4993
J . Phys. Chem. 1986,90, 4993-4997 binding of N2- to Li+, but, on the other hand, raises intriguing questions concerning the mechanism of formation. Recall that under the same conditions (1% N 2 in Ar for example), this species is not formed without ethylene (see Figure 1). There are three facts which can provide some light on the mechanism: (i) the product is only seen if the concentration in ethylene is very low, (ii) there seems to be an increased yield relative to monolithium species (Li(C2H4)2N2)when increasing Li concentration, and (iii) the product disappears in favor of B and C upon annealing which means that further diffusion facilitates the additon of new ligands. Because of facts (i) and (iii), formation of D cannot involve a high number of nitrogen molecules and probably only one ethylene, and fact (ii) suggests the participation of more than one Li atom. I t is therefore consistent to suggest the following two-step mechanism: H '
c I
H
H I
e/
H H
c H
"I
H
H
Thus, the addition of the second lithium atom to a Li(C2H4)(N2) complex would lead to the formation of a Li(C2H4)complex and a Li+N2- ion pair. It is also clear from the strong bands near 2300 cm-' in Figure 1, c and d, that the presence of ethylene catalyses the complexation of nitrogen to lithium. The Li(C2H4),, n = 1 and 2, species add N2 much more readily than does lithium itself. Again, this is believed to be due at least in part to a small fractional positive charge on lithium in the binary Li(C2H4), species, which facilitates coordination of molecular N2.
Conclusions Reaction of lithium atoms with ethylene and nitrogen molecules in an inert medium produced two kinds of species not seen in
experiments performed with lithium and either molecular reactant under the same conditions. The first kind is a series of ternary complexes containing both ethylene and nitrogen coordinated to the metal center. These are characterized by strong IR absorptions in the v(N=N) stretching region and by ethylene submolecule [6,(CH2) vCc] modes closely resembling that of the Li(C2H4), binary species. The Li(C2H4),(N2) species has been identified and can be better described as a Li(C2H4)2complex slightly perturbed by addition of a N2ligand. An analysis of the isotopic shifts in the case of 13C2H4,C2D4, C2H2D2,and C2H4 C2D4 provides an estimate of the force field of the ethylene ligands in the same framework as developed with Li(C2H4)nbinary species. A transfer of some of the molecular parameters derived for the binary species yields an estimate of the global geometry; although experimental data do not provide a definitive determination of the binding arrangement of the nitrogen ligand, the observations suggest the end-on structure for the nitrogen ligand. Other complexes, Li(C2H4)(N2)",n = 1, 2, 3, and a very stable aggregate species containing more than one lithium, but displaying only small perturbations of the ligands, have been characterized. On the basis of the shift in the v(N=N) fundamental in the nitrogen ligand, the lithium-nitrogen interaction in these complexes is weaker than in transition-metal-nitrogen species. The second kind of product is the Li+N2- species observed earlier in pure nitrogen, which is not formed without ethylene in low concentration in the argon matrix. This catalytic role for ethylene is believed to first involve LiC2H4,which is formed directly and readily adds one N2 molecule to give (N2)Li(C2H4);a second lithium probably abstracts C2H4 leaving nearby Li+N2- and LiC2H4species in the argon matrix. It is clear from these experiments (Figure 1) that the presence of ethylene catalyses the fixation of nitrogen in lithium complexes.
+
+
Acknowledgment. We gratefully acknowledge financial support from the National Science Foundation. Registry No. N2, 7727-37-9; Li, 7439-93-2; CzH4,74-85-1; Ar, 7440-37-1.
Near-Infrared Triplet-Triplet Spectrum and Triplet-Triplet Circular Dichroism as Probes of Electron Delocalization In Blnaphthyis Catherine Tetreau Institut Curie, Section de Biologie, Centre Uniuersitaire, 91 405 Orsay. France (Received: February 12, 1986; In Final Form: April 29, 1986)
Triplet-triplet (TT) absorption and triplet-triplet circular dichroism (TT-CD) of chiral binaphthyls I-V have been measured in the spectral range 1OOEC-26OOO an-'.SCF CI dipole velocity calculationswere performed for several values of the interplanar dihedral angle 0. Satisfactory agreement with the experiment was obtained by assuming that 0 varies from about 4 5 O for I to almost 90° for IV and V. The comparison of the present calculations with the alternative description of binaphthyl in terms of two interacting naphthalene moieties allows one to rationalize the main experimental observations and to relate the evolution of the TT and TT-CD spectra within the series to the change in the molecular conformation. The previous predictions of the exciton model are supported.
