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Radiofrequency-microwave double resonance with a dipole moment component induced by asymmetric isotopic substitution: Microwave spectrum of C4O2Cl2
JOURNAL OF MOLECULAR SPECTROSCOPY
131, 154-160 (1988)
Radiofrequency-Microwave Double Resonance with a Dipole Moment Component Induced by Asymmetric Isotopic Substitution: Microwave Spectrum of C402C12 W. CAMINATI,* ,t A. C. FANTONI,* *j-S B. LuNELLI,-f~g AND F.
SCAPPINI?
* Dipartimentodi Chimica Fisica ed Inorganica dell’ Universita’,Viale Risorgimento4, 40136 Bologna,
Italy; tlstituto di SpettroscopiaMolecolarede1 C.N.R., Via de’ Castagnoli1, 40126 Bologna, Italy; $ Departamentode Fisica, UniversidadNational de La Plats, C.C. 67, 1900 La Plata, RepublicaArgentina; and #Dipartimentodi Chimica ‘%.Ciamician” dell’ Vniversita’, Via Selmi 2, 40126 Bologna, Italy The microwave spectrum of C.+O$& (1,2_dichlorocyclobuten-3,4dione) has been investigated for both the C.+OZ~~C~~ and the C40235C137C1 isotopomers. The spectrum of the latter has been studied in greater detail because a number of c(brotational transitions were measured with the radiofmquency-microwave double-resonance technique by radiofrequency pumping of some K-, doublets. This pumping has been made possible since a small cc0dipole moment component appears owing to the aS)Wnetrk isotopic substitution. Q 1988 Academic press, Inc. INTRODUCTION
The rotational spectra of asymmetric top molecules with only the clbdipole moment component different from zero are the most difficult to assign, while pa- and PC-type spectra, due to the near-prolate and near-oblate degeneracies, generally present systematic regularities which favor the investigation of their microwave spectra. Furthermore, the radiofrequency-microwave double-resonance technique (RFMWDR) ( I ) , largely used to assign the first rotational transitions (K , near-degenerate doublets), is not useful for a pb-type spectrum. Nevertheless, if two rotational constants have very similar values (near symmetric top) and if an asymmetric isotopic substitution causes a principal axis to change its direction, originally perpendicular to the direction of the active dipole moment component, then an appreciable pa or pc dipole moment component can be generated. Such a component could then be used to make radiofrequency pumping while observing the more intense fib-type transitions. C402C12 ( 1,2-dichlorocyclobuten3,4dione, see Fig. 1), a near-oblate top with the two chlorine atoms far away from both a and b principal axes, could be an appropriate model to investigate the above described effect, due to the high abundance (~25%) of the second isotope of chlorine in the natural mixture. Several compounds containing the cyclobutene ring are known to work, or to be good candidates as photovoltaic (2)) photoconductive (3), and optical storage (4) materials, but very little structural information concerning these and similar molecules is available in the literature. An electron diffraction study (5) was the only source of data for predicting the rotational spectrum. Very recently, an infrared analysis undertaken giving evidence for a considerable number of low-frequency (v < 0022-2852188 $3.00 Copyright 0 1988 by Academic F’res.s,Inc. All rights of reproduction in any form reserved.
154
MICROWAVE SPECTRUM OF C,O& b’\
155
,b
FIG. 1. Schematic drawing of 1,2dichlorocyclobuten-3,4dione. The principal axes II and b correspond to the normal species, while u’ and b’ are the principal axes for the 35Cl/37Clspecies (“Cl in position 6).
