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Microwave spectra of thionyl fluoride in vibrationally excited states
JOURNAL OF MOLECULAR
SPECTKOSCOP~
Microwave
Spectra
45, 114- 11’) (lYi3 J
of Thionyl Fluoride Excited States
1). 1;. RIMMER, J. G. SMITH, SD
in Vibrationally
D. H. WHIFFEN
Scl~ool of Clremislry, The l’nirlersity, Newcusfle upon Tyne, NJ:‘1 7RLr, United k’ingdo~lt
The microwave spectrum has been observed and rotational constants obtained for the “rjl= 1 and ~‘6= 1 vibrationallp excited states of thionyl fluoride. The perturbation of the C rotational constant is explained in terms of a Coriolis resonance between these levels. I. INTKODUCTIOK
The ground state rotational spectrum of thionyl fluoride was originally reported by Ferguson (1). Later a more complete treatment including centrifugal distortion was given by Lucas and Smith (2) in which the centrifugal distortion constants were used together with other information to obtain a harmonic force field. This paper contains an account of the assignment of rotational transitions belonging to excited vibrational states of F&O and their relevance in obtaining the parameters of the vibrational potential energy function. II. EXPERIMENTAL The sample of thionyl fluoride was purchased from K and K Laboratories Inc., Hollywood, California, and was used without further purification. No strong lines attributable to impurities were found in the spectrum. All spectra were recorded on a frequency modulated source spectrometer employing phase sensitive detection at twice the modulation frequency in order to give a second derivative presentation of the absorption signal. E.M.I. klystrons were used throughout and were phase-locked to Rohde and Schwarz XUC and ND 100 M frequency synthesizers fed by a standard 5 MHz crystal. The latter was checked against the 200 kHz signal from B.B.C. Droitwith. Spectra above 40 GHz were obtained by generating harmonics of the above klystrons. All measurements are thought to be accurate to 100 kHz. III. THEORY Thionyl fluoride is pyramidal, point group C, and has inertial axes in the directions indicated in Fig. 1. It has two low frequency vibrations at 392.5 cm-l and 377.8 cm-‘, which might be expected to have a reasonable population at the temperatures (appros 220’K) at which the microwave spectra were recorded. Pace and Samuelson (3) have assigned these to the A”, ~6, and A’, ~4, fundamentals, respectively. They may approsi114 Copyright
@
lY73 by Academic
AI1 rights of reproduction
Press. Inc.
in any form rc~~r\-~vI.
1 13
mately be described as wagging of the oxygen atom antis! mmetricali>~ to the plane antl the F-S-1; symmetrical bending mode. These two vibrations correlate with the two comIjonents of the degenerate bending mode v4 in the isoelectronic species I’FZ. In the later it tirst order (‘oriolis resonance characterized by { g,JbC exists which splits this vibrational degeneracy. In F60, which has a similar structure (see Fig. l.), an analogous situation exists in that the v4 and &+,vibrations are strongly coupled b!. a Coriolis mechanism characterised by laCc. Symmetry arguments alone also predict a coupling about the A ask, but since FkXI is still fairly close to a symmetric rotor (PI;,) structure for which 1he constant analogous to j-46” 1s . zero by symmetr!- it is perhaps not surprising that the harmonic force field (.?) gives [{4,y4~ = 0.077. The manner in which this Coriolis resonance appears in the rotational spectrum is best appreciated b!. writing the effective rotational constants S, for a given vibrational ;15: s,. = s,. - x tr.-‘--‘-(T,.+ 3) -c. 11) where S,, are the equilibrium rotational constants and the LY;~~~constants express the vibrational dependence of the rotational constants. Nielsen (J) has given expressions for these constants based on second order perturbation theory: (Y-.1.\- = -
2-Y” 1,, ~. I~~~~_~,.~,::vr? + ~h9 I f c wy)z 4 ,’ II.,.‘l” Vb2- v,?
