Search

AimsThe standardised mean difference (SMD) is one of the most used effect sizes to indicate the effects of treatments. It indicates the difference between a treatment and comparison group after treatment has ended, in terms of standard deviations. Some meta-analyses, including several highly cited and influential ones, use the pre-post SMD, indicating the difference between baseline and post-test within one (treatment group).MethodsIn this paper, we argue that these pre-post SMDs should be avoided in meta-analyses and we describe the arguments why pre-post SMDs can result in biased outcomes.ResultsOne important reason why pre-post SMDs should be avoided is that the scores on baseline and post-test are not independent of each other. The value for the correlation should be used in the calculation of the SMD, while this value is typically not known. We used data from an ‘individual patient data’ meta-analysis of trials comparing cognitive behaviour therapy and anti-depressive medication, to show that this problem can lead to considerable errors in the estimation of the SMDs. Another even more important reason why pre-post SMDs should be avoided in meta-analyses is that they are influenced by natural processes and characteristics of the patients and settings, and these cannot be discerned from the effects of the intervention. Between-group SMDs are much better because they control for such variables and these variables only affect the between group SMD when they are related to the effects of the intervention.ConclusionsWe conclude that pre-post SMDs should be avoided in meta-analyses as using them probably results in biased outcomes.

The bonding and geometries of H2Fe(CO)3, H2Fe(CO)3(C2H4), and HFe(CO)3(C2H5), which are species relevant to iron carbonyl-catalyzed olefin hydrogenation, are analyzed using density functional theory (DFT). These calculations indicate that H2Fe(CO)3 and H2Fe(CO)3(C2H4) are respectively triplet and singlet dihydrogen complexes with H2Fe(CO)3(C2H4) lower in energy than HFe(CO)3(C2H5) by a modest amount (10.2 kcal/mol with BP86/LACV3P**). Further, it is likely that when H2Fe(CO)3(C2H4) forms, it can rearrange to form the HFe(CO)3(C2H5) complex. The reductive elimination of C2H6 from HFe(CO)3(C2H5) is exothermic, whether it is accompanied by or precedes ethylene addition.

The reaction of C2H5I with photogenerated Fe(CO)4 was followed using time-resolved infrared spectroscopy. Three novel products were detected. On the basis of their carbonyl stretching frequencies, their kinetics, and calculations of their relative energies, these complexes are assigned as IFe(CO)4(C2H5), IFe(CO)3(η²-COC2H5), and IFe(CO)4(COC2H5). Addition of C2H5I to Fe(CO)4 produces IFe(CO)4(C2H5) as the kinetically preferred product, which then converts to IFe(CO)3(η²-COC2H5), the thermodynamically preferred complex. IFe(CO)4(COC2H5) is formed upon addition of CO to IFe(CO)3(η¹-COC2H5), a 16-electron complex that is proposed as a common intermediate leading to the three products. Temperature-dependent studies of the kinetics of the decay of IFe(CO)4(COC2H5), the longest lived of the products, indicate that it decarbonylates with an activation energy of ~20 kcal/mol.

The goal of this research was to investigate the formation and reactivity of charge-transfer complexes (CTCs) among a homologous series of chlorophenols on TiO2. We previously showed that 2,4,5-trichlorophenol (245TCP) forms a CTC with Degussa P25, a commercial preparation of TiO2. Here, we probe how light energy influences reactivity and product formation. Slurries of P25 containing 245TCP were irradiated at 360, 400, 430, 480, and 550 nm. At each wavelength, the amount of transformation of 245TCP correlates to the diffuse-reflectance absorbance of a 245TCP/P25 system, supporting the CTC as the cause of reaction. In addition, polymeric products are formed only under wavelengths that excite the CTC, indicating a different reaction mechanism for the CTC than for bandgap excitation of TiO2. We also found a higher quantum efficiency for CTC reactivity than for bandgap activation of the catalyst, suggesting that the photocatalytic efficiency and selectivity can be improved for certain compounds by designing catalytic materials that form CTCs with them. Furthermore, to determine how chlorine substitution patterns affected adsorption and sub-bandgap reactivity, P25 slurries containing phenol, 4-chlorophenol, 2,4-dichlorophenol, or 2,4,6-trichlorophenol were probed following dark contact or irradiation at 360, 430, or 550 nm. With respect to the extent of adsorption, complexation, reaction, and polymerization on P25, the behavior of 245TCP far exceeded that of the other chlorophenols. Among these chlorophenols, only 2,4-dichlorophenol produced a polymeric product. 245TCP is unique among this family of chlorophenols, which we attribute to a chlorine arrangement that leads to a favorable orbital overlap with TiO2 and sterically permits coupling reactions. Our results demonstrate the critical role that charge-transfer complexation can play in determining the rates and products of photocatalytic reactions.

