The use of SCF-MO calculations and experimental data in evaluating general quadratic force fields of light atom molecules

The semiempirical MOCIC method, in which SCF-MO calculations are combined with limited frequency data in evaluating general quadratic force fields, has been tested on several light atom molecules (F₂CO, H₂CO, FNO, HCCH, NCCN, and H₂CCCH₂). Both MNDO and available ab initio treatments have been employed to establish the MOCIC constraints. The MOCIC potential functions are found to be remarkably close to potential functions determined uniquely from a plethora of experimental data. In comparison with potential constants evaluated using SCF-MO methods, the MOCIC functions are in much better agreement with experimentally determined counterparts. In addition to improving the estimates of primary potentials, the MOCIC approach gives statistically significant improvement in the estimate of interaction constants for these multiply bonded molecules where stretch-stretch interactions are large. The MOCIC constraint procedure is shown to work extremely well for fluorinated molecules.     

The combined use of SCF-MO calculations and frequency data in the evaluation of general harmonic force fields for molecules containing third row elements

The general harmonic force fields of several small molecules containing third row elements have been evaluated using a semiempirical method which combines SCF-MO calculations and limited frequency data. MOCIC (molecular orbital constraint using interaction coordinates) potential fields have been generated using the SCF-MO MNDO method which has recently been parameterized for third row elements. The general harmonic force field (GHFF) of molecules studied here (SO₂, NOCl, PF₃, PH₃, CCl₄, SiCl₄, SiF₄, SiH₄, and SF₆) are all well defined from spectroscopic data. The MOCIC functions are in excellent agreement with the spectroscopic GHFFs. Comparisons of the MNDO and MOCIC functions show substantial improvement in both primary and interaction valence potential constants when the MOCIC constraint procedure is used.     

The microwave spectrum,harmonic force field,and ground state average structure of 1,1-dichloroethylene

The microwave spectra of several isotopic species of 1,1-dichloroethylene have been measured up to high J values and have been analyzed for rotational constants and quartic centrifugal distortion constants. An approximate harmonic force field for the molecule has been obtained by combining the centrifugal distortion constants with known vibrational data. The harmonic force field has been used together with the results of the present and other microwave studies to determine the ground state average molecular structure.     

The microwave spectrum,harmonic force field,and structure of formyl chloride

Extensive measurements were made of the microwave spectra of nine isotopic species of formyl chloride, HCOCl. Analysis of these spectra gave accurate rotational constants, chlorine nuclear quadrupole coupling constants, and centrifugal distortion constants. The distortion constants, together with the vibrational wavenumbers, were used to evaluate a valence harmonic force field. Effective, substitution, ground-state average, and estimated equilibrium structures are presented.     

The microwave spectrum,harmonic force field,and structure of carbonyl chlorofluoride,OCCIF

Rotational spectra of three isotopic species of carbonyl chlorofluoride, OCCIF, have been extensively measured, and have been analyzed for rotational constants, quartic centrifugal distortion constants, and chlorine nuclear quadrupole coupling constants. Ab initio calculations of the harmonic force field have been made using several different sets of basis functions, and their relative cost efficiency has been assessed. The measured distortion constants have been combined with vibrational wavenumbers (both from the literature and from the present work) and with the ab initio force constants to refine the force field. Ground state effective (r₀) and average (r_z) structures have been evaluated for the molecule.     

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.     

Comparison of empirical and ab initio force fields for phosphine

Existing experimental data on frequencies, Coriolis coupling constants, and centrifugal distortion constants for phosphine can be reconciled by means of a harmonic force field, provided substantial vibrational dependences of the distortion constants are permitted. Such dependences are indicated in a recent high-resolution study of PD₃. The resulting empirical force constants are then in excellent agreement with existing predicted values from a sophisticated ab initio study, including electron correlation.     

Rotational spectrum of ONCl in the (0, 0, 1) and (0, 1, 0) vibrational states and “b” type spectrum in the ground state. Comparison of force field obtained by different combinations of experimental data

From the rotational spectrum of ONCl in (0, 0, 1) and (0, 1, 0) excited vibrational states the inertia defects in these states have been determined. The “b” type rotational spectrum in the ground state has also been measured allowing the determination of Δ⁰ and of all the first-order centrifugal distortion constants.It is shown that by using differences of inertia defects Δ₂ and Δ₃ together with centrifugal distortion constants which do not involve the planarity relations and with harmonic vibrational frequencies of two isotopic species, the harmonic vibrational frequencies of two other isotopic species can be satisfactorily predicted.By using only inertia defects differences Δ₂ and Δ₃ as extra data, a definite choice can be made among the three sets of force constants which equally well reproduce the harmonic frequencies of four isotopic species.     

