46
J . Phys. Chem. 1988, 92, 46-50
Infrared Laser Spectroscopy of the Ethylene-HF and Allene-HF Binary Complexes Formed in a Molecular Beam Z. S. Huang and R. E. Miller* Department of Chemistry, University of North Carolina, Chapel Hill,North Carolina 2751 4 (Received: April 7 , 1987)
The optothermal detection method has been used to obtain near-infrared spectra of the ethylene-HF and allene-HF binary complexes. The spectrum of ethylene-HF has been assigned and fit to a rigid asymmetric rotor Hamiltonian, yielding accurate values for the rotational constants and vibrational origins. For allene-HF the spectrum is only partially resolved so that the spectroscopic constants are less accurately known. In both cases, the line widths of the transitions are observed to be much broader than the limit imposed by instrumental effects, indicating that the lifetime of the vibrationally excited state is relatively short. In the case of ethylene-HF, the electric dipole moment has been determined in the excited vibrational state by measuring the infrared Stark spectrum of several transitions.
Introduction The importance of hydrogen bonding in inter- and intramolecular interactions, from the simplest systems such as HF-HF to the most complex biological molecules, cannot be overemphasized and is attested to by the enormous literature devoted to the subject.'-" Of particular interest has been the study of isolated binary c~mplexes,~ which undoubtedly provides the most direct comparison with theory. Unfortunately, the spectroscopic study of these species has long been hampered by the difficulties associated with forming high enough gas-phase concentrations of the molecule of interest. With the advent of free jet expansion technology this problem has been essentially eliminated. In the microwave region of the spectrum, the study of hydrogen-bonded binary complexes using molecular beam methods has developed into a mature field, with many detailed and reliable structural determinations having been made.5,6 Although infrared spectroscopy has long &en recognized as having important advantages in the study of these species,'-5 the vast majority of such studies have provided only low-resolution spectra from which it is difficult or impossible to determine accurate molecular properties and structures. Very recently, however, there have been dramatic developments in the field of high-resolution gas-phase infrared spectroscopy of these weakly bound complexes. For example, several groups have used tunable high-resolution infrared lasers in conjunction with long path length gas cells to obtain rotationally resolved spectra of a number of small binary complexes such as (HF)2,7(HC1)2,8Rg-HF,9 HF-HCN,lo>" and OC-HF.I2 Even higher resolution is available with a number of infrared lasermolecular beam methods. In the case of the optothermal detection technique, the spectrum is obtained by measuring the laser-induced change in the molecular beam energy with a liquid helium cooled ( I ) Pimentel, G.C.; McClellan, A. L. The Hydrogen Bond W. H. Freeman: San Francisco and London, 1960. (2) Joesten, M. D.; Schaad, L. J. Hydrogen Bonding, Marcel Dekker: New York, 1974. (3) Green, R. D. Hydrogen Bonding by C-H Groups; Wiley: New York, 1974. (4) Schuster. P.; Zundel, G.; Sandorfy, C., Eds. The Hydrogen Bond North-Holland: New York, 1976; Vols. 1-111. ( 5 ) Boschke, F. L., Ed. Hydrogen Bonds; Topics in Current Chemistry; Springer-Verlag: Berlin, 1984, Vol. 120. (6) Legon, A. C.; Millen, D. J . Chem. Reo. 1986, 86, 635. (7) Pine, A. S.; Lafferty, W. J. J . Chem. Phys. 1983, 78, 2154. Pine, A . S.; Lafferty, W.J.; Howard, B. J. J . Chem. Phys. 1984, 81, 2939. (8) Ohashi, N.; Pine, A. S. J . Chem Phys. 1984, 81, 73. (9) Fraser, G. T.; Pine, A. S. J . Chem. Phys. 1986, 85, 2502. (10) Wofford, B. A.; Bevan, J. W.; Olson, W. B.; Lafferty, W. J. J . Chem. Phys. 1985, 83, 6188. ( I I ) Jackson, M. W.; Wofford, 8 . A,; Bevan, J. W.; Olson, W. B. Lafferty, W. J. J . Chem. Phys. 1986, 85, 2401. (12) Kyro, E. K.; Shoja-Chaghervand, P.; McMillan, K.; Eliades, M.; Danzeiser, D.; Bevan, J . W. J . Chem. Phys. 1983, 78, 7 8 .
