Entropic Effects on the Aqueous Microsolvation of Protonated Glycine and Protonated ß-Alanine. Hybrid Density Functional Theory Born-Oppenheimer Molecular Dynamics Studies.

Recent low-temperature infrared-based experimental studies provided information about the effects of aqueous microsolvation on the intramolecular hydrogen bond of protonated glycine and ß-alanine [J. Phys. Chem. A 2019, 123, 3355]. Here we address the temperature-dependent entropic effects on the aqueous microsolvation patterns of these protonated amino acids using the AAH+(H2O)n (n = 1-8) cluster model at 50 K and room temperature with Born-Oppenheimer molecular dynamics using a calibrated hybrid density functional. The CCOOH-Ow, N-Ow, and center-of-mass-Ow radial distribution functions provide accurate structural data and temperature-dependent water coordination numbers vs. solvation degree. The solvation patterns for protonated glycine at 50 K show structural features in agreement with previous static optimizations. However, entropic effects at room temperature play a crucial role in the evolution of the intramolecular HB strength vs. solvation degree for both protonated amino acids. With increasing hydration entropic effects favor the making of solvent hydrogen bond networks over full solvation of protonated glycine. At room temperature four water molecules are needed to build the first solvation shell for protonated glycine while five are required for protonated ß-alanine. A new statistical Cumulative Percentage of Structures (CPS) scheme is proposed; when the CPS data are analyzed in light of the empirical formula of Rozenberg et al. [Phys. Chem. Chem. Phys. 2000, 2, 2699] and the hydrogen bond relative strength (HBRS) criteria of Jeffrey [An Introduction to Hydrogen Bonding; Oxford University: 1997] we can provide a detailed molecular mechanism for the weakening of the intramolecular hydrogen bond based on the average dynamical structures, which clearly reveals the temperature dependence of this process. The new CPS-HBRS scheme proposed here can be utilized using any type of molecular dynamics trajectory (classical, BOMD, CPMD, etc.).

Ammonia Solvation vs Aqueous Solvation of Samarium Diiodide. A Theoretical and Experimental Approach to Understanding Bond Activation Upon Coordination to Sm(II).

Coordination-induced desolvation or ligand displacement by cosolvents and additives is a key feature responsible for the reactivity of Sm(II)-based reagent systems. High-affinity proton donor cosolvents such as water and glycols also demonstrate coordination-induced bond weakening of the O-H bond, facilitating reduction of a broad range of substrates. In the present work, the coordination of ammonia to SmI2 was examined using Born-Oppenheimer molecular dynamics simulations and mechanistic studies, and the SmI2-ammonia system is compared to the SmI2-water system. The coordination number and reactivity of the SmI2-ammonia solvent system were found to be similar to those of SmI2-water but exhibited an order of magnitude greater rate of arene reduction by SmI2-ammonia than by SmI2-water at the same concentrations of cosolvent. In addition, upon coordination of ammonia to SmI2, the Sm(II)-ammonia solvate demonstrates one of the largest degrees of N-H bond weakening reported in the literature compared to known low-valent transition metal ammonia complexes.

Theoretical determination of half-wave potentials for phenanthroline-, bipyridine-, acetylacetonate-, and glycinate-containing copper (II) complexes.

We report a protocol for the evaluation of theoretical half-wave potential (E1/2) using a set of 22 mixed chelate copper (II) complexes containing 1,10-phenanthroline and 2,2'-bipyridine derivatives as primary ligands, and acetylacetonate or glycinate as secondary ligands (formally from the Casiopeínas® family) for which accurate experimental values were determined in a 2/5 mixture of ethanol/water. We have calibrated the BP86, PBE, PBE0, B3LYP, M06-2X, and ω-B97XD functionals, using the Los Alamos LANL2DZ and Stuttgart-Köln SDDAll effective core potentials for the Cu and Fe atoms and the 6-311+G* basis set for the C, H, O, and N atoms. To address the solvent effects, we have saturated the first solvation shell with up to 9 water molecules for the explicit model and compared it with the Continuum Like-Polarizable Continuum Model (CPCM) implicit solvent scheme. We found that the PBE/LANL2DZ-6-311+G* protocol (with the CPCM implicit solvent scheme with an effective dielectric constant ε = 64.9121 for the 2/5 mixture of ethanol/water) yields the overall best performance. The theoretical values are compared with experimental data, three of which are reported here for the first time. We find good correlations between the theoretical and experimental E1/2 values for the 2,2'-bipyridine derivatives (R2 = 0.987, MAE = 86 mV) and 1,10-phenanthroline derivatives (R2 = 0.802, MAE = 58.4 mV). The correlation trends have been explained in terms of the copper atom's ability to be reduced in the presence of the ligands. The Gibbs free energy differences at 298 K obtained for the redox reactions show that the more flexible secondary ligands (acetylacetonate) lead to larger entropic contributions which, as expected, increase the average MAE values as compared with the more rigid ligands (glycine). The present protocol yields lower MAEs as compared with previous approaches for similar mixed and flexible Cu(II) complexes.

