Electronic spectra of the C 2n 1 H nä2 4 radicals in the gas phase

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1 JOURNAL OF CHEMICAL PHYSICS VOLUME 7, NUMBER 8 8 NOVEMBER 22 Electronic spectra of the C 2n H nä2 4 radicals in the gas phase H. Ding, T. Pino, F. Güthe, and J. P. Maier Department of Chemistry, University of Basel, Klingebergstrasse 8, CH-456, Basel, Switzerland Received 5 June 22; accepted 2 August 22 The visible electronic spectra of the linear l-c 2n H n 2 4 radicals have been measured in the gas phase. These have been obtained by means of a mass-selective resonant two-color two-photon ionization technique coupled to a supersonic plasma source. The observed spectra are assigned to the A 2 X 2, B 2 X 2, and C 2 X 2 electronic transitions arising from electron promotion. The assignments are based on ab initio calculations, wavelength dependence of the transition on size, and isotopic substitution. The lifetime broadening of the bands and effects due to vibronic coupling are associated with the carbon skeleton bending modes. The detection of these carbon chains in the diffuse interstellar medium appears to be more favorable by radio astronomy rather than by electronic spectroscopy. 22 American Institute of Physics. DOI:.63/.583 I. INTRODUCTION Unsaturated hydrocarbons play an important role in the chemistry of the interstellar medium ISM.,2 Carbon chains such as the C n H n 2 8 radicals were detected in dark interstellar molecular clouds and envelopes of evolved stars by radio astronomy. 3 5 Bare carbon clusters C 2 and C 3 have also been detected in diffuse interstellar clouds by optical absorption spectra. 6,7 The existence of such highly unsaturated molecules is essential for ISM chemistry. Unsaturated hydrocarbons are also important intermediates in combustion processes 8 and plasma chemistry. 9 The smallest member of the C 2n H homologous series, the C 3 H radical, has been studied by microwave,, vibrational, 2 and electronic spectroscopy. 3 The two isomers, linear (l C 3 H) and cyclic (c C 3 H), have been detected in the ISM. C 3 H is considered as a key molecule to understand the mechanisms of the carbon chain synthesis. 4,5 l-c 5 H and l-c 7 H are also known to be present in the ISM by their rotational spectra, 6 9 and have been characterized in the laboratory by microwave spectroscopy. 2,2 Microwave spectra of l-c 9 H and a cyclic-chain isomer of C 5 H have been reported. 4,22 The energetic stabilities of the C 2n H isomers radicals have been investigated by ab initio calculations These show that the two most stable forms are linear and cyclicchain, with the others at considerably higher energies. The linear form is predicted to be more stable than the cyclicchain by 7.5 kj/mol for C 3 H, kj/mol for C 5 H, 24 and 2.3 kj/mol for C 7 H. 26 The ground electronic state X 2 of the linear isomers splits into two components 2 /2 and 2 3/2. The 2 3/2 level lies 4.2 cm above the 2 /2 level for l-c 3 H, cm for l-c 5 H, cm for l-c 7 H, 4,7,2 and 25 cm for l-c 9 H. 4 Recently the electronic spectrum of l-c 3 H has been investigated by the mass-selective resonant two-color twophoton ionization R2C2PI spectroscopy. 3 In this paper the electronic spectra of l-c 5 H, l-c 7 H, and l-c 9 H are reported, detected via R2P2CI in a molecular beam. Ab initio calculations and a free-electron model are used to understand and assign the observed spectra. This mass selective experimental technique combined with a supersonic plasma source has proved to be a powerful tool to measure the visible electronic spectra of carbon chain radicals considered of interest in the interstellar medium. In particular, the availability of the gas phase spectra enables a direct comparison with the diffuse interstellar bands DIB s data to be made. 28 II. EXPERIMENT The experimental setup has been described. 