J. Phys. Chem. 1995,99, 5945-5949
5945
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Energetics and Dynamics of Intermolecular Proton-Transfer Processes. 1. Ab Initio NH3 NH4+ H20 Studies of the Potential Energy Surface for the Reaction H30'
+
+
Heinz-Hermann Bueker and Einar Uggerud" Department of Chemistry, University of Oslo, P.O. Box 1033 Blindem, N-0315 Oslo, Norway
Received: November 15, 1994; In Final Form: January 31, 1995@
+
-
+
Ab initio quantum chemical calculations of the reaction H3O+ NH3 H20 W+have been performed using the following procedures: MF%6-3 lG(d,p), HF/6-31G(d,p) and HF/3-21G. Two-dimensional representations of the potential energy functions were constructed at MP2/6-31G(d,p), and this is the most detailed and precise potential surface of this system to date. Particular features of the three surfaces which may be important in forthcoming ab initio direct dynamics calculations are discussed. Some significant structural and energetical differences are noticed. When compared with MP2/6-31G(d,p) the results shows that HF/6-3 lG(d,p) and HF/3-21G give slightly less accurate representations of the reaction.
Introduction
+
-
+
-
H,O
Proton transfer AH+ B A BH+ is one of the most important reactions in chemistry.' It is of particular relevance and interest to know how these reactions occur in the gas phase, e.g., inside a mass spectrometer. The reaction enthapy, given by the difference of the proton affinities of A and B, W = PA(A) - PA(B), puts an upper limit to the internal energy of the product ions BH+. Since this is the energy available for further reactions of BH+, e.g., in chemical ionization mass spectrometry (CI), it is very interesting to know its exact amount. However, up to now the internal energy of product ions has only been determined in a few cases.* To gain more insight into the energetics and dynamics of proton transfer, we have investigated the reaction
+
H ~ O + NH,
+ NH,+
(1)
as an example of a highly exothermic proton transfer. In this first paper in a series we investigate the potential energy surface of reaction 1 and the structures of the reaction partners and intermediates by ab initio methods. Subsequent papers will deal with the reaction dynamics. During the past two decades, increasingly efficient algorithms for direct integration of the classical trajectories on ab initio potential energy surfaces have been de~eloped.~ One great advantage of this direct approach is that because the ab initio energy, and its first and second derivatives are calculated en route, it becomes unnecessary to construct a complicated multidimensional analytical model potential energy function prior to the dynamical calculation. Despite this simplification some knowledge about the performance of the ab initio method chosen is needed, and for this reason investigation of the most important part of the potential energy surface is required. A general proton transfer reaction can be characterized as follows:
tions, however, can only be described appropriately by reaction paths containing at least two minima corresponding to the two stable intermediates (A-H *B)+and (A. *H-B)+, separated by a transition state (A. *H OB)+.^ Several investigations reported in the literature5 indicate that the proton transfer between H20 and NH3 (reaction 1) proceed along a path with a single global minimum corresponding to the complex H3NH+* GH2. In this paper we report the results from a survey of the potential energy surface of the proton-bound ammonia-water dimer, (H3N *H OH#, calculated at the MP2/6-31G(d,p) level. A two-dimensionalrepresentation of the potential energy was obtained as a function of the OH distance d(0-H) at different fixed heavy-atom distances d ( N - 0 ) by optimizing all the other geometrical parameters. Furthermore, the potential energies of the optimized proton bound dimer complexes (H3NH * .OH# and (H3N -H-OHz)+ as well as the transition state for proton transfer, (H3N * *H. QH*)#+, separating these complexes have been computed for several fixed monomer distances d ( N - 0 ) at the MF'2/6-31G(d,p),HF/6-31G(d,p), and HF/3-21G levels. The results of these calculations as well as molecular geometries and computational details are given in the following sections. Method of Calculation The ab initio calculations were performed using standard procedures of the program package GAUSSIAN 92.6 The basis set 3-21G and the split valence plus polarization basis set 6-31G(d,p) were used at the Hartree-Fock level. In addition Mqiller-Plesset perturbation theory to the second order (MP2)7 was applied for the 6-31G(d,p) basis set. The calculations of the potential energy surface were performed with complete optimization of all geometrical parameters except those that were fixed. The zero point contributions to the electronic energy were calculated from the MP2/6-3 1G(d,p) vibrational frequencies after scaling by a factor of 0.9. Results and Discussion
In the simplest case, a reaction has a reaction path with a single potential energy minimum corresponding to a proton bound intermediate complex (A. * *Ha OB)+. Numerous reac@
Abstract published in Advance ACS Absrracfs, March 15, 1995.
