Heat Capacity Estimation Using a Complete Set of Homodesmotic Reactions for Organic Compounds

Reliable information about isobaric heat capacities CP is necessary to determine the energies of organic compounds and chemical processes at an arbitrary temperature. In this work, the possibility of theoretical estimation of CP by the homodesmotic method is analyzed. Three cases of CP calculation applying the methodology of the complete set of homodesmotic reactions (CS HDRs) are considered: the gas- and liquid-phase CP of organic compounds of various classes at 298 K (the mean absolute value of reaction heat capacity, MA ΔCP = 1.44 and 2.83 J/mol·K for the gas and liquid phase, correspondingly); and the gas-phase CP of n-alkanes C2–C10 in the temperature range of 200–1500 K with an average error in calculating the heat capacity of 0.93 J/mol·K. In the latter case, the coefficients of the Shomate equation are determined for all n-alkanes that satisfy the homodesmoticity condition. New values of gas- and liquid-phase heat capacities are obtained for 41 compounds. The CS HDRs-based approach for estimating the CP of organic compounds is characterized by high accuracy, which is not inferior to that of the best CP-additive schemes and allows us to analyze the reproducibility of the calculation results and eliminate unreliable reference data

Conformational Transformations in Aromatic Nitroso Oxides

A systematic theoretical study on conformational transformations of monosubstituted (ortho- and para-) aromatic nitroso oxides R-C₆H₄NOO was performed. The existence of two rotation axes enables two types of conformational transitions in substituted arylnitroso oxides: trans/cis (rotation around the N–O bond) and syn/anti (rotation around the C–N bond, which is important in ortho isomers). The complete set of conformers was localized for R-C₆H₄NOO using four selected density functional (M06-L, mPWPW91, OLYP, and HCTH) and augmented polarization basis set of triple splitting. It was found that the activation enthalpy of the trans-cis conformational transition is nearly insensitive to the nature of R and ranges within 58–60 kJ/mol for para isomers. The ortho substituent has an insignificant effect on Δ<i>H</i>^≠_trans→cis: it increases this value by ∼5 kJ/mol in syn isomers and decreases it by ∼3 kJ/mol in anti isomers. On the contrary, the syn-anti conformational barrier is considerably affected by the substituent R; an increase in the electron-withdrawing properties of R decreases Δ<i>H</i>^≠_syn→anti. The activation enthalpies grow with increasing polarity of the solvent, as it was found using IEFPCM calculation. The values of relaxation time for all conformational equilibria were calculated and compared with known lifetimes of aromatic nitroso oxides. Our results suggest that syn/anti transitions occur fast enough in the scale of the experimental lifetime. However, trans/cis transformations proceed more slowly. And under certain conditions discussed in the paper, the rate of this conformational transition limits that of irreversible decay of nitroso oxide

Theoretical Models for Quantitative Description of the Acid–Base Equilibria of the 5,6-Substituted Uracils

The acidities of 18 5,6-substituted uracils have been numerically estimated as p<i>K</i>_a values in terms of three theoretical models. The first scheme includes the calculation of the gas-phase acidity of uracil with the G3MP2B3 method and taking into account the solvent effect using the polarizable continuum approximation PCM­(SMD)-TPSS/aug-cc-pVTZ. The second model is one step and implies calculation of the free Gibbs energies of the hydrate complex of uracil (and its anion) with 5 water molecules by the TPSS/aug-cc-pVTZ method. This model accounts for the solvent effect corresponding to both specific and nonspecific solvation. The third scheme required high time and computational resources and includes the strong features of the two previous schemes. Here, the theoretical estimation of p<i>K</i>_a is performed by the CBS-QB3 composite method. As in the second approach, both specific (as pentahydrate) and nonspecific solvent effects are determined. We have analyzed the advantages and model restrictions of the considered schemes for the p<i>K</i>_a calculations. All models have systematic errors, which have been corrected with the linear empirical regression relations. In the presented model, the absolute mean deviations of the p<i>K</i>_a values of uracils dissociating via the N1–H bonds diminish to 0.25, 0.28, and 0.23 p<i>K</i>_a units (respectively, for I, II, and III models), which corresponds to ∼0.3 kcal/mol on the energy scale. The applicability of our computational schemes to uracils dissociating via N3–H, O–H (orotic acids) and C–H bonds is discussed

