Ab initio calculations of the rotational barriers in formamide and acetamide: The effects of polarization functions and correlation

Ab initio calculations have been used to determine the gas-phase rotational barrier about the CN bond in formamide and acetamide. The results indicate that the inclusion of polarization functions in the basis set leads to a substantial decrease (ca. 5 kcal mol⁻¹) in the calculated barrier height at the SCF level. Electron correlation effects decrease the barrier by less than 1 kcal mol⁻¹, while the addition of zero point energy corrections changes the barrier height only slightly. Based upon the current calculations, the 0 K rotational barriers for isolated formamide and acetamide are predicted to be 14.2 and 12.5 kcal mol⁻¹, respectively.     

Potential energy scans and vibrational assignments of cyclopropanecarboxylic acid and cyclopropanecarboxamide

The structural stability and internal rotations in cyclopropanecarboxylic acid and cyclopropanecarboxamide were investigated by the DFT-B3LYP and the ab initio MP2 calculations using 6-311G** and 6-311+G** basis sets. The computations were extended to the MP4//MP2/6-311G** and CCSD(T)//MP2/6-311G** single-point calculations. From the calculations the molecules were predicted to exist predominantly in the cis (C=O group eclipses the cyclopropane ring) with a cis-trans barrier of about 4-6kcal/mol. The OCOH torsional barrier in the acid was estimated to be about 12-13kcal/mol while the corresponding OCNH torsional barrier in the amide was calculated to be about 20kcal/mol. The equilibrium constant k for the cis<-->trans interconversion in cyclopropanecarboxylic acid was calculated to be 0.1729 at 298.15K that corresponds to an equilibrium mixture of about 85% cis and 15% trans. The vibrational frequencies were computed at the DFT-B3LYP level. Normal coordinate calculations were carried out and potential energy distributions were calculated for the low energy cis conformer of the molecules. Complete vibrational assignments were made on the basis of normal coordinate calculations and comparison with experimental data of the molecules.     

Ab initio and density function theory computational studies of the CH4 + H → CH3 + H2 reaction

The activation barrier for the CH₄ + H → CH₃ + H₂ reaction was evaluated with traditional ab initio and Density Functional Theory (DFT) methods. None of the applied ab initio and DFT methods was able to reproduce the experimental activation barrier of 11.0-12.0 kcal/mol. All ab initio methods (HF, MP2, MP3, MP4, QCISD, QCISD(T), G1, G2, and G2MP2) overestimated the activation energy. The best results were obtained with the G2 and G2MP2 ab initio computational approaches. The zero-point corrected energy was 14.4 kcal mol⁻¹. Some of the exchange DFT methods (HFB) computed energies which were similar to the highly accurate ab initio methods, while the B3LYP hybrid DFT methods underestimated the activation barrier by 3 kcal mol⁻¹. Gradient-corrected DFT methods underestimated the barrier even more. The gradient-corrected DFT method that incorporated the PW91 correlational functional even generated a negative reaction barrier. The suitability of some computational methods for accurately predicting the potential energy surface for this hydrogen radical abstraction reaction was discussed.     

Thermochemistry of organosilicon compounds VII. Permethylcyclosilazanes and 1,1,3,3-tetramethyldisilazane

The enthalpy of formation (ΔH_f⁰), enthalpy of evaporation (ΔH_v⁰) and enthalpy of atomization (ΔH_a) of permethylcyclosilazanes (Me₂SiNH)_n (n = 3, 4) and 1,1,3,3-tetramethyldisilazane (Me₂SiH)₂NH have been determined. The enthalpies of formation of these compounds were compared with those calculated by the Benson-Buss-Franklin and Tatevskii additive schemes. In higher permethylcyclosilazanes the energy of the endocyclic Si---N bond is 306 ± 2 kJ mol⁻¹ (73 kcal mol⁻¹), that is 12 ± 2 kJ mol⁻¹ (3 kcal mol⁻¹) lower than the energy of the acyclic Si---N bond. The strain energy of the cyclotrisilazane ring is estimated to be 10.5 kJ mol⁻¹ (2.5 kcal mol⁻¹), whereas the energy of the ring Si---N bond is 295 kJ mol⁻¹ (70.5 kcal mol⁻¹).

The thermochemical data for permethylcyclosilazanes were compared with the corresponding values for permethylcyclosiloxanes calculated from the results of previously reported studies.     