DFT calculations were employed to investigate the mechanism of formation and the interconversion of reduced-hapticity (η^x-C6H6)Cr(CO)n complexes (n = 1−5, x = 1−6) on both singlet and triplet energy surfaces. A variety of functionals (mPW1k, B3LYP, B97-1, and BP86) and basis sets (SDD, SDB-cc-pVDZ, and LANL2DZ+P) were used. The same structures were consistently found for all functional/basis set combinations. The structures of the different singlet and triplet complexes and comparisons between their energies are described. On the basis of the structures and energies of the intermediates and the transition states found for singlet reduced-hapticity complexes, a mechanism for the decomposition of the (η⁶-C6H6)Cr(CO)3 complex to Cr(CO)6 is suggested. This mechanism, which is closely related to the arene exchange reaction mechanism, involves the formation of (η¹-C6H6)Cr(CO)4 and (η²-C6H6)Cr(CO)5 complexes as ring-slippage intermediates along the pathway for the complete replacement of the benzene ring by carbonyl ligands. The barriers found for the ring-slippage process are accessible under mild conditions. Triplet reduced-hapticity complexes have different structures; however, they are similar in energy to the analogous singlet complexes. Formation of triplet reduced-hapticity complexes might take place in reactions of (η⁶-C6H6)Cr(CO)3 initiated by photolysis, which have been reported to show no observable decomposition to Cr(CO)6.

Geometries and energies have been calculated for the pseudoaxial and pseudoequatorial isomers of singlet and triplet state Fe(CO)3(η²-C3H6) and three isomers of HFe(CO)3(η³-C3H5) using density functional theory, with the BP86, BLYP, B3P86, and B3LYP functionals. The triplet state pseudoaxial olefin isomer is lower in energy than the pseudoequatorial isomer, while for the allyl hydride species the facial isomers are lower in energy than the meridional isomer. The energy of the Fe−allyl bond in XFe(CO)3(η³-C3H5) (X = H, I) has been calculated, and an effective upper limit of 54 kcal/mol can be set for X = H and 45 kcal/mol for X = I. A comparison of the calculated ΔH values with estimates for the Fe−allyl BDE, obtained via thermodynamic cycles, emphasizes that bond energies are not necessarily transferable from one complex to another. Comparison of our calculations with data obtained from matrix experiments, known gas phase experimental data, and data for ligand addition reactions to iron carbonyl complexes indicates that the reaction Fe(CO)3(η²-C3H6) → HFe(CO)3(η³-C3H5) is expected to preferentially occur from the triplet state pseudoaxial mono-olefin isomer to either or both of the facial-exo and endo allyl product isomers.

The adsorptive behavior of TiO2 under various illumination conditions was investigated. In the dark, Degussa P25, which contains both the anatase and rutile phases of TiO2, adsorbed significantly greater amounts of 2,4,5-trichlorophenol (TCP) than either pure anatase or pure rutile. Results in the literature and in our laboratory that appeared to indicate “photoenhanced adsorption” under ambient fluorescent lighting on P25 and anatase are in fact due to the photoreaction of TCP. On pure-phase anatase, all reactions are due to trace ultra-band-gap energy light present in the fluorescent lighting. On P25, however, reaction also occurs at sub-band-gap energies. TCP forms a charge-transfer complex with P25 that is activated by light wavelengths as long as 520 nm. The trichlorophenoxyl radicals resulting from charge-transfer couple with each other to form a suite of polyaromatic chlorinated products with detected masses as high as 1200 D. These products are not formed under UV irradiation and in fact are destroyed by conventional UV photocatalysis. No reaction occurs on pure-phase anatase or rutile as a consequence of irradiation with sub-band-gap light. Carbon mass balance was closed for all catalysts under all lighting conditions. Our results show that the wavelength of light is an important factor in determining products on P25. This knowledge can be used to avoid charge-transfer complex activation when the resulting products are undesirable. Alternatively, charge-transfer complexes on P25 may be exploitable for polymer syntheses. The differences between surface reactions on P25 and those on pure-phase TiO2 may be explained in terms of the morphology of Degussa P25, wherein anatase−rutile interfaces give rise to active sites.

