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Electrostatic Interactions Are Key to CO n‑π* Shifts: An Experimental Proof
journal contribution
posted on 2016-06-01, 00:00 authored by Tapas Haldar, Sayan BagchiCarbonyl n-π* transitions are
known to undergo blue shift
in polar and hydrogen-bonding solvents. Using semiempirical expressions,
previous studies hypothesized several factors like change in dipole
moment and hydrogen-bond strength upon excitation to cause the blue
shift. Theoretically, ground-state electrostatics has been predicted
to be the key to the observed shifts, however, an experimental proof
has been lacking. Our experimental results demonstrate a consistent
linear correlation between IR (ground-state phenomenon) and n-π*
frequency shifts (involves both ground and excited electronic-states)
of carbonyls in hydrogen-bonded and non-hydrogen-bonded environments.
The carbonyl hydrogen-bonding status is experimentally verified from
deviation in n-π*/fluorescence correlation. The IR/n-π*
correlation validates the key role of electrostatic stabilization
of the ground state toward n-π* shifts and demonstrates the
electrostatic nature of carbonyl hydrogen bonds. n-π* shifts
show linear sensitivity to calculated electrostatic fields on carbonyls.
Our results portray the potential for n-π* absorption to estimate
local polarity in biomolecules and to probe chemical reactions involving
carbonyl activation/stabilization.