Introduction Extension of circular dichroism studies to the first excited triplet state is now in progress.'-3 In a first approach of the triplet optical activity of "dimerlike" molecules, we had compared several compounds in the binaphthyl and the spirobifluorene series.2 On the
basis of the results and of the predictions of the exciton model, a clear-cut difference between the triplet- and the ground-state optical activities had been emphasized: while the exciton coupling mechanism is mainly responsible for the optical activity observed So absorption spectrum, it does not contribute to that in the S, T, spectrum; triplet-triplet circular dichroism (Tof the T, T-CD) can only originate from interchromophoric electron exchange. Due to instrumental limitations, the first TT-CD measurements on binaphthyls were restricted to the visible region where only + -
+ -
(1) Lavalette, D.; Tetreau, C. J . Phys. Chem., 1983, 87, 3226. (2) Tetreau, C.; Lavalette, D.; Cabaret, D.; Geraghty, N.; Welvart, Z. J . Phys. Chem. 1983,87, 3234. (3) Tetreau, C.; Lavalette, D.; Balan, A. J . Phys. Chem. 1985,89, 1699.
0022-3654/86/2090-4993$01.50/00 1986 American Chemical Society
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The Journal of Physical Chemistry, Vol. 90, No. 21, 1986
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-
the naphthalene-like absorption develops; however, the T, T, spectrum of binaphthyl has k e n reported4 to present another band, in the near-infrared region. This band, which does not appear in the naphthalene spectrum, nor in perpendicular binaphthyl, may be used for probing the change in the amount of interchromophoric electron delocalization, and the concomitant variation of triplet circular dichroism. In the present work, the measurements are extended to the near-infrared region; the TT and TT-CD spectra are interpreted on the basis of SCF CI dipole velocity calculations; their evolution in shape and intensity within the series is related to the change in the molecular conformation.
Experimental Section Materials. Binaphthyls I-V have been investigated.
> z OCH, OCH,
.... -.
....
P Figure 1. The Cartesian coordinate system and geometric parameters of 1 ,I’-binaphthyl.
Their synthesis, resolution, and optical purity characterization have been previously described.2 Measurement of TT-CD. The principles of TT-CD measurements and the experimental arrangement have been described in full detail1 and will be summarized here only briefly. Circular dichroism is detected by a conventional modulation technique. Triplet species are produced in a photostationary regime by continuous excitation in an organic glass at cryogenic temperature. The artifact-free measurement of C D under such conditions requires the suppression of the linear anisotropy effects associated with (a) the matrix birefringence and (b) the photoselection of the solute. This can be achieved in practice by a careful adjustment of the matrix viscosity; the convenient range ( 107-109 P) is that which allows the solvent and solute molecules to reorient during the C D detection time, while maintaining the collisional quenching within limits compatible with a reasonable steady-state triplet concentration. It has been determined from the simultaneous measurements of the Brownian rotational diffusion time and the triplet-state lifetime of phenanthrene in various glassforming solvents. In the present work, the experiments were performed in dry 2-methyltetrahydrofuran (MTHF) at 96-98 K (7 = 1 X 108-3 X lo8 P). The absence of residual artifact or C D distorsion due to photoselection effects in the specific case of binaphthyls has been unambiguously established2 from the mirror-image relationship between the TT-CD spectra of R and S enantiomers, and from the constant value of the anisotropy factor, g, measured between 97 and 102 K . 5 The sample was contained in an Oxford Instrument DN704 cryostat. Optical excitation occurred using a high-pressure mercury arc (Philips, SPSOO), and appropriate optics and filters. The triplet-triplet absorption and CD were monitored at a right angle to the direction of excitation, using a highly stabilized quartz-iodine lamp (Osram, 100 W); quarter-wave modulation was provided by a photoelastic modulator (4) hie, M.; Yoshida, K . ; Hayashi, K. J . Phys. Chem. 1977, 81, 969. (5) Raising the temperature from 97 to 102 K caused the triplet-state lifetime and the rotation correlation time of phenanthrene to decrease from 1.6 to 1 s and 1.5 to 0.2 s, respectively, which indicates restoration of isotropy for a solute of moderate size during the CD detection time (1-3 5).
