J . Phys. Chem. 1990, 94, 5748-5752
5748
Theoretical Study of the Charge-Transfer Spectrum of the Indene-Tetracyanoethylene Electron Donor-Acceptor Complex W. Daniel Edwards,* Mei Du, J. Scot Royal, and J. L. McHale Department of Chemistry, University of Idaho, Moscow, Idaho 83843 (Received: October 26, 1989; I n Final Form: February 16, 1990) The ground-state intermolecular potential energy surface for the indenetetracyanoethylene electron donor-acceptor (EDA) complex was calculated, for three of the six intermolecular degrees of freedom, using semiempirical quantum mechanical techniques. The electronic absorption spectra (frequencies and intensities) were calculated at selected points on this surface. For all geometries, two low-lying charge-transfer transitions were found, having mostly the character HOMO (indene) LUMO (TCNE) and HOMO-1 (indene) LUMO (TCNE). Using a quasiclassical approach, the line shape of the charge-transfer spectrum was calculated from the vertical energies and oscillator strengths obtained at a large number (206) of intermolecular geometries. The results show that the two charge-transfer bands in the experimental indene-TCNE spectrum are not associated with distinct intermolecular conformations of the complex.
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Introduction The charge-transfer electronic transition of a typical electron donor-acceptor (EDA) complex has the character DA D+Aand is associated with a strong broad visible absorption band. In many cases, where the donor D is a substituted benzene compound, lifting the degeneracy of the donor HOMO leads to a splitting in the charge-transfer (CT) ~ p e c t r u m . l - ~Experimentally, the energies of the two maxima in the absorption spectrum are well correlated with the first and second ionization potentials of the donor. It has frequently been assumed that the two C T bands are also associated with two distinct complex geometries, which would optimize the orbital overlap of the acceptor LUMO with either the HOMO or HOMO-I of the donor.5 Both theoretical@ and studies have been aimed at investigating the possibility of multiple ground-state conformations. In this work, we investigated the complex indene (D) and tetracyanoethylene (TCNE) (A). The INDO optimized geometries for these molecules are shown in Figure 1, a and b, and the Cartesian coordinates of the numbered atoms are listed in Tables I and 11. The electronic absorption spectrum of this complex (in dichloromethane solution) has two distinct maxima at about 425 and 540 nm, as shown in Figure 2. The relative intensities of these two bands did not vary with temperature within the range 5-35 OC. This theoretical study was undertaken in order to learn whether the two CT bands are associated with two different complex geometries, for example, involving overlap of the TCNE LUMO with either the six- or five-membered ring of indene. It was found that, although two such minima in the intermolecular potential surface exist, the barrier separating them is only about 300 cm-I. More important, it was found that the orbitals involved in the C T transitions are delocalized to the extent that both the LUMO transitions carry HOMO LUMO and the HOMO-I significant oscillator strength at a large number of intermolecular geometries. Using a quasiclassical approach to calculating the Franck-Condon profile, we used the results of the INDO calculations to compute a C T line shape in good agreement with the experimental spectrum.
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( I ) Voigt, E. M.; Reid, C. J . Am. Chem. SOC.1964, 86, 3930. (2) Mobley, M. J.; Rieckhoff, K. E.; Voigt, E. M. J . Phys. Chem. 1978, 82, 2005. (3) Rossi, M.; Buser, U.; Haselbach, E. Helo. Chim. Acta 1976. 59, 1039. (4) Prochorow, J.; Tramer, A. J . Phys. Chem. 1967,47, 775. (5) Mulliken, R. S.;Berson, W. B. Molecular Complexes; Wiley-lnterscience: New York, 1969. (6) Herndon, W. C.; Feuer, J. J . Am. Chem. SOC.1968, 90, 5914. (7) Kuroda, H.; Amanov, T.; Ikemoto, 1. J . Am. Chem. SOC.1967, 89, 6056. ( 8 ) Cioslowski, J . Tetrahedron 1986, 42, 735. (9) Lippert, J. L.; Hanna, H. W.; Trotter, P. J. J . Am. Chem. SOC.1969,
91, 4035. ( I O ) Matsuo, T.; Higuchi, 0. Buff. Chem. SOC.Jpn. 1968, 41, 518. ( 1 1 ) Russell, T. D.; Levy, D. H. J . Phys. Chem. 1982, 86, 2718. (12) Mobley, M. J.; Rieckhoff, K. E.; Voigt, E.-M. J . Phys. Chem. 1977, 81, 809.
