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In Search of a Low Barrier Hydrogen Bond In Proton-Bridged Diamines



In Search of a Low Barrier Hydrogen Bond in Proton Bridged Diamines


Sepideh Yaghmaei

Doctor of Philosophy, Graduate Program in Chemistry

University of California, Riverside, December 2008

Dr. Thomas H. Morton, Chairperson

The term "Low Barrier Hydrogen Bond" (LBHB) is sometimes applied to describe short, strong hydrogen bonds, in which a proton is held between two basic sites having the same proton affinity. The occurrence of LBHBs in enzyme-catalyzed reactions and proton transfer mechanisms has been debated for many years. Creating small molecule mimics of enzymes is a well-characterized approach to probing mechanistic enzymology. In this thesis monoprotonation of diamines has been examined, and results from gas and crystalline phases are compared. Monoprotonated diamines can crystallize in three general motifs: salt-bridged, cyclic, or clustered. Primary diamines tend to form salt-bridged structures, in which one NH hydrogen bonds to the anion while another NH forms a strong hydrogen bond to the neutral amino group of another molecule. When both amines are tertiary, NH-N strong hydrogen bonds tends to predominate over the salt bridge to the anion. In linear tertiary diamines, the observed motif depends on chain length. The 1:1 salt between N,N,N',N'-tetramethyl- putrescine and triflic acid (I) forms a cyclic structure, while the corresponding salt of N,N,N',N'-tetramethylcadaverine (II) forms a dimeric cluster in a tête-bêche orientation. According to x-ray structures of (I) and (II), the triflate anion is ~ 4 Ǻ away from the proton between the nitrogen atoms. The distance between the nitrogens involved in hydrogen bonding are 2.66 Ǻ (I) and 2.75 Ǻ (II). In both cases the NHN bond angle is almost linear. A more accurate NH distance in (I) was found by measuring the natural abundance 15N-proton dipolar coupling constants using solid state NMR (SSNMR). To eliminate NH coupling with aliphatic protons of (I) all the hydrogen atoms except the one between the nitrogens were labeled with deuterium. The SSNMR result of I-d20 indicates that the NH distance is 1.324 Ǻ; therefore, the proton has its equilibrium position in the middle. The zero point energy wavefunctions of the one and two-dimensional potential energy surface for the transfer of the proton between two nitrogen atoms in the cation of (I) predicts the NH distance to be 1.320 and 1.328 Ǻ respectively. The vibrations associated with NHN hydrogen bond (asymmetric and symmetric stretch and the bending modes) have been observed by IR, Raman and Inelastic Neutron Scattering (INS), but their assignment is tentative.

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