Negative Activation Enthalpy Research Articles

Capability of TiO2 and Fe3O4 nanoparticles loaded onto Algae (Scendesmus sp.) as a novel bio-magnetic photocatalyst to degration of Red195 dye in the sonophotocatalytic treatment process under ultrasonic/UVA irradiation

In this study, the magnetic photocatalyst Scendesmus/Fe3O4/TiO2 was synthesized, and its sonophotocatalytic properties in relation to the degradation of the Red195 dye were evaluated. Particles were characterized using a scanning electron microscope (SEM), Fourier's transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), and a vibrating-sample magnetometer (VSM). At a pH of 5, a photocatalyst dosage of 100 mg, an initial R195 concentration of 100 mg/l, an ultrasound power of 38W, and an exposure time of 20 min, the maximum Red195 removal efficiency (100%) was achieved. After five cycles of recycling, the composite's sonophotocatalytic degradation stability for R195 remains above 95%. Experiments on scavenging indicate that electrons (h+) and hydroxyls (OH-) are indispensable decomposition agents. The removal of R195 by Scendesmus/Fe3O4/TiO2 is consistent with the pseudo-first-order kinetic, Freundlich, and Henderson's isotherm models, as determined by kinetic and isotherm investigations. The negative activation enthalpy of the standard (ΔH°) illuminates the exothermic adsorption mechanism. The increase in standard Gibbs activation free energy (ΔG°) with increasing temperature reveals the process is not spontaneous. As indicated by the negative value of the standard entropy of activation (ΔS°), activation of the reactants resulted in a loss of freedom.

Amino-Ene Click Reaction of Electron-Deficient π-Conjugated Molecules with Negative Activation Enthalpies.

An amino-ene click reaction is a type of aza-Michael addition reaction that is congruent with click chemistry in terms of its reaction efficiency and rate under mild conditions. The amino-ene click reaction is increasingly recognized as a prominent synthetic tool to form C-N bonds in the context of organic materials chemistry and polymer chemistry. Herein, an unconventional amino-ene click reaction with negative activation enthalpies, in which an electron-deficient π-conjugated molecule, such as a naphthalenediimide, reacts with an amine faster at lower temperatures is reported. The detailed study of the reaction mechanism reveals that the amino-ene click reaction proceeds via a pre-equilibrium reaction, the key to which is the formation of a stable reaction intermediate due to the solvation and charge delocalization on the π-core. By optimizing the reaction conditions, it was demonstrated that the amino-ene click reaction proceeded faster at 273 K than at 347 K, which was easily observed visually.

Kinetics and thermodynamic studies on Moringa oleifera oil extraction for biodiesel production via transesterification

Moringa oleifera produces oil which can be extracted and converted to biodiesel. However, the kinetics and thermodynamics of this viable fuel have been seldomly investigated. We examine the kinetics and thermodynamics of the oil extraction for biodiesel production. The biodiesel kinematic viscosity (3.75 ± 0.04 mm2/s), cetane number (67.12), oxidative stability (15.2 ± 0.5), acid value (0.012 mg/KOH), pour point (−9) and carbon residue (0.020 0.001) satisfy the American Society for Testing and Materials (ASTM) limits. Fourier transform infrared spectroscopy distinguishes the biodiesel and the base oil at 1435.77 cm−1 (-CH3 stretch), 1195.74 cm−1 (O-CH3) and 1377.67 cm−1 (O-CH2) because -CH3 and O-CH3 stretches are absent in the oil whereas the biodiesel lacks O-CH2 stretch. The highly negative activation entropy (−214.11 0.16 J mol−1 K −1) and greater activation enthalpy (30.39 0.05 kJ/mol) indicate a slower extraction rate due to the higher energy requirement and stiffer transition of the extraction, respectively. The slower extraction rate agrees with the lower mass transfer coefficients (0.0119–0.0210 min−1). The equilibrium constant (K) is positive whilst the Gibbs free energy (ΔG) is negative, indicating a forward and spontaneous process. This investigation of kinetics, thermodynamics and transesterification essentially provides in-depth knowledge on oil extraction and biodiesel production.

