Probing Antioxidant-Related Properties for Phenolic Compounds.

In this work, properties related to antioxidant-potential mechanisms (such as the bond dissociation enthalpy, BDE, for the homolytic cleavage of the O-H bond and ionization energies, IEs) were determined for phenol, pyrocatechol, and gallic acid (GA). Both the protonated and deprotonated forms of GA were investigated. The Feller-Peterson-Dixon (FPD) composite method was employed with a variety of computational approaches, i.e., density functional theory, Möller-Plesset perturbation theory, and coupled-cluster-based methods, in combination with large correlation consistent basis sets with extrapolation to the complete basis set limit and consideration of core electron correlation effects. FPD results were compared to experimental and computational data available in the literature, presenting good agreement. For example, the FPD BDE (298 K) obtained for phenol, which was based on valence-correlated MP2/CBS calculations with contributions from correlating all electrons, was determined to be 87.56 kcal/mol, a value that is 0.42 kcal/mol lower than the result obtained in the most recent experiments, 87.98 ± 0.62. Calibration against coupled-cluster calculations was also carried out for phenol. We expect that the outcomes gathered here may help in establishing a general protocol for computational chemists that are interested in determining antioxidant-related properties for phenolic compounds with considerable accuracy as well as to motivate future IE measurements (particularly for GA) to be accomplished in the near future.

A high level theory investigation on the lowest-lying ionization potentials of glycine (NH2CH2COOH).

In this work, an investigation on the ionization potentials (IPs) of the glycine molecule (NH2CH2COOH) is presented. IPs ranging up to ∼20 eV were probed for each of the six conformations considered, with the referred threshold being chosen based on both: (i) the observations by recent photoelectron-photoion coincidence (PEPICO) experiments and (ii) the energy range of relevance to the modeling of other photo-induced processes (e.g., photoionization). For computing the IPs, the equation-of-motion ionization potential coupled-cluster with single and double excitations method (EOMIP-CCSD) was employed with large correlation consistent aug-cc-pVXZ and aug-cc-pCVXZ (X = D, T, and Q) basis sets. Extrapolation to the complete basis set limit and consideration of core electron correlation effects were also taken into account. Subsequently, the Feller-Peterson-Dixon (FPD) approach was used for considering all the contributions and to obtain accurate IPs. In addition, coupled-cluster with single and double excitations as well as perturbative triples, CCSD(T), was also used with the aug-cc-pVTZ basis set. When compared to each other, results obtained through the use of these approaches yielded excellent agreement. In general, the outcomes from the present work provide additional information to the insights gathered from the recent PEPICO experiments as well as accurate IPs for all 6 conformations of glycine using an approach based on high levels of theory. Hence, it is expected that other investigations focusing on photo-induced processes originating from NH2CH2COOH (for instance, the computational modeling of its photoionization) will be motivated for study in the future.

Ionization potentials for the H2CO trimer.

In this work, a computational study on the ionization potentials (IPs) of the formaldehyde trimer, (H2CO)3, is presented. Twelve lowest-lying vertical IPs were determined through the use of the coupled-cluster level of theory using correlation consistent basis sets with extrapolation to the complete basis set limit and consideration of core electron correlation effects. Specifically, the equation-of-motion ionization potential coupled-cluster with single and double excitations method with the aug-cc-pVnZ and aug-cc-pCVnZ (n = D and T) basis sets was used. The Feller-Peterson-Dixon (FPD) composite approach was employed to provide accurate IPs, and eight conformations of (H2CO)3 were considered. The FPD IPs determined for (H2CO)3 were found to be systematically lower than those computed for the dimer and monomer of H2CO in the pattern IP(monomer) > IP(dimer) > IP(trimer) for a given IP. In addition, the IPs calculated when considering only the more stable conformation (C0) are in good agreement with those obtained using the eight conformations of the H2CO trimer, and thus, the actual conformation played only a minor role in determining such properties in the present case. By providing first accurate IP results for the H2CO trimer, we hope to motivate future experimental and computational investigations (e.g., studies involving photoionization) that rely on such quantities.

Benchmarking Antioxidant-Related Properties for Gallic Acid through the Use of DFT, MP2, CCSD, and CCSD(T) Approaches.

