Force-field Fitting Research Articles

Boosting Ensemble Refinement with Transferable Force-Field Corrections: Synergistic Optimization for Molecular Simulations.

A novel method combining the force-field fitting approach and ensemble refinement by the maximum entropy principle is presented. Its formulation allows us to continuously interpolate between these two methods, which can thus be interpreted as two limiting cases. A cross-validation procedure enables us to correctly assess the relative weight of both of them, distinguishing scenarios in which the combined approach is meaningful from those in which either ensemble refinement or force-field fitting separately prevails. The efficacy of their combination is examined for a realistic case study of RNA oligomers. Within the new scheme, molecular dynamics simulations are integrated with experimental data provided by nuclear magnetic resonance measures. We show that force-field corrections are in general superior when applied to the appropriate force-field terms but are automatically discarded by the method when applied to inappropriate force-field terms.

A Fast, Convenient, Polarizable Electrostatic Model for Molecular Dynamics.

We present an efficient polarizable electrostatic model, utilizing typed, atom-centered polarizabilities and the fast direct approximation, designed for efficient use in molecular dynamics (MD) simulations. The model provides two convenient approaches for assigning partial charges in the context of atomic polarizabilities. One is a generalization of RESP, called RESP-dPol, and the other, AM1-BCC-dPol, is an adaptation of the widely used AM1-BCC method. Both are designed to accurately replicate gas-phase quantum mechanical electrostatic potentials. Benchmarks of this polarizable electrostatic model against gas-phase dipole moments, molecular polarizabilities, bulk liquid densities, and static dielectric constants of organic liquids show good agreement with the reference values. Of note, the model yields markedly more accurate dielectric constants of organic liquids, relative to a matched nonpolarizable force field. MD simulations with this method, which is currently parametrized for molecules containing elements C, N, O, and H, run only about 3.6-fold slower than fixed charge force fields, while simulations with the self-consistent mutual polarization average 4.5-fold slower. Our results suggest that RESP-dPol and AM1-BCC-dPol afford improved accuracy relative to fixed charge force fields and are good starting points for developing general, affordable, and transferable polarizable force fields. The software implementing these approaches has been designed to utilize the force field fitting frameworks developed and maintained by the Open Force Field Initiative, setting the stage for further exploration of this approach to polarizable force field development.

Development and benchmarking of an open, self-consistent force field for proteins and small molecules from the open force field initiative.

Contemporary protein force fields model canonical proteins well, but parameters for covalently modified residues are challenging to develop. Additionally, protein-specific parameters are derived separately from those for small molecules, limiting the transferability of small molecule parameters to protein contexts. A flexible force field that provides self-consistent parameters for proteins and small molecules would advance research in drug design and molecular mechanisms of protein structure and function. The Open Force Field (OpenFF) Initiative develops open, reproducible force fields for atomistic simulations, delivering automated infrastructure and systematic methodology for force field fitting and validation as well as version-controlled force field releases. OpenFF force fields assign parameters using direct chemical perception via the SMIRKS cheminformatics language, providing coverage of a large chemical space with fewer parameters than conventional atom-typed force fields. The most recent force field release, Sage (OpenFF 2.0.0), provides valence parameters trained against quantum chemical (QC) data and Lennard-Jones parameters trained against condensed phase properties of pure liquids and binary mixtures. Sage achieves competitive accuracy on benchmarks of the energies and geometries of QC minima, solvation free energies, and protein-ligand binding free energies. Here we describe the extension of Sage to proteins, resulting in a self-consistent force field that can simulate canonical and covalently modified proteins and small molecules. We used protein-specific SMIRKS to train proper torsions to two-dimensional QC scans of the backbone and sidechain dihedrals of capped peptides. The parameters were validated using benchmarks of published NMR observables for small peptides, folded proteins, and disordered proteins. Beyond force field parameters, the OpenFF software infrastructure now supports loading proteins from PDB files, iterating over hierarchies such as residues, and exporting parametrized systems to common molecular dynamics formats.

AutoSolvate: A toolkit for automating quantum chemistry design and discovery of solvated molecules.

