Spectroscopic calculation of the bond-dissociation energy of CH bonds in fluoro derivatives of methane,ethane, ethene,propene, and benzene

The bond-dissociation energy of CH bonds in fluoro derivatives of methane, ethane, ethene, propene, and benzene is determined by spectroscopic and quantum chemical methods. The spectroscopic values of the bond-dissociation energy of CH bonds are calculated in terms of fundamental absorption bands in the anharmonic approximation by the variational method using the Morse anharmonic basis. The quantum chemical calculations are performed using the 6-311G(3df, 3pd)/B3LYP basis set. The obtained regularities in variations of the CH bond dissociation energy values upon changes in the molecule structure are discussed.     

Spectroscopic calculations of CH bond dissociation energies for ethene, propene, and benzene chlorine derivatives

CH bond dissociation energies have been determined by spectroscopic and quantum-chemical calculations for ethane, propene, and benzene chlorine derivatives. The spectroscopic values of CH bond dissociation energies were obtained from the fundamental absorption bands in an anharmonic approximation using the variation method and the Morse harmonic basis. Quantum-chemical calculations were carried out using the 6-311G(3df,3pd)/B3LYP basis. The resulting tendencies of variation of bond dissociation energies are discussed in relation to changes in the structure of the molecule.     

Spectroscopic calculation of CH bond dissociation energy in the series of chloro derivatives of methane, ethane, and propane

The bond-dissociation energy of CH bonds in chloro derivatives of methane, ethane, and propane has been determined by spectroscopic and quantum chemical methods. Spectroscopic values for CH bond dissociation energy were computed, basing on fundamental absorption bands in the anharmonic approximation, by the variational method with the use of the Morse anharmonic basis. Quantum chemical computations were performed using the basis 6-311G(3df, 3pd)/B3LYP. There are discussed the obtained regularities of changes in the bond dissociation energy when the structure of a molecule is changed.     

Ionization,atomization, and bond energies as functions of distances in inorganic molecules and crystals

The ionization potentials and dissociation energies of diatomic molecules are determined as functions of bond length, and the atomization energies of metals and crystalline compounds are determined as E = a/d ⁿ functions. In most cases, n ≥ 1; but for a number of metals and compounds, n < 1, as distinct from all known types of interatomic interactions. It is shown that the ratios of the bond energies and bond lengths of Group 1A and 1B metals to the respective molecular parameters have similar values, proving the identical valence states of the atoms of these metals in crystal structures.     

Spectroscopic calculations of CH and OH bond dissociation energies for aldehydes, ketones, acids, and alcohols

CH and OH bond dissociation energies were calculated by the spectroscopic and quantum-chemical methods for aldehydes, ketones, acids, and alcohols. The spectroscopic values of CH and OH bond dissociation energies were obtained from the fundamental absorption bands by the variational method in an anharmonic approximation using the Morse-anharmonic basis set. Quantum-chemical calculations were carried out using the 6-311G(3df,3pd)/B3LYP basis set. It is discussed how the bond dissociation energies change with the structure of the molecule.     

Evaluation of Acid Dissociation Constants in DMSO and DMF by Quantum-Chemical Methods

We have calculated total electronic energies (E) and Gibbs energies (G) of a large number of acids and their anions in water, dimethylsulfoxide, and dimethylformamide using the hybrid B3LYP functional DFT method in the 6-31++G(d,p) basis set, taking into account the solvent effect by the conductor-like polarizable continuum model method. A linear correlation has been found between the experimental values of acid dissociation constants (pK_a) of different nature and the difference between anion and acid E values, and between pK_a and the difference between anion and acid G values. The obtained correlations allowed us to evaluate the pK_a values of both inorganic and organic acids. Such an evaluation is of special importance for nonaqueous solvents as it is quite problematic to determine these dissociation constants.     

Density functional calculations of the physicochemical properties of formamidyl radicals

The geometric, spectroscopic, and thermodynamic parameters of the HNC(O)H radical were studied by the DFT B3L YP/6-311++G(3df, 3pd) method. The structure of its conformers was established. Electron and spin density distributions were analyzed. The potential function of internal hindered rotation was calculated. The enthalpies of dissociation were determined for the O-H bond in HNC(OH)H and N-H bond in H₂NC(O)H.     