The microwave spectrum of the pyramidal isomer of disulfur difluoride: SSF2

The pyramidal isomer of disulfur difluoride, SSF₂, has been prepared by passing gaseous disulfur dichloride through heated potassium fluoride. The microwave spectra of three isotopic species of SSF₂ have been measured in the 26- to 76-GHz frequency region. For the most abundant isotopic species transitions with J values up to 79 have been observed. These data have been combined with previous measurements of low-J lines by R. L. Kuczkowski [J. Amer. Chem. Soc. 86, 3617–3621 (1964)] and fit to obtain values for the rotational constants and centrifugal distortion constants. The quartic centrifugal distortion constants have been used together with existing vibrational data to determine an harmonic force field for the molecule. Finally, ground state average and equilibrium structures have been calculated for SSF₂.     

Use ofD J,DJK constants for the unique fixing of intramolecular forces inXY 3 pyramidal molecules

A mathematical formalism is developed to evaluate all the possible sets of intramolecular force fields fitting fundamental vibrational frequencies and centrifugal distortion constants in the case ofXY ₃ pyramidal type molecules. The method is applied to PF₃ molecule as an example. It is found that there exist as many as four sets of force field fitting all the above experimental data. A few general criteria are suggested to eliminate the suprious sets, thus making further experimental data unnecessary to fix the true physical force field.     

Use of calculated centrifugal distortion constants (Dν, Hν, Lν and Mν) in the analysis of the B ← X system of I2

Fourier spectroscopy measurements of 14 703 lines from the B-X band system of I₂ are analyzed using centrifugal distortion constants calculated from RKR curves by the perturbation theory method of Hutson [J. Phys. B14, 851–857 (1981)]. Only vibrational and rotational constants are varied in the least-squares fits, with the centrifugal distortion constants fixed at the calculated values. The fit obtained in the present work is as good as when the centrifugal distortion constants are treated as independent parameters, but the molecular constants obtained are physically more meaningful because the statistical correlation between parameters is reduced. The analysis includes B-state vibrational levels up to v′ = 72, which is bound by only 0.5% of the well depth; the method used for calculating centrifugal distortion constants remains reliable even this close to dissociation. It is found that centrifugal distortion constants up to fifth order make significant contributions to the observed line positions.     

Gas phase vibrational fundamentals and vib-rotational parameters of AsCl3

Contour simulation of the gas phase infrared fundamental bands of AsCl₃ led to estimated values for the first-order Coriolis constants, the vibrational frequencies, and the relative values of the vibrational transition moments. Improved values of the six independent force constants of a general quadratic force field were determined on the basis of the observed frequencies, mean amplitudes of vibration, and Coriolis and centrifugal distortion constants. The potential energy distribution over the normal modes is reported.     

Experimental and ab Initio Equilibrium Structure and Harmonic Force Field of 1,2,5-Oxadiazole

The equilibrium structure of 1,2,5-oxadiazole has been calculated ab initio at the CCSD(T) level using a polarized valence quadruple zeta basis set. The harmonic force field has also been calculated at the MP2/cc-pVTZ, B3LYP/6-311++G(3df, 2pd), and B3LYP/cc-pVQZ levels. These force fields have been subsequently scaled and further refined by fitting them to the experimental values of the vibrational fundamentals of three isotopomers and the centrifugal distortion constants of the parent molecule. The specific refinement of those scaled force constants particularly sensitive to the experimental data set was decisive for obtaining a more reliable harmonic potential. The resulting force fields are presented and used, together with the ground state rotational constants, to calculate an r(z) structure. The experimental r(0), r(s), and r(m) structures have also been determined. The different results have been compared and it is concluded that the ab initio structure is a good approximation of the equilibrium structure. It is also shown that the magnetic correction is not negligible, particularly for the inertial defect. Another interesting conclusion is that the anharmonicity of the C-H stretching might be unusually small. Copyright 2001 Academic Press.     

