Stabilities Of Different Isomers Research Articles

Theoretical study of solvent effects on the structures and isomerization of silylenoid H2SiLiF

The structures and isomerization of silylenoid H2SiLiF are investigated by density functional theory (DFT) at B3LYP/6-311+G(d,p) level. The solvent effects are modeled using the self-consistent reaction field (SCRF) method with Tomasi’s polarized continuum model (PCM). Five representative solvents, i.e. benzene, ether, THF, acetone, and DMSO, are chosen for this work. Special attention is paid to THF solvent. The results indicate that the polarity of solvents has played an important role on the structures and relative stabilities of different isomers. Contrary to the three-membered ring structure 1, the relative stability of the “classical” tetrahedron structure 4 increases with increasing dielectric constants (e) of solvents. The σ-complex isomer 3 is most unstable structure. Although the relative energies are reduced with increasing dielectric constants (e) of solvents, the p-complex structure 2 still has the lowest energy. The effects of solvents on the dipole moments and charge distributions are also discussed.

Density functional theory study of the hydrogen bonding interaction of 1:1 complexes of formamide with hydrogen peroxide

For the first time, the hydrogen bonding of 1:1 complexes formed between formamide and hydrogen peroxide molecule have been completely investigated in the present study using density functional theory (DFT), second-order Moller-Plesset Perturbation (MP2) and Hartree-Fock (SCF) method at varied basis set levels from 6-31g to 6-31++g(d,p). Five reasonable geometries on the potential energy hypersurface of formamide and hydrogen peroxide system are considered with the global minimum being a cyclic double-hydrogen bonded structure. The relative stability order of the five structures is FH1>FH2>FH3>FH4≈FH5. The infrared spectrum frequencies, IR intensities and the vibrational frequency shifts are reported. Finally the solvent effects on the geometries of the formamide–hydrogen peroxide complexes have also been investigated using self-consistent reaction-field (SCRF) calculations at the B3LYP/6-31++g(d,p) level. The results indicate that the polarity of the solvent has played an important role on the structures and relative stabilities of different isomers. In water, FH2 disappears and the relative stability order changes to FH3>FH1>FH5≈FH4 which is not in consistent with the gas phase one.

Density functional theory study of the hydrogen bonding interaction in formamide dimer

The hydrogen bonding of 1:1 complexes formed between formamide and formamide molecule has been completely investigated in the present study using density functional theory (DFT) method at varied basis set levels from 6-31G to 6-311++G(2d,2p). The large basis sets 6-311++G(d,p) and 6-311++G(2d,2p) have been employed to determine the equilibrium structure and vibrational frequencies of the interacting complexes. Five reasonable geometries on the potential energy hypersurface of formamide dimer system are considered with the global minimum, four of which are cyclic double-hydrogen bonded structure and one is one-hydrogen bonded structure. The optimized geometric parameters and interaction energies for various isomers at different levels are estimated. The infrared spectrum frequencies, IR intensities and the vibrational frequency shifts are reported. Finally the solvent effects on the geometries of the formamide dimer have also been investigated using self-consistent reaction-field (SCRF) calculations at the B3LYP/6-311++G** level. The results indicate that the polarity of the solvent has played an important role on the structures and relative stabilities of different isomers.

Density functional theory study of the hydrogen bonding interaction of 1:1 complexes of serine with water

AbstractThe hydrogen bonding of 1:1 complexes formed between serine and water molecules were completely investigated in the present study employing ab initio and Density Functional Theory (DFT) methods at varied basis set levels from 6‐31g to 6‐311++g (2d,2p). For comparison, we also used the second‐order Moller–Plesset Perturbation (MP2) method at the 6‐31+g(d) level. Twelve reasonable geometries on the potential energy hypersurface of serine and water system were considered with the global minimum, 10 of which are cyclic double‐hydrogen bonded structures and the other two are one‐hydrogen bonded structures. The optimized geometric parameters and interaction energies for various isomers at different levels were estimated. The infrared spectrum frequencies, IR intensities, and the vibrational frequency shifts are reported. Finally, the solvent effects on the geometries of the serine–water complexes were also investigated using self‐consistent reaction‐field (SCRF) calculations at the B3LYP/6‐311++g(d,p) level. The results indicate that the polarity of the solvent played an important role in the structures and relative stabilities of different isomers. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Conformational Analysis of Trans-3, 6-Dibutanal-1, 2, 4, 5-Tetroxane

We report the results of theoretical semiempirical AM1 and PM3 molecular orbital methods on the conformational analysis of the title compound. The relative stability of different isomers and conformers are discussed on the basis of well defined electronic and steric effects, which are rather helpful in order to understand the relative stabilities.

