DZP Basis Set Research Articles

Rearrangements on the C6H6Potential Energy Surface and the Topomerization of Benzene

The benzene potential energy hyperface was examined employing hybrid Hartree−Fock/density functional theory (B3LYP), second-order perturbation theory (MP2), and the coupled-cluster method with single, double, and perturbative triple excitations [CCSD(T)] in conjunction with DZP and TZ2P basis sets. All stationary points were characterized by harmonic vibrational frequency analyses; intrinsic reaction coordinates were calculated for all transition structures at B3LYP/DZP. Final energies were evaluated at the CCSD(T)/DZP//B3LYP/DZP level and corrected for T = 1373 K. There are three competing mechanisms for the high-temperature intramolecular topomerization of [1,2-13C2]benzene to [1,3-13C2]- and [1,4-13C2]benzene: (a) benzene ring contraction to benzvalene (ΔG ⧧ = 93.5 kcal mol-1) followed by ring opening to benzene; (b) degenerate rearrangement of benzvalene via a 1A‘‘ prefulvene TS (ΔG ⧧ = 95.0 kcal mol-1 relative to benzene) generating [1,4-13C2]benzene as a primary reaction product of [1,2-13C2]benzen...

On the electronic structure of dihalogenophosphenium cations

The bonding properties of the dihalogenophosphenium cations, PX 2 (+), X = F, Cl, Br, I, are evaluated by relativistic effective core potential calculations with a DZP basis set. Our investigations include an analysis of the singlet-triplet energy separation in the cations and the determination of the relative cation and anion stabilities in the gas phase, via corresponding group transfer reactions. A comparison is made with other stabilized, previously reported low-coordinated π-bonded phosphorus cations.

Ring Opening of Substituted Cyclopropylidenes to Cyclic Allenes

Density functional theory and ab initio quantum mechanical computations elucidated the ring opening of trans- and cis-2,3-dimethylcyclopropylidene (1b and 1c, respectively), bicyclo[4.1.0]hept-7-ylidene (3), and bicyclo[3.1.0]hex-6-ylidene (7). The B3LYP geometry optimizations employed a DZP basis set. Single-point energies were evaluated at B3LYP/TZP. The ring-opening barrier leading to allene, around 5 kcal mol-1 for the parent cyclopropylidene (1a), is lowered by 2,3-cis-dimethyl substitution to almost zero for 1c. The larger barrier, 4.2 kcal mol-1, for the 2,3-trans compound (1b) is due to repulsive H···H interactions in the ring-opening transition structure TS2. While isomerization of bicyclo[3.1.0.]hex-6-ylidene (7) to 1,2-cyclohexadiene (8) proceeds almost spontaneously, the analogous cyclopropylidene ring opening of bicyclo[4.1.0.]hept-7-ylidene (3) to 1,2-cycloheptadiene (4) has an unusually high activation energy of 14.6 kcal mol-1. This results from unfavorable conformational changes in the cyclohexane moiety of 3 during the reaction. Intramolecular carbene CH insertions to give tricyclo[4.1.0.02,7]heptane and tricyclo[4.1.0.03,7]heptane are characterized by lower barriers, 6.4 and 9.1 kcal mol-1, respectively, and these are the products observed experimentally. The geometries and vibrational frequencies of cyclic allenes 4 and 8 were computed with B3LYP, with second-order Møller−Plesset perturbation theory (MP2), and with the coupled-cluster method involving single and double excitations using the 6-31G* and DZP basis sets. Both Runge and Sander (νas = 1829 cm-1) (Tetrahedron Lett. 1986, 27, 5835) as well as Wentrup et al. (νas = 1886 cm-1) (Angew. Chem., Int. Ed. Engl. 1983, 22, 542) claimed to have spectroscopic evidence for 1,2-cyclohexadiene (8). The calculated values for νas(CC) (1718−1838 cm-1) favor the experimental data of Runge and Sander.

Predicting Absolute and Site Specific Acidities for Zeolite Catalysts by a Combined Quantum Mechanics/Interatomic Potential Function Approach

The approach used describes the Bronsted site by the Hartree−Fock method and a T(O)DZP basis set, while the periodic zeolite framework and the interaction between the active site and the framework are described by a shell model potential parametrized on the same type of ab initio data for cluster models. It is capable of reproducing the effect of the crystallographic position and of different framework structures on the properties and reactivity of zeolitic Bronsted sites. For H-faujasite (Si/Al = 47) protonation of all four crystallographically different oxygen positons is considered. In agreement with experiment protonation on O(1) and O(3) is preferred. For the orthorhombic form of H-ZSM-5 (Si/Al = 95) protonation of the Al(7)−O(17)H−Si(4) site proves more stable than protonation at the Al(12)−O(24)−Si(12) site located at the channel intersection. In agreement with experiments, the OH vibrational frequency is predicted to decrease according to O(1)H-FAU > H-ZSM-5 > O(3)H-FAU, and the 1H NMR chemical sh...