29 It consists of a molecular beam combined with a linear time-of-flight TOF mass-analyzer. The source used to produce the C 2n H radicals is a pulsed valve coupled to an electric discharge. A gas mixture pulse of.3% of acetylene or d 2 acetylene in Ar backing pressure 8 bars is expanded through the ceramic body of the source which holds two steel electrodes withammhole in the middle separated by a ceramic spacer of about 4 mm. The gas flow is computer controlled adjusting the opening time of the pulsed nozzle to keep the pressure in the expansion chamber constant to ensure stable source conditions. A 2 s long high-voltage pulse 6 9 V from a homebuilt pulse generator is applied between the electrodes. The current is limited to about ma. The emerging C 2n H radicals beam enters the ionization region through a 2 mm skimmer. All ions are removed by an electric field perpendicular to the molecular beam after the skimmer and before entering the extraction zone of the TOF analyzer. The neutral radicals are then ionized by the R2PI method and the ions extracted in a two-stage acceleration setup towards an multichannel plate MCP detector. The signal from the MCP detector is fed into an ultrafast oscilloscope after preamplification and transferred to a computer. The mass resolution of the instrument is 9. Gates are set on the mass spectrum and the ion signal is recorded 2-966/22/7(8)/8362/6/$ American Institute of Physics Downloaded 26 Oct 22 to Redistribution subject to AIP license or copyright, see

2 J. Chem. Phys., Vol. 7, No. 8, 8 November 22 Electronic spectra of the C 2n H 8363 FIG.. Electronic spectrum of C 5 H detected by a resonant two-color twophoton ionization technique. FIG. 3. Electronic spectrum of C 9 H detected by a resonant two-color twophoton ionization technique. versus the wavelength, enabling the scan of a large number of masses independently. Up to 2 gates have been used concurrently. R2C2PI spectra have been recorded in the nm range. Excitation photons came from the output of a dye laser bandwidth. cm ) pumped by a XeCl excimer or an YAG laser for the longer wavelength. The dye laser energy was typically 5 5 mj per pulse. The ionizing photons at 57 nm came from a F 2 excimer laser with the energy of a few mj/pulse. Both lasers are unfocused. The dye laser beam was anticollinear with the molecular beam while the F 2 laser beam was perpendicular to the molecular beam. The temporal sequence of the two-color lasers is optimized to maximize the R2C2PI signal and the F 2 laser is usually fired a few nanoseconds after the dye laser. Bands in the spectra due to one-color two-photon resonances are excluded because the signals required two colors. In order to assure that the spectra do not contain bands from two-color 2 resonances, the vibronic band at 6 89 cm of C 7 H was probed using the second harmonic nm of the exciting photons. No resonance signal was observed. This shows that the observed R2P2CI spectra are the result of an process. III. RESULTS The R2C2PI spectra of C 5 H, C 7 H, and C 9 H are shown in Figs. 3. These are the cold gas phase spectra and no vibrational hot bands were observed. The wavelength maxima of the observed vibronic bands are given in Tables I III. Due to the broadness of the bands, their maxima were determined visually, leading to % uncertainty of their width, i.e.,..3 nm. No rotational substructure could be recorded in the spectra, even when cavity-ring-down absorption spectroscopy, which avoids saturation effects, was used. 3 This implies that the band shapes are intrinsic