Potential functions of internal rotations in n-mononitroalkanes

Internal rotation is studied in n-mononitroalkanes, CH₃(CH₂) _n NO₂, n ?? 7, with the use of the B3LYP/6-311++G(3df, 3pd) and MP2/6-311++G(3df, 3pd) models. Conformations are investigated, and 36 potential functions (V(??)) for all rotations are found. It is shown that as the hydrocarbon chain becomes longer, the potential functions that describe particular molecular fragments change only negligibly beginning from a certain n. Such dependences of V(??) are presented in the form of generalized functions with averaged coefficients (V(??)_av). The V(??)_av values can be recommended for use in simulations. In molecules with n ?? 2, the phenomenon of ??top interactions?? (??movement interactions??) and the gauche effect (the equality between the total energies of gauche ⁺, gauche ^?, and trans-conformers) are observed.     

Internal rotation in mononitro-n-alkyl radicals

Internal rotation in the C·H₂(CH₂) _n NO₂ (n ≤ 7) type radicals has been studied. 44 potential functions of the internal rotation, V(φ), have been calculated taking advantage of the B3LYP/6-311++(3df,3pd) and MP2/6-311++(3df,3pd) methods. The trends observed in the series of parameters characterizing the internal rotation have been explained in view of the electron clouds conjugation, the inductive effect of the end groups, the gauche effect, and the rotation tops interaction. The coefficients of V(φ) have been shown to depend predominantly on the nearest surrounding of the rotation axis. Based on this, the generalized functions, V _av(φ), have been developed, their coefficients being dependent exclusively on the rotating bond position. Such functions are convenient for molecular modeling applications.     

Thermodynamic functions of lactams in the ideal gas state

Thermodynamic functions (enthalpy, entropy, free energy, and heat capacity) of azacycloalkan-2-ones with ring sizes n = 4–8 in the ideal gas state are calculated by means of quantum chemistry and statistical physics, using an anharmonic approximation in the range of 298–1500 K with allowance for all known conformers and optical isomers. Equilibrium structures and total energies of lactams are calculated using the B3LYP/6-311++G(3df, 3pd), B3LYP/aug-cc-pVQZ, and MP2/6-311++G(3df, 3pd) methods, and the anharmonic frequencies of the fundamental vibrations of all the investigated structures were found via B3LYP/6-311++G(3df, 3pd).     

On the additivity of basis set effects in some simple fluorine containing systems

The reactions F + H₂ → HF + H, HF → H + F, F → F⁺ + e^? and F + e^? → F^? were used as simple test cases to assess the additivity of basis set effects on reaction energetics computed at the MP4 level. The 6-31G and 6-311G basis sets were augmented with 1, 2, and 3 sets of polarization functions, higher angular momentum polarization functions, and diffuse functions (27 basis sets from 6-31Gd, p) to 6-31 ++ G(3df, 3pd) and likewise for the 6-311G series). For both series substantial nonadditivity was found between diffuse functions on the heavy atom and multiple polarization functions (e.g., 6-31 + G(3d, 3p) vs. 6-31 + G(d, p) and 6-31G(3d, 3p)). For the 6-311G series there is an extra nonadditivity between d functions on hydrogen and multiple polarization functions. Provided that these interactions are taken into account, the remaining basis set effects are additive to within ±0.5 kcal/mol for the reactions considered. Large basis set MP4 calculations can also be estimated to within ±0.5 kcal/mol using MP2 calculations, est. E_MP4(6-31 ++ G(3df, 3pd)) ≈ E_MP4(6-31G(d, p)) + E_MP2(6-31 ++ G(3df, 3pd)) – E_MP2(6-31G(d, p)) or E_MP4(6-31 + G(d, p) + E_MP2(6-31 ++ G(3df, 3pd)) – E_MP2(6-31 + G(d, p)) and likewise for the 6-311G series.     

The inductive effect of a radical center and transferability of the properties of functional groups in <Emphasis Type="Italic">n</Emphasis>-alkyl radicals

Electron density distribution in n-alkyl radicals (from ethyl to n-octyl) was studied by the B3LYP/6-311++G(3df,3pd) DFT method. The theory of atoms in molecules was used to show that the inductive effect of a free valence extends to two neighboring CH₂ groups. The electronegativities χ(C^?H₂) > χ(CH₃) > χ(CH₂) of groups and χ(C^?) > χ(H) > χ(C) atoms were qualitatively determined. The group method for calculating the enthalpies of formation of n-alkyl radicals Δ_f H°(n-C _n H_{2n+ 1}, n > 5) was substantiated.     

