CO-loss Product Research Articles

Pervasive influence of heavy metals on metabolic pathways is potentially relieved by hesperidin to enhance the phytoremediation efficiency of Bassia scoparia.

Hesperidin (HSP), a flavonoid, is a potent antioxidant, metal chelator, mediator of signaling pathways, and regulator of metal uptake in plants. The study examined the ameliorative effects of HSP (100 μM) on Bassia scoparia grown under excessive levels of heavy metals (zinc (500 mg kg-1), copper (400 mg kg-1), cadmium (100 mg kg-1), and chromium (100 mg kg-1)). The study clarifies the underlying mechanisms by which HSP lessens metabolic mayhem to enhance metal stress tolerance and phytoremediation efficiency of Bassia scoparia. Plants manifested diminished growth because of a drop in chlorophyll content and nutrient acquisition, along with exacerbated deterioration of cellular membranes reflected in elevated reactive oxygen species (ROS) production, lipid peroxidation, and relative membrane permeability. Besides the colossal production of cytotoxic methylglyoxal, the activity of lipoxygenase was also higher in plants under metal toxicity. Conversely, hesperidin suppressed the production of cytotoxic ROS and methylglyoxal. Hesperidin improved oxidative defense that protected membrane integrity. Hesperidin caused a more significant accumulation of osmolytes, non-protein thiols, and phytochelatins, thereby rendering metal ions non-toxic. Hydrogen sulfide and nitric oxide endogenous levels were intricately maintained higher in plants treated with HSP. Hesperidin increased metal accumulation in Bassia scoparia and thereby had the potential to promote the reclamation of metal-contaminated soils.

UV photodissociation action spectra of protonated formylpyridines.

The first ππ* transition for protonated 2-, 3-, and 4-formylpyridine (FPH+) (m/z 108) is investigated by mass spectrometry coupled with photodissociation action spectroscopy at room temperature and 10K. The photoproduct ions are detected over 35000-43000cm-1, and the major product channel for 3-FPH+ and 4-FPH+ is the loss of CO forming protonated pyridine at m/z 80. For 2-FPH+, the CO loss product is present but a more abundant photoproduct arises from the loss of CH2O to form m/z 78. Plausible potential energy pathways that lead to dissociation are mapped out and comparisons are made to products arising from collision-induced dissociation. Although, in all cases, the elimination of CO is the overwhelming thermodynamically preferred pathway, the protonated 2-FPH+ results suggest that the CH2O product is kinetically driven and competitive with CO loss. In addition, for each isomer, radical photoproduct ions are detected at lower abundances. SCS-CC2/aug-cc-pVTZ Franck-Condon simulations assist with the assignment of vibrionic structure and adiabatic energies (0-0) for 2-FPH+ at 36 560cm-1, 37 430cm-1 for 3-FPH+, and 36 140cm-1 for 4-FPH+, yielding an accurate prediction, on average, within 620cm-1.

Comparative study of the China and U.S. import trade structure based on the global chromium ore trade network

As a critical mineral with highly effective strategic uses, chromium is highly unevenly distributed, leading to frequent international chromium ore trading. As the two most important global economies, China and the U.S., the chromium ore importing market is essential to utilize different trading partner structures. This paper constructed chromium ore import trade networks using trade volume and trade increment. The research then tests the similarities of China's trading partner structures and the U.S. chromium ore importing market combined with a Kendall correlation analysis and Quadratic Assignment Procedure. We aim to verify the correlation and dependence of chromium ore import trade patterns between China and the U.S. while analyzing the chromium ore importing trading communities globally. The results show that the trading partner structure of China and the U.S. is different, which is respectively “small community with more partners” and “large community with fewer partners”. Their chromium ore importing markets operate and develop independently. China is more influential in global importing market. For development demand and structural stability, building more partnerships is necessary for both countries. China also needs to maintain colossal production and consumption, while the U.S. needs to improve trade security.

Exploring diverse roles of micro RNAs in banana: Current status and future prospective.