cm-‘) vibrational transitions (6). The rotational spectrum of C40zC12 is expected to be extremely complicated and dense due to (1) the high molecular weight, (2) the presence of three isotopomers in natural abundance ( 35Cl/35Cl, 35Cl/37Cl, and “Cl/ 37C1 in the ratio 9/6/ 1, respectively), (3) the fact that only the pb dipole moment component is different from zero in the normal isotopic species, (4) the nuclear quadrupole coupling of the two chlorine atoms, and ( 5) the presence of a large number of low-lying vibrational energy levels. Despite all these unfavorable factors, we decided to study the rotational spectrum, relying on the possibility that the RFMWDR technique would help to identify the &-type transitions. This should be possible by pumping the K1 doublets of C40235C137C1,which are connected by a pa dipole moment induced by the different mass of the two chlorine isotopes. EXPERIMENTAL
DETAILS
C402C12 was prepared following a procedure reported previously ( 7) and further purified by two sublimations and two crystallizations from cyclohexane: mp 54°C (lit. (8), 51-52°C). The rotational spectra have been recorded by a computer-controlled spectrometer in the frequency range 26.5-40 GHz (9). The absorption cell was cooled to about - 10°C which seemed the best compromise between reasonable vapor pressure (10 pm& = 1.33 Pa) and ground state Boltzmann population. Both Stark modulation and radiofrequency-microwave double-resonance ( I ) techniques were used for the analysis of the spectrum. The full width at half-maximum of the measured lines is around 2 MHz due to the hyperfine structure and blending of lines. The accuracy of the measurements is believed to be within 0.1-0.2 MHz for the sharpest lines, and 0.5- 1.O MHz for the lines broadened due to the quadrupole structure.
156
CAMINATI ET AL. RESULTS AND DISCUSSIONS
For an approximate prediction of the spectrum, we took the electron diffraction (ED) structure from Ref. (5). Since the molecule is a near-oblate top (K N 0.70), with only ,.&b different from zero, the low-resolution spectrum should exhibit two kinds of bands. One of them is similar to the so-called type II bands described by Borchert ( 10). He considered bands that were originated from the piling up of pa-type transitions like (J + I)oJ+~ + JO,J and (J + 1) I,_I+I * JI,J, due to the oblate degeneracy in the prolate range. We have &-type transitions, but as we are in the oblate range, the oblate degeneracy is more accentuated, and so we have piling up of transitions like + 10,~ as the levels JO,,,and JIJ are degenerate for (J+ ~)OJ+I +JI,Jand(J+ ~)I,J+I high J values. Furthermore, the transitions J1_.,_,+ (J - 1) 2+,_J-2 fall in the same region and overlap to Jz,~_~+ (J - 1) I+, , and so on. The frequency of these R-branch transitions is obtained from the expression for an oblate top ( 1 I ) E J(K-I,K+d= I/2(,4 + B)J(J+
I) + [C-
and to a first approximation
1/2(A + B)][K:,
+ c;b,, + c$bo + . - -1
(1)
is given by
vR = 2C(J+
l/2) + 1/2(/t + B).
(2)
The second type of bands is similar to that described in the case of pyrrolidine ( 12), where pa Q bands in an oblate top rotational spectrum were observed. The same considerations hold for the pb Q bands of an oblate top like C40$12 and we obtain the following expression for the band frequencies: VQ
=
(A
+
B
-
2C)(K+, + l/2).
(3)
We will label the two kinds of bands R bands and Q bands, respectively. As the molecule has a Cr, symmetry and a pair of fermions with nuclear spin quantum number $, a statistical weight ratio 10/6 is expected for the rotational transitions, depending on the parity of the starting rotational level. This holds only for the symmetrically substituted isotopic species. Low-resolution spectra were recorded at various Stark voltages. Under the best conditions (high voltages) two series of sharp weak bands with roughly an intensity ratio 9 /6 were observed. They showed progressions of weaker components toward higher frequencies. According to the above discussions, they were assigned to the R bands of the “Cl/ 35C1and 35C1/37C1isotopic species. The Q bands were not observed because their intensities are much lower. This set of bands, whose spacings are A + B - 2C, might be overlapped by the R bands (with spacings 2C) as from model values (A + B - 2C) N 2C. Table I lists the frequencies of the origins of these bands, that is, of the (J + l),,.,+l t JyJ transitions (x, y = 0, 1, x # y) , which enable the determination of Cand (A + B) (see Eq. (I)). The obtained values for these two parameters are reported in Table II. Figure 1 shows a sketch of C40&1z and the principal axis systems for the normal and 35Cl/37Clisotopic species. The effect of the asymmetric isotopic substitution causes a rotation of about 9” of the principal axes a and b with respect to the molecular frame. This rotation induces a dipole moment component p0 which is about 0.15 Pb.
