5076
J . Phys. Chem. 1988, 92, 5076-5079
Molecular Dynamics of 18-Crown-6 Complexes with Alkali-Metal Cations: Calculation of Relative Free Energies of Complexation Johan van Eerden, Sybolt Harkema, and Dirk Feil* Laboratory of Chemical Physics, University of Twente, P.O. Box 21 7 , 7500 AE Enschede, The Netherlands (Received: April 26, 1988)
Complexes of 18-crown-6with alkali-metal cations (Na', K', and Rb') in aqueous solution were studied with the molecular dynamics method. The relative free energies of complexation were derived from simulations according to the thermodynamic slow growth method. The free energy difference for the complexation by 18-crown-6 of K+ vs Rb' was calculated as -5 (2) kJ/mol (experimental value -2.7 (7) kJ/mol). For the complexation by 18-crown-6 of K' vs Na+ a relative free energy of -8 (4) kJ/mol was found (experimental value -7 (1) kJ/mol). Thus the relative strength of 18-crown-6complexes with alkali-metal cations in aqueous solution (K' > Rb+ > Na') is reproduced by computational methods.
Introduction The field of host-guest chemistry has received an increasing amount of interest in the past two decades, since the discovery of the crown ethers by Pedersen.' Numerous host molecules, such as the macrocyclic polyethers, have been designed and synthesized.2 The complexation properties of these hosts with a variety of guest molecules have been r e p ~ r t e d . ~Cram has stated two principles that govern the complexation process: complementarity and pre~rganization.~ Complementarity involves the steric and electrostatic fit of host and guest; for complexation of metal cations by macrocyclic polyethers, complementarity is reflected by a cavity-diameter cation-size relationship. Preorganization is defined as the absence of structural reorganization and desolvation of the host, upon complexation; application of this principle has led to the synthesis of the spherands, rigid hosts that are only weakly ~olvated.~ Some theoretical work on host-guest systems has been reported. A number of quantum-mechanical studies on complexes of simple crown ethers with alkali-metal cations were p e r f ~ r m e d . ~Molecular mechanics studies were reported on 18-crown-6 complexes with alkali-metal cations7 and neutral molecules,8 and on the ~ p h e r a n d s . ~All these studies pertain to isolated complexes and cannot be compared directly to experimental data, which refer to solution or solid state. Solvation effects can explicitly be included in the computations of the Monte Carlo and the molecular dynamics simulation rnethods.'O The hydration of several conformations of 18-crown-6 was simulated with the Monte Carlo method." With molecular dynamics a number of host-guest (1) Pedersen, C. J. J . Am. Chem. SOC.1967,89, 7017. (2) Introductory reviews: (a) Weber, E.; Vogtle, F. Top. Curr. Chem. 1981,98, 1. (b) Colquhoun, H. M.; Stoddart, J. F.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1986, 25, 487. (3) (a) Dietrich, B. J . Chem. Educ. 1985, 62, 954. (b) Izatt, R. M.; Bradshaw, J. S.;Nielsen, S. A.; Lamb, J. D.; Christensen, J. J.; Sen, D. Chem. Rev. 1985,85, 271. (c) Inoue, Y.; Hakushi, T. J . Chem. SOC.,Perkin Trans. 2 1985, 935. (4) (a) Cram, D. J.; Trueblood, K. N. Top. Curr. Chem. 1981,98,43. (b) Cram, D. J. Angew. Chem. 1986, 98, 1041. (5) (a) Pullman, A.; Giessner-Prettre, C.; Kruglyak, Y. V. Chem. Phys. Lett. 1975, 35, 156. (b) Yamabe, T.; Hori, K.; Akagi, K.; Fukui, K. Tetrahedron 1979, 35, 1065. (c) Rode, B. M.; Hannongbua, S. V. Inorg. Chim. Acta 1985, 96, 91. (6) Systematic name: 1,4,7,10,13,16-hexaoxacyclooctadecane. (7) Wipff, G.; Weiner, P.; Kollman, P. J . Am. Chem. SOC.1982,104, 3249. (8) (a) Uiterwijk, J. W. H. M.; Harkema, S.; Feil, D. J . Chem. Soc., Perkin Trans. 2 1987, 721. (b) Damewood, J. R., Jr.; Anderson, W. P.; Urban, J. J. J. Cornput. Chem. 1988, 9, 111. (9) Kollman, P. A.; Wipff, G.; Singh, U. C. J. Am. Chem. SOC.1985, 107, 2212. (10) van Gunsteren, W. F.; Berendsen, H . J. C. In Molecular Dynanics and Protein Structure; Hermans, J., Ed.; Polycrystal Book Service: Western SDrines. 1985: D 5. (1 r)'Ranghiho, G.; Romano, S.; Lehn, J. M.; Wipff, G. J . Am. Chem. SOC. 1985, 107, 7873.
