The Journal of Physical Chemistry, Vol. 85,No. 19, 1981 2717
Letters
Acknowledgment. The authors thank Dr. R. A. Bogomolni and Professor W. Stoeckenius for the S-9 mutant strain and some samples. We are also grateful to Professor P. D. Boyer and his group for the use of their facilities to prepare the sample, and to Professor M. W. Berns for
lending us the microscope objectives. Support provided by the Department of Energy (Office of Basic Energy Sciences), National Science Foundation, the University Research Committee, and Spectra-Physics, Inc. is gratefully acknowledged.
ARTICLES An Analysis of the Hard-Soft Lewis Acid-Base Concept and the Drago Equation Employing ab Initio Molecular Orbital Theory John Douglast and Peter Kollman” Depattment of PharmaceuticalChemistry, School of Pharmacy, Universi& of California, San Francisco, California 94 143 (Recelved: April 7, 1980; In Final Form: March 19, 1981)
We present ab initio SCF calculations employing the Morokuma component analysis on a number of Lewis acid-base interactions. Examples of hard and soft acids and bases have been studied in order to analyze the Pearson idea that hard acids interact preferentially with hard bases and that soft acids interact preferentially with soft bases. We conclude, on the basis of the noncovalent interactions studied here, that the empirical preference of soft acids for soft bases maybe largely a solvent effect rather than an intrinsic property of the direct interactions themselves. We also applied our calculations to a number of systems previously analyzed by the Drago equation, in which each molecule’s hardness and softness is taken into account in estimating experimental interaction enthalpies. Here the parallel between the predictions of the Drago equation and the calculations is reasonable for closely related complexes, but not so satisfactory when rather different complexes are compared. Unfortunately, it is not certain at this point whether deficiencies in the theory (431-G ab initio SCF), deficiences in the Drago equation, or solvation effects are the reason for the poor agreement.
Introduction The chemical literature contains a vast amount of experimental data on the stabilities of complexes formed by the association of electron donors and acceptors. In the past 30 years there have been a number of efforts to systematize and to understand this body of data from an empirical and a theoretical viewpoint. In 1958, Ahrland, Chatt, and Daviesl noted that electron acceptors fall into two classes: those that form the strongest complex with electron donors from the first row of the periodic table and those that form the strongest complex with second row donors. This idea was extended by Pearson2to the principle of hard and soft Lewis acids and bases. This principle states that hard acids bind more strongly to hard bases and that soft acids bind more strongly to soft bases. The original classification was based primarily on the Ahrland, Chatt, and Davies empirical criterion, although subsidiary criteria were also introduced. It is found that soft bases are those in which the donor is highly polarizable, and has low electronegativity, while hard bases have the opposite properties. Soft acids have properties similar to those of soft bases, i.e., high polarizability, low positive charge, and low electronegativity. Hard acids are the opposite. Important and inherent in the hard-soft principle is the fact what when the strength of a series of Lewis bases is ranked, reversals in strength may occur in the order, depending t Eastern
Washington University, Cheney, WA 99004. 0022-3654/81/2085-27 17$01.25/0
upon the Lewis acid which is being used. drag^,^ recognizing this interdependence of donor and acceptor properties, in 1965 proposed a four-parameter equation to represent the enthalpy of interaction of donor-acceptor pairs in poorly solvating media: -AH = EAEB C A C B (1) Here A and B represent the electron acceptor and donor, respectively, while E (“electrostatic”) and C (“covalent”) are empirically determined parameters for each. The Drago relationship provides a useful means of predicting enthalpies for the formation of complexes for which experimental values are not available. It relates to the hard-soft principle in that, for there to be a strong donor-acceptor interaction, the two molecules must “match” in the sense that both must have a large E parameter or a large C. It goes beyond the hard-soft principle in that the E and C parameters are not exclusive so that both the E and C parameters for a given molecule might be either large or small. The theoretical basis of the Drago equation has been of some interest. Klopman4 has shown that eq 1is consistent
+
(1)S. Arhland, J. Chatt, and N. Davies, Quart. R., Chem. Soc., 12, 265 (1958). (2)R.G.Pearson, J. Am. Chem. SOC.,85,3533(1963). See also R. G. Pearson, J. Chem. Educ., 45,481,643 (1968). (3)R.S.Drago and B. B. Wayland, J.Am. Chem. SOC.,87,3571(1965). For a more recent review and a table of parameters for 33 acids and 48 bases, see R. S. Drago, Struct. Bonding, 15,73 (1973).