(:_~____-+ .v<,,,,i; ' c-----' I?,,,&~~~ +_,iT I Id\ ,', 1 1I II (, 1.(2) [I[
The first term in this expression is the Coriolis contribution to CY,~.‘,and in the present case since (vd-VG) is onI\- 14.i c111-~we ma\. expect the rZIic term to dominate the SIIIW
mation in the Coriolis contribution to (Y*(‘~’ and cygcf’. i [da” 1 is calculated to be 0.654 so that the C rotational constants in the 2’4= 1 and ~6 = 1 excited states will be strongly affected by this resonance. We may note that the denominator changes sign in considering the contributions to (~4~~and aGcc, so that we may expect the large changes of the C rotational constants from the ground state value to be in opposite senses. Note that since {4oA is so small, the aqAA and &A constants will be largely unaffected and the A rotational constants may be considered nearly free of such Coriolis resonance perturbntions. The R rotational constant is unaffected as {4RHis zero b!- s!mmetr!,.
F,SO has two series of rotational transitions arising from the F,~ and pc components of the dipole moment. The frequencies of the PC transitions are strongly dependent on the A and B rotational constants but are largely independent of the value of C. The FA transitions depend strongly on the C rotational constant. Thus we may expect the PC transitions to be largely unaffected by this Coriolis resonance and hopefully to lie close to the ground state absorptions, whereas the PA transitions will be shifted by a relatively large amount. A preliminary scan in the region of the strong AJ = + 1, bccc type ground state lines at 33 and 51 GHz yielded several likely candidates. Double resonance techniques were then used to prove connectivity of levels since the ground state lines at 33 GHz and 51 GHz exhibit such connections. Apart from yielding assignments, this technique has the advantage of proving common vibrational excitation. The klJ.stron producing the 51 GHz radiation was modulated and was locked at one possible transition frequency, while the unmodulated 33 GHz klystron was swept over regions of likely connected resonances. Signals were observed as a sharp dip in the absorption trace. By assigning the two transitions connected in this fashion to the same transitions as the nearest ground state absorption each double resonance yielded a trial value of .1 and B. Pairs giving similar il and R values were then assigned to the same vibrational state. In this manner the PC transitions of two vibrational levels were readily assigned. In order to determine the more important C rotational constant, the signal klystron was locked on the pLctransition 3.‘.?+ 212 at 50936 MHz while the pump klystron was scanned in the region of 40 GHz. Eventually, a double resonance signal was observed at 40109.99 MHz which was assigned to the p.4, 322 +- 221 transition. This gave the value of the C rotational constant and this assignment was confirmed by observing the double resonance between 322 + 21s and the p.1, 31s +- 21Zat 33592 MHz. The other vibrational level was attacked in a similar fashion, the PA transitions 313 + 21Zand 32.’+ 221 being located at 32773.82 MHz and 39740.7-l MHz, respectivel\-, by double resonance with the 322 +- 212 transition. The use of double resonance in these ways greatly minimized the time and effort required to make a satisfactory assignment. The final least squares refinement of the rotational constants to the observed and assigned lines is given in Table I, while the constants obtained and their errors are given in Table 11. Note that the centrifugal distortion parameters, while not important for such low .I levels, have been constrained to the ground state values (3). Since the two vibrational frequencies V~and vg are onl!, 15 cn~-’ apart, it would IX impossible to distinguish between them on the variation of the intensity of their rot;l-
‘I‘HIOS\~I.
I‘I.I’OKIIJI~
51’1~:(“1‘1<
\Il(‘KO\\‘.\Vb:
\
1 Ii
()HSEHVELI 'I‘K.4NSITION i“REQCENClES \VITH r\SSIGNMEh.TS ASL) k';KKOKSYKCIM CALCULATED \'ALl-ESH.4~1) ON THE B VALUES OF TABLE II AND GKWND STATE CENTRIFW~L
clipole 1?_pr
~~0.00 lJ.05 0.0-k 0.02 - 0.0s O.(l):
.51170.67 32ii3.S? .3974o.i.l 674Y3.JO
0.l.i ~~0.w 0.01 0.05
6iiX8.35
C‘
OTX41.01) Oi842.20 67928. II) x.0.
