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B. The Nature of the Reaction Barrier — Atom Transfer

B. The Nature of the Reaction Barrier — Atom Transfer

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Yang and Harris



Figure 21 Transition-state structures computed at the DFT B3LYP level of theory.

˚ Also shown are the imaginary frequenThe Re –Cl and C–Cl bond lengths are in A.

cies associated with each transition state. (Adapted from Ref. 72.)



To better understand the reactivity, the transition states for the series

of reactions were studied using DFT methods. As illustrated in Fig. 21,

the results show that each transition state can be characterized by a single

imaginary frequency that involves simultaneous dissociation of the C–Cl

bond and formation of the Re –Cl bond. The structural variation along the

series of chlorinated methanes suggests that the transition state becomes

more product-like as the number of hydrogen n in CHn Cl4 n increases. For

example, the Re . . . Cl distance in CO 5 Re . . . Cl . . . CHn Cl3 n decreases

˚ n D 0 to 2.84 A

˚ n D 1 to 2.76 A

˚ nD2 ,

monotonically from 2.95 A

˚ for the product (CO)5 Re-Cl. The calculated electron

and finally to 2.52 A

density distribution also displays a similar trend as displayed in Table 1.



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Bond Activation Reactions



109



The DFT energy barriers E0 ‡ in CCl4 , CHCl3 , and CH2 Cl2 are, respectively, 0.86, 3.0, and 8.5 kcal/mol, where the solvent effects are treated

self-consistently in the reaction-field approximation. The current level of

theory, however, does not permit an immediate comparison of the experimental energy barriers to the calculated values. Nonetheless, comparison

of the experimental trend and the qualitative trend from the theoretical

calculations along the series of chloromethane provides greater insight in

understanding the reactivity. The observed trends in the barrier and the

transition-state structure are a clear demonstration of Hammond’s postulate, which associates a later transition state with a higher energy barrier

(73). This suggests that the valence-bonding picture of structure-reactivity

correlation, which has enjoyed much success in the field of physical organic

chemistry, may be applicable to this case (74–76). Figure 22 shows a linear

correlation model that utilizes this concept (77,78). In this model, the reaction proceeds along the C–Cl bond potential energy curve, passes through

the transition state, and forms the final product along the Re –Cl bond

potential energy curve. The position of the transition state is approximated



Figure 22 A correlation diagram for Cl atom abstraction from CHn Cl4

Re(CO)5 radical.



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n



by the



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by the crossing point of the two diabetic curves. Accordingly, the reaction

profile can be understood by three factors: the C–Cl and Re –Cl bond

strengths and the overall reaction enthalpy. The required parameters are

available in the literature, obtained either through experiments or by electronic structure calculations. While this simple model correctly predicts the

energetic and structural trends of the transition states, more case studies are

necessary in order to arrive at a better, quantitative model for the atomtransfer step (79,80).

VI. CLOSING REMARKS



Our understanding of the reactivity of complex chemical systems such as

organometallic compounds has greatly advanced in the past decade owing

to the vast development of experimental and computational techniques.

Basic principles born of these collective efforts have guided the thinking

of reactions in fields including chemistry, biological sciences, and material sciences. Yet the critical examination of the postulates in the realistic

solution phase has been a daunting challenge due to the intricate liquid

dynamics, which render the relevant time scales to spreading several orders

of magnitude. Femtosecond infrared spectroscopy, which is capable of

“real-time” observation and characterization of a chemical reaction, has

been utilized to follow oxidative-addition reactions of prototypical twoelectron C–H and Si–H bonds and one-electron Cl atom to organometallic

complexes. This technique affords the description of not only the static reaction mechanism but also the dynamics of each elementary step, including

vibrational cooling, geminate-pair recombination, morphological reorganization, solvation-assisted intersystem crossing, and solvent rearrangement.

The explicit reaction scheme composed of elementary steps allows an experimental assessment for the current understanding of the reactivity. For C–H

and Si–H bond activation, the present results corroborate the prevailing

picture of the reactivity. For Cl atom abstraction by the Re(CO)5 radical,

however, the present results caution the notion of a 19-electron species or

a charge-transfer complex as reactive intermediates. In addition, in each of

the reactions discussed, the rate-determining process has been determined

to be the bond-breaking step for C–H activation, solvent rearrangement for

Si–H activation, and atom-transfer step for C–Cl activation. They represent three stereotypical rate-limiting steps that are expected to be common

to solution-phase reactions. More examples are anticipated to emerge as

more efforts are devoted to the study of complicated chemical processes,

which, in time, will improve our picture of reactions in liquids.



