TRIR

Time-Resolved Infrared (TRIR) Studies of Organic Reactive Intermediates... 

Applications of TRIR Spectroscopy to the Study of Organic Reactive ... 

TIME-RESOLVED INFRARED (TRIR) STUDIES OF ORGANIC REACTIVE INTERMEDIATES... 

Indeed, time-resolved resonance Raman (TR ) spectroscopy has been successfully employed to study the structure and dynamics of many short-lived molecular species and is the topic of a separate chapter by D. L. Phillips in this book. Like TR spectroscopy, TRIR spectroscopy gives one the ability to monitor directly both the structure and dynamics of the reactants, intermediates, and products of photochemical reactions. The time-resolved Raman and IR experiments, along with their transient UV-VIS absorption predecessor, are of course all complementary, and a combination of these techniques can give a very detailed picture of a photochemical reaction. 

Experimental limitations initially limited the types of molecular systems that could be studied by TRIR spectroscopy. The main obstacles were the lack of readily tunable intense IR sources and sensitive fast IR detectors. Early TRIR work focused on gas phase studies because long pathlengths and/or multipass cells could be used without interference from solvent IR bands. Pimentel and co-workers first developed a rapid scan dispersive IR spectrometer (using a carbon arc broadband IR source) with time and spectral resolution on the order of 10 ps and 1 cm , respectively, and reported the gas phase IR spectra of a number of fundamental organic intermediates (e.g., CH3, CD3, and Cp2). Subsequent gas phase approaches with improved time and spectral resolution took advantage of pulsed IR sources. 

Weitz and co-workers extended gas phase TRIR investigations to the study of coordinatively unsaturated metal carbonyl species. Metal carbonyls are ideally suited for TRIR studies owing to their very strong IR chromophores. Indeed, initial TRIR work in solution, beginning in the early 1980s, focused on the photochemistry of metal carbonyls for just this reason. Since that time, instrumental advances have significantly broadened the scope of TRIR methods and as a result the excited state structure and photoreactivity of organometallic complexes in solution have been well studied from the microsecond to picosecond time scale. ... 

TRIR methods have also found utility in the elucidation of reaction mechanisms involved in biological systems, most notably photosynthetic and respiratory proteins. In addition, TRIR spectroscopy has also been used to enhance our understanding of the dynamics of protein folding processes. ... 

In contrast to gas phase, organometallic, and biological studies, until recently, relatively few organic systems had been examined by TRIR methods. This chapter will begin with a brief survey of experimental approaches to TRIR spectroscopy and will follow with a discussion of several representative studies of organic reactive intermediates that demonstrate the significant utility of this technique. 

Recent technical advances have greatly expanded the applicability of TRIR spectroscopy, making measurements over wide temporal and spectral ranges now feasible. The relative merits of different experimental approaches have been discussed previously. " ... 

Although very detailed, fundamental information is available from ultrafast TRIR methods, significant expertise in femtosecond/picosecond spectroscopy is required to conduct such experiments. TRIR spectroscopy on the nanosecond or slower timescale is a more straightforward experiment. Here, mainly two alternatives exist step-scan FTIR spectroscopy and conventional pump-probe dispersive TRIR spectroscopy, each with their own strengths and weaknesses. Commercial instruments for each of these approaches are currently available. 

In conventional nanosecond pump-probe dispersive TRIR experiments, also described previously, kinetic data are collected at one frequency at a time. These data can then be used to construct a series of time-resolved IR spectra. Thus, in the dispersive experiment kinetic data are used to construct spectra, and in the step-scan experiment spectral data are used to derive kinetics. 

One solution to this dilemma has been advanced by Hamaguchi and co-workers who have made use of a MoSi2 IR source newly developed by JASCO that provides approximately twice the emissive intensity of conventional globar sources. This probe source was incorporated into a dispersive TRIR spectrometer that allows access to the entire mid-IR spectrum with high sensitivity (A A < 10 ) and sufficient time (50 ns) and frequency (4-16 cm ) resolution to probe a wide range of transient intermediates in solution. 

Since modern FTIR spectrometers can operate in a rapid scan mode with approximately 50 ms time resolution, TRIR experiments in the millisecond time regime are readily available. Recent advances in ultra-rapid scanning FTIR spectroscopy have improved the obtainable time resolution to 5 ms. Alternatively, experiments can be performed at time resolutions on the order of 1-10 ms with the planar array IR technique, which utilizes a spectrograph for wavelength dispersion and an IR focal plane detector for simultaneous detection of multiple wavelengths. ... 

The following representative examples of TRIR studies are not meant to be an exhaustive treatment of the various organic reactive intermediates that have been investigated by TRIR methods, but rather to demonstrate the unique insight that such studies can provide. The direct observation of organic intermediates in solution at room temperature by IR spectroscopy can reveal fundamental information related both to bonding and structure of reactive intermediates as well to mechanisms of product formation. 

These observations have been rationalized in terms of a small energy gap between structure 1 and its more reactive, but higher energy counterpart 2, and are consistent with previous computational work. Since the IR signatures of structures 1 and 2 are expected to be quite different, TRIR spectroscopy was used to examine the influence of substituents (R) and solvent on the relative stability of 1 and 2 the results of these studies are summarized in this section. The related iminooxirane and a-lactam intermediates have also been recently examined by TRIR spectroscopy. ... 

Figure 4.1. TRIR difference spectra averaged over the timescales indicated following 355 nm laser photolysis of diazirine 8 (15.7mM) in C02-satnrated dichloromethane. Reprinted with permission from B. M. Showalter and J. P. Toscano, J. Phys. Org. Chem. 2004, 14, 743. Copyright 2004, John Wiley Sons Limited.

The observed TRIR data are consistent with Scheme 4.1. Depletion of the diazirine and formation of the carbene occurs within the time resolution (50 ns) of the experiment. Subsequent decay of the carbene (J osbd = 3.0 X 10 s ) is observed at the same rate within experimental error ( 10%) that the a-lactone is produced ( osbd = 3.2 X 10 s ) and the final decay of the a-lactone (A bsd = 2.0 X 10 s ) occurs at the same rate as the acid chloride product is formed (A sbd = 1-8 X 10 s ). The position of the a-lactone band at 1910 cm is clearly indicative of ring-closed form 1 and in very good agreement with the signal observed at 10 K (1920 cm ) by Sander and co-workers. ... 

The difference in structure between 16 and 17 is readily understood in terms of the addition of strongly electron-donating substituents, but the contrast between 16 and 20 is less easily rationalized. Photolysis of 19 was carried out in HFIP (dielectric constant (e) = 16.75), while TRIR experiments with diphenyl diazomethane (22) were carried out in dichloromethane (e = 9.08), suggesting that a-lactone structure may be dependent on solvent polarity. 

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