0.23 O.OT
0.O.i
33673.5-l .13974.90 .Ll241.76 50.52 I .0-l
0.00 11.05
0.04
50866. i3 50936.16 .i 1005.1 1 51372.W 33592.01 40109.94 07382.33 S.O. s.0. 6iX81 Al 68083.92
-0.01
0.02 0.02 0.01 -0.10 0. I i -0.Oi
~~O.Oti O.Oi ~-0.10
67927.3-b
sot ol,scr\ctl
tional spectral
lines with temperature.
with ;I model anharmonic
However,
force field in internal
the anharmonic o(.~~~~ constants
potential
constants
and these predictions
assi~nmcnt
force field (2) together
of the t>-pe used b!, Finn&an
transformation
to normal coordinates,
in Eq. (2). These have been used to calculate that the assignment
of these two particular
excited states to the 13~= 1 and 7~6= 1 levels is correct and that no other is likely is indicated
b!- comparison
with the rest of the predicted
;,(i z
i'l= 1
1
Ground
State
(21
8576.549
f
0.015
8631.524
i
B/MHz
8387.840
+ 0.013
8347.391
-f 0.01 1
8356.951
f
(‘ ‘MHz
4359.261
f
jO22.696
f- 0.013
4952.944
zk 0.001
(listortion
constnnts
0.016 wcrc constrained
0.011
CV,~~.~.
d,~MHZ
’ (‘mrrifuqrl
thr
are given in Table III, together with the observed
From this table we are confident
vibrationall\~
the harmonic
coordinates
cl (11. (5) has been used to yield, via a nonlinear
values.
0.03 -- 0.11
.50863, I 5 .5OSY2.60 .iO92 1 .i5
C’ C’ c
:z
~ 0.01
33i39.84 33935 .X0 341 Ii.10 50611.00
(‘ c C’ C‘ C‘ C’ c (‘ .I .I C’ C’
1.0.
0115 (‘al(~
01,s (MH/
X614.804 z!z 0.001
to the gt~u~~~l st:~te Y:LIUC.S(21.
0.001
Predicted (harmonic) -4.95 - 13.49 -2.75 -34.42 -9.23 -11.30
a~-‘-‘/ MHz &“/MHz afC/MHz
Predicted (harmonic + model anharmanic) terms - 14.68 23.03 4.03 26.31 10.44 16.44
5.49 7.25 5.87
17.3i 9.38 5.61
13.22 4.55 100.16
13.88 -27.14 102.24
4.98 -5.43 3.15
ObS
38.254 f - 30.888 f 93.683 f
0.016 0.014 0.018
55.07 21.82 27.61
- 15.78 -1.04 -90.06
xk 0.012 9.558 f 0.012 -69.750 f 0.014
- 16.720
Due to the unknown effects of anharmonicity we are unable fully to utilize the LY;~-~ constants obtained to derive the Coriolis coupling parameter [4~~. However, if we make the assumption that it is the resonant Coriolis term which dominates alcc and aGcc, then it is possible to obtain approximate values. The approach used is to correct the observed values by the small quantities appearing in Eq. (2) which do not involve ldec, using the harmonic force field and model anharmonicit)to estimate these corrections and then derive {4Ecfrom the resultant. If we write
c (’ cc’ and LYE then the model predicts the surprisingly small values for (~4ilnlillT,ll
t rib&ion of the j-,$6”term Lo CQ’( ;tIId ag ‘:I’ is + 86.53 MHz and - il.12 MHz, rcspec tively. The cxqcc value yields 1{4Ec/ = 0.631, which is in excellent agreement with that predicted from the harmonic force field 1[*ec 1 = 0.654. The agcc value yields / {4ecc = 0.583 which is still fair agreement. The low value in this instance could arise from one of two causes. First, the estimates of the anhnrmonicitj. cannot be regarded as reliable, as is obvious from the observed CI,‘~~values. Second. we must a1waj.s bear in mind in these calculations that the band centers of v.&and V~are not well known and that while the force field determined from the values given in (3) is probabl!. fairI> reliable, the value of IJ~?- vi2 used in the calculation of the cy,cc is ver\- sensitive to thr, prtxke location of the band centers. Indeed the calculations can be inverted to treat (~6 - ~4) as the unknown parameter when a best tit value of 17.5 cm-’ is obtained ii {ae”! is taken to be the force field value 0.65-I. AIlthough the infrared spectrum c.7) favors 14.7 cmP1, the accuracy is not high and the Kaman scattering (.3) of the liquid suggests
19.1 (.111--1.
