The crystal and molecular structure of bis-(2,3-pentanedionedioximate-N,N')nickel(II) [or nickel(II) ethylmethylglyoxime] has been determined from three-dimensional X-ray diffraction data. The complex crystallizes in the monoclinic system in space group P21/c with two molecules per unit cell. Unit-cell parameters are: a = 4"7471 + 0.0005, b = 11"7409 + 0.0030, e = 11"9895 + 0.0020/~; ~= 91 "611 + 0.016 °. The structural analysis of the complex, based on all reflections out to 50 ° in 20 with Mo Kct radiation (final R value for 802 observed reflections of 4.8 %) reveals an intramolecular hydrogen bond with an O---O distance of 2.454 A. Bond distances in the five-membered chelate ring indicate significant delocalization of the two conjugated carbon-nitrogen double bonds.

Page X. ACKNOWLEDGMENTS 112 XI. APPENDIX A 113 XII. APPENDIX B 118 XIII. APPENDIX C l6l distinguish between single and double well distributions in short hydro gen bonds by measuring the isotope effect upon deuterium substitution. 2. Spectroscopic studies, of hydrogen bonding Spectroscopic studies of hydrogen bonding have largely been restricted to infrared spectral studies of the asymmetrical X-H stretching and X-H bending vibrations in X-H***Y bonds. Recently, far infrared, Raman, NMR, and electronic spectral studies have shed some light on the nature of the hydrogen bond. These studies have been thoroughly reviewed by Ratajczak and Orville-Ihomas (63), Jakobsen, et al. CU2), and Rao and Murtby (62). Infrared spectroscopy provides the most straightforward technique for the detection and study of hydrogen bonding. Hydrogen bond formation results in a shift of the asymmetric X-H stretching mode (v^) to lower frequencies accompanied by an increase in integrated intensity and halfbandwidth. The shift in frequency (^v^) from the "free" X-H stretching frequency (3700 cm~^ for 0-H bonds) can be correlated with various parameters and properties including X Y distance and hydrogen bond energy. The bending frequency (v^) also shifts, generally to hi^er frequency, and this shift can also be correlated with various properties. Hydrogen bond formation also results in additional vibrational modes, the most significant being the asymmetrical stretching (v^) and bending (Vg) modes of the H***Y bond. The frequencies of the modes associated with the weaker H* • 'Y or hydrogen bond have been found to lie N(l)-Cu-0C2') K(2)-Cu-0(2') 103.8 Cl) 91.7 Cl) NC3)-Cu-0(2') N(U)-Cu-.0(2') 97.7 CD 102.8 (1) Cu-Il(l)-0(1) Cu-Il(2)-0(2) 123.8 C2) 122.6 (2) Cu-N(3)-0(1^) Cu-N(4)-0(3) 12k.9 (2) 122.9 (2) Cu-N(l)-C(l) Cu-N(2)-C(3) 116.1 (3) 115.8 (2) Cu-N(3)-C(5) Cu-N(U)-C(7) 116.3 (3) 116.2 (3) C(l)-N(l)-0(1) C(3)-N(2)-0(2) 120.0 C3) 121.6 (3) C(5)-K(3)-0(4) C(7)-K(4)-0(3) 118.6 (3) 120.8 (3)

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