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Millimeter and submillimeter wave spectrum of the deuterated isoformyl ion DOC+
JOURNAL
OF MOLECULAR
SPECTROSCOPY
(1986)
115,229-231
Millimeter and Submillimeter Wave Spectrum of the Deuterated lsoformyl Ion DOC+ By analogy with the high interstellar value of the (HNC)/(HCN) ratio, it has been expected that HOC+ should also be present in molecular clouds, in view of the HCO+ abundance in these clouds. Quantum mechanical calculations have indeed shown that this ion is stable to intramolecular rearrangement with a barrier to isomerization in the range 35-40 kcal/mole (I-3). A number of molecular structure calculations (I, 2, 4) finally led to the detection of this ion in the laboratory (5-6) and tentatively in the interstellar medium (7). From the observation of the isotopic variants H18QC+and HO13C+,a preliminary determination of the r, structure of this ion has also been obtained (5). Comparison with theory shows that the C-O distance is in good agreement with ab initio calculations, but that the O-H one is too short (5). This discrepancy has been ascribed to a very-low-frequency bending mode, a feature already observed in molecules such as HNC (5). CsOH, and RbOH (8). Kraemer and Bunker’s calculations (.?) have shown that the isoformyl ion is not quasilinear, a possibility which has been envisaged by Gudeman and Woods (5). but that the bending potential is actually very shallow. Owing to these problems, the prediction of the still nonobserved spectrum of DQC’ was difficult. and several significantly different values of& have been recently published: 37 900(9). 38 232(30)(1(I). 38 lOO(20) MHz (3). However. an experimental determination of the DQC+ B0 value was desirable, since it would provide the hydrogen atom coordinate by using Kraitchman’s equation, which gives more consistent results than the center of mass condition used in (5). To save time. the DQC+ spectrum has been first searched and successfully observed in the submillimeter wave range by using a phase-locked carcinotron (C.S.F. TH 4218D) operating in the 340- to 406-GHz frequency range. This system allows scanning of the large frequency range for the preliminary search of lines as well as signal averaging when high sensitivity is needed (II). Lower frequency transitions have been studied by using harmonic generation from phase-locked klystrons as millimeter wave sources. The ions are produced in a magnetically confined glow discharge (I,?) cooled at liquid nitrogen temperature. which has been previously used to study HrD+ (13) HCOt (14). and H30+ (II). The optimum gas mixture composition is rather critical (DZ:CO:Ar = 1:5:150 at a total pressure of 7 mTorr) and is identical to the optimum conditions needed to observe HOC+in the same apparatus. The current must be kept very low (I * I mA). with a high voltage of only I .3 kV. A magnetic field of 250 G is necessary to observe the lines, confirming that they are due to an ion. With these experimental conditions, a S/N ratio of 20 can be obtained on the ./ = 4 - 5 line, with a lock-in time constant of 3 msec. The four measured frequencies are reported in Table I. From these frequencies, the rotational and centrifugal distortion constants in the ground state have been derived: & = 38193.1966(15) MHz Do = 93.7 lO(49) kHz. TABLE I Millimeter and Submillimeter Wave Transitions of DQC+ (Frequencies in MHz)
1+2
152 769.793(20)
2+3
229
3+4 4+5
0.005
149.049(30)
- 0.010
305
521.581(50)
- 0.002
381
885.116(50)
0.005
‘29
0022-2852186 $3.00 Copyright 0 1986 by Academic f’res. Inc. All rights of rcprcdunion in any fom reserved
NOTES
230
TABLE II Theoretical and Experimental Structures of HOC+ and HNC (in A) r(X-Y)
r(H-X)
Reference
rs(expt)a' b
1.1595
0.9641
rs(expt)c
1.1595
0.9342
(5)
C.I.
1.159
0.976
(4)
C.I.(corr.)
1.155
0.987
(A?)
1.155
0.988
(2)
CISDtQ/DZtP
1.1747
0.9961
(16)
C.I.-SDQ
1.1584
0.9910
(3)
C.I.-SDQ-Core
1.158
0.991
(17)
HNC rs(ewtJb
1.171
0.987
(E)
r,(exptJc
1.1717
0.9731
(5)
C.I.(corr.)
1.167
0.996
(9)
S.D.Q.
