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Millimeter and submillimeter-wave spectrum of CHCl3. Determination of the h3 splitting constant
Volume 203, number 2,3
CHEMICAL PHYSICS LETTERS
19 February 1993
Millimeter and submillimeter-wave spectrum of CHC13. Determination of the h, splitting constant G. Cazzoli, G. Cotti and L. Dore Dipartimento di Chimica “G. Ciamician” dell’liniversitb di Bologna, via Selmi 2, 40126 Bologna, Italy
Received 18 November 1992
The millimeter and submillimeter-wave spectrum of the ground state of CHJSCl, has been observed and analyzed up to J= 106. Theresultingspectroscopicconstantsare(inMHz):~=3302.07587(12),0,=1.511716(66)~10~~,D,,=-2.51757(20)~10-~, L,=-0.178(55)x10-14. The millimeter-wave H~~~=O.l268(12)~10-*, HJJK= -0.5000(30)~10-‘, H~=O.652(11)~10-*, spectrum of CH”C1, has also been observed and analyzed providing the following values of the rotational constants (in MHz): B,=3l29.6loO7(57),D,=1.36571(64)x1O-3,D,,~-2.2769(51)x1O-’,H,,,=O.l35(2O)x1O-8,H,,~=-O.78(l8)XlO-g, H,=O.283(49) X lo-‘. The splitting of the K= 3 ground state lines of CH”C13has been observed starting from the 5~46-45 transition and the value of the splitting constant h, is determined to be 0.15007(25) x 1O-gMHz.
1. Introduction
The rotational spectrum of chloroform (CHC&) has been investigated in the centimeter- and millimeter-wave region [ l-71 but, to our knowledge, an accurate centrifugal distortion analysis has not been performed up to now (see table 1 below). Moreover this molecule provides a good chance to observe the K= 3 line splitting predicted by Watson [ 81, since the rotational transitions with values of J suitable to show the splitting occur in a frequency region covered by the apparatus available in our laboratory, and in addition because the DJKcentrifugal distortion constant produces a K structure adequately resolved to allow the observation of this K= 3 line splitting. The presence of this kind of splitting has recently been observed in our laboratory [ 9 ] in the rotational spectrum of trichloroacetonitrile.
[ lo]. The measurements in the 700 GHz frequency region have been performed using a sideband FIR laser spectrometer described elsewhere [ 111. Briefly, the radiation from a microwave source (2-l 8 GHz), which is phase locked to a computer-controlled frequency synthesizer, is mixed with the radiation from an optically pumped FIR laser using a comer cube mixer with a GaAs Schottky barrier diode. A polarizing Michelson interferometer has been used to adjust the coupling of the radiation onto the mixing diode and to separate the tunable sideband from the carrier. A more efficient separation of the sideband is performed by using a tunable Fabry-Perot interferometer. The detector is a liquid He cooled silicon bolometer. Both spectrometers are equipped with a glass free-space absorption cell.
3. Results 2. Experimental details The rotational spectrum of CHC& has been observed in the millimeter-wave region ( 120-430 GHz) by using a computer controlled spectrometer whose radiation source is a klystron-driven frequency multiplier. This spectrometer has been described in ref.
Tables 1 and 2 report the experimentally measured transition frequencies and the differences between the observed and calculated frequency for CH3%13 and CH3’C13. The investigation of the CH3’C1, ground state rotational spectrum has been limited to the millimeter-wave region due to the
0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
227
Table 1 Observed frequencies and residuals of CH”CIj Transition
J
K
18 18 18 18 18 18 18 18 27 21 27 27 27 27 27 27 27 27 27 27 27 27 27 36 36 36 36 36 36 36 36 36 36 36 36 36 36 36 36 36 36 36 36 36 36 36 36 45 45 45 45 45 45 45 45 45
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 2 2 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3, 3_ 4 6 12 18
Frequency
Obs. - talc.
