RESUMEN
The solid-state [4+2] cycloaddition of anthracene to bis(N-ethylimino)-1,4-dithiin occurs via a unique single-phase topochemical reaction in the intermolecular (1:1) charge-transfer crystal. The thermal heteromolecular solid-state condensation involves the entire crystal, and this rare crystalline event follows topochemical control during the entire cycloaddition. As a result, a new crystalline modification of the Diels-Alder product is formed with a crystal-packing similar to that of the starting charge-transfer crystal but very different from that of the (thermodynamically favored) product modification obtained from solution-phase crystallization. Such a single-phase transformation is readily monitored by X-ray crystallography at various conversion stages, and the temporal changes in crystallographic parameters are correlated with temperature-dependent (solid-state) kinetic data that are obtained by 1H NMR spectroscopy at various reaction times. Thus, an acceleration of the solid-state reaction over time is found which results from a progressive lowering of the activation barrier for cycloaddition in a single crystal as it slowly and homogeneously converts from the reactant to the product lattice.
RESUMEN
Solid-state irradiation of the crystalline inclusion complex of (E)-stilbene in gamma-cyclodextrin (gamma-CD) yields a single isomer of syn-tetraphenylcyclobutane stereoselectively in high yield. In contrast, the photodimerization of stilbene in solution is very inefficient and unselective, and no photodimer is observed even upon prolonged irradiation of pure crystals. The monosubstituted stilbenes form a pair of photodimers stereoselectively, viz. the syn head-to-head and syn head-to-tail isomers, in comparable yields. The photodimer yields of about 70% and the biphasic decay kinetics of the excited stilbene (as established by picosecond time-resolved diffuse-reflectance spectroscopy) indicate that the stilbene guests are located in at least two distinct sites in the gamma-CD crystal lattice, i.e., a dimerization site where excited stilbene is in close reach of another stilbene guest molecule and an isomerization site where excited stilbene does not find a close neighbor for dimerization and thus undergoes trans --> cis isomerization only.
RESUMEN
Crystalline electron donor-acceptor (EDA) complexes of various diarylacetylenes (DA) and dichlorobenzoquinone (DB) are isolated and structurally characterized by X-ray crystallography. Deliberate excitation of either the DB acceptor at lambda(DB) = 355 nm or the 1:2 [DA, 2DB] complex at lambda(CT) = 532 nm in the solid state leads to [2 + 2] cycloaddition and identical (isomeric) mixtures of the quinone methide products. Time-resolved (ps) diffuse reflectance spectroscopy identifies the ion-radical pair [DA(*+), DB(*-)] as the reactive intermediate derived by photoinduced electron transfer in both photochemical procedures. The effects of crystal-lattice control on the subsequent ion-radical pair dynamics are discussed in comparison with the same photocouplings of acetylenes and quinone previously carried out in solution.
RESUMEN
The kinetics of the flash-induced photodissociation and rebinding of carbon monoxide in cytochrome aa3-CO have been studied by time-resolved infrared (TRIR) and transient ultraviolet-visible (UV-vis) spectroscopy at room temperature and by Fourier transform infrared (FTIR) spectroscopy at low temperature. The binding of photodissociated CO to CuB+ at room temperature is conclusively established by the TRIR absorption at 2061 cm-1 due to the C-O stretching mode of the CuB(+)-CO complex. These measurements yield a first-order rate constant of (4.7 +/- 0.6) x 10(5) s-1 (t1/2 = 1.5 +/- 0.2 microseconds) for the dissociation of CO from the CuB(+)-CO complex into solution. The rate of rebinding of flash-photodissociated CO to cytochrome a(3)2+ exhibits saturation kinetics at [CO] > 1 mM due to a preequilibrium between CO in solution and the CuB(+)-CO complex (K1 = 87 M-1), followed by transfer of CO to cytochrome a(3)2+ (k2 = 1030 s-1). The CO transfer from CuB to Fe alpha 3 was followed by CO-FTIR between 158 and 179 K and by UV-vis at room temperature. The activation parameters over the temperature range 140-300 K are delta H++ = 10.0 kcal mol-1 and delta S++ = -12.0 cal mol-1 K-1. The value of delta H++ is temperature independent over this range; i.e., delta Cp++ = 0 for CO transfer. Rapid events following photodissociation and preceding rebinding of CO to cytochrome a(3)2+ were observed. An increase in the alpha-band of cytochrome a3 near 615 nm (t1/2 ca. 6 ps) follows the initial femtosecond time-scale events accompanying photodissociation. Subsequently, a decrease is observed in the alpha-band absorbance (t1/2 approximately 1 microsecond) to a value typical of unliganded cytochrome a3. This latter absorbance change appears to occur simultaneously with the loss of CO by CuB+. We ascribe these observations to structural changes at the cytochrome a3 induced by the formation and dissociation of the CuB(+)-CO complex. We suggest that the picosecond binding of photodissociated CO to CuB triggers the release of a ligand L from CuB. We infer that L then binds to cytochrome a3 on the distal side and that this process is directly responsible for the observed alpha-band absorbance changes. We have previously suggested that the transfer of L produces a transient five-coordinate high-spin cytochrome a3 species where the proximal histidine has been replaced by L. When CO binds to the enzyme from solution, these processes are reversed.(ABSTRACT TRUNCATED AT 400 WORDS)