0 198 1 American Chemical Society
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The Journal of Physical Chemistty, Vol. 85, No. 19, 1981
Douglas and Kollman
TABLE I: Interaction Energies of H’, Li+,and K+ with NH,,H,O, and H,S 431-GComponent Lewis Lewis base A EESe AEPOL? A EEXg AECTh acid ~~
~
H+ Li+
K‘
NH I b
H+
H,O‘ H,OC H,Ob H,Sd H,Sd H,Sd
Li+
K+ H+ Li‘
K+
- 110.1 - 56.9 - 26.7 - 85.6 - 51.1 - 26.2 - 34.3 - 18.7
NHeU NHI’”
-10.0
- 23.4
-7.0 - 1.9 - 27.3 - 7.8 - 2.1 -13.8 -4.3 - 2.5
15.1 5.6 - 12.7
5.7 4.2 1.5
AETOT’
%ESj
- 79.4 - 1.9
- 212.9 - 50.7
-1.2
-24.2 -163.4 -47.9 - 23.3 -137.9 - 26.0 - 11.8
52 86 90 53 84 90 25 62 75
- 50.6 - 1.7 -0.7 - 89.7 - 7.2 -0.8
Reference 16. Reference 17. Reference 7. This work R(H+...S)= 1.69A , e = 60”;R(Li+...S)= 2.49 A ;e = 60”; R(K+...S)= 3.30 A , e = 30”. e refers to the angle between the C,, axis of H,S, and the M+...S line. H,S is kept at its experPolarization energy. g Exchange repulsion. Charge transfer. imental geometry. e Electrostatic interaction energy. Total interaction energy. Percent of total attraction as electrostatic, i.e., [aEEs/(aEEs+ A E ~ o Lt AEcT)] X 100.
with his perturbation theory approach to intermolecular interactions with the E parameters corresponding to a charge-controlled contribution and the C parameters corresponding to frontier-controlled, or covalent, binding. Marks and Drago6have shown that by starting with the Mulliken charge-transfer formulation6an equation of the form of eq 1 can be obtained by introducing certain approximations. This formulation gives some support to relating the E parameters to ionic interactions and the C parameters to covalent. In the work reported here we attempt to understand these empirical relationships and reversals in donor strengths through the application of ab initio molecular orbital theory. Ab initio theory and component analyses have been successful in explaining the gas-phase interactions of a variety of species to form intermolecular complexes including hydrogen-bonded molecules, “chargetransfer” complexes, ion-molecule associations, and others. The Morokuma component analysis8 breaks up the total SCF interaction energy into components-electrostatic, polarization, charge transfer, and exchange repulsionwhich can be directly related to those which come from perturbation theory expansions for these interaction^.^ In particular, this analysis allows us to assess how much hard-hard interactions are electrostatic dominated and soft-soft are polarization and charge-transfer dominated. Is there a preference of hard bases for hard (or soft for soft) acids in the gas phase also? The Morokuma analysis also allows us to assess the validity of the Drago division into electrostatic and covalent parameters; these terms should correspond to the Morokuma electrostatic component and to the Morokuma polarization charge transfer terms. Solvation is a problem we will face in comparing ab initio results with experimental data in solution, even for the “inert” solvents upon which the Drago parameters are based. There is some evidence to suggest that this correction may be of the order of 1 or 2 kcal/mol for a given system although the correction may be less when comparing similar complexes. Grundes and Christian’O studied the association of trimethylamine and sulfur dioxide in the gas phase and in heptane. In the gas phase the association enthalpy is -9.7 kcal/mol while in heptane it is -11.0 (see ref 11). Drago and co-workers12 have obtained solvent
+
(4)G.Klopman, J . Am. Chem. SOC.,90,223 (1968). (5) A. P.Marks and R. S. Drago, J. Am. Chem. SOC.,97,3324(1975). (6)R. S. Mulliken and W. B. Person, “Molecular Complexes”, WileyIntersciences, New York, 1969. (7)P.Kollman, J. Am. Chem. SOC.,99,4875 (1977). (8)K. Morokuma, J. Chem. Phys., 55, 1236 (1971). (9)J. C.G.M. van Duijneveldt-van der Rijdt and F. B. van Duijneveldt, J. Am. Chem. SOC.,93,5644 (1971). (10)J. Grundnea and S. D. Christian, J. Am. Chem. SOC.,90,2239 (1968).
