J. Phys. Chem. 1991, 95. 10582-10586
- 600 -500
0 00
0.2
' 0.4
06 O
a0 -L
1.000
c (molal)
Figure 7. Isothermal (T = 295 K) concentration dependence of the nitrogen-14 spin-lattice relaxation rate of the solute Pe4N+cations and the solvent CD3CN molecules.
largely unrelated at low solute concentration, both relaxation times show an increasingly similar behavior at high concentrations. Conclusions
Dynamic information about Pe4N+ ions dissolved in CD3CN has been gathered with NMR. Self-diffusion coefficients and proton and carbon-13 spin-lattice relaxation rates have been determined over a large concentration and temperature range. These transport coefficients show a simple Arrhenius temperature dependence, indicating that significant ion pairing is unimportant
in the whole temperature range. From chemical shift measurements it has been concluded that ion pairing would polarize the positive charge density which is centered at the nitrogen core and further smeared out over the neighboring a-methylene groups. This suggests that ion pairing would also affect the relaxation behavior of the a-methylene protons and carbon-13 nuclei. This has not been observed, however. Thus acetonitrile effectively solvates Pe4N+ ions in the whole range of concentrations and temperatures. The relaxation rate maximum detected in the 1 m solution clearly demonstrates the strong slowing down of the whole-ion reorientations in this system. The observation that the a-methylene group proton and carbon-1 3 relaxation is dominated by the overall tumbling mode shows that the segmental mobility of these groups is strongly reduced. Hence these local motions of the alkyl groups exhibit correlation times similar to those for whole-ion tumbling motions. A large spatial restriction of these segmental motions is indicated by order parameters S, close to 1 and is consistent with these conclusions also. Self-diffusion coefficients of the organic ions have been found to differ from those deduced from QENS data. It is argued that the difference arises from the contribution of a hydrodynamic and a relaxational mode to the diffusion process. Only the former is detected by QENS experiments because of the shorter time scale involved, whereas both contributions are detected by the slower NMR method. It is further argued that this effect may only be observed in low-viscosity liquids.
Acknowledgment. Two of the authors (E.W.L. and S.B.) thank the DFG for supporting this work. The skillful technical help of S. Heyn, R. Knott, and E. Treml made this work feasible. Registry No. Pe4NBr, 866-97-7; PelN+, 15959-61-2; acetonitrile, 75-05-8.
Translational Energy Dependence of Gas-Phase Reactions of Halides with Halogenated Alkanes C.E.C.A. Hop and T.B. McMahon* Department of Chemistry and Guelph- Waterloo Centre for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ontario, N2L 3GI Canada (Received: June 24, 1991)
-
-
-
The gas-phase bimolecular nucleophilic substitution reactions Br- + CC14 BrCC13 + Cl-, Br- + CF2C12 BrCFzCl + Cl-, and C1- + CBr4 ClCBr3 + Br- were studied as a function of the center-of-mass energy with Fourier transform ion cyclotron resonance spectrometry. From the energy dependence and the threshold energies of these reactions, conclusions were drawn concerning the mechanism involved.
Introduction
Bimolecular nucleophilic displacement reactions ( h 2 ) reactions have played a fundamental role in the de%elopment of mechanistic organic chemistry. These reactions have been studied theoretically' and experimentally,2-13both in the gas phase2-8 and in (1) (a) DeTar, D. F.; McMullen, D. F.; Luthra, N. P. J . Am. Chem. SOC. 1978,100,2484.-(b) Carrion, F.; Dewar, M. J. S. J . Am. Chern. SOC.1984, 106, 3531. (c) Cernusk, I.; Urban, M. Collecf. Czech. Chem. Commun. 1988,53,2239. (d) Tucker, S. C.; Truhlar, D. G . J . Phys. Chem. 1989.93, 8138. (e) Vetter, R.; Ziilicke, L. J . Am. Chem. Soc. 1990, 112, 5136 and references therein. ( f ) Shi, Z.; Boyd, R. J. J. Am. Chem. Soc. 1990, 112,6789. (8) Shi, Z.; Boyd, R. J. J . Am. Chem. SOC.1991, 113, 1072.
