J . Phys. Chem. 1989, 93, 135-139
135
coupling scheme used in the analysis of this experimental data. Finally, the dependence on the inverse of reduced mass of the nonadiabatic correction points at a further advantage of using D2 rather than Hz in investigating molecular hydrogen interactions.
Acknowledgment. This work has been supported by grants from the Italian Minister0 della Pubblica Istruzione and from the Italian Consiglio Nazionale delle Ricerche.
180
1' I
1
,
1
1
1.0
,
I
1.5
2.0
, 2.5
1 3.0
VELOCITY/ kms-1
Figure 6. Absolute integral cross sections Q for the D2-02 system as a function of beam velocity. The full curve is the cross section calculated by using a MSV potential model (see Appendix). TABLE V MSV Potential Parameters for the 02-H2 System
c/meV RmIA
P C,/(meV A6) C,/(meV AB) XI
5.9 3.60 6.3 1.853 X lo4 9.267 X lo4 1.10
XZ
bl b2 b3 b4
1.42 -0.7815 1.7903 -4.2073 4.7350
case (c), while for O-CH4 the maxima of P ( R ) occur at distances nearer to the minima locations. Therefore the 0-CH4 system presents a more prominent molecular character at the distance around the minima of the potentials. The P(R) terms are also useful in estimating nonadiabatic effects and in obtaining the first-order nonadiabatic corrections to the adiabatic curves derived in section IV from eq 4. From the values in Table IV it can be seen that these corrections, which are (h2/2fi)P2,are well within the uncertainties stated for the present analysis and confirm the adequacy of the adiabatic de-
Appendix Integral Cross Sections for D 2 - 4 Collisions and the Spherical Interaction between Hydrogen and Oxygen Molecules. In Figure 6 the absolute integral cross sections Q of the Dz-02 system (plotted, as usual when in the glory region, as Qv21S)are reported as a function of the beam velocity u. These measurements have been performed mostly to obtain a calibration for the absolute scale of the cross sections of this work (see section I1 and also ref 12). For the analysis a MSV potential model is used (see eq 7 in section IV). The quality of the data allows one to take into account also the long-range dipole-quadrupole interaction described by C8/R8in addition to the dipole-dipole interaction described by a C6/R6term. The potential parameters are reported in Table V and can be shown to fit into the picture of the weak intermolecular forces that is known at present. The capability of a spherical potential model to reproduce the large and well developed glory structure observed for this system (Figure 6 ) indicates that, under the present experimental conditions, namely at a relatively high rotational temperature of the particles involved, a substantially isotropic interaction drives the collision between these molecules. This can be taken as empirical evidence for excluding contributions from molecular rotations to the anisotropy effects discussed in the paper and attributed to the open shell structure of the oxygen atom. Registry No. 0, 17778-80-2; Hz,1333-74-0; CH4, 74-82-8; 02, 7782-44-7.
Deuterium Isotope Effects on S, Radiationless Decay in Phenol and on Intermolecular Vibrations in the Phenol-Water Complex Robert J. Lipert and Steven D. Colson* Sterling Chemistry Laboratory, Yale University, New Haven, Connecticut 0651 I (Received: April I I , 1988)
Replacement of the phenol OH hydrogen with deuterium has been found to reduce the rate of internal conversion in the supersonic jet cooled molecule by over 2 orders of magnitude. A similar but less pronounced effect was previously observed when water is hydrogen-bonded to the same hydrogen. It is likely that both effects are caused by a reduction in the effectiveness of the OH vibration in acting as an accepting mode in radiationless transitions due to a lowering of its vibrational frequency. The multiphoton ionization (MPI) spectra of phenol-water complexes containing one, two, or three deuteriums confirm the assignments of two low-frequency intermolecular vibrations. Successive shifts of the hydrogen bond stretching vibration as deuteriums are substituted into the complex have been used to determine the sites at which the substitution is occurring. Fermi resonances involving the hydrogen bond stretching vibration were also found in some isotopic species.
