Reactions of manganese (1+) with isobutane, neopentane, acetone

J . Phys. Chem. 1990, 94, 3589-3597

3589

Reactions of Mn+ with i-C4Hlo, neo-C,H,,, (CH,),CO, cyclo-C,H,, and cyclo-C,H,O: Bond Energies for MnCH,', MnH, and MnCH, L. S. Sunderlin and P. B. Armentrout*>+ Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (Received: June 12, 1989; In Final Form: October 30, 1989)

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 26, 2015 | http://pubs.acs.org Publication Date: May 1, 1990 | doi: 10.1021/j100372a042

The reactions of Mn+ with isobutane, neopentane, acetone, cyclopropane, and ethylene oxide are studied as a function of translational energy in a guided ion beam tandem mass spectrometer. The electronic state of the Mn+ ions is varied by altering the ionization technique. The a5S and aSD excited states are found to be more reactive than the a7D ground state, although formation of MnCH3 shows less of an effect than the other major reactions. The mechanisms for these processes are discussed and reaction thresholds are used to determine Do(Mn+-CH3) = 2.23 f 0.10 eV, Do(Mn-H) = 1.31 & 0.19 eV, Do(Mn-0) = 3.71 f 0.26 eV, and Do(Mn+-CH2) = 3.06 f 0.13 eV. Only lower and upper limits for Do(Mn-CH3) of 0.55 f 0.17 and I .3 eV can be obtained. These bond strengths are compared to previous determinations and to theoretical calculations. The results for MnH, MnCH2+, MnCH3, and MnCH3+ are found to be consistent with promotion energy-bond strength correlations. Introduction Knowledge of gas-phase metal-ligand bond strengths has expanded rapidly in the past decade through both experimental' and theoreticalz3 means. This information is of importance as a model for condensed-phase organometallic thermochemistry.'~~Comparatively little thermochemistry has been derived for Mn-containing gas-phase species. Of the first-row transition metal-methyl bond strengths, cationic and neutral, the only one remaining to be measured experimentally is Do(Mn-CH,). The Mn-H bond strength is also not well established. These values are of particular importance as a test of models that correlate bond strength to the energy needed to promote the metal atom into a state suitable Because of its stable s2ds ground-state for bonding configuration, Mn has the highest E, of the first-row neutral transition-metal atoms (2.53 eV) other than Zn. Mn is thus expected to have low metal-methyl and metal-hydride bond strengths. In contrast, Mn+(sd5)does not have a high promotion energy (EP= 0.52 eV), consistent with strong Mn+-H and Mn+-CH3bonds.' However, it does have a high second promotion energy (1.72 eV). This makes the Mn+-CH2 double bond another good test of the promotion energy model. Mn-containing species are also a challenge for theoretical computation because of the large number of unpaired electrons. This means that properly accounting for electron correlation is critically i m p ~ r t a n t . ~ . ~ Mn+ has been found to be relatively unreactive.s Studies of the reaction of Mn+ with small hydrocarbons6 and propylamine' show no exothermic reactivity. Mn+ does react exothermically with something as large as 4-octyne,* although the observed reactivity may be due to excited states. The only previous studies that have characterized the electronic state dependence of Mn+ reactions have dealt with the reactions of Mn+ with H2,9ethane,1° and ICI." The first two studies find that the a7S state has very low reactivity at its thermodynamic threshold but reacts more efficiently at much higher energies. In contrast, the aSD and a5S excited states of Mn+ react promptly at their respective thermodynamic thresholds with relatively high efficiency. Thus, these studies determined the thermochemistry of MnH+ and MnCH3+ from excited-state reactions only. These works demonstrate that determining the reactivity of different electronic states is important both for mechanistic understanding of the reactions and for determining the correct thermochemistry. (Ep).19495

Experimental Section A complete description of the apparatus and experimental procedures is given elsewhere.12 Briefly, the apparatus comprises three differentially pumped vacuum chambers. In the first chamber, ions are produced as described below. The resulting

'

N S F Presidential Young Investigator 1984-1989; Alfred P. Sloan Fellow; Camille and Henry Dreyfus Teacher-Scholar, 1988-1993.

0022-3654/90/2094-3589$02.50/0

ions are focused into a magnetic sector momentum analyzer for mass analysis. In the second vacuum chamber, the mass-selected ions are decelerated to a desired kinetic energy and focused into an octopole ion guide that traps ions over the mass range studied. The velocity of the ions parallel to the axis of the guide is unchanged. The octopole passes through a static gas cell into which reactant gases are introduced. Pressures are maintained at a sufficiently low level (CO.1 mTorr) that multiple ion-molecule collisions are improbable. After leaving the octopole, product and unreacted beam ions are focused into a quadrupole mass filter for product mass analysis. Ions are detected with a secondary electron scintillation ion detector, and the signal is processed by pulse-counting techniques. Raw ion intensities are converted to absolute cross sections as described previously.I2 The accuracy of our absolute cross sections is estimated to be f20%. Uncertainties at low cross-section values are generally about *I O-I9 cm2, primarily because of random counting noise. Uncertainties for MnH+ cross sections are larger because of mass overlap with the intense Mn+ reactant signal. The absolute energy as well as the energy distribution of the ions in the interaction region is measured by using the octopole as a retarding field analyzer. The fwhm of the energy distribution is typically 0.6 eV in the laboratory frame. Uncertainties in the absolute energy scale are *0.05 eV lab. Translational energies in the laboratory frame of reference are related to energies in the center-of-mass (CM) frame by ECM = E,,,m/(M + m ) , where M and m are the masses of the incident ion and neutral reactant, respectively. The data obtained in this experiment are broadened by the ion energy spread mentioned above and Doppler broadening, which has a width (eV) in the C M frame of about 0.4E1/2for the reactions discussed here.13 When the model cross sections are compared to experimental data, the calculated cross sections are convoluted with both sources of experimental energy broadening.I2 (Note that the distributions in the ion and neutral kinetic energies are just that, distributions that are well characterized; they are not uncertainties in the energy. Any effects that result from these ( I ) Armentrout, P. B.; Georgiadis, R. Polyhedron 1988, 7, 1573-1581. (2) Schilling, J. B.; Goddard, W. A,; Beauchamp, J. L. J . Am. Chem. SOC. 1987, 109, 5565-5573, 5573-5580. Schilling, J. B.; Goddard, W. A.; Beauchamp, J . L. J . Phys. Chem. 1987, 91, 5616-5623. (3) Pettersson, L. G. M.; Bauschlicher, C. W.; Langhoff, S. R. J . Chem. Phys. 1987.87, 481-492. (4) Carter, E. A.; Goddard, W. A. J . Phys. Chem. 1988,92, 5679-5683. ( 5 ) Armentrout, P. B.; Halle, L. F.; Beauchamp, J . L. J . Am. Chem. SOC. 1981, 103, 6501-6502. (6) Freas, R. B.; Ridge, D. P. J . Am. Chem. SOC.1980, 102,7129-7131. (7) Babinec, S. J.: Allison. J . J . Am. Chem. SOC.1984, 106. 7718-7720. (8) Schulze, C.; Schwarz, H. J . Am. Chem. SOC.1988, 110, 67-70. (9) Elkind, J . L.; Armentrout, P. B. J . Chem. Phys. 1986,84,4862-4871. (10) Georgiadis, R.; Armentrout, P. B. Int. J . Mass Specrrom. fon Processes 1989, 91, 123-133. (11) Strobel, F.; Ridge, D. P. J . Phys. Chem. 1989, 93, 3635-3639. (12) Ervin, K . M.; Armentrout, P. B. J . Chem. Phys. 1985,83, 166-189. (13) Chantry, P. J. J . Chem. Phys. 1971, 55, 2746-2759.

