Efficiency of post-Hartree-Fock force field for the interpretation of vibrational spectra ofN,N-dimethylnitramine

Harmonic force fields for the molecule ofN,N-dimethylnitramine were calculated in the RHF/6-31G^* and MP2/6-31G^** approximations. Scaling of the force fields obtained made it possible to reliably interpret the vibrational spectra of light and perdeuterated compounds reported in the literature. The assignment is confirmed by good reproducibility of experimental isotope shifts upon¹⁵N-amino- and¹⁵N-nitrosubstitution. The frequencies of intramolecular vibrations in far IR and Raman spectra as well as in neutron inelastic scattering spectra for the light and perdeuterated samples of solidN,N-dimethylnitramine were identified using the force field calculated with the inclusion of electron correlation (MP2). Although general structures of the force fields calculated in the RHF and MP2 approximations are similar, considerable differences in the force constants of the NO and NN stretching vibrations and especially in the constants of the NO_str/NO_str and NO_str/NN_str interactions remain even after scaling the force fields.     

FT-IR spectra and normal coordinate analysis of the [Cl2FeS2MoS2FeCl2]2− dianion

The FT-IR spectra of the [Cl₂FeS₂MoS₂FeCl₂]^2? dianion were measured in the range 500 to 100 cm^?1. A simplified general valence force field was used in a normal coordinate analysis. The calculated frequencies agree well with the observed values, with a mean deviation of less than 1.0%, thus confirming the assignments of the vibrational spectrum. Altogether 16 force constants, including stretching, bending, torsion and stretching-stretching interactions were obtained. The rationality and the reliability of the results were discussed.     

A vibrational molecular force field of model compounds with biological interest—III. Harmonic dynamics of α- and β-d-galactose in the crystalline state

The infrared spectra of α- and β-d-galactose were recorded, both in the mid-IR range (4000-500 cm⁻¹) and in the far-IR (500-50 cm⁻¹). The Raman spectra were also obtained. These spectra constitute the basis of a crystalline-state force field established for these two molecules through a normal coordinate analysis. A modified Urey—Bradley—Shimanouchi force field was combined with an intermolecular potential energy function which includes van der Waals interactions, electrostatic terms and an explicit hydrogen bond function. The force constants were varied, so as to obtain an agreement between the observed vibrational frequencies and the calculated ones of α-d-galactose. The force field obtained was then applied to α-d-galactose O-d5 and β-d-galactose, in order to test its transferability. The computed potential energy distribution was found to be compatible with previous assignments for d-glucose, particularly for the modes involving C6 and COH groups. For β-d-galactose the same force field was used with changing the force constants due to the C1 and C6 groups.     

A theoretical study of the vibrational spectra,geometry and force field of thiourea

A theoretical force field for the molecular vibrations of thiourea has been determined from ab initio calculations at the Hartree-Fock level using the 3-21G* basis set. The reliability of the force field is analyzed by calculating the vibrational frequencies for the deuterated and ¹⁵N isotopomers. Frequencies calculated from the force field are utilized to critically examine the experimental assignments for thiourea and deuterated thiourea. Theoretical geometry, the calculated IR and Raman band intensifies are analyzed.     

Studies on nitramide and its methyl derivatives with ab initio calculations. IV. The harmonic force field and vibrational spectra of dimethylnitramine and its isotopic derivatives

The complete harmonic vibrational force field of dimethylnitramine has been calculated at the Hartree-Fock level using 4–21G basis set. The harmonic force field was then scaled with scale factors previously derived from N-methylnitramine, and the vibrational spectrum of dimethylnitramine was computed. This a priori prediction, made with no reference to observations on dimethylnitramine, agrees with the experimental IR spectrum in gas phase with a mean deviation of 8.4 cm^?1. Some of the scale factors were reoptimized by fitting of the computed force field to experimental data. The new set of scale factors reduced the mean deviation to 4.5 cm^?1, and was used to predict the vibrational spectrum of deuterated form of dimethylnitramine(-6D). Dipole moment derivatives were also calculated and used to predict infrared intensities which are comparable with experimental values.     

