Structure of Contact Ion Pairs in the Ground and First Excited States Aromatic Closed-shell Anions containing a Five-membered Ring BY H. W. Vos,* C. MACLEANAND N. H. VELTHORST Chemical Laboratory of the Free University, De Lairessestraat 174, Amsterdam, The Netherlands Received 6th May, 1975 The absorption and fluorescence spectra of the carbanion of indene have been recorded as a function of temperature, solvent and counter ion. In ethereal solvents indenyl-lithium forms contact and solvent-separated ion pairs, depending on the temperature, whereas only contact ion pairs occur when Na+ and K+are the counter ions. The absorption spectra exhibit a blue shift on forma- tion of contact ion pairs, but in the fluorescence spectra a red shift is observed. The results for the indenyl anion are compared with those for the carbanion of fluorene and the anion of carbazole.The differences in cation-anion attraction and fluorescence shift are discussed with the aid of energy level diagrams, and it is concluded that these differences are ultimately determined by the r-electron charge distribution of the anion in the ground and first excited states. Theoretically calculated charge densities conflrm this conclusion ; an exception is the carbazolyl anion, where the nitrogen lone pair electrons play an important role, leading to the formation of a o-complex. The structures of the contact ion pairs of the two carbanions indenyl and Auorenyl in the ground and first excited states are inferred from the calculations. An apparent discrepancy with proton n.m.r.results is resolved by a theoretical examination of the effects of the cationic field on the proton chemicalshifts. It is well known that ion pairs of aromatic anions and alkali metal cations can exist in two forms: contact ion pairs (Ar-, M+) and solvent-separated ion pairs (Ar-11 M+); the latter is spectroscopically indistinguishable from the free solvated ions, which are present in strongly polar solvents. From many investigations 2-9 it has become clear that the change from solvent- separated ion pairs to contact ion pairs invariably leads to a displacement of the absorption spectrum to higher energies. This phenomenon (which we shall call the " ion-pair shift ") has been explained by the redistribution of the negative charge of the anion on excitation; it leads to a relative destabilization of the first excited (S,) state with respect to the ground (So) ~tate.~-~ Until now ion-pair structures in the S, state, which can be studied by emission spectroscopy, have hardly been investigated.Both high energy 2* * and low energy lo ion-pair shifts have been observed in the fluorescence spectra of aromatic ion pairs. In this paper we present a detailed investigation of the fluorescence spectrum of the indenyl anion (InH-), which has been reported to be temperature de~endent.~ It will be shown that InH- can form, depending on solvent and temperature, contact and solvent-separated ion pairs in the S, state, and that there is an ion-pair shift to low energy. These results, together with those for some other carbanions and nitrogen-containing anions reported previously,' are analysed with the aid of energy level diagrams, which in turn will be correlated with MO charge density calculations. The discrepancy with a reported structure of the ion pair indenyl-lithium is discussed and resolved.63 CONTACT ION PAIRS EXPERIMENTAL The absorption and fluorescence spectra of InH- have been recorded for solutions in 2-methyltetrahydrofuran(MTHF) from +20 to -180°C and in 1,Zdimethoxyethane (DME) from +20 to -80°C, with the cations Li+, Na+ and K+. The preparation of the carbanions l2and other experimental details have been described el~ewhere.~~ The fluorescence measurements have been performed with an improved apparatus, which makes possible the control of the cell temperature from room temperature down to liquid nitrogen tempera- ture.RESULTS AND DISCUSSION SPECTRA OF THE INDENYL ANION The results of the present investigation are summarized in table 1. Some of the absorption and fluorescence spectra of InH- are shown in fig. 1 and 2. 0/103cm-I FIG.1.-The absorption spectra of indenyl-lithium in MTHF at three different temperatures. TABLETH THE ABSORPTION AND FLUORESCENCE MAXIMA OF INDENYL ANION @/lo3cm-') absorption fluorescence solvent cation +20°C -80°C -16OOC +20°C -8OOC -16OOC MTHF Li+ 29.4 28.8 26.7a 19.7 20.0 21.8f Na+ 28.1 27.9 28.2 20.4 20.7 20.9 K+ 27.6 27.6 27.6 20.4 20.6 20.7 DME Li+ 29.1 26.5 -19.8 21.7 -Na+ 28.1 26.5 -20.4 20.8g -K' 27.8 27.4' -20.4 20.4 -4shoulders at 24.4, 25.5 and 27.2 x lo3cm-' ; shoulders at 26.0, 27.5 and 29.0x lo3cm-' ; c shoulders at 25.4, 26.9 and 28.3 x lo3cm-' ;dshoulders (see a) visible, but badly resolved ;e from ref.