Equilibrium structure of methylcyanide

The quadratic and cubic force fields of methylcyanide have been calculated at the MP2 and CCSD(T) levels of theory employing a core-valence basis set of triple-zeta quality. Semi-experimental equilibrium structures have then been derived from the experimental ground-state rotational constants available for various isotopologues and the corresponding vibrational corrections calculated from the ab initio force fields. These structures have been found in excellent agreement with the pure ab initio structure calculated at the CCSD(T) level of theory using a basis set of sextuple-zeta quality and including core correlation corrections.     

Ab initio anharmonic force field and equilibrium structure of vinyl bromide

The quadratic, cubic, and semi-diagonal quartic force field of vinyl bromide has been calculated at the MP2 level of theory employing a basis set of triple-ζ quality including a relativistic pseudopotential on bromine. A semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. This structure is in excellent agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of quadruple-ζ quality and an offset correction. The experimental mass-dependent r_m structures are also determined and their accuracy is discussed.     

Ab initio anharmonic force field and equilibrium structure of propene

The quadratic, cubic and semi-diagonal quartic force field of propene has been calculated at the MP2 level of theory employing a basis set of triple-ζ quality. A semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. This structure is in excellent agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of quintuple-ζ quality and a core correlation correction. The experimental mass-dependent r_m structures are also determined and their accuracy is discussed. The use of isolated CH stretching frequencies is shown to be a good method to determine CH bond length.     

Torsional barrier and equilibrium structure of ethyl cyanide

The quadratic, cubic and semi-diagonal quartic force field of ethyl cyanide has been calculated at the B3LYP level of theory employing a basis set of triple-ζ quality. A semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. This structure is in excellent agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of quadruple-ζ quality and a core correlation correction. The empirical structures are also determined and their accuracy is discussed. The potential barrier V₃ hindering internal rotation of the methyl group has been calculated from 23 rotational transitions of CH₃CH₂C¹⁵N which were found split into doublets, giving V₃ = 3074(27) cal mol⁻¹.     

Ab initio anharmonic force field and equilibrium structure of vinyl fluoride and vinyl iodide

The quadratic, cubic and semi-diagonal quartic force field of vinyl fluoride and vinyl iodide have been calculated at the MP2 level of theory employing a basis set of triple-ζ quality including a relativistic pseudopotential on iodine. A semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. This structure is in excellent agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of quadruple-ζ quality. Finally, the structure of different vinyl derivatives is compared.     

PuO的基态分子结构与相对论有效原子实势

用Pu的三种不同相对论有效原子实势 (RECP)和密度泛函 (B3LYP)、HF、MP2方法对PuO基态X5Σ-的分子结构、势能函数进行了计算 ,得到了相应的平衡几何构型和谐振频率 .采用最小二乘法拟合得到了PuO分子的Murrell Sorbie势能函数 ,在此基础上推导出光谱数据和力常数并与实验值做对比 ,结果表明 ,在三种RECP有效势中 ,对于平衡结构和离解能De 的计算 ,6 0个中心电子的SDD/B3LYP给出的结果较好 .而力常数f2 、f3 和 f4,以及光谱数据 ,则 14个价电子的 1987RECP/MP2的结果优于其它有效势和计算方法 .计算还给出了相应的电荷布居、自旋密度、电偶极矩和能量特征 .     

Ground state rotational spectrum,K = 3 splittings,ab initio anharmonic force field and equilibrium structure of trifluoroamine

The submillimetre-wave spectrum of ¹⁴NF₃ has been measured and the ground state rotational spectrum has been reanalysed, including the K=3 splittings. The quadratic, cubic and semidiagonal quartic force field has been calculated at the CCSD(T) level of theory employing a basis set of at least polarized valence triple-zeta quality. This force field has been used to predict the spectroscopic constants, including the parameters specific to the doubly degenerate vibrational states. The calculated values are found to be in good agreement with the available experimental data. The equilibrium structure has been derived from the experimental ground state rotational constants and either the ab initio or the experimental rovibrational interaction parameters. These experimental and semiexperimental structures are in excellent agreement with the ab initio equilibrium geometry.     

Theoretical prediction of vibrational spectra: The out-of-plane force field and vibrational spectra of pyridine

The harmonic force field for the out-of-plane vibrations of pyridine has been calculated from ab initio Hartree-Fock wavefunctions obtained with a 4–21 basis set of contracted Gaussians. To account for systematic errors, the calculated force constants were scaled, using only two independent sclae factors which were transferred unchanged from benzene. The resulting scaled quantum mechanical force field, which is strictly a priori in that it is not based on any experimental data on pyridine, predicts the 64 out-of-plane fundamental frequencies of pyridine and its deuterated isotopomers of C_2v symmetry with a mean deviation from experiment of only 8.5 cm^?1. Addition of polarization functions to the basis set for the nitrogen atom and refinement of the two scale factors by fitting them to the observed pyridine spectra produce no significant improvement in the fit. Assignments of the vibrational spectra are discussed.     

