Fourier transform infrared spectroscopy study of small transition-metal carbide clusters

Abstract

Small molecular clusters consisting of transition metals and carbon, which are the focus of the research, are important for their potential as astrophysically significant species and for the information they may provide about the structures and bonding mechanisms of larger transition-metal carbide structures, such as metallocarbohedrenes. Additional applications exist for semiconductors and quantum dots. The dissertation research is concerned with a new investigation of the structures and infrared signatures of novel transition-metal carbide clusters using Fourier transform infrared (FTIR) spectroscopy and density functional theory (DFT).^FTIR absorption spectra were obtained by trapping the vapor produced by the simultaneous evaporation of transition metal and carbon rods with Nd:YAG lasers in an Ar matrix maintained at ~10 K in a vacuum at ~10-7 Torr.^Comparison of the vibrational fundamentals and isotopic shifts observed in 13C-enriched vibrational spectra to the isotopic spectra predicted by DFT has enabled the identification of vibrational fundamentals of linear NiC3Ni, cyclic TiC3, and cyclic ScC3. The ?3(su) asymmetric carbon stretching mode of linear NiC3Ni has been observed at 1950.8 cm-1. Although other small nickel-carbon clusters have been investigated theoretically, this is the first experimental or theoretical study of the structure and infrared signature of NiC3Ni.^Two vibrational fundamentals of cyclic TiC3 (a fanlike geometry with a transannular metal-carbon bond) were observed at ?3(a1)=624.3 and ?5(b2)=1484.2 cm-1, corresponding to symmetric metal-carbon and asymmetric carbon-carbon stretching modes, respectively. A third frequency at 573.8 cm-1 has provisionally been assigned to the ?4(b1) fundamental.^Three vibrational fundamentals of cyclic ScC3 were observed including the ?5(b2)=1478.0 cm-1 asymmetric carbon stretch, the ?3(a1)=557.0 cm-1 symmetric metal-carbon stretch, and the ?1(a1)=1190.7 cm-1 symmetric carbon stretch. The results for TiC3 and ScC3 provide the first unambiguous evidence of the cyclic structures suggested by earlier photoelectron spectroscopy studies, significantly improve the accuracy of the ?3(a1) measurements, and identify three new vibrational fundamentals.^FTIR measured isotopic spectra for all of the fundamentals are in good agreement with the predictions of DFT simulations using the B3LYP hybrid functional. Innovative sample preparation techniques have also been developed in this investigation.