11628
J. Phys. Chem. 1993,97, 11628-1 1634
Spin Change Induced in Vanadium(1) by Low-Field Ligands: Binding Energies of V+(H*),, Clusters (n = 1-7) John E. Bushnell, Paul R. Kemper, and Michael T. Bowers' Department of Chemistry, University of California, Santa Barbara, California 93106 Received: July 9, 1993; In Final Form: September 1 , 19936
Dissociation energies for H2 loss from V+(H2)n clusters have been determined via temperature-dependent equilibrium measurements for clusters containing up to seven H2 ligands (D.E. = - W o = 10.2, 10.7,8.8,9.0, 4.2,9.4, C2.5 kcal/mol for n = 1-7, respectively). The V+(H2)" clusters for n = 1 and 2 are less strongly bound than the corresponding Co+(H& clusters, in agreement with recent theoretical results. We find an anomalously high binding energy for the sixth hydrogen ligand on vanadium and argue this is due to a change in spin state from a quintet to a triplet. The clustering entropies were also measured. Results are also presented for V+(HzO)(H&, with binding energies of 9.9, 8.5, and 6.9 kcal/mol for n = 1-3, respectively, and V+(H20)2(Hz) with a binding energy of 6.7 kcal/mol. The present results are compared with recent calculations on the V+(H2),, systems.
extensively.' A brief overview is given here with emphasis on the differences encountered in the vanadium experiments. Activation of u bonds by transition-metal ions is currently of TbeInstrument. The V+ ions are formedvia surface ionization great interest due to its importance in the conversion of alkanes of VOCl3 on a hot filament (Re ribbon T 2500 K). The nascent to more chemically valuable substances. The simplest of these ions are accelerated to 5 keV, mass selectedwith a double focusing, systems to study is a single transition-metal ion interacting with reversegeometry mass spectrometer,deceleratedto approximately H2. Previously, we have reported bond dissociation energies and 3-5 eV, and injected into a reaction cell containing H2 at about entropies for Co+ with up to seven H2 ligands1 (as well as for Co+ 1 X 1017 molecule/cm' (3 Torr at 300 K.) The ions are quickly with up to three CH4 or C2Hs ligands.*) Results are obtained thermalized via collisionswith the H2 and move through the 4-cmby measuring equilibrium constants as a function of temperature long reaction cell under the influence of a small, uniform electric for the various clustering reactions: field. The electric field is kept small enough that the ion thermal energy is not significantly perturbed. The H2 pressure in the ~ + ( ~ 2 ) n+_H, l = M+(H2)n (1) reaction cell is monitored directly with a capacitance manometer. Bond dissociation energies (BDE) are then extracted from Cell temperatures are varied using a flow of heated or cooled N2, statistical mechanical fitting of the data. and temperatures are measured using a thin-film platinum resistor suspended in the bath gas. Ions exiting the cell are accelerated In this paper we report results for the correspondingvanadium clusters V+(H& (n = 1-7). Also included are results for H2 slightly (2-5 eV), quadrupole mass analyzed, and collected using standard ion counting techniques. The quadrupole is computer binding with the V+(H20)1,2cluster ions. These systems provide scanned over the mass range of interest and the baseline resolved auseful comparison tobonding of Hz withother transitionmetals. peaks are then integrated to give the relative ion intensities. First, the most basicquestionconcerns the typeof bonding present. The Equilibrium Experiment. After the temperature and H2 The Co+-H2 bonds are not inserted but rather are largely electrostatic with a relatively small amount of electron t r a n ~ f e r . ~ . ~ pressure are observed to be stable within the reaction cell, product/ parent ion ratios are measured as a function of reaction time. In the Sc+-H2 ion, however, the Sc+ does insert into the H-H bond.5 Vanadium ions are believed to insert in the C2H2 ?r bond This time is varied by changing the drift voltage across the cell. As the drift time is increased, the product/parent ion ratios and could conceivably insert into the H-H bond also.6 This eventually become constant, indicating that equilibrium has been question needs to be answered. Further questions arise in comparing V+ bonding to dihydrogen with that of Co+. For reached. With the smaller clusters (V+(H2)1,2)this occurred at the shortest