In all these calculations the ground state structural parameters (2) have been used in I he absence of reliable equilibrium values. l’inall>., wt’ mtv note that this resonance is predicted to be stronger in F2S’“0 as thv VL- VRseparation is predicted Lo be much smaller. It might be possible to obtain more accurate (‘oriolis constants and force field from a stud\. of this molecule.
12 e wish lo thank the Science Research Council for the award oi a studentship to D. 1:. Ii. amI ;L liesearch Assistantship II) J, G. S., as well as fnr a major qnnt for micrrwavr equipment KIXWVI~:
I. 2. 3. +. 5.
K. N. E. H. 1).
July 27, 1072
(‘. J“EKCCSON, J. .4~er. C'henz.SK-. 76, 850 (1954). J. D. Luc.~ AND J. G. SMITH, J. .VoZ.Spectrosc.43, 32i (1072 1, I,. PACE AND H. V. SAMVELSON, J. Ckwz. P//y\.44. 3682 il9601. H. NIELSEN, Re:'.Mod. Ploys.,23, 90 (1951). J. FINNIGAN, -4. ‘2. Cos, A. H. RKITT.AIS, .ANO J. G. SMITH, ./.C'lrcw.SW., l.‘tz~~&~?‘r,r~r.~ II. 68, 548 (1972).
SPECTKOSCOP~
Microwave
Spectra
45, 114- 11’) (lYi3 J
of Thionyl Fluoride Excited States
1). 1;. RIMMER, J. G. SMITH, SD
in Vibrationally
D. H. WHIFFEN
Scl~ool of Clremislry, The l’nirlersity, Newcusfle upon Tyne, NJ:‘1 7RLr, United k’ingdo~lt
The microwave spectrum has been observed and rotational constants obtained for the “rjl= 1 and ~‘6= 1 vibrationallp excited states of thionyl fluoride. The perturbation of the C rotational constant is explained in terms of a Coriolis resonance between these levels. I. INTKODUCTIOK
The ground state rotational spectrum of thionyl fluoride was originally reported by Ferguson (1). Later a more complete treatment including centrifugal distortion was given by Lucas and Smith (2) in which the centrifugal distortion constants were used together with other information to obtain a harmonic force field. This paper contains an account of the assignment of rotational transitions belonging to excited vibrational states of F&O and their relevance in obtaining the parameters of the vibrational potential energy function. II. EXPERIMENTAL The sample of thionyl fluoride was purchased from K and K Laboratories Inc., Hollywood, California, and was used without further purification. No strong lines attributable to impurities were found in the spectrum. All spectra were recorded on a frequency modulated source spectrometer employing phase sensitive detection at twice the modulation frequency in order to give a second derivative presentation of the absorption signal. E.M.I. klystrons were used throughout and were phase-locked to Rohde and Schwarz XUC and ND 100 M frequency synthesizers fed by a standard 5 MHz crystal. The latter was checked against the 200 kHz signal from B.B.C. Droitwith. Spectra above 40 GHz were obtained by generating harmonics of the above klystrons. All measurements are thought to be accurate to 100 kHz. III. THEORY Thionyl fluoride is pyramidal, point group C, and has inertial axes in the directions indicated in Fig. 1. It has two low frequency vibrations at 392.5 cm-l and 377.8 cm-‘, which might be expected to have a reasonable population at the temperatures (appros 220’K) at which the microwave spectra were recorded. Pace and Samuelson (3) have assigned these to the A”, ~6, and A’, ~4, fundamentals, respectively. They may approsi114 Copyright
@
lY73 by Academic
AI1 rights of reproduction
Press. Inc.
in any form rc~~r\-~vI.