Introduction Through the study of radiationless transitions in supersonic jets, it has recently been established that the rate of internal conversion from the SI to the ground state of the phenol-H20 complex is much lower than in free It has been suggested' that this is because a phenol OH vibration or vibrations are less effective accepting modes due to the hydrogen bonding of water to the hydrogen of the phenol OH. These vibrations are attractive ( 1 ) Sur, A,; Johnson, P. M. J . Chem. Phys. 1986, 84, 1206. (2) Lipert, R. J.; Bermudez, G.: Colson, S. D. J. Phys. Chem. 1988, 92,
3801.
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candidates for accepting modes since they are either of high frequency or undergo large frequency shifts between the ground and SI states. For example, the 3656-cm-' ground-state OH stretching frequency is the highest frequency vibration in pheno13s4 and may therefore be the most important accepting mode. It is well established that the O H stretching frequency is lowered by hydrogen bonding in which phenol acts as the proton donor,5 as (3) Bist, H. D.; Brand, J. C. D.; Williams, D. R. J . Mol. Spectrosc. 1967, 24, 402. (4) Bist, H.D.;Brand, J. C. D.; Williams, D. R. J. Mol. Spectrosc. 1967, 24. 413.
0 1989 American Chemical Society
136 The Journal of Physical Chemistry, Vol. 93, No. I , 1989
it does in the phenol-water complex. In contrast, hydrogen bonding increases the frequency of the OH bending and torsion modes. In the case of bonding with water, the decrease in the stretching frequency is probably around 300-400 cm-I. If this vibration is an accepting mode, this will result in more quanta of this vibration being required in order to take up the large amount of electronic energy that must be converted to vibrational energy during SI to So internal conversion (complexation results in only a small change in the So-S1 energy gap). This would lead to smaller Franck-Condon factors between isoenergetic levels of the two electronic states and hence to a lower rate of internal conversion. It is therefore plausible, that the OH stretch is the dominant accepting mode and that hydrogen bonding lowers the rate of internal conversion by lowering its frequency. On the other hand, it is conceivable that the five C H stretching modes act as accepting modes and that other effects of hydrogen bonding cause the drop in internal conversion rate. Other factors that are known to be important in determining the rate of radiationless decay of vibrationless excited electronic states are the density of states in the manifolds coupled to the initially prepared level and the magnitude of the electronic factors In the case of internal conversion, the that couple the levels are coupled via the nuclear kinetic energy operator.',* Upon forming the complex, six low-frequency intermolecular vibrations, in addition to the three high-frequency intramolecular vibrations of the water molecule, are added to the system. These low-frequency vibrations, which are known to couple to the intramolecular vibrations of phenol, will greatly increase the density of states at 35 997 cm-' above the vibrationless level of the ground state, Le., at the energy of the vibrationless level of the S1 state. Thus a large decrease in the rate of internal conversion is observed in spite of this increase in the density of states. The electronic factors of the interaction matrix elements involve the normal coordinate dependence of the electronic wave functions.' A large degree of uncertainty is involved in the evaluation of these factors and in assessing how they are influenced by hydrogen bonding or other perturbations. What has often been done to circumvent this difficulty is to study relative transition rates where it is hoped the electronic factors are constant and therefore cancel, so that the importance of the other parameters can be evaluated. Thus, a large number of studies of the effect of deuterium substitution on the rates of radiationless transitions in aromatic hydrocarbons have helped clarify thinking on the density of states and Franck-Condon aspects of the processes.bs Here it is assumed isotopic substitution has only a small influence on the electronic factor. In benzene, the high-frequency C H stretching vibrations have been established to be the dominant accepting modes for radiationless transitions out of vibrationless electronic states. The principal effect of deuterium substitution is to reduce the frequency of these vibrations and therefore also to reduce the Franck-Condon factors between the vibrationless excited electronic state and the isoenergetic, highly vibrationally excited levels of the ground state. The consequences of isotopic substitution on internal conversion rates of perdeuteriobenzene were further studied by Knee et al.' In the aspect of their work most related to the phenol issue, they found that the rate of internal conversion from the vibrationless SI state of C,D6 to be about one-third the rate in C6H6. Though substantial, this decrease is less than the almost 2 orders of magnitude drop of internal conversion rate found in phenol upon complexation with water. As a step toward determining the mechanism whereby the rate of internal conversion in phenol is lowered by complexation, we ( 5 ) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; W. H. Freeman: San Francisco, 1960; Chapter 3. (6) Wright, W. R.; Frosch, R. P.; Robinson, G. W. J . Chem. Phys. 1960, 33, 934. (7) Robinson, G. W.; Frosch, R. P. J . Chem. Phys. 1962,37, 1962; 1963, 38, 1187. (8) Avouris, P.; Gelbart, W. M.; El-Sayed, M. A. Chem. Rev. 1977, 77, 793. (9) Knee,J. L.; Otis, C. E.; Johnson, P. M. J . Chem. Phys. 1984,81,4455.
