J. Phys. Chem. 1981, 85.3742-3746
3742
only through vibrational excitation. The combination of a somewhat enhanced nonradiative decay with this much-decreased radiative rate produces a fluorescence quantum yield which is much reduced from that of the monomer. Apparently, the T-shaped dimer S1 state initially populated by the pump laser is only a local minimum on the upper state hypersurface. The excited dimer is then free to rearrange over a low barrier (less than the zero point energy) into the deeper excimer well. In fact, the excitation profile of the dimer 0; bands does appear to be broadened to -4 cm-' (fwhm) by such a nonradiative process occurring on a picosecond timescale (see Figures 1-3). Since the (cGH&+ ion is also likely to have a parallel sandwich geometry, the Franck-Condon factors for the ionizing laser transition near the ionization threshold are expected to be far higher when the S1 state is in the excimer well as compared to the T-shaped van der Waals well, thus explaining the higher two-color ionization cross section for the dimer compared to the trimer. The most likely configuration of the van der Waalsbound trimer is a triangular pinwheel, if one assumes the same sort of bonding interactions are active in the trimer as in the dimer. Such a structure must have a substantial activation barrier toward excimer formation since one of the three major van der Waals bonds must be broken to allow rotation of one ring until it is parallel to another.
Absence of sufficient activation energy prohibita the trimer from entering the excimer region of the S1 hypersurface (at least for 0: excitation). Similar arguments will apply to the tetramer and higher clusters. The strong activity of progressions in the van der Waals modes of the higher clusters-particularly in the trimer-indicates the SI surface in these clusters is somewhat different from the ground-state surface. Even though these higher clusters may adopt a slightly different geometry in the S1state, the measurements of the fluorescence lifetime and quantum yield as well as the large difference in ionization cross section near threshold compared to the dimer suggest quite strongly that the larger clusters do not relax into a sandwich excimer well. Acknowledgment. We thank D. H. Levy and P. R. R. Langridge-Smith of the University of Chicago for helpful and stimulating discussions, particularly with regard to their fluorescence data on the benzene clusters and the possibility (and consequences) of excimer formation. The initial single-color photoionization observation of benzene clusters was performed in our group by M. A. Duncan and T. G. Dietz. This research was supported by the National Science Foundation and The Robert A. Welch Foundation. Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.
Ultraviolet Spectra of Benzene Clusters Patrick R. R. Langridge-Smith,+ Donald V. Brumbaugh, Christopher A. Haynam, and Donald H. Levy" The James Franck Institote and The Department of Chemlstry, The Unlverslty of Chlcago, Chicago, Illinols 60637 (Recelved: August 26, 1981; I n Flnal Form: October 19, 1981)
-
Clusters of benzene have been prepared in a supersonic expansion, and their lBzu lAl, ultraviolet spectra have been observed. The fluorescence excitation spectra of the clusters have features at the 6; vibronic band and at the origin. Dimers, trimers, and tetramers show sharp structure in the fluorescence excitation spectrum, while larger clusters produce a broad, unresolved band. The dispersed fluorescence spectra of the clusters is the same for excitation to either the 6' or the origin level. This is due to intramolecular vibrational relaxation from the mode into the low-frequency cluster vibrations. The dispersed fluorescence spectrum is broad even following excitation to the origin, and this broadening is interpreted as evidence for relaxation to an excimer state following excitation to a more weakly bound van der Waals state. Benzene shows anomalously little vibrational cooling in a supersonic expansion.
Introduction In a recent paper we described the fluorescence excitation spectrum of the gas-phase dimer of dimethyltetrazine (DMT).l The origin of the dimer spectrum was shifted by 548 cm-' to the low-energy side of the origin of the monomer spectrum, and a new progression with w,' = 78.7 cm-l, w p i = 1.21 cm-l was observed. The spectral shift gives the increase in dimer binding energy when the dimer is electronically excited, and the new vibrational progression was interpreted as the stretching motion of the two halves of the dimer. The overall interpretation was that in the excited electronic state the dimer was somewhat more tightly bound with a somewhat shorter bond distance. ~~
~~
NATO Postdoctoral Fellow.