No Longer a Complex, Not Yet a Molecule: A Challenging Case of Nitrosyl <i>O</i>‑Hydroxide, HOON

HOON might be an elusive intermediate of atmospheric photochemical reactions of HONO or recombination of the parent nitrene HN and molecular oxygen. However, no reliable data on HOON structure and stability are available, and the nature of the O–O bond is not well understood. In this study, we used high-level single- [CCSD­(T) and, CCSDTQ] and multireference [CASPT2, MR-AQCC] ab initio calculations to determine properties of HOON: geometry, harmonic and anharmonic vibrational frequencies, thermodynamic stability, and electronic structure. HOON has bonding minima only in the ¹A′ electronic state that correspond to cis- and trans-conformers; <i>trans</i>-HOON is more stable by 6.4–8.5 kJ/mol. The O–O bond in <i>trans</i>-HOON is unusually long, <i>R</i>(O–O) = 1.89 Å, and weak, <i>D</i>(O–O) = 33.3 kJ/mol; however, <i>trans</i>-HOON might be stable enough to be identified in cryogenic matrices. Though the electronic structure of the NO moiety in HOON most resembles nitric oxide, some nitrene character as well nitrosyl cation character are also important; therefore, the current name of HOON, hydroperoxynitrene, is misleading; instead, we propose the name “nitrosyl <i>O</i>-hydroxide” or “isonitrosyl hydroxide”

Nature of Lewis Base Catalysis of 1,3-Dipolar Cycloaddition of Methyl Diazoacetate to Methyl Acrylate; NMR Kinetic Spectroscopy and DFT Study

The effect of Lewis base (LB) in the domino reaction between methyl diazoacetate and methyl acrylate has been studied. This domino process is initialized by a [3+2]-cycloaddition reaction to generate 3<i>H</i>-pyrazoline followed by a subsequent 1,3-H shift reaction forming 1<i>H</i>-pyrazoline as the more stable isomer. The rate of the first step is not sensitive to the presence of LBs (THF, Py, DMAP, DBU, and triphenylphosphine) as it was evidenced by kinetic nuclear magnetic resonance spectroscopy and quantum chemical modeling. LBs manifest remarkable catalytic effect on the second step of the reaction only acting as proton acceptor. DFT calculations reveal fine correlation between enthalpy of proton transfer from <i>trans</i>-3<i>H</i>-pyrazoline to LB and basic strength of the latter described in terms of BF₃-affinity scale. Under conditions of LB catalysis the reaction rate of the first step (methyl diazoacetate and methyl acrylate interaction) limits the rate of 3<i>H</i> → 1<i>H</i> pyrazoline isomerization and, therefore, restricts catalytic efficiency of LB. An alternative mechanism for catalysis of the 1,3-dipolar cycloaddition through formation of triazene-like intermediate during reaction of diazo acetate with LB and following Michael addition of intermediate to alkene was carefully analyzed. This reaction scheme was not confirmed in our experiments

Optical Configuration Effect on the Structure and Reactivity of Diastereomers Revealed by Spin Effects and Molecular Dynamics Calculations

The peculiarities of spin effects in photoinduced electron transfer (ET) in diastereomers of donor-acceptor dyads are considered in order to study the influence of chirality on reactivity. Thus, the spin selectivity&mdash;the difference between the enhancement coefficients of chemically induced dynamic nuclear polarization (CIDNP)&mdash;of the dyad&rsquo;s diastereomers reflects the difference in the spin density distribution in its paramagnetic precursors that appears upon UV irradiation. In addition, the CIDNP coefficient itself has demonstrated a high sensitivity to the change of chiral centers: when one center is changed, the hyperpolarization of all polarized nuclei of the molecule is affected. The article analyzes the experimental values of spin selectivity based on CIDNP calculations and molecular dynamic modeling data in order to reveal the effect of optical configuration on the structure and reactivity of diastereomers. In this way, we succeeded in tracing the differences in dyads with L- and D-tryptophan as an electron donor. Since the replacement of L-amino acid with D-analog in specific proteins is believed to be the cause of Alzheimer&rsquo;s and Parkinson&rsquo;s diseases, spin effects and molecular dynamic simulation in model dyads can be a useful tool for investigating the nature of this phenomenon