The availability of large, high-quality datasets is crucial for artificial intelligence design and discovery in chemistry. Despite the essential roles of solvents in chemistry, the rapid computational dataset generation of solution-phase molecular properties at the quantum mechanical level of theory was previously hampered by the complicated simulation procedure. Software toolkits that can automate the procedure to set up high-throughput explicit-solvent quantum chemistry (QC) calculations for arbitrary solutes and solvents in an open-source framework are still lacking. We developed AutoSolvate, an open-source toolkit, to streamline the workflow for QC calculation of explicitly solvated molecules. It automates the solvated-structure generation, force field fitting, configuration sampling, and the final extraction of microsolvated cluster structures that QC packages can readily use to predict molecular properties of interest. AutoSolvate is available through both a command line interface and a graphical user interface, making it accessible to the broader scientific community. To improve the quality of the initial structures generated by AutoSolvate, we investigated the dependence of solute-solvent closeness on solute/solvent identities and trained a machine learning model to predict the closeness and guide initial structure generation. Finally, we tested the capability of AutoSolvate for rapid dataset curation by calculating the outer-sphere reorganization energy of a large dataset of 166 redox couples, which demonstrated the promise of the AutoSolvate package for chemical discovery efforts.

Molecular simulation of polymers with a SAFT-γ Mie approach

ABSTRACTWe review the group contribution Statistical Associating Fluid Theory with Mie interaction potentials (SAFT-γ Mie) approach for building coarse-grained models for molecular simulation of polymeric systems. In this top-down method, force field parameters for coarse-grained polymer models can be derived from thermodynamic information on constituent monomer units using the SAFT-γ Mie equation of state (EoS). This strategy can facilitate high-throughput computational screening of polymeric materials, with a corresponding states correlation expediting the force field fitting. Accurate and transferable non-bonded parameters linked to macroscopic thermodynamic data allow for calculation of properties beyond those obtainable from the EoS alone. To overcome limitations of SAFT-γ Mie regarding polymer chain stiffness and branching, hybrid top-down/bottom-up approaches have combined non-bonded parameters from SAFT-γ Mie with bond-stretching and angle-bending potentials from higher-resolution force fields. Our review critically evaluates the performance of recent SAFT-γ Mie polymer models, highlighting the strengths and weaknesses in the context of other equation of state and coarse-graining methods.

Machine learning coarse grained models for water

An accurate and computationally efficient molecular level description of mesoscopic behavior of ice-water systems remains a major challenge. Here, we introduce a set of machine-learned coarse-grained (CG) models (ML-BOP, ML-BOPdih, and ML-mW) that accurately describe the structure and thermodynamic anomalies of both water and ice at mesoscopic scales, all at two orders of magnitude cheaper computational cost than existing atomistic models. In a significant departure from conventional force-field fitting, we use a multilevel evolutionary strategy that trains CG models against not just energetics from first-principles and experiments but also temperature-dependent properties inferred from on-the-fly molecular dynamics (~ 10’s of milliseconds of overall trajectories). Our ML BOP models predict both the correct experimental melting point of ice and the temperature of maximum density of liquid water that remained elusive to-date. Our ML workflow navigates efficiently through the high-dimensional parameter space to even improve upon existing high-quality CG models (e.g. mW model).

POSSIM: Parameterizing Complete Second-Order Polarizable Force Field for Proteins

Previously, we reported development of a fast polarizable force field and software named POSSIM (POlarizable Simulations with Second order Interaction Model). The second-order approximation permits the speed up of the polarizable component of the calculations by ca. an order of magnitude. We have now expanded the POSSIM framework to include a complete polarizable force field for proteins. Most of the parameter fitting was done to high-level quantum mechanical data. Conformational geometries and energies for dipeptides have been reproduced within average errors of ca. 0.5 kcal/mol for energies of the conformers (for the electrostatically neutral residues) and 9.7° for key dihedral angles. We have also validated this force field by running Monte Carlo simulations of collagen-like proteins in water. The resulting geometries were within 0.94 Å root-mean-square deviation (RMSD) from the experimental data. We have performed additional validation by studying conformational properties of three oligopeptides relevant in the context of N-glycoprotein secondary structure. These systems have been previously studied with combined experimental and computational methods, and both POSSIM and benchmark OPLS-AA simulations that we carried out produced geometries within ca. 0.9 Å RMSD of the literature structures. Thus, the performance of POSSIM in reproducing the structures is comparable with that of the widely used OPLS-AA force field. Furthermore, our fitting of the force field parameters for peptides and proteins has been streamlined compared with the previous generation of the complete polarizable force field and relied more on transferability of parameters for nonbonded interactions (including the electrostatic component). The resulting deviations from the quantum mechanical data are similar to those achieved with the previous generation; thus, the technique is robust, and the parameters are transferable. At the same time, the number of parameters used in this work was noticeably smaller than that of the previous generation of our complete polarizable force field for proteins; thus, the transferability of this set can be expected to be greater, and the danger of force field fitting artifacts is lower. Therefore, we believe that this force field can be successfully applied in a wide variety of applications to proteins and protein–ligand complexes.

