Andrzej Nowek

Post‐Hartree–Fock study on Ar–HCO+ and He–HCO+ complexes: A critical examination of experimental data

The second order Mo/ller–Plesset (MP2) to the fourth order many‐body perturbation theory including single, double, triple, and quadruple substitutions (MP4(SDTQ)), coupled cluster with single and double excitations (CCSD), CCSD with perturbative triple excitations [CCSD(T)], quadratic configuration interaction with single and double substitutions (QCISD), and QCISD including noniterative triples contributions [QCISD(T)] ab initio correlated levels of theory have been employed in studies on molecular geometry and stability of the Ar–HCO+ and He–HCO+ complexes. Triple‐zeta split‐valence (6‐311G) and correlation consistent (cc‐pVTZ) basis sets augmented with diffuse and polarization functions were used. At applied levels of theory the predicted interaction energies (corrected for the basis set superposition error and zero‐point vibrational energy) for Ar–HCO+ range from −3.21 kcal/mol (QCISD/6‐311G(2df,2pd) to −4.21 kcal/mol (MP4(SDTQ)/aug‐cc‐pVTZ), and for He–HCO+ vary from −0.12 kcal/mol (−42 cm−1, MP2/cc‐...

Abinitio study on the stability and properties of XYCO⋅ ⋅ ⋅HZ complexes. III. A comparative study of basis set and electron correlation effects for H2CO⋅ ⋅ ⋅HCl

Ten basis sets of double and triple‐zeta quality augmented by polarization and diffusion function in conjunction with density functional theory (DFT, with the Becke–Lee–Yang–Parr exchange‐correlation potential), Mo/ller–Plesset MP2 to MP4 (SDTQ), coupled cluster with single and double excitations (CCSD) and CCSD with perturbative triple excitation [CCSD(T)] levels of theory were applied in studies of the molecular geometry and stability of the H2CO...HCl complex. Interaction energy (corrected for the basis set superposition error and zero‐point vibrational energy contributions) predicted at three highest levels used; the MP4(SDTQ)/6‐311++G(2df,2pd)//MP2/6‐311++G(2df,2pd), CCSD(T)/6‐311G(2d,2p)//CCSD/6‐311G(2d,2p), and MP2/aug‐cc‐pVTZ (augmented correlation consistent polarized valence triple‐zeta)//MP2/6‐311+G(2df,2pd) amounts to −2.65, −2.61, and −3.88 kcal/mol, respectively, while the DFT/6‐311++G(2df,2pd) level gives −2.86 kcal/mol. It appears that within a given computational method (e.g., MP2, DFT) i...

Theoretical study on the kinetics and mechanism of the hydrogen atom abstraction reactions of CF3O+H2O and CF3OH+OH

Quantum mechanical ab initio calculations have been performed at various levels of theory to study kinetics of the reactions leading to formation/decay of CF3OH in the gas phase. It is shown that two considered reactions i.e., CF3O+H2O (reaction (1)) and CF3OH+OH (−1) proceed via formation of intermediate complexes. Mechanism of the reactions appears to be more complex, and may consist of three consecutive processes. Calculated rate constants are in excellent agreement with available experimental data. Derived expressions: k1=2.5×10−13×(T/300)1.4×exp(−3130/T)cm3molec−1s−1 and k−1=1.9×10−12×(T/300)1.0×exp(−3650/T)cm3molec−1s−1 allow a description of the kinetics of the reactions under investigation in the temperature range 300–1000 K.

A post-Hartree-Fock quantum chemical investigation of biuret planarity

Abstract The structures and energies of planar and nonplanar biuret were studied using post Hartree-Fock MP2, DFT, and MP4(SDQ) levels of theory. The optimized planar conformations are each characterized by at least one imaginary harmonic vibrational frequency of large magnitude, i.e. one (198i cm−1) at the DFT/6-311G(df,pd) level, two (428i and 245i cm−1) at the MP2/6-311G(d,p) level and two (339i and 37i cm−1) at the MP4(SDQ)/6-31G(d,p) level of theory. Only real frequencies were predicted at all applied levels of theory for nonplanar conformations. Planar biuret lies above its nonplanar isomer by 1.19 kcal mol−1 (MP2/6-311G(d,p)), 0.58 kcal mol−1 (MP4(SDQ)/6-31G(d,p)), 0.74 kcal mol−1 (CCSD[T]/6-31G(d,p)//MP4(SDQ)/6-31G(d,p)), and 0.08 kcal mol−1 (DFT/6-311G(df,pd)) at the indicated levels of theory.

Ab initio investigation on stability and properties of XYCO... HZ complexes. II: Post hartree-fock studies on H2CO... HF

Ab initio MP2 level of theory in conjunction with three basis sets of a triple-zeta quality was applied to study the molecular geometry and stability of the H2CO... HF complex. An interaction energy predicted for this system at the highest, MP4(SDTQ)/6-311 + +G(2df, 2pd)//MP2/6-311 + +G(2df, 2pd), level corrected for the BSSE and ZPE contributions amounts to -4.85 kcal/ mol. BSSE contributes significantly to the interaction energies at all applied levels. Reliable MP2/ 6-311 + +G(2df, 2pd) level harmonic vibrational frequencies, IR intensities, and the predicted isotopic shifts upon deuteration and18O substitution are presented in order to facilitate experimental studies on the IR spectrum of the title complex.