Lipert and Colson have measured the rate of internal conversion in phenol-d, by using the pump-probe photoionization technique. The replacement of the O H hydrogen with deuterium lowers the stretching frequency3 from 3656 to 2699 cm-I, a greater change than could be produced through hydrogen bonding. Thus, if this vibration is an important accepting mode, the rate of internal conversion will be much reduced in phenol-d,. This observation would then be consistent with the mechanism proposed by Sur and Johnson to explain the effects of hydrogen bonding on the rate of internal conversion. On the other hand, little or no change in the internal conversion rate upon isotopic substitution would strongly suggest that O H vibrations are not important accepting modes. Therefore hydrogen bonding could not be suppressing internal conversion by reducing Franck-Condon factors since other phenol vibrations are probably not greatly affected by hydrogen bonding. The most likely source of the effect would then appear to be an alteration of the electronic factors. Phenol-d, was generated by exchange with D 2 0 within the sample holder of our beam machine. This approach also resulted in the generation of phenol-water 1:l complexes in which one to three of the readily exchangeable hydrogens had been replaced by deuterium. Therefore, in addition to the pump-probe study of the decay of phenol-d,, we also report one-color MPI wavelength scans of the low-energy region of the SI state of these complexes. These spectra provide insight into the nature of the intermolecular vibrations of the phenol-water complex. Experimental Section Phenol-d, and the various deuterium-substituted phenol-water complexes were produced from a solution of phenol (B&A) and deuterium oxide (MSD Isotopes, 99.8 atom % D). An excess amount of phenol was added to DzO, and.the immiscible organic layer that formed on top was used as the sample. A minimum amount of this solution was used to moisten a plug of glass wool located in the carrier gas line behind the 1 2 . 5 - ~ mcontinuous nozzle. The nozzle plus sample was placed in an oven located inside the vacuum chamber and heated to approximately 50 "C. The complexes were formed as the He carrier gas at 15 atm passed over the sample and expanded into the vacuum chamber maintained at lo4 Torr. The pumpprobe experiment was performed by crossing the free jet expansion, 1 cm downstream, with the unfocused, frequency-doubled output of a Nd:YAG pumped dye laser (Quanta Ray DCR-2, PDL, WEX) to excite the S1 state. The electronically excited molecules were then ionized with 193-nm radiation from a Questek 2200 excimer laser, counterpropagating to the pump beam and focused with a 1-m lens. One-color ionization by the pump beam was minimized by reducing the laser intensity by rotation of a linear polarizer, which provided a convenient means of continuously adjusting the light intensity without greatly modifying the beam characteristics. A repelling field of 150-200 V/cm accelerated the resulting ions into a time-of-flight mass spectrometer. Ion signal was monitored as a function of the delay between the firing of the two lasers. The firing of both lasers was triggered by a Stanford Research Systems DG535 digital delay/pulse generator interfaced to an LSI-11/23 computer. Delays were incremented in 2.04s steps. At each delay setting, 30 laser shots were averaged and the entire decay was scanned three times and averaged. The jitter in both lasers is estimated to be 3 ns. The one-color photoionization spectra of the deuteriated complexes were collected by simultaneously monitoring the ion intensity in the appropriate mass channels of the timeof-flight mass spectrometer as the wavelength of the unfocused laser beam was varied. The spectrum of the fully protonated complex was collected in a separated experiment since there was little of this complex present in the deuteriated sample. Results and Discussion Since isotopic substitution of the OH hydrogen results in only a 2.5-cm-' shift in the S , origin of phenol: it was desirable to obtain additional evidence that exchange had occurred. This was done by scanning to lower energies from the origin where hot bands and sequence bands were detected. The location of the relatively