MICROWAVE
157
SPECTRUM OF C,O,Cl, TABLE I
Experimental Transition Frequencies of 1,2-Dichlorocyclobuten-3,4dione *
1
J’(K+,‘,<--J”(K+,
",
35,35
36,3T
J',K,;,c--J-(K+,"~
3
35,35
18757.2 20257.4
35,37
19950.6
21757.5
21428.2
24758.4
24394.0
27770.9
J'lK.,',K+,,,c--J',K_,",K+,',
35,37
21352.46
=
25871.2
24652.29
a
27349.5
27476.6,
23266.0
17256.3
(MHz)
28834.9 29271.1
27523.39 305T9.31
26258.8
25861.4
28827.3
30604.08
29259.3
28817.5
30312.5
32861.0,
30759.8
30294.9
30305.~
33188.9,
32260.1
31772.9
31T91.2
33675.98
33760.5
33250.4
35260.8
34T27.6
36761.2
36205.4
38261.6
37683.3
39761.9
39161.2
30771.3
32272.0
33773.2
35273.3
36775.‘
31183.1
33689.88
33270.1
3m2*.92
33261.7
36212.69
34149.5
35771.49
34739.0
367T8.12
36227.2
39864.5,
36218.2
39867.88
3TTO5.3 38276.0
37696.7 39184.4
39177.5
39174.7
Note. Column I: R-band frequencies (transitions (f + I)oJ+~ + Jr, overlapped to (J + I) ,,+, + Jo,); Column 2: Higher frequency components of the R band [transitions Jr,_, + (J - 1)2,_2 overlapped to Jz,-, c (J- l)t,~--2, and (J- 1)2~--a6 (J - 2)3,-d overlapped to (J - l)~,-~ + (J - 2)2J_4]; Column 3: Transitions of C40235C137C1 measured by the RFMWDR technique. ’ Not included in the fit.
This value was not large enough to allow the direct observation of the p&pe transitions, but it allowed the selective observation and measurement of several pb-type transitions (also reported in Table I) with a RFMWDR experiment (I ). These types of transitions are very important for determining the rotational constants. The pumping takes place through high K-r doublets, due to the near-prolate degeneracy extended to the oblate range. The radiofrequencies used were in the range 2 to 40 MHz. The transitions have been fitted within the I’ representation of the rigid rotor Hamiltonian (II ) and the results of the fitting are reported in Table II. We then tried to assign the transitions located at higher frequencies within each R band, that is, transitions with higher Km1values. Due to the smaller line strengths, the large number of lines belonging to the low-energy vibrational satellites, the overlapping with the Q bands mentioned earlier, the quadrupole broadening, and the presence of numerous Stark lobes, this was not an easy task. Tentatively assigned transitions are reported in Table I for both isotopomers. Fittings including all transitions were performed with the Watson quartic Hamiltonian ( 13). The results, reported in Table II, are satisfactory except for the fact that the inertial defect and the centrifugal distortion
158
CAMINATI ET AL. TABLE I1 Spectroscopic Constants for C402C12
C/MHZ (A.*B)/HHZ 35/35
A/d2 Nd e= ,
Jmax
1545.738(8,
1017.32(0 739.21(20
1645.746(5) 1417.293(35) 738.872(2)
15d6.5,(6) 1,19.70(44) 738.82(2)
8.3(16) -3015, 35/37
2114) 4.1(B) 4.0(3) a/u?.2
0.u
Nd we
14
Jmax
26
0.28
0.15(25) 14
0.3. 13
0.46
1.09
28 0.32 26
45 0.71 26
Note. Column 1: C and (A + B) from the Km1= 0,1 transitions (R bands); Column 2: A, B, C from a rigid rotor htting ofthe rotational transitions measured by RFMWDR for the 3sCl/37Cl isotopomer; Column 3: A, E, C from a rigid rotor fitting combining the rotational transitions measured by RFMWDR with the K-! = 0, 1 transitions (R bands) for the 35Cl/3’CI isotopomer; Column 4: Results of a fitting including tentatively assigned higher K_, R-branch transitions. ’ Centrifugal distortion constants undetermined from the fit; fixed to zero. b Standard error in parentheses in units of the last digit. ’ Centrifugal distortion constants fixed at the 37Clspecies values. d Number of transitions in the fit. ’ Fit standard deviation.
constants are somehow larger than the values we could expect for such a planar molecule. These large values could however be due to the low-frequency in-plane motions. According to Herschbach and Laurie ( 14) if one in-plane vibration has a frequency much lower than all the other vibrations, the value of the inertial defect is given by AvibN 4klw
(~67.4510
u A2 if o is in cm-‘).
