The C7-C10 cycloalkanes revisited

The conformers of cycloheptane through cyclodecane have been examined at the B3LYP/6-311+G* and MP2/6-311+G* theoretical levels, with some additional calculations at the CCD/6-311+G* and CCSD(T)/6-311++G** levels. With cyclooctane, B3LYP predicts that the boat-chair and crown conformers have similar energies, whereas MP2 and CCSD(T) predict that the crown conformer is 2 kcal/mol higher in energy. The latter is in agreement with the electron diffraction data. With cyclononane, B3LYP predicts that two of the higher-energy conformers found in molecular mechanics calculations should convert to one of the lower-energy conformers. However, MP2/6-311+G* optimizations find them to be true minima on the potential energy surface. B3LYP systematically predicts larger C-C-C bond angles for these compounds than either MP2 or CCD. The results of molecular mechanics MM4 calculations are generally in good agreement with those obtained using MP2.     

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.     

An ab initio correlated study of the potential energy surface for the HOBr.H2O complex

The potential energy surface (PES) for the HOBr.H(2)O complex has been investigated using second- and fourth-order M?ller-Plesset perturbation theory (MP2, MP4) and coupled cluster theory with single and doubles excitations (CCSD), and a perturbative approximation of triple excitations (CCSD-T), correlated ab initio levels of theory employing basis sets of triple zeta quality with polarization and diffuse functions up to the 6-311++G(3dp,3df ) standard Pople's basis set. Six stationary points being three minima, two first-order transition state (TS) structures and one second-order TS were located on the PES. The global minimum syn and the anti equilibrium structure are virtually degenerated [DeltaE(ele-nuc) approximately 0.3 kcal mol(-1), CCSD-T/6-311++G(3df,3pd) value], with the third minima being approximately 4 kcal mol(-1) away. IRC analysis was performed to confirm the correct connectivity of the two first-order TS structures. The CCSD-T/6-311++G(3df,3pd)//MP2/6-311G(d,p) barrier for the syn<-->anti interconversion is 0.3 kcal mol(-1), indicating that a mixture of the syn and anti forms of the HOBr.H(2)O complex is likely to exist.     

Structural studies on ethyl isovalerate by microwave spectroscopy and quantum chemical calculations

We observed the microwave spectrum of ethyl isovalerate by molecular beam Fourier transform microwave spectroscopy. The rotational and centrifugal distortion constants of the most abundant conformer were determined. Its structure was investigated by comparison of the experimental rotational constants with those obtained by ab initio methods. In a first step, the rotational constants of various conformers were calculated at the MP2/6-311++G** level of theory. Surprisingly, no agreement with the experimental results was found. Therefore, we concluded that in the case of ethyl isovalerate more advanced quantum chemical methods are required to obtain a reliable molecular geometry. Ab initio calculations carried out at MP3/6-311++G**, MP4/6-311++G**, and CCSD/6-311++G** levels and also density functional theory calculations using the B3LYP/6-311++G** method gave similar results for the rotational constants, but they were clearly distinct from those obtained at the MP2/6-311++G** level. With use of these more advanced methods, the rotational constants of the lowest energy conformer were in good agreement with those obtained from the microwave spectrum.     

Theoretical Studies on Intermolecular Hydrogen-bond Interactions between Hexamethylenetetramine and Nitric Acid

The structures of the complexes generated by hexamethylenetetramine and nitric acid have been fully optimized by B3LYP method at the 6-311++G** and aug-cc-pVTZ levels. The intermolecular hydrogen-bonding interactions have been calculated by the B3LYP/6-311++G**, B3LYP/aug-cc-pVTZ, MP2(full)/6-311++G** and CCSD(T)/6-311++G** methods, respectively. The NBO (nature bond orbital), AIM (atom in molecule), temperature effect and solvation effect have been analyzed to reveal the origin of the interactions. The results indicate that the stable hydrogen-bonded complexes could be generated by hexamethylenetetramine and nitric acid. The interactions follow the order of (a)>>(e)>(b)>(c)>(d)>(f)>(g). The C-N bonds which are adjacent to the methylene involving the hydrogen bonds tend to break in the chemical reaction. Due to the exothermic process, low temperature is conducive to the formation of the composition, which tallies with the experimental result.     