Electronic structures and electronic spectra of the linkage isomers NSO and SNO

The relative stabilities and electronic structures of the linkage isomers NSO⁻ and SNO⁻ have been determined by the MNDO and ab initio Hartree—Fock—Slater methods. Both approaches predict a higher stability for SNO⁻ by ca. 100 kcal mol⁻¹, but an overlap population analysis indicates substantially higher bond orders for NSO⁻ compared to SNO⁻. The calculations also reveal a low energy pathway with a barrier of ca. 6 kcal mol⁻¹ for the isomerization process NSO⁻ → SNO⁻. Good agreement was found between the observed UV-visible absorption bands for NSO⁻ (λ_max 379 nm) and SNO⁻ (λ_max 340 nm) and calculated values of the electronic transition energies.     

Theoretical study of the N---H tautomerism in free base porphyrin

The N---H tautomerism of free base porphyrin is investigated at the semiempirical spin-unrestricted AM1 (UAM1) and ab initio RHF/3-21G levels. The UAM1 method provides delocalized geometries for all stationary structures without imposing any symmetry constraint. RHF/3-21G geometry optimizations have to be performed under symmetry restrictions to ensure that realistic delocalized structures are obtained. Both the semiempirical and the ab initio calculations predict that the interconversion between trans tautomers proceeds in an asynchronous two-step process via intermediate cis tautomers. The cis tautomers are characterized as minima in the potential energy surface and are 8–10 kcal mol⁻¹ higher in energy. The activation energy for the trans → cis interconversion is calculated to be approximately 23 kcal mol⁻¹ at the 3-21G level. The activation energy for the synchronous trans → trans interconversion is higher and has a value of 30.5 kcal mol⁻¹. The activation energies obtained at the semiempirical UAM1 level are twice as large as the ab initio values.     

Semiempirical and ab initio calculations on the alkaline hydrolysis of the β-lactam ring Influence of the solvent

We used semiempirical and ab initio calculations to investigate the nucleophilic attack of the hydroxyl ion on the β-lactam carbonyl group. Both allowed us to detect reaction intermediates pertaining to proton-transfer reactions. We also used ab initio calculations and the PM3 semiempirical method to investigate the influence of the solvent on the process. The AMSOL method predicts the occurrence of a potential energy barrier of 20.7 kcal mol⁻¹ due to the desolvation of the hydroxyl ion in approaching the β-lactam carbonyl group. Using the supermolecular approach and a water solvation sphere of 20 molecules around the solute, the potential energy barrier is lowered to 17.5 kcal mol⁻¹. Ab initio calculations using the SCRF method predict a potential energy barrier of 13.6 kcal mol⁻¹. These three values, especially the last two, are very close to the experimental value of 16.7 kcal mol⁻¹.     

Theoretical investigations of structure and enzymatic mechanisms of aspartyl proteinases : Part I. Ab-initio calculations on an active site model: hydrogen diformiate with H2O and H3O

The active site of aspartyl proteinases (Asp) was modelled as two formiates connected with a proton and set in geometry corresponding to Asp 32 and Asp 215 side chain carboxylate groups of endothiapepsin. The shared solvent molecule was alternatively H₂O and H₃O⁺. Their positions and those of hydrogen-bonded protons were optimized using the STO-3G basis set. Full geometry optimizations were made of the hydrogen diformiate complexes with H₂O and H₃O⁺. Asymmetric hydrogen-bonded structures resulted from these calculations, except for the fully optimized complex with H₂O. In the complexes with H₃O⁺, one proton moved consistently to the proximate carboxylic oxygen yielding a neutral, hydrated formic acid dimer. Interaction energies and proton potential energy curves were calculated using the 4-31G basis set. The interaction energy with H₂O was found to be 20.49 kcal mol⁻¹ and 202.75 kcal mol⁻¹ with H₃O⁺.     

MNDO study of the proton affinity of fluorinated formaldehydes and acetones

The MNDO molecular orbital method is employed to calculate the proton affinities of fluorinated formaldehydes and acetones. Agreement with experimentally reported proton affinities is good. In the acetone series a decrease in proton affinity is calculated for each successive fluorine substituent. The calculated strength of the intramolecular hydrogen bond in the protonated fluoro-formaldehydes and acetones is 0.6—2.7 kcal mol⁻¹, in good agreement with the experimental value of 2—3 kcal mol⁻¹ in the protonated fluoroacetones. Examination of the calculated charge distribution shows that the trends in proton affinity can be understood qualitatively both in terms of initial-state and final-state effects caused by the fluorine substituents. Protonation at the fluorine atom is less stable by about 25 kcal mol⁻¹ than protonation at the oxygen atom for monofluoroacetone.     

