J. Phys. Chem. 1993,97, 4344-4353
4344
Electronic Substituent and Amino Torsional Coupling Effects on Intramolecular Proton Tunneling in 5-Aminotropolone Frederick A. Ensminger, Jason Plassard, and Timothy S. Zwier' Department of Chemistry, Purdue University, West hfayette, Indiana 47907- 1393 Received: January 20, 1993
Fluorescence excitation, dispersed fluorescence,and population labeling spectra of six isotopesof 5-aminotropolone (-OH/-NH2, -OH/-NHD, -OH/-ND2, -OD/NH2, -OD/-NHD, and -OD/-ND2) are reported. A primary purpose of this study is to investigate the effect of the strong electron donation of the amino group on the proton tunneling. Single vibronic level tunneling splittings in the SI SOtransition of the supersonic-jet cooled molecules are used to probe these effects. Amino substitution produces a modest increase in the origin tunneling splitting but has a much more dramatic effect on the 26"o progression involving a low-frequency out-of-plane vibration of the molecule. This progression, which dominates the spectrum of tropolone itself, is much reduced in intensity in 5-aminotropolone. At the same time, the excited-state frequency of V26 is increased by almost a factor of 2 relative to tropolone. Both features point to a stiffening of the pseudoaromatic ring induced by the amino group. Accompanying these changes is a much reduced sensitivity of the tunneling splitting to Q26 excitation. D20 concentration studies and population labeling spectra show unusual secondary isotope effects in -OH/-NHD and -OD/-NHD which produce a 7.8-cm-I asymmetry splitting in the ground state, large enough to effectively quench proton tunneling. A model two-dimensional (2D) potential along proton tunneling and amino torsional coordinates is presented which accounts for the observations. In the ground state where the amino group is nonplanar, intramolecular proton tunneling between equivalent minima must be accompanied by internal rotation of the amino group through 180°, efficiently coupling the motions. By contrast, in the excited state, -NHD isotopic substitution has no effect on the tunneling splittings. It is postulated that the lack of quenching in the excited state is a consequence of the planarity of the -NHD group which removes the major sources of asymmetry in the minima of the 2D torsion/tunneling potential energy surface.
-
I. Introduction Studies of intramolecular proton transfer carried out at a vibrationally state-selected level free from solvent effects lay an essential foundation for a molecular scale description of proton transfer in bulk solution.Is2 The hope for such studies is that they can determinein significant detail which electronic and vibrational properties of an isolated polyatomic inhibit or enhance proton tunneling. A simple-minded treatment of intramolecular proton or hydrogen-atom transfer equates the reaction coordinate with the hydrogen motion between donor and acceptor moieties in which, due to its light mass, tunneling provides a mechanism for circumventing energetic barriers to reaction. A schematic diagram of this one-dimensional tunneling process in tropolone is shown in Figure 1, together with the symmetries and electric dipole-allowed transitions between vibronic levels in the S p 9 , transition of tropolone. Throughout the paper we refer to such tunneling as proton tunneling, but whether a proton or hydrogen atom is actually transferred in tropolone is largely an open question. Despite the appeal of such a one-dimensional picture, thecombinedwisdom of previous studies of intramolecularproton transfer is that proton tunneling is properly described only as a truly multidimensional process which includes both heavy atom and light atom motions.2-'0 Malonaldehydehas a special experimental' I J * and theoreticals7 legacy as a prototypical polyatomic capable of intramolecular tunneling between symmetric minima. More recently, malonaldehyde's larger cyclic analog tropolone has received increasing attention, due in part to its pseudoaromaticity. Several groups have studied the single vibronic level spectroscopy of tropolone with an eye toward addressing the multidimensional character of intramolecular proton tunneling.*-IOJ3-16 Significant progress has been made in assigning the vibrational structure observed in
* Author to whom correspondenceshould be addressed
Tunneling Reaction Coordinate Figure 1. Schematic one-dimensionalpotential energy curvesfor groundand excited-state tropolone along the intramolecular proton tunneling reaction coordinate. The tropolone molecule is shown at one of its tunneling minima with the inertial axes drawn in (the x axis is out of the plane of the page).
