J. Phys. Chem. 1987, 91, 6097-6099
OH rotational energy decreases monotonically. It is noteworthy that only less than 7% of the available energy is disposed to the fragment OH rotational energy even when Eava,= 8900 cm-'. The population of v" = 1 level of the X211 state is too small to be measured at any dissociation energy and is estimated to be less than 5% of the v = 0 fragment from the signal-to-noise ratio in our experiments. From the present experiment it was found that the extent of the rotational and vibrational energies disposed to the OH fragment is very small. This result indicates that the C-0-H angle of formic acid is close to 90' in the S1 state and also the 0-H bond distance is not changed by the SI So excitation. Since the Sl(nr*) So absorption of formic acid is thought to be the relatively localized transition in the -C=O chromophore, this transition is expected not to seriously affect the C-0-H angle and the 0-H bond distance. Actually, in the ground state the C-0-H angle is obtained to be 106' by the measurement of the microwave spectrum,* and this angle is relatively close to 90'. Also, the 0-H bond distance is measured to be 0.97 A, which is equal to the equilibrium bond distance of the O H radical in the X211 state (0.969 66 A),9and the Franck-Condon principle
-
-
-
(8) Kwei, G. H.; Curl, R. F., Jr. J . Cfiem.Phys. 1960, 32, 1592. (9) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure IV; Van Nostrand Reinhold: New York, 1979.
6097
explains the observation of vibrationally cold O H fragments. Furthermore, since the center of mass of the OH radical is close to the 0 atom, the repulsive force will be more effective in promoting the translational motion of the O H fragment but not the rotational motion, which explains the low fraction of the available energy to the rotational motion. On the other hand, this n T* transition appreciably changes the C=O bond length in the SI state and the counterpart H C O radical should be very excited in the CO stretching mode (YJ. In this sense, this problem may be close to the case of HONO dissociation. The SI So absorption of HONO is also localized in the -N=O chromophore, and the O H fragment was found to be rotationally and vibrationally cold but translationally hot.1° To confirm the above consideration, it is necessary to measure the Doppler width of the OH LIF spectrum and also the LIF detection of the HCO radical. Besides the laser-induced fluorescence, we have found a weak emission by shining the photodissociation laser light. It is not clear whether this weak emission is due to the formic acid dimer or monomer, and the question is now under investigation.
-
-
Acknowledgment. We thank Dr. N. Mikami for his interest in this work and for many helpful discussions. (IO) Vasudev, R.; Zare, R. N.; Dixon, R. N . J . Chem. Pfiys. 1984, 80, 4863.
Mechanism of Dissociation of Sodium Monobenzo-I5-crown-5' in the Solvent Nitromethane Kathleen M. Briere and Christian Detellier* Ottawa-Carleton Chemistry Institute, Ottawa University Campus, Ottawa, Ontario K l N 9B4, Canada (Received: June 23, 1987)
Exchange kinetics for the decomplexation reaction of sodium tetraphenylboratecomplexed by the ligand monobenzo-15-crown-5 in nitromethane have been studied by *"a nuclear magnetic resonance. The predominant mechanism for the sodium exchange is bimolecular and is characterized by the following activation parameters: & = 28 f 3 lcJ.mol-', LIS' = -57 f 10 J.mol-'-K-'. and PGt3,,,, = 45 & 3 kJmol-I. A coalescence was observed for a sodium concentration of 4.0 X M (300 K). At low sodium concentrations the unimolecular decomplexation mechanism becomes competitive. It is characterized by AGt300 = 6 2 & 6 kJ.mol-'.
Introduction The number of studies dealing with the properties of supermolecules has increased exponentially.2 The first synthesis of crown ethers 20 years ago3 started the coordination chemistry of alkali-metal cations. Despite their structural simplicity, crown ethers can recognize cationic species with high selectivity. In the case of crown ethers or cryptands in nonaqueous solutions, the rate of complexation is generally diffusion-controlled and, consequently, the complexation selectivity is governed by the decomplexation rate.4,5 In that context, the understanding of decomplexation mechanisms and kinetics is particularly important. Recently, it has been shown6-I0 that two mechanisms must be (1) Monobenzo-l5-crown-5 (B1 9 2 5 ) : 2,3,5,6,8,9,11,12-octahydro1,4,7,10,13-pentaoxabenzocyclopentadecene. (2)Lehn, J. M. Science 1985, 227, 849-856. (3) Pedersen, C. J. J. Am. Cfiem. SOC.1967,89, 7017-7036. (4) Lockhart, J. C.J . Cfiem.Soc., Faraday Trans. 1 1986.82, 1161-1167. (5) Cox, B.G.; Garcia-Rosas, J.; Schneider, H. J . Am. Chem. SOC.1981, 103, 1054-1059. (6) Schmidt, E.; Popov, A. I. J . Am. Chem. SOC.1983, 105, 1873-1878. Hallenga, K.; Popov, A. I. J . Am. Cfiem. SOC.1985, (7) Strasser, B. 0.; 107, 789-792. ( 8 ) Strasser, B. 0.;Shamsipur, M.; Popov, A. I. J . Phys. Chem. 1985, 89, 4822-4824.