Microwave spectrum of 3-butyne-1-thiol: evidence for intramolecular S-H...pi hydrogen bonding

The microwave spectrum of 3-butyne-1-thiol has been studied by means of Stark-modulation microwave spectroscopy and quantum-chemical calculations employing the B3LYP/6-311++G(3df,2pd), MP2/aug-cc-pVTZ, MP2/6-311++G(3df,2pd), and G3 methods. Rotational transitions attributable to two conformers of this molecule were assigned. One of these conformers possesses an antiperiplanar arrangement of the atoms S-C1-C2-C3, while the other is synclinal and stabilized by the formation of an intramolecular hydrogen bond between the H-atom of the thiol group and the pi-electrons of the C[triple bond]C triple bond. The energy difference between these conformers was estimated to be 1.7(4) kJ mol(-1) by relative intensity measurements, with the hydrogen-bonded conformer being lower in energy. The spectra of five vibrationally excited states of the synclinal conformer were observed, and an assignment of these states to particular vibrational modes was made with the aid of a density functional theory (DFT) calculation of the vibrational frequencies at the B3LYP/6-311++G(3df,2pd) level of theory.     

Conformational energies for 2-substituted butanes

The conformational free energies for some 2-substituted butanes where X = F, Cl, CN, and CCH were calculated using G3-B3, CBS-QB3, and CCSD(T)/6-311++G(2d,p) as well as other theoretical levels. The above methods gave consistent results with free energies relative to the trans conformers as follows: X = CCH, g+ = 0.77 +/- 0.05 kcal/mol. g- = 0.88 +/- 0.05 kcal/mol; X = CN, g+ = 0.85 +/- 0.05 kcal/mol, g- = 0.75 +/- 0.05 kcal/mol; X = Cl, g+ = 0.70 +/- 0.05 kcal/ml, g- = 0.80 +/- 0.05 kcal/mol; and X = F, g+ = 0.53 +/- 0.05 kcal/mol, g- = 0.83 +/- 0.05 kcal/mol. The conformational free energies also were estimated using the observed liquid phase IR spectra and intensities calculated using B3LYP/6-311++G** and MP2/6-311++G**. The rotational free energy profiles for all of the compounds were estimated at the G3-B3 level.     

Dimers of boroglycine and methylamine boronic acid: a computational comparison of the relative importance of dative versus hydrogen bonding

Boronic acids are widely used in materials science, pharmacology, and the synthesis of biologically active compounds. In this Article, geometrical structures and relative energies of dimers of boroglycine, H2N-CH2-B(OH)2, and its constitutional isomer H3C-NH-B(OH)2, were computed using second-order M?ller-Plesset perturbation theory and density functional theory; Dunning-Woon correlation-consistent cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, and aug-cc-pVTZ basis sets were employed for the MP2 calculations, and the Pople 6-311++G(d,p) basis set was employed for a majority of the DFT calculations. Effects of an aqueous environment were incorporated into the results using PCM and COSMO-RS methodology. The lowest-energy conformer of the H2N-CH2-B(OH)2 dimer was a six-membered ring structure (chair conformation; Ci symmetry) with two intermolecular B:N dative-bonds; it was 14.0 kcal/mol lower in energy at the MP2/aug-cc-pVDZ computational level than a conformer with the classic eight-centered ring structure (Ci symmetry) in which the boroglycine monomers are linked by a pair of H-O...H bonds. Compared to the results of MP2 calculations with correlation-consistent basis sets, DFT calculations using the PBE1PBE and TPSS functionals with the 6-311++G(d,p) basis set were significantly better at predicting relative conformational energies of the H2N-CH2-B(OH)2 and H3C-NH-B(OH)2 dimers than corresponding calculations using the BLYP, B3LYP, OLYP, and O3LYP functionals, particularly with respect to dative-bonded structures.     

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.     

Infrared spectroscopy of ozone-water complex in a neon matrix

Matrix isolation infrared spectroscopy has been applied to study an ozone-water complex of atmospheric interest. The complex was identified in the spectral region of three normal modes of ozone and water. Ab initio calculation at MP4(SDQ), QCISD, and CCSD(T) levels indicates the existence of only one stable conformer, which accords with the present experimental result. This conformer belongs to the Cs symmetry group where two molecular planes of ozone and water are perpendicular to the Cs symmetry plane. The binding energy was calculated to be 1.89 kcal/mol at the CCSD(T)/6-311++G(3df,3pd)//CCSD(T)/6-311++G(d,p) level of theory. The formation constant and atmospheric abundance of the ozone-water complex are estimated using the thermodynamic and spectroscopic data obtained.     