Deuterium Isotope Effects in Phenol
The Journal of Physical Chemistry, Vol. 93, No. I , 1989 137
L
120 180 240 300 Time (nsec) Figure 1. Pumpprobe photoionization decay of phenol-d,. Solid curve is a nonlinear least-squaresfit to a convolution of a Gaussian excitation pulse with a biexponential decay yielding T~ = 16 ns and Qi,= 0.48. 0
60
strong 1 1 band, whose frequency undergoes a 9-cm-I shift,4 confirmed that the major species with a mass to charge ratio of 95 was phenol-d, rather than, for example, phenol-h, containing carbon-1 3. Internal Conversion in Phenol-dl. Figure 1 shows the excited-state decay of phenol-d, initially excited to the SI vibrationless level. Since the probe laser operates at 193 nm, we see ionization of both the singlet and isoenergetic vibrationally excited triplet levels populated through intersystem crossing. The relatively rapid fall-off in ion signal at early times reflects the decay of the singlet state, while the more gradual decay at longer times corresponds to the decay of the triplet state. At sufficiently high probe laser intensity the ionization of both singlet and triplets will be saturated. In this regime their relative intensities reflect the relative populations of these states.I0 One can then obtain the quantum yield for intersystem crossing, as well as singlet and triplet state lifetimes, from an biexponential fit of the data.1° Therefore, also shown in Figure 1 is a nonlinear least-squares fit to convolution of a Gaussian excitation pulse with a biexponential decay.2 The fit yields a singlet state lifetime, T,, of 16 ns and a quantum yield for intersystem crossing, Qiso of 0.48. These can be compared to our previous results’ for phenol-h6 of 2 ns and 0.10.’ As previously discussed, due to the limitations of our apparatus it was not possible to collect a long enough period of the triplet decay to provide a reliable value for the triplet lifetime.’ The fact that both the singlet-state lifetime and the quantum yield for intersystem crossing have increased suggests that deuterium substitution has cut the rates of internal conversion. The magnitude of the effect can be estimated if we assume that the radiative rate of phenol-d, is approximately the same as that of phenol-h,, which in turn is approximately equal to the cyclohexane solution value.” If we then write k, = l/r, = kf ki, ki, and ki, = Qi,/r,, where kf is the radiative rate, ki, is the rate of internal conversion, and ki, is the rate of intersystem crossing, we can then calculate kic. The results for phenol-d, are k, = 6.2 X lo7 s-l, kf = 3.3 X lo7 s-l, and ki, = 3.0 X lo7 s-I, yielding a small negative number for kic. Thus, to within the uncertainty in our approximations and measurements, internal conversion has been eliminated by isotopic substitution. For comparison, we previously found2 that in phenol-h, kisc= 5.0 X lo7 s-’ and kic = 42 X lo7 s-’, Therefore the major effect of deuterium substitution has been to reduce internal conversion, which is by far the dominant relaxation process in phenol-h6, to a level that is insignificant relative to fluorescence and intersystem crossing. The reduction is by at least 2 orders of magnitude. The rate of intersystem crossing is also reduced, but here the difference is less than a factor of 2. These results are qualitatively what one would expect if the OH stretching vibration is the dominant accepting mode in phenol. At first the magnitude of the change brought about by isotopic
+ +
(10) Dietz, T. G.; Duncan, M. A,; Srnalley, R. E. J . Chem. Phys. 1982, 76, 1227. (1 1) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic Molecules; Academic: New York, 1965.
