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The 2644 Å band system of fluorobenzene
Volume 59, number1
CIiEblIC~L PHYSICS LETIYERS
1 November 1978
THE 2644 JkBAND SYSTEM OF FLUOROBENZENE Carl J. SEJXXAR
and Elmer D. LIPP
Depart?nentof chemimy, UniversQ of Cincinnati, Cimhmti, Ohio 45221. CBA
Received26 June 1978 Revisedmanuscriptreceived1 August 1978 A successfulrovibronicband analysisof the ‘B2 flAp electronictransitionin isotopic fluorobenzeneshas been performed.Salientfeaturesof the aaaIysisof the transitionare presentedand includediscussions of prominent sequence bands, hot and cold bands and comparisonof theseresultswith new infmred results.
1. lhtroduction The analysis of the vibrational structure in the 2644 W (‘B2 + ‘Al) transition of fluorobenzene has persisted as an active problem for a period of three decades [l-6]. That fluorobeuzene should not yield to numerous attempts to understand its spectrum is surprising especially since isoelectronic molecules aniline and phenol have been successfully analyzed _ [7-IO] for some time. The content of this communication is a short description of the salient features of the 2644 A system the details of which are to appear in publications to follow_
bands have been found in the spectrum although they could appear on the basis of symmetry arguments. The estimated accuticy of measurements for sharp bands is kO.2 cm -I. Where analogy is possible between the present measurements and the early measurements of Wollman [l] which were taken at much lower resolution, agreement is consistently good (+ 2 cm-l). However, in many instances, Wolhuau’s measurements did not resolve all vibronic bands especially in dense clusters of intensity.
3. Results and discussion 2. Experimental Samples of commercially available fluorobenzene, FC6H5 (Matheson, Coleman and Bell) and pentadeuterofluorobenzene, FC6Ds (Aldrich) were used after distillation. The pdeuterofluorobenzene isotope, FC6H& was prepared [1 11 by a Grignard technique_ A-bsorption spectra were photographed in the 21st to 23rd order of a Jar&l-Ash 3/4 m Ebert spectrograph fitted with a 570~line Harrison grating_ All spectra were obtained on fluorobenzene isotopic vapors at about 23°C and pressure-path-lengths of 0.005-5.0 torr m. Measurements of vibronic bands were made from = 3cm- l/cm dispersion photographic prints calibrated by use of iron/neon hollowcathode lines. Band measurements presented are for the blueedge of the central sba~~ rotational feature [S] for both A- and B-type rotational band envelopes. No C-type
An important factor in the success of the analysis of the 26&I A band system was our recently completed infrared spectroscopy [1 l] on these three isotopes of fluorobenzene. The result is that several fundamental are reassigned and most of the a2 and bl vibrations in all three isotopes were located experimentally. It turns out that activity in non-totally symmetric vibrations accounts for a large fraction of the total intensity iu the II32 + ‘A1 electronic transition- Comparison of the ground state in plane fundamentals obtained from infrared measurements [l l] with those deduced from hot-band structure lying at v’ < &JOin the electronic spectrum is given in table 1. Under the prevaiiing experirren+%rlconditions, hotband intensity is negligible for 3 < 388 -15CO cm-’ and, thus, not all fundamentals can be compared Iuspecdon of table 1 shows that in this rather limited comparison the agreement is excellent. 47
Volume 59, number 1
CHEMICAL
PHYSKS
Tabte 1 Compzuison of ground state fundamentals from UV and LR meanuements (cm-‘, Mode
bz
vacuum)
FC6-D
=6&s
~Gs~S
uv
IR
uv
iR
w
fR
9a l&i 1 12 6a
1238.7 1156.3 1022.4 1008.7 809.9 517.3
1238.4 1156.3 1022.7 1008.6 808.7 517-t
1238.8 1021.2 991.2 808.2 512.4
1239.2 1156.1 1021.6 990.9 806.6 Sf3.2
1173.3 965.1 877.9 820.6 753.6 503.7
1172.4 9639 877.2 819.8 752.5 502.8
3 9b 15 6b 18b
13008 1228.3 1065.9 614.1 403.7
1300-7 1127-7 1066.1 400.4
1033.9 608.1 395.5
i288.9 1093.2 (1036jb)
1034.7 842.9 805.7 5915 385.0
1035.0 843.2 806.7 384.9
72)
at
1 November 1978
LETTERS
&o,b,
‘I Vibration 7a is in Fermi resonance 11f ] with the combination band 1Oa f f6a in both FQHS uted represent the perfzurbed levelsb, AppiOzcimatevalueS..
