Insight into global reaction mechanism of [C2, H4, O] system from ab initio calculations by the scaled hypersphere search method

A detailed computational study is performed on the singlet potential energy surface (PES) for possible isomerization and dissociation reactions of CH(3)CHO at the DFT (B3LYP/6-311++G(d,p)) and CCSD(T)/cc-pVTZ//B3LYP/6-311++G(d,p) levels. The pathways around the equilibrium structures can be discovered by the scaled hypersphere search (SHS) method, which enables us to make a global analysis of the PES for a given chemical composition. Fourteen isomers inclusive of 11 single-molecules and three "non-stabilized" oxygen-based ylides, 5 energetically favored complexes, and 79 interconversion transition states have been found on the singlet PES. Four lowest lying isomers with thermodynamic stability are also kinetically stable with respect to metastable intermediates. It was revealed that vinyl alcohols, which could be generated by the tautomerization of acetaldehyde, could undergo dissociation to form acetylene and water. In addition, recombination channels between some fragments, such as H(2)CO + (1)CH(2) and (1)CHOH + (1)CH(2), are energetically accessible via collision complex or oxygen-based ylides. Most of available unimolecular decompositions are found to be responsible for favorable hydrogen abstraction processes.     

Structure of the boronic acid dimer and the relative stabilities of its conformers

Despite the widespread use of boronic acids in materials science and as pharmaceutical agents, many aspects of their structure and reactivity are not well understood. In this research the boronic acid dimer, [HB(OH)(2)](2), was studied by second-order M?ller-Plesset (MP2) perturbation theory and coupled cluster methodology with single and double excitations (CCSD). Pople split-valence 6-31+G*, 6-311G**, and 6-311++G** and Dunning-Woon correlation-consistent cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, and aug-cc-pVTZ basis sets were employed for the calculations. A doubly hydrogen-bonded conformer (1) of the dimer was consistently found to be lowest in energy; the structure of 1 was planar (C(2h)) at most computational levels employed but was significantly nonplanar (C(2)) at the MP2/6-311++G** and CCSD/6-311++G** levels, the result of an intrinsic problem with Pople-type sp-diffuse basis functions on heavy atoms. The dimerization energy, enthalpy, and free energy for the formation of (1) from the exo-endo conformer of the monomer were -10.8, -9.2, and +1.2 kcal/mol, respectively, at the MP2/aug-cc-pVTZ level. Several other hydrogen-bonded conformers of the dimer were local minima on the potential energy surface (PES) and ranged from 2 to 5 kcal/mol higher in energy than 1. Nine doubly OH-bridged conformers, in which the boron atoms were tetracoordinated, were also local minima on the PES, but they were all greater than 13 kcal/mol higher in energy than 1; doubly H-bridged structures proved to be transition states. MP2 and CCSD results were compared to those from the BLYP, B3LYP, OLYP, O3LYP, PBE1PBE, and TPSS functionals with the 6-311++G** and aug-cc-pVTZ basis sets; the PBE1PBE functional performed best relative to the MP2 and CCSD results. Self-consistent reaction field (SCRF) calculations predict that boronic acid dimerization is less favorable in solution than in vacuo.     

The importance of dihydrogen complexes HnGe(H2)+ (n=0,1) to the chemistry of cationic germanium hydrides: advanced theoretical and mass spectrometric analysis

Investigations of [Ge,Hn]-/0/- (n = 2,3) have been performed using a four-sector mass spectrometer. The results reveal that the complexes HnGe(H2)+ (n = 0,1) play an important role in the unimolecular dissociation of the metastable cations. Theoretical calculations support the experimental observations in most instances, and the established view that the global minimum of [Ge,H2]+ is an inserted structure may need reexamination; CCSD(T,full)/cc-pVTZ//CCSD(T)/6-311 ++ G(d,p) and B3LYP/cc-pVTZ studies of three low-lying cation states (2A1 HGeH+, 2B2 Ge(H2)+ and 2B1 Ge(H2)+) indicate a very small energy difference (ca. 4 kcal mol(-1)) between 2A1 HGeH+ and 2B2 Ge(H2)+; B3LYP favours the ion-molecule complex, whereas coupled-cluster calculations favour the inserted structure for the global minimum. Single-point multireference (MR) averaged coupled-pair functional and MR-configuration interaction calculations give conflicting results regarding the global minimum. We also present theoretical evidence indicating that the orbital-crossing point implicated in the spin-allowed metastable dissociation HGeH+* --> Ge(H2)+* --> Ge+ + H2 lies above the H-loss asymptote. Thus, a quantum-mechanical tunneling mechanism is invoked to explain the preponderance of the H2-loss signal for the metastable ion.     