(8) ;fshoulder at 23.2 x lo3cm-' ; 20.5 x lo3cm-' at -6OOC. H. W. VOS, C. MACLEAN AND N. H. VELTHORST ABSORPTION SPECTRA The absorption spectra of the samples with Li+ in MTHF and DME and with Na+ in DME show a conversion from a contact to a solvent-separated ion pair as the temperature is lowered. Those with Na+ in MTHF and with M+in both solvents display only contact ion pairs. The "conversion temperature " (i.e. the temperature at which 50 % of both types of ion pair are present) is approximately -120°C for Lif in MTHF, -30°C for Li+ in DME and -50°Cfor Na+ in DME.The spectra of the various solvent-separated ion pairs are almost identical. The absorption spectrum of the free solvated anion in hexamethylphosphoric triamide (HMPT) has a maximum at 26.7 x lo3crn-l.* The absorption maxima of the contact ion pairs shift to higher energy as the cation becomes smaller ;the magnitude of the ion pair shift is slightly dependent on solvent and temperature. The results are in agreement with those found for other carbanions:6-9 the tendency to form solvent-separated ion pairs increases when a more polar solvent is used (DME is more polar than MTHF), the temperature is lowered or the cation is smaller. I .f? 24 22 20 18 16 403cm-I FIG.2.-The fluorescence spectra of indenyl-lithium in MTHF at three different temperatures.The intensities of the spectra at 3-20 and -100°C are multiplied by factors 16 and 8, respectively. FLUORESCENCE SPECTRA Inspection of table 1 shows that the shifts observed in the absorption spectra have counterparts in analogous, but opposite shifts in the fluorescence spectra. The " conversion temperatures " are approximately -110°C with Li+ in MTHF and -50°C with Li+ in DME; these temperatures are comparable with those found in the absorption spectra. The ion pair InH-,Na+ in DME seems to behave differently: a conversion to InH-11 Na+ is reflected in the absorption but not in the fluorescence spectrum. However, in view of the fact that the fluorescence maximum of this ion pair undergoes a shift of 300 cm-l in going from -60" to -8o"C, we suppose that the conversion 11-3 CONTACT ION PAIRS has just begun at -8O"C, the lowest temperature attainable in this solvent.The " conversion temperature " will be about -9O"C, which is 40°C lower than deter- mined from the absorption spectra. Below -160°C the fluorescence maxima of all three ion pairs in MTHF show a further shift to high energy (400-800cm-l at -18OOC), which is accompanied by a considerable change in band structure. We ascribe this effect to the increased viscosity of MTHF at these low temperatures, which may cause incomplete stabiliza- tion of the Franck-Condon excited state ;the result is a blue shift of the fluorescence spectrum.In our previous work 'we could not distinguish between this viscosity shift and the fluorescence ion-pair shift, hence the latter was not recognized. COMPARISON WITH THE SPECTRA OF RELATED SYSTEMS From the results in the foregoing section we may conclude that the tendency to form contact ion pairs is roughly equal for the So and S, states of the indenyl anion. This does not agree with the results for some benzo-condensed indenyl systems :the fluorenyl anion (FlH-) and the 4,5-methylenephenanthrenylanion (MH-) have much greater tendencies to form solvent-separated ion pairs in their excited states than in their ground states.'' lo Moreover, the absolute value of the ion-pair shift for FlH- is considerably smaller in the fluorescence than in the absorption spectrum,' O whereas there is only a small difference for InH-.Nevertheless, in both carbanions a fluores-cence ion-pair shift to low energy has been observed. The results for InH- do not agree either with those for the nitrogen-containing analogues of FlH- and MH-, namely the carbazolyl anion (Cb-) and 4,5-iminophenan- threnyl anion (Im-).' These systems do not show ion-pair conversion when the temp- erature is varied over the usual range. In ethereal solvents contact ion pairs are formed exclusively, which means that the cation-anion attraction is even greater than in the InH-contact ion pairs. The most remarkable difference is encountered in the fluorescence ion-pair shift of Cb- and Im-, which is in the same direction as the absorption ion-pair shift : to high energy. The absorption and fluorescence ion-pair shifts of Cb-and Im- are approximately equal in magnitude.ENERGY LEVEL DIAGRAMS To account for