Harmonic Force Fields from Scaled SCF Calculations: Program ASYM40

We report an extended version of our normal coordinate program ASYM40, which may be used to transform Cartesian force constants from ab initio calculations to a force field in nonredundant internal (symmetry) coordinates. When experimental data are available, scale factors for the theoretical force field may then be optimized by least-squares refinement. The alternative of refining an empirical force field to fit a wide variety of data, as with the previous version ASYM20, has been retained. We compare the results of least-squares refinement of the full harmonic force field with least-squares refinement of only the scale factors for an SCF calculated force field and conclude that the latter approach may be useful for large molecules where more sophisticated calculations are impractical. The refinement of scale factors for a theoretical force field is also useful when there are only limited spectroscopic data. The program will accept ab initio calculated force fields from any program that presents Cartesian force constants as output. The program is available through Quantum Chemistry Program Exchange. Copyright 2000 Academic Press.     

Mg2Ni分子的结构和势能函数

采用密度泛函B3LYP方法在6-311+g(d,p)基组水平上对MgNi、Mg2和Mg2Ni分子的各种可能的结构进行优化,得到了它们的几何构型、平衡核间距、离解能和谐振频率.采用最小二乘法拟合MgNi和Mg2分子的Murrell-Sorbie势能函数,在此基础上导出光谱数据和力常数.通过多体项展式理论导出Mg2Ni分子的解析势能函数,并绘出了Mg2Ni分子的等值势能面,其等值势能面正确地反应了基态Mg2Ni分子的平衡构型特征.     

Pure rotational spectrum, ab initio anharmonic force field, and equilibrium structure of silyl chloride

The ground state rotational spectra of H₃Si³⁵Cl, H₃Si³⁷Cl, and D₃Si³⁵Cl have been measured from the microwave to the submillimeterwave ranges and accurate rotational parameters have been determined. For H₃Si³⁷Cl, they are in good agreement with the values obtained from the ground state combination differences. The quadratic, cubic, and semi-diagonal quartic force field has been calculated at the MP2 level of theory employing a basis set of polarized valence quadruple-zeta quality. This force field has been used to predict the spectroscopic constants. The calculated values are found to be in good agreement with the available experimental data. The equilibrium structure has been derived from the experimental ground state rotational constants and either the ab initio or the experimental rovibrational interaction parameters. These experimental and semi-experimental structures are in excellent agreement with the ab initio equilibrium geometry.     

Equilibrium Structure and Spectroscopic Constants of Difluorovinylidene: An ab Initio Study

Highly correlated ab initio calculations with large basis sets are reported for difluorovinylidene, F(2)CC. Based on CCSD(T)/aug-cc-pVQZ results and taking core correlation effects properly into account, a reliable theoretical equilibrium geometry is derived: r(e)(CC) = 134.74(10) pm, r(e)(CF) = 131.00(10) pm, and angle(e)(FCC) = 123.23(10) degrees. The error bars are estimated from analogous comparative calculations on the equilibrium structures of the CF(2), C(2), and C(3) species. Correlated harmonic [CCSD(T)/aug-cc-pVQZ] and anharmonic [CCSD(T)/TZ2Pf] force fields provide theoretical values for the fundamental vibrational wavenumbers which are in excellent agreement with those measured previously in an argon matrix. Many spectroscopic constants of F(2)CC are predicted. In addition, the energy of F(2)CC relative to difluoroethyne (FCCF) and the barrier to isomerization from F(2)CC to FCCF have been reinvestigated by means of the present high-level ab initio calculations. Copyright 2001 Academic Press.     

PuN基态和低激发态的分子结构与相对论有效原子实势

用Pu的三种不同相对论有效原子实势 (RECP)和密度泛函(B3LYP)方法对PuN基态和低激发态(4Σ+、6Σ+、8Σ+、10Σ+)的分子结构进行了计算,得到了相应的平衡几何构型和谐振频率.采用最小二乘法拟合得到了PuN分子的Murell-sorbie势能函数,在此基础上推导出完整的力常数和光谱数据,并与实验值作对比.结果表明:PuN的基态为X6Σ+,其余为低激发态;在三种RECP中,对于ωe的计算,60个中心电子的SDDRECP/B3LYP给出的结果与实验值符合得比较好(如X6Σ+:ωe=840.77 cm-1,ωeχe=5.73·"cm-1,De=5.880 6 eV,Re=0.176 3 nm,Be=0.408 4 cm-1,αe=3.3E-03 cm-1).计算还给出了相应的电荷布居、自旋密度、电偶极矩和能量特征等系列分子的性质.     

Scaled DFT Force Constants and Vibrational Spectrum of Cyclopropylamine

Abstract

The molecular structure and harmonic vibrational frequencies of cyclopropylamine have been calculated using the B3LYP density functional method with the 6‐31G(2d,2p) basis set. The scaled DFT force field gives very good reproduction of the experimental vibrational frequencies. Several of the vibrational fundamental modes assigned previously are reassigned on the basis of the B3LYP/6‐31G(2d,2p) method and the scaled force field calculations. The optimized scaling factors were used to scale the B3LYP/6‐31G(2d,2p) force field of cyclopropane and cyclopropylamine (–ND₂) molecules.     

