Vibrational spectra and harmonic force fields of pyrrolidine derivatives: comparison between HF, MP2 and DFT force fields

Infrared and Raman spectra are reported for the isotopic species of pyrrolidine-d₀ (PY) and -d₁ and for N-methylpyrrolidine-d₀ (NMP), -d₂, -d₃ and -d₈. A complete assignment of the experimentally observed bands to normal modes is presented and discussed in particular in the CH/CD stretching region. The molecular structures and harmonic force fields were calculated ab initio at the Hartree–Fock (HF), the second order Møller–Plesset (MP2) and the density functional theory (DFT) level with the 6-31G* basis set. The force fields were fitted by use of 7 (PY) and 4 (NMP) independent scale factors. The spectra calculated with the DFT force fields are in better agreement with the experiment than those calculated by the MP2 and HF force fields. Though some scaled fundamental frequencies show larger deviations from the experimental ones, the mean percentage deviations of calculated frequencies from experimental fundamentals are less than 2.6% for all isotopic species of PY and NMP under study. The results indicate that density functional theory is a reliable tool to get a deeper insight in the assignment of vibrational spectra and the nature of normal modes of pyrrolidine derivatives.     

Vibrational anharmonicity constants for PuF6

The fundamental vibrations of PuF₆ cluster in two distinct group-vibrational, near-local modes at 589±50 cm^?1 for the stretching modes and 195 ± 15 cm^?1 for the bending modes. This clustering results in a corresponding “bunching up” of the density of states. One of the consequences of this behavior is that “hot band” absorption intensity pools into stronger absorption features appearing shifted at regular intervals from the fundamental. This observed structure in the 16μm region for PuF₆ has been interpreted to yield values for two empirical anharmonicity constants X_3j (j = 1, 2, 3) and X_3k (k = 4, 5, 6).     

Model harmonic force fields of inorganic molecules and calculations of thermodynamic values of chemical compounds

In this article are discussed the methodical principles of the selection of inorganic groups of compounds and finding their force fields, the main feature of which is the requirement for a monotonic variation of each force constant of the force fields of groups of molecules in the K_i(ij)–N_E(X) coordinates, where K_i(ij) is the force constant, N_e(X) is the atomic number of the central (E) or peripheral (X) atom of the molecules in a given subgroup of the Periodic system. It was shown that the monotonicity of these dependences is maintained in isostructural and isoelectronic groups of molecules. Numerous data are given on the force constants and of normal vibration frequencies of molecules formed by atoms of the III, IV, VI, and VII groups of the Periodic system.GIPKh Scientific-Production Association. State Institute of Applied Chemistry. Moscow Municipal Association Tekhnokhim. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 1, pp. 83–93, January–February, 1993.     

Ab initio calculations of the structure and harmonic force fields for the amine forms of dinitramine and methyldinitramine. Vibrational spectra and their interpretation using a scaling procedure

Ab initio geometries and vibrational spectra have been calculated for the amine structures of dinitramine and methyldinitramine, HN(NO₂)₂ and CH₃N(NO₂)₂. It is shown at the RHF and MP2 levels with the use of the 6-31G^* and 6-31G^** basis sets that these molecules have different symmetries in their equilibrium states,C _sandC ₁ respectively. The quantum chemical RHF/6-31G^* force fields were scaled with the set of transferable factors previously obtained by the authors to assign the available experimental vibrational bands and predict the positions of bands for the unmeasured spectral regions. Some common patterns of the geometrical parameters, vibrational spectra, and force fields of the simplest nitramines are discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2135–2147, November, 1995.The authors are grateful to the Russian Foundation for Basic Research (Project No. 93-03-4410) and to the International Science Foundation (Grant No. MQXOOO) for financial support of works fulfilled at the Department of Chemistry, M. V. Lomonosov Moscow State University. The authors also acknowledge the support of the Scientific Technical Program Universities of Russia.     