(MHz)
(MHz)
Transition
‘) ‘) 0.001 0.000 -0.004 -0.011
0.000 0.001 a) n’ - 0.006 -0.007 -0.008 - 0.008
184788.695 184790.521 184792.629 184795.025 184797,693 184800.644 184803.900 184807.369 184811.199 244047.849 ‘) 244048.034 ‘) 244048.618 244049.541 244050.822 244052.496 244054.534 244056.932 244059.707 244062.849 244066.369 244070.249 244074.512 244079.166 244084.200
- 0.004 - 0.005 - 0.006 0.000 - 0.004 - 0.006 0.015 - 0.032 0.000
244089.604 244095.345 244101.455 244107.947 244114.796 244122.075 244129.653 244137.636 244145.969 303203.963 a) 303204.193 a) 303204.874 303205.845 a) 303206.2 15 I) 303207.641 303212.226 303237.055 303278.424
0.041 0.031 0.018 0.015 - 0.004 0.034 -0.001 -0.005 -0.032
0.028 0.025 0.009 0.015 0.014 0.003 - 0.002 -0.011 -0.013 - 0.026 -0.028 -0.010 0.017
-0.008
0.003 -0.006 0.007 -0.015
Ohs.-talc.
(MHz)
(MHz)
303336.428 303411.174 362226.106 ‘) 362226.379 a) 362221.194 362228.116 ‘) 362229.021 ‘) 362230.479 362235.947 362265.481 362314.832 362383.941 362472.937 362581.897 427618.000”’ 427618.322 *) 427619.305 427619.853 ‘) 427621.940 a) 427623.147 427629.590 427664.354 427722.355 427803.638 427908.300 428036.450 428188.285 680120.57 a) 680121.07 n)
-0.024 0.022
K
J 125437.426 125437.522 125437.809 125438.286 125438.951 125439.804 125440.866 125442.109 184783.639 184783.779 184784.195 184784.897 184785.879 184787.144
Frequency
45 45 54 54 54
24 30 0 1 2
54 54 54 54 54 54 54 54 54 64 64 64 64 64 64 64 64 64 64 64 64 64 103 103 103 103 103 103 103 103 106 106 106 106 106 106 106 106 106 106 106 106 106 106 106 106 106 106 106 106
3, 3_ 4 6 12 18 24 30 36 0 1 2 3, 3_ 4 6 12 18 24 30 36 42 0 1 2 3, 3_ 4 5 6 0 1 2 3, 3_ 4 5 6 7 8 9 10 11 12 21 27 33 39 45 48
680122.93 680114.13”’ 680136.03 a) 680128.38 680132.94 680138.55 699340.98 a) 699341.50 ‘) 699342.82 699332.99 ‘) 699358.23 ” 699348.39 699353.15 699359.50 a) 699366.30 699373.90 699382.70 699392.60 699403.50 699415.20 699568.29 6997 16.69 699902.7 1 700126.50 700388.45 700533.10
-0.006
-0.004 -0.009 -0.033 0.003 0.007 0.005 -0.006
0.018
- 0.002 0.004 -0.002 0.005 0.004 0.000 -0.016 0.006
0.358
-0.206 -0.157 -0.061
-0.221
-0.822 -0.691 0.114 -0.002 0.052 0.176 0.270 0.134 0.246 0.068 0.041 0.097 0.365 0.05 I
Volume 203, number 2,3
CHEMICAL PHYSICS LETTERS
Table 2 Observed frequencies and residuals of CH”Cl,
Frequency (MHz)
Transition
J
Obs. - talc. (MHz)
K 1 1 2 3 4 5 6
19 19 19 19 19 19 19 19 29 29 29 29 29 29 29 29 29 29 29 29 39 39 39 39 39
125140.726 ‘) 125140.817 a)
1 0
1 4 5 6 7 8 9
10 11 12 13 0 1 2 3 4
39 39 39 39 39 39 39
5 6 1 8
-0.013 0.004 0.001 0.011 0.003 -0.008
187631.473 187632.700 187634.196 187635.957 187638.009 187640.318 187642.907 187645.768 187648.880 187652.298 250020.016 ‘) 250020.197 =) 250020.718 250021.630 250022.89 1
