Comparative theoretical study of transition structures,barrier heights,and reaction energies for the intramolecular tautomerization in acetaldehyde/vinyl alcohol and acetaldimine/vinylamine systems

The transition structures associated with the possible intramolecular tautomerization for acetaldehyde/vinyl alcohol and acetaldimine/vinylamine systems as models of keto/enol and imine/enamine interconversion processes, respectively, were characterized. The relative stabilities of the tautomers and the associated barrier heights were calculated. Ab initio analytical gradients and second derivatives at the HF level of theory and 3-21G, 6-31G, 6-31G**, 6-31++G**, and 6-311++G** basis-set, DFT (BP86/6-311++G** and BLYP/6-311++G**), and semiempirical (AM1 and PM3) procedures were used to identify the stationary points. Correlation effects were estimated using the perturbational approach at MP2/6-31G**, MP2/6-311++G**, and MP2/6-311++G (3df,2p) levels. The geometry, electronic structure, harmonic vibrational frequencies, and transition vector associated with the transition structures as well as the relative stabilities of different isomers and barrier heights were analyzed. The dependence of these properties upon theoretical methods is analyzed and discussed. The transition structures are four-membered rings and the corresponding transition vectors are associated to collective fluctuations. The 1,3 intramolecular hydrogen migration is much more advanced than are the hybridization changes on donor and acceptor centers at the transition structure. The corresponding barrier heights can be related to the change of bond orders and acid/base properties of these centers. A comparison of the results obtained with different methods renders that the nature of the transition structure seems to be a rather robust entity. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 9–24, 1998     

Halogen switching of azacarbenes C2NH ground states at ab initio and DFT levels

Relative stabilities and singlet–triplet energy differences are calculated for 24 C₂NX azacarbenes (where X is H, F, Cl, and Br). Three skeletal arrangements are employed including azacyclopropenylidene, [(imino)methylene]carbene, and cyanocarbene. Halogens appear to alternate the electronic ground states of C₂NH azacarbenes, from triplet to singlet states, at MP3/6‐311++G**, B1LYP/6‐311++G**, B3LYP/6‐311++G**, MP2/6‐311++G**, MP4(SDTQ)/6‐311++G**, QCISD(T)/6‐311++G**, CCSD(T)/6‐311++G**, CCSD(T)/cc‐pVTZ, G1, and G2 levels of theory. The aromatic characters of singlet cyclic azacyclopropenylidenes are measured using GIAO–NICS calculations. Linear correlations are found between the B3LYP/6‐311++G** calculated LUMO–HOMO energy gaps (ΔE_{HOMO ‐ LUMO}) of the singlet carbenes versus their corresponding singlet–triplet energy separations (ΔE). Electrophilic characters are found for all singlet azacarbenes in their addition reactions to alkenes with the highest electrophilicity being exhibited for X = F. © 2008 Wiley Periodicals, Inc. Heteroatom Chem 19:377–388, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20442     

Density functional efficiency in the calculations of vibrational frequencies and molecular structures of β-diketones

Density functional theory (DFT) levels are employed to calculate the vibrational frequencies and geometrical data of β-diketones. We evaluate the relative performance of the different levels by comparing theoretical results to experimental values. The applied DFT levels in this work are B3LYP, BLYP, B3P86, B3PW91, BPW91, G96LYP, BP86, and G96PW91 with the standard 6-31G, 6-31G*, 6-31G**, 6-31+G**, 6-31++G**, 6-311G**, 6-311++G** basis sets. The best results are obtained at the B3LYP, B3PW91, and B3P86 levels.     

硝酸甲与硝酸乙酯的结构、振动频率和热力学性质的密度函数 理论(DFT)研究

用密度函数理论(DFT)的BLYP和B3LYP方法,取6-31G,6-31G^*,6-31G^*^*,6-311G,6-311G^*和6-311G^*^*六种基组,对硝酸甲酯和硝酸乙酯的几何构型和红外振动频率进行了计算研究.结果表明,B3LYP方法在采用极化基组(6-31G^*,6-31G^*^*,6-311G^*和6-311G^*^*)时计算得到的结果均较好,适用于硝酸酯类化合物的研究.而BLYP方法无论采用何种基组均不适用;运用校正后的B3LYP/6-31G^*频率(校正因子0.975)计算得到的热力学性质(C^o~p,H^o和S^o)与实验结果较吻合。     

Ab initio and DFT energetics of silylenic X–CNSi (X=H, F, Cl, and Br)