The stability of the η²-acyl−Fe(CO)3X (acyl = CH3C(O), C2H5C(O), C(CH3)3C(O); X = H, I, CH3) complexes relative to the respective η¹-acyl−Fe(CO)3X complex and their alkyl isomers has been studied using density functional theory (DFT). The effect of an acetyl versus a propionyl group, and the effect of the nature of the substituent, X, on the stability of various isomers is considered. For the CH3 substituent the effect of a t-valeryl ((CH3)3CC(O)) group was also probed. The η²-(CH3)3CC(O)Fe(CO)3CH3 complex is the most stable of the iron complexes studied. The effect of the substituents on the energies of the molecular orbitals of the complexes provides insight into the energy ordering of the various isomeric structures. DFT calculations were also performed on the η²-CH3C(O)Mn(CO)4 complex and its isomers. Good agreement is achieved with regard to these calculations and available experimental and theoretical data. Predictions are presented for the stability of η²-acyl−Cr complexes.

The chromium−olefin complex Cr(CO)5(DMB) (DMB = 3,3-dimethyl-1-butene) has been studied in the gas phase using transient infrared spectroscopy. This complex forms by addition of DMB to photogenerated Cr(CO)5 with a rate constant, kL = (7.0 ± 1.5) × 10^-11 cm³ molecule^-1 s^-1. The bond enthalpy for the DMB−Cr(CO)5 bond has been determined from the kinetics for the decay of Cr(CO)5(DMB) to be 20.1 ± 1.7 kcal/mol at 298 K. An energy decomposition analysis has been performed for a series of Cr(CO)5(olefin) complexes (olefin = DMB, ethylene, propene, 1-butene, 1-hexene, cis-2-butene, trans-2-butene, isobutene, and tetramethylethylene (TME)) using density functional theory. These calculations provide insights into trends in the chromium−olefin bond energy. The results reveal that the trend in bond energies in these complexes correlates with the number and the nature of the alkyl groups around the double bond, and that the dominant factor in this trend is the deformation energy of the olefin and Cr(CO)5, where the deformation energy is the energy required to deform the olefin ligand and the unsaturated metal centered moiety from their isolated ground-state geometries to the geometry they adopt in the bound complex.

The iron−olefin bond energies for the monoolefin iron tetracarbonyl complexes Fe(CO)4(C2X4) (X = H, F, Cl, Br, I, CN) have been determined using density functional theory (DFT), with the BP86 functional. An energy decomposition analysis of the bonding interactions demonstrate that, as predicted by current models of metal−olefin bonding, the attractive electronic interactions of the haloolefins and percyanoethylene with iron are stronger than those of ethylene. However, in addition to these electronic interactions the net bond energy depends on the energy needed to deform the Fe(CO)4 and olefin moieties from their equilibrium geometries to the geometrical conformation they adopt in the complex. This energy is termed the deformation energy. As a result of the deformation energy, the bond energies for the substituted olefins are similar to or smaller than that of the Fe−C2H4 bond. More than half of the total deformation energy involves deforming the olefin, principally as a result of a change in hybridization of the carbon atoms from sp² in the free olefin toward an sp³-like carbon in the bound olefin. The deformation of Fe(CO)4 involves mainly the axial CO ligands, which bend away from the olefin as a result of a repulsive interaction with the olefin substituents. In addition, the increase in the C−X bond length, upon bonding of the olefin to Fe(CO)4, correlates well with the exothermicity of the oxidative addition reaction, Fe(CO)4(C2X4) → XFe(CO)4(C2X3), indicating that the deformation of the bound olefin lowers the energy of the C−X bond.

The transient spectra of Fe(CO)3(3,3-dimethyl-1-butene) and Fe(CO)3(3,3-dimethyl-1-butene)2 have been obtained in the carbonyl stretching region of the infrared. Fe(CO)4(3,3-dimethyl-1-butene) has also been monitored. The bond dissociation enthalpy for loss of 3,3-dimethyl-1-butene (DMB) from Fe(CO)3(DMB)2 has been determined as 15.2 ± 3.8 kcal mol^-1. The rate constants for the reactions of Fe(CO)3 + DMB, Fe(CO)3DMB + DMB, Fe(CO)4 + DMB, and Fe(CO)3DMB + CO have been measured as (9.6 ± 1.4) × 10^-11, (2.3 ± 0.8) × 10^-12, (5.0 ± 0.5) × 10^-13, and (3.9 ± 0.6) × 10^-12 cm³ molecule^-1 s^-1, respectively. The effects of the polarizability and steric interaction energy of the ligand on the bimolecular rate constants for the reactions of Fe(CO)3L + L (L = olefin) complexes are discussed. A correlation between these rate constants and the steric interaction energy divided by the magnitude of the polarizability of the ligand is observed. An estimate for the rate constant for addition of 1-pentene to Fe(CO)3(1-pentene), based on data and correlations from this study, does not lead to a significant change in calculated thermodynamic parameters for processes relevant to the isomerization of pentene. Factors that could influence the stability of Fe(CO)3L2 complexes are also discussed.

