Sachie Sakurai

Two-dimensional vibrational potential energy surface for phthalan: The effect of large coupling on vibrational quantum states

The low-frequency analysis of the phthalan ring-puckering vibration shows a highly irregular pattern of energy levels which cannot be adequately described by a one-dimensional potential energy function. Because the ring-puckering mode interacts strongly with the ring-flapping vibration of the same symmetry, a two-dimensional analysis involving these two motions is required. Two-dimensional kinetic energy (reciprocal reduced mass) expansions were calculated for the puckering, flapping, and interaction terms and these were utilized in the calculation of the two-dimensional potential energy surface. This surface does an excellent job of reproducing the irregular pattern of observed energy spacings for the ring-puckering vibration in both the ground and excited flapping states. The potential-energy surface has a barrier to planarity of 35 cm−1 and energy minima at x1=0.09 A, x2=−0.03 A, and x1=−0.09 A, x2=0.03 A where x1=puckering and x2=flapping. Although the minima correspond to puckered conformations with ...

Raman spectroscopy of vapors at elevated temperatures

The most effective way to obtain high quality vapor-phase Raman spectra is to heat the samples to increase their vapor pressure. Many samples can be heated to 350 °C and higher without decomposition. We have designed a simple Raman cell to allow these high temperature studies to be carried out. The high-temperature Raman spectra of nine molecules will be presented and discussed. Most of these are non-rigid molecules containing aromatic rings for which vibrational potential energy surfaces have been determined from their spectra. Two molecules (p-cresol and 3-methylindole) are model compounds for amino acids and their vapor-phase spectra are characteristic of environments with no hydrogen bonding.

Far-infrared and combination-band spectra of the ring-puckering and ring-flapping vibrations of phthalan: A failure of the one-dimensional model

The far-infrared spectra of phthalan showing bands resulting from the ring-puckering and ring-flapping vibrations have been recorded and analyzed. The observation and assignment of the puckering fundamental at 30.9 cm−1 and of numerous hot-band, double quantum jump, and triple quantum jump transitions have provided the basis for generating a detailed and self-consistent energy map for the vibrational states. In addition, sum, difference, and hot bands associated with the C–O symmetric stretching mode also provide confirmation of the assignments. Previous far-infrared, electronic, and dispersed fluorescence spectra assignments are shown to be erroneous whereas a previous microwave study is consistent with the present results. The ring-puckering energy spacings show considerable irregularities and these can be ascribed to the puckering/flapping interaction. Consequently, a one-dimensional potential energy function can only do a mediocre job of fitting the observed vibrational data.

Fluorescence and electronic absorption spectra of phthalan: Two-dimensional vibrational potential energy surface for the ring-puckering and flapping in the S1(π,π*) state

The ring-puckering and ring-flapping vibrations of phthalan in its S1(π,π*) electronic excited state have been studied using fluorescence excitation spectroscopy of jet-cooled molecules, dispersed fluorescence spectroscopy, and ultraviolet absorption spectroscopy. This electronic state has A1 symmetry resulting from a B2→B2 orbital transition. Thus type A absorption bands result from A1→A1 and B2→B2 transitions to the S1 vibronic levels. The ring-puckering levels for the S1(π,π*) electronic state were determined for both the flapping ground (vF=0) and excited states (vF=1) and these were used to calculate both one- and two-dimensional potential energy surfaces which fit the observed spectra. In the S1(π,π*) state phthalan was found to be planar and more rigid than in the ground state in terms of the puckering coordinate. However, the molecule is less rigid along the flapping coordinate. This study shows how several types of spectroscopy and computations must be used in conjunction with each other to attai...