Bond Energy Scheme for Estimating Heats of Formation of Monomers and Polymers. VI. Sulfur Compounds

The bond energy scheme is extended to sulfur compounds and heats of formation and atomization energy terms derived from thermochemical data reviewed to 1977, for bonds of sulfur with carbon, hydrogen, halogens, and oxygen atoms. A precision of ± 1 kcal/mole was attainable for the covalent bonds of divalent sulfur in the lowest oxidation state S(± II). The higher valency states: S(IV) and S(VI) involve polar contributions depending upon the electrouegativity of the combining atom as well as (d^π -p^π) orbital promotion energies which are specific to the compound and transferable to other molecules only with a limited precision, no better than about ± 3 kcal/mole. The atomization energy terms (E_a ^25°C) of various bonds of sulfur a are found consistent with the experimental bond dissociation energies and bear a relationship with bond lengths and force constants as observed in the previous work. Heats of polymer-forming reactions and heats of formation of sulfur-containing monomers and polymers are estimated from the newly derived bond energy terms.     

Ab initio computation of force constants: III. A simple procedure for the evaluation of X-H bond dissociation energies

A simple expression derived from the Morse potential allows X-H bond dissociation energies to be estimated from the quadratic stretching force constants obtained in ab initio computations. Agreement with experiment is excellent for C-H bonds, except when the radical is stabilized by π-conjugative effects, in which case the bond dissociation energy is overestimated. The procedure also fails to estimate ionic dissociation energies.     

Rationale for the acidity of Meldrum's acid. Consistent relation of C-H acidities to the properties of localized reactive orbital

Detailed investigation on the origin of the acidity of the alpha-protons of a set of the carbonyl molecules was carried out on the basis of properties of the localized molecular orbital. An anomalously high acidity of Meldrum's acid, as compared with those of dimedone and dimethyl malonate, is one of the well-known but unresolved issues. The well-localized sigma orbitals of the C-H bonds at the alpha-position of the carbonyl groups can be obtained with the reactive hybrid orbital (RHO) theory. We found that the energy levels of the unoccupied RHOs of the C-H moiety of Meldrum's acid and other carbonyl compounds showed a good linear correlation with the experimental deprotonation energies. This is probably because the deprotonation reaction to form the proposed naked anions in a polar solvent is a highly endothermic process, in which the thermodynamic energy differences between the neutral molecules and the corresponding anions approximately coincide with the activation energies. We also investigated the effect of the conformational change upon deprotonation on the electron-accepting energy level of the relevant C-H bonds of cyclic/acyclic and monocarbonyl/dicarbonyl compounds. A conformational change occurs in the cases of cyclic six-membered compounds, but its influence on the reactivity of the C-H bond is small. The acidity of dicarbonyl compounds, including Meldrum's acid, showed a good correlation with the deviations from the perpendicular position of the dihedral angles of the relevant C-H bond with respect to the adjacent carbonyl C=O bond. This angle parameter can be related to the magnitude of the in-phase orbital interaction between the sigma(CH) and pi(C)(=)(O) orbitals, which facilitate electron acceptance. These results indicated that the acidity of the alpha-proton of carbonyl compounds can be represented in terms of the electron-accepting orbital levels of the unoccupied RHO of the C-H moiety. All the linear relationships found in the present work strongly suggested that the acidity of Meldrum's acid, which is conventionally regarded as an anomaly, is consistent with those of the other carbonyl compounds.     

Free radical abstraction of H atom from peroxides with concerted dissociation of O-O bond

Quantum chemical calculations of the dissociation energy of the C-H bond in the ??-hydroperoxide fragment of Me₂CHOOH were carried out. It was shown that abstraction of H atom is accompanied by dissociation of the O-O bond. Density functional calculations of transition states of the reactions of ^·CH₃, CH₃OO^·, and HO₂ ^· radicals with the C-H bond in the ??-hydroperoxide fragment of Me₂CHOOH were carried out. It was established that H atom abstraction is accompanied by concerted dissociation of the O-O bond. For 45 peroxides R¹R²CHOOH, R¹R²CHOOR³, and R¹R²CHOOC(O)R³ (R¹, R² = H, Me, Et, Ph, H₂C=CH), the enthalpies of H atom abstraction from the C-H bond in the a-hydroperoxide fragment with fragmentation of the peroxides at the O-O bond were calculated. The kinetic parameters for 12 classes of radical abstraction reactions with fragmentation of molecules were calculated from experimental data within the framework of the model of intersecting parabolas. The activation energies and reaction rate constants of H atom abstraction from C-H bonds of a-peroxide fragments involving peroxyl and alkyl radicals were determined for 45 peroxides of different structure.     