Proton donor effects on the reactivity of SmI2. Experimental and theoretical studies on methanol solvation vs. aqueous solvation.

Proton donors are important components of many reactions mediated by samarium diiodide (SmI2). The addition of water to SmI2 creates a reagent system that enables the reduction of challenging substrates through proton-coupled electron-transfer (PCET). Simple alcohols such as methanol are often used successfully in reductions with SmI2 but often have reduced reactivity. The basis for the change in reactivity of SmI2-H2O and SmI2-MeOH is not apparent given the modest differences between water and methanol. A combination of Born-Oppenheimer molecular dynamics simulations and mechanistic experiments were performed to examine the differences between the reductants formed in situ for the SmI2-H2O and SmI2-MeOH systems. This work demonstrates that reduced coordination of MeOH to Sm(ii) results in a complex that reduces arenes through a sequential electron proton transfer at low concentrations and that this process is significantly slower than reduction by SmI2-H2O.

Solving the CH_{4}

Several types of experiments showed the existence of negative methane ions CH_{4}^{-} over a period of 50 years but the nature of this elusive species remains unknown. A benchmark study has shown that the experimentally observed species cannot be described by the attachment of an electron in the doublet ground state of CH_{4}^{-}. Here we find CH_{4}^{-} as being a metastable species in its lowest quartet spin state, a CH_{2}^{-}:H_{2} exciplex with three open shells lying ca. 10 eV above the methane singlet ground state but slightly below the dissociation fragments. The formation of charged high-spin exciplexes is a novel mechanism to explain small molecular anions with implications in a plethora of basic and applied research fields.

Experimental and Theoretical Studies on the Aqueous Solvation and Reactivity of SmCl2 and Comparison with SmBr2 and SmI2.

Water addition to Sm(II) has been shown to increase reactivity for both SmI2 and SmBr2. Previous work in our groups has demonstrated that this increase in reactivity can be attributed to coordination induced bond weakening enabling substrate reduction through proton-coupled electron transfer. The present work examines the interaction of water with samarium dichloride (SmCl2) and illustrates the importance of the Sm-X interaction and bond distance upon water addition critical for the reactivity of the reagent system. Born-Oppenheimer molecular dynamics simulations identify substantial variations among the reductants created in solution upon water addition to SmI2, SmBr2, and SmCl2 with the latter showing the least halide dissociation. This results in a lower water coordination number for SmCl2, creating a more powerful reducing system. As previously shown with the other SmX2-water systems, coordination-induced bond-weakening of the O-H bond of water bound to Sm(II) results in significant bond weakening. In the case of SmCl2, the bond weakening is estimated to be in the range of 83 to 88.5 kcal/mol.

Bi- and tridentate stannylphosphines and their coordination to low-valent platinum.