The structures of the reactants, products and intermediate of reaction 1 computed at the different levels are shown in Figure 1. The bond lengths and angles are those from the Mp2/631G(d,p) calculations, with the values from the HF/6-31G(d,p) and HFl3-21G calculations given in parentheses.
0 1995 American Chemical Society QO22-3654/95/2Q99-5945$Q9.QQ/Q
Bueker and Uggerud
5946 J. Phys. Chem., Vol. 99, No. 16, 1995
5 (C,)
Figure 1. Geometrical structures of reactants, products and intermediate computed at MP2/6-3 lG(d,p). The corresponding values computed at HF/3-21G and HF/6-31G(d,p) are given in parentheses. Bond lengths are given in angstroms and valence bond angles in degrees.
As expected, the results of the MP2/6-31G(d,p) calculations give the best agreement with experimental determined structures* as far as these are known. For water the deviations from the experimental OH bond distmce d(0-H) = 0.958 8, and angle LHOH = 104.5" are small. The same holds for ammonia with experimental values d(N-H) = 1.012 A and angle LHNH = 106.7'. For H3O+ the MP2 and HF calculations with the 6-31G(d,p) basis set give the expected C3" geometry with angles LHOH = 112.5 and 114.7", respectively, whereas at the lowest HF/3-21G level H3O+ is planar with an angle LHOH = 120" (point group D3h). The equilibrium structure of the complex NH4+. OH2 corresponding to the global minimum of the (H3N *H- .OH# potential surface is also shown in Figure 1 (structure 5 ) . At all levels of theory this complex has a structure with a directed ion-dipole hydrogen bond,g which is almost coincident with the dipole moment vector of H20 rather than one of the oxygen lone pairs. The different levels of theory lead to distinctly different NO distances. The MP2/6-3 lG(d,p) calculation results in d(N-0) = 2.676 A, whereas the HF/6-31G(d,p) and HF/321G calculations lead to longer and shorter NO distances with d(N-0) = 2.752 and 2.593 A, respectively. The computed MP2/6-3lG(d,p) electronic energies and zeropoint vibrational energies (Table 1) give a binding enthalpy of the N%+.-.OHz complex of 95.1. kJ mol-' relative to the compounds N&+ and H20. This agrees reasonably with the experimental value of 83.3 kJ mol-', which was adjusted to 86.2 kJ mol-' by Meot-Ner.'O The corresponding value of 81.9 kJ mol-' based on HF/6-31G(d,p) is in even better agreement with the experimental value, whereas the HF/3-21G value of 131.3 kJ mol-' shows a larger deviation. Figure 2 defines the geometrical parametrs of the protonbound ammonia-water dimer with C, symmetry (structure I).
- -
6 (Cs)
Figure 2. Geometrical structures of various complexes formed during proton transfer from water to ammonia (see text for reference).
