Knowledge Bank

Helium nanodroplet isolation and a tunable quantum cascade laser are used to probe the fundamental CO stretch bands of Aluminum Carbonyl complexes, Al-(CO) ($n\le 5$). The droplets are doped with single aluminum atoms via the resistive heating of an aluminum wetted tantalum wire. The downstream sequential pick-up of CO molecules leads to the rapid formation and cooling of Al-(CO) clusters within the droplets. Near 1900 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, rotational fine structure is resolved in bands that are assigned to the CO stretch of a $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi 1/2$ linear Al-CO species, and the asymmetric and symmetric CO stretch vibrations of a planar C Al-(CO)$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ complex in a $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}B1$ electronic state. Bands corresponding to clusters with $n\ge 3$ lack resolved rotational fine structure; nevertheless, the small frequency shifts from the n=2 bands indicate that these clusters consist of an Al-(CO)$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ core with additional CO molecules attached via van-der-Waals interactions. A second n=2 band is observed near the CO stretch of Al-CO, indicating a local minimum on the n=2 potential consisting of an "unreacted" Al-CO-(CO) cluster. The linewidth of this band is $\sim$0.5cm$^{-1}$,whichisover50timesbroaderthantransitionswithintheAl-COband.TheadditionalbroadeningisconsistentwithahomogeneousmechanismcorrespondingtoarapidvibrationalexcitationinducedreactionwithintheAl-CO-(CO)clustertoformthecovalentlybondedAl-(CO)$_{2}$ complex. For the n=1,2 complexes, CCSD(T) calculations and Natural Bond Orbital (NBO) analyses are carried out to investigate the nature of the bonding in these complexes. The NBO calculations show that both $\pi$ "back" donation (from the occupied aluminum p-orbital into the $\pi$ antibonding CO orbital) \textit{and} $\sigma$ donation (from CO into the empty aluminum p-orbitals) play a significant role in the bonding, analogous to transition metal carbonyl complexes. The large redshift of the CO stretch vibrations is consistent with this bonding analysis.