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J. Yang1, M. Wagner1, K. Okuyama2, K. Morris1, Z. Arp1, J. Choo3, N. Meinander4, Ohyun Kwon5, and J. Laane1*
1Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255
2Department
of Chemical Engineering & Materials, College of Engineering, Nihon
University, Koriyama, Japan 963-8642.
3Department of Chemistry, Hanyang University, 425-791 Ansan, Korea.
4Department
of Technology, National Defence College, FIN-00861 Helsinki, Finland.
5Materials
& Devices Research Center, Samsung Advanced Institute of Technology,
Suwon, Korea 440-600.
The fluorescence excitation (jet-cooled), single vibrational level fluorescence, and the ultraviolet absorption spectra of coumaran associated with its S1(π,π*) electronic excited state have been recorded and analyzed. The assignment of more than seventy transitions has allowed a detailed energy map of both the S0 and S1 states of the ring-puckering (υ45) vibration to be determined in the excited states of nine other vibrations, including the ring-flapping (υ43) and ring-twisting (υ44) vibrations. Despite some interaction with υ43 and υ44, a one-dimensional potential energy function for the ring-puckering very nicely predicts the experimentally-determined energy level spacings. In the S1(π,π*) state coumaran is quasi-planar with a barrier to planarity of 34 cm-1 and with energy minima at puckering angles of ± 14°. The corresponding ground state (S0) values are 154 cm-1 and ± 25°. As is the case with the related molecules indan, phthalan, and 1,3-benzodioxole, the angle strain in the five-membered ring increases upon the π®π* transition within the benzene ring and this increases the rigidity of the attached ring. Theoretical calculations predict the expected increases of the carbon-carbon bond lengths of the benzene ring in S1, and they predict a barrier of 21 cm-1 for this state. The bond length increases at the bridgehead carbon-carbon bond upon electron excitation to the S1(π,π*) state gives rise to angle changes which result in greater angle strain and a nearly planar molecule.
The S0 Ring-Puckering Potential Energy Function for Coumaran
J. Yang1, K. Okuyama2, K. Morris1, and J. Laane1*
1Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255
2Department of Chemical Engineering & Materials, College of Engineering, Nihon University, Koriyama, Japan 963-8642.
With the aid of a reported inversion splitting value, the far-infrared spectrum resulting from the ring-puckering vibration of coumaran has been reassigned and the one-dimensional potential energy function has been determined. The barrier to planarity is 155 ± 4 cm-1 and the dihedral angle is 25°. These results agree well with the millimeter wave spectra values of 152 cm-1 and 23° which utilized different data and a different type of potential function for the calculations. The MP2/6-31G* ab initio values of 258 cm-1 and 26.5° agree more poorly. The puckering potential functions for the ring-flapping and ring-twisting vibrationally excited states were also determined and the barriers were found to be 149 and 156 cm-1, respectively.
C. Mlynek1, H. Hopf1, J. Yang2, and J. Laane2*
1Institut für Organische Chemie, Technische Universität, D-38106 Braunschweig, Germany
2Department of Chemistry, Texas A&M University, College Station, TX, USA 77843-3255
The bicyclic bisether molecule 3,7-dioxabicyclo[3.3.0]-1,5-ene (DOBO) molecule has been synthesized in small quantities and has been shown to be analytically pure by NMR and mass spectra. The vapor-phase Raman spectrum has been analyzed and compared to the predicted spectra from DFT calculations. This spectrum further confirms the structure of the molecule and shows the skeleton of both rings to be planar with D2h symmetry. Ab initio calculations were carried out to predict the bond distances and angles of the molecule, and these also showed the molecule to be planar.
Vibrational Frequencies and Structure of Cyclopropenone from Ab Initio Calculations
J. Yang, K. McCann, and J. Laane
Several calculations with different basis sets have been carried out to better understand the unusual vibrational frequencies of cyclopropenone. It is shown that the bands at 1840, 1483, 1026 cm-1 are predominantly the C=O, C=C, and symmetric C-C stretches. However, for the first and last of these there is strong interaction between the C=O and C-C stretches. The results differ quantitatively from a previous normal coordinate calculation and interpretation.
Vibrational Spectra and DFT Calculations of Tetralin and 1,4-Benzodioxan
D. Autrey, J. Yang, and J. Laane
The infrared and Raman spectra of vapor-phase and liquid-phase tetralin (TET) and 1,4-benzodioxan (14BZD) have been recorded and assigned. Calculations for the structures were carried out using the MP2/cc-pvtz (triple zeta) basis set, and the twisting angles were calculated to be 31.4° for TET and 30.1° for 14BZD. The barriers to planarity were calculated to be 4809 and 4093 cm-1, respectively. Density functional theory calculations for both planar (C2v) and twisted (C2) structures were carried out to predict the vibrational frequencies. After scaling, the agreement with the experimental values was excellent for C2 symmetry. Almost all frequencies below 1350 cm-1 were calculated to be within 10 cm-1 of the experimental values. Frequencies calculated for the C2v structures preclude the possibility of many vibrational interactions and hence agree more poorly.
