An ab initio anharmonic force field of SiHCI3

An ab initio quartic force field of SiHCl₃ is derived using the second-order M?ller-Plesset perturbation theory and Dunning's correlation consistent triple-zeta basis set. After a minor empirical adjustment for the six diagonal quadratic force constants, most fundamentals of SiHCl₃ and SiDCl₃ agree with the experimental values within 1 cm^?1. Additionally the observed overtones, combinations and hot band centres can also be well reproduced. Vibrational analysis based on the second-order perturbation theory is carried out with the calculated force constants. Two sets of spectroscopic constants are predicted for ²⁸SiH³⁵Cl₃ and ²⁸SiD³⁵Cl₃, respectively.     

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.     

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.     

Microwave and high resolution infrared spectra of vinyl chloride, ab initio anharmonic force field and equilibrium structure

The quadratic, cubic, and semi-diagonal quartic force field of vinyl chloride has been calculated at the MP2 level of theory employing a basis set of triple-ζ quality. The spectroscopic constants derived from this force field are compared with the experimental values. To make this comparison more complete, the rotational constants of the lowest excited state, v₉ = 1 at 395 cm⁻¹ have been determined by microwave spectroscopy and the ν₁₂ band (around 618 cm⁻¹) has been investigated by high-resolution infrared Fourier transform spectroscopy. 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-ζ quality and a core correlation correction. The experimental mass-dependent r_m structures are also determined and their accuracy is discussed. The recommended equilibrium geometry is: r (CC) = 1.3262(10), r (CCl) = 1.7263(10), r (CH_g) = 1.0784(10), r (CH_c) = 1.0795(10), r (CH_t) = 1.0797(10), ∠(CCCl) = 122.77(10)°, ∠(CCH_g) = 123.86(10)°, ∠(CCH_c) = 121.80(10)°, ∠(CCH_t) = 119.29(10)°.     

Ground-state constants, ab initio anharmonic force field, and equilibrium structure of F2BOH

The millimeterwave spectra of F₂¹⁰BOH and F₂¹¹BOH (difluorohydroxyborane) have been measured in their ground vibrational state. Accurate rotational and centrifugal distortion constants have been determined. The equilibrium geometry and anharmonic force fields have been calculated at the CCSD(T) level of theory. The ab initio centrifugal distortion constants and rotation-vibration interaction constants are compared to the experimental values. Some discrepancies are found and discussed. Particularly, it is explained why the semi-experimental structure is not reliable. The best equilibrium structure is: r_e(BF_cis) = 132.29 pm, r_e(BF_trans) = 131.29 pm, r_e(BO) = 134.48 pm, r_e(OH) = 95.74 pm, ∠_e(FBF) = 118.36°, ∠_e(F_cisBO) = 122.25°, and ∠_e(BOH) = 113.14°.     

HeNO分子光谱常数和非谐振力场的从头算研究

在本论文中,我们在Gaussian 03软件下,采用密度泛函理论（B3LYP）以及二阶微扰理论（MP2）的方法结合Dunning相关一致基组cc-pVNZ (N=T,D,Q)以及6-311G、6-311G**、6-311G（2df,2pd）、6-311G（3df,3pd）基组优化了HeNO分子的几何结构,然后在此基础上计算了它们的光谱常数和非谐振力场。通过计算得到了分子的平衡几何结构、基频、转动常数、四次和六次离心畸变常数等,并与相关的实验值和理论值进行了比较;预测了部分光谱常数,其中包括谐振频率、非谐性常数、振转相互作用常数、三次力常数、四次力常数和科里奥利耦合常数。结果表明,在MP2方法下计算的结果要优于B3LYP计算的结果;基组6-311G、6-311G**、6-311G（2df,2pd）、6-311G（3df,3pd）下的结果普遍优于cc-PVNZ (N=T,D,Q)得到的结果。结果还表明,MP2理论方法结合基组6-311G、6-311G**、6-311G（2df,2pd）、6-311G（3df,3pd）基组计算的结果非常接近实验值,对实验测量某些光谱数据有较好的预测作用     

Ab initio anharmonic force field and equilibrium structure of carbonyl chlorofluoride

The quadratic, cubic, and semi-diagonal quartic force field of OCFCl has been calculated at the MP2 level of theory employing a basis set of triple-zeta quality. The spectroscopic constants derived from the force field are in excellent agreement with those from previous and new experiments. 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. This good agreement indicates that the derived structure is accurate. The equilibrium geometry is: r_e(CO)=1.173(1) Å; r_e(C-F)=1.323(1) Å; r_e(C-Cl)=1.721(1) Å; ∠_e(OCF)=124.0 (1)°; and ∠_e(OCCl)=126.4(1)°.     

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.     

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.     