Global optimization of parameters in the reactive force field ReaxFF for SiOH

We have used unbiased global optimization to fit a reactive force field to a given set of reference data. Specifically, we have employed genetic algorithms (GA) to fit ReaxFF to SiOH data, using an in-house GA code that is parallelized across reference data items via the message-passing interface (MPI). Details of GA tuning turn-ed out to be far less important for global optimization efficiency than using suitable ranges within which the parameters are varied. To establish these ranges, either prior knowledge can be used or successive stages of GA optimizations, each building upon the best parameter vectors and ranges found in the previous stage. We have finally arrive-ed at optimized force fields with smaller error measures than those published previously. Hence, this optimization approach will contribute to converting force-field fitting from a specialist task to an everyday commodity, even for the more difficult case of reactive force fields.

First principles derived, transferable force fields for CO2 adsorption in Na-exchanged cationic zeolites

The development of accurate force fields is vital for predicting adsorption in porous materials. Previously, we introduced a first principles-based transferable force field for CO2 adsorption in siliceous zeolites (Fang et al., J. Phys. Chem. C, 2012, 116, 10692). In this study, we extend our approach to CO2 adsorption in cationic zeolites which possess more complex structures. Na-exchanged zeolites are chosen for demonstrating the approach. These methods account for several structural complexities including Al distribution, cation positions and cation mobility, all of which are important for predicting adsorption. The simulation results are validated with high-resolution experimental measurements of isotherms and microcalorimetric heats of adsorption on well-characterized materials. The choice of first-principles method has a significant influence on the ability of force fields to accurately describe CO2-zeolite interactions. The PBE-D2 derived force field, which performed well for CO2 adsorption in siliceous zeolites, does not do so for Na-exchanged zeolites; the PBE-D2 method overestimates CO2 adsorption energies on multi-cation sites that are common in cationic zeolites with low Si/Al ratios. In contrast, a force field derived from the DFT/CC method performed well. Agreement was obtained between simulation and experiment not only for LTA-4A on which the force field fitting is based, but for other two common adsorbents, NaX and NaY.

Improved intermolecular force field for molecules containing H, C, N, and O atoms, with application to nucleoside and peptide crystals

AbstractA new intermolecular force field for nitrogen atoms in organic molecules was derived from a training dataset of 76 observed azahydrocarbon crystal structures and 11 observed heats of sublimation. The previously published W99 force field for hydrogen, carbon, and oxygen was thus extended to include nitrogen atoms. Nitrogen atoms were divided into four classes: N(1) for triply bonded nitrogen, N(2) for nitrogen with no bonded hydrogen (except the triple bonded case), N(3) for nitrogen with one bonded hydrogen, and N(4) for nitrogen with two or more bonded hydrogens. H(4) designated hydrogen bonded to nitrogen. Wavefunctions of 6‐31g** quality were calculated for each molecule and the molecular electric potential (MEP) was modeled with net atomic and supplementary site charges. Lone pair electron charge sites were included for nitrogen atoms where appropriate, and methylene bisector charges were used for CH2 and CH3 groups when fitting the MEP. XH bond distances were set to standard values for the wave function calculation and then foreshortened by 0.1 Å for the MEP and force field fitting. Using the force field optimized to the training dataset, each azahydrocarbon crystal structure was relaxed by intermolecular energy minimization. Predicted maximum changes in unit cell edge lengths for each crystal were 3% or less. The complete force field for H, C, N, and O atoms was tested by intermolecular energy relaxation of nucleoside and peptide molecular crystals. Even though these molecules were not included in any of the training datasets for the force field, agreement with their observed crystal structures was very good, with predicted unit cell edge shifts usually less than 2%. These tests included crystal structures of representatives of all eight common nucleosides found in DNA and RNA, 15 dipeptides, four tripeptides, two tetrapeptides, and a pentapeptide with two molecules in the asymmetric unit. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1154–1166, 2001