This behavior is to be contrasted to that of aromatic hydrocarbons whose solution spectra have been studied as a function of concentration. Essentially all aromatic hydrocarbons are observed to form excimers a t higher concentration, these excimers being observed as strongly red-shifted bands in the fluorescence spectrum.2 Excimers have been described as dimers which are unbound (neglecting van der Waals forces) in their ground electronic state but rather tightly bound in their excited electronic state. The red shift in the fluorescence spectrum provides a measure of the excited-state binding energy, and this binding energy is thought to be several thousand ~ m - l , ~ (1) D. V. Brumbaugh, C. A. Haynam, and D. H. Levy,J. Chern. Phys., 73, 5380 (1980). (2) J. B. Birks, Rep. h o g . Phys., 38, 903 (1975).
0022-3654/81/2085-3742$01.25/00 1981 American Chemical Society
Letters
The Journal of Physical Chemistty, Vol. 85,No. 25, 1981 3743 T
X 156
l
l
,
I
38450
I
l
/
/
,
38500
,
l
38550 V,
I
,
I
I
l
t
38600
(crr?)
Flgure 1. The fluorescence excitation spectrum of a mixture of 0.4% benzene in helium at a total pressure of 8 atm expanded through a 0.050-mm nozzle. Features marked with a vibrational assignment (e.g., 6;) are due to the benzene monomer. Features marked D and T are due to the benzene dimer and trimer, respectively. The feature at 38562 cm-' is an overlap of a monomer and a dimer band. The sensitivity is decreased by a factor of 12 for the 6; monomer band.
substantially larger than that in the gas-phase DMT dimer. The electronic transition observed in the gas-phase DMT dimer was an T* n excitation,while the transitions observed in solution hydrocarbon work were T* a. As n excimer has never been obfar as we know, an ?r* served in solution. To explore the range of possibilities for cluster binding, it would be helpful to investigate the gas-phase T* T spectra of small polymers of aromatic hydrocarbons. Benzene is the simplest of the aromatic hydrocarbons, and this paper is a report of our investigation of the gas-phase T* T spectrum of benzene clusters.
-
-
-
-
Experimental Section Benzene clusters were prepared in a supersonic expansion of benzene and helium, and their fluorescence spectra were excited with a pulsed, frequency-doubled, Nd:YAG pumped dye laser with a bandwidth of 0.5-1 cm-'. The supersonic free-jet apparatus was new, but its design is similar to an older apparatus which has been previously described. The major change in the apparatus was the use of a high-powered frequency-doubled pulsed laser to produce ultraviolet light, and the pulsed detection and signal processing equipment which this required. Although the laser was pulsed, the supersonic free jet was continuous. The nozzle was a 0.050-mm diameter circular pinhole in a 0.040-mm thick nickel substrate. The sample was prepared by passing high-pressure helium through liquid or solid benzene. The benzene was contained in a stainless steel sample bulb, and the concentration of benzene in a mixture could be controlled by immersing the sample bulb in a constant temperature bath. The resulting mixture passed through some room temperature tubing and out the nozzle. The nozzle and gas mixture could be heated.
Figure 2. The fluorescence excitation spectrum of a mixture of benzene and helium taken under the same conditions as those of Figure 1. Features marked M, T, and TET are assigned to the monomer, trimer, and tetramer, respectively. The expected position of the dimer peak is under the monomer feature at 38 050 cm-'. The dimer intensity is estimated to be less than 20% that of the trimer feature at 37975 cm-'. This spectrum was taken at 13 times the sensitivity of the cluster part of Figure 1 and 156 times the sensitivity of the 62,monomer feature In Flgure 1. The calculated position of the forbidden monomer origin (03is marked.
0.07%
h I uu \
,
37900
38000
38100
I
"38400
38500
38600
v,(cm?) Flgure 3. The fluorescence excitation spectra of mixtures of 1.3 (top), 0.4 (middle), and 0.07 % (bottom) benzene in 8 atm of helium expanded through a 0.050-mm nozzle. The sensitivity is higher for the lower concentrations and for the lower energy (origin) regions of the spectrum.