Theoretical study on the structures, isomerization and stability of SiC3H isomers

The calculations of the geometry optimizations, energies, dipole moments, vibrational spectra, rotational constants, and isomerization of doublet SiC₃H species were performed using density functional theory and ab initio methods. Four types of isomers, a total of 18 minima, connected by 16 interconversion transition states, were located on the potential energy surface (PES) at the B3LYP/6-311G (d, p) level. More accurate energies were obtained at the CCSD(T)/6-311G(2df, 2p), and G3(MP2) levels. With the highest isomerization barrier, the lowest lying structure, linear A1 possesses the largest kinetic stability. Besides, the isomerization barriers of A2, A4, C2, F1, F4 and F5 are over 10 kcal/mol, and these isomers are also considered to be higher kinetically stable. Other isomers cannot be kinetically stabilized with considerably low isomerization barriers. Investigation on the bonding properties and the computations of vibrational spectra, dipole moments, and rotational constants for SiC₃H isomers are helpful for understanding their structures and also valuable for their detections in the interstellar space and laboratory.     

Theoretical Study on Dissociation Potential Energy Surface of Peroxynitric Acid

The lowest energy structures of peroxynitric acid have been studied with B3LYP/6-311+ G(2d,2p) method. The potential energy surfaces (PES) along the O-N and O-Obonds have been scanned at CCSD(T)/aug-cc-pVDZ level, respectively. The calculated results show that on the O-N PES, the O3-N4 bond length of the loose transition state is 2.82 ? and the corresponding energy barrier is 25.6 kcal/mol, while on the O-O PES, the loose transition state with of O2-O3 bond length of 2.35 ? has the energy barrier of 37.4 kcal/mol. Thus the primary reaction path for peroxynitric acid is the dissociation into HO₂ and NO₂.     

On the mechanism of intramolecular nitrogen‐atom hopping in the carbon chain of C6N radical: A Plausible 3c−4e crossover Long‐Bond

Linear isomers of C₆N radical differ in the position of the nitrogen atom in the carbon chain of C₆N. Reaction routes, involving intramolecular nitrogen atom insertion at varying position in the carbon chain of C₆N, are analyzed for the isomerisation between linear isomers of C₆N. Through an automated and systematic search performed with global reaction route mapping of the potential energy surface, thermal isomerisation pathways for C₆N radical are proposed based on the computations carried out at CASSCF/aug‐cc‐pVTZ, and CCSD(T)/6‐311++G(d,p)//B3LYP/6‐311++G(d,p) levels of the theory. Notably, a high lying linear isomer, centrosymmetric with respect to the nitrogen atom, is observed to be stabilized by a unique crossover three center‐four electron π long bond between the carbon atoms that are spatially separated by a nitrogen atom in a natural bond orbital. This long bond is concluded to be responsible for the predicted thermal isomerisation to be more feasible than the dissociation during the isomerisation pathway of a linear isomer of C₆N. © 2014 Wiley Periodicals, Inc.     

Theoretical study on the potential energy surface of NC<Subscript>3</Subscript>P isomers

Density functional theory (DFT) calculations have been used to study the isomerization process in the NC₃P system. At the DFT/B3LYP/6-311G(d) level, 28 triplet and 28 singlet minima were obtained on their respective potential energy surfaces. The linear triplet ³NCCCP is the lowest-energy structure among the isomers. On the triplet PES, only linear isomers ³NCCCP, ³CNCCP, ³CCCNP, and ³CCNCP possess great kinetic and thermodynamic stabilities to exist under low-temperature conditions (such as in the dense interstellar clouds). At the same time, one chain-like and four three-membered-ring isomers on the singlet PES have been located with high kinetic and thermodynamic stabilities. Further CCSD(T)/6-311G(2df)//QCISD/6-311G(d), CCSD(T)/cc-pVTZ//DFT/B3LYP/cc-pVTZ, and CASPT2(14,12)/cc-pVQZ//CASSCF(14,12)/cc-p VQZ calculations are performed on the structures, frequencies, and energies of the relevant species. The bonding natures were analyzed and the results were compared with the analogous NC₃N and NC₂P molecules so as to aid their future experimental or astrophysical detection.     

