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Nuclear magnetic resonance (NMR) spin relaxation is the most informative approach to experimentally probe the internal dynamics of proteins on the picosecond to nanosecond timescale. At the same time, molecular dynamics (MD) simulations of biological macromolecules are steadily improving through better physical models, enhanced sampling algorithms, and increasing computational power, and they provide exquisite information about flexibility and its role in protein stability and molecular interactions. Many examples have shown that MD is now adept in probing protein backbone motion, but improvements are still required towards a quantitative description of the dynamics of side chains, for example probed by the dynamics of methyl groups. Thus far, the comparison of computation with experiment for side chains has primarily focused on the relaxation of 13C and 2H nuclei induced by auto-correlated variation of spin interactions. However, the cross-correlation of 13C-1H dipolar interactions in methyl groups offers an attractive alternative. Here, we establish a methodological framework to extract cross-correlation relaxation parameters of methyl groups in proteins from all-atom MD simulations. To demonstrate the utility of the approach, cross-correlation relaxation rates of ubiquitin are computed from MD simulations performed with the AMBER99SB*-ILDN and CHARMM36 force fields. The simulation results were found to agree well with those obtained by experiment. Moreover, the data obtained with the two force fields are highly consistent.

A major challenge in understanding ligand binding to biomacromolecules lies in dissecting the underlying thermodynamic driving forces at the atomic level. Quantifying the contributions of water molecules is often especially demanding, although they can play important roles in biomolecular recognition and binding processes. One example is human carbonic anhydrase II, whose active site harbors a conserved network of structural water molecules that are essential for enzymatic catalysis. Inhibitor binding disrupts this water network and changes the hydrogen-bonding patterns in the active site. Here, we use atomistic molecular dynamics simulations to compute the absolute entropy of the individual water molecules confined in the active site of hCAII using a spectrally resolved estimation (SRE) approach. The entropy decrease of water molecules that remain in the active site upon binding of a dorzolamide inhibitor is caused by changes in hydrogen bonding and stiffening of the hydrogen-bonding network. Overall, this entropy decrease is overcompensated by the gain due to the release of three water molecules from the active site upon inhibitor binding. The spectral density calculations enable the assignment of the changes to certain vibrational modes. In addition, the range of applicability of the SRE approximation is systematically explored by exploiting the gradually changing degree of immobilization of water molecules as a function of the distance to a phospholipid bilayer surface, which defines an "entropy ruler". These results demonstrate the applicability of SRE to biomolecular solvation, and we expect it to become a useful method for entropy calculations in biomolecular systems.

Molecular simulations of intrinsically disordered proteins (IDPs) are challenging because they require sampling a very large number of relevant conformations, corresponding to a multitude of shallow minima in a flat free energy landscape. However, in the presence of a binding partner, the free energy landscape of an IDP can be dominated by few deep minima. This characteristic imposes high demands on the accuracy of the force field used to describe the molecular interactions. Here, as a model system for an IDP that is unstructured in solution but folds upon binding to a structured interaction partner, the transactivation domain of c-Myb was studied both in the unbound (free) form and when bound to the KIX domain. Six modern biomolecular force fields were systematically tested and compared in terms of their ability to describe the structural ensemble of the IDP. The protein force field/water model combinations included in this study are AMBER ff99SB-disp with its corresponding water model that was derived from TIP4P-D, CHARMM36m with TIP3P, ff15ipq with SPC/Eb, ff99SB*-ILDNP with TIP3P and TIP4P-D, and FB15 with TIP3P-FB water. Comparing the results from REST2-enhanced sampling simulations with experimental CD spectra and secondary chemical shifts reveals that the ff99SB-disp force field can realistically capture the broad and mildly helical structural ensemble of free c-Myb. The structural ensembles yielded by CHARMM36m, ff99SB*-ILDNP together with TIP4P-D water, and FB15 are also mildly helical; however, each of these force fields can be assigned a specific subset of c-Myb residues for which the simulations could not reproduce the experimental secondary chemical shifts. In addition, microsecond-timescale MD simulations of the KIX/c-Myb complex show that most force fields used preserve a stable helix fold of c-Myb in the complex. Still, all force fields predict a KIX/c-Myb complex interface that differs slightly from the structures provided by NMR because several NOE-derived distances between KIX and c-Myb were exceeded in the simulations. Taken together, the ff99SB-disp force field in the first place but also CHARMM36m, ff99SB*-ILDNP together with TIP4P-D water, and FB15 can be suitable choices for future simulation studies of the coupled folding and binding mechanism of the KIX/c-Myb complex and potentially also other IDPs.

