J. Phys. Chem. 1988, 92, 5449-5455 bandwidth of the laser is sufficient to excite the entire ensemble of levels incoherently. The most rapid collisional relaxation process is rotational relaxation with a rate constant3* -5 X lo7 Torr-’ s-l. Assuming this process establishes the level width, we calculate that at an O2pressure of 500 Torr the mean lifetime of a MES is -40 ps, corresponding to a level width of 0.1-0.15 cm-’. By comparison to Table I, it can be seen that this width is on the order of the spacing of MES associated with the zero-order levels 6ll I , 6l 13, and 7’. Similarly, it can be determined that an O2 pressure of 2000 Torr will produce a level width -0.5 cm-l, sufficient to overlap the MES associated with all of the zero-order levels in this study. Thus, the spectra taken at O2pressures above 2000 Torr (0.1 mol/dm3) are associated with the dynamic evolution of coherent superpositions of states. For spectra taken below these pressures, we note that the average quenching interval in these cases is >60 ps, whereas the IVR dynamics typically occur on a time scale of + a I I
H
\ \ \
I
Y
HA
O ,,.
I H
is
/'"
(c 1 (d) Figure 8. Schematic representation of AM1 optimized geometries of 4-ABN-H20 complexes. In (b), (c), and (d) the water molecule lies below the plane of the aromatic ring.
different widths. The rotational profiles depend on the rotational temperature of the complex and on the geometry of the complex (specifically the ground- and excited-state rotational constants). The fact that the three excitation features have different rotational contours indicates that they are due to three different geometrical isomers, rather than being vibronic transitions of a single complex. A recent study of aniline-rare gas complexes34 showed that the rotational temperature of the complexes did not differ from that of the parent molecule. In the present case, with a more strongly bound polyatomic ligand such as water, the rotational temperature of the complexes might be expected to be somewhat higher than that of the bare molecule. However, all three complex peaks exhibit contours narrower than that of the bare molecule and the width of the contours decreases with increasing spectral shift (or binding energy). These observations are contrary to what would be expected if rotational temperature were the dominant factor in determining the width of the contours. Water may hydrogen bond to the cyano group of 4-ABN, to the amino nitrogen, or to the amino protons. As discussed above, the latter type of interaction is expected to give rise to a red shift whereas the former two can both lead to blue shifts. AM1 calculations were performed to assess the relative stabilities of some probable complex geometries involving interaction with the cyano group or the amino nitrogen. Four examples of stable geometries are shown schematically in Figure 8. As depicted in Figure 8a,b
5454 The Journal of Physical Chemistry, Vol. 92, No. 19. 1988
there are two obvious alternative structures for H 2 0 interacting with the cyano group; the linear structure shown in (a) is predicted to be the more stable of the two. The water molecule may be able to take up a large number of configurations in interacting with the amino nitrogen. Two possible geometries are illustrated in Figure 8c,d; both are predicted to be stable, with structure (d), in which the water molecule is swung round toward the ring, being the more stable. For all four structures shown the calculated hydrogen-bond length lies in the range 2.6-2.8 A. The complex exhibiting the smallest spectral shift (84 cm-l) has the broadest rotational contour, most similar to that of the bare molecule. It seems likely, therefore, that this complex has a structure of the form shown in Figure 8a. The intermolecular interaction responsible for the blue shift in this case is assumed to be dipole-dipole in nature, analogous to that discussed for the nitrile complexes discussed above. (Little change in the electron density on the cyano nitrogen is expected on excitation and hence little change in the strength of the hydrogen-bonding interaction.) On the basis of the relative dipole moments of water (1.85 D)36 and the nitriles (4.0 D)P6 a shift of approximately 110 cm-' would be predicted for this complex. The other two 4-ABN-H20 isomers probably involve association of the water molecule with the amino lone pair, with structures similar to those shown in Figure 8c,d. Further AM1 calculations and rotational contour simulations are currently in progress and together are expected to yield more detailed structural information. 