Simple Force Field Research Articles

Vibrational spectra of dimethyl sulphone and dimethyl sulphone-d6

The infrared spectra of dimethyl sulphone and dimethyl sulphone-d6in the solid state have been recorded for the ordinary and polarized radiations. The Raman spectra of dimethyl sulphone-d6 in the solid state and in the D2O solution have been recorded. From these data, all the fundamental frequencies have been assigned except the C-S-C antisymmetric stretching frequency of dimethyl sulphone-d6, the a2 and b2 CD3 rocking frequencies, and the CH3 and CD3 torsional frequencies. The normal co-ordinate analysis has shown that the simple Urey-Bradley force field is not satisfactory for explaining the observed frequencies. A modified Urey-Bradley force field including three additional constants has been proposed to improve the fit between the observed and calculated frequencies.

Infrared spectra of bis-biuret cadmium chloride, bis-biuret mercuric chloride and their deuterated compounds

Abstract The infrared spectra between 4000 and 250 cm −1 of the CdCl 2 and the HgCl 2 complexes of biuret and their deuterated compounds have been recorded. The fundamental frequencies of the biuret molecule in these complexes have been assigned by taking account of isotopic frequency shift and by referring to the infrared spectra of urea, diacetamide and biuret-hydrate. Normal co-ordinate analysis of the in-plane vibrations of biuret and biuret- d 5 in the CdCl 2 complex has been carried out by using the molecular model of the planar trans-cis conformation. A simple Urey-Bradley force field has been employed and the force constants have been refined by the least squares method. The calculation of the potential energy distributions has shown that the NH in-plane deformation and the CN stretching vibrations couple appreciably with each other and give the imide II and III bands at 1579 and 1362 cm −1 , respectively. The fact that the imide II frequency of the CdCl 2 complex is much higher than that of biuret-hydrate can be explained by the increase of the CN stretching force constant on the complex formation.

Schwingungsspektren und Kraftkonstanten einiger Tertiärbutylacetylene

Abstract A normal co-ordinate analysis of the vibrational spectra of the (CH3)3C-C = C-H, - D, -Cl, -Br and -J compounds has been carried out, and force constants of three slightly different simplified valence force fields have been obtained. Both the iteration procedure described in the preceding paper, and a well-known least-squares technique were used in the computations. The first method proved successful for all species and all force fields. The least-squares method yielded converging results in the case of tertiary butyl acetylene, when the two isotopic frequency sets were refined simultaneously and when a sufficient amount of constraint was introduced into the force field. The force constants belonging to the same force field as obtained by the two independent methods are in good agreement. The valence constants of the acetylenic chain seem to reveal some expected distinctions in the bond properties of the pure acetylene and the halogenated compounds.

Infrared spectrum and force field of methylthioethyne

Abstract The far infrared spectrum (400-50 cm −1 ) of methylthioethyne, (CH 3 SCCH), was recorded in both the vapour phase and in solutions; two absorption bands were found in this frequency range. Also, the infrared spectrum of the crystalline compound at liquid nitrogen temperature was recorded from 4000 to 400 cm −1 . The assignment of the fundamental frequencies made by moritz [1] could be completed and a simplified force field has been proposed.

A simplified CH out-of-plane force field for methylpyridines and their N-oxides

Normal coordinate treatments were carried out for CH out-of-plane vibrations of pyridine, pyridine- N-oxide, and their methyl-derivatives on the basis of the simplified method presented previously, where the motions of only the ring hydrogen atoms were taken into account. A simplified and transferable force field was introduced and five force constants were refined for each of the two series of methylpyridines and their N-oxides. The assigned frequencies corresponding to the umbrella mode were reproduced with a root-mean-square difference of about 0.5% for each series.

Low-Temperature Infrared Spectra between 1200 and 20 cm−1 and Normal-Coordinate Analysis of Silicates with Olivine Structure

Infrared spectra of powdered samples of Mg2SiO4 (olivine) and γ-Ca2SiO4 (space group D2h16, Z + 4) were measured at liquid-N2 and liquid-He temperature between 1200 and 20 cm−1. The low-temperature spectra do not show noticeable band sharpening. Using a simple force field with five stretching-type force constants and symmetry-adapted coordinates, a normal-coordinate analysis is carried out for both crystal compounds. It is shown that satisfactory agreement between observed band frequencies and normal frequencies may be obtained through few iteration cycles. A number of features of the conventionally used group-vibration approach are confirmed by the normal-coordinate result, which in turn leads to a complete and fairly detailed assignment of the olivine-type infrared absorption spectra.