the different results obtained for the fluorescence ion-pair shifts of a number of structurally related carbanions and nitrogen-containing anions, we give a detailed discussion of the various factors which may influence the position of the fluorescence bands of contact ion pairs. We focus our attention on three typical systems, namely InH-, FlH- and Cb-. The spectral positions of the bands of these anions are indicated schematically in fig. 3. In this figure the maxima in different solvents have been averaged for each type of ion pair. THE ABSORPTION ION-PAIR SHIFT In spite of the differences in fluorescence shift and cation-anion attraction, all ion pairs investigated thus far resemble each other in one important respect: the ion-pair shift is always to high energy.Apparently, the formation of a contact ion pair invariably results in a greater stabilization of the ground state with respect to the Franck-Condon excited state which the system is in immediately after excitation [see fig. 4(a)]. The relative destabilization of the latter state has been explained by several authors 3-6 (although somewhat differently formulated) in the following way. In the ground state the cation is located near that part of the anion which has the H. W. VOS, C. MACLEAN AND N. H. VELTHORST highest charge density.Upon excitation the charge distribution changes, while the cation retains its position. Consequently, the electrostatic cation-anion interaction diminishes. THE FLUORESCENCE ION-PAIR SHIFT For our discussion of the various possibilities for the fluorescence ion-pair shift, we start from the incomplete energy level diagram of fig. 4(a). In this diagram arbitrary values have been given to the absorption energies of both ion pairs and the s K+ ~4+Li+ abs. InH' fi. I I I Li+ N~+.K+ s abs. S \ K+k* Li' \I / FlH- f I. Ill /I\\Li+ I& K+ S s~ K+ Li+. _. Cb' abs. m . 20 .2:. 2.4 1 *6j 1. 20 . M. I1 I fl. II 1S K' Li* wavenumber/1O3cm-I FIG.3.-The absorption and fluorescence spectral positions of InH-, FlH-and Cb-.The positions of the spectral maxima (InH-and Cb-) or the 0-0transitions (FIH-) are indicated for the solvent- separated ion pairs (denoted by s)and the contact ion pairs (denoted by the cation involved). energy --so S C S C (Model S C ( intermediate) FIG.4.-Energy level diagrams which describe the absorption and fluorescence ion-pair shifts. S and C denote solvent-separated and contact ion pairs, respectively. fluorescence energy of the solvent-separated ion pair. For convenience we have made the (not strictly necessary) supposition that the Franck-Condon (FC) de-stabilization energies of the ground and excited state of a certain system are equal. CONTACT ION FAIRS Obviously the fluorescence energy of the contact ion pair may have different values.We can distinguish two extreme cases, which we shall refer to as models 1 and 2. In model 1 [fig. 4(b)] we assume that the FC destabilization energies of the contact ion pair are equal to those of the solvent-separated ion pair. This implies that the S1 lowest vibronic state is less stabilized by the cation than the So lowest vibronic state : the equilibrium stabilization energy of the ion pair is larger in the ground state than in the excited state. On the other hand, in model 2 [fig. 4(c)] we suppose that the equilibrium (lowest vibronic state) stabilization energies of the So and S, states are equal, so that the FC energies in the contact ion pair should be larger than they are in the solvent-separated ion pair.These two models represent extreme cases, an ion pair will generally conform to an intermediate model [fig. 4(d)], in which both the equilibrium stabilization energies and the Franck-Condon energies are different for the two types of ion pairs (see table 2). TABLE2.-sUMMARY OF THE RELATIONS BETWEEN THE GROUND AND FIRST EXCITED ST.4TE PROPERTIES AND THE FLUORESCENCE ION-PAIR SHIFT model 1 model 2 general extent of charge localization in So and S1depth of potential minimum equilibrium stabilization energy 1 different equal different in So and S11position of potential minimum area of charge localization Franck-Condon energy equal different different fluorescence ion-pair shift blue red either direction, may be small example Cb- InM- FlH- The fluorescence ion-pair shift can easily be derived from fig.4(b)-(4 : in model 1 it is equal in magnitude to and in the same direction as the absorption ion-pair shift (blue shift). In model 2 the fluorescence ion-pair shift is also equal in magnitude but has the opposite direction (red shift). In the intermediate case the fluorescence