Potential energy curves and calculations of spectroscopic constants for the ground state of AlC and AlN

The density functional theory method with B3LYP/6-311++G(df,pd), B3LYP/6-311++G(2df, 2pd), and B3LYP/6-311++G(3df,3pd) basis sets is used to compute the geometrics and single point energy of aluminum carbide (AlC) and aluminium nitride (AlN) in their ground state. The Level 8.0 program is used to calculate spectroscopic constants and fit the energy potential curves. The effect of a basis set on the spectroscopic constants is discussed. The results show that the calculated potential curve matches well with the Level 8.0 fitting curve, and the calculated values of spectroscopic constants become more reliable with the improvement of the quality of basis sets. The spectroscopic constants are in good agreement with the existing experimental and theoretical values. For the first time, the reliable anharmonicity constant data of AlC are reported, which agrees so well with the experimental value.     

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.     

Correlated one‐electron wave functions

Using four basis sets, 6‐311G(d,p), 6‐31+G(d,p), 6‐311++G(2d,2p), and 6‐311++G(3df,3pd), the optimized structures with all real frequencies were obtained at the MP2 level for dimers CH₂O? HF, CH₂O? H₂O, CH₂O? NH₃, and CH₂O? CH₄. The structures of CH₂O? HF, CH₂O? H₂O, and CH₂O? NH₃ are cycle‐shaped, which result from the larger bend of σ‐type hydrogen bonds. The bend of σ‐type H‐bond O…H? Y (Y?F, O, N) is illustrated and interpreted by an attractive interaction of a chemically intuitive π‐type hydrogen bond. The π‐type hydrogen bond is the interaction between one of the acidic H atoms of CH₂O and lone pair(s) on the F atom in HF, the O atom in H₂O, or the N atom in NH₃. By contrast with above the three dimers, for CH₂O? CH₄, because there is not a π‐type hydrogen‐bond to bend its linear hydrogen bond, the structure of CH₂O? CH₄ is a noncyclic shaped. The interaction energy of hydrogen bonds and the π‐type H‐bond are calculated and discussed at the CCSD(T)/6‐311++G(3df,3pd) level. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Long‐range π‐type hydrogen bond in dimers CH2?HF,CH2O?H2O,and CH2O?NH3

Using four basis bets, (6‐311G(d,p), 6‐31+G(d,p), 6‐31++G(2d,2p), and 6‐311++G(3df,3pd), the optimized structures with all real frequencies were obtained at the MP2 level for the dimers CH₂O? HF, CH₂O? H₂O, CH₂O? NH₃, and CH₂O? CH₄. The structures of CH₂O? HF, CH₂O? H₂O, and CH₂O? NH₃ are cycle‐shaped, which result from the larger bend of σ‐type hydrogen bonds. The bend of σ‐type H‐bond O…H? Y (Y?F, O, N) is illustrated and interpreted by an attractive interaction of a chemically intuitive π‐type hydrogen bond. The π‐type hydrogen bond is the interaction between one of the H atoms of CH₂O and lone pair(s) on the F atom in HF, the O atom in H₂O, or the N atom in NH₃. In contrast with the above three dimers, for CH₂O? CH₄, because there is not a π‐type hydrogen bond to bend its linear hydrogen bond, the structure of CH₂O? CH₄ is noncyclic shaped. The interaction energy of hydrogen bonds and the π‐type H‐bond are calculated and discussed at the CCSD (T)/6‐311++G(3df,3pd) level. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

A study of the electronic structure and properties of the propargyl radical

By means of B3LYP/6-311++G(3df,3pd) the electron density distribution in the propargyl radical CH₂CCH is obtained. Within the Quantum Theory of Atoms in Molecules the phenomenon of conjugation and the spin density distribution of the unpaired electron in CH₂CCH are studied at the qualitative level. Characteristics of the electronic structure of CH₂CCH and its parent molecules CH₃–C≡CH and CH₂=C=CH₂ are compared. With the use of the rigid rotator-anharmonic oscillator model the thermodynamic properties of the propargyl radical and enthalpies of bond cleavage in propyne and allene are calculated in the temperature range 298-1500 K. The relationship between the electronic and thermodynamic properties of CH₂CCH is considered and its conjugation energy is calculated.     

A quantum-chemical study of the thermodynamic properties of the CNOH<Subscript>3</Subscript> isomers

The B3LYP/6-311++G(3df,3pd) DFT method was used to determine the geometry and vibrational frequencies for the formamide, formaldoxime, nitrosomethane, oxaziridine, and formimide. The potential energy functions of the hindered internal rotations were calculated. The conformers of formaldoxime and formimide were determined. For all the molecules, including the conformers, the thermodynamic characteristics Δ_f H° (298 K), S°(298 K), C _p(298 K), and Δ_f G° (298 K) were determined. The temperature dependences C _p(T) within 298–1500 K were represented in the form of cubic polynomials.     