0 1990 American Chemical Society

Sunderlin and Armentrout

3590 The Journal of Physical Chemistry, Vol. 94, No. 9, 1990

ENERGY IeV. Lob)

TABLE I: Electronic States of Mn+ state a's a 3 a5D other states

energy," confie eV 3dS4s 3d54s 3d6

0.000 1.175 1.808d >3.41

SI

0.000

IO. 0

E l pop.c

P0P.b

0.9977 (-15, +5) 0.0019 ( + l l , -4) 0.0004 (+4, -1)

0.7 ( i 0 . 2 ) 0.05 (+4, -2) 0.017 (+22, -9) ..oe

"Values are from: Sugar, J.; Corliss, C . J . Phys. Chem. Ref Dara 1985, 14, Supplement 2. Populations from a Maxwell-Boltzmann distribution a t 2300 K. Uncertainties in the last digit (in parentheses) are indicated for +200/-100 K . CPopulationsa t 30 eV estimated in ref 9. Error limits (in parentheses) are &30% plus the variation in the SI population due to uncertainty in the temperature. dEnergy is a statistical average over the J levels. e Reference 9 suggests that the population of these states is zero since there is no evidence of reactivity from such highly excited states. However, since the sum of the populations of the lower lying states may not sum to I , the population of the states above 3.4 eV may be appreciable but the reactivity of these states with H 2 may be very small. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 26, 2015 | http://pubs.acs.org Publication Date: May 1, 1990 | doi: 10.1021/j100372a042

5. 0

0.0

distributions are explicitly accounted for by the convolution analysis.) The data shown and analyzed in this paper involve the reactions of Mn+ produced in a surface ionization (SI) source and in an electron impact (El) source. I n the SI source, MnC1,.4H20 (Mallinkrodt) is vaporized in an oven and directed toward a resistively heated rhenium filament where the metal halide decomposes and the resulting metal atoms are ionized. It is generally assumed that a Maxwell-Boltzmann distribution accurately describes the populations of the electronic states of ions produced by SI. Evidence supporting this assumption has been discussed previ~usly.'~Table I gives these populations for Mn+ at 2300 K, the filament temperature generally used for these experiments. A conservative estimate of the uncertainty of the absolute temperature is +200/-100 K. In the E1 source, gaseous Mn2(CO)lo(Alfa) is dissociated and ionized by bombardment with electrons with 30 eV of energy, 10 eV above the appearance potential of Mn+.I5 E1 produces a non-Boltzmann distribution of electronic states with more excited ions. An estimate of the electronic state distribution of ions made by electron impact has been made previo~sly,~ Table I, and will be used in this paper. These results are consistent with other recent work from our groupIOand with recent results that suggest that 22.4 f 1.4% of the ions formed by E1 at 70 eV are in reactive excited states." The population distribution of ref 9 is deduced from comparisons of absolute cross sections for SI- and EI-produced ions and therefore represents the absolute populations. Conservative error limits on the E1 populations are provided in Table I and are largely due to the absolute SI population uncertainties. Note that the relative ratio of the two excited states is less uncertain than the absolute values because the effect of different temperatures is similar for both excited states. Neutral reagents are obtained from Baker (acetone, 99.6%) and Matheson (ethylene oxide, 99.7%; cyclopropane, 99.0%; isobutane. 99.5%; and neopentane, 99.8%).

Results Isoburane. The products seen in the reaction of Mn+ with isobutane (2-methylpropane) are given in reactions 1-4, and their cross sections are shown in Figure 1, In contrast to the reactivity Mn+ + (CH3)3CH MnCH3+ + (CH3)*CH (I) MnCH, + (CH3)2CH+ (2) MnH+ + (CH3),C (3) MnH + (CH3)3C+ (4) of Mn+, reactions of Fe+,6Co+, and Ni+ l6 with isobutane show exothermic dehydrogenation and demethanation, as well as other products not seen here, while excited-state Cr+ exothermically

E

(14) Sunderlin, L. S.; Armentrout, P. B. J . Phys. Chem. 1988, 92, 1 209-1 2 19. (15) Winters, R. E.; Kiser, R. W . J . Phys. Chem. 1965, 69, 1618-1622. (16) Georgiadis, R.; Fisher, E. R.; Armentrout, P. B. J . Am. Chem. SOC. 1989, 1 1 1 , 4251-4262.

lo-2r"PvPv.'

0.0

1

3

1.0

I

"

1

'

L O '

2.0

'

1

3.0

"

'7

" "

4.0

EVESGY ie V.

I

" "

5.0

1 '

"

6.0

'

I

7.0

CM)

Figure 1. Variation of the cross sections for reactions of Mn+ with isobutane as a function of translational energy in the center-of-mass frame (lower scale) and laboratory frame (upper scale). Closed symbols represent SI data and open symbols represent E1 data.

dehydrogenates i~obutane.~J'Thus, the neighbors of Mn+ in the periodic table are generally more reactive with isobutane than Mn+. In the SI data, the dominant processes below 6 eV are reactions 1 and 3. The cross section for reaction 2 is similar in shape to reactions 1 and 3 but shifted to higher energies by about 1.5 eV. The cross section for reaction 1 peaks at 4 eV, such that reactions 2 and 3 are dominant at higher energies. Only a very small amount (less than 0.01 A2) of reaction 4 is seen. Compared to the SI data, the apparent thresholds in the E1 data for reactions 1 and 3 are shifted to lower energies by -2.0 eV, while, for reaction 2, the apparent threshold shift is less distinct. This shift is consistent with the increased population of the two low-lying quintet states in the E1 beam, Table I. The relative magnitudes of the various cross sections also change under E1 conditions. E1 data for reaction 1 show a somewhat higher (by a factor of 1.5) maximum cross section and greatly enhanced reactivity at energies below =4 eV, while the maximum cross sections for reaction 2 in the SI and E1 data are nearly the same. The maximum cross section for reaction 3 in the E1 data is 5 times higher than in the SI data, and reaction 4 shows an even more dramatic dependence on the source conditions. Thus, the efficiencies of reactions 1, 3, and 4 are greatly enhanced for the excited quintet states, while, for reaction 2, the excited states are not significantly more reactive than the ground state. We can quantify this by comparing cross-section maxima and correcting the excited-state populations by the factors in Table I. This yields relative reactivities for the quintet states vs the septet states of about 20, 6, 30, and 2300 for reactions 1-4, respectively. Neopentane. The products seen in the reaction of Mn+ with neopentane (dimethylpropane) are given in reactions 5-7. Cross sections for these reactions with ions created by SI and E1 are shown in Figure 2. Again, there is no exothermic reactivity, in Mn+

+ (CH3)&

E

M n C H 3 + + (CH3)3C M n C H 3 (CH3)3C+

+ MnH+ + C S H l l

(5) (6) (7)

contrast to the exothermic demethanation seen in the reactions of Fe+, Co+, and Ni+ with neopentane.16,'s Neither are several other products observed for these elements seen here with Mn+.16 SI cross sections for reactions 5-7 are qualitatively similar to the analogous cross sections for the isobutane system, although the cross sections in the neopentane system are somewhat larger. (17) Recent work in our laboratory (Fisher, E. R.; Armentrout, P. B. Work in progress) shows that ground-state Cr+ undergoes no exothermic reactions with isobutane. (18) Halle, L . F.; Armentrout, P. B.: Beauchamp, J. L. Organometallics

1982. I . 963-968.

The Journal of Physical Chemistry, Vol. 94, No. 9, 1990 3591

Bond Energies for MnCH2+, MnH, and MnCH3

ENERGY (eV. Lab)

ENERGY (eV. Lab)

0. 0

5.0

10.0

0. 0

10.0

5.0

100

n;'

8 2 9

L

6

2 LJ ?4

10-1

3s 10-2

0.0

1.0.

4.0

3.0

2.0

ENERGY (eV.

5.0

6.0

:0-2

7.0

0.0

W


neopentane as a function of translational energy in the center-of-mass frame (lower scale) and laboratory frame (upper scale). Closed symbols represent SI data and open symbols represent El data. TABLE 11: Literature Thermochemistry

B

I-CdH,

H

CHI CH, CH, CHI' CH2' OC OC

,

2-C3H7

t-CdH9 CH,COb C2H4 CHZO C2H4

C3H6

DoZ9,(A-B),' eV 4. I5 f 0.03 3.81 f 0.03 3.75 f 0.03 3.51 f 0.02

IE(B),beV 6.70 f 0.03 7.36 f 0.02 6.70 f 0.03 7.02 f 0.06

3.99 f 0.04 3.42 f 0.04 3.67 f 0.02 5.04 f 0.02d

Mn

10.51 f 0.01 7.432

Except where noted, bond strengths derived from heats of formation for closed shell species in: Pedley, J. M.; Naylor, R. D.; Kirby, S. P. Thermochemical Data of Organic Compounds: Chapman and Hall: London, 1986. Radical heats of formation listed in ref 16. bIonization potentials and A,H(CH,CO) = -0.25 f 0.02 eV from: Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G . J . Phys. Chem. ReJ Dara 1988, 17, Supplement I . cArHfor CH2 and 0 from: Chase, M. W., Jr.; Davies, C. W.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N . J . Phys. Chem. Ref Data 1985, 14, Supplement 1 (JANAF Tables). dThis "bond strength" is the energy required to form 0 + propene from acetone. The difference between the thresholds of the neutral and ionic manganese-methyl channels (reactions 5 and 6) is less in the neopentane system than in the isobutane system, reflecting the 0.66-eV difference in ionization potentials of (CH3)& and (CH,),CH, Table 11. As with the analogous isobutane channels, the apparent thresholds for reactions 5-7 in the E1 data are 1-2 eV lower than in the SI data, and the MnCH3+channel again shows a larger dependence on ion source conditions. Comparison of corrected cross-section magnitudes yields relative reactivities for the quintet states vs the septet states of about 7, 2, and 30 for reactions 5-7, respectively. The relatively small amount of MnH+ in both the SI and E1 data in the neopentane system compared to the isobutane system is presumably due to the existence of a tertiary hydride in isobutane, which is readily removed compared to primary hydrides. N o MnH ( + C 5 H I I + )was seen in this reaction, in contrast to the case for isobutane. This is attributable to the low ionization potential of the tert-butyl radical vs that for the primary C S H l lradical. Acetone. The products seen in the reaction of Mn" with acetone (dimethyl ketone) are given in reactions 8-1 1. Cross sections for these reactions are shown in Figure 3. Again, MnCH3 and Mn+

