Application of self-consistent-field ab initio calculations to organic molecules

The local symmetry force constants of propene have been calculated from extended (4–31 G) self-consistent-field ab initio energies. The previously developed scaling method was used to adjust seven scale factors on 84 observed frequencies of propene and deuterated analogues. The latter were taken from the literature. The average difference between observed and calculated frequencies amounted to 8·3 cm^-1 or 0·63 per cent. The stretching force constant of the carbon-carbon single bond was found to be 0·3–0·4 mdyn Å^-1 larger than those of ethane and propane. Force constants of the CH₃ groups in the series: ethane, propane, propene, methanol and dimethyl ether, written in local symmetry coordinates, seem to be transferable within about 2 per cent. Individual CH bond stretching force constants, written in internal coordinates, show significant chemical variations.     

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.     

Ab initio calculation of force constants for the linear molecules HCN,FCN, (CN)2 and the ion N2F+

Force constants have been calculated from ab initio Hartree-Fock wave-functions by the force method, using 7s3p/1 and 5s2p/1 gaussian basis sets for HCN, FCN, C₂N₂ and FN₂ ⁺. Agreement of the quadratic and some cubic force constants with experiment is good for HCN and FCN. The influence of anharmonicity upon the l-type doubling constant of FCN is estimated. Both the experimental l-type doubling constant and the ab initio calculations indicate that the quadratic stretch, stretch coupling constant is positive in FCN, contrary to recent results of Wang and Overend, obtained from the Anderson potential function. There is good agreement for the CN, C′N′ coupling in C₂N₂ but the calculated CN, CC coupling, although positive, is much lower than in two recent experimental force fields. The calculated FN, NN coupling in FN₂ ⁺ is small and positive. The predicted geometry of FN₂ ⁺ is r _NF = 1·28 Å, r _NN = 1·10₅ Å. The validity of the Anderson potential function is discussed.     

Evidence for slow exchange in E.S.R. spectra of nitroxide biradicals

Force constants have been calculated for H₂NF and HNF₂ from ab initio Hartree-Fock wavefunctions by the force method, using 7s3p/1 gaussian basis sets. The force field of HNF₂, obtained from a combination of the theoretical results with the experimental frequencies, can be considered as the most reliable one at present. Previous force fields are discussed critically. From a full optimization the predicted geometry of H₂NF is: NF = 1·399 Å, NH = 1·018 Å, HNF = 102·9° and HNH = 105·9°.     

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 study of spectroscopic properties of RgCl (Rg=He,Ne, Ar,Kr) and their anions

Spectroscopic properties of HeCl, NeCl, ArCl, KrCl and their anions HeCl^?, NeCl^?, ArCl^? and KrCl^? in their ground state have been studied in detail using ab initio MP2, CCSD and CCSD(T) methods. These neutral molecules and their anions are weakly bound and their spectroscopic constants have been estimated using a method developed recently for calculating the spectroscopic constants of weakly bound molecule in Lennard–Jones potential. The net attractive force and the distance at which the net attractive force is greatest, have been calculated to get the physical picture. Most of the spectroscopic constants are first predicted. The calculated equilibrium bond length, dissociation energy and harmonic frequency agree very well with the experimental and theoretical values wherever available.     

Ab initio calculation of the spin-orbit coupling constant from gaussian lobe SCF molecular wavefunctions

The spin-orbit coupling constants of BH⁺ and CH have been calculated using ab initio molecular SCF wavefunctions with the gaussian lobe function basis set. It is demonstrated that fair agreement with experimental values can be achieved even with a relatively small basis set, provided that no terms are neglected in calculating the matrix elements.     

Anharmonic force field for methanol

An ab initio quartic anharmonic force field for methanol has been calculated at the equilibrium position using the CCSD(T) method for the structure and the harmonic potential energy surface, and the MP4(SDQ) method for the anharmonic part of the surface. A triple zeta basis set was employed with symmetrized curvilinear internal valence coordinates in all calculations. The internal coordinate force field constants have been transformed into force constants in the dimensionless normal coordinate representation for various isotopomers. Vibrational term values for CH₃OH, CH₃OD, CD₃OH, and CD₃OD have been obtained using second order perturbation theory. Particular care has been devoted to the inclusion of Fermi resonance interactions between different vibrational states. A good accuracy has been achieved in the calculation of the fundamentals for all the isotopomers, the mean absolute error being 5.8 cm^?1.     

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.     

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.     

Theoretical study of the fine-structure coupling constants in the 2p 3Π u state of H2

The one and two-electron fine-structure constants for the 2p ³Π _u state of the H₂ molecule have been calculated using all-integral, ab initio methods for a variety of molecular wavefunctions. The results have been averaged over the first three vibrational states and are compared with previous calculations and with experiment.     