Semirigid bifunctional tin-substituted o-tolylphosphines of general formulae [Ph2P(o-C6H4CH2)SnR3] (R = Ph, 1; R = Me, 2) and [{Ph2P(o-C6H4CH2)}2SnPh2] (3) were synthesized and isolated in good yields. The new compounds were fully characterized by single-crystal X-ray diffraction and multinuclear solution NMR spectroscopic techniques. The observed J(119Sn,31P) values in solution NMR spectroscopy as well as the PSn distances in the solid state and DFT calculations (B3LYP) on compounds 1 and 3 do not support the existence of intramolecular P → Sn bond interactions in either of the three compounds. 1 and 2 reacted with stoichiometric amounts of tristriphenylphosphine platinum(0) [Pt(PPh3)3] under toluene refluxing conditions leading to formation of Pt(ii) distorted square-planar complexes [Ph2P(o-C6H4CH2)Pt(SnR3)(PPh3)], (R = Ph, 4; R = Me, 5), each bearing a five-membered carbometallated ring resulting from Pt coordination to P and the benzylic C sp3 atom of the ligand architecture rather than from activation of the terminal Sn-C carbon bonds of the phenyl or methyl substituents which would have rendered six-membered rings. Additionally, the fragment SnR3 also binds to the metal centre disposing cis to the cyclometalated carbon atom and to the single remaining PPh3. This carbometallation takes place affecting the integrity of the ligand skeleton. NBO calculations show the Sn fragment coordinates to the metal as X-type stannyl, SnR3. The analogous reaction of [Pt(PPh3)3] towards the stannyldiphosphine 3 leads to the quantitative formation of complex [(Ph2P-o-C6H4CH2)Pt(Ph2P-o-C6H4CH2SnPh3)], 6, which exhibits five- and six-membered metallacycles at the expense of the ligand frame. All compounds were characterized exhaustively by solution spectroscopic measurements and by single crystal X-ray diffraction analysis. DFT computations corroborate the higher stability of the observed products over those resulting from preservation of the ligand backbone.

Symmetric dissociation of the water molecule with truncation energy error. A benchmark study.

We use selected configuration interaction with truncation energy error (SCI-TEE) and CI by parts (CIBP) to study the symmetric dissociation of the water molecule with Roos' triple-ζ double polarization basis set and with the Dunning cc-pVTZ basis. The calculations comprise CISDTQ (CI-4x) through CI-8x for H2O at its equilibrium geometry (Req) and up to fifteen times Req. With the Dunning basis our SCI-TEE-8x energies differ from full CI by less than 0.01 mHartree (0.006 kcal mol-1) at all O-H distances, representing the best upper bounds for this system outside Req. We compare our results with those of other relevant ab initio methods finding good agreement with recent DMRG calculations. The non-parallelity error (NPE) for SCI-TEE-6x remains stable below 0.1 mHartree when moving from the Roos to the Dunning orbitals. For the present system, CBS energy errors at the experimental equilibrium geometry and at dissociation can accurately be evaluated as the difference between non-relativistic total electronic energies taken from the literature, and our SCI-TEE-8x energies obtained with Dunning's or Roos' orbitals. In both cases, the difference between CBS energy errors at the equilibrium geometry and dissociation is not smaller than 10 mH, showing that chemically accurate NPE values do not guarantee a chemically accurate potential energy surface.

Experimental and Theoretical Studies on the Implications of Halide-Dependent Aqueous Solvation of Sm(II).

The addition of water to samarium(II) has been demonstrated to have a significant impact on the reduction of organic substrates, with the majority of research dedicated to the most widely used reagent, samarium diiodide (SmI2). The work presented herein focuses on the reducing capabilities of samarium dibromide (SmBr2) and demonstrates how the modest change in halide ligand results in observable mechanistic differences between the SmBr2-water and the SmI2-water systems that have considerable implications in terms of reactivity between the two reagents. Quantum chemical results from Born-Oppenheimer molecular dynamics simulations show significant differences between SmI2-water and SmBr2-water, with the latter displaying less dissociation of the halide, which results in a lower coordination number for water. Experimental results are consistent with computational results and demonstrate that the coordination sphere of SmBr2 is saturated at lower concentrations of water. In addition, coordination-induced bond-weakening of the O-H bond is demonstrably different for water bound to SmBr2, leading to an estimated O-H bond-weakening of at least 83 kcal/mol, nearly 10 kcal/mol larger than the bond-weakening observed in SmI2-H2O. Experimental results also demonstrate that the use of alcohols in place of water with SmBr2 leads to substrate reduction, albeit several orders of magnitude slower than for SmBr2-water. The difference in rates resulting from the change in proton donor is attributed to a rate-limiting proton-coupled electron transfer in SmBr2-water and a sequential electron transfer then proton transfer in SmBr2-alcohol systems, where electron transfer is rate-limiting.

Hydration of CH3HgOH and CH3HgCl compared to HgCl2, HgClOH, and Hg(OH)2: A DFT microsolvation cluster approach.