+ (CH3)2C0

€

2.0

3.0

4.0

5.0

6.0

7.0

ENERGY lev, CW

Figure 2. Variation of the cross sections for reactions of Mn+ with

A

1.0

MnCH3+ + C H 3 C 0 MnCH, + CH3CO+ MnO+ + C3H6 MnH+ + C,H50

(8) (9) (10) (11)

Figure 3. Variation of the cross sections for reactions of Mn+ with

acetone as a function of translational energy in the center-of-mass frame (lower scale) and laboratory frame (upper scale). Closed symbols represent SI data and open symbols represent E1 data. MnCH3+ are major products, with the latter starting at lower energy. The results contrast with the results for the reaction of Fe+,19320 Co+, and N P 2 I with acetone, where MCO+ and MC2H6+ are formed exothermically, although M C H 3 and MCH3+ are major products at higher energies for M = Fe and CO.~O In the Mn system, a small amount of reaction 10 is seen at higher energies. This product appears at much lower energies in the E1 data, but the cross-section magnitude is only moderately enhanced. The E1 data also show enhanced cross sections for reactions 8 and 9 at low energies. As in the alkane systems, MnH+ is the dominant product at high energies in the E1 data. No SI data for reaction 11 was obtained, but by analogy with the other systems studied, it is likely that this cross section is also significantly smaller and shifted up in energy in comparison to the E1 data. Comparison of corrected cross-section magnitudes yields relative reactivities for the quintet states vs the septet states of about 10, 2, and 25 for reactions 8-10, respectively. Cyclopropane. The major product seen in the reaction of Mn+ with cyclopropane is given in reaction 12, where the neutral product is assumed to be ethene on thermodynamic grounds. Mn+ + c-C3H6

c

MnCH2++ C2H4 MnH+ + C3H5

(12) (13)

MCH2+ is also the dominant product in the reaction of cyclo~~ propane with M = Cr,22Fe,20C0,23924Ni,24925and C U ,although other products including MCzH4+,MC2HZ+,C3H5+,and C3H3+ are also seen for these other metals. While small amounts of C3H5+and C3H3+are seen here (maximum cross sections of =0.4 A2 at 6 eV in the E1 data), the Mn-ethene and Mn-ethyne ions are not. Figure 4 shows the SI and E1 cross sections for reaction 12 and the E1 cross section for reaction 13. The SI data for reaction 13 is much smaller than the E1 data and is not plotted since it obscures the other cross sections. (The cross section has an apparent threshold between 2 and 3 eV and rises to a plateau of 0.15 A2 at 7 eV.) The SI data for reaction 12 shows both a small exothermic cross section and an obvious endothermic feature (19) Burnier, R. C.; Byrd, G. D.; Freiser, B. S . J . Am. Chem. SOC.1981, 103, 4360-4367. (20) Schultz, R. H.; Armentrout, P. B. Work in progress. (21) Halle, L. F.; Crowe, W.E.; Armentrout, P. B.; Beauchamp, J. L. Organometallics 1984, 3, 1694-1 706. (22) Georgiadis, R.; Armentrout, P. B. Int. J . Mass Spectrom. lon Processes 1989, 89, 227-247. (23) Armentrout, P. B.; Beauchamp, J. L. J . Chem. Phys. 1981, 74, 2819-2826. (24) Fisher, E. R.; Armentrout, P. B. J . Phys. Chem., in press. (25) Halle, L. F.; Armentrout, P. B.; Beauchamp, J. L. Organometallics 1983, 2, 1829-1833.

3592 The Journal of Physical Chemistry, Vol. 94, No. 9, 1990 ENERGY (PV.

10-ZLj"' 10-1

Lob)

I

1

1no ENERGY (oV.


Sunderlin and Armentrout

ENERGY

CM)

rev.

CMI

Figure 4. Variation of the cross sections for reactions of Mn+ with cyclopropane as a function of translational energy in the center-of-mass frame (lower scale) and laboratory frame (upper scale). Closed circles represent SI data and open circles and diamonds represent El data. The line is El data ( E , = 25 eV) for formation of MnCH2+scaled to the SI data. Open squares represent the SI data after subtracting the scaled El data.

Figure 5. Variation of the cross sections for reactions of Mn' with ethylene oxide as a function of translational energy in the center-of-mass frame (lower scale) and laboratory frame (upper scale). Closed symbols represent SI data and open circles, triangles, and diamonds represent E1 data. The line is E1 data (E, = 25 eV) for formation of MnCH2+scaled to the SI data. Open squares represent the SI data after subtracting the scaled E1 data.

starting at =3 eV. In the E1 data, the exothermic feature is much larger and there is no endothermic feature, probably because it is obscured by the enhanced exothermic feature. The dramatic difference in the size of the exothermic feature indicates that the low-energy cross section is due to excited electronic states. Quantification of the relative reactivities for reaction 12 is problematical since these reactions are endothermic for the ground state and exothermic for the excited states. To gain a rough comparison, the cross sections can be compared to the Langevin -Gioumousis-Stevenson (LGS) cross section.26 This shows that the excited states undergo reaction 12 for all collisions ( ~ 1 0 0 % efficiency), while Mn+(a7S)has reaction efficiencies of 0.1% and 2% at the peaks of the first and second endothermic features, respectively. Ethylene Oxide. The main products seen in the reaction of Mn+ with ethylene oxide ( C 2 H 4 0 )are given in reactions 14-18. The SI and El cross sections for these reactions are shown in Figure 5, except for the SI cross section for reaction 16 (which has a

at the lowest energies to about 30:70 at about 1 eV). For the ground state, the large endothermic features for these reactions have maximum efficiencies of 1% and 3%, respectively; the small endothermic feature in reaction 14 is only 0.1% efficient. The reactions of Mn+ again differ from the wide variety of reactivities exhibited by Cr+,22Fe+,20Cof,23924and Ni+ with ethylene oxide. All these metal ions undergo processes analogous to reactions 14, 15, and 17, although CHO+ formation is only seen for Fe+. MC2H,+, MC2H4+,MCO', MOH2+, C2H30+,and other minor products are seen with these metals, although not all products are seen with every metal.

Mn+ + c-C2H40

E

MnCH2++ C H 2 0 MnO+ + C2H4 MnH+ + C2H30 C2H4++ MnO CHO+ + (Mn CH3)

+

(14) (15)

(16) (17) (18)

maximum of less than 0.2 A2 at =6 eV) and the E1 cross sections for reactions 17 and 18 (which rise to cross sections of 0.03 and 0.09 A*, respectively, at 5 eV). Small amounts of C2H3' are also seen in the E1 data. This product has an apparent threshold of less than 1 eV and a maximum cross section of less than 0.05 A2 at 5 eV. The maximum SI cross section for C2H3+ is less than 0.01 A2. Again, the dramatic difference between the S I and E1 cross sections for reactions 14 and 15 at low energy indicates that the low-energy cross section is due to excited electronic states. Comparison to the Langevin cross section indicates that the total for the quintet states (where the reaction probability is ~ 1 0 0 % branching ratio between reactions 14 and 15 varies from 5050 ( 2 6 ) When the long range interaction between an ion and a neutral molecule is the ion-induced-dipole potential, the LGS model (Gioumousis, G . ; Stevenson, D. P.J . Chem. Phys. 1958,29,294-299) predicts a cross section for close collisions of uL = r e ( 2 a / E ) ' / * ,where e is the electronic charge, a is the polarizability of the neutral (5.66 A3 for cyclopropane from: Nenner, T.; Tien, H.; Fenn, J . B. J . Chem. Phys. 1915, 63, 5439-5444. 4.43 A' for ethylene oxide from: Ramaswamy, K. L. Proc. Ind. Acud. Sci. Sect. A 1936, 4 , 675). and E is the relative translational energy of the reactants. This gives close collision cross sections in A2 of 40.1 [ E (eV)]'/2 for cyclopropane and 3 5 . 5 [ E (eV)]'/* for ethylene oxide.