-0.005 -0.001 0.000 -0.006 0.007 0.004 0.007 0.009 -0.012 -0.002
250024.516 250026.510 250028.878 250031.576 250038.066 250045.992 250060.7 I 1 250078.561 250099.938
10 12 15 18 21
39 39
125141.077 125341.548 125142.181 125143.009 125144.001 125145.172 187629.305 a) 187629.441 a)
- 0.020 -0.009 - 0.009 - 0.007 0.004 0.027 0.019 0.010 -0.016 0.042 - 0.068 0.027
19 February 1993
weakness of its spectrum when observed in natural abundance. Table 3 gives the results obtained by a least-squares analysis of the observed spectra of the two isotopomers, compared with those available in literature. The fitting procedure does not include the K=O, 1 transitions since these are not resolved, and the K= 3 transitions with J higher than 39 for the reasons that will be discussed later. In the fitting procedure the assigned line frequencies were weighted according to the inverse square of the experimental uncertainty which is found to be i 15 kHz for the spectrometer with klystron-driven frequency multiplier, and + 500 kHz for the side-band spectrometer. Accurate values of the rotational constant Be and of the quartic and sextic centrifugal distortion constants have been obtained for CH35C13and, as regards CH3’C13,the values of the centrifugal distortion constants have been determined for the first time. The standard deviations of the tit are 0.15 1 and 0.021 MHz for CHS5Cljand CH3’ClJ, respectively. The higher value for the first isotopomer is due to the higher uncertainty of those fitted experimental lines frequencies which are obtained by using the side-band laser FIR spectrometer. In trigonal symmetric-top molecules higher-order vibration-rotational interaction may split the otherwise degenerate Al and A2 components of the level with K=3n [ 81. This effect is dominant for K=3, and in the first-order approximation the splitting is due to: E_3,3= +h,J(Jt
l)[J(Jt
l)-21
x[J(J+l)-61.
(1)
‘) Predicted value. Table 3
Spectroscopic constantsof CHWl,and
CHW,
cH~scl 3
Bo(MHz) n, (kHz) &K (k=) hJJ
(Hz
)
HJJK (Hz ) &KJ (Hz) 4 @Hz 1
CH”C1 3
this work
ref. [5]
this work
ref. [4]
3302.07587( 12) 1.511716(66) -2.51757(20) 0.001268(11) -0*005000( 30) 0.00652( 11) -0.178(55)x lO-5
3302.083(3) 1.52(l) -2.50( 2)
3129.61007(57) 1.36571(64) -2.27688(51) 0.00135(20) -0.0078(18) 0.0283(49)
3129.54
u(unweighted)=0.151
a=0.02 1
229
Volume 203, number 2,3
Jsl IKI=
CHEMICAL PHYSICS LETTERS
3
19 February 1993
AA-F
J=40-39 250.019 GHz MHZ
J=46-45
303.203 GHz
A-
MHZ
A+ b, =2h,J(J+l)[J(Jtl)-Z][J(J+l)-61
u_-Vt=dv,,Av,
J=55-54 362.2'25GHz MHZ
Fig. 1. Energy level diagram of nondegenerate K=3 states of a trigonal symmetric top molecule.