Sequencebandswithv'- U" = 0 and B-type rotational contour in non-totally symmetric vibrations account for nearly half of the total room temperature intensity in the ’ B2 + ‘At transition. A smah portion of the spectrum of the normaI isotope in the region of the electronic origin, as shown in fig 1, illustrates this point. Principally active modes are I6a(a2), 16b(bl), Il(bl), 18bfb2) and 6b(b,z)- These vibrations also appear in ground and excited electronic
2670
2660 WAVELEXGTH
2650
26-S0
th
Fig. 1. The tmnsmission spectrum of the normal isotope of ffuorobenzene vapor in the region of the efectronk or&~. The dasfied Jine denotes the transmission of the gas ceU &one. 48
and FGE&D_
The values indi-
state combination and afford ready comparison with infrared data. Vibration 11 alone accounts for t.he major fraction of the total sequence band intensity. As shown in table 2 and fig. I sequence bands f I$ up to n = 5 appear with surprisingly high intensity. The ground (l Al) state value of vibration 1I is 248.6 cm-l in the normal isotope. Based on an analogy with isoeiectronic phenol [9, lo] , one fully expects a Her&erg-Teller component in the total intensity of the t B2 + ’ A, transition. While our analysis here is not ftiy complete, it is clear that the Hen&erg-Telier system is present and consists of relatively strong A-type bands in odd quanta of both excited and ground state vibrations 6b, 18b and 14, ail of which are b2 fundamentals. Conditions in the excited ’ B2 electronic state are such that the most prominent cold bands are in totally symmetric (al) vibrations 1,7a, 12 and 18a and these appear in multiple quanta and in combiiation in ail three isotopes_ Many of these bands in the normal isotope were noted by Wolhnan fl] and by others in condensed media [3,4,6] but precise assignments have not been made. There is considerable congestion in the normal isotope in the region too + 920 cm-l_ This problem does not persist in the deuterated isotopes- The two-photon absorption spectrum of the normal isotope [13] reveals the same situation_ Apparently, a large resonance interaction occurs here
Volume 59, number 1
CHEMICAL
Table 2 Sequence bands, Au = 0, and fit in rotational contour
PHYSICS
differences in vibration 11 (bl)
11;
=-d%
n=O
37816.2(;ooj
attached to origin band in cm-’
ZJ-FWW
750_4-
2
684.3
3
619.4
4
554.2
5
488.9
64.8
63.1
783.3
927s
66.1
64.6
62.2
718.6
865.3
64.9
64.0
62.3
654.6
803.0
*
63.9
61.7
590.7
741.3
65.3
63.8 526.9
involving l& but we do not yet fiilly understand the details. As the analysis becomes more complete, the perturbation should become apparent. There remain three pertinent points in the analysis tich might be made in response to recent literature assignments of the vibronic band structure in the 1 B2 + ‘A1 transition. Firstly, the band appearing at 38203.4 cm-l (&JO f 387.2 cm-‘) in the normal isotope has been assigned as 6aA in the literature [4,6] _ Assignment as 6aA requires that the rotational profile of the band be B-type. Our specrra show clearly that the &JO f 387.2 cm-l band has A-type contour and therefore cannot be 6a& The vibronic component must be of b2 symmetry overall- That this band persists in the low temperature absorption spectra Table 3 Vibration
(vacuumj. All bands are B-type
37990.6(&)
37848.1(;ooj
65.2
1978
FW’s
65.8 1
1 November
LE-ITERS
of fluorobcnzene [3,4,6] indicates that lower state in the transition is the vibrationless level of the ground electronic state. The only acceptable assignment of this band is 18bh and this is further substantiated by finding numerous sequence bands 18bi , n = 1,2, perpetuated throughout the entirety of the spectrum. Vibronic activity in 18b homologous to that in the normal isotope may be found in the p-deutero- and pentadeutero-isotopes_ A brief illustration of this homology is indicated in table 3 for the spectral region near the origin (Coo). Secondly, the absorption band appearing at 38228.7 cm-’ (600 + 412.5 cmL1) in the normal isotope is moderately strong and B-type in rotational contour. Its counterparts in the other two isotopes
18b (b2) intensity in the region of the electronic origin Band
Band
Band position (cm-‘,
vacuum)
type =X&i
@G&D
FG&
B
18b;
37799.3 (-16.9jaj
37831.6 (-16.5)
37973.8 (-16.8)
B
lSb?,
37783.3 (-32.9)
-
37957.8 (-32.8)
A
18b:
38186.9 (+370.7)
-
38342.8 (+352.2)
A
18b’:
374125 (403.7)
37446.4 (-401.7)
37605.6 (-385.0)
A
18b:
38203.4 (+387.2)
38234.3 (+386_2)
38358.6 (~368.0)
aj Wavenumbers in parentheses denote band positions relative to ioo_ 49
Volume
59.