Microwave and Infrared Spectra,ab InitioCalculation, and Two-Dimensional Model of Amino Group Inversion and Ring Puckering in 2,5-Dihydropyrrole

The microwave spectra of 2,5-dihydropyrrole and 2,5-dihydropyrrole-1-d₁have been measured with Stark and Fourier transform spectrometers in the range 10–39 GHz. Rotational constants, centrifugal distortion constants, and¹⁴N quadrupole coupling constants have been determined from the observed transition frequencies for the ground vibrational state. In addition, two satellites of the normal species and one satellite of the deuterated species have been identified and measured. Splittings of the rotational transitions due to amino group inversion tunneling have been observed and analyzed. Infrared transitions of the amino group inversion mode have been measured in the range 490–720 cm⁻¹. The effect of ring puckering on the inversion motion of the amino group in 2,5-dihydropyrrole and 2,5-dihydropyrrole-N-d₁has been investigated byab initiocalculations and two-dimensional flexible model calculations from the results of microwave and infrared spectroscopy. The observed molecular properties have been reproduced by a model which involved adjustable parameters for the potential energy surface and the structural relaxation of the CCC valence angles. Additional parameters have been transferred from theab initiocalculations. The adjustment of the model to the experimental data has yielded an equatorial equilibrium conformation with slightly larger CCC valence angle than in the most stable axial conformation. Excitation of the first ring puckering state has been found to enhance the inversion tunnel splittings.     

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 study on the GVFF of CHF3, CH2F2, and CH3F

The complete GVFF of CHF₃, CH₂F₂, and CH₃F has been calculated from self-consistent-field ab initio energies, using a 4–31 G basis set. The larger part of the interaction force constants is close to those of the best available force fields from experimental data. Only one interaction term in CH₃F and the interaction force constants of the A₁ species in CH₂F₂ differ appreciably from the experimental ones. Using constraints from the ab initio studies we have improved the GVFF of CH₃F and CH₂F₂. It is shown that all comparable stretch-stretch interaction terms are of the same order of magnitude in the three molecules. The sign of all stretch/bend force constants are in accordance with those predicted by the hybrid orbital force field.     

Raman and infrared,microwave spectra,conformational stability,adjusted r0 structural parameters,and vibrational assignments of cyclopentylamine

Fourier transform microwave spectrum of cyclopentylamine, c–C₅H₉NH₂ has been recorded, and seven transitions have been assigned for the most abundant conformer, and the rotational constants have been determined: A = 4909.46(5), B = 3599.01(4), and C = 2932.94(4). From the determined microwave rotational constants and ab initio MP2(full)/6‐311 + G(d,p) predicted structural values, adjusted r₀ parameters are reported with distances (Å): rC_α–C_β = 1.529(3), rC_β–C_γ = 1.544(3), rC_γ–C_γ = 1.550(3), rC_α–N = 1.470(3), and angles (°) ∠CCN = 108.7(5), ∠C_βC_αC_β = 101.4(5), and τC_βC_αC_β′C_γ′ = 42.0(5). The infrared spectra (4000–220 cm⁻¹) of the gas have been recorded. Additionally, the variable temperature (−60 to −100 °C) Raman spectra of the sample dissolved in liquefied xenon was recorded from (3800–50 cm⁻¹). The four possible conformers have been identified, and their relative stabilities obtained with enthalpy difference relative to t‐Ax of 211 ± 21 cm⁻¹ for t‐Eq ≥ 227 ± 22 cm⁻¹ for g‐Eq ≥ 255 ± 25 cm⁻¹ for g‐Ax. The percentage of the four conformers is estimated to be 53% for the t‐Ax, 11 ± 1% for t‐Eq, 20 ± 2% for g‐Ax and 16 ± 2% for g‐Eq at ambient temperature. The conformational stabilities have been predicted from ab initio calculations by utilizing several different basis sets up to aug‐cc‐pVTZ from both MP2(full) and density functional theory calculations by the B3LYP method. Vibrational assignments have been provided for the observed bands for all four conformers, which are predicted by MP2(full)/6‐31G(d) ab initio calculations to predict harmonic force constants, wavenumbers, infrared intensities, Raman activities, and depolarization ratios for all of the conformers. The results are discussed and compared to the corresponding properties of some related molecules. Copyright © 2014 John Wiley & Sons, Ltd.     

Ab initio NMR parameters of BrCH3 and ICH3 with relativistic and vibrational corrections

This study is focused on two effects identified when NMR parameters are calculated based on first principles. These effects are 1. vibrational correction of properties when using ab initio optimized equilibrium geometry; 2. relativistic effects and limits of using the Flygare equation. These effects have been investigated and determined for nuclear spin-rotation constants and nuclear magnetic shieldings for the CH₃Br and CH₃I molecules. The most significant result is the difference between chemical shieldings determined based on the ab initio relativistic four-component Dirac-Coulomb Hamiltonian and chemical shieldings calculated using experimental values and the Flygare equation. This difference is approximately 320 ppm and 1290 ppm for ⁷⁹Br and ¹²⁷I in the CH₃X molecule, respectively.     

