John Overend

Transferability of Urey‐Bradley Force Constants. I. Calculation of Force Constants on a Digital Computer

An algorithm for the systematic calculation of Urey‐Bradley force constants has been programed for a digital computer (the Datatron 204). The secular equation is set up and solved in internal coordinates, the potential energy being transformed from Urey‐Bradley space to internal‐coordinate space by a matrix Z. This same matrix is also used to transform the Jacobian of λ with respect to the force constants from internal‐coordinate to Urey‐Bradley space, thereby allowing the direct determination of Urey‐Bradley force constants. A method is described whereby the Z matrix and Wilsons G matrix may be set up by the computer from the geometrical parameters of the molecule.

The application of the Urey-Bradley force field to the in-plane vibrations of benzene

Abstract C alifano and C rawford have recently demonstrated that a basic Urey-Bradley force field reproduces the ingold assignment of the planar B2u vibration fundamentals of benzene but does not give acceptable agreement with the M air -H ornig assignment. We have proposed a model which removes these objections to the M air -H ornig assignment, and the normal modes for this model are discussed and compared with those obtained for the I ngold assignment. This model gives a slightly better frequency fit for certain vibrational modes and, although the ingold assignment cannot be excluded, a slight preference for the M air -H ornig assignment is indicated.

Transferability of Urey‐Bradley Force Constants. II. Carbonyl Halides and Related Molecules

Urey‐Bradley force constants have been fitted to the observed vibrational frequencies of COF2, COCl2, and COBr2 by the method described in the previous paper. These force constants have been used to calculate the frequencies of COClF, COBrF, and COBrCl. Normal coordinates and the potential energy distribution among the various Urey‐Bradley coordinates have also been calculated. It is found that, despite the fact that the carbonyl stretching frequencies fall over a range of 100 cm—1, the carbonyl stretching force constant is essentially the same for all molecules in the carbonyl halide series.

Acetyl halides—I: Infrared and Raman spectra and vibrational assignment of CH3COCl, CD3COCl and CH2DCOCl

Abstract The infrared and Raman spectra of acetyl chloride, acetyl chloride-d3 and acetyl chloride-d1 have been studied and a vibrational assignment has been made on the basis of an approximate normal-co-ordinate calculation with a Urey-Bradley field. It is proposed that the two intense bands at 1109 and 958 cm−1 in the infrared spectrum of acetyl chloride correspond to vibrational modes which are mixtures of the C-C stretching co-ordinate and the CH3 rocking co-ordinate, and that the similar pair of intense bands in the CD3COCl spectrum arise from modes which are mixtures of CC stretching and CD3 deformation co-ordinates. Thermodynamic functions were calculated for acetyl chloride in the ideal gaseous state.

Transferability of Urey‐Bradley Force Constants. V. Bromoethylenes

Urey‐Bradley force constants (UBFCs), calculated by a least‐squares method, were obtained for ethylene, vinyl bromide, vinylidene bromide, trans‐dibromoethylene, cis‐dibromoethylene, tribromoethylene, and tetra bromoethylene. Even with complete isotopic data it was found impossible to determine all the force constants for an individual bromoethylene; moreover, from the viewpoint of giving stability to the calculation, values have been supplied by transfer from other molecules within this series and these force constants have been constrained in the least‐squares refinement. By inspection of the correlation matrix, the uncertainties in the various force constants were, in many cases, found to be intimately related, and, it was usually found unnecessary to constrain both force constants of an indeterminate pair; in this manner we have kept the number of transferred force constants to a minimum. A comparison of the remaining UBFCs shows that they are moderately transferable. Although the flexible bond para...

Vibrational spectra of sym-trichlorobenzene and the Mair-Hornig assignment of benzene

The vibrational spectra of the deuteroisotopic mixtures of sym-trichlorobenzene have been measured and analyzed and complete assignments of the H3 and D3 molecules and partial assignments of the H2D and D2H molecules were made. It is demonstrated that a basic Urey-Bradley Force Field is inadequate to describe certain of the observed B2-fundamentals of sym-trichlorobenzene-d. In the instances where the basic UBFF fails, the Kekule model proposed by scherer and overend accounts for the discrepancies between observed and calculated frequencies. The success of this model is submitted as evidence for the validity of the mair-hornig assignment of the B2u-fundamentals of benzene. The value of the Kekule constant for sym-trichlorobenzene is found to be 0.400 ± 0.018 mdyn/A compared with 0.351 ± 0.019 mdyn/A for benzene.

Transferability of Urey‐Bradley Force Constants. IV. Methyl Halides

Urey‐Bradley force constants have been fitted by least squares to the vibrational frequencies of CH3F, CH3Cl, CH3Br, CH3I, and their fully deuterated isotopes. Both observed and zeroth‐order frequency data have been used and the corresponding sets of force constants have been compared.

The contributions of mass and force-constant effects to the carbonyl-stretching frequency of acetyl, carbonyl and benzoyl halides

Abstract The value of the carbonyl-stretching frequency in molecules of the type COXY is determined not only by the CO force constant, but also by the masses of X and Y, the molecular configuration, and the remaining force constants. A numerical analysis of the dependence of νco on these factors allows us to estimate a correction to the observed carbonyl frequency which takes into account these extraneous effects and leads to a frequency νco∗ which truly reflects the strength of the CO bond. In the acetyl, carbonyl and benzoyl halides the value of νco∗ has been found to be almost independent of the nature of the halogen substituent.

Correlations of force constants for carbonyl out-of-plane deformations with substituent constants

Abstract It is shown that the out-of-plane bending force constants of twenty-two molecules of the type satisfy an empirical relation, where σ I and σ R are respectively Tafts inductive and resonance substituent constants.

Repulsive Potential between Nonbonded Methyl Groups

The vibrational analysis of many molecules depends critically on our being able to supply values for some of the force constants. In particular, for many chemically interesting molecules it is important to know the repulsive force constants between two methyl groups bonded to the same atom, and we have used the known frequencies of tetramethyl carbon, silicon, germanium, tin, and lead to determine this force constant as a function of the carbon-carbon internuclear distance. Our results indicate that FCC=∂2V(rCC)/∂(rCC)2=196.5 rCC−6.8 md/A.