Photochemistry of (η5-cyclopentadienyl)-tricarbonylmethylchromium in gas matrices and polymer films at 12—298 K: infrared spectroscopic evidence for reversible carbon monoxide ejection and α-H elimination from the methyl group

UV-Vis and infrared spectroscopic evidence, including 13CO labelling and energy-factored force-field fitting, is presented which shows that photolysis of(η5-C5H5)Cr(CO3)CH3 at high dilution in frozen gas matrices (Ar, CH4, N2, and CO) and poly(vinyl chloride) films at ca. 12 K affords the α-elimination product, trans-η(5-C5H5)Cr(CO)2(CH2)H, via the 16-electron species (η5-C5H5)Cr(CO)2CH3. These reactions are thermally reversible at temperatures as low as 30 K with conversion of (η5-C5H5)Cr(CO)2(CH2)H to (η5-C5H5)Cr(CO)2(L)CH3 complexes (L = CO and tetrahydrofuran) in poly(vinyl chloride) films. Irradiation of (η5-C5H5)Cr(CO)3CH3 in poly(vinyl chloride) films at 298 K yielded the dimer (η5-C5H5)2Cr2(CO))6 analogously to the solution photochemistry. The results are discussed in relation to the occurrence of α-H elimination reactions in general and the reactions of (η5-C5H5(Cr(CO)3CH3 in particular. It is suggested that the formation of the dimer (η5-C5H5)2Cr2(CO)6 in solution arises not from initial CrCH3 bond cleavage or α-H elimination but from reactions of (η5-C5H5)Cr(CO)2CH3 or (η5-C5H5)Cr(CO)2(solvent)CH3 species with unreacted (η5-C5H5Cr(CO)3CH3.

Photochemistry of some (η5-cyclopentadienyl)carbonylchloro complexes of molybdenum, tungsten, iron, and ruthenium in frozen gas matrices at ca. 12 K and in poly(vinyl chloride) films at ca. 12–298 K

Infrared spectroscopic evidence including 13CO substitution and energy-factored force-field fitting is presented to show that photolysis of [M(η5-C5H5)(CO)3Cl] complexes (M = Mo or W) in argon, methane, nitrogen, carbon monoxide, and 5% C2H4–CH4 matrices at ca. 12 K yields ejection of CO and formation of the 16-electron species [M(η5-C5H5)(CO)2Cl] as the primary process. This process is reversible. In nitrogen and 5% C2H4–CH4 matrices the [M(η5-C5H5)(CO)2Cl] species react with N2 and C2H4 to give cis-[Mo(η5-C5H5)(CO)2(N2)Cl] and cis-[M(η5-C5H5)(CO)2(C2H4)Cl](M = Mo or W) respectively. In carbon monoxide matrices the ejection of CO is suppressed and the photolysis products are proposed to be the species [M(η5-C5H5)(CO)3]+Cl–(M = Mo or W), indicative of metal–chlorine bond heterolysis. Irradiation of [M(η5-C5H5)(CO)2Cl] complexes (M = Fe or Ru) in methane, nitrogen, and carbon monoxide matrices and in poly(vinyl chloride) films at ca. 12 K produced only the 16-electron species [M(η5-C5H5)(CO)Cl] and CO indicating that dissociation, rather than cleavage of the metal–chlorine bond, is the principal reaction pathway. The primary process can be reversed thermally in the polymer films by virtue of their wider working temperature range (ca. 12–298 K). For the iron complex reversal occurred by ca. 60 K, whereas the ruthenium complex needed a higher temperature of annealing (ca. 80 K). The results of the low-temperature studies are discussed in relation to the thermal and photochemical reactions of complexes of Mo, W, Fe, and Ru in solution at ambient temperatures.