An energy levels diagram showing the frequency splitting of a generic rotational transition Jt 14, 1KI = 3 is shown in fig. 1, where, by convention, the positive sign of hSincrease the energy of the A_ level while the negative one reduces that of the A+. Since the constant h3 depends on the cubic potential constants so that its magnitude is comparable to that of the sextic centrifugal distortion constant HJJJ, these energy level splittings are usually negligible. Moreover the frequency difference between the two LU= 1, K= 3 rotational transitions, labelled V+and u+ in fig. 1, depends, in agreement with the rotational selection rules, on the difference between the energy splittings of the J+ 1 and Jlevels. This occurrence clearly makes it more difficult to obtain experimental evidence of this splitting by means of rotational spectroscopy. For these two reasons no splitting of the K= 3 rotational transitions is usually observed unless the Jvalue is sufficiently high to show it. As concerns CH3’C13,this effect has been observed starting from J= 45. Fig. 2 shows the recorded spectra of six transitions of CH3’C15with J values included between 39 and 106. The J=40-39, K= 3 transition is unsplit, while all the K= 3 transitions with higher values of J show the splitting. A least-squares analysis of the observed splitting with the calculated values obtained by using eq. ( 1) gives a value of h3=0.00015007(25 ) Hz. Since the K=6 and K=3_ components of the J= 107-106 230
J=65-64 427.617 GHr
1.00
_I K
3,
Ok12
4
5
6
J=107-106 699.329 GHz 000 0.00
1O.C.J
20.00
moo
MHZ
Fig. 2. Recorded ground state rotational transitions of CH”CII showing the K= 3 line splitting at increasing J.
transition overlap each other and it was impossible to resolve them, it has been decided to exclude from the lit the K=3 splitting of this transition. Table 4 reports the results of the fit. An independent analysis of the splittings has been preferred to the possibility of adding the h3 constant to the parameters of the line frequencies fitting procedure, since the side-band spectrometer presents an uncertainty in the measurement of the absolute value of a line frequency of + 500 kHz, while in a recorded spectrum the frequency difference between two lines is accurate within * 5 kHz. In fact, as pointed out in ref. [ 111, the short time (few minutes) frequency
Volume 203, number 2,3
CHEMICAL PHYSICS LETTERS
Acknowledgement
Table 4 Splitting of the K= 3 rotational transitions of CHCIJ Splitting (MHz)
Transition I
J”
K
obs.
obs. - talc.
46 55 65 104
45 54 64 103
3 3 3 3
0.419 0.937 2.109 21.895
0.049 0.032 0.022 - 0.004
hl=0.15007(25)x
19 February 1993
We would like to thank Mr. G. Tasini for technical assistance. This work was financially supported by the Minister0 della Universith e della Ricerca Scientifica e Tecnologica (MURST 40% and 60%) and by CNR.
10-3Hz
stability of the free running side-band laser spectrometer source is of the order of 5 x 10w9while the uncertainty of the laser absolute frequency, due to its broad gain profile, is orders of magnitude higher. This is not particularly inconvenient if, as in the present situation, the aim of the experiment is to measure the frequency splitting of an absorption line.
4. Conclusions An accurate centrifugal distortion analysis of the millimeter and submillimeter-wave spectrum of CH35C13and of the millimeter-wave spectrum of CH3’C13has been performed. The observation of K= 3 rotational transitions, which are split because of higher-order vibration-rotational interaction, allowed the determination of the hXsplitting constant.
[ I ] P. Kisliuk and C.H. Townes, NBS Circular 518 ( US GPO, Washington, 1952). [2] P.N. Wolfe, J. Chem. Phys. 25 (1956) 976. 131M.W. Long, J, Chem. Phys. 32 (1960) 948L. [4] M. Jen and D.R. Lide, J. Chem. Phys. 36 (1962) 2525. [ 51P.G. Favero and A.M. Mirri, Nuovo Cimento 30 ( 1963) 502. [6] P.B. Reinhart, Q. Williams and T.L. Weatherly, J. Chem. Phys. 53 (1970) 1418. [ 71 D.B. McLay and G. Winnewisser, J. Mol. Spectry. 44 ( 1972) 32. [8] J.K. Watson, in: Vibrational spectra and structure, Vol. 6, ed. J. Durig (Elsevier, Amsterdam, 1977) p. 1. [9] G. Cazzoli and Z. Kisiel, J. Mol. Spectry. submitted for publication. [lo] G. Cazzoli and L. Dore, J. Mol. Spectry. 141 (1990) 49. [ 111G. Cazzoli, L. Cludi, C. Degli Esposti and L. Dore, J. Mol. Spectry. 152 (1992) 185.