In that case an in-plane vibration at about 60 cm-’ could account for the large positive inertial defect. From the empirical force field the lowest in-plane motion is calculated to be 158 cm-’ (6). The quadrupole coupling effect due to the two chlorine atoms was calculated (15) by transferring the quadrupole coupling constants of phosgene ( 16) to our molecule while taking into account the required rotation of the principal axis system. The resulting hyperline structure, for the measured RFMWDR lines, is centered almost symmetrically around the unperturbed frequency, and within the WHM of the lines,
159
MICROWAVE SPECTRUM OF C,02C12 TABLE III Comparison of the Structure Adjusted to the Rotational Constants to the ED Structure from
ref.
[5]
this
work
c-o
1.191
1.1906
bond
c=c
1.362
1.3478
lengths
cl-c3b
1.516
1.5133
ci,
c3 -c4
1.578
1.5689
C-Cl
1.682
1.6813
bond
o=c-c
94.0
94.19
angles
c=c-Cl
133.4
133.27
ldeg)
C2-C3=O
135.7
135.45
a
a Adjusted parameters with respect to Ref. (5). All seven independent parameters have been allowed to vary within a confidence interval set at the corresponding uncertainty in the ED structure. The differences between observed and calculated values are 0.08, -0.09, -0.46, -0.53, and +3 MHz for A, B, C (35C1/ %I), and C and (A + B) (“5Clz), respectively. ‘See Fig. 1 for atom numbering.
while for the other measured lines it is much more compact and centered on the unperturbed frequency. Table II shows the spectroscopic constants obtained for four different fits. When only (J + 1)0,~+1 * JLJ and(J + ~)o,(J+I) + J,+, transitions have been used, still it has been possible to obtain a value for the inertial defect, as it was almost insensitive to the assumed (A - B) value. A small adjustment of the structural parameters has been performed in order to reproduce the observed rotational constants. The final plausible structure is compared in Table III with the ED structure (5). In conclusion, we point out the advantage of the RFMWDR, made possible by an asymmetric isotopic substitution induced dipole component, in assigning ~&pe spectra. We also stress once again that in the present case there are no doubts about the assignment of the transitions measured with the RFMWDR technique, whereas for the remaining transitions, due to the reasons mentioned above, the assignments are tentative. ACKNOWLEDGMENTS We thank Dr. R. Danieli and Mr. S. Rose for technical assistance with the electronic equipment. WC. thanks Dr. R. Danieli, Director of the Istituto di Spettroscopia Molecolare de1 C.N.R., for hospitality in his Institute and also for proposing him as “Incaricato di ricerca.”
Note added in proof From an approximate force field calculation using the data from Refs. (5,6) a ground vibrational state inertial defect of 0.2 1 uAzwasobtained. This value is in agreement with the value listed in column 2 of Table II.
RECEIVED:
March 10, 1988
160
CAMINATI ET AL. REFERENCES
1. F. J. W~DARCZVK AND E. B. WILSON,J. Mol. Spectrosc.37,445-463 (1971). 2. A. P. PIECHOWSKI, G. R. BIRD,D. L. MOREL,ANDE. L. STOGRYN,J. Phys. Chem. 88,934-950(1984), and references therein. 3. K. Y. LAW ANDF. C. BAYLEY,U.S. Patent 4508803, cf. Infruredl’hys. 26(l), 4. A. H. WIRER, Appl. Opt. 23,2738-2743 (1984).
5. 6. 7. 8.
V (1986).
K. HAGEN, K. HEDBERG,B. LUNELLI,AND M. G. GIORGINI,J. Phys. Chem., in press. B. LUNELLIAND M. G. GIORGINI,Spectrochim.Ada, Part A 43,829-835 (1987). R. C. DE SELMS,C. J. Fox, AND R. C. RINDON, TetrahedronLett., 781-782 (1970). G. MAAHS, Liebigs Ann. Chem. 686,55-63 (1965).