We address the aqueous microsolvation of the CH3HgCl and CH3HgOH molecules using a stepwise hydration scheme including up to 33 water molecules and compare our results with the previously studied HgCl2, HgClOH, and Hg(OH)2 complexes. Optimized geometries and Gibbs free energies were obtained at the B3PW91/aug-RECP(Hg)-6-31G(d,p) level. At least 33 water molecules were required to build the first solvation shell around both methylmercury compounds. Optimized geometries were found having favorable interactions of water molecules with Hg, Cl, and the OH moiety. Born-Oppenheimer molecular dynamics simulations were performed on the largest CH3HgX(X = Cl, OH)-(H2O)33 clusters at the same level of theory. Born-Oppenheimer molecular dynamics simulations at T = 300 K (ca. 0.62 kcal/mol) revealed the presence of configurations with hydrogen-bonded networks that include the OH moiety in CH3HgOH and exclude both the Hg and Cl in CH3HgCl, favoring a clathrate-type structure around the methyl moiety. The comparison to the microsolvated HgClOH, Hg(OH)2, and HgCl2 molecules showed that, in all cases, the water molecules easily move away from Cl, thus supporting the idea that HgCl2 behaves as a non-polar solute. The theoretical (LIII edge) X-ray absorption near edge structure spectra are obtained and found in good agreement with experimental data, especially for the CH3HgCl species.

Aqueous Solvation of SmI3: A Born-Oppenheimer Molecular Dynamics Density Functional Theory Cluster Approach.

We report the results of Born-Oppenheimer molecular dynamics (BOMD) simulations on the aqueous solvation of the SmI3 molecule and of the bare Sm3+ cation at room temperature using the cluster microsolvation approach including 37 and 29 water molecules, respectively. The electronic structure calculations were done using the M062X hybrid exchange-correlation functional in conjunction with the 6-31G** basis sets for oxygen and hydrogen. For the iodine and samarium atoms, the Stuttgart-Köln relativistic effective-core potentials were utilized with their associated valence basis sets. When SmI3 is embedded in the microsolvation environment, we find that substitution of the iodine ions by water molecules around Sm(III) cannot be achieved due to an insufficient number of explicit water molecules to fully solvate the four separate metal and halogen ions. Therefore, we studied the solvation dynamics of the bare Sm3+ cation with a 29-water molecule model cluster. Through the Sm-O radial distribution function and the evolution of the Sm-O distances, the present study yields a very tightly bound first rigid Sm(III) solvation shell from 2.3 to 2.9 Å whose integration leads to a coordination number of 9 water molecules and a second softer solvation sphere from 3.9 to 5 Å with 12 water molecules. No water exchange processes were found. The theoretical EXAFS spectrum is in excellent agreement with the experimental spectrum for Sm(III) in liquid water. The strong differences between the solvation patterns of Sm(III) vs Sm(II) are discussed in detail.

Spin Density Distribution in Open-Shell Transition Metal Systems: A Comparative Post-Hartree-Fock, Density Functional Theory, and Quantum Monte Carlo Study of the CuCl2 Molecule.

We present a comparative study of the spatial distribution of the spin density of the ground state of CuCl2 using Density Functional Theory (DFT), quantum Monte Carlo (QMC), and post-Hartree-Fock wave function theory (WFT). A number of studies have shown that an accurate description of the electronic structure of the lowest-lying states of this molecule is particularly challenging due to the interplay between the strong dynamical correlation effects in the 3d shell and the delocalization of the 3d hole over the chlorine atoms. More generally, this problem is representative of the difficulties encountered when studying open-shell metal-containing molecular systems. Here, it is shown that qualitatively different results for the spin density distribution are obtained from the various quantum-mechanical approaches. At the DFT level, the spin density distribution is found to be very dependent on the functional employed. At the QMC level, Fixed-Node Diffusion Monte Carlo (FN-DMC) results are strongly dependent on the nodal structure of the trial wave function. Regarding wave function methods, most approaches not including a very high amount of dynamic correlation effects lead to a much too high localization of the spin density on the copper atom, in sharp contrast with DFT. To shed some light on these conflicting results Full CI-type (FCI) calculations using the 6-31G basis set and based on a selection process of the most important determinants, the so-called CIPSI approach (Configuration Interaction with Perturbative Selection done Iteratively) are performed. Quite remarkably, it is found that for this 63-electron molecule and a full CI space including about 10(18) determinants, the FCI limit can almost be reached. Putting all results together, a natural and coherent picture for the spin distribution is proposed.

Electronic charge density analysis of Li-doped polyacetylene: molecular vs periodic descriptions and nature of Li-to-chain bonding.

