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| - Computational and Experimental Studies of the Effect ofSubstituents on the Singlet−Triplet Energy Gap inPhenyl(carbomethoxy)carbene
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| - The effect of aromatic substitution on the singlet−triplet energy gap in substituted phenyl(carbomethoxy)carbene (X−Ph−C−CO2CH3, PCC) has been explored by time-resolved infrared(TRIR) spectroscopy and gas-phase computational methods. The ground state of para-substitutedPCC is calculated to change from the triplet state in p-NO2-PCC (ΔGST = 6.1 kcal/mol) to thesinglet state in p-NH2-PCC (ΔGST = −2.8 kcal/mol). The absence of solvent perturbation in theTRIR spectra of p-N(CH3)2-PCC (which should have electronic properties similar to p-NH2-PCC)and parent PCC is consistent with their ground states lying >±2 kcal/mol from the next availableelectronic state, in line with the computational results. The observation of solvent perturbation inthe TRIR spectra of p-OCH3-PCC and p-CH3-PCC implies that their ground states lie <±1 kcal/mol from their next available electronic state. This is in agreement with our computational results,which predict a gas-phase ΔGST of −0.8 and 1.6 kcal/mol for p-OCH3-PCC and p-CH3-PCC ascompared to ΔGST values of −3.9 and −1.3 kcal/mol from polarizable continuum model (PCM)calculations with acetonitrile as a solvent. Gas-phase computational results for the meta- and ortho-substituted PCC species are also presented, along with selected linear free energy (LFE)relationships for the para and meta species.
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