ion-pair shift may be small and have either direction. Comparing these results with the data of fig. 3 it is clear that Cb- agrees well with model 1, InH- largely satisfies model 2 whereas FlH- appears to be an example of the intermediate case. FACTORS DETERMINING THE ENERGY TERMS To answer the question why the spectra of different aromatic anions are described by different energy level diagrams one should know which factors influence the equilibrium stabilization and FC energy terms.The equilibrium (lowest vibronic state) stabilization energy of the contact ion pair is largely determined by the electrostatic attraction between the cation and the anion. The cation will be located at a position of minimum potential energy in the electrostatic field of the anion. The position of the potential minimum will be near that part of the anion at which the major part of the negative charge is localized. The equilibrium stabilization energy is equal to the depth of the potential minimum, which will depend on the extent of charge localization in its neighbourhood. It is therefore required for model 1 that the charge of the anion is more localized in the 13.W. VOS, C. MACLEAN AND N. H. VELTHORST So state than in the S1state, whereas for model 2 the charges must be equally localized (see table 2). The FC destabilization energy is related to the difference in geometry between the lowest vibronic So and S, states. The FC energy arising from the change in anion geometry will not increase appreciably on the formation of contact ion pairs. The difference in the FC energies of the two ion pairs in model 2 should therefore arise from an important change in the equilibrium position of the cation with respect to the anion on excitation. This will be caused by a displacement of the position of the potential minimum, which implies a displacement of the area of charge localization.Such a displacement will be absent in model 1, since the FC energies of both ion pairs are equal in this model (see table 2). So far no attention has been paid to the polarizing influence of the cationic field on the charge densities of the anion. The cation will induce such a change of the charge distribution that the electrostatic stabilization of the ion pair increases. As this effect arises in all possible cation-anion conformations, the transition energies will hardly be affected. We will show in a following section, however, that this effect may account for the anomalous chemical shift differences found in the proton n.m.r. spectrum of indenyl-lithium.ll CALCULATIONS METHOD In the foregoing section it has become clear that the energy terms which SUM up to the ion-pair shift are ultimately determined by the charge distributions in the ground and first excited states.To explain the direction of the fluorescence ion-pair shift it is therefore necessary to calculate these charge distributions theoretically. We have used the semiempirical x-electron Variable Electronegativity SCF method,14 which is an extension of the familiar Pariser-Parr-Pople method and gives a more realistic description of the charge distribution.15 We have applied the following modification in the VESCF-method: the one-centre repulsion integrals have been taken proportional to, instead of quadratically dependent on the Slater effective charge. The atomic valence state parameters have been taken from Hinze and Jaff6.17 The two-centre repulsion integrals have been calculated according to Mataga and Nishimoto.* The two-centre core integrals have been given the following values : pCc = -2.26 eV,19 PCN= -2.40 eV. The latter parameter is chosen somewhat arbitrarily, but its value is close to those used in the literature.20 Since the actual geometries of the anions are unknown we have assumed planar structures consisting of regular polygons with sides of 1.39 A. Our program includes an option to incorporate the influence of the electrostatic field of the cation into the calculation. This has been done in a way analogous to the method of McClelland,*l by adding a term to the diagonal elements Hppof the core matrix. These elements represent the energy of an electron located on an atom p in the field of the positive core : Ip denotes the valence state ionization potential of atom p.The second term on the right-hand side gives the energy of an electron on atomp in the field of all other core centres q(#p) ;Zq is the positive core charge of q and ypq is the repulsion integral between electrons on atoms p and q. Analogously, the third term represents the energy of an electron on p in the field of the cation rn. The metal-carbon repulsion integrals yplrl are calculated by the multipole expansion method. 