On the flexibility of beta-peptides

The full conformational space was explored for an achiral and two chiral beta-peptide models: namely For-beta-Ala-NH2, For-beta-Abu-NH2, and For-beta-Aib-NH2. Stability and conformational properties of all three model systems were computed at different levels of theory: RHF/3-21G, B3LYP/6-311++G(d,p)//RHF/3-21G, B3LYP/6-311++G(d,p), MP2//B3LYP/6-311++G(d,p), CCSD//B3LYP/6-311++G(d,p), and CCSD(T)//B3LYP/6-311++G(d,p). In addition, ab initio E = E(phi, micro, psi) potential energy hypersurfaces of all three models were determined, and their topologies were analyzed to determine the inherent flexibility properties of these beta-peptide models. Fewer points were found and assigned than expected on the basis of Multidimensional Conformational Analysis (MDCA). Furthermore, it has been demonstrated, that the four-dimensional surface, E = E(phi, mu, psi), can be reduced into a three-dimensional one: E = E[phi, f(phi), psi]. This reduction of dimensionality of freedom of motion suggests that beta-peptides are less flexible than one would have thought. This agrees with experimental data published on the conformational properties of peptides composed of beta-amino acid residues.     

Computational study of the aminolysis of anhydrides: effect of the catalysis to the reaction of succinic anhydride with methylamine in gas phase and nonpolar solution

Different possible pathways of the aminolysis reaction of succinic anhydride were investigated by applying high level electronic structure theory, examining the general base catalysis by amine and the general acid catalysis by acetic acid, and studying the effect of solvent. The density functional theory at the B3LYP/6-31G(d) and B3LYP/6-311++G(d,p) levels was employed to investigate the reaction pathways for the aminolysis reaction between succinic anhydride and methylamine. The single point ab initio calculations were based on the second-order M?ller-Plesset perturbation theory (MP2) with 6-31G(d) and 6-311++G(d,p) basis sets and CCSD(T)/6-31G(d) level calculations for geometries optimized at the B3LYP/6-311++G(d,p) level of theory. A detailed analysis of the atomic movements during the process of concerted aminolysis was further obtained by intrinsic reaction coordinate calculations. Solvent effects were assessed by the polarized continuum model method. The results show that the concerted mechanism of noncatalyzed aminolysis has distinctly lower activation energy compared with the addition/elimination stepwise mechanism. In the case of the process catalyzed by a second methylamine molecule, asynchronous proton transfer takes place, while the transition vectors of the acid-catalyzed transition states correspond to the simultaneous motion of protons. The most favorable pathway of the reaction was found through the bifunctional acid catalyzed stepwise mechanism that involves formation of eight-membered rings in the transition state structures. The difference between the activation barriers for the two mechanisms averages 2 kcal/mol at various levels of theory.     

Theoretical study on structures and stability of triplet SiC<Subscript>3</Subscript>O isomers

Various levels of calculations are carried out~for exploring the potential energy surface (PES) of triplet SiC₃O, a molecule of potential interest in interstellar chemistry. A total of 38 isomers are located on the PES including chain-like, cyclic and cage-like structures, which are connected by 87 interconversion transition states at the DFT/B3LYP/6-311G(d) level. The structures of the most relevant isomers and transition states are further optimized at the QCISD/6-311G(d) level followed by CCSD(T)/6-311+G(2df) single-point energy calculations. At the QCISD level, the lowest lying isomer is a linear SiCCCO 1 (0.0 kcal/mol) with the ³ ∑ electronic state, which possesses great kinetic stability of 59.5 kcal/mol and predominant resonant structure . In addition, the bent isomers CSiCCO 2 (68.3 kcal/mol) and OSiCCC 5 (60.1 kcal/mol) with considerable kinetic stability are also predicted to be candidates for future experimental and astrophysical detection. The bond natures and possible formation pathways in interstellar space of the three stable isomers are discussed. The predicted structures and spectroscopic properties for the relevant isomers are expected to be informative for the identification of SiC₃O and even larger SiC _n O species in laboratory and interstellar medium. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Theoretical study of the conformational energy hypersurface of cyclotrisarcosyl

