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)°.     

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.     

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°.     

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.     

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.     

Anharmonic force field and equilibrium structure of nitric acid

The quadratic, cubic and semi-diagonal quartic force field of nitric acid has been calculated at the CCSD(T) 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. It is found that the A and B semi-experimental equilibrium rotational constants of the ¹⁸O isotopologues (for which the rotation of principal axes is large) cannot be accurately reproduced. This problem is discussed and a remedy is proposed. Finally, the semi-experimental structure is in agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of at least quadruple-ζ quality and a core correlation correction, except for the long NO single bond for which the CCSD(T) value is too short due to inadequate treatment of electron correlation. The empirical structures are also determined and their accuracy is discussed. The best equilibrium structure is: r_e(NO_syn) = 1.209(1) Å, r_e(NO_anti) = 1.194(1) Å, r_e(NO) = 1.397(1) Å, r_e(OH) = 0.968(1) Å, ∠(ONO_syn) = 115.8(1)°, ∠(ONO_anti) = 114.2(1)° and ∠(NOH) = 102.2(1)°.     

Rotational spectra of isotopic species of silyl fluoride. Part II: Theoretical and semi-experimental equilibrium structure

The equilibrium structure of silyl fluoride, SiH₃F, has been reinvestigated using both theoretical and experimental data. With respect to the former, quantum-chemical calculations at the coupled-cluster level have been employed together with extrapolation to the basis set limit, consideration of higher excitations in the cluster operator, and inclusion of core correlation as well as relativistic corrections (r(Si-F) = 1.5911 Å, r(Si-H) = 1.4695 Å, and ∠FSiH = 108.30°). A semi-experimental equilibrium structure has been determined based on the available rotational constants for the various isotopic species of silyl fluoride (²⁸SiH₃F, ²⁸SiD₃F, ²⁹SiH₃F, ²⁹SiD₃F, ³⁰SiH₃F, ³⁰SiD₃F, ²⁸SiH₂DF, and ²⁸SiHD₂F) together with computed vibrational corrections to the rotational constants (r(Si-F) = 1.59048(6) Å, r(Si-H) = 1.46948(9) Å, and ∠FSiH = 108.304(9)°).     

Experimental,semi-experimental and ab initio equilibrium structures

The determination of equilibrium structures of molecules by spectroscopic methods or by quantum mechanical calculations is reviewed. The following structures are described in detail: experimental equilibrium structures, empirical structures, semi-experimental structures and ab initio structures. The approximations made by the different methods are discussed and their accuracies are compared.     

Equilibrium structure of protonated cyanogen, HNCCN

The cubic force field of protonated cyanogen, HNCCN⁺, has been calculated at the CCSD(T) level of theory employing correlation consistent bases of quadruple-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 theoretical force fields. While a good agreement has been found with the pure theoretical best estimate of equilibrium geometry, computed at the CCSD(T) level of theory accounting for basis set truncation as well as including core correlation corrections, large discrepancies have been noted with the experimental substitution, r_s, as well as effective, r₀, structures.     

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.     

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.     

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.     

NeNO分子光谱常数和非谐振力场的从头算研究简

本论文分别用从头算方法（B3LYP、MP2）结合基组cc-pVNZ (N ∈ {T,D,Q}) 及 6-311G、6-311G（2df,2pd）计算了NeNO分子的平衡几何结构、光谱常数和非谐振力场.结果表明：在MP2方法下计算的结果略优于B3LYP计算的结果;基组6-311G、6-311G（2df,2pd）下的结果相对优于cc-PVNZ (N=T,D,Q)得到的结果;MP2理论方法结合基组6-311G、6-311G（2df,2pd）基组计算的结果非常接近实验值,这些计算结果对实验测量某些光谱数据有较好的预测作用.     

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.     

NeNO分子光谱常数和非谐振力场的算法研究

本论文分别用从头算方法（B3LYP、MP2）结合基组cc-pVNZ (N ∈ {T,D,Q}) 及 6-311G、6-311G（2df,2pd）计算了NeNO分子的平衡几何结构、光谱常数和非谐振力场.结果表明：在MP2方法下计算的结果略优于B3LYP计算的结果;基组6-311G、6-311G（2df,2pd）下的结果相对优于cc-PVNZ (N=T,D,Q)得到的结果;MP2理论方法结合基组6-311G、6-311G（2df,2pd）基组计算的结果非常接近实验值,这些计算结果对实验测量某些光谱数据有较好的预测作用.     

ArNO分子光谱常数和非谐振力场的理论研究

本论文分别用从头算方法（B3LYP、B3PW91、MP2）结合基组（aug）cc-pVNZ（N∈{T,D,Q}）及6-311G、6-311G^**、6-311G（2df,2pd）、6-311G（3df,3pd）计算了ArNO分子的平衡几何结构、光谱常数和非谐振力场.结果表明:在MP2方法下计算的结果要优于B3LYP和B3PW91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;MP2理论方法结合基组6-311G、6-311G^**、6-311G（2dr,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.     

Rotational spectrum of silyl chloride: hyperfine structure and equilibrium geometry

The Lamb-dip technique was employed to record the rotational spectra of two isotopic species of silyl chloride, namely ²⁸SiH₃Cl and ²⁹SiH₃Cl, in order to investigate their hyperfine structure. High-accuracy quantum-chemical computations were employed to predict the hyperfine parameters involved and to support the experimental investigation. Analysis of the experimental spectra led to an improvement in the accuracy of the known spectroscopic constants as well as allowed us to determine additional spectroscopic parameters for the first time. Furthermore, the equilibrium structure of silyl chloride was reinvestigated using both theoretical and experimental data. The best theoretical and semi-experimental geometries were found to agree within their stated accuracy, leading to the following recommended structure: r(Cl–Si)?=?2.046(1)Å, r(Si–H)?=?1.469(1)Å, and ∠ClSiH?=?108.43(1)^○.     

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,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.