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A. The Dynamics of Reaction Intermediates — Vibrational Relaxation and Molecular Morphology Change

A. The Dynamics of Reaction Intermediates — Vibrational Relaxation and Molecular Morphology Change

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Figure 3



Yang and Harris



Alkane C–H bond activation by Á3 -TpŁ Rh(CO).



Figure 4 Transient difference spectra in the CO stretching region for

Á3 -TpŁ Rh(CO)2 in room-temperature alkane solution at various time delays

following 295 nm photolysis. Panel (e) is an FTIR difference spectrum before and

after 308 nm photolysis. A broad, wavelength-independent background signal from

CaF2 windows has been subtracted. (Adapted from Ref. 29.)



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Bond Activation Reactions



87



a non-Boltzmannian population distribution such that higher vibrational

levels may also be occupied to the extent that they become observable.

As a result, the v D 1 ! 2 and v D 2 ! 3 CO transitions appear at 1958

and 1945 cm 1 , respectively. The two hot bands gradually decay away

while the v D 0 band 1972 cm 1 rises as the system approaches thermal

equilibrium. The time scale for such intramolecular relaxation (IVR) is

measured by monitoring the population dynamics of the v D 0 state. The

1972 cm 1 band shows a fast rise of ¾23 ps, attributed to the above IVR

process, and a slower decay of ¾200 ps (Fig. 5a). The ¾200 ps decay



Figure 5 Ultrafast kinetics (dots) of Á3 -TpŁ Rh(CO)2 in room-temperature alkane

solution after 295 nm photolysis at (a) 1972 cm 1 , the CO stretch of the solvated

Á3 -TpŁ Rh(CO) (alkane) intermediate, and (b) 1990 cm 1 , the CO stretch of the

arm-detached Á2 -TpŁ Rh(CO) (alkane) intermediate. (Adapted from Ref. 29.)



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88



Yang and Harris



indicates that the Á3 -TpŁ Rh(CO)(S) solvate reacts to form another species

on this time scale. Indeed, another peak appears at 1990 cm 1 at later time

delays (Fig. 4c and d). The correlation between Á3 -TpŁ Rh(CO)(S) and the

1990 cm 1 species is established by the ¾200 ps rise of the 1990 cm 1

band (Fig. 5b) (29).

The 1990 cm 1 band is attributed to an Á2 -TpŁ Rh(CO)(S) solvate

where one of the three chelating pyrazolyl ligands detaches itself from

the Rh center. To verify this assignment, the fs-IR spectra following UV

photolysis of Á3 -TpŁ Rh(CO)2 are compared to those of an analogous

compound Á2 -BpŁ Rh(CO)2 (BpŁ D H2 B-PzŁ2 ) (Fig. 6). Photolysis of Á2 BpŁ Rh(CO)2 in alkane solutions results in the Á2 -BpŁ Rh(CO)(S) solvate

that also exhibits a single CO-stretching band at ¾1990 cm 1 , thereby

providing experimental evidence for the assignment (30). Theoretically,

Zaric and Hall computed the reaction using a CH4 to model the alkane

solvent (31). Their DFT calculations show that the Á2 -TpRh(CO)(CH4)

(Tp D HB-Pz3 , Pz D pyrazolyl complex is energetically more stable than

the Á3 -TpRh(CO)(CH4 complex by 7.7 kcal/mol. In addition, the DFT

frequency of the model Á2 complex is found 22 cm 1 higher than that of

the Á3 complex. The calculated frequency shift in the model systems is

consistent with the observed 18 cm 1 blue shift from Á3 -TpŁ Rh(CO)(S)

(1972 cm 1 ) to Á2 -TpŁ Rh(CO)(S) (1990 cm 1 ). With the above-discussed

evidence, it can be concluded that the 1990 cm 1 intermediate should

be assigned to Á2 -TpŁ Rh(CO)(S). Therefore, the observed 200 ps time

constant measures the free energy barrier G‡ ³ 4.1 kcal/mol for the Á3 to-Á2 isomerization.Ł

B. The Activation Barrier — The Bond-Breaking Step



The fact that the final product Á3 -TpŁ Rh(CO)(H)(R) does not appear on the

ultrafast time scale (<1 ns,) (Fig. 4) indicates a free energy barrier greater

than 5.2 kcal/mol for the alkane C–H bond activation. Nanosecond stepscan FTIR experiments on the Á3 -TpŁ Rh(CO)2 /cyclohexane system show

that the remnant of the Á2 -TpŁ Rh(CO)(S) peak persists for ¾280 ns after

photoexcitation, while the product CO stretch at 2032 cm 1 rises with a



Ł



The free-energy barrier G‡ is estimated from the transition-state theory:

the reaction rate k D 1/ D kB T/h exp G‡ /RT , where is the measured

lifetime, kB Boltzmann constant, h Plank’s constant, R the ideal gas constant,

and T temperature in Kelvin.



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Bond Activation Reactions



Figure 6



89



Photodissociation and subsequent solvation of Á2 -BpŁ Rh(CO)2 .



Figure 7 Nanosecond kinetics (dots) of Á3 -TpŁ Rh(CO)2 in room-temperature

alkane solution after 295 nm photolysis at 1990 cm 1 , the Á2 intermediate, and

2032 cm 1 , the final product. (Adapted from Ref. 30.)



time constant of 230 ns (Fig. 7) (30). Notice that the previously detached

pyrazole ring rechelates back in the final product Á3 -TpŁ Rh(CO)(H)(R).

Since no other transient intermediates appear on the time-resolved IR

spectra prior to formation of the final product, it is concluded that the

rate-limiting step consists of both the C–H bond cleavage and the pyrazole



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