transfer energies between CHZCIzand CCll and other such solvent pairs. Generally these run 2 kcal/mol or less.
Methods of Procedure We used the program GAUSSIAN 70 in these calculations.13 The calculations employed a 431G basis,14 with the exception of those involving K+, where we used a double zeta basis described previously, and Li+, where we used the 5-21G15basis. The Morokuma component analysis used was that previously described,8 in which the total SCF energy was broken down into electrostatic, polarization, charge-transfer, and exchange components.
Results and Discussion The Hard-Soft Concept and the Lewis Acidity of H+, Li+, and K+. We report here the affinities of H+, Li+, and K+ with HzS and use previous studies by Kollman7J6J7on the interaction of these Lewis acids with H20 and NH3. In Table I we summarize these results, showing both total energies and the electrostatic energy as percent of total attractive terms (excluding exchange repulsion). In the series of Lewis acids, H+,Li’, K+, all are expected to be hard, but with increasing tendency to softness because of increasing size. The hard-soft acid-base principle suggests that an increased interaction with HzS, relative to NH, and HzO, might be observed as one proceeds along this series of acids. The data in Table I fail to provide any support for this supposition. As expected, H,S interacts more weakly and with a smaller percentage contribution from electrostatic energy than does NH3 and HzO. In addition, the percent electrostatic is largest for all three bases with the softest Lewis acid (K+). No unique effect can be seen for the K+-H2S interaction. It may be that there is simply not enough difference in the hardness of this series of ions. It would be most interesting to calculate the interaction energies of a truly soft cation such as Ag+ with this series of bases. We hope that in time pseudopotential methods will allow this calculation to be done. The “lone pair” directionalities (amount of off Czvaxis (11)According to R. Drago (personal communication), the gas-phase enthalpy contains very large uncertainties that could make this similarity in gas phase and hexane AE fortuitous. (12)R. S. Drago, M. S. Nozari, and G. C. Vogel, J. Am. Chem. SOC., 94,90(1972);R. S.Drago, J. A. Nusz, and R. C. Courtright, ibid.,96,2082 (1974). (13)W.Hehre, W.A. Lathan, R. Ditchfield, M. D. Newton, and J. A. Pople, Quantum Chemistry Program Exchange, No. 236. (14)R. Ditchfield, W.J. Hehre, and J. A. Pople, J. Chem. Phys., 54, 7241 (1971). (15)J. D. Dill and J. A. Pople, J. Chem. Phys., 62,2921 (1975). (16)P.A. Kollman and S. Rothenberg, J.Am. Chem. SOC.,99,1333 (1977). (17)P. Kollman, Chem. Phys. Lett., 55,555 (1978).
Analysis of the Hard-Soft Lewis Acid-Base Concept
The Journal of Physical Chemistry, Vol. 85,
No.
19, 1981
2719
TABLE 11: Ab Initio Energies for BH, Complexesa
to
energy to form complex net from distorted monomers
~
total A E % electrostatic
BH,-CO~ BH,-NH,‘ BHi(tetrahedra1 PH.(106.7”Ld R =’2.10 BH,(115”)PH,(101”), R = 2.18e
-28.5 -44.7 - 35.2
47.8 70.6 57.0
~
monomers - 10.8 - 27.7 - 3.6
0 NHrHNCO
4
~
3l
- 22.9
57.7
-9.95
a Energies in kcal/mol. Reference 18. Monomers described as “pyrimidal” but angle not specified, see ref Kept monomers fixed at angles identical with those 18. for BH,-NH, complex. e Optimized e(HBH), e(HPH), and R(P-B) with the constraint of C,, symmetry. Component analysis done at R = 2.10 A for most direct comparison with fixed geometry calculations in row above.