9. G. CAZZOLI, A. DAL BORGO, D. G. LISTER,AND D. DAMIANI,J. Mol. Spectrosc.95,45-50 (1982). 10. S. J. BORCHERT,J. Mol. Spectrosc.57, 312-315 (1975). 11. C. H. TOWNESAND A. L. SCHAWLOW,“Microwave Spectroscopy,” McGraw-Hill, New York, 1955. 12. W. CAMINATI, H. OBERHAMMER,G. F?=AFFEROTT, R. R. FILGUEIRA,AND C. H. GOMEZ, J. Mol. Spectrosc. 106,217-226 (1984). 13. J. K. G. WATSON, in “Vibrational Spectra and Structure” (J. R. Durig, Ed.), Elsevier, New York, 1977. 14. D. R. HERSCHBACH AND V. W. LAURIE,J. Chem. Phys. 40,3142-3149 (1964). 15. W. C&RDYANDR. L. COOK, “Microwave Molecular Spectra,” Interscience, New York, 1970; computer program by Dr. H. Gunther, Tubingen, West Germany. 16. G. W. ROBINSON, J. Chem. Phys. 21, 1741-1745 (1953).
131, 154-160 (1988)
Radiofrequency-Microwave Double Resonance with a Dipole Moment Component Induced by Asymmetric Isotopic Substitution: Microwave Spectrum of C402C12 W. CAMINATI,* ,t A. C. FANTONI,* *j-S B. LuNELLI,-f~g AND F.
SCAPPINI?
* Dipartimentodi Chimica Fisica ed Inorganica dell’ Universita’,Viale Risorgimento4, 40136 Bologna,
Italy; tlstituto di SpettroscopiaMolecolarede1 C.N.R., Via de’ Castagnoli1, 40126 Bologna, Italy; $ Departamentode Fisica, UniversidadNational de La Plats, C.C. 67, 1900 La Plata, RepublicaArgentina; and #Dipartimentodi Chimica ‘%.Ciamician” dell’ Vniversita’, Via Selmi 2, 40126 Bologna, Italy The microwave spectrum of C.+O$& (1,2_dichlorocyclobuten-3,4dione) has been investigated for both the C.+OZ~~C~~ and the C40235C137C1 isotopomers. The spectrum of the latter has been studied in greater detail because a number of c(brotational transitions were measured with the radiofmquency-microwave double-resonance technique by radiofrequency pumping of some K-, doublets. This pumping has been made possible since a small cc0dipole moment component appears owing to the aS)Wnetrk isotopic substitution. Q 1988 Academic press, Inc. INTRODUCTION
The rotational spectra of asymmetric top molecules with only the clbdipole moment component different from zero are the most difficult to assign, while pa- and PC-type spectra, due to the near-prolate and near-oblate degeneracies, generally present systematic regularities which favor the investigation of their microwave spectra. Furthermore, the radiofrequency-microwave double-resonance technique (RFMWDR) ( I ) , largely used to assign the first rotational transitions (K , near-degenerate doublets), is not useful for a pb-type spectrum. Nevertheless, if two rotational constants have very similar values (near symmetric top) and if an asymmetric isotopic substitution causes a principal axis to change its direction, originally perpendicular to the direction of the active dipole moment component, then an appreciable pa or pc dipole moment component can be generated. Such a component could then be used to make radiofrequency pumping while observing the more intense fib-type transitions. C402C12 ( 1,2-dichlorocyclobuten3,4dione, see Fig. 1), a near-oblate top with the two chlorine atoms far away from both a and b principal axes, could be an appropriate model to investigate the above described effect, due to the high abundance (~25%) of the second isotope of chlorine in the natural mixture. Several compounds containing the cyclobutene ring are known to work, or to be good candidates as photovoltaic (2)) photoconductive (3), and optical storage (4) materials, but very little structural information concerning these and similar molecules is available in the literature. An electron diffraction study (5) was the only source of data for predicting the rotational spectrum. Very recently, an infrared analysis undertaken giving evidence for a considerable number of low-frequency (v < 0022-2852188 $3.00 Copyright 0 1988 by Academic F’res.s,Inc. All rights of reproduction in any form reserved.
