THE FLUORESCENCE EXCITATION SPECTRUM OF 1-NAPHTHOIC ACID AT ROTATIONAL RESOLUTION - S-0 AND S-1 POTENTIAL-ENERGY SURFACES ALONG THE R-COOH TORSIONAL COORDINATE

Several vibronic bands that appear within 300 cm(-1) of the electronic origin in,the S-1<--S-0 fluorescence excitation spectrum of 1-naphtho ic acid (1NA) and carboxyl-deuterated 1NA have been examined at full r otational resolution. The data show that all bands belong to the s-cis isomer of 1NA. They also show that all bands are torsional in-nature; i.e., that they involve displacements along either the S-0 or the S-1 carboxyl torsional coordinate, phi, or both. Unambiguous assignments of the bands follow from the observed inertial defects, from which the torsional potential energy surfaces of both electronic states are der ived. In S-0 s-cis-1NA, the naphthalene and carboxyl groups are not co planar. The s-cis minima are at phi= +/- 17 degrees; the barrier to pl anarity is 13.2 cm(-1). In contrast, S-1 s-cis-1NA is a completely pla nar molecule, with phi=0 degrees. The barrier for s-cis to s-trans iso merization is similar to 1450 cm(-1) in the S-0 state and similar to 2 200 cm(-1) in the S-1 state. The validity of the two derived potential energy surfaces of s-cis-1NA has been tested by comparing the observe d inertial defects with those computed using the torsional eigenfuncti ons. Excellent agreement is obtained, confirming the one-dimensional a pproach em ployed.