, J. Chem. Phys. 47 (1967) 863. [1’7] V.N.Sarin and H.D.Bist. unpublfshed results. [18] W. A; Seth-Paul and H. de Meyer. J. Mot. Structurs 3 (1969) 11.
475 :
-_.
_-
. .
PR
CHEMICAL PHYSICS LETTERS
SEPARATIONS
IN INFRARED
AND
BAND
Department
RELATIVE
CONTOURS
dF
Q-BRANCH
I Eeptember 1970
INTENSITIES
MONO-SUBSTITUTED
V. N. SARIN,. M. M. RAI, H. D. BIST Indian Institute Gf Technology, Kanpur-16,
of Physics,
BENZENES
Xndta
and Department
of Physics,
D. P. KHANDELWAL A?B. TechnoEogicaL Institute,
Kanpur, India
Received 14 May 1970 In this communication we report the PR separations (AU& and relative Q-branch intensities /Itotal) in infrared band envelopes of benzaldehyde, chlorobenzene. phenol-&. phenol-4 and phe49 molecules and compare them with values calculated on the basis of semi-empirical formuIae.
For a linear molecule the PR separation, 6, in a vibration rotation vapour phase band contour at T°K is given under well known approximations r11 by 6 = (aBkT/Jzc)1’2, (1) where B is the rotational constant and other symbols have their usual meanlug. For prolate symmetric top molecules Gerhard and DeMison [Z] deduced the following expressions for PR separation (A-R) and the relative intensity of Q branch (IQ) with respect to that of the whole band (Itotal) for the parallel type bands (2) and
-=IQ Itotal
lop&@
+ (l+@]
- [P/(l+s)]1’2
@/(l+s)ll’z
(j)
where B = (A/B) - 1; and S(8) is given by the empirica1 relation log@@)]
= 0.721/(4 +&13
(4)
For asymmetric top molecules, us&g the work of Badger and.ZumwaIt [3], Seth-Paul and coworkers [4,5] have given general empirical formulae for PR separations of both the parallel and perpendicular, type bands. For the near prolate molecules, which are.our Present interest, these formulae are. AbpRA(,,)
,‘S@)g;
:
(5)
hUpRB(Lj = 5,
(6)
AuPRC(I)
(7)
= (3/2)S(BW,
where 8 and fi are given by same formulae as 6 and S respectively, except for replacing B, in effect, by B = B(BC)/(BcC). However, in accordance with general formulae for near asymmetric top molecules [6] we consider it more desirable to use B = (B+C)/2. For computing the intensity ratio (IQ/&& in A-type bands we use Gerhard and Dennison’s [Z] formula repIacing Sbyi?. The rotational constants for the molecules chosen are available from microwave data [?-lo]. Using these we have deduced the branch separations AuPR for A(a), B(I) and C(l) type bands; and also the ~Q&&l for the A(111bands using our modified s value in formula (3). These cakulated values
are summarized
in table
I. Values
of
Aupp. C(l! are not given in the table since they are just 3/2 times the AVPR A(R) under the appraRimationS used. The vapour -phase infrared spectra
of the five molecules were recorded on a Perkin-Elmer 621 spectrophotometer fitted with a frequency marker accessory and a&o equipped with a 10 meter variable path gas cell. They show a large number of bands ‘all of which could hot be examined for their Avp~ and iQ/ltotaI values due either Jo the complex structure resulting from overtones, binarg combiiions, Fernhi resonances; hot bands etc.; or rjue to their very feebIe intensities - es._ -. 473. _. . - .,: _, ,.,
Volume 6, number5
CHEMSCALPWSICS_LET'TERS
lSeptember1970
Table1 Bohtfonalparam~ters and calcuktedPR separations at300°Kfortl~einfraredbandcontours of mono-substituted benzene8 Molecule
(T&&j
&zj.
