Computational Studies of the 13C and 1H NMR Isotropic Chemical Shifts Using Density Functional Optimized Geometries. Adamantane and 2,4-Methano-2,4-dehydroadamantane (a [3.1.1]Propellane) as Case Studies

The 13C and 1H Chemical shift values computed at HF, BLYP and B3LYP/6-311G(d,p) levels of theory, for the BLYP/6-31G(d,p) optimized geometries of adamantane and 2,4-methano-2,4-dehydroadamantane, are reported and compared with the available experimental data. Except for the »inverted« carbon atoms, the HF values are superior to the DFT ones when the isotropic shifts with respect to TMS are in question. However, in case of the relative shifts computed with respect to the most deshielded center within the molecule, the DFT methods yield significantly better agreement with the experiment than the HF. The most probable reason for these findings may be the cancellation of errors arising from the inappropriate description of the paramagnetic contributions to the overall shielding tensor within the Kohn-Sham approach when an internal standard (within a molecule) is chosen, instead of an external one. The CSGT relative shift values correlate better with the experiment than the GIAO ones, the correlations being significantly superior at DFT than at the corresponding HF level of theory

An Iterative Perturbation Theory-Based Algorithm for Calculation of Diagonal Force Constants of the ν3 Modes in Quasi-Tetrahedral Systems with Low Anharmonicity

A novel, stationary perturbation theory – based iterative algorithm for calculation of the diagonal force constants of the ν3 modes in tetrahedral and quasi-tetrahedral systems with low anharmonicity is developed. The diagonal elements in the potential energy expression are calculated on the basis of experimentally measured fundamental and second-order transition wavenumbers, in a self-consistent manner. Perturbation corrections up to the second order are included in the model. The procedure is rapidly convergent, simple, and may be easily implemented within computer programs. The calculated diagonal force constants are consistent with the measured fundamental and second-order transition wavenumbers. The proposed model is applied to several isomorphously isolated (distorted-tetrahedral) sulfate impurities in selenate and chromate matrices. Experimental data for these systems were obtained by FT-IR spectroscopy

Fourier Transform Infrared Study of Dichlorodioxochromium(VI), Dichlorodioxomolybdenum(VI) and Dichlorodioxotungsten(VI) Complexes with 2,2’-Bipyridine

The Fourier transform infrared spectra of the 2,2’-bipyridine complexes of dichlorodioxochromium(VI), dichlorodioxomolybdenum(VI) and dichlorodioxotungsten(VI) were studied in the 4000 to 400 cm–1 region. The assignment of the bands originating from vibrations mainly localized in the bipyridine ligand is relatively straightforward. The essentially pure MO2 stretching vibrations give rise to very strong bands, the separation of which increases in the order Cr < Mo < W as a consequence of the greater sensitivity of the antisymmetric stretching O–M–O vibration to the changes in the mass of the M atoms. For this to be true, the symmetric stretching vibration should have a higher frequency than the antisymmetric one. The results of the preliminary ab initio HF SCF calculations on simple model structures are in line (at least qualitatively) with such a conclusion

A Quantum Theoretical Basis for Some Spectra – Structure Correlations in Crystalline Hydrates

Starting with the perturbation theory and the Hellmann-Feynmann theorem, an attempt was made to derive a fundamental theoretical basis for some frequency – structure correlations in crystalline hydrates. It was found that within a few reasonable approximations, a satisfactory theoretical background may be found for the &nu;OH(OD) versus RO⋅⋅⋅O, as well as for the –2XOH(OD) versus &nu;OH(OD) correlations (&nu;OH(OD) is the spectroscopically measured wavenumber of the OH(OD) stretching vibration, RO⋅⋅⋅O is the hydrogen bond distance, and X is the anharmonicity of vibration). The OH(OD) oscillators were treated as mixed cubic – quartic anharmonic systems. The influence of hydrogen bonding on these oscillators was introduced through the changes in the harmonic diagonal force constants (as proposed by Sceats and Rice1), the other diagonal terms in the potential energy expression being regarded as practically unchanged in the course of the hydrogen bonding. The parameters obtained by empirical correlations, within the proposed model, describe the dependence of the intramolecular potential of the uncoupled OH(OD) species on the hydrogen bond strength

Crystal Structures of Members in Isostructural Series: Prediction of the Crystal Structure of Cs2MnO4 - a p-K2SO4 Type Isomorph

Recent studies have shown that the crystal structure of members within a group of isostructural compounds may be successfully predicted. The P-K2SO4 group isomorphs of the general formula M2XO4 were chosen as a family of closely related compounds for which accurately refined crystal structures exist. It is shown that the unit cell parameters and the fractional atomic coordinates exhibit systematic variations with both cation and anion size, as well as with the Mulliken charge of the oxygen atom in the tetrahedral anion. It is thus possible to predict the crystal structure of a given member in the series only on the basis of its chemical composition. The structure of Cs2MnO4 predicted in this way is compared with the one refined earlier by X-ray diffraction. The agreement be- tween the structural parameters in both structures is very good

Computational Studies of Chemical Shifts Using Density Functional Optimized Geometries. II. Isotropic 1H and 13C Chemical Shifts and Substitutent Effects on 13C Shieldings in 2-Adamantanone

The 1H and 13C isotropic Chemical shifts and the substituent effects thereof (with respect to adamantane), computed at the HF, BLYP, B3LYP/6-3llG(d,p) as well as at MPW1PW91/6-3ll+G(2d,p) levels of theory with CSGT, GIAO and IGAIM algorithms, for the BLYP/6-31G(d,p) and B3LYP/6-31G(d,p) optimized geometries of 2-adamantanone are reported and compared with the experimental data. When absolute values of isotropic Chemical shifts (with respect to TMS) are in question, the MPW1PW91/6-3ll+G(2d,p) level leads to excellent agreement with the experiment, while the HF approach is superior to the BLYP and B3LYP ones. However, the substituent effects on 13C shieldings are better reproduced at the BLYP and B3LYP levels than at the HF level, while the MPW1PW91 approach is again significantly superior to ali the others, leading to excellent agreement with experimental data. The most probable reason for these findings may be the cancellation of errors arising from the inappropriate description of the paramagnetic contributions to the overall shielding tensor within the Kohn-Sham approach, and the more systematic nature of errors in DFT approaches. The isotropic Chemical shift values at all levels of theory, however, correlate excellently with the experimental data, the correlation being superior for DFT to the HF level of theory

The Vibrational Stark Shifts of Sulfate Internal Modes in SO42- Doped Potassium, Rubidium and Cesium Selenates. A Quantum Model for Measurement of Crystalline Fields

A quantum theoretical basis for the experimentally observed vibrational Stark shifts of the sulfate internal modes in SO42- doped K2SeO4, Rb2SeO4 and Cs2SeO4 is presented. Analytical first order perturbation theoretical expressions are derived for the field-dependent wavenumbers of the 1 &#x2190; 0 and 2 &#x2190; 0 transitions, harmonic wavenumbers, as well as for the Stark tuning rate and the electrostatic field strength at the Cs sites of the host lattices. It is shown that the local field differences are the factor dominating over the differences in the anharmonicities of the guest anions in various host lattices, and are thus responsible for the experimentally observed trends. The proposed method allows calculation of the local crystalline field strength if the anharmonic potential energy parameters of the dopant anions are known. The calculated values for the studied series of matrices range from 229 to 259 V nm-1, which are approximately 3 times larger than those reported for water molecule sites in several clathrate hydrates and for the N2O adsorbed in the NaA zeolite cavities. The model also successfully explains the greater bond length distortions than the angular ones of the dopant anions observed in ali studied cases