substitution was a little surprising, being significantly larger than the effect of complete deuteriation on internal conversion in benzene. However, our upper limit on the internal conversion rate in phenol-d, of approximately 0.3 X lo7 s-l is not unreasonable, since C H vibrations are now the highest frequency vibrations in the molecule and should be the best accepting modes, as they are in benzene. Therefore, to the extent that their electronic factors and promoting modes are comparable, we should expect the rate of internal conversion in phenol-d, to be similar to that in benzene or somewhat higher due to the difference in SI-Soenergy gaps. The rate observed in benzene” of 0.16 X lo7 sd is well below our upper limit on the rate of internal conversion in phenol-d,. A similar comparison of intersystem crossing rates is more difficult since the S,-T, energy gaps in the two molecules are appreciably different, 8600 cm-’ in benzene and 7500 cm-’ in phenol. Our result for the rate of intersystem crossing of 3.0 X lo7 s-l compared to the benzene value” of 0.62 X lo7 is at least consistent with the smaller energy gap in phenol. These findings demonstrate that O H vibrations are very important in the radiationless transitions in phenol. While other vibrations are affected by isotopic substitution at the O H hydrogen, these changes are considerably less than those experienced by the O H vibrations. The reduction of internal conversion following isotopic substitution indicates that the O H vibrations exert their influence by producing favorable Franck-Condon factors between the SI and ground states. Isotope effects on the electronic factor and on the matrix element of the promoting mode or modes are expected to be small. This is especially true with regard to the promoting modes because none of the O H vibrations of phenol are of the proper symmetry to act as promoting modes between the B2 SI state and AI ground state. In particular, it appears likely that it is the high-frequency (3656 cm-I) OH stretching vibration that is responsible for phenol’s relatively short single-state lifetime and low fluorescence quantum yield. The high frequency of this vibration makes it an exceptionally good accepting mode. This results in internal conversion being the fastest decay route for the vibrationless level of the SI state. In light of these results it seems very probable that the explanation of Sur and Johnson for the effect of hydrogen bonding on internal conversion in phenol is correct. Our findings demonstrate that this rate is highly sensitive to OH vibrational frequencies and that isotopic substitution results in a greater drop in internal conversion than that produced by hydrogen bonding, in parallel with their effect on O H stretching frequencies. The two other O H vibrations are probably not significantly involved in the hydrogen bonding effect because they cannot be promoting modes and hydrogen bonding raises their frequencies. Therefore, our results provide additional data that is consistent with the Sur and Johnson mechanism and with current thinking on radiationless transitions. MPI Wavelength Scans of Deuteriated Phenol- Water Complexes. In Figure 2 are shown the one-color MPI wavelength scans of phenol-water 1:l complexes in which zero, one, two, or all three labile hydrogens have been replaced by deuterium. First, it should be noted that in the case of species containing one or two deuteriums, there are more than one possible nonequivalent ways of arranging the deuterium atoms, regardless of the structure of the complex. These products are not expected to show the same isotopic shifts. However, the origin bands of all the spectra are of approximately the same width, and no other significant peaks appear in the origin region. This suggests that the spectrum obtained in each mass channel is dominated by a single species. In other words, we find that when one deuterium is present in the complex, it is located at only one of the various possible nonequivalent, easily exchangeable sites. Similarly, when two deuteriums are present, only one of a number of possible isomers appears to be formed. Since, as we will show later, the formation of the d2 complex is equivalent to the substitution of a deuterium for a hydrogen in the d, complex, and similarly for the formation (12) Otis, C. E.; Knee, J. L.; Johnson, P. M. J . Phys. Chem. 1983, 87, 2232.