0022-3654/91/2095-10582$02.50/0
and the "back side attack" mechanism has become widely accepted? The potential energy surface for gas-phase SN2reactions (2) Riveros, J. M.; Jose, S. M.; Takashima, K. Adu. Phys. Org. Chem. 1985, 21, 197 and references therein. (3) (a) Bohme, D. K.; Young, L. B. J . Am. Chem. SOC.1970.92, 7354. (b) Young, L. B.; Lee-Ruff, E.; Hohme, D. K. J . Chem. SOC.,Chem. Commun. 1973.35. (c) Bohme, D. K.; Mackay, G. I.; Payzant, J. D. J. Am. Chem. SOC.1974,96,4027. (d) Tanaka, K.; Mackay, G. I.; Payzant, J. D.; Bohme, D. K. Can. J . Chem. 1976,541643. (e) Bohme, D. K.; Raksit, A. B. J. Am. Chem. Soc. 1984,106, 3447. ( f ) Bohme, D. K. In Ionic Processes in the Gas Phase; Almoster Ferreira, M. A., Ed.; D. Reidel Publishing Company: Dordrecht, 1984; pp 1 1 1-134. (4) Barlow, S. E.; Van Doren, J . M.; Bierbaum, V. M. J . Am. Chem. SOC. 1988, 110, 7240.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 26, 1991 10583
Reactions of Halides with Halogenated Alkanes
-
Reaction Coordinate
+Y Figure 1. Potential energy surface for gas-phase SN2reactions. X- t AY
K-AY XA-Y XA 9
9
is shown in Figure 1. The reactions proceed in one step with the formation of the new bond occurring synchronously with cleavage of the old bond. The rates of exothermic SN2reactions range from almost collision controlled to too slow to be observed, and they depend strongly on the height of the internal barrier. A feature of sN2 reactions is their sensitivity to repulsion between the various substituents associated to the central carbon atom in the transition state, Le. steric ~ t r a i n . ~ vIn ~ *solution '~ the ease of displacement of a halogen atom in an alkyl halide is dramatically reduced by the replacement of an a-hydrogen atom by another halogen atom or an alkyl g r o ~ p . ~ ~Moreover, ,~J~ hydrolysis of haloforms and carbon tetrahalides in solution no longer displays sN2 ~ h a r a c t e r . ~It~ has , ~ been shown both experimentally6a and theoretically l a that an increase in the steric strain increases the activation energy of SN2reactions in the gas phase. Steric strain is therefore expected to be an important feature of the C1- CC14 reaction in the gas phase. A theoretical studyIb suggested that this reaction does not take place via a mechanism similar to that depicted in Figure 1, but via a covalent complex of the type [C13C-Cl-Cl]-. A binding energy of 134 kEmol-I was calculated for the complex; however, it was stated that MNDO calculations overestimate the stability of these complexes. Experimentally a crude binding energy of 59 klsmol-I was obtained.8a The barrier for the overall nucleophilic displacement reaction was calculated to be 108 W-mol-l,lb The same study also indicated that for less substituted halocarbons the covalent complexes are stable as well, but less stable than the [ M Y ] - ion-dipole complexes. To explain an exponential increase of the rate coefficient from 0.4 to 2.0 eV for the 37Cl- CH335C1 35Cl- + CH337ClSN2 reaction, Barlow et ala4suggested a mechanism corresponding to anionic attack at chlorine, leading to a [CH3.C1~]intermediate complex in which the chlorine atoms are equivalent.