0022-3654/88/2092-5076$01.50/0
systems in aqueous solution have been studied.I2J3 A very interesting application of these simulation methods is the thermodynamic perturbation method,I4 which can yield free energy differences. This method has been applied to the computation of the relative free energies of solvation, of ethane and methanol,15 of the noble gas atoms Ne and Xe;I6 and of the relative free energies of complexation of C1- and Br- by a macrotricyclic recept0r.I' Most of these simulations involve only small conformational changes in the host molecules; the focus is on changes in the solvation structure and energy upon changing the solvated system. 18-Crown-6 is a rather flexible molecule, which is known to adopt different conformations when complexing different guest species.I8 This means that conformational changes can be expected when changing the guest in a complex with 18-crown-6. It was decided to study the complexation of alkali-metal cations by 18-crown-6, with the molecular dynamics method, in order to test the applicability of the thermodynamic perturbation method to these systems.
Computational Method Molecular dynamics simulations were performed with the GROMOS package.19 The force field consists of bonded (covalent bonds and angles and dihedral angles) and nonbonded (Coulomb and Lennard-Jones) interactions. Parameters for 18-crown-6 were taken from the AMBER force field,20with the CH2 group as a united atom and with 50% scaling of the nonbonded 1,4 interactions. The parameters for the alkali-metal cations were those used in a molecular mechanics study of alkali-metal cation complexes of 18-crown-6.7 For water the SPC model2' was used, with (12) Teleman, 0.: Ahlstrom, P. J. Am. Chem. SOC.1986, 108, 4333. (13) (a) Wong, C. F.; McCammon, J. A. Isr. J. Chem. 1986,27,211. (b) Bash, P. A.; Singh, U. C.; Brown, F. K.; Langridge, R.; Kollman, P. A. Science (Washington, D.C.)1987, 235, 574. (14) (a) Zwanzig, R. W. J. Chem. Phys. 1954,22, 1420. (b) Tembe, B. L.; McCammon, J. A. Comput. Chem. 1984,8,281. (c) Singh, U. C.; Brown, F. K.; Bash, P. A,; Kollman, P. A. J . Am. Chem. SOC.1987,109, 1607. (d) van Gunsteren, W. F.; Berendsen, H. J. C. J. Computer Aided Mol. Des. 1987, 1, 171. (15) Jorgensen, W. L.; Ravimohan, C. J . Chern. Phys. 1985, 83, 3050. (16) Straatsma, T. P.; Berendsen, H. J. C.; Postma, J. P. M. J . Chem. Phys. 1986,85, 6720. (17) (a) Lybrand, T. P.; Ghosh, I.; McCammon, J. A. J. Am. Chem. SOC. 1985, 107, 7793. (b) Lybrand, T. P.; McCammon, J. A,; Wipff, G. Proc. Null. Acad. Sci. U.S.A. 1986, 83, 8 3 3 . (18) (a) Dalley, N. K. In Synthetic Multidentate Macrocyclic Compounds; Izatt, R. M., Christensen, J. J., Eds.; Academic: New York, 1978; p 207. (b) Goldberg, I. In Inclusion Compounds 2; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Eds.; Academic: London, 1984; p 261. (19) van Gunsteren, W. F.; Berendsen, H. J. C. Groningen Molecular Simulation Library, 1987. (20) Weiner, S. J.; Kollman, P. A,; Nguyen, D. T.; Case, D. A. J . Comput. Chem. 1986, 7, 230.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 18, 1988 5077