numh3t
CHEMICAL PHYSICS LEEERS
1
Table 4
VJhratio~~ 16a (as) JntensJty &Ithe regionof the ekctronic origin-AB handsare B-type 5tIrotationaLcontour Baud
Bandposition(cm--‘, vacuum) FC6Ds
P-FcSJ%D
FC6DS
X6a$
36990-4 (-8’c_S)a)
370220 (-826-l)
37270.5 (-720-X)
lSa$
38228.7 (+412-s)
38262.7 0414.6)
38348.8 0-358-2)
Ifiat
37610.2 (-206.0)
37643.0 (-2051)
37808.5 (-l&2.1)
16a$
374029 (-4123)
374369 (-411-Z)
376294 f-361.2)
1 November 1978
assignment,64 is indeed very w?ak, but the sequence band is strong as expected from the relatively tow value of 6a i? the ground state and from Boltzmarm
considerations, In conclusion, we have performed a successful rotibronic band ailalysisof the ’ B2 c ’ A1 transition in isotopic fbrorobenzenes. Details of the analysis ti appear in subsequent pubkations.
AdmowIe~ment The authors thauk Profaor J-CD- Brand for permitting us to pbofograpb spectra in his laboratory at University of Western Ontario.
a1Wavenumbersin parenthesesdenote handpositionsrelative to zoo_
Refertsnces are 38262.7 cm- J (i& t4f4.6 cm-‘, FCgH4D)ZUId 383488 cm-l (i&, f 3582 CII+, FCBDS)_This band has previously been assigned as a fundamental El,31 _ One assignmentmight appear to be 6ag especially since isoelectronic phenol and aniline 17-101 do indeed have prominent components of ibis sort. However, this assignment is incorrect_ No 6~12 sequence bands calculated on this basis are to be found in any of the three isotopes. The correct assignmentis l&s and sequence bands 16ag support this conclusion in all three isotopes. Table 4 gives a
brief illustration of this relationship for the spectral region near the origin &-& Eastly, tie pkcement of the 6ah band in fluorobenzene is still somewhat in question. ‘Ibat this band should be of miuimai intensity is;not surprising, since it is very weak in the spectrum ofchlorobenzene [13]. A tentative assignment is 6ai = 38275’7 Cm1 (&J + 459.5 cm-“) with 6ai at 38758.8 cm-l (too - 57-4 cmwf) in the normal isotope. Wi$b this
50
J. Chem-Phys. f4 (1946) X23,. ill S-EL4voflmaq, I21 A.M. Bsa and EJ.Sponer,J- Ont. So= Aa 40 (195Oj
389[31 C. Bachiuachand J. Kahane-Mllous,J. Chim.Phys. 62 (1965) 1243. r41 G-V. Klimushew,AF. Prikhotkoand G.M. Soroka, Opt. Spectry.25 (1968) 197. IS1 GJI. Kirby, hfoL Phys-$9 (X970) 289. @I M- Pierre,J- Chea phus. 66 (1977) 3814_ f71 ~.C.D.Brand, D.R.WiBiamsandT_J-&ok, J. Mot Slpectry-20 (1966) 359. Cal LCD. Brand, V_‘F-Jones,BJ. Forrest and R_J_ Pi&k, J- MoL Spectry_39 (1971)352. [91 H-D-Bist,J-CD. Brandand D-R tvilhsms,J. Mol. Speciqr- 21(1966176H-D-B&t,J.CXk BzznrJand D-R. wiliiam+,J_ Ml. lw Spedy. 24 (1967)413s J. MoL Spectry.,to he 1111 E-D. Upp and CJ. SeJiskar, pubBshed_ J. Mol. Spectry.,to he Cl21 R. Vasudevand J.C.D.Braud, published. I131 H-D. Bist,V.N. Sat& k OJaaand Y.S. fain, APpL Spectry.24 (1970) 292-
CIiEblIC~L PHYSICS LETIYERS
1 November 1978
THE 2644 JkBAND SYSTEM OF FLUOROBENZENE Carl J. SEJXXAR
and Elmer D. LIPP
Depart?nentof chemimy, UniversQ of Cincinnati, Cimhmti, Ohio 45221. CBA
Received26 June 1978 Revisedmanuscriptreceived1 August 1978 A successfulrovibronicband analysisof the ‘B2 flAp electronictransitionin isotopic fluorobenzeneshas been performed.Salientfeaturesof the aaaIysisof the transitionare presentedand includediscussions of prominent sequence bands, hot and cold bands and comparisonof theseresultswith new infmred results.