+
-
+
(5) (a) Brauman, J. I.; Olmstead, W. N.; Lieder, C. A. J. Am. Chem. Soc. 1974, 96, 4030. (b) Olmstead, W. N.; Brauman, J. I. J . Am. Chem. SOC. 1977, 99, 4219. (c) Asubiojo, 0. I.; Brauman, J. I. J . Am. Chem. Soc. 1979, 101, 3715. (d) Pellerite, M. J.; Brauman, J. I. J . Am. Chem. Soc. 1980, 102, 5993. (6) (a) Caldwell, G.; Magnera, T. F.; Kebarle, P. J . Am. Chem. Soc. 1984, 106,959. (b) Magnera, T. F.; Kebarle, P. In Ionic Processes in the Gas Phase; Almoster Ferreira, M. A., Ed.; D. Reidel Publishing Company: Dordrecht, 1984; pp 135-157. (7) (a) Henchman, M.; Paulson, J. F.; Hierl, P. M. J . Am. Chem. SOC. 1983, 105, 5509. (b) Henchman, M.; Hierl, P. M.; Paulson, J. F. J . Am. Chem. Soc. 1985,107,2812. (c) Hierl, P. M.; Ahrens, A. F.; Henchman, M.; Viggiano, A. A.; Paulson, J. F.; Clary, D. C. J . Am. Chem. SOC.1986, 108, 3142. (8) (a) Dougherty, R. C.; Dalton, J.; Roberts, J. D. Org. Mass Spectrom. 1974.8, 77. (b) Dougherty, R. C.; Roberts, J. D. Org. Mass Spectrom. 1974, 8, 81. (c) Dougherty, R. C. Org. Mass Spectrom. 1974, 8, 85. (9) (a) Hine, J. J . Am. Chem. SOC.1950, 72, 2438. (b) Hine, J.; Dowell, A. M., Jr. J . Am. Chem. SOC.1954,76,2688. (c) Hine, J.; Thomas, C. H.; Ehrenson, S.J. J . Am. Chem. SOC.1955, 77, 3886. (d) Hine, J.; Ehrenson, S. J.; Brader, W. H., Jr. J . Am. Chem. SOC.1956, 78, 2282. (10) De la Mare, P. B. D.; Fowden, L.; Hughes, E. D.; Ingold, C. K.; Mackie, J. D. H. J . Chem. SOC.1955, 3200. (11) Parker, A. J. Chem. Rev. 1969, 69, 1. (12) Streitwieser, A., Jr. Solvolytic Displacement Reactions; McGrawHill: New York, 1962. (13) Amis, E. S.; Hinton, J. F. Solvent Effect on Chemical Phenomena; Academic Press: New York, 1973.
Most gas-phase studies of SN2reactions have been performed with the flowing afterglow technique,)^^ ion cyclotron resonance,5 or high-pressure mass spectrometry.6 Only the last technique routinely allows for the measurement of rate constants as a function of temperature. Even in this case the maximum attainable temperature is usually -600 K, and thus one is limited to study endothermic sN2 reactions with a barrier of less than ca. 50 kl-mol-I. In ion beam experiments reactions between ions and a stationary target gas can be studied as a function of the translational energy of the ions. Henchman et aL7 used this technique to measure energy barriers for exothermic SN2reactions and to study higher energy pathways. Fourier-transform ion cyclotron resonance (FT-ICR) is also well suited to perform energy-resolved experiments.I4-l7 In recent studies from this laboratoryI7 low-energy collision-induced dissociation (CID) has been used to accurately determine bond dissociation energies in a variety of species. Where available, the threshold energies obtained were, within experimental error, equal to the literature values, confirming that rf excitation yields ions with a well-defined energy. In this paper we set out to study endothermic bimolecular nucleophilic displacement reactions between halide ions and fully halogenated alkanes (CC14, CF2C12,and CBr4) as a function of the translational energy of the nucleophilic anion (Cl- and Br-) in a FT-ICR spectrometer, to obtain information about the reaction mechanisms involved, and to determine barrier heights for these reactions.
Experimental Section All experiments were performed with a Bruker Spectrospin CMS 47 FT-ICR spectrometerlsa equipped with a high-pressure external ion source.lsb Mixtures of CC14/CH4and CHBr3/CH4 (15000) were introduced into the external ion source to generate C1- and Br-, respectively. The ion source pressure was 3.0-4.0 mbar and the ion source temperature was -30 OC. Gas-flow restrictions and differential pumping provided a pressure differential of lo9 between the external ion source and the ICR cell.Isb The product ions were transferred from the high-pressure ion source to the ICR cell and trapped, followed by a 2-s delay to thermalize the ions via ion-molecule collisions. Ejection of all other ions from the ICR cell resulted in isolation of the ion of interest. An rf pulse at the exact cyclotron frequency was then used to increase the translational energy of this ion to a known value. In our experiments the amplitude of the rf pulse was kept constant and the length of the rf pulse was the parameter used to vary the translational energy of the ion.17a In the subsequent delay (tR), reactions between the halide ions and halocarbons were allowed to take place. CC14, CF2C12,or CBr4 was present in the ICR cell at a pressure of (3.5-4.0) X lo-* mbar (after correction of the read-out pressure for the sensitivity of the ionization gauge for the target gas and the background pressure). The product ions were detected in both broad band mode (full mass spectrum) and band mode (single ion detection). For threshold energy measurements the product ion intensity was measured in the narrow band mode as a function of the center-of-mass energy of the parent ion. The