Transferability of quantum mechanical force field scale factors between conjugated hydrocarbons (original) (raw)
Related papers
Journal of Structural Chemistry, 1998
The effect of scaling of an ab initio quantum mechanical force field on the frequencies and forms of normal vibrations are studied in terms of first-and second-order perturbation theory. Scaling the force constant matrix according to Pulay using certain assumptions in first-order perturbation theory is equivalent to scaling vibration frequencies and does not modify the form of vibrations. In this case, the second-order corrections to the frequencies and forms of vibrations become zero. The first-order perturbation theory formulas are used to verify the assumptions by calculating the frequencies and matrices of transition to perturbed forms of vibrations of ethane, propane, ethylene, cyclopropene, and isobutene molecules from quantum mechanical force fields found with the 6-31G basis set. It is shown that the vibration frequencies calculated by the formulas of first-order perturbation theory are in good agreement with exact values; the matrix of transition to perturbed eigenvectors is rarefied, with only =1% of its elements being markedly nonzero.
Journal of the American Chemical Society, 1983
Fully optimized geometries, complete in-and out-of-plane force fields, and dipole moment derivatives have been calculated for the title compounds at the ab initio HartreeFock level using the 4-21 Gaussian basis set. The theoretical information is combined with experimental data by fitting the calculated force constants through a few parameters to the observed frequencies to obtain the final, scaled quantum mechanical (SQM) force fields. Recommendations for a standard procedure of this type are given. The SQM force fields give excellent reproduction of the fundamental frequencies and are considered as approaching the best accuracy which can be achieved in a harmonic treatment. The infrared intensities obtained at this level of theory are only qualitative estimates, but they are still useful for making assignments more reliable.
Scaled quantum mechanical (SQM) force field and vibrational assignment for hexatriene
Journal of Molecular Structure: THEOCHEM, 1987
The geometry and complete harmonic force field for the all-trans form of 1,3,5-hexatriene have been determined from ab initio Hartree-Fock calculations. To account for systematic errors, the theoretical force field was scaled by empirical factors taken over fixed from butadiene. The scaled quantum mechanical (SQM) force field obtained this way reproduces the fundamental vibrational frequencies of hexatriene within 20 cm-' in most cases, which gives a good basis for analyzing the spectral assignment. In the light of a slight discrepancy between theory and experiment for the C=C stretching frequencies and bond lengths, the question of how different the two kinds of double bonds in hexatriene are is raised.
Journal of Chemical Physics, 1981
The quadratic and the most important cubic force constants of benzene have been determined from ab initio Hartree-Fock calculations with a double-zeta basis set. Some constants have also been recalculated using other basis sets, including a polariZed one. A few empirical scale factors, applied to the ab initio force field, allow the reproduction of a large number of observed vibrationa1 frequencies, isotope shifts, and Coriolis constants within the uncertainties of experiment and the harmonic model. It is shown that the simultaneous utilization of ab initio and spectroscopical information is sufficient for the conclusive resolution of the uncertainties and alternatives in previous empirical force fields. The resulting scale factors can be used directly to obtain force fields for other aromatic hydrocarbons from ab initio calculations. Reproduction of the observed infrared intensities is only moderately successful, even with the polarized basis set. The calculated vibronic coupling constants show qualitative agreement but important deviations from previous simpler calculations. The predicted vibrational patterns confirm Lindholm's assignment of the photoelectron spectrum of benzene.
Derivation of Class II Force Fields. III. Characterization of a Quantum Force Field for Alkanes
Israel Journal of Chemistry, 1994
Recently, a quantum mechanical Class II force field (QMFF) was derived from a fit of HF/6-3IG* ab initio energy and energy derivative data for alkanes, and a comparison of this quantum force field and the ab initio energy and energy derivatives was presented. In this work, the quantum force field is further evaluated with regard to its accuracy, and, more importantly, transferability. A detailed comparison between structures, frequencies, and energies calculated from quantum mechanics and from the classical analytical form is given for a set of molecules selected from both those used in the original training set and molecules selected from outside the training set. None of these properties were used directly in the original derivation of the force field. In order to assess the importance of anharmonic and coupling interactions that occur in and contribute to molecular energy surfaces, the results are compared to a diagonal quadratic force field. It is demonstrated that the QMFF functional form is capable of calculating the ab initio bond lengths, bond angles, torsion angles, and conformational energy differences to an rms accuracy of 0.003 A, 0.4°, 1.2°, and 1.0 kcal/mol, respectively. This compares quite well to corresponding deviations of 0.006 A, 0.8 A, 2.3°, and 3.3 kcaVmol for a harmonic diagonal force field. Excluding three-or four-membered rings, the QMFF rms frequency deviations were 24 em", which again is much better than the-100 cm' deviations for the harmonic diagonal force field. Larger average rms frequency deviations of 36 and 71 em" were found with QMFF for molecules with three-and four-memberedrings. An in
Chemical Physics, 2000
A systematic method to fit calculated to observed vibrational frequencies has been developed and implemented in a computer program. The procedure consists of the refinement of a scaled quantum mechanical force field (SQMFF) previously obtained according to Pulay's method. The key step in the process is the generation of an intermediate matrix, CΛCT, which is then refined. The above step produces only small corrections to the scaled force constants, yielding a considerable improvement of the fitted frequencies. This scheme of refinement can be carried out using any kind of coordinates. To show the reliability and performance of the proposed method, the force fields of two very different systems, as benzene and tetranitromethane, have been chosen as example tests.