Metal−N2 bond energies have been calculated for the Fe(CO)5-n(N2)n (n = 1−5) and Cr(CO)6-n(N2)n (n = 1−6) complexes using density-functional theory (DFT). Bond enthalpies calculated using the gradient corrected BP86 functional are in good agreement with the available experimental data. An energy decomposition procedure and a population analysis were performed for all of the complexes to quantitatively characterize the interactions of N2 and CO with the relevant coordinatively unsaturated metal species. In all cases, the metal−N2 bond is weaker than the metal−CO bond because CO is both a better donor and a better acceptor of electron density. Calculated bond energies for Cr−N2 bonds for the lowest energy isomers of the chromium complexes are 24, 23, 22, 21, 20, and 25 kcal/mol for n = 1−6, respectively. The trend of decreasing bond energy with added N2 ligands is a result of weaker orbital interactions. The exception is Cr(N2)6, which is predicted to be more stable than the CO containing complexes. This increase in stability is ascribed to the absence of a CO trans effect. In contrast, the Fe−N2 bond energies for the lowest energy isomers in the series are 24, 17, 14, 10, and 5 kcal/mol for n = 1−5, respectively. Although iron has a larger orbital interaction with dinitrogen ligands than chromium, the 16-electron iron complexes have to deform substantially when going from their ground triplet states to their final pentacoordinated singlet geometries. An energy cost that increases as the number of N2 ligands increases is associated with this deformation. For chromium complexes, this deformation term does not significantly decrease the bond energy, but the magnitude of this term becomes the dominant factor in the differences in bond energies in the dinitrogenated iron complexes.

Time-resolved infrared absorption spectroscopy has been used to study the gas-phase reactions of Fe(CO)3, Fe(CO)3N2, and Fe(CO)4 with N2, where Fe(CO)3 is generated by 308 nm laser photolysis of Fe(CO)5. The heretofore unknown complex Fe(CO)3(N2)2 forms by addition of N2 to Fe(CO)3N2 with a rate constant of (5.4 ± 1.8) × 10^-16 cc molecule^-1 s^-1. This rate constant is much smaller than is typical for the addition of small ligands to coordinately unsaturated metal carbonyls, and data are consistent with this reaction being activated. The bond dissociation energy (BDE) for the loss of a N2 ligand from Fe(CO)4N2 is 17.6 ± 1.8 kcal mol^-1. The activation energy for the loss of N2 from Fe(CO)3(N2)2 is 14.1 ± 5.2 kcal mol^-1. The kinetics of the system are consistent with a model that involves equilibria between Fe(CO)3, Fe(CO)3N2, and Fe(CO)3(N2)2 as well as reactions of coordinatively unsaturated species with Fe(CO)5. Using this kinetic model, an upper limit for the BDE for the Fe−N2 bond in Fe(CO)3N2 has been estimated and the BDE for the Fe−N2 bond in Fe(CO)3(N2)2 has been determined under the assumption that one of the relevant reactions has a minimal activation energy.

The gas-phase reactions of Fe(CO)3 and Fe(CO)4 with perchloroethylene (C2Cl4) have been investigated using transient infrared spectroscopy. The addition of C2Cl4 to Fe(CO)3 produces Fe(CO)3(C2Cl4) with a rate constant of (3.0 ± 0.8) × 10^-11 cm³ molecule^-1 s^-1. A second olefin can add to Fe(CO)3(C2Cl4) with a rate constant of (1.9 ± 0.3) × 10^-13 cm³ molecule^-1 s^-1 to form the novel bisolefin complex Fe(CO)3(C2Cl4)2. Absorptions of this complex were identified at 2084 and 2057 cm^-1. C2Cl4 reacts with Fe(CO)4 with a rate constant of (1.2 ± 0.3) × 10^-13 cm³ molecule^-1 s^-1 to produce Fe(CO)4(C2Cl4), which is identified by its absorptions at 2125, 2069, and 2039 cm^-1. This product isomerizes to a novel chloride complex via an oxidative addition process, with Arrhenius parameters Ea = 21 ± 2 kcal/mol and ln A = 28 ± 2 in the 297−315 K temperature range. The chloride complex is best assigned as ClFe(CO)4(C2Cl3), and possible mechanisms for this isomerization reaction are discussed. ClFe(CO)4(C2Cl3) can also be produced by the photolysis of Fe(CO)4(C2Cl4), and a mechanism for this process is proposed. Absorptions of ClFe(CO)4(C2Cl3) were identified at 2166, 2109, and 2089 cm^-1. Where possible, the measured rate constants and the observed infrared absorptions are compared to those for analogous C2H4 and C2F4 complexes. Finally, simulations of a “global” mechanism for the kinetics of this system are in good agreement with experimental data. From these simulations, ΔG for the isomerization of Fe(CO)3(C2Cl4) to ClFe(CO)3(C2Cl3) is estimated to be ≥4 kcal/mol at 297 K.