shape of the curve representing the centerof-mass energy, E,,, dependence of the product ion intensity was analyzed with an empirical model, eq 1,19 where E, is the threshold (14) Bensimon, M.; Houriet, R. Int. J. Mass Spectrom. Ion Processes 1986, 72, 93. (1 5 ) Forbes, R. A,; Lech, L. M.; Freiser, B. S. Int. J . Mass Spectrom. Ion Processes 1987, 77, 107. (16) (a) Katritzky, A. R.; Watson, C. H.; Dega-Szafran, Z.; Eyler, J. R. J. Am. Chem. Soc. 1990, 112, 2471. (b) Katritzky, A. R.; Watson, C. H.; Dega-Szafran, 2.;Eyler, J. R. J . Am. Chem. Soc. 1990, 112, 2479. (17) (a) Hop, C. E. C. A.; McMahon, T. B.; Willett, G. D. Int. J . Mass Spectrom. Ion Processes 1990, 101, 191. (b) Hop, C. E. C. A,; McMahon, T. B. J . Am. Chem. Soc. 1991,113, 355. (c) Hop, C. E. C. A.; McMahon, T. B. Inorg. Chem., in press. (d) Hop, C. E. C. A.; McMahon, T. B. J . Am. Chem. SOC.,submitted. (18) (a) Allemann, M.; Kellerhals, H.-P.; Wanczek, K. P. Inr. J . Mass Specrrom. Ion Phys. 1983, 46, 139. (b) Kofel, P.; McMahon, T. B. Int. J . Mass Spectrom. Ion Processes 1990, 98, I .
10584 The Journal of Physical Chemistry, Vol. 95, No. 26, 1991
Hop and McMahon
TABLE I: The Ratio of the Probabilities for CI- and Br- Ions Undergoing One (Two) and Zero (One) Collisions with the Reagent Gas, Q1/Qo ( Q J Q , ) , during the 10-ms CID Delay after Acceleration of the Ions by an rf Pulse of Length td at Its Exact Cyclotron Frequency
---
reaction
+ CCL B r C C h + CI+ CF2CI2 BrCF,CI + CICP + CBr4 CICBr, + BrBrBr-
+ CCll BrCCI, + C1Br- + CF2CI2 BrCF2CI + C1CI- + CBr4 CICBr, + BrBr-
trf (FS) 44.0 46.0 26.0 26.0
E,, tev)
44.0 46.0 26.0
10
11.2
10 10
8.3
11.14
11.14
20
1 1.07
1000 1000
330
1000
325
11.07 11.14
energy of the endothermic reaction, uo is an energy-independent scaling factor, and n and m are variables.
In this study we restricted analysis to m = 1 while the other parameters, uo, E,, and n, were optimized by using a nonlinear least-squares analysis to give the best fit to the experimental data. Armentrout et al.19,20and previous experiments from this labohave shown that the form with m = 1 is one of the most useful in deriving accurate thermochemistry. In addition, this form has been predicted theoretically for translationally driven reactions.2' All compounds were of commercial origin and showed no detectable impurities. Collision Frequency of Ions in the ICR Cell To obtain meaningful threshold energies via energy-resolved CID of polyatomic ions, single-collision conditions are a necessity.15,i7,22Increasing the CID delay time reduces the observed threshold energies obtained for dissociation, which is due to a higher degree of conversion of translational energy to internal (vibrational and/or rotational) energy with each successive collision. The efficiency of the sN2 reactions studied here is such that the signal-tenoise ratio of the threshold curves obtained under single-collision conditions is rather weak (see below). However, since conversion of translational energy to internal energy is not possible for monatomic anions (Cl- and Br-), multiple-collision conditions are permitted.Is Although the slope of the intensity versus energy curve in the threshold region increases with reaction time, the intercept does not change. Thus, to obtain threshold curves a long reaction delay was used (tR= lo00 ms). In contrast, to obtain information about the collision efficiency (the percentage of reactive collisions) well-defined, single-collision conditions are necessary ( t R = 10 ms). The signal-to-noise ratio a t a short reaction delay only allowed accurate measurements of the collision efficiency at energies significantly above threshold. The time between collisions is a distribution and the probability Qn that the ion encounters n target gas molecules during the reaction delay, tR,can be approximated by the Poisson distribut i ~ n . Using ~ ~ the kinetic theory of gases and the above approximation, the ratio of the probabilities for an ion undergoing one and no collisions, QI/Qo, and two and one collisions, Qz/Ql, during tR can be obtained from eqs 2 and 3, re~pectively,~~
Q2 = -QI
QI
2Qo
(3)
(19) Sunderlin, L.; Aristov, N.; Armentrout, P. B. J . Am. Chem. SOC. 1987, 109, 78 and references therein. (20) (a) Boo, B. H.; Armentrout, P. B. J. Am. Chem. Soc. 1987,109,3549. (b) Aristov, N.; Armentrout, P. B. J . Chem. Phys. 1987, 91, 6178. (c) Sunderlin, L. S.; Armentrout, P. B.J . Am. Chem. SOC.1989, J J J, 3845. (d) Boo, B. H.; Elkind, J. L.; Armentrout, P. B. J . Am. Chem. SOC.1990, 112, 2083 and references therein. (21) Chesnavich, W. J.; Bowers, M. T. J . Phys. Chem. 1979, 83, 900. (22) This is in marked contrast with the approach in a recent threshold
CID study,I6 which allowed multiple collisions to occur. (23) Kim, M. S. Int. J. Mass Spectrom. Ion Phys. 1983, 50, 189. trf was (24) In the manuscript in which these formulas were
absent from the formulas due to an oversight in proof-reading.