Micro RNAs (miRNAs) are 20-24 nucleotides long non-coding RNA sequences identified and characterized in multiple plant and animal systems. miRNAs play multifarious roles ranging from plant development to stress tolerance by synchronizing physiological processes at the level of transcription and translation. Banana is a major horticultural crop with colossal production worldwide. Despite the recent encouraging developments, the information on functions of miRNAs in banana plants is still in its infancy. The available literature pertaining to miRNAs in banana plants hints towards their contribution as master regulators in crucial physiological processes for instance abiotic stress responses, pathogenic defence response, fruit ripening and so on. This review is focused on biogenesis of miRNAs, their identification and deciphering their respective roles in banana plants with special emphasis on abiotic stress responses, plant immune responses, fruit ripening and storage. Based on the prior reports, we identified a few miRNAs with prospective roles in stress tolerance and illustrated the potential applications of miRNAs in banana crop improvement utilizing recent biotechnological tools such as CRISPR cas9, RNAi and the nano particle based foliar spray of miRNAs. The review briefly explained the future directions in banana research with a special emphasis on miRNA regulatory networks and agronomic traits improvement. Finally, future domains in miRNA research in plants and their possible applications towards crop improvement in agriculture are described briefly.

Dissociative Ionization Mechanism and Appearance Energies in Adipic Acid Revealed by Imaging Photoelectron Photoion Coincidence, Selective Deuteration, and Calculations.

Adipic acid, a model compound for oxygenated organic aerosol, has been studied at the VUV beamline of the Swiss Light Source. Internal energy selected cations were prepared by threshold photoionization using vacuum ultraviolet synchrotron radiation and imaging photoelectron photoion coincidence spectroscopy (iPEPICO). The threshold photoelectron spectrum yields a vertical ionization energy (IE) of 10.5 eV, significantly above the calculated adiabatic IE of 8.6 eV. The cationic minimum is accessible after vertical ionization by H-transfer from one of the γ-carbons to a carbonyl oxygen and is sufficiently energetic to decay by water loss at the ionization onset. The slope of the breakdown curves, quantum chemical calculations, and selective deuteration of the carboxylic hydrogens establish the dissociative photoionization mechanism. After ionization, one γ-methylene hydrogen and the two carboxylic hydrogens are randomized prior to H2O loss. On the basis of the deuteration degree in the H2O + CO-loss product at higher energies, a direct water-loss channel without complete randomization also exists. The breakdown diagram and center of gravity of the H2O + CO-loss peak were modeled to obtain 0 K appearance energies of 10.77, 10.32, and 11.53 eV for H2O + CO loss, CH2COOH loss, and H2O + CH2COOH loss from adipic acid. These agree well with the CBS-QB3 calculated values of 10.68, 10.45, and 11.57 eV, respectively, which shows that threshold photoionization can yield energetics data as long as the dissociation is statistical, even when the parent ion cannot be observed. The results can be used as a starting point for a deeper understanding of the ionization and low-energy fragmentation of organic aerosol components.

Excited state evolution towards ligand loss and ligand chelation at group 6 metal carbonyl centres.