Structure and Reactivity of Copper Complexes Supported by a Bulky Tripodal N4 Ligand: Copper(I)/Dioxygen Reactivity and Formation of a Hydroperoxide Copper(II) Complex

A copper(I) complex, [CuI(TIPT2aped)(CH3CN)]ClO4 (1), and two copper(II) complexes, [CuII(TIPT2aped)(Cl)]PF6 (2) and [CuII(TIPT2aped)(ClO4)]ClO4 (3), were synthesized and characterized. The reaction of 1 with dioxygen in acetone at –90 °C gave an end‐on superoxide copper(II) complex 4. The superoxide copper(II) complex 4 reacted with 2,2,6,6‐tetramethylpiperidin‐1‐ol (TEMPO‐H) and 2,6‐di‐tert‐butyl‐4‐methoxyphenol (DTBMP) to generate a hydroperoxide copper(II) complex 6. These copper‐oxygen adducts were thoroughly characterized by UV/Vis, resonance Raman, EPR, and NMR spectroscopic techniques. Complex 6 was also synthesized from the copper(II) complex 3 in the presence of hydrogen peroxide and triethylamine. The reaction of 4 with TEMPO‐H showed a relatively large kinetic deuterium isotope effect (kH/kD = 5.8) and a largely negative activation enthalpy (ΔH‡). Based on the kinetic data, the reaction mechanism for the conversion of 4 to 6 is discussed.

Role of Hydrogen Bonding in Photoinduced Electron-Proton Transfer from Phenols to a Polypyridine Ru Complex with a Proton-Accepting Ligand.

Electron-proton transfer (EPT) from phenols to a triplet metal-to-ligand charge transfer (MLCT)-excited Ru polypyridine complex containing an uncoordinated nitrogen site, 1(T), can be described by a kinetic model that accounts for the H-bonding of 1(T) to phenol, 1(T) to solvent, and phenol to solvent. The latter plays a major role in the kinetic solvent effect and commonly precludes simultaneous determination of the EPT rate constant and 1(T)-phenol H-bonding constant. A number of these quantities previously reported for similar systems are shown to be in error due to inconsistent data analysis. Control experiments replacing either 1(T) by its structural isomer with a sterically screened nitrogen site or phenol by its H-bonding surrogate, trifluoroethanol, and the observation of negative activation enthalpies for the overall reactions between 1(T) and phenols lend support to the proposed model and provide evidence for the formation of a precursor H-bonded complex between the reactants, which is a prerequisite for EPT.

Perspective View on Sorption Thermodynamics: Basic Dye Uptake on Southern Nigerian Clay

Macroscopic phenomenon like adsorption has a mechanistic tie typical of thermodynamics and its principles. This work examined the thermodynamic parameters for methylene blue (MB) uptake onto modified Ekowe clay (EC). The purified clay was calcined for 4hrs. at 750 oC to obtain Natural Ekowe Clay (NEC). The purified clay was activated (1.6M H2SO4 (aq)) and calcined for 4hrs. at 750 oC obtaining Activated Ekowe Clay (AEC). Thermodynamic study applied the equilibrium data in determining the activation and heats of adsorption parameters. The concave Eyring plot suggests more than one rate-limiting steps coexisting in the sorption. For temperatures: 25, 30 and 40 oC, activation energies (3) for NEC and AEC lie between 2 - 29kJ/mol inferring physisorption. Negative activation enthalpies (ΔH * ) values confirm exothermic activations. The less negative ΔH * values, in compliance with the significant k2 values varying inversely with temperature, suggests high sorption rate. The negative activation entropy explained an associative uptake and the less negative values are attributive to a physical uptake. Negative free activation enthalpy, ΔG * indicated that uptake on the modified EC is spontaneous. High negativity of ΔG * values suggest strong physic-sorption bond. The negative ΔH * , ΔS * and ΔG * values characterize the physisorption of MB onto modified EC. Values of the isoexcess heats (qisox) obtained: 2.67kJ/mol. (NEC) and 2.47kJ/mol. (AEC) agreed with the value range of <80kJ/mol. typical to physic-sorption. This work opines that sorption of MB onto modified EC is a spontaneous exothermic multilayer phenomenon that progresses heterogeneously with continuous decrease in sorption potential and fall in isosteric heat.