A detailed analysis of the electronic structure and charge distribution around the trigonal site of Li-doped polyacetylene is reported using finite chain and periodic descriptions of the polymer. Atoms-in-molecules (AIM) analysis is done to characterize the nature of the bond between Li and the polymer backbone through the location of the bond critical points and computation of the total charge on the atomic basins around the doping site. We find that the Li atom donates practically one electron to the π-system, in accordance with the classical Su-Schriffer-Heeger model. However, despite that the Li atom is equidistant from the three closest C atoms in the geometric soliton, a single Li-C bond critical point is found. The AIM quantitative analysis of the electronic density reveals that the Li(+) ion is immersed into the polymer π-cloud in a way that resembles a metallic bonding interaction.

Theoretical study of the solvation of HgCl2, HgClOH, Hg(OH)2 and HgCl3(-): a density functional theory cluster approach.

The determination of the solvation shell of Hg(II)-containing molecules and especially the interaction between Hg(II) and water molecules is the first requirement to understand the transmembrane passage of Hg into the cell. We report a systematic DFT study by stepwise solvation of HgCl(2) including up to 24 water molecules. In order to include pH and salinity effects, the solvation patterns of HgClOH, Hg(OH)(2) and HgCl(3)(-) were also studied using 24 water molecules. In all cases the hydrogen bond network is crucial to allow orbital-driven interactions between Hg(II) and the water molecules. DFT Born-Oppenheimer molecular dynamics simulations starting from the stable HgCl(2)-(H(2)O)(24) structure revealed that an HgCl(2)-(H(2)O)(3) trigonal bipyramid effective solute appears and then the remaining 21 water molecules build a complete first solvation shell, in the form of a water-clathrate. In the HgCl(2), HgClOH, Hg(OH)(2)-(H(2)O)(24) optimized structures Hg also directly interacts with 3 water molecules from an orbital point of view (three Hg-O donor-acceptor type bonds). All the other interactions are through hydrogen bonding. The cluster-derived solvation energies of HgCl(2), HgClOH and Hg(OH)(2) are estimated to be -34.4, -40.1 and -47.2 kcal mol(-1), respectively.

Bond breaking and bond making in tetraoxygen: analysis of the O2(X3Sigma(g)-) + O2(X3Sigma(g)-) <==> O4 reaction using the electron pair localization function.

We study the nature of the electron pairing at the most important critical points of the singlet potential energy surface of the 2O2 <==> O4 reaction and its evolution along the reaction coordinate using the electron pair localization function (EPLF) [Scemama, A.; Chaquin, P.; Caffarel, M. J. Chem. Phys. 2004, 121, 1725]. To do that, the 3D topology of the EPLF calculated with quantum Monte Carlo (at both variational and fixed-node-diffusion Monte Carlo levels) using Hartree-Fock, multiconfigurational CASSCF, and explicitly correlated trial wave functions is analyzed. At the O4 equilibrium geometry the EPLF analysis reveals four equivalent covalent bonds and two lone pairs on each oxygen atom. Along the reaction path toward dissociation it is found that the two oxygen-oxygen bonds are not broken simultaneously but sequentially, and then the lone pairs are rearranged. In a more general perspective, the usefulness of the EPLF as a unique tool to analyze the topology of electron pairing in nontrivial chemical bonding situations as well as to visualize the major steps involved in chemical reactivity is emphasized. In contrast with most standard schemes to reveal electron localization (atoms in molecules, electron localization function, natural bond orbital, etc.), the newly introduced EPLF function gives a direct access to electron pairings in molecules.

Calculation of electric dipole (hyper)polarizabilities by long-range-correction scheme in density functional theory: a systematic assessment for polydiacetylene and polybutatriene oligomers.

The long-range correction (LC) for treating electron exchange in density functional theory, combined with the Becke-Lee-Yang-Parr (BLYP) exchange-correlation functional, was used to determine (hyper)polarizabilities of polydiacetylene/polybutatriene oligomers. In comparison with coupled-cluster calculations including single and double excitations as well as a perturbative treatment of triple excitations, our values indicate that the tendency of conventional functionals to result in a catastrophic overshoot for these properties is alleviated but not eliminated. No clear-cut preference for LC-BLYP over Hartree-Fock values is obtained. This analysis is consistent with the calculations of Sekino et al. [J. Chem. Phys. 126, 014107 (2007)] on polyacetylene and molecular hydrogen oligomers. Thus, the performance of LC-BLYP with regard to (hyper)polarizabilities of quasilinear conjugated systems is now well characterized.

