Knowledge Bank

A thorough examination of the microwave spectrum of mercaptoethanol ($HSCH2CH2OH$) has led to the identification of the stable rotameric conformation. Because of the three degrees of internal rotation-conformational freedom, an a priori estimate of the stable molecular conformation(s) is a very challenging task. By means of data from five isotopic species, our experimental results lead unambiguously to the correct molecular structure, which will be described in detail. Briefly, the stable conformation shows that the overall potential energy minimum occurs when each of the individual rotameric moieties occupies a gauche position. The most interesting feature is that the hydroxyl proton is folded back over the heavy atoms so chat it achieves a rather close proximity to the ``backside” of the sulfur atom. Our structural data for the $H\cdots S$ distance yield a value of 2.56 $\backslash AA$, which is considerably shorter than the van der Waals’ non-bonding distance of 3.05 $\backslash AA$. Thus the results argue strongly for stabilization via a non-linear hydrogen bond-type interaction between the sulfur atom and the hydroxyl proton. A careful spectral search has failed to turn up evidence for a second stable mercaptoethanol conformation. However, because of the high ground state spectral-line density and the presence of several sets of satellite spectra from low-lying vibrational states, it would be very difficult to observe any species lying more than about 800-900 cal/mole above the observed rotamer. For the normal isotopic form of the observed rotamer, the rotational constants (in MHz) are: $A=12152.64,B=3467.08,C=2965.55$. Other data will be presented and discussed in detail. In particular, we shall describe the use of radio frequency-microwave double resonance experiments, which were essential for assignment of the various isotopic species.