Accurate equilibrium inversion barrier of ammonia by extrapolation to the one-electron basis set limit

A scheme based on treating uniform singlet-pair and triplet-pair interactions is suggested to extrapolate electron correlation energy of ammonia, calculated at two basis-set levels of ab initio theory in the infinite one-electron basis-set limit. The dual-level method is tested on the extrapolation of the full correlation in coupled-cluster singles and doubles and in the case also a noniterative perturbative correction for connected triple energies for the C3v and D3h structures of ammonia, with correlation-consistent basis sets of the type cc-pVXZ （X = D, T, Q, 5, 6） and aug-cc-pVXZ （X = D, T, Q, 5）. For testing and comparison purposes, the energies reported by Klopper [J. Comput. Chem. 22 1306 （2001）] have been taken. From a corresponding extrapolation of CCSD（T）/AVXZ energies for X = 4, 5, we obtain total inversion barriers of 1833.87 cm-1/1832.33 cm^-1 for the two/three-parameter extrapolation rules, which are in good agreement with other theoretical extrapolation and empirical values in the literature.     

Gaussian basis set of triple zeta valence quality for the atoms from K to Kr: Application in DFT and CCSD(T) calculations of molecular properties

A contracted basis set of triple zeta (TZ) valence quality for the atoms from K to Kr was constructed from fully-optimized Gaussian basis sets generated in this work. Gaussian polarization functions (d, f, and g symmetries), which were optimized at the second-order Mφller–Plesset level, were added to the TZ set. This extends earlier work on segmented contracted TZ basis set for atoms H-Ar. This set along with the BP86 non-hybrid and B3LYP hybrid functionals were used to calculate geometric parameters, dissociation energy, harmonic vibrational frequency, and electric dipole moment of a sample of molecules and, then, comparison with results obtained with other basis sets and with experimental data reported in the literature is done. CCSD(T) atomic excitation energies and bond lengths, dissociation energies, and harmonic vibrational frequencies of some diatomics were also evaluated. Using density functional theory and gauge-including atomic orbitals, ⁵⁷Fe and ⁷⁷Se nuclear magnetic resonance chemical shifts in Fe(C₅H₅)₂, H₂Se, (CH₃)SeH, CSe₂, SeCO, H₂CSe, and SeF₆ were calculated. Comparison with theoretical and experimental values previously published in the literature was done. It is verified that in general these results give good agreement with experimental and benchmark values.     

The gas-phase pyrolysis of methyl azidoformate in the absence and presence of water: a theoretical study

The potential energy surface profiles for the gas-phase pyrolysis of methyl azidoformate (MA, CH₃OC(O)N₃) in the absence and presence of one water molecule have been investigated by ab initio methods at CCSD(T)/6-311++G(2df,2pd)//MP2(full)/6-311++G(d,p)+0.95×ZPE levels of theory. Three types of mechanisms are discussed for the gas-phase decomposition of CH₃OC(O)N₃. Ab initio calculations show that a four-membered-ring intermediate can be formed by the stepwise routes. The resulting intermediate can undergo two competitive decomposition channels to generate the major products CO₂?+?CH₂?=?NH and HNCO?+?HC(O)H. The calculated results are in qualitative agreement with the observed experimental data. However, CH₃ONCO can be produced from the Curtius-type rearrangement route. This is an intriguing finding in this study. Moreover, the effect of one water molecule on the gas-phase pyrolysis of MA has been also explored. We find that the relative energy of the hydrated transition states is effectively lowered when water is added to the reaction. However, the estimated rate constant at 625?K for the naked reaction is about 30 times faster than the reaction with water. Thus, a single water molecule cannot play an important role in the thermal decomposition of MA.     

渤海J油田储层核磁共振测井孔隙度影响因素分析及校正

渤海J油田沙河街组储层核磁共振（NMR）测井孔隙度和岩心NMR孔隙度均低于岩心氦孔隙度,这种现象影响了NMR测井的应用效果．通过开展岩心NMR实验,对该研究区仪器采集参数、井眼环境以及储层流体性质等因素进行分析,发现造成储层NMR测井孔隙度偏低的主要原因是高矿化度泥浆滤液侵入．基于饱和不同矿化度盐水对T₂谱的影响规律,确定了需要对T₂谱进行形态校正的矿化度下限值,并建立了对应不同矿化度的T₂谱形态校正模型及NMR孔隙度校正方法．应用结果表明,校正后的NMR测井孔隙度与岩心氦孔隙度的平均相对误差从13.56%下降至2.81%,有效提高了NMR测井孔隙度的精度．