1.1719
0.9979
(I)
HOC+
MP3/6-311G* *
l
a) This work. b) Calculated using a complete substitutionmethod. c) Calculated using the center of mass condition.
These new data, together with the results of Gudeman and Woods (5), have provided a complete substitution structure. The results are given in Table II, where they are compared with the structure determined by using the center of mass condition (5) and with ab initio calculations. From this comparison, it appears that the O-H bond length shortening is much less dramatic than previously claimed (5) (0.026 instead of 0.056 A). However, this shortening is still two times greater than in the isoelectronic molecule HNC (Table II), a feature which could be ascribed to the difference in the bending mode frequency for these two molecules [Q = 378 cm-’ for HOC+ (.?) compared to Ye= 477 cm-’ for HNC (IS)]. Actually, observation of vibrational satellites is needed to obtain the true equilibrium structure with good accuracy. ACKNOWLEDGMENTS This work was partially supported by the Centre National de la Recherche Scientifique (ATP 93% 12) and by the Etablissement Public Regional, Nord-Pasde-Calais. It has been made possible by the loan of the carcinotron from the Centre Commun de Mesures de I’Universite de Lille I. We are grateful to M. Denis and P. Rosseels for experimental assistance. REFERENCES 1. P. J. BRUNA, S. D. PEYERIMHOFF,AND R. J. BUENKER, Chem. Phys. 10,323-334 (1975). 2. R. H. NOBES AND L. RAWM, Chem. Phys. 60, I-10 (1981). 3. W. P. KRAEMER AND P. R. BUNKER, J. Mol. Spectrosc. 101, 379-394 (1983).
NOTES
231
4. E. HERBST,J. M. NORBECK,P. R. CERTAIN, AND W. KLEMPERER,Astrophys. J. 207, 110-I 12 (1976). 5. C. S. GUDEMAN AND R. C. WOODS, Phys. Rev. Left. 48, 1344-1348 (1982). 6. G. A. BLAKE, P. HELMINGER, E. HERBST,AND F. C. DE LUCIA, Astrophys. J. 264, L69-L70 (1983). 7. R. C. WOODS, C. S. GUDEMAN, R. L. DICKMAN, P. F. GOLDSMITH,G. R. HUGUENIN, W. M. IRVINE, A HJALMARSON,L. 8, NYMAN, AND H. OLOFSSON,Astrophys. J. 270, 583-588 (1983). 8. D. R. LIDE AND C. MATSUMURA, J. Chem. Phys. 50,3080-3086 (1969). 9. W. P. KRAEMER, P. HENNING, AND G. H. F. DIECKSEN,in “LRs spectres des molkules simples au
laboratoire et en astrophysique,” 21st International Colloquium on Astrophysics, pp. 87-98. 1977. 10. C. S. GUDEMAN, PhD thesis, University of Wisconsin, 1982. 11. M. BOGEY,C. DEMUYNCK, M. DENIS, AND J. L. DESTOMBES, Astron. Astrophys. 148, Ll l-L13 (1985). 12. F. C. DE LUCIA, E. HERBST, G. M. PLUMMER, AND G. A. BLAKE, J. Chem. Phys. 78, 2312-2316
(1983). 13 M. BOGEY, C. DEMUYNCK, M. DENIS, J. L. DESTOMBES,AND B. LEMOINE, Astron. Astrophys. 137, Ll5-L16 (1984). 14. M. BOGEY, C. DEMUYNCK, AND J. L. DESTOMBES,Astron. Astrophys. 138, Ll l-L12 (1984). 15. R. A. CRESWELLAND A. G. ROBIETTE,Mol. Phys. 36,869-876 (1978). 16. D. J. DE FREES,G. H. LOEW, AND A. D. MC LEAN, Astrophys. J. 257, 376-382 (1982). 17. D. A. DIXON, A. KOMORNICKI, AND W. P. KRAEMER, J. Chem. Phys. 81, 3603-36 1 I ( 1984). 18. R. A. CRESWELL,E. F. PEARSON,M. WINNEWISSER,AND G. WINNEWISSER,Z. Natwforsch 31a, 22 l-
224 (1976). M. BOGEY C. DEMUYNCK J. L. DESTOMBES UniversitP de Lille I Laboratoire de Spectroscopic Hertzienne associe’ au CNRS 59655 VILLENEUVE D’ASCQ, Cedex, France Received May 24, 1985
OF MOLECULAR
SPECTROSCOPY
(1986)
115,229-231