Interaction of anions with perfluoro aromatic compounds

The complexes formed by a variety of anions with perfluoro derivatives of benzene, naphthalene, pyridine, thiophene, and furan have been calculated using DFT (B3LYP/6-31++G**) and MP2 (MP2/6-31++G** and MP2/6-311++G**) ab initio methods. The minimum structures show the anion interacting with the pi-cloud of the aromatic compounds. The interaction energies obtained range between -8 and -19 kcal mol(-1). The results obtained at the MP2/6-31++G** and MP2/6-311++G** levels are similar. However, the B3LYP/6-31++G** results provide longer interaction distances and smaller interaction energies than do the MP2 results. The interaction energies have been partitioned using an electrostatic, polarization, and van der Waals scheme. The AIM analysis of the electron density shows a variety of topologies depending on the aromatic system considered.     

A quantum-chemical study of the LiH + LiH⋅− ⇄ LiH2− + Li⋅ reaction

Parts of the potential energy surface of the title process and related processes have been investigated at the SCF /6-31G **, SCF /6-31++G **, and MP 2/6-31++G ** levels. The investigated reaction is exothermic (?6.23 kcal/mol, MP 4/6-31++G **//MP 2/6–31++G** level, ZPE included): A linear intermediate radical anion, Li? H? Li? H??, is significantly stabilized with respect to LiH + LiH?? (?38.74 kcal/mol, the same level as above). The BSSE at MP 2/6–31++G **//MP 2/6–31++G ** amounts to 1.8 kcal/mol. The title process seems to be suitable for experimental study in molecular beams.     

Comparison of levels of electronic structure theory in direct dynamics simulations of C2H5F --> HF + C2H4 product energy partitioning

Direct dynamics simulations at the MP2/6-311++G** level of theory were performed to study C(2)H(5)F --> HF + C(2)H(4) product energy partitioning. The simulation results are compared with experiment and a previous MP2/6-31G* simulation. The current simulation with the larger basis set releases more energy to HF vibration and less to HF + C(2)H(4) relative translation as compared to the previous simulation with the 6-31G* basis set. The HF rotation and vibration energy distributions determined from the current simulation are in overall very good agreement with previous experimental studies of C(2)H(5)F dissociation by chemical activation and IRMPA. A comparison of the simulations with experiments suggests there may be important mass effects for energy partitioning in HX elimination from haloalkanes. The transition state (TS) structures and energies calculated with MP2 and the 6-31G* and 6-311++G** basis sets are compared with those calculated using CCD, CCSD, CCSD(T), and the 6-311++G** basis set.     

Conformational analysis of carbonic anhydrase inhibitors using ab initio molecular orbital methods. 1. Rotational isomerism in methane sulfonamide anion, CH3-SO2-NH−

Ab initio calculations have been performed on methane sulfonamide anion. Geometries have been optimized using Hartree-Fock basis sets up to 6-31+G*, and single-point calculations employing those Hartree-Fock geometries have been performed at levels up to MP2/6-311++G**. In addition, geometry optimizations for the 0°, 90°, 150°, and 180° conformers have been carried out at the MP2/6-31G*, MP2/6-31+G*, and MP2/6-311++G** levels. Vibrational frequencies have been calculated using the HF/4-31G*, MP2/6-31G*, and MP2/6-31+G* geometries. All calculations at or above the 4-31G* level agree that H—N—S—C ˜90° is the global minimum. The H—N—S—C = 180° conformer is clearly higher in energy although the relative energy of this conformer varies from 0.36 to 1.03 kcal/mol for the post-HF calculations depending on basis set. The H—N—S—C = 180° conformer appears to be a very shallow local minimum. However, the potential energy surface is quite flat in this region, and the highest-level calculations, including MP2 optimizations and vibrational frequency analysis, are ambiguous on this point. The conformer with an H—N—S—C torsion of 0° is a transition state with a relative energy ˜8 kcal/mol. Received: 3 December 1996 / Accepted: 2 January 1997     

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).     

Thermochemistry of Lewis adducts of BH3 and nucleophilic substitution of triethylamine on NH3BH3 in tetrahydrofuran