The anharmonic force fields of PH3, PHF2, PF3, PH5, and H3PO

The cubic and quartic force fields of the title compounds are determined from ab initio SCF calculations using 6-31G^** and TZP/TZ2P basis sets. The computed geometries, vibration-rotation interaction constants, l-doubling constants, anharmonicity constants, and vibrational wavenumbers are compared with the available experimental data, especially for PH₃ and PF₃. Many experimentally unknown spectroscopic constants are predicted. A scaling procedure based on calculated harmonic and anharmonic force fields is proposed for predicting the vibrational wavenumbers of unknown molecules such as PH₅.     

A combined empirical-ab initio determination of the general harmonic force field of ketene

The general harmonic force field (GHFF) of ketene has been determined through a joint empirical-ab initio investigation. Perturbations in the infrared spectra of all isotopic species render experimental frequency data of limited discriminatory value in the empirical determination. Microwave/infrared determined quartic distortion constants are found to be mutually incompatible, both within and between isotopic species. The sensitivity of the distortion constants to truncation and constraints made in their determination is established in order to make a realistic estimate of their reliability in the force constant calculations. Ab initio calculations performed at various different levels of sophistication predict consistent values for interaction constants, some of which are markedly different from previously reported empirical values. The joint empirical-ab initio GHFF reproduces all observed and perturbation-corrected data well over five isotopic species. Coriolis interaction constants are calculated for ketene-H₂, -HD, and -D₂, which will be of assistance to future analyses, particularly of the strongly interacting four-level systems below 1000 cm⁻¹. Scaled ab initio force constants, calculated around the experimental ground state geometry as reference, are in excellent agreement with the empirical values, with one exception, which arises due to neglect of configuration interaction.     

Ab initio anharmonic force field,spectroscopic parameters and equilibrium structure of trifluorosilane

The quadratic, cubic and semidiagonal quartic force field of trifluorosilane has been calculated at the MP2 level of theory employing a basis set of polarized valence triple-zeta quality. This force field has been used to predict the spectroscopic constants, including the parameters specific of the double degenerate vibrational states. The calculated values are found to be in good agreement with the available experimental data, and explanations are offered for discrepancies. This confirms the accuracy of the ab initio force field and the validity of the theory of the reduction of the rovibrational Hamiltonian of a doubly degenerate vibrational state. The equilibrium structure has been derived from the experimental rotational constants and the ab initio rovibrational interaction parameters. This semiexperimental structure is in excellent agreement with the ab initio equilibrium geometry.     

UN、UN2分子的结构和势能函数

用相对论有效原子实势(RECP)和密度泛函(B3LYP)方法对UN和UN2分子的结构进行优化,得到了它们的平衡几何构型和谐振频率.采用最小二乘法拟合出UN 分子的Murrell-Sorbie势能函数,在此基础上推导出光谱数据和力常数;并通过多体展式理论导出UN2分子的势能函数,正确地反应了其平衡构型特征.     

Submillimeterwave spectrum of CH2PH and equilibrium structures of CH2PH and CH2NH

The rotational spectrum of phosphaethene (CH₂PH) was reinvestigated. One hundred and nineteen new lines were measured in the submillimeter range from 500 to 650 GHz. The determination of the centrifugal distortion constants is significantly improved. As the molecule is close to symmetric prolate top, both reduction A and S were compared. The equilibrium structure has been derived from experimental ground state rotational constants and ab initio rovibrational interaction parameters. This semi-experimental structure is in excellent agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of quintuple-zeta quality and a core correlation correction. The structure of CH₂PH was compared to that of CH₂NH which was also determined for this goal. It is found that the semi-experimental structure of CH₂NH is less accurate than the ab initio structure. It is also found that the methylene group is much more asymmetric in CH₂NH than in CH₂PH.     

Experimental study of the vibrational spectra of (CH3)3GeBr supported by DFT calculations

The infrared and Raman spectra of bromotrimethylgermane (BTMG) were recorded afresh to complete the assignment of its vibrational spectra. The vibrational spectrum of BTMG has been predicted from hybrid density functional theory calculations (B3LYP) with several basis sets. The resulting harmonic wavenumbers were scaled by Pulay's scaled quantum mechanical (SQM) and the wavenumber‐linear scaling (WLS) methods to obtain accurate force fields which could aid in the vibrational assignment. Low‐temperature infrared techniques together with Fourier self‐deconvolution (FSD) on the Raman spectrum were used to improve the resolution of the spectra for the modes that could not be observed before. An SQM analysis was carried out to obtain the valence force constants and a set of scale factors that best reproduced the experimental data. Copyright © 2008 John Wiley & Sons, Ltd.