and due to internal conversion. The electronic spectrum of C 5 H consists of six strong vibronic bands Fig.. The observed origin is at nm. The profile of this band shows a shoulder about 24 cm to the red. In the spectrum of C 5 D three weak vibronic bands are observed Table I. The spectrum of C 7 H Fig. 2 exhibits four vibronic bands. The profile of the first band in the lowenergy part of the spectrum reveals a shoulder about 26.5 cm to the red. One deuterated vibronic band of C 7 Dis observed Table II. The spectrum of C 9 H Fig. 3 is composed of five vibronic bands. The bands are extremely broad full width at half maximum FWHM 8 cm for band. One vibronic band of C 9 D is observed Table III. TABLE I. Maxima of vibronic bands observed in the electronic spectrum of C 5 H and the assignment. Values observed for C 5 D are given in parentheses. Label nm (cm ) (cm ) Assignment FIG. 2. Electronic spectrum of C 7 H detected by a resonant two-color twophoton ionization technique. A 2 X B 2 X Downloaded 26 Oct 22 to Redistribution subject to AIP license or copyright, see

3 8364 J. Chem. Phys., Vol. 7, No. 8, 8 November 22 Ding et al. TABLE II. Maxima of the vibronic bands observed in the electronic spectrumofc 7 H and the assignment. Values observed for C 7 D are given in parentheses. Label nm (cm ) (cm ) Assignment A 2 X B 2 X C 2 X IV. THEORETICAL CALCULATIONS A. Ground states Ab initio calculations were carried using the GAUSSIAN 98 suite of programs 3 at the B3LYP/6-3G* level of theory. The relative energies of the isomers are inferred from the optimized ground-state geometries of the linear and cyclicchain isomers listed in Table IV. These calculations show that the linear structure is more stable than the cyclic-chain c-c 2n H) by 4. kj/mol for C 5 H, 57.3 kj/mol for C 7 H and 66.3 kj /mol for C 9 H zero-point energy corrections are included. The results are in agreement with those of Takahashi. 26 The linear chains become more stable compared to the cyclic-chain with increasing size. The electronic configuration of the X 2 ground state in the l-c 2n H homologous series is (n ) 2 (m ) 4 n 2 m, with n 7 and m 2 for l-c 3 H, n and m 3 for l-c 5 H, n 5 and m 4 for l-c 7 H, and n 9 and m 5 for l-c 9 H. The calculated harmonic vibrational frequencies are listed in Table V. Due to the high symmetry of these radicals, C v, the calculations could not treat correctly the symmetry of the X 2 ground state and the bending motions are found nondegenerate, even at different levels of theory. However the values are considered as satisfactory for an estimation of the mode frequencies. The results for l-c 5 H are in agreement with those obtained with the coupled cluster method. 24 The electronic configurations of the X 2 B 2 ground states in the c-c 2n H homologous series are TABLE III. Maxima of the vibronic bands observed in the electronic spectrum of C 9 H and the assignment. Values observed for C 9 D are given in parentheses. The asterisk indicates extrapolated values see text for details. Label nm (cm ) (cm ) Assignment A 2 X 2 66* 5 52* B 2 X 2 595* 6 87* C 2 X D 2 X F 2 X TABLE IV. Calculated bond lengths in Å in the ground states of the C 2n H n 4 radicals at the B3LYP/6-3G* level. The bond angles given are unique angle of the isosceles triangle ring of c-c 2n H. a Values taken from Ochsenfeld et al. Ref. 27. l-c 9 H l-c 7 H l-c 5 H l-c 3 H a c-c 9 H c-c 7 H c-c 5 H c-c 3 H C -H C -C C 2 -C C 3 -C C 4 -C C 5 -C C 6 -C C 7 -C C 8 -C b 2 a 2 4b 2 for c-c 5 H, 3b ] 2 4a 2 5b 2 for c-c 7 H, and 8a 2 4b 2 6b 2 for c-c 9 H. B. Excited states Ab initio calculations of the excited states were undertaken using the MOLPRO package. 