22 We shall refer CONTACT ION PAIRS to the method with inclusion of the cationic field as the "polarizable anion " (PA) approximation ; without this inclusion we have the "rigid anion " (RA) approxi-mation.The cation-anion attraction energy in the ground state Eigis calculated according to Ei, = En-E," +E,,,,. Enis the total n-electron energy of the contact ion pair, E,"is the corresponding energy of the undisturbed anion and E,,,, is the repulsion between the cation and the anion core; all core-centres are assumed to be point charges. The attraction energy in the excited state E$, is derived from ET, = Eip+AE-BEo AE and AEo are the excitation energies of the contact ion pair and the free anion, respectively. The excitation energies and the excited state charge distributions are obtained using the virtual orbital approximation. It has been shown by 'Li n.m.r. chemical shift measurernent~,~~ that in contact ion pairs with carbanions like InH- and FlH-, the Li+ cation is located above the ring system.We have therefore performed the calculation, both with the PA and RA approximations, for a number of cation positions at a constant distance of 3.0 A from the anion (3.0 A is probably a good estimate for the Na+-carbanion distance 6). Equipotential lines have been obtained by interpolation of fitting polynomials. COMPARISON OF THE THEORETICAL AND EXPERIMENTAL RESULTS The theoretical n-electron charge densities in the So and S, states of the free anions InH-, FIH- and Cb- are shown schematically in fig. 5. The equipotential lines of InH- and F1H- in both states, obtained with the PA approximation, are shown in fig. 6 and 7.The calculated transition energies of the free ions InH- and FlH- and of their contact ion pairs with the cation located in the So and S, state potential minima, are presented in table 3. TABLE3.-THE CALCULATED TRANSITION ENERGIES OF InH-AND FlH-AE"/lOJcm-1 AE(s0)a/103 cm-1 AE(S1)b/103 cm-1 InH- 25.7 27.8 23.6 FlH- 23.2 25.4 22.3 aThe transition energy when the cation is located in the ground state potential minimum and b in the excited state. The negative charge of the indenyl anion is largely localized on the five-membered ring in the ground state, whereas most charge has been displaced to the six-membered ring in the excited state (fig.5, top). This is reflected by a corresponding displacement of the potential minimum on excitation (fig.6); we may conclude that the cation will be located above the five-membered ring in the So state and above the six- membered ring in the SI state. The potential minima in both states are approxi- mately equally deep (-4.90 eV in the So state and -4.92 eV in the S1 state). In view of these results InH- may be expected to satisfy model 2, which is in good agreement with the experimental results. The calculated (absolute) values for the absorption and fluorescence ion-pair shifts of InH- are both 2.1 x lo3cm-l (see table 3) ;they are somewhat larger than the experi- mental shifts of the sodium ion pairs, which amount to 1.5 and 1.4 x lo3 cm-l, respectively (see fig. 3). H. W. VOS, C. MACLEAN AND N. €1. VELTHORST In fluorenyl anion (fig.5, middle) the negative charge is spread over three rings instead of two, it is therefore not surprising that the tendency to form contact ion pairs is smaller than in InH-. From fig. 6 and 7 it appears that the cation-anion attraction energy in the ground state is indeed smaller for FIH-(-4.68 eV) than for InH-. In the ground state the charge is slightly localized in the central ring, and consequently the potential minimum is located there (fig. 7) ; we may expect that the cation is positioned above this ring.24 In the excited state the negative charge is InH-s1 -.-SO 1.04 1.17 FLH' 110 I .07 1.17 1.2I i 0.86 104 so ! 116 Cb- 1.096 1 0 7 I/ 1.05- 1.00 107 ! 117 1.06 I. 10 134l 101 FIG.5.-~-Electron charge densities of InH-, FlH-and Cb- in the ground (So) and first excited (St) states.FIG.6.--Equipotential lines for a cation in a plane at a distance of 3A above the ring system of InH-: bottom, ground state ;top, excited state. Potential energies are indicated in eV. predominantly localized in the two side-rings. In the PA approximation there are now two potential minima (fig. 7), which are less deep (-4.56 eV) than the one in the So state. It is likely that the cation will jump back and forth between the two CONTACT ION PAIRS side-ring minima. The changes in ion-pair structure and cation-anion attraction on excitation agree with the experimental result that FlH- satisfies the intermediate model. In the RA approximation the minimum still remains above the central ring, although it has become very broad.The relative importance of the FC energy term in FlH-, which follows from the observed red shift, is not well explained by the RA result. FIG.7.