Near Infrared Investigation of Interamide Hydrogen Bonding in Aqueous Solution.

Various methods have been used by different investigators t.o estimate the stability of amide to amide hydrogen bonds in aqueous solution. Schellman (1) and Kauzmann (2) have discussed the thermodynamic properties of t)he C=O--HN bond in water wit,h the use of aqueous urea solutions as a model system. From studies on heats of dilution of urea in water, Schellman concluded that this bond has an enthalpy of 1.5 kcal per mole in aqueous solution (1). Hibberd and Alexander (3) concluded from monolayer studies of octadecylacetamide that the energy of the C==O--HN hydrogen bond in the presence of water is close to 2 kcal per mole. Klotz and Franzen (4) studied N-methylacetamide in aqueous solution by near infrared methods and concluded that the enthalpy of amide to amide hydrogen bond formation in aqueous solution is close to zero. NBmethy and Scheraga (5) have suggested that the possible presence of a hydrophobic bond furnishes an alternative explanation for the latter observation, because the positive enthalpy of formation of the hydrophobic bond may cancel a negative enthalpy contribution made by the hydrogen bond. A considerable amount of near infrared work, mostly in the 3 p spectral region, has been carried out in nonaqueous solutions. Tsuboi calculated an enthalpy of approximately -10 kcal per mole for the dimerization of b-valerolactam in carbon tetrachloride solution (6), -3.5 kcal per mole for N-methylacetamide in carbon tetrachloride solution (7), and 1.6 kcal per mole in chloroform solution (7). The latter result, agrees well with conclusions by Klemperer et al. (8). Mizushima et al. (9) have pointed out that in ether solution the stability of amide to amide hydrogen bonds is very low because of hydrogen bonding between the amide and ether. Lord and Porro, and Isaacson have reinvestigated the dimerization of lactams with more recent instrumentation (10, 11). Because of the absence of a t,endency to form long chain polymers, lactams are particularly suited as model compounds for hydrogen bonding studies (6, 8, 10). Lord and Porro suggest (10) t.hat earlier results (6) might be in error and report a somewhat lower AH value for the dimerization of +caprolactam in carbon tetrachloride solution. The present communication describes and discusses results obtained in aqueous solution with &valerolactam as a model compound. Spectroscopic measurements were carried out in the spectral region around 1.5 p, which offers experimental advantages over the 3 p region. EXPERIMENTAL PROCEDURE