154
MICROWAVE SPECTRUM OF C,O& b’\
155
,b
FIG. 1. Schematic drawing of 1,2dichlorocyclobuten-3,4dione. The principal axes II and b correspond to the normal species, while u’ and b’ are the principal axes for the 35Cl/37Clspecies (“Cl in position 6).
cm-‘) vibrational transitions (6). The rotational spectrum of C40zC12 is expected to be extremely complicated and dense due to (1) the high molecular weight, (2) the presence of three isotopomers in natural abundance ( 35Cl/35Cl, 35Cl/37Cl, and “Cl/ 37C1 in the ratio 9/6/ 1, respectively), (3) the fact that only the pb dipole moment component is different from zero in the normal isotopic species, (4) the nuclear quadrupole coupling of the two chlorine atoms, and ( 5) the presence of a large number of low-lying vibrational energy levels. Despite all these unfavorable factors, we decided to study the rotational spectrum, relying on the possibility that the RFMWDR technique would help to identify the &-type transitions. This should be possible by pumping the K1 doublets of C40235C137C1,which are connected by a pa dipole moment induced by the different mass of the two chlorine isotopes. EXPERIMENTAL
DETAILS
C402C12 was prepared following a procedure reported previously ( 7) and further purified by two sublimations and two crystallizations from cyclohexane: mp 54°C (lit. (8), 51-52°C). The rotational spectra have been recorded by a computer-controlled spectrometer in the frequency range 26.5-40 GHz (9). The absorption cell was cooled to about - 10°C which seemed the best compromise between reasonable vapor pressure (10 pm& = 1.33 Pa) and ground state Boltzmann population. Both Stark modulation and radiofrequency-microwave double-resonance ( I ) techniques were used for the analysis of the spectrum. The full width at half-maximum of the measured lines is around 2 MHz due to the hyperfine structure and blending of lines. The accuracy of the measurements is believed to be within 0.1-0.2 MHz for the sharpest lines, and 0.5- 1.O MHz for the lines broadened due to the quadrupole structure.
156
CAMINATI ET AL. RESULTS AND DISCUSSIONS
For an approximate prediction of the spectrum, we took the electron diffraction (ED) structure from Ref. (5). Since the molecule is a near-oblate top (K N 0.70), with only ,.&b different from zero, the low-resolution spectrum should exhibit two kinds of bands. One of them is similar to the so-called type II bands described by Borchert ( 10). He considered bands that were originated from the piling up of pa-type transitions like (J + I)oJ+~ + JO,J and (J + 1) I,_I+I * JI,J, due to the oblate degeneracy in the prolate range. We have &-type transitions, but as we are in the oblate range, the oblate degeneracy is more accentuated, and so we have piling up of transitions like + 10,~ as the levels JO,,,and JIJ are degenerate for (J+ ~)OJ+I +JI,Jand(J+ ~)I,J+I high J values. Furthermore, the transitions J1_.,_,+ (J - 1) 2+,_J-2 fall in the same region and overlap to Jz,~_~+ (J - 1) I+, , and so on. The frequency of these R-branch transitions is obtained from the expression for an oblate top ( 1 I ) E J(K-I,K+d= I/2(,4 + B)J(J+
I) + [C-
and to a first approximation
1/2(A + B)][K:,
+ c;b,, + c$bo + . - -1
(1)
is given by
vR = 2C(J+
l/2) + 1/2(/t + B).
(2)
The second type of bands is similar to that described in the case of pyrrolidine ( 12), where pa Q bands in an oblate top rotational spectrum were observed. The same considerations hold for the pb Q bands of an oblate top like C40$12 and we obtain the following expression for the band frequencies: VQ
=
(A
+
B
-
2C)(K+, + l/2).