0022-3654/88/2092-0046$01.50/0
bolometer detector. This method has been used to obtain highresolution spectra for systems such as (HF)2,13Ar-HF,14 NZ-HF,I5 OC-HF,I6 C2HZ-HF," (CzH4),,18,19and (C02)z.z0~21Direct absorption in pulsed free jet expansions has also emerged as a sensitive method for obtaining spectra of this type. A number of tunable laser sources have been used in this way to study systems such as Ar-OCS,2z Ar-HF,z3 and (HCCH)3.z4 Clearly, infrared spectroscopy is fast becoming a method of similar flexibility in the study of these species to that of microwave spectroscopy. In the case of the infrared spectroscopic studies there is the added incentive to understand the vibrational predissociation dynamics associated with the excited vibrational states of these complexes. In a recent paper, we reported the sub-Doppler resolution infrared spectrum of the v I band of C2HZ-HF.l7 This study clearly showed that the equilibrium structure (established from nuclear spin statistical arguments) has the H F molecule hydrogen bonded to the A electrons of the acetylene in the T-shaped configuration. This is in agreement with the earlier microwave which obtained the vibrationally averaged structure from the rotational constants of several isotopic forms of C2H2-HF. From the width of the observed transitions it was also possible to estimate the lifetime of the excited vibrational state. Another hydrogen-bonded complex that has been observed in both the infraredz6 and mic r o w a ~ regions e ~ ~ ~of~ the ~ spectrum is ethylene-HF. The microwave results clearly show that the structure of this complex is T-shaped with the H atom of H F pointing toward the A electrons of ethylene. In view of the previous low-resolution infrared study
(13) Huang, Z . S.; Jucks, K . W.; Miller, R.E. J . Chem Phys. 1986, 85, 3338. (14) Huang, Z . S.; Jucks, K. W.; Miller, R. E. J . Chem. Phys. 1986,85, 6905. (15) Huang, 2.S.; Jucks, K. W.; Miller, R. E. J . Chem. Phys. 1987, 86, 1098. (16) Jucks, K. W.; Miller, R. E. J. Chem. Phys. 1987, 86, 6637. (17) Huang, 2.S.; Miller, R. E. J . Chem. Phys. 1987, 86, 6059. (18) Snels, M.; Fantoni, R.; Zen, M.; Stolte, S.; Reuss, J. Chem. Phys. Lett. 1986, 124, 1. (19) Watts, R. O., private communication. (20) Miller, R. E.; Watts, R. 0. Chem. Phys. Lett 1984, 105, 409. (21) Jucks, K. W.; Huang, Z. S.; Dayton, D.; Miller, R. E ; Lafferty, W. J. J . Chem. Phys. 1987, 86, 4341. (22) Hayman, G. D.; Hodge, J.; Howard, B. J.; Muenter, J. S.; Dyke, T. R . Chem. Phvs. Lett. 1985. 118. 12. (23) Lovejoy, C. M.; Schuder, M. D.; Nesbitt, D. J. Chem. Phys. Lert. 1986, I27, 374; J . Chem. Phys. 1986, 85, 4890. (24) Prichard, D.; Muenter, J. S. Chem. Phys. Letr., in press. (25) Read, W. G.; Flygare, W. H. J . Chem. Phys. 1982, 76, 2238 (26) Kolenbrander, K. D.; Lisy, J. M. J . Chem. Phys. 1986, 85, 2463. (27) Read, W. G.; Flygare, W. H. J . Chem. Phys. 1982, 76, 4857. (28) Nelson, D. D.; Fraser, G.T.; Klemperer, W. J . Chem. Phys. 1985, 82, 4483.