Two chiral derivatives of 18-crown-6, namely the host molecules 2,3-diphenyl- and 2-phenyl-18c6, serve as model systems to investigate whether VCD spectroscopy can be used to monitor conformational changes occurring upon complexation of guests. Host-guest complexes of both crown ethers were prepared by addition of KNO

Solution-state NMR relaxation experiments are the cornerstone to study internal protein dynamics at an atomic resolution on time scales that are faster than the overall rotational tumbling time τR. Since the motions described by NMR relaxation parameters are connected to thermodynamic quantities like conformational entropies, the question arises how much of the total entropy is contained within this tumbling time. Using all-atom molecular dynamics simulations of the T4 lysozyme, we found that entropy buildup is rather fast for the backbone, such that the majority of the entropy is indeed contained in the short-time dynamics. In contrast, the contribution of the slow dynamics of side chains on time scales beyond τR on the side-chain conformational entropy is significant and should be taken into account for the extraction of accurate thermodynamic properties.

In Expanded Ensemble (EXE) or Simulated Tempering simulations, the system's (effective) temperature is frequently updated to enhance configurational sampling. We investigated how short the EXE state update interval τ can become before too frequent updates impede Boltzmann sampling. Simulating alanine dipeptide in explicit water, we show that a hybrid MC/MD integrator reliably yields Boltzmann-distributed configurations regardless of τ. However, in MD-driven EXE simulations with short τ, configurational ensembles depend on the thermostat settings.

Solvent fluctuations have been explored in detail for idealized and rigid hydrophobic model systems, but so far it has remained unclear how internal protein motions and their coupling to the surrounding solvent affect the dynamics of ligand binding to biomolecular surfaces. Here, molecular dynamics simulations were used to elucidate the solvent-mediated binding of a model ligand to the hydrophobic surface patch of ubiquitin. The ligand's friction profiles reveal pronounced long-time correlations and enhanced friction in the vicinity of the protein, similar to idealized hydrophobic surfaces. Interestingly, these effects are shaped by internal protein motions. Protein flexibility modulates water density fluctuations near the hydrophobic surface patch and smooths out the friction profile of ligand binding.

Enhanced sampling techniques are a promising approach to obtain reliable binding free energy profiles for flexible protein-ligand complexes from molecular dynamics (MD) simulations. To put four popular enhanced sampling techniques to a biologically relevant and challenging test, we studied the partial dissociation of an antigenic peptide from the Major Histocompatibility Complex I (MHC I) HLA-B*35:01 to systematically investigate the performance of Umbrella Sampling (US), Replica Exchange with Solute Tempering 2 (REST2), Bias Exchange Umbrella Sampling (BEUS, or replica-exchange umbrella sampling), and well-tempered Metadynamics (MTD). With regard to the speed of sampling and convergence, the peptide-MHC I complex (pMHC I) under study showcases intrinsic strengths and weaknesses of the four enhanced sampling techniques used. We found that BEUS can handle best the sampling challenges that arise from the coexistence of an enthalpically and an entropically stabilized free energy minimum in the pMHC I under study. These findings might be relevant also for other flexible biomolecular systems with competing enthalpically and entropically stabilized minima.