(Jobin-Yvon), operated at 18 kHz. The spectral range investigated was 10000-260000 cm-’; the detection was performed by using either a red-sensitive R446 Hamamatsu or an EM1 65258 photomultiplier (respectively below and above 16000 cm-I). The ac component of the signal was received by a PAR HR8 lock-in amplifier, while the dc component was sent on a X-Y recorder. The signal-to-noise ratio was about 5 X lo4,using a time constant of 1 s. The method used for determining TT extinction coefficients has been previously described.6 Experimental dipole (D)and rotational ( R ) strengths were obtained from the appropriate band areas by the expressions’ D = 91.8 X 10-40s(c/v) dv R = 22.9
X
10-40s[(c1- t,)/v] dv
in which D and R are in cgs units, the frequency, v, is in cm-’, and the extinction coefficient, t, is in L mol-’ cm-I. Calculations. The dipole and rotational strengths of the binaphthyl TT transitions were calculated by using the dipole velocity method developed by Moscowitz.* Charge displacements along the bonds between adjacent atoms are assumed to provide the major contribution to the transition moments. In spite of its semiempirical character, the approximation has been shown to give a satisfactory description of the ground-state optical activity of helicenes”’ and benzotriptycenes,12J3and of the triplet optical activity of h e l i ~ e n e . ~ (6) (a) Lavalette, D. J . Chim. Phys. Phys. Chim. Biol. 1969,66, 1845. (b) Lavalette, D. Ibid. 1969, 66, 1853. (c) Lavalette, D. Ibid. 1969, 66, 1860. (7) Mason, S . F. Q. Rev. Chem. SOC.1963, 17, 20. (8) Moscowitz, A. Tetrahedron, 1961, 13, 48. (9) Kemp, C. M.; Mason, S . F. Tetrahedron, 1966, 22, 629. (10) Brown, A.; Kemp, C. M.; Mason, S . F. J . Chem. SOC.A . 1971, 751. ( 1 1 ) Brickell, W. S . ; Brown, A.; Kemp, C.; Mason, S . F. J . Chem. SOC. A 1971, 756. (12) Harada, N.; Tamai, Y . ;Takuma, Y.; Uda, H. J . Am. Chem. SOC. 1980, 102, 501. (13) Harada, N.; Tamai, Y . ;Uda, H. J . Am. Chem. SOC.1980, 102, 506.
The Journal of Physical Chemistry, Vol. 90, No. 21 1986 4995
Electron Delocalization in Binaphthyls
I
~
D
5
15
20
25
15
20
I
I
I,
1
I
+A51
25 I ,
15
20
1
I
-3 -1 VxlO c m
-3
-1 VxlO c m
25
>
-3
VxlO
I
-1 cm
6
0
I
II
Ip
Figure 2. Triplet-triplet absorption (top) and triplet-triplet circular dichroism (bottom) of binaphthyls I, 11, and IV in MTHF at 96 K. The spectra shown are for the R enantiomers; they have been normalized to 100%optical purity. c and A6 are in L mol-’ cm-’. The calculated transitions (bars) for 0 = 45’ (left) and 0 = 89’ (right) are also for the R enantiomers; they have been shifted arbitrarily 5000 cm-’ to the red for an easier comparison. The units for D and R are as in Table I. (0) Calculated transitions with D E 0 or R < 15 X 1040 cgs units.