Photochemistry of σ-allyl complexes of manganese and tungsten in frozen gas matrices at ca. 12 K. Infrared spectroscopic evidence for photodecarbonylation and σ → π conversion processes

Infrared spectroscopy, together with 13CO enrichment and energy-factored forcefield fitting of terminal CO stretching bands, has been employed to study the photochemistry of Mn(η 1-C 3H 5)(CO) 5 and W(η 1-C 3H 5)(η 5-C 5H 5)(CO) 3 isolated at high dilution ( 1 2000 to 1 5000 ) in various gas matrices at ca. 12 K. For the Mn complex ejection of a CO ligand was concomitant with σ → π rearrangement to afford Mn(η 3-C 3H 5)(CO) 4 which lost a further CO reversibly to give Mn(η 3C 3H 5(CO) 3. In N 2 matrices the unsaturated species picked up an N 2 ligand to yield Mn(η 3-C 3H 5)(CO) 3(N 2) while in CO matrices its reaction with CO was so facile that no Mn(η 3-C 3H 5)(CO) 3 could be detected. In contrast, the W complex confirmed that CO ejection and σ → π rearrangement occurred in two steps by virtue of the isolation of the 16-electron species W(η 1-C 3H 5)(η 5-C 5H 5)(CO) 2. For W, however, the η 3-allyl complex did not seem to undergo further CO loss to give W(η 3-C 3H 5)(η 5-C 5H 5)(CO). The two step process seen for W, together with the facile reversal W(η 1-C 3H 5)(η 5-C 5H 5)(CO) 2 ⇌ W(η 5C 3H 5)(η 5-C 5H 5)(CO) 2, indicates that η 1 → η 3-allyl rearrangements in general proceed in solution by two step reactions even when the intermediate stages cannot be observed. Such a two step process will have implications for kinetic studies. Attempts to enrich Mn(η 1-C 3H 3)(CO) 5 with 13CO in solution and in 13CO dop 13CO incorporation. It is suggested that Mn has a preference for an η 3-allyl ligand and that σ → π rearrangement is irreversible in this case.

Photochemistry of dicarbonyl(η5-cyclopentadienyl)-methyl- and -ethyl-iron and -ruthenium complexes in solutions at –30 °C and in frozen gas matrices at 12 K

The photolysis reactions of [M(CO)2R(η5-C5H5)](M = Fe or Ru, R = Me or Et) complexes have been studied by a combination of solution (–30 °C) and matrix-isolation (12 K) techniques. In pentane solutions (–30 °C) photolysis of [M(CO)2Me(η5-C5H5)](M = Fe or Ru) alone yields the dimers [{M(CO)2(η5-C5H5)}2], whereas that of [M(CO)2Et(η5-C5H5)](M = Fe or Ru) leads to the known dimers, [{M(CO)2(η5-C5H5)}2], and the new dimer[{Ru(CO)(η5-C5H5)}2(µ-H)2]. Photolysis of [M(CO)2Me(η5-C5H5)](M = Fe or Ru) in pentane (–30 °C) in the presence of C2H4 results in the formation of the new complexes [M(CO)(C2H4)Me(η5-C5H5)]. The ethyl complex [Fe(CO)2Et(η5-C5H5)] reacts photochemically with C2H4 to produce [Fe(CO)2H(η5-C5H5)], [{Fe(CO)2(η5-C5H5)}2] and the new dimer[{Fe(CO)(C2H4)(η5-C5H5)}2], while the complex [Ru(CO)2Et(η5-C5H5)] dealkylates to give only [Ru(CO)2H(η5-C5H5)]. In gas matrices (12 K) the trapping of [Ru(12Co)Me(η5-C5H5)] and [Ru(13CO)Me(η5-C5H5)] species provided direct evidence for the CO-dissociation pathway. The [M(CO)Me(η5-C5H5)] species readily recombined with ejected CO or reacted with N2 or C2H4 to give the complexes [Fe(CO)(C2H4)Me(η5-C5H5)], [Ru(CO)(N2)Me(η5-C5H5)], and [Ru(CO)(C2H4)Me(η5-C5H5)]. Photolysis of [M(CO)2Et(η5-C5H5)](M = Fe or Ru) in CH4 matrices afforded the β-elimination products [M(CO)2H(η5-C5H5)] together with [Ru(CO)(C2H4)H(η5-C5H5)]. The photolysis of [Ru(CO)2Et(η5-C5H5)] in N2 and C2H4-doped CH4 matrices yielded [Ru(CO)(N2)H(η5-C5H5)] and [Ru(CO)(C2H4)H(η5-C5H5)], respectively. Using 13CO-doped CH4 matrices and energy-factored force-field fitting to investigate the possibility of competing reactions, it was found that the initial process was β elimination which was followed by CO exchange. Photolysis in CO matrices revealed the existence of a further pathway possibly involving ring-slippage products, e.g. [Fe(CO)3R(η3-C5H5)](R = Me or Et) and [Ru(CO)3H(η3-C5H5)]