both ground and excited states.8.9315J6 Startling mode-specific effects have been observed in which the tunneling splitting has increased or decreased by large factors depending on the vibrational mode@)excited. In an effort to further characterize the normal modes in terms of specific heavy and light atom
0022-365419312097-4344%04.00/0 0 1993 American Chemical Society
Intramolecular Proton Tunneling
The Journal of Physical Chemistry, Vol. 97, No. 17, 1993 4345
motions, several of tropolone's isotopesI7J8have also been studied (TrOD, 5-deuteriotropolone,andTr180H). Ab initiocalculations of the ground-state vibrational frequencies have also been performed.10 From these studies, it is obvious that the proton tunneling splitting is extremely sensitive to the regions of the multidimensional potential energy surface sampled by the nuclei. An alternative approach to the study of the interplay between structure, vibrational motion, and tunneling dynamicsis to study proton tunneling in a series of symmetrically substitutedtropolone derivative~.~~Jo To that end, we report here the first elements of a detailed study of 5-aminotropolone,in which substitution involves
0
0
I
'0
. . . . . . . . . . . . . . . . . . . . .. ... .. .. . .. . . .. . ..
0
NH* 5-aminolropdone
the stronglyelectron donating NH2 substitutent. This work thus provides a contrast with the recent study of Sekiya and co-workers on the electron-withdrawinghalogen substituents.20 In addition to its strong electron donation, amino substitution also provides avenues for studying the interplay between the vibrational modes of the spatially well-separated amino group and the tunneling coordinate. Already at the S d l origin, such effects are readily apparent in the large secondaryisotope effects observed in the -NHD species. We will see that these effects are large enough to quench the tunneling in the ground state of the -NHD species. In the process of exploring these effects, we have recorded spectra of all six isotopically substituted derivatives ranging from -NHz/-OH to -ND2/-OD. The extension of our studies to regions of the spectrum well above the S& origin, where direct excitation of -NH2 vibrations can occur, will be discussed in a forthcoming paper.
11. Experimental Section Fluorescence excitation and dispersed fluorescence spectra were recorded using an apparatus which has been described previously.2' A sample of 5-aminotropolone was resistively heated to 100 OC in a housing placed directly before the pulsed nozzle. Helium gas at 2-10 bar of pressure was passed over the heated sample and expanded through a pulsed valve (General Valve, 0.5-mm diameter) into a chamber pumped by two vapor booster pumps operating in parallel. The output of an excimer-pumped dye laser (Lambda EMG50E/FL2001) crossed the expansion 1-2 cm downstream from the nozzle. In fluorescence excitation, truly unsaturated spectra required unfocused laser pulse energies of about 10 pJ/pulse. Dispersed fluorescence spectra utilized a 0.75-m monochromator operatingat a resolution of 10cm-I fwhm. Fluorescencesignals were recorded using either a gated integrator or, in low light level spectra, gated photon counting. Population labeling scans were recorded by monitoring the decrease in fluorescence from a particular vibronic transition induced by a second,high-power excimer-pumpeddye laser (-0.2 mJ/pulse). This high-power pump laser is capable of saturating or partially saturating many of the SI SOtransitions, thereby removing population from the ground-state level involved in the transition. This pump laser produces a dip in the fluorescence induced by the probe laser only when the probe laser has been fixed on a transition out of the same ground-state level as the pump laser. The outputsof the twolasers are spatially overlapped. Typical delays between pump and probe lasers were 100 ns. Deuterium isotopic substitutionis achieved in 5-aminotropolone by entraining a small flow of D20 in the helium gas which serves as buffer gas for the 5-aminotropolone. The amount of D20 was controlled by diverting a small fraction of the total helium flow through a water sample cooled to 0 OC. The flow through this
-
200
-
600 Relative Frequency (cm ') 400
. . . . . . . . . . . . 800 1000
Figure 2. Overview fluorescence excitation scan of the first 1000 cm-I of the SI SOtransition of 5-aminotropolone. The frequency scale is relative to the origin H++transition at 23 883 cm-I. The asterisks denote transitions due to the ATr-H20 complex. The scan was taken under unsaturated conditions and is power-normalized.