The potential energy of this complex has been calculated at the MP2/6-31G(d,p) level for various NO distances with 2.2 8, 5 d(N-0) 5 3.4 8, in steps of 0.2 8, and different OH distances with 0.9 8, 5 d(0-H) 5 2.4 8, in steps of 0.1 8, (for each NO distance). All the other geometrical parameters were fully optimized. Furthermore, the critical points on the potential surface, the intermediate complexes (H3N * -H-OHo+, (H3NHe .OH2)+ and (H3N * .He .OH*)#+ have been optimized for the different NO distances at all three computational levels. In addition, the reactant and product complexes were calculated at the NO distances d(N-0) = 5.0, 8.0, and 10.0 A, respectively. Figure 3 shows the two dimensional potential energy map of the optimized (H3N*-.H*..OH2)+ complex as a function of the d(N-0) and d(0-H) distances at the MP2/6-31G(d,p) level. Similar calculations on this system at the MP2/6-3 lG(d) level have already been published in the literatures5 However, in all of these publications, simplifications and restrictions in the geometry optimization, such as averaging and keeping bond lengths and angles constant, are made. In this work, however, a full optimization of all geometrical parameters is performed for each fixed d(N-0) and d(0-H) distance. A particular restriction which has been lifted in the present approach is that the transferred proton is not confined to move on the NO line, so that the complex (H3N..*H...OH2)+ is free to adopt a
TABLE 1: Energies Obtained from the Quantum Chemical Calculations
compound Hz0 (1) H@+ (2) NH3 (3) NH4+ (4) H3NH+. .OH2 (5)
HF/3-2 1G (hartrees) -75.585 -75.891 -55.872 -56.233 -131.873
96 23 20 85 88
HF/6-31G(d,p) (hartrees) -76.023 62 -76.310 33 -56.195 55 -56.545 53 - 132.603 46
MP2/6-3 lG(d,p) (hartrees) -76.219 -76.506 -56.383 -56.733 -132.992
79 11 22 68 04
E(z.p.v) at MP2/6-3 lG(d,p) (kT mol-') 51.7 83.4 83.8 120.3 178.2
J. Phys. Chem., Vol. 99, No. 16, I995 5947
Intermolecular Proton-Transfer Processes TABLE 2: Relative Energies in kJ mol-' complex
+
H3O+ NH3 H3NH+* *OH2( 5 )
NH4+
+ H20
d(0-H)
1
HF/3-21G
HF/6-3 1G(d,p)
MP2/6-3 1G(d,p)
0.W-d -259.3" -280.0b -283.9' -290.0d -146.3" -149.0b - 148.9' - 148.0d
0.0"-d -224.5" -245.9' -249.9' -256.2d - 160.7" -165.6' -166.2' -166.1d
O.O"-d -237.3" -259.2' -263.3' -269.7d -161.8" - 167.5' -168.1' -168.4d-
1.5
OVdThelabels a, b, c, and d mean that talculation refers to points with d(N-0) distances equal to 5.0,8.0, 10 A and completely separated, respectively.
TABLE 3: Relative Enthalpies at 298 K (kJ mo1-l)" complex
+ +
H30' NH3 H3NH+*.OH;! ( 5 ) NH4+ H20
HF/ 3-21G
HF/ 6-31G(d,p)
MP2/ 6-31G(d,p)
exp
0.0 -279 - 143
0.0 -245 -161
0.0 -259 -164
0.0 -250b -161'
a Calculated for separated reactants and products, taking differences in Ezpvplus rotational and translational components of the enthalpy into account. From the experimental valuelo of the binding enthalpy of 5 relative to the components NH4+ and H2O. 'From proton affinity difference APA = PA(H2O) - PA(NH3); values from ref 14.
nonlinear N-H-0 geometry. This is important with respect to our further investigations, because this includes the possibility of rotations of the H3N and OH2 subunits relative to their positions in a forced linear complex. The calculations at the MP2/6-3 lG(d,p) and HF/6-3lG(d,p) levels show that at short d(0-H) distances the corresponding most stable complex (H3N =H-OHz)+with C, symmetry has a geometry as shown in structure I of Figure 2, except for very small NO distances d(N-0) 5 2.2 8, (see below). The angle LHON is close to zero with deviations of less than 2" for NO distances 2.4 8, 5 d(N-0) I3.4 A, so that the complex is almost linear, with the two H atoms of OH2 in the antiposition to the two out-of-mirror-plane H atoms of H3N. At long NO distances 5.0 5 d(N-0) 5 10.0 A, however, the optimized (H3N *H-OH2)+complex has a slightly nonlinear structure with 5.0" 5 LHON I 7.0". At the lowest HF/3-21G level the complex (H3N.**H-OH2)+is linear with a planar HOH2+ geometry at distances d(N-0) 2 2.4 A. Increasing d(0-H) at a fixed NO distance corresponds to transfer of the proton from OH2 to NH3. The structural changes in bond length and angles connected with this proton transfer correspond essentially to those described in detail in the literature" and will therefore not be discussed further here. After the proton transfer, the resulting optimized complex (H3NH* OH2)+ with optimized OH and NH distances d(0-H) and d(N-H), respectively, has the linear structure I1 (Figure 2) at all three levels of theory and for all NO distances investigated except for the shortest with d(N-0) I2.4 8,. If d(0-H) is increased further after passing through this energy minimum to values with d(0-H) > d(N-0) - d(NH), geometry optimization forces the transferred proton to move out of the NO line. This leads to two possible structures I11 and IV (Figure 2), which exist in separate energy minima. The formation of 111 and IV corresponds to a left-hand and righthand rotation, respectively, of H3N-H+ relative to its positon in the (H3N-H. OH$ complex 6. Which of the structures 111 and IV is most stable, depends on the values of the fixed distances d(N-0) and d(0-H). At the MP2/6-31G(d,p) level IV is more stable than I11 at NO distances d(N-0) 1 2.6 8,
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Figure 3. MP2/6-31G(d,p) two-dimensional representation of the potential energy function of the reaction H30+ NH3 H20 N b + . The energy surface was obtained by interpolation through geometry optimized points calculated at fixed values for d(N-0) and d(0-H). The grid in d(N-0) and d(0-H) consists of a total of 112 points as explained in the text. For selected energy values consult Table 4.