approach of the electrophile) for H2S are significantly greater than those for H20, which is consistent with our previous analysis of (HF), vs. (HCl)2.7 The Interaction of BH3 and NH,, CO and PH,. BH, and its derivatives are interesting Lewis acids in many respects. BH, is generally considered an “intermediate” species ( C / E < 0.1 for a hard acid) but the E and C parameters for B(CH3I3are both large, being 6.14 and 1.70, respectively. BH3 is planar, but becomes pyrimidal in its complexes, so that a reorganization energy enters into the net energy of complex formation. Umeyama and Morokurnal8 (UM) have reported component analyses for BH3 with the hard base NH3 and the soft base CO. We investigated the system BH3-PH, and show our and the UM results in Table 11. We first optimized the P-B distance, assuming fixed monomer geometries of B(HPH) = 106.7’ (as found in NH,) and B(HBH) = 109.47’. Then we optimized the geometry to maximize the net interaction energy to form the complex from the monomers in their experimental geometry. First the monomer angles were optimized by using the fixed P-B distance of 2.10 A,then with the newly optimized B(HPH) = 101’ and B(HBH) = 115’ the P-B distance was reoptimized. Since reoptimization changed the P-B distance by only 0.1 A,it is unlikely that further optimization of monomer angles would result in a significant change. It can be seen that optimizing the net interaction energy results in a weaker complex from the viewpoint of the distorted monomers, but a smaller reorganization energy of the monomer is required. Although the Drago parameters are not available for PH3 and BH,, we may get some insight into their properties by using the parameters for the completely methylated bases. For the P(CH,),-B(CH,), interaction the total Drago energy is 16.29 kcal (68.4% EAEB) while for the comparable N(CH3), interaction the Drago energy is 20.38 kcal (77.7% E A E B ) . The ratio of the Drago energies for these system is very close to the ab initio calculations (using BH3(109.5”)-PH3(106.7’) for comparison) for the distorted monomers. When reorganization energy is added, the ratios become quite different. Fisher and Dragolg have pointed out that this “reorganization” energy could most reasonably be incorporated into the two-parameter eq 1 by noting that the (18) H. Umeyama and K. Morokuma, J. Am. Chem. SOC.,98, 7208 (1976). (19) K. Fisher and R. Drago, Inorg. Chem., 14, 2804 (1975).
~
~
$
$
2
’-
“
~
NHsSO2 ,@MMA.SO~ DMA-SO2
oMe~S-HNCO OMePHrSO2
TMA.SO2
0 Me2SSOZ 1
1
I
1
I
I
I
I
I
I
2720
The Journal of Physical Chemistty, Vol. 85, No. 19, 1981
Douglas and Kollman
TABLE HI: Drago Terms and 431G Components for a Number of Complexesa ~
EAEBb 4.38 1.25
R
complex
NH,-HNCO' ",-SO,'
2'63 2.45 2.45 2.45 2'45 2.36
(cH,)NH,-so,I: (CH,),NH-SO,' (CH,),N-SO,'
-A&$
19.13
tt:::
1.20 1.00 0.74
(CH,),S-SOZh (CH, ),S-HNCO~ (CH3)PH,-SOZm
33.09 32.52
:;:;;
0.32 1.10 0.77
- AECT t
- A EESd+
4.21 5.21 4.49
A E E ~ 6.69 3.47 - 2.29 - 2.64 - 3.45 -4'20 - 9.51 1.06 2.68 1.25
AH^
aEpoL 5.51 8.24 13.53 16.30 17.79