Hydrogen bonding of water, hydrogen sulphide and related acceptors with electron donors

Properties of hydrogen bonds formed by 1:1 interaction of H₂O with oxygen, nitrogen, sulphur and other electron donors have been evaluated by extended Hückel and CNDO methods and the results are discussed in relation to the experimental data. A detailed analysis of the variation of the dissociation energies and charge densities with bond distances is presented for the amine-water system. 1:2 complexes of water with donors are found to contain weaker hydrogen bonds than 1:1 complexes. Results of molecular orbital calculations on the hydrogen bonding of H₂S and CH₃SH with some donors are presented. The theoretical value of hydrogen bond dissociation energy varies linearly with the overlap population, and stretching force constant of the hydrogen bond as well as with the experimental O—H frequency shift.     

Simulant molecules with trivalent or pentavalent phosphorus atoms: bond dissociation energies and other thermodynamic and structural properties from quantum chemical models

The CBS-QB3 and G4 thermochemical models have been used to generate energetic, structural, and spectroscopic data on a set of molecules with trivalent or pentavalent phosphorus atoms that can serve as simulants of chemical warfare agents. Based on structural data, the conformational stabilities of these molecules are explained in terms of the anomeric interaction within the OPOC and OPSC fragments. For those cases where experimental data are available, comparisons have been made between calculated and previously reported vibrational frequencies. All varieties of bond dissociation energies have been examined except those for C-H and P═O bonds. In trivalent phosphorus molecules, the O-C and S-C bonds have the lowest dissociation energies. In the pentavalent phosphorus set, the S-C bonds, followed by P-S bonds, have the lowest dissociation energies. In the fluorinated simulant molecules, the P-F bond is strongest, and the P-C or O-C bonds are weakest.     

Experimental and theoretical studying the thermolysis of nitrate

This paper presents calorimeter measurement for the thermal decomposition of n-propyl nitrate (NPN), isopropyl nitrate (IPN) and 2-ethylhexyl nitrate (EHN). Similar experimental results of triethylene glycol dinitrate (tri-EGDN) and tetraethylene glycol dinitrate (tetra-EGDN) are included for comparison. The potential energy surfaces (PESs) along O-NO₂ bond stretch are investigated using the DFT (B3P86, B3PW91 and B3LYP), ab initio Hartree-Fock and PM3 methods. The good coincidence of experimental with theoretical results indicates that initial stage in the thermal decomposition of five nitrates is only unimolecular homolytical dissociation of the O-NO₂ bonds and the activation energies of thermolysis by DSC correspond to the energies of O-NO₂ bond scission of nitrates.     

Theoretical investigation on cyclohexane dehydrogenation catalyzed by V<Subscript>2</Subscript><Superscript>+</Superscript> in gas-phase

The dehydrogenation reaction mechanism of cyclohexane catalyzed by dimer transition metal cluster V₂⁺ has been investigated at the B3LYP/6-31G (d, p) level of density functional theory. Density of states (DOS) graph is used to understand more deeply the roles of the front molecular orbital of the initial complexes. After the first molecular dehydrogenation, the reaction mainly consists of two competition mechanisms. First, the C-H bonds of cyclohexane can be effectively activated by the V₂⁺ cation, yielding the same-face dehydrogenation products. Second, the C-C bonds are activated, forming the different-face dehydrogenation products. Our calculations indicate that the reaction takes place more easily along the low-spin potential energy surface on the same-face and is a low-barrier or even barrier-free transformation. Carbon-carbon single bonds are nonpolar and generally far less reactive. A comparison of the reaction mechanism of V₂⁺ and congener Ti₂⁺ with cyclohexane has been presented. The bond dissociation energies (BDEs) of V₂⁺ are greater than that of Ti₂⁺, leading to difficulties in forming sandwich complexes in the different-face dehydrogenation of cyclohexane, and the same-face dehydrogenation is an important reaction channel.     

