INFRARED SPECTRUM AND STRUCTURE OF THE $AlC_{2}H_{4}$ MOLECULE IN SOLID ARGON

Abstract

$AlC_{2}H_{4}$, aluminum monoethylene, has been synthesized by cocondensation of A1 atoms and ethylene molecules in solid argon. Analysis of the isotopic shifts (using $C_{2}H_{4}, C_{2}D_{4}, ^{13}C_{2}H_{4}$ and $CH_{2}CD_{2}$) for the 7 observed fundamentals, shows that the metal forms a symmetrical $\pi$ complex of $C_{2v}$ symmetry. A normal-coordinate analysis for the Al block motions estimates the perturbations of the $C=C$ and $CH_{2}$ bending and stretching force constants, and allows quantitative comparisons with other metal complexes of transition metals and lithium with ethylene. If these complexes present structural similarities and comparable perturbations of the $C=C$ bond, $AlC_{2}H_{4}$ presents two noteworthy characteristics: i) a relatively large Al-C force comstant and ii) the relative intensities of the observed fundamentals are very different from others complexes. This second point is analysed through estimates of some of the electro-optical parameters of $AlC_{2}H_{4}$, linked to the variation of charge flow between the metal center and the - bond through CC distance or $CH_{2}$ bond angle variations. Comparison with the same parameters in $LiC_{2}H_{4}$ reveals a significant difference. This fact might be more indicative of differences in the nature of the bonding between the metals and the $C=C$ double bond, than is a simple comparison of the vibrational frequencies or even $C=C$ force constant perturbations.