Singlet–triplet energy splitting for 24 silylenic reactive intermediates, X–CNSi (where X=H, F, Cl and Br), are compared and contrasted at 11 levels of theory: B1LYP/6-31++G**, B3LYP/6-31++G**, B1LYP/6-311++G**, B3LYP/6-311++G**, MP3/6-31G*, MP3/6-311++G**, MP2/6-31+G**, MP2/6-311++G**, MP4 (SDTQ)/6-311++G**, QCISD(T)/6-311++G** and CCSD(T)/6-311++G**. Each X-substituted silylenic species may either be singlet (s) or triplet (t), with one of the following three structures: 3-X-2-aza-1-silacyclopropenylidene (1_s-X, 1_t-X); [(X-imino)methylene]silylene (2_s-X, 2_t-X); and X-cyanosilylene (3_s-X, 3_t-X). For all X–CNSi species studied, orders of singlet–triplet energy separations (ΔE_s-t,X), appear as a function of electro-negativity (F>Cl>Br>H). For the six H–CNSi isomers (X=H), stability order is: 3_s-H>1_s-H>2_t-H>3_t-H>2_s-H>1_t-H. Likewise, stability order for the six isomers with X=F, is: 3_s-F>3_t-F>1_s-F>1_t-F>2_s-F>2_t-F. For X=Cl, the order of stability is: 3_s-Cl>1_s-Cl>3_t-Cl>2_t-Cl>1_t-Cl>2_t-Cl. Finally, the order of stability for six isomers of Br–CNSi is: 3_s-Br>3_t-Br>1_s-Br>2_s-Br>2_t-Br>1_t-Br. The lowest energy minimum, among all 24 species scrutinized, appears to be the singlet acyclic 3_s-X. Triplet silylene 2_t-H is suggested to be more stable than its corresponding 2_s-H at MP3, MP2 and DFT levels of theory. Comparisons between relative stabilities; multiplicities and geometrical parameters of 1–3 are discussed.     

Theoretical study of light-metal tetrahydroborates

Ab initio calculations of the structural, energetic, vibrational, and magnetic characteristics of the lowest-lying structures for isolated molecules and ions of light-metal tetrahydroborates (Li, Na, Be, Mg, and Al) have been performed by the perturbation theory (MP2), quadratic configuration interaction (QCISD(T)), coupled cluster (CCSD(T)), and density functional theory (B3LYP) methods using the 6-31G*, 6-31G**, 6-311+G**, and 6-311++G** basis sets. The trends in the behavior of the structural parameters, the energies of different decomposition pathways, barriers to internal rotation of BH₄ groups, normal mode frequencies, magnetic shielding constants, and spin density distribution (in radicals) have been analyzed in various related series of these compounds. The results obtained by ab initio methods and at the DFT level are compared. The economical approximation B3LYP/6-311++G**//B3LYP/6-311+G** adequately reproduces the results obtained at the higher level of theory CCSD(T)/6-311++G*s*//MP2/6-31G* even though it requires considerably shorter CPU times and smaller amounts of memory.     

Theoretical study of light-metal tetrahydroaluminates

Ab initio calculations of the structural, energetic, vibrational, and magnetic characteristics of the lowest-lying structures for isolated molecules and ions of light-metal tetrahydroaluminates (Li, Na, Be, Mg, and Al) have been performed by the perturbation theory (MP2), quadratic configuration interaction (QCISD(T)), coupled cluster (CCSD(T)), and density functional theory (B3LYP) methods using the 6-31G*, 6-31G**, 6-311+G**, and 6-311++G** basis sets. The trends in the behavior of the molecular characteristics have been analyzed in various related series of these compounds. The results are compared with the data on analogous light-metal tetrahydroborates calculated at the same levels of theory. The differences in structure and stability between analogous hydroborate and alanate complexes are examined. The economical approximation B3LYP/6-311++G**//B3LYP/6-311+G** has been shown to adequately reproduce the results obtained at the higher level of theory CCSD(T)/6-311++G**//MP2/6-31G* even though it requires considerably shorter CPU times and smaller amounts of memory.     

Post Hartree–Fock and density functional theory studies on Di-Protonated Allopurinol

Post Hartree–Fock and density functional theory (DFT) methods have been employed to study the molecular properties of Di-Protonated Allopurinol²⁺ tautomers in gaseous and aqueous phase environments. The tautomers in gaseous phase have been optimized at MP2/6-311G(2d,2p) and B3LYP/6-311G(2d,2p) levels of theory. The self-consistent reaction field theory (SCRF) has been employed to optimize the tautomers in aqueous phase (ε = 78.5) at B3LYP/6-311G(2d,2p) level of theory and the solvent effect has been studied. The structure, energetics and relative stabilities of the tautomers have been analyzed both in gaseous and aqueous phases. The principle of maximum hardness (MHP) has been tested at B3LYP/6-311G(2d,2p) level of theory. The condensed Fukui functions have been calculated using the atomic charges obtained through Natural population analysis to identify the relative change in the most reactive site of the optimized structures. NMR studies have been carried out, on the basis of Cheeseman coworker’s method, to analyze the molecular environment as well as the delocalization activities of electron clouds.     