32 The X 2 ground state of l-c 2n H is dominated by the 4 2 electronic configuration. The electron promotion leads to the 4 2 configuration, resulting in 4, 2, 2, and 2 excited states. The electron promotion yields the configuration, giving 2, 2, and 4 excited states. Vertical electronic excitation energies were calculated at the ground-state geometry for the doublet multiplicity only. These were obtained at the CASSCF/cc-PVD T Z level TABLE V. Ab initio calculated harmonic vibrational frequencies (cm )of l-c 2n H n 2 4 in their ground states at the B3LYP/6-3G* level. Stretching and bending modes are indicated by s and b labels. l-c 9 H l-c 7 H l-c 5 H 3484(s) 348(s) 3479(s) 2 295(s) 254(s) 273(s) 3 246(s) 267(s) 977(s) 4 233(s) 94(s) 467(s) 5 89(s) 588(s) 776(s) 6 648(s) 9(s) 77(b) 7 264(s) 845(b) 696(b) 8 928(b) 732(b) 588(b) 9 874(s) 697(b) 396(b) 823(b) 639(b) 346(b) 75(b) 572(s) 27(b) 2 67(b) 565(b) 5(b) 3 657(b) 43(b) 5(b) 4 588(b) 39(b) 5 536(b) 36(b) 6 486(b) 28(b) 7 45(b) 23(b) 8 398(b) 88(b) 9 36(b) 84(b) 2 242(b) 2 23(b) 22 36(b) 23 34(b) 24 5(b) 5(b) 25 Downloaded 26 Oct 22 to Redistribution subject to AIP license or copyright, see

4 J. Chem. Phys., Vol. 7, No. 8, 8 November 22 Electronic spectra of the C 2n H 8365 TABLE VI. Calculated vertical excitation energies ev and oscillator strength in parentheses of the - bands of the electronic transitions for the l-c 2n H n 4 radicals. State l-c 3 H l-c 5 H l-c 7 H l-c 9 H A ) ) ) ) B ) ) ) ) C ) ) ) ) D ) ) ) ) of theory, in the C 2v group of symmetry. The calculations included electrons in 2 orbitals for l-c 5 H, electrons in 3 orbitals for l-c 7 H, and 9 electrons in orbitals for l- C 9 H. No convergence could be reached for a larger active space in the case of l-c 9 H. The calculated vertical excitation energies are listed in Table VI together with that of l-c 3 H. 3 The data show that the three lowest excited states are dominated by electron promotion and the fourth one originates from. These four excited states are located within ev in the visible and near-uv spectral range. It appears that the excitation energies of the homologous electronic transition decrease with the carbon chain size Fig. 4. Furthermore, the vertical excitation energies of the three electronic states arising from the 4 2 electronic configuration are converging with the chain length Table VI, perhaps because the electron correlation decreases with the size of the chain. The vertical electronic excitation energies and the transition dipole moments of c-c 2n H n 4 were also calculated at the ground-state geometry at the CASSCF/6-3G** level of theory in the C 2v group of symmetry. The calculations included the active spaces with all 3 valence electrons in 3 orbitals for c-c 3 H Ref. 3, electrons in orbitals for c-c 5 H and c-c 7 H, and 9 electrons in 9 orbitals for c-c 9 H. These are listed in Table VII. It is shown that the dipole-allowed electronic transitions of c- C 2n H n 2 4 are located between.37 and.5 ev and in the range 3.35 ev. FIG. 4. The observed solid lines and calculated dashed lines wavelength of - bands of three lowest electronic transitions of l-c 2n H n 2 4 series. V. DISCUSSION A. Electronic transitions The bands in Figs. and 2 reveal small shoulders about 24 cm for C 5 H and 26.5 cm for C 7 H to the red. These are consistent with the spin orbit splitting in the ground state of the linear isomer C 5 H cm ) and C 7 H 26.7 cm ). 