1 13
mately be described as wagging of the oxygen atom antis! mmetricali>~ to the plane antl the F-S-1; symmetrical bending mode. These two vibrations correlate with the two comIjonents of the degenerate bending mode v4 in the isoelectronic species I’FZ. In the later it tirst order (‘oriolis resonance characterized by { g,JbC exists which splits this vibrational degeneracy. In F60, which has a similar structure (see Fig. l.), an analogous situation exists in that the v4 and &+,vibrations are strongly coupled b!. a Coriolis mechanism characterised by laCc. Symmetry arguments alone also predict a coupling about the A ask, but since FkXI is still fairly close to a symmetric rotor (PI;,) structure for which 1he constant analogous to j-46” 1s . zero by symmetr!- it is perhaps not surprising that the harmonic force field (.?) gives [{4,y4~ = 0.077. The manner in which this Coriolis resonance appears in the rotational spectrum is best appreciated b!. writing the effective rotational constants S, for a given vibrational ;15: s,. = s,. - x tr.-‘--‘-(T,.+ 3) -c. 11) where S,, are the equilibrium rotational constants and the LY;~~~constants express the vibrational dependence of the rotational constants. Nielsen (J) has given expressions for these constants based on second order perturbation theory: (Y-.1.\- = -
2-Y” 1,, ~. I~~~~_~,.~,::vr? + ~h9 I f c wy)z 4 ,’ II.,.‘l” Vb2- v,?
(:_~____-+ .v<,,,,i; ' c-----' I?,,,&~~~ +_,iT I Id\ ,', 1 1I II (, 1.(2) [I[
The first term in this expression is the Coriolis contribution to CY,~.‘,and in the present case since (vd-VG) is onI\- 14.i c111-~we ma\. expect the rZIic term to dominate the SIIIW
mation in the Coriolis contribution to (Y*(‘~’ and cygcf’. i [da” 1 is calculated to be 0.654 so that the C rotational constants in the 2’4= 1 and ~6 = 1 excited states will be strongly affected by this resonance. We may note that the denominator changes sign in considering the contributions to (~4~~and aGcc, so that we may expect the large changes of the C rotational constants from the ground state value to be in opposite senses. Note that since {4oA is so small, the aqAA and &A constants will be largely unaffected and the A rotational constants may be considered nearly free of such Coriolis resonance perturbntions. The R rotational constant is unaffected as {4RHis zero b!- s!mmetr!,.