The multidimensional Conformational Potential Energy Hypersurface (PEHS) of cyclotrisarcosyl was comprehensively investigated at the DFT (B3LYP/6-31G(d), B3LYP/6-31G(d,p) and B3LYP/6-311++G(d,p)), levels of theory. The equilibrium structures, their relative stability, and the Transition State (TS) structures involved in the conformational interconversion pathways were analyzed. Aug-cc-pVTZ//B3LYP/6-311++G(d,p) and MP2/6-31G(d)//B3LYP/6-311++G(d,p) single point calculations predict a symmetric cis-cis-cis crown conformation as the energetically preferred form for this compound, which is in agreement with the experimental data. The conformational interconversion between the global minimum and the twist form requires 20.88 kcal mol-1 at the MP2/6-31G(d)//B3LYP/6-311++G(d,p) level of theory. Our results allow us to form a concise idea about the internal intricacies of the PEHSs of this cyclic tripeptide, describing the conformations as well as the conformational interconversion processes in this hypersurface. In addition, a comparative analysis between the conformational behaviors of cyclotrisarcosyl with that previously reported for cyclotriglycine was carried out     

NMR spectra, GIAO and charge density calculations of five-membered aromatic heterocycles

The B3LYP/6-31+G(d) molecular geometry optimized structures of 17 five-membered heterocycles were employed together with the gauge including atomic orbitals (GIAO) density functional theory (DFT) method at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p) and B3LYP/6-311+G(2d,p) levels of theory for the calculation of proton and carbon chemicals shifts and coupling constants. The method of geometry optimization for pyrrole (1), N-methylpyrrole (2) and thiophene (7) using the larger 6-311++G(d,p) basis sets at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p), B3LYP/6-31+G(2d,p) and B3LYP/cc-pVTZ levels of theory gave little difference between calculated and experimental values of coupling constants. In general, the (1)H and 13C chemical shifts for all compounds are in good agreement with theoretical calculations using the smaller 6-31 basis set. The values of nJHH(n=3, 4, 5) and rmnJ(CH)(n=1, 2, 3, 4) were predicted well using the larger 6-31+G(d,p) and 6-311++G(d,p) basis sets and at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p), B3LYP/6-31+G(2d,2p) levels of theory. The computed atomic charges [Mülliken; Natural Bond Orbital Analysis (NBO); Merz-Kollman (MK); CHELP and CHELPG] for the B3LYP/6-311++G(d,p) geometry optimized structures of 1-17 were used to explore correlations with the experimental proton and carbon chemical shifts.     

气相离子-分子反应B2H3-+CS2——B2H3S-+CS的理论研究

用密度泛函方法B3LYP/6-311++G(d,p)和高级电子相关的偶合簇法CCSD(T)/6-311++G(d,p)研究了气相离子-分子反应B₂H₃^-+CS₂B₂H₃S^-+CS的机理.结果表明,B₂H₃最可能进攻CS₂中碳原子形成三元环中间体,随后通过氢迁移和最终消除CS的反应步骤形成硫原子转移产物H₃BBS^-+CS,反应大量放热且不需要活化能.B₂H₃直接对CS₂中硫原子进攻夺取硫原子的反应方式存在一定能垒阻碍.计算结果有助于深入了解B₂H₃,B₃H-6和B₄H₇^-等缺电子硼氢负离子的反应行为.     

A theoretical study of the H-abstraction reactions from HOI by moist air radiolytic products (H, OH, and O (3P)) and iodine atoms (2P(3/2))

The rate constants of the reactions of HOI molecules with H, OH, O ((3)P), and I ((2)P(3/2)) atoms have been estimated over the temperature range 300-2500 K using four different levels of theory. Geometry optimizations and vibrational frequency calculations are performed using MP2 methods combined with two basis sets (cc-pVTZ and 6-311G(d,p)). Single-point energy calculations are performed with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (pertubatively) electron excitations CCSD(T) using the cc-pVTZ, cc-pVQZ, 6-311+G(3df,2p), and 6-311++G(3df,3pd) basis sets. Reaction enthalpies at 0 K were calculated at the CCSD(T)/cc-pVnZ//MP2/cc-pVTZ (n = T and Q), CCSD(T)/6-311+G(3df,2p)//MP2/6-311G(d,p), and CCSD(T)/6-311++G(3df,3pd)//MP2/6-311G(d,p) levels of theory and compared to the experimental values taken from the literature. Canonical transition-state theory with an Eckart tunneling correction is used to predict the rate constants as a function of temperature. The computational procedure has been used to predict rate constants for H-abstraction elementary reactions because there are actually no literature data to which the calculated rate constants can be directly compared. The final objective is to implement kinetics of gaseous reactions in the ASTEC (accident source term evaluation code) program to improve speciation of fission products, which can be transported along the reactor coolant system (RCS) of a pressurized water reactor (PWR) in the case of a severe accident.     