(3)
We will label the two kinds of bands R bands and Q bands, respectively. As the molecule has a Cr, symmetry and a pair of fermions with nuclear spin quantum number $, a statistical weight ratio 10/6 is expected for the rotational transitions, depending on the parity of the starting rotational level. This holds only for the symmetrically substituted isotopic species. Low-resolution spectra were recorded at various Stark voltages. Under the best conditions (high voltages) two series of sharp weak bands with roughly an intensity ratio 9 /6 were observed. They showed progressions of weaker components toward higher frequencies. According to the above discussions, they were assigned to the R bands of the “Cl/ 35C1and 35C1/37C1isotopic species. The Q bands were not observed because their intensities are much lower. This set of bands, whose spacings are A + B - 2C, might be overlapped by the R bands (with spacings 2C) as from model values (A + B - 2C) N 2C. Table I lists the frequencies of the origins of these bands, that is, of the (J + l),,.,+l t JyJ transitions (x, y = 0, 1, x # y) , which enable the determination of Cand (A + B) (see Eq. (I)). The obtained values for these two parameters are reported in Table II. Figure 1 shows a sketch of C40&1z and the principal axis systems for the normal and 35Cl/37Clisotopic species. The effect of the asymmetric isotopic substitution causes a rotation of about 9” of the principal axes a and b with respect to the molecular frame. This rotation induces a dipole moment component p0 which is about 0.15 Pb.
MICROWAVE
157
SPECTRUM OF C,O,Cl, TABLE I
Experimental Transition Frequencies of 1,2-Dichlorocyclobuten-3,4dione *
1
J’(K+,‘,<--J”(K+,
",
35,35
36,3T
J',K,;,c--J-(K+,"~
3
35,35
18757.2 20257.4
35,37
19950.6
21757.5
21428.2
24758.4
24394.0
27770.9
J'lK.,',K+,,,c--J',K_,",K+,',
35,37
21352.46
=
25871.2
24652.29
a
27349.5
27476.6,
23266.0
17256.3
(MHz)
28834.9 29271.1
27523.39 305T9.31
26258.8
25861.4
28827.3
30604.08
29259.3
28817.5
30312.5
32861.0,
30759.8
30294.9
30305.~
33188.9,
32260.1
31772.9
31T91.2
33675.98
33760.5
33250.4
35260.8
34T27.6
36761.2
36205.4
38261.6
37683.3
39761.9
39161.2
30771.3
32272.0
33773.2
35273.3
36775.‘
31183.1
33689.88
33270.1
3m2*.92
33261.7
36212.69
34149.5
35771.49
34739.0
367T8.12
36227.2
39864.5,
36218.2
39867.88
3TTO5.3 38276.0
37696.7 39184.4
39177.5
39174.7
Note. Column I: R-band frequencies (transitions (f + I)oJ+~ + Jr, overlapped to (J + I) ,,+, + Jo,); Column 2: Higher frequency components of the R band [transitions Jr,_, + (J - 1)2,_2 overlapped to Jz,-, c (J- l)t,~--2, and (J- 1)2~--a6 (J - 2)3,-d overlapped to (J - l)~,-~ + (J - 2)2J_4]; Column 3: Transitions of C40235C137C1 measured by the RFMWDR technique. ’ Not included in the fit.
This value was not large enough to allow the direct observation of the p&pe transitions, but it allowed the selective observation and measurement of several pb-type transitions (also reported in Table I) with a RFMWDR experiment (I ). These types of transitions are very important for determining the rotational constants. The pumping takes place through high K-r doublets, due to the near-prolate degeneracy extended to the oblate range. The radiofrequencies used were in the range 2 to 40 MHz. The transitions have been fitted within the I’ representation of the rigid rotor Hamiltonian (II ) and the results of the fitting are reported in Table II. We then tried to assign the transitions located at higher frequencies within each R band, that is, transitions with higher Km1values. Due to the smaller line strengths, the large number of lines belonging to the low-energy vibrational satellites, the overlapping with the Q bands mentioned earlier, the quadrupole broadening, and the presence of numerous Stark lobes, this was not an easy task. Tentatively assigned transitions are reported in Table I for both isotopomers. Fittings including all transitions were performed with the Watson quartic Hamiltonian ( 13). The results, reported in Table II, are satisfactory except for the fact that the inertial defect and the centrifugal distortion
158
CAMINATI ET AL. TABLE I1 Spectroscopic Constants for C402C12
C/MHZ (A.*B)/HHZ 35/35
A/d2 Nd e= ,
Jmax
1545.738(8,
1017.32(0 739.21(20
1645.746(5) 1417.293(35) 738.872(2)
15d6.5,(6) 1,19.70(44) 738.82(2)
8.3(16) -3015, 35/37
2114) 4.1(B) 4.0(3) a/u?.2
0.u
Nd we
14
Jmax
26
0.28
0.15(25) 14
0.3. 13
0.46
1.09
28 0.32 26
45 0.71 26
Note. Column 1: C and (A + B) from the Km1= 0,1 transitions (R bands); Column 2: A, B, C from a rigid rotor htting ofthe rotational transitions measured by RFMWDR for the 3sCl/37Cl isotopomer; Column 3: A, E, C from a rigid rotor fitting combining the rotational transitions measured by RFMWDR with the K-! = 0, 1 transitions (R bands) for the 35Cl/3’CI isotopomer; Column 4: Results of a fitting including tentatively assigned higher K_, R-branch transitions. ’ Centrifugal distortion constants undetermined from the fit; fixed to zero. b Standard error in parentheses in units of the last digit. ’ Centrifugal distortion constants fixed at the 37Clspecies values. d Number of transitions in the fit. ’ Fit standard deviation.
