Potential Energy Surface Sections Research Articles

Specificity of internal rotation in aldehydes with three-membered rings

The results of anharmonic calculations of low-frequency vibrations of aldehyde molecules containing a three-membered cycle with a π-bond are presented. The theoretical exploration of the conformational behavior and geometrical structure of two related molecules is carried out and potential energy surface sections along the coordinates of internal rotation and non-planar vibration close in frequency are constructed and analyzed. The possibility of kinematic coupling of these two vibrations and the complexity of their forms are especially considered.

Electrochemical Reduction of Trichlorobiphenyls: Mechanism and Regioselectivity

The regioselectivity of electrochemical reduction of four trichlorobiphenyls (PCB 28–30 and PCB 37) was studied by cyclic voltammetry and bulk electrolysis. The number of stages and mechanism of electrochemical reduction of each of the examined substrate were inferred on the basis of the experimental electron transfer coefficients and calculated (DFT) bond lengths and potential energy surface sections. GC/MS analysis of the controlled potential electrolysis products showed that chlorine atom in the disubstituted ring of trichlorobiphenyls is reduced more readily than in the monosubstituted ring and that the rate of chlorine reduction changes in the series o-Cl > p-Cl > m-Cl.

On large amplitude motions of simplest amides in the ground and excited electronic states

For the formamide, acetamide, N-methylformamide and N-methylacetamide molecules in the ground (S0 ) and lowest excited singlet (S1 ) and triplet (T1 ) electronic states equilibrium geometry parameters, harmonic vibrational frequencies, barriers to conformational transitions and conformer energy differences were estimated by means of MP2, CCSD(T), CASSCF, CASPT2 and MRCI ab initio methods. One-, two- and three-dimensional potential energy surface (PES) sections corresponding to different large amplitude motions (LAM) were calculated by means of MP2/aug-cc-pVTZ (S0 ) and CASPT2/cc-pVTZ (S1 ,T1 ). For these molecules, in each excited electronic state six minima were found on 2D PES sections. Using PES sections, different anharmonic vibrational problems were solved and the frequencies of large amplitude vibrations were determined.

Position-dependent fragmentation mechanism for radical anions of fluorinated benzoates

Abstract DFT calculations were used to study the potential energy surfaces (PESes) of a full series of fluorinated benzoate radical anions (RAs). The sections of PESes along the C–F bond cleavage coordinates in polar media were built, and the transition states for RA fragmentation with fluoride anion elimination were located. The estimated reaction barrier heights let us interpret the experimental regularities of the RA decay including the process regioselectivity. The fragmentation mechanism was shown to depend on the position of the leaving fluorine atom. When defluorination occurs at an ortho - or meta - position to the CO 2 − group, the reaction coordinate involves pseudorotation as a way for odd electron density transfer to the breaking C–F bond. Additional gas phase calculations were performed to confirm the pseudorotational architecture of the PESes of polyfluorinated benzoate RAs. The results obtained clearly demonstrated that the multihole PES structure gives rise to the multichannel mechanism of RA cleavage.

Conformational analysis of N-methylformamide in ground S0 and excited S1 and T1 electronic states

For conformers of the N-methylformamide (HCONHCH3) molecule, calculations of equilibrium geometry parameters, harmonic vibration frequencies, energy differences and potential barriers to conformational transitions were performed in the ground (S0) and lowest excited singlet (S1) and triplet (T1) electronic states. In the S0 state, the molecule exists in trans and cis stable conformations (having Cs symmetry). Our calculations show that the electronic excitations T1←S0 and S1←S0 cause changes in the structure of conformers: both HCON and HNCC fragments become pyramidal and rotate around the CN bond. As a result, in each excited electronic state under consideration, there are 12 minima forming six pairs of equivalent conformers separated by relatively small potential barriers. One- and two-dimensional potential energy surface sections corresponding to different intramolecular large-amplitude motions were calculated using the MP2/aug-cc-pVTZ (S0) and CASPT2/cc-pVTZ (S1 and T1) methods. Anharmonic vibrational problems for large-amplitude motions were solved, and the corresponding frequencies were estimated.