Letters a fixed internal geometry, maintained by the SHAKE routine,22 which also constrained the covalent bonds in 18-crown-6,to remove the internal bond vibrations. Thus a time step of 0.002 ps could be used in the integration of the equations of motion. The systems studied were isolated (gas-phase) complexes of a molecule of 18-crown-6 with an alkali-metal cation, the same complexes in aqueous solution, and single cations in solution. To create an aqueous solution, a cation or a complex was placed in the center of a truncated octahedron, which was then filled with water molecules. Periodic boundary conditions were applied to this computational box. The size of the box was chosen so as to give two complete solvation layers around the central solute and to allow a cutoff distance of 8.5 A for the calculation of the nonbonded interactions. The simulations were of the isothermal-isobaric type.23 The temperature was maintained at 298 K (or 0 K, for energy minimization) by velocity scaling with a time constant of 0.1 ps, whereas for constant pressure, position scaling with a time constant of 0.5 ps was applied. Data were stored every 5 or 10 steps during the simulations, for further analysis. For the calculation of free energy differences the thermodynamic perturbation rnethodl4 was used. This method is related to the following thermodynamic cycle, in the case of complexation of two different cations MI' and M2+by 18-crown-6, in aqueous solution. 18-crown-6aq
+ MI,,^*
18-crown-6aq
+ M2,Pq*
AG
-
(18 crown-6 M,+)Pq
A02
(16-crown-6*M~),,
The free energies of complexation, AGl and AG2, are experimentally known; simulation of these processes would require very long computing times. The nonphysical perturbations M1+ M2+,with corresponding free energies AG3 and AG4 for the uncomplexed and the complexed cation, respectively, however, can be simulated in manageable times (10-100 ps), if MI+ and M2+ are not too different. The free energy difference AAG, defined as AGI - AGz, can then be computed from AG3 - AG4, for this closed thermodynamic cycle. The perturbation is defined in terms of a coupling parameter X, which gives an intermediate potential energy function Vi, between the two extremes V , and V2, which characterize the systems with M I + and M2+, respectively:
-
v,
= V,
+ X(V2 - VI)
The free energy of perturbation is derived as the integral of the ensemble-averaged derivative of the potential energy, with respect to A: AG = S ( 6 V / 6 X ) , dX
This expression can be evaluated in two ways, in a molecular dynamics simulation. The first method involves the computation of the ensemble average of the derivative at a limited number of X values and subsequent summation of the contributions of the X intervals. The second method is the slow growth (or thermodynamic integration) method, in which X is changed continuously, with every time step of the simulation, in very small increments (with rate of change A). In a simulation, with discrete time steps, the integral is then replaced by a summation, using the instanteneous values of the derivative:
-
-
-
-
TABLE I: Free Energy Changes (ACJ for tbe Perturbations Na+ K+, K+ Rb+ (A = 0 1) and Reverse (A = 1 O), in Aqueous Solution
x 0.0-1 .o 0.0-1 .o 0.0-0.4, 0.4-1.0
time, ps
K+ 5 10 20 (10, 10)
-
25.7 26.5 25.7
Na+ 0.0-1.0 0.0-0.25, 0.25-0.5, 0.5-1.0 0.0-0.25, 0.25-0.5, 0.5-1.0
10 20 (IO, 5, 5) 30 (15, 7.5, 7.5)
94.2 89.7
AG, kJ/mol Rb+
-
Rb+
-
K+
-25.6
K+
K+
-
Na+
-93.2 -90.2 -90.6
This expression is only valid if the perturbation is slow enough and the system remains essentially in equilibrium.