Results and Discussion The fluorescence excitation spectrum of benzene and its clusters is shown in Figures 1and 2. This spectrum was taken with the benzene sample held at 0 "C (equilibrium vapor pressure 21 torr) and a total stagnation pressure of 8 atm (6000 torr). The spectra were digitally smoothed by using the method of Savitzky and Go1ay.l' A 9-point -
quadratic smoothing function was used, the frequency interval per point being 0.1 cm-l. The strongest feature in the spectrum is the 6; band of benzene monomer at 38 611 cm-l. To the red of this strong feature we observe weaker features due to other vibronic transition of the benzene monomer as well as weaker features assigned to the benzene dimer, trimer, and tetramer. The assignment of the benzene monomer features is based on the work of Atkinson and Parmenter? while the assignment of the identity of the clusters is based on the mass-selective photoionization spectra of Hopkins, Powers, and Smalley.s The features assigned to clusters fall into two groups, one group (Figure 1)just to the red of the 6; feature of the monomer, and one group (Figure 2) displaced to the red by 522 cm-', the v g frequency of the excited lBzUstate of benzene. The origin of the lBzu lAl, transition in the benzene monomer is forbidden and the transition is only allowed by Herzberg-Teller coupling to nontotally symmetric vibrations such as vg. However, if the sixfold sym-
(3)J. Ferguson, J. Chem. Phys., 28,765 (1958);R.M. Hochstrasser, ibid., 36,1099(1962);40,2559(1964);J. M. Murrell and J. Tanaka, Mol. Phys., 7,363 (1964);T.Azumi and S. P. McGlynn, J. Chem. Phys., 41, 3131 (1964);M. T. Vela, Jr., I. H. Hillier, S. A. Rice, and J. Jortner, ibid., 44, 23 (1966).
(4)G. H. Atkinson and C. S. Parmenter, J. Mol. Spectrosc., 73,20,31, 52 (1978). (5)J. B.Hopkins, D. E. Powers, and R. E. Smalley, J. Phys. Chem., preceding letter in this issue.
-
3744
Letters
The Journal of Physical Chemistry, Vol. 85, No. 25, 1981
I
38000
,
/
37000
,
/
,
1
35000
36000
,
I
34000
VF
Fpure 4. The dispersed fluorescence spectra obtained by exciting the 6,, band of the benzene trimer (top), the origin band of the benzene trimer (middle), and the 6; band of the benzene monomer (bottom. All spectra were taken with an instrumental resolution of 54 cm-
1.
metry of the benzene monomer is broken by cluster formation, the origin transition of the clusters becomes weakly allowed. Therefore, in the red-shifted group we observe the origin transitions of the clusters even though the corresponding transition in the monomer is not observed. The blue-shifted group is the 6; vibronic transition of the clusters. In Figure 3 we show the fluorescence excitation spectrum of benzene and its clusters taken with different concentrations of benzene. For the lower trace the benzene sample was held at -26 OC (vapor pressure 4.2 torr), for the middle trace the sample was at 0 "C (21 torr), and for the upper trace the sample was at room temperature (76 torr). In addition to the sharp dimer, trimer, and tetramer features that grew in at increasing benzene concentration, there are also broad underlying background features in both the 6; and the origin regions. Both the relative intensity and the position of the maxima of these broad features depend on benzene concentration and on total pressure, and these features are thought to be due to mixtures of higher polymers of benzene. The peaks of the broad features are displaced 170 cm-I to the red of the corresponding monomer bands as compared to a gas to CI'J'Sd red Shift Of 248 Cm-' for C6H6 in a c&3 Crystal6and a gas-to-solution red shift of 275 f 25 cm-l for benzene in various cryogenic liquid^.^ The dispersed fluorescence spectra that are obtained when the 0; and 6; bands of the trimer are excited are shown in Figure 4. Also shown in this figure is the dispersed fluorescence spectrum resulting from excitation of the 6; transition in the benzene monomer. The spectra were digitally smoothed with a 9-point quadratic smoothing function,17each point being 3 cm-l. All spectra were taken with 54-cm-I resolution. There are two striking differences between the monomer and the trimer fluorescence spectra. First, the trimer spectra resulting from excitation of 6l and Oo are identical while the 6l and Oo monomer spectra are known to be quite different.* The fluorescence spectra must obey selection rules Au6 = f l , and, therefore, in the monomer the Oo spectrum shows only the single strong progression 6!1$ while the 6l spectrum shows two strong progressions, 601: and 6!j1:. In the case (6)E. R. Bernstein, S. D. Colson, R. Kopelman, and G . W. Robinson, J. Chem. Phys.,48,5596 (1968). (7)E.R. Bernstein and J. Lee, J. Chem. Phys., 74,3159 (1981). (8)C. S. Parmenter in "Advances in Chemical Physics", Vol. 22,I.