High-level electron correlation calculations on some tautomers of cytosine

Five isomers of cytosine, representing three tautomers and rotamers for two of them, have been investigated by ab initio techniques. Complete geometry optimizations were carried out at the Hartree-Fock (HF) and second-order perturbation theory, MBPT(2), levels. Our highest-level calculations used the coupled cluster method with single and double excitations to obtain energies at the MBPT(2) geometries. In the HF calculations basis sets up to 6-311G(2 d,2 p) were tested. The electron correlation calculations applied a DZP basis set. In the geometries, bond lengths reflect the varying formal bond orders of the tautomers, specifically the aromatic character of the pyrimidine ring in the amino-oxo tautomer. This latter form is found to be the most stable one in all calculations, in accordance with recent related literature results. It is found that its rotamer pair lies only about 0.8 kcal higher in energy. A qualitatively new result of the CCSD/DZP//MBPT(2)/DZP calculations is that the imino-oxo tautomer may be much more stable than thought previously, at only 0.2–0.3 kcal relative to the ground state. The order of energies in these high level calculations shows the pattern: amino-hydroxy < imino-oxo < amino-oxo.

On the structure of hexaaquacopper(II) complexes

Using standard ab initio methods the optimal geometries and corresponding electronic structures of hexaaquacopper(II) complexes [Cu(H 2O) 6] q, q = + 1 and + 2, of T h , D 2 h and C i symmetries are investigated at the SCF level using DZP basis sets. Contrary to [Cu(H 2O)6] +, [Cu(H 2O) 6] 2+ in the ground electronic state has planar H 2O ligands and their tilting might be caused by the contribution of excited 2A u electronic states within configurational interaction. This contribution also causes nonvanishing CuO bond strength in [Cu(H 2O) 6] 2+. H 2O ligand tilting is not caused by the Jahn-Teller effect.

The vibrational and NMR spectra, conformations and ab initio calculations of methoxymethylene- and 1-methoxyethylidene- propanedinitrile

The IR and Raman spectra of methoxymethylene-propanedinitrile [H 3COCHC(CN) 2] and 1-methoxyethylidene-propanedinitrile [H 3COC(CH 3)C(CN) 2] as a solid, liquid and solute in various solvents have been recorded in the region 4000-50 cm −1. NMR spectra of both compounds in various solvents at different temperatures were also obtained. Semiempirical (AM1, PM3, MNDO, MINDO3), ab initio using DZP basis sets and DFT calculations using B3LYP with 6-31G∗∗ basis sets were carried out for both compounds. These calculations support the idea on the existence of two conformers with a planar CCOC moiety and with the methyl group oriented as anti and syn towards the CC double bond. The calculated small energy difference between them suggests the presence of both in the samples at room temperature. In contrast with the calculations, only one conformer has been found in the liquid vibrational and NMR spectra. Complete assignments of the vibrational spectra for the compounds mentioned were made with the aid of normal coordinate calculations employing scaled ab initio and DFT force field constants. The scaled ab initio and DFT frequencies indicate that the conformer present in the liquid (solutions) and solid phases is anti.

Protonated High Energy Density Materials: N4 Tetrahedron and N8 Octahedron

Previous theoretical studies have demonstrated that tetrahedral N4 should be an extraordinarily effective high energy density material. But this species has thus far resisted laboratory efforts directed to its synthesis. Ab initio electronic structure methods have been used to examine the Td N4 and Oh N8 nitrogen clusters, including their protonated forms. We have optimized geometries using DZP, TZ2P, and TZ2P(f,d) basis sets with the Hartree−Fock self-consistent-field (SCF) method, second-order Moller−Plesset perturbation theory (MP2), single and double excitation configuration interaction (CISD), coupled-cluster (CCSD and CCSD(T)) methods, and three DFT/Hartree−Fock hybrid (B3LYP, B3P86, BHLYP) methods. Harmonic vibrational frequencies and infrared intensities have been obtained at the SCF, MP2, B3LYP, B3P86, and BHLYP levels of theory. The vertex protonated Oh N8 and Td N4 structures are found to represent minima on their respective potential energy surfaces. The bond protonated Td N4 molecule was dete...