Synthesis and antiviral activity of bis-spirocyclic derivatives of rhodanine

Bis-spiro heterocycles containing spiro units at the 1,3-positions of the pyrrolizidine (isothiapyrrolizidine) moiety were synthesized by the reaction of unstabilized azomethine ylides, which were generated in situ from isatin and proline (isothiaproline), with hetarylidene-substituted rhodanines. Quantum chemical calculations of potential energy surface sections and descriptors controlling the regioselectivity of cycloaddition were carried out. For a number of compounds in vitro activity against the influenza virus A/California/07/09 (H1N1)pdm2009 was experimentally established.

Theoretical study of the features of the nuclear-chemical synthesis of phenyl-substituted picoline derivatives

Sections of potential energy surfaces (PES) of phenyl cation addition to picolines were constructed using the DFT/B3LYP and MRCI methods. In the framework of the SA-MCSCF method energies of excited states of the products and intermediates in the reaction of phenyl cation with 4-picoline were determined. The equilibrium geometry and the dissociation energy of intermolecular complexes of benzene and picolines were calculated by the DFT/B3LYP method. The experimentally observed decrease in the yield in the reaction of phenyl cation addition to the 4-picoline as compared with 2- and 3-picolines was substantiated.

Spin-orbit coupling effects in the gas-phase reaction Se + O2

The potential energy surface sections of the ground and excited states of the SeOO (Cs) superoxo complex have been constructed on the basis of spin-orbit coupling calculations. The activation barriers to the forward and reverse reactions—superoxide SeOO ↔ peroxide Se(O2) ↔ dioxide SeO2—have been calculated. Changes in the spectral and spin properties of oxygen (zero-field splitting) and possible channels of formation of active forms of 1O2 oxygen (1Δg, 1Σg+) upon dissociation of excited states of selenium oxo complexes have been scrutinized.

A computational study on the excited state properties of a cationic cyanine dye: TTBC

The ground and excited state behaviors of a biologically important mitochondria selective dye, 5,5′,6,6′-tetrachloro-1,1′,3,3′-tetraethyl-benzimidazolo carbocyanine (TTBC), were investigated in order to give useful information related to the enhancement of its electronic and optical properties. DFT and TDDFT levels of theory with different functionals and basis sets were used. The results obtained by using PBEPBE/6-31+G(d,p)/C-PCM are in good agreement with the experiments. The potential energy surface sections of TTBC were explored and the geometry optimized in the first excited state, S1, to analyze the fluorescence spectrum of the dye. There are two distinct minima on the S1 surface, one located near the ground state geometry and an energetically favorable twisted geometry, having a charge transfer character. The observation of similar ground and excited state dipoles as well as solvent-independent fluorescence and absorption wavelengths reveals that there is no competition between the two minima; the emission is dominantly coming from the more planar configuration.

Quantum-chemical study of the photoisomerization and photocyclization reactions of styrylquinolines: Potential energy surfaces

The cross sections of potential energy surfaces (PES) for the S0 and S1 states were calculated by the semiempirical PM3 and PM3-CI (8 × 8) methods, respectively, along the reaction coordinate of the isomerization and cyclization of 2- and 4-styrylquinolines (SQ). The PES of the S0 state exhibits three minima separated by the transition-state barriers of isomerization and cyclization corresponding to three isomeric SQ forms, the E- and Z-isomers and the dihydrogenated cyclic product. On the PES of the S1 state, the “perpendicular minimum” at dihedral angle values of ∼ 90° corresponds to the transition state of the isomerization reaction and the pericyclic minimum with a distance of 1.7–2.0 A between the atoms involved in cyclization corresponds to the transition state of the cyclization reaction. With simultaneous scanning of the PES of the S1 state along the isomerization and cyclization reaction coordinates, the minimal-energy path was found for 4SQ, which makes it possible to explain the formation of the photocyclization product in the single-photon process upon irradiation of the E-isomer. It was found that the PM3 method overestimates the stability of the structures in which the aromatic ring is oriented perpendicular to the plane of the molecule, resulting in virtual minima on the PES of the S1 states.