Calculations were performed for binaphthyl, as a function of the dihedral angle between both naphthalene moieties, for the values 8 = 45,60, 75, 85, and 89”. As compared to the angular dependence, the substitution effects were expected to be of minor importance; accordingly, the atoms located on the bridges were not introduced in the present calculations. This simplification is supported by the fact that the TT spectrum of binaphthyl strongly resembles those of compounds I-V. The Cartesian coordinates of the carbon atoms were calculated by using the geometric parameters of Post et aI.l4 shown in Figure 1, which are in close agreement with the X-ray crystallographic data of bina~hthy1.l~ MO were calculated in the PPP S C F CI approximation, using the following parameters, isc< = -2.35 eV, Wp-c = -1 1.22 eV, and yc-c = 10.53 eV. The resonance integral of the 1,l’ bond = -1.97 cos 8, which takes was calculated by the equation into account the dependence of upon both the length of the 1,l‘ bond and the dihedral angleI4 8. The bicentric Coulombic integrals yw were estimated according to the Pariser and Parr equationI6 on the basis of the distances calculated from the structural model; the angular dependence of yp9could be neglected, since it has been previously shown that the change of the relative orientation of the 2p, functions had no significant consequence for the MO’s.I4 The “del” value of the aromatic C-C bond (V,,) = 6.44 X lo7 cm-’, estimated from the UV spectrum of benzene,’O was adopted. In view of the great sensitivity of the calculated rotational strengths to the amount of configuration interaction (CI),) calculations were performed by including progressively new configurations until no appreciable changes of the rotational strengths could be noticed. C I was consequently extended to 76 configurations, including both singly and doubly excited configurations with respect to the lowest triplet state. F. M.; Eweg, J. K.; Langelaar, J.; Van Voorst, J. D. W.; Ter Maten, G.Chem. Phys. 1976, 14, 165. (15) Kerr, K. A.; Robertson, J. M. J . Chem. SOC.B 1969, 1146. (16) Pariser, R.; Parr, R. G. J . Chem. Phys. 1953, 21, 767. (14) Post, M.
TABLE I: Triplet-Triplet Transitions Energies (Wavenumber, cm-’), Dipole Strengths (0,lo-%cgs Units or 1.11 X SI Units), and Rotational Strengths ( R , lo4 cgs Units or 3.33 X SI Units) of the R Enantiomers of Binaphthyls I-Vu band D band N compd u D R V L P V R 1 14300 11.2 +12 23400 9.1 +69.0 -82 11 14300 5.7 -23 23500 8.2 23200 +29.9 111
14000
4.4
-18.6
23000
IV
13900
2.6
-3.0
22900
V
14500
1.4
0
22400 23600
naphthalene
25 500 -10.6 7.7 23200 +20.6 24 500 -12.2 6.4 23000 +3.6 24 700 -3.6 9.3 0 10.7 0
‘The data for naphthalene (unpublished results) are given for comparison. bThe values in this column were determined from extrapolation of the profile of band N, and therefore must be regarded just as orders of magnitude.
Results Experimental TT and TT-CD Spectra. In order to appreciate the evolution of TT and TT-CD spectra, Figure 2 displays a few typical examples. A more extensive summary of the results is given in Table I. Two bands were observed between 12 000 and 26 000 cm-l. In addition to band N which had been previously observed in the near-UV region, the triplet state also developed absorption in the near-infrared (band D). The evolution of both bands within the series differs markedly: band N shows only minor changes from one compound to another; its intensity does not vary by more than 40%; it resembles the well-known naphthalene T T band around )B2”+transition;” in contrast, 23 800 cm-’, attributed to a 3Bl; band D has no counterpart in the naphthalene chromophore; its
-
(17) Pariser, R. J . Chem. Phys. 1956, 24, 250.