0 1988 American Chemical Society
Ethylene-HF and Allene-HF Binary Complexes of Lisy and wworkersZ6on the v 1 band of C2H4-HF, which shows that rotational fine structure can be observed for this system, we have carried out a high-resolution study of this molecule. The spectrum is accurately fit by a rigid rotor Hamiltonian yielding accurate upper state rotational constants and band origin. In addition, we have obtained for the first time an infrared spectrum of the allene-HF binary complex. In this case, the spectrum is only partially resolved due to the short lieftime of the excited vibrational state. For both of these systems, an accurate value of the excited-state lifetime has also been obtained from the width of the observed transitions. In the case of C2H4-HFthe resolution is sufficiently high so that the excited-state diple moment can also be determined from Stark measurements. Experimental Section
The apparatus used in this study has been discussed in detail el~ewhere.l~-'~ As a result, only a brief description of it is given here. The molecular beam source consists of a conventional free jet expansion nozzle which is operated continuously. A simple two-stage differential pumping system is used in order to maintain a low background pressure in the detection chamber and to collimate the molecular beam. A liquid helium cooled bolometer is positioned so as to intercept the molecular beam, thus giving a monitor of its total energy. Located midway between the source and the detector is a multipass reflection cell29which is used to increase the interaction time between the molecules in the beam and the infrared laser. By applying an electric field between the two gold-coated mirrors of this cell, it can also be used to carry out infrared Stark measurements from which electric dipole moments can be determined. The gases used in this study were mixed on line in order to facilitate easy optimization of the source conditions. In practice a 1% mixture of HF in He was mixed with the gas of interest by adjusting the flow rate through two needle valves. The spectra reported here were obtained by using a gas mixture consisting of approximately 10% ethylene or allene expanded from a source pressure of 350 P a . Under these conditions there was no evidence, within the spectral region studied, for contributions to the spectrum from higher clusters. As discussed elsewhere,I3 the F-center laser used in this study has been interfaced to a computer in order to facilitate long single mode scanning. In order to obtain continuous frequency scans, all three laser tuning elements, namely, the grating, cavity end mirror, and the intracavity etalon, must be varied. Two extracavity confocal etalons (FSR = 734.4 MHz and 7.5 GHz) are used as spectrum analyzers to ensure that the laser scans continuously while a third (FSR = 149.58 MHz) is used as a frequency marker. Absolute calibration of the spectrum is obtained by simultaneously recording the spectrum of interest and that of Doppler limited water vapor. In this way, it is possible to obtain an absolute frequency calibration for the observed transitions to approximately 0.005 cm-' and a relative calibration of 0,0005 cm-'. The ethylene-HF spectrum reported in the next section was recorded in this way. In the case of the allene-HF spectrum, the observed transitions are so broad, in comparison with the frequency spacing between laser cavity modes, that only the grating and intracavity etalon had to be scanned. In this mode of operation the laser hops from one cavity mode to the next rather than scanning continuously. The spacing between laser cavity modes is approximately 300 MHz. In order to test the reliability of this method in recording the molecular spectrum, several regions were recorded by using both the continuous and mode hop scanning methods. In all cases, the spectra obtained by using the two methods were in excellent agreement. The advantage of the mode hop method is that large regions of the spectrum can be covered very quickly, a feature that is particularly useful when searching for a new spectrum. This method is clearly only applicable to spectra whose transition are much broader than the cavity FSR. In this mode of operation (29) Stewart, G . M.; Ensminger, M. D.; Kulp, T. J.; Ruoff, R. S.; McDonald, J. D. J . Chem. Phys. 1983, 79, 3190.
The Journal of Physical Chemistry, Vol. 92, No. 1, 1988 41
I
EXPERIMENTAL
1
1
1
1
5. 0 FREQUENCY (CM - 1 )
Figure 1. Calculated and experimental ethylene-HF Y, spectra. Spectra B and A were calculated with and without nuclear spin statistics included,
respectively. the 7.5-GHz confocal etalon was used as a frequency marker. Once again, water vapor transitions observed in a low-pressure gas cell were used to obtain the absolute frequency calibration. As is now standard in these experiments, spectra are recorded by amplitude modulating the laser at 204 Hz with a chopper, while using phase-sensitive detection of the bolometer signal. In this way, bolometer signals are only obtained when the laser is in resonance with the molecules in the beam so that the base line is always flat. Ethylene-HF
Figure 1 shows the experimental spectrum obtained for the ethylene-HF binary complex. The spectrum shows partially resolved P, Q, and R branch lines which are characteristic of a parallel band for a slightly asymmetric rigid rotor. In fact, the splitting of the P and R branch lines results from the asymmetry of the molecule. Two calculated spectra are also shown in the figure. These spectra were calculated by using an asymmetric rigid rotor program, the constants having been determined from a fit to the assigned transitions in the experimental spectrum. The fit was obtained by fixing the ground-state rotational constants at the microwave values and fitting to the upper vibrational state rotational constants and the vibrational origin. In both of the simulated spectra a rotational temperature of 4 K was used. Calculation A shows the result of a spectral simulation in the absence of nuclear spin statistics while in spectrum B the spin statistics characteristic of the T-shaped hydrogen-bonded equilibrium geometry have been included. For this geometry the even to odd K , spin degeneracy is 10:6. Although the effects of spin degeneracy are less pronounced than in the acetyleneHF complex reported earlier,17 it is clearly evident when one compares the experimental and calculated spectra. As a result, the spin statistics alone firmly establish that the C2H4-HF binary complex has a C, symmetry axis which lies along the A inertial axis, as expected for the T-shaped geometry. It is interesting to note that while rotational constants give information on the vibrationally averaged structure the nuclear spin statistics are determined by the equilibrium structure. The observed transition frequencies for all of the individually assignable lines in the spectrum, along with the assignments and residuals obtained from the fitted spectrum, are listed in Table I. There is clearly excellent agreement between the experimental and calculated spectra. The molecular constants used to generate the calculated spectrum are summarized in Table 11. As is now, apparently, a general characteristic of the H F containing binary complexes, the B and C rotational constants increase due to vibrational excitation. This indicates that the hydrogen-bond length becomes shorter upon vibrational excitation of the H F subgroup.
