2Ba08S
SPECTRA AND STRUCTURE OF SOME FLOPPY MOLECULES COOLED IN
SEEDED JET
Mihir Chowdhury
Department of Physical Chemistry
Indian Association for the Cultivation of Science
Jadavpur, Kolkata 700 032, India
The pioneering works of Raman and Mizushima in the area of vibrational spectroscopy
have been widely exploited by physical chemists for obtaining structural information aboutmolecules. This lecture, dedicated to the two great scientists, reviews our work on low-frequency
vibronic spectra of jet-cooled large organic molecules.
A chemical change involves hopping of the system from one well in the multidimensional potential energy surface (PES) to another. A simplified model for this process whichlends itself to further detailed treatment is large-amplitude vibration along the soft mode of a
floppy molecule. It is therefore pertinent that we collect more information about the groundand/or excited state PES corresponding to motions like aryl rotation, ring-puckering, H-bondstretching and intermolecular dissociation.
In the condensed phase the perturbation by the surrounding environment is large enoughto cause deformation and blurring of conformer identities. It is necessary, therefore, to work witha set of collisionless, cold, uniformly moving molecules as provided in a jet. The vibronic
features of the laser-induced fluorescence (LIF) and fluorescence excitation (FE) spectra of thecold molecular beam are often amenable to analysis in terms of a fundamental frequency and its
higher harmonics, which in turn can be correlated with the nature of excited and ground PESs.
The molecules of interest to us are listed in the table. In particular, the power of twocolour IR-UV double resonance technique in determining structures of clusters is illustrated withfive hydrates of tetrahydroisoquinoline.
Acknowledgement: The works have been carried out by my collaborators with DST support. I
thank Professor O. Kajimoto and Prof. Y. Yoshihara for the invitation to deliver this talk.
References:
1. T. Chakraborty, M.Chowdhury; Chem. Phys. Letts. 171, 1990, 25
2. N. Guchait, R. Dutta,P.Dutta,T.Chakraborty, Ind. J. Phys. 66B, 1992, 241
3. T. Chakraborty, M.Chowdhury; Chem. Phys. Letts. 177, 1991, 223
4. T. Chakraborty, D. N. Nath, M.Chowdhury; J. Chem. Phys. 96, 1992, 6456
5. N. Guchhait, T. Chakraborty, D.Majumdar, M.Chowdhury, J.Phy. Chem. 98,1994, 9227
6. A. Ckaraborty et al. J. Chem.Phys. 114, 2001, 865
7. S. Banerjee et.al. Chem.Phys.Letts. 366, 2002, 211
8. N. Guchhait et.al. J. Mol.Struct. 327, 1994, 161
9. A.Chakraborty et.al. J. Chem.Phys. 115, 2001, 1
10. N.Guchhait et. al. J. Chem. Phys. (Communicated)

Molecules

Referen-ce
numberTable I
Vibrational modes (cm-1)
GroundExcited1-28Butterfly (Puckering)(symmetry-equivalent
double well, low barrier)

2-53

Butterfly (Puckering)(symmetry-equivalent
double well, low barrier)

3-90

Torsion(symmetry-equivalent
double well, low barrier)

4

135

26

Butterfly (Puckering) (softmode with barrier less thanzero-point energy)

5105
160

95
179

Puckering Twist
Puckering Bend

6-66, 87
128

Puckering (in phase andout-of-phase boat and chairform)

7

188
81

129
60

Ring Puckering twist[in phase and out-of-phase](Sym and antisym twist
conformer)
Puckering coupled to Ninversion

DihydroanthraceneO

Description of modes andpotential energy surface

Xanthene

Dihydrophenanthrene
FFFFFFFF

Octafluoronaphthalene

TetralinHexahydropyrene

Octahydroanthracene
845119NHIndoline

9N

H

95
144

92
98

3717
3377
3352

-

Tetrahydroisoquinoline
N. H 2OH

THIQ (H2O)1 cluster10

Out-of-plane N-puckering(axial and equatorial form)
Free OH
H-bonded OH
H-Bonded NH(Five clusters with five
distinct IR-signatures)