4996 The Journal of Physical Chemistry, Vol. 90, No. 21, 1986
Tetreau
TABLE II: Calculated Triplet-Triplet Transition Energy ( E , cm-I), Dipole Strength ( D , Units) of R-Binaphthyl at Various Values of the Dihedral Angle 8 symstatemetry 1 B 2 B 3 A 4 5 6 7
8 = 45' E
D
0 14400 0.2 +26 19970 34.6 -44 25810 8.0 -6 25760 3.4 +12 26800 13.1 +120 30060 6.4 -81
{I
0 = 85'
8 = 75' R
E
D
cgs Units), and Rotational Strength (R, lo-" cgs
R
E
D
0 = 89'
R
0
0 13430 0 +5 13310 0 +1 21 180 28.9 -18 21 510 20.6 -6 25240 9.5 + 1 25160 11.2 0 25310 0 0 25280 0 0 27560 15.8 +218 27900 15.3 +261 28270 10.0 -234 27970 13.2 -269
D
E 0 13300 21 580 25150 25280 27940 28020
R
main configurations
0.7( V t - Vy6') 0 0 O.S( VS7 - Vd6) - 0.5( Vy" - V4r6') 8.9 0 0.7(Vs> Vsa') 11.7 0 0.4(V3"V48)+0.5(Vj,7'-Vvq,'') 0 0 0.4( VI7 - V4') - 0.5( V3," - Vvq,':) 15.0 +277 0.5(V36- V5') -0.5(Vy6'- V5.") 14.7 -280 O.S(V:- V,') 0.5(Vjj6'- V 5 J 8 )
+
+
"Rightmost column: truncated binaphthyl (8 = 89') wave functions showing the most significant configurationsof the two interacting naphthalene moieties, The SCF M O s of naDhthalene are denoted bv 1 to 10 in the first moiety, and 1' to 10' in the second one. The numbering of the binaphthyl triplet states is according to the order of increasing energies for 8 = 89' intensity decreases continuously by almost an order of magnitude from I to V (Table I). For both bands, the triplet anisotropy factor, g, decreases upon increasing the number of methylene groups in the bridge; for band N, a negative C D contribution appears at the same time on the high-energy side. No TT-CD could be detected for nonbridged binaphthyl V. The evolution of intensity of band D and of triplet anisotropy of both bands must reflect the change in molecular conformation within the series. Several previous s t ~ d i e s ~ .of' ~binaphthyl .~~ have shown that electronic factors are favorable to an almost perpendicular arrangement of the two naphthalene moieties in the ground state, and to a more planar geometry in the first excited singlet and triplet states. It has been reported4 that upon excitation in fluid solution at room temperature, binaphthyl exhibited a broad TT absorption around 6 10 nm; this band, which clearly corresponds to band D of I-V, was attributed to the absorption of the relaxed, more planar, conformation of the triplet state; it has been also observed that rotation around the intraannular bond was suppressed below 110 K in a M T H F matrix; the naphthalene-like absorption at 420 nm of the unrelaxed-almost perpendicularconformation of the triplet was then only observed. In view of the high ambient viscosity of the M T H F matrix at 96-98 K, binaphthyl V is not expected to undergo important conformational changes in its triplet state. The ground-state molecular conformation must be almost perpendicular for this nonbridged derivative. For bridged binaphthyls, the interplanar angle is mainly determined by the length and the stereochemical conformation of the bridges; in the absence of any structural data, it could be estimated from molecular models to about 45' for three to nearly 90' for five atoms in the bridge. We had previously observed that the anisotropy g factor measured at the peak of band N correlated reasonably well with the specific rotation in the ground state;2 Figure 3 shows that such a correlation also holds for band D. Therefore, independently of the electronic transition involved, the triplet and the ground state optical activities follow the same trends upon increasing the dihedral angle. Moreover, the intensity of band D also correlates with the specific rotation (Figure 3); thus, it must be also governed by the same angular parameter. Calculations. The results of the dipole velocity calculations for 8 = 45,75, 85, and 89' are given in Table 11. For an easier comparison, the theoretical results calculated for 8 = 45' and 8 = 89' are compared in Figure 2 with the experimental spectra of I and IV, respectively. Six sign-allowed transitions have been calculated in the spectral range 10000-38000 cm-'. Satisfactory agreement with the main experimental observations is obtained by assigning band D to the T, TI transition, and by assuming a contribution of the four transitions toward states 4 to 7 to band N. In particular, the relative intensity of bands D and N and its change from I to IV are then well accounted for; moreover, the calculation satisfactorily
-
(18) Hochstraaser, R. M. Can. J . Chem. 1961, 39, 459. (19) Post, M. F. M.; Langelaar, J.; Van Voorst, J. D. W. Chem. Phys. Left. 1975, 32, 59.