F,SO has two series of rotational transitions arising from the F,~ and pc components of the dipole moment. The frequencies of the PC transitions are strongly dependent on the A and B rotational constants but are largely independent of the value of C. The FA transitions depend strongly on the C rotational constant. Thus we may expect the PC transitions to be largely unaffected by this Coriolis resonance and hopefully to lie close to the ground state absorptions, whereas the PA transitions will be shifted by a relatively large amount. A preliminary scan in the region of the strong AJ = + 1, bccc type ground state lines at 33 and 51 GHz yielded several likely candidates. Double resonance techniques were then used to prove connectivity of levels since the ground state lines at 33 GHz and 51 GHz exhibit such connections. Apart from yielding assignments, this technique has the advantage of proving common vibrational excitation. The klJ.stron producing the 51 GHz radiation was modulated and was locked at one possible transition frequency, while the unmodulated 33 GHz klystron was swept over regions of likely connected resonances. Signals were observed as a sharp dip in the absorption trace. By assigning the two transitions connected in this fashion to the same transitions as the nearest ground state absorption each double resonance yielded a trial value of .1 and B. Pairs giving similar il and R values were then assigned to the same vibrational state. In this manner the PC transitions of two vibrational levels were readily assigned. In order to determine the more important C rotational constant, the signal klystron was locked on the pLctransition 3.‘.?+ 212 at 50936 MHz while the pump klystron was scanned in the region of 40 GHz. Eventually, a double resonance signal was observed at 40109.99 MHz which was assigned to the p.4, 322 +- 221 transition. This gave the value of the C rotational constant and this assignment was confirmed by observing the double resonance between 322 + 21s and the p.1, 31s +- 21Zat 33592 MHz. The other vibrational level was attacked in a similar fashion, the PA transitions 313 + 21Zand 32.’+ 221 being located at 32773.82 MHz and 39740.7-l MHz, respectivel\-, by double resonance with the 322 +- 212 transition. The use of double resonance in these ways greatly minimized the time and effort required to make a satisfactory assignment. The final least squares refinement of the rotational constants to the observed and assigned lines is given in Table I, while the constants obtained and their errors are given in Table 11. Note that the centrifugal distortion parameters, while not important for such low .I levels, have been constrained to the ground state values (3). Since the two vibrational frequencies V~and vg are onl!, 15 cn~-’ apart, it would IX impossible to distinguish between them on the variation of the intensity of their rot;l-
‘I‘HIOS\~I.
I‘I.I’OKIIJI~
51’1~:(“1‘1<
\Il(‘KO\\‘.\Vb:
\
1 Ii
()HSEHVELI 'I‘K.4NSITION i“REQCENClES \VITH r\SSIGNMEh.TS ASL) k';KKOKSYKCIM CALCULATED \'ALl-ESH.4~1) ON THE B VALUES OF TABLE II AND GKWND STATE CENTRIFW~L
clipole 1?_pr
~~0.00 lJ.05 0.0-k 0.02 - 0.0s O.(l):
.51170.67 32ii3.S? .3974o.i.l 674Y3.JO
0.l.i ~~0.w 0.01 0.05
6iiX8.35
C‘
OTX41.01) Oi842.20 67928. II) x.0.
0.23 O.OT
0.O.i
33673.5-l .13974.90 .Ll241.76 50.52 I .0-l
0.00 11.05
0.04
50866. i3 50936.16 .i 1005.1 1 51372.W 33592.01 40109.94 07382.33 S.O. s.0. 6iX81 Al 68083.92
-0.01
0.02 0.02 0.01 -0.10 0. I i -0.Oi
~~O.Oti O.Oi ~-0.10
67927.3-b
sot ol,scr\ctl
tional spectral
lines with temperature.
with ;I model anharmonic
However,
force field in internal
the anharmonic o(.~~~~ constants
potential
constants
and these predictions
assi~nmcnt
force field (2) together
of the t>-pe used b!, Finn&an
transformation
to normal coordinates,
in Eq. (2). These have been used to calculate that the assignment
of these two particular
excited states to the 13~= 1 and 7~6= 1 levels is correct and that no other is likely is indicated
b!- comparison
with the rest of the predicted
;,(i z
i'l= 1
1
Ground
State
(21
8576.549
f
0.015
8631.524
i
B/MHz
8387.840
+ 0.013
8347.391
-f 0.01 1
8356.951
f
(‘ ‘MHz
4359.261
f
jO22.696
f- 0.013
4952.944
zk 0.001
(listortion
constnnts
0.016 wcrc constrained
0.011
CV,~~.~.
d,~MHZ
’ (‘mrrifuqrl
thr
are given in Table III, together with the observed
From this table we are confident
vibrationall\~
the harmonic
coordinates
cl (11. (5) has been used to yield, via a nonlinear
values.
0.03 -- 0.11
.50863, I 5 .5OSY2.60 .iO92 1 .i5
C’ C’ c
:z
~ 0.01
33i39.84 33935 .X0 341 Ii.10 50611.00
(‘ c C’ C‘ C‘ C’ c (‘ .I .I C’ C’
1.0.
