Robert A. R. Pearce

Absolute infrared intensities of cyclopropane‐d0 and cyclopropane‐d6: Dipole moment derivatives and polar tensors

Integrated infrared absorption intensities for the fundamental vibrations of gas‐phase cyclopropane‐d0 and cyclopropane‐d6 were experimentally determined using nitrogen pressure‐broadening techniques. Although the experimental data are insufficient to resolve the relative sign combinations for the (∂P/∂Qi) and (∂P/∂Sj) charge distribution parameters in the A2′′ symmetry species, a definitive relative sign choice is indicated for the E′ dipole moment gradients. The normal coordinate transformations required for the dipole derivative analysis are based on the highly reliable force field derived by Duncan and Burns [J. Mol. Spectrosc. 30, 253 (1969)]. For ease both in computing the atomic polar tensor elements of cyclopropane and in comparing cyclopropane force constants and dipole derivatives to related systems, the Duncan and Burns potential function is recast in terms of a set of more convenient internal valence force constants. (∂P/∂Qii) dipole gradients determined by semiempirical CNDO/2 molecular orbit...

Vibrational spectra and normal coordinate analysis for trans‐2‐butene‐d0 and trans‐2‐butene‐d8

The vibrational spectra of trans‐2‐butene‐d0 and trans‐2‐butene‐d8 were recorded in the region 4000–100 cm–1. In conjunction with a normal coordinate analysis, vibrational assignments are proposed on the basis of Raman depolarization ratios for the liquid samples, isotopic shift ratios, and group coordinate considerations. Both assignments and vibrational coupling effects are discussed in terms of the calculated potential energy distributions. A 21‐parameter modified internal valence force field was determined by least squares procedures from the observed polycrystalline frequencies of the two isotopic species.

Raman spectra and vibrational potential function of polycrystalline azomethane and azomethane‐d6

Raman spectra of polycrystalline azomethane (CH3N=NCH3) and azomethane‐d6 were examined at liquid nitrogen temperatures using argon ion laser excitation. These data, in conjunction with a normal coordinate analysis, provide a basis for a modification of the vibrational assignments for the azomethane system. A 17 parameter molecular force field, based upon the solid state vibrational frequencies of azomethane and its deuterated analog, was determined by least‐squares procedures. The unusual coupling characteristics between the X–N stretching and X–N=N bending coordinates are discussed for both azomethane and trans‐1,2‐difluorodiazine, FN=NF.

Vibrational Raman spectra and intramolecular potential function of solid solutions of dimethyl ether‐d0 and dimethyl ether‐d6

Vibrational Raman spectra of dilute solid solutions of dimethyl ether‐d0 and dimethyl ether‐d6 were recorded at liquid nitrogen temperatures. These data were used to develop a variety of modified internal valence force fields for specifically reflecting intermethyl potential energy coupling terms. A final 18 parameter force field was generated by applying a backward elimination statistical procedure to the least‐squares refinement computation for the force constants. This force field emphasizes the importance of a methyl–methyl interaction term between out‐of‐plane H–C–O bending displacement coordinates. Vibrational assignments for the dimethyl ether‐d0, ‐d3, and ‐d6 isotopic species were determined from the potential energy distributions calculated from the 18 parameter force field. The effects of both the tilt of the methyl group and inequivalence of the Hin‐plane–C–O and Hout‐of‐plane–C–O angles upon the normal coordinate analysis frequency fits were assessed.

Intramolecular potential function and methyl—methyl coupling of crystalline dimethyl sulfide

Abstract Raman spectra of both polycrystalline and dilute solid solutions of dimethyl sulfide-d0 and dimethyl sulfide-d6 were observed at liquid nitrogen temperatures using argon ion laser excitation. Mixed isotopic crystals of the unique molecular species were prepared in molar ratios of 3/100. Liquid phase Raman spectra of the -d6 isotopic species were also determined. The solid solution frequency data for both isotopic species provided the basis for a detailed vibrational assignment from which various model internal valence force fields were developed. The final 14 parameter force field, representative of the vibrations of dimethyl sulfide in a static crystal field, emphasizes the contribution to the potential function of a small, but significant methyl—methyl coupling term between the anti HCS angular displacement coordinates.

Vibrational analysis of the Raman spectra of polycrystalline cis-2-butene

Abstract The vibrational assignments and potential energy distributions for cis -2-butene were examined by transferring appropriate force constants from trans -2-butene and the model systems, cis -1,2- and trans -1,2-difluoroethylene. The model fluoroethylene systems provided both the crucial interaction force constants, which account for effects across the double bond in cis -2-butene, and the skeletal out-of-plane force constants. The zero-order frequencies calculated for cis -2-butene from the transferred potential function agreed quite well with the low temperature Raman spectra of the polycrystalline material.

Vibrational coupling effects and potential functions for cis ‐ and trans ‐1,2‐difluorodiimide and cis ‐ and trans ‐1,2‐difluoroethylene

Separate six parameter internal valence force fields were determined for cis‐ and trans‐1,2‐difluorodiimide (F–N=N–F), using a least‐squares procedure. For the cis‐ and trans‐1,2‐difluoroethylenes (F–CH=CH–F), a 17 parameter overlay internal valence force field was fitted to the in‐plane vibrational frequencies, and separate four parameter general valence force fields were calculated for the out‐of‐plane vibrations. For both sets of molecules, potential energy distributions are reported and discussed, with particular reference to the unusual couplings between the F–X stretching and F–X=X bending coordinates. These couplings are investigated in detail, especially for the 1,2‐difluorodiimides, and it is concluded that they are predominately due to kinetic effects.

Normal coordinate treatment for propane: Force constant selection through a backward elimination regression–refinement procedure

A 26 parameter internal valence force field for propane and four deuterium isotopic species was determined by applying a backward elimination regression–refinement technique for selecting and adjusting statistically significant force constants. In comparison to computations based upon fits to harmonic frequencies, this regression–adjustment procedure generated a more satisfactory set of force constants in a fit to observed vibrational frequencies. The regression analysis for the final minimum parameter force field retains a long range methyl‐methyl interaction force constant involving H–C–C in‐plane bending displacement coordinates for each methyl rotor. The general valence force field for propane, whose methyl groups are tilted such that the out‐of‐plane CH3 hydrogen atoms approach one another, is compared to force fields for other two rotor systems in which the methyl groups are tilted in an oppostie manner.

Role of redundant coordinates in evaluating skeletal force constants in simple cyclic systems

Abstract The treatment of redundant vibrational coordinates and their effect on force field determinations often presents difficulties for the vibrational spectroscopist. W hiffen [17], for example, approached the redundancy problem in the force field for the planar vibrations of benzene by transforming a set of redundant internal valence force constants ƒij, to a non-redundant set of tilde force constants ƒ ij, where ƒ ij represents appropriate linear combinations of the original ƒij. We use the concept of tilde force constants to eliminate the redundancies among the internal valence displacement coordinates in cyclopropane C3H6 and phosphorus P4 and to clarify the precise meaning of the resulting skeletal force constants for these systems.