1. lhtroduction The analysis of the vibrational structure in the 2644 W (‘B2 + ‘Al) transition of fluorobenzene has persisted as an active problem for a period of three decades [l-6]. That fluorobeuzene should not yield to numerous attempts to understand its spectrum is surprising especially since isoelectronic molecules aniline and phenol have been successfully analyzed _ [7-IO] for some time. The content of this communication is a short description of the salient features of the 2644 A system the details of which are to appear in publications to follow_
bands have been found in the spectrum although they could appear on the basis of symmetry arguments. The estimated accuticy of measurements for sharp bands is kO.2 cm -I. Where analogy is possible between the present measurements and the early measurements of Wollman [l] which were taken at much lower resolution, agreement is consistently good (+ 2 cm-l). However, in many instances, Wolhuau’s measurements did not resolve all vibronic bands especially in dense clusters of intensity.
3. Results and discussion 2. Experimental Samples of commercially available fluorobenzene, FC6H5 (Matheson, Coleman and Bell) and pentadeuterofluorobenzene, FC6Ds (Aldrich) were used after distillation. The pdeuterofluorobenzene isotope, FC6H& was prepared [1 11 by a Grignard technique_ A-bsorption spectra were photographed in the 21st to 23rd order of a Jar&l-Ash 3/4 m Ebert spectrograph fitted with a 570~line Harrison grating_ All spectra were obtained on fluorobenzene isotopic vapors at about 23°C and pressure-path-lengths of 0.005-5.0 torr m. Measurements of vibronic bands were made from = 3cm- l/cm dispersion photographic prints calibrated by use of iron/neon hollowcathode lines. Band measurements presented are for the blueedge of the central sba~~ rotational feature [S] for both A- and B-type rotational band envelopes. No C-type
An important factor in the success of the analysis of the 26&I A band system was our recently completed infrared spectroscopy [1 l] on these three isotopes of fluorobenzene. The result is that several fundamental are reassigned and most of the a2 and bl vibrations in all three isotopes were located experimentally. It turns out that activity in non-totally symmetric vibrations accounts for a large fraction of the total intensity iu the II32 + ‘A1 electronic transition- Comparison of the ground state in plane fundamentals obtained from infrared measurements [l l] with those deduced from hot-band structure lying at v’ < &JOin the electronic spectrum is given in table 1. Under the prevaiiing experirren+%rlconditions, hotband intensity is negligible for 3 < 388 -15CO cm-’ and, thus, not all fundamentals can be compared Iuspecdon of table 1 shows that in this rather limited comparison the agreement is excellent. 47
Volume 59, number 1
CHEMICAL
PHYSKS
Tabte 1 Compzuison of ground state fundamentals from UV and LR meanuements (cm-‘, Mode
bz
vacuum)
FC6-D
=6&s
~Gs~S
uv
IR
uv
iR
w
fR
9a l&i 1 12 6a
1238.7 1156.3 1022.4 1008.7 809.9 517.3
1238.4 1156.3 1022.7 1008.6 808.7 517-t
1238.8 1021.2 991.2 808.2 512.4
1239.2 1156.1 1021.6 990.9 806.6 Sf3.2
1173.3 965.1 877.9 820.6 753.6 503.7
1172.4 9639 877.2 819.8 752.5 502.8
3 9b 15 6b 18b
13008 1228.3 1065.9 614.1 403.7
1300-7 1127-7 1066.1 400.4
1033.9 608.1 395.5
i288.9 1093.2 (1036jb)
1034.7 842.9 805.7 5915 385.0
1035.0 843.2 806.7 384.9
72)
at
1 November 1978
LETTERS
&o,b,
‘I Vibration 7a is in Fermi resonance 11f ] with the combination band 1Oa f f6a in both FQHS uted represent the perfzurbed levelsb, AppiOzcimatevalueS..
