Ab initio study of the vibrational assignment and force field of thiosemicarbazide-d0 and -d5

The complete harmonic force field and optimized geometry of thiosemicarbazide have been calculated at the ab initio Hartree—Fock level using the 3-21G basis set. On the basis of this, the frequencies of thiosemicarbazide-d₀ and -d₅ and their ¹⁵N isotopic molecules have been calculated. The calculated frequencies and their band assignments are utilized to critically examine our previous experimental assignments which were based on normal coordinate calculations. The theoretical IR and Raman intensities, together with qualitative experimental band intensities, are also presented.     

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.     

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.     

Constrained force constants for octahedral molecules

A set of constrained force constants has been derived from experimental vibrational frequency data for eighty three octahedral molecules. Superimposing the condition that the larger value for f_d—f_dd′ be used when f_dα—f_dα″ is a maximum on the six equations relating vibrational frequencies to force constants generates a seventh. This provides a uniform set of results for all 83 molecules. The values of the force constants have a simple rationale in terms of chemical bonding theory. Some preliminary calculations for SF₆ show that these force constants are suitable for use in generating reliable molecular dynamical trajectory data.     

Calculation of the frequencies and intensities in the infrared and raman spectra of cyclopropenone

By using the values of the vibrational frequencies of normal and deuterated cyclopropenone (II-d₀ , II-d₂ ) and ¹⁶ O-and ¹⁸ O-substituted dimethylcyclopropenone (III) as -well as the infrared and Raman intensities of II a consistent set of force constants has been derived for the cyclopropenone skeleton. The derived values show that the zwitterionic form makes a substantial contribution to the electronic ground state of the molecule. The combined frequency and intensity calculation - simulation of the infrared and the Raman spectrum - is shown to be a good method for making a proper assignment of calculated and observed vibrations and deriving realistic sets of force constants.     

Accurate theoretical IR and Raman spectrum of Al2O2 and Al2O3 molecules

The accurate harmonic vibration frequencies together with the infrared (IR) and Raman intensities of the most stable conformers of Al₂O₂ and Al₂O₃ molecules have been calculated by the density functional theory (DFT) method with B3LYP exchange–correlation potential and using a set of the augmented correlated consistent basis sets up to quintuple order. The anharmonic vibration frequencies of the non-linear Al₂O₂ molecule have also been calculated. The obtained equilibrium geometrical parameters, harmonic and anharmonic vibration frequencies along with the IR and Raman intensities good converge to their limits with increasing the size of the used basis set. A comparison of the calculated harmonic and anharmonic vibrational frequencies with the available experimental ones points out that the small differences between the calculated harmonic and experimental frequencies can be further substantially reduced when calculations of the anharmonic vibrational frequencies will be available for all types of molecular geometries.     

Franck—condon factors of the T1 ↷ S0 radiative and nonradiative transitions of naphthalene-h8 and -d8-a correlation function analysis

Franck—Condon factors for the T₁ ? S₀ transition in naphthalene-h₈ and naphthalene-d₈ are calculated employing the correlation function approach which allows us to investigate the distribution of the released electronic energy among the normal modes of the Final ground state. The relevant coupling parameters relating to geometry, frequency and anharmonicity changes due to excitation are included. Those related to geometry changes are obtained from the vibronic intensities of the phosphorescence spectrum as well as from a calculation implementing a semi-empirical relation between bond order and bond length. The calculated nonradiative rates compare well with the experimental rates in terms of absolute magnitude and deuterium effect. The semi-empirical calculations of the ribtonic intensities provide detailed information about force fields that are otherwise indistinguishable on the basis of their ability to reproduce infrared frequencies.     

Spectra and structure of silicon compounds. XVII Raman and infrared spectra and vibrational assignment for hexamethyldisilane and ab initio calculations for disilane and hexamethyldisilane

The IR spectra of gaseous and solid hexamethyldisilane between 4000 and 25 cm⁻¹ and the far-IR spectrum of the liquids from 450 to 25 cm⁻¹ have been recorded. The Raman spectra have been recorded from 3500 to 10 cm⁻¹ for all three physical phases. Assisted by ab initio calculations, the vibrational spectrum of hexamethyldisilane has been assigned under D_3d symmetry and the results of a normal coordinate analysis are discussed. No spectral features indicative of free internal rotation have been observed. Gradient ab initio calculations have been carried out for the disilane and hexamethyldisilane molecules using different types of basis sets. The structural parameters, rotational constants, unscaled and scaled frequencies and harmonic force constants have been reported for both disilane and hexamethyldisilane.     