Why are the Homoleptic Diyl Complexes M(InR)4 with M = Ni and Pt Stable Compounds while only Ni(CO)4 but not Pt(CO)4 can become Isolated? A Theoretical Study of M(EMe)44 and M(CO)4 (M = Ni,Pd, Pt; E = B,Al, Ga,In, Tl)

The equilibrium geometries and first bond dissociation energies of the homoleptic complexes M(EMe)₄ and M(CO)₄ with M = Ni, Pd, Pt and E = B, Al, Ga, In, Tl have been calculated at the gradient corrected DFT level using the BP86 functionals. The electronic structure of the metal‐ligand bonds has been examined with the topologial analysis of the electron density distribution. The nature of the bonding is revealed by partitioning the metal‐ligand interaction energies into contributions by electrostatic attraction, covalent bonding and Pauli repulsion. The calculated data show that the M‐CO and M‐EMe bonding is very similar. However, the M‐EMe bonds of the lighter elements E are much stronger than the M‐CO bonds. The bond energies of the latter are as low or even lower than the M‐TlMe bonds. The main reason why Pd(CO)₄ and Pt(CO)₄ are unstable at room temperature in a condensed phase can be traced back to the already rather weak bond energy of the Ni‐CO bond. The Pd‐L bond energies of the complexes with L = CO and L = EMe are always 10 — 20 kcal/mol lower than the Ni‐L bond energies. The calculated bond energy of Ni(CO)₄ is only D_o = 27 kcal/mol. Thus, the bond energy of Pd(CO)₄ is only D_o = 12 kcal/mol. The first bond dissociation energy of Pt(CO)₄ is low because the relaxation energy of the Pt(CO)₃ fragment is rather high. The low bond energies of the M‐CO bonds are mainly caused by the relatively weak electrostatic attraction and by the comparatively large Pauli repulsion. The σ and π contributions to the covalent M‐CO interactions have about the same strength. The π bonding in the M‐EMe bonds is less than in the M‐CO bonds but it remains an important part of the bond energy. The trends of the electrostatic and covalent contributions to the bond energies and the σ and π bonding in the metal‐ligand bonds are discussed.     

Comparative estimation of the energies of intramolecular C-H…O,N-H…O,and O-H…O hydrogen bonds according to the QTAIM analysis and NMR spectroscopy data

The energies of intramolecular C-H…O, N-H…O, and O-H…O hydrogen bonds in model compounds are empirically estimated based on the values of the hydrogen bond induced weak-field shift of the bridging hydrogen atom signal in the ¹H NMR spectrum. It is supported by a theoretical estimation of these energies based on the electron density value at the hydrogen bond critical point calculated within the QTAIM method. Good agreement between the empirical and theoretical estimates is found, which gives evidence of their reliability. It is shown that from the standpoint of their strength the intramolecular N-H…O and O-H…O hydrogen bonds can be classified as moderate whereas the intramolecular C-H…O hydrogen bonds must be classified as very weak interactions similar in their energy significance to van der Waals interactions.     

Ab initio study of the transition-metal carbene cations

The geometries and bonding characteristics of the first-row transition-metal carbene cations MCH_2~ were investigated by ab initio molecular orbital theory (HF/LANL2DZ). All of MCH_2~ are coplanar. In the closed shell structures the C bonds to M with double bonds; while in the open shell structures the partial double bonds are formed, because one of the σ and π orbitals is singly occupied. It is mainly the π-type overlap between the 2p_x orbital of C and 4p_x, 3d_(xz), orbitals of M~ that forms the π orbitals. The dissociation energies of C—M bond appear in periodic trend from Sc to Cu. Most of the calculated bond dissociation energies are close to the experimental ones.     

Computational studies of the structure and cation-anion interactions in 1-ethyl-3-methylimidazolium lactate ionic liquid

Quantum chemical calculations of the structures and cation-anion interaction of 1-ethyl-3-methylimidazolium lactate ([Emim][LAC]) ion pair at the B3LYP/6-31++G** theoretical level were performed. The relevant geometrical characteristics, energy properties, intermolecular H-bonds (H-bonds), and calculated IR vibrations with respect to isolated ions were systematically discussed. The natural bond orbital (NBO) and atoms in molecule (AIM) analyses were also employed to understand the nature of the interactions between cation and anion. The five most stable geometries were verified by analyzing the relative energies and interaction energies. It was found that the most of the C-H···O intermolecular H-bonds interactions in five stable conformers have some covalent character in nature. The elongation and red shift in IR spectrum of C-H bonds which involve in H-bonds is proved by electron transfers from the lone pairs of the carbonyl O atom of [LAC] to the C-H antibonding orbital of the [Emim]+. The interaction modes are more favorable when the carbonyl O atoms of [LAC] interact with the C2-H of the imidazolium ring and the C-H of the ethyl group through the formation of triple H-bonds.     