A simplified CH out-of-plane force field for benzene, methylbenzenes, and chlorobenzenes

Normal coordinate treatments were carried out for CH out-of-plane vibrations of benzene, methylbenzenes, and chlorobenzenes, where the motions of only the ring hydrogen atoms were taken into account. A simplified and transferable force field was introduced, force constants were refined, and 68 assigned frequencies were reproduced with a root-mean-square difference of 0.62 percent. All the CH out-of-plane fundamental frequencies were unequivocally assigned for these compounds in CS 2 solution, by the aid of analysis of their combination frequencies.

The normal vibrations of formic acid and methyl formate

Normal coordinate calculations have been carried out for methyl formate and formic acid on the basis of three different simplified harmonic force fields. The infrared spectra of DCOOCD3 were obtained and the fundamental modes were assigned. The isotopic species HCOOH, DCOOH, HCOOD, DCOOD, HCOOCH3, DCOOCH3, HCOOCD3 and DCOOCD3 were included in least squares refinements of force constants. Force constants associated with the H(CO)O grouping were assumed to be transferable from the acid to the ester. Calculations were carried out with a Urey-Bradley force field, a valence-type force field derived solely from observed frequencies, and a valence-type force field with carbon-oxygen stretching constants evaluated from bond length data. The average error for 90 observed frequencies was 8 cm−1 (~0.7%) for the Urey-Bradley force field and below 6 cm−1 (~0.6%) for the valence-type force fields. The calculations revealed that the use of deuterated species does not provide sufficient data to obtain values for important skeletal interaction constants. Utilization of bond length data led to a reasonably complete force field, to acceptable values of interaction constants and to a good agreement between observed and calculated frequencies. Application of the Urey-Bradley approximation resulted in CO and C-O stretching constants with apparently too low values. The calculated potential energy distribution as well as the Cartesian displacements for several modes are sensitive to the nature of the approximate force field, despite the good frequency agreement obtained with all three employed approximations. The results suggest that consideration must be given to the nature of an approximate force field before the calculated form of a group vibration is accepted as being physically meaningful.

Infrared Spectra of Biuret-hydrate and Its Deuterated Compound

Abstract The infrared spectra between 4000 and 33 cm−1 of biuret-hydrate and between 4000 and 250 cm−1 of its deuterated compound have been recorded. The vibrational assignments have been made by referring to the infrared spectra of the related compounds and the isotopic frequency shift. Normal coordinate analysis of the in-plane vibrations of biuret and biuret-d5 as twelve-body problem has been carried out by using the molecular model of the planar trans-cis conformation. A simple Urey-Bradley force field has been employed and the force constants have been refined by the least squares method. The calculation of potential energy distributions has shown that the NH in-plane deformation and the CN stretching vibrations couple appreciably with each other and give the imide II and III bands at 1515 and 1365 cm−1, respectively, just as in the case of diacetamide.

Calculations of Frequencies and Intensities of the Lattice Vibrations of Thiourea and Thiourea-d4

Frequencies of the lattice vibrations of the room-temperature modification of thiourea and deuterated thiourea have been calculated assuming a simple force field. The calculated frequencies are in good agreement with the observed frequencies in the Raman and far-infrared spectra. Approximate relative intensities of the lattice vibrations in both spectra have been calculated and used to check the assignments given by the frequency calculation. One of the intermolecular force constants, 0.173 mdyn/Å, seems to represent a fairly strong S···H–N hydrogen bond.

Vibrational spectra and bonding in metal carbonyls. V. New data for XMn(CO)5 molecules and further examination of simplified force fields

Vibrational spectra and bonding in metal carbonyls. V. New data for XMn(CO)5 molecules and further examination of simplified force fields

Normal Coordinate Analysis of Trigonal Bipyramidal XY5 Type Molecules

Abstract Normal coordinate analysis has been carried out for trigonal bipyramidal XY5 molecules such as PF5, SbF5, PCl5 and SbCl5. It has been found that the simple Urey-Bradley force field does not provide satisfactory results for the bending modes of these molecules. The results have been improved considerably by introducing an interaction constant between equatorial and axial angle coordinates. In carrying out normal coordinate analysis, the repulsive force constants were first estimated from a Lennard-Jones type potential. Then the stretching and bending force constants were adjusted to fit the observed frequencies. Finally, the new interaction constant was introduced to minimize the errors in the bending frequencies. The force constants thus obtained are consistent through the four molecules studied. Some conflicting band assignments made by previous investigators have been discussed on the basis of the present calculations.