Building a database of force constants based on scaled ab initio (SQM) results. I. Chlorobenzenes

As part of a project for developing a database of harmonic force constants for organic molecules, the complete force fields for chlorobenzene, ortho-, meta-, para-dichlorobenzene and sym-trichlorobenzene have been determined, on the basis of ab initio Hartree—Fock calculations combined with empirical adjustments. The latter serve to correct for systematic errors in the theory, and are applied at two stages: the geometry is corrected by using empirical offset forces during the optimization; force constants are corrected by a few scale factors according to the SQM (scaled quantum mechanical) force field procedure. With scale factors taken over fixed from benzene and only two new scale factors introduced for the chlorobenzenes, experimental frequencies are reproduced with mean deviations of about 10 cm⁻¹. Some controversial assignments, still present in the deuterated derivatives, are discussed. Theoretical IR and Raman intensities have also been calcuated and used as semiquantitative information to assist assignments.     

A general quadratic force field for the pyridazine molecule: out-of-plane B1 normal modes

On the basis of Raman and infrared data for pyridazine-d_o′, −3, 6-d₂, and −d₄ we are carrying out a normal coordinate treatment using a general quadratic force field (GQFF) on a non-redundant basis. The BB^* matrix has been set up and diagonalized to obtain the linear redundancy relations. Afterwards, using the Schmidt's process, we constructed the independent coordinates used in our calculations. To date we have completed the refinement of the calculations of the force constants for the B₁ specie.     

Crystallographic analysis of atomic structures and the phenomenological mechanism of crystallization

A theoretical study of the structure and vibrational spectrum of methyl-β-D-glucopyranoside is performed with allowance for the hydrogen bond effect on them. At the density functional theory level with the use of the B3LYP functional in the 6–31G(d) basis set the structural dynamic models of a free molecule of methyl-β-D-glucopyranoside and its simplest complexes with hydrogen bonding in the form of dimers with different structures are constructed. Energies are minimized; structures, electro optical parameters, force constants, and normal vibrational frequencies in the harmonic approximation and their intensities in IR spectra are calculated; the hydrogen bond energy is estimated. Based on the calculation, the conclusions are drawn about the structure of the methyl-β-D-glucopyranoside sample, the formation and interpretation of its IR spectrum, and the possibilities of the used density functional theory method.     

Vibrational spectra,force constants and thermodynamic functions of cycloheptasulfur,S71

Raman and IR spectra of four allotropes of cycloheptasulfur are reported. All 15 fundamental vibrations of the S₇ molecule have been observed and were assigned in accordance with the molecular point group C_s. A normal-coordinate analysis using an extended Urey—Bradley force field with 10 independent force constants resulted in very good agreement between observed and calculated wavenumbers. Entropy, heat capacity and other functions of gaseous S₇ have been calculated by statistical methods and evidence for pseudorotation of S₇ in the vapor state is presented. The molecular structure as well as the valence force constants of S₇ are rationalized by a qualitative molecular orbital treatment.     

Vibrational spectra,normal modes,force field and barrier to inversion of phosphiran,phosphiran-1-d1 and phosphiran-2,3-d4

The infrared spectra over the range of intramolecular fundamentals of polycrystalline phosphiran and phosphiran-1-d₁ at 77 °K have been obtained and the observed frequencies assigned. Correlation field effects have been observed and analyzed in terms of the intermolecular forces operative in the unit cell. The intramolecular vibrations of phosphiran, phosphiran- 1-d₁, and phosphiran-2,3-d₄ have been subjected to simultaneous analysis in the approximation of harmonic forces and the resulting eigenvectors and potential energy distributions are presented. An estimate from the force field of the periodic barrier with inversion at phosphorus of 33 kcal mole^?1 has been obtained and compared to results for similar molecules. Thermodynamic functions for phosphiran have been calculated in the rigid rotor harmonic oscillator approximation.     

A DFT analysis of the vibrational spectra of nitrobenzene

Raman and FTIR, spectra of nitrobenzene, nb, and its isotopomers, nb-¹⁵N, nb-¹³C₆ and nb-d₅, were obtained and the fundamental vibrational modes assigned with the aid of a B3LYP/6-311+G** calculation, without the need for scaling of the force constants. The changes in vibrational coupling between the nitro and benzene groups upon certain isotopic substitutions are well modelled by the calculation, which is able to reproduce the isotopic shifts in frequencies for the nitro vibrations, as well as changes in IR intensities.     