The photochemistry and photophysics of three model "half-sandwich" complexes (η(6)-benzophenone)Cr(CO)3, (η(6)-styrene)Cr(CO)3, and (η(6)-allylbenzene)Cr(CO)3 were investigated using pico-second time-resolved infrared spectroscopy and time-dependent density functional theory methods. The (η(6)-benzophenone)Cr(CO)3 complex was studied using two excitation wavelengths (470 and 320 nm) while the remaining complexes were irradiated using 400 nm light. Two independent excited states were detected spectroscopically for each complex, one an unreactive excited state of metal-to-arene charge-transfer character and the other with metal-to-carbonyl charge transfer character. This second excited state leads to an arrested release of CO on the pico-second time-scale. Low-energy excitation (470 nm) of (η(6)-benzophenone)Cr(CO)3 populated only the unreactive excited state which simply relaxes to the parent complex. Higher energy irradiation (320 nm) induced CO-loss. Irradiation of (η(6)-styrene)Cr(CO)3, or (η(6)-allylbenzene)Cr(CO)3 at 400 nm provided evidence for the simultaneous population of both the reactive and unreactive excited states. The efficiency at which the unreactive excited state is populated depends on the degree of conjugation of the substituent with the arene π-system and this affects the efficiency of the CO-loss process. The quantum yield of CO-loss is 0.50 for (η(6)-allylbenzene)Cr(CO)3 and 0.43 for (η(6)-styrene)Cr(CO)3. These studies provide evidence for the existence of two photophysical routes to CO loss, a minor ultrafast route and an arrested mechanism involving the intermediate population of a reactive excited state. This reactive excited state either relaxes to reform the parent species or eject CO. Thus the quantum yield of the CO-loss is strongly dependent on the excitation wavelength. Time-dependent density functional theory calculations confirm that the state responsible for ultrafast CO-loss has significant metal-centred character while the reactive state responsible for the arrested CO-loss has significant metal-to-carbonyl charge-transfer character. The CO-loss product (η(6)-allylbenzene)Cr(CO)2 formed following irradiation of (η(6)-allylbenzene)Cr(CO)3 reacts further with the pendent alkenyl group to form the chelate product (η(6),η(2)-allylbenzene)Cr(CO)2.

The Aboriginal voice in Baz Luhrmann's left-leaning Australia (2008)

Arguing that Baz Luhrmann's Australia (2008) is a big-budget, non-independent film espousing a left-leaning political ideology in its non-racist representations of Aborigines on film, this paper suggests the addition of a ‘fourth formation’ to the 1984 Moore and Muecke model is warranted. According to their theorizing, racist ‘first formation’ films promote policies of assimilation whereas ‘second formation’ films avoid overt political statements in favour of more acceptable multicultural liberalism. Moore and Muecke's seemingly ultimate ‘third formation films’, however, blatantly foreground the director's leftist political dogma in a necessarily low budget, independent production. Australia, on the other hand, is an advance on the third formation because its left-leaning feminized Aboriginal voice is safely backed by a colossal production budget and indicates a transformation in public perceptions of Aboriginal issues. Furthermore, this paper argues that the use of low-cost post-production techniques such as voice-over narration by racially appropriate individuals and the use of diegetic song in Australia work to ensure the positive reception of the left-leaning message regarding the Stolen Generations. With these devices Luhrmann effectively counters the claims of right-wing denialists such as Andrew Bolt and Keith Windschuttle.

Time-Resolved Vibrational Spectroscopy of [FeFe]-Hydrogenase Model Compounds

Model compounds have been found to structurally mimic the catalytic hydrogen-producing active site of Fe-Fe hydrogenases and are being explored as functional models. The time-dependent behavior of Fe(2)(μ-S(2)C(3)H(6))(CO)(6) and Fe(2)(μ-S(2)C(2)H(4))(CO)(6) is reviewed and new ultrafast UV- and visible-excitation/IR-probe measurements of the carbonyl stretching region are presented. Ground-state and excited-state electronic and vibrational properties of Fe(2)(μ-S(2)C(3)H(6))(CO)(6) were studied with density functional theory (DFT) calculations. For Fe(2)(μ-S(2)C(3)H(6))(CO)(6) excited with 266 nm, long-lived signals (τ = 3.7 ± 0.26 μs) are assigned to loss of a CO ligand. For 355 and 532 nm excitation, short-lived (τ = 150 ± 17 ps) bands are observed in addition to CO-loss product. Short-lived transient absorption intensities are smaller for 355 nm and much larger for 532 nm excitation and are assigned to a short-lived photoproduct resulting from excited electronic state structural reorganization of the Fe-Fe bond. Because these molecules are tethered by bridging disulfur ligands, this extended di-iron bond relaxes during the excited state decay. Interestingly, and perhaps fortuitously, the time-dependent DFT-optimized exited-state geometry of Fe(2)(μ-S(2)C(3)H(6))(CO)(6) with a semibridging CO is reminiscent of the geometry of the Fe(2)S(2) subcluster of the active site observed in Fe-Fe hydrogenase X-ray crystal structures. We suggest these wavelength-dependent excitation dynamics could significantly alter potential mechanisms for light-driven catalysis.