Scaled quantum chemical force fields for 1,1-difluorocyclopropane and the influence of vibrational anharmonicity

Potential functions and harmonic (omega(i)) and anharmonic (nu(i)) fundamental frequencies have been calculated for 1,1-difluorocyclopropane (DFCP) and its d4 and d2 isotopomers using the program Gaussian 03. B3LYP and MP2 models were employed, each with the bases 6-311++G** and cc-pVTZ. Anharmonicity corrections Delta(i) = omega(i) - nu(i) are listed and shown to be different for symmetric and antisymmetric CH stretching modes in situations where Fermi resonance appears to be absent. The same effect is missing in C2H4, for which similar calculations were made. The quadratic force fields for DFCP have been scaled in symmetry coordinate space with 15 scale factors both to observed frequencies nu(obsd)and also to omega (obsd), where omega(obsd) = nu(obsd) + Delta. With nu(obsd) especially, different scale factors are needed for the symmetric and antisymmetric CH stretching force constants due to their differing anharmonicities. The source of the latter in the quartic and cubic force field is explored. MP2 calculations of valence interaction force constants involving the stretching of bonds on a common carbon atom are preferred to those from a B3LYP model. In either model, scaling to omega(obsd) rather than to nu(obsd) does not remove the necessity of varying scale factors for differing types of motion in the same group. Theoretical values of the five quartic centrifugal distortion constants are listed for the normal species and compared with new experimental data. The predictions are sufficiently good to be useful in fitting pure rotational transitions. A weakness is identified in the current Gaussian 03 code for the calculation of vibration-rotation quantities, and limitations are noted in the manner in which Fermi resonance is handled.     

Vibrational spectra,force fields and electronic structures of sydnones

Normal vibrational analyses of 3-methylsydnone and 3-methylsydnone-d₄ have been performed. On the basis of the set of force constants obtained, the electronic distribution within the mesoionic ring is evaluated and compared to the results of MO calculations. The π-bond order of the sydnone carbonyl group is shown to be lower than that in alicyclic esters and the unusually high “carbonyl stretching” frequency is due to the contributions from other coordinates to this mode. The splitting of the “carbonyl stretching” band observed in the spectra of 3-methylsydnone and of related derivatives is explained by the strong kinematic coupling between mesoionic bonds.     

Vibrational frequencies and spectroscopic constants from quartic force fields for cis-HOCO: the radical and the anion

The use of accurate quartic force fields together with vibrational configuration interaction recently predicted gas phase fundamental vibrational frequencies of the trans-HOCO radical to within 4 cm(-1) of experimental results for the two highest frequency modes. Utilizing the same approach, we are providing a full list of fundamental vibrational frequencies and spectroscopic constants for the cis-HOCO system in both radical and anionic forms. Our predicted geometrical parameters of the cis-HOCO radical match experiment and previous computation to better than 1% deviation, and previous theoretical work agrees equally well for the anion. Correspondence between vibrational perturbation theory and variational vibrational configuration interaction for prediction of the frequencies of each mode is strong, better than 5 cm(-1), except for the torsional motion, similar to what has been previously identified in the trans-HOCO radical. Among other considerations, our results are immediately applicable to dissociative photodetachment experiments which initially draw on the cis-HOCO anion since it is the most stable conformer of the anion and is used to gain insight into the portion of the OH + CO potential surface where the HOCO radical is believed to form, and we are also providing highly accurate electron binding energies relevant to these experiments.     

Metallomethanes: XIII. Vibrational spectra and force fields of cyanomercuriomethanes

Vibrational spectra with assignments and results of normal coordinate calculations for cyanomercuriomethanes CH_4−n(HgCN)_n (2 ≤ n ≤ 4) are discussed. The valence force constants of the central CHg bonds are 2.149, 1.944, and 1.798 N cm⁻¹,while those of the HgCN bonds are 2.204, 2.123, and 2.162 N cm⁻¹, for n = 2, 3 and 4, respectively. All these force constants are lower than the corresponding constants for methylmercury cyanide (2.445 and 2.379 N cm⁻¹). The overall behaviour of these force constants as a function of the degree of mercuration n is quite similar in both the cyanomercuriomethanes CH_4−n(HgCN) and methylmercuriomethanes CH_4−n(HgCH₃)_n series with the difference that there are variations in the constants at higher values in the former series. The potential energy distributions indicate that the valence vibrations of the CH, CN, and HgCN bonds are almost independent of all other vibrations, which in turn are more or less strongly coupled.     