Computational mechanistic study of methanol and molecular oxygen reaction on the triplet and singlet potential energy surfaces

The reaction of CH₃OH with the O₂ on the triplet and singlet potential energy surfaces (PES) was carried out using the B3LYP, MP2, and CCSD(T)//B3LYP theoretical approaches in connection with the 6-311++G(3df–3pd) basis set. Three pre-reactive complexes, ¹C1, ¹C2, and ³C1, on the singlet and triplet PES were formed between methanol and molecular oxygen. From a variety of the complexes, seven types of products are obtained, of which four types are found to be thermodynamically stable. Results reveal that there exists one intersystem crossing between triplet and singlet PES. For P4 adduct that is the main and kinetically the most favorable product, the rate constants are calculated in the temperature range of 200–1,000 K in the reliable pathway.     

Ab initio investigation on stability and properties of XYCO... HZ complexes. II: Post hartree-fock studies on H2CO... HF

Ab initio MP2 level of theory in conjunction with three basis sets of a triple-zeta quality was applied to study the molecular geometry and stability of the H₂CO... HF complex. An interaction energy predicted for this system at the highest, MP4(SDTQ)/6-311 + +G(2df, 2pd)//MP2/6-311 + +G(2df, 2pd), level corrected for the BSSE and ZPE contributions amounts to -4.85 kcal/ mol. BSSE contributes significantly to the interaction energies at all applied levels. Reliable MP2/ 6-311 + +G(2df, 2pd) level harmonic vibrational frequencies, IR intensities, and the predicted isotopic shifts upon deuteration and¹⁸O substitution are presented in order to facilitate experimental studies on the IR spectrum of the title complex.     

Theoretical study of the reaction of ethynyl radical with acetonitrile

A detailed computational study has been performed on the mechanism and kinetics of the C₂H + CH₃CN reaction. The geometries were optimized at the BHandHLYP/6–311G(d, p) level. The single-point energies were calculated using the BMC-CCSD, MC-QCISD and QCISD(T)/6–311+G(2df, 2pd) methods. Five mechanisms were investigated, namely, direct hydrogen abstraction, C-addition/elimination, N-addition/elimination, C₂H–to–CN substitution and H-migration. The kinetics of the title reaction were studied using TST and multichannel RRKM methodologies over a wide range of temperatures (150–3,000 K) and pressures (10^?4–10⁴ torr). The total rate constants show positive temperature dependence and pressure independence. At lower temperatures, the C-addition step is the most feasible channel to produce CH₃ and HCCCN. At higher temperatures, the direct hydrogen abstraction path is the dominant channel leading to C₂H₂ and CH₂CN. The calculated overall rate constants are in good agreement with the experimental data.     

Does CH prefer a C2v rather than a Cs structure?

The closely related C_s ( 1 ) and C_2v ( 3 ) structures of CH have been reinvestigated at many ab initio levels using MP2/6-31G** and MP2/6-311 + + G(2df, 2pd) geometries. The largest basis sets employed were 6-311G(3df, 2p), 6-311 + + G(3df, 3pd), and the Dunning “correlation consistent” polarized triple-split valence basis set (cc-pVTZ). Electron correlation was probed at the MP4 level, but the QCISD method was also used with the largest basis sets. While electron correlation favors 3 over 1 by about 2 kcal/mol, the correlated relative energies with all basis sets employed range from 0.36–1.03 kcal/mol in favor of 1 . The best estimate of this difference, 0.86 kcal/mol, is essentially identical with the (scaled) zero-point energy difference, 0.84 kcal/mol, favoring 3 over 1 . These results indicate that 1 and 3 have almost exactly the same energy at 0 K. Our best value for the dissociation energy of CH is 42.0 kcal/mol [QCISD(T)/6-311 + + G(3df, 3pd)//MP2(fu)/6-311 + + G(2df, 2pd), corrected to 298 K], which agrees very well with the experimental value. © 1992 by John Wiley & Sons, Inc.     

Potential Energy Surface of Cooperative Proton Tunneling in the C2H+ 3 Cation

The nine-dimensional potential energy surface for proton tunneling in the nonrigid C₂H⁺ ₃ cation was constructed from quantum-chemical data [MP4SDQ(T)/6-311++G(3df,3pd)] on the equilibrium geometry, energy, frequencies, and eigenvectors of the normal vibrations at the stationary points and transition states using the theory of isodynamic symmetry groups along the tunneling path.     

Counterion effects on the isotopic properties of the Cl<Superscript>−</Superscript> and Li<Superscript>+</Superscript> ions in aqueous solutions

The reduced partition function ratios between isotopic forms (β-factors) were calculated by the ab initio RHF/6-311++G**(3df, 3p) and MP2/6-311++G**(3df, 3p) quantum-chemical methods for hydrated chloride ion and ion pair hydrates NaCl·nH₂O and LiOH · nH₂O. The influence of the Na⁺ cation on the β-factor value and the chlorine isotope separation factor in the precipitation of NaCl from concentrated aqueous solutions was found to be substantial. At the same time, the presence of OH^? counterions had no noticeable effect on the β-factor of the hydrated Li⁺ cation.     