Multireference quantum Monte Carlo study of the O4 molecule.

Fixed-node diffusion quantum Monte Carlo (FN-DMC) calculations are performed to obtain the most accurate dissociation barrier and heat of formation with respect to dissociation into molecular oxygen for the chemically bound tetraoxygen molecule. Multireference trial wave functions were used and built from truncated CASSCF(16,12) through a weight-consistent scheme allowing to control the fixed-node error. Results are compared with the previous ab initio benchmark Complete Active Space SCF Averaged Coupled Pair Functional/aug-cc-pVQZ (CASSCF-ACPF/AVQZ) results. The FN-DMC barriers to dissociation and heat of formation obtained are 11.6+/-1.6 kcal/mol and 98.5+/-1.9 kcal/mol, respectively. These thermochemical energies should be taken as the theoretical references when discussing the relevance of tetraoxygen in a variety of experiments and atmospheric chemical processes.

A new nonsymmetric as (OH)3 species. Comparison with the known C3 species and themochemistry at the HF, DFT(B3LYP), MP2, MP4, and CCSD(T) levels of theory.

A new nonsymmetric As(OH)(3) species that is more stable than the C(3) structure is found at HF, Density Functional Theory (B3LYP), MP2, MP4 and CCSD(T) levels with the Stuttgart RECP-basis for As and the aug-cc-pvdz/pvtz extended basis sets. Transition state (TS) geometries are close to the C(3) one. Energy differences and interconversion barriers become smaller with increasing inclusion of electronic correlation. However, for MP4 and CCSD(T) descriptions, these differences increase by more than 100% when basis set goes from the AVDZ to AVTZ quality. Zero point energy (ZPE) corrections are essential and have been taken into account at all levels of theory; although this leads to barrier collapse at the B3LYP, MP2, MP4 and CCSD(T) levels, the C(1) isomer remains more stable than the C(3) one. MP2/AVTZ infrared spectra are also given for the C(1) and C(3) isomers as guiding data for future IR studies in the gas phase.

Towards accurate all-electron quantum Monte Carlo calculations of transition-metal systems: spectroscopy of the copper atom.

In this work we present all-electron fixed-node diffusion Monte Carlo (FN-DMC) calculations of the low-lying electronic states of the copper atom and its cation. The states considered are those which are the most relevant for the organometallic chemistry of copper-containing systems, namely, the (2)S, (2)D, and (2)P electronic states of Cu and the (1)S ground state of Cu(+). We systematically compare our FN-DMC results to CCSD(T) calculations using very large atomic-natural-orbital-type all-electron basis sets. The FN-DMC results presented in this work provide, to the best of our knowledge, the most accurate nonrelativistic all-electron correlation energies for the lowest-lying states of copper and its cation. To compare our results to experimental data we include the relativistic contributions for all states through numerical Dirac-Fock calculations, which for copper (Z=29) provide almost the entire relativistic effects. It is found that the fixed-node errors using Hartree-Fock nodes for the lowest transition energies of copper and the first ionization potential of the atom cancel out within statistical fluctuations. The overall accuracy achieved with quantum Monte Carlo for the nonrelativistic correlation energy (statistical fluctuations of about 1600 cm(-1) and near cancelation of fixed-node errors) is good enough to reproduce the experimental spectrum when relativistic effects are included. These results illustrate that, despite the presence of the large statistical fluctuations associated with core electrons, accurate all-electron FN-DMC calculations for transition metals are nowadays feasible using extensive but accessible computer resources.

Systematic ab initio calculations on the energetics and stability of covalent O4.

Ab initio calculations with highly correlated methods together with extensive basis sets have been used to obtain the most accurate heat of formation and stability with respect to dissociation (into molecular oxygen) for the chemically bound tetraoxygen molecule. Our calculations show that the heat of formation is significantly smaller and that the barrier to dissociation is larger than previously assumed. In particular, we have shown that the previous theoretical estimate for the heat of formation of tetraoxygen was in error by a significant amount (18%-24%) owing to lack of accuracy in the theoretical method then used. Our best estimates places that value in the range 93-95 kcal/mol and this should be taken into consideration when discussing the possible relevance of tetraoxygen in a variety of experiments, as well as in the fundamental atmospheric chemical processes where oxygen species participate.