Theoretical study of the interaction mechanism of single-electron halogen bond complexes H3C⋯Br-Y (Y = H, CN, NC, CCH, C2H3)

The characteristics and structures of single-electron halogen bond complexes [H₃C?Br-Y (Y = H, CCH, CN, NC, C₂H₃)] have been investigated by theoretical calculation methods. The geometries were optimized and frequencies calculated at the B3LYP/6-311++G** level. The interaction energies were corrected for basis set superposition error (BSSE) and the wavefunctions obtained by the natural bond orbital (NBO) and atom in molecule (AIM) analyses at the MP2/6-311++G** level. For each H₃C?Br-Y complex, a single-electron Br bond is formed between the unpaired electron of the CH₃ (electron donor) radical and the Br atom of Br-Y (electron acceptor); this kind of single-electron bromine bond also possesses the character of a “three-electron bond”. Due to the formation of the single-electron Br bond, the C-H bonds of the CH₃ radical bend away from the Br-Y moiety and the Br-Y bond elongates, giving red-shifted single-electron Br bond complexes. The effects of substituents, hybridization of the carbon atom, and solvent on the properties of the complexes have been investigated. The strengths of single-electron hydrogen bonds, single-electron halogen bonds and single-electron lithium bonds have been compared. In addition, the single-electron halogen bond system is discussed in the light of the first three criteria for hydrogen bonding proposed by Popelier.     

An INDO-LMO and 13C NMR spectroscopic study of the C-H bonds in tetracyclo[3.2.0.02,7.04,6]heptane (quadricyclane) and tricyclo[3.1.0.02,4]hexane

Using an INDO approximation to Von Niessen's charge density localization method, the percent s character of the C atom hybrid orbitals engaged in C-H bonds is computed for a series of open-chain and cyclic saturated and unsaturated hydrocarbons. A semi-empirical linear relationship is found between these quantities and the experimental nuclear spin-spin coupling constants J_CH (standard deviation 4·98 Hz). The experimental values of J_CH were measured for quadricyclane (1) and tricyclo[3.1.0.0^2,4]hexane (2) and were shown to be in fair agreement with the theoretical predictions. An analogous relationship is also established between the percent s character of the involved C atom hybrid orbital and the C-H bond distance, the standard deviation being 0·0040Å. This relation is used in order to predict the different C-H bond lengths in the vibrational ground state of 1 and 2, for which equal CH distances were assumed in the analysis of electron diffraction experiments. The results indicate that molecular geometries can be refined with the use of semi-empirical correlations of the above mentioned type.     

Significant effects of phosphorylation on relative stabilities of DNA and RNA sugar radicals: remarkably high susceptibility of h-2' abstraction in RNA

The roles of nucleic acid radicals in DNA and RNA damage cannot be properly understood in the absence of knowledge of the C-H bond strengths depicting the energy cost to generate each of these radicals. However, previous theoretical studies on the relative energies of different nucleic acid radicals are not fully convincing mainly because of the use of oversimplified model compounds. In the present study we chose nucleoside 3',5'-bisphosphates as model compounds for DNA and RNA, in which the effects of both the nucleobase and phosphorylation were taken into consideration. Using the newly developed ONIOM-G3B3 methods, we calculated the gas-phase bond dissociation enthalpies and solution-phase bond dissociation free energies of all the carbohydrate C-H bonds in the model compounds. It was found that the monoanionic phosphate group (OPO3H-) was a better radical stabilization group than the OH group by 1.3 kcal/mol, whereas the neutral phosphate group (OPO3H2) was a significantly worse radical stabilization group than OH by 4.4 kcal/mol. Due to these reasons, the relative thermodynamic susceptibility of H-abstraction from deoxyribonucleotides and ribonucleotides varied considerably depending on the phosphorylation state and the charge carried by the phosphate groups. Strikingly, the bond dissociation free energy of C2'-H in ribonucleotides was dramatically lower than that of all the other C-H bonds by 5-6 kcal/mol regardless of the phosphorylation state and the charge carried by the phosphate group. This explained the previous experimental finding that radiation damage of RNA occurs mainly via H-abstraction at H-2'. A model study suggested that the strength of the hydrogen bonding interaction between the 2'-OH and 3-phosphate groups should dramatically increase from ribonucleoside 3',5'-bisphosphate to its C2' radical. The strengthened hydrogen bonding stabilized the C2' radical, rendering the C2'-H bond of RNA extraordinarily vulnerable to H-abstraction.     