Theoretical study of helical structure caused by chirality of cysteine dimer

In this work we report a theoretical study of the helix structure and chiral discrimination on the interactions between the chiral cysteine–cysteine. Two reasonable geometries on the potential energy hypersurface of the cysteine–cysteine system are considered with the global minimum. Accurate geometric structures, relative stabilities, harmonic vibrational frequencies, and infrared (IR) intensities were investigated. To take into account the water solvation effect, the Onsager model within the self-consistent reaction field (SCRF) method and the polarized continuum (PCM) method were used to evaluate the interaction energy, ΔGsolv at the same level employed in the gas phase. The results indicate that the polarity of the solvent plays an important role in the structures and relative stabilities of different isomers. Computational results indicate that the global minimum should be conformer I regardless of whether in the gas phase or in aqueous solution, which differs from previous theoretical reports. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Theoretical study of chiral discrimination in the hydrogen bonding complexes of the hydrazine dimer

AbstractIn this work, the discrimination of different chiral forms of the hydrazine dimer were investigated using Density Functional Theory (DFT) and second‐order Moller–Plesset Perturbation (MP2) theory at basis set levels from 6‐31g to 6‐31++g(d,p). Four chiral structures were studied. The optimized geometric parameters, interaction energies, and chirodiastatic energy for various isomers at different levels were estimated. Finally, the solvent effects on the geometries of the hydrazine dimers were also investigated using self‐consistent reaction‐field (SCRF) calculations at the B3LYP/6‐31++g(d,p) level. The results indicate that the polarity of the solvent has played an important role in the structures and relative stabilities of different isomers. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Density Functional Theory Study of the Hydrogen-Bonding Interaction of 1:1 Complexes of Alanine with Water

The hydrogen bonding of 1:1 complexes formed between alanine and water molecules has been completely investigated in the present study using density functional theory, method B3LYP at varied basis set levels from 6-31G to 6-311++G(d,p) and the second-order Møller−Plesset perturbation method at the 6-31++G(d,p) level. Eight reasonable geometries on the potential energy hypersurface of the alanine and water system are considered with the global minimum. The optimized geometric parameters and interaction energies for various isomers at different levels are estimated. The infrared spectrum frequencies, IR intensities, and vibrational frequency shifts are reported. Finally, the solvent effects on the geometries of the alanine−water complexes have also been investigated using self-consistent reaction-field calculations at the B3LYP/6-311++G(d,p) level. The results indicate that the polarity of the solvent plays an important role in determining the structures and relative stabilities of different isomers.

Theoretical study on the hydrogen bond interaction between diacetamide and methanol

The hydrogen bonding of 1:1 complexes formed between diacetamide and methanol molecule have been completely investigated in the present study using density functional theory and second order Moller–Plesset perturbation (MP2) method. Various basis sets from 6-31G to 6-311++G** have been employed to determine the equilibrium structure of the interacting complexes. Three reasonable geometries on the potential energy hypersurface of diacetamide with methanol system are considered with the global minimum being a cyclic double-hydrogen bonded structure. The optimized geometric parameters and interaction energies for various isomers at different levels are estimated showing that the two carbonyl groups have about the same proton acceptor ability. Special attention was paid to the basis set superposition error and zero-point energy corrections that were not given appropriate attention in previous studies. Finally, the solution phase studies are also carried out using the Onsager reaction field model at B3LYP/6-311++G** level for the hydrogen-bonded complex of diacetamide with methanol. The results indicate the polarity of the solvent has played an important role on the structures and relative stabilities of different isomers or the proton acceptor ability of two carbonyl groups.

Study of the diacetamide–water dimer with ab initio and density functional theory methods

Abstract The hydrogen bonding of 1:1 complexes formed between diacetamide and water molecule have been completely investigated in the present study using density functional theory and second order Moller–Plesset perturbation (MP2) method. The large basis sets 6-311++g(d,p) and 6-311++g(2d,2p) have been employed to determine the equilibrium structure of the interacting complexes. All the results reveal a planar configuration of the amide groups and a tendency of the CH 3 group to eclipse the CO bond for the geometry of the isolated diacetamide molecule. Calculations at different theoretical levels indicate that cis – trans configuration is the most stable isomer in both gas phase and solution phase. Three reasonable geometries on the potential energy hypersurface of diacetamide with water system are considered with the global minimum being a cyclic double-hydrogen bonded structure. The optimized geometric parameters and interaction energies for various isomers at different levels are estimated and a result that two carbonyl groups have about the same proton acceptor ability has been obtained. Finally, the solution phase studies are also carried out using the onsager reaction field model at B3LYP/6-311++g** level for the isolated diacetamide molecule and the hydrogen-bonded complex of diacetamide with water. The results indicate the polarity of the solvent has played an important role on the structures and relative stabilities of different isomers.