Ab initio study of Ba+Arn (n = 1–4) clusters: spectroscopic constants and vibrational energy levels

This study is interested in the illustration of ab initio potential energy curves for Ba⁺Ar_n (n = 1–4) clusters. The electronic structures of these molecules are calculated using [Ba²⁺] and [Ar] non-empirical core pseudo-potentials complemented by the core polarisation operators for both atoms, which allow the consideration of core valence correlation effects. The structure and stabilities of Ba⁺Ar_n (n = 1–4) clusters are investigated. These molecules are treated as one-electron active system. Spectroscopic constants and vibrational energy levels have been derived from their potentials. The analysis of the geometric forms, basing on the potential energy curves and the spectroscopic constants, clearly shows the importance of rare gas induced dipole. We also show that the dipolar interactions can influence the coupling between atoms.     

Electronic structure and molecular spectroscopic constants of ScN and ScP investigated by several quantum chemistry methods

The electronic structure of the ScN and ScP molecules is a subject of controversy and turns out to be a challenging problem in quantum chemistry. We show that the ground-state electronic structure for both molecules depends critically on the choice of methods used which incorporate different ways of accounting for electron correlation. A parallel ab initio, DFT and TD-DFT study is performed for this purpose and uses sufficiently flexible basis sets able to reproduce accurate electronic structures, as well as correct spectroscopic constants.

In the ab initio methodology, results have been obtained with methods such as Hartree-Fock (HF), M?ller-Plesset perturbation theory (MPn), direct configuration interaction (CI), quadratic configuration interaction (QC), coupled cluster configuration interaction (CC), complete active space self-consistent field (CASSCF) and multireference configuration interaction (CIPSI) methods. In the DFT methodology, various ‘pure’ and ‘hybrid’ density functionals are used and the corresponding results are compared to sophisticated ab initio methods and to available experimental data.

All the methods used show that the ground state of both molecules is ¹Σ⁺, but two electronic structure natures, ¹Σ⁺ open-shell or ¹Σ⁺ closed-shell, are competitive and depend on the method employed. All the ab initio methods based on a single determinant wavefunction suffer seriously in predicting clearly the exact nature of the ground state or its correct structural and spectroscopic parameters. However, the ab initio methods based on a multiconfigurational wavefunction appear to be successful in describing correctly, within one shot, the electronic structure and the molecular spectroscopic constants. The ground state, particularly for the ScN molecule, presents an unusual electronic structure: the presence of degenerate determinants, quasidegeneracy with other states and one avoided crossing in the region around the equilibrium distances. The bonding of the ground state is a two open-shell ¹Σ⁺ state described as a π double bond and a Σ dative bond; the real triple bond ¹Σ⁺ state, i.e. closed-shell state, is found to lie higher in energy. The potential energy curves of the lowlying electronic states, the derived electronic structures and various molecular spectroscopic constants are presented and discussed for each method employed.     

Calculation of spectroscopic parameters and vibrational overtones of methanol

A high-level quartic ab initio potential energy surface of methanol has been used to calculate spectroscopic constants of the ¹²CH₃OH molecule. These include coefficients of quartic anharmonic resonance terms, Darling-Dennison constants, for stretching states. A model expressed in terms of dimensionless normal coordinates has been employed in the calculation of O—H and C—H stretching vibrational states in high-overtone regions. Both cubic Fermi and quartic Darling-Dennison anharmonic coupling terms have been included in the model in order to take into account strong resonances between different states. The nonlinear least-squares method has been used to optimize some of the model parameters employing experimental term values of ¹²CH₃OH as data. Vibrational assignments are suggested for the first C—H stretching overtone region.     

Tetrafluoroberyllate(2-) Ions as Hydrogen-Bond Proton Acceptors: Spectroscopic Evidence

In the case of isostructural compounds it is easy to intuitively assume that the hydrogen bonds involving fluorine atoms as proton acceptors should be stronger than those in which oxygen atoms play the role of proton acceptors. Such an assumption was tested by comparing the infrared spectra of three tetrafluoroberyllate compounds with those of the isostructural sulfates and/or selenates. The experimental results do not support the basic hypothesis, although the elementary calculations based on the concept of equalized electro negativities [1, 2] give higher values for the partial relative electric charge on the fluorine atoms in the idealized BeF^2- ₄ anions than are those estimated for the oxygen atoms in either SO₄ ^2- or SeO₄ ^2- ions. On the other hand, the values obtained by ab initio methods are more in line with the spectroscopic observations.