constants are somehow larger than the values we could expect for such a planar molecule. These large values could however be due to the low-frequency in-plane motions. According to Herschbach and Laurie ( 14) if one in-plane vibration has a frequency much lower than all the other vibrations, the value of the inertial defect is given by AvibN 4klw
(~67.4510
u A2 if o is in cm-‘).
In that case an in-plane vibration at about 60 cm-’ could account for the large positive inertial defect. From the empirical force field the lowest in-plane motion is calculated to be 158 cm-’ (6). The quadrupole coupling effect due to the two chlorine atoms was calculated (15) by transferring the quadrupole coupling constants of phosgene ( 16) to our molecule while taking into account the required rotation of the principal axis system. The resulting hyperline structure, for the measured RFMWDR lines, is centered almost symmetrically around the unperturbed frequency, and within the WHM of the lines,
159
MICROWAVE SPECTRUM OF C,02C12 TABLE III Comparison of the Structure Adjusted to the Rotational Constants to the ED Structure from
ref.
[5]
this
work
c-o
1.191
1.1906
bond
c=c
1.362
1.3478
lengths
cl-c3b
1.516
1.5133
ci,
c3 -c4
1.578
1.5689
C-Cl
1.682
1.6813
bond
o=c-c
94.0
94.19
angles
c=c-Cl
133.4
133.27
ldeg)
C2-C3=O
135.7
135.45
a
a Adjusted parameters with respect to Ref. (5). All seven independent parameters have been allowed to vary within a confidence interval set at the corresponding uncertainty in the ED structure. The differences between observed and calculated values are 0.08, -0.09, -0.46, -0.53, and +3 MHz for A, B, C (35C1/ %I), and C and (A + B) (“5Clz), respectively. ‘See Fig. 1 for atom numbering.
while for the other measured lines it is much more compact and centered on the unperturbed frequency. Table II shows the spectroscopic constants obtained for four different fits. When only (J + 1)0,~+1 * JLJ and(J + ~)o,(J+I) + J,+, transitions have been used, still it has been possible to obtain a value for the inertial defect, as it was almost insensitive to the assumed (A - B) value. A small adjustment of the structural parameters has been performed in order to reproduce the observed rotational constants. The final plausible structure is compared in Table III with the ED structure (5). In conclusion, we point out the advantage of the RFMWDR, made possible by an asymmetric isotopic substitution induced dipole component, in assigning ~&pe spectra. We also stress once again that in the present case there are no doubts about the assignment of the transitions measured with the RFMWDR technique, whereas for the remaining transitions, due to the reasons mentioned above, the assignments are tentative. ACKNOWLEDGMENTS We thank Dr. R. Danieli and Mr. S. Rose for technical assistance with the electronic equipment. WC. thanks Dr. R. Danieli, Director of the Istituto di Spettroscopia Molecolare de1 C.N.R., for hospitality in his Institute and also for proposing him as “Incaricato di ricerca.”
Note added in proof From an approximate force field calculation using the data from Refs. (5,6) a ground vibrational state inertial defect of 0.2 1 uAzwasobtained. This value is in agreement with the value listed in column 2 of Table II.
RECEIVED:
March 10, 1988
160
CAMINATI ET AL. REFERENCES
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