Unusually Slow Photodissociation of CO from (η6-C6H6)Cr(CO)3 (M = Cr or Mo): A Time-Resolved Infrared, Matrix Isolation, and DFT Investigation

The photochemistry of (η6-C6H6)M(CO)3 (M = Cr or Mo) is described. Photolysis with λexc. > 300 nm of (η6-C6H6)Cr(CO)3 in low-temperature matrixes containing CO produced the CO-loss product, while lower energy photolysis (λexc. > 400 nm) produced Cr(CO)6. Pulsed photolysis (λexc. = 400 nm) of (η6-C6H6)Cr(CO)3 in n-heptane solution at room temperature produced an excited-state species (1966 and 1888 cm−1) that decays over 150 ps to (η6-C6H6)Cr(CO)2(n-heptane) (70%) and (η6-C6H6)Cr(CO)3 (30%). Pulsed photolysis (λexc. = 266 nm) of (η6-C6H6)Cr(CO)3 in n-heptane produced bands assigned to (η6-C6H6)Cr(CO)2(n-heptane) (1930 and 1870 cm−1) within 1 ps. These bands increase with a rate identical to the rate of decay of the excited-state species and the rate of recovery of (η6-C6H6)Cr(CO)3. Photolysis of (η6-C6H6)Mo(CO)3 at 400 nm produced an excited-state species (1996 and 1898 cm−1) and traces of (η6-C6H6)Mo(CO)2(n-heptane) within 1 ps. For the chromium system CO-loss can occur following excitation at both 400 and 266 nm via an avoided crossing of a MACT (metal-to-arene charge transfer) and MCCT/LF (metal-to-carbonyl charge transfer/ligand field) states. This leads to an unusually slow CO-loss following excitation with 400 nm light. Rapid CO-loss is observed following 266 nm excitation because of direct population of the MCCT/LF state. The quantum yield for CO-loss in the chromium system decreases with increasing excitation energy because of the competing population of a high-energy unreactive MACT state. For the molydenum system CO-loss is a minor process for 400 nm excitation, and an unreactive MACT state is evident from the TRIR spectra. A higher quantum yield for CO-loss is observed following 266 nm excitation through both direct population of the MCCT/LF state and production of a vibrationally excited reactive MACT state. This results in the quantum yield for CO-loss increasing with increasing excitation energy.

Density Functional Theory and Low-Temperature Matrix Investigations of CO-Loss Photochemistry from [(C5R5)Ru(CO)2]2 (R = H, Me) Complexes

The photochemical CO-loss products of the diruthenium complexes [CpRu(CO) 2]2 (5; Cp = η5-C5H5), [Cp*Ru(CO)2]2 (5*; Cp* = η5-C5(CH3)5) and CpCp*[Ru(CO)2]2 (5′) have been studied experimentally in low-temperature (96 K) matrices in 3-methylpentane by using IR spectroscopy. It is proposed that all three complexes undergo single-CO-loss chemistry but that the products have different structures. The single-CO-loss product from 5 is proposed to have one bridging and two terminal carbonyl ligands, whereas 5* and 5′ generate triply bridged CO-loss products similar to that observed from [CpFe(CO)2]2 and [Cp*Fe(CO)2]2. Double-CO-loss from 5* and 5* 9′ is also apparently observed. Relativistic DFT calculations have been carried out on various isomers of the starting materials and on potential CO-loss products from 5. The calculations suggest that the triply bridged product Cp2Ru2(μ-CO)3 (6) might have a singlet ground state in contrast to the corresponding diiron complex Cp2Fe2(μ-CO)3 (3), which has a triplet ground state.