CACB~ 0.89 2'80 4.75 7.05 9.32 6.03 1.92 5.29
A E T ~ T ~ 12.1
5.27 4.05 5.95 8.05 10.06 6.35 3.02 6.06
1;2:
2.75 3.40 2.93
1':;
13.67 14.34 3.80 6.07 4.18
'
a Energies in kcal/mol. Product of Drago electrostatic parameters. Morokuma electrostatic energy. Morokuma electrostatic and exchange energies. e Drago covalent term. Morokuma charge-transferand polarization energy. g Drago equation A H . Total ab initio interaction energy. R(N-.H) = 1.88 A ; HNCO experimental geometry (J. M. Schoenberg, R. Shulman, and D. Yost, J. Chem. Phys., 1 8 990 (1950)); cis and trans HNCO (relative to N-H bond) gave essentially identical results. See ref 20. Optimized geometry for H,S-SO, and used this geometry for the methyl substituent. R(S-.S) = 3.42 A ; p(ti1t of SO, from S-S axis) = 90" and 01 (tilt of H,S from S-S axis) = 70". For (CH,),S used geometry from "Table of Interatomic Distances and Angles". For H,S...SO, we found the complexation energy of the cis geometry (H cis to 0)= -4.07 kcal/mol, for the trans A E = - 2.59 kcal/mol. Because of steric effects, we only studied the (CH,),S-.SO, complex in a trans geometry, where A E = -4.18 kcal/mol. R(S...H) = 2.59 A , 01 (tilt of H,S off S...H axis)= 70". R(P...S) = 3.51 A , p (tilt of SO, off S.-P axis) = 75", (CH,),PH geometry used PH, geometry with tetrahedral CH, added along P-H bond direction R(P-C) = 1.78 A . J
TABLE IV: Drago Terms and 431G Components as Percent of Total Interaction Energy
R
complex
NH,-HNCO ";-SO,
%EAEB 83.1 30.9 20.2 12.4 7.4
2.63' 2.45 2.45' 2.45' 2.45' 2.36'
(CH, )NH,-SO, (CH; ),NH-SO; (CH,),NH-SO, ( CH 3 )zs-soz
5.0 36.4 12. 7 d
(CH,),S-HNCO (CH,)PH,-SO, H,S-H,S a
As percent of attractive term.
I
I
2
4
-
I
6
I
8
I
I
As percent of total E .
I
I
10 12 14 16 -fAEpot t AEcT)
%AEE t EE$ 54.5 29.6 - 20.4 - 19.3 - 24.1 - 28.5 - 63.9 27.8 44.1 29.9 25.2
%AEES~ 77.6 72.9 71.2 67.0 64.6 62.7 62.1 60.5 60.5 60.5 50.0
I
I
I
I
18
20
22
I
2425
Figure 2. Comparison of Drago's C,Cs with the quantum mechanically A€c T calculated polarization plus charge transfer energies, A€= (kcaVmol).
+
energies. Here, the trend between ammonia and the amines with HNCO and SOz is generally reproduced, although the CACB term is smaller than in the 431G calculations. However, for the sulfide and phosphine bases the CACB term is much greater than that given by the 431-G method. Figure 3 is a comparison of total energies calculated from eq 1 vs. the ab initio results. One striking feature of this plot is that for the case of methyl sulfide and methylphosphine with SOz the CAC, product is much higher that the ab initio result. As another means of comparing the Drago parameters with the ab initio results we have computed the percentage contribution to the total energy from the terms linked with electrostatic effects from the two approaches, and likewise
% AEPOL % CACB
16.9 69.1
' Minimum energy distance.
0
2
+
%A E ~ ~ L A ECTb
22.4 27.1 28.8 33.0 35.4 37.3 37.1 39.5 39.5 39.5 50.0
79.8 87.6 92.6 95.0 63.6 87.3d
Ill
+
A ECTa
45.5 70.4 120.3 119.2 124.1 128.5 163.9 72.2 55.9 70.1 74.8
Parameters are for (CH,),P.