4. 4-ABN-CH30H. As shown in Figure 6b addition of a small partial pressure (- 5 mbar) of methanol to the expansion is accompanied by the appearance of six new features to the highenergy side of the bare molecule features. All of these features appeared to be due to complexes of 1:l stoichiometry. The bands at 92,210, and 305 cm-I probably correspond to complexes similar in structure to the three formed between 4-ABN and water discussed above. The rotational contours of these peaks are similar in shape to these observed for 4-ABN-H20 and show a similar decreasing trend in width with increasing spectral shift. The weaker features between 210 and 305 cm-' may be due to transition involving low-frequency vibrations of the 210-cm-' complex. Further work, both experimental and theoretical, is being conducted to test the validity of the above interpretation. D. DMABN Complexes. The results of earlier studies of DMABN-CH30H complexes in this laboratory7 and of DMABN-H20 and DMABN-CF3H by Kobayashi et a1.8 are summarized here and discussed in the light of the above results for 4-ABN. The fluorescence excitation spectra of 1:l DMABN-CH,OH, DMABN-H20, and DMABN-CF,H complexes were shifted to the blue of the bare molecule spectrum by 19, 18, and 69 cm-l, respectively. The complex spectra displayed the same low-frequency structure as observed for the bare molecule, indicating interaction of the solvent molecule with the benzonitrile moiety rather than the dimethylamino group. The hypsochromic spectral shift could be accounted for in terms of the calculated change in electron distribution on excitation, as discussed above. These complexes did not exhibit anomalous a* emission. Increasing the partial pressure of complexing agent in the expansion resulted in the formation of complexes of higher stoichiometry, as manifested by the decrease in intensity of the excitation features due to the bare molecule and 1:l complexes. However, no new excitation features due to complexes of 1:2 (or higher) stoichiometry could be found. Kobayashi et a1.18 attributed this behavior to the possession of a small absorption coefficient by the 1:2 complexes. Our interpretation, on the other hand, is that such complexes are nonemissive (or too weakly emissive to be observed). Methanol, water, and trifluoromethane are all protic solvents and thus expected to form hydrogen-bonded complexes with DMABN. The results obtained for 4-ABN suggest that stable hydrogen-bonded complexes may be formed by interaction with either the cyano group or the amino nitrogen. In the case of DMABN, only 1:l complexes with the solvent molecule bound to the cyano group appear to fluoresce, implying that interaction
Gibson et al. of a solvent molecule with the dimethylamino group results in efficient nonradiative decay. This nonradiative decay process may correspond to formation of the a* state. Solution-phase experiments and quantum chemical calculations show that the a* state lies -5000 cm-' below the initially excited b* state.* In solution, this excess energy is rapidly dissipated and emission occurs from levels low in the a* manifold. In the jet such vibrational relaxation is impossible, and conversion to the a* state will leave the DMABN-solvent complex in possession of some 5000 cm-' of excess vibrational energy. It would not be surprising if the decay of such a highly vibrationally excited state were dominated by nonradiative processes, so that the a* state is nonemissive under jet-cooled conditions. (Even in solution phase the quantum yield of a* fluorescence is an order of magnitude less than that of unquenched b* emission.) The dark a* state invoked above cannot be a TICT state, since the hydrogen-bonding interaction between the amino nitrogen and the solvent molecule will not stabilize such a state. An alternative explanation for the anomalous behavior of DMABN in polar solvents has been proposed by Visser, Varma, and c o - ~ o r k e r s . ~ ~ * ~ ~ They attribute the a * emission to a DMABN-solvent exciplex which is bound by overlap of the lone-pair orbitals on the amino group and the solvent molecule. Formation of the exciplex involves twisting of the dimethylamino group in order to localize the lone pair on the amino nitrogen. The behavior of DMABN-solvent complexes is consistent with the formation of such an exciplex which is nonemissive under jet-cooled conditions. Thus, although the TICT hypothesis appears to account for the anomalous fluorescence of DMABN and related molecules, under those conditions where specific solutesolvent interaction at the amino group occurs exciplex formation may contribute to the observed behavior. 