Richard H. Sullivan

QUANTUM MECHANICAL AND 13C DYNAMIC NMR STUDY OF 1,3-DIMETHYLTHIOUREA CONFORMATIONAL ISOMERIZATIONS

Abstract We present 13 C dynamic NMR results for relative free energies of equilibrium structures and free energies of activation for conformational transformations of 1,3-dimethylthiourea in aqueous solution, and we compare the results to theoretical predictions. The latter are based on ab initio gas-phase electronic structure calculations of the geometries, dipole moments, and energies combined with semiempirical molecular orbital calculations of the free energies of solvation in three different solvents. The gas-phase electronic structure calculations were performed using Moller-Plesset second-order (MP2) perturbation theory with a correlation-consistent polarized valence-double-zeta basis set; we calculated relative energies for the three minima Z , Z , E , Z , and E , E and for three transition states on the potential energy surface. The solvation energy calculations were carried out using the SM5.4/AM1-aqueous, -chloroform, and -organic solvation models; these solvation models are based on semiempirical molecular orbital theory with class-IV charges and geometry-based first-solvation-shell effects. The relative energies of the conformers and transition states are compared to experiment in water and five organic solvents.

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.

Vibrational infrared spectra of biuret and its thioanalogs. An ab initio SCF/3-21G study

Abstract The results of ab initio SCF/3-21G * calculations are reported for vibrational IR spectra (harmonic wavenumbers, absolute intensities) of biuret, thiobiuret, dithiobiuret and their model systems, formamide and thioformamide. The predicted vibrational spectra of both biurets and model systems compare well with the available experimental data, and the observed shifts of the IR wavenumbers upon deuteration of the molecules are correctly predicted by the calculations. The positions and relative intensities of characteristic IR bands of formamide and thioformamide correlate well with the corresponding data for the biurets.

Abinitio prediction of the structure, harmonic vibrational frequencies, and dissociation energy of the H2–GeH+3–H2 cluster ion

Ab initio predictions of the molecular geometry, harmonic vibrational frequencies, and dissociation energies are reported for the germanium hydride cluster ion GeH+7. Seven stationary points were located on the potential energy surface (PES) of GeH+7 using the self‐consistent field (SCF), configuration interaction including single and double excitations (CISD), and coupled cluster including single, double, and perturbatively included triple excitations [CCSD(T)] methods in conjunction with a double‐ζ plus polarization (DZP) and a triple‐ζ plus polarization TZ(3d1f,1p) quality basis set. The most stable structure has a C2 symmetry with the two H2 subunits rotating freely about the symmetry axis of the GeH+3 fragment. Our best estimate of the dissociation energy for GeH+7, taking into account the zero point vibrational energy (ZPVE) is 3.10 kcal/mol compared to 4.6 and 1.2 kcal/mol obtained, respectively, for the SiH+7 and CH+7 cluster ions.

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.

Structures of anti,Z-4,4-dimethyl-3-thiosemicarbazide, syn,E,Z-2,4-dimethyl-3-thiosemicarbazide and syn,E-1-cyclopentano-3-thiosemicarbazone

The structures of three alkyl derivatives of thiosemicarbazide are described: anti,Z-4,4-dimethyl-3-thiosemicarbazide (1), syn,E,Z-2,4-dimethyl-3-thiosemicarbazide (2), and syn,E-1-cyclopentano-3-thiosemicarbazone (3). Crystal data: for 1: triclinic, P-1 (#2), a = 5.802(1)Å, b = 6.935(1)Å, c = 8.104(2)Å, α = 78.35(1)°, β = 82.13(1)°, γ = 70.71(1)°, and Z = 2; for 2: orthorhombic, Pbca (#61), a = 9.417(3)Å, b = 8.624(2)Å, c = 15.169(3)Å, and Z = 8; for 3: triclinic, P-1 (#2), a = 6.068(3)Å, b = 8.145(4)Å, c = 8.666(5)Å, α = 83.75(4)°, β = 86.16(5)°, γ = 74.07(4)°, and Z = 2. In general, molecules are linked by N–H···S hydrogen bonds with sulfurs accepting two or three hydrogen bonds. Structures 2 and 3, which adopt the syn conformation, form N–H···N intramolecular hydrogen bonds. The solid-state structures are consistent with their infrared and proton nuclear magnetic resonance spectra.

Quantum chemical conformational analysis and X-ray structure of 4-methyl-3-thiosemicarbazide

Abstract We present X-ray crystallographic results and gas-phase electronic structure calculations of the geometry of 4-methyl-3-thiosemicarbazide. Using the Hartree-Fock theory with a 6-31G ∗ basis set, we calculated relative energies for eight different conformations. For the lowest-energy conformations of each of the four possible combinations of rotamers about the two CN bonds, we also included electron correlation by Moller-Plesset second-order (MP2) perturbation theory with the same basis set. From these calculations, we selected the lowest-energy structure and calculated structural parameters at the MP2 level of theory with the larger correlation-consistent cc-pVDZ basis set. The geometry of the minimum-energy gas-phase structure is in good agreement with the structure observed experimentally in the crystal.

Cyclic-AlO2 revisited

Abstract In this report, we present the results of ab initio electronic structure calculations of the structures, harmonic vibrational frequencies and the dissociation energy of cyclic AlO2. At the highest level of theory employed [CCSD(T)/6-311++G(3df)] the 2A2 state is lower than the 2A1 state by roughly 7 kcal mol−1. The molecular structure for the 2A2 state is predicted to have r e (AlO) = 1.940 A and r e (OO) = 1.354 A . The AlO2 dissociation energy is computed to be 65 kcal mol−1 using full counterpoise corrections. The ω2 mode (AlO symmetric stretch) of 529 cm−1 calculated at the CCSD(T) level is about 6% higher than the experimental value of 496 cm−1.

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.

Structure and stability of the N-hydroxyurea dimer: Post-Hartree–Fock quantum mechanical study

The potential energy surface (PES) search of the N-hydroxyurea dimer was searched with second-order Moller–Plesset perturbation theory (MP2) and the 6-31G(d,p) basis set. Eight local minimum energy structures have been found. Four of them have relatively strong (ΔE∼−10 to −13 kcal/mol) intermolecular interactions and the others are moderately strongly interacting species (ΔE∼−3 to −7 kcal/mol). Final estimation of interaction energies was performed using the larger 6-311G(df,pd) and 6-311G(2df,2pd) basis sets. The predicted interaction energies are ΔE=−14.26 kcal/mol and −3.43 kcal/mol for the strongest and the weakest interacting forms of the studied complex, respectively, at the MP2/6-311G(2df,2pd)//MP2/6-31G(d,p) level of theory. The self-consistent field (SCF) interaction energy decomposition indicates the important influence of the deformation term magnitude on ΔE(SCF). The calculated electron correlation contribution to ΔE(MP2) depends on the geometry of the system and varies from −0.5 to −5 kcal/mo...