A combined semiempirical-DFT study of oligomers within the finite-chain approximation, evolution from oligomers to polymers

A computationally cheap approach combining time-independent density functional theory (TIDFT) and semiempirical methods with an appropriate extrapolation procedure is proposed to accurately estimate geometrical and electronic properties of conjugated polymers using just a small set of oligomers. The highest occupied molecular orbital-lowest unoccupied molecular orbital gap (HLG) obtained at a TIDFT level (B3PW91) for two polymers, trans-polyacetylene--the simplest conjugated polymer, and a much larger poly(2-methoxy-5-(2,9-ethyl-hexyloxy)-1,4-phenylenevinylene (MEH-PPV) polymer converge to virtually the same asymptotic value than the excitation energy obtained with time-dependent DFT (TDDFT) calculations using the same functional. For TIDFT geometries, the HLG is found to converge to a value within the experimentally accepted range for the band gap of these polymers, when an exponential extrapolation is used; however if semiempirical geometries are used, a linear fit of the HLG versus 1/n is found to produce the best results. Geometrical parameters are observed to reach a saturation value in good agreement with experimental information, within the length of oligomers calculated here and no extrapolation was considered necessary. Finally, the performance of three different semiempirical methods (AM1, PM3, and MNDO) and for the TIDFT calculations, the performance of 7 different full electron basis sets (6-311+G**, 6-31+ +G**, 6-311+ +G**, 6-31+G**, 6-31G**, 6-31+G*, and 6-31G) is compared and it is determined that the choice of semiempirical method or the basis set does not significantly affect the results.     

A Computational View of PATO and its Tautomers

1, 2, 4‐Triazole and 3‐amino‐1, 2, 4‐triazole are useful starting materials for the synthesis of many 1, 2, 4‐triazole‐based explosives. Electronic properties and Kamlet–Jacobs detonation performances of PATO (3‐picrylamino‐1, 2, 4‐triazole) (a relatively inexpensive, insensitive, and thermally stable explosive) and its 1, 3‐ and 1, 5‐ tautomers are investigated computationally using PM3, HF/6‐31G(d, p), and B3LYP/6‐31G(d, p) methods. The relationships between the thermodynamic and kinetic stabilities and the aromaticities are examined for all structures and their relative sensitivities are compared in terms of bond dissociation energies. The use of both PM3 and isodesmic methods in the calculation of heat of formation of PATO is discussed.     

A computational study of intramolecular proton transfer in gaseous protonated glycine

The minimum energy paths for intramolecular proton transfer between the amino nitrogen and carbonyl oxygen atoms in gaseous protonated glycine were estimated at the Hartree-Fock (HF) and second-order M?ller-Plesset Perturbation (MP2) levels of theory. Potential energy profiles and their associated reactant, transition state, and product species calculated at the MP2/6-31G* level were shown to differ significantly from those obtained at the HF/6-31G* level. Effects of electron correlation and basis functions on the calculated geometries and energies of relevant species were examined at the HF, MP2, MP4, CCSD, and B3LYP levels using the 6-31G*, 6-31G**, 6-31+G**, 6-311+G**, 6-31+G(2d,2p), 6-311+G(3df,2p), cc-pVDZ, aug-cc-pVDZ, and cc-pVTZ basis sets. The HF and MP2 optimized levels with the 6-31G*, 6-31G**, 6-31+G**, and 6-311+G** bases were used to calculate the thermodynamic and kinetic properties of the proton transfer reaction at 298.15 K and 1 atm, which include enthalpy, entropy, Gibbs free energy, equilibrium constant, potential energy barriers, tunneling transmission coefficients, and rate constants. Results indicate that the proton in a carbonyl O-protonated glycine undergoes a rapid migration to the amino nitrogen atom, while the reverse process is extremely unfavorable. The objective of this work is to develop practical theoretical procedures for studying proton transfer reactions in amino acids and peptides and to assemble physical data from these model calculations for future references.     

