Kinetics of the photolysis of benzenetricarbonylchromium(0) in chloroform

In contrast to the photolysis of Cr(CO)3(C6H6) in nonhalogenated solvents, in which the products are CrL(CO)2(C6H6) in the presence of a donor L, or Cr(CO)6 and C6H6 if no donor is present, the photo-reaction in chloroform yields CrCl3. No significant portion of the reaction occurs through absorption of 254nm light by CHCl3• The quantum yield is 1.4, consistent with a mechanism in which several radicals are formed upon chlorination of the chromium, which then cause further decomposition of the reactant. In 24% CCl4, the reaction still occurs primarily through the excited state metal complex, but there is a solvent-initiated contribution, which is more significant the lower the reactant concentration. The quantum yield for the solvent-initiated pathway is 0.3 in 24% CCL4

Kinetics of the photosubstitution of cis-bis(benzonitrile)dichloroplatinum(II) in chloroform

Under 254 nm irradiation cis-[Pt(C6H5CN)2Cl2] is converted to H2PtCl6. Absorption of light by both the metal complex and the solvent contribute to the first step of this process, suggested to form HPt(C6H5CN) Cl3. A linear dependence of the reaction rate on light intensity appears to rule out chlorination by trichloromethyl radicals. However, at higher light intensities a higher order dependence on intensity develops, and under 313 nm irradiation is dominant, and a reaction between trichloromethyl radical and the excited state complex is proposed to account for this

A kinetic study of the photolysis of tris(2,4-pentanedionato)cobalt(III) and bis(2,4-pentanedionato)cobalt(II) in chloroform

Under 254nm irradiation in chloroform, Co(acac)3 (Hacac = 2,4-pentanedione) is converted to Co(acac)2 and then to CoCl2. The metal complex is the primary photoactive species in the photoreduction of Co(acac)3, but the photosubstitution of Co(acac)2 appears to occur primarily through absorption of light by the solvent, followed by a chain reaction in which chlorine atoms displace pentanedionyl radicals. The photosubstitution rate law is complex, and the apparent quantum yield (based on total light absorbed) varies with incident light intensity and Co(acac)2 concentration, reaching values as high as 16 under the conditions of this study. Referred only to the light absorbed by CHCL3, the highest quantum yield measured was 150. An observed partial inverse dependence of the photosubstitution rate on the initial concentration of Co(acac)2 is explained in terms of a mechanism in which the pentanedione product competes with Co(acac)2 for an intermediate

A kinetic study of the photolysis of ethylferrocene in chloroform

The photooxidation of ethylferrocene to ethylferricinium ion and tetrachloroferrate in CHCl3 under 254 nm irradiation proceeds through light absorption by both ethylferrocene and chloroform. The products remain in solution at concentrations below 10-3 M. The fraction occurring through a solvent-initiated pathway increases during the course of the reaction. A secondary thermal reaction is responsible for generating tetrachloroferrate from ethylferricinium ion. The rate of the reaction increases during the early stages, and the data throughout the course of the reaction are consistent with the rate law ( afs + bfR)/ (1 + c[R]/[P]-d[R]/[Cl ]), where [R] and [P] are the concentrations of ethylferrocene and ethylferricinium ion, respectively, and ƒs and ƒR are the fractions of light absorbed by the solvent and ethylferrocene, respectively

Solvent‐initiated photochemistry of transition metal complexes

Although it is generally assumed that photoreactions of transition metal complexes proceed through metal complex excited states, some reactions in halogenated solvents occur instead as radical processes, following carbon-halogen bond homolysis. Oxidation, substitution, or oxidative addition can occur by either a metal-centered or a solvent-initiated photoreaction, and they can be hard to distinguish. Under some circumstances even the kinetic rate laws can be the same. However, with proper choice of irradiation wavelength the dependence of the initial rate on light intensity and metal complex concentration suffices to discriminate between the two possibilities

Integrating single crystal x-ray diffraction in the undergraduate curriculum

Aspects of single crystal X-ray crystallography have been introduced into all four years of the chemistry curriculum at Santa Clara University. The laboratory components consist of (a) the determination of a molecular structure from a data set in year one, (b) the redetermination of that molecular structure and the determination of an ionic structure in year two, (c) the determination of another ionic structure from a data set in year three, along with the experimental determination of unit cell dimensions for crystals synthesized by students, and (d) a complete experimental structure determination in year four. Together with supporting lecture components, this program attempts, in part, to address problems in spatial reasoning skills common to chemistry students

Angular Overlap Model parameters

Since the introduction of the Angular overlap model (AOM) in the mid-1960s, expressing d orbital energies in terms of the σ- and π-antibonding parameters e σ and eπ , the AOM has failed to supplant crystal field theory as the standard model to explain structure and electronic spectra in transition metal complexes. This is so despite the much more obvious connection in the AOM between structure and d orbital energies, the pictorial simplicity of the AOM approach, and the more consistent transferability of AOM parameters from one complex to another. The main reason is probably that AOM parameters cannot be determined uniquely when all the ligands are on the Cartesian axes. The scales for e σ and e π must then be fixed arbitrarily, as is done automatically in the crystal field model. A number of experimental approaches have evolved to solve, or at least evade, the nonuniqueness problem, including: (a) the assignment of e π for saturated amines to zero, reflecting their inability to π-bond; (b) the simultaneous use of magnetic and spectroscopic data; (c) the inclusion of data from sharp, spin-forbidden lines in Cr(III) spectra, along with application of the exact geometry and full d n configuration interaction in computations, or any combination of these; (d) the use of charge transfer bands involving dπ orbitals to determine eπ values. As of yet, the level of consistency among different techniques leaves something to be desired, and even with a common technique, reported AOM parameter values for particular metal-ligand combinations show a much higher variability than one would like, given even minimum expectations for transferability. Some of the variability can be ascribed to differences in the other ligands present. Even with these variations, AOM parameter sets can be usefully correlated with kinetic and thermodynamic data from both photochemical and thermal reactions

Trying a case on ethics in scientific research

While most prepared exercises for ethics in science programs—including an excellent AAAS video series—present a complete account of the relevant facts, a role-playing exercise is described here in which the participants are provided with differing reports of events. The exercise is based on a true case involving a student who was convicted of grand theft for withholding laboratory notebooks. The participant materials consist of published accounts retrieved from the Internet. After reviewing the assigned materials from the perspective of a stakeholder in the dispute, small-group discussions take place.During the whole-group discussions that follow, participants often find that their positions change. This exercise reinforces several instructional goals from two core areas enunciated by NIH concerning the responsible conduct of research. More generally, students also learn and have reinforced the importance of accurate information availability to effectively make ethical decisions

Approximating the product spectrum and product concentrations in continuous photochemical reactions

Several approximations to a common photochemical rate law, in which the rate is proportional to the fraction of light absorbed by the reactant chromophore, have been developed to permit the product spectrum to be determined from a sequence of spectra during irradiation that exhibit isosbestic points. The methods were tested on the photolysis of [Cr(NH3)6]3+ and [Cr(en)3]3+ (en = ethylenediamine) in water, and [Fe(Et2dtc)3], tris(diethyldithiocarbamato)iron(III), in CHCl3