Photochemistry of tricarbonyl(η5-cyclopentadienyl)hydrido complexes of molybdenum and tungsten and of dicarbonyl(η5-cyclopentadienyl)-(ethylene)hydridotungsten in solution and in frozen gas matrices at 12 K

The photochemical reactions of [MH(CO)3(η5-C5H5)] complexes (M = Mo or W) and of trans-[WH(CO)2(C2H4)(η5-C5H5)] have been studied by a combination of conventional solution and low-temperature (12 K) matrix isolation techniques. In pentane solution the photolysis of [MH(CO)3(η5-C5H5)] complexes proceeds with the formation of the dimeric complexes [{M(CO)3(η5-C5H5)}2], [{M(CO)2(η5-C5H5)}2], and [{WH(CO)2(η5-C5H5)}2] together with evolution of H2 and CO. The photo-induced reactions with N2 and C2H4 lead to monosubstitution products but only [WH(CO)2(C2H4)(η5-C5H5)] is stable enough at room temperature for it to be fully characterised. The olefin–hydrido-complex [WH(CO)2(C2H4)(η5-C5H5)] exists only as the trans isomer at room temperature. Under thermal and especially photochemical reaction conditions the olefin ligand in [WH(CO)2(C2H4)(η5-C5H5)] can be replaced by CO, N2, PMe3, and tetrahydrofuran (thf) ligands leading to substitution products with different thermal stabilities. The photolysis of [WH(CO)2(C2H4)(η5-C5H5)] in pentane solution in the absence of potential ligands leads to the loss of the olefin and the formation of [{WH(CO)2(η5-C5H5)2}2]. In Ar and CH4 matrices at 12 K u.v. irradiation results in dissociative loss of one of the CO ligands to give the co-ordinately unsaturated 16-electron species[MH(CO)2(η5-C5H5)](M = Mo or W). This process was reversed using visible light, i.e. there is an equilibrium (i). The reactivity of the [MH(CO)2(η5-C5H5)] species was apparent in their reactions with N2 and C2H4[MH(CO)3(η5-C5H5)] [graphic omitted] [MH(CO)2(η5-C5H5]+ CO (i) at 12 K to produce trans-[MH(CO)2(N2)(η5-C5H5)] complexes and both cis and trans isomers of [WH(CO)2(C2H4)(η5-C5H5)] but only the trans isomer of [MoH(CO)2(C2H4)(η5-C5H5)]. In CO matrices the formation of the radicals [M(CO)3(η5-C5H5)]˙ and HCO˙ is indicative of photo-induced metal–hydrogen bond cleavage. The photolysis in 13CO matrices results initially in the formation of all possible [MH(13CO)3–n(CO)n(η5-C5H5)](n= 0–3) derivatives indicating easy CO exchange. Ultimately bands of H13CO˙ and 13CO-enriched [M(CO)3(η5-C5H5)]˙ species were observed. The identities of [MH(CO)2(η5-C5H5)], [MH(CO)2{N2)(η5-C5H5)], and [M(CO)3(η5-C5H5)]˙ species were confirmed by energy-factored force-field fitting procedures for the 13CO-enriched molecules. Upon u.v. irradiation of trans-[WH(CO)2(C2H4)(η5-C5H5)] in CH4 matrices the primary product is the cis isomer and this is followed by insertion of C2H4 into the W–H bond to generate the 16-electron species [W(CO)2(C2H5)(η5-C5H5)]. A monocarbonyl complex, possibly [WH(CO)(C2H4)(η5-C5H5)], is the final product. In CO matrices, however, the olefin is substituted by CO, and the corresponding hydrido-carbonyl complex [WH(CO)3(η5-C5H5)] is formed. The cis⇄trans isomerisation and the insertion of C2H4 into a M–H bond are relevant to the understanding of hydroformylation reactions catalysed by transition metal hydrides.