0022-3654/90/2094-5748$02.50/0
TABLE I: Cartesian Coordinates for INDO-Optimized Indeneu X Y z atom no. c1 1.357 1.195 0.000 2.176 1.393 -1.203 -2.4 15 -2.445 -1.261 -0.040 -0.009 1.574 1.574 3.269 1.722 -1.194 -3.357 -3.410 -1.301
-0.036 -1.139 -1.455 -0.766 0.628 1.367 0.694 -0.728 1.822 1.822 -0.000 -2.181 -2.551 -1.326 1.148 2.463
0.000 0.000 0.000 0.000 0.000
0.000 0.000 0.000 0.879 -0.879 0.000 0.000 0.000 0.000 0.000 0.000
c2 c3 c4 c5 C6 c7 C8 c9 h10 HI 1 H12 h13 h14 h15 h16 h17
"See Figure l a for numbering scheme. TABLE 11: Cartesian Coordinates for INDO-Optimized TCNEu X
Y
Z
atom no.
0.680 -0.680 1.446 1.446 -1.446 - 1.446 2.098 2.098 -2.098 -2.098
0.000 0.000 1.197 -1.197 1.197 -1.197 2.203 -2.203 2.203 -2.203
0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
CI c2 c3 c4 C5 C6 N7 N8 N9 N10
"See Figure Ib for numbering scheme.
Experimental Study of IndeneTCNE The indene-TCNE C T spectrum was measured in dichloromethane for a range of concentrations and at 5, 20, and 35 "C. Standard regression analysis of the concentration dependent data, using the Benesi-Hildebrand e q u a t i ~ n , gave ' ~ an equilibrium constant of 1.09 M-l at 20 OC. The enthalpy of the complex was found to be 30.5 kJ/mol. The molar absorptivity of the complex at 25 OC was found to be about 1100 M-' cm-I at 420 nm and 1700 M-' cm-' at 540 nm. The energy difference between the two CT maxima at 425 and 540 nm is 0.62 eV, which compares favorably with the separation of 0.80 eV between the first and second ionization potentials of indene.I4,l5 This comparison lends credence to the idea that the two bands in the visible spectrum ~
~
~~
(13) Benesi, H. A.; Hildebrand, J. H. J . Am. Chem. Soc. 1948, 70, 3978. (14) Eland, J. H. D ; Danby, C. J. Z. Naturforsch. 1968, 23a, 355. ( 1 5 ) Glisten, H , Klasnic, L. Z. Naturforsch. 1976, 310, 1051
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 15, I990 5749
CT Spectrum of Indene-TCNE Complex
I
Figure 1. INDO optimized geometries for (a, top) indene and (b, bottom) TCNE.
0.01
350
I
400
I
I
I
I
I
450
500
550
600
650
Wavelength, nm Figure 2. Electronic absorption spectrum of 0.50 M indene and 0.002 M TCNE in dichloromethane at these temperatures.
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are assignable to the transitions HOMO (indene) LUMO (TCNE) and HOMO-I (indene) LUMO (TCNE). However, the questions remains whether the two transitions belong to the same or different EDA complexes. Using multiwavelength linear regression of concentration dependent absorbance data,16 we found no evidence of significant concentrations of 2: 1 (D2A) complexes and linear regression of concentration-dependent data cannot ( 1 6) Gribaudo, M. L.; Knorr, F. J.; McHale, J. L. Spectrochim. Acto 1985, 41a, 419.
distinguish the existence of two different 1 :1 isomers. Therefore, a limited temperature study was done in order to learn whether the relative intensity of the two CT bands would change, as it might if there are two 1: 1 complexes in equilibrium. As shown in Figure 2, there is no noticeable change in the relative intensity of the two bands with temperature. These results, however, could also be explained by the existence of two 1:l isomers with the same enthalpy of association. Based on the overlap and orientation principle of Mulliken5 (vide infra), there might exist orientational isomers of this complex involving overlap of the TCNE LUMO with either the HOMO or HOMO-I of indene. The enthalpies of two such species might turn out to be the same due to the similarity of the potential energy as a function of donor-acceptor distance. It was therefore decided to undertake a quantum mechanical study of the intermolecular potential surface and CT spectra of indene-TCNE using semiempirical molecular orbital methods.