An approach is presented to directly simulate the dynamics of methyl groups in protein side-chains, as accessible via NMR spin relaxation measurements, by all-atom MD simulations. The method, which does not rely on NMR information or any system-specific adjustable parameters, is based on calculating the time-correlation functions (TCFs) of the C-H bonds in methyl groups and explicitly takes the truncation of the TCFs due to overall tumbling of the molecule into account. Using ubiquitin as a model protein, we show (i) that an accurate description of the methyl dynamics requires reparametrization of the potential energy barriers of methyl group rotation in the AMBER ff99SB*-ILDN force field (and related parameter sets), which was done with CCSD(T) coupled cluster calculations of frequency isolated dipeptides as reference, and (ii) that the TIP4P/2005 solvation model yields overall tumbling correlation times that are in close agreement with experimental data. The methyl axis squared order parameters S-axis(2) and associated correlation times tau(f), obtained within the Lipari-Szabo formalism, are in good agreement with the values derived from NMR deuterium relaxation experiments. Importantly, the relaxation rates and spectral densities derived from MD and NMR agree as well, enabling a direct comparison without assumptions inherent to simplified motional models.

The internal dynamics of proteins occurring on time scales from picoseconds to nanoseconds can be sensitively probed by nuclear magnetic resonance (NMR) spin relaxation experiments, as well as by molecular dynamics (MD) simulations. This complementarity offers unique opportunities, provided that the two methods are compared at a suitable level. Recently, several groups have used MD simulations to compute the spectral density of backbone and side chain molecular motions and to predict NMR relaxation rates from these. Unfortunately, in the case of methyl groups in protein side chains, inaccurate energy barriers to methyl rotation were responsible for a systematic discrepancy in the computed relaxation rates, as demonstrated for the AMBER ff99SB*-ILDN force field (and related parameter sets), impairing quantitative agreement between simulations and experiments. However, correspondence could be regained by emending the MD force field with accurate coupled cluster quantum chemical calculations. Spurred by this positive result, we tested whether this approach could be generally applicable, in spite of the fact that different MD force fields employ different water models. Improved methyl group rotation barriers for the CHARMM36 and AMBER ff15ipq protein force fields were derived, such that the NMR relaxation data obtained from the MD simulations even now display very good agreement with the experiment. Results herein showcase the performance of present-day MD force fields and manifest their refined ability to accurately describe internal protein dynamics.

The collective behaviour of individual lipid molecules determines the properties of phospholipid membranes. However, the collective molecular motions often remain challenging to characterise at the desired spatial and temporal resolution. Here we study collective vibrational motion on picosecond time scales in dioleoylphosphatidylcholine lipid bilayers with varying cholesterol content using all-atom molecular dynamics simulations. Cholesterol is found to not only laterally compact the lipid bilayer, but also to change the velocity of longitudinal density fluctuations propagating in the plane of the membrane. Cholesterol-induced reduction of the area per lipid alters the collective dynamics of the lipid headgroups, but not of the lipid tails. The introduction of cholesterol reduces the number of water molecules interacting with the lipid headgroups, leading to a decrease in the velocity of the laterally-propagating sound mode. Thus, the stiffening effect of cholesterol is found to be indirect: decreasing the area per lipid weakens the interactions between the lipid headgroups and water. The collective modes characterised in this work can enable the membrane to dissipate excess energy and thus maintain its structural integrity, e.g., under mechanical stress.

Correlated vibrational motion on the sub-picosecond timescale and associated collective dynamics in a protein–membrane environment are characterized using molecular dynamics simulations. We specifically analyze correlated motion of a membrane-associated protein and a lipid bilayer for distinct separation distances. Correlated vibrations persist up to distances of 25 A between both biomolecular surfaces. These correlations are mediated by separating layers of water molecules, whose collective properties are altered by the simultaneous presence of protein and lipid bilayer interfaces.