Results and Discussion Initial calculations were performed on gas-phase complexes of 18-crown-6 with the alkali-metal cations Na+, K+, and Rb', starting from the conformations found in crystal structures of the corn pound^.^^*^^ In complexes of 18-crown-6 with K' or Rb+, the crown ether macrocycle always adopts a conformation with approximate D3d symmetry; all six ether oxygens coordinate equatorially to the cation in the center of the macrocyclic cavity. Above and below the macrocyclic plane there is room for cation coordination by the anion or, e.g., water molecules. Molecular dynamics simulations of isolated Kf and Rb' complexes of 18crown-6 at 298 K showed no conformational changes during 200-ps runs. Simulations in which K+ was changed into Rb' by slow growth were also run. Again no conformational changes were observed. In the Na+ complex 18-crown-6 also can adopt the D3d conformation, but the Na+-O coordination distances are larger than those normally found,Iga showing that the cavity of 18-crown-6 is too large for Na+. This conformation, however, leaves room for two additional coordinating molecules at shorter distances, as observed in several crystal s t r ~ c t u r e s .To ~ ~decrease the coordination distances to normal values, the crown ether adopts an irregular conformation with C1 symmetry, by wrapping itself around the cation, as observed in another crystal In this C 1 conformation the Na+ cation has only one additional coordinating molecule. Energy minimization (at 0 K) of the isolated 18-crown-6.Na+ complex, starting from the two observed conformations, resulted in an energy difference of 25 kJ/mol, in favor of the C1conformation, whereas a value of 18 kJ/mol was reported for an earlier version of the AMBER force field.' Also two molecular dynamics runs (at 298 K) of 400 ps each were performed, starting from the D3dand the C1 conformation. In both runs numerous dihedral transitions were observed, indicating that there is not one clear global minimum-energy conformation for the 18-crown-6.Na' complex. Starting from the 400-ps configurations, slow growth perturbations of Na+ to K+ were performed; in 200-ps runs AG values of 94 and 103 kJ/mol were found for the two configurations, compared with -84 kJ/mol for the reverse perturbation (K+ Naf), indicating a considerable amount of hysteresis. During the perturbations conformational changes occurred. For simulation of aqueous solutions of the alkali-metal cations Na', K+, and Rb', systems with one cation and 179 water molecules were used. All systems were equilibrated for at least 40 ps, before starting the simulations to calculate AG3. Results are given in Table I. First the smaller of the two perturbations (K' Rb') was studied with slow growth simulations. From
-
-
AG = z(SV/SX)h dt (21) Berendsen, H. J. C.; Postma, J. P. M.; van Gunsteren, W. F.; Hermans, J. in Intermolecular Forces; Pullman, B., Ed.;Reidel: Dordrecht, 1981; p 331. (22) (a) Ryckaert, J. P.; Ciccotti, G.; Berendsen, H. J. C. J . Comput. Phys. 1977, 23, 327. (b) van Gunsteren, W. F.; Berendsen, H . J. C. Mol. Phys. 1977, 34, 1311. (23) Berendsen, H. J. C.; Postma, J. P. M.; van Gunsteren, W. F.; DiNola, A.; Haak, J. R. J . Chem. Phys. 1984, 81, 3684.
(24) (a) Dobler, M.; Dunitz, J. D.; Seiler, P. Acta Crystallogr., Sect. B 1974, 30, 2741. (b) Seiler, P.; Dobler, M.; Dunitz, J. D. Acta Crystallogr., Sect. B 1974,30, 2744. (c) Dobler, M.; Phizackerley, R. P. Acta Crystallogr., Sect. B 1974, 30,2746. (25) (a) Sheldrick, W. S.; Kroner, J.; Zwaschka, F.; Schmidpeter, A. Angew. Chem. 1979,91,998. (b) Cooper, M. K.; Duckworth, P. A,; Henrick, K.; McPartlin, M. J . Chem. Soc., Dalton Trans. 1981, 2357. (c) Bailey, S.
I.; Engelhardt, L. M.; Leung, W.-P.; Raston, C. L.; Ritchie, I. M.; White, A. H. J . Chem. Soc., Dalton Trans. 1985, 1747. (d) Darensbourg, D. J.; Bauch, C. G.; Rheingold, A. L. Inorg. Chem. 1987, 26, 977.