accessible drift times (about 150 p). The larger example, the V+ ground electronic state is a SD[Ar]3d4configuration, while the Co+ is a 3F[Ar]3d8 configuration.7 Since the clusters required more time and in all cases the reactions were first H2 will probably approach the unoccupied d orbital (to probed out to 3.5 ms to ensure equilibrium had been established. This measurement also insures that the drift-field is not signifminimize repulsion), the effect of an empty du orbital (in V+) can be compared with that of a half-filled du orbital (in Co+). icantly perturbing the ion thermal kinetic energy. For selected In the Co+-H2 complexes, back donation from the filledd r orbitals experiments, the pressure of H2 was varied by a factor of 2 with had a large effect on the bonding.3.4 By comparing the V+ and no significant change observed in AGOT. Co+ clusters, the effect of half-filled vs filled d r (and d6) orbitals The ion ratios are converted to equilibrium constants using eq can be seen. Finally, the V+ ion is significantly larger than the 2, where P H is~ the hydrogen pressure in Torr and V+(H2)" and Co+ ion,8 and one wonders how this size change will affect the bonding. Comparison of the V+ and Co+ systems provides qualitative (and some quantitative) answers to these questions.
I. Introduction
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11. Experimental Technique
Both the instrument and data analysis methods have been described previously.9JO Sourcesof error have also been discussed
V+(H2),1 are the measured intensities of the cluster ions of interest. The standard free energy change is calculated using eq 3, where R is the gas constant, and T is the temperature. Free
Abstract published in Advance ACS Abstracts, October 15, 1993.
AGO = -RT In K O ,
0022-365419312097-11628$04.00/0
0 1993 American Chemical Society
(3)
Binding Energies of V+(H2)nClusters
The Journal of Physical Chemistry, Vol. 97, No. 45, 1993 11629
energies are measured at a variety of temperatures until a satisfactory range of data has been collected. A number of potential sources of error are present in these experiments, e.g., pressure and temperature inaccuracies, mass discrimination and resolution, and the presence of electronically excited metal ions. The effect of these factors on our dissociation energies and entropies are discussed in ref 1. The net result is that the bond dissociation energies are essentially unaffected by these uncertainties. The entropies are affected to a small extent by any mass discrimination in the quadrupole mass analyzer, but in these experiments such mass discrimination should be very small since the parent/product mass difference is only 2 AMU. Two separate effects lead to nonlinear AGO vs T plots: lack of mass resolution (leading to overlapping of peaks) and the presence of V+ excited electronic states. Care was taken to baseline resolve the different product masses and to avoid overlap of adjacent peaks. Electronic states with 4s3d"' configurations are much less strongly bound than the states with 3dn configurations due to the increased repulsion of the 4s orbital. Consequently, the 3dn states cluster preferentially. Since V+ has a 3d4ground state, the presence of any long lived V+ 4s3d3 excited states would perturb the first clustering equilibrium by causing an excess of V+. The presence of long-lived 4s3d7 excited states created a problem in the Co++ H2 experiments, making it difficult to accurately measure K p for addition of the first Hz ligand.' The effect of excited 3d34s1electronic states is not a problem in the V+ experiments reported here, however, since these excited V+ states are effectively quenched in collisions with H2. This quenching was demonstrated in three ways. First, equilibrium was attained rapidly in the V+ + H2 V+(H2) reaction, showing that the residual amounts of excited V+ 5F (4s3d3) and 3F (4s3d3) were far smaller than the true equilibrium amount of V+ SD (3d4) ground stateafter about 104colli~ion~ with H2. This result implies that the quenching rate constant for excited 3d34sl electronic states is k > 5 X l@I4cm3s-I. In contrast, the Co+ ions required roughly 10times more collisions for equilibrium to be established.' A second check on the deactivation of excited V+ was made by forming V+ by 100 eV electron impact (EI) instead of surface ionization (SI). While SI produces