channel is controlled by a needle valve and monitored by a flow meter. Total concentrationsof DzO in the expansion varied from 0.004 to 0.1% in helium at a total backing pressure of about 3 bar. The careful control of D20 flows allowed the isotopic composition of the expansion to be controlled reversibly and with good reproducibility. 5-Nitrotropolone was synthesized from tropolone (Aldrich) according to the procedure of Doering and K ~ o x The . ~ ~precipitate was filtered, washed, dried, and verified by mp (195 OC), NMR, and MS. The nitro group was then reduced by Na2S204(aq) using a procedure similar to that of Cook, Loudon, and Steel.23 The reduction of NO2 to NH2 was carried out in basic solution and was neutralized with HzS04 before extraction with ethyl acetate. After solvent evaporation, the 5-amino derivative was verified by mp (175 "C), NMR, and MS.
III. Results A. 5-Aminotropolone (-OH/-NH*). Figure 2 presents an overview fluorescence excitation spectrum of the first 1000 cm-I of the SI-SO transition of 5-aminotropolone. The spectrum has been corrected for changes in laser power and was taken under unsaturated conditions. Theorigin of 5-aminotropolone (23 883 cm-I) is shifted over 3000 cm-I to the red of its position in tropolone (27 017 cm-I). The tunneling doublet at the origin is clearly observed even in this overview scan. The features marked by an asterisk are due to the ATr-H20 complex. Above the origin, the spectrum of Figure 1 is remarkably complex, especially in the highly congested regions near 350 and 700 cm-I above the origin. In the present paper we restrict attention to the region extending no more than about 200 cm-I above the origin. Throughout this discussion, the vibrational numberingof tropolone (TrOH) itself will be used where appropriate to facilitate comparison with the parent compound. An expanded view of the region near the SI SOorigin is shown in Figure 3b, with the corresponding region of tropolone's spectrum directly above it for comparison (Figure 3a). Previous workers have shown that the doublet at tropolone's origin is a tunneling splitting (18.9 cm-I) arising from transitions involving lower and upper members of the ground- and excited-state tunneling doublets, as shown in Figure 1. Recent microwave measurements have determined that TrOH's ground-state tunneling splitting is 0.98 cm-I, indicating that the observed splitting at the S&, origin is largely due to the excited state.24 The low-frequency region of tropolone's spectrum (Figure 3a) is dominated by a long progression in even overtones of a very low frequency,out-of-planering foldingvibration,v26 (azsymmetry).
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Intramolecular Proton Tunneling
The Journal of Physical Chemistry, Vol. 97, No. 17, I993 4347
TABLE I: Tunneling Splittings, U26 Vibrational Frequencies, ~ in the SIState of TrOH and the and 2 6 2 Intensities 5-Aminotropolone Isotopes. TrOHb 00 splitting
18.9 7.2 38.6
262 splitting v26'
1(26*0)/W'o)
H++ H-_ 00 splitting 262H++ 262H-_ 262 splitting 2~26~ v26
5-aminotropolone (-OH/-NH# 22.0 16.8 64.7 0.09
-0.45
-OH/ -NH2
-OH/ -NHD
-OH/ -ND2
-OD/ -NHI
-OD/ NHD
-OD/ -ND2
Od
0 22.1 22.1 131.7 148.7 17.0 129.1 64.5
0 21.6 21.6 131.1 147.8 16.7 128.6 64.3
0 2.8 2.8 128.5 130.1 1.6 127.9 63.9
0 2.8 2.8 126.8 128.9 2.1 126.4 63.2
0 2.5 2.5 126.6 128.4 1.8 126.2 63.1
22.0 22.0 132.0 148.8 16.8 129.4 64.7
All vibrational frequencies and tunneling splittings are in wavenumbers. Reference 8. ? Present work. d Frequencies relative to H++. e Calculated as 2 ~ = 2 l/1(26~H+ ~ 262H- - OoH+ - OOH-).