Prigogine and S. A. Rice, Ed., Wiley, New York, 1972. See especially Figure 3.
of the trimer, the strong 6y1: progression appears in emission regardless of whether the original excitation was to the 6l or Oo level. The 6A1: progression is absent or very weak as may be seen in Figure 4. A weaker 1: progression (including the origin band 0;) appears in the trimer spectrum although this progression is symmetry forbidden in the monomer spectrum. Excitation of the 6l level of the dimer also produces a fluorescence spectrum characteristic of emission from the Oo level. In the dimer the :1 progression is relatively much weaker than it is in the trimer. We believe that the fact that the fluorescence spectrum is insensitive to the initially excited state is an indication of intramolecular vibrational relaxation?JO When clusters are formed, new low-frequency vibrational modes are generated. These modes describe the motion of the benzene subunits with respect to each other. Because the new modes are of much lower frequency than the ring modes, it is useful to think of the vibrational degrees of freedom of the cluster as being described by the highfrequency normal modes of the separated benzene subunits plus the new low-frequency cluster modes. The excitation of the 6l level of the trimer is to a state which in zero order can be described as containing one quantum of excitation in V6 with all other modes, including the trimer modes, in their zero-point levels. However, there are other isoenergetic zero-order states consisting of all ring modes in their zero-point levels plus a given amount of energy distributed among the trimer modes. When the trimer contains 522 cm-l of excess vibrational energy (one 1 6 quantum) the density of excited states consisting of only excited trimer modes may be quite high. If these states mix with the 6l state, this will lead to a relaxation of energy from the v6 modes into the trimer modes.'l Of course, if such relaxation takes place, emission will be from the relaxed zero-point level of the ring vibrations. A second striking difference between the dispersed fluorescence spectra of the monomer and the trimer is the width of the bands. When the monomer is excited the bands in the dispersed fluorescence spectrum are instrumentally narrow, in our case 54 cm-l fwhm. When the trimer is excited the bands are broadened to 200 cm-' fwhm, well beyond the instrumental width. This broadening is observed for excitation to both the excited 6l state and the zero-point Oo state, and it is also observed in other bands attributed to benzene clusters: dimer, trimer, tetramer, and the broad background. It should be noted that although there is broadening in the emission (dispersed fluorescence) spectrum, no broadening is observed in the absorption (fluorescence excitation) spectrum even though the experimentalresolution is far better in absorption than in emission. Broadening in emission without broadening in absorption has been previously ob~erved.~The phenomenon requires a sparse set of excited-state levels which have nonvanishing Franck-Condon factor with the populated levels of the ground state. In a cold jet the only populated level may be the zero-point vibrational level. Also required are a dense set of levels which cannot be reached optically (9)P. S.H.Fitch, C .A. Haynam, and D. H. Levy, J. Chem. Phys., 74, 6612 (1981). (10)J. B.Hopkins, D. E. Powers, and R. E. Smalley, J. Chem. Phys., 71,3886(1979);72,5039(1980);J. B.Hopkms, D. E. Powers, S. Mukamel, and R. E. Smalley, ibid., 72,5049 (1980);J. B. Hopkins, D. E. Powers, and R. E. Smalley, ibid., 73,683 (1980);72,2905 (1980). (11)Whether there is a time-dependent relaxation or simply a sharing of the absorption strength among many levels depends on whether the excited state is or is not prepared as a coherent superposition of a l l levels containing the zero order state 6l. For a more detailed discussion of OUT understanding of this process see ref 9.