Jaebum Choo1, Sunghwan Kim1, Stephen Drucker2, and Jaan Laane3*
1Department of Chemistry, Hanyang University, Ansan 425-791, South Korea
2Department of Chemistry, University of Wisconsin-Eau Claire, Eau Claire, WI 54702-4004
3Department of Chemistry, Texas A&M University, College Station, TX 77843-3255
Density functional calculations have been carried out on the S0, S1(n,π*), and T1(n,π*) states of 2-cyclopenten-1-one (2CP) to complement the experimental study of the triplet state using cavity ringdown spectroscopy described in the previous paper. Structures and vibrational frequencies were calculated for each state using both the BLYP/6-31+G(d,p) and BLYP/6-311+G(d,p) basis sets. The structural information was used to obtain the kinetic energy part of the ring-bending Hamiltonian, for analysis of triplet-state spectral data. The density functional calculations show the molecule in its S0 and S1 states to be planar, but to have a small barrier to planarity in the T1(n,π*) triplet state. This is in line with potential-energy fits to the experimental ring-bending levels for each state. The calculated barrier for the T2(π,π*) state is 999 cm-1. This provides further confirmation that the cavity ringdown data, from which a 43-cm-1 barrier was determined, correspond to the T1(n,π*) state. The calculated vibrational frequencies are in excellent agreement with the experimental data for the S0 state and also for the most part for the S1 and T1 states. Notably, the frequency calculated for the very anharmonic ring-bending vibration cannot be expected to be very accurate.
Nathan R. Pillsbury1, Jaebum Choo2, Jaan Laane3, and Stephen Drucker1*
1Department of Chemistry, University of Wisconsin-Eau Claire, Eau Claire, WI 54702-4004
2Department of Chemistry, Hanyang University, 425-791 Ansan, Korea
3Department of Chemistry, Texas A&M University, College Station, TX 77843-3255 USA
The room-temperature cavity ringdown absorption spectra of 2-cyclopenten-1-one (2CP) and deuterated derivatives were recorded near 385 nm. The very weak (e < 1 M -1 cm-1) band system in this region is due to the T1 ¬ S0 electronic transition, where T1 is the lowest-energy 3(n,π*) state. The origin band was observed at 25,963.55(7) cm-1 for the undeuterated molecule and at 25,959.38(7) and 25,956.18(7) cm-1 for 2CP-5-d1 and 2CP-5,5-d2 , respectively. For the –d0 isotopomer, about 50 vibronic transitions have been assigned in a region from –500 to +500 cm-1 relative to the origin band. Nearly every corresponding assignment was made in the –d2 spectrum. Several excited-state fundamentals have been determined for the d0/d2 isotopomers, including ring-twisting ( υ'29 = 238.9/227.8 cm-1), out-of-plane carbonyl deformation (υ'28 = 431.8/420.3 cm-1), and in-plane carbonyl deformation ( υ'19 = 346.3/330.2 cm-1). The ring-bending (υ'30 ) levels for the T1 state were determined to be at 36.5, 118.9, 213.7, 324.5, and 446.4 cm-1 for the undeuterated molecule. These drop to 29.7, 101.9, 184.8, 280.5, and 385.6 cm-1 for the –d2 molecule. A potential-energy function of the form V = ax4 + bx2 was fit to the ring-bending levels for each isotopic species. The fitting procedure utilized a kinetic-energy expansion that was calculated based on the structure obtained for the triplet state from density functional calculations. The barrier to planarity, determined from the best-fitting potential-energy functions for the –d0, –d1, and –d2 species, ranges from 42.0 to 43.5 cm-1. In the T1 state, electron repulsion resulting from the spin flip favors nonplanarity. The S0 and S1 states have planar structures that are stabilized by conjugation.
Feature Article: Raman Spectroscopy of Vapors at Elevated Temperatures
Jaan Laane1*, Kristjan Haller2, Sachie Sakurai1, Kevin Morris1, Daniel Autrey1, Zane Arp1, Whe-Yi Chiang1, and Amanda Combs1
1Department of Chemistry, Texas A&M University, College Station, TX 77843-3255
2Institute of Physics, University of Tartu, Tartu, Estonia
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.
