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Using Wilson's FG matrix technique, general valence force constants and Urey—Bradley force constants of As(CH3)3, AsCl3 and AsBr3, all molecules of the type XY3, have been calculated from published Raman data and structural parameters. It appears however that the molecule As(CH3)3 cannot be approached by the Urey—Bradley force field. So the comparative study of the bonding characteristics is limited to the general valence force field data.

A 36 parameter valence force field useful for the vibrational analysis of paraffins has been derived from 308 observed vibrational frequencies of the following 17 molecules: propane, trans n-butane, trans n-pentane, gauche n-butane, gauche n-pentane, iso-butane, neo-pentane, 2-methylbutane, 2,2-dimethylbutane, 2,3-dimethylbutane, 2,2,3-trimethylbutane, 2,2,3,3-tetramethylbutane, adamantane, cyclohexane, κ-methylcyclohexane, 1,1-dimethylcyclohexane and trans-decalin. After refinement the force field leads to calculated frequencies which differ from observed frequencies by 0·95 per cent. A trend to significantly larger errors is found for molecules with quaternary carbon atoms. The infrared spectra of all the paraffins have been measured with the samples at −196°C. Vibrational assignments and potential energy distribution are presented. It is concluded that only the low symmetry (C1) form of 2-methylbutane is present between 35° and −196°C.

The planar vibrations of pyrazine, cis -pyrazine- d 2 and pyrazine- d 4 have been calculated using an Urey-Bradley and a valence force field. The calculations were performed on a Data Control computer using a perturbation method. The agreement between the observed and calculated frequencies is satisfactory for both types of fields but considerably better in the case of the valence force field. The normal coordinate analysis confirms that one of the CH bending modes (species B 2 g ) should occur around 1350 cm −1 as postulated previously. The force constants obtained are then compared with those obtained for similar molecules.

A normal coordinate treatment of the tertiary butyl halides has been made and the constants of a valence force field are obtained by an iteration method. The assignments are made by use of potential energy distributions. The tendency of change of the carbon—halogen stretching constants in H 3 CX and (CH 3 ) 3 CX type molecules is discussed.

The Raman and infrared spectra of CI4 are reported for the first time. The fundamental frequencies are, for Td symmetry: a1 178 cm−1; e, 90 cm−1; f2, 555 and 123 cm−1. Force constants are given for a simple valence force field, simple central force field and Urey-Bradley force field.

The calculation of force constants of a valence force field including interactions from molecular vibration frequencies is a problem which in general has no unique solution. Therefore a number of different potential functions, all of which are in agreement with the observed molecular frequencies, are calculated for some planar molecules of the type XZY 2 under different assumptions, and the resulting sets of force constants are compared and discussed, in order to find solutions which probably are close to the true nature of the molecules. The following molecules are treated: CO(CH 3 ) 2 ; CO(NH 2 ) 2 ; CH 3 CO 2 − ; CH 3 C + (NH 2 ) 2 ; OHC + (NH 2 ) 2 ; COF 2 COCl 2 and COBr 2 .

A 9-parameter transferable Urey-Bradley force field (UBFF) reproduces the 36 vibrational frequencies of the series of molecules GeH4, GeH3Cl, GeHCl3, GeCl4 and the fully deuterated derivatives with a mean frequency deviation of 10 cm−1. The use of an individual UBFF for each molecule reduces this deviation to 4 cm−1. The 9-parameter transferable UBFF is superior to a 12-parameter transferable valence force field for these molecules. Similar results obtain for the series of analogous compounds of silicon.

A method is discussed for imposing any desired constraint on the force field obtained in a force constant refinement calculation. The application of this method to force constant refinement calculations for the methyl halide molecules is reported. All available data on the vibration frequencies, Coriolis interaction constants and centrifugal stretching constants of CH 3 X and CD 3 X molecules were used in the refinements, but despite this apparent abundance of data it was found that constraints were necessary in order to obtain a unique solution to the force field. The results of unconstrained calculations, and of three different constrained calculations, are reported in this paper. The constrained models reported are a Urey—Bradley force field, a modified valence force field, and a constraint based on orbital-following bond-hybridization arguments developed in the following paper. The results are discussed, and compared with previous results for these molecules. The third of the above models is found to reproduce the observed data better than either of the first two, and additional reasons are given for preferring this solution to the force field for the methyl halide molecules.

From the vibration frequencies of the molecules BF3, BCl3, BBr3, BJ3, BO33−, CO32−, NO3− und C(NH2)3+ the force constants of a valence force field are calculated using an electronic analog computer. The solutions are presented graphically. If available the frequencies of isotopic molecules are used to find a unique solution, but in the case of BBr3 and BJ3 the isotope effect is too small to be of any practical value, whereas for BF3 and BCl3, it is just sufficient to set rather broad limits for the true potential function of the molecules.

By a least-square iteration process and following the analytical criteria recently proposed by A ldous and M ills the recalculation of the quadratic potential energy function for fluoro-, chloro- and bromo-forms in a modified valence force field approximation was carried out, adopting actual interbond angles. The assignment of ν 2 fundamental for CF 3 H necessary to obtain convergence was found to be different from that used in previous calculations. The values of stretch—bend interaction constants which lead to a well-conditioned convergence suggest that the hybrid orbital force field approximation is applicable for the present molecules. Coriolis coupling constants, rotation distorsion constants, mean-square amplitudes of vibration and thermodynamic properties were also calculated.

Using W ilson's F - and G -matrix method and data in the literature, force constants have been calculated for the three thionyl halides F 2 SO, Cl 2 SO and Br 2 SO. A valence force field with six independent constants was used. The stretching constants for the SO bonds follow closely the “SO stretching frequencies” and both vary as would be expected qualitatively assuming that increased electronegativity of the halogen atoms favors greater p π- d π SO bonding.

A valence force field, including selected interaction constants, has been used to compute the thirty-three planar vibration frequencies of each of naphthalene, naphthalene-α-d4, naphthalene-β-d4 and naphthalene-d8. Starting with benzene-like force constants, the method of steepest descents was used in an iterative calculation which forced agreement with seven observed Ag-type fundamentals of the undeuterated compound, using (in the more successful of two such calculations) the Ag assignments of Luther. The final force field gave encouraging results when used to calculate all the other frequencies, and appears to be particularly suitable for the contentious low frequencies. The only really large and unavoidable disagreement was found in the case of the particular B2u vibration which carries naphthalene between two Kekule structures. The normal modes are depicted; they are strikingly similar to the modes found by Schmid in a calculation which led to considerably different numerical frequencies.

The infrared and Raman spectra of s-triazine and s-triazine-d3 have been taken and an assignment of the fundamental vibrational modes has been made. Nearly all the remaining observed bands are accounted for as combinations or overtones. A simple valence force field normal co-ordinate analysis was made, and the values of some force constants calculated.

Normal coordinate analysis of the in-plane vibrations of C 5 H 5 − , from the vibrational spectrum of the potassium salt, has been carried out using both a simple valence force field and a Urey—Bradley force field. The results confirm the planar D 5 h structure of cyclopentadienide anion and the aromatic nature of the species. The C C stretching force constant from the Urey—Bradley treatment has a value (5·39 mdyn/A) very close to that reported previously for benzene.