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Probing nuclear quadrupole interactions in the rotationally resolved S1]S0 electronic spectrum of 2-chloronaphthalene

Published

Author(s)

David F. Plusquellic, F -. Jahanmir

Abstract

Rotationally resolved S1 So fluorescence excitation spectra of 2-chloronaphthalene (2CIN) are measured using a newly constructed UV laser/molecular beam spectrometer. More than 1000 well-resolved rotational lines are recorded for the two overlapping band origins of the 35Cl and 37Cl isotopomers (Δ nuorigin~3 Ghz) over a 4 cm-1 spectral region and at a rotational temperature of 9 K. Both spectra are a,b-hybrid bands having a 18% a-/82% b0type character in accordance with 1La1A type excitations observed for other 2-substituted naphthalenes. For most spectral regions, the rotational lineshapes are Voigt with Δ nu FWHM=6.5 plus or minus 0.1 MHZ, consistent with the natural Lorentzian linewidth of the S1 state (~4.5 MHZ) and the Doppler width of the instrument (~3 MHZ). However, for low J transitions, additional splittings are observed from Cl quadrupole interactions, with linewidths in excess of 20 MHZ. Accurate quadrupole coupling constants for the S1 state of 2CIN are obtained from non-linear least squares fits of these profiles when combined with high precision FT microwave measurements made on the ground state. The So and S1 values of eQqzz are -71.198(5) MHZ and -67.8(40) MHZ, and the asymmetry parameters, eta [eQqyy - eQqxx) / eQqzz], are 0.07638(3) and 0.13(5), respectively. These measurements represent the first determination of ht nuclear quadrupole coupling constants in the excited singlet state of an aromatic molecule. Following the formalism of Townes-Dailey-Gordy, the C-Cl sigma bond increases in ionic character by 3% in S1. The eta values provide a measure of the relative differences in the in-plane and out-of-plane quadrupole components and within this same model, represents an >60% increase in pi bond character on electronic excitation. The ground state coupling constants from additional FTMW measurements of 1CIN are eQqzz=-771.568(4) and eta=0.09499(5). The So values of 1CIN and 2CIN are in good agreement with the relative trends predicted from ab initio theory using 6-31G*/MP2 basis set and provide a framework to interpret excited state relaxation properties of both 1CIN and 2CIN in terms of these tensor quantities.
Citation
Journal of Chemical Physics
Volume
115
Issue
1

Keywords

heavy atom effects, intersystem crossing, quadrupole structure, rotationally resolved UV spectrum

Citation

Plusquellic, D. and Jahanmir, F. (2001), Probing nuclear quadrupole interactions in the rotationally resolved S1]S0 electronic spectrum of 2-chloronaphthalene, Journal of Chemical Physics (Accessed February 25, 2024)
Created July 1, 2001, Updated February 19, 2017