Tautomeric and conformational stabilities of methylphosphonic dicyanide, methoxydicyanophosphine, and their isocyano analogs: an ab initio calculation

The relative energies and structural parameters of the equilibrium forms and the potential functions of internal rotation of methylphosphonic dicyanide, CH₃(=O)(CN)₂, methoxydicyanophosphine, CH₃OP(CN)₂, and their isocyano analogs, CH₃P(=O)(NC)₂ and CH₃OP(NC)₂, have been calculated at the RHF/6-31G* level. The total energy of the more stable oxo forms CH₃P(=O)(CN)₂ and CH₃P(=O)(NC)₂ are 10–20 kcal mol⁻¹ lower than the energies of the aci forms CH₃OP(CN)₂ and CH₃OP(NC)₂. The relative stabilities of the cyano and isocyano isomers are almost the same in the case of the oxo forms, but for the aci forms the energies of the cyano isomers are 8 kcal mol⁻¹ lower than those of the isocyano isomers. The potential curves for internal rotation in the aci forms are characterized by a deep minimum corresponding to the trans arrangement of the methyl group and the lone pair of electrons on the phosphorus atom. Two less pronounced minima are symmetrically situated with respect to relative maximum corresponding to the transition cis form. The potential curves of internal rotation in the oxo form possess three minima corresponding to staggered configurations of the methyl group and phosphorus atom bonds. The energy characteristics and geometrical parameters of the studied molecules are compared with known data for similar compounds.     

Ab initio calculations of the vibrational spectra of the ammonia and fluoramide complexes with HF

The structures, energetics, vibrational frequencies and IR intensities of the H₃N HF, H₃N F₂ and NH₂FHF (three isomers) complexes were examined using the self-consistent field method within the 6-311G^** basis set. The interaction energies were calculated using the MP2 approach. The results are compared with monomer calculations and experimental data. The complex NH₂FHF was found to exist in three forms: one with the HF molecule hydrogen bonded to the nitrogen lone pair of NH₂F (D₀ =7.403 kcal mol⁻¹), another a complex formed through the F atom lone pair (D₀=4.698 kcal mol⁻¹) and third a cyclic structure (D₀=5.644 kcal mol⁻¹).     

Ab initio study on the mechanism of forming a silapolycyclic compound between silylidene and formaldehyde

The mechanism of the cycloaddition reaction of forming a silapolycyclic compound between singlet silylidene and formaldehyde has been investigated with MP2/6-31G* method, including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. The energies of the different conformations are calculated by CCSD(T)//MP2/6-31G* method. From the potential energy profile, it can be predicted that the cycloaddition reaction process of forming the silapolycyclic compound (P₂) for this reaction consists of four steps: (I) the two reactants first form a semi-cyclic intermediate INT1a through a barrier-free exothermic reaction of 32.5 kJ mol⁻¹; (II) this intermediate then isomerizes to an active four-membered ring intermediate INT1 via a transition state TS1a with an energy barrier of 30.8 kJ mol⁻¹; (III) INT1 further reacts with formaldehyde to form an intermediate INT2, which is also a barrier-free exothermic reaction of 30.1 kJ mol⁻¹; (IV) INT2 isomerizes to a silapolycyclic compound P₂ via a transition state TS2 with a barrier of 50.6 kJ mol⁻¹. Comparing this reaction path with other competitive reaction paths, we can see that this cycloaddition reaction has an excellent selectivity.     