Lipert and Colson
138 The Journal of Physical Chemistry, Vol. 93, No. 1, 1989
il
36300
36200
3sioo
36000
Frequency (cm-1) Figure 2. MPI spectra in the S , origin region of phenol-water 1:l complexes in which (a) zero, (b) one, (c) two, and (d) three of the nonring hydrogens have been replaced by deuteriums.
of the d3 complex, we will refer to this series of compounds as arising from a sequence of isotopic substitutions. The origin bands of these complexes were found to shift to higher energy by 2, 6, and 9 cm-I when one, two, and three deuteriums are present, respectively, relative to the fully protonated complex origin band. Toward higher energies the spectra show interesting differences relative to the nondeuteriated complex and among themselves. The assignment of the major features of the spectrum of the nondeuteriated complex has been given by Oikawa et al.13 They attribute the strongest peak, at 156 cm-I above the origin, to the totally symmetric hydrogen bond stretch. The next most intense band, at 121 cm-I, is assigned to a bending vibration. The third totally symmetric intermolecular vibration has not been previously assigned, and overtones of the observed transitions are not seen. The other major feature that appears in all of our spectra as well as in the spectrum of the fully protonated complex is the broad transition around 36250 cm-I. This is most likely due to the fragmentation of clusters containing more than one water molecule.14 The same is true for the other peaks between 36 200 and 36 300 cm-I. The small peaks between about 36090 and 36 150 cm-I occurring in the various spectra, other than those discussed below, were not reproducible. Isotopic substitution greatly alters the relative intensities of these transitions. In the complex containing one deuterium, there is little evidence for any transition in the region of the bending vibration. However, in the species containing two and three deuteriums, the bending vibration apparently reappears at about 86 cm-I above the origin. Th,is assignment results in a ratio of reduced masses, given by (121/86)2 = 1.98, indicating that this vibration consists mainly of the motion of hydrogen (deuterium) atoms. If the structure and vibrations of the complex are assumed (13) Oikawa, A,; Abe, H.; Mikami, N.; Ito, M. J . Phys. Chem. 1983, 87, 5083. (14) Fuke, K.; Kaya, K. Chem. Phys. Letf. 1983, 94, 97.
to be analogous to those of the water dimer,lSthen this vibration would correspond to the in-plane wagging of the water hydrogens (deuteriums). In the region of the hydrogen bond stretching transition, a new peak appears in the spectra, most noticeably in the moncdeuterium complex spectrum. It is improbable that the hitherto unobserved intermolecular vibration, the in-plane hydrogen bond bend or shear, which should occur at a considerably higher frequency, is shifted into this region. Also, there are no intramolecular phenol vibrations that shift to this frequency. It therefore appears that the stretching transition is involved in a Fermi resonance with a previously unseen level that has been shifted into this region by isotopic substitution. A Fermi resonance can only occur between vibrational levels of the same overall symmetry. Since we have excluded the one remaining totally symmetric intermolecular vibration from participating in the interaction and there are no intramolecular vibrations of a frequency this low, then the interacting level must correspond to either an overtone of the wagging vibration or to a not totally symmetric intermolecular vibration in which an even number of quanta have been excited. This would require the fundamental frequency of the not totally symmetric vibration to be roughly 75 cm-I. However, this value is highly uncertain due to the large degree of anharmonicity expected in these vibrations. The out-of-plane hydrogen bond bending vibration is probably too high in frequency to be involved. This leaves out-of-plane wagging and torsion as possible candidates. These vibrations should also show the largest isotopic shifts. Regardless of this identity, it is apparent that in the Fermi doublet, the level mixing with the Stretching vibration is acquiring all of its intensity through the interaction. Therefore we can estimate the perturbation matrix element and the frequencies of the unperturbed levels from the splitting of the doublet and the ratio of the intensities of the two components. The ratio of fundamental to overtone intensities is equal tot6
