Kinetics of Neutral Transition Metal Atoms in the Gas Phase

We present a broad survey of the reactivity of ground-state, neutral transition metal atoms from ... constants for the reactions of 9 ground-state met...
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10636

J. Phys. Chem. 1992, 96, 10636-10645

Kinetics of Neutral Transition Metal Atoms in the Gas Phase: Reactions of Sc through Cu with Alkanes and Alkenes David Ritter? John J. Carroll, and James C. Weisshaar* Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706 (Received: July 28, 1992; In Final Form: September 25, 1992)

We present a broad survey of the reactivity of ground-state, neutral transition metal atoms from the 3d series (Sc through Cu) with 11 alkanes and alkenes. Effective bimolecular rate constants are measured in 0.5-0.8 Torr of He in a fast flow reactor at 300 K. Reaction of specific spin-orbit levels is monitored by laser-induced fluorescence. None of the nine metal atoms reacts with linear alkanes. Sc, Ti, and V react moderately rapidly with larger alkenes. Ni reacts quite efficiently

with alkenes, even ethylene. From the absence of a measurable pressure dependence of the rate constant over the limited range 0.5-0.8Torr of He, we infer that bimolecular elimination chemistry occurs. For the Ti + C3H6and Ti + isobutene reactions, we have used photoionization mass spectrometryto try to identify the products. The results at 157 nm are consistent with H2eliminationin both cases. We interpret the pattern of reactivity with alkenes using a donoracceptor model of Malkene bonding. In favorable cases, electron promotion and sd or sp hybridization can relieve the long-range repulsive interaction with the 4s2outer-shell configuration of the ground-state metal atom. Apparently those neutral transition metal atoms that overcome long-range barriers and achieve close contact with alkenes p r d to break C-H or C-C bonds, much as transition metal cations do.

I. Introduction Bare transition metal cations, both M+ and M2+,are remarkably aggressivechemicals that can break C-H or C-C bonds of alkanes and alkenes, eliminating H2 and smaller alkanes in gas-phase collisions at room Due to the concentrated positive charge and the complete absence of ligands, the electronic structure of M+ and M2+is very different from that of a solvated metal cation or a surface atom of a metallic solid. As a counterpoint to M+ and M2+chemistry, we have begun a survey of the chemical reactivity of neutral transition metal atoms (M) with hydrocarbons! We hope to infer to what extent the unusual reactivity of M+ and M2+is due to the positive charge, which stabilizes d" and d"Is configurations relative to dr2s2 and provides attractive long-range force^.^ In earlier work, neutral metal atoms have been cocondensed with hydrocarbons in cryogenic matrices. The reaction products were studied by infrared and VisibleUV absorption spectroscopy.6 In the gas phase, we briefly reported on the reactivity of Fe, Co, Ni, and Cu with several alkanes and alkenes! Mitchell, Hackett, and co-workers' have studied association reactions of Cr,* Fe,9 Ni,Io and Cull with a variety of small molecules including ethylene. Increasingly powerful ab initio electronic structure techniques have recently been applied to the problem of transition metal atom association with ethyleneI2and insertion into the C-H bond of CH4 and the C-C bond of C2H6.I3 Here we present measurements of effective bimolecular rate constants for the reactions of 9 ground-state metal atoms, Sc through Cu, with 11 hydrocarbons a t 300 K in the presence of 0.5-0.8 Torr of He buffer gas. The neutral 3d-series metal atoms are much less reactive than their cationic counterparts. N o 3dseries neutral metal atom reacts with any linear alkane. Sc, Ti, and V react moderately rapdily with the larger alkenes. Ni reacts more efficiently with alkenes, including ethylene. The products are not yet identified, but we can infer that both bimolecular elimination and termolecular association reactions occur. The relative inertness of the neutral transition metal atoms is due to the 3dx-24s2ground-state config~rations.~~ The 4s orbital is much larger than 3d, so the 4s2 configuration presents a closed-shell appearance at long range, typically resulting in potential energy bamers to the close approach of an alkane or alkene. We interpret the observed pattern of reactivity with alkenes in terms of donoramptor models of Malkene bonding,I5including electron promotion and sd or sp hybridization to relieve repulsive Present address: Department of Chemistry, Southeast Missouri State University, Cape Girardeau, MO 63701-4799.

0022-3654/92/2096-10636$03.00/0

interactions at long range.I2J6 In addition to promotion energies, electron spin plays an important role in determining the height of adiabatic potential energy barriers to formation of long-lived M(a1kene) complexes. While the picture is .far from complete, our data suggest that those neutral transition metal atoms that can overcome long-range barriers and achieve close contact with alkenes proceed to break C-H or C-C bonds, much as M+ and M2+do. 11. Experimental Section

The kinetics technique uses a sputtering source to produce gas-phase transition metal atoms (M) in a fast flow of He buffer gas. Laser-induced fluorescence (LIF) monitors the decay of the metal atom concentration vs hydrocarbon number density. The flow reactor has been used for transition metal ion kinetia studies1' and more recently for neutral metal atom studies.'* Further details are given e1se~here.I~ The flow reactor (Figure 1) is a 7.3 cm i.d. stainless steel tube made of modular sections joined by end flanges sealed by O-rings and evacuated by a Roots blower. For this study, reaction lengths from the hydrocarbon gas inlet to the detection region were 54, 74, and 84 cm. The longer reaction lengths increase the reaction time for slow reactions. Gas-phase metal atoms produced in a hollow cathode source are thermalized in the buffer gas and carried down the flow tube. The He gas is purified by passage through a series of three traps containing zeolite sieve material at 77 K. To facilitate sputtering, about 100 STP cm3/min of Ar (Matheson, Prepurified, 99.998% minimum, used directly from bottle) is mixed into the He immediately before passage through the cathode into the high pressure region. All of the buffer gas mixture of 99% He and 1% Ar passes through the 2-mm hole in the center of the cathode. A stainless steel diaphragm with a 1 cm diameter hole in its center is positioned 8 cm downstream of the cathode to create a highpressure region at the head of the flow tube. The total pressure is -3 Torr upstream of the diaphragm. Total pressure in the reaction zone is either 0.5 or 0.8 Torr. Pressure is measured with a capacitance manometer (MKS Baratron, Model 310BH-10). The average bulk flow speed is 6240 f 160 cm/s in the reaction region. A thermistor indicates that the temperature in the reaction zone 74 cm downstream of the source is 300 f 5 K. A series of seven monel meshes floated at +lo0 V dc between the metal source and the reactant gas inlet strips ions from the flow while allowing neutral species to pass through to the reaction region. The hydrocarbon gas is added midstream through showerhead inlets oriented upstream relative to the He flow. The reactant inlet is always more than 74 cm downstream of the hollow cathode 0 1992 American Chemical Society

Kinetics of Neutral Transition Metal Atoms

DYE GLASS TUBE?

LASER

ti

I

U

Figure 1. Schematic of flow reactor showing hollow cathode source, laser-induced fluorescence detection, and photoionization mass spectrometric detection. PMT, photomultiplier; RB,Roots blower: DP, diffusion pump; QMS, quadrupole mass spectrometer.

source. Flow of hydrocarbon is regulated by a needle valve or a flow controller and monitored by a mass flow meter (Tylan). The hydrocarbon gases, ethylene (Matheson, >99.5%), propene (Matheson, >99.0%), hexafluoropropene (Matheson, >99.5%) , propened6 (Cambridge Isotope Laboratories, d6 >98%), 1-butene (Matheson, >99.0%), trans-2-butene (Matheson, >95%), cis-2butene (Matheson, >95%), isobutene (Matheson, >99.0%), 1,3butadiene (Matheson, >99.0%), propane (Matheson, >99.0%), n-butane (Matheson, >99.0%), and cyclopropane (Matheson, >99.0%) are used directly from the bottle. Because flow meter response is not linear with gas flow, we have calibrated the flow meters with each reactant gas. Due to the cost of C3D6,we use the C3Hs calibration curve modified by factors for density and heat capacity.20 Metal atom reactant number density is monitored by unsaturated LIF using a homemade pulsed dye laser pumped by an excimer laser (Lumonics EX-520, XeCl, 10 Hz, 10 ns fwhm). Available tunable pulse energies are -0.5 mJ/pulse for fundamentals and -0.1 mJ/pulse when the dye laser is frequency doubled (KD*P) into the near-UV. The laser beam enters and exits the system through Suprasil windows at the Brewster angle, traveling through 63 cm long baffled side arms to minimize scattered light. Total fluorescence is collected by a 5 cm diameter f / 2 quartz lens system and focused through a 5 mm wide slit before striking an EM1 photomultiplier tube (PMT, 9235 for UV excitation and 9658R for visible). The fluorescence is not spectrally resolved. The PMT current passes through a 50-ohm resistor; the resulting voltage pulse is integrated by an SRS Boxcar Integrator (Model SR-250).The integration gate opens 20 ns prior to the laser pulse. Scattered light is not a significant fraction of the signal. Typical gate widths are 100-150 ns. We detect as many as 1.5 X le photons/shot and estimate that each laser pulse creates as many as 3.5 X 10" excited-state atoms in the 3 mm X 3 mm X 3 mm detection region. The low-lying electronic states of each metal atom are listed in Table I. Metal atoms are detected with spin-orbit level resolution. Table I1 summarizes the atomic transitions used to study each metal atom. A wavelength scan checks the purity of the spectrum; lines are then selected for kinetics studies. We observe clean, sharp lines. Saturated LIF spectra indicate that the populations of spin-orbit levels within the ground term are roughly Boltzmann at 300 K. Except for Ni*(3D3),all of the intense lines in the spectrum are due to transitions from the ground term. In Ni, spin-orbit levels of the two lowest energy terms interleave. Both Ni(3F4,d8s2)and Ni*CD3,d9s1)are easily observed and studied; they behave identically in our kinetics measurements. 111. Kinetics Models The simplest kinetics mechanism is a single step