Structures and conformational dynamics of monomethylated derivatives of acrolein: A quantum-chemical study

For all monomethylated derivatives of acrolein: methyl vinyl ketone СH 3C(O)CH CH 2, methacrolein CHO C(CH 3) CH 2, trans- and cis-crotonaldehydes CHO CH CH CH 3 in the ground electronic state, the conformer energy differences, barriers to internal rotation, geometric parameters of minima and transitions states corresponding to the barriers of internal rotation were studied by means of various quantum-chemical methods (B3LYP, MP2, QCISD, CCSD(T), CASSCF and others). The conformer energy differences were also estimated using the extrapolative technique VFPA. The vibrational frequencies were calculated at MP2/6-311G( d,p) level in harmonic and different anharmonic approximations. The coupling of two internal rotation motions was investigated by constructing one- and two-dimensional potential energy surface sections and by solving respective vibrational problems.

Vibronic spectra, ab initio calculations, and structures of conformationally non-rigid molecules of oxalyl halides in the ground and lowest excited electronic states. Part II: Theoretical investigation of oxalyl chloride

The structures of the oxalyl chloride molecule (COCl) 2 in the ground and the four lowest (two singlet and two triplet) excited electronic states were investigated by means of the CASPT2(8-6)/cc-pVTZ technique. The equilibrium geometric parameters, harmonic vibrational frequencies, and adiabatic energies of the electronic transitions were determined for all states under investigation. The calculations predicted the existence of the trans- and gauche- conformers in the ground state and the trans- and cis-conformers in all excited states. For the ground electronic state, the conformer energy difference and the barriers to conformational transitions were estimated using extrapolation to the complete basis set within a Valence Focal-Point Analysis procedure. The internal rotation in the excited electronic states was found to be strongly coupled with the non-planar symmetric CCOCl wagging. Two-dimensional potential energy surface sections along internal rotation and non-planar coordinates were constructed, and the corresponding anharmonic vibrational problems for the trans-conformer were solved.

The dynamic structure of cis and trans conformers of substituted phosphinous acids

The potential energy surfaces of bis-(trifluoromethyl)-phosphinous (III), diemthylphosphinous (IV), and bis-(pentafluorophenyl)-phosphinous (V) acids, the geometric parameters of the cis and trans conformers of these acids, and their P-O and O-H stretching vibration frequencies were calculated by the density functional theory method (PBE, 4Z). The potential energy surface sections corresponding to internal rotations about the P-O bonds were constructed, and dynamic problems for such a motion of the proton in a cyclic potential were solved by the Ritz method using a basis set of 100 trigonometric functions. According to calculations, the energy differences between the cis and trans conformers of acids III–V were of 4–7 kJ/mol, and the heights of potential barriers separating these conformers were of from 11 to 16 kJ/mol. In acids III and V, the cis, and, in IV, the trans conformer was stabler. At 298 K, only the ground vibrational states were populated for both rotamers of IV and V and the cis conformer of III. The special features of the potential of III were such that, at 298 K, the first excited vibrational level of its trans conformer was also noticeably populated.

Connection between Chemisorption Kinetics and Adsorption Equilibria of Organic Compounds on Oxide Surfaces

Quantum chemical computations of potential energy surface sections along the reaction pathway for interaction of some molecules with silica surface OH groups in a cluster approach have been performed. Two minimums on the potential curves which are due to formation of linear hydrogen-bonded and following cyclic donor-acceptor complexes were observed prior to formation of the transition state for proton transfer in the cyclic complexes. Some tendencies between quantum chemical indexes of organic molecules and heats of the complex formation or chemisorption activation energies on the silica surface have been established.

A quantum-chemical study of the mechanism of nucleophilic addition to carbonyl compounds

1. MINDO/2 calculations have been used to develop potential energy surface sections for nucleophilic (H2O) addition to carbonyl compounds (H2CO) in systems with various types of protonation. 2. The increase of the reaction rate resulting from acid-base catalysis can be largely explained in terms of reactant interaction at distances ranging up to 3 A. 3. The activation energy for aqueous solution reactions between reactants, one of which is charged, is determined by the energy of deformation of the hydration spheres, the process which opens the possibility for reactant approach.