Infrared matrix isolation evidence for the formation of (η 5-cyclopentadienyl)tricarbonyl-molybdenum and -tungsten radicals in carbon monoxide matrices at 12 K

Infrared spectroscopic evidence including 13CO labelling and energy-factored force-field fitting is presented for the first time to show that the radicals (η 5-C 5H 5)-M(CO) 3 • (M = MO, W) and HCO are produced on photolysis of (η 5-C 5H 5)M(CO) 3H complexes in CO matrices at 12 K.

Photochemistry of tricarbonyl(η4-cyclo-octatetraene)iron in a methane matrix at 12 K. Infrared spectroscopic evidence for photoisomerisation and carbon monoxide ejection reactions

Fourier-transform infrared spectroscopic evidence, including 13CO labelling and energy-factored CO force-field fitting, is presented to show that photolysis of [Fe(‘tub’-η4-C8H8)(CO)3] in a CH4 matrix at 12 K leads to the formation of a [Fe(‘chair’-η4-C8H8)(CO)3] species via the [Fe(η2-C8H8)(CO)3] intermediate. An additional photoproduct is the co-ordinatively unsaturated species [Fe(η4-C8H8)(CO)2] formed by photoelimination of a CO ligand. The results are related to the fluxional behaviour of [Fe(η4-C8H8)(CO)3] in solution.

Matrix isolation evidence for reversible α-elimination of hydrogen from a chromium methyl compound to give a chromium methylene hydride species

I.r. spectroscopic evidence, including 13CO labelling and energy-factored force-field fitting, is presented to show that photolysis of [(η5-C5H5)Cr(CO)3CH3] in gas matrices at 12 K leads to a methylene hydride species, [(η5-C5H5)Cr(CO)2(CH2)H], following CO dissociation and α-H elimination reactions and that this process is reversible.

Photochemistry of acetylacetonato-, trifluoroacetylacetonato-, and hexafluoroacetylacetonato-dicarbonyl-rhodium and -iridium complexes in frozen gas matrices at 12 K. Infrared spectroscopic evidence for carbon monoxide dissociation and chelate ring-opening processes

Infrared spectroscopic evidence, including 13CO labelling and energy-factored CO force-field fitting, is presented to show that on photolysis of [M(CO)2L](L = RCOCHCOR′; R,R′= CH3 or CF3; M = Rh or Ir) complexes at high dilution in CH4, Ar, CO, and N2 matrices at 12 K new species [M(CO)L], [M(CO)2(L*)](L*= unidentate form of L), and [M(CO)(N2)L] are produced. The results are discussed in relation to the mechanisms of thermal reactions in solution. In particular, the reversibility of the reaction [M(CO)2L]⇌[M(CO) L]+ CO and the dominance of this reaction indicates that the hitherto discounted dissociative pathway should be reconsidered. Some evidence is also found for the chelate ring opening to give [ M(CO)2(L*)] species, but no evidence was found for expanded-co-ordination-number species corresponding to the previously proposed associative mechanism for ligand-exchange and substitution reactions.

Photochemistry of tricarbonyl(η-cyclopentadienyl)methylmolybdenum in frozen gas matrices at 12 K. Infrared spectroscopic evidence for the formation of dicarbonyl(η-cyclopentadienyl)methylmolybdenum and dicarbonyl(η-cyclopentadienyl)(dinitrogen)methylmolybdenum

Infrared spectroscopic evidence, including 13CO lebelling and energy-factored CO force-field fitting, is presented to show that on photolysis of [Mo(η5-C5H5)(CO)3(CH3)] at high dilution in CH4, Ar, CO, and N2 matrices at 12 K new species [Mo(η5-C5H5)(CO)2(CH3)] and [Mo(η5-C5H5)(CO)2(N2)(CH3)] are produced. The reversibility of the reaction [Mo(η5-C5H5)(CO)3(CH3)]⇌[Mo(η5-C5H5)(CO)2(CH3)]+ CO is interpreted as confirming the existence of a reduced co-ordination number species as the intermediate in the substitution reactions of [Mo(η5-C5H5)(CO)3(CH3)]. The results are discussed in relation to the mechanisms of thermal and photochemical solution reactions.