Theoretical studies of structure, ground state vibrations and force field of methyldiborane

Molecular geometry, force field and infrared absorption intensities of methyldiborane have been determined from ab initio calculations at the SCF and MP2 levels utilizing a DZP basis set. The theoretical geometry reveals slight distortion of the diborane ring, by 1°–2°, on methylation. The calculated value of the three-fold barrier for methyl torsion by both the methods is nearly 8.5 kJ mol −1 in satisfactory agreement with the experimental value of 10.5 kJ mol −1. The ground state effective force constants of methyldiborane have been obtained by the RECOVES procedure which we proposed recently. The RECOVES-SCF/DZP force constants of methyldiborane are very close to those of the RECOVES-MP2/DZP. Normal coordinate analyses of methyldiborane and its various 2H isotopic molecules and the predicted infrared intensities have provided reliable vibrational assignment for methyldiborane. The effects of methylation on the vibrational frequencies and force constants of diborane have been discussed.

Theoretical study of thermal decomposition of azomethane

Thermal one- and two-bond dissociation processes ofcis- andtrans-azomethane were studied byab initio computation with DZP and TZ2P basis sets, using thed(N-C) bond lengths as the reaction coordinates. The geometries were optimized at the MP2 level, and the dissociation energies obtained exploiting a single-point, fourth-order Moller-Plesset calculations [MP4SDTQ/TZ2P]. At this level of theory including zero-point energies, thetrans-isomer is by 9.3 kcal/mol more stable than thecis-isomer. The results show that the energetically more favourable one-bond cleavage proceeds without transition state with the predicted bond dissociation energyDo of 47.8 kcal/mol fortrans-azomethane and 38.5 kcal/mol forcis-azomethane. With calculated barrier heights the unimolecular dissociation rate constants have been determined by means of the RRKM theory. The second-order saddle points localized for synchronous decomposition pathways lie 13 (trans)-23(cis) kcal/mol above the one-bond dissociation energies [MP2/DZP].

The vibrational and NMR spectra, conformations and ab initio calculations of aminomethylene, propanedinitrile and itsN-methyl derivatives

The IR and Raman spectra of aminomethylene propanedinitrile (AM) [H2N-CH=C(CN)2], (methylamino)methylene propanedinitrile (MAM) [CH3NH-CH=C(CN)2] and (dimethylamino)methylene propanedinitrile (DMAM) [(CH3)2N-CH=C(CN)2] as solids and solutes in various solvents have been recorded in the region 4000-50 cm−1. AM and DMAM can exist only as one conformer. From the vibrational and NMR spectra of MAM in solutions, the existence of two conformers with the methyl group orientedanti andsyn toward the double C=C bond were confirmed. The enthalpy difference δH 0 between the conformers was measured to be 3.7±1.4 kJ mol−1 from the IR spectra in acetonitrile solution and 3.4±1.1 kJ mol−1 from the NMR spectra in DMSO solution. Semiempirical (AM1, PM3, MNDO, MINDO3) and ab initio SCF calculations using a DZP basis set were carried out for all three compounds. The calculations support the existence of two conformersanti andsyn for MAM, withanti being 7.8 kJ mol−1 more stable thansyn from ab initio and 8.6, 13.4, 11.6, and 10.8 kJ mor−1 from AM1, PM3, MNDO, and MINDO3 calculations, respectively. Finally, complete assignments of the vibrational spectra for all three compounds were made with the aid of normal coordinate calculations employing scaled ab initio force constants. The same scale factors were optimized on the experimental frequencies of all three compounds, and a very good agreement between calculated and experimental frequencies was achieved.

Searching for the ylide structure. An ab initio study of the H 2O…CCl 2 complex

The potential energy surface (PES) for the H 2O…CCl 2 complex has been investigated at the self-consistent field level with a DZP basis set in order to search for stationary points and to verify the possible formation of an ylide species. Six stationary points were located on the PES, being one minimum, three first-order transition states (TS) and two second-order TS structures. A stable ylide species was not found, and an explanation for this is given based on electrostatic grounds. The unique minimum corresponds to an H-bond structure, with a dissociation energy of 845.5 cm −1, calculated including zero point energy correction, a more extended basis set, electron correlation effects at the MP2 level and taking into account basis set superposition errors employing the counterpoise method.

Cyclopropyne and Silacyclopropyne: A World of Difference

The lowest singlet and triplet states of cyclopropyne and silacyclopropyne have been investigated using ab initio electronic structure models. Employing DZP and TZ(2df,2pd) basis sets, optimum geometries and harmonic vibrational frequencies have been obtained with the following methods: restricted Hartree-Fock or self-consistent-field (SCF), two-configuration self-consistent-field (TCSCF), single and double excitation configuration interaction (CISD) and coupled cluster (CCSD), and CCSD incorporating perturbative estimates of connected triple excitations [CCSD(T)]. Although silacyclopropyne has been observed via matrix isolation, cyclopropyne remains a high-lying saddle point on the C{sub 3}H{sub 2} potential energy surface. Structural and electronic differences between these two molecules are explored. The triplet states of cyclopropyne and silacyclopropyne are minima on their potential energy surfaces and lie higher in energy than the corresponding singlet states, by 10 and 40 kcal mol{sup -1}, respectively. 60 refs., 7 figs., 1 tab.