+
+
for each d(0-H) > d(N-0) - d(N-H) within the range investigated. This holds also at d(N-0) = 2.4 8, for OH distances with 2.0 8, < d(0-H) < 2.4 A, whereas for 1.4 A < d(0-H) < 2.0 8, structure 111 is more stable. At the shortest distance d(N-0) = 2.2 8, the MP2/6-31G(d,p) optimization results always in a nonlinear structure of the (H3N *E .OHz)+ complex, the structure I11 being more stable than IV for all OH distances 0.9 8, < d(0-H) < 2.4 8, investigated. The geometry optimized complex structure 6 corresponding to the energy minimum at d(N-0) = 2.2 8, is shown in Figure 2 for the three levels of theory. At MP2/6-31G(d,p) structure 6 corresponds to I11 with d(0-H) = 1.894 A, which is more stable by 9.3 kJ mol-' than the minimim energy structure IV with d(0-H) = 1.893 A. In contrast, the HF/6-31G(d,p) minimum energy structure of the (H3N *W .OH# complex at d(N-0) = 2.2 8, is still almost linear with an optimized d(0-H) = 1.172 8, and LHON = 0.1". The HF/3-21G structure is nonlinear like the MP2/6-31G(d,p) structure and corresponds also to structure 111, but with a smaller angle LHON = 19.4" (compared with LHON = 27.4'). Because structure IV is more stable than I11 except for the shortest NO distances d(N-0) < 2.4 8,, it has been chosen to represent the whole area of the MP2/6-3lG(d,p) potential surface in Figure 3 with d(0-H) > d(N-0) - d(N-H) at d(N-0) 2 2.4 A and for all investigated OH distances 0.9 8, < d(0-H) < 2.4 8, at d(N-0) = 2.2 8,. In this way the continuity of changes in structure and potential energy of the (H3N---H--.OH2)+ complex with changing parameters d(N-0) and d(0-H) is warranted. Consequently, the surface in Figure 3 always represents the most stable (H3N *He OH$ complex with Cs symmetry with exception only of the smallest NO distances d(N-0) I2.4 A.
5948 J. Phys. Chem., Vol. 99, No. 16, 1995
Bueker and Uggerud
TABLE 4: Relative Energies (kl mol-') of Complexes and Activation Energies for Proton Transfer at Different NO Distances from MP2/6-31G(d,p), HF/6-31G(d,p), and HF/3-21G Calculations (H3N. * .H-OH*)+ (H3N * * H GH#+ (H3N-He * *OH2)+ AE#"
d(N-0), 8, 5.0 3.4 3.2 3.0 2.9
MP2/6-3 lG(d,p) HF/6-3 lG(d,p) HF13-21G
MP2/6-3 lG(d,p) HF/6-31G(d,p) HF/3-21G
0 0 0 -79.1 -69.5 -88.1 -99.2 -86.8 -112.5 -122.8 -106.3 - 142.2 -136.3 -116.6 - 159.3
+3.1 +53.5 f7.2 -54.8 -9.3 -64.1 -108.8 -69.6 -129.5 -133.0 -97.1 -157.6
-127.2
-121.3
2.8 2.6 2.4 2.2
a
MP2/6-3 lG(d,p) HF/6-3 lG(d,p) HF/3-21G -161.8 - 160.7 -146.3 -206.9 -201.9 -206.3 -217.0 -210.7 -221.5 -227.3 -219.1 -237.1 -23 1.9 -222.3 -244.7 -235.4 -224.3 -251.6 -236.3 -220.9 -259.3 -218.4 -197.1 -245.8 -178.1 -127.6 -187.2
MP2/6-3lG(d,p) HF/6-3 lG(d,p) HF/3-2 1G
82.2 123.0 95.3 44.4 77.5 48.4 14.0 36.7 12.7 3.6 19.5 1.7 5.9
Classical (non-zero-point) activation barrier heights relative to the reactant complex at fixed d(N0).