Millimeter and Submillimeter Wave Spectrum of the Deuterated lsoformyl Ion DOC+ By analogy with the high interstellar value of the (HNC)/(HCN) ratio, it has been expected that HOC+ should also be present in molecular clouds, in view of the HCO+ abundance in these clouds. Quantum mechanical calculations have indeed shown that this ion is stable to intramolecular rearrangement with a barrier to isomerization in the range 35-40 kcal/mole (I-3). A number of molecular structure calculations (I, 2, 4) finally led to the detection of this ion in the laboratory (5-6) and tentatively in the interstellar medium (7). From the observation of the isotopic variants H18QC+and HO13C+,a preliminary determination of the r, structure of this ion has also been obtained (5). Comparison with theory shows that the C-O distance is in good agreement with ab initio calculations, but that the O-H one is too short (5). This discrepancy has been ascribed to a very-low-frequency bending mode, a feature already observed in molecules such as HNC (5). CsOH, and RbOH (8). Kraemer and Bunker’s calculations (.?) have shown that the isoformyl ion is not quasilinear, a possibility which has been envisaged by Gudeman and Woods (5). but that the bending potential is actually very shallow. Owing to these problems, the prediction of the still nonobserved spectrum of DQC’ was difficult. and several significantly different values of& have been recently published: 37 900(9). 38 232(30)(1(I). 38 lOO(20) MHz (3). However. an experimental determination of the DQC+ B0 value was desirable, since it would provide the hydrogen atom coordinate by using Kraitchman’s equation, which gives more consistent results than the center of mass condition used in (5). To save time. the DQC+ spectrum has been first searched and successfully observed in the submillimeter wave range by using a phase-locked carcinotron (C.S.F. TH 4218D) operating in the 340- to 406-GHz frequency range. This system allows scanning of the large frequency range for the preliminary search of lines as well as signal averaging when high sensitivity is needed (II). Lower frequency transitions have been studied by using harmonic generation from phase-locked klystrons as millimeter wave sources. The ions are produced in a magnetically confined glow discharge (I,?) cooled at liquid nitrogen temperature. which has been previously used to study HrD+ (13) HCOt (14). and H30+ (II). The optimum gas mixture composition is rather critical (DZ:CO:Ar = 1:5:150 at a total pressure of 7 mTorr) and is identical to the optimum conditions needed to observe HOC+in the same apparatus. The current must be kept very low (I * I mA). with a high voltage of only I .3 kV. A magnetic field of 250 G is necessary to observe the lines, confirming that they are due to an ion. With these experimental conditions, a S/N ratio of 20 can be obtained on the ./ = 4 - 5 line, with a lock-in time constant of 3 msec. The four measured frequencies are reported in Table I. From these frequencies, the rotational and centrifugal distortion constants in the ground state have been derived: & = 38193.1966(15) MHz Do = 93.7 lO(49) kHz. TABLE I Millimeter and Submillimeter Wave Transitions of DQC+ (Frequencies in MHz)
1+2
152 769.793(20)
2+3
229
3+4 4+5
0.005
149.049(30)
- 0.010
305
521.581(50)
- 0.002
381
885.116(50)
0.005
‘29
0022-2852186 $3.00 Copyright 0 1986 by Academic f’res. Inc. All rights of rcprcdunion in any fom reserved
NOTES
230
TABLE II Theoretical and Experimental Structures of HOC+ and HNC (in A) r(X-Y)
r(H-X)
Reference
rs(expt)a' b
1.1595
0.9641
rs(expt)c
1.1595
0.9342
(5)
C.I.
1.159
0.976
(4)
C.I.(corr.)
1.155
0.987
(A?)
1.155
0.988
(2)
CISDtQ/DZtP
1.1747
0.9961
(16)
C.I.-SDQ
1.1584
0.9910
(3)
C.I.-SDQ-Core
1.158
0.991
(17)
HNC rs(ewtJb
1.171
0.987
(E)
r,(exptJc
1.1717
0.9731
(5)
C.I.(corr.)
1.167
0.996
(9)
S.D.Q.
1.1719
0.9979
(I)
HOC+
MP3/6-311G* *
l
a) This work. b) Calculated using a complete substitutionmethod. c) Calculated using the center of mass condition.