An activation energy approach for viscous flow: A complementary tool for the study of microstructural evolutions in sheared suspensions

The dynamic viscosity of low-molecular-weight polypropylene glycol-based SiO2 nanofluids was measured in the temperature range 10−50°C, with varying solid volume concentrations (0.07−0.10), and a shear rate of 0.1−300s−1. A weak shear-thinning region followed by temperature- and solid concentration-dependent shear-thickening behavior was observed. The onset of shear-thickening behavior moved to higher shear rate values when the temperature increased and the particle concentration decreased. The temperature dependence of the viscosity at given shear rate values was fitted by the Andrade-Eyring equation. An analogy to chemical reactions, considered to be processes that change from reactants to products, with sheared particle suspensions that change from initial to final microstructure state, is proposed. This activation energy approach for a viscous flow is used to elucidate the shear-thickening mechanism (order-disorder or hydroclusters) shown by hydrophobic fumed silica/low-molecular-weight polypropylene glycol suspensions. Activation enthalpy is interpreted as a measure of the potential energy barrier that has to be overcome when a suspension changes from the initial to the final microstructural state. Positive (negative) activation enthalpy indicates adsorption (liberation) of energy in chemical reactions, which is interpreted as a phase transition from a more to less (less to more) structured state in particle suspensions. Activation entropy is interpreted as a measure of order in the suspension microstructure. Positive (negative) activation entropy is an expression of a dissociative (associative) mechanism between reactant molecules, which can be interpreted as the microstructure breaking down (buildup) in particle suspensions.

DNA Strand Replacement Mechanism in Molecular Beacons Encoded for the Detection of Cancer Biomarkers.

Signaling properties of a fluorescent hairpin oligonucleotide molecular beacon (MB) encoded to recognize protein survivin (Sur) mRNA have been investigated. The process of complementary target binding to SurMB with 20-mer loop sequence is spontaneous, as expected, and characterized by a high affinity constant (K = 2.51 × 10(16) M(-1)). However, the slow kinetics at room temperature makes it highly irreversible. To understand the intricacies of target binding to MB, a detailed kinetic study has been performed to determine the rate constants and activation energy Ea for the reaction at physiological temperature (37 °C). Special attention has been paid to assess the value of Ea in view of reports of negative activation enthalpy for some nucleic acid reactions that would make the target binding even slower at increasing temperatures in a non-Arrhenius process. The target-binding rate constant determined is k = 3.99 × 10(3) M(-1) s(-1) at 37 °C with Ea = 28.7 ± 2.3 kcal/mol (120.2 ± 9.6 kJ/mol) for the temperature range of 23 to 55 °C. The positive high value of Ea is consistent with a kinetically controlled classical Arrhenius process. We hypothesize that the likely contribution to the activation energy barrier comes from the SurMB stem melting (tm = 53.7 ± 0.2 °C), which is a necessary step in the completion of target strand hybridization with the SurMB loop. A low limit of detection (LOD = 2 nM) for target tDNA has been achieved. Small effects of conformational polymorphs of SurMB have been observed on melting curves. Although these polymorphs could potentially cause a negative Ea, their effect on kinetic transients for target binding is negligible. No toehold preceding steps in the mechanism of target binding were identified.

Kinetic and Thermodynamic Analysis of Cholesterol Transfer between Phospholipid Vesicles and Nanodiscs.

We investigated interparticle transfer of cholesterol (Chol) between large unilamellar vesicles (LUVs) and phospholipid bilayer nanodiscs. The Chol transfer rate from LUVs to nanodiscs was decreased by an increase in the Chol content or incorporation of sphingomyelin in donor phosphatidylcholine/Chol LUVs but was not influenced by the lipid composition of acceptor particles. These results suggest that Chol dissociation from the lipid bilayer into aqueous phase is the rate-limiting step of the transfer and that the process depends on the fluidity of the donor membranes. The Chol dissociation rate from nanodiscs was faster than that from LUVs with similar lipid composition. Chol preferably partitioned to LUVs rather than nanodiscs, which is consistent with the faster dissociation rate from nanodiscs. The activation energy of Chol dissociation from nanodiscs was 1.7 kJ/mol lower than that from LUV, which was brought by increased (less negative) activation entropy and enthalpy. In addition, fluorescence lifetime and anisotropy data revealed that the lipid bilayer of nanodiscs is more tightly packed than that of LUVs. These results suggest that the tighter lipid packing in nanodiscs destabilizes the Chol-containing bilayer by reducing the entropy, which facilitates Chol dissociation.

The role of loop stacking in the dynamics of DNA hairpin formation.

