Electronic structure calculations were carried out to study the various isomers of formula C3H6O, as a part of our current quantum chemical and dynamical approaches to intra- and intermolecular kinetics for the CnH2nO (n = 1, 2, 3) molecules. The usefulness of the GRRM (global reaction route mapping) program developed by Ohno and Maeda in predicting the structure of all isomers and of the transition states connecting them is fully exploited. All the isomers are identified as local minima on the MP2/CC-PVDZ potential energy surface. Acetone is the most stable isomer. In increasing order of stability the others are propanal, 2-propenol, 1-propenol, allyl alcohol, methyl vinyl ether, cyclopropanol, propylene oxide, and oxetane. Various isomerization paths connecting them are identified. All the transition states are fully characterized using intrinsic reaction coordinate calculations. The isomerization reactions may proceed through a single step or involve an intermediate species which is either a carbene or...

Quantum Chemistry of C3H6O Molecules: Structure and Stability, Isomerization Pathways, and Chirality Changing Mechanisms†

The chiral monosubstituted derivatives of spiropentane, spiropentylcarboxylic acid methyl ester, 1, and spiropentyl acetate, 2, have been synthesized in optically active form. Configurational and conformational analysis of 1 and 2 has been carried out using infrared (IR) and vibrational circular dichroism (VCD) spectroscopies. Analysis of the experimental IR and VCD spectra has been carried out using ab initio density functional theory (DFT). For both 1 and 2, DFT predicts two populated conformations. Comparison to experiment of the conformationally averaged IR and VCD spectra of 1 and 2, predicted using DFT, provides unequivocal evidence of the predicted conformations and yields the absolute configurations R(−)/S(+) for 1 and R(+)/S(−) for 2. These absolute configurations are consistent with the R(−)/S(+) absolute configuration of spiropentylcarboxylic acid, assigned previously via X-ray crystallography of its α-phenylethylammonium salt.

Configurational and conformational analysis of chiral molecules using IR and VCD spectroscopies: spiropentylcarboxylic acid methyl ester and spiropentyl acetate.

Photoelectron spectra of cyclopropane and cyclopropene compounds.

Two conformers of cyclohexanol and the cyclohexanol–water adduct have been characterized in a jet expansion using rotational spectroscopy. In the gas phase, cyclohexanol adopts an equatorial position for the hydroxyl group, with the two conformers differing in the orientation of the hydroxylic hydrogen, either gauche or trans with respect to the aliphatic hydrogen at C(1). Axial cyclohexanol was not detected in the jet. The transitions of the gauche conformer are split into two component lines due to the tunneling effect of the O–H internal rotation, which connects two equivalent gauche minima. The tunneling splitting in the vibrational ground state has been determined to be ΔE0+0− = 52(2) GHz. From this splitting, the inversion barriers connecting the two equivalent gauche conformers have been determined using a flexible model to be B2 = 377 cm−1. A single isomer is detected for the cyclohexanol–water dimer, in which the water molecule acts as a proton donor to the equatorial gauche ring. The presence of torsional tunneling in the adduct suggests a concerted large-amplitude-motion in which the internal rotation in the ring is accompanied by a torsion of the water molecule, to produce an equivalent enantiomer. The torsional tunneling in the adduct is reduced to ΔE0+0− = 32.7(4) GHz and the potential barrier in the complex increases to B2 = 494 cm−1.