Infrared and Raman spectra of terephthalonitrile and terephthalonitrile-15N2. Force field for out-of-plane vibrations

A general assignment of the vibrational spectra of terephthalonitrile and terephthalonitrile-¹⁵N₂ is proposed on the basis of their infrared and Raman spectra. The relevant symmetry is found to be D_2h. The force field for the out-of-plane vibrations of these molecules was calculated by refining the general quadratic force field obtained by the semi-empirical MINDO/3 method, starting with a geometry optimized by this method. The refined force field reproduces the observed frequencies of the out-of-plane vibrations to better than ±0.5 cm⁻¹.     

Equilibrium structure and spectroscopic constants of maleic anhydride

The quadratic, cubic, and semi-diagonal quartic force fields of maleic anhydride have been calculated at the MP2 level of theory employing the cc-pVTZ basis set. The spectroscopic constants derived from the force field are in excellent agreement with the corresponding experimental values. The semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational corrections calculated from the cubic force field. This semi-experimental equilibrium structure is in excellent agreement with the ab initio structures computed at the CCSD(T) level of theory and it is closer to the ab initio structure than the purely experimental (or empirical) structures r ₀, r _m⁽¹⁾, and r _m⁽²⁾ obtained by microwave spectroscopy as well as the equilibrium structure derived from gas-phase electron diffraction data.     

Vibrational frequencies and assignments for some isotopomers of urcail using a scaled ab initio force field

Vibrational frequencies and IR band intensities for 18 isotopomers of uracil, including deuterated ¹⁵N and ¹⁸O species, have been calculated using the scaled ab initio force field of Ref. 1. The results obtained are compared with available experimental data, and a number of refinements in former assignments are proposed. The good agreement between the calculated and experimental frequencies confirms the reliability of the scaled quantum mechanical-force field.     

Scaled quantum mechanical (SQM) force field and theoretical vibrational spectrum for benzonitrile

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 systematically over-estimated 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-d₅. 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⁻¹ for each isotopomer. Theoretical i.r. intensities reproduce the main features of the spectra semiquantitatively.     

A CNDO/2 study of the OH torsion in phenol and its hydrogen bonded complex with pyridine

The CNDO/2 molecular orbital method has been applied to the study of the OH torsion, in phenol and phenol—pyridine hydrogen bonded complex. The calculated torsional barrier (13.58 kJ mol^?1) and force constant (5.4 × 10^?20 J rad^?2) of phenol agree well with the experimental quantities. The calculated force constant of the corresponding vibration in phenol—pyridine is increased sixfold, reproducing closely the rise in the torsional frequency observed when phenol is complexed to strong acceptors. It is shown that according to CNDO theory, most of the increase can be attributed to the influence of the intermolecular force field and not to a major change in the torsional force constant.     

Force field for in-plane vibrations of isophthalonitrile

Summary The general quadratic force field for the in-plane vibrations of isophthalonitrile was calculated by the semi-empirical MINDO/3 method. This force field was refined to the frequencies observed experimentally for isophthalonitrile and isotopic shifts of isophthalonitrile-¹⁵N₂. The normal coordinates and the force field in internal coordinates were also calculated from the refined field.

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Kräftefeld für die nichtebenen Schwingungen von Isophthalsäuredinitril

Zusammenfassung Das allgemeine quadratische Kräftefeld für die nichtebenen Schwingungen von Isophthalsäuredinitril wurde mit der halbempirischen MINDO/3-Methode berechnet. Das Kräftefeld wurde aufgrund der experimentalen Frequenzen von Isophtalsäuredinitril und der Isotopen-Verschiebungen von Isophtalsäuredinitril-¹⁵N₂ verfeinert. Die Normalkoordinaten und das Kräftefeld in inneren Koordinaten wurden auch vom verfeinerten Kräftefeld berechnet.