We study the dynamics of DNA hairpin formation using oxDNA, a nucleotide-level coarse-grained model of DNA. In particular, we explore the effects of the loop stacking interactions and non-native base pairing on the hairpin closing times. We find a nonmonotonic variation of the hairpin closing time with temperature, in agreement with the experimental work of Wallace et al. (Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 5584-5589). The hairpin closing process involves the formation of an initial nucleus of one or two bonds in the stem followed by a rapid zippering of the stem. At high temperatures, typically above the hairpin melting temperature, an effective negative activation enthalpy is observed because the nucleus has a lower enthalpy than the open state. By contrast, at low temperatures, the activation enthalpy becomes positive mainly due to the increasing energetic cost of bending a loop that becomes increasingly highly stacked as the temperature decreases. We show that stacking must be very strong to induce this experimentally observed behavior, and that the existence of just a few weak stacking points along the loop can substantially suppress it. Non-native base pairs are observed to have only a small effect, slightly accelerating hairpin formation.

Designing processive catalytic systems. Threading polymers through a flexible macrocycle ring.

The translocation of polymers through pores is widely observed in nature and studying their mechanism may help understand the fundamental features of these processes. We describe here the mechanism of threading of a series of polymers through a flexible macrocyclic ring. Detailed kinetic studies show that the translocation speed is slower than the translocation speed through previously described more rigid macrocycles, most likely as a result of the wrapping of the macrocycle around the polymer chain. Temperature-dependent studies reveal that the threading rate increases on decreasing the temperature, resulting in a negative activation enthalpy of threading. The latter is related to the opening of the cavity of the macrocycle at lower temperatures, which facilitates binding. The translocation process along the polymer chain, on the other hand, is enthalpically unfavorable, which can be ascribed to the release of the tight binding of the macrocycle to the chain upon translocation. The combined kinetic and thermodynamic data are analyzed with our previously proposed consecutive-hopping model of threading. Our findings provide valuable insight into the translocation mechanism of macrocycles on polymers, which is of interest for the development of processive catalysts, i.e., catalysts that thread onto polymers and move along it while performing a catalytic action.

Kinetic Evidence of an Apparent Negative Activation Enthalpy in an Organocatalytic Process

A combined kinetic and computational study on our tryptophan-based bifunctional thiourea catalyzed asymmetric Mannich reactions reveals an apparent negative activation enthalpy. The formation of the pre-transition state complex has been unambiguously confirmed and these observations provide an experimental support for the formation of multiple hydrogen bonding network between the substrates and the catalyst. Such interactions allow the creation of a binding cavity, a key factor to install high enantioselectivity.

DNA hybridization kinetics: zippering, internal displacement and sequence dependence

Although the thermodynamics of DNA hybridization is generally well established, the kinetics of this classic transition is less well understood. Providing such understanding has new urgency because DNA nanotechnology often depends critically on binding rates. Here, we explore DNA oligomer hybridization kinetics using a coarse-grained model. Strand association proceeds through a complex set of intermediate states, with successful binding events initiated by a few metastable base-pairing interactions, followed by zippering of the remaining bonds. But despite reasonably strong interstrand interactions, initial contacts frequently dissociate because typical configurations in which they form differ from typical states of similar enthalpy in the double-stranded equilibrium ensemble. Initial contacts must be stabilized by two or three base pairs before full zippering is likely, resulting in negative effective activation enthalpies. Non-Arrhenius behavior arises because the number of base pairs required for nucleation increases with temperature. In addition, we observe two alternative pathways—pseudoknot and inchworm internal displacement—through which misaligned duplexes can rearrange to form duplexes. These pathways accelerate hybridization. Our results explain why experimentally observed association rates of GC-rich oligomers are higher than rates of AT- rich equivalents, and more generally demonstrate how association rates can be modulated by sequence choice.

Kinetic Study on Michael-Type Reactions of β-Nitrostyrenes with Cyclic Secondary Amines in Acetonitrile: Transition-State Structures and Reaction Mechanism Deduced from Negative Enthalpy of Activation and Analyses of LFERs

A kinetic study is reported for the Michael-type reactions of X-substituted β-nitrostyrenes (1a-j) with a series of cyclic secondary amines in MeCN. The plots of pseudo-first-order rate constant k(obsd) vs [amine] curve upward, indicating that the reactions proceed through catalyzed and uncatalyzed routes. The dissection of k(obsd) into Kk2 and Kk3 (i.e., the rate constants for the uncatalyzed and catalyzed routes, respectively) revealed that Kk3 is much larger than Kk2, implying that the reactions proceed mainly through the catalyzed route when [amine] > 0.01 M. Strikingly, the reactivity of β-nitrostyrene (1g) toward piperidine decreases as the reaction temperature increases. Consequently, a negative enthalpy of activation is obtained, indicating that the reaction proceeds through a relatively stable intermediate. The Brønsted-type plots for the reactions of 1g are linear with β(nuc) = 0.51 and 0.61, and the Hammett plots for the reactions of 1a-j are also linear with ρX = 0.84 and 2.10 for the uncatalyzed and catalyzed routes, respectively. The reactions are concluded to proceed through six-membered cyclic transition states for both the catalyzed and uncatalyzed routes. The effects of the substituent X on reactivity and factors influencing β(nuc) and ρX obtained in this study are discussed.