W ) / W (X104)
CID delay (ms)
11.07 11.07
Q ~ Q tX102) o
(X10-7
Q2/QI
5.3
4.6 4.6 5.9
2.3 2.3 3.0
10.8
11.9
5.9
190
where C,, is the number density of the target gas, rpis the average radius of the parent ion, r, is the average radius of the target molecule, Ed is the strength of the rf pulse, trf is the length of the rf pulse, e is the electronic charge, and mpis the mass of the parent ion. Table I shows the QI/Qo and Q2/Q, values for the sN2 reactions studied here with a center-of-mass interaction energy for the halide ion and halocarbon of 11 eV and a reaction delay of 10 ms. Note that the Q2/Qi values do not exceed 0.03, which indicates that these are essentially single-collision conditions.
Results and Discussion The following nucleophilic substitution reactions25,26were examined: Br-
+ CC14
BrC1-
-
BrCC13 + C1-
+ CF2C12
+ CBr4
BrCF2Cl
AH',,,
+ C1-
= 4 0 kJ-mol-'
(4)
= 26 kEmol-I
AH',,,
(5)
ClCBr3 + Br-
AH",,, = -21 kEmol-l
(6)
C1- was the only product ion observed upon interaction of accelerated Br- anions with CC14and CF2Cl, up to center-of-mass energies of 10 eV (see reactions 4 and 5). For the C1- CBr4 pair the adduct, [Cl + CBr4]-, was also observed, but its intensity was independent of the translational energy of C1-. These latter adduct ions were generated during the 2-s relaxation delay. The abundance of the product ions as a function of the center-of-mass energy of the reactant ions is presented in Figure 2. A comparison of the experimental threshold profile with the enthalpy change of the reactions clearly indicates that, as expected, these reactions do not take place a t their thermodynamic thresholds. The CUNS corresponding with reactions 4 and 5 have sharp onsets and rise linearly thereafter. Threshold values of 0.74 f 0.05 (n = 1.97 f 0.02) and 0.91 f 0.05 eV (n = 2.05 f 0.02), respectively, can be derived. Thus, the transition states are 71 and 88 kl-mol-', respectively, higher in energy than the reactants and 3 1 and 62 kJ.mol-', respectively, higher in energy than the products. The absence of any discontinuities in the threshold curves indicates that one mechanism must be responsible for the generation of C1- in reactions 4 and 5. The rapid and linear increase in cross-section at energies slightly above the threshold suggests nonstatistical behavior and a collision complex which is either not bound or very weakly b ~ u n d . ~ ' - The ~ ' latter is in keeping with the failure to observe the [Br + CC14]- and [Br CF2C12]- adducts. For these fully substituted alkanes the conventional sN2 mechanism shown in Figure 1 no longer applies. A mechanism similar to that proposed for the ,'CI- + CH335C1 "Cl- + CH337Clreaction at center-of-mass energies above 0.4 eV4 could be in operation. This mechanism would involve structures A and B as "intermediates" for reactions 4 and 5, respectively. Note
-
+
+
-
..GI mc:. * 1A
-
Br
.GI [FpCIC::
*
*
er
1-
B
(25) All thermochemical data from: Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W.G. J . Phys. Chem. Ref.Dura 1988, 17, Suppl. 1 unless otherwise stated. (26) A.Hf(CICBr,) = 49 k J T I " l from Kudchadker, S. A.; Kudchadker, A. P. J. Phys. Chem. ReJ Data 1978, 7 , 1285. (27) Chesnavich, W. J.; Bowers, M. T . J . Chem. Phys. 1978, 68, 901.