Theoretical study of the reaction mechanism of HCN+ and CH4 of relevance to Titan's ion chemistry

Titan is the largest satellite of Saturn. In its atmosphere, CH4 is the most abundant neutral after nitrogen. In this paper, the complex doublet potential-energy surface related to the reaction between HCN+ and CH4 is investigated at the B3LYP/6-311G(d,p), CCSD(T)/6-311G++(3df,2pd)(single-point), and QCISD/6-311G(d,p) computational levels. A total of seven products are located on the PES. The initial association of HCN+ with CH4 is found to be a prereaction complex 1 (HCNHCH3(+)) without barrier. Starting from 1, the most feasible pathway is the direct H-abstraction process (the internal C-H bond dissociation) leading to the product P1 (HCNH++CH3). By C-C addition, prereaction complex 1 can form intermediate 2 (HNCHCH3(+)) and then lead to the product P2 (CH3CNH++H). The rate-controlling step of this process is only 25.6 kcal/mol. It makes the Path P2 (1) R --> 1 --> TS1/2 --> 2 --> TS2/P2 --> P2 another possible way for the reaction. P3 (HCNCH3(+) + H), P5 (cNCHCH2(+) + H2), and P6 (NCCH3(+) + H2) are exothermic products, but they have higher barriers (more than 40.0 kcal/mol); P4 (H + HCN + CH3(+)) and P7 (H + H2 + HCCNH+) are endothermic products. They should be discovered under different experimental or interstellar conditions. The present study may be helpful for investigating the analogous ion-molecule reaction in Titan's atmosphere.     

Conformational inversion-topomerization processes of ethylcyclohexane and 1,2-dimethylcyclohexane: A computational investigation

To deeply reveal the impact of the substituents and their special orientations in ring on conformational behaviors for substituted cyclohexanes, a comprehensive study of ethylcyclohexane, cis-, and trans-1,2-dimethylcyclohexanes has been carried out. All conformational structures for them were captured by the accurate ab intio method, that is, B3LYP/6-311++G(d,p) method was used for geometry optimizations, and MP2/6-311++G(d,p), G4, and CCSD(T)/6-311++G(d,p) methods were applied for the high-level single point energy refinements. Based on CCSD(T)/6-311++G(d,p) quantum results, the conformational populations of minima for these three substituted cyclohexanes were calculated by Boltzmann distribution over 300-2500 K. Additionally, the conformational inversion-topomerization pathways for them were thoroughly investigated. The complete characterization involved in their potential energy surfaces are clearly presented by three or two-dimensional schemes.     

Ab Initio Study on the Potential Energy Surfaces of B4 Cluster

The singlet and triplet potential energy surfaces (PES) for the isomerization and dissociation reactions of B₄ isomers have been investigated using ab initio methods. Ten B4 isomers have been identified and of these 10 species, 4 have not been reported previously. The singlet rhombic structure ¹1 is found to be the most stable on the B₄ surface, in agreement with the results of previous reports. Several isomerization and dissociation pathways have been found. On the singlet PES, the linear ¹3b can rearrange to rhombus ¹1 directly, while ¹3c rearranges to ¹1 through two‐step reactions involving a cyclic intermediate. On the triplet PES, the capped triangle structure ³2 undergoes ring opening to the linear isomer ³3b with a barrier of 34.8 kcal/mol and 44.9 kcal/mol, and the latter undergoes ring closure to the square structure ³1 with a barrier of 30.4 kcal/mol and 33.0 kcal/mol at the MP4/6–311+G(3df)//MP2/6–311G(d) and CCSD/aug‐cc‐pVTZ//MP2/6–311G(d) levels of theory, respectively. The direct decomposition of singlet B₄ yielding to B₃+B is shown to have a large endothermicity of 87.3 kcal/mol (CCSD), and that producing 2B₂ to have activation energy of 133.4 kcal/mol (CCSD).     

