+++Ph.D Dissertation

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Prapansiri, V. Ph.D Dissertation, Northeastern University, Boston, MA, 1989

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Bibliographic Information

Deuterium isotope effects in amines. Prapansiri, Vichukorn. Northeast. Univ., Boston, MA, USA. Avail. Univ. Microfilms Int., Order No. DA9007735. (1989), 218 pp. From: Diss. Abstr. Int. B 1990, 50(12, Pt. 1), 5630. Dissertation written in English. CAN 113:211048 AN 1990:611048 CAPLUS

Deuterium isotope effects in amines
by Prapansiri, Vichukorn, Ph.D., Northeastern University, 1989, 218 pages; AAT 9007735

Abstract (Summary)
Conformational equilibrium isotope effects (CEIEs) associated with various $N$-substituted-3-azabicyclo(3.2.2) nonanes were studied. Conformations with deuterium located in a gauche rather than anti alignment with the nitrogen lone pair are favored. The primary source of these effects is attributed to n-$\sigma\sp\*$ negative hyperconjugation which weakens and lengthens the C-H(D) bonds situated anti to the lone pair. The magnitude of this interaction depends upon the energy of the nitrogen lone pair which is directly affected by the change of character of $N$-substituents. The electronegative substituent increases the s character of the lone pair, and increases the energy gap between the lone pair and the $\sigma\sp\*\sb{\rm C-H}$ orbital resulting in a decrease in the magnitude of the CEIE. In contrast, an increase in the p character of the lone pair by the conjugative interaction between the lone pair and the $\sigma\sp\*$ orbital of C$\sb1$,-X bond accounts for larger CEIE.

The angular dependence of isotope effects was also observed as expected from hyperconjugative origins. The change of the dihedral angle between the lone pair and C-H(D) bonds results in changing of CEIEs.

Intrinsic isotope effects on $\sp{15}$N NMR spectra were also detected. A low temperature $\sp{15}$N NMR spectrum of a deuterated $N$-methyl-3-azabicyclo(3.2.2) nonane also demonstrated an angularly dependent intrinsic isotope effect.

The interaction of C-D bonds with the amine lone pair expected if the isotope effect arises solely from zero point energy differences associated with the stretching vibration was also detected in IR spectra, where separated C-D stretching frequencies are found for C-D(anti) and C-D(gauche). This separation between the two IR bands allows estimation of the CEIE.

Conformational equilibrium isotope effects were applied to conformational analyses of the $N$-alkyl side chain of $N$-alkylpiperidines. Introduction of a single deuterium at C$\sb1$$\sp\prime$ of the side chain causes a gauche $\rightleftharpoons$ gauche equilibrium to be non-degenerate and observable by the separation of signals for C$\sb2$ and C$\sb6$ in $\sp{13}$C NMR spectra at low temperature. MM2 calculations using Allinger's parameters for amines, and intrinsic isotope shifts on $\sp{15}$N NMR spectra also support the conclusion that both gauche and anti conformers are present, with the proportion of gauche conformers declining in the sequence N-ethyl- $>$ N-propyl-, N-butyl- $>$ N-benzylpiperidine. (Abstract shortened with permission of author.)

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