M

+ alkene

ki

productl

(1) where kl could include both bimolecular and termolecular components. By measuring the logarithmic attenuation of metal atom number density (M) vs hydrocarbon number density (hc) at fixed

The Journal of Physical Chemistry, Vol. 96, No.26, 1992 10637 TABLE I: Low-Lying Electronic Term Energies (1v cm-') of 3d-Series Transition Metal Atoms" 3dx-24s2 3d" 3dr14s 3dX-24s4p atom high low high low high low high low (x) spin spin spin spin spin spin spin spin Sc 2D 'F 2D 'F 2F 4F 2D (3) 0.1 33.8 36.3 11.6 15.0 15.8 16.0 Ti 'F ID 5D 'P 5F 'F 5G 'F (4) 0.2 7.3 28.9 34.3 6.7 11.6 16.2 19.4 V 4F 2G 6S 'F 6D 'D 6G 'D (5) 0.2 11.0 20.2 37.0 2.2 8.5 16.7 20.8 Cr sD 'P 5D 7S 7F sP (6) 8.0 23.6 35.5 0.0 7.6 25.3 29.6 Mn 6S 4G 4F 6D 4D 8P 6P (7) 0.0 25.3 45.0 17.4 23.6 18.5 24.8 Fe 'P 'F 5F 'F 7D 5D (8) 0.6 19.3 33.4 7.6 12.5 19.7 26.3 Co 'F 2G 2D 'F 2F 6F 'F (9) 1.0 16.8 28.0 4.3 7.9 24.5 29.0 Ni 'F ID IS 'D ID 5D 'G (10) 1.2 13.5 14.7 0.9 3.4 27.2 31.3 Cu 2D 2 s 'P 2F (11) 12.2 0.0 40.0 44.3 "Energies in lo3 cm-I; entries are the mean of the energies of the spin-orbit levels for each term. In each case, the lowest energy term of the specified configuration and electron spin is given. The number of valence electrons is x. Data from ref 14.

TABLE II: Laser-Induced Fluorescence Transitions" energy atom atom transition (cm-I) transition SC y2P512 a2D3/2 25 585 V x4GoSl2 a4Fs12 Sc y 2 P V 2 a2DSl2 25 556 Cr z 7 P 3 a7S3 Ti y'D, + a 3 F 2 25644 Mn @7/2 a6S5/2 Ti w3G? a3F2 31 374 z5Do4 aSD4 Fe Ti W ' G ~ ~ - ~ ' F ~ 31319 c o z4D0,12 a4FgI2 Ti w 3 G S 0 6 a 3 F 4 31242 z3P3 a3F4 Ni V y4P712 a4F712 25 799 Ni z'P4 a3D3 V y4F'Sl2 a4F512 25 867 Ni z3G05 a3F4 V y4FdSl26 a4F3/, 26 004 cu Z 2 P I / 2 a2s,/2 "Assignments from ref 14.

-

--

-+

+

+

+

energy (cm-l) 31 261 23 499 24 802 25 900 29 295 29 321 29 216 31 031 30 535

-n

trans-2-butene cis-2-butene

0.01

0

2

4

6

B

1 0 1 2 1 4

Alkene Flow (Std. cm3-min-' 1 Figure 2. Kinetics plots for reactions of Sc with various butenes.

mean reaction time t,,, = zIxn/uM,we obtain the effective bimolecular rate constant kl from the pseudo-first-order expression In [(M)/(M),] = -kl(hc)t,,,. The ratio (M)/(M)o is measured as the ratio of time-integrated metal atom fluorescence signal with and without hydrocarbon reactant flow; I,,, is the length of the reaction zone; and uM is the mean axial speed of the metal atoms, which is larger than the bulk flow velocity due to loss of metal atoms at the walls. At 0.8 Torr of He, uy = 9070 f 250 cm cl, as determined earlier17J8by timing the amval of different neutral metal atom packets from a pulsed laser ablation source to the LIF zone. On the basis of those results, we expect all metal atoms

Ritter et al.

10638 The Journal of Physical Chemistry, Vol. 96, No. 26, 1992

TABLE III: Effective Bimolecular Rate Constants k, (IO-'* cm3 s-') for Reactions of Transition Metal Atoms with Hydrocarbons at 0.80 f 0.05 Torr of He and 300 f 5 K" reactant Sc 2D3/2 Ti IF2 V 4Fp/2 Cr 'S3 Mn 6S5/2 Fe 5D4 Co 4F9/2 Nib 'F4 Cu 2S112 NR NR NR NR NR 0.5 f 0.05 NR NR NR ethene 9.6 f 1 NR NR NR NR 11 1 4 NR 9.5 f 1 6.2 1 0.6 propene propene-d, 8.3 & 0.8 5.0 1 0.5 9.6 f 1 21 * 2 NR NR NR NR NR NR 0.3 f 0.1 NR C3F6 NR 14 f 2 NR NR 14f2 7.1 f 0.7 NR 0.09 f 0.03 140 & 30 NR I-butene 22 f 2 NR NR NR NR 160 f 16 NR 20 f 2 7.5 f 0.7 trans-2-butene cis-2-butene 33 f 3 1415 3013 NR NR NR NR 155 f 16 NR isobutene 71 1 7 52f5 6817 NR NR NR NR 67 i 9 NR 1,3-butadiene 29 f 3 10f1 15f2 0.15 f 0.02 NR NR 0.35 f 0.04 110 f 11 NR cyclopropane 0.01 f 0.02 NR 0.02 i 0.02 NR NR NR NR 10f 1 NR propane NR NR NR NR NR NR NR NR NR n-butane NR NR NR NR NR NR NR NR NR " N R means no reaction observed, i.e., k, I cm3 s-I; dash means no information on this reaction. Uncertainties refer to the precision of the measurements; they are the larger of f l O % or f 2 standard deviations of the mean. Absolute accuracies estimated as f30%. bThe reaction rates of the Ni('D3) level were identical to those of Ni('F4) within experimental precision.

of the exponential decay of (M)/(M)o vs (hc), which still measures kl. Inclusion of a back reaction, i.e., product,

5

0

10

Alkene Flow (Std. crn3-rnin-')

Figure 3. Kinetics plots for reactions of Ni with C3H6and C,H6, showing inverse isotope effect.

to diffuse similarly in He and to stick to the flow tube walls with unit probability. Figures 2 and 3 show examples of the logarithmic plots. The semilog plots typically show good linearity over two decades; some of the lower signal-to-noiseplots and some of the slower reactions are linear over only 1.5 decades. We see no evidence of systematic curvature in any of the plots within the signal-to-noise of the data. Linear least-squares fitting produces the effective bimolecular rate constant k , , which could include both bimolecular and termolecular components. Collisional cascade of electronically excited states in the reaction zone has negligible effect on the ground-state rate constants. With the laser pitioned 75 cm downstream of the source, the intensities of lines due to M* excited states are always less than 1% of the ground-state intensities, except for Ni*. Even if the total excited-state population were appreciable, excited states may well react faster than the ground term.z1 Most importantly, quenching to the ground state on a time scale similar to that of ground-state reaction would lead to nonlinear semilog plots, which are not observed. Within the ground term we observe no discernible effect of initial spin-orbit level on the reaction rate constant. Either all the spin-orbit levels probed react at the same rate, or collisional interconversion of spin-orbit levels is fast compared with chemical reaction in our experimental conditions. More complicated mechanisms can be consistent with exponential decay of (M). The simplest extension is a sequential reaction: product,

+ alkene

kz

productz

(2)

