Low-Lying Potential Energy Surfaces - American Chemical Society

Chapter 19

Spectroscopic Determination of Potential Energy Surfaces for the Out-of-Plane Ring Vibrations of Indan and Related Molecules in Their S and S (Π, Π*) States Downloaded by STANFORD UNIV GREEN LIBR on September 14, 2012 | http://pubs.acs.org Publication Date: August 14, 2002 | doi: 10.1021/bk-2002-0828.ch019

0

1

1

1

1

1

1

2

J. Laane , Z. Arp , S. Sakurai , K. Morris , N. Meinander , T. Klots , E. Bondoc , K. Haller , and J. Choo 1

1

1

1

Department of Chemistry, Texas A&M University, College Station, TX 77843 SC Johnson Polymer, 8310 16th Street, Sturtevant, WI 53177 2

The laser induced fluorescence excitation spectra of jet-cooled indan and related moelcules along with their ultraviolet absorption spectra have been used to study their S (π,π*) excited states. Far-infrared, Raman, and dispersed fluorescence were utilized to obtain the vibrational data for the S ground states. This allowed the potential energy surfaces (PESs) of these molecules to be determined in terms of the ring-puckering and ring-flapping coordinates for both states. These PESs provide barriers to planarity and conformational structures for these bicyclic molecules. Phthalan has a barrier of 35 cm i n S but no barrier for S . Coumaran has an S barrier of 279 cm , while 1,3-benzodioxole has barriers of 171 and 264 cm for S and S , respectively, due to the anomeric effect. The barriers for indan are 1077 cm for S and 698 cm for S . Ab initio calculations in general provide good barrier values for the ground state. 1

-1

0

-1

1

0

-1

0

-1

-1

0

380

1

0

© 2002 American Chemical Society

In Low-Lying Potential Energy Surfaces; Hoffmann, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

381

Introduction Conformational changes in molecules often proceed along vibrational pathways. In selected cases these pathways can be represented by one- or twodimensional vibrational potential energy surfaces (PESs). In our laboratory we have for many years investigated the PESs of cyclic and bicyclic molecules in their electronic ground states using far-infrared and Raman spectroscopy. " In recent years we have also used fluorescence spectra of jet-cooled molecules and ultra violet absorption spectra to determine the PESs i n electronic excited states. " In the present paper we present our results for the ground (S ) and excited [ β ^ π , π * ) ] electronic states of several molecules in the indan family which are shown in Scheme I.

Downloaded by STANFORD UNIV GREEN LIBR on September 14, 2012 | http://pubs.acs.org Publication Date: August 14, 2002 | doi: 10.1021/bk-2002-0828.ch019

1

5

3

5

0

@> m

90.9 νρ=0 vf=0

10.9 0

508.4 932

i

a 325.2

383.7

I5.9

•30.6 236.8 79.5i

vp=2 472.6

88.9I

415J 90.0

563.5

ΊΓ 90.9

297.8 267.2

vp=l

157.3

f%2

99.1 89.5 9.6 0

vp=0

vp=0

Figure 10. Ring-puckering quantum states of 1,3-benzodioxole in differ flapping and electronic states.



0

Figure 11. Two-dimensional potential energy surface of 1,3-benzodioxole in its S state.



Figure 12. The η-σ* anomeric interaction for 1,3-benzodioxole. The planar structure (left) has no interaction but puckering (right) allows overlap to occur.


395 1

nearly doubled to 264 cm" . This value is much more in line with the 1,3-dioxole result where there is not competition from benzene ring orbital interactions with the oxygen non-bonded orbitals. Thus, the π - π * transition clearly decreases the suppression of the anomeric effect. The parent molecule in this group of molecules is indan. Its far-infrared spectrum shows a group of closely spaced bands originating at 143 cm" arising from the ring-puckering while bands near 250 cm" are due to the ring-flapping. The ring-twisting can be seen in the vapor-phase Raman spectrum at 178 cm" . Without additional information, however, the assignment of the levels is not unambiguous. Figure 13 shows the FES and ultraviolet absorption spectra of indan. The ring-puckering, twisting, and flapping bands can be observed at 116, 137, and 176 cm" for the S ^ ^ T i * ) state, all at lower values reflecting the reduced rigidity of the molecule. Figure 14 shows the dispersed (SVLF) spectra resulting from excitation of the 0 ° band. This along with the dispersed spectra from the other vibronic quantum states in S not only provide energy level data for the S ground state, but also help with the assignment of the excited state. Vibronic levels associated with puckering levels in S tend to give rise to fluorescence transitions to puckering levels in S , twisting vibronic states fluoresce to twisting S levels, and flapping vibronic states to flapping S levels. This type of analysis then is invaluable for making both the S and β ^ π , π * ) energy level assignments. Figure 15 presents the one-dimensional potential energy fonctions for the ringpuckering for both electronic states based on the experimental data. The barrier for the ground state is 1077 cm* ; this drops to 698 cm" for the excited state. For indan, as for coumaran, the barrier arises from - C H - C H - torsional interactions which tend to bend the five-membered ring into a non-planar conformation. Since there are three of these interactions as opposed to two for coumaran, the barrier is considerably higher in the ground state. Ab initio calculations were also carried out to compare predicted barrier heights with experimental values. The results are summarized i n Table 1. The agreement for the ground state is remarkably good except for indan where the computed barrier is too low. For excited state calculations the computational methodology is not as well established and the calculated values are less satisfactory. 1

1


1

1

0

x

0

{

0

0

0

0

1

1

2

2

Conclusions The spectroscopic methods described here provide a powerful way for accurately determining PESs for both ground and excited electronic states. The work on the indan family of molecules provided a number of unusual results.