The thermochemistry of the formation of Lewis base adducts of BH(3) in tetrahydrofuran (THF) solution and the gas phase and the kinetics of substitution on ammonia borane by triethylamine are reported. The dative bond energy of Lewis adducts were predicted using density functional theory at the B3LYP/DZVP2 and B3LYP/6-311+G** levels and correlated ab initio molecular orbital theories, including MP2, G3(MP2), and G3(MP2)B3LYP, and compared with available experimental data and accurate CCSD(T)/CBS theory results. The analysis showed that the G3 methods using either the MP2 or the B3LYP geometries reproduce the benchmark results usually to within ~1 kcal/mol. Energies calculated at the MP2/aug-cc-pVTZ level for geometries optimized at the B3LYP/DZVP2 or B3LYP/6-311+G** levels give dative bond energies 2-4 kcal/mol larger than benchmark values. The enthalpies for forming adducts in THF were determined by calorimetry and compared with the calculated energies for the gas phase reaction: THFBH(3) + L → LBH(3) + THF. The formation of NH(3)BH(3) in THF was observed to yield significantly more heat than gas phase dative bond energies predict, consistent with strong solvation of NH(3)BH(3). Substitution of NEt(3) on NH(3)BH(3) is an equilibrium process in THF solution (K ≈ 0.2 at 25 °C). The reaction obeys a reversible bimolecular kinetic rate law with the Arrhenius parameters: log A = 14.7 ± 1.1 and E(a) = 28.1 ± 1.5 kcal/mol. Simulation of the mechanism using the SM8 continuum solvation model shows the reaction most likely proceeds primarily by a classical S(N)2 mechanism.     

高级量子化学从头计算法研究N2和H2O分子间相互作用

在MP2/6-311++G(3d,3p)电子相关校正水平上,对N₂和H₂O分子间可能存在的氢键复合物进行全自由度能量梯度优化,发现了一个接近于直线的弱氢键总能量极小结构(1),进一步在高级电子相关校正的MP4SDTQ和CCSD(T)水平,用6-311++G(3d,3p)基组加上(3s3p2d1f)键函数,用MP4和CCSD(T)计算的结构1的结合能分别为-5.061kJ/mol和-4.715kJ/mol.     

Stabilisation energy of C(6)H(6)...C(6)X(6) (X = F, Cl, Br, I, CN) complexes: complete basis set limit calculations at MP2 and CCSD(T) levels

Stabilisation energies of stacked structures of C(6)H(6)...C(6)X(6) (X = F, Cl, Br, CN) complexes were determined at the CCSD(T) complete basis set (CBS) limit level. These energies were constructed from MP2/CBS stabilisation energies and a CCSD(T) correction term determined with a medium basis set (6-31G**). The former energies were extrapolated using the two-point formula of Helgaker et al. from aug-cc-pVDZ and aug-cc-pVTZ Hartree-Fock energies and MP2 correlation energies. The CCSD(T) correction term is systematically repulsive. The final CCSD(T)/CBS stabilisation energies are large, considerably larger than previously calculated and increase in the series as follows: hexafluorobenzene (6.3 kcal mol(-1)), hexachlorobenzene (8.8 kcal mol(-1)), hexabromobenzene (8.1 kcal mol(-1)) and hexacyanobenzene (11.0 kcal mol(-1)). MP2/SDD** relativistic calculations performed for all complexes mentioned and also for benzene[dot dot dot]hexaiodobenzene have clearly shown that due to relativistic effects the stabilisation energy of the hexaiodobenzene complex is lower than that of hexabromobenzene complex. The decomposition of the total interaction energy to physically defined energy components was made by using the symmetry adapted perturbation treatment (SAPT). The main stabilisation contribution for all complexes investigated is due to London dispersion energy, with the induction term being smaller. Electrostatic and induction terms which are attractive are compensated by their exchange counterparts. The stacked motif in the complexes studied is very stable and might thus be valuable as a supramolecular synthon.     

Protonated acetylene revisited

The potential energy surface for protonated acetylene has been re-examined with large basis sets and highly correlated methods. The energy difference of 3.6–3.8 kcal/mol between the classical structure and non-classical (bridged) structure computed with CCSD (T)/cc-pVQZ, CCSD(T)/6-311+G(3df,2pd), BD(T)/cc- pVQZ, BD(T)/6-311+G(3df,2pd) and CBS-APNO methods is in very good agreement with the best previous calculations, 3.7–4.0 kcal/mol. In contrast, BLYP, B3LYP, PW91, PBE and TPSS density functional methods do rather poorly, yielding −0.52. 0.29, 1.81, 2.16 and 0.62 kcal/mol, respectively, with the 6-311+G(3df,2pd) basis. MP2 calculations predict the classical structure to be a transition state; however, frequency calculations at the CCSD/6-311+G(3df,2pd) level of theory show that the classical structure is a local minimum. CCSD(T), BD(T) and CBS-APNO energy calculations along the MP2/6-311+G(3df,2pd) reaction path indicate that the classical structure is a shallow local minimum separated from the non-classical structure by a very small barrier of 0.11–0.13 kcal/mol. Because the barrier for proton exchange between the non-classical isomers via the classical structure is broad and nearly flat at the top, the tunneling splitting should be reduced, possibly accounting for the 15% difference between the calculated and experimental barrier heights. Contribution to the Fernando Bernardi Memorial Issue.