Again, k2 could include both bimolecular and termolecular components. Sequential reactions could well occur in our experiments. Since we monitor only the reactant M,we are blind to its ultimate fate. However, sequential steps alone do not alter the meaning

-% M + alkene

(3) could be important if termolecular stabilization of M(a1kene) adducts dominates the decay of M. If k2 = 0 and if k-, is significant relative to k,(alkene), then the decay of (M)/(M)ovs (hc) would be nonexponential. The system approaches an equilibrium mixture of M, alkene, and M(alkene). If k2 = 0, then the linearity of the semilog plots precludes substantial back reaction in all cases. If both sequential reaction (k2) and back reaction (k-,) are important, then the decay of (M)/(M)o vs (hc) is complicated unless the steady-state approximation is made for product,. We have examined the exact solutions numerically in some detail. We conclude that exponential decay of (M) vs (hc) over two decades implies that we indeed determine k , from the metal atom decay within the f30% absolute accuracy of the measurements themselves. IV. Results A. Metal Atom Kinetics in 0.8 Torr of He. We have studied the reactions of 9 3d-series neutral transition metal atom ground states with 11 hydrocarbons at 0.80 f 0.05 Torr of He and 300 f 5 K. The specific spin-orbit states probed (Table 11) are S ~ ( ~ D , / , , d l s ~Ti(3F2,dZsz), ), V(4F3/2,d3s2),Cr(7S3,d5s'), Mn(6S3,z,dss2), Fe(SD4,d6sz), C O ( ~ F ~ / ~ ,Ni(3F4,d*s2), ~~S~), Ni+(3D3,d9s'),and Cu(2S1/2,d'os').An atomic state14 is labeled by its electron configuration, total electron spin S,orbital angular momentum L,and total angular momentum J, as embodied in the symbol zs+lLk Russell-Saunders coupling is a good approximation in the 3d series, where electron spin is a useful quantum number. The effective bimolecular rate constants are collected in Table cm3 s-I. 111. The sensitivity limit of the apparatus is k , > The error estimates in Table I11 refer to the precision of the 0.8 Torr rate constants. They represent the larger of &lo% or f 2 standard deviations of the mean of multiple measurements. The absolute accuracy of the observed rate constants is estimated as f30%, limited by the uncertainties in t , and gas flow calibrations, by incomplete reagent mixing, and by source instability and random noise in the data. We observe no reaction for any of the 3d-series transition metal atoms with the linear alkanes propane and butane. Cyclopropane reacts moderately rapidly with Ni(3F4)and Ni*(3D3) and very slowly with Sc and V. Sc, Ti, V, and Ni atoms are moderately reactive with alkenes larger than C2H,. The reactivity of Sc with various butenes is illustrated in Figure 2. The other five metal atoms are generally inert to alkenes. Sc, Ti, and V exhibit the same pattern of relative reactivity with each alkene. Ni reacts faster than Sc, Ti, or V with every alkene except isobutene. The two low-energy levels N ~ ( ~ * s ~at, ~0 Fcm-l ~ ) and Ni*(d9s',3D3) at 205 cm-l exhibit

The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 10639

Kinetics of Neutral Transition Metal Atoms TABLE I V Effective Bimolecular Rate Constants k l ( 10-l2cm3 8-l) for Reactions of Transition Metal Atoms with Hydrocarbons at 0.50 f 0.05 Torr of He and 300 + 5 C reactant Sc 2D3/2 Ti )F2 V 4F3/2 Ni 3F4 ethene propene 1-butene trans-2-butene cis-2-butene isobutene 1,3-butadiene

-

12+2

-

6 4 + 13

-

6.2 1 7.6+2 8*2 15i3 47+9 11+2

12+2

-

7 2 * 14

-

+

0.5 0.1 12 2 175+35 180 + 36

-

7 2 + 14

-

"Dash means no information on this reaction. Uncertainties are +20%, our estimate of the precision. Absolute accuracies estimated to be +30%%.

identical kinetics. This indicates either that they react at the same rate or that collisional interconversion of the two terms in He is rapid on the time scale of the chemical reactions. Both Co and Cr react slowly with butadiene, and Co reacts very slowly with 1-butene. The reactivity of C3D6and C3F6was investigated for each metal atom that reacts with C3H6. We observe no significant isotope effect in the C3D6vs C3H6rate constants for the reactions of Sc, Ti, and V. For both Ni(3F4)and Ni*(3D3),the C3D6rate constant is twice as large as the C3H6rate constants (Figure 3 and Table 111). This inverse isotope effect will be discussed below. The C3D6 rate constants could be systematically in error relative to the C3H6 rate constants by as much as 10% because we used the C3H6 calibration with a correction for density and heat No reaction of Sc, Ti, or V was observed with C3F6. The Ni(3F4) and Ni*(3D3)reaction rates with C3F6are comparable to those with C3H6. B. Kinetics Measurements at 0.5 Torr of He: Evidence of Bimolecular Chemistry. In an attempt to determine whether third-body stabilization contributes significantly to the observed rate constants, we carried out measurements at the lower He pressure of 0.5 Torr. The hollow cathode discharge is less stable at the lower pressure and cannot be used below 0.5 Torr in the main flow tube. Rate constants measured at 0.5 Torr are listed in Table IV. Each entry is a single determination of kl from 10 to 15 data points of metal atom LIF signal vs flow of hydrocarbon. Comparison with the 0.8 Torr rate constants in Table I11 shows no systematic change in rate constant with pressure within the estimated relative experimental uncertainty of f2Wo. For the 15 reactions for which we have measured rate constants at both 0.5 and 0.8 Torr, the two results agree within 10% in 11 cases and within 20% in 14 cases. In every case but one, the metal atom LIF decay at 0.5 Torr was exponential over more than one decade within the scatter of the data. In the case of Ni*(3D3,dgs1) C2H+ we observe an initial increase of about 10% in the LIF signal with flow of C2H4 at 0.5 Torr of He; subsequent exponential decay of the metal atom LIF signal with additional C2H4 then parallels the higher pressure

+

data. We interpret this as arising from fast quenching of Ni* excited states and subsequent slower reaction of Ni*()D3,d9sl). The weaker signal for Ni(3F4,dss2)did not allow study at 0.5 Torr. If a tennolecular reaction were in the linear,low-pressure regime of effective bimolecular rate constant vs He pressure, the measured rate constant kl would increase by a factor of 1.6 as the He pressure increased from 0.5 to 0.8 Torr. We can estimate a lower limit on the fraction of the measured k, due to bimolecular, as opposed to termolecular, reaction at 0.8 Torr of He by writing the ratio R = k1(0.8 Torr)/kl(0.5 Torr) = [kbi, k,,(0.8 Torr)]/[kbim k,,,(O.5 Torr)]. Here kbimis the bimolecular component and k1,,,(He) is the termolecular component of the measured k l . The experimental estimate R I1.4, which allows a generous 40% uncertainty in the ratio of measured rate constants at 0.8 and 0.5 Torr, implies that at least one-quarter of the measured kl is due to the bimolecular component kbim.It is much more likely that R I1.2, which implies that at least one-halfof k, is due to kbm. The data are consistent with the possibility that kl is dominated by bimolecular chemistry at 0.8 Torr of He. Table V includes the estimated lower limits on kbimfor each of the reactions studied at both 0.5 and 0.8 Torr. Mitchell and co-workers7J0have studied Ni C2H4 and Ni C3H6over the pressure range 5-100 Torr of COP Extrapolating their results to lower pressure, it is clear that both reactions are in the linear regime at C 0 2 pressures below 1 Torr. Since He is likely a less efficient third body than C02, we expect the linear regime to extend to higher pressure in He than in C02. Our inference of bimolecular chemistry in 0.8 Torr of He does not conflict with the dominance of a tennolecular mechanism at higher pressures. The lower limits on the bimolecular rate constants in Table V are about 50 times smaller than Mitchell's rate constants at 5 Torr of C02. The bimolecular component would be insignificant in the higher pressure experiments. Comparison of our data at 0.8 Torr of He with the low-pressure extrapolation of the falloff curve fitted to the data above 5 TOITof C 0 2indicates that He stabilizes Ni(a1kene) adducts at least 10 times less efficiently than C02. C. Photoionization of RoducQ. Identification of the products of neutral metal atom reactions is important but very difficult. We have attempted to probe the products using photoionization and quadrupole mass spectrometry. In these experiments, an excimer laser beam (Lumonics EX-520, 10 Hz, 10 ns fwhm, 0.5-30 mJ/pulse) is focused 2 mm upstream of the 1 mm pinhole in the molybdenum disk which samples ions into the differentially pumped chamber containing the quadrupole mass spectrometer (QMS, Figure 1). For the photoionization studies, 0.03 Torr of nitrogen (Matheson, Prepurified, >99.998'%) is added through a showerhead inlet 5 cm downstream of the source diaphragm to quench Ar and He metastables and minimize production of Ti+ and its reaction products in the reaction zone. Our initial efforts used multiphoton ionization at 308, 248, and 193 nm (0.5-30 mJ/pulse). Extensive fragmentation of products occurred. For example, we observed the photoion Tic2+from the products of the Ti C3H6reaction. The dominant photoion was