24925

Thermochemical Analysis Theory and experiment indicate that cross sections can be parametrized in the threshold region by function 19.27 Here, Eo is the threshold for reaction of the a7S state of Mn+, E is the u ( E ) = Cgp,(E - Eo i

+ E,)"/,"

(19)

relative translational energy of the reactants, and n and m are adjustable parameters. Eiis the electronic excitation of each specific electronic state, giis the population of that state, and uio is a scaling factor for that state. It is assumed that n and m in eq 19 are the same for all states. As in previous studies, we have utilized eq 19 with m = 1. This form is expected to be the most appropriate for translationally driven reactions28 and has been found to work exceptionally well in a number of previous studies of ion-molecule reactions.29 The parameters ai,,,n, and EO are optimized by using a nonlinear least-squares analysis to give the best fit to the data. Error limits for Eo are calculated from the range in these threshold values for different fitting parameters, the deviations for different data sets, and the error in the absolute energy scale. The threshold energies for endothermic reactions are converted to thermochemical values of interest by assuming that Eo represents the enthalpy difference between reactants and products. This assumes that there are not activation barriers in excess of the endothermicity. This assumption is generally true for ionmolecule reactions and has been explicitly tested a number of times.30 Do(Mn-CH3),Do(Mn+-CH3),and Do(Mn+-CH2) were (27) See references in: Sunderlin, L. S.; Armentrout, P. B. J . Am. Chem. SOC.1989, I l l , 3845-3855.

( 2 8 ) Chesnavich, W. J.; Bowers, M. T. J . Phys. Chem. 1979,83,90&905. (29) BOO,B. H.; Armentrout, P. B. J . Am. Chem. SOC.1987, 109, 3549-3559.

The Journal of Physical Chemistry, Vol. 94, No. 9, 1990 3593

Bond Energies for MnCH2+, MnH, and MnCH,

TABLE 111:

Fitting Parameters

reaction MnCH,' + C3H7 Mn+ + C4H,,, MnCH, + C3H7+ MnH + C4H9+ MnH + C4H9+ Mn'

Ec

+ CSHI2

:A:;+:;:;:

Mn+ + E-

C3H60 Mn+ + c-C3H6 Mn+ + c-C2H40

E

MnCH,' + C2H30 MnCH, + C2H30+ MnO+ + C3H6 MnCH2+ + C2H4 MnCH2+ + H2C0 MnO+ + C2H4 C2H4++ MnO

an

n

Eo

Do(Mn-R)

source

4.3 (1.1) 0.08 (0.06) 2.5 (1.0) 1.7 (0.3) 3.4 (1.5) 0.36 (0.22) 0.008 (0.002) 0.09 (0.05) 0.013 (0.004) 0.061 (0.006) 0.38 (0.48) 1.7 (0.7) 0.025 (0.01 1)

2.5 (0.2) 4.0 (0.5) 1.7 (0.5) 2.2 (0.2) 3.1 (0.1) 3.4 (0.6) 2.0 (0.1) 3.4 (0.4) 2.0 (0.2) 0.8 (0.2) 1.3 (0.9) 1.4 (0.2) 2.0 (0.5)

1.59 (0.13) 3.24 (0.19) 2.10 (0.19) 1.58 (0.05) 1.59 (0.06) 2.44 (0.18) 1.22 (0.05) 2.55 (0.30) 2.93 (0.21) 0.90 (0.10) 0.39 (0.19) 1.97 (0.09) 3.04 (0.26)

2.22 (0.14) 0.50 (0.19) 1.31 (0.19) 1.83 (0.07)b 2.16 (0.07) 0.58 (0.18) 2.29 (0.05) 0.54 (0.31) 2.11 (0.21)b 3.09 (0.1 1) 3.03 (0.19) 1.70 ( O . l O ) b 3.71 (0.26)

El SI E1

aSS

states"

E1 E1 SI E1 SI SI SI SI SI SI

ass a5S

a7S a5S,a5D a7S a5S a7S a7S a7S a7S a7S all


'States considered in the analysis. bValues believed to be incorrect and not used in determining thermochemistry. measured with more than one reaction in order to minimize the possibility of kinetic shifts or barriers leading to incorrect results. This was not possible for Do(MnH) and DO(Mn0). We also presume that the products formed at the threshold of an endothermic reaction are characterized by a temperature of 298 K in all degrees of freedom. Thus, we make no correction for the energy available in internal modes of the neutral reactant. Literature thermochemistry needed below is given in Table 11. MnCH3+. Do(Mn+-CH3) has been previously reportedlo to be 2.23 f 0.17 eV. On the basis of this value, reactions 1 , 5, and 8 should be endothermic for the a7S and a5S states of Mn+ but exothermic for the aSD and higher states. The small amount of exothermic reactivity seen in the E1 data for all three systems, Figures 1-3, can therefore be assigned to these latter excited states. To find a threshold for the endothermic reactions, we first subtract a model for the exothermic cross section from the E1 data for reactions I , 5, and 8. The remaining cross sections should be due to the a7S and aSS states. However, the SI data for reactions 1 and 5 demonstrate that the a7S state does not react readily at its thermodynamic threshold (1.58 f 0.1 7 and 1.52 f 0.17 eV for reactions 1 and 5, respectively) but is delayed to over 2 eV. By modeling the E1 cross sections below 2.0-2.5 eV, where only the a5S state contributes to the cross sections, we can determine a threshold for this state exclusively. We find a threshold for reaction 1 of 0.42 f 0.13 eV, which when corrected for ,!?,(ass) = I . 175 eV gives a ground-state threshold of Eo = 1.59 f 0.13 eV. The latter value is listed in Table 111 along with the other optimized parameters of eq 19. Analysis of reaction 5 gives an a5S threshold of 0.42 f 0.06 eV, or Eo = 1.59 f 0.06 eV. For reaction 8, we find an aSS threshold of 0.04 f 0.05 eV, or Eo = 1.22 f 0.05 eV. These thresholds yield MnCH3+bond energies of 2.22 f 0.14 eV, 2.16 f 0.07 eV, and 2.29 f 0.05 eV, respectively. All three values are in good agreement with the previous value, 2.23 f 0.17 eV. The present results can be averaged with the previous results from the ethane systemlo to provide a more precise value for Do(Mn+-CH3) of 2.23 f 0.10 eV ( 5 1.4 f 2.3 kcal/mol). For comparison, a theoretical value of I .96 eV has been calculated for this bond energy.,' After accounting for the a5S contributions to the SI data, we can then analyze this data for thresholds for the a7S state. We find that the Eovalues derived for reactions 1 , 5, and 8 are higher than the thermodynamic thresholds for the a7S state by 0.7 f 0.2, 0.9 f 0.2, and 0.7 f 0.5 eV, respectively. This type of behavior is directly analogous to that seen in the reaction of Mn+ with H 2 and C2H6.9-i0 MnCH,. Analyses of the thresholds for reactions 2, 6 , and 9 for Mn+ (SI) are given in Table 111. Since the Mn+ produced by SI is 99.8% in the a7S state, contributions from the excited (30) Ervin, K. M.; Armentrout, P. B. J . Chem. Phys. 1987,86,2659-2673. Elkind, J . L.; Armentrout, P. B. J . Phys. Chem. 1984.88, 5454-5456. Armentrout, P. B. In StructurelReactiuity and Thermochemistry of Ions; Ausloos, P., Lias, S. G.,Eds.; Reidel: Dordrecht, The Netherlands, 1987; pp 97-164. (31) Bauschlicher, C. W.; Langhoff, S . R.; Partridge, H.; Barnes, L. A. J . Chem. Phys. 1989, 91. 2399-2411.