231
CHEMICAL PHYSICS LETTERS
19 February 1993
Millimeter and submillimeter-wave spectrum of CHC13. Determination of the h, splitting constant G. Cazzoli, G. Cotti and L. Dore Dipartimento di Chimica “G. Ciamician” dell’liniversitb di Bologna, via Selmi 2, 40126 Bologna, Italy
Received 18 November 1992
The millimeter and submillimeter-wave spectrum of the ground state of CHJSCl, has been observed and analyzed up to J= 106. Theresultingspectroscopicconstantsare(inMHz):~=3302.07587(12),0,=1.511716(66)~10~~,D,,=-2.51757(20)~10-~, L,=-0.178(55)x10-14. The millimeter-wave H~~~=O.l268(12)~10-*, HJJK= -0.5000(30)~10-‘, H~=O.652(11)~10-*, spectrum of CH”C1, has also been observed and analyzed providing the following values of the rotational constants (in MHz): B,=3l29.6loO7(57),D,=1.36571(64)x1O-3,D,,~-2.2769(51)x1O-’,H,,,=O.l35(2O)x1O-8,H,,~=-O.78(l8)XlO-g, H,=O.283(49) X lo-‘. The splitting of the K= 3 ground state lines of CH”C13has been observed starting from the 5~46-45 transition and the value of the splitting constant h, is determined to be 0.15007(25) x 1O-gMHz.
1. Introduction
The rotational spectrum of chloroform (CHC&) has been investigated in the centimeter- and millimeter-wave region [ l-71 but, to our knowledge, an accurate centrifugal distortion analysis has not been performed up to now (see table 1 below). Moreover this molecule provides a good chance to observe the K= 3 line splitting predicted by Watson [ 81, since the rotational transitions with values of J suitable to show the splitting occur in a frequency region covered by the apparatus available in our laboratory, and in addition because the DJKcentrifugal distortion constant produces a K structure adequately resolved to allow the observation of this K= 3 line splitting. The presence of this kind of splitting has recently been observed in our laboratory [ 9 ] in the rotational spectrum of trichloroacetonitrile.
[ lo]. The measurements in the 700 GHz frequency region have been performed using a sideband FIR laser spectrometer described elsewhere [ 111. Briefly, the radiation from a microwave source (2-l 8 GHz), which is phase locked to a computer-controlled frequency synthesizer, is mixed with the radiation from an optically pumped FIR laser using a comer cube mixer with a GaAs Schottky barrier diode. A polarizing Michelson interferometer has been used to adjust the coupling of the radiation onto the mixing diode and to separate the tunable sideband from the carrier. A more efficient separation of the sideband is performed by using a tunable Fabry-Perot interferometer. The detector is a liquid He cooled silicon bolometer. Both spectrometers are equipped with a glass free-space absorption cell.
3. Results 2. Experimental details The rotational spectrum of CHC& has been observed in the millimeter-wave region ( 120-430 GHz) by using a computer controlled spectrometer whose radiation source is a klystron-driven frequency multiplier. This spectrometer has been described in ref.
Tables 1 and 2 report the experimentally measured transition frequencies and the differences between the observed and calculated frequency for CH3%13 and CH3’C13. The investigation of the CH3’C1, ground state rotational spectrum has been limited to the millimeter-wave region due to the
0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
227
Table 1 Observed frequencies and residuals of CH”CIj Transition
J
K
18 18 18 18 18 18 18 18 27 21 27 27 27 27 27 27 27 27 27 27 27 27 27 36 36 36 36 36 36 36 36 36 36 36 36 36 36 36 36 36 36 36 36 36 36 36 36 45 45 45 45 45 45 45 45 45
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 2 2 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3, 3_ 4 6 12 18
Frequency
Obs. - talc.