Table 4 shows the relative energies of the critical points of the MP2/6-3 lG(d,p) as well as of the HF/6-3 lG(d,p) and HF/ 3-21G potential surface for various fixed d(N-0) distances. The classical (non-zero-point) activation barrier heights for proton transfer relative to the reactant complex (H3N. * .HOH2)+ at fixed d(N-0) are also listed. It can be seen that the relative energies of the reactant and product complexes and also of the activated complexes depend on the level of calculation. For distances d(N-0) 5 3.2 A, the relative energies calculated at the HF/6-3 lG(d,p) level are somewhat higher than that of the MP2/6-31G(d,p) level, whereas the HF/3-21G values are distinctly lower. For the reactant and product complexes, the relative energies in Table 4 result from the ion-molecule attraction potential energy. Consequently, the ion-molecule attraction resulting from HF/6-31G(d,p) calculations is lower than that from the MP2/6-3 lG(d,p) calculations, whereas that at the HF/3-21G level is higher. It is of interest to note that in several studiesI2 the ion-molecule attraction potential energy calculated by ab initio methods has been found to be lower at short and medium distances, corresponding to a stronger ion-molecule attraction, than the long-range ion-dipole/ion-induced dipole potential employed by the ADO theory.I3 Table 4 also shows that the relative energies of the transition state complexes (H3N. * -Ha OH2)#+ and the corresponding activation barrier heights AE# for proton transfer relative to the reactant complex (HsN. *H-OH2)+depend very strongly on the donor-acceptor distance d(N-0) and also on the level of calculation. The dependence on the donor-acceptor distance has already been observed and discussed in the literature5 so that further discussion will be kept short here. At the MP2/6-31G(d,p) level, the barrier drops from A?,? = 82.2 kJ mol-' at d(N-0) = 3.4 A to 3.6 kJ mol-' at 2.9 A and disappears completely at 2.8 A. Similar results are obtained at
the HF/3-2!G level. In contrast the activation barriers calculated at the HF/6-31G(d,p) level are substantially higher. At d(N0)= 2.8 A, where the barriers for the other two levels disappear, HF/6-31G(d,p) gives a barrier of 5.9 kJ mol-'. Even at the equilibrium distance d(N-0) = 2.753 A of the global minimum energy complex N b + . OH2 there exists a stable reactant complex (H3N- *H-OH2)+, which is separated from the global minimum by a small activation barrier hE# = 1.6 kJ mol-'. The higher activation barriers at the HF/6-3 lG(d,p) level mean that stretching of the OH bond in the reactant complex at fixed d(N-0) in order to reach the transition state for proton transfer requires more energy than for the MP2/6-31G(d,p) and HF/321G levels. This finding is in line with the HF/6-31G(d,p) geometries found for the reactant and product complexes. The equilibrium bond distance d(0-H) in a reactant-like complex, or d(N-H) in a product-like complext at fixed d(N-0) are significantly smaller than those calculated by MP2/6-3 lG(d,p) and HF/3-21G. The latter two methods give similar values. To test the quality of the ab initio results, relative enthalpies AH at 298 K of the minimum energy complex N&+.*.OHz ( 5 ) and of the products relative to the reactants were calculated at the the different ab initio levels and compared with the available experimental data. It can be seen from Table 3 that the relative enthalpies of 5 calculated at the MP2/6-31G(d,p) and HF/6-3 lG(d,p) levels agree reasonably with the value obtained from the experimental binding enthalpy of 5 , whereas the HF/3-21G value is too low by 29 M mol-'. The MP2/631G(d,p) and HF/6-31G(d,p) enthalpy of the products relative to the reactants, which corresponds to the proton affinity difference APA of H20 and NH3, is also in a good agreement with the experimental APA,I4 whereas the HF/3-21G value is too low by 18 kJ mol-'.