Hybrid all-atom/coarse-grained (AA-CG) simulations in which AA solutes are embedded in a CG environment can provide a significant computational speed-up over conventional fully atomistic simulations and thus alleviate the current length and time scale limitations of molecular dynamics (MD) simulations of large biomolecular systems. On one hand, coarse graining the solvent is particularly appealing, since it typically constitutes the largest part of the simulation system and thus dominates computational cost. On the other hand, retaining atomic-level solvent layers around the solute is desirable for a realistic description of hydrogen bonds and other local solvation effects. Here, we devise and systematically validate fixed resolution AA-CG schemes, both with and without atomistic water layers. To quantify the accuracy and diagnose possible pitfalls, Gibbs free energies of solvation of amino acid side chain analogues were calculated, and the influence of the nature of the CG solvent surrounding (polarizable vs nonpolarizable CG water) and the size of the AA solvent region was investigated. We show that distance restraints to keep the AA solvent around the solute lead to too high of a density in the inner shell. Together with a long-ranged effect due to orientational ordering of water molecules at the AA-CG boundary, this affects solvation free energies. Shifting the onset of the distance restraints slightly away from the central solute significantly improves solvation free energies, down to mean unsigned errors with respect to experiment of 2.3 and 2.6 kJ/mol for the polarizable and nonpolarizable CG water surrounding, respectively. The speed-up of the nonpolarizable model renders it computationally more attractive. The present work thus highlights challenges, and outlines possible solutions, involved with modeling the boundary between different levels of resolution in hybrid AA-CG simulations.

Nuclear magnetic resonance (NMR) spin relaxation has become the mainstay technique to sample protein dynamics at atomic resolution, expanding its repertoire from backbone 15N to side-chain 2H probes. At the same time, molecular dynamics (MD) simulations have become increasingly powerful to study protein dynamics due to steady improvements of physical models, algorithms, and computational power. Good agreement between generalized Lipari-Szabo order parameters derived from experiment and MD simulation has been observed for the backbone dynamics of a number of proteins. However, the agreement for the more dynamic side-chains, as probed by methyl group relaxation, was much worse. Here, we use T4 lysozyme (T4L), a protein with moderate tumbling anisotropy, to showcase a number of improvements that reduce this gap by a combined evaluation of NMR relaxation experiments and MD simulations. By applying a protein force field with accurate methyl group rotation barriers in combination with a solvation model that yields correct protein rotational diffusion times, we find that properly accounting for anisotropic protein tumbling is an important factor to improve the match between NMR and MD in terms of methyl axis order parameters, spectral densities, and relaxation rates. The best agreement with the experimentally measured relaxation rates is obtained by a posteriori fitting the appropriate internal time correlation functions, truncated by anisotropic overall tumbling. In addition, MD simulations led us to account for a hitherto unrealized artifact in deuterium relaxation experiments arising from strong coupling for leucine residues in uniformly 13C-enriched proteins. For T4L, the improved analysis reduced the RMSD between MD and NMR derived methyl axis order parameters from 0.19 to 0.11. At the level of the spectral density functions, the improvements allow us to extract the most accurate parameters that describe protein side-chain dynamics. Further improvement is challenging not only due to force field and sampling limitations in MD, but also due to inherent limitations of the Lipari-Szabo model to capture complex dynamics.