Theoretical Study on the Hydrogen Bonding Interactions in Complexes of 5‐Hydroxytryptamine with Water

The energies, geometries and harmonic vibrational frequencies of 1:1 5‐hydroxytryptamine‐water (5‐HT‐H₂O) complexes are studied at the MP2/6‐311++G(d,p) level. Natural bond orbital (NBO), quantum theory of atoms in molecules (QTAIM) analyses and the localized molecular orbital energy decomposition analysis (LMO‐EDA) were performed to explore the nature of the hydrogen‐bonding interactions in these complexes. Various types of hydrogen bonds (H‐bonds) are formed in these 5‐HT‐H₂O complexes. The intermolecular C4H5^5‐HT···O^w H‐bond in HTW3 is strengthened due to the cooperativity, whereas no such cooperativity is found in the other 5‐HT‐H₂O complexes. H‐bond in which nitrogen atom of amino in 5‐HT acted as proton donors was stronger than other H‐bonds. Our researches show that the hydrogen bonding interaction plays a vital role on the relative stabilities of 5‐HT‐H₂O complexes.     

AM1 calculations of bond dissociation energies. Allylic and benzylic C-H bonds

AM1 method and correlation dependence between electronic relaxation energy and valence change on the C atom of the breaking bond were used to calculate the bond dissociation energies in 50 compounds with allylic or benzylic C-H bonds. The average calculation error is 0.8 kcal/mol.     

Theoretical predictions of the nitrogen heterocyclic compounds with metal and noble gas (metal = Cu,Ag, Au)

Here, we report a new type of Ng-containing compounds formed between the Ng-M group and nitrogen heterocyclic compounds, (CH₂)_nHNCuNg⁺ (n = 2, 3), (CH)₄NMNg, and (CH)₅NCuNg⁺ (M = Cu, Ag, Au; Ng = Ar, Kr, Xe). Quantum chemistry computations were carried out to optimize their geometric structures and calculate the dissociation energies, dissociation enthalpy, and dissociation free energy change. The stability of these Ng-bonding complexes was inspected by investigating the three dissociation processes of the these compounds into (a) Ng, M, and nitrogen heterocycle C_nN; (b) C_nN + MNg⁺; and (c) C_nNM + Ng, which are all endothermic and nonspontaneous, these dissociation processes are also turned out to be endergonic in nature at standard state. The natural bond orbital, atoms in molecules, and energy decomposition analysis based on the molecular wavefunction show that the M-Ng and M-N bonds have some covalent and electrostatic characters.     

Chemical bonding in phosphane and amine complexes of main group elements and transition metals

The geometries and bond dissociation energies of the main group complexes X3B-NX3, X3B-PX3, X3Al-NX3, and X3Al-PX3 (X = H, Me, Cl) and the transition metal complexes (CO)5M-NX3 and (CO)5M-PX3 (M = Cr, Mo, W) have been calculated using gradient-corrected density functional theory at the BP86/TZ2P level. The nature of the donor-acceptor bonds was investigated with an energy decomposition analysis. It is found that the bond dissociation energy is not a good measure for the intrinsic strength of Lewis acidity and basicity because the preparation energies of the fragments may significantly change the trend of the bond strength. The interaction energies between the frozen fragments of the borane complexes are in most cases larger than the interaction energies of the alane complexes. The bond dissociation energy of the alane complexes is sometimes higher than that of the borane analogues because the energy for distorting the planar equilibrium geometry of BX3 to the pyramidal from in the complexes is higher than for AlX3. Inspection of the three energy terms, DeltaE(Pauli), DeltaE(orb), and DeltaE(elstat), shows that all three of them must be considered to understand the trends of the Lewis acid and base strength. The orbital term of the donor-acceptor bonds with the Lewis bases NCl3 and PCl3 have a higher pi character than the bonds of EH3 and EMe3, but NCl3 and PCl3 are weaker Lewis bases because the lone-pair orbital at the donor atoms N and P has a high percent s character. The calculated DeltaE(int) values suggest that the trends of the intrinsic Lewis bases' strengths in the main-group complexes with BX3 and AlX3 are NMe3 > NH3 > NCl3 and PMe3 > PH3 > PCl3. The transition metal complexes exhibit a somewhat different order with NH3 > NMe3 > NCl3 and PMe3 > PH3 > PCl3. The slightly weaker bonding of NMe3 than that of NH3 comes from stronger Pauli repulsion. The bond length does not always correlate with the bond dissociation energy, nor does it always correlate with the intrinsic interaction energy.