states are ignored in this analysis. (If the excited states are included and assumed to have equal reactivity as the ground state, the values of Eo increase by only 0.05 f 0.03 eV. The SI and E1 data for these reactions indicate that excited states are not drastically more or less reactive than the ground state.) The MnCH, bond energies derived from the three systems are in g o d agreement, Table 111. The average bond energy derived from all data sets (two each for acetone and isobutane and four from neopentane) is Do(Mn-CH,) = 0.55 f 0.17 eV. While these analyses reveal no obvious systematic errors, the data for MnCH3+ indicate that the ground state does not react efficiently until 0.7-0.9 eV above the thermodynamic thresholds. As discussed below, we expect that the dynamics for formation of MnCH, + R+ and MnCH3+ R (where R = 2-C&7, t-C,H,, CH,CO) should be similar for the reaction of Mn+(a7S). Thus, the 0.55 f 0.17 eV bond energy is probably a lower bound, with the true value -0.8 eV higher. In our previous work on Mn+ reaction^^,^^ (and for most of the other reactions in this work), this behavior of ground-state Mn+ meant that thermochemistry was derived from a measurement of the thresholds for excited-state reactions. Unlike these other reactions, however, the formation of MnCH, is not appreciably enhanced for reaction of excited-state Mn+. This prohibits an accurate determination of the bond strength from the E1 data. An upper limit for Do(Mn-CH3) of 1.3 eV has been reported previously from a study of the reaction of Mnz+ with ethane.32 Note A theoretical calculation of this bond energy gives 1.30 that this bond energy suggests that the ground-state thresholds for reactions 2, 6, and 9 are shifted up in energy by 0.75 eV, consistent with the shift seen for reactions 1, 5, and 8. MnH. The ground state of Mn+ does not react with isobutane to form MnH. Thus, the E l data for reaction 4 is analyzed including only excited states (i.e., uio= 0 for the ground state in eq 19). Given the thermochemistry in Tables I and 11, reaction 4 must be exothermic for the states above the aSD,no matter what the value of Do(Mn-H). Since only a small amount of exothermic reactivity is seen, we can neglect the exothermic contribution to the reactivity by not fitting below 0.5 eV (subtracting a model for the exothermic contribution leads to identical results). The question of whether reaction of the a5D state is also exothermic cannot be determined unequivocally from the data alone. Therefore, we interpret the data in two ways: ( I ) only the a5S state contributes to the endothermic reactivity (while the aSDstate reacts exothermically), and (2) reaction of both the a5S and aSD states are endothermic. In the latter case, we assume that both states have equal reactivities. This assumption is consistent with the data in ref 9. Further, in the analysis of data for formation of MnH+ in reaction 3, this assumption leads to thresholds consistent with the known bond energy, Do(Mn+-H) = 2.10 f 0.15 eV.9 (The detailed analysis of these cross sections are not reported since the data are insufficiently precise to provide new thermodynamic information.) Finally, the a5S and a5D states are expected

+

(32) Armentrout, P. B. In Laser Applications i n Chemistry and Biophysics. El-Sayed, M. A., Ed. Pror. SPIE 1986, 620, 38-45.


3594

The Journal of Physical Chemistry, Vol. 94, No. 9, I990

to adiabatically mix,33leading to similar reactivity, as discussed below. The analyses of the threshold for reaction 4 are given in Table I l l . The thresholds indicate D"(Mn-H) = 1.83 f 0.07 eV (a% alone) or 1.31 f 0.19 eV (a% and a5D). While the assumption of equal reactivity is probably a good one, it is possible that both states react endothermically but the a5S is much more reactive than the a5D or vice versa. In the former case (for the limit where the a5S is infinitely more reactive than the a5D), the bond energy is 1.83 f 0.07 eV (the a5S alone result). In the limit where the a5D state is much more reactive than the a5S, the bond energy derived is 1.20 f 0.07 eV. Note that the latter result falls within the error limits of the 1.31 f 0.19 eV bond energy. Some experimental data on D"(Mn-H) has been previously published, but the results are contradictory. The work of Kant and Moon,34which has generally proven to be reliable,' indicates that D"(Mn-H) 5 1.39 eV. On the basis of this work, the endothermic reactivity for reaction 4 cannot be due primarily to the a5S. It appears that our value of 1.31 f 0.19 eV is the correct interpretation of the data with error bars sufficiently large to include alternate interpretations of the data. In contrast, Huber and H e r ~ b e r gquote ~ ~ as "uncertain data" a 0 K value of 2.5 eV for Do(Mn-H). Although no reference is given, this value is apparently from a linear Birge-Sponer analysis.36 It is known that this method tends to give high values for bond energies.36 Several theoretical values for Do(Mn-H) have also been published. The most recent report37gives several values from 1.62 to 1.67 eV, depending on method and basis set. Other values are 1.5 1 38 and 1.85 eV.39 Thus, the experimental value is somewhat lower than the theoretical values. This is in contrast to the case of D"(Mn+-H). where the theoretical values ( 1 .76,2 1 .93,3 and 1.8 I eV40) are lower than the experimental value (2. I O f 0.15 eV9). MnCH2+. The magnitude of the exothermic portion of the SI cross section for reaction 12 at 0. I eV is equal to 0.4% of the close-collision cross section.26 This value is larger than but within experimental error of the combined populations of the aSS and a5D states (since the population of the excited states at the upper limit of error on the temperature, 2500 K, is 0.39%) but is much too large to be accounted for exclusively by the a5D state and higher lying states (0.08% at 2500 K). Thus, reaction 12 must be exothermic for the a5Sstate, which means that Do(Mn+-CH2) L 2.82 eV. A further indication that excited states are responsible for the exothermic reactivity is that the exothermic cross section in the El data is larger than in the SI data by a factor of =35. This increase matches within experimental error the relative populations of the excited states in the SI and El data of Table 1.41

I f we scale the El cross section4*to the SI data at low energies, Figure 4, it is clear that the exothermic part (attributable to the a5S and a5D states) of the SI reactivity is reproduced. By subtracting this scaled El cross section from the SI data, we are left with the a7S cross section also shown in Figure 4. Two features (33) Elkind, J . L.: Armentrout, P. B. J . Phys. Chem. 1987, 91,2037-2045. (34) Kant, A.; Moon, K. A. High Temp. Sci. 1981, 14, 23; 1979, 11, 5 5 . (35) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Slructure I V Consranfs of Diatomic Molecules; Van Nostrand-Reinhold: New York, 1979. (36) Gaydon, A. G. Dissociation Energies and Spectra of Diatomic Molecules; Wiley: New York, 1947. (37) Chong, D. P.: Langhoff, S . R.; Bauschlicher. C. W.: Walsh. S. P.: Partridge, H. J. Chem. Phys. 1986, 85, 2850-2860. (38) Bagus. P. S.: Schaefer, H. F.. 111. J . Chem. Phys. 1973, 58, 1844-1848. (39) Das, G . J . Chem. Phys. 1981, 7 4 , 5766-5774. (40) Vincent, M. A.; Yoshioka, Y.; Schaefer, H. F.. 111. J . Phys. Chem. 1982, 86,3905. (41) Although the absolute fraction is again larger than the estimated population of excited states in the E1 beam, this is to be expected since the E1 populations were estimated from a comparison of SI and El cross sections. (42) The scaled El data is taken at an electron energy of 25 eV. The 25-eV data are not greatly different from the 30-eV data but are preferred for this purpose since the relative populations of the two excited states at this electron energy (3% 5S and 0.6% SD. ref 9) are nearly the same as the populations for the SI data. Table I . This is similar to the procedure used in ref 9 .

Sunderlin and Armentrout are evident in the data, an observation similar to that made for the reaction of Mn+(a7S) with H2.9 In that system, the small low-energy feature was found to begin a t the thermodynamic threshold, while the large second feature was delayed to substantially higher energies. W e presume a similar result here. Analysis of the low-energy feature, Table 111, gives Do(Mn+-CH2) = 3.09 f 0.1 I eV.43 In contrast, the high-energy feature has a threshold of -3.0 eV, about 2 eV higher than the apparent thermodynamic threshold. The reaction of Mn+ with ethylene oxide is similar to the reaction of Mn+ with cyclopropane in that there is an exothermic feature and an endothermic feature in the SI data for formation of MnCH2+. As with the cyclopropane data, the relative and absolute magnitudes of the cross sections in the SI and El data for reaction 14 indicate that the exothermic feature is due primarily to the a5S and a5D states while the higher energy reactivity is due to the ground state. If we again subtract a scaled El cross section42from the SI data, the remaining cross section exhibits a low-energy feature and a high-energy feature, both attributable to the ground state. The threshold for the lower energy endothermic channel indicates Do(Mn+-CH2) = 3.03 f 0.19 eV.43 The threshold for the higher energy feature is delayed = I eV above the lower energy feature. Averaging the results for ethylene oxide and cyclopropane gives 3.06 f 0.13 eV as our best value for DO(Mn+-CH,). This value is considerably lower than the only previous determination, 4.1 f 0.3 eV.5 This value was derived from analysis of reaction 20,5 Mn+