Organic Matrix Photochemical Studies of Rhenacarborane Nitrosyl Complexes. Evidence for Linkage Isomeric Nitrosyl Photointermediates

The photochemistry of the rhenacarborane nitrosyl compounds [1,2-R2-3,3-(CO)2-3-NO-closo-3,1,2-ReC2B9H9], where R = H, I, and R = Me, II, has been studied in Nujol and methytetrahydrofuran (MeTHF) frozen glasses at ca. 90K. Visible light photolysis gives rise to a new species that has been assigned to the isonitrosyl linkage isomer, while ultraviolet photolysis gives rise to a second species believed to be the η 2 -NO linkage isomer. In MeTHF high-energy photolysis also gives rise to a CO-loss product. Photolysis of derivatives of I, [3-L-3-L'-3-NO-closo-3,1,2-ReC 2 B 9 H 1 1 ], where L = CO, L' = PMe 3 , III; L = CO, L' = CNC 6 H 3 -Me 2 -2, 6, IV, and L = L' = CNC 6 H 3 Me 2 -2,6,, V, all give rise to a photoproduct believed to be the η 2 -NO lnkage isomer. There was no evidence of the isonitrosyl isomer in these cases. Complexes III and IV also undergo CO loss. Ultraviolet photolysis of [3,3,3-(CO) 3 -closo-3,1,2-RuC 2 B 9 H 1 1 ], VI, in MeTHF results in CO loss and formation of the corresponding [3,3-(CO) 2 -3-MeTHF-closo-3,1,2-RuC 2 B 9 H 1 1 ] derivative. In frozen Nujol, ultraviolet photolysis of VI gives rise to a CO-loss species. Upon visible light back-photolysis or annealing of the sample, this photoproduct gives rise to a second dicarbonyl species of unknown structure.

Influence of Metal Core Composition on Redox Properties and Photoreactivity of the Clusters [H4-xRu4-xRhx(CO)12] (x = 0, 2, 3, 4)

Electrochemical properties of tetrahedral clusters [H2Ru2Rh2(CO)12], [HRuRh3(CO)12] and [Rh4(CO)12] were investigated in order to evaluate the influence of metal core composition in the series [H4-xRu4-xRhx(CO)12] (x = 0-4). The cluster [H3Ru3Rh(CO)12] was not available in sufficient quantities. As reported for [H4Ru4(CO)12], electrochemical reduction of the hydride-containing clusters [H2Ru2Rh2(CO)12] and [HRuRh3(CO)12] also results in (stepwise) loss of hydrogen, producing the anions [HRu2Rh2(CO)12]-, [Ru2Rh2(CO)12]2- and [RuRh3(CO)12]-. These anions can also be prepared from the neutral parent clusters via chemical routes. Electrochemical reduction of [Rh4(CO)12] does not result in the formation of any stable tetranuclear anion. Instead, [Rh5(CO)15]- and [Rh6(CO)15]2- are the major reduction products detected in the course of IR spectroelectrochemical experiments. Most likely, these cluster species are formed from the secondary CO-loss product [Rh4(CO)11]2- by fast redox condensation reactions. Their reoxidation regenerates parent [Rh4(CO)12], together with some [Rh6(CO)16]. Unlike [H4Ru4(CO)12] that undergoes photochemical CO-dissociation, [H2Ru2Rh2(CO)12] and [Rh4(CO)12] are completely photostable in neat hexane and dichloromethane as well as in the presence of oct-1-ene.