Kinetic study of the reaction H2O2+H→H2O+OH by ab initio and density functional theory calculations

Theoretical investigations on the kinetics of the elementary reaction H₂O₂+H→H₂O+OH were performed using the transition state theory (TST). Ab initio (MP2//CASSCF) and density functional theory (B3LYP) methods were used with large basis set to predict the kinetic parameters; the classical barrier height and the pre-exponential factor. The ZPE and BSSE corrected value of the classical barrier height was predicted to be 4.1 kcal mol⁻¹ for MP2//CASSCF and 4.3 kcal mol⁻¹ for B3LYP calculations. The experimental value fitted from Arrhenius expressions ranges from 3.6 to 3.9 kcal mol⁻¹. Thermal rate constants of the title reaction, based on the ab initio and DFT calculations, was evaluated for temperature ranging from 200 to 2500 K assuming a direct reaction mechanism. The modeled ab initio-TST and DFT–TST rate constants calculated without tunneling were found to be in reasonable agreement with the observed ones indicating that the contribution of the tunneling effect to the reaction was predicted to be unimportant at ambient temperature.     

Ab initio study of the two competitive channels HO2 + H → H2 + O2 and HO2 + H → H2O + O

Saddle point geometries and barrier heights have been calculated for the H abstraction reaction HO₂(²A″)+H(²S) → H₂(¹Σ⁺_g)+O₂(³Σ⁻_g) and the concerted H approach-O removing reaction HO₂ (²A″)+H(²S) → H₂O(¹A₁)+O(³P) by using SDCI wavefunctions with a valence double-zeta plus polarization basis set. The saddle points are found to be of C_s symmetry and the barrier heights are respectively 5.3 and 19.8 kcal by including size consistent correction. Moreoever kinetic parameters have been evaluated within the framework of the TST theory. So activation energies and the rate constants are estimated to be respectively 2.3 kcal and 0.4×10⁹ ℓ mol⁻¹ s⁻¹ for the first reaction, 20.0 kcal and 5.4.10⁻⁵ ℓ mol⁻¹ s⁻¹ for the second. Comparison of these results with experimental determinations shows that hydrogen abstraction on HO₂ is an efficient mechanism for the formation of H₂ + O₂, while the concerted mechanism envisaged for the formation of H₂O + O is highly unlikely.     

Pyramidane 2. Further computational studies: potential energy surface, basicity and acidity, electron-withdrawing and electron-donating power, ionization energy and electron affinity, heat of formation and strain energy, and NMR chemical shifts

The novel cycloalkane pyramidane (tetracyclo[2.1.0.0^1,30^2,5]pentane, [3.3.3.3]fenestrane), C₅H₄, with a pyramidal carbon atom, was investigated further. Calculations at the B3LYP/6-31G^* and G2(MP2) levels supported earlier conclusions from QCISD(T)/6-31G^*//MP2(FC)/6-31G^* energies that pyramidane lies in a deep well (ca. 100 kJ mol⁻¹) on the potential energy surface. The pyramidal carbon is predicted to have a lone electron pair, and calculations (CBS-4) indicate that pyramidane is remarkably basic for a saturated hydrocarbon (proton affinity 976, cf. 922 and 915 kJ mol⁻¹ for pyridine and aniline, respectively). The calculated (CBS-4) acidity is similar to that of tetrahedrane and toluene; the pyramidyl group (C₅H₃) attached to an atom bearing a lone electron pair appears to be much more strongly electron-withdrawing than the phenyl group. The infrared CO stretching frequency and C–CHO rotational barriers of pyramCHO, PhCHO and cyclopropylCHO indicate that the pyramidyl group is comparable to phenyl and cyclopropyl in its ability to donate electrons to an electron-deficient carbon. The adiabatic ionization energy of pyramidane is ca. 9.0 eV (MP2/6-31G^*, energy differences and Koopmans’ theorem), similar to that of typical cycloalkanes. The heat of formation of pyramidane was calculated by the G2(MP2) method and isodesmic reactions to be to be 585 kJ mol⁻¹ and the strain energy was estimated to be 622 kJ mol⁻¹; pyramidane is 122 kJ mol⁻¹ more strained than its isomer spiropentadiene. Application of the NMR NICS method, varying the position of the probe nucleus, gave no evidence for benzenoid-type aromaticity in the potentially cyclobutadiene cation-like base of pyramidane.     