4. Conclusion The spectroscopic behavior of 4-ABN reported here closely resembles that of aniline. The excitation spectrum of 4-ABN is dominated by four fundamental vibrations, 6a, 12, 1, and 13 (in Varsanyi's notation), and the amino inversion mode is active in excitation and dispersed fluorescence spectra. The inversion frequencies observed for 4-ABN suggest that the amino group geometry is similar to that of aniline. In contrast, the fluorescence excitation spectra of DMABN and DMA are dominated by a characteristic progression of low-frequency transitions which can be attributed to torsional motion of the dimethylamino group. This suggests that the SIstate of DMABN has a twisted geometry, although the widely accepted TICT model assumes both the ground and SI states to be planar with twisting only occurring on formation of the a * state. In agreement with Kobayashi et ales and results recently published by Peng et al.?9 we find that the DMABN bare molecule does not exhibit a * emission. Fluorescence excitation spectra of 1:l complexes between DMABN and CH30H,7 H20,8339and NH339show small hypsochromic shifts ( 1 2 1 cm-I). Such complexes do not exhibit a* emission, and the similarity of their low-frequency spectral structure with that of the bare molecule suggests that the ligand is interacting with the benzonitrile moiety. Complexes of higher stoichiometry could be formed but appeared to be nonfluorescent. By comparison with results obtained from 4-ABN complexes, it can be inferred that interaction of a protic solvent molecule with the dimethylamino group of DMABN facilitates efficient nonradiative decay to the extent that these complexes cannot be observed by LIF. This behavior may be due to formation of a solute-solvent exciplex which is nonemissive under jet-cooled conditions. Very recently, and subsequent to the initial submission of this paper, Warren et aLa have reported an investigation of jet-cooled (37) Visser, R. J.; Varma, C. A. G. 0.;Konijnenberg,J.; Bergwerf, P. J . Chem. SOC.,Faraday Trans. 2 1983, 79, 341. (38) Visser, R. J.; Weisenborn, P. C. M.; Varma, C. A. G. 0.Chem. Phys. Lett. 1985, 113, 330. (39) Peng, L. W.; Dantus, M.; Zewail, A. H.; Kemnitz, K.; Hicks, J. M.; Eisenthal, K. B. J . Phys. Chem. 1987, 91, 6162.
J. Phys. Chem. 1988, 92, 5455-5466 DMABN by two-color time-of-flight mass spectrometry (TOFMS). This has revealed absomtion features due to two
(40) Warren, J. A.; Bernstein, E. R.; Seeman, J. I. J . Chem. Phys. 1988, 88, 871.
5455
theory that a specific type of solute-solvent interaction induces efficient nonradiative decav of iet-cooled DMABN.
untimely demise. Registry No. 4-ABN, 873-74-5; Ar, 7440-37-1; CH,CN, 75-05-8; CjH,CN, 109-74-0; H20, 7732-18-5; CH,OH, 67-56-1.
High-Resolutlon Interferometric Fourier Transform Infrared Absorption Spectroscopy in Supersonic Free Jet Expansions: Carbon Monoxide, Nitric Oxide, Methane, Ethyne, Propyne, and Trifluoromethane Andreas Amrein, Martin Quack,* and Ulrich Schmitt Laboratorium fur Physikalische Chemie, ETH- Zurich (Zentrum), CH-8092 Zurich, Switzerland (Received: January 29, 1988; In Final Form: April 18, 1988)
The infrared absorption spectra of neat CO, NO, CHI, CzH2,CH,CCH, and CHF, in supersonic free jet expansions have been recorded at apodized resolutions of up to 0.004 cm-' (fwhm) and wavenumbers ranging from 1000 to 3400 cm-'. The line widths are found to be essentially Doppler limited. Rotational temperatures as low as 6 K for CO and 10-50 K for the other gases have been determined from the spectra. Electronic (NO), vibrational (CH,CCH), and nuclear spin (CH,, C2H2)relaxation is discussed as well. In these cases rotational and electronic relaxation is fast, vibrational relaxation incomplete, and nuclear spin symmetry conservation perfect to within our accuracy. The aspect of simplification of infrared spectra by jet cooling is investigated, in particular for the extremely complex bands of CHF,.