Methods All electronic structure calculations were of the INDO type, based on the methods of Zerner et al.17.'8 Ground-state potential energy surfaces were generated by using the original Pople, Santry, and Segal parametrization (geometry parameter^).'^ The starting geometries for the isolated molecules were derived from the X-ray crystal structure in the case of TCNE20and from microwave data in the case of indene.2' These starting geometries were refined by using the gradient based optimization method of Head and &merZ2 and the resulting optimized geometries are listed in Table I and Table 11. For calculations on the complexes, the donor and acceptor geometries were frozen at the optimized values and only the relative orientation was allowed to vary. The energy of the complex was calculated as a function of relative position and the resulting cuts through the potential energy surfaces were constructed. Due to the tendency for INDO calculations to underestimate intermolecular separation^,^^ the indene-TCNE distance was frozen at 3.2 A in all calculations. This is comparable to the experimentally observed distance for the hexamethylbenzeneTCNE complex.24 In addition to the ground-state energy calculations, a series of configuration interaction (CI) calculationsZ5were performed at selected points on the potential energy surface. These calculations were done using INDO/S spectroscopic parameters (spectroscopy parameters) and excited states were generated from single-electron excitations only. From these CI calculations, the transition energy between the ground and excited state was obtained at the different intermolecular geometries. The excited-state potential energy surfaces were constructed by adding this energy difference to the previously constructed ground-state potential energy surface. Transition moments were calculated by using the dipole length operator and keeping all one-center charge and polarization terms. Intermolecular Potential Surface As mentioned previously, the geometries of the donor and acceptor molecules were frozen (see Table 11) and the relative position and orientation of TCNE was varied with respect to indene. There are six intermolecular degrees of freedom, but it was not considered practical to vary all of these independently in this work. We focused instead on the in-plane translations (x, y ) and relative orientation 4. The two planar molecules were held (17) Ridley, J.; Zerner, M. C. Theor. Chim. Acta 1973, 32, 1 1 1. ( 1 8) Ridley, J.; Zerner, M. C. Theor. Chim. Acta 1976, 42, 223. (19) Pople, J. A.; Beveridge, D. L.; Dobosh, P. A. J. G e m . Phys. 1976, 47, 2026. (20) Little, R. G.; Poulter, D.; Coppens, P. Acta Crystallogr. 1971, 27,
1493. (21) (22) (23) (24)
Li, Y. S.; Jalilian, R. J.; Durig, J. R. J . Mol. Struct. 1979.51. 171. Head, J. D.; Zerner, M. C. Chem. Phys. Lett. 1985, 122, 264. Carreira, L. A.; Person, W. B. J. Am. Chem. SOC.1972, 94, 1485. Saheki, M.; Yamada, H.; Yoshioka, H.; Nakatsu, K.Acta Crysrallogr., Sect. B 1976, 32, 662. (25) Bacon, A. D.; Zerner, M. C. Theor. Chim. Acta 1979, 53, 21.
5750 The Journal of Physical Chemistry, Vol. 94, No. 15, 1990
Edwards et al.
TCNE LUMO
H
H H indene HOMO
H H
I H
H -2.5
-0.5
-1.5
X, H H H
indene HOMO-1
0.5
1.5
2.5
3.5
ANGSTROMS
Figure 4. Potential energy as a function of position of x of TCNE center of mass for y = 0 and for x axes of the two molecules (a) parallel (-*-) and (b) perpendicular (-+-), The energies plotted in Figures 5 and 6 are relative to -144 hartrees.
H I
H
H -0.031
Figure 3. ?r-orbital coefficients of (a, top) TCNE LUMO, (b, middle) indene HOMO,and (c, bottom) indene HOMO-I.