Photochemical studies of Co 2(CO) 6(acetylene) complexes and their phosphine derivatives in frozen Nujol matrices

Photolysis of Co 2(CO) 6(alkyne), where alkyne=H 2C 2 ( I), (C 2H 5) 2C 2 ( II) (C 6H 5) 2C 2 ( III) and C 6H 5C 2H ( IV) in frozen Nujol at ca. 90 K results in loss of CO to give Co 2(CO) 5(alkyne) in which CO-loss appears to be from the axial position. Complete conversion to a second isomer, presumed to have a vacancy in an equatorial position, or to starting material is observed for all compounds upon annealing the matrices to ca. 140 K. The second isomer is also formed by Co 2(CO) 5[(C 6H 5) 2C 2] upon photolysis at λ<280 nm. Photolysis of Co 2(CO) 6As 2 ( V), gave photochemical and annealing products whose spectra were analogous to those of the alkyne compounds. Photolysis of phosphine substituted derivatives such as axial-Co 2(CO) 5(PR 3)(C 2R′ 2), where R=Bu; R′=H ( VI), R=Ph; R′=H ( VII), R=OPh, R′=H ( VIII), R=Ph; R′=Ph (M) yields two isomeric CO-loss products. Annealing samples of V– IX to ca. 140 K results in simple reaction reversal. Solution photolysis of IX with PPh 3 gives Co 2(CO) 4(PPh 3) 2(Ph 2C 2). Photolysis of Co 2(CO) 4(PPh 3) 2(H 2C 2) ( X) yields two products. Annealing the sample reforms the starting material and also generates a new species. Photolysis of Co 2(CO) 4(Ph 2P) 2CH 2][(C 2H 5) 2C 2] ( XI) forms a single product that reforms XI upon annealing.

Density functional calculations of dinuclear organometallic carbonyl complexes. Part I: metalmetal and metalCO bond energies

One of the most fundamental properties in chemistry is the bond dissociation energy (BDE), the energy required to break a specific bond of a molecule. In this paper we apply gradient-corrected density functional theory (DFT) to the calculation of the BDEs of three prototypical organometallic complexes, Mn 2(CO) 10, Fe 2(CO) 9, and Co 2(CO) 8, along with the CO-loss products Mn 2(CO) 9 and Mn 2(CO) 8. We consider the dissociation of both the metalmetal bond and a metalcarbonyl bond. For Mn 2(CO) 10 and Fe 2(CO) 9, the calculated metalmetal BDE is within the error of the experimental measurements. However, the calculated metalmetal BDE for Co 2(CO) 8 is not within the errors of the measurements, but is improved greatly with an unrestricted wavefunction compared with a restricted wavefunction. For the first carbonyl BDE, the calculations agree within a few kcal mol −1 for each complex.

Matrix isolation Raman spectroscopy and photochemistry of ( η5-C 5H 5)(OC) 2Fe–SiH 2R (R=H, CH 3) in comparison with density functional calculations

The compounds ( η 5-C 5H 5)(OC) 2Fe–SiH 2R with RH ( 1) and CH 3 ( 2) have been isolated in krypton or nitrogen matrices and subsequently detected by Raman spectroscopy, followed by a detailed vibrational analysis. To study the photochemistry of the iron silyl complexes 1 and 2, Raman spectra were also taken after UV irradiation. Resonance Raman spectroscopy enabled the detection of newly generated species in low concentrations. Due to the observed differences in the spectra, a reaction mechanism for the photochemical conversion of 1 and 2 in matrices is suggested. The experimental spectra of 1 are compared with the vibrational data of density functional calculations. These calculations propose a CO-loss product with a (Fe, H, Si) three-centre bond as a stable intermediate for the photochemical conversion of 1. Infrared measurements confirm the photochemical CO elimination.

Thermochemistry and Photodissociation Studies of [CoL] and [CoL2], L=Pyrrole, Furan, Thiophene and Selenophene