Metallomethanes: XII. Vibrational spectra and force fields of methylmercurimethanes

The vibrational spectra data, the assignments, and the results of normal coordinate calculations for CH_{4 − n}(HgCH₃)_n molecules (2 ⩽ n ⩽ 4) are reported. The central CHg valence force constants are 1.870, 1.653, and 1.582 N cm⁻¹ while the terminal ones are 2.121, 2,101, and 2.160 N cm⁻¹ for n = 2, 3 and 4, respectively. The latter values are 12, 21, and 27% higher than the central CHg force constants, but all of them are substantially lower than those in dimethylmercury (2.379 N cm⁻¹). These findings can be accounted for in terms of increasing shift of electron density towards the periphery of these molecules and increasing non-bonded metal-metal interaction. The nature of the normal modes is discussed.     

The formate anion: the harmonic and anharmonic force fields

Harmonic and anharmonic symmetry force constant matrices have been obtained for six isotopically substituted species of the formate anion. The valence force fields derived from the above matrices were used to recalculate the vibrational frequencies. These force fields are compared with others previously reported and obtained via other methods.     

Some aspects of scaling factor calculations for quantum-mechanical molecular force fields

Stages of the scaling procedure for correcting molecular force fields obtained in rough quantum chemical calculations are discussed. The procedure includes selection of the number of scaling factors and their determination using experimental vibration frequencies of isotopomers and related molecules. It is stated that the difference in anharmonicity corrections for light molecules and their heavy analogs is to be taken into account along with possible errors in vibration frequency assignments of both isotopomers and more complex related molecules.     

Vibrational anharmonicity and Fermi resonances in CHF2Cl

The infrared spectrum of CHF₂Cl has been recorded between 15 000 and 350 cm⁻¹. The Fermi resonance between levels involving ν₄ and 2ν₆ is analysed in bands extending from 800 cm⁻¹ to 7000 cm⁻¹ leading to a best value of k₄₆₆ = ± 14.98 cm⁻¹. In conjunction with the recent results of Amrein, Dubal and Quack, Molec. Phys. 56,727 (1985); estimates are reported for 38 out of 45 possible x_ij constants. A variation in the relative intensity of the two Q branches associated with ν₁, on cooling the gas cell, indicates that a hot band contributes to the upper branch at 3024.55 cm⁻¹. However, other evidence suggests that the latter arises also from the combination ν₂ + ν₇ + ν₉, in a very weak, close resonance with ν₁ at 3021.27 cm⁻. A number of anomalous band contours are reported.     

Use of semiempirical molecular orbital calculations for the evaluation of force fields

Various methods, employing molecular orbital calculations of varying approximations, for evaluation of force fields of polyatomic molecules have been reviewed. Applications ofcndo/force method for the force field calculations are specially dealt with in detail because of its ease of operation and being economically more viable in terms of computer time. The calculated C=O stretching force constants for a series of organic molecules are shown to have linear relationship with substituent constants.     

FSGO calculation of harmonic force fields of selected polyatomic hydrides

Harmonic force fields for several polyatomic hydrides of first-row atoms have been determined using the FSGO method and its extensions. The obtained results suggest that for larger polyatomic systems, the use of small, but completely optimised basis sets composed mainly of bond functions may be more economical than the conventional large basis set LCAO approach.     

Vibrational and quantum-chemical study of push-pull chromophores for second-order nonlinear optics from rigidified thiophene-based pi-conjugating spacers