mostly ground state V+ (80%) with small amounts of the first two excited states (SF 20% and 3F< l%), E1 is known to produce very little ground state, along with large amounts of highly excited V+.ll Despite this high initial excited-state population, the equilibrium results were identical with the surface ionization experiment, showing that even very high energy states are deactivated under our experimental conditions. Finally, the arrival time distributions of V+ in pure He and in He with small amounts of D2 added were collected. The ATD in He of V+ formed by SI shows the three lowest V+ electronic states in the ratio expected for SI (Figure la). As H2 (or Dz) is added in small amounts to the He bath gas, deactivation of the excited states begins (Figure lb). In Figure IC it is shown that the arrival time distribution of V+(Dz)' can essentially be superimposed on the ATD for ground-state V+. The inescapable conclusions are that the V+ excited electronic states are rapidly quenched and that we are observing clustering of ground state V+. The presence of deactivation, of course, implies that activation occurs as well. This is important because it implies that a thermal distribution of spin-orbit (J) states is also present. This affects our calculation of the electronic degeneracy and is discussed below.
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111. Data Analysis and Results
The observed AGO vs T data for the V+(H2)n/V+(H,)n., equilibria are plotted in Figure 2. The corresponding data for the V+(H,O)(Hz),/V+(H20)(H2)~l are shown in Figure 3. This functional form, rather than the more usual van't Hoff plot, is used to facilitate the statistical mechanical analysis. The
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Drift Time ( p s ) Figure 1. Arrival time distributions(ATDs), in arbitrary intensity units, for (a) V+ formed by surface ionization with 5 Torr of pure He in the reaction cell. The three peaks have been previously assigned.I1l Peaks I and I1 correspond to the excited states 'F (3d34s1)and SF (3d34sl), respectively, and peak I11 is ground state 5D (3d4). Note that the peaks are baseline resolved, indicating essentially no deactivation is occurring. The relative intensities of the peaks correspond to those expected for a Boltzmann distributionat the filament temperature used in the S I source. (b) V+ formed by S I with 3 Torr of He and 6 X l e Torr of D2 in the reaction cell. Note the filling in between peaks I1 and 111, indicating deactivation has begun. Note also the relative intensity of peak I1 has decreased relative to peak 111. This process continues with increasing D2 pressure until complete coalescenceof peak I1 with peak I11 has occurred. A similar process occurs for peak I but is difficult to observe due to the low intensity of this peak. (c) V+(D2) ATD under the same conditions as panel b. Note that the ATDs of peak I11 for V+ and the V+(D2) peak can be nearly superimposed,indicating ground state sD V+ (3d4) is the exclusive source of V+(D2). association enthalpies ( M O T ) and entropies (&So.,) aregiven by the intercepts and slopes of the AGO vs T plots. These are listed in Table I. To obtain the desired cluster bond dissociation energies, the A H O T must be converted to A H o o (= -BDE). This is done by calculating AGO as a function of temperature using statistical mechanics. The bond lengths, frequencies, and dissociation energy used in the calculation are varied until the experimental and calculated functions agree. It might appear that given the large number of molecular parameters, that a wide range of dissociation energies would result. This is not the case. The majority of the association entropy is due to the loss of H2 translational and rotational modes. These values are known exactly. The experimental slope thus serves to tightly fix the possible range of the cluster vibrational frequencies which appear in the product ion. This, in turn, tightly fixes the rangeof thevibrational heat capacity. Since both the translational and rotational heat capacities of the product and reactants are known exactly, the uncertainty in the calculated value of M o o is small. A subtle point in the analysis concernsthe electronicdegeneracy. The V + ( H Z )clusters ~ are all calculated to have SA symmetry12 (except V+(H2)6,7, see below) and the value of ASEL is obvious
Bushnell et al.