A
+
as it was in TrOH itself. Note that in retaining the vibrational numbering in TrOH in making these assignments, ~ 2 possesses 5 a lower ground-state frequency than V26, at odds with conventional vibrational numbering schemes. B. DeuteriumIsotopic Substitution. Figure 6 presents a series of fluorescence excitation spectra in theorigin region withvarying D20 concentrations in the expansion. As outlined earlier, the labile hydrogens on 5-aminotropolone, namely, the tunneling hydrogen and the two hydrogens on the -NH2 substituent, can be exchanged by D2O in this way. We expect six isotopes to be formed: -OH/-NH2, -OH/-NHD, -OH/-ND2, -OD/NH2, -OD/-NHD, and -OD/-ND2. As Figure 6a shows, with no D 2 0present, the single tunneling doublet due to the -OH/-NH2 isotope is present at the origin. At the highest D2O flows (Figure 6g), the spectrum is also dominated by a doublet of splitting 2.5 cm-I, which is reasonably assigned as the H++and H-- transitions of the -OD/-ND2 isotope. These transitions are shifted 44.6 and 47.3 cm-' to the blue of the -OH/-NH2 H++ transition. At D 2 0flows between these extremes, the spectrum near the origin is muchmorecomplex. However, at all flows, the transitions seem to cluster into three groups near the 0-, 22-, and 45-cm-I regions. The two sets to the red always have matching transitions separated by 22 cm-I, suggesting that they belong to isotopes containing-OH. The transitions near +45 cm-I are much more closely spaced,consistent with their being blue-shifted transitions due to -OD-containing isotopes with a much reduced tunneling splitting. Parts b andf of Figure 6 further support this assignment scheme. Under the lowest D2O flows of Figure 6b, the first set of transitions to appear beside the -OH/-NH2 transitions is a doublet at 46.3 and 49.1 cm-I. Since the-OH proton is expected to be the most easily exchanged, this blue-shifted doublet with 2.8-cm-' splitting is assigned to the -OD/-NH2 isotope. In the same way, the last remaining peaks in the -OH region to survive the high D20 flows of Figure 6f comprisea doublet with 2 1.6-cm-' splitting at -2.6 and +18.9 cm-I. These peaks scale together with changing D2O flow and carry a natural assignment as the H++and H-- transitions of -OH/-NDz. Figure 6c-e presents spectra with intermediate D2O flows in which eight additional transitions appear. Some of these transitions overlap or partially overlap with those due to other isotopes. Nevertheless, both the D20 flow study and the positions of the transitions indicate that the eight transitions are composed of two quartets, one in the -OH region, the other in the -OD region, suggesting assignmentsas -OH/-NHD and-OD/-NHD. As can be seen in Figure 6d, within the -OH/-NHD quartet, there are two sets of 22-cm-I split doublets which are separated
7.8
I
A.7.0
7.0
k
-
7.8
-
a
A 2.5
-
-OD'-NDz
0) /*
I""~""I""I""I""I""I"'~
20 40 60 Relative Frequency (c"') Figure 6. Fluorescence excitation scans near the S l S o origin of 5-aminotropolone taken with varying concentrations of D20 in a 3-bar helium expansion. (a) No D 2 0 (b) 0.002% D2O; (c) 0.007% DzO;(d) 0.02% D20; (e) 0.03% D2O; (f) 0.04% D2O; (g) 0.1% D20. The assignment of the transitions to isotopic species and the tunnelingsplittings are indicated. The -OH/-NHD and -OD/-NHD transitions are numbered to correspond to the energy level diagram of Figure 9. See the text for further discussion. 0
by 7.8 cm-I. Similarly, in the -OD/-NHD quartet, there are two sets of doublets with 2.8- and 2.7-cm-I splitting, which are again separated from one another by 7.8 cm-I. In both cases, backing pressure studies indicate that the 7.8-cm-I red-shifted transitions are hot bands. The assignment suggested by these characteristics is that in -OH/-NHD and -OD/-NHD the asymmetry in the amino group leads to nonzero intensity not only in the H++ and H-- transitions but also in transitions which in the symmetrically substitutedisotopes have zero intensity, namely, the H-+ and H+- transitions. To test this assignment, we have recorded population labeling scans monitoring fluorescence from each of the nonoverlapped bands in the origin region of the six isotopes. Examples from the symmetricallysubstituted-OH/-NHz and -OD/-ND2 molecules are shown in Figure 7. By monitoring the H++(H--) transitions with the delayed probe laser, fluorescence dips are only observed when the saturation laser is resonant with a transition which removes population from the lower (upper) member of the tunneling doublet. In particular, the 2620H++and 2620H-transitions are clearly observed and distinguishedfrom one another in each of the isotopes. Note that since in the symmetric isotopes the lower and upper members of the tunneling doublet have different symmetries (aland b2, respectively, in the Gd molecular
The Journal of Physical Chemistry, Vol. 97, No. 17, 1993
4348
'
1
0
'
"
'
I
.