+ C2H4

-

MnCHz+

+ CH2

(20)

but the state-specific reactivity of Mn+ was not accounted for. The authors mention "difficulties in interpretation of the data" for other Mn+ reaction^,^ and it is now obvious that explicit consideration of the reactivity of the different electronic states of Mn+ is necessary in order to derive reliable thermc~hemistry.~J~ MnO+ and MnO. D"(Mn+-O) has been determined to be 2.85 f 0.06 eV.# Reaction 15 should thus be exothermic for all excited states and endothermic by 0.82 eV for the ground state. The assignment of the exothermic portion of the observed SI cross section, Figure 5, to excited states and the endothermic feature to the ground state is confirmed by the dependence of the cross-section features on the source conditions. However, the threshold for the endothermic feature is measured to be 1.97 f 0.09 eV, substantially higher than the thermodynamic threshold. A similar observation is made for formation of MnO+ from reaction with acetone. Here, the measured threshold is above the thermodynamic threshold by 0.74 f 0.22 eV, assuming the neutral product is propene (the lowest energy form of C3H6). These observations are in keeping with the delayed thresholds in other reactions for the ground state. The threshold for reaction 17 (fit assuming all states have equal reactivity, which is consistent with the data) indicates that D"(Mn-0) = 3.71 f 0.26 eV. This compares well to previous values of 3.70 f 0.1 7 eV from mass spectrometric methods45and 3.8 f 0.2 eV from previous ion beam studies.32 It is somewhat less than values derived from spectroscopic measurements of equilibrium concentrations, 4.16 f 0.1346 and 4.0 f 0.4 eV.47 Bond Strength-Promotion Energy Correlations. A recent bond strengths depicts m ~ d e l ' . ~of, ~transition-metal-hydride .~ metal-hydrogen (and by extension metal-methyl) bonding as being primarily an interaction between hybridized valence s and d o orbitals of the metal and the Is orbital of H (or an sp3 radical orbital of methyl). This model is qualitatively consistent with a b (43) The final data analysis also includes line-of-centers model ( n = m = I ) fits to the derived ground-state data for formation of MCH2+ from cy-

clopropane and ethylene oxide because the derived data was relatively scattered. This functional form gives good fits to the data. For comparison, fits to cross sections analogous to reactions 12 and 14 for Co+, Ni+, and Cu+ use values of n from 0.3 to 1.7, ref 24. (44) Loh, S. K.; Armentrout, P. B. Unpublished work. (45) Burns, R. P. Quoted in ref 46. (46) Padley, P. J.; Sugden, T. M . Trans. Faraday Soc. 1959, 55, 2054-206 I. (47) Huldt. L., Lagerqvist, A . Ark. Fys. 1954, 3, 525-531.

Bond Energies for MnCH2+, MnH, and MnCH3

The Journal of Physical Chemistry, Vol. 94, No. 9, I990 3595

TABLE IV: Experimental Thermochemistry (eV) of Manganese-Ligand Bonds' svecies

D O TOP

Mn-H Mn+-H M n+-C H Mn-CH, Mn+-C H, Mn-0 M n+-0

1.31 f 0.19 2.10 f 0.15b 3.06 f 0.13 0.4 - 1.30' 2.23 f 0.10 3.71 f 0.26 2.85 f 0.06d

2

IE

5.6 - 6.50 8.30 f 0.27


initio calculation^.^*^ In order to form a covalent bond, the metal must be promoted into a state where a single electron is in the bonding orbital and its spin is decoupled from the remaining d electrons. The energy required to induce this promotion is E,. High values of E, correlate with lower bond energies. For neutral Mn, the energy of the bonding state is an average of the energies of the a6D(4s3d6) and a4D(4s3d6) states, which gives E, = 2.53 eV. For cationic Mn, the pertinent states are a7S(4s3d5) and a5S(4s3d5). For convenience, we use the value calculated and compiled by Carter and Goddard, Ep(Mn+) = 0.52 eV.4 To correlate these promotion energies to the bond energies for single metal-ligand bonds of the first-row transition metals, we use the extremely good correlation found for the M+-H bond energies.' This correlation suggests that Mn-R single bond strengths should be 1.2 eV while Mn+-R bonds should have a higher strength of -2.3 eV. These values are in good agreement with the experimental values derived here of 1.3 eV for Do(Mn-H) and of 2.23 eV for Do(Mn+-CH3). The correlation suggests that Do(Mn-CH3) is closer to the upper limit of 1.3 eV than to the lower limit of -0.5 eV, in agreement with the published theoretical value.3' For double bonds, derivation of the promotion energy is more complex but has recently been calculated for many metal ions.4 In the case of Mn+, E,(double) = 2.24 eV. On the basis of this value, an analysis of the periodic trends in the first-row transition metal methylidene ions suggests that the Mn+-CH2 bond energy should be about 2.9 eV.' Using a more complete data set, we recently reanalyzed the trends in M+-CH2 bond energies4*and found that the present value of 3.06 f 0.13 eV is well within experimental error of the best fit to the bond energies for the first-row metals. Thus, the M-H and M - C bond strengths derived in this paper are all consistent with the predicted effects of promotion energy. The difference between D o ( M n + 4 ) and Do(Mn+-CH2) is only 0.21 f 0.1 5 eV. This small difference suggests that the M n + 4 bond is similar to the Mn+=CH2 double bond. However, Do(Mn-0) is greater than Do(Mn+-O) by 0.86 f 0.27 eV. This is surprising considering that for forming two bonds, E,(Mn) > E , ( M I I + ) . ~ ~ The Mn-0 bond presumably has more ionic character than the Mn+-O bond or the Mn+-CH2 bond. This This effect could easily increase the bond energy ~ignificantly.~~ is not accounted for in the promotion energy-bond strength model, which is based on covalent M-H and M-C bonds. Finally, since we have derived bond energies for both ion and neutral species, we can also derive the ionization energies (IEs) for the neutral species by the use of eq 21 where IE(Mn) = 7.43 IE(MnR) Do(Mn+-R) = IE(Mn) Do(Mn-R) (21) eV and R = any ligand. Table IV gives the results for MnH, MnCH,, a n d MnO. The low value for MnCH3 is consistent with calculations that show that the electron removed from MnCH3

-

+

(48) Armentrout. P. B.; Sunderlin, L. S.;Fisher, E. R. Inorg. Chem. 1989, 28, 4436-4431. (49) The first excited state (and the lowest energy component of the bonding state of Mn) has an excitation energy of 2.14 eV, greater than the promotion energy of Mn+, I .72 eV. (50) Pauling's electronegativity calculations (see: Allred, A. L. J . Inorg. Nucl. Chem. 1961, 17, 215-221) suggests that there is an ionic contribution to the bond of 3.43 eV because of the differing electronegativities of Mn and 0.

+ R' ('A)

+ RL

6.65 f 0.24

'All values derived here except as noted. bReference 9. < S e e text for a discussion of this result. dReference 44.

+

MnL ('X)

Mn+('D)

MnL' (6X)

%

Mnt('S)

+ RL

Mn+('S)

+ RL

+ R (2A)

z

;' UJ

0

Figure 6. Qualitative potential energy surface for the reaction of Mn+ with RCH,. Solid lines represent diabatic surfaces, and dashed lines represent an avoided crossing (adiabatic surfaces). Similar surfaces exist for formation of M n H and MnH+.

