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

Published

July 1, 2001

Author(s)

David F. Plusquellic, F -. Jahanmir

Abstract

Rotationally resolved S₁ S_o 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 nu_origin~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 ¹L_a¹A 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 S_o and S₁ values of eQq_zz are -71.198(5) MHZ and -67.8(40) MHZ, and the asymmetry parameters, eta [eQq_yy - eQq_xx) / eQq_zz], 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 eQq_zz=-771.568(4) and eta=0.09499(5). The S_o 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

Pub Type

Journals

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 April 19, 2024)