Chelation of a proton by an aliphatic tertiary diamine.
A series of monoprotonated aliphatic diamines has been examined, which crystallize in three general motifs: salt-bridged, cyclic, or clustered. The monoprotonated triflic acid salt of Me2N(CH2)4NMe2 forms a proton-bridged cyclic cation. The internal N-N distance is 2.66 A, with the bridging proton in the middle, having an NHN angle >/=172 degrees. The triflate oxygens lie more than 4 A away from the midpoint between the nitrogen atoms, indicating that a salt bridge does not form. The average NH distance in a solid sample was determined by measuring the 15N-H dipolar coupling in the triflic acid salt of the completely deuterated diamine (CD3)2N(CD2)4N(CD3)2. The value of the dipolar coupling constant, 5250 +/- 90 Hz, corresponds to an average NH distance of 1.32 A, nearly half-the NN distance. That result agrees with DFT calculations, which give a double-well potential minimum for proton transit between the two amino groups, having a zero-point vibrational level close to the barrier top. Theory predicts that the maximum value of the zero point vibrational wave function is almost coincident with a local potential energy maximum, consistent with the experimental findings.
Yaghmaei
Sepideh
S
Department of Chemistry, University of California, Riverside, California 92521-0403, USA.
Khodagholian
Sevana
S
Kaiser
J Michael
JM
Tham
Fook S
FS
Mueller
Leonard J
LJ
Morton
Thomas Hellman
TH
D002614:Chelating Agents / Q000737:chemistry*; D018360:Crystallography, X-Ray; D003959:Diamines / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D015394:Molecular Structure; D011522:Protons
D002614:Chelating Agents; D003959:Diamines; D011522:Protons