Summary. Azrak and Wilson measured the microwave spectra of 9 isotopomers of 2-chloroethanol and observed only the gauche-gauche conformer. They inferred from the well defined r_s structure and measurements of the chlorine quadrupole coupling constant that the conformation is a consequence of a an interaction between the chlorine and the hydroxyl hydrogen and suggested that a dipole-dipole model accounts for the coupling. Ammeningen, Fernholt, and Kveseth measured its electron diffraction at 5 temperatures. Although they did not characterize its structure with the same precision as Azrak and Wilson, they observed two conformers: the energetically more stable gauche-gauche conformer and a second conformer with a anti (s-trans) conformation with respect to the O-C-C-Cl dihedral angle. From an integration of characteristic peaks in the radial distribution function, they showed that the standard enthalpy and entropy of the second conformer was higher by 2.4 +/- 0.2 kcal/mol and 2.8+/- 0.3 ca/K-mole, respectively. They did not determine the orientation of the hydroxyl group in the second conformer. The spectroscopic results are supported by modeling calculations. A conformation search with the Merck Molecular Force Field (MMFF94) showed that the gauche-gauche conformer is energetically the stablest. The next conformer whose H-O-C-C and O-C-C-Cl dihedral angles are both anti (s-trans) is higher in energy by 1.4 kcal/mole.

Azrak and Wilson cautiously propose that 2-chloroethanol exhibits hydrogen bonding and cite the following structural evidence taken almost verbatim from Table XVI in their thorough paper:

• the gauche-gauche conformer is the dominant species in the gas phase
• the O-C-C-Cl dihedral angle is smaller than the corresponding dihedral angle in 1,2-dichloroethane
• the O...Cl distance is approximately the sum of the van der Waals radii
• the distance between the hydroxyl hydrogen and the Cl atom is about 0.4 Å less than the sum of the van der Waals radii
• the O-H bond is 5-6% shorter than in ethanol and methanol
• the O-H and C-Cl bond are parallel to within 2º
• the C-O-H bond angle is about 3º less than in methanol
• the CO torsional frequency is higher than in methanol and ethanol (the torsional frequency was obtained from measurements of the intensities of assigned vibrational satellites in the microwave spectrum)

References

1. R. G. Azrak and E. B. Wilson, Jr., "Microwave Spectra and Intramolecular Hydrogen Bonding in the 2-Haloethanols: Molecular Structure and Quadrupole Coupling Constants for 2-Chloroethanol and 2-Bropmoethanol", J. Chem. Phys., 52, 5299-5316 (1973).

2. K. Almenningen, L. Fernholt, and K. Kveseth, "An Electron Diffraction Study of the anti-gauche Raio as a Function of Temperature for Ethylene Chlorohydrin, 2-Chloroethanol", Acta Chem. Scand., Part A, 31, 297-305 (1977).

SYBYL mol2 and Brookhaven pdb coordinate files of structures generated by various means
(You will be able to view these structures with Netscape or Explorer if you have installed Chime and click on the entry for the pdb file.)

• the microwave structure of Azrak and Wilson.
  Brookhaven pdb file of the same
  They assumed a C-Cl distance of 1.789 Ångstrom. This result was transferred from the microwave structure of chloroethane. They also assume standard values for the C-H bond length and the H-C-H C-C-H bond angles of 1.093 Ånstrom, 108.67º, and 111.40º, respectively.

• electron diffraction structure of the gauche-gauche conformer
  Brookhaven pdb file of the same
  Almmeningen et al. assumed a H-O-C-C dihedral angle of 60º. Their structure of the second conformer can be gnerated by changing the O-C-C-Cl torsional angle to 180º. Their value for the H-O-C bond angle of 125º is physically unreasonable and should be treated as an estimate.

• structure calculated from molecular mechanics with the MMFF94 force field
  .Brookhaven pdb file of the same
  This is the structure of the lowest energy conformer. It compares very well with the microwave result.

• molecular mechanics structure with the Tripos force field
  Brookhaven pdb file of the same

• structure from semi-empirical quantum mechanics and the AM1 Hamiltonian
  Brookhaven pdb file of the same
  Reasonable agreement is obtained for the bond lengths and bond angles. However, the dihedral angles differ significantly from the microwave values. As a result, the AM1 value for the distance between the hydroxyl hydrogen and the chlorine is off by 0.2 Å.

• result from ab initio quantum mechanics, Hartree-Fock with a 3-21G* basis set
  Brookhaven pdb file of the same
  The torsion angles are better but still differ from the microwave result by several degrees. Replicating the fine details of the microwave structure will be a challenge for the quantum mechanic.

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  Tabular Results of Structural Parameters for the Gauche-Gauche Conformer from Various Methods

  method	microwave	electron diffraction	MMFF94	Tripos	AM1	HF/3-21G*
  O-C-C-Cl dihedral angle	-63.2º	-67.3º	-65.8º	-59.7º	-73.8º	-65.8º
  C-C-O-H dihedral angle	58.4º	ca. 60º	56.8º	60.0º	62.0º	65.7º
  Cl..O distance in Å	3.119	3.158	3.129	3.055	3.244	3.163
  Cl...H distance in Å	2.611	2.967	2.634	2.619	2.841	2.781
  C-O-H bond angle	105.8º	125º	106.8º	110.0º	107.4º	110.4º
  C-C-O bond angle	112.8º	111.5º	111.2º	110.2º	112.4º	111.7º
  C-C-Cl bond angle	110.1º	110.4º	110.3º	110.1º	112.0º	110.1º
  O-H bond distance in Å	1.008	1.034	0.976	0.950	0.966	0.967
  C-O bond distance in Å	1.411	1.421	1.425	1.432	1.412	1.432
  C-C bond distance in Å	1.519	1.516	1.522	1.547	1.517	1.525
  C-Cl bond distance in Å	1.789	1.768	1.785	1.768	1.756	1.820
  dihedral angle between H-O and C-Cl bonds	-1.9º	º-13.9	-5.7º	0.3º	-7.4º	-0.4º
  electric dipole moment in D	1.78			1.23	1.82

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  Observed Rotational Constants in MHz for the Isotopomers of 2-Chloroethanol
  (from the paper by Azrak and Wilson, uncertainties range from 0.02 to 0.06 MHz)
  (35-12-16-1 stands for ³⁵Cl¹²CH₂¹²CH₂¹⁶O¹H, etc.)

  Isotopomer	A0 (MHz)	B0 (MHz)	C0(MHz)
  35-12-12-16-1	12747.976	3505.777	2981.276
  37-12-12-16-1	12705.937	3423.524	2919.510
  35-12-12-16-2	12160.334	3480.553	2930.052
  35-12-12-18-1	12432.189	3364.191	2863.503
  35-13-12-16-1	12449.228	3503.936	2966.171
  35-12-13-16-1	12631.650	3465.039	2949.643
  37-12-12-16-2	12121.532	3398.344	2869.546
  37-12-12-18-1	12388.274	3284.147	2803.200
  35-12-12-18-2	11902.826	3341.751	2818.484

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  Radial Distribution Functions from the Paper by Almenningen et al.
  • Experimental radial distribution curves at 37º (A), 125º (B), 170º (C), 200º (D), and 250º (E). The feature at 4.0 Å that grows with an increase in temperature is assigned to the anti conformer.

    

  • The radial distribution curve calculated from the structural parameters for the case of 15 mole-% of the anti conformer.

    

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  last changed 3 Jan. 2003, WES