(MHz)
(MHz)
Transition
‘) ‘) 0.001 0.000 -0.004 -0.011
0.000 0.001 a) n’ - 0.006 -0.007 -0.008 - 0.008
184788.695 184790.521 184792.629 184795.025 184797,693 184800.644 184803.900 184807.369 184811.199 244047.849 ‘) 244048.034 ‘) 244048.618 244049.541 244050.822 244052.496 244054.534 244056.932 244059.707 244062.849 244066.369 244070.249 244074.512 244079.166 244084.200
- 0.004 - 0.005 - 0.006 0.000 - 0.004 - 0.006 0.015 - 0.032 0.000
244089.604 244095.345 244101.455 244107.947 244114.796 244122.075 244129.653 244137.636 244145.969 303203.963 a) 303204.193 a) 303204.874 303205.845 a) 303206.2 15 I) 303207.641 303212.226 303237.055 303278.424
0.041 0.031 0.018 0.015 - 0.004 0.034 -0.001 -0.005 -0.032
0.028 0.025 0.009 0.015 0.014 0.003 - 0.002 -0.011 -0.013 - 0.026 -0.028 -0.010 0.017
-0.008
0.003 -0.006 0.007 -0.015
Ohs.-talc.
(MHz)
(MHz)
303336.428 303411.174 362226.106 ‘) 362226.379 a) 362221.194 362228.116 ‘) 362229.021 ‘) 362230.479 362235.947 362265.481 362314.832 362383.941 362472.937 362581.897 427618.000”’ 427618.322 *) 427619.305 427619.853 ‘) 427621.940 a) 427623.147 427629.590 427664.354 427722.355 427803.638 427908.300 428036.450 428188.285 680120.57 a) 680121.07 n)
-0.024 0.022
K
J 125437.426 125437.522 125437.809 125438.286 125438.951 125439.804 125440.866 125442.109 184783.639 184783.779 184784.195 184784.897 184785.879 184787.144
Frequency
45 45 54 54 54
24 30 0 1 2
54 54 54 54 54 54 54 54 54 64 64 64 64 64 64 64 64 64 64 64 64 64 103 103 103 103 103 103 103 103 106 106 106 106 106 106 106 106 106 106 106 106 106 106 106 106 106 106 106 106
3, 3_ 4 6 12 18 24 30 36 0 1 2 3, 3_ 4 6 12 18 24 30 36 42 0 1 2 3, 3_ 4 5 6 0 1 2 3, 3_ 4 5 6 7 8 9 10 11 12 21 27 33 39 45 48
680122.93 680114.13”’ 680136.03 a) 680128.38 680132.94 680138.55 699340.98 a) 699341.50 ‘) 699342.82 699332.99 ‘) 699358.23 ” 699348.39 699353.15 699359.50 a) 699366.30 699373.90 699382.70 699392.60 699403.50 699415.20 699568.29 6997 16.69 699902.7 1 700126.50 700388.45 700533.10
-0.006
-0.004 -0.009 -0.033 0.003 0.007 0.005 -0.006
0.018
- 0.002 0.004 -0.002 0.005 0.004 0.000 -0.016 0.006
0.358
-0.206 -0.157 -0.061
-0.221
-0.822 -0.691 0.114 -0.002 0.052 0.176 0.270 0.134 0.246 0.068 0.041 0.097 0.365 0.05 I
Volume 203, number 2,3
CHEMICAL PHYSICS LETTERS
Table 2 Observed frequencies and residuals of CH”Cl,
Frequency (MHz)
Transition
J
Obs. - talc. (MHz)
K 1 1 2 3 4 5 6
19 19 19 19 19 19 19 19 29 29 29 29 29 29 29 29 29 29 29 29 39 39 39 39 39
125140.726 ‘) 125140.817 a)
1 0
1 4 5 6 7 8 9
10 11 12 13 0 1 2 3 4
39 39 39 39 39 39 39
5 6 1 8
-0.013 0.004 0.001 0.011 0.003 -0.008
187631.473 187632.700 187634.196 187635.957 187638.009 187640.318 187642.907 187645.768 187648.880 187652.298 250020.016 ‘) 250020.197 =) 250020.718 250021.630 250022.89 1
