Conformational Equilibrium in (Cyclopropylmethyl)acetylene: a Microwave Spectroscopy and ab Initio Calculation Study

Abstract

The microwave spectrum of (cyclopropylmethy1)acetylene (a newly prepared compound) was investigated in the frequency range 28-40 GHz. Both gauche and cis conformers were observed, the latter being 64 f 30 cm-' lower in energy. The geometries of these conformers were obtained combining the ab initio and microwave spectroscopy results. A large-amplitude motion (the methylacetylene group torsion) was studied by assigning the rotational spectra of several torsionally excited states.

Conformational Studies of Cyclopropylmethylacetylene from Temperature Dependent FT-IR Spectra of Xenon Solutions and Ab Initio Calculations

“…This value should be near the value for the gas − because both conformers have similar sizes and polarities. The value of the enthalpy implies at room temperature a gas-phase equilibrium mixture of 48% cis conformer and 52% gauche rotamer, which is a slightly smaller amount of the gauche conformer than previously suggested from the microwave data 3 Temperature and Intensity Ratios from Conformational Study of Cyclopropylmethylacetylene T (°C)1000/ T (K) I 1052 / I 1045 I 1021 / I 1019 I 848 / I 828 I 575 / I 502 −55 4.58 0.758 0.778 − 0.236 −60 4.69 0.786 0.826 0.850 0.233 −65 4.80 0.786 0.856 0.876 0.241 −70 4.92 0.788 0.881 0.890 − −75 5.05 0.806 0.874 0.886 0.241 −80 5.18 0.817 0.920 0.898 0.268 −85 5.31 0.878 0.955 0.906 0.288 −90 5.46 0.894 0.995 0.922 0.308 −95 5.61 0.908 1.008 0.970 0.300 −100 5.78 0.932 1.029 1.033 − Δ H a (cm -1 ) 124 ± 10 157 ± 10 103 ± 15 203 ± 30 a Average value of Δ H is 147 ± 14 cm -1 (1.76 ± 0.16 kJ/mol) with the cis conformer the more stable form. …”

“…As a continuation of our conformational studies of these three-membered rings, we have investigated the vibrational spectrum of cyclopropylmethylacetylene, c -C 3 H 5 CH 2 CCH. There has been a recent conformational study of this molecule from a microwave investigation and ab initio calculations . It was concluded that the cis conformer was more stable than the gauche rotamer by 64 ± 30 cm -1 (0.77 ± 0.36 kJ/mol) from relative intensity measurements of the microwave transitions .…”

Conformational stability, r0 structural parameters, barriers to internal rotation, vibrational spectra and ab initio calculations of c-C3H5SiH2CH3

“…However, later studies [4][5][6] conclusively showed that the predominant form was the gauche conformer and from variable temperature studies of the infrared spectra of xenon solutions [7] the enthalpy difference was determined to be 274 ± 21 cm À1 (3.28 ± 0.25 kJ mol À1 ) with only 12% of the cis form present at room temperature. With other X substituents such as cyano ACN [8,9], and ethynyl AC"CH [10,11] groups the percent of the gauche form present at ambient temperature dropped to 72% and 52%, respectively. Some investigators [10] concluded that the abundance of the cis conformers parallels the increase of the electronegativity [12] of the substituents for the monosubstituted methylcyclopropane molecules.…”

“…With other X substituents such as cyano ACN [8,9], and ethynyl AC"CH [10,11] groups the percent of the gauche form present at ambient temperature dropped to 72% and 52%, respectively. Some investigators [10] concluded that the abundance of the cis conformers parallels the increase of the electronegativity [12] of the substituents for the monosubstituted methylcyclopropane molecules. However, more recent conformational studies [13] of fluoromethylcyclopropane where only 12% of the cis form was found at ambient temperature indicate that the electronegativity can not be the dominant factor controlling the abundance of the cis form.…”

Conformational stability, r0 structural parameters, barriers to internal rotation, vibrational assignments and ab initio calculations of c-C3H5GeH2CH3

“…Initially, it was proposed [1] that the electronegativity of the substituent has the most pronounced effect on the relative amounts of the conformational isomers present since the substitution of the ethynyl and cyano-groups had a relatively larger amount of the cis (syn) form present compared to the amount of the gauche conformers. These groups have considerably larger electronegativities [2] i.e.…”