-Equipotential lines for a cation in a plane at a distance of 3 above the ring system of FIH-: left-hand side, ground state ;right-hand side, excited state. Potential energies are indicated in eV. The calculated values for the absorption and fluorescence ion-pair shifts of FlH- are 2.2 and 0.9 x lo3crn-l, respectively (see table 3). The magnitudes of the corres- ponding experimental shifts of the sodium ion pairs are 1.4 and 0.3 x lo3cm-l, in the same order (see fig. 3). It appears that the theoretical results qualitatively account for the differences between the absorption and the fluorescence ion-pair shifts of FlH-.However, the calculated values are consistently too large, just as for InH-. Plodinec and Hogen-Esch lo have explained the red fluorescence ion-pair shift of FlH- in terms of an energy level diagram which is essentially identical with that of model 2. These authors, however, did not note that the red shift in the fluorescence spectra, expressed in energy units, is much smaller than the blue shift in the absorption spectra, whereas model 2 requires that these shifts are (approximately) equal. Also this model does not account for the greater tendency to form solvent-separated ion pairs in the excited state than in the ground state. The pure model 2 is therefore inadequate to describe the energy levels of F1H-.The charge distributions in the carbazolyl anion (fig. 5, bottom) resemble those of FlH- to a great extent, with the difference that more charge is located on the more electronegative nitrogen atom and less on the neighbouring carbon atoms. The result is that the equipotential lines of Cb- are very similar to those of F1H-. It is obvious that the large experimental difference in cation-anion attraction between these two anions cannot be explained by considering the n-electron charge distribution only. The 7Li n.m.r. chemical shifts of Cb-, Li+ indicate that the cation is located near the nitrogen atom, probably in the plane of the anion.25 The strong association H. W. VOS, C.MACIrEAN AND N. Ii. VELTHORST between Cb- and alkali metal cations may therefore arise from an interaction between the cations and the nitrogen lone pair electrons, leading to the formation of a 0-complex rather than a n-complex: We are now able to explain the experimental result that Cb- satisfies model 1 with the assumption that the cation is co-ordinated to the nitrogen atom lone pair both in the Soand S1states, so that there is no displacement of the cation on excita-tion. The smaller cation-anion attraction in the S, state is caused by the greater delocalization of the negative charge. COMPARISON WITH PROTON N.M.R. RESULTS The structures of some contact ion pairs of InH-and FIH-have been inferred from proton chemical shift data by Van der Kooij et aZ.ll*26 They concluded from the chemical shift differences between InH-, Li+ and InH- [I Lif in THF (measured at +40 and -37"C, respectively) that the Li+ cation is located near the six-membered ring of InH-in the contact ion pair, as the differences are larger for the protons 4-7 attached to this ring (see fig.8). On formation of a contact ion pair with the cation positioned above the n-system, the electron density in each C-H bond will be dis- placed somewhat towards the C-atom, resulting in a smaller shielding of the proton ; the proton resonance will consequently be shifted downfield. This shift will be larger the closer is the cation to a particular C-H bond. We shall call this effect the "direct action " of the cation on the proton chemical shift.4 expartrnantal:O.29 0 22 017 0.12 calculatod 10.66 039 0.31 022 diruct action : h-indiract action : --rosult ( f told -) FIG.