Conformational Stabilization by Intramolecular OH···S and CH···O Interactions in 2-(Methylthio)ethanol. Matrix-Isolation Infrared Spectroscopy and ab Initio MO Calculations

Abstract

The conformational stability of 2-(methylthio)ethanol has been studied by matrix-isolation infrared spectroscopy and ab initio MO calculations. In an argon matrix, the conformer with gauche-gauche-gauche′ (GGg′) around the CS-C-C-OH bonds is the most stable and the G′Gg′ conformer is the second most stable. These and the TGg′ conformers are stabilized by intramolecular hydrogen bonding between the hydroxyl hydrogen atom and the sulfur atom. The relative strength of the hydrogen bonding in these conformers is in the order GGg′ > TGg′ > G′Gg′, as estimated from the nonbonded OH‚‚‚S distance. In the G′Gg′ conformer, an additional intramolecular interaction between the methyl hydrogen atom and the hydroxyl oxygen atom is involved. The relative strength of this 1,5-CH‚‚‚O interaction in the G′Gg′ conformer is the least among the three relevant conformers with G′G around the CS-C-COH bonds. The calculated results indicate that the geometry of this conformational form is considerably distorted so that it is simultaneously accessible to both of the interactions. This geometry is, however, not best suited for the respective interactions to be the most effective. The results for 2-(methylthio)ethanol have been compared with those for 2-methoxyethanol, in which the analogous intramolecular interactions are involved. The present study emphasizes the importance of the intramolecular interactions in the conformational stabilization of 2-(methylthio)ethanol and other relevant compounds.

Strength of the CαH··O Hydrogen Bond of Amino Acid Residues

“…This elongation behavior contrasts markedly with the contractions that occur in the CH⅐⅐O H-bonds of all amino acids as well as F 2 HCH. This seemingly opposite behavior, observed in a number of CH⅐⅐O bonds (46,(62)(63)(64)(65)(66)(67), has nevertheless been demonstrated to be consistent with the characterization of the CH⅐⅐O interaction as a true H-bond (54). There is no clear pattern concerning the magnitude of this contraction.…”

Raman microspectroscopy study of structure, dispersibility, and crystallinity of poly(hydroxybutyrate)/poly(l-lactic acid) blends

“…One of the most important findings in our studies is that crystalline structures of PHB and its copolymers, poly(hydroxybutyrate-co-hydroxyhexanoate) (P(HB-co-HHx)) have C-H/OaC hydrogen bonds between CH 3 groups of one helical structure and CaO groups of another helical structure. This unique C-H/OaO hydrogen bond was investigated by IR and Raman spectroscopy and quantum chemical calculations [38][39][40][41]. We also investigated interactions of poly(Llactide) (PLLA) and poly(L-lactide)/poly(D-lactide) (PLLA/ PDLA) stereocomplex by means of IR spectroscopy [42].…”

Rotational spectra, conformational structures, and dipole moments of thiodiglycol by jet-cooled FTMW and ab initio calculations

“…An additional -CHÁ Á ÁO intramolecular interaction between the methyl hydrogen and the hydroxyl oxygen atom will also be possible for the 'compact' conformers, and their cooperative effect may give rise to high stability of this particular conformation. Such 'compact' conformer structures were observed in the matrix isolation experiment for MTE [3] and were predicted by ab initio calculations for HOEES as low-energy conformers and yet we failed to detect these in the FTMW jet condition for HOEES. One explanation for this could be the uncertainty in calculated ab initio electronic energy for large compounds like HOEES.…”

“…In this conformer, one of the hydroxyl hydrogen forms a hydrogen bond to one of the lone pairs of electrons on the sulfur atom (-OHÁ Á ÁS), while the hydrogen on the second hydroxyl forms another type of hydrogen bond (-OHÁ Á ÁO) with the oxygen atom on the first hydroxyl group. Because thiodiglycol has a longer backbone chain in comparing with HOEES [4] and MTE [3], the -OHÁ Á ÁO interaction becomes much stronger, which is indicated by the smaller -OHÁ Á ÁO hydrogen-bonding distance of only about 1.90 A at MP2 ¼ Full/6-311G** level ab initio calculation. This strong -OHÁ Á ÁO hydrogen-bonding interaction is thought to explain why this ''closed'' structure lies at the lowest energy at our current calculation (MP2 ¼ Full/6-311G**).…”

“…The conformation of the molecules in which an intramolecular hydrogen bond is formed between a hydroxyl hydrogen atom and a sulfur atom has been studied for several compounds that contain the -SCH 2 CH 2 OH group. Previous microwave spectroscopic studies have shown that the molecules of 2-mercaptoethanol (HSCH 2 CH 2 OH) [1] and 2-(methylthio)ethanol (MTE) (CH 3 SCH 2 CH 2 OH) [2,3] adopt a conformation in the gas phase that is stabilized by intramolecular -OHÁ Á ÁS hydrogen bond. This characteristic has also been confirmed by our recent studies of 2-(ethylthio)ethanol (HOEES) (CH 3 CH 2 SCH 2 CH 2 OH).…”