&?.j
1564.6
(Id&
1205.6"a)
-’ s
MJP~(~,
S@)
Aycmfmj
_
IQ&otal
C6H5CHO
-5224.6
1385
2.7:'2 1.211
.8.8
10.6
0.180
C6H5C1
5672.950 1576.774 1233.672b) 1405
3.037 1.201
8.8
10.6
0.173
C6D50H .C6H50D
4602.677 2422.815 1596.93Oc) 2010 5603.25 2528.43 1743.15d) 2136
1.330 '1.285 1.626 1.266
10.6 10.9
13.6 13.8
0.232 0.216
C6H50B
5650.450 2619.190 17W.843c)
1.563 1.270
11.1
14.1
0.222
2205
* The rotational constants A, Band Ctabulatedhere are from microwavedata:a)ref.[7];b) ref.[8);c) rsf.PI and d) ref.[lo]. Table2 TheQbandpeaks. observedandcalculat.edPR separations andl /&,talforA(H).B(L)and C(1)bandsoifewprolateasymmetrictopmoecties B (cmY-1) c&&HO
450.3,c
Iii 10.7 10.5
15.9
0.31
-
10.6 10.6
0.18 0.22
0.18 Q.18
10.8 (l071.O),B 8.4 1167.0.A 10.3
10.6 8.8 10.6
0.17 0.18
0.18 0.18
825.3,A 1003.0.A 1024.8,A
C6H5Cl
1313.1,B* 1386.6.B*
10.0 9.8
8.8 8.8
0.18 0.18
-
(1459.4),B
8.8
8.8
-
-
466.9
706.0,A 740.6,C 806.0,A 1025.4,A 1091.6.A 1175.5.A
15 11 15 10.6 10.6 10.5 9.8
1208.0.A 10.0 1232.0,A 11.0 (1270.5),B*-10.0 (129&3).B* 11.0 (1448.0).B* 10.5 1483.5,A 10.8 *indicates thehyhridband:
15.9
0.25
-
10.6 15.9
0.16 0.23
0.17 -
10.6 0.17 10.6 0.17. 10.6 0.18 10.6 0.16
0.17 0.17 0.17 o.i7
10.6 0.17 0.17 10.6 0.19 0.17 8.8 8.8 8.8 10.6 0.18 0.17
C&.OH
C6H5OD
C6H50H
Avp~(crn-1) @&otal obs. talc. obs. cak.
307.4,c
19
336.4,B* 551.3,c 753.7,A
13.8 19
20.4 13.5
20.4 0.31 10.8 0.24 20.4 0.31 13.6 0.22 0.23 10.6 0.20 13.6 0.20 0.23 10.6 0.19 10.6 0.20 20.7 0.26 13.8 0.19
13.2 12.5 21
13.8 13.8 21.2
13.6 19.7 20.3
11.1 21.2 21.2
0.25 0.22 0.24 0.25
20.4 13.0 11.4 13.5 13.3 14.5 22.1
21.2 14.1
0.25 0.19
13.2
1020.8,B 1178.6.A
13.4 12.3
1236.7.B 1372.5,B* 686.7,C
10.2 13.1
917.3.A* 1499.6,A 2699.8.A* 309.6.C 402.9.B* 686.0.c 751.2,C 881.2.C 1025.6,A* 1069.5.B 1343.5.B* 1471.5,,B* 1501.2,A 3655.5,A+
0.20 0.18
0.22 11.1 0.15 11.1 0.15 11.; 0.20 14.1 0.22 0.22 14.1 0.25 0.22
Volume 6, number 5
CHEMICAL
PHYSlCS
pecially in isotopically Iabelled molecules [U-15]. For our present purpose we have chosen only those band&of medium intensity which either exhibit no overlapping with other bands or which could easily be analysed into components. In the latter case we were guided by the typical contours of A, B and C-type bands. The Q positions given in table 2 are usually accurate within f 0.2 cm-l. The P and R maxima are rather broad and hence measurement of PR separation has larger uncertainty. However, in the distinct bands chosen by us the AvpR values were reproducible within f 0.8 cm-l in A and B-type bands and within k 1.5 cm-l in C-type bands. For deducing ZQ/Ztotd, linear absorbance plots were used and Q branch contribution was deduced by a method analogous to that of Franks and Innes [16]. These intensity ratios ar’c reproducible within f 20% The letters A, B and C in front of the Q positions indicate the type of band based on earlier assignments both in phenol and its isotopic species [ll, 121 and a!so in chlorobenzene [13-151. The benzaldehyde assignments will be discussed in detail shortly [l?]