is in a high-energy 4s4p hybridized orbitaL3' A similar result should hold for MnH. The value determined for I E ( M n 0 ) can be compared with a literature value of 8.65 f 0.2 eV,5' which is based upon an earlier (and less reliable) value of Do(Mn+-O). Reaction Mechanisms Formation of MnL and MnL+ ( L = H , C H J . The relative efficiencies of the quintet excited states vs the septet ground state in the first three systems discussed in this paper are 2-6 for formation of MnCH3, 7-20 for formation of MnCH3+,25-30 for formation of MnH+, and 1 3 0 0 for formation of MnH. For comparison, formation of MnH+ in the reaction with H, is a factor of 40 more efficient for the quintet state^,^ and formation of MnCH3+ in the reaction with ethane is a factor of 10 more efficient for the excited statesi0 Thus, the state-specific reactivities in the systems studied here are similar to those seen previously with smaller neutral reactants. However, the branching ratios within a particular system are distinctly different for different electronic states. While Mn+(a7S) forms nearly equal amounts R+ and MnCH3+ + R in these three systems, of MnCH, Mn+(a5S,a5D)are noticeably more efficient at forming MnCH3+ (by factors of 2-8). All states are much more efficient at forming MnH+ than MnH (by factors of 7 or more). One key consideration in explaining this state-specific reactivity is conservation of spin. MnL has a septet ground ~ t a t e , ~while ',~~ R+ is a singlet. MnL+ has a sextet ground ~ t a t e , ~ *and ~.R ~ ' is a doublet. Thus, formation of MnL + R+ leads to products with septet spin overall, while formation of MnL' + R leads to products having an overall septet or quintet spin. The Mn+(a7S) ground state therefore correlates to both sets of products, while the excited quintet states correlate only to MnL'. Since a spin change is needed in order for the quintet states to form the ground state of MnL, this explains why the excited states preferentially form MnL+. Spin conservation fails, however, to explain why the excited-state reactivity is so much higher than that for the ground state. This requires the use of molecular orbital arguments discussed in detail previously.33 The previous discussion of the interaction of metal ions with H2 serves as a model for insertion into C-H, C-C, and C - 0 bonds and is outlined here for neutral reactants R-L. Consider the interactions of Mn' with R-L as it approaches in a perpendicular (insertion) geometry. The potential energy surfaces (PESs) involved in this reaction are shown in Figure 6 and resemble those previously discussed for the interaction of Cr+ and Mn+ with CH4.i',53 Initially, all PESs are attractive due

+

(51) Armentrout, P. B.; Halle, L. F.; Beauchamp, J. L. J . Chem. Phys. 1982, 76, 2449-2457.

(52) Chong, D. P.; Langhoff, S. R.; Bauschlicher, C. W.; Walch, S.P.; Partridge, H. J . Chem. Phys. 1986, 85, 2850-2860. (53) Georgiadis. R.; Armentrout, P. B. J . Phys. Chem. 1988, 92, 7067-7074.


3596

The Journal of Physical Chemistry, Vol. 94, No. 9, I990

to the long-range ion induced dipole potential. As the reactants approach more closely, the PESs for the a7S(4s3d5)and asS(4s3d5) states are repulsive because of the interaction of the half-occupied 4s orbital of Mn with the fully occupied bonding orbital of R-L. The aSD(3d6) state, on the other hand, will not have such a repulsive interaction and can proceed to form the R-Mn+-L intermediate. This reaction is spin-allowed because R-Mn+-L should have a quintet ground state if both metal-ligand bonds are covalent. Since the a5S state is lower in energy than the a5D state and has a more repulsive interaction, these two surfaces will cross. This interaction will lead to extensive mixing, since the two states have the same spin. Thus, both excited states can react via an insertive geometry. Spin-orbit interactions can cause mixing of the ground-state septet surface and the quintet surfaces, leading to insertive reaction for the ground state, although such interactions are very inefficient with H, and ethane. Because the interaction of ground-state Mn+ is highly repulsive, it is most apt to react via an impulsive (hard-sphere-like) c o l l i ~ i o n . The ~ ~ ~energy ~ available for a purely impulsive reaction is not ECMbut rather the "pairwise" interaction energy between the ion and the ligand t r a n ~ f e r r e d .This ~ results in a shift of the observed reactivity to higher kinetic energies in the center-of-mass frame. It is also conceivable for reaction to occur along a collinear pathway (SN2 reaction). This pathway reduces the repulsive interaction of an occupied 4s orbital with the RL bonding orbital. Steric considerations may make such a pathway unfavorable for abstraction of a methyl group, although no such problem exists for hydrogen or hydride abstraction. Since the metal-methyl ion (and neutral products) is less likely to be produced via a direct, collinear reaction because of the directionality of the methyl group, reaction of excited states probably m u r s through C - C bond insertion followed by homolytic or heterolytic bond cleavage. The ground state has a delayed threshold for reactions 1, 5 , and 8 (and probably for reactions 2, 6, and 9). indicating impulsive behavior, but the delays in the thresholds are not large enough to be consistent with pure pairwise behavior, which is seen in the reactions of Mn+(a7S)with H, and C2H6. This is consistent with the "floppier" neutral reactants used in this study as compared to the diatomic H, and the pseudodiatomic C2H6. In reaction 4, hydride extraction from isobutane, the ground state is essentially unreactive, while excited states react with reasonable efficiency. This difference in reactivity is dramatically larger (2300) than for the other reactions in the isobutane system (factors of 30 or less). This is surprising, since reaction by the quintet states cannot lead directly to ground-state MnH(7Z+)S2 + C4H9+. One possible explanation for this observation is that reaction 4 is proceeding through a different mechanism than formation of other products (e.g., a direct pathway for MnH while other reactions proceed through insertion). Alternatively, we note that reactions 3 and 4 are nearly isoenergetic, E0(3) = 2.05 0.15 eV and E0(4) = 2. I O f 0.19 eV, since the ionization potentials of MnH and t-C,H9 are nearly the same, Table IV. This may enhance the coupling between these two surfaces such that the excited states form more MnH + C4H,+ than might otherwise be expected. Finally, we note that the strong state-specific reactivity observed here contrasts with the results for Ti+,s4where larger reactants (such as those studied here) showed virtually no state-specific reactivity. For Ti+, this was explained by noting that long-lived complexes are more likely to be formed with larger molecules than with smaller ones. This will lead to greater mixing of states and less state-specificreactivity. In the case of Mn+, the a7S interaction with the neutral reactant is strongly repulsive, and the aSD state, which leads directly to the ground-state intermediate, is very high in energy. This means that the crossing between the surfaces of different spin occurs at an energy above that of ground-state reactants (while in the case of Ti' it is below) and also that the

*

(54) Sunderlin, L. S.; Armentrout, P. B. Int. J. Mass Spectrom. Ion Processes 1989. 94, 149-177.

Sunderlin and Armentrout SCHEME I Mn'

+

A-

M n (

1'

,,,,..,

-

~ ~ - - Mn+=CH, - ~ ~ ~ ~ MnCH,' ~ ~ -+ -

C2H,

7 '-

M - CH,-: niI

" .CH,

Mn'

---

-

7 7 -

"j

bond-insertion intermediate is much less stable in the case of Mn+. Thus, long-lived complexes probably do not form, and strong state-specific reactivity is retained even for large reactants. Formation of MnCH2+and Mn@. Reactions 12, 14, and 15 (formation of MnCH2+ and MnO+ from cyclopropane and ethylene oxide) are presumed to proceed through the same mechanism, since the reactants and products are isoelectronic, the cross sections are similar (for both ground and excited states), and Do(Mn+-CH2) = Do(Mn+-O). All three reactions are 20-50 times more efficient for the quintet excited states than for the septet ground state. This can again be explained by spin conservation. If the Mn-C and Mn-0 bonds are double bonds, as suggested by the periodic trends analysis above, then the ionic products must have quintet ground states, while the accompanying neutrals are singlets. Thus, these three reactions are spin-allowed from the quintet excited states and spin-forbidden from the a7S ground state. This explains the very inefficient reactivity of Mn+(a7S)at the thermodynamic threshold. The large endothermic features observed for reactions 12, 14, and 15 at higher energies could again be due to impulsive behavior or possibly the formation of excited MnCH2+and MnO+ states in spin-allowed reactions. The mechanism typically invoked for these reactions is the upper path in Scheme I, where replacing one of the CH, groups with 0 gives the analogous reactions of ethylene oxide. The first step is formation of the metallacyclobutane (I) by metal insertion into a C-C bond, which has a strength of only 2.6 eV because of ring strain.55 MCH2+ is formed when this intermediate undergoes a (2 + 2) reaction to form 11 followed by loss of ethene. Spectroscopic evidence for such intermediates has been obtained in matrix studies of Fe reacting with cyclopropane and ethylene oxide.56 The (2 + 2) reaction step is part of the mechanism for olefin metathesis and is well established for condensed-phase reactions of early transition metakS7 In the gas phase, reaction 22 (and the analogous reactions to form MCH,+ and MO+ from M+ c-C,H, MCH,+ C2H, (22) ethylene oxide) proceeds efficiently and apparently without a * Fe,,O Co, Ni,24and the excited states barrier for M = S C , ~Cr,,, of Mn. Cu+, however, does exhibit an activation barrier for reaction 22, although not for the analogous reaction to form CuCH,' from C - C ~ H ~ O . ~ ~ Other (2 + 2) reactions that have been seen for gas-phase transition-metal ions include P-H migrations9 and four-center dehydrogenation of methane,'4ss3@reactions 23 and 24. Although

+

-

M+-CH,-CH,-H

+

+

H-M+-

II

(23)

+

M+=CH2 H2 (24) H-M+-CH3 reaction 23 occurs for both early and late transition metals, there is a barrier to dehydrogenation of ethane for some late transition metals,'6i61indicating that there may be a barrier to reaction 23 for these metals. Reaction 24 has been seen only for early transition metals (Sc+-Cr+), indicating that there is also a barrier +

(55) Doering, W. v. E. Proc. Nat. Acad. Sci. U.S.A.1981, 78. 5279-5283. (56) Kafafi, 2. H.; Hauge, R. H.; Fredin, L.; Billups, W. E.; Margrave, J . L. J. Chem. Sor., Chem. Comm. 1983, !23C-1231. Kafafi, Z. H.; Hauge, R. H.; Billups, W. E.; Margrave, J. L. J. Am. Chem. SOC.1987, 109. 4775-4780. (57) Steigerwald, M. L.; Goddard, W . A. J. Am. Chem. SOC.1984, 106, 308-311. Upton, T. H.; Rappe, A. K . J. Am. Chem. SOC.1985, 107, 1206-1 2 18.

