J . Pkys. Ckem. 1984,88, 6014-6018
6014
appears entirely in the SO(%) producta5 This implies that the reaction occurs by a stripping mechanism, in which case vibrational excitation is expected for the SO('Z), but not the CO, in agreement with the observed excitation." Conversely, the product distribution for reaction 2a is not consistent with a stripping mechanism. The results are more consistent with the suggestion by van Veen et al.9 that OCSz is a potentially stable molecule having C,, symmetry analogous to cyclopropanone or C 0 3 . They point out that the progression of C-0 bond lengths in oing from OCS to cyclopropanone to CO is 1.16, 1.19, and 1.13 ,respectively. The energy distribution of the products is then to be correlated with the slope of the potential energy in the region where the reaction products are first formed. The vibrational excitation of COt suggests that the slope is attractive with respect to the C-0 bond length, as is plausible from the sequence of bond lengths.
1
Appendix Kinetic Model for the Vibrational Energy Disposal. Vibrational excitation is initially formed by reactions j which produce COT in an average quantum state vi. The redistribution of this energy is defined by the following energy-transfer processes which have rate constants kj and convert an energy difference Aej into translational energy: CO(1)
+ OCS(OO0) * C O ( 0 ) + OCS(OO1)
+ M * OCS(OO1) + M + M * OCS(lO0) + M OCS(Ol0) + M a OCS(OO0) + M
(Al)
OCS(040)
('42)
OCS(020)
(A31 ('44)
The time-dependent concentration of each species n, and the vibrational excitation of each of the selected classes of vibrational states defined below is calculated by numerical integration of the following equations3' from known initial conditions:
dni/dt = CSjtj J
('45)
du,/dt = vjtj/nco - kIlVm(uc + 1) - uc(um + 1) e x ~ ( - A e ~ / k T J l(Ab) dvc/dt = kl(n,o/nocdhn(uc + 1) - uc(um + 1) exP(-A.El/kT2)1 + k2bb4 - 5vc e x P ( - k / k T ) ) dv,/dt = k3(vb2- 30, exp(-Ae3/kT))
(A71 (A81
dUb/dt = -k&b(t) - Ub(t,)] - 4k,{Vb4 - 5Uc exp(-Ae,/kT)) 2k3bbz - 3 4 exp(-AdkT)I (A91 The terms are defined as follows: 6, = f l if ni is a product or reactant of reaction j and is 0 otherwise;,tj = kjm,m, is the extent of reaction j during time interval dt; v, is the quanta of excitation of CO per molecule of CO; v, is the quanta of CS stretching mode of OCS per molecule of OCS; ub is the quanta of bending mode of OCS per molecule of OCS; uc is the quanta of CO stretching mode of OCS per molecule of OCS; Ob, = 5 exp(-4hvb/kTb)(l - exp(-hv/kTb)I2 is the fraction of molecules in the OCS(040) level; and Ub2 = 3 exp(-2kvb/kTb)(l - eXp(-kv/kTb))' is the fraction of molecules in the OCS(020) level. The equations apply to arbitrary distributions of C O and the C O and CS stretching modes of OCS in the harmonic oscillator approximation. The bending mode is taken to be equilibrated at a bending-mode temperature Tb, where ub = 2/(exp(hv/kTb) - 1). The kinetic temperature T, calculated from the heat capacities and heats of reaction, is nearly constant in these systems. Registry No. OCS, 463-58-1;CO,630-08-0;CS, 2944-05-0;N2, 7727-37-9; 02,7782-44-7; S,7704-34-9; Ar, 7440-37-1; He, 7440-59-7; 0,17778-80-2. (33) Herzfeld, K. F.;Litovitz, T. A. "Absorption and Dispersion of U1trasonic Waves"; Academic Press: New York, 1959;pp 83-90 and 110-1 16.
Kinetlcs of the Reactions of n-Donor Bases wlth Mn,'.
Absolute Mn+ Affinities1
Barbara S. Larsen,* R. B. Freas, 111,s and D. P. Ridge* Department of Chemistry and Center for Catalytic Science and Technology, University of Delaware, Newark, Delaware 19716 (Received: May 7, 1984)
Rate constants determined by ion cyclotron resonance methods for reaction of Mnz+formed by electron impact on Mnz(CO)lo with n-donor bases are reported. The principal product is a complex of Mn+ with the reactant base. Reactions are efficient for species with relatively high affinity for H+,Li+, and Mn+ and inefficient otherwise. On the assumption that the efficient reactions are exothermic and unobservable reactions are endothermic and using previously reported values for D(Mn2+)and relative Mn+ affinities we tentatively determine absolute Mn+ affinities. The Mn+ affinities are compared to the corresponding Li+ and cyclopentadienylnickel cation affinities. The results suggest that the bonds in CH3-Mn-OH+ are weaker by 20-30 kcal/mol than the bonds in MnCH3+and MnOH+.
Introduction The Mn2+ion formed by electron impact on M ~ I ~ ( C reacts O)~~ in the gas phase with a number of n-donor bases, B, according to eq 1. I In this paper are reported rate constants for reaction Mn2+ + B MnB+ Mn (1)
-
+
1 for a number of bases, B. A scale of relative values of D(Mn+-B) is available,2 and bases with the largest relative values of DBased in part on: Larsen, Barbara S.Ph.D. Dissertation, University of Delaware, Newark, DE, 1983. *Current address: Mid-Atlantic Center for Mass Spectrometry, Johns Ho kins Medical School, Baltimore, MD. !Current address: Naval Research Laboratories, Washington, DC.
