Measurement of theg-factor of the 21 + state of192Pt in an external magnetic field

Theg-factor of the 2₁ ⁺ state of¹⁹²Pt has been measured by the IPAC technique in an external magnetic field as:g(2₁ ⁺,¹⁹²Pt)=+0.287(17). An additional IPAC experiment with an¹⁹²IrFe sample was performed with the same level in order to investigate the hyperfine field. The result:ω _L τ(2₁ ⁺,¹⁹²PtFe)=0.1115(9) gives the hyperfine field:B _hf ^4.2k (PtFe)=126.8(71) T. The result of an LTNO experiment with the same level is compatible with the assumption that 100% of the¹⁹²Ir atoms were on unique sites.     

A simple method for calculating enantiomer ratio and equilibrium constants in biocatalytic resolutions

A computer programme for determination of equilibrium constant (K) and enantiomer ratio (E) in biocatalytic resolutions has been developed. The programme utilises experimental data, ee_s and ee_p measured at more than one conversion, and determines both K and E no matter whether the reaction is irreversible (K=0) or reversible (K>0). An estimation of errors in the calculations indicates that errors in E does not show a Gaussian distribution, while errors in K does. The usefulness of the programme has been tested in a lipase-catalysed transesterification of 1-phenoxy-2-propanol at various concentrations of acyl donor, with different solvents and at different water activities.     

Single-site absorption spectroscopy of pheophytin-a and chlorophyll-a in a n-octane matrix

Site-selection experiments have been performed on pheophytin-a and chlorophyll-a in n-octane at 4.2 K. Single-site absorption spectra of both compounds were recorded, but single-site emission spectra could not be obtained. The absorption spectra show sharp lines and vibrational frequencies of the lowest excited singlet state correspond with resonance Raman frequencies. A remarkable feature of the emission is the presence of a strong 0-0 transition but extremely weak vibronic transitions.     

Characterization of Carotenoid Triplet States in the Light-Harvesting Complex B800–850from the Purple Bacterium Rubrivivax gelationsus

The carotenoid triplet states in the light-harvesting complex B800–850from purple bacterium Rubrivivax gelatinosus were characterized by absorption-detected magnetic resonance in zero magnetic field (ADMR) spectroscopy. Detailed HPLC analysis of carotenoids from B800–850demonstrated the presence of several carotenoids bound to the complex: the major ones are hydroxyspheroidene and spheroidene (together 80%), followed by neurosporene and hydroxyneurosporene (7%), spheroidenone and hydroxyspheroidenone (7.5%) and two other minor carotenoids that could be 3,4-dihydrospheroidenone and 3,4-dihydrohydroxyspheroidenone (5.5%). Three triplet states originating from carotenoids present in the B800–850were observed. The identical T-S spectra recorded at selectively chosen 2|E| transitions of carotenoids indicated that all these triplet states can be attributed to three different populations of one carotenoid family, probably to spheroidene and to hydroxyspheroidene, with different out-of-plane distortions of their polyene chain due to a different protein environment. Triplet states of the neurosporene and the spheroidenone families are probably not observed because of the low signal amplitude.     

Increased reactivity of the *Cr(CO)3(C5Me5) radical with thiones versus thiols: a theoretical and experimental investigation