Cold-Induced Precipitation of a Monoclonal IgM: A Negative Activation Enthalpy Reaction

Cold-induced precipitation of a monoclonal IgM cryoglobulin isolated from a patient with Waldenström's macroglobulinemia was observed to have a negative activation enthalpy. The rate of the reaction increased, as the temperature decreased. Differential scanning calorimetry of the monoclonal IgM showed precipitation as an inverted peak during a downward temperature scan. The transition temperature was between 14 and 15 °C and was possibly concentration dependent. At temperatures below the transition the precipitation was best described by second-order kinetics. The difference in change in enthalpy between precipitation and disassociation suggests that cold-induced precipitation had a fast precipitation stage followed by a slower consolidation reaction. Negligible curvature of the Eyring plot suggested the precipitation reaction was dominated by van der Waal forces and hydrogen bonding. Conversely, during an upward temperature scan, disassociation was observed as a positive enthalpy peak. This reaction had two stages, a reaction undoing consolidation followed by heat-induced disassociation that had first-order kinetics.

The element effect revisited: factors determining leaving group ability in activated nucleophilic aromatic substitution reactions.

The "element effect" in nucleophilic aromatic substitution reactions (S(N)Ar) is characterized by the leaving group order, F > NO(2) > Cl ≈ Br > I, in activated aryl halides. Multiple causes for this result have been proposed. Experimental evidence shows that the element effect order in the reaction of piperidine with 2,4-dinitrophenyl halides in methanol is governed by the differences in enthalpies of activation. Computational studies of the reaction of piperidine and dimethylamine with the same aryl halides using the polarizable continuum model (PCM) for solvation indicate that polar, polarizability, solvation, and negative hyperconjugative effects are all of some importance in producing the element effect in methanol. In addition, a reversal of polarity of the C-X bond from reactant to transition state in the case of ArCl and ArBr compared to ArF also contributes to their differences in reactivity. The polarity reversal and hyperconjugative influences have received little or no attention in the past. Nor has differential solvation of the different transition states been strongly emphasized. An anionic nucleophile, thiolate, gives very early transition states and negative activation enthalpies with activated aryl halides. The element effect is not established for these reactions. We suggest that the leaving group order in the gas phase will be dependent on the exact combination of nucleophile, leaving group, and substrate framework. The geometry of the S(N)Ar transition state permits useful, qualitative conceptual distinctions to be made between this reaction and other modes of nucleophilic attack.

An Outer Membrane Protein Undergoes Enthalpy- and Entropy-Driven Transitions

β-Barrel membrane proteins often fluctuate among various open substates, yet the nature of these transitions is not fully understood. Using temperature-dependent, single-molecule electrophysiology analysis, along with rational protein design, we show that OccK1, a member of the outer membrane carboxylate channel from Pseudomonas aeruginosa, features a discrete gating dynamics comprising both enthalpy-driven and entropy-driven current transitions. OccK1 was chosen for the analysis of these transitions, because it is a monomeric transmembrane β-barrel of a known high-resolution crystal structure and displays three distinguishable, time-resolvable open substates. Native and loop-deletion OccK1 proteins showed substantial changes in the activation enthalpies and entropies of the channel transitions, but modest alterations in the equilibrium free energies, confirming that the system never departs from equilibrium. Moreover, some current fluctuations of OccK1 indicated a counterintuitive, negative activation enthalpy, which was compensated by a significant decrease in the activation entropy. Temperature scanning of the single-channel properties of OccK1 exhibited a thermally induced switch of the energetically most favorable open substate at the lowest examined temperature of 4 °C. Therefore, such a semiquantitative assessment of the current fluctuation dynamics not only demonstrates the complexity of channel gating but also reveals distinct functional traits of a β-barrel outer membrane protein under different temperature circumstances.