We present a benchmark investigation on the O-H bond dissociation enthalpies (BDEs) and ionization potential (IP) for gallic acid (GA), a widely known polyphenolic antioxidant. These properties were determined in the gas-phase and in water through the use of density functional theory (DFT), second-order Møller-Plesset perturbation theory (MP2), coupled-cluster with single and double excitations (CCSD), and coupled-cluster with single and double excitations as well as perturbative inclusion of triples (CCSD(T)). The 6-311++G(df,p), cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, and aug-cc-pVTZ basis sets were used. Regarding DFT functionals, the M06-2X provided the best agreement for the BDEs when compared to the corresponding CCSD(T)/aug-cc-pVTZ results; M06-2X was also found to be the most suitable for probing the IP for the protonated forms of GA while LC-ωPBE was the most reliable in the case of deprotonated GA. Given that these properties represent important descriptors for examining mechanisms related to the antioxidant potential of a given polyphenol, we hope that the present work can serve as a guide for computational chemists venturing in the field.

Probing the ionization potentials of the formaldehyde dimer.

In this work, we present a computational investigation on the ionization potentials (IPs) of the formaldehyde dimer, (H2CO)2. Twelve lowest lying IPs (corresponding to the entire valence orbitals) for both C2h and Cs symmetry conformers have been computed at the coupled cluster level of theory using large correlation consistent basis sets with extrapolation to the complete basis set limit and consideration of core electron correlation effects. Specifically, the equation-of-motion ionization potential coupled-cluster with single and double (EOMIP-CCSD) excitations method with the aug-cc-pVXZ and aug-cc-pCVXZ (X = T, Q, and 5) basis sets combined with the Feller-Peterson-Dixon approach was employed, as well as CCSD with perturbative triples [CCSD(T)] with the aug-cc-pVTZ basis sets. In general, excellent agreement was observed from the comparison between the results obtained through the use of these approaches. In addition, the IPs for the formaldehyde monomer were also obtained using such methodologies and the results compared with existing experimental data; excellent agreement was also observed in this case. To the best of our knowledge, this work represents the first of its kind to determine the IPs for all these systems using a high level theory approach and is presented to motivate experimental investigations, e.g., studies involving photoionization, particularly for the formaldehyde dimer. The equilibrium binding energy of the C2h dimer is calculated in this work at the CCSD(T)/aug-cc-pVTZ level of theory to be -4.71 kcal/mol. At this same level of theory, the equilibrium isomerization energy between C2h and Cs conformers is 0.76 kcal/mol (Cs conformer being more stable).

Thermal rate constants for the O(3P) + HBr and O(3P) + DBr reactions: transition-state theory and quantum mechanical calculations.

The O((3)P) + HBr → OH + Br and O((3)P) + DBr → OD + Br reactions are studied on a recent high-quality ab initio-based potential energy surface. Thermal rate constants over the 200-1000 K temperature range, calculated using variational transition-state theory (VTST) with the small-curvature tunneling (SCT) correction and quantum mechanical methods with the J-shifting approximation (QM/JS) for zero total angular momentum (J = 0), are reported. These results are compared to the available experimental data, which lie in the ranges of 221-554 and 295-419 K for O + HBr and O + DBr, respectively. The rate constants, in cm(3) molecule(-1) s(-1) and at 298 K, for the O + HBr reaction are 3.66 × 10(-14) for VTST, 3.80 × 10(-14) for QM/JS, and 3.66 × 10(-14) for the average of eight experimental measurements.

Accurate ab initio potential energy surfaces for the 3A'' and 3A' electronic states of the O(3P)+HBr system.

In this work, we report the construction of potential energy surfaces for the (3)A('') and (3)A(') states of the system O((3)P) + HBr. These surfaces are based on extensive ab initio calculations employing the MRCI+Q/CBS+SO level of theory. The complete basis set energies were estimated from extrapolation of MRCI+Q/aug-cc-VnZ(-PP) (n = Q, 5) results and corrections due to spin-orbit effects obtained at the CASSCF/aug-cc-pVTZ(-PP) level of theory. These energies, calculated over a region of the configuration space relevant to the study of the reaction O((3)P) + HBr → OH + Br, were used to generate functions based on the many-body expansion. The three-body potentials were interpolated using the reproducing kernel Hilbert space method. The resulting surface for the (3)A('') electronic state contains van der Waals minima on the entrance and exit channels and a transition state 6.55 kcal/mol higher than the reactants. This barrier height was then scaled to reproduce the value of 5.01 kcal/mol, which was estimated from coupled cluster benchmark calculations performed to include high-order and core-valence correlation, as well as scalar relativistic effects. The (3)A(') surface was also scaled, based on the fact that in the collinear saddle point geometry these two electronic states are degenerate. The vibrationally adiabatic barrier heights are 3.44 kcal/mol for the (3)A('') and 4.16 kcal/mol for the (3)A(') state.