TRIR spectroscopy

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

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. 

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. 

APPLICATIONS OF TRIR SPECTROSCOPY TO THE STUDY OF ORGANIC REACTIVE INTERMEDIATES... 

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. ... 

As demonstrated in the two previous sections, TRIR spectroscopy can be used to provide direct structural information concerning organic reactive intermediates in solution as well as kinetic insight into mechanisms of prodnct formation. TRIR spectroscopy can also be used to examine solvent effects by revealing the inflnence of solvent on IR band positions and intensities. For example, TRIR spectroscopy has been used to examine the solvent dependence of some carbonylcarbene singlet-triplet energy gaps. Here, we will focns on TRIR stndies of specific solvation of carbenes. 

Since the most direct evidence for specihc solvation of a carbene would be a spectroscopic signature distinct from that of the free carbene and also from that of a fully formed ylide, TRIR spectroscopy has been used to search for such car-bene-solvent interactions. Chlorophenylcarbene (32) and fluorophenylcarbene (33) were recently examined by TRIR spectroscopy in the absence and presence of tetrahydrofuran (THF) or benzene. These carbenes possess IR bands near 1225 cm that largely involve stretching of the partial double bond between the carbene carbon and the aromatic ring. It was anticipated that electron pair donation from a coordinating solvent such as THF or benzene into the empty carbene p-orbital might reduce the partial double bond character to the carbene center, shifting this vibrational frequency to a lower value. However, such shifts were not observed, perhaps because these halophenylcarbenes are so well stabilized that interactions with solvent are too weak to be observed. The bimolecular rate constant for the reaction of carbenes 32 and 33 with tetramethylethylene (TME) was also unaffected by THF or benzene, consistent with the lack of solvent coordination in these cases. °... 

Further studies were carried out with halocarbene amides 34 and 357 Although again no direct spectroscopic signatures for specifically solvated carbenes were found, compelling evidence for such solvation was obtained with a combination of laser flash photolysis (LFP) with UV-VIS detection via pyridine ylides, TRIR spectroscopy, density functional theory (DFT) calculations, and kinetic simulations. Carbenes 34 and 35 were generated by photolysis of indan-based precursors (Scheme 4.7) and were directly observed by TRIR spectroscopy in Freon-113 at 1635 and 1650 cm , respectively. The addition of small amounts of dioxane or THF significantly retarded the rate of biomolecular reaction with both pyridine and TME in Freon-113. Also, the addition of dioxane increased the observed lifetime of carbene 34 in Freon-113. These are both unprecedented observations. 

In either neat dioxane or THF, carbene-ether ylides are observed as a broad IR absorption band between 1560 and 1610 cm , distinct from the IR bands of the free carbenes. With discrete spectroscopic signatures for the free carbene and its corresponding ether ylides, TRIR spectroscopy was used to confirm that the effects described above with dilute ether in Freon-113 were due to specific solvation of the carbene (Scheme 4.6, Reaction 2) rather than a pre-equilibration with the coordinating solvent (Scheme 4.6, Reaction 3) or reactivity of the ylide itself (Scheme 6, Reaction 4). In Freon-113 containing 0.095M THF simultaneous TRIR observation of both the free carbene (x = ca. 500 ns) and the carbene-THF ylide (x = ca. 5ps) was possible7 The observation that lifetimes of these species were observed to be so different conclusively demonstrates that the free carbene and the carbene-THF ylide are not in rapid equilibrium and that Reaction 3 of Scheme 4.6 is not operative. By examining the kinetics of the carbene 34 at 1635 cm directly in Freon-113 with small amounts of added dioxane, it was observed that the rate of reaction with TME was reduced, consistent with Reaction 2 (and not Reaction 4) of Scheme 4.6. 

Intermediate a-nitrosobenzaldehyde 36 was generated in solution by laser photolysis of 3,5-diphenyl-l,2,4-oxadiazole-4-oxide 37 and its time-resolved infrared (TRIR) spectroscopy has been recorded <2003JA1444>. The second-order rate constants for reaction with diethylamine and 1,3-cyclohexadiene were determined to be (1.3 0.5) x s ... 

Nanosecond time resolved infrared (TRIR) spectroscopy has recently become available to physical organic chemists. This spectroscopy is an attractive tool for studying carbonyl nitrenes. Such work is in progress in several laboratories ... 

Tsao has used TRIR spectroscopy to determine that the lifetime of azirine in 43 at ambient temperature is 2.6 which is in excellent agreement with the work of Shrock and Schuster, who studied the same system years earlier by LFP with UV-vis detection. 

No transient absorption >350 nm is detected upon LFP of 1-naphthylazide. A band with absorption maxima at 370 nm is formed with a time constant of 2.8 ps after the laser pulse. The carrier of the 370-nm absorption reacts over >100 ps to form azonaphthalene. The carrier of the 370-nm absorption is identified as triplet 1-naphthylnitrene that has previously been characterized as a persistent species at 77 K by UV-vis (A,nmx = 367 nm) and EPR spectroscopy. Azirine 43, detected by TRIR spectroscopy must not absorb significantly >350 nm, a fact that was established later by the matrix isolation studies of Wentrup s and Rally s groups. Assuming a rapidly equilibrating mixture of azirine and nitrene, and given that kisc = 1 X 10 s (determined by Tsao by LFP at 77 K and assumed to have the same value at 298 then K = [singlet nitrene]/[azirine 43] = 0.038 at 298 K. 

In the absence of amine, the ketenimine-azirine singlet nitrene species can equilibrate and, eventually, the singlet nitrene can cychze to form carbazole. Berry and co-workers independently monitored the growth of carbazole ( max = 289.4 nm) by this process. In cyclohexane, some carbazole was formed this way with an observed rate constant of 2.2 x 10 at 300 K over a barrier of 11.5 kcal/mol. Tsao and co-workers recently used TRIR spectroscopy to show that ketenimine decay equals the rate of carbazole formation. 

Bergman and co-workers used IR laser-based TRIR spectroscopy to investigate alkane C—H activation with ( 7 -C5Me5)Rh(CO)2 in liquid Kr and liquid Xe in the presence of cyclohexane 42-44) and neopentane 45). This permitted the detection of the noble gas intermediates. 

More recently, Weitz and co-workers studied the bonding of Xe and Kr atoms to M(CO)5 fragments [M = Cr and Mo (Xe only) and W] in the gas phase using TRIR spectroscopy (52). The IR v(C—O) bands of metal carbonyls in the gas phase are much broader than those in solution or... 

In addition to UV/visible flash photolysis and TRIR spectroscopy, other techniques have been used for the detection of transition metal-noble gas interactions in the gas phase. The interaction of noble gases with transition metal ions has been studied in detail. A series of cationic dimeric species, ML" " (M = V, Cr, Fe, Co, Ni L = Ar, Kr, or Xe), have been detected by mass-spectroscopic methods (55-58). It should be noted that noble gas cations L+ are isoelectronic with halogen atoms, therefore, this series of complexes is not entirely unexpected. The bond dissociation energies of these unstable complexes (Table IV) were determined either from the observed diabatic dissociation thresholds obtained from their visible photodissociation spectra or from the threshold energy for collision-induced dissociation. The bond energies are found to increase linearly with the polarizability of the noble gas. 

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