&-Top, experimental and calculated differences in proton n.ni.r. shifts (in p.p.m.) between contact and solvent-separated ion pairs ;bottom, schematic indication of the effects on the proton chemical shifts arising from a cation located above the five-membered ring. The above conchsion disagrees with the present result that in the ground state the cation is located over the five-membered ring. This discrepancy may be resolved by taking into account another effect, which we shall call the '' indirect action " of the cation on the chemical shift. It is well known that the n-electron charge densities strongly influence the proton chemical shifts of an aromatic system. A cation above the five-membered ring will influence the n-electron charge distribution of the car- banion: a part of the negative charge will be displaced from the six- to the five- membered ring.By a mechanism similar to that described above the decrease of the negative charge in the six-membered ring will cause a further downfield shift of the protons 4-7, whereas the increase of the negative charge in the five-membered ring 74 CONTACT LON PAIRS results in an upfield shift of the protons 1-3. The effects of the direct and the indirect action of the cation are shown schematically by arrows in fig. 8 ;it appears that the sum of these contributions, dependent on their magnitudes, may result in a greater downfield shift for the protons 4-7 than for the protons 1-3.The effect of the cation on the charge distribution of the carbanion has been calculated by the semiempirical MO method described. The cation was assumed to be located in the potential minimum of fig. 6, 3.0A above the ring system. The influence of the charge distribution in the whole ion pair on the proton chemical shift has been obtained using a formula given by M~sher.~' The results are also indicated in fig. 8. It appears that the calculated shifts are of the right order, which justifies our supposition that the inclusion of the indirect action of the cation resolves the apparent discrepancy between the optical and the n.m.r.spectroscopic results. However, the theoretical shifts are about twice the experimental ones. A theoretical overestimate of ion-pairing effects is often encountered in calculations of this type 26p 28 (see also the foregoing section). It may arise from partial solvation of the cation in the contact ion pair, which screens the cation field to some extent. To account for this effect Takeshita and Hirota 28 have added a screening factor E to the de- nominators of the cation-anion electrostatic interaction terms. Following their suggestion, we have obtained a quantitatively more satisfactory agreement with E = 2. Another parameter which greatly affects the results of the calculations is the position of the cation, which is not known precisely. In view of the purely empirical nature of E, the uncertainty of the cation position and the approximate nature of the theoretical method we feel that attempts to obtain quantitative agreement of theory and experiment have little significance.Similar reasoning as has been given for indenyl-lithium resolves an analogous discrepancy for fluorenyl ion pairs. In this carbanion formation of a contact ion pair results in chemical shift displacements which are approximately equal for all 29* 30 This need not be explained by jumping of the cation between the side rings 26 when the indirect action discussed above is included. CONCLUSIONS It has been shown that the various experimental results for the fluorescence ion- pair shift in the systems InH-, FlH- and Cb- may be interpreted by a careful analysis of the different energy contributions to this shift.There are two important factors which determine the absolute value and the direction of fluorescence ion-pair shift : first, the equilibrium stabilization energies in the So and S1states, which depend on the extents of charge localization of the negative charge in both states, and may be enhanced by cation-lone pair interactions ; secondly, the Franck-Condon de-stabilization energies, which are determined by the difference in the areas of charge localization in the So and S1states. The positions and depths of the potential energy minima for the cation, which correspond with the extents and areas of charge localization, respectively, may be predicted theoretically by n-electron MO calculations for carbanions.More satisfactory results are obtained when the polarization of the anion by the cation is included. In nitrogen-containing anions like Cb- the specific interaction of the cation with the nitrogen lone pair electrons is important. Ion-pairing effects on proton chemical shifts may be described qualitatively with the results of similar calculations, provided the polarization of the n-system by the cation is taken into account. H. W. VOS, C. MACLEAN AXD N. H. VELTHORST The present investigations have been carried out under the auspices of the Nether- lands Foundation for Chemical Research (S.O.N.) and with financial aid from the Netherlands Organization for the Advancement of Pure Research (Z.W.O.).The authors thank Mr. A. 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