5078
The Journal of Physical Chemistry, Vol. 92, No. I%, 1988
Letters
'd 0 0
. I W 0
0 I
0.00
I
0.20
I
I
0.40
I
I
I
0.60
1
I
I
1.00
0.80
1 ambda
1
I
1
0.00
I
I
0.20
I
1
0.40
I
I
I
0.60
1 1.00
I
0.80
-
lambda
Figure 1. Free energy changes vs A, for the K+ -+ Rb+ slow growth perturbation, in aqueous solution (AG,, lower two curves, 20-ps runs, forward (-) and backward simulation) and in the complex with 18-crown-6 (AG4, upper two curves, 40-ps runs with additional equilisimulation). bration at X = 0.4, forward (-) and backward
Figure 2. Free energy changes vs A, for the Na+ K+ slow growth perturbation, in aqueous solution (AC3, 20-ps runs, forward (-) and backward simulation) and in the complex with 18-crown-6 (AG4, 75-ps run, backward (- - -) simulation).
the 5- and 10-ps runs the approximate curve of the free energy change vs X was determined. Thereupon the X interval was divided in two parts (0.0-0.4 and 0.4-1.0), to give approximately equal simulation times for equal free energy changes in two consecutive 10-ps runs. It seems that a 20-ps run for a complete perturbation is satisfactory, as the hysteresis is only 0.1 kJ/mol and the shorter runs give similar results. In Figure 1 the 20-ps forward and backward simulations are shown; over the complete X interval the curves almost completely coincide. Thus we find a value of -25.7 (1) kJ/mol for the free energy difference of solvation of a potassium and a rubidium ion, whereas an experimental value of -23 kJ/mol is reported.26 The perturbation Na+ K+ was studied analogously. From 10-ps runs the free energy change was roughly determined. Subsequently in 20 ps the perturbation was applied in three intervals (see Table I), resulting in a free energy difference of -90 (1) kJ/mol for the solvation of a sodium and a potassium ion. The error in this value is derived from the maximum deviation between the 20-ps forward and backward simulations, shown in Figure 2. The experimental value of -72 Na+ kJ/mo126 is somewhat smaller. A 30-ps run of the K+ perturbation gave within the estimated error the same result as the 20-ps runs, indicating that these simulation times are sufficient for describing the changes in solvation. The alkali-metal cation complexes of 18-crown-6 were studied in a box with 225 water molecules. All systems were equilibrated for at least 30 ps, before starting the simulations to calculate AG,. Results are given in Table 11. Again, first the perturbation K+ Rb' was studied. The slow growth perturbation was first performed in two 5-ps runs, starting from two different, equilibrated configurations of the solvated 18-crown-6.K' complex. The resulting free energies differed by 3 kJ/mol, which gives an impression of the statistical error. Subsequently a 20-ps run was done, with the X range split up in two parts, as before. The computed AG was higher than in the two short runs, indicating that the perturbation was still too large. Thereupon the time for the perturbation was increased to 40 ps. Forward and backward slow growth simulations resulted in free energy changes of 30.5 and -33.3 kJ/mol, respectively. One of the possible reasons for the hysteresis in this free energy is a lagging of the system behind the perturbation. To remove part of this, an additional equili-
.+ K+,
(-a)
(-e)
-
(-e)
-
-
-
(26) Marcus, Y . Ion Soluation; Wiley: Chichester, 1985.
TABLE 11: Free Energy Changes (AC,) for the Perturbations Na+ K+ Rb+ (A = 0 1) and Reverse (A = 1 0 ) , in the Complex with 18-Crown-6 in Aqueous Solution +
+
time, ps
h
0.0-1.0
0.0-0.4, 0.4-1.0 0.0-0.4, 0.4-1.0 0.0-0.4, 0.4-1 .Ob
-
AG, kJ/mol
K+ Rb+ 31.6, 34.6"
5
20 (10, 10) 40 (20, 20) 40 (20, 20)
Rb+
-
K+
35.3
30.5 29.4
-33.3
-
-32.8
-
Na+ K+ K+ Na+ 0.0-0.35, 0.35-0.65, 75 (35, 22.5, 17.5) 90.6,' 101.9' -81.5 0.65-1 .O "Starting from a different configuration, reached after another 5 ps of equilibration. *Including I O ps of equilibration at h = 0.4. 'Starting with a &-like conformation of 18-crown-6. dStarting with the C, conformation of 18-crown-6.
