Reports: ND648740-ND6: Experimental Probes of Key Organometallic Reactive Intermediates: Metal Carbenes

Scott Allen Reid , Marquette University

Narrative Report

ACS PRF 48740-ND6 (PI: Scott A. Reid, Marquette University)

During this project period, significant progress has been made in the study of reactive intermediates using a combination of matrix isolation and gas-phase spectroscopic methods.  In particular, we continue to explore the use of pulsed discharge methods for the generation of organometallic intermediates, and develop new late mixing discharge sources.  Use of Ni-enriched electrodes in our discharge nozzle have afforded new spectra of the NiX (X=F,Cl,Br,I) radicals, which were characterized by Laser-Induced Fluorescence (LIF) and Dispersed Fluorescence (DF) spectroscopy.  Complete characterization by DF spectroscopy of the five low-lying electronic levels associated with the 3d9 configuration of Ni+ have been completed for NiCl, NiBr, following our earlier study of NiI.  The figure at left shows DF spectra of NiBr obtained at lower (bottom panel) and higher (upper panel) resolution.  Term energies and a complete set of vibrational parameters were derived for all five states, and compared with Description: Figure_2_newrecent high level ab initio calculations.  In contrast to NiI, few perturbations observed in the vibronic structure of these levels for either NiBr or NiCl.  The data set derived in this work afforded a detailed analysis of periodic trends in the Nickel monohalide series.  To date the search for larger Ni containing intermediates has been less successful, and we are designing a dual ablation-discharge source for enhanced production of polyatomic intermediates. 

We are also interested in the study of pre-reactive complexes of metal atoms with organic species.  Using discharge sampling matrix isolation methods, we have successfully characterized radical-molecule complexes involving reactive halogen species, reporting studies of the Br—BrCH2X (X=H,Br,Cl) species.  These species exhibit characteristic charge transfer bands in the near-UV, and the observed band maxima in the matrix are consistent with solution phase data accounting for the significant solvochromatic shifts.  These species were characterized theoretically using DFT and TDDFT methods.

Our matrix isolation studies have extened to the iso-polyhalomethanes, which are a class of known reactive intermediates that play a pivotal role in the photochemistry of halomethanes in condensed phases.  Thus, iso-bromoform (iso-CHBr3) and its deuterated isotopomer were characterized by matrix isolation infrared and UV/visible spectroscopy, supported by ab initio and density functional theory calculations.  The isomer is bound by some 60 kJ/mol in the gas phase with respect to the CHBr2 + Br asymptote.  Intrinsic reaction coordinate calculations confirmed the existence of a first-order saddle point connecting the two isomers, which lies energetically below the threshold of the radical channel.  Natural bond orbital analysis and natural resonance theory were used to characterize the important resonance structures of the isomer and related stationary points, which demonstrate that the isomerization transition state represents a crossover from dominantly covalent to dominantly ionic bonding. In condensed phases, the ion-pair dominated isomerization transition state structure is preferentially stabilized, so that the barrier to isomerization is lowered.

In the past year we have also successfully implemented Resonantly Enhanced MultiPhoton Ionization (REMPI) spectroscopy in our laboratory, using a linear time-of-flight mass spectrometer.   Our studies to date have focused on characterization of this system using neutral molecules and complexes; however, we plan in near future to add a discharge source to this system for the study of gas-phase reactive intermediates.

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