Theoretical Study of Rotational Isomerism in Ethyl Pseudohalides

The structure and conformational stability of ethyl pseudohalides CH3CH2 — XCN (X = O, S, Se) were investigated using ab initiocalculations at the MP2 level of theory with a triple-ζ basis set augmented with polarization and diffusion functions. Full optimization was performed on the minimum energy structures as well as on the transition state forms. The relative stabilities of rotational conformers were calculated at the MP4 level using MP2 optimized reference geometries. The nature of all considered stationary points was verified by calculation of the harmonic vibrational frequencies. The calculated bond lengths, bond angles, dipole moments, and rotational constants of optimized global minima structures agree very well with the corresponding experimental data obtained from microwave spectroscopic studies. Also, available experimental frequencies are in good accord with the theoretical values. For ethyl cyanate CH3CH2 — OCN, the antiperiplanar (trans) form is predicted to be more stable than the synclinal (gauche) form, and the synperiplanar (cis) form corresponds to the transition state. For both ethyl thiocyanate CH3CH2 — SCN and ethyl selenocyanate CH3CH2 — SeCN, the gaucheform is the global minimum while the trans-conformer is a local minimum and the cis-form is a transition state.

An ab initio study of the electronic structure and relative stability of the halogenated thiophosphorus compounds SPX (X = Cl, F, Br) and their isomers

Abstract The electronic structure and relative stability of the halogenated thiophosphorus compounds SPCl, SPF, and SPBr and their isomers ClSP, FSP, and BrSP were investigated using ab initio post-Hartree-Fock methods. Molecular geometries of all these structures together with the transition states between isomers, have been optimized at the SCF, MP2, and CCSD levels. Single-point CCSD(T) and MP4 calculations have been performed at the optimal CCSD and MP2 geometries. All calculations have been done using the standard 6–311G(2d) basis set. Harmonic vibrational frequencies and IR intensities for all species were calculated at the correlated levels, and they are in good agreement with the available data from matrix-isolated IR spectroscopy. Because the isomers ClSP, FSP, and BrSP have not yet been experimentally observed, we extended our study by calculating of equilibrium constants of isomerization using Eyring transition state theory, and we have found that at sufficiently high temperatures (≈ 1000 K) the equilibrium constants are large enough for the possible detection of these isomers.

An ab initio study on HXC(double bond)O ? HY molecular complexes (X, Y = F, Cl)

Post-Hartree-Fock calculations were carried out to predict the stabilities and properties of four HClCO … HCl, HClCO … HF, HFCO … HCl, and HFCO … HF molecular complexes. Full geometry optimizations and vibrational frequency calculations were performed for all systems using standard 6-311G(d, p) and 6-311G(2d,2p) basis sets at the MP2 level of theory. Single-point calculations of the interaction energies were carried out for all complexes at the MP4(SDTQ) level with the 6-311G(d, p) basis set. All systems were found to be stable and their predicted molecular parameters match well available (very scarce) experimental data.

Molecular complexes of nitric acid with N2: a post Hartree–Fock quantum mechanical study

The structure of the molecular 1:1 complexes of nitric acid with N2 were optimized at the MP2/6-311G(df,pd) level of theory revealed a nearly linear planar O-H…N hydrogen bonded form and a nonplanar and non-hydrogen bonded form. The global minimum energy structure corresponds to a hydrogen-bonded species: −1.31 kcal/mol at the MP4(SDTQ)/6-311+G(df,pd)//MP2/6-311G(df,pd) and −1.27 kcal/mol at the CCSD[T] and QCISD[T]/6-311+G(df,pd)//MP2/6-311G(df,pd) levels. The nonplanar form is predicted to be a very weakly bonded system at both the MP4(SDTQ)/6-311+G(df,pd)//MP2/6-311G(df,pd) (−0.34 kcal/mol) and at the CCSD[T]/6-311+G(df,pd)//MP2/6-311G(df,pd) (−0.17 kcal/mol) levels of theory. Additionally, a decomposition of the SCF interaction energy was performed.

Theoretical probation of FClCO⋯Cl2 molecular complex: Post Hartree-Fock studies on molecular structures, stabilities and vibrational harmonic frequencies

Abstract Potential energy surfaces of the weak FClCO⋯Cl2 complex were studied using ab initio post Hartree-Fock theory at the MP2 and MP4 levels with 6-311G(d) and 6-311G(2d) basis sets. Two minimum energy conformations, first-order transition state and two second-order transition structures were found. The minimum energy forms (C and T) are separated by a small (less than 0.5 kcal mol ) barrier. At the MP2/6-311G(d) level their relative stabilities amount to −0.63 and −0.60 kcal mol for C and T conformers, respectively, where at the MP2/6-311G(2d) approximation stabilities of both forms rise to −1.09 and −1.21 kcal mol . The harmonic vibrational frequencies were calculated and compared with available experimental data.