The reaction dynamics of photofragments produced by UV photolysis of neat CH3I films, adsorbed on defect-free TiO2(110), have been investigated using postirradiation temperature programmed desorption (PITPD). At low fluences, the film structure remains frozen, retaining photofragments and reaction products in the film. CH4, C2H6, I2, CH2I2, and C2H5I are detected in PITPD. CH4 is a product of translationally hot methyl radicals reacting with CH3I. Desorption of the CH4 trapped in the CH3I films is limited by diffusion in the films. The I-containing products detected in PITPD come predominantly from reactions of trapped I, CH2I, and CH3 radicals, which occur during the TPD process. The desorption of I-containing products in PITPD follows desorption-limited kinetics on the TiO2(110) surface. For C2H6 detected in PITPD, it is not clear whether the dominant reaction pathway involves hot or cold radicals. Electrons produced in the TiO2 substrate upon UV irradiation do not induce significant dissociation of CH3I adsorbed on the (110) surface and thus do not contribute significantly to fragment reactions.

A case of early postoperative inferior mesenteric arteriovenous fistula is de scribed in a sixty-three-year-old man who underwent an anterior resection of the rectum for carcinoma.Six weeks after surgery, he developed increasingly severe rectal bleeding. A single incident of copious bleeding caused him to go into shock and require resuscitation. A selective angiography revealed an arteriovenous fistula of the inferior mesenteric artery, which was managed by embolization of the artery by three spring steel coils.These rare postoperative arteriovenous fistulae are probably caused by the incorporation of an artery and vein in a single stitch or ligature and usually present many years after surgery.

The bond dissociation energy for loss of C2H4 from Fe(CO)3(C2H4)2, produced by the reaction of C2H4 + Fe(CO)3(C2H4), has been determined as 21.3 ± 2.0 kcal/mol. An estimate is made for a lower limit for the bond dissociation energy of Fe(CO)4(C2H4), which can be formed by reaction of CO + Fe(CO)3(C2H4) or Fe(CO)4 + C2H4 with rate constants of (4.3 ± 0.8) × 10^-12 and (1.7 ± 0.2) × 10^-13 cm³/(molecule s) at 24 °C, respectively. The values for these bond dissociation energies are compared with those determined in prior studies of these systems. A new compound with infrared absorptions at 2147, 2091, and 2068 cm^-1 is best assigned as Fe(CO)3(C2F4)2. A rate constant of (5.4 ± 1.7) × 10^-12 cm³/(molecule s) at 24 °C is reported for the reaction of C2F4 with Fe(CO)3(C2F4) to form Fe(CO)3(C2F4)2. Fe(CO)4(C2F4) can be formed by reaction of C2F4 and Fe(CO)4, with a rate constant of (1.8 ± 0.4) × 10^-14 cm³/(molecule s) at 24 °C. Infrared absorptions observed at 2135, 2074, and 2043 cm^-1 are assigned to this species. The relative stabilities of the mono- and bisethylene and perfluoroethylene compounds of iron are compared. Where possible, they are also compared to the corresponding chromium compounds and are discussed in the context of current concepts regarding metal−olefin bonding.

Density functional theory (DFT) methods, with three different gradient-corrected functionals (BP86, BLYP, and B3LYP), have been used to calculate molecular geometries and energies along the pathway for oxidative addition of H2 to Fe(CO)4 leading to H2Fe(CO)4. The geometry of H2Fe(CO)4, optimized using each of the different functionals, is in good agreement with experimental results. The enthalpies for reductive elimination of H2 from H2Fe(CO)4, referenced to both the first excited singlet and the triplet ground state of Fe(CO)4, have been calculated using the BP86, BLYP, and B3LYP functionals and are compared to the experimental result of 21 ± 2 kcal mol^-1. The mechanism for the oxidative addition of H2 to Fe(CO)4 is discussed and compared with experimental observations. The geometry of a dihydrogen intermediate, (η²-H2)Fe(CO)4, and the transition state between (η²-H2)Fe(CO)4 and H2Fe(CO)4, along the reaction path, has also been optimized.