396


36904

35904

36404

-1000 cm

Figure 13. Fluorescence excitation and ultraviolet absorption spectra of in

ο

UV absorption

FES

It Ί

37400

u 37200

37000 cm"

36800

1

Figure 14. Dispersedfluorescence(SVLF) spectra ofjet-cooled indan.



397

Figure 15. One-dimensional potential energyfunctionsfor the ring-pucker indan in its ground and excited electronic states.

Table L Comparison of Experimental and Ab Initio Barriers 1

(cm ) for Molecules in the Indan Family Ground State

Ab Initio^

ε ^ π , π * ) Excited State

Molecule

Exp.

Exp.

PHT

35

91

0

0

BZD

164

171

264

369

COU

279

258

—

IND

488

662

441

-

Ab Initio

—

841

Ground: MP2/6-31G* basis set Excited: CIS/6-31 l+G(2s.p)//CIS/6-31+G* basis set


398 Phthalan has a low but unexpected barrier to planarity i n S which disappears as the five-membered ring becomes more rigid in the excited state. 1,2-Benzodioxole is non-planar i n S due to the anomeric effect. In Sj the anomeric effect and barrier increase as suppression by interactions with the benzene ring is decreased. Indan has a much higher barrier to planarity than the similar cyclopentene. This barrier decreases in Ab initio calculations do a remarkably good job i n predicting barriers for the electronic ground sate (less so for indan), but the methodology for excited state calculations is not as well established. 0


0

Acknowledgements The authors wish to thank the National Science Foundation, the Texas Advanced Research Program, and the Robert A . Welch Foundation for financial assistance.

References 1. 2.

3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Laane, J. Pure&Appl.Chem. 1987, 59, 1307. Laane, J. In Structures and Conformations of Non-Rigid Molecules, J. Laane, M. Dakkouri, B . van der Veken, and H . Oberhammer, Eds., Amersterdam, 1993; Chapter 4. Laane, J. Annu. Rev. Phys. Chem. 1994, 45, 179. Laane, J. Int. Rev. Phys. Chem. 1999, 18, 301. Laane, J. J. Phys. Chem,, 2000, 104, 7715. Malloy, T. B . J. Mol. Spectrosc., 1972, 44, 504. Laane, J.; Harthcock, M. A.; Killough, P. M.; Bauman, L. E . ; Cooke, J. M . J. Mol. Spectrosc., 1982, 91, 286. Harthcock, Μ. Α.; Laane, J. J. Mol. Spectrosc., 1982, 91, 300. Schmude, R. W.; Harthcock, Μ. Α.; Kelly, M. B . ; Laane, J. J. Mol. Spectrosc., 1987, 124, 369. Tecklenburg, M. M.; Laane, J. J. Mol. Spectrosc., 1989, 137, 65. Strube, M. M.; Laane, J. J. Mol. Spectrosc., 1988, 129, 126. Cheatham, C. M.; Huang, M . - H . ; Laane, J. J. Mol. Struct., 1996, 377, 93. Cheatham, C. M.; Huang, M . - H . ; Meinander, N.; Kelly, M. B . ; Haller, K . ; Chiang, W . - Y . ; Laane, J. J. Mol. Struct., 1996, 377, 81. Morris, K . Ph.D. Thesis, Texas A&M University, 1999; Morris, K.; Laane, J. to be published.


399 15. 16. 17. 18. 19.


20.

Bondoc, E.; Klots, T.; Laane J. J. Phys. Chem., 2000, 104, 275. Klots, T.; Sakurai, S.; Laane, J. J. Chem. Phys., 1998, 108, 3531. Sakurai, S.; Meinander, N.; Laane, J. J. Chem. Phys., 1998, 108, 3537. Cortez, E . ; Verastegui, R.; Villarreal, J. R.; Laane, J. J. Amer. Chem. Soc., 1993, 115, 12132. Sakurai, S.; Meinander, N.; Morris, K.; Laane, J., J. Amer. Chem. Soc., 1999, 121, 50. Laane, J..; Bondoc, E . ; Sakurai, S.; Morris, K.; Meinander, N.; Choo, J. J. Amer. Chem. Soc., 2000, 122, 2628.


Low-Lying Potential Energy Surfaces - American Chemical Society

Recommend Documents