Study of conformational and optical rotation for the alaninamide

Six stationary points of alaninamide have been located on the potential surface energy (PES) at the B3LYP/6‐311++G(2d,2p) level of theory both in the gas phase and in aqueous solution. In the aqueous solution, to take the water solvent effect into account, the polarizable continuum model (PCM) method has been used. Accurate geometric structures and their relative stabilities have been investigated. The results show that the intramolecular hydrogen bond plays a very important role in stabilizing the global minimum of the alaninamide. Moreover, the consistent result in relative energy using high‐level computations, including the MP2 and MP3 methods with the same basis set [6‐311++G(2d,2p)], indicates that the B3LYP/6‐311++G(d,p) level may be applied to the analogue system. More importantly, the optical rotation of the optimized conformers (both in the gas phase and in aqueous solution) of alaninamide have been calculated using the density functional theory (DFT) and Hartree–Fock (HF) method at various basis sets (6‐31+G*, 6‐311++G(d,p), 6‐311++G(2d,2p) and aug‐cc‐pvdz). The results show that the selection of the computation method and the basis set in calculation has great influence on the results of the optical rotations. The reliability of the HF method is less than that of DFT, and selecting the basis set of 6‐311++G(2d,2p) and aug‐cc‐pvDZ produces relative reliable results. Analysis of the computational results of the structure parameters and the optical rotations yields the conclusion that just the helixes in molecules caused the chiral molecules to be optical active. The Boltzmann equilibrium distributions for the six conformers (both in the gas phase and in the aqueous solution) are also carried out. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Theoretical investigation of the tautomerism of isoorotic acid in gaseous and aqueous phases

In the present investigation, the tautomeric and conformational equilibrium of isoorotic acid have been studied using Møller–Plesset second‐order (MP2) and density functional theory (DFT) methods in the gas phase and aqueous solution (ε = 78.5) using the IPCM model. The relative energies of these tautomers have been calculated at the two levels of theory using 6‐311++G** basis set. Energetics and relative stabilities of the tautomers were compared and analyzed in both the gaseous and aqueous phases. The results indicate that the diketo tautomer (iso) is the most stable form in the gas phase and water. The carboxylic substitution in the uracil ring does not alter its relative stability order of the tautomers. The proton affinity of the oxygen atoms and the deprotonation enthalpy of the NH bonds of isoorotic acid have been compared with recent data of uracil. The relative stability of both syn‐ and anti‐conformations was investigated and the syn form was found to be more stable by 17.65 kcal/mol. It was determined in ab initio calculations that an electron can attach to isoorotic acid, forming a stable anion better than uracil. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Study of metal ions (Na+, K+) interaction with different conformations of glycine molecule

Interaction of metal ions (Na⁺, K⁺) with different binding sites, such as amino nitrogen, hydroxyl oxygen, and carbonyl oxygen for all gaseous conformers of glycine molecule were investigated using Density Functional Theory (B3LYP/6‐311++G**, B3PW91/6‐311++G**) methods. It was found that the order of stability of the conformers was changed due to the binding of the metal ion. The relative energy values show that the 7p conformer is more stable than the 1p conformer when a metal ion binds with the carbonyl oxygen. The intensity of interaction on hydroxyl oxygen is very low due to the low basicity of hydroxyl oxygen. The binding affinities of the complexes were calculated using the thermochemical properties. The relative energy and chemical hardness values predicted the most stable complex. The calculated condensed Fukui functions predict the favorable reactive site among the three binding sites. It is concluded that the reactivity of each binding site varies for each conformation due to the presence of intramolecular hydrogen bonding. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Kinetic and thermodynamic study of inter‐ and intramolecular proton transfer in N′‐acetyl formohydrazide tautomers

Nine tautomers and eleven possible tautomeric interconversions of N′‐acetyl formohydrazide have been studied at B3LYP/6‐311++G** level of theory. From these calculations, optimized geometries, molecular parameters, IR frequencies, NMR chemical shifts, and energetic results are obtained. In all tautomers except tautomers 4, E isomer is more stable than Z isomer. Energetic data were used to calculate the energy barriers of tautomeric interconversions and very high energy barriers were obtained for all tautomeric interconversions. Moreover, study of solvent effects on relative stabilities of tautomers and transition states showed that they are similar to those in the gas phase. In addition, intermolecular proton transfer with the assistance of one to three water molecules has been studied and the results showed that activation barriers in water‐assisted tautomerism are in general lower than those in the gas phase. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Transitions to high tautomeric states can be induced in adenine by interactions with carboxylate and sodium ions: DFT calculation data