The Journal of Physical Chemistry, Vol. 95, No. 26, 1991 10585
Reactions of Halides with Halogenated Alkanes
16W
C C1' + CBr, -+ ClCBr, + Bf c:
11w
i
-
-
E
I
1200
-
1000
-
sw
-
-
,E (ev)
-
-
BrCCl, + C1- (A), Br- t CFIClz BrCF,Cl + C1- (B), and CI- + CBr4 ClCBr, + Br- (C) as a function of the center-of-massenergy with a 1000-ms reaction delay. They axis represents the ratio of the intensities of the product and parent ions normalized to 1000 for the ratio at a center-of-massenergy of 11.0 eV.
Figure 2. Abundance of the product halide ion from the bimolecular nucleophic displacement reactions Br- + CC1,
that the sum of the enthalpies of Cl3C*and ClBr-25*28 is ca 90 kJ-mol-I higher than the transition state energy of reaction 4. The latter difference is a reasonable number for the ion-neutral interaction energy between Cl3C*and ClBr-. Similarly, the sum of the enthalpies of CF2CL' and ClBr-25a is ca. 100 kl-mol-l higher than the transition-state energy of reaction 5. Covalent complexes analogous to the [Cl,C-Cl-CI]-, proposed by Carrion et al.Ib for the Cl- CCl, nucleophilic displacement reaction, could represent the first step for the displacement reactions 4 and 5. The threshold curve for reaction 6 contrasts with those of reactions 4 and 5 in that it is a very gradual curve. Using the
+
~~
~
~
~~
(28) AHdCIBr') = -226 kJ.mo1-l was calculated from AHf(CIBr) = 15 kJ.mol-lZ5 and an estimated electron affinity (EA) for ClBr of 2.5 eV. [EA(CI ) = 2.4 eV, EA(C1I) = 2.4 eV, EA(Br,) = 2.5 eV and EA(Br1) = 2.5 eV.']
empirical model described in the Experimental Section, a threshold of 1.51 f 0.15 eV is obtained, but the corresponding value for n is remarkably high, 2.41 f 0.05. Our experience so far is that with m = 1 and n = 1.7-2.0 the best fit is obtained." This could imply that a different mechanism is involved than that proposed for reactions 4 and 5. The fact that during the 2-s relaxation delay a large amount of the [Cl + CBr4]- adduct ion is generated, suggests that the latter complex is not weakly bound. (Without activation of C1- the ratio Cl-:[CI + CBrJ is ca. 100:24.) Two explanations for this observation are plausible. Either a complex mechanism is involved in this nucleophilic substitution reaction with [Cl CBr4]- as a long-lived intermediate or more than one mechanism is implicated. If the latter is the case, analysis of the data with eq 1 will be meaningless. The product versus reactant ion intensity ratios, I(P)/Z(R), obtained under single-collisionconditions (tcID= 10 ms) and with
+
J. Phys. Chem. 1991, 95, 10586-10592
10586
a center-of-mass interaction energy of 11 eV for the s ~ reactions 2 4, 5, and 6 are presented in Table I. Increasing the CID delay time from 10 to 20 ms doubles the I(P)/Z(R) ratio, which c o n f i i that the contribution from multiple-collisions events is negligible. A comparison of these Z(P)/I(R) ratios with the ratios yields the collision efficiencies for the reactions 4, 5 , and 6. For reaction 4 one in 41 collisions results in nucleophilic displacement and for reaction 5 this ratio is similar: one in 55. Reaction 6 is considerably less efficient since only one in 110 collisions is reactive. The latter result is in keeping with the above proposal that a different mechanism is operative for reaction 6. Barlow et aL4 determined the efficiency of the 37Cl- CH33SCI 35ClCH337C1sN2 reaction to be one reaction in 175 collisions at a center-of-mass energy of 2.0 eV, and numerous data are available for the efficiencies of sN2 reactions at temperatures between 300 and 600 K.3” However, no experiments for systems and energies
el/&
+
-
+
.
similar to ours have been performed, Further studies of this and other displacement reactions are in progress29to assess the nature of the potential energy surfaces of these Drocesses.