Ab initio anharmonic force field and equilibrium structure of the sulfonium ion

The semi-experimental equilibrium structure of the sulfonium ion, , has been obtained from the experimental ground-state rotational constants available for five isotopologues and the corresponding vibrational corrections computed at the CCSD(T)/cc-pwCVQZ level of theory. This geometry has been found in very good agreement with the pure ab initio equilibrium structure calculated at the CCSD(T) level of theory using a basis set of sextuple-zeta quality and including core correlation corrections. The anharmonic force field has been used for deriving spectroscopic properties: in particular, in addition to the vibrational corrections, the rotational parameters of the SH₂D⁺ isotopic species, not yet experimentally observed, have been predicted to a guessed good accuracy.     

HNCS分子的光谱常数和非谐振力场的理论研究

采用Gaussian09数据包,以密度泛函理论（B3LYP、B3PW91）和二阶微扰（MP2）方法,分别采用基组6-311+G、6-311++G(2d,2p)、cc-PVNZ(N=T,Q)以及加入弥散函数的高角动量基组aug-cc-PVNZ(N=T,Q)等计算了HNCS分子的光谱常数和非谐振力场。结果表明：B3LYP、B3PW91和MP2计算的结果与实验结果都比较接近;当基组由VTZ增大到VQZ或加弥散函数对结果的影响均不明显;其中B3PW91\cc-PVTZ的各项计算结果相对于其他计算结果更接近实验值。通过计算,可以在前人基础上进一步预测HNCS分子其它的光谱常数（如:非谐性常数、科里奥利耦合常数等）。     

HNCS分子的光谱常数和非谐振力场的理论研究

采用Gaussian09数据包,以密度泛函理论(B3LYP、B3PW91)和二阶微扰(MP2)方法,分别采用基组6-311+G、6-311++G(2d,2p)、cc-PVNZ(N=T,Q)以及加入弥散函数的高角动量基组aug-ccPVNZ(N=T,Q)等计算了异磺氰酸(HNCS)分子的光谱常数和非谐振力场.结果表明:B3LYP、B3PW91和MP2计算的结果与实验结果都比较接近;当基组由VTZ增大到VQZ或加弥散函数对结果的影响均不明显;其中B3PW91/cc-PVTZ的各项计算结果相对于其他计算结果更接近实验值.通过计算,可以在前人基础上进一步预测HNCS分子其它的光谱常数(如:非谐性常数、科里奥利耦合常数等).     

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.     

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.     

Millimetre-wave and diode laser spectroscopy of IC 15N: anharmonic force field of cyanogen iodide from spectroscopic data and ab initio calculations

(Molecular Physics, 1998, 93, 95–106)     

An anharmonic force field for carbonyl sulphide

A 16-parameter force field for OCS based on curvilinear coordinates is presented in table 2 and the agreement between observed and calculated transitions and rotational constants is indicated in tables 4, 5 and 6. The computational approach differs from that of Morino and Nakagawa [2] and similar work on other molecules by other authors, in that perturbation theory is eschewed and the calculated vibrational transitions and rotational constants are derived via the numerical solution of a large matrix. The usual parameters x_ij , y_ijk , α _i ^B and γ _ij ^B are thereby rendered inappropriate intermediates. The approach also leads to estimates of B _e - B ₀ for each isotopic species and thence suggests equilibrium distances r _CO = 1·155386(21) Å and r _CS = 1·562021(17) Å.     

A comparative ab initio study of the structures,dipole moments,force fields,and anharmonic frequencies of formamide and thioformamide

The structures, dipole moments, force fields, and anharmonic frequencies for the planar conformation of formamide and thioformamide were calculated using the unscaled 4-31G basis set, augmented with a full set of d functions on the sulfur, and full geometry optimization. Extensive comparison of the geometries are made, especially the CO and CS bond lengths, with both the experimental values for the amides and values calculated in previous studies on the acids and other carbonyl compounds. Comparison of the dipole moments calculated using the optimized and experimental geometries with the experimental values suggest there is some inconsistancy in the experimental geometry for thioformamide. Quadratic, cubic, and quartic force constants are calculated for both amides, and hence the fundamental vibration frequencies. Critical comparisons are made with the assignments based on experimental observations. Differences in the bond lengths and stretching force constants for the two NH bonds are shown to be consistent with a hydrogen-bonding type of interaction between the proximal NH and CO and CS groups, like that in the acids.     

The anharmonic force field and equilibrium structure of methane

The anharmonic force field of methane has been refined to fit spectroscopic data from the isotopic species ¹²CH₄, ¹³CH₄, ¹²CH₄, ¹²CH₃D, ¹²CHD₃ and ¹²CH₂D₂. Six of the thirteen cubic force constants have been determined experimentally, the remaining cubic constants being fixed at values derived from ab initio calculations. The quartic force field is very crude, in that only f_rrrr has been refined. It is concluded however that the cubic and quartic force fields, even though they are subject to limitations, provide a considerable improvement in the experimental determination of the r _e structure and the quadratic force field. The equilibrium bond length is found to be r _e(CH) = 1·085₈ ± 0·001 Å.