(58) Sunderlin, L. S.; Armentrout, P. B. Unpublished results. (59) Allison, J.; Ridge, D. P.J. Am. Chem. SOC.1976, 98, 7745-7747. (60) Aristov, N.; Armentrout, P. B. J. Phys. Chem. 1987, 91,6178-6188. Sunderlin, L. S.;Armentrout, P. B. J. Am. Chem. SOC.1989, 1 1 1 , 3845-3855. (61) Schultz, R. H.; Elkind, J . L.; Armentrout, P. B. J. A m . Chem. SOC. 1988, 110, 41 1-423.

J. Phys. Chem. 1990, 94, 3597-3601


for this reaction for late transition metals. Note that barriers for reactions 22 and 23 are energetically mediated by ?r donation from the newly formed C-C double bond to the metal and, for reaction 22, relief of cyclopropane strain energy. This may explain why reaction 24 is nor observed for the late transition metals and also why barriers for reaction 22 are not apparent. Theoretical considerations indicate that processes like reactions 22-24 can occur with little or no barrier for metals where the bonding primarily involves a metal d ~ r b i t a l . ~ ' Calculations ,~~ suggest that this is true for Sc+-Cr+ but not for Mn+ and later transition metals, where the bonding has mainly s character.* This suggests that concerted pathways for reactions 22-24 should occur with larger barriers for later transition metals than for early transition metals. If this suggestion is true, then the upper pathway in Scheme I may not be a viable route to products at their thermodynamic threshold. An alternative mechanism for reaction 22 that involves a nonconcerted radical pathway is also shown in Scheme 1. This involves formation of 111, either directly from reactants or via Mn-C bond cleavage in 1. Formation of 111 should be only -0.4 ( 6 2 ) Reactions 22 and 24 are different from reaction 23 in that the metal-carbon bond formed is a T bond rather than a u bond. Direct formation of a metal-carbon T bond is not symmetry-allowed. Thus, for these reactions to occur in a concerted manner, interactions between the u and T bonds presumably occur.

3597

eV endothermic, assuming the Mn-C bond strength in Ill is equal to Do(Mn+-CH3). Movement of the radical center results in IV, which has an energy compared to reactants of 1.8 eV Do(H2CMn+-C2H4). Since Do(Fe+-C2H4) Z 1.4 eV,6' Do(Co+-C2H4) = 2.0 f 0.3 eV, and Do(Ni+-C2H4) = 2.1 eV,63 formation of IV is probably near thermoneutral (C0.4 eV endothermic). If the radical center is low-spin-coupled (quintet spin) to the nonbonding electrons on the metal, IV can collapse immediately to 11, which can then lose ethene to form ground-state products. If the electrons are high-spin-coupled (septet spin), the spin-orbit interactions may be necessary for the eventual formation of the Mn-C 7~ bond in 11. In either case, intermediate I1 is lower in energy than IV due to formation of the M-C 7~ bond. Overall, since reaction 12 is endothermic by 0.9 eV, this nonconcerted reaction pathway is thermodynamically plausible since it does not appear to lead to a barrier in excess of the reaction endothermicity. In the case of ethylene oxide, similar energetics hold, except that if the initial insertion is into a C - 0 bond, the energy of intermediates I and Ill can be lowered by donation from the lone-pair electrons of 0 to Mn+. Acknowledgment. This research is funded by the National Science Foundation. CHE-89 17980. (63) Hanratty, M. A.; Beauchamp, J. L.; Illies, A. J.; van Koppen, P. A. M.; Bowers, M. T . J . Am. Chem. SOC.1988, 110, 1-14.

I n Vitro Photodynamic Activity of Diprotonated Sapphyrin: A PP-?r-Eiectron Pentapyrroiic Porphyrin-like Macrocycle Bhaskar G. Maiya,t Mike Cyr,t Anthony Harriman,*,*and Jonathan L. Sessler*'t Department of Chemistry and Center for Fast Kinetics Research, The University of Texas at Austin, Austin, Texas 78712 (Received: July 3, 1989; In Final Form: October 10, 1989)

Optical and IH N M R spectroscopy indicates that diprotonated 3,8,12,13,17,22-hexaethyl-2,7,18,23-tetramethylsapphyrin (SAP2+), a 22-*-electron pentapyrrolic porphyrin-like macrocycle, dimerizes at quite low concentration in polar solvents such as acetonitrile or methanol but exists in a monomeric form in chloroform. Fluorescence quantum yields, excited singletand triplet-state lifetimes, quantum yields for formation of the triplet state and of singlet molecular oxygen 02('Ag), and rate constants for triplet quenching by ground-state oxygen have been determined in each of the above solvents. It IS seen that, in acetonitrile solution, both monomer and dimer species appear to retain similar photodynamic properties, both species exhibiting modest efficienciesfor generation of 02(IAg) upon illumination with visible light. The pentapyrrolic macrocycle binds in the form of an aggregate to human serum albumin and to liposomes, and under such conditions, the quantum yields for formation of triplet state and of 02(1$)are minimal. The suitability of sapphyrin as a photosensitizer for in vivo photodynamic therapy is discussed in terms of its in vitro photodynamic properties and by comparison to previously studied tetrapyrrolic macrocycles.

Introduction Macrocyclic ligands capable of complexing metal cations are finding an increasing number of applications in biomedical research, most specifically as photosensitizers in photodynamic therapy and as targets for magnetic resonance imaging processes. One approach to extending the range of compounds available for such studies involves the use of expanded porphyrins in which the basic ring structure is enlarged beyond the normal 18-*-electron periphery. Recently, the "texaphyrin" family of expanded porphyrins was introduced and shown to have useful photosensitizing properties.' We now extend this work to include "sapphyrin", a pentapyrrolic 22-*-electron "expanded porphyrin" first prepared by the groups of Johnson and Woodward2 a number of years ago *To whom correspondence should be addressed.

'Department of Chemistry.

'Center for Fast Kinetics Research.

but essentially unexplored in the years since.3 The sapphyrins possess two properties that make them of potential interest for biomedical applications. First, they contain an unusually large central cavity which could provide an effective means of complexing large lanthanide cations for use in magnetic resonance imaging. Second, they absorb strongly around 680 nm. This latter ( I ) (a) Sessler, J. L.; Murai, T.; Lynch, V.;Cyr, M. J . Am. Chem. SOC. 1988, 110, 5586. (b) Sessler, J. L.; Murai, T.; Lynch, V. Inorg. Chem. 1989,

28, 1333. (c) Harriman, A.; Maiya, B. K.; Murai, T.; Hemmi, G.; Sessler, J. L.; Mallouk, T. E. J . Chem. SOC.,Ckem. Commun. 1989, 314. (d) Maiya, B. K.; Harriman, A,; Sessler, J. L.; Hemmi, G.; Murai, T.; Mallouk, T. E. J . Phys. Chem. 1989, 93, 81 1 I . (2) Woodward, R. B. Presented at the Aromaticity Conference, Sheffield, U. K., 1966 (see ref 3a). ( 3 ) (a) Broadhurst, M. J.; Grigg, R. J . Chem. Soc., Perkin Trons. I 1972, 21 11. (b) Bauer, V . J.; Clive, D. L. J.; Dolphin, D.; Paine 111, J. B.; Harris, F. L.; King, M. M.; Loder, J.; Wang, S.-W. C.; Woodward, R. 8 . J . Am. Chem. SOC.1983, 105, 6429.

0022-3654/90/2094-3597$02.50/00 1990 American Chemical Society

Reactions of manganese (1+) with isobutane, neopentane, acetone

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