where W is the perturbation matrix element and 6 is the separation of the unperturbed levels. The separation of the perturbed levels is just (41w2 rJ2)'l2. Thus Wand 6 can be found. Then, since the mean of the observed level energies equals the mean of the unperturbed level energies, the energies of the unperturbed levels can be found. In addition, since the more intense of the two peaks in the doublet must correspond to the stretching vibration, we then know the isotopic shifts of this vibration as successive deuteriums are substituted into the complex. The unperturbed hydrogen bond stretching frequencies are then found to be 154, 150, and 149 cm-I for one, two, and three deuteriums, respectively. The perturbation matrix element is determined to be about 4 cm-I. Comparing the ratio of the hydrogen bond stretching frequencies to the square root of the inverse of the ratio of reduced masses of the nondeuteriated vs the fully deuteriated species establishes that the hydrogen bond stretch can be thought of as the vibration of the water molecule relative to the phenol molecule. These ratios are vH/uD = 156/ 149 = 1.047 and (pD/pH)1/2= 1.046. Therefore, the magnitude of the successive shifts gives an indication of where the deuterium are located in the complexes. The smallest isotopic shift is expected to occur when the phenol hydrogen is replaced by deuterium. The smallest frequency change occurs when the last deuterium is added to the complex. It therefore appears that the phenol hydrogen is the last hydrogen to exchange. Additional evidence for this sequence of deuterium substitution is perhaps provided by the absence of a detectable hydrogen wagging transition when only one deuterium is present in the complex (Figure 2b). If the first hydrogen to exchange was on the phenol, then the disappearance of this transition is difficult
+
(15) Curtiss, L. A,; Pople, J. A., J . Mol. Spectrosc. 1975, 55, 1. (16) Herzberg, G. Molecular Spectra and Molecular Structure II. In-
frared and Ramon Spectra of Polyatomic Molecules; Van Nostrand Reinhold: New York, 1945; Chapter 2.
J . Phys. Chem. 1989, 93, 139-144 to explain. However, if this deuterium is located on the water molecule, then isotopic mode mixing17J8 in the asymmetrical complex could account for the observed behavior. The transition frequency is the same when both two and three deuteriums are present, and its shift, relative to the fully protonated complex, corresponds to a nearly pure hydrogen motion. This suggests that when two deuteriums are present in the complex, both are bonded directly to the water. This again is consistent with the isotopic shifts of the stretching vibration. Conclusions Measurements of the effects of isotopic substitution on the radiationless decay of the vibrationless level of the phenol SI state (17) Wiberg, K. B.; Walters, V. A.; Wong, K. N.; Colson, S. D. J . Phys. Chem. 1984. 88. 6067. ( 1 8) Rava, R: P.; Philis, J. G.; Krogh-Jespersen, K.; Goodman, L. J. Chem. Phys. 1983, 79, 4664.
139
show a dramatic reduction in the rate of internal conversion and a lesser reduction in the rate of intersystem crossing. Similar but smaller changes are observed upon hydrogen bonding of water to the OH hydrogen. It is likely that these two effects have the same origin, namely, reduction in the effectiveness of the O H stretching mode as an acceptor mode for radiationless transitions. The low-frequency, phenol-water "complex" modes are discussed in terms of the effects of substitution. Successive isotopic substitution is found to first replace the water hydrogens and then that of the phenol OH. Isotopic shifts are used to confirm the assignments of the lowest two observed modes in phenol-HzO (156 cm-' for the totally symmetric hydrogen bond stretch and 121 cm-I for a totally symmetric hydrogen wagging mode). Some isotopic species also show Fermi resonances, which are analyzed.
Acknowledgment. This research was supported by the National Institutes of Health. Registry No. HzO, 7732-18-5; D2,7782-39-0; phenol, 108-95-2.