0022-3654/87/2091-6097$01.50/0
considered in the decomplexation process of a crown ether (C)-alkali-metal cation (M') complex: unimolecular dissociative (eq 1) and bimolecular cation interchange (associative) eq 2). k
C
+ M+ & (C,M)+ k-1
(C,M)+ + M+* (C,M*)+ + M+ (2) In the cases of dibenzo-24-crown-8 (DB24C8)I0 and dibenzo18-crown-6 (DB18C6)" the exchange is predominantly bimolecular (eq 2) in solvents with low donicity number (DN)12 such as nitromethane (DN = 2.7). We report here the evidence that the bimolecular cation interchange mechanism is also predominant in the case of the complex monobenzo- 15-crown-5 (B15C5)-Na+ in nitromethane. (9)Stover, H.D. H.; Delville, A.; Detellier, C. J . Am. Chem. SOC.1985, 107, 4167-4171. (10) Delville, A,: Stover. H. D. H.; Detellier, C. J . Am Chem. SOC.1985,
IO?, 4172-4175.
(11) Delville, A.; Stover, H. D. H.; Detellier, C. J . Am. Cfiem. Soc., in press. (12) Gutmann, V.; Wychera, E. Inorg. Nucl. Chem. Lett. 1966, 2,
257-260.
0 1987 American Chemical Society
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The Journal of Physical Chemistry, Vol. 91, No. 24, 1987
t
Letters T
I
1400
P Figure 1. 23Natransverse (i = 2; 0)and longitudinal (i = 1; 0)relaxation rates as a function of the ratio p = [BlSCS]/[NaBPh,] in nitromethane. ijo = 79.35 MHz; T = 301.5 f 0.5 K; [NaBPh,] = 8.00 X M.
Experimental Part Monobenzo-15-crown-5 was obtained from Aldrich and was vacuum-dried over PzOs (30 "C) for at least 20 h prior to use. Sodium tetraphenylborate (Aldrich, 99%) was vacuum-dried (60 "C) over P205for at least 24 h prior to use. Nitromethane (NM) (Aldrich, 96%, Gold Label) was dried, under reflux, over calcium hydride for 3 h, distilled under nitrogen, and stored under argon. All the spectra were recorded within 48 h after the preparation of the samples. The N M R tubes containing the solutions were purged with argon and sealed with parafilm. 23Na N M R spectra were obtained at 79.35 MHz (Varian XL-300) as described previou~ly;~ TI measurements were done using the inversion-recovery 180°-7-900 pulse sequence and were obtained from a three-parameter, nonlinear regression analysis. In the case of Lorentzian line shapes, the transverse relaxation rates, T2-l,were obtained directly from the line widths. The temperature in the probe was measured with a thermocouple N M R tube. dipped into nitromethane in a nonspinning IO-" The temperature of the sample was estimated to be reliable at f0.5K. (The measured range of temperature was between 240 and 310 K). Pseudo-first-order rate constants were obtained as described previously.l0 Results and Discussion Figure 1 shows the variation of the 23Na longitudinal and transverse relaxation rates for a solution of sodium tetraM) in nitromethane, with increasing phenylborate (8.0 X amounts of B15C5. The longitudinal relaxation rates ( T l - ' ) increased steadily from 26 Hz ( p = 0; p = [B15C5]/[NaBPh4]) to 660 Hz ( p = 1.O). For p > 1.O, TI-' decreased slightly to 620 Hz a t p = 2.0. The variation of the chemical shifts paralleled the longitudinal relaxation rates variation, with a linear relationship from -14.6 ppm ( p = 0) to -3.8 ppm ( p = l.O), followed by a slight shift to -5.2 ppm ( p = 2.0). The chemical shift variation had previously been reported by Lin and POPOV,'~ who attributed the observed behavior to a two-step reaction, the formation of a stable (Na+,C) complex (log Kf> 4 ) followed by the formation of a sandwich (C,Na+,C) complex (Kf= 0.8 & 0.3).13 The longitudinal relaxation rates measurements reported here confirm their interpretation. The transverse relaxation rates ( T2-l) increased from 31 Hz ( p = 0) to a maximum of 1260 Hz ( p = 0.6) (13) Lin, J. D.; Popov, A . I . J . Am. Chem. SOC.1981, 103, 3773-3777
1
0'
2 .o
[Na'],
1
I
4.0
6.0
L,
80
(IO-' M )
Figure 2. ( k A + k B )as a function of [NaBPh,] in nitromethane. [B15C5]/[NaBPh4]= 0.50 in all cases; T = 301.5 f 0.5 K.