Letters
from the populated ground-state levels but which can mix weakly with the sparse absorbing levels. This dense set of states is dark when one considers absorption from (or emission to) the zero-point level of the ground electronic state that originally had most of the population in the cold jet. All levels must have nonvanishing Franck-Condon factors to some vibrational levels of the ground electronic state, and therefore the dense set of levels will contribute to the emission spectrum. The width of the absorption spectrum is determined by the strength of coupling between the level carrying the absorption Franck-Condon factors and the dark levels which mix with it. The fact that the absorption line is narrow is an indication that this coupling is weak. The width of the emission line is determined by the change in vibrational structure between the ground and excited electronic states. If the vibrational structure of these states is identical, all emission lines will be at the same frequency regardless of the state initially excited. If the vibrational structure is very different, each excited state will have its own characteristic emission spectrum, and the composite spectrum from the several states that are mixed with a single absorbing level will have intensity spread over a wide frequency range. If the benzene trimer emission spectrum showed broadening only from the excited 6l level, one would be inclined to attribute the broadening to the intramolecular vibrational relaxation discussed above. In this case the chemically bound ring vibrations (including vg) would provide the sparse set of states having allowed absorption transitions, and the dense set of trimer vibrational levels isoenergetic with 6l would provide the other states. However, we also observe broadening in the trimer emission spectrum when the Oo level is excited, and this requires some additional discussion. If, in this case, we were truly exciting the zero-point vibrational level of the excited electronic state, there would be no dense set of isoenergetic states to produce the broadening. In the benzene trimer, a relatively tightly bound excimer state seems most likely to be responsible for the required dense set of levels, and we must consider the effect that such a state will have on the spectrum. Formation of an excirner requires that two aromatic rings lie in a stacked geometry where the planes of two rings are parallel. In solution where excimers are usually observed,the geometry of the absorbing species is not relevant because collisions with the surrounding medium can allow relaxation to the stacked plate geometry and the excimer binding energy can be transferred to the surrounding medium. However, in the gas phase in the absence of collisions the geometry of the absorbing species is more important. It is likely that the geometry of the ground electronic state of benzene clusters resembles that of the crystal where the planes of nearest-neighbor benzene rings are perpendicular, not stacked parallel.12 That being the case, the Franck-Condon principle would require that optical excitation be to a vibronic state where there is some appreciable probability of having a T-shaped geometry during the vibrational period. If the only excited electronic state were an excimer state, this would mean excitation to highly excited vibrational levels of the cluster, and the absorption (fluorescenceexcitation) spectrum would show long progressions or, more likely at the required energy, (12) E. C. Cox, D. W. J. Cruickshank, and J. A. S. Smith, Proc. R . SOC. London, Ser. A, 247,1(1958);G.E.Bacon, N. A. Curry, and S. A.Wilson, ibid., 279,98 (1964);K. C. Janda, J. C. Hemminger, J. S. Winn, S. E. Novick, S. J. Harris, and W. Klemperer, J.Chem. Phys., 63, 1419 (1975); J. M.Steed, T. A. Dixon, and W. Klemperer, ibid., 70, 4940 (1979).
The Journal of Physical Chemistty, Vol. 85,No. 25, 798 1 3745
a broad absorption feature produced by many closely spaced excited vibrational levels. The observed trimer absorption bands are sharp and all within 40 cm-l of each other, and this means that the excited trimer state is not a simple excimer. Our spectroscopic data would indicate that in the benzene clusters there are two kinds of excited states produced by different configurations, a weakly bound T-shaped van der Waals state and a more deeply bound parallel plate excimer state, and that these two kinds of state are only weakly interacting. By weakly interacting we mean that isoenergetic vibrational wave functions can be largely localized in separate parts of the multidimensional potential surface. For example, a simple case of separable wave functions could occur if we consider only one geometric variable, the angle 8 between the planes of the two rings in a dimer. If there were a barrier between the deep excimer well (parallel plates, 8 = 0) and the shallow van der Waals well (perpendicular plates, 8 = goo), then there could be two separate sets of states below the barrier that would only mix be tunneling. In this case, if the tunneling rate was much faster than the fluorescence rate, absorption could be to a single van der Waals state, the system could relax to a dense set of isoenergetic excimer states, and the molecule could emit as an excimer. In the real, multidimensional molecule a true barrier would not be necessary to isolate the van der Waals absorbing states from the excimer emitting states. All that would be necessary would be a classically allowed path that was tortuous enough to provide a small overlap between the two states. We believe that the broadening in the fluorescence spectra of the trimer, tetramer, and higher polymers excited at the origin indicates that there is relaxation from a van der Waals to an excimer state between the time of absorption and the time of emission. Finally, there is an interesting property of seeded supersonic molecular beams of benzene that should be noted; there is an anomalously small amount of vibrational cooling that occurs in the expansion. Three hot bands originating from the vibrational excited state 6 are a 6: doublet at 38 522.5 and 38 529.3 cm-l and the 6,ll; d. combination band a t 38 517.2 cm-l. These three bands have relative fluorescence excitation intensities 3,8, and 3% in the room temperature static gas, and 3, 7, and 3% in a supersonic jet consisting of 1.5% benzene in 8 atm of He expanded throu h a 50-pm nozzle. These intensities are relative to the 6,Bcold band intensity defined to be 100%. Thus, the ~ ~ ( 6 cm-') 0 8 vibration appears not to cool a t all under these expansion conditions. A doublet due to the transition 6h16a: occurs at 38448.9 and 38452.0 cm-' and each component has a room temperature relative intensity of 13%. In the supersonic free jet the relative intensity is 4% and therefore the lower frequency v16(399 cm-') vibration appears to be slightly cooled in the expansion. If the vibrational distribution were Boltzmann, the hot band intensity would be produced by a vibrational temperature of -200 K. This lack of vibrational cooling seems to be unique to benzene. In the case of iodine13 and chromyl chloride,14 excited vibrational states were depopulated, and hot band transitions were very weak in the jet. The heteronuclear aromatic molecule tetrazine is probably the closest analogue to benzene that we have studied in a supersonic jet, and here again vibrational cooling a pears to be very efficient. The O,! 17b:, 6a:, 6b:, 16al, P and 16b: bands of (13)R.E.Smalley, D. H. Levy, and L. Wharton, J. Chem. Phys., 64, 3266 (1976). (14)J. A. Blazy and D. H. Levy, J . Chem. Phys., 69, 2901 (1978).