Molecular structure and mechanisms of unimonomolecular decomposition of primary <Emphasis Type="Italic">N</Emphasis>-nitramines

By means of nonempirical and the density functional methods the geometrical parameters, the enthalpies of formation of the compounds and radicals, and the dissociation energies of the N-NO₂ bond in primary and secondary N-nitramines were evaluated. The tendencies to the variation of spatial arrangement, of the formation enthalpies, and of the dissociation energies in the series of simplest N-nitramines were analyzed. Alternative mechanisms of the initial stage of the gas phase unimolecular decomposition were considered. It is noted that among all the processes of unimolecular decomposition the formation and destruction of aci-form according to the complex multy-stage mechanism was the most energetically favored.     

Potential functions of internal rotation in <Emphasis Type="Italic">n</Emphasis>-alkanes and its contribution to thermodynamic properties

The potential functions of braked internal rotation V(?) in n-alkanes (ethane, propane, butane, n-pentane, n-hexane, n-heptane) were calculated by ab initio and DFT methods with the 6-311++G(3df,3pd) basis set. The functions were approximated as a series of six cosines. The dependences of V(?) on the length of the hydrocarbon chain in n-alkanes were analyzed. The heights of the trans-cis and trans-gauche barriers and the differences between the energies of the trans and gauche conformers were calculated and compared with the experimental data. From the calculated geometric parameters and V(?), the contributions of the braked internal rotation to the enthalpy, entropy, heat capacity, and Gibbs free energy at 298 K were determined. The contributions of internal rotations are transferable within the framework of additive approaches. The generalized function V _av(?) for n-alkanes and averaged contributions of internal rotation of the C-C bonds and CH₃- and -CH₂- tops to the thermodynamic properties were suggested.     

<Emphasis Type="Italic">N</Emphasis>-allyl and <Emphasis Type="Italic">N</Emphasis>-propargyl trifluoromethanesulfonimides. Theoretical analysis

Tautomers of N-allyl- and N-propargyl-substituted trifluoromethanesulfonimides (CF₃SO₂)₂NR (R = CH₂CH=CH₂, Z/E-CH=CHMe, CH₂C≡CH, CH=CH=CH₂, C≡CCH₂) were calculated by the DFT (B3LYP, wB97XD, PBE1PBE), MP2, and CBS-QB3 methods. The results were compared with the theoretical data for the corresponding amines and amides NHRR¹ (R¹ = H, CF₃SO₂). It was shown that there is no conjugation between the nitrogen atom and C=C bond and that conjugation exists with the C≡C bond with electron density displacement toward the nitrogen atom. The calculations of anions derived from N-allyl- and N-propargyl-trifluoromethanesulfonimides revealed the possibility of their rearrangement with elimination of trifluoromethanesulfinate anion and formation of its H-complex with N-(prop-2-en-1-ylidene)trifluoromethanesulfonamide or N-(prop-2-yn-1-ylidene)trifluoromethanesulfonamide.     

Structural and electronic properties of heptachlor

In this work, the molecular geometry of heptachlor is investigated using ab initio HF, DFT, LDA, and GGA methods. The natural bond orbital (NBO) analysis is performed at the B3LYP/6-311++G(d,p) level of theory. The first order hyperpolarizability β_total, the mean polarizability Δα, the anisotropy of the polarizability Δα, and the dipole moment μ, are calculated by B3LYP/6-311++G(d,p) and HF/6- 311++G(d,p) methods. The first order hyperpolarizability (β_total) is calculated based on the finite field approach. UV spectral parameters along with HOMO, LUMO energies for heptachlor are determined in vacuum and the solvent phase using HF, DFT, and TD-DFT/B3LYP methods implemented with the 6-311++G(d,p) basis set. Atomic charges and electron density of heptachlor in vacuum and ethanol are calculated using DFT/B3LYP and TD-DFT/B3LYP methods and the 6-311++G(d,p) basis set. In addition, after the frontier molecular orbitals (FMOs), the molecular electrostatic potential (MEP), the electrostatic potential (ESP), the electron density (ED), and the solvent accessible surface of heptachlor are visualized as a results of the B3LYP/6-311++G(d,p) calculation. Densities of states (DOS), the external electric field (EF) effect on the HOMO-LUMO gap, and the dipole moment are investigated by LDA and GGA methods.     