+

+

+

+

+

TABLE V Reaction Collision Number kb/k1, Upper Bound E,, on Activation Energy (kcal mol-'), and Lower Bound kMm(10-l2 cm3 8-l) on Pure Bimolecular Rate Constant for Sc, Ti, N, and Ni Reactions" SC(2D3/2) Ti('F2) V(4F3/2) Ni('F.,, 3D3) reactant n kbim n kbim n kbim n kbim ethene NR NR NR 500 3.7 0.1 2.1 24 1.9 2.8 propene 38 2.2 52 2.4 1.6 34 C3F6 NR NR NR 700 3.9 1-butene 24 1.9 3.5 44 2.3 2 21 1.8 3.5 2 0.4 35 trans-2-butene 17 1.7 42 2.2 2 14 1.6 2 0.3 40 10 1.4 2 0.3 cis-2-butene 10 1.4 23 1.9 3.5 isobutene 5 1.0 18 6 1.1 13 4 0.9 17 4 0.8 17 1,3-butadiene 12 1.5 32 2.1 2.5 20 1.8 2.2 05 "Collision number ( n ) is khs/kl, the ratio of the estimated hard-spheres collision rate constant to the observed effective bimolecular rate constant. E,,, is the maximum activation energy (kcal mol-I) consistent with the measured k ,(T=300 K), assuming Arrhenius behavior with prccxponential factor estimated as the hard-spheres rate constant. kb,m is the minimum pure bimolecular rate constant (10-l2 cm3 s-l) consistent with the observed ratio of effective bimolecular rate constants at 0.8 and 0.5 Torr of He (Tables 111 and IV). This is 25% of the measured k,. NR means no reaction; dash means no information.

Ritter et al.

10640 The Journal of Physical Chemistry, Vol. 96, No. 26, 1992

+

Ti+ from the Ti isobutene reaction and Ni+ from several Ni+ alkene reactions. Studies vs alkene flow show that much of the M+ signal is indeed due to a neutral product. Apparently three photons are absorbed. The first photolyzes the neutral product to produce neutral M and two more photons ionize M to produce M+. Laser-produced M+ may then react with the alkene in the time required to be sampled from the flow reactor to the mass spectrometer chamber. These complications led to attempts at 157 nm. One-photon ionization using the F2 laser at 157 nm (7.9 eV, roughly 0.5 mJ/pulse) should minimize fragmentation, since the photon energy is a good match to likely ionization energies of molecular products containing Ti. This experiment requires a N2-purged tube between laser and flow tube. The two reactions studied most carefully were Ti C3Hs and Ti isobutene. The most interesting laser-related mass spectral peaks occurred at m / e = 88 for Ti C3H6(the TiC3H4+cation) and at m / e 5 102 for Ti isobutene (the TiC4H6+cation). In both casea, the mass of the observed ion corresponds to elimination of one H2 molecule from the Ti(a1kene) complex. In both cases, ion signal vs alkene flow rises and then falls in a manner consistent with sequential reaction (eqs 1 and 2) with the same value of kl as in Table 111, and with k2 > kl. Significantly, no signals are observed at the mass of Ti(alkene)+, the parent ion of the product of termolecular association. It is tempting to claim that we have gently ionized the H2 elimination products of Ti C3H6and Ti isobutene without fragmentation, and thus directly demonstrated that bimolecular chemistry occurs. However, there are two possible problems. Fit, ionization occurs in the high-pressure zone of the flow tube, 2 mm upstream of the sampling d i c e . The ionizing laser could produce Ti+ from unreacted Ti. Ti+ photoions may subsequently react rapidly with alkene to eliminate H2 and produce false background signals at the masses we observed. Second, even if reactions of Ti+ photoions could be ruled out, the observed ions might still be due to dissociative ionization of termolecular products, e.g., Ti(C3H6) hu TiC3H4+ H2. Careful analysis shows that it is difficult to completely rule out either of these possibilities. While the photoionization results are inconclusive, they are at least consistent with the inference of bimolecular elimination chemistry from the rate constant measurements at 0.5 and 0.8 Torr of He. Future ionization work will place the laser beam in the low-pressure region downstream of the sampling orifice and use time-of-flight mass spectrometry to maximize detection sensitivity. This will minimize bimolecular ion-molecule reactions subsequent to photoionization.

+

+

+

+

+

+

v.

-

+

+

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IliSawion

The central results of our kinetics measurements are threefold. First, none of the ground electronic terms of the 3d-series neutral transition metal atoms reacts with linear alkanes at 300 K. Seoond, Sc, Ti, V, and Ni react with alkenes at 300 K. The other five atoms are inert except for very slow reactions of Cr and Co with larger alkenes. Third, the equality of the rate constants at 0.5 and 0.8 Torr within experimental uncertainty indicates that bimolecular elimination reactions occur with alkenes. These are presumably H2 or CH4 elimination reactions analogous to those observed in M+ alkene chemistry. In Ti C3H6and Ti isobutene, the photoionization results suggest H2 elimination chemistry. The discussion compares our gas-phase results with previous gas-phase and matrix isolation experiments and with ab initio theory. We also attempt to understand how the electronic structure of bare neutral metal atoms controls their chemical reactivity. A. Overview of Previous Experiments. 1. Reactioas in Cryogenic MaMccs. Neutral M + alkene interactions have been extensively studied in cryogenic matrices, particularly for the right-hand side of the 3d series. Typically metal atoms are cocondensed with reactant molecules in an inert-gas matrix or a matrix of the reactants themselves. The advantage of matrix work is that the resulting M(a1kene) complexes or the products of bimolecular reactions can be characterized spectroscopically in

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the infrared and visible-UV. The disadvantage is the difficulty of identifying the carrier of an observed spectrum. Klotzbucher, Mitchell, and O h z cocondensed vanadium with a variety of normal, branched, and cyclic alkanes. They found no reaction in the temperature range 10-100 K. Even V + cyclopropane, for which we observe a slow reaction at 300 K, was unreactive in the colder matrix. There has been controversy concerning the interaction of iron atoms with ethene. Parker et al.23cocondensed Ar/C2H4 gas mixtures with iron vapor at 16-20 K. They interpreted IR and Mossbauer spectra to indicate that the major product at low concentration is a r-bonded form of Fe-C2H,. However, Kafafi et al.24identified a set of product absorption bands in the 7001800-cm-l region for Fe ethene in argon matrix at 14 K. No shift typical of *-interaction was detected in the v ( C 4 ) frequency of ethene. Perturbation of the CHI stretching spectral region suggests a hydrogen-bonded complex (Fe-HC2H3). The complexes studied by Parker et al. were probably Fe(C2H4), w it! n > 1. In a cocondensation study of Co C2H4, Hanlan et al. observed several new product bands which they assign to Con(C2H4), with n = 1, 2 and m = 1, 2. The largest body of work on M ethene systems appears for Ni. In a series of studies, HisatsuneZ6reports that propene, 1-butene, cis-2-butene, and rranr-2-butene form unstable r-complexes when cocondensed with Ni atoms. The complexes undergo double-bond isomerization when slowly warmed from liquid nitrogen temperatures to sublimation; spectral signals indicate interconversion between the various isomers. The complexes eventually decompose back to the alkene metal atoms. Ni 1,3-butadiene shows no spectral change when cocondensed. Huber, Ozin, and Power27have published a series of reports on various Ni alkene complexes in cold matrices. They conclude in all cases that they are investigating simple r-type complexes of neutral Ni. In the IR, they observe a substantial reduction in the @pc stretching frequency v ( M ) from 1612 cm-'for pure ethene to 1496 cm-' for Ni(C2H4). There is a small body of work for Cu ethylene. In a matrix isolation ESR experiment, Kasai et ala2*identify a Cu-C2H4 complex. Huber et observe product peaks in the IR region which they attribute to Cu(C2H4). The stretching frequency v ( M ) is 1504,1496, and 1475 cm-' for Co(C2H4),Ni(C2H4), and Cu(C2H4),respectively. The value for free C2H4 in argon matrix is u(C==C) = 1623 c ~ - ' . ~ O They interpret the decrease in u(C-42) from Co to Ni to Cu as evidence of decreasing M(dr) CZH4(**) back-bonding in moving to the right in the 3d series. Most recently, the same Cu(C2H4)experiments were extended to include resonant Raman spectroscopy, confirming the earlier results.31 The matrix work appears reasonably consistent with our gasphase survey. There is little precedent for our observation of fast Sc,Ti, and V reactions with alkenes, but it was already clear from matrix work that Ni is a lively chemical. The gas-phase kinetics data support the conclusion that Fe atoms are inert to alkenes. The matrix literature suggests that Co and Cu form substantially bound binary complexes with ethylene, while no reaction is observed in our gas-phase conditions. However, termolecular reactions with an alkene as small as ethylene may often go undetected in the gas phase at 0.8 Torr and 300 K. 2. Gm-Phuse Repctio~. There are few studies of neutral gas-phase transition metal atom reactivity with organic molecules. Mitchell, Hackett, and co-workers7-" have studied Cr, Fe, Ni, and Cu reactions with small alkenes and other ligands in kinetics experiments detected by LIF. They use a static gas cell with total pressure variable from 1 to 700 Torr, a significant advantage over our flow tube experiment for distinguishing bimolecular from termolecular reactions. They found no reactions for Cr('S3,dSs1) with CO, C 2 K , benzene, C02, OCS, and CH30H.* Fe does not react with CO or with alkenes? Ni'v'O and Cull react with both CO and alkenes. In favorable cases,metal-ligand bindw energies can be extracted by fitting calculated rate constants from statistical theory to the observed dependence of effective bimolecular rate constant on third-body pressure.