NHjSOz
0 Mo2SHNCO
4
I
I
6
8
,
IO -A ETOT
,
I
12
14
16
I
,
18
20
Figure 3. Comparison of AH (Drago equation) with quantum mechanically calculated interaction energies, A,ElOl (kcal/mol).
those associated with covalent-like effect. The computation was done in two ways. In the first, only the attractive contributions to the energy were considered, omitting the exchange repulsion component. For ammonia and amines there is the same trend in both the EAEB term and the electrostatic energy. However, for the sulfide and phosphine cases the relationships are badly represented. While the contribution of EAEB term ranges from 5.0% (CH3)zS-S0zto 36.4% is (CH3)2S-HNC0,the electrostatic energy contribution is a remarkably constant 60.5%. The expected soft-soft pair, H2S-H2S, expectedly shows the lowest electrostatic contribution of 50.5%. The relationship of the covalent-like terms (CACBand polarization +
Analysis of the Hard-Soft Lewis Acid-Base Concept
charge transfer) is complementary to those of the electrostatic terms. As a second form of analysis we considered the combined electrostatic plus exchange repulsion energies as a fraction of the total energy. In some cases the exchange repulsion is greater than the electrostatic attraction. It can be seen that this approach properly orders the terms for the sulfide and phosphine bases but does not do well for the amines. an advantage of the first method is that the contributions from the ab initio components are insensitive to donoracceptor internuclear distance, while the second method drastic changes occur. The foregoing analysis shows that certainly no close general correlation has been found between the Drago parameter and the ab initio components. However, those systems considered to be hard-hard (e.g., NH,-HNCO) show a greater contribution from the electrostatic term than the soft-soft systems such as the sulfide bases. A discrepancy between the ab initio calculations and eq 1 arises in the underestimation of the polarization and charge-transfer terms in the ab initio calculations. In the case of ammonia and the three methylamines interacting with SOz, the ab initio calculations underestimate the relative contribution of covalent effects, compared to the Drago equation. However, the ratio between the relative contribution of covalent effects compared to the percent of the total Drago equation energy from the CAC, term is quite constant at about 2.5 regardless of the extent of methylation. This may reflect an inadequacy of the 431G basis set to represent electron mobility for elements of the second row of the periodic table. The 431G basis is also known to overestimate electrostatic effects,’@ so that this clearly affects the comparison between the absolute interaction energies predicted by theory and experiment. We expect, however, that the calculations will systematically overestimate electrostatic and underestimate polarization effects, so that the relative energies for species of similar polarity and polarizability will be adequately represented. Clearly, calculations using more extensive basis sets are desirable, but for many of the complexes studied here ((CH3)3N-S0z),are well beyond our computer budget. Another possible deficiency in the theory is our representation of charge-transfer effects, which in the version of the Morokuma component analysis used here contains both charge-transfer and higher order (e.g., second-order exchange) energies. In addition, an alternative method for decomposing the SCF interaction energy has been proposed by Mehler.21 For H-bonding interactions both the Morokuma and the Mehler energy decompositionslead to attractive interactions in the order electrostatic > charge transfer > polarization. One of the most interesting result of this analysis is the predicted differences in the series NH3-.HNC0, NH3.-S02, (CH3)2S.-HNC0,and ( C H J 2 S 4 0 2by the Drago eq 1 and the 431G calculations (Figure 3). Both the Drago equation and the 431G calculations predict that the AI3 (AH) for NH3-HNC0 will be more exothermic than that for NH3-..SO2. However, the ab initio calculations predict the same relative Lewis acidity for Me2Sas the base, whereas the Drago equation predicts reversal by a significant amount: AH(Me2S-.S02) is predicted to be -6 kcal/mol and AH(Me2S-.HNCO) is predicted to be -3 kcal/mol. Thus, we feel these are very important systems to study experimentally in the gas phase and solution, both for the purpose of testing the validity of the Drago equation in the gas phase and for assessing the ability of the ab initio (20) J. Douglas and P. Kollman, J . Am. Chem. SOC.,100,5226 (1975). (21) E. Mehler, J. Am. Chem. SOC.,102, 4051 (1980).