[CoL]+ and [CoL2]+ (L=pyrrole, furan, thiophene and selenophene) were generated in a prototype Nicolet-FTMS 1000 Fourier transform mass spectrometer, where they were irradiated with light from a 2.5 kW Xe arc lamp. Photodissociation thresholds, obtained using energy cut-off filters, yielded the bond energies D0(Co+−pyrrole)=59±3 kcal mol-1, D0(Co+−furan)=57±3 kcal mol-1, D0(Co+−thiophene)=61±3 kcal mol-1 and D0(Co+−selenophene)=64±3 kcal mol-1 (1 kcal=4.184 kJ), as well as D0([Co(pyrrole)]+−pyrrole)⩽48±3 kcal mol-1, D0([Co(furan)]+−furan)⩽46±3 kcal mol-1, D0([Co(thiophene)]+−thiophene)⩽49±3 kcal mol-1 and D0([Co(selenophene)]+−selenophene)⩽51±3 kcal mol-1. The photoappearance threshold for [CoC3H4]+, the CO loss product ion from [Co(furan)]+, yielded D0(Co+−C3H4)=either 33±6 or 32±6 kcal mol-1, depending on whether C3H4 was allene or propyne, respectively. Where possible, these bond energy measurements were further corroborated by ligand displacement reactions and competitive collision-induced dissociation experiments.

Thermochemistry and Photodissociation Studies of [CoL]+ and [CoL2]+, L=Pyrrole, Furan, Thiophene and Selenophene

[CoL]+ and [CoL2]+ (L=pyrrole, furan, thiophene and selenophene) were generated in a prototype Nicolet-FTMS 1000 Fourier transform mass spectrometer, where they were irradiated with light from a 2.5 kW Xe arc lamp. Photodissociation thresholds, obtained using energy cut-off filters, yielded the bond energies D0(Co+−pyrrole)=59±3 kcal mol-1, D0(Co+−furan)=57±3 kcal mol-1, D0(Co+−thiophene)=61±3 kcal mol-1 and D0(Co+−selenophene)=64±3 kcal mol-1 (1 kcal=4.184 kJ), as well as D0([Co(pyrrole)]+−pyrrole)⩽48±3 kcal mol-1, D0([Co(furan)]+−furan)⩽46±3 kcal mol-1, D0([Co(thiophene)]+−thiophene)⩽49±3 kcal mol-1 and D0([Co(selenophene)]+−selenophene)⩽51±3 kcal mol-1. The photoappearance threshold for [CoC3H4]+, the CO loss product ion from [Co(furan)]+, yielded D0(Co+−C3H4)=either 33±6 or 32±6 kcal mol-1, depending on whether C3H4 was allene or propyne, respectively. Where possible, these bond energy measurements were further corroborated by ligand displacement reactions and competitive collision-induced dissociation experiments.

Photochemistry of (η6-2,6-X2C5H3N)Cr(CO)3 (X = H, CH3, (CH3)3Si). First Example of a Photoinduced Ring-Slip at an (η6-Arene)M(CO)3 Center. Molecular Structures of (η6-2,6-(CH3)2C5H3N)Cr(CO)3 and (η6-2,6-((CH3)3Si)2C5H3N)Cr(CO)3

The photochemistry of (η6-2,6-X2C5H3N)Cr(CO)3 was investigated both in low-temperature matrices (X = H or (CH3)3Si) and in room-temperature solution (X = H, CH3, or (CH3)3 Si). Room-temperature photolysis (λexc > 410 nm) in CO-saturated methanol or acetonitrile produced (η1-2,6-X2C5H3N)Cr(CO)5 which subsequently formed Cr(CO)6 in a secondary photochemical process (X = H or CH3). The efficiency of pentacarbonyl formation is lower in CO-saturated cyclohexane and follows the order X = H > X = CH3. Photolysis in low-temperature matrices resulted in an η6 to η1 pyridine ring-slippage (λexc = 460 nm; X = H). Visible irradiation in a CO-doped methane matrix produced (η1-C5H5N)Cr(CO)5, while in an N2 matrix fac-(η1-C5H5N)(N2)2Cr(CO)3 is formed. Irradiation with λexc = 308 nm produced both the ring-slippage product and also the CO-loss product (η6-C5H5)Cr(CO)2, which in a N2 matrix is trapped as (η6- C5H5N)Cr(CO)2(N2). Time-resolved infrared spectroscopy in cyclohexane revealed only the CO-loss product (λexc = 308 nm; X = H). The apparent difference in room-temperature and low-temperature photochemistry is explained by a rapid regeneration of (η6-C5H5N)Cr(CO)3 from the η1-intermediate. This explanation was supported by laser flash photolysis experiments (λexc = 355 nm) in CO-saturated cyclohexane (Sol), where the recovery of the (η6-C5H5N)Cr(CO)3 absorption follows a biphasic time profile, whereby the faster process was assigned to the η1 to η6 transformation and the slower to the reaction of (η6-C5H5N)Cr(CO)2(Sol) with CO. Crystals of (η6-2,6-(CH3)2C5H3N)Cr(CO)3 and (η6-2,6-((CH3)3Si)2C5H3N)Cr(CO)3 were characterized by X-ray diffraction.