p
=
and were close to the values of T1-' for p 2 1.0. The observed difference between Tl-l and T2-l in the range 0 < p < 1 is typical of a system undergoing moderately rapid e x ~ h a n g e . ~ Under .'~ these conditions, the chemical exchange does not affect the longitudinal relaxation time.14 The exchange contribution to the observed transverse relaxation rate ( T2,e;1)can be easily obtainedL0 and is given by eq 3, valid for an uncoupled two-site case (eq 4). PA and PB are the fractional populations of sites A [solvated sodium, Na+ (s)] and B [complexed sodium, (Na+,C)], and v A and vB are their chemical shift^.'^,'^ T2,ex-l = 4PAPBaZ(vA- V B ) ' ( ~ A + k ~ ) - ' A&B
(3)
(4)
ks
+
The values of the mean lifetime, ( k A kB)-', did not depend upon the concentration of B15C5 under the conditions of Figure 1 ([NaBPh4] = 8.0 X M; ( k A + kB) = (8.9 f 0.4) X IO3 s-I; T = 301.5 f 0.5 K). The dependence of ( k A k B )upon [BlSCS] or [NaBPh4] permits the discrimination between the two mechanisms shown in eq 1 and 2. Since the formation constants for the complexes (Na+,C) and (C,Na+,C) are respectively >lo4 and 0.8,13 the formation of the 1:l complex can be considered as quantitative, and PB = p with PA = (1 - p ) . Consequently, the experimental mean lifetime, ( k A + kB)-l, is related to kl and k , by eq 5 and 6 under the hypothesis of an unimolecular dissociative mechanism (eq 1 ) or of a bimolecular cation interchange mechanism (eq 2), respectively. ( [ N ~ + ]isT the total sodium concentration.I5)
+
The exchange slowed down when the total concentration of sodium was lowered. A coalescence was observed for a sodium M. Below the coalescence, a complete concentration of 4.0 X line-shape analysis was carried out by using the Bloch equations modified for an uncoupled spin system undergoing chemical ex(14) Woessner, D. E. J . Chem. Phys. 1961, 35, 41-48. ( I 5) Another mechanism, the anion-assisted unimolecular dissociation, could be considered even if it is not very probable here since ion pairing is not favored in the case of the tetraphenylborate anion.9 Under this hypothesis, ( k A + k,) would be proportional to p(1 - p ) - ] (for [Na+ITconstant)." This relationship does not account for our experimental data.
J. Phys. Chem. 1987, 91, 6099-6102
t
6099
TABLE I: Activation Parameters of the Bimolecular Cation Interchange Mechanism (k2,Eq 6) for the Systems Crown-NaBPb-Nitromethane
10.01
crown B15C5 DB18CE" DB24C8'
AH*,
As*,
AG'bi 300,
AG'uni
3009
e
kJ.mol-'
J.K-l.mo1-l
kJ.mol-l
kJ-mo1-l
28 f 3 d 31 f 3
-51 f 10
45 f 3 48f4 41 f 3
62 f 6 6 0 f 3 -65
d -32 f 10
"Reference 11. 'Reference 10. cAG*jOOof the unimolecular decomplexation mechanism ( k ]eq , 5 ) . dNot determined. 6.01
h
1.0
0-
2.0
3.0
4.0
5.0
(l-pl-' Figure 3. ( k A + kB) as a function of (1 - p)-' for various concentrations of sodium tetraphenylborate in nitromethane at 301.5 f 0.5 K: (0),8.0 X M; ( a ) ,4.0 X M; (A),2.0 X lo-' M; (0),8.0 X M; (VI,4.0 x 10-3 M.