3746
J. Phys. Chem. 1981, 85, 3746-3748
tetrazine have relative fluorescence excitation intensities of 100, 2, 0.3, 0.8, 33, and 41%, respectively, at room temperature.16 In a supersonic expansion at 4 atm through a 100-pm nozzle the corresponding relative intensities are 100, 0.6, 0.04, 0.08, 2, and 2%. Rice and co-workers16have noticed similar behavior in the excited electronic state vibrational relaxation in benzene. In other molecules, the vibrational relaxation (15) Room temperature fluorescence excitation intensities of tetrazine were obtained from our own measurements, the absorption measurements of K. K. Innes, L. H.Franks, A. J. Merer, G. K. Vemulapalli, T. Cassen, and J. Lowry, J. Mol. Spectrosc., 66, 465 (1977), and the lifetime measurements of J. Langelaar, D. Bebelaar, M. W. Leeuw, J. J. F. Ramaekers, and R. P. H. Rettachnick in “Proceedings of the 2nd International Conference on Picosecond Phenomena”, Springer-Verlag, Berlin, 1980, p 171. The room temperature fluorescence excitation intensities of benzene were obtained from the absorption intensities of ref 4 and the quantum yields of K. G. Spears and S. A. Rice, J. Chem. Phys., 56, 5561 (1971). (16) C. Jouvet, M. Sulkes,and S. A. Rice, Chem. Phys. Lett., in press. (17) A. Savitzky and M. J. F. Golay, Anal. Chem., 36, 1627 (1964).
cross section increases with decreasing temperature and becomes very large at the low translational temperature in the downstream part of the supersonic expansion. This produced a much faster vibrational-translational energy transfer than is usually observed in room temperature static gases. In the case of benzene, no increase in the vibrational relaxation cross section was observed as the temperature was lowered. This is consistent with our lack of vibrational cooling of ground electronic state benzene. At this time we have no explanation for this anomalous behavior of benzene. Acknowledgment. This material is based upon work supported by the National Science Foundation under Grant CHE-7825555, by the US.Public Health Service under Grant 5-R01-GM25907, and by the donors of the Petroleum Research Fund, administered by the American Chemical Society. C.A.H. was supported by the Fannie and John Hertz Foundation.
Additivity of Fluorine Substituent Effects in the Gas-Phase Basicities of Fluorinated Acetones D. F. Drummond and T. B. McMahon” Department of Chemistw, UniversiV of New Brunswick, Fredericton, New Brunswick, Canada E3B 6E2 (Received: July 27, 1981)
Gas-phase basicities of six fluorinated acetones have been determined from proton transfer equilibrium and bracketing experiments with ion cyclotron resonance spectroscopy. A very regular decrease in proton affinity of 6.1 f 0.4 kcal mol-1 for each successive fluorine substituent is found. This result is interpreted in terms of a substituent effect which is almost purely inductive and correlates extremely well with electronegativity of the methyl substituents. In addition, a small stabilizing interaction of 2-3 kcal mol-l is revealed in each of the protonated fluoroacetones due to formation of an intramolecular hyrogen bond. Intr