0115 (‘al(~
01,s (MH/
X614.804 z!z 0.001
to the gt~u~~~l st:~te Y:LIUC.S(21.
0.001
Predicted (harmonic) -4.95 - 13.49 -2.75 -34.42 -9.23 -11.30
a~-‘-‘/ MHz &“/MHz afC/MHz
Predicted (harmonic + model anharmanic) terms - 14.68 23.03 4.03 26.31 10.44 16.44
5.49 7.25 5.87
17.3i 9.38 5.61
13.22 4.55 100.16
13.88 -27.14 102.24
4.98 -5.43 3.15
ObS
38.254 f - 30.888 f 93.683 f
0.016 0.014 0.018
55.07 21.82 27.61
- 15.78 -1.04 -90.06
xk 0.012 9.558 f 0.012 -69.750 f 0.014
- 16.720
Due to the unknown effects of anharmonicity we are unable fully to utilize the LY;~-~ constants obtained to derive the Coriolis coupling parameter [4~~. However, if we make the assumption that it is the resonant Coriolis term which dominates alcc and aGcc, then it is possible to obtain approximate values. The approach used is to correct the observed values by the small quantities appearing in Eq. (2) which do not involve ldec, using the harmonic force field and model anharmonicit)to estimate these corrections and then derive {4Ecfrom the resultant. If we write
c (’ cc’ and LYE then the model predicts the surprisingly small values for (~4ilnlillT,ll
t rib&ion of the j-,$6”term Lo CQ’( ;tIId ag ‘:I’ is + 86.53 MHz and - il.12 MHz, rcspec tively. The cxqcc value yields 1{4Ec/ = 0.631, which is in excellent agreement with that predicted from the harmonic force field 1[*ec 1 = 0.654. The agcc value yields / {4ecc = 0.583 which is still fair agreement. The low value in this instance could arise from one of two causes. First, the estimates of the anhnrmonicitj. cannot be regarded as reliable, as is obvious from the observed CI,‘~~values. Second. we must a1waj.s bear in mind in these calculations that the band centers of v.&and V~are not well known and that while the force field determined from the values given in (3) is probabl!. fairI> reliable, the value of IJ~?- vi2 used in the calculation of the cy,cc is ver\- sensitive to thr, prtxke location of the band centers. Indeed the calculations can be inverted to treat (~6 - ~4) as the unknown parameter when a best tit value of 17.5 cm-’ is obtained ii {ae”! is taken to be the force field value 0.65-I. AIlthough the infrared spectrum c.7) favors 14.7 cmP1, the accuracy is not high and the Kaman scattering (.3) of the liquid suggests
19.1 (.111--1.
In all these calculations the ground state structural parameters (2) have been used in I he absence of reliable equilibrium values. l’inall>., wt’ mtv note that this resonance is predicted to be stronger in F2S’“0 as thv VL- VRseparation is predicted Lo be much smaller. It might be possible to obtain more accurate (‘oriolis constants and force field from a stud\. of this molecule.
12 e wish lo thank the Science Research Council for the award oi a studentship to D. 1:. Ii. amI ;L liesearch Assistantship II) J, G. S., as well as fnr a major qnnt for micrrwavr equipment KIXWVI~:
I. 2. 3. +. 5.
K. N. E. H. 1).
July 27, 1072
(‘. J“EKCCSON, J. .4~er. C'henz.SK-. 76, 850 (1954). J. D. Luc.~ AND J. G. SMITH, J. .VoZ.Spectrosc.43, 32i (1072 1, I,. PACE AND H. V. SAMVELSON, J. Ckwz. P//y\.44. 3682 il9601. H. NIELSEN, Re:'.Mod. Ploys.,23, 90 (1951). J. FINNIGAN, -4. ‘2. Cos, A. H. RKITT.AIS, .ANO J. G. SMITH, ./.C'lrcw.SW., l.‘tz~~&~?‘r,r~r.~ II. 68, 548 (1972).