Conformational analyses of 2,3-dihydroxypropanoic acid as a function of solvent and ionization state as determined by NMR spectroscopy

Determining the Ionization Constants of Organic Acids Using Fluorine Gauche Effects

Using NMR spectroscopy, the conformational studies of two fluoroethylsulfonamides (N-(2-fluoroethyl)-p-tolylsulfonamide (1) and N-(2-fluoroethyl)trifluoromethanesulfonamide (2)) revealed that fluorine gauche effects are a function of ionization. While acids 1 and 2 exhibited gauche effects (with gauche populations of 87% and 92% in DMSO-d₆, respectively), their anions, on the other hand, preferred the anti conformer (with gauche populations of 35% and 55%, respectively). The ability of these compounds to undergo conformational changes as a function of ionization enabled their application as molecular probes (standards) for determining the acidity (pK_a) of organic compounds in DMSO, which was achieved with the aid of the equation K_rel = [(³J_AH - ³J_obs)/(³J_obs - ³J_A)]², where K_rel is the ratio of ionization constants of two acids (standard and test acids), ³J_AH and ³J_A are the proton-fluorine vicinal coupling constants of the standard acid and its anion, respectively, and ³J_obs represents the proton-fluorine vicinal coupling constant observed at the midpoint of an acid-base equilibrium. As a means of demonstrating its utility, this equation accurately calculated the ionization constants (K_a) of several organic compounds in DMSO. Taking advantage of fluorine's unique gauche effect as a strategy for molecular design has the potential to open a new frontier in structural chemistry.

Conformational analysis of N,N,N-Trimethyl-(3,3-dimethylbutyl)ammonium iodide by NMR spectroscopy: a sterically hindered trans-standard

A predominantly trans-1,2-disubstituted ethane system - N,N,N-trimethyl-(3,3-dimethylbutyl)ammonium iodide - is of particular interest for conformational analysis, because it contains both an organic and a highly polar substituent, making it soluble and thus applicable to study in a large variety of solvents. The fraction of the trans conformer of this molecule in a wide range of protic and aprotic solvents was determined by the nuclear magnetic resonance proton couplings to be approximately 90%, in contrast to the previously assumed 100%. The consistently strong preference of the trans conformation should establish N,N,N-trimethyl-(3,3-dimethylbutyl)ammonium iodide as a possibly useful 'trans-standard' in conformational analysis, much more so than 1,2-ditert-butylethane, which has a poor solubility in many solvents.

Conformational preferences of 3-(dimethylazinoyl)propanoic acid as a function of pH and solvent; intermolecular versus intramolecular hydrogen bonding

The conformational equilibrium of 3-(dimethylazinoyl)propanoic acid (DMAPA, azinoyl = N(+)(O(-)) has a weak pH-dependence in D(2)O, with a slight preference for trans in alkaline solutions. The acid ionization constants of the protonated amine oxide and carboxylic functional groups as determined by NMR spectroscopy were 7.9 x 10(-4) and 6.3 x 10(-6), respectively. The corresponding value of K(1)/K(2) of 1.3 x 10(2) is not deemed large enough to provide experimental NMR evidence for a significant degree of intramolecular hydrogen bonding in D(2)O. Conformational preferences of DMAPA are mostly close to statistical (gauche/trans = 2/1) in other protic solvents, e.g., alcohols. However, the un-ionized form of DMAPA appears to be strongly intramolecularly hydrogen-bonded and gauche in aprotic solvents.

Conformational analysis of 3-(trimethylsilyl)propionic acid by NMR spectroscopy: an unusual expression of the beta-silyl effect

The rotational freedom of the carbon-carbon single bonds of 1,2-disubstituted ethanes affords the possibility of these compounds existing as a rapidly interconverting mixture of conformers in solution. The conformational preferences of one such compound, 3-(trimethylsilyl)propionic acid, and its anion were studied in water, dimethyl sulfoxide, methanol, ethanol, isopropyl alcohol, tert-butyl alcohol, tetrahydrofuran, and toluene with 1H NMR spectroscopy. The conformational preferences were determined from the vicinal proton-proton coupling constants between the hydrogen nuclei of the CH(2)CH(2) group with the aid of the Altona equations to derive the equilibrium anti and gauche percentages of rotamers from the averaged NMR-time scale couplings. Conformational analyses of 4,4-dimethylpentanoic acid and its anion as well as 2-(trimethylsilyl)ethanesulfonate anion were also conducted to compare the relative structural influences on the conformational preferences of silicon and carbon.