The Journal of Physical Chemistry, Vol. 92, No. I, 1988
48
Huang and Miller
TABLE I: Summary of the Frequencies and Assignments for the Transitions Used in the Spectral Fit“ J’ 9 8 7 6 5 4 3 2 1 0 1 2 3 4
5 6 9 8 8 7 7 6 5
5 4 3 3 4 3 4 5 8
K,‘ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2
K,’
J”
9 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 9 7 8 6 7 6 4
8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 7 7 6 6 5 4 4 3 4 4 5 4
5 4 3 2 4 2 3 4 6
K/ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1
1
5
1 1 1 2 2
6 7
2 2
K,“ 8 7 6 5 4 3 2 1 0 1 2 3 4
5 6 7 8 6 7 5 6
5 3 4 3 4 3
5 3 4
5 5
obsd, cm-l 3784.4805 3784.1575 3783.8349 3783.5199 3783.2083 3782.9028 3782.6018 3782.3063 3782.0164 3781.4580 3781.1904 3780.9293 3780.6756 3780.4290 3780.1912 3779.9596 3784.4155 3784.2488 3784.0946 3783.9119 3783.7764 3783.4664 3783.2522 3783.1615 3782.8619 3780.6955 3780.6368 3780.4534 3780.6551 3780.4047 3780.1599 3784.1877
residuals, cm-’ 0.0014 0.0024 -0.0002 00005 0.0001
0.0008 0.0007 0.0003 -0.0006 -0.0007 0.0004 0.0007 0.0009
0.0005 0.0012 0.0002 -0.0008 -0.0010 0.0003 0.0005
-900.0 -300.0 300.0 900.0 FREQUENCY (MHz) Figure 2. Comparison between the line widths for the HF monomer and
the ethylene-HF complex. The solid line through the C,H,-HF spectrum is a Voigt line shape.
-0.0015 -0.0009 -0.0016 -0.0011 -0.0018 -0.0006 -0.0020 -0.0007 0.0037 0.0016 -0.0019 -0.0007
“Residuals were calculated by using the constants given in Table 11. TABLE 11: Summary of the Molecular Constants for C2H4-HF in the Ground and Excited Vibrational States
mol const A, cm-’ B, cm-I C, cm-’ P? D uo, cm-’
lower state
upper state
0.804 6326
0.80277 (30) 0.145 69926 0.14952 (5) 0.130 04626 0.13300 ( 5 ) 2.383 917 2.610 (8) 3781.735 (5)
This is consistent with the large red shift in the H F stretching frequency due to formation of the complex which indicates that the well depth in the excited vibrational state is greater than that of the ground state. As expected, the A rotational constant, which is essentially independent of the length of the hydrogen bond, is relatively unchanged by the HF vibrational excitation. As is the case for many of the binary complexes we have recently studied, the line widths of the observed transitions are considerably broader than the limit imposed by instrumental effects. Indeed, Figure 2 shows a comparison between the R(0) transition of the C2H4-HF complex and the P(1) transition of the HF monomer. In the case of the latter the width is primarily due to the residual Doppler broadening resulting from the nonorthogonal crossing of the laser and the molecular beam. The solid line through the R(0) transition of the complex is a Voigt fit in which the monomer line width was used as the Gaussian component. This fit gives a Lorentzian component of the line width of 480 MHz fwhm. The R(0) transition of the complex was used in the line width determination since it is known from the spectral assignment to be a single line. In view of the fact that an excellent fit to the observed spectrum can be obtained in the conventional way, there is obviously no “new” s t r u c t ~ r in e ~the ~ ~spectrum ~~ that might be related to coupling to other vibrational states. In the case where many such (30) Stewart, G.; Ruoff, R.; Kulp, T.; McDonald, J. D. J . Chem. Phys. 1984, 80, 5353, 5359.