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Bond Activation Reactions



111



ACKNOWLEDGMENT



The following individuals have made indispensable contributions

in bringing this project to the current state: T. Lian, K. T. Kotz,

M. C. Asplund, H. Frei, S. E. Bromberg, P. T. Snee, W. J. Wilkens,

C. K. Payne, J. S. Yeston, B. K. McNamara, and R. G. Bergman. The use

of the static FTIR spectrometer of the C. B. Moore group and that of a UVVis spectrometer of the A. P. Alivisatos group are gratefully acknowledged.

The use of specialized equipment under the Office of Basic Energy Science,

Chemical Science Division, U.S. Department of Energy contract DE-AC0376SF00098 is also acknowledged. This project is supported by a grant from

the National Science Foundation.



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3

Applications of Broadband Transient

Infrared Spectroscopy

Edwin J. Heilweil

National Institute of Standards and Technology, Gaithersburg, Maryland



I. INTRODUCTION



The application of ultrafast transient infrared (IR) spectroscopy to the study

of chemical, biochemical, and related physical phenomena has dramatically increased over the last decade. This surge of interest has largely been

inspired by the increased availability of commercial, solid-state, table-top

pulsed laser equipment and nonlinear frequency conversion methods for

generating high-power, broadly tunable infrared pulses in the near- to midinfrared wavelength regime (ca. 2–25 µm). Indeed, the accessibility and

flexibility of these modern optical systems has far exceeded the expectations

of many researchers working in this field. Early methodologies for generating ultrafast IR pulses used simple multipass optical parametric amplifiers

(1) or visible difference frequency mixing techniques (2) to access the

mid-IR spectral region. Dynamical measurement of vibrational energy flow

within solute and adsorbate species begun in the late 1970s (3) has given

way to highly sophisticated ultrafast methods (4), studies of ultrafast molecular reaction mechanisms (5), energy transfer within short-chain amino

acids (6), at surfaces (7,8), and in liquids (9,10) and solids (11), to name a

few examples.

Early ultrafast transient infrared measurements were typically

performed using identical frequency picosecond infrared pulses for both

pumping and probing a vibrational mode of condensed-phase molecules



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Heilweil



(12). In this scenario, an intense, narrowband tunable infrared pump pulse

excites population from the v D 0 to the v D 1 level of a molecular

vibration (producing increased sample transmission), and a weaker probe

pulse interrogates the recovery of ground state absorption to yield the

v D 1 population relaxation time (T1 lifetime). These simple “single-color”

measurements were quickly superceded by “two-color” techniques since

it was recognized that monitoring the transient infrared spectrum as a

function of time yields much more detailed information about the system

dynamics. For example, detection of mode-specific vibrational solute-tosolvent energy transfer (13) or the generation of new transient reaction

intermediates (14) necessarily dictates that a tunable infrared probe pulse

be used to observe new spectral features removed in frequency from

the parent molecular absorptions. However, using tunable, narrowband

probe pulses to obtain broadband spectra involves the arduous task of

taking frequency-scanned spectra with single element detectors at multiple

pump-probe time delays. This generic approach suffered from extended

data acquisition times for low-repetition-rate laser systems and potential

long-term drift problems that potentially distorted the spectral intensity

information.

To circumvent these difficulties, broadband infrared detection using

multichannel arrays was employed by our group (15–17) and others

(18,19). In many respects this approach is complementary to the

accepted technique for performing ultraviolet-visible transient absorption

spectroscopy. By generating broadband mid-infrared probe pulses a few

picoseconds or less in duration and detecting a portion of an infrared

spectral region for each laser pulse, data acquisition rates are vastly

enhanced and system stability issues are diminished. It should also be

pointed out that picosecond or femtosecond broadband probe pulses can

be used to detect narrowband transient absorption features as long as the

transient absorber has lived on the order of the coherence T2 lifetime of

the interrogated state (20). These typically narrowband features (3–15 cm 1

FWHM) are spectrally resolved by an up-converter crystal (with CCD

detection) or through the spectrograph resolution independent of the

inherent time-bandwidth characteristics of the probe pulse. Demonstrations

of the general broadband IR detection approach were first made using

nonlinear IR frequency down- and up-conversion with visible spectrographCCD detection (15,16). With Defense Department declassification and

increased commercial availability of infrared focal-plane arrays for midinfrared imaging use, direct broadband infrared spectroscopic detection

covering the 1–12 µm range is now practical (21).



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