bration of 10 ps was performed at X = 0.4, after which the second half of the perturbaton was continued, in both directions (see Figure 1). The free energy change for this second half was indeed reduced, with 1.1 and 0.5 kJ/mol in the forward and backward directions, respectively, but the hysteresis was not reduced. Obviously there are longer simulation times needed for that. As a check, simulations in a number of X intervals were performed. At intermediate X values, first 5 ps of equilibration was done, after which 5 ps of averaging was performed. This resulted in an accumulated free energy change of 32 (2) kJ/rnol,*' consistent with the slow growth result of 31 (2) kJ/mol. Combining the results of the slow growth perturbations K+ Rb+ of the single cation and the 18-crown-6 complex, in aqueous solution, gives a free energy difference (AG3 - AG,) of -5 (2) kJ/mol, in favor of K+ complexation. The experimental value is -2.7 (7) kJ/mo1.28 In view of the relatively large uncertainties and the fact that the present parameter set was not optimized for this kind of calculation, the agreement is at least qualitatively satisfactory. Then the larger perturbation K+ Na+ in the complex with 18crown-6, starting from the D3d conformation, was performed in a long, 75-ps run, with the X range split up in three parts (see Table
-
+
(27) Error estimated according to the method described in: Straatsrna, T. P.; Berendsen, H. J. C.; Stam, A. J. Mol. Phys. 1986, 57, 89. (28) Izatt, R. M.; Terry, R. E.; Haymore, B. L.; Hansen, L. D.; Dalley, N. K.; Avondet, A. G.; Christensen, J. J. J . A m . Chem. SOC.1976, 98. 7620.
J . Phys. Chem. 1988, 92, 5079-5080
I1 and Figure 2). A free energy change of -82 (3) kJ/mol was found. The error in this result is a rough estimate of the statistical error, based on runs at X = 0 and X = 1. A preliminary analysis of the structural data indicates that during the perturbation K+ Na+ the mobility of the macrocycle encapsulating the cation increases, but that no permanent deviation from the D3d conformation occurs. During equilibration of the system reached after the K+ Na+ perturbation, however, dihedral transitions were observed, indicating that the D3dconformation is not kinetically stable for the Na+ complex. After this equilibration the perturbation was applied in the reverse direction, starting from a D3,,Jike conformation. Now numerous conformational changes were observed, but the macrocycle did not return to the expected D3dconformation, as the cation changed back to K+. The resulting free energy change of 90.6 kJ/mol is obviously not related to a perturbation of a system in or near equilibrium. The perturbation was also performed starting from the 18-crown-6-Na+complex in the C,conformation. During the perturbation the Cl conformation turned out to be kinetically stable. This conformation is, however, not thermodynamically stable, as it is approximately 50 kJ/mol higher in energy than a D3hlike conformation: the lower internal energy of the 18-crown-6.Na+ complex in the C1 conformation is more than compensated by a more favorable solvation energy of the complex in a D3hlike conformation. Thus of these perturbations, only the K+ Na+ run starting from the D3,, conformation, which is thermodynamically stable for K', seems to meet the requirement of approximate equilibrium during the perturbation; only the last part of the run ("near" Na') will
-
-
-
5079
be somewhat less reliable due to conformational fluctuations. The free energy difference for the complexation by 18-crown-6 of sodium vs potassium is now calculated as 8 (4) kJ/mol (Le,, -8 (4) kJ/mol for potassium vs sodium complexation), which compares well with the experimental value of 7 (1) kJ/moL2* Additional simulations of the solvated 18-crown-6-Na+complex are, however, necessary to study its conformational equilibrium.
Conclusions The results show that molecular dynamics simulations with the thermodynamic slow growth method qualitatively can reproduce the experimentally observed preference of 18-crown-6 for the complementary potassium cation, in aqueous solution. The perturbations of the single cation in solution give in 20-ps runs good agreement between the forward and backward slow growth simulations. For the 18-crown-6 complex obviously longer simulation times are necessary, possibly due in part to longer (dihedral) relaxation times. Further simulations on these systems and on more rigid host molecules, which can show an effect of preorganization, will be performed. Acknowledgment. This work was supported by the Netherlands Organization for the Advancement of Pure Research (ZWO) through a grant for computer time on the CYBER-205 at SARA (Amsterdam). We thank Professors H. J. C. Berendsen and W. F. van Gunsteren for supplying the GROMOS package and for helpful discussions, Dr. W. J. Briels for useful comments, and Dr. P. F. W. Stouten for help in getting started on the CYBER-205.