2-
-0.031
in a cofacial geometry; i.e., the two degrees of freedom which correspond to tilting one plane with respect to another were ignored. The donor-acceptor distance R,as discussed above, was not varied but was kept constant at 3.2 A due to limitations of the semiempirical approach employed here. Subsequent calculations at different intermolecular separations showed similar overall features. The overlap and orientation principle of MullikenS was considered in order to limit the number of calculations to those most likely to be energetically favorable. According to this principle, the charge-transfer stabilization is maximal when the overlap of the donor HOMO and the acceptor LUMO is optimized. It is now well-known that electrostatic and exchange repulsion forces can also play a strong role in determining the minimum-energy g e ~ m e t r y . ~ ~The . ~ ' .Ir-orbitalcoefficients of the relevant orbitals are shown in Figure 3a-c. For CT interaction based on overlap of the TCNE LUMO with either the HOMO or the HOMO-1 of indene, favorable overlap could be obtained in a geometry which places the center of TCNE over the six-membered ring of indene. The x axes of the two molecules would be expected to make an angle of about 45'. In addition, the HOMO-LUMO overlap might also allow for a geometry which places the center of TCNE near the five-membered ring of indene, with an acute angle between the x axes. These considerations invoke the question of a possible barrier separating translational isomers associated with the five- and six-membered rings of the donor. We will refer to these as the five-ring and six-ring complexes. At the same time, if a sufficient barrier to rotation exists, orientational isomers would result. We therefore varied the position x,y of the center of TCNE with respect to the indene coordinate system, and the angle 4 between the x axes. The x axis of TCNE contains the carbon-carbon double bond while that of indene bisects the C5-C6 bond, as defined in Figure 1. In Figure 4 are shown the results of a number of calculations in which the TCNE center was moved along the x axis of indene, keeping y equal to zero and the angle C#J equal to 0 and 90°, respectively. In both cases, there are indeed two minima associated with the two ring moieties, but the barrier separating them is quite ~
~~~~~
(26) Kollman, P. J. Am. Chem. Soc. 1977, 99,4875. (27) Lathan, W. A.; Morokuma, K. J. Am. Chem. SOC.1975, 97, 3617.
\
1,
f I
E-0.031 >
/ Z W -0.032k
36
72
ANGLE,
108
144
la0
DEGREES
Figure 5. Potential energy as a function of angle cp for TCNE center of mass (a, *) near first minimum in Figure 4, x = -1.23 A, y = 0 and (b, +) near second minimum in Figure 4, x = 0.50 A, y = 0.
Figure 6. Potential energy as a function of position x,y of TCNE center of mass with x axes of both molecules parallel.
small (- 30-200 cm-'), especially for translations from positive to negative values of x. The six-ring isomer is predicted to be slightly more stable than the five-ring isomer. At two positions near the minima in Figure 4, the energy was calculated as a function of angle 4 between the x axes and is shown in Figure 5. The barriers to rotation were observed to be somewhat higher than the barriers to translation. The most stable angles for the six-ring complex are about 10' and 95" (with energies of -144.032 338 and -144.032 442 hartrees, respectively), while the minima for the five-ring species are found at about 80"
CT Spectrum of Indene-TCNE Complex
The Journal of Physical Chemistry, Vol. 94, No. 15. 1990 5751
I I ‘ I
>
F H
>
H
+ a
8ooo 7000
1
8000 6000
a 0 cn
4000
m a
a000
a
2000
1 0 E
1000
a
n 000 358
374
389
405
420
436
451
467
020 a40 080 080 400 420 440 480
482
WAVELENGTH - nm Figure 7. Calculated stick spectrum a t 25 OC for I = 3.2 A. The full lines are contributions from the six-ring geometries and the dashed lines are contributions from the five-ring geometries. Note that an 8-nm interval has been used to coarse grain the spectrum.
and 160’ (energies of -144.031 746 and -144.032 158 hartrees, respectively). Thus, the conclusion that the six-ring isomer is more stable is still true when the orientational coordinate is optimized. The energy was also calculated at a grid of x,y points, keeping 4 fixed at 0 or 90°. The result for 4 equal to 0 is shown in Figure 6 as a three-dimensional plot. The result for 4 equal to 90’ is qualitatively similar to Figure 6 and is not reproduced here. Again, it is apparent that there are two wells corresponding to stable translational isomers, but the barrier separating them is only a little bigger than k T at room temperature.
Calculated Charge-Transfer Spectrum Spectroscopic INDO/S calculations were performed in order to simulate the line shape of the C T spectrum and to determine a possible correlation between complex geometry and oscillator strengths of the first and second CT transitions. At 206 selected geometries, the ground-state energy E6 and the vertical frequencies ugC of the low-lying excited states were calculated by using geometry and spectroscopy parameters, respectively. As in the calculation of the potential energy surface, only the coordinates x y and 4 were varied, the two molecules being kept in a cofacial geometry at a separation of 3.2 8,. The 206 geometries were chosen somewhat arbitrarily to cover a range of energetically accessible points xy and a number of discrete angles 4 at selected positions. At all geometries sampled, both thefirst and second CT transition carried significant oscillator strength. The results from these INDO calculations were then employed to approximate the line shape of the total spectrum using a semiempirical approach to the Franck-Condon approximation28 as follows:
480 100
Wavelength, nm Figure 8. Calculated charge-transfer spectrum of indene-TCNE at 298 K, averaged over donor-acceptor distance z .