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The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 10641

B. Limits on Activation Energies from 300 K Reaction Efficiencies. Since we study reactions from a Boltzmann distribution of translational energies at 300 K and a specific electronic state of M, the reactant energy distribution is well-defined. We can use reaction efficiencies to set limits on the activation energies of each observed reaction. For those reactions that occur in the flow tube at 300 K, we first estimate the hard-spheres collision rate constant kb and calculate the average number of hard-spheres collisions required for chemical reaction. The results are collected in Table V. Typical values of khsare about 3 X cm3 s-'.~~ The observed rate constants range from 0.01 X to 160 X 10-l2cm3 s-l. The lower limit of cm3 s-' on the measured rate constants means we cannot observe reactions that requires more than 3 X lo4 hard-spheres collisions. Because kBT is 0.6 kcal mol-' at 300 K, only reactions with activation energies in a narrow range will be observed in our flow tube. Assuming the Arrhenius dependence k , ( T ) = A X exp(-E/kB7') and using the estimated hard-spheres collision rate constant ke as an upper bound on the preexponential factor A, we can convert kl/kh,into an upper bound E, on the activation energy. For example, when k l / k h ,= 0.1, the activation energy can be no larger than E,,, = 1.4 kcal mol-'; when k l / k b = 0.01, E,,, = 2.8 kcal mol-'; and when kl/khs = 0.001, E, = 4.2 kcal mol-'. Table V includes these calculated upper limits on activation energy for each observed reaction. Since we have not measured k( T), the true activation energy might be substantiallysmaller than the calculated E,,,. It could be zero. This would mean that the pre-exponential factor is much smaller than khs,Le., that there is a steric requirement for reaction cm3 s-I), to occur. For those reactions not observed (k < either the activation energy is >5 kcal mol-' or the preexponential factor is quite small. If we loosely identify activation energy with a barrier height along a potential energy surface (section V.C.5 below), then the 300 K kinetics experiment selects M + hydrocarbon reactions with barrier heights below about 5 kcal mol-'. Temperature-dependent measurements that would distinguish large activation energy from small preexponential factor are desirable. C. EIeetronic Effects 011 Metal Atom Reactivity. 1. Low-Lying Electronic States of 3d-Series Metal Atoms. Both the size and energy of atomic orbitals influence the strength of metal-ligand chemical bonding.I6 Moreover, the pattern of low-lying electronic states affects chemical reactivity by determining the energetics of key potential surface intersection^.^^^ In a somewhat different language, the promotion energy from the ground state to certain excited states determines whether or not hybridization and strong chemical bonding can occur during the M + alkene collision. The low-lying electronic states of bare metal atoms are wellknown experimentally for M, M+, and M2+ in the 3d and 4d series.14 The energetic order of low-lying states results from a competition between electron configuration stability (orbital energies) and electron exchange effects that favor high-spin states.16 For a given atomic charge (0, +1, or +2), the effective nuclear charge felt by a 3d or 4s electron increases from left to right across a transition series, differentially stabilizing 3d relative to 4s. This lowers the energy of 3dx relative to 3dx-'4s' and of 3drW relative to 3dP24s2as nuclear charge increases. At the same time, electron exchange interactions stabilize high-spin terms relative to low-spin terms. Figure 4 graphs the energies of the low-lying electronic terms for the neutral 3d series metal atoms. In neutral M, most of the ground states (Sc, Ti, V, Mn, Fe, Co,and Ni) have 3dr24s2 configurations; the lowest excited states are high-spin 3dx-'4s1. In Cr and Cu, exchange interactions overcome orbital energetics so that the 3d54s1and 3dI04s1configurations become the ground states. Table VI compares some electronic properties of the neutral 3d-series metal atoms. From left to right (Sc to Cu), the first ionization energy generally increases. The mean orbital radii ( r ) & and ( r ) Mfrom HartreeFock calculations decrease as the effective nuclear charge increases from left to right. The 4s orbital contracts less rapidly than the 3d orbital; ( r)4s/ ( r )3d smoothly increases from 2.4 in Sc to 3.2 in C U . ~ ~

35

t

1

3d-SERIES NEUTRAL TRANSITION METAL ATOMS

3

4

5

6

7

0

9

101'1

Sc Ti V Cr Mn Fe Co Ni Cu Figwe 4. Selected low-lying terms of neutral transition metal atoms from the 3d series. Data are the mean of the spin-orbit-level energies for each term. Low and high refer to the relative spin within each atom; see Table I for term symbols and exact energies.

TABLE VI: First Ionization Ewrgy and Mean Orbital Radii (A). for 3d Series Transition Metal Atoms atom sc Ti

V Cr Mn Fe co Ni cu

I, (eV)

(r)&

(r)3d

(f)4s/(f)3d

6.54 6.82 6.74 6.77 7.44 7.87 7.87 7.64 7.73

2.09 2.00 1.92 1.85 1.79 1.72 1.67 1.62 1.57

0.89 0.77 0.70 0.64 0.60 0.57 0.54 0.51 0.49

2.4 2.6 2.7 2.9 3.0 3.0 3.1

"Expectation value ( r ) in Fock calculations, ref 32.

A for 3d and 4s orbitals

3.2 3.2

from H a r t r t t

2. Reactions with Aucpaea: Brief Comparison of M, M+, W+, and M- Chemistry. The long-range potential gives strong clues to the overal reactivity of transition metal atoms of various ~ h a r g e s . ~ JTransition metal cations M+ and M2+ are lively chemicals that dehydrogenate or even demethanate linear alkanes in low energy collisions.'J We have s e n that the neutral metal atoms are much more inert. The atomic data in the previous section explain why. The size disparity between 4s and 3d gives 3dx-24s2states a closed-shell appearance at long range. The long-range interaction with alkanes, which are closed-shell molecules, is repulsive. In contrast, the M+ ground states and low-lying excited states are either 3dP'4s' or 3dx; the 3dr24s2 states lie high in energy. Accordingly, the M+ ground states are highly reactive. The dipositive cations M2+have 3dx ground states, and these are also highly reactive. The anions M-have mostly 3dF24s2ground states, and the available data show that M- atoms are quite inert; they do react with certain sulfur-containingcompound~.~~ A more quantitative understanding of the short-range interaction between neutral M and alkane is emerging from ab initio calculations. In a recent study, Blomberg et ai." find potential barriers of 40-45 kcal mol-' for insertion of Fe, Co, or Ni into the C-C bond of C2H6. They find barriers of 20-26 kcal mol-' for insertion of the same three atoms into a C-H bond of CH,. The absence of M + alkane chemistry at 300 K is consistent with these calculated barriers. 3. Metal Atom Boading to Alkenes: D e w w C W - I h m a w a a Model. The chemical interaction between a transition metal atom

10642 The Journat of Physical Chemistry, Vol. 96, No. 26, 1992

Ritter et al.

TABLE VII:

Promotioll M e a for sd and ID HvbridiutioaP

sd hybridization spin

atom

ma (kcal mol-')

change?