<Emphasis Type="Italic">Ab initio</Emphasis> study of scandium fluoride molecules: ScF,ScF<Subscript>2</Subscript>, AND ScF<Subscript>3</Subscript>

Equilibrium geometric parameters, normal mode frequencies and intensities in IR spectra, atomization enthalpy, and relative energies of low-lying electronic states of scandium fluoride molecules (ScF, ScF₂, and ScF₃) are calculated by the coupled-cluster method (CCSD(T)) in triple-, quadruple, and quintuple-zeta basis sets with the subsequent extrapolation of the calculation results to the complete basis set limit. The ScF molecule is also studied by the CCSDT technique. The error in the approximate calculation of triple excitations in the CCSD(T) method does not exceed 0.002 Å for the equilibrium internuclear distance R _e, 4 cm^?1 for the vibrational frequency, and 0.2 kcal/mol for the dissociation energy of the molecule. In the ground electronic state \(\tilde X^2 \) A ₁(C _2ν ) of ScF₂ molecules, R _e(Sc-F) = 1.827 Å and α_e(F-Sc-F) = 124.2°; the energy barrier to bending (linearization) h = E _min(D _g8h ) ? E _min(C_2ν) = 1652 cm^?1. The relative energies of Ã²Δ _g and \(\tilde B^2 \)Π _g electronic states are 3522 cm^?1 and 14633 cm^?1 respectively. The bond distance in the ScF₃ molecule (\(\tilde X^1 \) A′₁, D _3h ) is refined: R _e(Sc-F) = 1.842 Å. The atomization enthalpies Δ_at H ₂₉₈ ⁰ of ScF _k molecules are 139.9 kcal/mol, 289.0 kcal/mol, and 444.8 kcal/mol for k = 1, 2, 3 respectively.     

Electronic structure of crystalline uranium nitride: LCAO DFT calculations

The results of electronic structure calculations performed for the first time for crystalline uranium nitride and using a LCAO basis are discussed. For calculations we used the density functional method with the PW91 exchange correlation potential and a variety of relativistic core potentials for the uranium atom. The calculated atomization energy of the crystal agrees well with the experimental data and with the results of calculations with the plane wave basis. It is shown that a chemical bond in crystalline uranium nitride is a metal covalent bond. The metal component of the bond is due to the 5f electrons localized on the uranium atom and having energies near the Fermi level and the bottom of the conduction band. The covalent component of the chemical bond results from an overlap between the uranium 6d and 7s valence orbitals and the nitrogen 2p atomic orbitals. Inclusion of the 5f electrons in the core of the uranium atom introduces relatively minor changes in the calculated binding energy and electron density distribution.     

Influence of Linkage Stereochemistry and Protecting Groups on Glycosidic Bond Stability of Sodium Cationized Glycosyl Phosphates

Energy-resolved collision-induced dissociation (ER-CID) experiments of sodium cationized glycosyl phosphate complexes, [GP _x +Na]⁺, are performed to elucidate the effects of linkage stereochemistry (α versus β), the geometry of the leaving groups (1,2-cis versus 1,2-trans), and protecting groups (cyclic versus non-cyclic) on the stability of the glycosyl phosphate linkage via survival yield analyses. A four parameter logistic dynamic fitting model is used to determine CID_50% values, which correspond to the level of rf excitation required to produce 50% dissociation of the precursor ion complexes. Present results suggest that dissociation of 1,2-trans [GP _x +Na]⁺ occurs via a McLafferty-type rearrangement that is facilitated by a syn orientation of the leaving groups, whereas dissociation of 1,2-cis [GP_x+Na]⁺ is more energetic as it involves the formation of an oxocarbenium ion intermediate. Thus, the C1?C2 configuration plays a major role in determining the stability/reactivity of glycosyl phosphate stereoisomers. For 1,2-cis anomers, the cyclic protecting groups at the C4 and C6 positions stabilize the glycosidic bond, whereas for 1,2-trans anomers, the cyclic protecting groups at the C4 and C6 positions tend to activate the glycosidic bond. The C3 O-benzyl (3 BnO) substituent is key to determining whether the sugar or phosphate moiety retains the sodium cation upon CID. For 1,2-cis anomers, the 3 BnO substituent weakens the glycosidic bond, whereas for 1,2-trans anomers, the 3 BnO substituent stabilizes the glycosidic bond. The C2 O-benzyl substituent does not significantly impact the glycosidic bond stability regardless of its orientation.