.
"
I
"
50 100 Relative Frequency (cm")
.
'
I
'
'
150
Figure 7. Population labeling scans and the correlation with transitions in the fluorscence excitation scans for two of the symmetric isotopes, (a) -OH/-NH2 and (b) -OD/-ND2. The upper (middle) traces in (a) and (b) are taken with the probe laser tuned to the OooH++(OOoH--) transition for the species while tuning the saturation laser.
symmetry group), population labeling scans in these molecules might be thought of as symmetry labeling scans. This feature can be put touse in assigningvibronic transitions in the congested regions further above the origin where tunneling doublets are less readily assigned, a task with which we are currently engaged. Figure 8 presents population labeling scans monitoring transitions ascribed to -OH/-NHD and -OD/-NHD. In these asymmetric isotopes two transitions at the origin are observed out of each monitored ground-state level (H+ or H-). The splittings of these transitions are very close to those in the symmetric -OH or -OD isotopes, namely, 22.1 and 2.8 cm-I, respectively. Several conclusionscan immediately be drawn from these spectra. First, the spectra confirm our assignments of the origins of the -OH/-NHD and -OD/-NHD isotopesas quartets. Thus, the symmetry-forbidden nature of the H-+ and H+transitions is removed in these asymmetric -NHD isotopes. Second, the excited-state tunneling splittings are virtually unchanged in the -NHD isotopes from that in -NH2 or -ND2. Third, the 7.8-cm-' shift of the doublets is a ground-state splitting characteristic of the -NHD substituent (Le., irrespectiveof -OH or-OD isotopicsubstitution) which is not present in the symmetric isotopes. Wesurmise on this basis that the intramolecularproton tunneling is largely quenched by the asymmetric deuterium substitution in thegroundstate of-OH/-NHD and-OD/-NHD with an asymmetry splitting of 7.8 cm-I but is unaffected by the NHD asymmetry in the excited state. The population labeling scans of Figures 7 and 8 also provide the v26 frequencies and 262 splittings in the isotopes. These are listed in the lower portion of Table I for comparison with - O H / -NHz and TrOH. In the aminotropolone isotopes, little change in either the frequency or excited-state tunneling splittings of 262 is observed with changing isotopic composition. However, there is some indication that the V26 frequency is more sensitive to OH/OD isotopic substitution than the change from -NH2 to
Ensminger et al. a) -OH/-NHD
I
b) -OD/-NHD
I
1
,
'
0
"
1
'
"
'
"
'
'
50 100 Relative Frequency (cm")
.
1
.
150
Figure 8. Population labeling scans and the correlation with transitions in the fluorescence excitation scans for the two asymmetric isotopes, (a) -OH/-NHD and (b) -OD/-NHD. The upper (middle) trace in (a) is taken with the probe laser turned to the [1]([3]) transition identified below it. The upper (middle) trace in (b) is taken with the probe laser tuned to the [2]([3]) transition.
-NHD to -ND2. This, in turn, provides some evidence that the out-of-plane motion of Q26 in the excited state of 5-aminotropolone involves larger motion of the OH0 group than at the 5-position in the seven-membered ring.
