# Hydration and Conformation of 2‑Ethylfuran Explored by Microwave Spectroscopy

## TL;DR

This study uses microwave spectroscopy to explore the structure and hydration of 2-ethylfuran, revealing how water interacts with the molecule.

## Contribution

The paper reports new structural insights into the hydration of 2-ethylfuran and its conformational changes.

## Key findings

- The hydrogen bond between water and 2-ethylfuran is 2.0950(42) Å in length.
- The hydrogen bonding angle is 167.69(16)°, indicating a weak, non-linear interaction.
- Hydration causes a reversal in the energy ordering of 2-ethylfuran conformers.

## Abstract

Rotational spectra of one conformer of a 2-ethylfuran···H2O complex and two conformers of the isolated 2-ethylfuran
molecule have been recorded by chirped-pulse Fourier transform microwave
spectroscopy. The species were probed while entrained within a gas
sample undergoing supersonic expansion. The spectra of five isotopologues
of the complex have been analyzed to yield rotational (A
0, B
0, C
0) and centrifugal distortion constants (D

J
, D

JK
, d
1) allowing structural parameters
to be determined by fitting to the experimentally determined moments
of inertia. Quantum chemical calculations have been performed to support
the interpretation of the experimental results and gain further insights.
2-Ethylfuran is shown to adopt C1 symmetry within the observed
conformer of 2-ethylfuran···H2O with the
length of the hydrogen bond, r(Hb···O1),
which connects H2O with 2-ethylfuran determined to be 2.0950(42)
Å in the r
0 geometry. The geometry
of the hydrogen bonding interaction deviates from linearity such that
the ∠(Ow–Hb···O1)
angle (where Ow and O1 are the oxygen atoms of water and
furan, respectively) is 167.69(16)° in the r
0 geometry. The experimental and theoretical results thus
imply the presence of a weak interaction between the oxygen of H2O and the ethyl group within the observed conformer of 2-ethylfuran···H2O. Evidence is presented to suggest that the Cs conformer of the isolated 2-ethylfuran molecule is lower in energy
than the C1 conformer implying that the energy ordering
of the two lowest-energy conformers of 2-ethylfuran reverses when
the isolated molecule is hydrated by a single H2O molecule.