8 The spin orbit splitting in the excited states is expected to be small ( 2 ) or zero ( 2 ). From the intensity ratio between the 2 /2 and 2 3/2 components of the spinorbit splitting, the rotational temperatures are estimated to be 25 K for l-c 5 H and 45 K for l-c 7 H. The observed spectra are assigned to electronic transitions of the linear isomers. The assignment is also supported by the results from calculations. The calculated electronic transition energies Table VI of the linear isomers are located within the range of the observed spectra, while the values Table VII of cyclicchain isomers are in near infrared and UV and are out of range of the observed spectra. Electronic spectra of many carbon chains show a linear or quasilinear dependence between the wavelength of the origin band and the number of carbon atoms. 33 This can be understood within the frame of a free-electron model which has been useful in predicting the origin bands of the spectra of carbon chains. 34 It is based on the assumption that the electrons move in a potential that is relatively constant along the chain of N carbon atoms. Thus the value of the model is mostly in correlating electronic spectra for a homologous series. In case of the l-c 2n H series this model predicts the relationship for the wavelength n of the origin bands and n: n (2n ), where and are constants. The ab initio calculations show that the excited states of l-c 3 Hdo not fit the pattern followed by the larger members. This is due to electron correlation effects, as is also the case for the C 2 H radical in the l-c 2n H series. For l-c 2n H, n 2 4, the theoretical calculations reveal a similar constant for the states dominated by the same electronic configurations, 49 4 for and 38 4 for electron promotion. It is not straightforward to assign the transitions of l-c 2n H based on calculated vertical excitation energies because the four lowest electronic excited states are located within the range of the observed spectra. The linear relationship n (2n ) was used as a cross-reference through the assignment. The origin bands at longest wavelength are located at nm for l-c 5 H(C 5 D and nm for l-c 7 H(C 7 D Tables I and II. These are assigned to the A 2 X 2 transition. The small isotope shift supports Downloaded 26 Oct 22 to Redistribution subject to AIP license or copyright, see

5 8366 J. Chem. Phys., Vol. 7, No. 8, 8 November 22 Ding et al. TABLE VII. Calculated vertical excitation energies ev and transition dipole moment in parentheses, unit in Debye for the c-c 2n H n 4 radicals. a Values taken from Ref. 3. b Values taken from Ref. 4. c Values taken from Ref. 4. State c-c 3 H a c-c 3 H b c-c 3 H c c-c 3 H c-c 5 H c-c 7 H c-c 9 H 2 A B A B A A B this assignment. The ab initio predictions are.3 and. ev above the experimental values. This is a comparable difference to that found for l-c 3 H, using a similar level of theory. 3 According to this assignment, the experimental constant is estimated to be 32 for the electron promotion. Band 4 of l-c 5 H(C 5 D at nm and band 2ofl-C 7 H at nm are assigned as the origin transitions of the B 2 X 2 system. The ab initio calculations predict that the transition dipole moment of the latter system is comparable to that of A 2 X 2, in agreement with the intensity of the observed bands. The calculated vertical excitation energies are.4 ev and.2 ev above the experimental values. This assignment yields 28. The nm band of l-c 7 H and the nm bands of l-c 9 H(C 9 D are assigned as origins of the C 2 X 2 electronic transition. The calculated vertical excitation energies are within. ev of the experimental values. The origin of this system in l-c 5 H is predicted to be at 44 nm by the ab initio calculations. The origin bands of the A 2 X 2 and B 2 X 2 transitions in l-c 9 H are expected to lie at 66 and 595 nm, respectively, consistent with the ab initio prediction at 68 and 6 nm. They were not detected in the spectrum. The reason is that the ionization potential IP is probably too high