interval. The partition function Z is also estimated by using a Monte Carlo approximation to the integral:
The result of this calculation is a stick spectrum which approximates the Franck-Condon profile due to the translational ( x y ) and orientational (4) degrees of freedom. In Figure 7 is shown the result of this calculation. The intensity at wavelengths greater than 400 nm is entirely due to the first C T transition, HOMO LUMO, while that at lower wavelengths is due to the second CT transition, HOMO- 1 LUMO. The maximum molar absorptivity at 25 ‘C was found to be 5280 M-l cm-’ at about 440 nm. The full lines represent the contribution from x,y values associated with the six-ring geometry and the dashed lines represent the contribution of the five-ring geometries. Clearly, the two C T transitions are allowed in each of the two types of structures associated with the separate wells of Figure 6, although the intensity of the higher energy band is somewhat diminished in the five-ring geometries. It has been previously shown that the dissociation coordinate R (also referred to as z in this work) makes a large contribution to the line width of the CT spectrum.mJOJ1In order to approximate the contribution of the donor-acceptor stretching motion to the Franck-Condon profile, a semiclassical approach was employed to average the spectrum over R. The potential energy was taken to be a harmonic function of R, with a force constant k calculated on the basis of an estimated donor-acceptor stretching frequency of 90 cm-I. This vibration has been observed at 80-100 cm-l for a number of complexes of TCNE with benzene derivative^.'^^^^ The vertical C T energy was estimated from that calculated at R = 3.2 8, on the basis of the change in Coulombic energy of the excited state:
-
-
vg,(R)
= ugc(RO)+ e2/Ro - e2/R
The transition moment M~~ was considered to be proportional to R, so that the oscillator strength, proportional to ugyet2, could be corrected by referencing to the value at R. The resulting molar absorptivity was averaged over R according to
where c is the molar absorptivity at wavelength A, No is Avogadro’s number, me and e are the mass and charge of the electron, and PA is the wavelength interval for coarse graining the spectrum. The integral, which is estimated in a Monte Carlo fashionB from the 206 points in Q-space, where Q represents a set of coordinates xy.4 and depends on the oscillator strength& and the wavelength A, of the vertical charge-transfer transitions. The exponential function represents the Boltzmann probability of a given ground-state geometry, and the delta function 6(A, - A) ensures that the absorption intensity is added into the correct wavelength
The choice of Ro = 3.2 A as the equilibrium value of the donor-acceptor distance is completely arbitrary, and a different value would result in a wavelength shift of the entire spectrum.
(28) Lax, M.J . Chem. Phys. 1952, 20, 1752. (29) Press, W. H.; Flannery, B. P.;Teukolsky. S. A.; Vetterling, W.T. Numerical Recipes; Cambridge University Pres: Cambridge, 1986, Chapter 7.
(30) McHale, J.; Simons, J. J . Chem. Phys. 1979, 70, 4974. (31) McHale. J.; Banerjee, A.; Simons, J. J . Chem. Phys. 1978,69, 1406. (32) Smith, M. L.; McHale, J. J. Phys. Chem. 1985, 89, 4002. (33) McHale, J.; Merriam, M. J. J . Phys. Chem. 1989, 93, 526.
t(A,,R) exp[-k(R - R0)2/2kBT16(AR- A) d R (€(A)),
=
1 exp[-k(R
- R0)2/2kBq d R
J . Phys. Chem. 1990, 94, 5152-5156
I
0 4
ooo
f
-'A
1'
a 2 0 o40 a50
\
not very different from that shown in Figure 9. A slight broadening with increasing temperature leads to a decrease in the maximum molar absorptivity with temperature. The calculated molar absorptivity at 440 nm varies from 8940 to 5280 M-l cm-' on going from 5 to 35 OC. The spectrum calculated at 77 K, shown in Figure 8, is considerably sharper than the room temperature spectrum, in agreement with experiments on related systeme4 The calculated molar absorptivities at 356 and 440 nm are 7414 and 9270 M-I cm-I, respectively.