     

Development of modified force field for cation–amino acid interactions: Ab initio-derived empirical correction terms with comments on cation–π interactions

The modeling of voltage-gated ion-channel proteins is a continuing challenge for force-field calculations because of the diverse range of interactions involved. In particular, current force fields are not parameterized for either ion–amino acid or amino acid–electric field interactions. To address the parameterization of ion–amino acid interactions, we have tested the use of empirical correction terms, derived from ab initio calculations of single amino acids (representing the peptide backbone) interacting with K⁺ ions. Having demonstrated the utility of such an approach, we then extended the application to the amino acid side chains. The calculation of the interaction of K⁺ with serine, cysteine, methionine, lysine, arginine, aspartate, histidine (uncharged), tyrosine, tryptophan, and phenylalanine, both completes the parameterization of the molecular environments contained in the amino acids, and allows specific comment on these ion–functional group interactions. The cation–π interactions were of particular interest, given recent proposals in the literature and the fear that force fields would not be able to treat such interactions. We present a comprehensive comparison of the ab initio (DFT [BLYP], 6-311 G**) and force field (CHARMm22.0) assessments of these interactions. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1515–1525, 1998     

Effect of the substituents on the conformational behavior of five-membered rings: Application to the cis- and trans-2,5-dimethoxytetrahydrofuran

A model is proposed which assumes that the pseudorotational potential in five-membered rings is given by the combination of contributions from the unsubstituted ring, from the individual substituents and from interactions between pairs of substituents. The application of this model to the potentials calculated by the MM2 force field for the cis and trans-2,5-dimethoxytetrahydrofuran shows that the contributions from the individual substituents explain the main features of the potentials of these disubstituted rings. The pseudorotational analysis from vicinal proton spin-spin coupling constants ³J_HH confirms the realibility of the MM2 potentials.     

Vibrational spectra and force field of terephtalonitrile

The IR and Raman spectra of the two molecules terephtalonitrile and terephtalonitrile-¹⁵N were recorded to permit the general assignment of the vibrational bands observed, in agreement with a D_2h symmetry for these molecules. The general quadratic force field was calculated by the semi-empirical MINDO/3 method from an optimized geometry obtained by the same method. The resulting force field was refined by employing the experimental vibrational frequency data of the two molecules and those of terephtalonitrile-d₄. The final differences between the calculated end experimentally observed frequencies for B_2g and B_3u terephtalonitrile species were within the range ± 0.1 cm⁻¹.     

Theoretical prediction of vibrational spectra. Scaled quantum mechanical (SQM) force field for fluorobenzene

The complete harmonic force field of fluorobenzene has been determined from ab initio Hartree-Fock calculations using the 4–21 Gaussian basis set. As force constants are systematically overestimated at this level of theory, the directly calculated force field was scaled by empirical factors taken over from benzene and methylfluoride. Except for a slight overestimation of the CF stretching frequency, the scaled quantum mechanical (SQM) force field obtained in this way reproduces the experimental fundamental frequencies of the parent molecule and two deuterated isotopomers within 20 cm⁻¹ (with mean deviations below 12 cm⁻¹), and experimental assignments are analyzed on this basis. Theoretical i.r. intensities reproduce the main features of the spectra fairly well.     

Theoretical study on conformational features and cation-binding properties of a diquinone calix[4]arene

Theoretical studies of a diquinone calix[4]arene and its interactions with the cations Li⁺, Na⁺, K⁺ and Ag⁺ have been performed. Conformational features and cation-binding properties were evaluated with the restricted hybrid Becke three-parameter exchange functional method using the 6-31G(d) basis set and its relativistic effective core potentials. To model the effect of medium, the polarisable continuum model was also used. Four typical conformations of the parent diquinone calix[4]arene were studied. The calculated results show that the most stable conformers are 1,3-alternate and partial cone in the gas phase and in CH₂Cl₂ solution, respectively. The optimised geometric structures were used to perform natural bond orbital analysis. The two main types of driving force metal–ligand and cation–π interactions are investigated. The calculated binding energy for cations (Li⁺, Na⁺, K⁺ and Ag⁺) is discussed. The calculated results indicate that cone complexes are the most stable.     

