William F. Maddams

Conformations of some αβ-unsaturated carbonyl compounds. Part III. Infrared solution spectra of methyl, [2H3]methyl, ethyl, and [2H5]ethyl acrylates and trans-crotonates

Infrared spectra of methyl, [2H3]methyl, ethyl, and [2H5]ethyl acrylates and trans-crotonates in solution in carbon disulphide are reported. Band intensity measurements at temperatures between –110 and 35 °C together with changes in the appearance of these bands on solidification of the solution are interpreted in terms of conformational equilibria between high- and low-energy forms. The additional enthalpies (ΔH°) of the high-energy forms are remeasured for methyl acrylate as 326 ± 18 cal mol–1 and for methyl trans-crotonate as 306 ± 30 cal mol–1. The additional entropy (ΔS°) of the high-energy form of methyl trans-crotonate is estimated as 1·5 ± 1·0 cal mol–1 K–1.

Fourier Self-Deconvolved Infrared Spectra as an Aid to Conformational Studies on Poly(Vinyl Chloride)

Fourier self-deconvolution has been used to enhance the detail present in the carbon-chlorine stretching region of the infrared spectra of three samples of polyvinyl chloride) covering a range of syndiotacticities. The results are of interest both as a good example of the scope and limitations of Fourier self-deconvolution and because they complement the information on the conformational structure of these polymers obtained previously by other peak-finding methods. In the former context, the results show that the use of varying degrees of over-deconvolution is advantageous in dealing with systems of overlapping bands having a range of half widths. With respect to the conformational structure of poly(vinyl chloride), the most significant conclusion is that the frequency of the B1 mode of long syndiotactic sequences is dependent on both polymer tacticity and the form in which the sample is examined. The implications of this finding for the use of curve fitting are discussed briefly.

Aggregation and association of polar molecules in low temperature matrices and in the gaseous phase

Abstract In the compounds: comparisons are made between the infrared spectra in the gas and matrix isolated phases. The alkene out-of-plane vibrations are sensitive to rotation, aggregation and conformational change. The twist and wag modes show consistent differences between the gas and matrix phases which are discussed in terms of different anharmonicities. These compounds were examined as proton acceptors in order to determine their mode of interaction with HCl, as indicated by the shift in the HCl stretching vibration. Some shifts in the vibrations of the base were also noted. Weak Van der Waals interactions were measured for CH 3 |CH 2 |CH 3 . Moderate strength hydrogen bonded complexes associated with σ and Π electrons were measured in other cases. Correlations for bifunctional molecules of the type: suggested that σ-bonded forms were preferred to Π-bonded forms although in certain cases the Π-bonded complex revealed a larger shift in the HCl stretching mode of the complex than the σ-bounded complex. In corresponding mixtures of monofunctional compounds σ and Π-complexes were simultaneously formed.

Matrix Isolation of Monomeric and Polymeric Forms of Acrylonitrile

The infrared spectrum of a sample isolated at low temperature in an inert matrix is simplified by certain factors but complicated by others. Studies of band splittings directed toward information on symmetry changes, rotational effects (gross or internal), isotope effects, or molecular interactions may be bedevilled by band multiplicites arising from self-aggregation to polymers or from trapping of samples in differing sites. There is an urgent need for better experimental and theoretical grasp of these phenomena.

Conformations of some αβ-unsaturated carbonyl compounds. Part V. Assignments of Raman and infrared spectra of methyl and [2H3]methyl acrylates and trans-crotonates

Raman and i.r. spectra of methyl and [2H3]methyl acrylates and trans-crotonates in the liquid state at room temperature and as frozen solids are presented together with vibrational assignments for particular conformers. Intensities of selected Raman bands at different temperatures lead to enthalpy differences (ΔH0± standard deviation) of 377 ± 56 cal mol–1 between the high and low energy forms of methyl trans-crotonate, in very good agreement with a previous result based on i.r. measurements.