2-pyridinethione (2-mercaptopyridine, H-2mp) undergoes rapid oxidative addition with 2 mol of the 17-electron organometallic radical *Cr(CO)3Cp (where Cp*=C5Me5), yielding hydride H-Cr(CO)3Cp* and thiolate (eta1-2mp)Cr(CO)3Cp*. In a slower secondary reaction, (eta1-2mp)Cr(CO)3Cp* loses CO generating the N,S-chelate complex (eta2-2mp)Cr(CO)2Cp* for which the crystal structure is reported. The rate of 2-pyridine thione oxidative addition with *Cr(CO)3Cp* (abbreviated *Cr) in toluene best fits rate=kobs[H-2mp][*Cr]; kobs(288 K)=22 +/- 4 M(-1) s(-1); DeltaH++=4 +/- 1 kcal/mol; DeltaS++=- 40 +/- 5 cal/mol K. The rate of reaction is the same under CO or Ar, and the reaction of deuterated 2-pyridine thione (D-2mp) shows a negligible (inverse) kinetic isotope effect (kD/kH=1.06 +/- 0.10). The rate of decarbonylation of (eta1-2mp)Cr(CO)3Cp* forming (eta2-2mp)Cr(CO)2Cp* obeys simple first-order kinetics with kobs (288 K)=3.1x10(-4) s(-1), DeltaH++=23 +/- 1 kcal/mol, and DeltaS++=+ 5.0 +/- 2 cal/mol K. Reaction of 4-pyridine thione (4-mercaptopyridine, H-4mp) with *Cr(CO)3Cp* in THF and CH2Cl2 also follows second-order kinetics and is approximately 2-5 times faster than H-2mp in the same solvents. The relatively rapid nature of the thione versus thiol reactions is attributed to differences in the proposed 19-electron intermediate complexes, [*(S=C5H4N-H)Cr(CO)3Cp*] versus [*(H-S-C6H5)Cr(CO)3Cp*]. In comparison, reactions of pyridyl disulfides occur by a mechanism similar to that followed by aryl disulfides involving direct attack of the sulfur-sulfur bond by the metal radical. Calorimetric data indicate Cr-SR bond strengths for aryl and pyridyl derivatives are similar. The experimental conclusions are supported by B3LYP/6-311+G(3df,2p) calculations, which also provide additional insight into the reaction pathways open to the thione/thiol tautomers. For example, the reaction between H* radical and the 2-pyridine thione S atom yielding a thionyl radical is exothermic by approximately 30 kcal/mol. In contrast, the thiuranyl radical formed from the addition of H* to the 2-pyridine thiol S atom is predicted to be unstable, eliminating either H* or HS* without barrier.     

Gyromagnetic ratio of the 159 keV 3/2+ state of117Sn

TheF-spin formalism is employed to extract scalar boson effective charges from theB(E2, 2 ₁ ⁺ →0_g) systematics in rare-earth nuclei.     

Molybdenum chalcogenobenzimidato complexes: radical synthesis and nitrile extrusion via beta-EPh (E = S, Se, and Te) elimination

Molybdenum chalcogenobenzimidates of formula (Ph[PhE]C=N)Mo(N[t-Bu]Ar)(3) (Ar = 3,5-C(6)H(3)Me(2)) have been obtained by treatment of Mo(N[t-Bu]Ar)(3) sequentially with benzonitrile and 0.5 equiv of PhEEPh (E = S, Se, and Te). Molecular structure determinations have been carried out for the S and Se variants. The Te variant extrudes PhCN forming structurally characterized (PhTe)Mo(N[t-Bu]Ar)(3) with facility assessed via stopped-flow kinetic measurements, while the Se and S analogues exhibit increasing stability. Quantum chemical calculations and solution calorimetry have been employed as an aid to interpretation of the PhCN extrusion reaction.     

Thermochemistry of molybdenum tricarbonyl complexes of arenes and cyclic polyolefins

The heats of reaction of arene, cycloheptatriene, and cyclooctatetraene complexes of molybdenum tricarbonyl with pyridine yielding (py)₃Mo(CO)₃ have been measured by solution calorimetry. Reaction of toluene molybdenum tricarbonyl with cyclopentadiene yielding HMo(CO)₃C₅H₅ and with tetrahydrofuran yielding (THF)₃Mo(CO)₃ have also been studied thermochemically. These measurements yield accurate values for the enthalpy of arene exchange in methylene chloride solution for a number of organomolybdenum complexes. The order of stability: benzene < toluene < cyclooctatetraene < mesitylene < hexamethylbenzene < cycloheptatriene < (tris)-tetrahydrofuran < η⁵-cyclopentadienylhydrido < (tris)-pyridine spans a range of 31 kcal/mol.The enthalpy of isomerization of cycloheptatriene to toluene is reduced by 7.1 ± 1.2 kcal/mol upon coordination to molybdenum tricarbonyl, indicative of a loss of “resonance” energy for the complexed arene. The MoH bond strength in HMo(CO)₃C₅H₅ is estimated as 66 ± 8 kcal/mol. The importance of entropic factors in arene exchange is discussed.     

Sublevel decay kinetics of the triplet state of bacteriochlorophyll a and b in methyltetrahydrofuran at 1.2 K

The decay rates of the triplet sublevels of monomeric BChl a and b have been reexamined with pulsed microwave excitation. Our values of k_x and k_y are significantly higher than published values. The new results are discussed in relation to the exciton model of the special pair making up the primary electron donor in photosynthetic purple bacteria.