In-plane vibrational modes of cytosine from an ab initio MO calculation

Harmonic force constants, in-plane vibrational frequencies, and in-plane vibrational modes of cytosine were calculated by an ab initio Hartree—Fock SCF MO method. The force contants were calculated by the use of an energy gardient method with the STO-3G basis set, and then they were corrected into “4-31G force constants” by the scaling factors given by us previously for the case of uracil. The corrected set of force constants can produce a calculated vibrational spectra of cytosine and cytosine-1,amino-d₃, that can be well corrected with the observed Raman and infrared spectra of these compounds, with little ambiguity. Thus, the assignments of all the in-plane vibrations are now practically established. The calculated vibrational modes, in addition, can account for the recently published resonance Raman effects of cytosine residue.     

Vibrational spectrum of ethynol: AB initio calculation and matrix isolation studies

Ab initio calculation of the harmonic force field of ethynol have been performed. A 6-31G^** basis set was used and electron correlation was taken into account by applying perturbation theory carried to second order (MP2). After application of experimental scaling factors, vibrational frequencies have been computed. From numerical differentiation of the dipole moments, intensities of the vibrational transitions have been derived. With the help of the harmonic force field the quartic centrifugal distortion constants have been obtained. The reactions OH + CH₃CCH, OH + HC₄H, H+ C₃O₂ and Ar^* + HCCCOOH have been tried to produce ethynol. The reaction products were frozen in an argon matrix and their IR spectrum recorded. Many new absorption lines appeared but there was no evidence for ethynol.     

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 spectra of 1,3-dideuterioallene and normal mode calculations for allene

Infrared and Raman spectra of 1,3-d₂-allene were measured and fundamental frequencies were assigned. The frequencies were used with previously published data for the d₀, d₄, d₁ and 1,1,-d₂ isotopomers of allene in force field calculations. A new ab initio quantum mechanical force field calculated at the 6-31G** level was scaled to fit the anharmonic fundamental frequencies and used to provide some of the interaction constants for a second, conventionally refined force field.     

Ab initio studies on vibrational spectra of XSO2NCO (X = F,Cl): harmonic force fields and frequency assignments

The harmonic vibrational force fields and the IR spectrum of XSO₂NCO (X= F, C1) molecules have been studied usingab initio HF/SCF method with the 6-31G’ basis set. Theab initio harmonic force fields are scaled empirically using the scaled quantum mechanical (SQM) method of Pulay. A set of scale factors are optimized by the least-squares fitting to the experimental frequencies of FSO₂NCO and then are transferred to CISO₂NCO to give ana priori prediction of its fundamental frequencies. The average deviations between the theoretical frequencies and the experimental values for FSO₂NCO and C1SO₂NCO are 3 and 5 cm^-1, respectively. The assignments of the fundamentals for these two molecules are also made atcording to the potential energy distributions and theab initio IR intensities Project supported by the National Natural Science Foundation of China (Grant No. 29673029)     

Theoretical study on the structures,force field,and vibrational spectra of cyclooctatetraene and cyclooctatetraene-d8

The structures and force field of 1,3,5,7-cyclooctatetraene (COT) have been studied using ab initio theory at the SCF level with the 4-21G basis set. The quadratic force field of the D_2d structure obtained by systematic scaling of the ab initio force constants successfully reproduces the observed frequencies of COT and COT-d₈ with a mean deviation of less than 10 cm⁻¹ for non-CH stretching modes. On the basis of the calculated results, assignments of the fundamental vibrations are examined. The normal mode υ₅ is reassigned to a weak band at 758 cm⁻¹ in the Raman spectrum of COT and to a weak band at 591 cm⁻¹ in the Raman spectrum of COT-d₈. The calculations favor the assignment of υ₂₆ given by Lippincott et al. [J. Am. Chem. Soc. 73, 3370 (1951)] over the revised assignment of Perec [Spectrochim. Acta 47A, 799 (1991)]. The calculations also furnish reliable prediction for the inactive A₂ fundamentals of COT and COT-d₈. The fundamental frequencies and IR and Raman intensities of ¹³CC₇H₈, which constitutes about 9% of COT in natural abundance, are also calculated. Only ν₁₀ (calculated at 908 cm⁻¹) of the formal inactive A₂ modes has appreciable Raman intensity (0.23 Å⁴/amu). A spectral feature due to this fundametal is identified in the liquid Raman spectrum of Tabacik and Blaise [C. R. Acad. Sci. Ser. II 303, 539 (1986)] as a weak peak at 908 cm⁻¹.