Control of conformers combining cooling by supersonic expansion of seeded molecular beams with hexapole selection and alignment: experiment and theory on 2-butanol

Abstract

Selection and alignment of rotamers and, more in general, of conformers in the gas phase is a challenge that we tackle experimentally by supersonic expansion of seeded molecular beams and hexapolar electrostatic fields with quadrupole mass detection. The studied system involves the nine conformers of the asymmetric-top molecule 2-butanol, which coexist because of nearly free rotations around a CC and a CO bond. From the measured time-of-flight of a 2-butanol supersonic molecular beam seeded in either He or Ar, the corresponding velocity distributions are obtained. The different nature and masses of the seeding gas decrease selectively the vibrational temperature and determine the population of the conformers, which is assessed on the basis of their statistical distribution, derived from high level accompanying quantum mechanical calculations. The use of a hexapolar electrostatic field permits us to induce a variation of the population distribution as a function of the applied voltage and of the selective focusing and alignment of the conformers. A technique, recently developed for treating asymmetric tops and involving extensive trajectory simulations, is applied to obtain the link between the focusing curves, i.e. the dependence of the beam intensity on the hexapole voltage, and the conformers' populations and alignment. Perspectives are provided for photo- and stereo-dynamics experiments, particularly appealing also on account that 2-butanol is the simplest chiral alcohol.

Rotational state-selection and alignment of chiral molecules by electrostatic hexapoles

“…This molecule presents a complex conformer manifold because of the torsion around the second and the third carbon, which originates rotamers T, G+ and G-, and the torsion around the second carbon and the oxygen, originating rotamers t, g+ and g-; the resulting conformers (nine) are given by the combination of these rotamers (for details see reference [14]). Each conformer possesses characteristic rotational constants, dipole moments and related components, determining different aligning process, as evidenced by the focusing curves (Figure 2, central panel) of the nine conformers of the molecular beam seeded in He (5 % in 2-butanol).…”

“…Electrostatic hexapole is composed by six rods in aluminum, of length typical included between 50 cm to 2 m. In Figure 1a, we report a scheme of the experimental apparatus used to align propylene oxide 9 and 2-butanol 14 . FIGURE 1. a) A typical scheme of the experimental setup.…”

“…The picture in the left shows the permanent dipole moment parallel to the molecular axis, while in the picture in the right we illustrate of the perpendicular arrangement. (Adapted from references [9,14]). …”

“…Figure 2c shows the change of intensity of detected Br photofragment with (red line) and without (black line) application of the orienting field. (Adapted from references [9,12,14])…”

“…They were initially applied to linear and symmetric-top molecules, while more recently they have been successfully applied to molecules of higher complexity such as the chiral molecules propylene oxide 9,10 (for a characterization of the reaction pathways of propylene oxide see reference [11]) and 2-bromobutane 12 (see also reference [13]). Hexapolar technique has been also employed for the first time in the selection of conformers of 2-butanol 14 . An important development is the combination of hexapolar electric fields (non-homogenous) with DC (Direct Current) orienting fields (homogenous) in order to obtain molecular orientation.…”