500
1500
2500
3500
4500
FREQUENCY (MHz) Figure 3. Stark shifts for the R(0) and P(1) transitions of C,H,-HF. In both cases, the laser is scanned from right to left. TABLE 111: Summary of the Stark Effect Measurements Used To Determine the Upper Vibrational State Electric Dipole Moment app voltage P(l) freq shift, MHz R(0) freq shift, MHz P,
D
E , V/cm
1
2
3
-2520 1860 2.610 8442
-2949 2189 2.603 9435
-3601 2597 2.618 10760
states are involved, this effect is often referred to as intramolecular vibrational relaxation (IVR).31s32 Although there is little doubt that the width of the observed transitions is associated with the lifetime of the excited vibrational state, we are still left to speculate whether this lifetime is due to vibrational predissociation or IVR. In view of the fact that the broadening is independent of J , which suggests that the coupling is to a very uniform density of states, vibrational predissociation is perhaps the better explanation. In any case, the excited-state lifetime can be estimated by t = 1 / 2 ~ A v , giving 0.33 ns. As expected, on the grounds that C2Hp-HF has a larger number of vibrational degrees of freedom, the excited-state lifetime of this complex is considerably shorter than that of C2H,-HF (0.80 ns”). (31) Hoffbauer, M. A,; Gentry, W. R.; Giese, C. F. In Laser-Induced Processes in Molecules, Kompa, K.; Smith, S . D., Eds.; Springer Series in Chemical Physics 6; Springer-Verlag: West Berlin, 1978); p 252. (32) Hoffbauer, M . A.; Liu, K.; Giese, C. F.; Gentry, W. R. J . Chem. Phys. 1983,78, 5567.
Ethylene-HF and Allene-HF Binary Complexes I
The Journal of Physical Chemistry, Vol. 92, No. 1, 1988 49 TABLE I V Summary of the Constants for C3H4-HF in the Ground and Excited Vibrational States
ground vibratl state
excited vibratl state
calcd from ground struct in vibratl state Figure (micr~wave'~) A B
mol const A, cm-' 0.309' 0.312" 0.3130 E , cm-I 0.125 (17) 0.127 (17) 0.124169 C, cm-' 0.091 (17) 0.092 (17) 0.089977 yo, cm-I 3769.299 (5)
3767 0 3769 0 3771 0 3773 0
Although A"and A'are poorly determined from this parallel band, the spectrum is sensitive to AA. As a result, the number of significant figures listed represents the accuracy in AA and not the individual A's.
FREQUENCY (CM-1)
THEORETICAL STICK SPECTRUM
Figure 4. Calculated (A and B) and experimental (C) allene-HF v I spectra. Spectra A and B are calculated with the constants given in Table IV and TR = 8 K and TR = 5 K, respectively.
Stark measurements have also been carried out for this complex by using the same method discussed previously for the acetylene-HF complex." This was done by measuring the zero-field and field-on positions of the R(0) and P(1) transitions with the same electric field. These transitions were chosen for the Stark measurements since they are not overlapped by other transitions in the spectrum. The spectra obtained in this way are shown in Figure 3. Summarized in Table I11 are the frequency shifts for these transitions measured at three different electric fields. When combined with the ground-state dipole moment (determined from the microwave measurementszs to be p,, = 2.3839 (45) D), these splittings can be used to determine an upper state dipole moment for the complex of p , = 2.610 (8) D. The Stark shifts were calculated by diagonalizing the total Hamiltonian matrix which was built by using a prolate symmetric rotor basis set. For the energy levels involved in this study, the matrix was truncated after J = 5 , giving results which are converged to much better than the experimental uncertainty. As indicated previously for CzHz-HF," the large increase in the dipole moment upon vibrational excitation of the complex is believed to result from a decrease in the H F bending angle as well as charge transfer between the two monomer units. A detailed understanding of this phenomenon will require a theoretical calculation at a level considerably more sophisticated than the simple multipole treatment reported previo~sly.~~
Allene-HF In order to further explore the dependence of the vibrationally excited state lifetime on the complexity of the partner in these H F binary complexes, we have also recorded the vl spectrum of the allene-HF complex, which is shown in Figure 4. As indicated under Experimental Section, this spectrum was recorded by scanning the laser in a mode hop fashion. Clearly line congestion is a serious problem in this spectrum due to the large line width associated with the observed transitions. Nevertheless, there is still considerable structure in the spectrum that can be used to obtain at least reasonable estimates of the rotational constants. Indeed, this spectrum looks very much like a parallel band for an asymmetric rotor. When the analysis of this spectrum was first done, we were unaware of any microwave data for this system. As a result, both the ground- and excited-state constants had to be varied in the fit. Since then, we have obtained preliminary ground-state constants (obtained from a fit to seven transitions) from Legod4 that are in good agreement with those obtained here. It was not possible to fit individual transition frequencies in this case due to the large amount of line congestion. As a result, the constants were simply varied until the overall features in the spectrum were reproduced. The constants used to generate the two calculated spectra shown in Figure 4, along with the microwave values for the ground state, are summarized in Table IV. (33) Dixon, T. A,; Joyner, C. H.; Baiocchi, F. A,; Klemperer, W. J . Chem. Phys. 1981, 75, 2041. (34) Legon, A. C., private communication.