Vibrational Spectra of Triethylfluorosilane

Abstract The infrared and the Raman spectra of triethylfluorosilane have been measured, and the normal vibrations for the skeletal modes have been calculated by Wilson’s FG matrix method, using a simple Urey-Bradley force field and assuming point group C3v for the molecular symmetry. The skeletal vibrations have been assigned quantitatively on the basis of the normal coordinate treatment, and the hydrogen vibrations have been tentatively assigned. The vibrational spectra of this molecule have also been discussed in comparison with those of triethylchlorosilane and methyltrimethoxysilane.

The calculation of Franck-Condon factors for the electronic transitions of the polyacenes

Progressional intensities have been calculated for the 1La band of the polyacenes which give good agreement with experiment. The Franck-Condon factors are calculated on the assumption that the frequencies and normal modes in the ground and excited states are identical. Our results, based on a simple force field, also show that the assumption (by McCoy and Ross) that the C-C stretching frequencies are degenerate is satisfactory as far as the calculation of the shape of the overall vibrational envelope is concerned.

Infrared Spectrum and Thermodynamic Properties of Gaseous Sulfur Trioxide

The infrared spectrum of sulfur trioxide was obtained under prism resolution in the gaseous state and in xenon matrices at liquid-nitrogen temperature. In both cases a satisfactory vibrational assignment of the monomeric SO3 spectrum could be made on the basis of a simple D3h symmetry model. The observed gas phase fundamentals were v3(e′) = 1391, v4(e′) = 529, and v2(a2″) = 495 cm—1. The matrix fundamentals were essentially the same as those of the gas phase except that the v2(a2″) mode was shifted to 464 cm—1. A normal coordinate treatment of the SO3 molecule was carried out and potential constants were determined for both Urey-Bradley and simplified valence force fields. Three weak absorption bands of S3O9 were also detected in the gas-phase spectrum. From the temperature and pressure variation of the intensities of these bands it was estimated that for the trimerization reaction at 298°K, Kp was approximately 1 atm—2 and ΔH298° was approximately 30 kcal/mole of S3O9. The values of the thermodynamic properties of monomeric SO3 were computed for the ideal gaseous state using the rigid rotor harmonic oscillator approximation at 1 atm from 100° to 1500°K.

The vibrational spectrum and force constants of carbon tetraiodide

The Raman and infrared spectra of CI 4 are reported for the first time. The fundamental frequencies are, for T d symmetry: a 1 178 cm −1; e, 90 cm −1; f 2, 555 and 123 cm −1. Force constants are given for a simple valence force field, simple central force field and Urey-Bradley force field.

Vibrational Spectrum of Borine Carbonyl

The infrared spectra of both solid and gaseous BH3CO, 10BH3CO, BD3CO, and 10BD3CO have been obtained. In addition some observations have been made on the Raman spectra of the gaseous compounds. Fundamental frequencies have been assigned to all normal modes and the vibrational potential constants for borine carbonyl have been approximated by means of a simplified force field.

Force Constants of Triatomic Molecules

In the general expression of the potential energy of the triatomic molecule XYZ there are six force constants c1, c1′ (valence bond), c2 (angle deformation), c3, c3′ (angle bond interaction), and c4 (bond-bond interaction), of which only three can be calculated from the three experimentally determined fundamental frequencies. For the isosceles triatomic molecule YX2 the number of force constants reduces to four (c1, c2, c3, and c4). From a study of all the real values of these four force constants, it is seen that they are related to one another by ellipses and straight lines. The graphs of these ellipses indicate the possibility of choosing a certain singular point (near one end of the major axis) which gives the following relation between the force constants: dc4/dc2=0 or dc4/dc3=0.This condition and the equations usually obtained from the theory of small vibrations permit the calculation of a singular set of four force constants of the isosceles triatomic molecule. When these force constants are applied to isotopic molecules, satisfactory checks between the calculated and experimentally determined frequencies are obtained. The ellipses showing the relation between the force constants indicate clearly why the simple central force field (c1, c2, c3) or the simple valence force field (c1, c2, c3), containing one interaction constant or crossterm, must yield imaginary values for c2 and c3 in certain cases.