Twelve triple complexes of nine adenine tautomers with carboxylate ion of acetic acid CH₃COO⁻ and sodium ion Na⁺ of the CH₃COO⁻:Ade:Na⁺ type were studied by the B3LYP/6-311++G(d,p)//B3LYP/6-31G(d,p) quantum chemical method. It was established that three rare adenine tautomers generate more stable complexes than the ground state one. The B3LYP/6-311++G(d,p) full optimization was applied to four most stable triple complexes in order to refine on their stability order. To evaluate contributions of each ligands for stabilization of these complexes, calculations of energies and dipole moments of respective binary complexes of the CH₃COO⁻:Ade and Ade:Na⁺ types were performed at the B3LYP/6-31++G(d,p) level of theory. At the same level, there were computed energies and dipole moments of isolated tautomers. Two of them, first calculated in this work, appeared to be zwitter-ions. Highly cooperative interplay of interactions of both ligands with adenine tautomers was observed in triple complexes. Preferable sites of coordination of Na⁺ with tautomers were ascertained. Biological importance of the results obtained is discussed.     

An Ab Initio Molecular Orbital Study of Valence Bond Isomers of Silabenzene and Phosphabenzene

Ab initio molecular orbital calculation at HF/6-31G*, HF/6-31G**, HF/6-311G**, HF/6-311++G**, RMP2-FC/6-31G*, and B3LYP/6-31G* levels of theory for geometry optimization and MP4(SDQ)/6-31G* for a single point total energy calculation are reported for silabenzene ( 7 ), phosphabenzene ( 8 ) and 16 valence bond isomers of silabenzene and phosphabenzene ( 9-24 ). The calculated energy difference (19.78 kcal mol m 1 ) between silabenzene and the most stable valence bond isomer of silabenzene (1-silabenzvalene, 9 ) is much smaller than the difference (73.60 kcal mol m 1 ) between benzene and benzvalene ( 2 ). The energy difference between phosphabenzene and the most stable valence bond isomer of phosphabenzene (1-phosphabenzvalene, 17 ) is calculated to be 43.29 kcal mol m 1 .     

An SCF and CI Study of the 1,3 Shift in the HX?CHY⇌XCH?YH Isoelectronic Series: X,YCH2, NH,and O

Ab initio molecular orbital calculations at SCF level with the 3-21G, 6-31G, and 6-31G** basis sets and CI level with the 6-31G basis set have been carried out for an isoelectronic series HX? CH?Y and X?CH? YH, where X, Y can be CH₂, NH, and O. Optimized structures (3-21G and 6-31G**) for both tautomers and the 1,3 hydrogen shift transition states are reported. The relative stabilities of the isomers and the barriers of the 1,3 shift are discussed in terms of proton affinities and bond orders. It is shown that both the relative stabilities of the tautomers and the relative barrier heights can be explained qualitatively using simple proton affinity arguments and that the barrier heights are quantitatively related to bond orders.     

Theoretical study of molecular interaction between tirapazamine enzymatic catalysis metabolites and water

The weakly hydrogen‐bonded complexes, between tirapazamine enzymatic catalysis metabolites and water, have been investigated by density functional theory (DFT), using the B3LYP hybrid functional. The geometries of these complexes have been fully optimized at the B3LYP/6‐31G(d) and B3LYP/6‐311+G(d) levels. The stabilization energies and charge changes of some atoms have been calculated and analyzed. The results indicate that the catalysis metabolites and water can form stable hydrogen‐bonded complexes. Nine complexes are identified. It is important and necessary to add zero‐point vibrational energy (ZPVE) and basis set superposition error (BSSE) corrections for calculating stabilization energy. The results also reveal an important relationship between the relative stabilities of hydrogen‐bonded complexes and the final products of tirapazamine medication. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Comprehensive study of the effects of methylation on tautomeric equilibria of nucleic acid bases

Minor tautomers of nucleic acid bases can result by intramolecular proton transfer. These rare tautomers could be stabilized through the addition of methyl groups to DNA bases. A comprehensive theoretical study of tautomers of methylated derivatives of guanine, adenine, cytosine, thymine, and uracil was performed. Molecular geometries of all tautomers were obtained at the density functional theory and MP2 levels with the 6-31G(d,p) basis set, and single-point calculations were performed at the CCSD(T)/6-311G(d,p) level. Tautomers obtained by protonation at the preferred protonation site for methylated isolated bases were compared to their nonmethylated counterparts. The effects of methylation on the relative stabilities of nucleic acid base tautomers are also studied and discussed in this work. The results suggest that some sites on the bases may not be mutagenic and may even stabilize the canonical Watson-Crick form. The results also indicate that a number of methylation sites can stabilize the tautomers, suggesting possible mechanisms for mutagenic changes.