Negative hyperconjugation in methyl and silyl anions and its effect on the acidities of CH(4-n)Fn CH(4-n)C1n SiH(4-n)Fn and SiH(4-n)Cln

Group separation reactions calculated using an ab intio molecular orbital calculation at the MP4/6-31 + + G(d,p) level of theory, show the negative hyperconjugation between fluorine atoms to be larger in methanes than in silanes. Stabilisation due to negative hyperconjugation is larger in anions than in identically substituted neutral molecules, e.g. 43.1 kcal mol⁻¹ in CF₃⁻ compared with 26.7 kcal mol⁻¹ in CHF₃. By contrast, in chloro-substituted methanes, silanes, methyl anions and silyl anions, group separation energies are approximately zero, indicating no appreciable negative hyperconjugation. An -chloro substituent is more effective than an -fluoro one at delocalising the negative charge of an anion and, as a consequence, the chloromethanes and chlorosilanes are all more acidic than the identically substitued fluoromethanes and fluorosilanes. For chloro-substituted molecules the acidity is linearly dependent on the number of chlorine atoms; for fluoro-substituted molecules stabilisation by negative hyperconjugation results in each additional fluorine atom increasing the acidity by larger increments.     

Can chlorine anion catalyze the reaction of HOCl with HCl?

The reaction of HOCl + HCl → Cl₂ + H₂O in the presence of chlorine anion Cl⁻ has been studied using ab initio methods. The overall exothermicity is 15.5 kcal mol⁻¹ and this reaction has been shown to have a high activation barrier of 46.5 kcal mol⁻¹. Cl⁻ is found to catalyze the reaction via the formation of HOCl·Cl⁻, ClH·HOCl·Cl⁻ and Cl⁻·H₂) intermediate ion-molecule complexes or by interacting with a concerted four-center transition state of the reaction of HOCl + HCl.     

Potential energy distributions and potential scans for the internal rotation of two rotors in 3,3-dichloro and 3,3,3-trichloropropanals.

The conformational behavior and structural stability of 3,3-dichloropropanal and 3,3,3-trichloropropanal were investigated by ab initio calculations. The 6-311 + + G** basis set was employed to include polarization and diffuse functions in the calculations at B3LYP level. From the calculation, the trans conformer of 3,3,3-trichloropropanal was predicted to be the predominant conformer with about 2 kcal mol(-1) of energy lower than the cis form. Additionally, 3,3 dichloro-propanal was predicted to exist as a mixture of three stable conformers. The potential function scans were calculated for the two molecules from which the rotational barriers could be estimated. The vibrational frequencies were computed at B3LYP level and complete vibrational assignments were made based on normal coordinate calculations for the conformers of the two molecules. Vibrational Raman and infrared spectra of the mixture of the stable conformers were computed at 300 K.     

Rotational barriers in monomeric CH2=CX-COOH and CH2=CX-CONH2 (X is H or CH3) and vibrational analysis of methacrylic acid and methacrylamide

The internal rotations in acrylic and methacrylic acids CH2=CX-COOH and their amides CH2=CX-CONH2 (X is H or CH3) were investigated by DFT-B3LYP calculations with 6-311+G** basis set. The potential energy curves were consistent with two minima that correspond to planar cis and trans conformation in the case of the acids (or cis and near-trans forms in the case of the amides). Acrylic acid and acrylamide were predicted to have the cis form as the low and predominant conformation of the molecules. In the case of the methacrylic acid and methacrylamide, the conformational relative stability was predicted to reverse as going from the acrylic to the metha compounds. The trans conformer in methacrylic acid or the near-trans in methacrylamide were predicted to be thermodynamically low energy structures of the molecules. The CCCO rotational barrier was calculated to vary from 4 to 6kcal/mol in the four molecules. The OCOH and OCNH torsional barriers were calculated to be about 13 and 22kcal/mol in the acids and the amides, respectively. The vibrational frequencies of methacrylic acid and methacrylamide were computed at the DFT-B3LYP/6-311+G** level and reliable vibrational assignments were made on the basis of normal coordinate analyses and comparison with experimental data of both molecules in their low energy conformations.     

An ab initio SCF study of conformational isomerism in HNO2

The structures and complete force fields of cis and trans nitrous acid have been calculated with a (7, 3) basis set. The differences between the two stable isomers are reproduced well. The dipole moments, centrifugal distortion constants, vibrational frequencies and isotope shifts also agree satisfactorily with observed values. The fully optimized structures of rotamers corresponding to intermediate values of internal rotation around the N-O bond have been calculated to investigate structural changes during internal rotation. The considerable changes show the strong influence of π electron delocalization in the planar forms. The barrier is calculated to be 8.7 kcal mol⁻¹ in reasonable agreement with experimental values. Both the cis and trans barriers are calculated to be attractive dominant in contradiction to earlier work.