In bundled SPC water models, the relative motion of groups of four water molecules is restrained by distance-dependent potentials. Bundled SPC models have been used in hybrid all-atom/coarse-grained (AA/CG) multiscale simulations, since they enable to couple atomistic SPC water with supra-molecular CG water models that effectively represent more than a single water molecule. In the present work, we systematically validated and critically tested bundled SPC water models as solvent for biomolecular simulations. To that aim, we investigated both thermodynamic and structural properties of various biomolecular systems through molecular dynamics (MD) simulations. Potentials of mean force of dimerization of pairs of amino acid side chains as well as hydration free energies of single side chains obtained with bundled SPC and standard (unrestrained) SPC water agree closely with each other and with experimental data. Decomposition of the hydration free energies into enthalpic and entropic contributions reveals that in bundled SPC, this favorable agreement of the free energies is due to a larger degree of error compensation between hydration enthalpy and entropy. The Ramachandran maps of Ala3, Ala5, and Ala7 peptides are similar in bundled and unrestrained SPC, whereas for the (GS)2 peptide, bundled water leads to a slight overpopulation of extended conformations. Analysis of the end-to-end distance autocorrelation times of the Ala5 and (GS)2 peptides shows that sampling in more viscous bundled SPC water is about two times slower. Pronounced differences between the water models were found for the structure of a coiled-coil dimer, which is instable in bundled SPC but not in standard SPC. In addition, the hydration of the active site of the serine protease α-chymotrypsin depends on the water model. Bundled SPC leads to an increased hydration of the active site region, more hydrogen bonds between water and catalytic triad residues, and a significantly slower exchange of water molecules between the active site and the bulk. Our results form a basis for assessing the accuracy that can be expected from bundled SPC water models. At the same time, this study also highlights the importance of evaluating beforehand the effects of water bundling on the biomolecular system of interest for a particular multiscale simulation application.

Monoclonal antibody (mAb)-based therapeutics often require high-concentration formulations. Unfortunately, highly concentrated antibody solutions often have biophysical properties that are disadvantageous for therapeutic development, such as high viscosity, solubility limitations, precipitation issues, or liquid-liquid phase separation. In this work, we present a computational rational design principle for improving the thermodynamic stability of mAb solutions through targeted point mutations. Two publicly available IgG1 monoclonal antibodies that exhibit high viscosity at high concentrations were used as model systems. Guided by a computationally efficient approach that combines molecular dynamics simulations with three-dimensional reference interaction site model theory, point mutations of charged residues were introduced in the variable Fv regions in such a manner that the hydration free energy was optimized. Two selected point mutants were then produced by transient expression and characterized experimentally. Both engineered mAbs have reduced viscosity at high concentration, less negative second virial coefficient, and improved solubility compared to the respective wild-types. The results obtained with the suggested straightforward design principle underline the relevance of solvation effects for understanding, and ultimately optimizing, the properties of highly concentrated mAb solutions, with possible implications also for other biomolecular systems.

We present a refined version of the polarizable Martini water model - coined refPOL - designed specifically for the use with long-range electrostatics. The refPOL model improves the agreement with the experimentally measured dielectric constant and the mass density of water at room temperature compared to the original polarizable Martini water force field when particle mesh Ewald electrostatics are employed. Our study reveals that the model remains applicable with various commonly used settings for the non-bonded interactions, including reaction field electrostatics. The oil/water partitioning behavior of uncharged Martini bead types is thoroughly investigated: Lennard-Jones interactions between the refPOL model and the remaining Martini beads are adjusted to reproduce the hydration free energies obtained with the original polarizable water model, while free energies of solvation in apolar media remain unchanged. The cross-interactions with charged bead types are parameterized to agree with the experimentally observed area per lipid of a fully solvated dipalmitoylphosphatidylcholine bilayer. We additionally verify the model by analyzing the potentials of mean force between different sample pairs in refPOL water and comparing the results to reference data obtained using the original polarizable Martini water model as well as fully atomistic simulations. Based on the results, we suggest to replace the original polarizable Martini water model with the new refPOL model for future applications.