Internal dynamics of cyclohexanol and the cyclohexanol–water adduct

Context. Theories of the origins of life propose that early cell membranes were synthesised from amphiphilic molecules simpler than phospholipids, such as fatty alcohols. The discovery in the interstellar medium (ISM) of ethanolamine, the simplest phospholipid head group, raises the question whether simple amphiphilic molecules are also synthesised in space. Aims. We investigate whether precursors of fatty alcohols are present in the ISM. Methods. To do this, we have carried out a spectral survey at 7, 3, 2 and 1mm towards the Giant Molecular Cloud G+0.693-0.027 located in the Galactic centre using the IRAM 30m and Yebes 40m telescopes. Results. Here, we report the detection in the ISM of the primary alcohol n -propanol (in both conformers Ga - n -C 3 H 7 OH and Aa - n - C 3 H 7 OH), a precursor of fatty alcohols. The derived column densities of n -propanol are (5.5 ± 0.4) × 10 13 cm − 2 for the Ga conformer and (3.4 ± 0.3) × 10 13 cm − 2 for the Aa conformer, which imply molecular abundances of (4.1 ± 0.3) × 10 − 10 for Ga – n -C 3 H 7 OH and of (2.5 ± 0.2) × 10 − 10 for Aa - n -C 3 H 7 OH. We also searched for the AGa conformer of n -butanol [ AGa - n -C 4 H 9 OH] without success, yielding an upper limit to its abundance of ≤ 4.1 × 10 − 11 . The inferred CH 3 OH:C 2 H 5 OH:C 3 H 7 OH:C 4 H 9 OH abundance ratios are 1:0.04:0.006: ≤ 0.0004 towards G+0.693-0.027, that is, they decrease roughly by one order of magnitude for increasing complexity. We also report the detection of both syn and anti conformers of vinyl alcohol, with column densities of (1.11 ± 0.08) × 10 14 cm − 2 and (1.3 ± 0.4) × 10 13 cm − 2 , and abundances of (8.2 ± 0.6) × 10 − 10 and (9.6 ± 3.0) × 10 − 11 , respectively. Conclusions. The detection of n -propanol, together with the recent discovery of ethanolamine in the ISM, opens the possibility that precursors of lipids according to theories of the origin of life, could have been brought to Earth from outer space.

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Precursors of fatty alcohols in the ISM: Discovery of n-propanol

The Microwave spectra of cyclopropanol and cyclopropan-[2H1]-ol slow the gauche-conformation is the dominant species present in the vapour at 293K. No evidence for the existence of the trans-conformation has been obtained. The spectra of this rotamer show transitions associated with dipole components in all three inertial axes, the b-type transitions, which form an appreciable part of the spectra, being split into doublets separated by ≈ 8200 MHz in the normal form and by ≈ 326 MHz in the [2H1] form an account of contributions of the torisonal frequence 0–â†� 0+ to these absorptions. Many lines of the normal form depart from rigid rotor behaviour. An analysis has been made in terms of nine parameters for this form, and confirmed by numerous double reasonances. The 0–â†� 0+ excitation energy for this species is 4115.25(42) MHz. Spectra of gauche-cyclopropan-[2H1]-ol can be fitted to a single set of rotational constants common to the 0+ and 0– states and a 0–â†� 0+ excitation energy of 163.74(1.80) MHz. The dipole moment components of the [2H12] from are determined from Stark effect measurements as : µa= 1.58(10)× 10–30 C m, [0.48(3)D]′µb= 3.75(50)× 10–30 C m, [1.12(15) D]; µc= 2.70(57)10–30 C m, [0.81(17) D]; and µtotal= 4.88(67)× 10–30 C m, [1.46(20) D];. Substitution coordinates of the hydroxyl hydrogen accord with an equilibrium dihedral angle of internal rotation, α, of 106°± 5° relative to a zero at the trans-conformation. The 0–â†� 0+ energy differences are used in conjunction with this value of α and the assumption that no stable trans-from occurs, to yield an approximate potential function for internal rotation. The cis-barrier separating the gauche-minima is 7.9(1.5)kJ mol–1, while the barrier via the trans-conformation is 17(6) kJ mol–1, while the barrier via the trans-conformation is 17(6) kJ mol–1.

David Norbury

Papers

Microwave spectrum, dipole moment and internal rotation potential function of gauche-cyclopropanol

Microwave spectrum, dipole moment and internal rotation potential function of gauche-cyclopropanol