Two types of push-pull chromophores built around thiophene-based pi-conjugating spacers rigidified by either covalent bonds or noncovalent intramolecular interactions have been analysed by means of IR and Raman spectroscopical measurements in the solid state as well as in a variety of solvents. Comparison of the Raman features of NLO-phores based on a covalently rigidified dithienylene (DTE) spacer with those of their open chain DTE analogues shows that the bridging of the central double bond of DTE with the nearest beta-positions of the thienyl units through two ethylene bridges significantly improves the intramolecular charge transfer. This also occurs for NLO-phores based on a 2,2'-bi(3,4-ethylenedioxythiophene) (BEDOT) spacer as compared with their corresponding parent compounds based on an unsubstituted bithiophene (BT) spacer. For NLO-phores based on a BEDOT spacer, noncovalent intramolecular interactions between sulfur and oxygen atoms are responsible for the rigidification of the spacer. The Raman spectra of these NLO-phores obtained in the form of solutes in dilute solutions reveal two different behaviours: i) chromophores based on covalently bridged or open chain DTE spacers display Raman spectral profiles in solution quite similar to those of the corresponding solids, with a very little dependence on the polarity of the solvent, while ii) larger spectral changes are noticed for NLO-phores built around BEDOT or BT spacers on going from solids to solutions. In the second case, spectral changes must be ascribed not solely to conformational distortions of the donor and acceptor end groups with respect to the pi-conjugated backbone mean-square-plane (as for the DTE-based NLO-phores) but also to distortions of the thienyl units of the pi-conjugating spacer from coplanarity. The insertion of vinylenic bridges between the thienyl units of the pi-conjugating spacer and between the spacer and the donor and acceptor end groups is a suitable strategy to reach a fairly large intramolecular charge transfer both in polar and nonpolar solvents. Density functional theory (DFT) calculations have been carried out to assign the relevant electronic and vibrational features and to derive useful information about the molecular structure of these NLO-phores.     

Vibrational spectrum,force Field calculations,thermodynamic functions and barrier to internal rotation for benzoyl fluoride

The i.r. and Raman spectra of benzoyl fluoride have been studied in solid, liquid and vapour phases. All of the 36 normal modes of vibration have been assigned in the light of normal coordinate analysis. The carbonyl group stretching mode exhibits a broad band group at ∼ 1800 cm⁻¹, with three bands of comparable intensities. The appearance of the triplet is explained by the existence of strong Fermi resonance. The CO stretch, CF stretch and CFO wag modes have been confidently assigned at 1810, 1014 and 682 cm⁻¹ (in solid/liquid state spectra), respectively. The former two of these show an upward shift and the latter one shows a downward shift in going from solid phase to vapour phase. These results evidence the presence of strong intermolecular interactions in solid and liquid phases. Thermodynamic functions and barrier to internal rotation of the CFO group have also been computed using the fundamental frequencies. The barrier height is found to be ∼4.91 kCal/mole.     

Developing ab initio quality force fields from condensed phase quantum-mechanics/molecular-mechanics calculations through the adaptive force matching method

A new method called adaptive force matching (AFM) has been developed that is capable of producing high quality force fields for condensed phase simulations. This procedure involves the parametrization of force fields to reproduce ab initio forces obtained from condensed phase quantum-mechanics/molecular-mechanics (QM/MM) calculations. During the procedure, the MM part of the QM/MM is iteratively improved so as to approach ab initio quality. In this work, the AFM method has been tested to parametrize force fields for liquid water so that the resulting force fields reproduce forces calculated using the ab initio MP2 and the Kohn-Sham density functional theory with the Becke-Lee-Yang-Parr (BLYP) and Becke three-parameter LYP (B3LYP) exchange correlation functionals. The AFM force fields generated in this work are very simple to evaluate and are supported by most molecular dynamics (MD) codes. At the same time, the quality of the forces predicted by the AFM force fields rivals that of very expensive ab initio calculations and are found to successfully reproduce many experimental properties. The site-site radial distribution functions (RDFs) obtained from MD simulations using the force field generated from the BLYP functional through AFM compare favorably with the previously published RDFs from Car-Parrinello MD simulations with the same functional. Technical aspects of AFM such as the optimal QM cluster size, optimal basis set, and optimal QM method to be used with the AFM procedure are discussed in this paper.     

Vibrational spectra of 1,1,3,3-tetramethyl-2-nitroguanidine and their interpretation with the use of scaling of quantum-chemical force field

The IR (4000–50 cm⁻¹) and Raman (3500–170 cm⁻¹) spectra of solid 1,1,3,3-tetramethyl-2-nitroguanidine (TMNG) were obtained. The spectra were interpreted using the scaling of the TMNG quantum-chemical force field in the B3LYP/6-311G(d,p) approximation. Transferable scale factors necessary for the interpretation of spectra of more complex related compounds were determined. The scaled harmonic force field is supposed to be used in the analysis of the available gas-phase electron diffraction data for TMNG. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 495–498, March, 2008.