Daniel Autrey1, Zane Arp1, Jaebum Choo2, and Jaan Laane1*
1Department of Chemistry, Texas A&M University, College Station, TX 77843-3255
2Department of Chemistry, Hanyang University, 425-791 Ansan, Korea
The laser-induced fluorescence spectra and dispersed fluorescence spectra of jet-cooled 1,2-dihydronaphthalene have been analyzed to investigate the ring inversion process in both the S0 and S1(π,π*) excited states. Ultraviolet absorption, infrared, and Raman spectra were also recorded to complement the analyses. Ab initio calculations predict the inversion process to involve four out-of-plane ring motions, and linear combinations of these were made to model the inversion process. The data show the barrier to inversion in the ground state to be 1363 ± 100 cm-1(the triple-zeta ab initio value is 1524 cm-1). The experimental data indicate that the barrier increases substantially in the excited state, for which the calculated barrier is 1526 cm-1 with a CIS/6-311+G(d) basis set.
Zane Arp1, Niklas Meinander2, Jaebum Choo3, and Jaan Laane1*
1Department of Chemistry, Texas A&M University, College Station, TX 77843-3255
2Department of Physics, University of Helsinki, Finland
3Department of Chemistry, Hanyang University, 425-791 Ansan, Korea
The vapor-phase far-infrared, mid-infrared, ultraviolet, Raman, and laser-induced fluorescence spectra of indan have been recorded and analyzed. The far-infrared spectra, which are very similar to those previously reported, together with the Raman and dispersed fluorescence (SVLF) spectra of the jet-cooled molecules were used to reassign the ring-puckering and ring-flapping energy levels for the S0 ground state. These were then utilized to calculate a two-dimensional vibrational potential energy surface (PES) which nicely fits all of the assigned puckering and flapping levels. The PES has a barrier of 488 cm-1 as compared to a previously reported value of 1979 cm-1, which was based on a one-dimensional analysis and earlier assignments. The dihedral angle of puckering is ±30°. Fluorescence excitation spectra of jet-cooled indan together with ultraviolet absorption spectra were used to assign the flapping and puckering levels in the S1(π,π*) electronic excited state. The PES for this state has a barrier of 441 cm-1 and the energy minima correspond to puckering angles of ±39°. The flapping frequency and the stiffness of the PES along the flapping coordinate both decrease substantially in the excited state. The barriers to planarity for both states are higher than those for analogous molecules due to the two -CH2-CH2- torsional interactions. Ab initio calculations do a fairly good job of predicting the experimental barriers for indan and related molecules in their S0 and S1 states.
Niklas Meinander1 and Jaan Laane2*
1Department of Physics, University of Helsinki, Finland
2Department of Chemistry, Texas A&M University, College Station, TX 77843-3255
The formalism involved in the solution of the Schrödinger equation for two-dimensional vibrational potential energy surfaces for large-amplitude low-frequency motions is reviewed. The performance of two different bases, the prediagonalized harmonic basis (PHB) and the prediagonalized distributed Gaussian basis (PDGB), is investigated. The calculated energy levels obtained with the two basis sets are in excellent agreement with one another.
Jaan Laane
Department of Chemistry, Texas A&M University, College Station, TX 77843-3255 USA
For more than three decades far-infrared and Raman spectroscopy, along with appropriate quantum mechanical computations, have been effectively used to determine the potential energy functions which govern the conformationally important large-amplitude vibrations of non-rigid molecules. More recently, we have utilized laser-induced fluorescence (LIF) excitation spectroscopy and ultraviolet absorption spectroscopy to analyze the vibronic energy levels of electronic excited states in order to determined the potential energy surfaces and molecular conformations in these states. Transitions from the ground vibrational state in an S0 electronic state can typically be observed only to several excited vibronic levels. Hence, the LIF of the jet-cooled molecules generally provides data on only a few excited state levels. Ultraviolet absorption spectra recorded at ambient temperatures, however, often provide data on many additional excited vibronic levels. However, these can only be correctly interpreted if the electronic ground state levels have been accurately determined from the far-infrared, Raman, and dispersed fluorescence studies. In this article we will first present our results for four bicyclic molecules in the indan family in their S0 and S1(π,π*) electronic states. Two-dimensional potential energy surfaces in terms of the ring-puckering and ring-flapping vibrations were utilized for the analyses. Next we review our work on trans-stilbene in its S0 and S1(π,π*) states and examine the data from which two-dimensional potential energy surfaces were determined for the phenyl torsions and one-dimensional functions were calculated for the torsion about the C=C bond, which governs the photoisomerization. Finally, we consider seven cyclic ketones in their S0 and S1(n,π*) states. The carbonyl wagging vibration of each was studied in its electronic excited state in order to determine the barrier to inversion and the wagging angle. Conformational changes between the ground and excited electronic states were also examined in terms of the out-of-plane ring motions. The barrier to inversion was found to increase with angle strain.