. The observed Z /.Ztohl values are given in column 4 for all those B ands which exhibit a Q branch. The calculated values of this ratio are given in column 5 for A-type bands alone, since no formulae are available at present for other bands. For most of the A and B-type bands the calculated and observed PR separations agree within the experimental limitations. In some cases the divergence is, however, large and one must either re-examine the assignment or look for hybridization effects. Those of the A and B-type bands which show large divergence in PR separations have been marked with an asterisk (*) in table 2. On examining their detailed assignments [ll-151 we find that many of these bands pertain to localized modes in which the direction of the change in dipole moment is parallel neither to A nor B axis, and the deviations of Avp~ values would largely be explained by considering the AB hybridization effects [la]. The AYPR deviations in non-localized modes could be used to estimate the direction of the change in effective dipole moment. Present study also indicates that in some cases where the AupR value of band assigned as B-type is close to that expected for an A-type band, the older assignments [12] would need to be re-examined. The band at 1021 cm-l in C8D50H falls in this last category [l2]_ Empirical extension of Dennison and Gerhard formula for ZQ/ZtoM of parallel type bands in symmetrical top tiolecules to the asymmetrical molecules appears to be quite s@isfactory - at
LETTERS
1 September 1970
least in these large molecules with moderate asymmetry. However, the large inherent uncertainty (i 20%) in evaluatiori of ZQ/Ztotd would hardly justify the use of such ratio to deduce the asymmetry parameter K, especially by choosing a solitary band from a complex spectrum [IS]. For C(L) type bands we note that the ratio is larger than for A(s) bands. Fur all Localized modes (e.g. the stretching and bending modes of OH groups in phenols), if one assumes that the change in the magnitude of the dipole moment occurs along the bond for a stretching mode and perpendicular to the bond for a bending mode, all the reported deviations in PR separations are satisfactorily explained. Details will be published elsewhere. Thanks are due to Council Industrial
Research
(India)
of Scientific and for. financial assist-
ance. One of us (H.D.B.) is thankful to Professor 3. C. D. Brand (London, Canada) for useful
discussions.
REFERENCES [I] G. Herzberg. Molecular spectra and molecular
structure. I. Diatomic molecules (Prentice-Hall, New York, 1939). [Z] S. L. Gerhard and D. M. Dennison, Phys. Rev. 43 (l933) 197. [3] R. M-Badger and L. R. Zumwalt. J.Chem. Phys. 6 Q938) 711. [4] W. A. Seth-Paul and G. Dijkstra. Spectrochim. Acta 23A (i967) 3861. [S] W. A-Seth-Paul. J. Mol.Structure 3 (l969) 403. [6] H. C. Allen and P. C. Cross, Molecular vib-rotors (Wiley, New York, 1963). [ 71 T.Kojima. C. R.Quade and C. C. Lin, Bull. Am. Phys.Soc., Ser. II, 7 (1962) 44: L.Bemstein. Numerical data, group 2. vol. 4 (Springer. Berlin, 1967). [E] R. L. Poynter. J. Chem. Phys. 39 (1963) 1962. [9] H. Forest and B. P.Dailey. J. Chem. Phys. 45 (l966) 1736. [lo] T.Kojima. private communication. [ll] H. D. Bisr. J. C. D-Brand and D.R. Williams. J. Mol.Spectry. 21 (1966) 76. [12J EL D. Bist. J. C. D-Brand and D. R.WiLKams, J. Mol.Spectry. 24 (1967) 413. [13] Ii. D. Bisr, V. N. Sarin, A.Ojha and Y. S.Jain. Spectrocbim. Acta. to be published. 1141V. N. Sarin, H. D. Bist and D. P. Khandelwal, Spectrochim.Acta. to be published. (151 Y.S. Jain arid H. D:Bist. Spectrochim.Acta, to be published. [16] L. A. Franks and K. K. tines, J. Chem. Phys. 47 (1967) 863. [1’7] V.N.Sarin and H.D.Bist. unpublfshed results. [18] W. A; Seth-Paul and H. de Meyer. J. Mot. Structurs 3 (1969) 11.
475 :
-_.
_-
. .