change between two nonequivalent sites.16 This analysis gave the same results as eq 3 for the systems above the coalescence concentration. The rate constants are shown in Figure 2 for to 4.0 X M, p being [Na+IT decreasing from 8.0 X constant at 0.50.The relationship is linear, extrapolating to zero, and is in agreement with eq 6. Figure 3 shows the relationships between (kA kB) and (1 - p)-l for five different sodium concentrations. At low concentrations, there is evidence for a residual unimolecular process: the relationships are linear, parallel, and
+
(16) Sandstram, J. Dynamic NMR Spectroscopy; Academic: New York, 1982.
not horizontal. Their slope, indicative of a residual slow process, provides k-l (k-l = (1.1 f 0.1) X 10, s-l at 301.5 f 0.5 K). This process is slow compared to the bimolecular one, even at the lowest sodium concentration investigated (4.0 X M; k, = 1.1 X lo5 s-'.M-'). At higher sodium concentrations, the errors preclude any useful determination of k-l. The slight curvature observed in Figure 2 could originate from the residual unimolecular decomplexation mechanism shown in Figure 3. A temperature study was done on a 6.0 X lo-, M sodium solution (for p = 0.50). At that concentration, the unimolecular dissociative mechanism is negligible and one can assume that kz is given directly by eq 6. The Eyring plot was linear in the temperature range investigated (six temperatures, 253.0 K S T S 301.3 K) and gave the values of the activation parameters for the bimolecular process: AH* = 28 f 3 kJ-mol-' and AS* = -57 f 10 J.K-'-mol-'. (AG*300= 45 f 3 kJ.mol-I.) Table I gives the activation parameters for the decomplexation of Na+ complexed by B15C5, DB18C6, and DB24C8 in nitromethane (anion: BPhJ. In the case of DB18C6, the mechanism was predominantly unimolecular, which did not permit the accurate determination of AH* and AS* for the bimolecular process. Both the bimolecular and the unimolecular decomplexation mechanisms are characterized by AG*3M,values which are almost identical for the three crown ethers. The values of AH* and AS* are also close. This constancy of the activation parameters for the three crown ethers suggests that the rate-determining step of the decomplexation process may have a common origin in the three systems, plausibly a partial sodium desolvation" accompanying a conformational rearrangement of the ligand. Acknowledgment. Natural Science and Engineering Research Council of Canada (NSERCC) is gratefully acknowledged for financial support.
Temperature Effects on Fluorescence in Dlphenylpolyene Derivatives; Structure- and Substituent-Dependent Changes In Mechanisms and Rates for Nonradlative Decay Mary T. Allen, Laerte Miola, and David G. Whitten* Department of Chemistry, University of Rochester, Rochester, New York 14627 (Received: July 9, 1987; In Final Form: September 9, 1987)
The activation energy for nonradiative decay has been determined to be 5.3 kcal/mol for diphenylbutadiene (DPB) and 4.5 kcal/mol for diphenylhexatriene (DPH). Much lower values were obtained for the corresponding 4,4'-dialkyl-~ubstituted molecules, 4B4A and 4H4A. Evidence for S, emission and the effect of solvent polarizability on this process have been obtained from fluorescence spectra and lifetimes. The results are discussed in terms of the relationship between the two low-lying excited singlet states and differential solvent and substituent effects on them. Introduction The arenes, trans-stilbene (ts), trans,trans- 1,4-diphenyl- 1,3butadiene (DPB), and trans,trans,trans-l,6-diphenyl-l,3,5-hexatriene (DPH) comprise a series of molecules which exhibit formally similar photophysical and photochemical behavior. All three compounds show intense and structured absorption and fluorescence spectra and each compound undergoes trans-cis
photoisomerization upon irradiation into its long wavelength absorption. Fluorescence and excited-state rotation have been Shown to be Strongly coupled in trans-Sti1bene.l Although the fluorescence intensities of both DPB and DPH increase with (1) Saltiel, J.; Charlton, J. L. In Rearrangements in Ground and Excited Stafes; deMayo, P., Ed.; Academic: New York, 1980; Vol. 3, pp 25-89.
0022-3654/87/2091-6099$01.50/0 0 1987 American Chemical Society