-0.005 -0.001 0.000 -0.006 0.007 0.004 0.007 0.009 -0.012 -0.002
250024.516 250026.510 250028.878 250031.576 250038.066 250045.992 250060.7 I 1 250078.561 250099.938
10 12 15 18 21
39 39
125141.077 125341.548 125142.181 125143.009 125144.001 125145.172 187629.305 a) 187629.441 a)
- 0.020 -0.009 - 0.009 - 0.007 0.004 0.027 0.019 0.010 -0.016 0.042 - 0.068 0.027
19 February 1993
weakness of its spectrum when observed in natural abundance. Table 3 gives the results obtained by a least-squares analysis of the observed spectra of the two isotopomers, compared with those available in literature. The fitting procedure does not include the K=O, 1 transitions since these are not resolved, and the K= 3 transitions with J higher than 39 for the reasons that will be discussed later. In the fitting procedure the assigned line frequencies were weighted according to the inverse square of the experimental uncertainty which is found to be i 15 kHz for the spectrometer with klystron-driven frequency multiplier, and + 500 kHz for the side-band spectrometer. Accurate values of the rotational constant Be and of the quartic and sextic centrifugal distortion constants have been obtained for CH35C13and, as regards CH3’C13,the values of the centrifugal distortion constants have been determined for the first time. The standard deviations of the tit are 0.15 1 and 0.021 MHz for CHS5Cljand CH3’ClJ, respectively. The higher value for the first isotopomer is due to the higher uncertainty of those fitted experimental lines frequencies which are obtained by using the side-band laser FIR spectrometer. In trigonal symmetric-top molecules higher-order vibration-rotational interaction may split the otherwise degenerate Al and A2 components of the level with K=3n [ 81. This effect is dominant for K=3, and in the first-order approximation the splitting is due to: E_3,3= +h,J(Jt
l)[J(Jt
l)-21
x[J(J+l)-61.
(1)
‘) Predicted value. Table 3
Spectroscopic constantsof CHWl,and
CHW,
cH~scl 3
Bo(MHz) n, (kHz) &K (k=) hJJ
(Hz
)
HJJK (Hz ) &KJ (Hz) 4 @Hz 1
CH”C1 3
this work
ref. [5]
this work
ref. [4]
3302.07587( 12) 1.511716(66) -2.51757(20) 0.001268(11) -0*005000( 30) 0.00652( 11) -0.178(55)x lO-5
3302.083(3) 1.52(l) -2.50( 2)
3129.61007(57) 1.36571(64) -2.27688(51) 0.00135(20) -0.0078(18) 0.0283(49)
3129.54
u(unweighted)=0.151
a=0.02 1
229
Volume 203, number 2,3
Jsl IKI=
CHEMICAL PHYSICS LETTERS
3
19 February 1993
AA-F
J=40-39 250.019 GHz MHZ
J=46-45
303.203 GHz
A-
MHZ
A+ b, =2h,J(J+l)[J(Jtl)-Z][J(J+l)-61
u_-Vt=dv,,Av,
J=55-54 362.2'25GHz MHZ
Fig. 1. Energy level diagram of nondegenerate K=3 states of a trigonal symmetric top molecule.