Structurally-Rich Potential Energy Surface of the Alagallylyne (AlGaH2) Molecule

Ab initio molecular quantum mechanics has been used to study the potential energy surface of the AlGaH2 system, utilizing DZP basis sets in conjunction with five correlated methods, the most highly...

Reaction Paths for Aqueous Decomposition of CCl2

The potential energy surface (PES) for the H2O + CCl2 reaction was investigated at the ab initio SCF and MP2 levels of theory, employing the DZP basis set, in order to determine the mechanism of basic aqueous decomposition of CCl2. Several possible pathways were considered, including reactions with other H2O molecules and OH-. We have found that the first step corresponds to insertion of CCl2 into the O−H bond of water, resulting in the CHCl2OH species. This molecule loses HCl in one elimination reaction catalyzed by OH-, forming ClCHO. Again, OH- catalyzes the elimination of other HCl, resulting in CO, the decomposition product. The first step is the slow one, and we have used transition-state theory to estimate the rate constant for the aqueous decomposition of CCl2. The obtained rate constant was used for building a general picture of CHCl3 decomposition in basic aqueous solution. The results of the present study are in agreement with experimental observations.

A geometry optimization benchmark using highly correlated wavefunctions (FCI and MRD-CI)

Full configuration interaction (FCI) geometry optimizations have been performed for the X3B1, a1A1, b1B1 and c1A1 electronic states of CH2, the X2B1 and A2A1 electronic states of NH2 and the X1A′1 electronic state of BH3 using a DZP basis set. The results are compared with those obtained using the MRD-CI method at different levels of theoretical treatment. The agreement between the geometrical parameters optimized with the FCI and MRD-CI methods is very good.

Dimethylcarbene: A Singlet Ground State?

Ab initio molecular electronic structure theory has been used to determine the energy separation between the lowest {sup 3}B{sub 1} and {sup 1}A states of dimethylcarbene. The geometries of both states have been optimized at the self-consistent Field(SCF) and the single and double excitation configurationinteraction (CISD) levels of theory using various basis sets. Single point energies at the coupled cluster with single and double excitations (CCSD) and the CCSD with perturbative triple excitations [CCSD(T)] levels of theory were determined at the CISD equilibrium geometries with the same basis set. Harmonic vibrational frequencies and infrared (IR) intensities were determined for both states at the SCF level of theory using all three basis sets and at the CISD level of theory - using the DZP and TZ2P basis sets. The energy separation between the lowest triplet state ({sup 3}B{sub 1}) and the lowest singlet state ({sup 1}A) for dimethylcarbene decreases with increasing basis set size and electron correlation. At the highest level of theory employed in this research, TZ2P+fCCSD(T), the singlet state is predicted to be lower in energy than the triplet state by 0.8 kcalmol{sup -1}. This energy separation becomes 1.4 kcal mol{sup -1} with the inclusion of zero-point vibrational energy(ZPVE)more » corrections. 35 refs., 2 figs., 4 tabs.« less

Ab initio study of the cyclic isomers of N2S4

Abstract An ab initio study of the three possible isomers of cyclic N2S4 is reported. Calculations of minima and transition states at the SCF and MP2 levels of theory are presented, using a DZP basis set with and without model potentials on the sulfur atom. The lowest energy structure for the 1,3-isomer is an asymmetric chair structure, consistent with experimental studies. For the 1,4-isomer, an asymmetric chair is the minimum at the SCF level and there are two minima at the MP2 level; a symmetric chair and a twisted-planar structure. The 1,2-isomer was the most stable structure at the SCF level, having two minimum conformations; a twisted-chair and a symmetrical boat, however these could not be confirmed at the MP2 level. The two experimental IR bands which have been attributed to the 1,4-isomer are compared with the harmonic bands from our MP2 calculations for the 1,4-chair singlet isomers of N2S4.

Vibrational analysis of orthoboric acid H 3BO 3 from ab initio second-order perturbation calculations

The vibrational spectrum of molecular B(OH) 3 is strongly medium-dependent as a result of hydrogen bonding interactions, and the vapor phase assignment has not been completely elucidated. In order to predict and remove any remaining doubt in the interpretation of the observed data, we calculated IR intensities and vibrational frequencies for orthoboric acid and six of its isotopomers from MP2 calculations using a DZP basis set. The study is completed by anharmonic corrections for the A′ mode.

The uncatalyzed hydrosilation reaction

The reactions of ethylene with silane, chlorosilane, fluorosilane, and dichlorosilane, and of propylene with silane are considered usingab initio electronic structure calculations. At both the MP2 and MP4 levels of theory with both DZP and TZP quality basis sets, the energy barriers for all reactions are found to be in excess of 50 kcal/mol.