for ions to be produced using this R2C2PI scheme. The IP of l-c 9 H is expected to be higher than that of l-c 7 H Ref. 35 and thus.23 ev. Bands 2 and 5 of l-c 9 H Table III are assigned as origins of the D 2 X 2 and F 2 X 2 (E 2 X 2 is dipole forbidden transitions, respectively, based on the ab initio calculations. In Fig. 4 calculated or extrapolated and observed origin bands of the three electronic transitions in the l-c 2n H n 2 4 series versus the number of carbon atoms are plotted. A qualitative agreement between the theory and assignment is seen. The inferred experimental constants are comparable, following the trend revealed by the ab initio results. B. Vibronic bands The vibronic bands apparent in the spectra are assigned by comparison to the calculated harmonic frequencies in the ground state Table V. IntheA 2 state of l-c 5 H the 9 and 2 modes are excited with frequencies of 66 and 456 cm. The 9 mode is a carbon skeleton bending motion with a frequency of 478 cm in l-c 5 D. In the B 2 X 2 system, the and 2 transitions are identified, resulting in 356 cm, a carbon skeleton bending motion. The vibronic band at nm in the spectrum of l-c 7 H is assigned to the 3 transition. The 3 mode has then a frequency of 457 cm. The nm band of l-c 9 H is associated to the excitation of the mode, with frequency of 745 cm. The next nm band is assigned as the 7 transition. The unambiguous assignment to individual vibrational modes of electronic excited states requires the knowledge of the potential energy surface of the excited states. This is not available at present. However, carbon skeleton bending motions appear to be the most active coordinates of the l-c 2n H n 2 4 species. VI. CONCLUDING REMARKS The electronic spectra of the l-c 2n H n 2 4 radicals have been observed for the first time in the visible and near-uv spectral ranges. The ab initio calculations of the structures in electronic excited states of these radicals have also been carried out. No signature of large geometry changes on electronic excitation is seen, but all the vibronic bands are broad. It is interesting to compare the difference between the odd l-c 2n H and even l-c 2n H chains. For the latter, n 2 4, both the ground and excited states are linear and electronic spectra are at least partially rotational resolved lifetime broadening is found for n 4, 36 while the l-c 2n H n 2) spectra have a diffuse character, although lying in the same energy region. This is due to a stronger nonadiabatic coupling between the states, perhaps related to the nature of the promoting modes and electronic configurations. The visible electronic cold spectra in the gas phase provide a database for an astronomical search. However, the observed electronic spectra do not match any known DIB s, 37 except for the origin of A 2 X 2 transition of l-c 7 H which is coincidental because the widths of the laboratory.5 Å and DIB band Å are too different. An upper limit of the column density can be estimated for the l-c 2n H n 2 4 molecules in diffuse clouds. The oscillator strengths of the - band of the observed electronic transitions have been calculated to be in the range..3 Table VI. Assuming an equivalent width of må current sensitivity limit of DIB s detection, the column densities in such diffuse clouds would be 2 cm For Downloaded 26 Oct 22 to Redistribution subject to AIP license or copyright, see