0.3182 0.2965 0.1256 0.1254 0.0918 0.0897
I
I
3767.0 3769.0 377?.0 3 7 7 3 . 0 F R E Q U E N C Y (CM-1) Figure 5. Calculated spectrum for allene-HF using the instrumental Gaussian line width of 0.0003 cm-I. The constants summarized in Table IV were used in this calculation.
H
H
F F Figure 6. Two possible structures for the allene-HF binary complex.
The vibrational dependence of the rotational constants is precisely what we have come to expect to these HF-containing systems, namely, that the B and C rotational constants increase upon vibrational excitation. It should be stressed here that the A rotational constants are poorly determined since the parallel band spectrum is rather insensitive to A. AA, on the other hand, is much more accurately determined (AA = 0.003 (1) cm-I) since it greatly affects the relative frequencies of the transitions. The problem of line congestion can be appreciated by considering the calculated spectrum shown in Figure 5. This spectrum was generated by using the fitted rotational constants and a line width corresponding to the instrumental resolution (0.0003 cm-I). It is clear from this that the features observed in the experimental spectrum result from the overlapping of many individual transitions. Two calculated spectra have been included in Figure 4, corresponding to rotational temperatures of 5 K (spectrum A) and 8 K (spectrum B). This has been done in order to show that the experimental spectrum is not well represented by a single rotational temperature. Clearly the low J transitions are best represented by a rotational temperature which is lower than that of the high J states. This is a result of the fact that the energy gap between states with large J is large so that cooling in the jet is slower than that for small J. In view of the complexity of this system, and the limited number of accurately determined rotational constants, a complete structural determination is clearly not possible. However, in view of the similarity between this system and C2H4-HF at least some
J . Phys. Chem. 1988, 92, 50-56
50
progress can be made. To begin with, there seems little doubt that the H F will bind somewhere along the C=C=C backbone, giving rise to an essentially T-shaped structure. This is consistent with the fact that the spectrum observed for the H F stretch is a parallel band. Figure 6 shows two possible structures of this type with the HF bonded to the center of a carbon-carbon bond, in analogy with C2H,-HF, and to the central carbon atom. Unfortunately, with the A constant being so poorly determined, we are unable to differentiate between these two structures or any that lie between these extremes. Work is in progress to obtain a spectrum for the C-H stretches of this complex. Since these bands will be perpendicular, a much more accurate A value should be obtained. In both of the calculated spectra a Lorentzian line width of 1400 MHz fwhm was used. The lifetime of the excited vibrational state is clearly shorter than that of the ethylene-HF complex discussed above. Once again, we are unable to associate the width of the observed transitions with the vibrational predissociation lifetime of the complex, since it is not clear whether IVR is important or not. However, it is interesting to note that in all of the systems studied with the optothermal detection method, there is not a single example of a system in which there is unambiguous homogeneous broadening of the lines and yet a positive bolometer signal is observed. In principle, of course, if IVR is fast and dissociation is slow on the time scale of the molecular flight time from the laser crossing point to the bolometer, this situation could occur. This indicates, therefore that when the lifetime of the excited
vibrational state is short enough to observe homogeneous broadening of the transitions, the vibrational predissociation lifetime of the complex is at least short enough to ensure that the signal on the bolometer is negative. Unfortunately, this upper limit on the predissociation lifetime is rather long, namely, 3 X s.