The Journal of Physical Chemistry, Vol. 85, No. 19, 1981 2721
TABLE V : Solvation Energies Estimates’” -AE-
molecule “3
N(CH313 SO, HZS PH3 (CH3)2S SO,-”, S02-N(CH3)3
SO,-H,S (cis) SO,-H,S (trans) SO,-(CH3),S (trans) SO,-PH3
uC
Pb
(solv) A ( A E )
2.0 1.80 2.30 1.24 2.89 0.19 2.79 2.03 2.83 1.78 2.39 0.71 1.04 2.05 0.38 1.83 2.31 0.83 4.69 2.44 4.61 3.17 2.68 5.30 1.55 2.92 0.29 (4.57) (2.92) 2.56 4.82 3.19 2.18 3.71 2.22 3.84
+0.52 +0.34 +3.25 +0.98 +1.82
+0.63
a We used e = 2.1, an average of that of CCl, and cyclohexane. This leads to a reaction field equation2’ for solvation of a dipole of -AESolv = 3.05 hard-soft > soft-soft. Thus we feel, based on the limited number of systems we studied, that the attraction of soft-soft noted in the HSAB concept is largely a solvent effect. However, the Morokuma component analysis does validate the greater percent contribution of polarization and charge transfer to the total energy in the case of softer acids and bases. A comparison of the results of the ab initio calculations with those of the Drago equation shows some agreement. However, there are some areas of disagreement,which may come from deficiencies in the theory (most likely basis set insufficiency) deficiencies in the Drago equation, or solvation effects (the ab initio calculations relate directly to “gas-phase” interactions and the Drago equation t o low dielectric solvents.) In view of the differences predicted (22) G. Schnuelle and D. Beveridge, J.Phys. Chem., 79,2562 (1975). (23) However, dispersion energy would contribute more to the gasphase dimerization energy than to the solution reaction and increase this solvation energy difference. For the complexes we consider here, this contribution can be estimated to be 1-2 kcal/mol using standard van der Waals parameters. We thank Professor R. Drago for pointing this out.
-
2722
J. Phys. Chem. 1981, 85, 2722-2725
by the two approaches, we suggest four complexes for further detailed experimental and theoretical study: (CH3)2S-HNC0, (CH3)2S-S02,H3N-HNCO, and H3N-S02.
Acknowledgment. We thank the National Science Foundation (CHE-76-81718and 80-26560) for support of this research. Very helpful comments have been made by Professor R. Drago.
Competition between Isomerization and Fragmentation of Gaseous Ions. 1. Kinetic and Thermodynamic Control for C4H,+ Ions Tacheng Hsieh, Jerome P. Gilman, Morris J. Welss, and G. G. Meisels" Department of Chemistry, Universlfy of Nebraska, Lincoln, Nebraska 68588 (Recelved: March IO, 198 1; In Final Form: June 22, 198I )
Breakdown graphs of the six C4H8isomers have been determined by threshold photoelectron-coincident photoion (TPE-CPI) mass spectrometry. When internal energy is expressed relative to the most stable isomer structures, the breakdown graphs of the four olefins are identical, indicating that all ions rearrange to the thermodynamically most stable structure before they fragment. However, at higher internal energies, methylcyclopropane and cyclobutane show different behavior consistent with a scheme in which direct fragmentation from a structurally specific transition state begins to compete with isomerization followed by dissociation.
Introduction Structure elucidation of gaseous ions is one of the important tasks in mass spectrometry. It is complicated by the frequent incidence of rearrangement of molecular ions before fragmentation to a structure totally different from that of the neutral precursor. In general, the fragmentation thresholds of odd-electron ions are lower than those of even-electron ions. Therefore, in odd-electron hydrocarbon ions the isomerization barrier is usually below that for fragmentation, often leading to complete randomization between isomeric species.l The above argument, based on the stability of even- or odd-electron ions, should also be valid for unsaturated hydrocarbon ions; however, it is well-known that the double bond migrates readily. The situation is particularly complicated for small unsaturated hydrocarbons. The isomeric C4H8 molecules constitute a unique structurally representative system which can be used to probe the structural behavior of molecular olefin and cyclic ions before and during fragmentation. As precursor ions for intermediate states, the different possible fragmentation paths taken by each isomer can, in principle, be correlated with the structure of the intermediate. The low-energy electron impact spectra of the isotopically labeled C4H8isomers2 suggest that the open-shell C4H8+ions have completely equilibrated to a mixture of interconverting structures before fragmentation although it was recognized that some of the isomers showed dissimilarities in their mass spectra taken at 70 eV. Subsequent studies of C4H8+by metastable ion f~rmation?~ field ionization kinetics,6and charge-ex~hange~~' measurements (1) Karsten Levsen, "Fundamental Aspects of Organic Mass Spectrometry", Verlag Chemie, Weinhein, West Germany, 1978. (2) (a) G. G. Meisels, J. Y. Park, and B. G. Giessner, J. Am. Chem. SOC.,91, 1555 (1969), and references cited therein; (b) ibid., 92, 254 (1970); (c) M. S. H. Lin and A. G. Harrison, Can. J . Chem., 62, 1813 (1974). (3) G. A. Smith and D. H. Williams, J. Chem. SOC.B, 1529 (1970). (4) J. L. Holmes, G. M. Weese, A. S. Blair, and J. K. Terlouw, Org. Mass Spectrom., 12, 424 (1977). (5) R. P. Morgan and P. J. Derrick, Org. Mass Spectrorn., 10, 563 (1975).