13C NMR and force field investigations of hydrindane conformations

¹³C NMR shifts of trans- and cis-annelated bicyclo[4.3.0]nonanes with substituents R in position 8 (R ? H, OH, Cl, Br) and 1-hydroxy derivatives were analysed on the basis of force field calculated torsional angles using Allinger's MM1 program. Shielding increments for the 6 membered ring agree with corresponding cyclohexane values within ± 0.8 ppm maximal deviation. ¹³C NMR line shape analysis with cis-hydrindane between 148 and 180 K yielded ΔH* = 37.0 ± 0.4 kJ mol^?1 and ΔS* = 28 J mol^?1 K^?1 for the topomerization. The force field calculated reaction profile showed ΔH* = 37 kJ mol^?1, in close agreement.     

A spectroscopically effective molecular mechanics model for the intermolecular interactions of the hydrogen-bonded N-methylacetamide dimer

An MP2/6-31+G¹ calculation of the N-methylacetamide dimer shows that it has two minimum energy structures, both hydrogen bonded with peptide planes roughly perpendicular to each other. A complete molecular mechanics optimization of the dimer has been done, using a model for the intermolecular interactions consisting of charges, atomic dipoles, and van der Waals interactions and the methodology of our spectroscopically determined force field for the intramolecular interactions. The two structures are satisfactorily reproduced, as are their interaction energies, their dipole moments, and, from the point of view of our goal of a spectroscopically accurate force field, their six intermolecular normal mode frequencies.     

Molecular mechanics study of zirconocenium catalyzed isospecific polymerization of propylene

Stereocontrol energy (ΔE⁰) is investigated as a measure of enantioselectivity of ansa-zircoocenium catalyst in propylene polymerization; it was calculated with MM2 (molecular mechanics) force field using π complex (°C) and transition state (TS) geometries obtained by ab initio molecular orbital methods. Both rac-ethylenebis (1-η⁵-indenyl) - ( 1 ) and rac-ethylenebis (1-η⁵-4,5,7,8-tetrahydroindenyl) ( 2 ) zirconocenium species are isospecific in either the π-complexes or the transition states. The stereoselectivity is greater if there is α-agostic interaction; it is lowered in the cases of β and γ agostic interactions. The ¹³C-NMR steric pentad distribution indicates the poly(propylene) to be much less stereoregular than that predicted by ΔE⁰. Following the occurrence of a regiochemical insertion error, the addition of another monomer via any mode is prohibitively unfavorable. The catalyst suffers loss of stereospecificity as temperature of polymerization increases. Insertion via transition states involving different agostic interactions could be one explanation for the observed loss. © 1995 John Wiley & Sons, Inc.     

Theoretical prediction of vibrational spectra. The a priori scaled quantum mechanical (SQM) force field and vibrational spectra of pyrimidine

The complete harmonic force field of pyrimidine has been computed at the ab initio Hartree—Fock level using a 4–21 Gaussian basis set. In order to compensate the systematic overestimations of the force constants at the aforementioned level of quantum mechanical approximation, the theoretical force constants were empirically scaled by using nine scale factors. (The values of all these scale factors were previously determined by fitting the theoretical force field of benzene to the observed vibrational spectra of benzene.) The resulting a priori scaled quantum mechanical (SQM) force field is regarded as the most accurate and physically the most correct harmonic force field for pyrimidine. This force field was then used to predict the vibrational spectra of pyrimidine-h₄ and pyrimidine-d₄. On the basis of these a priori vibrational spectra uncertain assignments have been confidently resolved. After a few reassignments, the mean deviations between the experimental and calculated frequencies are below 9 and 18 cm⁻¹ for the non-CH stretching in-plane and the out-of-plane vibrations, respectively. Computed IR intensities are generally in agreement with experiments at a qualitative level.