0022-3654/84/2088-6014$01.50/0
(Mn+-B) react rapidly. Those with small relative values of D(Mn+-B) react slowly. Explored below is the possibility that these kinetic results provide the basis for a quantitative relationship between the scale of relative Mn+ affinities (AH for separating MnB' into Mn+ and B) and D(Mn+-Mn). Such a relationship either can provide a means of determining absolute Mn+ affinities from the relative scale or can provide a means of evaluating D(Mn+-Mn) in the event that absolute Mn+ affinities can be determined in some other way. (1)Ridge, D.P. "Lecture Notes in Chemistry"; Hartmann, H., Wanczek, K. P., Eds.; Springer-Verlag: West Berlin, 1982;Vol. 31, pp 140-152. (2) Uppal, J. S.; Staley, R. H. J. Am. Chem. SOC.1982,104,1238-1243.
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 24, 1984 6015
Reactions of n-Donor Bases with Mn2+ TABLE I: Product Distributions for Reactions of Mnz+with ROH ionic uroduct ROH
Mn(ROH)+
Mn20H+
CH30H CZHe,OH
1.oo 0.80
n-C9HTOH
0.90
0.20 0.10
t-C4HgOH
0.80
0.07
MnH20S
0.13
At present the cyclopentadienylnickel cation is the only ionic transition-metal species for which a scale of absolute metal ligand bond strengths have been reported., There are also two conflicting recent reports of values for D ( M ~ + - M I I ) . ~There ~ ~ are, therefore, few unequivocal data on which to base firm thermochemical conclusions at this time. As new data appear, however, the relationships discussed here will be available to assist in interpreting the data and further elucidating the interaction of transition metals with ligand molecules. Even in the absence of conclusive thermochemical data, a number of interesting conclusions can be drawn about the interaction of Mn+ with the molecules studied. Of considerable value in making these conclusions are values of D(M+-H20) for several metals derived from gas-phase ion hydration studies.6-10 Experimental Section The rate constants were measured by using a conventional ion cyclotron resonance apparatus. A three-region "flat" drift cell with a 1.27 X 2.54 cm cross section was used to assure proper drift motion.' A previously described capacitance bridge detector was used to obtain the cyclotron resonance spectra of ions in the cell.12 In a typical experiment, was admitted to a pressure of 1 X lo4 torr and the base to a pressure of 0.5 X 104-1 X torr. Drift spectra were taken at pressures corresponding to low extent of conversion of reactant to product so that rate constants could be simply deduced from the reactant and product ion signal intensities.13 Each reported rate constant is the average of three or more determinations. Pressures were determined with an ionization gauge. The ionization gauge readings were corrected for the various n-donor bases by using ionization cross sections from the 1iterat~re.l~The accuracy of the absolute rate constants is estimated at *40%, limited primarily by the estimated accuracy of the pressure determination. The relative values of the rate constants are reproducible to *20%. All the reactions were verified by double The n-donor bases reacted with MnCO+ to form complexes of the bases with Mn'. The relative amount of the Mn+-base complex coming from MnCO+ was readily determined by double-resonance ejection experiments. The MnCO+ peak is only 20% the intensity of the Mn2+peak in the mass spectrum of Mn2(CO)lo,so its contributions to the Mn+-base complex were small and easily corrected
for by using the double-resonance results. Where more than one product was observed, the branching ratios were determined by double-resonance ion ejection experiments. Upper limits on rate constants for reactions not observed were determined from estimates of the minimum detectable product signal. was obtained commerically and purified by vacuum distillation. Its mass spectrum was found to agree with the literature. l5 The measurements were done on spectra taken at 80-eV ionizing energy. Reducing the energy to 30 eV did not change the results significantly. Reducing the electron energy further reduced the signal to noise so that reliable measurements could not be obtained. Results The n-donor bases react with Mnz+ predominantly to form a complex of the base with Mn+. The alcohols react to give other minor products as indicated in Table I. The Mn20H+ product, in particular, may be pertinent to the bonding mode in the complexes of the alcohols with Mn+ as discussed below. The atomic ion, Mn+, reacts with methyl, ethyl, n-propyl, and tert-butyl alcohols to form MnOH' at rates of approximately (1-2) X cm3 s-l. The t-C4HgOH also reacts with Mn+ to give a small cm3 s-l). amount of MnC4H8+( k < 1 X The rate constants for reaction 1 are listed in Table 11. They cm3 s-l, those fall into three categories: those on the order of for which no reaction product could be detected, and those of intermediate rate constant. As indicated in Table 11, the reactive bases tend to be relatively strong bases toward a proton16 and metal ions such as Li+ l7 and Mn+.2 The unreactive bases are weaker bases. The bases of intermediate reactivity also have intermediate basicity. In the case of CH,SH, which has relatively high proton affinity but relatively low Mn+ affinity, the reactivity follows the Mn+ affinity. This suggests that reaction 1 proceeds efficiently ( k / k , g 1) if exothermic, inefficiently ( k / k