43 33 24 22 67 34 20 6 -

same same same diff diff diff diff diff

sc Ti V

Cr Mn Fe

co Ni cu

-

sp hybridization P spin (kcal mol-') change? 45 same 55 same 59 same 85 diff 71 same 73 same 80 same 86 same 127 same 4

and AE,, are promotion energies to lowest term of proper configuration and spin to form sd- and sphybrid orbitals (Table I). Entry under 'Spin Change?" denotes whether the ground state and the relevant excited state have the same or different electron spins. See text. O

M

a

as discussed by Langhoff and BauschlicherI6and Siegbahn and m ~ o r k e r s . ' ~First, J ~ promotion from 3dr24s2 to 3dX-'4s' permits sd hybridization in which the orbitals Isd,) = (4su) (3du) and Ish) = 1 4 s ~-) 13do) are formed. With the sign of the 4su wave function chosen to match that of the two 3du lobes directed along the fz axis (Figure 5 ) , Isd,) concentrates electron density in an extended orbital directed along the intermolecular approach axis, which provides a good a-acceptor orbital. The (sd-) orbital concentrates density in a smaller, torus-shaped orbital concentrated near the xy plane, perpendicular to the approach axis, which decreases repulsion between the two nonbondmg 4sz electrons and the approaching alkene. As a rough measure of the energy required for promotion and sd hybridization, we use the excitation energy to the lowest electronic term with 3d"W configuration and the m e c t electron spin to allow double occupancy of the sb orbital (Figure 4). We call this energy aEd. Using the entries in Table I, aEd = 67 kcal mol-' for Mn; 34 kcal mol-' for Fe; 20 kcal mol-' for Co; and 6 kcal mol-l for Ni. Table VI1 collects these promotion energies. In Mn, Fe, Co, and Ni, the electron spin must change from the ground state to the state prepared for sd hybridization. Alternatively, we can consider sp hybridization, with (sp,) = 1 4 s ~ ) 14pu) and Isp-) = 1 4 s ~ ) 14pu). The Isp,) orbital is polarized along the +z axis, toward the incoming alkene, so it is the good acceptor orbital. The Jsp-) orbital is polarized along the -z axis, away from the incoming alkene, so it provides for lowenergy storage of the 4s2 nonbonding 'pair. The corresponding promotion energies are estimated as the excitation energy from the ground term to the lowest term of 3dP24s4p configuration and the appropriate spin to allow double occupancy of (sp-) (Figure 4). These energies, which we denote aEs (Table VII), are quite large on the right-hand side: 71 kcal mof' in Mn, 73 kcal mol-' in Fe, 80 kcal mol-l in Co, and 86 kcal mol-' in Ni. While the real atoms can optimize the mixing of 3du, 4su, and 4pu orbitals during the collision to minimize the potential energy as alkene and metal atom approach, the simple sd- and sp-hybridization picture provides insight into why Ni is reactive at 300 K while Mn, Fe, and Co are not. The promotion energy aEsp increases from 7 1 kcal mol-' for Mn to 86 kcal mol-l for Ni. These energies are apparently too large to permit sp hybridization from the 3dx-24s2ground states. Recent ab initio calculations by Widmark and roo^'^ find bound FeCzH4 complexes of both quintet and triplet spin, but both lie well above ground-state Fe(d6s2,5D) C2H4reactants. This is consistent with the absence of Fe + alkene chemistry in our work and in an earlier study by Mitchell and co-w~rkers.~ On the other hand, the sd-hybridizationscheme apparently is effective for Ni. The promotion energy AEd decreases rapidly from 67 kcal mol-I for Mn to 6 kcal mol-l for Ni. We infer that the sd-hybridizationscheme, which costs little promotion energy in Ni, permits formation of a strong M-alkene bond from Ni(3d84s2)reactants with little or no activation barrier. This occurs efficiently during the Nialkene collision, in spite of the need to

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Figure 5. Schematic of Dewar-Chatt-Duncanson donor-aoctptor model of M-alkene bonding. (a) C,, approach gmmetry. (b) Metal (3dJ alkene (r*)donor-acceptor interaction. (c) Alkene ( x ) metal ( 4 s q or a 4s-3d-3p hybrid orbital) donor-acceptor interaction.

and an alkene is often described in terms of the Dewar-ChattDuncanson (DCD) donoracceptor model.'s In the DCD model, bonding occurs via the simultaneous formation of two donoracceptor, 'dative" bonds, as pictured schematically in Figure 5 . The first bond involves donation of electrons from the alkene 2 p ~ orbital to the metal 4s orbital, forming a u bond that is axially symmetric about the axis of approach of M to the alkene (zaxis). The second bond involves 'back-donation" from the metal 3d,, orbital to the empty alkene 2pr' orbital, forming a bond of T symmetry with respect to the z axis. The donor and acceptor orbitals on the metal need not be pure atomic orbitals; it will often be advantageous to hybridize the metal atom orbitals. Accordingly, Chatt and Duncanson generalized Dewar's original prescription to allow for dsp hybridization (aacceptor) and dp hybridization ( r donor) of the metal atom's orbitals. The DCD model with hybridization and the energies of low-lying atomic states (Figures 4 and 5 ) can begin to explain why the ground states of only Sc, Ti, V, and Ni react with alkenes. 4. Promotion lhergks and Hybridizatiaa Scbesres. We begin on the right-hand side of the 3d series. In Mn through Ni, the ground state is 3dP24s2. Since the outermost orbital, 49, is doubly occupied and has the proper u symmetry to interact with the alkene T orbital, hybridization or promotion is required to relieve longrange repulsion and permit alkene metal donation to occur. Two simple promotion/hybridization schemes are of interest,

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Kinetics of Neutral Transition Metal Atoms

change electron spin from the triplet ground state to the singlet Ni(a1kene) adduct, as discussed below. Recent ab initio calculations2*clearly demonstrate the importance of sd hybridization in the 'Al ground state of NiC2H4. Cu is inert in 0.8 Torr of He because no low-lying state can form the fulldonoritcaptor bond with the alkene. The 3d'O4~(~S) ground state can form a half-bond (three electrons, two orbitals) with the alkene, which may be responsible for the complexes observed in mat rice^.^+^' In a core-pseudopotentialcalculation, Nicolas and B a ~ t h e l afind t ~ ~that ground state Cu(d'Os') C2H4 yield a weakly-bound (8.3 kcal/mol) complex with the ethylene geometry essentially unperturbed. Apparently such a low binding energy leads to complex lifetimes too short to be stabilized by third-body collisions in 0.8 Torr He at 300 K. Mitchell and wworkersl' do observe Cu(alkene) complex formation in the gas phase at higher pressure. The 3d94s4p(2F)term of Cu, which could form strong bonds, lies at 127 kcal mol-', far too high to be useful for ground state chemistry. On the left-hand side of the 3d series, Sc, Ti, and V react similarly with alkenes, while Cr is quite inert. Formation of the full alkene Cr donor-acceptor bond requires promotion to a 3d54sterm of low spin, such as 5Sat 22 kcal mol-'. This is almost identical to the situation in Co, which must change spin and pay the promotion energy Ud= 25 kcal mol-' in order to hybridize. Accordingly, Cr and Co are quite inert, although each reacts very slowly with one or two of the larger alkenes. While no single mechanism is obviously responsible for the reactivity of Sc, Ti, and V, we can point to two important distinctions. First, in Sc, Ti, and V, sp and sd hybridization from the ground state require no change of electron spin (Table I). For a given promotion energy, this results in lower adiabatic potential barriers, as discussed below. Second, the promotion energy Us,, is relatively small in Sc, Ti, and V (45-59 kcal mol-', Table VII). In the other atoms, sd hybridization requires a change of electron spin (Table I) and Us, is much larger (71-127 kcal mol-'). Sc, Ti, and V react remarkably similarly with both alkenes and oxidants.18 Based on the trends in promotion energy, we suggest that both sp and sd hybridization are important as the Sc, Ti, and V ground states approach alkene. If sd hybridization dominated, we would expect V to react faster than Ti and Ti to react faster than Sc, since Ud decreases from Sc (43 kcal mol-') to Ti (33 kcal mol-') to V (24 kcal mol-'). At the same time, Uvincreases from Sc (45 kcal mol-') to Ti (55 kcal mol-') to V (59 kcal mol-'). The similarity in reactivity among the three metal atoms thus suggests that both hybridization schemes contribute. While A&,, = 45-59 kcal mol-' seems quite a large promotion energy, sp hybridization has the advantage over sd hybridization of providing a polarized a-acceptor (sp+) orbital and a polarized nonbonding Isp-) orbital. The polarization improves overlap between Isp,) and the alkene u orbital and relieves electron-electron repulsion between the nonbonding pair in Isp-) and all other electrons. A more quantitative view of the binding between Sc, Ti, V, and alkenes awaits ab initio calculations. While our discussion deemphasizesthe metal alkene (3d,, u * ) component of the bonding, calculations on NiC2H4find substantial charge transfer from metal to ligand.I2 This interaction probably occurs at shorter range than the repulsive 4s2-u2 interaction that must be overcome to enable ground-state chemistry. This suggests that as Ni and alkene approach, electron density first moves from alkene to Ni at long range (u sd+ donation and concurrent s2 sd-2 promotion/hybridization) and subsequently from metal to alkene at shorter range (3d,, d').In the low-lying states of Sc, Ti, and V, any 3d orbital is at most singly occupied, which diminishes the importance of the backdonation into u*# 5. Model Potential Energy Surfaces. The orbital interactions of the previous section are the underlying cause of the potential energy barriers we infer from inefficient M alkene reaction rates. In Sc, Ti, and V,neither sp nor sd hybridization requires a change of electron spin from the ground state. This suggests the potential surface model of Figure 6, which assumes only sd hybridization for simplicity. M(3dp24s2) alkene initially approach on repulsive