all led to similar conclusions. The role of the C4H8+ions as intermediates in unimolecular fragmentation has also been investigated in photoionization: y i r r a d i a t i ~ n , ~ photodissociation,1° and ion-molecule rea~tions?J~-'~ All of the various experiments mentioned above suffer from a drawback in that the precursor ions do not possess a well-specified amount of internal energy but have an energy distribution determined by the ionization process. This is thought to be largely responsible for the differences in the 70-eV mass spectra of C4Hs. A more elegant method, which enables one to observe fragmentation as a function of energy deposition, is the technique of threshold photoelectron-coincident photoion (TPE-CPI) mass spectrometry. Since photoions are detected in coincidence with threshold photoelectrons, the internal energy of the ion is uniquely determined by the photon energy. Although Baer et applied this technique to C4H8+ions, they only addressed the threshold for the dissociation process to C3H6+and CH3 and the unimolecular dissociation rate leading to the formation of C3H6+fragment ion. They find that the butenes and methylcyclopropane are not distinguishable; cyclobutane was not included in their investigation. (6) (a) T. 0. Tiernan and L. P. Hills, presented at the 19th Annual Conference on Mass Spectrometry and Allied Topics, Atlanta, GA, May 2-7, 1971; (b) C. Lifshitz and T. 0. Tiernan, J. Chem. Phys., 66,3555 (1971). (7) J. Sunner, Int. J.Mass Spectrom. Ion Phys., 32,285 (1980). (8) (a) L. W. Sieck, S. K. Searles, and P. Ausloos, J. Am. Chern. SOC., 91,7627 (1969); (b) L. W. Sieck, S. G. Lias, L. Hellner, and P. Ausloos, J. Res. Natl. Bur. Stand., Sect. A, 76, 115 (1972). (9) S. G. Lias and P. Ausloos, J. Res. Natl. Bur. Stand., Sect. A, 75, 591 (1971). (10) (a) J. M. Kramer and R. C. Dunbar, J. Chem. Phys., 59, 3092 (1973); (b) M. Riggin, R. Orth, and R. C. Dunbar, ibid., 66,3365 (1976). (11) (a) 2.Herman, A. Lee, and R. Wolfgang, J.Chem. Phys., 51, 462 (1969); (b) A. Lee, R. L. Leroy, Z. Herman, R. Wolfgang, and J. C. Tully, Chem. Phys. Lett., 12, 569 (1972). (12) T. Huntress, Jr., J. Chern. Phys., 56, 5111 (1972). (13) P. R. LeBreton, A. D. Williamson, and J. L. Beauchamp, J.Chern. Phys., 62, 1623 (1975). (14) W. J. Chesnavlch and M. T. Bowers, J. Am. Chem. SOC.,98,8301 (1976). (15) T. Baer, D. Smith, B. P. Tsai, and A. S.Werner, Ado. Mass Spectrom., A7, 56 (1978). Professor Baer has reinterpreted their data with a single lifetime distribution.
0022-3654/81/2085-2722$01.25/00 1981 American Chemical Society
