Matrix sites

The bands of matrix-isolated molecules are frequently observed at the wavelengths which differ from those in gas-phase spectra. These matrix shifts are induced by the repulsive and attractive forces between the isolated molecules and the atoms which form the matrix site. Repulsions lead to small increases (1-15 cm ) of vibrational frequencies, and attractions decrease them. Matrix shifts depend on the type of matrix gas they rise in the sequence from neon to xenon. In general, the shifts are positive (the... 

A few methods produce reactive molecules by reactions in solid matrices. The most widely used consists of irradiating already isolated precursors with UV light (including vacuum UV light at A < 200 nm), y- or X-rays. In this case, the fragments which are formed as products of the precursor s dissociation must not recombine in the matrix site. To achieve this effect, one of the fragments should be either chemically inactive [e.g. N2, CO2 see (la)] or able to diffuse easily from the site [e.g. hydrogen atoms as in (lb)]. 

Determination of QMT effects often rests upon the temperature or isotope dependence of rates, as described above. Thus, the matrix site dispersity presents an immediate dilemma Which matrix sites should be compared at different temperatures or for different isotopes There have been different approaches to this problem. The most simple has been to compare the first 10-20% of the decay curves after irradiation is shut off First-order plots are generally linear in those time frames. However,... 

Figure 10.3. First-order plots of ln[Intensity] versus time, showing idealized exponential decay (dotted line), and nonexponential decay from statistical distribution of matrix sites (solid line).

It has been shown that the rate constants obtained from the slopes of In [intensity] versus plots approximate the rates of the highest-probability matrix sites. Hence, workers have utilized the temperature dependence of these values, or other empirically derived stretched exponential time dependencies, to estimate low temperature Arrhenius plots. The validity of such methods, however, depends critically on obtaining accurate time-dependence data on the fastest matrix sites, which is increasingly difficult as temperatures are raised. 

Final resolution of these problems, particularly the complications from multiple matrix sites, came from investigations using spectroscopic methods with higher time resolution, viz. laser flash photolysis. Short laser pulse irradiation of diazofluorene (36) in cold organic glasses produced the corresponding fluorenylidene (37), which could be detected by UV/VIS spectroscopy. Now, in contrast to the results from EPR spectroscopy, single exponential decays of the carbene could be observed in matrices... 

Deactivates active matrix sites (to facilitate the release of analytes)... 

Site-selection spectroscopy Maximum selectivity in frozen solutions or vapor-deposited matrices is achieved by using exciting light whose bandwidth (0.01-0.1 cm-1) is less than that of the inhomogeneously broadened absorption band. Lasers are optimal in this respect. The spectral bandwidths can then be minimized by selective excitation only of those fluorophores that are located in very similar matrix sites. The temperature should be very low (5 K or less). The techniques based on this principle are called in the literature site-selection spectroscopy, fluorescence line narrowing or energy-selection spectroscopy. The solvent (3-methylpentane, ethanol-methanol mixtures, EPA (mixture of ethanol, isopentane and diethyl ether)) should form a clear glass in order to avoid distortion of the spectrum by scatter from cracks. 

Other possible carbene precursors (vicinal dihalides, peresters, ketenes, carbene adducts with stable hydrocarbons, etc.) cannot generally be used for in situ generation of carbenes, because the fragments are likely to recombine. However, they can be used, for example, in experiments involving pyrolysis or other forms of external carbene generation where the fragments get a chance to separate in the gas phase and become trapped in distant matrix sites. AU conceivable halo- and dihalocar-benes were made and smdied in this way (see, e.g., the 1993 review by Sander et al." ) However, such methods can only be applied to carbenes which resist thermal rearrangement to more stable products. 

The concept of tunneling has recently been invoked to explain the mechanism of photodissociation of matrix-isolated molecules. Previously, photodissociation was customarily accounted for by the fact that transla-tionally hot photofragments escape from the cage and stabilize in separate matrix sites, thereby avoiding recombination. Using the time-dependent self-consistent field approximation for molecular dynamics simulations,... 

If the photolysis takes place in an inert gas matrix, both the homolytic splitting of the metal-metal bond and the breaking of a metal-nitrogen bond will be followed by a fast backreaction to the parent compound. The radicals formed by homolysis of the metal-metal bond can not diffuse from the matrix site and will recombine to the parent compound. Moreover, the photoproduct obtained by breaking of a metal nitrogen bond, will not be stabilized by a coordinating solvent molecule and therefore react back to the parent compound. Because of this the photochemistry of some of these complexes has also been studied in a Cl-k-matrix at 10K and for comparison in a PVC film, which is a less rigid medium than the matrix especially at room temperature. 

Figure 2. ESR spectrum of pseudorotating Ks in an Ar matrix at 34.2 K. The stick spectrum shows the predicted intensity distribution for three equivalent nuclei. Also indicated are K atom resonances for several matrix sites. He = 3312.3 G is the resonance field of a free electron...

Discussion. Copper in Krypton. The absorption spectra of copper atoms Isolated in rare gas matrices have been extensively studied (15-25) and the triplet of bands at 310nm attributed to a number of different causes. These include (1) spin orbit splitting and static axial site distortion (17), (2) multiple matrix sites (18), (3) exciplex formation between the metal and a single matrix atom (19), (4) long range metal-metal interactions (2 ), and (5) Jahn-Teller (JT) effect resulting from matrix cage atom vibrations on the excited metal (21,22,23). The MCD of Cu atoms in the rare gas matrices has recently been reported (24,25) and the results interpreted as consistent with either the distorted site or JT hypotheses (39). 

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