11630 The Journal of Physical Chemistry, Vol. 97, No. 45, 1993
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10.2f 0.5 10.7 f 0.5 8.8 f 0.4 9.0f 0.4 4.2f 0.5 9.6 f 0.5 9.9 f0.5 8.5h0.4 6.9f 0.7 6.7f 1.5
19.1 f 1.1 22.2f 1.3 21.7 f 1.4 26.1 i 1.5 26.1 f 1.5 40.9f 2.8 21.5 f 1.4 22.2f 1.3 24.3 f 2.1 20.9f 4.4
18.2f 1.0 17.0f 0.7 9.6f 0.5 9.6f 0.6 4.3 f 0.7 4.0 f 0.7
20.6f 1.5 24.5 f 1.5 20.5f 1.5 25.2 f 1.5 21.9 f 1.5 23.8f 1.5
In units of kcal/mol. In units of cal/mol.K.
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dependence, as do the AS,l and AH4 values calculated from it. The derived value of M O O refers to V+(5Do) H2 V+(H#Al. Figure 2. Plots of AGO vs temperature for the equilibria V+(H2),+,+ Ab initio calculations for the V+(H2),,clusters have been done Hz P V+(H& for n = 1-6. The AGO were obtained from the measured by Maitre and Bauschlicher.12 A relatively low level of theory values of K p as described in the text. (UMP2 with a relatively small basis set) was used to determine structures and BDEs for n = 1-5. Calculations at the same level of theory were also done on CO+(Hdn: The resulting structures and bond lengths for Co+(H& were nearly identical to those calculated using MCPF wave functions and a very large basis set. This result indicates that the corresponding V+(H2)" (n = 1-5) structures and bond lengths are probably quite reliable. Thus, our calculated ASRoTvaluesare expected to be similarly accurate. The Co+-H2 stretching frequencies, which were calculated a t the UMPZ level, were about 35% lower than those from the higher level calculation. The change in the bending and internal rotation modes was not determined. Thus, there is greater uncertainty in the cluster frequencies and in and A&B. In our data analysis, the A S O T O T A L calculated using the UMPZ frequencies agreed well with our experimental entropy values for the second through fifth clusters. For the first association, however, significantly lower frequencies were needed to match the exper-6 -7 ! I I imental entropy. The reason for this is unknown, but since only 200 300 400 500 the first cluster is affected, it may have to do with the V+ electronic state distribution. In all cases, the vibrational frequencies were Temperature(K1 Figure 3. Plots of AGO vs temperature for theequilibriaV+(H~O)(H~),+I varied over a wide range to see the effect on the resulting W o ( B D E ) and these uncertainties are included in the error + H2 P V+(HzO)(H2),for n = 1-3. The AGO were obtained from the measured values of Kpas described in the text. limits. The bond lengths and frequencies used in thedata analysis are listed in Tables 111 and IV. TABLE I: Experimental A P T and A P T Values for In contrast to the first five clusters, it is clear from the data in Figure 2 and Table I that formation of V+(H2)6 is a special case. The M O T of association declines roughly pairwise for the first four clusters (from -1 1.5 for n = 1 and 2 to -9.9 kcal/mol temp rangec m n -MOP for n = 3 and 4). For n = 5, M O T = -5.6 kcal/mol, but jumps 11.8f0.3 19.5f 0.2 330-550 0 1 to -1 1.7 kcal/mol for n = 6. Similarly, S O T of association is 21.7 f 0.4 0 2 285-525 11.2f0.3 far more negative for the sixth cluster than for any of those 9.9f 0.3 0 3 22.1 f 0.5 250-400 preceding (-36.7 vs --25 cal/mol K). We can conceive of no 9.8 f 0.3 240-350 0 4 25.5 f 0.6 impurity or artifact that could possibly mimic the AGO vs Tdata 200-290 0 5 5.6 f 0.2 24.5 f 0.5 200-270 0 6 11.7 0.3 36.8 1.9 shown in Figure 2 and thus are convinced that the data are valid. 0 7
