Whe-Yi Chiang

Fluorescence spectra and torsional potential functions for trans‐stilbene in its S0 and S1(π,π*) electronic states

The fluorescence excitation spectra and dispersed fluorescence spectra of trans‐stilbene have been recorded and analyzed. Vibrational assignments for the eight low‐frequency modes have been made for both the S0 and S1(π,π*) electronic states, and these differ substantially from those of previous workers. Two‐dimensional kinetic and potential energy calculations were carried out in order to determine the potential energy surfaces for the two phenyl internal rotations ν37 and ν48. The function V(φ1,φ2)= 1/2V2(2+cos 2φ1+cos 2φ2)+V12 cos 2φ1 cos 2φ2 +V12’ sin 2φ1 sin 2φ2, with V2=1550 cm−1, V12=337.5 cm−1, and V12’ = 402.5 cm−1 for the S0 state and with V2=1500 cm−1, V12=−85 cm−1, and V12’ = −55 cm−1 for the S1(π,π*) state fits the observed data (nine frequencies for S0 and six for S1) extremely well. The barriers to simultaneous internal rotation of both phenyl groups are given by twice the V2 values. The fundamental frequencies for these torsions are ν37=9 cm−1 and ν48=118 cm−1 for the S0 state and ν37=35 c...

Jet‐cooled fluorescence excitation spectra, conformation, and carbonyl wagging potential energy function of cyclopentanone and its deuterated isotopomers in the S1 (n,π*) electronic excited states

The jet‐cooled fluorescence excitation spectra of cyclopentanone and its 2,2,5,5‐d4 isotopomer have been recorded in the 305–335 nm region. In addition, the spectra of d1, d2, and d3 species were obtained from isotopic mixtures. The electronic band origin of the d0 molecule for the S1 (n,π*) state of A2 symmetry occurs at 30 276 cm−1, while that of the d4 molecule is at 30 265 cm−1. More than 100 fluorescence bands were assigned for each species. These arise from combinations of ν3 (C=O stretch), ν11 (ring‐angle bending), ν18 (ring twisting), ν25 (C=O out‐of‐plane wag), ν26 (ring bending), and ν36 (C=O in‐plane wag) and their vibrational excited states. The vibrational frequencies for ν3, ν11, and ν36 are significantly lower in the S1 state than the S0 ground state. However, the out‐of‐plane ring modes ν18 and ν26 are only slightly shifted. A progression observed for ν26 does indicate that in the S1 state, the bent ring conformation lies about 500 cm−1 above the ring‐twisting minimum and corresponds to a ...

HIGH-TEMPERATURE VAPOR-PHASE RAMAN SPECTRA AND ASSIGNMENT OF THE LOW-FREQUENCY MODES OF TRANS-STILBENE AND 4-METHOXY-TRANS-STILBENE

Abstract The vapor-phase Raman spectra of trans -stilbene at 330°C and 4-methoxy- trans -stilbene at 350°C have been recorded using a specially constructed high temperature cell. Together with the dispersed fluorescence spectra, these spectra have been used to assign the eight low-frequency vibrational modes in each molecule. For trans -stilbene, strong polarized bands at 273 and 152 cm −1 were ascribed to the totally symmetric ν 24 (C e -phenyl bend) and ν 25 (in-plane phenyl wag), respectively. A depolarized band at 207 cm −1 was assigned to the B g phenyl flap ( ν 47 ), and a polarized shoulder near 125 cm −1 is believed to result from both the B g phenyl torsion ( ν 48 ) and the overtone of the A u phenyl flap ( ν 36 ). For 4-methoxy- trans -stilbene the C e -phenyl bend and in-plane phenyl wag were shifted to 234 and 144 cm −1 , respectively. These assignments were critical for understanding the fluorescence spectra associated with the torsional motions.

Jet‐cooled fluorescence excitation spectra and carbonyl wagging and ring‐puckering potential energy functions of cyclobutanone and its 2,2,4,4‐d4 isotopomer in the S1(n,π*) electronic excited state

The jet‐cooled fluorescence excitation spectra of cyclobutanone and its 2,2,4,4‐d4 isotopomer have been recorded in the 305–335 nm region. The electronic band origin of the d0 molecule for the S1(n,π*) state of A2 symmetry occurs at 30 292 cm−1 (30 265 cm−1 for the d4 molecule). The observed spectra consisting of more than 50 bands for each isotopomer involve ν7, ν8, and ν9 (the three A1 ring vibrations) as well as ν20(C=O in‐plane wag), ν26 (C=O out‐of‐plane wag), and ν27 (ring puckering). Five bands associated with the excited vibrational states of ν26 in the S1(n,π*) electronic state were observed for each isotopic species, and these were used to determine the one‐dimensional potential energy functions for the C=O out‐of‐plane wagging. The C=O wagging angle was determined to be 39° and the barrier to inversion is 2149 cm−1 (2188 cm−1 for the deuteride). For the ring‐puckering in the S1 state the lowest three vibrational energy spacings were found to be 106, 166, and 185 cm−1 as compared to values of 35...

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.

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...

A computer-controlled apparatus for laser-induced fluorescence spectroscopy in a supersonic jet

Abstract An experimental apparatus for laser-induced fluorescence excitation spectroscopy (FES) and for dispersed fluorescence studies in a supersonic jet has been constructed and optimized for the collection of spectra from weakly fluorescing samples. A detailed description of the hardware and software is presented here.

Jet-cooled fluorescence excitation spectra and carbonyl wagging potential energy functions of several cyclic ketones in their S1(n, π*) electronic excited states

The jet-cooled fluorescence excitation spectra of 2-cyclopenten-l-one and its 5,5-d2 isotopomer have been recorded in the 370 to 340 nm region. The electronic origin for the undeuterated species occurs at 27210 cm-1 for the S 1(n,π*) electronic excited state. The vibrational frequencies for the three carbonyl motions and the nine ring modes were observed for the excited state. Bands at 67, 158, and 256 cm-1 for the do species, at 63, 147, and 240 cm-1 for the 5-d1 isotopomer, and at 59,138, and 227 cm-1 for the d2 species were assigned to the ring-puckering motion in the S 1 state. A single one-dimensional potential energy function accurately fits the data for all three isotopomers. This function is nearly purely quartic in character and shows the ring to be planar in the electronic excited state. However, it has become less rigid, and this is ascribed to a decrease in initial angle strain within the ring. The C=O and C=C stretching frequencies occur at 1418 and 1357 cm-1 for the do molecule. The ring-twisting frequency for the S 1 state occurs at 274 cm-1. Previous electronic absorption measurements had resulted in a misassignment for this motion.