Structural Chemistry, 2008
The ab initio-based, scaled quantum-mechanical molecular force field (SQM-FF) analysis of the vibrational spectra of the s-trans and s-gauche conformers of 2-methylbuta-1,3-diene (isoprene), reported previously at the HF/6-31G//HF/6-31G computational level [Bock, et al. J Mol Struct 160: 337, 1987], has been updated in this article using a more complete set of experimental data on the s-gauche conformer along with revised results for the s-trans conformer obtained in the gas phase, in a lowtemperature matrix, and in neat crystals. Geometrical parameters and the calculated wavenumbers derived from the SQM-FF at the MP2(FC)/aug-cc-pVDZ//MP2(FC)/aug-cc-pVDZ level are compared to experiment. The analyses performed are consistent with the presence of a twisted high-energy s-gauche conformer of isoprene.
Scaled quantum mechanical (SQM) force field and theoretical vibrational spectrum for benzonitrile
Spectrochimica acta, 1989
The complete harmonic force field of benzonitrile has been determined by ab initio Hartree-Fock calculations using a 4-21 Gaussian basis set. As force constants are systematidally overestimated at this level, the directly calculated force field was scaled by empirical factors previously optimized for benzene and HCN. Frequencies calculated from this scaled quantum mechanical (SQM) force field confirm the published experimental assignments for benzonitrile, benzonitrile-p-d and benzonitrile-d,. Aside from the C-H (and C-D) stretching frequencies, which are strongly affected by anharmonicity, the mean deviation between the observed and calculated frequencies is below 9 cm-' for each isotopomer. Theoretical i.r. intensities reproduce the main features of the spectra semiquantitatively.
Journal of Computational Chemistry, 2001
A class II valence force field covering a broad range of organic molecules has been derived employing ab initio quantum mechanical "observables." The procedure includes selecting representative molecules and molecular structures, and systematically sampling their energy surfaces as described by energies and energy first and second derivatives with respect to molecular deformations. In this article the procedure for fitting the force field parameters to these energies and energy derivatives is briefly reviewed. The application of the methodology to the derivation of a class II quantum mechanical force field (QMFF) for 32 organic functional groups is then described. A training set of 400 molecules spanning the 32 functional groups was used to parameterize the force field. The molecular families comprising the functional groups and, within each family, the torsional angles used to sample different conformers, are described. The number of stationary points (equilibria and transition states) for these molecules is given for each functional group. This set contains 1324 stationary structures, with 718 minimum energy structures and 606 transition states. The quality of the fit to the quantum data is gauged based on the deviations between the ab initio and force field energies and energy derivatives. The accuracy with which the QMFF reproduces the ab initio molecular bond lengths, bond angles, torsional angles, vibrational frequencies, and conformational energies is then given for each functional group. Consistently good accuracy is found for these computed properties for the various types of molecules. This demonstrates that the methodology is broadly applicable for the derivation of force field parameters across widely differing types of molecular structures.
Journal of Computational Chemistry, 1994
A new method for deriving force fields for molecular simulations has been developed. It is based on the derivation and parameterization of analytic representations of the ab initio potential energy surfaces. The general method is presented here and used to derive a quantum mechanical force field (QMFF) for alkanes. It is based on sampling the energy surfaces of 16 representative alkane species. For hydrocarbons, this force field contains 66 force constants and reference values. These were fit to 128,376 quantum mechanical energies and energy derivatives describing the energy surface. The detailed form of the analytic force field expression and the values of all resulting parameters are given. A series of computations is then performed to test the ability of this force field to reproduce the features of the ab initio energy surface in terms of energies as well as the first and second derivatives of the energies with respect to molecular deformations. The fit is shown to be good, with rms energy deviations of less than 7% for all molecules. Also, although only two atom types are employed, the force field accounts for the properties of both highly strained species, such as cyclopropane and methylcyclopropanes, as well as unstrained systems. The information contained in the quantum energy surface indicates that it is significantly anharmonic and that important intramolecular coupling interactions exist between internals. The representation of the nature of these interactions, not present in diagonal, quadratic force fields (Class I force fields), is shown to be important in accounting accurately for molecular energy surfaces. The Class I1 force field derived from the quantum energy surface is characterized by accounting for these important intramolecular forces. The importance of each of the interaction terms of the potential energy function has also been assessed. Bond anharmonicity, angle anharmonicity, and bond/angle, bond/ torsion, and angle/angle/ torsion cross-term interactions result in the most significant overall improvement in distorted structure energies and energy derivatives. The implications of each energy term for the development of advanced force fields is discussed. Finally, it is shown that the techniques introduced here for exploring the quantum energy surface can be used to determine the extent of transferability and range of validity of the force field. The latter is of crucial importance in meeting the objective of deriving a force field for use in molecular mechanics and dynamics calculations of a wide range of molecules often containing functional groups in novel environments.