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Figure 6. Schematic potential energy curves showing promotion energy AEd and avoided crossing of diabatic curves (light lines) to form adiabatic curves (heavy lines). (a) In Sc + C2H4, hEd is large but crossing is strongly avoided because ground and excited state have same spin. (b) In Ni + C2H4,AEd is smaller, but crossing is weakly avoided due to change of spin. See text and Table VII.

potentials due to long-range interaction of the closed 4s subshell with the doubly occupied u orbital of the alkene. In favorable geometries similar to the C, approach of Figure 5, the repulsive, diabatic 3dx-24s2surface intersects attractive, diabatic surfaces of the same spin correlating to excited-state reactants, M*(3dp14s) alkene. Here diabatic means conserving both electron spin of the full system and orbital occupancy of the metal atom. The crossing of two diabatic surfaces of the same spin will be strongly avoided. The lowest-energy adiabatic surface changes electron configuration, but not electron spin, along the reaction coordinate. This is the sd-hybridization mechanism described in the language of potential energy surfaces. The height of the barrier on the lowest energy adiabatic surface correlating to ground-state reactants (lower heavy line in Figure 6a) should be roughly proportional to the atomic excitation energy Ud(and perhaps also Us,,) from Table VII, as discussed above. Apparently the resulting barrier is sufficiently small for Sc, Ti, and V + alkene to allow reaction at 300 K. Collisions with sufficient kinetic energy and favorable approach geometry overcome the barrier and form long-lived M(a1kene) adducts. Internally hot adducts may eventually break C-H or C-C bonds and evolve to H2 or CH4 elimination products, or they may be collisionally stabilized by the third bcdy He. For Cr through Ni, formation of the bound M(alkene) adduct requires a change of electron spin, as depicted in Figure 6b for Ni + alkene. Now the repulsive, diabatic surface from groundstate reactants intersects attractive, diabatic surfaces from excited-state asymptotes having lower electron spin (Table I). Ground-state reactants must surmount the barrier on the lowest energy adiabatic surface (lower solid curve), whose height is due to both the promotion energy AEd and the need to change spin. This is sd hybridization with a concomitant change of electron spin, described in the language of potential energy surfaces. Quantitative information is emerging for the Ni + C2H4 potential surfaces pictured in Figure 6b. Mitchell and co-workers7J0 estimate the binding energy of NiC2H4to be 35 kcal mol-' relative to ground-state Ni(3F) + C2H4. Apparently such a substantial binding energy is necessary for us to ohserve collisional stabilization in M + C2H4at only 0.8 Torr of He. Recall that the Cu + C2H4 reaction is not observed at 0.8 Torr. Recent ab initio calculatiod2

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10644 The Journal of Physical Chemistry, Vol. 96, No. 26, 1992

find a 'Al ground state of NiC2H4bound by only 10 kcal mol-' relative to ground-state Ni CzH4. The calculated diabatic singlet-triplet crossing point lies 10 kcal mol-' above Ni(3F) C2H4, Our 300 K experiments show that the activation energy on the adiabatic surface is at most 4 kcal mol-'. For Mn and Fe, and perhaps for Co, the M(a1kene) complex may lie at higher potential energy than ground-state M alkene. The promotion energies are too large compared to the resulting bond energies. The calculations of Widmark and Roos34indicate this is the case for Fe C2H4. While parts a and b of Figure 6 are qualitatively similar, they differ quantitatively in an important way. We expect the spinchanging diabatic surface intersections of Figure 6b to be more weakly avoided than the spin-allowed intersections of Figure 6a, since spin is a useful quantum number in the 3d series. For a given promotion energy A&,, the adiabatic barrier height should be larger for spin-changing M alkene addition (Cr through Ni) than for spin-allowed addition (Sc through V). The kinetics data appear to support this picture. Cr (A& = 22 kcal mol-') and Co (A& = 20 kcal mol-') are quite inert at 300 K; they require a change of spin in order to hybridize. Sc (A& = 43 kcal mol-') and Ti (A& = 33 kcal mol-') are quite reactive; they have larger promotion energies but require no change of spin. The only efficient reactions requiring a change of spin are those of Ni, which is a particularly favorable case. Ni has the smallest aE, and the strongest spin-orbit coupling between diabatic surfaces of Merent spin. Of course, parts a and b of Figure 6 are highly oversimplified. Even in C, geometry, many potential energy surfaces arise from each orbitally degenerate metal atom asymptote as the alkene approaches. In general, a few surfaces may be attractive and many will be repulsive. Our discussion focuses only on the key electronic considerations. Finally, we have assumed that formation of the *-bonded M(alkene) complex determines M alkene reactivity, in spite of evidence (section 1V.B and Table V) that bimolecular elimination chemistry occurs for Sc, Ti, V, and Ni. Even if the ratelimiting chemical step were insertion of the metal into a C-H or C-C bond of the alkene, the electronic requirements are very similar to those for M(a1kene) formation. In order to insert, a 4s2 ground state must form sd hybrids, and in addition the electron spin must change for Cr through Ni. The deuterium isotope effects in M = C3H6also suggest an indirect bimolecular mechanism in which formation of M(C3H6)precedes bond insertion, as discussed next. 6. Deuterium Isotope Effects in M C&. For the reactions of Sc, Ti, V, and Ni with C3H6,we measured the effect of complete deuteration of the alkene (Table I11 and Figure 3). Sc, Ti, and V showed no isotope effect, while Ni showed an inuerse isotope effect (reaction with C3H6a factor of two slower than with C3D6). In any mechanism whose overall rate is determined by the dissociation lifetime of internally hot M(alkene)* adducts back to M alkene reactants, we expect a moderate inverse isotope effect. The binding energies of M(a1kene) adducts can be substantial; Mitchell and co-workers7J0find 35 kcal mol-' for NiC2H,. Long-lived complexes should form with wntially the same probability per collision for both isotopes, since the transition state does not involve motion of hydrogen atoms. The density of vibrational states of M(alkene)* is larger for the deuterated complex, so the statistical dissociation lifetime will be longer. In the lowpressure limit of a termolecular mechanism, longer lifetime means greater probability of collisional stabilization. The inverse isotope effect for Ni C3& is thus consistent with a termolecular process. However, the same inverse isotope effect is also consistent with a bimolecular H2 or CHI elimination reaction which proceeds through a long-lived Ni(C3H6) complex. In this indirect bimolecular mechanism, the initial step is formation of the long-lived complex (k,), after which elimination (k,) and dissociation back to reactants ( k d )compete. With the steady-state approximation applied to the complex, the net bimolecular reaction rate is kl = k,k,/(k, kd). We expect kd to decrease on deuteration; k, may decrease or remain constant. If k, is smaller than or comparable

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+

to kd for Ni + C3H6 at 300 K, then the net rate of product formation will increase on deuteration if kd decreases by a larger factor than k,. In this picture, the 5% reaction efficiency of Ni + C3H6(Table V) is due to a slow elimination step. The absence of a deuterium isotope effect in the Sc, Ti, and V reactions can also be consistent with bimolecular elimination, even if H2 is the product. In the indirect bimolecular mechanism, if k, is determined by the rate of insertion in a C-H bond, we expect a normal isotope effect for that step. The overall rate constant kl can still be independent of deuteration if k, >> kd for both MC3H6and MC3D6. In this limit, all collisions that reach the deep well live there long enough to eliminate, with or without deuteration. The small reaction efficiencies (2-3%) then require that k, be small, suggesting that the Sc, Ti, and V reaction rates with C3H6and C3D6are limited by the fraction of collisions at 300 K that reach the M(C3H6) or M(C3D6)wells. This initial step has little or no deuterium isotope effect. If this picture is correct, increased kinetic energy will accelerate the Sc, Ti, and V reactions. On the other hand, the absence of a deuterium isotope effect in Sc,Ti, and V + C3H6seems inconsistent with a predominantly termolecular mechanism. The density of states of the complex should increase from C3H6to C3D6,producing a measurable inverse isotope effect in the low-pressure limit.