J. Phys. Chem., Vol. 99, No. 16, 1995 5949
Intermolecular Proton-Transfer Processes Conclusion The findings reported above show that all three ab initio methods give reasonable potential energy surfaces with grossly similar characteristics. For this reason we would expect that MP2/6-31G(d,p), HF/6-31G(d,p), and HF/3-21G provide wave functions useful for further direct dynamics calculations. If we assume that MP2/6-3 lG(d,p) is the most realistic description, discrepancies are noticed for the two other methods. HF/631G(d,p) gives correct relative energies for the intermediate and the products, but too large barriers for proton transfer at a given N - 0 distance. HF/3-21G gives slightly incorrect relative energies but correct barriers. Acknowledgment. This study is a part of the EU project “Fundamental studies in gas-phase ion chemistry and mass spectrometry” (Contract CHRX-CT93-0291 (DG 12 COMA)). The key role of Dr. Steen Hammerum, University of Copenhagen during its establishment is gratefully acknowledged. References and Notes ( 1 ) (a) Proton-Transfer Reactions; Caldin, E., Gold, V., Eds.; Chapman and Hall: London, 1975. (b) Uggerud, E. Mass Spectrom. Rev. 1992, 11,
389. ( 2 ) (a) Zwier, T. S.; Bierbaum, V. M.; Ellison, G. B.; Leone, S. R. J. Chem. Phys. 1980, 72, 5426. (b) Weisshaar, J. C.; Zwier, T. S.; Leone, S. R. J. Chem. Phys. 1981, 75,4873.(c) Bowen, R. D.; Harrison, A. G. Org. Mass Spectrom. 1981, 16, 159. ( 3 ) (a) Wang, I. S. Y; Karplus, M. J. Am. Chem. SOC.1973, 95, 8160. (b) Leforestier, C. J. Chem. Phys. 1978,68,4406. (c) Helgaker, T.; Uggerud,
E.; Jensen, H. J. Aa. Chem. Phys Lett. 1990, 173, 145. (d) Uggerud, E.; Helgaker, T. J. Am. Chem. SOC.1992, 114, 4265. (e) Chen, W.; Hase, W. L.; Schlegel; H. B. Chem. Phys. Lett. 1994, 228, 436. ( 4 ) (a) Fameth, W. E.; Brauman, J. I. J. Am. Chem. SOC.1976, 98, 7891. (b) Olmstead, W. N.; Brauman, J. I. J . Am. Chem. SOC. 1977, 99, 4219. (c) Jasinski, J. M.; Brauman, J. I. J. Am. Chem. SOC.1980,102,2906. ( 5 ) (a) Scheiner, S. Acc. Chem. Res. 1985, 18, 174. (b) Jaroszewsky, L.; Lesyng, B.; McCammon, J. A. J. Mol. Struct. (THEOCHEM) 1993, 283, 57. (c) Jaroszewsky, L.; Lesyng, B.; Tanner, J. J.; McCammon, J. A. Chem. Phys. Lett. 1990, 175, 282. (6) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley,J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees,D. J.; BakerJ.; Stewart, J. J. P.; Pople, J. A. GAUSSIAN 92; GAUSSIAN Inc.: Pittsburgh, PA, 1992. Further reference to basis sets and quantum chemical procedures are given in the GAUSSIAN 92 User’s Guide. (7) Moller, C.; Plesset, M. S. Phys. Rev. 1934, 4 6 , 618. ( 8 ) (a)CRC Handbook of Chemistry and Physics, 73rd ed.; CRC Press: Boca Raton, FL, 1992. (b) Tables of Interatomic Distances and Configurations of Molecules and Ions; Special Publication No. 18; Royal Chemical Society: London, 1958, 1965. (9) Del Bene, J. E. J. Phys. Chem. 1988, 92, 2874. (10) Meot-Ner (Mautner), M. J. Am. Chem. SOC.1984, 106, 1265. (1 1 ) Roszak, S.; Kaldor, U.; Chapman, D. A,; Kaufman, J. J. J. Phys. Chem. 1992, 96, 2123. (12) (a) Vande Linde, S. C.; Hase,W. L. J. Phys. Chem. 1990,94,2778. (b) Vande Linde, S. C.; W. L. Hase,W. L. J. Chem. Phys. 1990, 93, 7962. (c) Lim, K. F.; Brauman, J. I. J. Chem. Phys. 1991, 94, 7164. (13) Su,T.; Bowers, M. T. Int. J. Mass Spectrom. lon Phys. 1973, 12, 347. (14) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. Gas-Phase Ion and Neutral Thermochemistry. J. Phys. Chem. Re& Data 1988, 17 (Supplement Nr. 1).
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