Summary We have reported here the infrared-molecular beam spectra for ethylene-HF and allene-HF corresponding to the excitation of the HF stretching vibration. In both cases the spectra are well represented by a conventional rigid rotor, asymmetric top Hamiltonian facilitating the accurate determination of rotational constants and vibrational origins. For the case of allene-HF the transitions are heavily blended owing to extensive homogeneous broadening. As a result, the constants obtained for this system are more poorly determined than those for ethylene-HF so that a completely reliable structure cannot be obtained. Future work on the C-H stretching vibrations of this molecule should be of considerable help in clarifying this issue. Acknowledgment. We are grateful to A. C. Legon for providing us with the microwave results prior to publication and to the following agencies for their support of this research: the National Science Foundation (CHE-86-03604), the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the Research Corp. Registry No. HF, 7664-39-3; allene, 463-49-0; ethylene, 74-85-1.
Laser-Induced Optical Emission Studies of Eu3+ Sites in Polycrystalline Powders of Monoclinic and Body-Centered Cubic Eu,03 K. C. Sheng and G . M. Korenowski* Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 121 80-3590 (Received: April 8, 1987; In Final Form: July 1 , 1987)
Laser-induced emission from the visible wavelength f-f transitions of Eu3+is used to determine the number of distinct europium ion sites and their site symmetries in powdered monoclinic and body-centered cubic (bcc) Eu203. Identical studies are also carried out for a higher surface area catalytic bcc Eu203material. For the monoclinic sample, the f-f emission spectrum is found to result from three distinct europium sites, all of which possess C, symmetry. Individual emission lines are assigned to the separate emitting sites for the 'Do 'FJ series for J = 0, 1, and 2. Emission spectra from the regular and higher surface area bcc Euz03materials are found to originate from two distinct Eu3+sites in the oxide lattice. The strongest emitting Eu3+site is assigned to the known C2symmetry site of the bcc lattice. No emission was observed from the known S6 symmetry site. The second and weaker emitting site, which is observed in the spectra of both the regular and higher surface area materials, is determined to result from an Eu3+ site with either C, or C, symmetry.
-
Introduction The unique physical and chemical properties of the lanthanide oxides make these materials useful in a variety of diverse applications. Some well-known areas of application for the oxides include use their as laser materials' and phosphors.z A lesser known application and an area of growing general interest is the use of the oxides as heterogeneous chemical catalyst^.^.^ Besides the practical importance of developing new catalysts, systematic studies of catalytic reactions on these oxides are also possible, and such studies promise to yield fundamental insights into the basis of catalytic activity. The ability to perform these studies with the lanthanide oxides stems from the regular decrease in surface basicity of the oxides as the lanthanide atomic number increases (a result of the lanthanide contraction). This regular variation of basicity, coupled with the fact that relatively few structures are found for the oxides but that these structures are exhibited *Address correspondence to this author
0022-3654/88/2092-0050$01.50/0
throughout the series, enables one to use a series of oxides to determine the importance of surface basicity versus structural effects on the catalytic activity. It is toward providing structural information that will be of future use in such systematic studies of catalysis that the spectroscopic studies of this paper are directed. The site symmetries occupied by the lanthanide ions, interaction between neighboring lanthanide ions, and interactions with the oxide lattice are important factors with respect to the observed physical and chemical behavior of the oxides. For the sesquioxides ( 1 ) Weber, M. J. In Handbook on the Physics and Chemistry of Rare Earrhs; Gschneider, K . A,, Jr.; Eyring, L., Eds.; North-Holland: New York, 1979; Vol. 4, Chapter 35, pp 275-315. (2) Blesse, G. In Handbook on the Physics and Chemistry of Rare Earths; Gschneider, K. A,, Jr.; Eyring, L., Eds.; North-Holland: New York, 1979; Vol. 4, Chapter 34, pp 237-274. (3) Netzer, F. P.; Bertel, E. In Handbook on the Physics and Chemistry of Rare Earths, Gschneider, K. A,, Jr.; Eyring, L., Eds.: North-Holland: New York, 1983; Vol. 5, Chapter 43, pp 217-320. (4) Rosynek, M . P.Catal. Rev. Sci. Eng. 1977, 16, 1 1 1 .
0 1988 American Chemical Society
