Uttam Pal - Cα-H Carries Information of Hydrogen Bond Involving Geminal Hydroxyl Group: A Case Study With Hydrogen Bonded Complex of HFIP and Tertiary Amines

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      Pal, Uttam, Sen, Sudeshna, Maiti, Nakul Chandra -

      Abstract: Experimental measurement of   contribution of H-bonding to intermolecular and intramolecular   interactions that provide specificity to biological complex   formation is an important aspect of macromolecular chemistry and   structural biology. However, there are a very few viable methods   available to determine the energetic contribution of individual   hydrogen bond to binding and catalysis in biological systems.   Therefore, the methods that use secondary deuterium isotope   effects analyzed by NMR or equilibrium or kinetic isotope effect   measurements are attractive ways to gain information on the   H-bonding properties of an alcohol system, particularly in   biological environment. Here, we explore the anharmonic   contribution to the C-H group when the O-H group of   1,1,1,3,3,3-hexafluoroisopropanol (HFIP) form intermolecular   H-bond with the amines by quantum mechanical calculations and by   experimentally measuring the H/D effect by NMR. Within the   framework of density functional theory, ab initio calculations   were carried out for HFIP in its two different conformational   states and their H-bonded complexes with tertiary amines to   determine the 13C chemical shielding, change in their vibrational   equilibrium distances and the deuterium isotope effect on 13C2   (secondary carbon) of HFIP upon formation of complexes with   tertiary amines. When C2-OH involved in hydrogen bond formation   (O-H as hydrogen donor), it weakened the geminal C2-H bond; it   was reflected in the NMR chemical shift, coupling constant and   the equilibrium distances of the C-H bond. The first derivative   of nuclear shielding at C2 in HFIP was -48.94 and -50.73 ppm Å-1   for anti and gauche conformations, respectively. In the complex,   the values were -50.28 and -50.76 ppm Å-1, respectively. The C-H   stretching frequency was lower than the free monomer indicating   enhanced anharmonicity in the C-H bond in the complex form. In   chloroform HFIP formed complex with the amine; δC2 was 69.107 ppm   for HFIP-tryethylamine and 68.766 ppm for HFIP-d2-tryethylamine   and the difference in chemical shift, the ?δC2 was 341 ppb. The   enhanced anharmonicity in the hydrogen bonded complex resulted   larger vibrational equilibrium distance in C-H/D bonds. An   analysis with Morse potential function indicated that the   enhanced anharmonicity encountered in the bond was the origin of   larger isotope effect and the equilibrium distances. Change in   vibrational equilibrium distance and the deuterium isotope   effect, as observed in the complex, could be used as parameters   in monitoring the strength of the H-bond in small model system   with promising application in bio-macromolecules.

      Address (URL): http://dx.doi.org/10.1021/jp411488a