Energy-Resolved Phosphorescence of the Lowest Triplet State of Pyrazine in a Pulsed Planar Supersonic Jet Abraham Penner and Aviv Amirav* Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University, Ramat Aviu 69978 Tel Aviv, Israel (Received: March 7 , 1988; In Final Form: June 8. 1988)
The energy-resolved phosphorescence spectrum of pyrazine was obtained in a pulsed planar supersonic jet. A comparison of this spectrum with the phosphorescence excitation spectrum of ref 1 suggests a reduction of the nonradiative lifetimes at higher triplet vibrational energies.
A direct spectroscopic study of the lowest triplet state of pyrazine is of considerable interest as it may reveal important features pertaining to the dynamics of intersystem crossing in the SI(lB3") state. The triplet state of pyrazine was studied in supersonic jets by Villa et aL2 using the multiphoton ionization technique and very recently by Tomer et al.' and by Penner et al.' using the phosphorescence excitation (PE) technique. In spite of the fact that the So-Tl oscillator strength is -lo-* and the emission quantum yield is 104/ps, it proved easy to obtain PE spectra3s4 by using pulsed slit nozzle and delayed imaging. In Figure 1 we demonstrate that with the current unoptimized experimental situation it is even possible to obtain energy-resolved phosphorescence (ERP) spectra which are involved in a further reduction of the number of detected photons by more than 2 orders of magnitude. In order to obtain this E R P spectrum, we used an excimer pumped dye laser (Lambda-Physik EMG53MSC and FL2002E) tuned to the Q branch of the TI electronic origin (Aex 3728 A).
-
=
(1) Tomer, J. L.; Holtzclaw, K. W.; Pratt, D. W.; Spangler, L. H. J. Chem. .~ Phys. 1988,88, 1528. (2) Villa, E.; Terazima, M.; Lim, E. C. Chem. Phys. Lett. 1986, 194, 336. (3) Penner, A,; Oreg, Y . ;Villa, E.; Lim, E. C.; Amirav, A. Chem. Phys. Lerr., in press. (4) Villa, E.; Amirav, A,; Chen, W.; Lim, E. C. Chem. Phys. Lett., in press.
0022-3654/88/2092-5079$01.50/0
The pyrazine sample at 50 "C was seeded with argon (80 Torr) and expanded through a 35-mm X 0.22-mm pulsed slit n ~ z z l e . ~ The laser intersected the beam 10 mm from the nozzle. The phosphorescence was imaged after 15-ps time of flight by using a conventional lens system onto a 0.3-m McPherson monochromator (10-8, resolution at 800-bm slit width). The phosphorescence was detected by a photomultiplier (Hamamatsu R-292) and processed by using a 15-ps gate with 8-ps delay. This 8-pus time delay has totally eliminated any stray light or stray fluorescence. The ERP displayed in Figure 1 is dominated by a long progression with -606-cm-' intervals which may be a superposition of more than one vibration under our resolution. The observed ERP spectrum is very similar to the ERP spectrum of Goodman and Kasha6 obtained at 77 K in rigid glass solution, except for narrower emission line width. The ERP spectrum can be compared with the phosphorescence excitation spectrum of Tomer et al.' We note that the expected mirror image relationship is obeyed in the vibrational energy range E, < 2000 cm-'. The continuation of the ERP spectrum in the range E, > 2000 cm-l, Le., the 2423and 3035-cm-I bands are totally missing in the PE spectrum. This
-
~
(5) Amirav, A,; Horwitz, C.; Jortner, J. J. Chem. Phys. 1988.88, 3092. Amirav, A. "Pulsed Linear Nozzle", unpublished (available upon request). (6) Goodman, L.; Kasha, M. J. Mol. Spectrosc. 1958, 2, 5 8 .
0 1988 American Chemical Society