Conformations of conjugated hydrocarbons. Part 1. A spectroscopic and thermodynamic study of buta-1,3-diene and 2-methylbuta-1,3-diene (isoprene)

Complete vibrational spectra of both buta-1,3-diene and 2-methylbuta-1,3-diene (isoprene) have been recorded over a range of temperatures. It is indicated by spectroscopic and thermodynamic methods that both compounds exhibit an equilibrium (s-trans⇌ s-cis) and the gauche-conformers are not present. Measurement of intensities of selected Raman bands of isoprene over a range of temperatures leads to a value of ΔH⊖± standard deviation of 4.59 ± 0.16 kJ mol–1 and a value of ΔS⊖ of –0.8 J K–1 mol–1. The complex methyl torsions of both conformers of isoprene have been assigned and the heights of the three-fold barriers calculated. Complete vibrational assignments for both conformers of isoprene have been made except for the s-cis C–C torsional frequency. Comparison between thermally derived thermodynamic functions and those derived by statistical mechanics leads to a value of ΔH⊖ of 10.4 kJ mol–1 for butadiene, and a concentration of less than 3% s-cis-butadiene at normal temperatures. Using the experimental value of ΔH⊖ obtained for isoprene new values of thermodynamic functions have been calculated over a range of temperatures, indicating a concentration of 11% s-cis-isoprene at normal temperature.

Conformations of some saturated carbonyl compounds. Part III. Infrared spectra of isomeric propyl and butyl formates and acetates

I.r. spectra of n-propyl, isopropyl, n-butyl, isobutyl, s-butyl, and t-butyl formates and acetates are reported. The effects on these spectra of cooling liquid films and of solidification are interpreted in terms of conformational equilibria. Evidence for the presence of rotational equilibria has been obtained in all the esters except for isopropyl formate, isopropyl, and t-butyl acetate. Conformers are considered to arise from rotation about the HCO–O bond in t-butyl formate (for which ΔH° has been estimated as 150 ± 20 cal mol–1) and from rotation about the O–alkyl bond in isobutyl formate (for which ΔH° has been estimated as 745 ± 40 cal mol–1).

Conformations of some saturated carbonyl compounds. Part II. Infrared spectra of ethyl formate, acetate, propionate, and n-butyrate, and [2H5]ethyl n-butyrate

Infrared spectra of ethyl formate, acetate, propionate, and n-butyrate, and [2H5]ethyl n-butyrate are reported. The effects on these spectra of cooling solutions in carbon disulphide to –100 °C and of solidification are reported and are interpreted in terms of conformational equilibria. Evidence for the presence of one pair of conformers, derived probably from rotation about the O–Et bond, has been found in all the esters examined; an enthalpy difference (ΔH°) of 315 ± 25 cal mol–1 between high- and low-energy conformers has been obtained for ethyl acetate. Evidence has also been obtained for the presence of an additional equilibrium in ethyl and [2H5]ethyl n-butyrates associated with rotations within the n-butyl chain.

Matrix effects in the infrared spectra of cyanoethene (acrylonitrile) and 2-chlorobuta-1,3-diene (chloroprene)

I.r. spectra of matrix-isolated forms of acrylonitrile and chloroprene are reported. The twisting mode of the vinyl group in the 970–990 cm–1 region reveals considerable splitting in each compound which is attributable to high and low energy forms. Measurements are reported on rate of change from high to low energy forms compared between 1 : 500 and 1 : 5 000 dilutions in argon in both compounds at 35 K. The results confirm the low energy form is a polymeric aggregate in the case of acrylonitrile but arises from an intramolecular effect in chloroprene. First-order rate constants were estimated for acrylonitrile and chloroprene at low and high dilutions. Assignments to high and low energy forms are presented between 400 and 3 500 cm– in each case and the structures of these forms are discussed.

Conformations of conjugated hydrocarbons. Part 2. A spectroscopic and thermodynamic study of cis- and trans-penta-1,3-diene

Vibrational spectra of both cis- and trans- penta-1,3-diene have been recorded over a range of temperatures for three phases. Complete vibrational assignments for both compounds have been made and new thermodynamic functions calculated over a range of temperatures by statistical methods. No spectroscopic or thermodynamic evidence of a high energy conformer for either compound has been found. Assignment of torsions leads to values (in kJ mol–1) for cis- and trans- penta-1,3-diene respectiveiy of 12.3 and 10.4 for the three-fold barrier height V3 and 133 and 120 for the two-fold barrier parameter V*.