VI. Summary and Conclusion Our broad survey of reactivity provides some new insight into the mechanisms by which neutral transition metal atoms in the 3d series interact with alkanes and alkenes, but further work is necessary. Although we believe the M + alkene reactions have a substantial bimolecular component at 300 K and 0.8 Torr of He, we have not clearly identified the elimination products. Further experimental studies at variable temperature and pressure such as those of Mitchell and co-workers7-" will provide valuable new information. For example, the absence of M alkene reaction at 300 K could mean that M(alkene) lies at higher energy than ground-state reactants (as calculations suggest for Fe + C2H4) or that M(alkene) lies below reactants but is separated from them by a bamer in excess of 5 kcal mol-'. Variabletemperature data can distinguish these possibilities. In our labs, we plan to explore the reactivity of the 4d-and 5d-series neutral atoms, which in effect provide the opportunity to vary the size and energy of the valence nd and ( n + 1)s orbitals and measure the effect on chemical reactivity.16 Ab initio theory is increasingly able to provide bond energies and barrier heights of useful accuracy as well as quantitative insights into transition metal-hydrocarbon bonding mechanisms. We hope that our inference of bimolecular chemistry in the reactions of Sc,Ti, v, and Ni with alkenes will inspire searches for low-energy paths to C-H and C-C bond-insertion intermediates in these systems. On the left-hand side of the 3d series, little is known about the optimal hybridization scheme for binding to alkanes or alkenes. Quantitative understanding of these electronically complicated systems will come only from the interplay of experiment and theory.

+

Acknowledgment. We thank the National Science Foundation (CHE-9000503) and the donors of the Petroleum Research Fund for generous support of this work. References and Notes (1) For recent reviews of M+chemistry, see: Eller, K.;Schwarz, H. Chem. Rev. 1991,91, 1121. Armentrout, P. B. In Gas Phase Inorganic Chemistry; Russell, D. H., Ed.; Plenum: New York, 1989; p 1. Buckner, S. W.; Freiser, B. S. In Gas Phase Inorganic Chemistry; Russell, D. H., Ed.; Plenum: New York, 1989; p 219. Ridge, D. P.;Meckstroth, W. K.In Gas Phase Inorganic Chemistry; Russell, D. H., Ed.; Plenum: New York, 1989; p 93. Armentrout, P. B.; Beauchamp, J. L. Acc. Chem. Res. 1989, 22, 315. Allison, J. Prog. Inorg. Chem. 1986,34,621. Armentrout, P. 8. Annu. Rev. Phys. Chem. 1990, 41, 313. (2) Weisshaar, J. C. In State-Selected and State-to-State Ion-Molecule Reaction Dynamics, Part I; Ng, C.-Y., Ed.; Wiley: New York, 1992. (3) For a review of M2+chemistry, see: Roth, L. M.; Freiser, B. S . Mass Spectrom. Reu. 1991, 10, 303. (4) Ritter, D.; Weisshaar, J. C. J. Am. Chem. SOC. 1990, 112, 6426.

J. Phys. Chem. 1992, 96, 10645-10653 ( 5 ) Weisshaar. J. C. In Gas Phase Metal Reactions; Fontijn, A., Ed.; ACS Symposium Series; Elsevier: Amsterdam, 1992. (6) For reviews of matrix isolation spectroscopy, see: Hauge, R. H.; Margrave, J. L. In Chemistry and Physics of Matrix-Isolated Species; Andrews, L., Moskovits, M., Eds.;North Holland: New York, 1989. Ball, D. W.; Kafafi, Z. K.; Fredin, L.; Hauge, R. A Bibliography ofMatrix Isolation Spectroscopy: 1954-1985; Rice University Press: Houston, 1988. Ozin, G. A. ACC.Chem. Res. 1977, I O , 21. (7) Mitchell, S.A. In Gas Phase Metal Reactions; Fontijn, A., Ed.; ACS Symposium Series; Elsevier: Amsterdam, 1992. (8) Parnis, J. M.; Mitchell, S.A.; Hackett, P. J. Phys. Chem. 1990, 94, 8 152. (9) Mitchell, S. A.; Hackett, P. J. Chem. Phys. 1990, 93, 7822. (10) Brown, C. E.; Mitchell, S.A,; Hackett, P. Chem. Phys. Lett. 1992, 191, 175. (11) Blitz, M. A.; Mitchell, S.A,; Hackett, P. J. Phys. Chem. 1991, 95, 8719. (12) Widmark, P. 0.;Roos, B. 0.;Siegbahn, P. E. M. J. Phys. Chem. 1985,89, 2180. (13) Blomberg, M. R. A.; Siegbahn, P. E. M.; Nagashima, U.; Wennerberg, J. J. Am. Chem. Soc. 1991, 113,424. (14) Moore, C. E.NBS Circ. No. 467 (US.Dept. of Commerce, Washington, D.C., 1949), Vol. I. ( I S ) Dewar, M. J. S. Bull. Soc. Chim. Fr. 1951, 18, C71. Chatt, J.; Duncanson, L. A. J. Chem. Soc. 1953, 2939. (16) Langhoff, S.R.; Bauschlicher, C. W., Jr. Annu. Rev. Phys. Chem. 1988, 39. 181. (17) Tonkyn, R.; Weisshaar, J. C. J. Phys. Chem. 1986,W. 2305. Tonkyn, R.; Ronan, M.; Weisshaar, J. C. J . Phys. Chem. 1988,92,92. Tonkyn, R.; Weisshaar, J. C. J. Am. Chem. Soc. 1986, 108, 7128. (18) Ritter, D.; Weisshaar, J. C. J. Phys. Chem. 1990, 94, 4907. (19) Ritter, D. Ph.D. Thesis, Department of Chemistry, University Wisconsin-Madison, 1990. (20) Instruction manual, Tylan mass flowmeters, July 1981. Chao, J.; Zwolinski, B. J. J. Phys. Chem. Ref. Dara 1975, 4, 251. Pamidimukkala, K.

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Transient EPR of the Spin-Polarized Triplet States of Chlorophyll and Retinal in Liquid Crystaiilne Matrlx: Characterization of the Overall and Librational Pigment Dynamics Andreas Miinzenmaier, Norbert Riisch, Stefan Weber, Christine Feller, Ernst Ohmes, and Cerd Kothe* Department of Physical Chemistry, University of Stuttgart, Pfaffwaldring 55, D- 7000 Stuttgart 80, Germany (Received: July 31, 1992; In Final Form: September 24, 1992) The molecular dynamics of chlorophyll a and retinal, dissolved in liquid crystalline phase 85-1084, has been studied by transient EPR following pulsed laser excitation. The time evolution of the transverse magnetization of the photoexcited triplet states is monitored at various static and microwave magnetic fields. Analysis of the EPR experimentshas been achieved by employing a relaxation model based on the stochastic Liouville equation. Simulation of the transients at high microwave magnetic fields provides the parameters for the overall pigment motions. From the EPR response at low microwave power detailed information about the librational dynamics is obtained. The evaluated correlation times range from s in the slow motional to lo-” s in the fast rotational time regime. 1. Introduction The transient nutation technique, introduced by Torrey,’ is a well established method in time-resolved EPR. Combination of this technique with pulsed laser excitation has provided valuable infonnation about short-lived paramagnetic~ p e c i e s .In~ contrast to steady-state methods, transient EPR measures the full time evolution of the magnetization. This property is of particular importancefor the study of photoinduced species where large initial electron spin polarization occurs due to selective population processes. Under proper conditions, the time resolution of transient EPR compare8 favorably with pulsed methods? Moreover, the full time development of the magnetization is obtained, undiminished by any microwave detection dead time. Finally, no restriction exists with respect to excitation bandwidth. Apparently, transient EPR is particularly suited for the study of light-induced triplet states in viscous environments, where large anisotropic magnetic interactions d ~ m i n a t e . ~ J + ~ However, while considerableinterest has centered on the molecular organization of the photoexcited triplets and the mechanism of their formation and decay, relatively little attention has been

focused on the dynamic behavior of the species.lWI2 In this paper we present a transient EPR study of molecular dynamics in liquid crystals, using spin polarized chlorophyll and retinal triplets. Both pigments play an important role in biophysical processes such as photosynthesis and vision. EPR time profiles with oscillatory (transient nutations) and nonoscillatory behavior are observed. Analysis of the EPR experiments is achieved by employing a relaxation model based on the stochastic Liouville equation. Simulation of the transients at high microwave magnetic field provides the parameters for the overall pigment motions. From the EPR response at low microwave power detailed information about the librational dynamics is obtained. The results clearly show that transient EPR is a powerful method for studying molecular motions over an extremely broad dynamic range, extending from the picosecond to the millisecond time regime. 2. Theory

In this section we develop a dynamic EPR model for S = 1 spin systems in transient nutation experiments,employing the density operator formalism. Specifically we consider a sudden, light-in-

0022-3654/92/2096-lO645%03.00/00 1992 American Chemical Society