Comparison of large basis set DFT and MP2 calculations in the study of the barrier for internal rotation of 2,3,5,6‐tetrafluoroanisole

The barrier for internal rotation around the ? OCH₃ bond in 2,3,5,6‐tetrafluoroanisole was calculated using the density functional theory (DFT) and second‐order Møller–Plesset (MP2) methods with Pople's basis sets up to 6‐311++G(3df,2p) and Jensen basis sets up to pc‐3. The results are converged only if fairly large basis sets are used (at least 6‐311++G(3df,2pd) or pc‐2). Both the DFT and MP2 potential energy curves show internal structure. Two minima and three maxima are observed on the curves, arising from the interplay between lone‐pair delocalization and changes in the hybridization around the oxygen atom, together with the attraction between the positively polarized hydrogens in the methyl group and the negatively polarized fluorine atom at the ortho position. These critical points are somehow ironed out by the addition of zero‐point and thermal corrections to the energy curves. At this level, the MP2 method can describe reasonably well the previously determined single‐well experimental rotational barrier, 2.7 ± 2.0 kcal/mol at 298 K, while all DFT methods yield a much smaller result. As observed experimentally, the ? OCH₃ group is perpendicular to the aryl ring in the equilibrium structure, although two very close minima with an intermediate hump at 90° are still observable. The theoretical free energy barrier of rotation at the MP2(full)/pc‐2 level is 2.0 ± 1.0 kcal/mol, in reasonable agreement with the experimental determination. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

The structure of 1-chloro-1-silabicyclo(2.2.2)octane as determined by gas-phase electron diffraction

The structure of 1 -chloro-1 -si labicyclo( 2.2.2 )octane is determined by gas-phase electron diffraction. The molecule is found to have a large amplitude twisting motion with a double minimum quartic potential function of the form V(φ) = V_o[1 + (φ/φ_o)⁴ - 2(φ/φ_o)²]. Least-squares analysis of the experimental data gives values of 1.4(0.8) kcal mole^? for V_o and 17.5(2.5)° for φ_o. Other structural parameters for the “quasi-C_3v” cage-like molecule include: r_g(Si-Cl) = 2.061(3) Å, r_g(Si-C) = 1.863(3) Å, r_g(C-C_av) = 1.559(2) Å, and r_g(C-H_av) = 1.098(7) Å. Several valence angles exhibit large deviations from tetrahedral values, e.g. ∠Cl-Si-C₂ = 114.6(0.2)°, ∠Si-C₂-C₃ = 105.8(0.4)°, ∠C₂-C₃-C₄ = 114.2(1.2)°, ∠C-₃-C₄-C₅ = 111.4(0.8)° and ∠C₂-Si-C₆= 103.9(0.2)°. Many of the structural features in this strained polycyclic compound. Including the nature of the quartic potential function, can be rationalized in terms of a simple molecular mechanics model. A new method for the calculation of an analytical Jacobian of the intensity function with respect to parameters of the potential function is also discussed.     

Tautomerism of monochalcogenosilanoic acids CH3Si(O)XH (X = S,Se, Te) in the gas phase and in the polar and aprotic solution: An ab initio computational investigation

Computational investigations by an ab initio molecular orbital method (HF and MP2) with the 6‐311+G(d,p) and 6‐311++G(2df, 2pd) basis sets on the tautomerism of three monochalcogenosilanoic acids CH₃Si(?O)XH (X = S, Se, and Te) in the gas phase and a polar and aprotic solution tetrahydrofuran (THF) was undertaken. Calculated results show that the silanol forms CH₃Si(?X)OH are much more stable than the silanone forms CH₃Si(?O)XH in the gas‐phase, which is different from the monochalcogenocarboxylic acids, where the keto forms CH₃C(?O)XH are dominant. This situation may be attributed to the fact that the Si? O and O? H single bonds in the silanol forms are stronger than the Si? X and X? H single bonds in the silanone forms, respectively, even though the Si?X (X = S, Se, and Te) double bonds are much weaker than the Si?O double bond. These results indicate that the stability of the monochalcogenosilanoic acid tautomers is not determined by the double bond energies, contrary to the earlier explanation based on the incorrect assumption that the Si?S double bond is stronger than the S?O double bond for the tautomeric equilibrium of RSi(?O)SH (R?H, F, Cl, CH₃, OH, NH₂) to shift towards the thione forms [RSi(?S)OH]. The binding with CH₃OCH₃ enhances the preference of the silanol form in the tautomeric equilibrium, and meanwhile significantly lowers the tautomeric barriers by more than 34 kJ/mol in THF solution. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008