These new data, together with the results of Gudeman and Woods (5), have provided a complete substitution structure. The results are given in Table II, where they are compared with the structure determined by using the center of mass condition (5) and with ab initio calculations. From this comparison, it appears that the O-H bond length shortening is much less dramatic than previously claimed (5) (0.026 instead of 0.056 A). However, this shortening is still two times greater than in the isoelectronic molecule HNC (Table II), a feature which could be ascribed to the difference in the bending mode frequency for these two molecules [Q = 378 cm-’ for HOC+ (.?) compared to Ye= 477 cm-’ for HNC (IS)]. Actually, observation of vibrational satellites is needed to obtain the true equilibrium structure with good accuracy. ACKNOWLEDGMENTS This work was partially supported by the Centre National de la Recherche Scientifique (ATP 93% 12) and by the Etablissement Public Regional, Nord-Pasde-Calais. It has been made possible by the loan of the carcinotron from the Centre Commun de Mesures de I’Universite de Lille I. We are grateful to M. Denis and P. Rosseels for experimental assistance. REFERENCES 1. P. J. BRUNA, S. D. PEYERIMHOFF,AND R. J. BUENKER, Chem. Phys. 10,323-334 (1975). 2. R. H. NOBES AND L. RAWM, Chem. Phys. 60, I-10 (1981). 3. W. P. KRAEMER AND P. R. BUNKER, J. Mol. Spectrosc. 101, 379-394 (1983).
NOTES
231
4. E. HERBST,J. M. NORBECK,P. R. CERTAIN, AND W. KLEMPERER,Astrophys. J. 207, 110-I 12 (1976). 5. C. S. GUDEMAN AND R. C. WOODS, Phys. Rev. Left. 48, 1344-1348 (1982). 6. G. A. BLAKE, P. HELMINGER, E. HERBST,AND F. C. DE LUCIA, Astrophys. J. 264, L69-L70 (1983). 7. R. C. WOODS, C. S. GUDEMAN, R. L. DICKMAN, P. F. GOLDSMITH,G. R. HUGUENIN, W. M. IRVINE, A HJALMARSON,L. 8, NYMAN, AND H. OLOFSSON,Astrophys. J. 270, 583-588 (1983). 8. D. R. LIDE AND C. MATSUMURA, J. Chem. Phys. 50,3080-3086 (1969). 9. W. P. KRAEMER, P. HENNING, AND G. H. F. DIECKSEN,in “LRs spectres des molkules simples au
laboratoire et en astrophysique,” 21st International Colloquium on Astrophysics, pp. 87-98. 1977. 10. C. S. GUDEMAN, PhD thesis, University of Wisconsin, 1982. 11. M. BOGEY,C. DEMUYNCK, M. DENIS, AND J. L. DESTOMBES, Astron. Astrophys. 148, Ll l-L13 (1985). 12. F. C. DE LUCIA, E. HERBST, G. M. PLUMMER, AND G. A. BLAKE, J. Chem. Phys. 78, 2312-2316
(1983). 13 M. BOGEY, C. DEMUYNCK, M. DENIS, J. L. DESTOMBES,AND B. LEMOINE, Astron. Astrophys. 137, Ll5-L16 (1984). 14. M. BOGEY, C. DEMUYNCK, AND J. L. DESTOMBES,Astron. Astrophys. 138, Ll l-L12 (1984). 15. R. A. CRESWELLAND A. G. ROBIETTE,Mol. Phys. 36,869-876 (1978). 16. D. J. DE FREES,G. H. LOEW, AND A. D. MC LEAN, Astrophys. J. 257, 376-382 (1982). 17. D. A. DIXON, A. KOMORNICKI, AND W. P. KRAEMER, J. Chem. Phys. 81, 3603-36 1 I ( 1984). 18. R. A. CRESWELL,E. F. PEARSON,M. WINNEWISSER,AND G. WINNEWISSER,Z. Natwforsch 31a, 22 l-
224 (1976). M. BOGEY C. DEMUYNCK J. L. DESTOMBES UniversitP de Lille I Laboratoire de Spectroscopic Hertzienne associe’ au CNRS 59655 VILLENEUVE D’ASCQ, Cedex, France Received May 24, 1985