6 J. Chem. Phys., Vol. 7, No. 8, 8 November 22 Electronic spectra of the C 2n H 8367 l-c 3 H the column density is determined as 5 2 cm 2 in a dark cloud TMC- and 8.7 cm 2 in diffuse clouds from millimeter-wave observations. 39 Similarly the column densities of l-c 5 H and l-c 7 H in TMC- have been reported as cm 2 and.5 cm 2, respectively. 3,7,9 The 2 cm 2 upper limit of the column densities in diffuse clouds implies that microwave rather than optical absorption should be used to detect the C 2n H n 2 4 chains in view of their large dipole moments. ACKNOWLEDGMENTS Professor H. Huber is thanked for helpful discussions about the ab initio calculations. This work has been supported by the Swiss National Science Foundation Project No T. Henning and F. Salama, Science 282, H. W. Kroto, J. R. Heath, S. C. O Brian, R. F. Curl, and R. E. Smalley, Astrophys. J. 34, M. B. Bell, P. A. Feldman, J. K. G. Watson, M. C. McCarthy, M. J. Travers, C. A. Gottlieb, and P. Thaddeus, Astrophys. J. 58, , and references therein. 4 M. C. McCarthy, M. J. Travers, A. Kovacs, C. A. Gottlieb, and P. Thaddeus, Astrophys. J., Suppl. Ser. 3, M. C. McCarthy, W. Chen, M. J. Travers, and P. Thaddeus, Astrophys. J., Suppl. Ser. 29, 6 2, and references therein. 6 E. F. van Dishoeck and J. H. Black, Astrophys. J., Suppl. Ser. 62, J. P. Maier, N. M. Lakin, G. A. H. Walker, and D. A. Bohlender, Astrophys. J. 553, K.-H. Homann, Angew. Chem. Int. Ed. Engl. 37, T. Fujii and M. Kareev, J. Appl. Phys. 89, P. Thaddeus, C. A. Gottlieb, A. Hjalmarson, L. E. B. Johansson, W. M. Irvine, P. Friberg, and R. A. Linke, Astrophys. J. Lett. 294, L S. Yamamoto, S. Saito, M. Ohishi, H. Suzuki, S. Ishikawa, N. Kaifu, and A. Murakami, Astrophys. J. Lett. 322, L Q. Jiang, C. M. L. Rittby, and W. R. M. Graham, J. Chem. Phys. 99, H. Ding, T. Pino, F. Güthe, and J. P. Maier, J. Chem. Phys. 5, R. I. Kaiser, C. Ochsenfeld, M. Head-Gordon, Y. T. Lee, and A. G. Suits, Science 274, E. Buonomo and D. C. Clary, J. Phys. Chem. A 5, J. Cernicharo, C. Kahane, J. Gomez-Gonzalez, and M. Guélin, Astron. Astrophys. 64, L J. Cernicharo, C. Kahane, J. Gomez-Gonzalez, and M. Guélin, Astron. Astrophys. 67, L J. Cernicharo, M. Guélin, and C. M. Walmsley, Astron. Astrophys. 72, L M. Guélin, J. Cernicharo, M. J. Travers, M. C. McCarthy, C. A. Gottlieb, P. Thaddeus, M. Ohishi, S. Saito, and S. Yamamaoto, Astron. Astrophys. 37, L C. A. Gottlieb, E. W. Gottlieb, P. Thaddeus, and J. M. Vrtilek, Astrophys. J. 33, M. J. Travers, M. C. McCarthy, C. A. Gottlieb, and P. Thaddeus, Astrophys. J. Lett. 465, L A. J. Apponi, M. E. Sanz, C. A. Gottlieb, M. C. McCarthy, and P. Thaddeus, Astrophys. J. 547, L D. E. Woon, Chem. Phys. Lett. 244, T. D. Crawford, J. F. Stanton, J. C. Saeh, and H. F. Schaefer, J. Am. Chem. Soc. 2, S. J. Blanksby, S. Dua, and J. H. Bowie, J. Phys. Chem. A 3, J. Takahashi, Publ. Astron. Soc. Jpn. 52, C. Ochsenfeld, R. I. Kaiser, and Y. T. Lee, J. Chem. Phys. 6, T. Motylewski, H. Linnartz, O. Vaizert, J. P. Maier, G. A. Galazutdinov, F. A. Musaev, J. Krelowski, G. A. H. Walker, and D. A. Bohlender, Astrophys. J. 53, T. Pino, H. Ding, F. Güthe, and J. P. Maier, J. Chem. Phys. 4, Unpublished data from this laboratory. 3 M. J. Frisch, G. W. Trucks, and H. B. Schlegel, GAUSSIAN 98, Revision A.7, Gaussian, Inc., Pittsburgh, PA, MOLPRO is a package of ab initio programs written by H.-J. Werner and P. J. Knowles, with contributions from others: see molpro 33 J. P. Maier, Chem. Soc. Rev. 26, J. K. G. Watson, Astrophys. J. 437, S. B. H. Bach and J. R. Eyler, J. Chem. Phys. 92, H. Linnartz, T. Motylewski, and J. P. Maier, J. Chem. Phys. 9, S. Ó. Tuairisg, J. Cami, B. H. Foing, P. Sonnentrucker, and P. Ehrenfreund, Astron. Astrophys. J., Suppl. Ser. 42, L. Spitzer, Physical Processes in the Interstellar Medium Wiley, New York, R. Lucas and H. S. Liszt, Astron. Astrophys. 358, S. Ikuta, J. Chem. Phys. 6, R. K. Chaudhuri, S. Majumder, and K. F. Freed, J. Chem. Phys. 2, Downloaded 26 Oct 22 to Redistribution subject to AIP license or copyright, see