A&
E
4 10
t 50
0
0
- 1000'
Figure 3. Circular dichroism of band D (A)and N (m),and extinction coefficient of band D ( 0 )of pure R enantiomers of binaphthyls I-IV20 vs. specific rotation in the ground state [ a ] 2 5 5(see 4 6 ref 2). c and Ac are in L mo1-l cm-'.
reproduces the sign of the main CD component measured in each band, and its decrease in band D upon increasing 8; finally, the evolution in shape and intensity of the CD of band N within the series may be also well explained with the given assignment: for I, the positive rotational strength is seen to result from the T, TI transition (Table 11, 8 = 45'); the negative CD component associated with the T, TI transition does not give rise to a negative signal detectable below 26 000 cm-I, but it is likely to induce strong cancellation of the positive CD, in agreement with the decrease of the g value noticed at shorter wavelengths. According to the calculation (Table 11), their mutual cancellation is expected to increase until becoming complete for 0 N 90'; the observation of a couple of small, opposite CD bands for IV, and the absence of any detectable CD for V are then well understood. The several points discussed above support the proposed assignment. Two discrepancies must, however, be noticed. First, all the calculated transition energies are overestimated by roughly 5000 cm-I; such an overestimation has been often reported in T-SCF calculations of nonplanar systems." Moreover, the calculation does not account satisfactorily for the small positive CD observed in the low-energy side of band D for I; the single possible assignment is to the T2 TI transition, but its calculated energy is 5000 cm-' lower than that of the T3 TI transition, while the experimental splitting is smaller than 2000 cm-'. A possible explanation for the failure to account for small details in the spectra may be the neglect in the present calculations of the inductive and mesomeric effects introduced by the substituents.
-
+-
-
-
J . Phys. Chem. 1986, 90, 4997-5000 Discussion The purpose of the following discussion will be to establish a connection between the present calculations of binaphthyl and its alternative description in terms of two interacting naphthalene moities. This will allow a better understanding of the evolution of the spectral characteristics within the series. For an almost perpendicular conformation of binaphthyl, no interchromophoric electron delocalization can occur, since there is no orbital overlap at the 1,l’ bond. The S C F M O s of binaphthyl are thus simply the plus and minus combinations of two sets of localized naphthalene orbitals. The electronic states of binaphthyl can then be transposed in terms of the configurations of the two interacting naphthalene moities. Exciton (ER) and charge (CR) resonance states are obtained ac~ording1y.l~ Examination of the rightmost column in Table I1 reveals that all excited states, except state 3, may be described as nearly pure ER states; in contrast, state 3 has a pronounced charge resonance character. This single difference explains all the main experimental observations. On the basis of the results presented in Table 11, the parentage suggested by the data between the N band of I-V and the naphthalene TT band can be established. The 3B1; state of naphthalene which is involved has been reported to develop mainly as 2 - ’ 4 l/j6 - V5s);22Table I1 shows that states 6 and 7, which provide the major contributions to the dipole and rotational strengths of band N, are respectively the “+” and “-” exciton levels resulting from the naphthalene 3B1; state. Using the classical exciton model, it was predicted2 that each monomer transition would give rise in the dimer to a pair of transitions with opposite rotational strengths; moreover, both will be degenerate, due to the vanishing of the exciton splitting term by spin selection rules. Obviously, these conclusions remain valid whatever the molecular conformation. The calculations in Table I1 corroborate these predictions and show moreover that the strict degeneracy of states 4-5, on one hand, and states 6-7, on the other, imposed by the exciton model is in fact released by the interchromophoric electron delocalization, even for very small deviations from 90’. (20) The theoretical considerations developed in the discussion below show that, upon increasing 0, AeN, AeD, and ED must decrease toward zero for 0 = 90°,and then increase again. Consequently, one cannot include in the linear correlations compound V for which 0 is likely to be greater than 90°.*’ (21) Hanazaki, I.; Akimoto, H. J . Am. Chem. SOC.1972, 94, 4102. (22) Tetreau, C.; Lavalette, D.; Peradejordi, F. Chem. Phys. Lett. 1973, 20, 319. (23) Fitts, D. D.; Siegel, M.; Mislow, K. J. Am. Chem. SOC.1958,80,480.