The harmonic force field and structure of chloryl fluoride

The microwave spectra of three isotopic species of chloryl fluoride, FClO₂, previously published by Parent and Gerry (J. Mol. Spectrosc., 49, 343–364 (1974)), have been refit to rotational constants and centrifugal distortion constants using Watson's Hamiltonian in both its A and S reductions. The quartic distortion constants have been combined with the vibrational data of Smith, Begun, and Fletcher (Spectrochim. Acta, 20, 1763–1770 (1964)) to calculate a refined harmonic force field. The rotational constants and force field have been used to calculate a zero-point average structure and an approximate equilibrium structure. Both the force field and structures are in essential agreement with those published earlier.     

The harmonic force fields of methylene chloride and dichlorosilane from combined microwave and infrared data

The harmonic force fields of methylene chloride and dichlorosilane have been obtained by combining the vibrational wavenumbers and centrifugal distortion constants of several isotopic species. Although enough data were available from earlier work for dichlorosilane, it was first necessary for methylene chloride to determine its distortion constants from microwave spectra. Transitions were measured up to J = 80 and J = 90 for CH₂Cl₂ and CD₂Cl₂, respectively, and the analysis gave accurate rotational constants, and quartic and sextic distortion constants. Ground-state effective, substitution, ground-state average and approximate equilibrium structures have been obtained for both molecules.     

Microwave spectra and centrifugal distortion constants of formic acid containing 13C and 18O: Refinement of the harmonic force field and the molecular structure

Pure rotational spectra of H¹³COOH, HC¹⁸OOH, and HCO¹⁸OH have been measured in the frequency region 8–185 GHz. Analysis of the spectra has given improved rotational constants and quartic and sextic centrifugal distortion constants. The quartic distortion constants have been combined with previously published distortion constants of four other isotopic species, and with the vibrational wavenumbers of seven isotopic species, to produce a refined harmonic force field. An improved substitution structure and the ground state average structure have been obtained. Some unmeasured transition frequencies which may be of importance in radioastronomy are also presented.     

Kinetic Constants Method for the Vibrational Analysis of Thallous Halide Dimers

The kinetic constants method is applied to the case of thallous halide dimers to evaluate the molecular constants such as, potential constants, compliance constants, mean amplitudes of vibration, coriolis coupling constants and centrifugal distortion constants, using the recent vibrational frequencies¹. The values of mean amplitude of vibration obtained by this method are in good agreement with the experimental values². The thermodynamic functions of thallous flouride for eleven temperatures ranging from 100K to 1000K are calculated on the basis of rigid-rotor, harmonic-oscillator approximation and are reported.     

The Degenerate Vibrations of BCl3, BBr3 and BI3 with Application of Keating Coordinates

The experimental works of vibrational spectroscopy and normal coordinate analyses for BCl₃, BBr₃ and BI₃ are reviewed extensively. Harmonic force fields of the E' species are produced using isotopic frequencies and Coriolis constants as additional data, respectively. The usefulness of Keating coordinates versus valence coordinates as basis of force field approximations is discussed. The conclusions are not unequivocal, but they go in favour of the Keating coordinates when the reliability of the different computations is taken into account. Boron trichloride is treated specifically in some detail. Final force fields are proposed for the title molecules with the aid of the mass influence on Coriolis constants.     

The microwave spectrum, structure, chlorine nuclear quadrupole coupling constants, dipole moment, and centrifugal distortion constants of dichlorosilane

The microwave spectra of three isotopic species of dichlorosilane, SiH₂Cl₂, in its ground vibrational state, have been measured in the frequency region 8–40 GHz. The spectra have yielded values for the rotational constants, centrifugal distortion constants, and chlorine nuclear quadrupole coupling constants, as well as the molecular dipole moment, 1.13 ± 0.02 D. The molecule has C_2v symmetry, and the bond lengths and angles r(Si---Cl=2.033±Å, r(Si---H)=1.480±0.015Å, (Cl---Si---Cl)=109°43′±20±, (H---Si---H)=111°18′±40′ The centrifugal distortion constants have been compared with those calculated using a published force field.     

Calculation of molecular constants for the X 1Σ g + , A 1Σ u + , B 1Πu, and a 3Σ u + electronic states of 39K2

The molecular constants are calculated for the X ¹Σ _g ⁺ , A ¹Σ _u ⁺ , B ¹Π_u, and a ³Σ _u ⁺ and electronic states of a potassium dimer. The wave functions and vibrational energies necessary for calculating the molecular constants are determined by solving the radial wave equation with the use of potential energy curves constructed by the semiempirical method. The vibrational terms, the rotational constants, and the centrifugal distortion constants calculated from the potential curves are compared with those determined from the experimental data.