Eugene Bondoc, Sachie Sakurai, Kevin Morris, Whe-Yi Chiang, and Jaan Laane*
Department of Chemistry, Texas A&M University, College Station, TX 77843-3255
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 6 B2 orbital transition. Thus type A absorption bands result from A1 6 A1 and B2 6 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 attain a comprehensive analysis of the electronic excited state.
Jaan Laane1*, Eugene Bondoc1, Sachie Sakurai1, Kevin Morris1, Niklas Meinander2, and Jaebum Choo3
1Department of Chemistry, Texas A&M University, College Station, TX 77843-3255
2Department of Physics, University of Helsinki, FIN-00014 , Finland
3Department of Chemistry, Hanyang University, 425-791 Ansan, Korea
The electronic absorption spectra and the laser induced fluorescence spectra of supersonic jet-cooled 1,3-benzodioxole molecules have been investigated in order to map out the vibronic energy levels in the S1(π, π*) electronic excited state. These were used to determine a two- dimensional potential energy surface in terms of the ring-puckering and ring-flapping vibrational coordinates, and the molecule was found to be puckered with a dihedral angle of 22°. The barrier to planarity in the excited state is 264 cm-1 (3.16 kJ/mole) as compared to 164 cm-1 (1.96 kJ/mole) in the ground state. This increase is attributed to reduced suppression of the anomeric effect by the benzene ring resulting from decreased π bonding character in the S1(π, π *) state. As expected, the motion along the flapping coordinate is governed by a more shallow potential energy well. Ab initio calculations carried out for both the ground and excited states support the experimental conclusions.
Spectroscopic Determination of Ground and Excited State Vibrational Potential Energy Surfaces
Jaan Laane
Department of Chemistry, Texas A&M University, College Station, TX 77843-3255 USA
Far-infrared spectra, mid-infrared combination band spectra, Raman spectra, and dispersed fluorescence spectra of non-rigid molecules can be used to determine the energies of many of the quantum states of conformationally important vibrations such as out-of-plane ring modes, internal rotations, and molecular inversions in their ground electronic states. Similarly, the fluorescence excitation spectra of jet-cooled molecules, together with electronic absorption spectra, provide the information for determining the vibronic energy levels of electronic excited states. One- or two-dimensional potential energy functions, which govern the conformational changes along the vibrational coordinates, can be determined from these types of data for selected molecules. From these functions the molecular structures, the relative energies between different conformations, the barriers to molecular interconversions, and the forces responsible for the structures can be ascertained. This review describes the experimental and theoretical methodology for carrying out the potential energy determinations and presents a summary of work that has been carried out for both electronic ground and excited states. The results for the out-of-plane ring motions of four-, five-, and six-membered rings will be presented, and results for several molecules with unusual properties will be cited. Potential energy functions for the carbonyl wagging and ring modes for several cyclic ketones in their S1(n,π*) will also be discussed. Potential energy surfaces for the three internal rotations, including the one governing the photoisomerization process, will be examined for trans-stilbene in both its S0 and S1(π,π*) states. For the bicyclic molecules in the indan family, the two-dimensional potential energy surfaces for the highly interacting ring-puckering and ring-flapping motions in both the S0 and S1(π,π*) states have also been determined using all of the spectroscopic methods mentioned above. Here, the effect of the electronic transition on the potential energy surface and hence the molecular structure can be ascertained.
S. Sakurai1, N. Meinander2, K. Morris1, and J. Laane1*
1Department of Chemistry, Texas A&M University, College Station, TX 77843-3255
2Department of Physics, University of Helsinki, FIN-00014 , Finland
The far-infrared and Raman spectra of 1,3-benzodioxole vapor have been recorded and analyzed. Forty-one infrared and six Raman bands were assigned to transitions between the various ring-puckering energy levels in the ground and excited ring-flapping states. The determination of the energy levels was assisted by analysis of the single vibronic level fluorescence spectra of the jet cooled molecules. The puckering levels change substantially in the flapping excited state indicating substantial interaction between the two vibrational modes. From the spectroscopic data a two-dimensional vibrational potential energy surface was determined. This has a barrier to planarity of 164 cm-1 and energy minima at puckering and flapping angles of ±24E and K3E, respectively. This molecule has a lower barrier to planarity than 1,3-dioxole reflecting the influence of the benzene ring on the anomeric effect. Nevertheless, the anomeric effect is clearly the origin of the non-planarity of this bicyclic ring system.