t
Ll -
I
$80 400 420 440
450 480 500
Wavdmqth, nm Figure 9. Calculated charge-transfer spectrum of indene-TCNE at 77
K, averaged over donor-acceptor distance z . The resulting approximation to the line shape is shown in Figure 8 for a temperature of 25 OC. This spectrum can be compared to the experimental solution phase spectra of Figure 1, which are additionally influenced by the tilting motion of the molecular planes as well as the inhomogeneous broadening of the solvent. The calculated maxima in Figure 8 are at 356 and 449 nm, and the corresponding calculated molar absorptivities are 5260 and 6530 M-'cm-I. The theoretical R-averaged spectrum was also calculated at temperatures of 5 and 35 O C , but the resulting line shapes were
Conclusions In this work we have exdored the orientation deDendence of spectroscopic electron transf& in the indene-TCNE EDA complex. By calculating the energy and CT spectra as a function of complex geometry, it has been shown that the first and second chargetransfer transitions are allowed in a wide range of energetically accessible conformations. Investigation of the ground-state intermolecular potential also shows that fairly small energy barriers of the order of kT at room temperature separate the minima associated with possible orientational and translational isomers. Thus, for indene-TCNE, there is no correspondence between the two CT bands and distinct complex geometries. Acknowledgment. W.D.E. acknowledges a seed grant from the University of Idaho Research Council and generous start-up funds needed to acquire the components of The Computational Facility for Theoretical Chemistry. W.D.E. and J.L.M. thank the National Science Foundation's EPSCoR program for grant R11-8902065. J.L.M. acknowledges the support of N S F grant 8605079.
AMI and INDO/S Calculatbns on Electronic Slnglet and Triplet States Involved in ExcitebState Intramolecular Proton Transfer of 3-Hydroxyfiavone Bernhard Dick Max- Planck- Institut f u r biophysikalische Chemie, Abteilung Laserphysik, D- 3400 Gottingen, Federal Republic of Germany (Received: November 21, 1989)
Energies and geometries of the electronic ground states and vertical transition energies and oscillator strengths of singletsinglet and triplet-triplet transitions have been calculated for both tautomer forms of 3-hydroxyflavone. The resulting energy level diagram and calculated excited-state spectra yield a consistent interpretation of the data from transient absorption measurements. It is concluded that all long-lived transient absorptions are due to triplet species and that an intrinsic barrier to proton transfer exists in the lowest triplet state.
Introduction Among the molecules which display an excited-state intramolecular proton transfer (ESIPT) reaction, 3-hydroxyflavone (3-HF) is one of the most extensively studied. However, as more and more experimental information on this system is collected, it appears that a simple four-level picture involving the ground states and the first excited singlet states of the normal form and the tautomer form of this molecule cannot explain all the observations. In particular, the observation of long-lived transient absorptions'4 has stimulated hope to find an intrinsic barrier to the backward (ground-state) intramolecular-proton-transfer reaction, but the tautomer ground state could not be trapped at low temperatures. ( 1 ) Itoh, M.; Fujiwara, Y. J . Phys. Chem. 1983, 87, 4558. (2) Itoh, M.; Tanimoto, Y.; Tokumura, K.J . Am.Chem. SOC.1983,105, 3339. (3) Itoh, M.; Fujiwara, Y.; Sumitani, M.; Yoshihara, K. J . Phys. Chem. 1986, 90, 5612. (4) Brewer, E. W.; Studer, S. L.; Standiford, M.; Chou, P.-T. J . Phys. Chem. 1989. 93. 6088.
0022-36S4/90/2094-5752%02.50/0
An impediment to the interpretation of the experimental data is the fact that the relative ordering of the singlet and triplet states of both tautomer forms of 3-HF is not known. Bouman et aL5 have performed ab initio calculations on the ground state and the first aa* excited state of 3-HF, but calculations on other states do not yet exist. This paper presents results of semiempirical molecular-orbital calculations for several singlet and triplet levels of both tautomer forms of 3-HF. For the triplet manifold of the zwitterionic tautomer these calculations predict rather unusual properties which yield a consistent interpretation of the experimental data available so far. These experimental findings will be briefly summarized in the following. We denote the singlet and triplet states of the normal form of the molecule by the symbols S, and T, and the corresponding states of the tautomer form by S,' and T,'. (In other molecules these forms are often termed enol and keto forms. In 3-HF, however, proton transfer changes the enol form to a ( 5 ) Bouman. T. D.; Knobeloch, M. A.; Bohan, S.J . Phys. Chem. 1985,89, 4460.
0 1990 American Chemical Society