Alkyl-dependent photochemistry of Mn (R)(CO) 3(R′-DAB) (R = Me, Bz; R′ = iPr, pTol): homolysis of the MnR bond for R = Bz and release of CO for R = Me

The spectroscopic properties and photochemistry of the complexes Mn(R)(CO) 3(R′-DAB) (R=Me, Bz; R′=iPr, pTol) are reported. The UV-photoelectron spectrum of Mn(Me)(CO) 3(iPr-DAB) shows that the σ(MnMe) orbital has a higher ionization potential than the D π(Mn) orbitals, which is of importance for the photochemical behavior of this complex. The R group bound to the metal determines the photochemistry of these complexes. The Me complexes lose CO upon irradiation into their MLCT bands. The CO-loss products react with Lewis bases, the final product being cis(CO,CO), trans(Me,L)-Mn(Me)(L)(CO) 2(R′-DAB). The structure of one of these products, viz. cis,trans-Mn(Me)(P(OMe) 3)(CO) 2(iPr-DAB)(C 14H 28N 2O 5PMn) has been determined by a single-crystal X-ray diffraction study ( T=200 K). The crystal is monoclinic, space group P2 1/ n with unit cell dimensions a=15.435(3), b=15.200(3), c=18.192(4) A ̊ , V=3915(2) A ̊ 3, Z=8 . The structure refinement converged to R=0.077 for 2829 observed reflections (total number of parameters: 319). Transient absorption spectroscopy shows that an equatorial CO ligand is lost upon excitation and that the final product is formed via different cis,cis-isomers. For R = Bz, visible excitation leads to efficient homolysis ( Φ= 0.4 for R′ =iPr) of the MnBz bond, resulting in the formation of radicals which are characterized by ESR spectroscopy. The different behavior of the Bz and Me complexes is attributed to a difference in relative energies of two reactive excited states. For R=Me, the complexes lose CO from the lowest MLCT state; for R=Bz they undergo homolysis of the MnBz bond from the lowest σπ ∗ state.

Metal-to-Ligand Charge Transfer (MLCT) Photochemistry offac-Mn(Cl)(CO)3(H-DAB): A Density Functional Study

The title compound has low-energy Mn−3d to 1,4-diaza-1,3-butadiene (H-DAB) π* metal-to-ligand charge transfer (MLCT) excited states, which are not, by their electronic nature, Mn−CO dissociative. Their potential energy curves (PEC) exhibit Mn−COeq and Mn−COax bonding minima around Re. Loss of an equatorial CO ligand upon MLCT excitation is explained by a radiationless transition from the MLCT states to the dissociative continuum of the electronic ground state. According to the calculated PEC of the ground state, the complex will undergo a strong structural rearrangement upon equatorial CO dissociation, during which the chloride shifts to the equatorial open site. This rearrangement explains the experimentally found formation of mer-Mn(Cl)(CO)3(α-diimine) complexes upon back-reaction of their CO-loss product with CO. This mechanism of equatorial CO dissociation is very different from the usual photochemical dissociation directly from a dissociative ligand-field state or through crossing of the photoactive excited state by such a ligand-field state. In contrast, axial CO dissociation, which does not occur readily, does not give rise to structural rearrangement and is predicted to produce the fac complex upon back-reaction with CO.