1. Introduction Infrared spectra provide a wealth of information on molecular energy levels and intramolecular dynamics. This information is of great importance for a thorough understanding of chemical reactions' and IR laser chemistry in particular? In order to obtain a reasonable molecular Hamiltonian from the experimental data, one not only needs high spectral resolution but also needs a proper assignment of the observed transitions. The latter is often a formidable task because at ambient temperatures only a small fraction of polyatomic gas-phase molecules exists in the quantum mechanical internal ground state. For large molecules with many heavy (non-hydrogen) atoms, e.g., practically all molecules of biological interest, the fraction of ground-state molecules at ordinary temperatures is quite negligible. The high density of spectral lines at room temperature often leads to continuous spectra, because many lines overlap within their natural or Doppler widths. This vibrational and rotational hot-band congestion can be greatly reduced by using cooled samples. Cooling samples in spectroscopic cells works very well at not too low temperatures until the vapor pressure becomes too low. A second technique is matrix isolation spectroscopy. With liquid helium as a coolant, very low temperatures can be achieved. However, the interaction of the sample molecules with the matrix leads to important perturbations. Cooling molecules in a supersonic jet expansion appears to be an ideal technique for isolated molecules at low internal temp e r a t u r e ~ . ~There is hardly any spectroscopic technique that has (1) (a) Garland, N. L.; Lee, E. K. C. Faraday Discuss. Intrarnol. Kinet. 1983, 75, 377. (b) Parmenter, C. S. Ibid. p 7. (c) Puttkamer, K.v.; Diibal,
H. R.; Quack, M. Ibid. p 197. (d) Rizzo, T. R.; Hayden, C. C.; Crim, F. F. Ibid. p 223. (e) Chuang, M. C.; Baggott, J. E.; Chandler, D. W.; Farneth, W. E.; Zare, R. N. Ibid. p 301. ( f ) Zewail, A. H. Ibid. p 315. (8) Faraday Discuss. Dyn. Mol. Photofragmentation 1986, 82. (2) Lupo, D. W.; Quack, M. Chem. Reu. 1987, 87, 181.
0022-3654/88/2092-5455$01.50/0
not been used in conjunction with supersonic jets. The very sensitive laser-induced fluorescence (LIF) method was the first one a ~ p l i e dand , ~ it is the one that is most frequently used. E. K. C. Lee and his co-workers have used it for a number of important investigationss Later efforts to make use of the advantages offered by cold molecular beams include UV absorption spect r o ~ c o p y ,IR ~ ~ emission spectroscopy,4d I R laser absorption spectroscopy with bolometric6 and standard7,*detection, coherent anti-Stokes Raman spectroscopy (CARS),98vgbstimulated Raman spectroscopy (SRS),"W conventional laser Raman spectroscopy,1° visible overtone spectroscopy," microwave spectroscopylZand time-domain Raman spectroscopy.13 (3) Smalley, R. E.; Wharton, L.; Levy, D. H. Acc. Chem. Res. 1977,10, 139. (4) (a) Liverman, M. G.; Beck, S. M.; Monts, D. L.; Smalley, R. E. J . Chem. Phys. 1979, 70, 192. (b) Ryali, S. B.; Fenn, J. B. Ber. Bunsen-Ges. Phys. Chem. 1984,88,245. (c) Amirav, A.; Even, U.; Jortner, J. Chem. Phys. Lett. 1981,83, 1. (d) Venkateshan, S. P.; Ryali, S. B.; Fenn, J. B. J. Chem. Phys. 1982, 77, 2599. (5) (a) MacDonald, B. G.; Lee, E. K. C. J. Phys. Chem. 1982,86, 323. (b) Noble, M.; Apel, E. C.; Lee,E. K. C. J. Chem. Phys. 1983, 78,2219. (c) Noble, M.; Lee, E. K . C. J . Chem. Phys. 1984, 80, 134. (6) Gough, T. E.; Miller, R. E.; Scoles, G. Appl. Phys. Lett. 1977,30, 338. (7) Mizugai, Y.; Kuze, H.; Jones, H.; Takami, M. Appl. Phys. B 1983,32, 43. ( 8 ) Veeken, K.; Reuss, J. Appl. Phys. B 1984, 34, 149. (9) (a) Duncan, M. D.; Byer, R. L. IEEE J. Quantum Electron. 1979, QE-25,63. (b) Huber-Walchli, P.; Guthals, D. M.; Nibler, J. W. Chem. Phys. Lett. 1979, 67, 233. (c) Valentini, J. J.; Esherick, P.; Owyoung, A. Chem. Phys. Lett. 1980, 75, 590. (d) Esherick, P.; Owyoung, A. In Aduances in Infrared and Raman Spectroscopy: Clark, R. J. H., Hester, R. E., Eds.; Heyden: London, 1982. (10) Luijks, G.; Stolte, S.; Reuss, J. Chem. Phys. 1981, 62, 217. (1 1) Douketis, C.; Anex, D.; Ewing, G.; Reilly, J. P. J . Phys. Chem. 1985, 110, A171 ... -. (12) Zivi, H.; Bauder, A.; Giinthard, Hs. H. Chem. Phys. Lett. 1981,83, 469. (13) Graener, H.; Lauberau, A.; Nibler, J. W. Opt. Lett. 1984, 9, 165. I ,
0 1988 American Chemical Society