Structure and Properties of C,H,+ Cations Alan Cameron, Jerzy Leszczynski, Michael C. Zerner,* Quantum Theory Project, Department of Chemistry, University of Florida, Gainesville, Florida 3261 1
and Brian Weiner Department of Physics, Penn State University, Dubois, Pennsylvania 15801 (Received: April 25, 1988)
Two possible mechanisms have been suggested for the formation of soot in fuel-rich flames and exhausts. One of these involves precursors that are small radicals, the other small carbocations. We begin here a quantum chemical study of the latter mechanism an ion observed in abundance in sooting flames. We uncover four stable singlet with an examination of the isomers of C3H3+, structures and suggest that structures computed stable at lower levels of theory are artifacts of those calculations. We calculate that the cyclic isomer is most stable, followed by the linear propargyl cation with an estimated heat of formation 27.7 kcal/mol greater. In order to characterize these tautomers for possible identification in sooting environmentswe calculate their vibrational and electronic spectra.
Introduction Two possible mechanism have been proposed for the formation of soot in fuel-rich flames. One mechanism invokes radical reactions, the other ion/molecule reactions involving small ions. The latter mechanism has been advanced by the observation that C3H3+ is the most intense ion observed in sooting flames'V2 and motivates this theoretical study along with experimental studies of ion/ molecule reactions involving small hydrocarbon ions using Fourier transform ion cyclotron resonance techniques3 A good characterization of possible C3H3+tautomers would be helpful in identifying their presence in sooting flames. The presence of such species is experimentally determined through mass spectroscopic techniques and the techniques presently being used cannot distinguish isomeric structures unambiguously. In the ion/molecule soot formation scheme condensation and condensation-elimination reactions of C3H3+with acetylene and polyacetylenes are postulated to occur rapidly with eventual formation of polycyclic aromatic hydrocarbon ions of mass 500-1000 d a l t o n ~ .Various ~ ~ ~ mass spectrometric studies suggest ( I ) Goodings, J. M.; Bohme, D. K.; Ng, C. W. Combust. Name 1979,36, 27. (2) Olson, D. B.; Calcote, H. F. Symp. (hit.) Combust., [Proc.] 1981, lath, 453. (3) See, for example: Ozturk, F.; Baykut, G.; Moini, M.; Eyler, J. R., J . Phys. Chem. 1987, 91, 4360. (4) Calcote, H. F. Combust. Flame 1981, 42, 215. (5) Olson, D. B.; Calcote, H. F. In Particulate Carbon Formation during Combustion; Siegler, D. C., Smith, G. W., Eds.; Plenum: New York, 1981.
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that C3H3+possesses reactive and nonreactive components. The most stable form of C3H3+,the cyclopropenyliumcation (I), Figure 1, is relatively unreactive toward acetylene and polyacetylenes as well as toward other small hydrocarbon^.^^^ The propargylium cation (11) is considerably more reactive toward C2H2and leads to the formation of C5H3+and C5H5+ions, both with reactive and unreactive components. The propargylium cation (11) is believed to lie some 25 kcal/mo17 higher in energy than the cyclopropenylium cation (I), but this species, as well as others potentially important, might be formed under the reaction conditions observed in sooting flames. We thus embark in this study to uncover possibly reactive species of C3H3+. Studies of larger ions suggested by the ion/molecule soot formation mechanism, C5H3+ and CSHS+,for example, are under way. Carbocations such as C3H3+have been of interest for some time. Cyclopropenylium ions represent the first members of the 2 4n aromatic series and as such have been the object of much theoretical discussion. A rather complete study of the relative stability of many C3H3+structures has been reported by Radom, Hariharan, Pople, and Schleyer.8 Although some of these earlier calculations are of somewhat lesser accuracy than those we report here, in general we agree with their ordering of stabilities and their
+
(6) Symth, K. C.; Lias, S.G.; Auslons, P. Combust. Sci. Technol. 1982, 28, 147. (7) Lossing, F. P. Can. J. Chem. 1972, 50, 3973. (8) (a) Radom, L.; Hariharan, P. C.; Psple, J. A.; Schleyer, P. v. R. J . Am. Chem. SOC.1976,98, 10. (b) See also: Raghavachari, K.; Whiteside, R. A.; Pople, J. A.; Schleyer, P. v. R. J. Am. Chem. SOC.1981, 103, 5649.
0 1989 American Chemical Society