Sequencebandswithv'- U" = 0 and B-type rotational contour in non-totally symmetric vibrations account for nearly half of the total room temperature intensity in the ’ B2 + ‘At transition. A smah portion of the spectrum of the normaI isotope in the region of the electronic origin, as shown in fig 1, illustrates this point. Principally active modes are I6a(a2), 16b(bl), Il(bl), 18bfb2) and 6b(b,z)- These vibrations also appear in ground and excited electronic
2670
2660 WAVELEXGTH
2650
26-S0
th
Fig. 1. The tmnsmission spectrum of the normal isotope of ffuorobenzene vapor in the region of the efectronk or&~. The dasfied Jine denotes the transmission of the gas ceU &one. 48
and FGE&D_
The values indi-
state combination and afford ready comparison with infrared data. Vibration 11 alone accounts for t.he major fraction of the total sequence band intensity. As shown in table 2 and fig. I sequence bands f I$ up to n = 5 appear with surprisingly high intensity. The ground (l Al) state value of vibration 1I is 248.6 cm-l in the normal isotope. Based on an analogy with isoeiectronic phenol [9, lo] , one fully expects a Her&erg-Teller component in the total intensity of the t B2 + ’ A, transition. While our analysis here is not ftiy complete, it is clear that the Hen&erg-Telier system is present and consists of relatively strong A-type bands in odd quanta of both excited and ground state vibrations 6b, 18b and 14, ail of which are b2 fundamentals. Conditions in the excited ’ B2 electronic state are such that the most prominent cold bands are in totally symmetric (al) vibrations 1,7a, 12 and 18a and these appear in multiple quanta and in combiiation in ail three isotopes_ Many of these bands in the normal isotope were noted by Wolhnan fl] and by others in condensed media [3,4,6] but precise assignments have not been made. There is considerable congestion in the normal isotope in the region too + 920 cm-l_ This problem does not persist in the deuterated isotopes- The two-photon absorption spectrum of the normal isotope [13] reveals the same situation_ Apparently, a large resonance interaction occurs here
Volume 59, number 1
CHEMICAL
Table 2 Sequence bands, Au = 0, and fit in rotational contour
PHYSICS
differences in vibration 11 (bl)
11;
=-d%
n=O
37816.2(;ooj
attached to origin band in cm-’
ZJ-FWW
750_4-
2
684.3
3
619.4
4
554.2
5
488.9
64.8
63.1
783.3
927s
66.1
64.6
62.2
718.6
865.3
64.9
64.0
62.3
654.6
803.0
*
63.9
61.7
590.7
741.3
65.3
63.8 526.9
involving l& but we do not yet fiilly understand the details. As the analysis becomes more complete, the perturbation should become apparent. There remain three pertinent points in the analysis tich might be made in response to recent literature assignments of the vibronic band structure in the 1 B2 + ‘A1 transition. Firstly, the band appearing at 38203.4 cm-l (&JO f 387.2 cm-‘) in the normal isotope has been assigned as 6aA in the literature [4,6] _ Assignment as 6aA requires that the rotational profile of the band be B-type. Our specrra show clearly that the &JO f 387.2 cm-l band has A-type contour and therefore cannot be 6a& The vibronic component must be of b2 symmetry overall- That this band persists in the low temperature absorption spectra Table 3 Vibration
(vacuumj. All bands are B-type
37990.6(&)
37848.1(;ooj
65.2
1978
FW’s
65.8 1
1 November
LE-ITERS
of fluorobcnzene [3,4,6] indicates that lower state in the transition is the vibrationless level of the ground electronic state. The only acceptable assignment of this band is 18bh and this is further substantiated by finding numerous sequence bands 18bi , n = 1,2, perpetuated throughout the entirety of the spectrum. Vibronic activity in 18b homologous to that in the normal isotope may be found in the p-deutero- and pentadeutero-isotopes_ A brief illustration of this homology is indicated in table 3 for the spectral region near the origin (Coo). Secondly, the absorption band appearing at 38228.7 cm-’ (600 + 412.5 cmL1) in the normal isotope is moderately strong and B-type in rotational contour. Its counterparts in the other two isotopes
18b (b2) intensity in the region of the electronic origin Band
Band
Band position (cm-‘,
vacuum)
type =X&i
@G&D
FG&
B
18b;
37799.3 (-16.9jaj
37831.6 (-16.5)
37973.8 (-16.8)
B
lSb?,
37783.3 (-32.9)
-
37957.8 (-32.8)
A
18b:
38186.9 (+370.7)
-
38342.8 (+352.2)
A
18b’:
374125 (403.7)
37446.4 (-401.7)
37605.6 (-385.0)
A
18b:
38203.4 (+387.2)
38234.3 (+386_2)
38358.6 (~368.0)
aj Wavenumbers in parentheses denote band positions relative to ioo_ 49
Volume
59.