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Homodesmotic method of determining the O–H bond dissociation energies in phenols

The dissociation energy of the O–H bond has been calculated by the homodesmotic reaction method for phenolic compounds, which are well-known antioxidants, including for natural phenols. Use of moderately complex computational levels, such as B3LYP/6-31G(d), is sufficient for reliably estimating the D(O–H) value for phenols within the homodesmotic approach. The O–H bond dissociation energy for monosubstituted phenols has been calculated, and the additive character of the effect of methyl groups on D(O–H) in methylphenols has been demonstrated: the introduction of a CH₃ group into the aromatic ring decreases the D value by 7.8 kJ/mol (ortho position), 1.8 kJ/mol (meta position), and 7.6 kJ/mol (para position). The O–H bond strength has been calculated for a number of ubiquinols, selenophens, flavonoids, and chromanols. The D(O–H) value recommended for α-tocopherol is 328.0 ± 1.3 kJ/mol.     

Reactivity of alkyl iodides in molecular decomposition reactions

The experimental results for the concerted molecular decomposition of alkyl iodides RI to olefin and HI were analyzed in terms of the intersecting parabolas model (IPM). The activation energies (E) and rate constants (k) of the earlier unstudied reactions of the concerted molecular decomposition of RI were calculated on the basis of the enthalpy of the reaction and IPM algorithms. The factors that influence on E of RI decay were established: the enthalpy of the reaction, the energy of stabilization of radical R*, the length and force constant of the C—I bond, and the size of the halogen atom. The values of E and k for the backward reactions of HI addition to olefins were estimated.     

Reaction of <Emphasis Type="Italic">N</Emphasis>-phenyltriflamide with 1,2-dibromoethane and propargyl bromide. Unexpected cleavage of С–С and С–N bonds

Reaction of N-phenyltriflamide with 1,2-dibromoethane under basic conditions in DMSO unexpectedly results in N-methyl-N-phenyltriflamide and 1,3-diphenylurea. The presumed reaction mechanism includes the formation of unstable intermediate disubstitution product TfN(Ph)CH₂CH2N(Ph)Tf that suffers the the С–С bond cleavage resulting in TfN(Me)Ph and N,N′-methanediylbis(N-phenyltriflamide). The latter reacts with K₂CO₃ releasing two molecules of potassium triflinate and after hydrolysis of diphenylcarbodiimide PhN=C=NPh gives 1,3-diphenylurea. With propargyl bromide, N-phenyltriflamide affords N-propargyl-Nphenyltriflamide in high yield. The bromination of the latter results in a mixture of Z,E-isomers of N-(2,3-dibromoprop-2-en-1-yl)-N-phenyltriflamide which undergo dehydrobromination giving first N-(3-bromopropanedienyl)-N-phenyltriflamide and then the products of the C–N bond cleavage: N-phenyltriflamide and 3,3-dimethoxyprop-1-yne.     

Tautomerism in the Sulfonamide Moiety: Synthesis,Experimental and Theoretical Characterizations

Following our previous work, we synthesized N-(7-methyl-5,6-diphenyl-2-m-tolyl-7H-pyrrolo[2,3-d]pyrimidin-4-yl)benzensulfonamides to study the sulfonylimine-sulfonamide tautomerism. This goal is performed using the density functional theory (DFT). Four plausible isomers including the keto and enol sulfonamide as well as Z and E sulfonimide are considered for each of compounds. The DFT calculations are carried out at the B3LYP/6-31+G(d,p) level of theory. The optimized geometric parameters such as bond lengths and bond angles are calculated. The computed IR vibrational frequencies and ¹H NMR chemical shifts are in good agreement with the experimental data. The structure of all compounds is confirmed on the basis of their full spectral data. In all three compounds, the Z-sulfonimide form is more stable than the other isomers. A high energy gap between the frontier orbitals confirms the stability of the compounds.     

[Ca(BH<Subscript>4</Subscript>)<Subscript>2</Subscript>]<Subscript> <Emphasis Type="Italic">n</Emphasis> </Subscript> clusters as hydrogen storage material: A DFT study

Calcium borohydride is widely studied as a hydrogen storage material. However, investigations on calcium borohydride from a cluster perspective are seldom found. The geometric structures and binding energies of [Ca(BH₄)₂]_n (n = 1–4) clusters are determined using density function theory (DFT). For the most stable structures, vibration frequency, natural bond orbital (NBO) are calculated and discussed. The charge transfer from (BH₄) to Ca was observed. Meanwhile, we also study the LUMO–HOMO gap (E_g) and the B–H bond dissociation energies (BDEs). [Ca(BH₄)₂]₃ owns higher E_g, revealing that trimer is more stable than the other forms. Structures don’t change during optimization after hydrogen radical removal, showing that calcium borohydride could possibly be used as a reversible hydrogen storage material. [Ca(BH₄)₂]₄ has the smallest dissociation energy suggesting it releases hydrogen more easily than others.