Structure and bonding in the isoelectronic series CnHnP5-n+: is phosphorus a carbon copy?

The relative stabilities of different isomers of the isoelectronic series C(n)H(n)P(5-n)(+) have been investigated using G3X theory. The results indicate that all species containing one or more phosphorus atom adopt a three-dimensional nido geometry, in marked contrast to the planar structure favoured by the all-carbon analogue. Within isomeric nido clusters, a strong correlation between total energy and the nucleus-independent chemical shift (NICS) indicates that three-dimensional aromaticity plays a significant role in determining the stability of the cluster. In the context of these nido clusters, the extent to which phosphorus is a carbon copy proves to be highly dependent on the global electronic environment. The first isolobal substitution of CH by P causes a complete switch from localised to delocalised bonding, accompanied by a transition from a two- to a three-dimensional structure, with the phosphorus atom showing a strong preference for the unique apical site. In contrast, further increasing the phosphorus content causes no further change in structure or bonding, suggesting that, at the basal sites, phosphorus is a rather better carbon copy. The low-energy pathways for interconversion of apical and basal atoms previously identified in C(2)H(2)P(3)(+) prove to be a general feature of all members of the series.

Study of the formamide-methanol dimer with ab initio and density functional theory methods

For the first time, the structures and energies for the hydrogen bonding of a 1:1 complex formed between formamide and methanol molecules have been computed with various pure and hybrid density functional theory (DFT) and ab initio methods at varied basis set levels from 6-31g to 6-31+g(d,p). Five reasonable geometries on the potential energy surface of methanol and formamide system are considered and their relative stability is discussed. The infrared (IR) spectrum frequencies, IR intensities, and vibrational frequency shifts are reported. From the systematic studies, it is found that all the DFT methods selected here correctly compute the dimerization energies and geometries, with the B3P86 method predicting the hydrogen bond lengths relatively shorter and BPW91 yielding the interaction energies relatively lower. Finally, the solvent effects on the geometries of the formamide–methanol complexes have also been investigated using self-consistent reaction field (SCRF) calculations with five different DFT methods at the 6-31+g(d,p) basis set level. The results indicate that the polarity of the solvent has played an important role on the structures and relative stabilities of different isomers. Moreover, the basis set superposition error correction is critical to the interaction energies in the polar solvents. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Density functional theory study of the hydrogen bonding interaction of 1:1 complexes of formamide with methanol

The hydrogen bonding of 1:1 complexes formed between formamide and methanol has been investigated using DFT and MP2 methods at varied basis set levels from 6-31g to 6-31++g(2d, 2p). Five reasonable geometries are considered with the global minimum being a cyclic double-hydrogen bonded structure. The IR intensities and vibrational frequency shifts are reported. The solvent effects on the geometries of the complex have also been investigated using SCRF calculations at the B3LYP/6-31+g(d, p) level. The results indicate the polarity of the solvent has played an important role on the structures and the relative stabilities of different isomers.

“True” Inorganic Heterocycles: Structures and Stability of Group 13—15 Analogues of Benzene and Their Dimers.

AbstractFor Abstract see ChemInform Abstract in Full Text.

"True" inorganic heterocycles: structures and stability of group 13-15 analogues of benzene and their dimers.

Group 13-15 inorganic analogues of benzene, [HMYH](3) (M = B, Al, Ga; Y = N, P, As), mixed heterocycles of the type [BAlGaNPAs]H(6) and their dimers have been theoretically examined at the B3LYP/TZVP level of theory. Six different isomers have been structurally characterized for the mixed compounds [BAlGaNPAs]H(6). B-N bonding strongly (about approximately 90-100 kJ mol(-)(1)) stabilizes the mixed heterocycles, followed by the preference of the Al-N bonded structures over Ga-N bonded ( approximately 30-40 kJ mol(-1)), while B-P bonding is slightly (5-10 kJ mol(-1)) more favorable compared to B-As. Thus, the bonding pattern is predicted to be the most stable, followed by the core. Processes of [HMYH](3) formation from donor-acceptor complexes H(3)MYH(3) are predicted to be thermodynamically favorable for all MY combinations. Dimerization reactions of the coordinationally unsaturated [HMYH](3) heterocycles yielding hexamer clusters [HMYH](6) are found to be exothermic, with the exception of borazine, for which, as for benzene, dimerization is strongly endothermic due to the aromaticity of C(6)H(6) and [HBNH](3). Despite the high endothermicity of [HBNH](3) dimerization, the B-N bond formation is the driving force of the dimerization of mixed species [BAlGaNPAs]H(6). The dimerization enthalpies of [BAlGaNPAs]H(6) may be both exo- and endothermic, depending on the bonding pattern of the isomers. A complete set of mean MY bond energies in four- and six-membered cycles of [HMYH](6) was derived. The MY energies were found to be transferable quantities and may serve for a qualitative prediction of the relative stability of different isomers of mixed cluster compounds. [BAlGaNPAs](2)H(12) clusters are promising synthetic targets, they are expected to serve as single-source precursors for the stoichiometry-controlled CVD processes of the group 13-15 composites. A strategy of their synthesis and the most suitable starting systems have been also predicted.