Tropospheric degradation of ethylene glycol monovinyl and divinyl ethers: A mechanistic and kinetic study

AbstractIn this work, we have performed a theoretical study on the mechanisms and kinetics of the •OH initiated oxidation of selected ethylene glycol vinyl ethers, using quantum chemistry and computational kinetics methods. We have found that the main reaction path is the •OH‐addition to vinyl double bonds, and especially to the vinyl terminal carbon atom. Branching ratios for addition at C1 are larger than 94%. Although divinyl ethers could be expected to react approximately twice as fast as monovinyl ethers, it was found that, in fact, •OH rate constants of monovinyl and divinyl ethers are very similar, in agreement with experimental results. This effect is attributed to the sharing of the oxygen lone pairs effect, when the molecule contains two vinyl groups. Thus, in EGMVE, the vinyl group is more reactive than the one in divinyl ethers. The energy of its highest occupied molecular orbital (HOMO) is closer to that of the OH radical, thus favoring the interaction with the single occupied molecular orbital (SOMO) of the radical. Negative activation enthalpies are obtained for the three molecules studied, implying that the rate constant decreases as temperature increases. This anti‐Arrhenius behavior is commonly observed in •OH addition to alkenes. An excellent correlation is observed between experimental and calculated rate constants. © 2012 Wiley Periodicals, Inc.

Influence of the methylation degree on the rate constants of the •OH addition to alkenes and its temperature dependence

AbstractThe increase of the rate constants with the degree of methylation and the negative temperature dependence in the·OH + alkenes reactions has been investigated using the CCSD(T)/6‐311++G(2d,2p)//BHandHLYP/aug‐cc‐pVTZ potential energy surface and the canonical transition state theory. Our results show that the pre‐exponential factor has positive temperature dependence for all the studied reactions. The experimental finding that the reaction rates augment as temperature decreases and the methylation degree increases is explained by the effective negative enthalpy of activation due to a complex mechanism, as first proposed by Singleton and Cvetanovic. The excellent agreement between the calculated and experimental rate constants at 298.15 K strongly supports our conclusions. © 2012 Wiley Periodicals, Inc.

Excited State Dynamics and Activation Parameters of Remarkably Slow Photoinduced CO Loss from (η6-Benzene)Cr(CO)3 in n-Heptane Solution: A DFT and Picosecond-Time-Resolved Infrared Study

The electronic structure of (η⁶-benzene)Cr(CO)₃ has been calculated using density functional theory and a molecular orbital interaction diagram constructed based on the Cr(CO)₃ and benzene fragments. The highest occupied molecular orbitals are mainly metal based. The nature of the lowest energy excited states were determined by time-dependent density functional theory, and the lowest energy excited state was found to have significant metal to carbonyl charge transfer character. The photochemistry of (η⁶-benzene)Cr(CO)₃ was investigated by time-resolved infrared spectroscopy with picosecond time resolution. The low energy excited state was detected following irradiation at 400 nm, and this exhibited ν(CO) bands at lower energy than the equivalent ν(CO) bands of (η⁶-benzene)Cr(CO)₃, consistent with metal to carbonyl charge transfer character, and is formed with excess vibrational energy, relaxing to the v = 0 vibrational state within 3 ps. The resulting "cold" excited state decays to form the CO-loss species (η⁶-benzene)Cr(CO)₂ in approximately 70% yield and to reform (η⁶-benzene)Cr(CO)₃ within 150 ps. The rates of relaxation from the vibrationally hot state to the cold excited state and its subsequent reaction to yield (η⁶-benzene)Cr(CO)₂ were measured over a range of temperatures from 274 to 320 K, and the activation parameters for both processes were obtained from Eyring plots. The vibrational relaxation exhibits a negative activation enthalpy ΔH(‡) (-10 (±4) kJ mol⁻¹) and a negative activation entropy ΔS(‡) (-50 (±16) J mol⁻¹ K⁻¹). A significant barrier (ΔH(‡) = +12 (±4) kJ mol⁻¹) was obtained for the formation of (η⁶-benzene)Cr(CO)₂ with a ΔS(‡) value close to zero. These data are used to propose a model for the CO-loss process to yield (η⁶-benzene)Cr(CO)₂ and to explain why low temperature irradiation of (η⁶-benzene)Cr(CO)₃ with light of wavelengths greater than 400 nm produced relatively minor amounts of (η⁶-benzene)Cr(CO)₂.