An energy levels diagram showing the frequency splitting of a generic rotational transition Jt 14, 1KI = 3 is shown in fig. 1, where, by convention, the positive sign of hSincrease the energy of the A_ level while the negative one reduces that of the A+. Since the constant h3 depends on the cubic potential constants so that its magnitude is comparable to that of the sextic centrifugal distortion constant HJJJ, these energy level splittings are usually negligible. Moreover the frequency difference between the two LU= 1, K= 3 rotational transitions, labelled V+and u+ in fig. 1, depends, in agreement with the rotational selection rules, on the difference between the energy splittings of the J+ 1 and Jlevels. This occurrence clearly makes it more difficult to obtain experimental evidence of this splitting by means of rotational spectroscopy. For these two reasons no splitting of the K= 3 rotational transitions is usually observed unless the Jvalue is sufficiently high to show it. As concerns CH3’C13,this effect has been observed starting from J= 45. Fig. 2 shows the recorded spectra of six transitions of CH3’C15with J values included between 39 and 106. The J=40-39, K= 3 transition is unsplit, while all the K= 3 transitions with higher values of J show the splitting. A least-squares analysis of the observed splitting with the calculated values obtained by using eq. ( 1) gives a value of h3=0.00015007(25 ) Hz. Since the K=6 and K=3_ components of the J= 107-106 230
J=65-64 427.617 GHr
1.00
_I K
3,
Ok12
4
5
6
J=107-106 699.329 GHz 000 0.00
1O.C.J
20.00
moo
MHZ
Fig. 2. Recorded ground state rotational transitions of CH”CII showing the K= 3 line splitting at increasing J.
transition overlap each other and it was impossible to resolve them, it has been decided to exclude from the lit the K=3 splitting of this transition. Table 4 reports the results of the fit. An independent analysis of the splittings has been preferred to the possibility of adding the h3 constant to the parameters of the line frequencies fitting procedure, since the side-band spectrometer presents an uncertainty in the measurement of the absolute value of a line frequency of + 500 kHz, while in a recorded spectrum the frequency difference between two lines is accurate within * 5 kHz. In fact, as pointed out in ref. [ 111, the short time (few minutes) frequency
Volume 203, number 2,3
CHEMICAL PHYSICS LETTERS
Acknowledgement
Table 4 Splitting of the K= 3 rotational transitions of CHCIJ Splitting (MHz)
Transition I
J”
K
obs.
obs. - talc.
46 55 65 104
45 54 64 103
3 3 3 3
0.419 0.937 2.109 21.895
0.049 0.032 0.022 - 0.004
hl=0.15007(25)x
19 February 1993
We would like to thank Mr. G. Tasini for technical assistance. This work was financially supported by the Minister0 della Universith e della Ricerca Scientifica e Tecnologica (MURST 40% and 60%) and by CNR.
10-3Hz
stability of the free running side-band laser spectrometer source is of the order of 5 x 10w9while the uncertainty of the laser absolute frequency, due to its broad gain profile, is orders of magnitude higher. This is not particularly inconvenient if, as in the present situation, the aim of the experiment is to measure the frequency splitting of an absorption line.
4. Conclusions An accurate centrifugal distortion analysis of the millimeter and submillimeter-wave spectrum of CH35C13and of the millimeter-wave spectrum of CH3’C13has been performed. The observation of K= 3 rotational transitions, which are split because of higher-order vibration-rotational interaction, allowed the determination of the hXsplitting constant.
[ I ] P. Kisliuk and C.H. Townes, NBS Circular 518 ( US GPO, Washington, 1952). [2] P.N. Wolfe, J. Chem. Phys. 25 (1956) 976. 131M.W. Long, J, Chem. Phys. 32 (1960) 948L. [4] M. Jen and D.R. Lide, J. Chem. Phys. 36 (1962) 2525. [ 51P.G. Favero and A.M. Mirri, Nuovo Cimento 30 ( 1963) 502. [6] P.B. Reinhart, Q. Williams and T.L. Weatherly, J. Chem. Phys. 53 (1970) 1418. [ 71 D.B. McLay and G. Winnewisser, J. Mol. Spectry. 44 ( 1972) 32. [8] J.K. Watson, in: Vibrational spectra and structure, Vol. 6, ed. J. Durig (Elsevier, Amsterdam, 1977) p. 1. [9] G. Cazzoli and Z. Kisiel, J. Mol. Spectry. submitted for publication. [lo] G. Cazzoli and L. Dore, J. Mol. Spectry. 141 (1990) 49. [ 111G. Cazzoli, L. Cludi, C. Degli Esposti and L. Dore, J. Mol. Spectry. 152 (1992) 185.
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