Spectroscopic Investigation of the Molecular Vibrations of 1,4-Dihydronaphthalene in Its Ground and Excited Electronic States

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Mohamed Z. M. Rishard,† Martin Wagner,† Jaebum Choo,‡ and Jaan Laane*,†
Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843-3255, and
Department of Chemistry, Hanyang UniVersity, 425-791 Ansan, Korea
ReceiVed: March 24, 2009; ReVised Manuscript ReceiVed: May 7, 2009
A comprehensive spectroscopic study of 1,4-dihydronaphthalene (14DHN) has been carried out for its ground
and S1(π,π*) electronic states using infrared, Raman, ultraviolet, and laser-induced fluorescence (LIF)
spectroscopic techniques. The experimental work was complemented by ab initio and DFT calculations. For
the ground state excellent agreement between observed and calculated values was attained. For the S1(π,π*)
excited state 19 of the vibrational modes were clearly determined and excited vibronic levels for a number
of these were also identified. A detailed energy map for the low-frequency modes in both electronic states
was established. 14DHN is very floppy in its S0 ground state but less so in its excited state. The floppiness
relaxes C2V selection rules for the S0 state.
Introduction
Recently we reported1 the laser-induced fluorescence (LIF)
and ultraviolet (UV) absorption spectra associated with the ringpuckering
vibration of 1,4-dihydronaphthalene (14DHN). From
these the ring-puckering potential energy function was determined
for both the S0 and S1(π,π*) states and this showed the
molecule to be planar in both electronic states. However, in
the S1(π,π*) state the molecule was found to be much more
rigid. In the present paper we report a comprehensive study of
the vibrations of 14DHN in both its S0 and S1(π,π*) states and
compare the results to theoretical calculations. These results
provide a deeper insight into the energetics of the 14DHN
molecule.
Experimental Section
The sample of 14DHN was purchased from TCI America
and purified by trap-to-trap distillation. However, the sample
still contained a few percent of 1,2-dihydronaphthalene, which
we had previously studied2 and for which the spectra were
thoroughly analyzed.
The experimental procedures for the acquisition of the LIF
spectra have been previously described.2-10 The UV absorption
spectra were recorded using a Bomem DA8.02 Fourier
transform spectrometer and the LIF spectra were obtained
using an apparatus based on a Continuum Sunlite OPO laser
system. The Raman spectra of 14DHN were acquired on an
SA Jobin-Yvon U-1000 spectrometer equipped with a liquidnitrogen-
cooled CCD (charge couple device) detector. Raman
scattering was achieved using a Coherent Radiation Innova
20 argon ion laser with excitation at 514.5 nm or a Verdi-10
at 532 nm. The liquid-phase spectra were recorded at room
temperature with lasing power of 0.2-0.6 W. Vapor-phase
Raman spectra were collected in the previously described
vapor Raman cell11 at a temperature of 300 °C. A laser power
of 5 W was used for the vapor studies. The mid-infrared
spectra of a thin capillary film of the pure sample between