IV. Discussion A. Electronic Effects of the -NH2 Group. As stated in the Introduction, a primary motivation of this work is to better understand the electronic effect on the intramolecular proton tunneling of the strongly donating -NH2 group substituted in a symmetric position spatially far-removed from the tunneling proton. The following comparisons between 5-aminotropolone and TrOH itself provide some insight to this question. First, the S d l origin of 5-aminotropoloneis shifted to the red by 3 130 cm-l from the origin of TrOH. The similar red shift in the S O S ,origin of aniline relative to benzene (4060 cm-I) suggests that the nitrogen lone pair in 5-aminotropoloneinteracts with the pseudoaromatic u cloud nearly as strongly as it does with the aromatic ring in aniline. Second, the origin and 2620 transitions of the excitation spectrum have rotational band contours which are like those of TrOH, namely, y-axis polarized (Figure 1). No Q-branch intensity is observed in any of the transitions. We conclude that in this low-energy region of the spectrum the transition is in all probability a nearly pure T-T* transition with little effect from either charge transfer (which would likely bez-polarized) or n-r* (x-polarized) transitions. Third, the 26"o progression in 5-aminotropolone is far weaker than in TrOH, indicating more nearly Au = 0 Franck-Condon factors along Q 2 6 following amino substitution. Finally, accompanying the decrease in the intensity of the excited-state v26 progression is a large increase in its vibrational
The Journal of Physical Chemistry, Vol. 97, No. 17. 1993 4349
Intramolecular Proton Tunneling
TABLE Ik G Molecular Symmetry Group Character Table AI’
1
1
Ai BI’ Bi
1 1 1 1 1
1 -1 -1 1 1 -1 -1
AI” AT BI” B2”
1 1
1 -1 -1 1 1 -1 -1
1
1 -1 1 -1
1 -1 1 -1
1 1 1 1 -1 -1 -1 -1
1 1 -1 -1 -1 -1 1 1
1 -1 -1 1 -1 1 1 -1
1 -1 1 -1 -1 1 -1 1
Reference 27. (12) is the transformation associated with the proton tunneling. (34) is the exchange of protons (deuterons)on the amino group. Correlation to G4 appropriate for the -NHD isotopes in which (34) is not feasible. tropolone molecule, proton tunneling is the only large-amplitude frequency (39 66 cm-I). A significant stiffening of the force motion in the molecule, producing a tunneling splitting at the constant along Q26 is indicated. zero point level of about 1 cm-1.22 By contrast, the lowest The interpretation of these data is clouded somewhat by an imprecise knowledge of the Q26 normal coordinate. In tropolone, frequency vibration of the molecule is 109 cm-I. Thus, the tunneling doublet at the zero-point level is comparatively isolated the very low frequency (39 cm-I) of this out-of-plane vibration points to an unusual floppiness of the excited-state molecule about from other motions of the molecule; Le., proton tunneling occurs its nominally planar equilibrium configuration.s-~0J5-~7Until on a time scaleslow by comparison to any other vibrational motions recently, the symmetry of this mode has been taken to be bl. in the molecule. However, on the basis of the large 1 8 0 and OD isotope effect in In 5-aminotropolone, -NH2 substitution brings with it two tropolonels-20and recent ab initio calculationsI0, Redington et other potentially feasible large-amplitude motions of the amino a1.IO have suggested that Q26 in tropolone is best thought of as group, namely, inversion (I) and torsion (T). By analogy to an a2 symmetry, out-of-plane wagging fundamental with large, aniline, it is quite likely that the amino group is pyramidal in the out-of-phase oxygen atom motion. In 5-aminotropolone, the ground state but becomes planar in the SIstate.27 Furthermore, greater sensitivity of U26 to isotopic substitution at -OH rather since the barriers to both inversion and torsion are expected to than -NH2 suggests that this general picture of e26 is likely be quite high, both modes will have two low-lying levels which preserved by aminosubstitution as well. On this basis, Redington in the limit of infinite barriers become degenerate. In aniline, et a1.I0 qualitatively explain the reduction in tunneling splitting the NH2 torsional splitting is thought to be very small at the zero with increasing 26” excitation as an increase in the average point level (