Solvation plays an important role in virtually all biomolecular recognition and binding processes. However, the consequences of changes in solvation conditions often remain elusive. In this work, we combined isothermal titration calorimetry (ITC) and molecular dynamics (MD) simulations to investigate the effect of solvent composition on the thermodynamics of protein-ligand binding. We studied the binding of p-aminobenzamidine (PAB) to trypsin in various water/methanol mixtures as a model system for a biomolecular complex. Our ITC experiments show that the free energy of binding changes only very modestly with methanol concentration, and that this small change is due to strong enthalpy-entropy compensation. The MD and free energy simulations not only reproduce the experimental binding free energies, but also provide atomic-level insights into the mechanisms behind the thermodynamic observations. The more favorable binding enthalpy at increased methanol concentrations (when compared to pure water) is attributed to stronger protein-ligand and intramolecular protein-protein interactions. The stronger protein-ligand interaction is linked to a small-scale conformational rearrangement of the L2 binding pocket loop, which senses the solvent environment. Remarkably, the stronger interactions do not substantially reduce the configurational entropy of the protein. Instead, the more disfavorable entropy contribution to the binding free energy at increased methanol concentrations is due to the desolvation of the ligand from the bulk, which is more favorable in pure aqueous solution than in the presence of methanol. Our work thus underpins the importance of including conformational flexibility, even for an overall rather rigid complex, since even small-amplitude motions can significantly alter the binding energetics. Furthermore, the ability of our combined ITC/MD approach to assign different thermodynamic contributions to distinct conformational states might contribute to an enhanced understanding of biomolecular binding processes in general.

The Martini coarse-grained force field has been successfully used for simulating a wide range of (bio)molecular systems. Recent progress in our ability to test the model against fully atomistic force fields, however, has revealed some shortcomings. Most notable, phenylalanine and proline were too hydrophobic, and dimers formed by polar residues in apolar solvents did not bind strongly enough. Here, we reparametrize these residues either through reassignment of particle types or by introducing embedded charges. The new parameters are tested with respect to partitioning across a lipid bilayer, membrane binding of Wimley White peptides, and dimerization free energy in solvents of different polarity. In addition, we improve some of the bonded terms in the Martini protein force field that lead to a more realistic length of a-helices and to improved numerical stability for polyalanine and glycine repeats. The new parameter set is denoted Martini version 2.2.

Light-driven molecular rotary motors derived from chiral overcrowded alkenes represent a broad class of compounds for which photochemical rearrangements lead to large scale motion of one part of the molecule with respect to another. It is this motion/change in molecular shape that is employed in many of their applications. A key group in this class are the molecular rotary motors that undergo unidirectional light-driven rotation about a double bond through a series of photochemical and thermal steps. In the present contribution we report a combined quantum chemical and molecular dynamics study of the mechanism of the rotational cycle of the fluorene-based molecular rotary motor 9-(2,4,7-trimethy1-2,3-dihydro-1H-inden-1-ylidene)-9H-fluorene (1). The potential energy surfaces of the ground and excited singlet states of I were calculated, and it was found that conical intersections play a central role in the mechanism of photo conversion between the stable conformer of 1 and its metastable conformer. Molecular dynamics simulations indicate that the average lifetime of the fluorene motor in the excited state is 1.40 +/- 0.10 ps when starting from the stable conformer, which increases to 1.77 +/- 0.13 ps for the reverse photoisomerization. These simulations indicate that the quantum yield of photoisomerization of the stable conformer is 0.92, whereas it is only 0.40 for the reverse photoisomerization. For the first time, a theoretical understanding of the experimentally observed photostationary state of 1 is reported that provides a detailed picture of the photoisomerization dynamics in overcrowded alkene-based molecular motor 1. The analysis of the electronic structure of the fluorene molecular motor holds considerable implications for the design of molecular motors. Importantly, the role of pyramidalization and conical intersections offer new insight into the factors that dominate the photostationary state achieved in these systems.

We investigated structural and dynamical properties of nanodiscs comprising dimyristoylphosphatidylcholine (DMPC) lipids and major scaffold protein MSP1Δ(1-22) from human apolipoprotein A-1 using combined all-atom and coarse-grained (CG) molecular dynamics (MD) simulations. The computational efficiency of the Martini-CG force field enables the spontaneous self-assembly of lipids and scaffold proteins into stable nanodisc structures on time scales up to tens of microseconds. Subsequent all-atom and CG-MD simulations reveal that the lipids in the nanodisc have lower configurational entropy and higher acyl tail order than in a lamellar bilayer phase. These altered average properties arise from rather differential behavior of lipids, depending on their location in the nanodisc. Since the scaffold proteins exert constrictive forces from the outer rim of the disc toward its center, lipids at the center of the nanodisc are highly ordered, whereas annular lipids that are in contact with the MSP proteins are remarkably disordered due to perturbed packing. Although specific differences between all-atom and CG simulations are also evident, the results obtained at both levels of resolution are in overall good agreement with each other and provide atomic level interpretations of recent experiments. Thus, the present study highlights the applicability of multiscale simulation approaches for nanodisc systems and opens the way for future applications, including the study of nanodisc-embedded membrane proteins.