Acknowledgment. T.B.M. is grateful to the Natural Sciences and Engineering Research Council of Canada (NSERC) and the donors of the Petroleum Research Fund, administered by the American Chemical Society, for financial support. The authors also thank R. H. Schultz and Prof. P. B. Armentrout for supplying the CRUNCH program for threshold energy analysis and Dr.G. D. Willett and C. Li for stimulating discussions. Registry No. CCI,, 56-23-5; CF2C12,75-71-8; CBr,, 558-13-4; Br-, 24959-67-9; CI-, 16887-00-6. (29) Li, C.;Hop, C.E.C.A.; McMahon, T.B.;Ross, P. To be submitted.
S1- and T,-State Properties of n-Butylamine Schiff Bases of Isomeric Retinylldeneacetaldehyde As Revealed by Transient Absorption and Transient Raman Spectroscopies and by HPLC Analysis of Triplet-Sensitized Isomerization Yumiko Mukai, Hideki Hashimoto, Yasushi Koyama,* Faculty of Science, Kwansei Gakuin University, Uegahara, Nishinomiya 662, Japan
Shoziro Kuroda, Junior College of Kinki University, Kowakae, Higashi-Osaka 577, Japan
Yoshinori Hirata, and Noboru Mataga Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka 560, Japan (Received: July 10, 1991)
-
Transient absorption spectroscopy upon direct photoexcitation of n-butylamine Schiff base of isomeric retinylideneacetaldehyde (C22 SB) showed that each isomer generates its own SI species showing a unique S, SIabsorption and lifetime (all-trans, 420 ps; 9-cis, 290 ps; 11-cis, 370 ps; and 13-cis, 330 ps); no time-dependent spectral change indicating isomerization in the SIstate was seen for each isomer. Transient absorption a few nanosecondsafter excitation showed that each isomer generates as a result of intersystem crossing, its own T1 species showing a unique T, TI absorption. Transient Raman spectroscopy upon direct photoexcitation of isomeric C22 SB showed that each isomer generates its own SIspecies with a unique contiguration. It showed also that the SI state of each isomer is actually the 2’A, state, which is vibronically coupled with the So.(lAg) state and gives rise to an SI,C==C stretching Raman line with an extremely high frequency (1738-1727 cm-I). The excitation profile of the particular Raman line evidenced that it is due to the SIstate probed by the transient absorption spectroscopy. Analysis by HPLC of triplet-sensitized isomerization of isomeric C22 SB showed efficient cis to trans isomerization. The quantum yields (defined as isomerization of the starting isomer per triplet species generated) were as follows: all-trans, 0.02; 9-cis, 0.5; 11-cis, 0.6; and 13-cis, 0.4. Transient Raman spectroscopy upon triplet-sensitized photoexcitation of isomeric C22 SB showed that the all-trans isomer generates its own TI species (“all-trans” Tl) and that each cis isomer generates both its own “cis” T, and the “all-trans” TI species. The above results indicate that no isomerization takes place in the SI (2IA,) state, and that cis to trans isomerization takes place in the T, state. The photophysical properties of C22 SB are compared with those of retinylideneacetaldehyde (C22 aldehyde). +
Introduction Retinal binds to a lysine residue in the form of a protonated Schiff base in retinal proteins, and its photoisomerization triggers the conformational changes of the apo-proteins to start their own physiological functions. Since the isomerization reactions are started by photoexcitation of the retinylidene chromophore, they should be attributed to its excited-state properties. From this viewpoint, extensive investigations have been done on model systems of the retinylidene chromophore (see reviews by Birge’ and by Becker2). (1) Birge, R. R. Annu. Rev. Biophys. Bioeng. 1981, 10, 315.
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Why has the protonated Schiff base of retinal with the particular length of the polyene chain been selected by nature? The excited-state properties are expected to depend on the length of the polyene chain and also on the terminal group in the form of the protonated Schiff base. In order to reveal the latter effect, it is ideal to compare the SI-and TI-state properties among retinal (C20 aldehyde), its Schiff base (C20 SB) and its protonated Schiff base (C20 PSB). Actually, efficient “cis” to “trans” isomerization of C20 aldehyde in the TI state has been detected by transient absorption spectroscopy of a set of its isomer^,^.^ but the S1state (2) Becker, R.S . Photochem. Photobiol. 1988, 48, 369.
0 1991 American Chemical Society