4997
When a small amount of electron delocalization is allowed by the molecular conformation, each electronic state retains almost pure ER or C R character; the dipole and rotational strengths of transition pairs such as T6 TI and T7 TI are only weakly redistributed (see Table 11, for 6 = 89, 85, and even 75’). At the same time, the release of the exact degeneracy allows the development of a pair of weak, opposite C D components, which are clearly of exciton character, except for their energy splitting term. If a large amount of electron delocalization can occur, the molecule cannot obviously be considered as a “dimer” any longer; real charge displacements over the whole molecule may detect its overall chirality and there must be no more restrictions on displaying TT-CD; this corresponds to an important mixing of E R and C R states, which leads to dissymmetrization of the energies and dipole and rotational strengths of the pairs of transitions (Table 11, 6 = 45’). As concerns band D, its absence in the naphthalene spectrum and its drastic loss of intensity as the dihedral angle tends toward 90’ suggests that the involved transition implies important charge displacements over the whole “dimer” molecule. This view is supported by the CR character of state 3; the T3 TI transition, to which band D has been assigned, is of the CR ER type, and thus is strictly forbidden for 6 = 90’; as the mixing between E R and C R states increases with deviation of 6 from 90°, increasing components of the C R C R and E R ER types can give both more dipole and rotational strengths to the transition. Thus, the intensity of the triplet-triplet absorption and of the triplet circular dichroism equivalently measures the amount of interchromophoric electron delocalization. Both are furthermore linearly related as can be easily inferred from the correlations shown in Figure 3. Finally, the theoretical considerations developed in this Discussion show that, upon increasing 6 from 45’ to 90°, the values of AtN, A t D , and tD must decrease toward zero, as the specific rotation is also expected to do.23 The correlations shown in Figure 3 are entirely consistent with these views: the three least-squares fits converge toward a common point on the abscissae and the corresponding value of [(Y]25546 = -170’ must be that associated with the perpendicular conformation.
-
-
--
+ -
+-
Acknowledgment. I am greatly indebted to Doctor D. Lavalette for his continuous interest and encouragement during the course of this investigation. I also thank Doctor D. Cabaret who has generously supplied optically active binaphthyls I-V, and Doctor 0. Chalvet for the use of the PPP program.
Emission Spectroscopy of Photodissociating CH,I and CD,I Michael 0. Hale, Gary E. Galica, Stella G. Glogover, and James L. Kinsey* Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 021 39 (Received: February 25, 1986)
Emission spectra from photodissociating CHJ and CD,I are recorded following excitation with 266-nm light. The emission wavelengths and relative intensities to the ground vibrational states are tabulated. A time-dependent formalism is used to interpret the data in terms of vibrational wave functions of the ground electronic state of methyl iodide and the dynamics of photodissociation on an electronic excited state. The observed intensities in the CD31spectra are perturbed by a Fermi resonance between two quanta of C-I stretch and one quantum of umbrella bend. Both CH31and CD31 photodissociate with initial motion in the C-I stretching coordinate and subsequent motion in the umbrella coordinate to form the planar CH3/CD3products. The qualitative mechanisms described here are consistent with a more detailed theoretical treatment presented in the following paper.
Introduction Spectroscopic investigation of intermediate species reveals a great deal of information about the dynamics of nuclear motion during chemical reactions and collision^.^-^ Recent studies of 0022-3654/86/2090-4997$01.50/0
emission from molecules during photodissociation show sharply resolved structure that yields dynamical inf~rmation.~-’ The (1) Foth, H. J.; Polanyi, J. C.; Telle, H. H. J . Phys. Chem. 1982,86, 5027.
0 1986 American Chemical Society