Ab initioconfiguration interaction calculations of the semiconductor ternary clustersGelSimCn(l+m+n<~6)

Ground-state structures, stabilization energies, HOMO-LUMO energy gaps, and vibrational frequencies of ${\mathrm{Ge}}_{l}{\mathrm{Si}}_{m}{\mathrm{C}}_{n}$ ternary microclusters $(s=l+m+nl~6)$ have been investigated using the configuration interaction with all single and double substitutions method, and ionization potentials and vertical detachment energies of trimers and tetramers predicted utilizing the outer valence Green-function frozen-core procedure. ${\mathrm{Ge}}_{l}{\mathrm{Si}}_{m}{\mathrm{C}}_{n}$ is found to follow structural patterns similar to corresponding ${\mathrm{Si}}_{l+m}{\mathrm{C}}_{n}$ binary clusters and most ternary species possess singlet ground states except ${\mathrm{GeSiC}}_{4},$ which prefers a triplet one. Trimers, tetramers, and ${\mathrm{GeSi}}_{2}{\mathrm{C}}_{2}$ are planar, and the C-rich ${\mathrm{GeSiC}}_{3}$ and ${\mathrm{GeSiC}}_{4}$ are linear, while all the other Si-rich or Si- and Ge-rich clusters with $sg~5$ atoms prefer three-dimensional structures. Formation of strong C=C bond(s) predominates the relative stabilities of different isomers for clusters with limited numbers of C atoms, while Si=C bonds play an important role for silicon-rich species or systems with close C and Si atomic ratios. Planar and linear semiconductor clusters possess delocalized multicenter\char21{}two-electron \ensuremath{\pi} bonds (aromatic) and follow the $(4n+2)$ electron counting rule. Frequency analyses indicate that most vibrational modes of small ternary clusters are carbon dominated in terms of amplitudes, while Si atoms vibrate in medium sizes and Ge atoms vibrate very weakly.

Gold and platinum microclusters and their anions: comparison of structural and electronic properties

We present a study of Au 2 to Au 5 and Pt 2 to Pt 5 clusters within density-functional theory in the neutral and anionic states. Results obtained using two exchange-correlation (xc) functionals, local spin density approximation and spin-polarized Becke–Lee–Yang–Parr (BLYP), are compared. The structural characteristics and relative stabilities of different isomers as well as electron affinities and vertical detachment energies are calculated. The latter compare generally well with experimental data, especially in the BLYP case. However, strong dependence of the vertical electron detachment energies on the isomer and on the xc-functional scheme indicates that special care must be taken for any comparison of theoretical results with experiment. Both for platinum and gold tetramers and pentamers, 3D geometries are unfavored. Triplet states are predicted to be more stable than singlets for platinum, for all sizes considered. The relative stability of different geometrical isomers is strongly affected by the addition of one electron. Marked differences emerge in the electronic structure between the clusters of these two metals.

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

An ab initio study of diarsacyclobutadienes

Møller-Plesset MP2/6-311G* and MP4SDQ/6-311G* calculations have been made for the various possible ground state conformers of the simple 1,2- and 1,3-diarsacyclobutadienes (AsCH)2, in order to determine their geometries, the nature of the bonding, and the relative stabilities of different isomers. These calculations revealed some qualitative differences between the analogous 1,2- and 1,3-diphosphacyclobutadienes. The most stable (gas phase) isomeric form of (AsCH)2 is predicted to be 1,2-diarsatetrahedrane. Additionally, the mono- and di-substituted compounds But(AsC)2H, But(AsC)2But, Ph(AsC)2H and Ph(AsC)2Ph have been optimized. These indicate that 1,3-diarsacyclobutadienes prefer a planar C2h-like ring structure with localized single and double As–C bonds.

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