OH-与CH2ClF反应的阴离子产物通道

理论研究了羟基负离子(OH^-)与氟氯代甲烷(CH₂ClF)反应的阴离子产物通道. 分别在B3LYP/6-31+G(d,p)和B3LYP/6-311++G(2d,p)水平上得到反应势能面上各关键物种的优化构型, 进而计算得到谐振频率和零点能. 基于CCSD(T)/6-311+G(3df,3dp)水平的相对能量, 描述了由质子转移和双分子亲核取代(S_N2)过程生成各阴离子产物的途径. 各阴离子产物途径势垒的计算结果表明质子转移过程是实验中的主要产物通道, 与以往实验测量的结论相符. 此外, 计算还显示双分子亲核取代过程得到了非典型的阴离子产物, 其中动力学效应可能会导致F^-的生成.     

Isomerization pathways from the norbornadiene to the cycloheptatriene radical cation by opening a bridgehead-methylene bond: a theoretical investigation

Three skeletal rearrangement channels for the norbornadiene (N*+) to the 1,3,5-cycloheptatriene (CHT*+) radical cation conversion, initialized by opening a bridgehead-methylene bond in N*+, are investigated using the quantum chemical B3LYP, MP2 and CCSD(T) methods in conjunction with the 6-311 +G(d,p) basis set. Two of the isomerizations proceed through the norcaradiene radical cation (NCD*+), either through a concerted path (N*+ - NCD*+), or by a stepwise mechanism via a stable intermediate (N*+ - I1 - NCD*+). At the CCSD(T)/6-311 +G(d,p)//B3LYP/6-311 +G(d,p) level, the lowest activation energy, 28.9 kcal mol(-1), is found for the concerted path whereas the stepwise path is found to be 2.3 kcal mol(-1) higher. On both pathways, NCD*+ rearranges further to CHT*+ with significantly less activation energy. The third channel proceeds from N*+ through I1 and then directly to CHT*+, with an activation energy of 37.1 kcal mol(-1). The multi-step channel reported earlier by our group, which proceeds from N*+ to CHT*+ via the quadricyclane and the bicyclo[2.2.1]hepta-2-ene-5-yl-7-ylium radical cations, is 4.6 kcal mol(-1) lower than the most favorable path of the present study. If the methylene group is substituted with C(CH3)2, however, the concerted path is estimated to be 5.6 kcal mol(-1) lower than the corresponding substituted multi-step path at the B3LYP/6-311+(d,p) level. This shows that substitution of particular positions can have dramatic effects on altering reaction barriers in the studied rearrangements. We also note that identical energies are computed for CHT*+ and NCD*+ whereas, in earlier theoretical investigations, the former was reported to be 6-17 kcal mol(-1) more stable than the latter. Finally, a bent geometry is obtained for CHT*+ with MP2/6-311 +G(d,p) in contradiction with the planar conformation reported for this cation in earlier computational studies.     

A new scheme for determining the intramolecular seven-membered ring N-H...O=C hydrogen-bonding energies of glycine and alanine peptides

In this paper a new scheme was proposed to calculate the intramolecular hydrogen-bonding energies in peptides and was applied to calculate the intramolecular seven-membered ring N-H...O=C hydrogen-bonding energies of the glycine and alanine peptides. The density-functional theory B3LYP6-31G(d) and B3LYP6-311G(d,p) methods and the second-order Moller-Plesset perturbation theory MP26-31G(d) method were used to calculate the optimal geometries and frequencies of glycine and alanine peptides and related structures. MP26-311++G(d,p), MP26-311++G(3df,2p), and MP2/aug-cc-pVTZ methods were then used to evaluate the single-point energies. It was found that the B3LYP6-31G(d), MP26-31G(d), and B3LYP6-311G(d,p) methods yield almost similar structural parameters for the conformers of the glycine and alanine dipeptides. MP2/aug-cc-pVTZ predicts that the intramolecular seven-membered ring N-H...O=C hydrogen-bonding strength has a value of 5.54 kcal/mol in glycine dipeptide and 5.73 and 5.19 kcal/mol in alanine dipeptides, while the steric repulsive interactions of the seven-membered ring conformers are 4.13 kcal/mol in glycine dipeptide and 6.62 and 3.71 kcal/mol in alanine dipeptides. It was also found that MP26-311++G(3df,2p) gives as accurate intramolecular N-H...O=C hydrogen-bonding energies and steric repulsive interactions as the much more costly MP2/aug-cc-pVTZ does.