numh3t
CHEMICAL PHYSICS LEEERS
1
Table 4
VJhratio~~ 16a (as) JntensJty &Ithe regionof the ekctronic origin-AB handsare B-type 5tIrotationaLcontour Baud
Bandposition(cm--‘, vacuum) FC6Ds
P-FcSJ%D
FC6DS
X6a$
36990-4 (-8’c_S)a)
370220 (-826-l)
37270.5 (-720-X)
lSa$
38228.7 (+412-s)
38262.7 0414.6)
38348.8 0-358-2)
Ifiat
37610.2 (-206.0)
37643.0 (-2051)
37808.5 (-l&2.1)
16a$
374029 (-4123)
374369 (-411-Z)
376294 f-361.2)
1 November 1978
assignment,64 is indeed very w?ak, but the sequence band is strong as expected from the relatively tow value of 6a i? the ground state and from Boltzmarm
considerations, In conclusion, we have performed a successful rotibronic band ailalysisof the ’ B2 c ’ A1 transition in isotopic fbrorobenzenes. Details of the analysis ti appear in subsequent pubkations.
AdmowIe~ment The authors thauk Profaor J-CD- Brand for permitting us to pbofograpb spectra in his laboratory at University of Western Ontario.
a1Wavenumbersin parenthesesdenote handpositionsrelative to zoo_
Refertsnces are 38262.7 cm- J (i& t4f4.6 cm-‘, FCgH4D)ZUId 383488 cm-l (i&, f 3582 CII+, FCBDS)_This band has previously been assigned as a fundamental El,31 _ One assignmentmight appear to be 6ag especially since isoelectronic phenol and aniline 17-101 do indeed have prominent components of ibis sort. However, this assignment is incorrect_ No 6~12 sequence bands calculated on this basis are to be found in any of the three isotopes. The correct assignmentis l&s and sequence bands 16ag support this conclusion in all three isotopes. Table 4 gives a
brief illustration of this relationship for the spectral region near the origin &-& Eastly, tie pkcement of the 6ah band in fluorobenzene is still somewhat in question. ‘Ibat this band should be of miuimai intensity is;not surprising, since it is very weak in the spectrum ofchlorobenzene [13]. A tentative assignment is 6ai = 38275’7 Cm1 (&J + 459.5 cm-“) with 6ai at 38758.8 cm-l (too - 57-4 cmwf) in the normal isotope. Wi$b this
50
J. Chem-Phys. f4 (1946) X23,. ill S-EL4voflmaq, I21 A.M. Bsa and EJ.Sponer,J- Ont. So= Aa 40 (195Oj
389[31 C. Bachiuachand J. Kahane-Mllous,J. Chim.Phys. 62 (1965) 1243. r41 G-V. Klimushew,AF. Prikhotkoand G.M. Soroka, Opt. Spectry.25 (1968) 197. IS1 GJI. Kirby, hfoL Phys-$9 (X970) 289. @I M- Pierre,J- Chea phus. 66 (1977) 3814_ f71 ~.C.D.Brand, D.R.WiBiamsandT_J-&ok, J. Mot Slpectry-20 (1966) 359. Cal LCD. Brand, V_‘F-Jones,BJ. Forrest and R_J_ Pi&k, J- MoL Spectry_39 (1971)352. [91 H-D-Bist,J-CD. Brandand D-R tvilhsms,J. Mol. Speciqr- 21(1966176H-D-B&t,J.CXk BzznrJand D-R. wiliiam+,J_ Ml. lw Spedy. 24 (1967)413s J. MoL Spectry.,to he 1111 E-D. Upp and CJ. SeJiskar, pubBshed_ J. Mol. Spectry.,to he Cl21 R. Vasudevand J.C.D.Braud, published. I131 H-D. Bist,V.N. Sat& k OJaaand Y.S. fain, APpL Spectry.24 (1970) 292-