Of course, Harry is also a great source of humor and perspective, and he has taught me much of what I thought I came to Caltech to learn. Of course, the men in the instrument shop are the epitome of quality and, at times, tolerance. The reactivity of the metal dimers is strongly regulated by the detailed nature of the ligands ringing the axial site, hydrogen-.
For substrates with the same C-H bond strength that do not have a hydroxyl group (e.g. benzyl hydrocarbons), the rate of quenching is higher. The shape and size of the axial site, as determined by the ligands, also differentiates the quenchers according to their stericity. Selected bond distances for BF2 Pt2 and Pt2 Selected bond angles for BF2Pt2 and Pt2.
Selected geometrical parameters for BF2Pt2 and other BF2-bridged square-planar, platinum phosphite complexes. Photophysical parameters for axial dihydrides Stern-Volmer quenching rate constants for the reaction of excited platinum dimers and organic H atom donors.
Chapter 2 Figure 1
Chapter 1
Cumene and ethylbenzene react rapidly with Pt2 in the presence of air with loss of Pt2, requiring mixing of the two during quench studies only after degassing. [TOA)4Pt2 in neat toluene also degrades rapidly in the presence of air. For activation parameter studies, temperature in the range soc to 800C was controlled by immersion of the extinguisher in a water bath. Thus, changes in the 3 Pt2 * lifetime as temperature varies, in the presence of a given quenching concentration, directly reflect changes in the quenching rate.
Since changes in the solvent medium do not significantly alter the natural lifetime of Pt2, such effects are negligible and lifetime changes can only be attributed. The dependence of the 3 Pt2 * quenching rate constant on the C-H bond strength for a series of organic H atom donors. In the triplet excited state of Pt2 responsible for H-atom transfer reactions, an electron has been promoted from a dcr*.
This position is analogous to the hole left in the oxygen-localized n orbital in the n1t· state of organic ketones, which also draw H atoms from suitable ones. The rate of quenching, 111:-1/-co, reaches a limiting value as more alcohol is added (ie, the Starn-Volmer plot becomes horizontal, Fig. 4).
Scheme 2
The slope of the Stern-Volmer plot at low alcohol concentrations is equal to Keqk1, which corresponds to the previously reported kq. The slope of the Michaelis-Menten linear graph is equal to (Keqk 1 )-1; the high-concentration cutoff is k1-1. The entropic barriers in this unimolecular quenching step are quite large and reflect the highly organized nature of
For the series of aliphatic alcohols, including isopropanol, 2-butanol, and 3-methyl-2-butanol, the increase in substituent size increases the quenching rate dramatically from the lower limits of the technique (1 o4 M-1 s-1) to rates close to those of the benzyl alcohols . The variation of the Stern-Volmer quenching rate kq as a function of the cone angle of α-substituents on benzyl alcohols. Although the quenching of 3 Pt2* with alcohols has been extensively studied, deactivation of the excited state is only the first step in a sequence of reactions leading to the observed stable products Pt2H2, H2, and ketones.
Photolyses with cumene and ethylbenzene also produced a small amount of the dihydride, but the kp values were not. The activation parameters for the equilibrium step are the enthalpy, £\Heq, and the entropy of the reaction, £\Seq.
Chapter 2
In contrast to differences of several orders of magnitude observed in electron transfer reactivity, 1• 12 quenching rates for 3Pt2* and 3BF2Pt2* with a given H atom donor are all within an order of magnitude 13 (Table 6 and Figure 5). As demonstrated in Chapter 1, the steric bulk of the substrate increases kq with small aliphatic alcohols but decreases kq with the larger benzyl alcohols. This same tendency is observed when steric bulk at the axial site is increased, as with the substitution of BF2+ groups for protons.
The similarity in quenching rates of alcohols with Pt2 and B F 2 Pt2 also implies that the ligands' oxygen atoms participate in the hydrogen bonding of the alcohols to Pt2, since the ligand protons have been removed. On the other hand, the large quenching rate for benzyl methyl ether14 with 3 Pt2 * indicates that the ligand protons, when present, also aid coupling. Reactivity differences with benzylic hydrocarbons are not easily rationalized since the quenching is not well understood (see Chapter 1).
However, an electron transfer mechanism can be ruled out since the kq change with BF2 substitution is much smaller than the large one. The efficiency of the dehydrogenation of isopropanol measured by the formation of BF2Pt2H2 (kp=1x1Q4 M-1s-1, F=0.1) is identical to that of the reaction of Pt2 with isopropanol, a counterintuitive result since the axial dihydride is meant. as a result of disproportionation of the monohydride.15 Changing the sterics and hydrophilicity of the axial site should affect the efficiency of dihydride formation if this mechanism reflects reality. The emissive product of the reaction between BF3 and Pt2 is a species with a general structure identical to Pt2 except for exchange of BF2+ groups for the eight ligand protons. significant changes in bond lengths resulting from this substitution are consistent with expectations and reflect an increase in the 7t-surity of the pyrophosphito ligands.
The degree of emission quenching with H atom donors also varies little between the two compounds, and the differences that exist are simple. rationalized based on previous results with the parent system.
Chapter 3
The energy of each of the two photons is equal to the rest mass of the electron (or positron). The positron affinities for the hydrogen atom and elements Li through F of the second row (Table 1) show that none of the free atoms should bind positrons. Localizing in the cr framework allows the positron to interact with both lobes of the 1t.
Calculations of the positron wave functions for fluorobenzene7 show that the most stable orbital has four nodes. Tungsten foil is held between stainless steel plates attached to a flange at the end of the vacuum chamber. A reliable calculation of the ion current produced by annihilation requires some knowledge of the positron attachment cross section.
A reagent gas for attachment enters the reaction region through a hypodermic needle directed toward the center of the two grids. Two forms of data are extracted from the positron annihilation mass spectra: the internal energy of the molecular cation when it is formed and the relative positron annihilation rates for gaseous reagents. The mass spectrum collected from the experiment is a weighted sum of the spectra from the individual PAMS experiments.
The destruction rate is dependent on both the cross-sectional area of the fastener and the life of the PAC. The following section contains a detailed description of the positron destruction mass spectrometer, which was designed but only partially built. This limits the resolution of the mass spectrum and the triggered P1-P2 difference must be kept to a minimum.
There is currently no way to measure the actual pressure in the reaction area; only the total pressure of the vacuum chamber is known. Data is collected by the computer through a Metrabyte MDB64 board inside the computer case, which talks to the outside. The two outputs are 200~s 5V pulses triggered by the rising edge of the PMT pulse.
This number, multiplied by the time between individual pulses, provides the time of flight of the ions. This voltage is approximately the induced bias across P2 and therefore approximately the kinetic energy of the ion.