We present the results of experimental and theoretical studies of the speed-dependent spin–orbit state distributions of chlorine photofragments produced in the photodissociation of thiophosgene (CSCl 2 ) at 235 nm. Three-dimensional imaging has been employed for observing chlorine photofragments in their ground (Cl) and excited (Cl * ) spin–orbit states. The kinetic energy distributions for Cl and Cl * fragments reflect excitation of several electronic states of the partner fragment CSCl. The spin–orbit branching ratio of P (Cl * )/[ P (Cl) + P (Cl * )] was found to depend on the kinetic recoil energy increasing from 0.1 for low kinetic energy to 0.8 for high kinetic energy. The theoretical interpretation is based on the computation of the CSCl 2 potential energy surfaces (PES) along the C–Cl bond. Two completely different methods of determination of the PES were applied for small and for large values of the C–Cl bond separation R . In case of small and intermediate R values time-dependent density-functional theory has been used. In case of large R values we used an asymptotic method of computation of the PES, which is a generalisation of the Heitler–London approach for many-electron systems. Basis molecular wavefunctions with definite values of the total spin S and the spatial and spin reflection symmetry σ v with respect to reflection of the total electronic wavefunction in the molecular plane were used. The developed theoretical approach was used for the assignment of the molecular states involved in the photodissociation and for the qualitative explanation of the non-statistical population of the spin–orbit states of the chlorine photofragments as function of the kinetic energy. The spin–orbit branching ratio of P (Cl * )/[ P (Cl) + P (Cl * )] predicted by the theory strongly depends on the quantum state of the CSCl fragment. It is large in case of the CSCl(X) + Cl and CSCl(A) + Cl channels and small in case of the CSCl(B) + Cl channel which explains the experimental results.

New theoretical and experimental results for the ultraviolet photodissociation dynamics of thionyl chloride (SOCl2) are presented and combined with existing data from a variety of sources in order to provide a unified view of the photodissociation dynamics of SOCl2. Time-dependent density functional theory on the basis of the hybrid-type B3LYP functional was employed to calculate vertical excitation energies for the SOCl2 parent molecule up to 6.3 eV. Three-dimensional (3D) imaging of photofragments was performed for a dissociation wavelength of 235 nm. Atomic chlorine fragments were observed in the 2P3/2 ground state [Cl] and the 2P1/2 excited spin–orbit state [Cl*] by employing resonance enhanced multi-photon ionization (REMPI) and time-of-flight (TOF) techniques. State-specific speed distributions and the speed dependence of the β anisotropy parameter were obtained from the full 3D momentum vector distribution by appropriate projection methods. Bimodal speed distributions for both spin–orbit states are evidence of a competition between the radical (SOCl2 → SOCl + Cl/Cl*) and the three-body decay channel (SOCl2 → SO + 2 Cl/Cl*). No evidence of the molecular fragmentation channel (SOCl2 → SO + Cl2) was found. With increasing fragment speed the β anisotropy parameter increases from 0.1 to 0.85 and 0.68 for Cl and Cl*, respectively, suggesting fragmentation via an excited A′ state for slow fragments and via an A″ state for fast fragments. The calculations allow for the first time to interpret all previous and new experimental data for the ultraviolet photodissociation of SOCl2 by assuming simultaneous excitation of several excited electronic states giving rise to competing dissociation channels.