investigations on vibrational overtones in thenir

Preface
Spectroscopy plays a key role in our understanding the basics of physical and life
sciences. This thesis covers the spectroscopy of several polyatomic organic molecules,
unravelling their structures, and electronic conﬁgurations.
Vibrational overtone spectroscopy of polyatomic molecules containing X-H
oscillators (X=C, N, 0, etc) provides valuable information regarding molecular structure,
conformation, intra- and inter—molecular interactions, radiationless transitions,
inlramolecular vibrational relaxations, multiphoton excitations, chemical reactivities,
anisotropic environment created by lone pair and / or TE electron interaction and the effect
of substitution on the strength of individual X-H bonds on the parent molecule. Local
mode model, in which X-H bonds are considered to be loosely coupled anharrnonic
oscillators, has been widely used to interpret the observed vibrational overtone spectra of
these molecules.
The central theme of this research concerns the study of vibrationally excited
molecules. We have used the local mode description of such vibrational states, and this
-model has now gained general acceptance. A central feature of the model is the
Wloealizaﬁon of vibrational energy. A study of these high—energy localized states provides
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new information about the structural and dynamical properties of molecules. For
example, becauseof this localization, overtone spectra, which measure the absorption of
T vibrational energy, are extremely sensitive to the properties of X-H bonds. We also use
-overtone spectra to study the conformation of molecules, i.e., the relative internal
orientation of their bonds. The thesis comprises six chapters.
The first chapter presents a review of the earlier works on vibrational overtone
spectroscopy of X-H containing molecules. This chapter also discusses an outline of the
. local mode theory and its reﬁnements including the theory of coupled local mode
oscillators. A description of the spectrophotometric measurements of the overtones is
given at the end of this chapter. The next two chapters (2 and 3) deal in detail the results
of experimental investigations on the vibrational overtone spectra of some organic
molecules in the liquid phase.
In the second chapter, the analysis of the NIR vibrational overtone spectra of
liquid phase six and five membered aromatic molecules— benzyl amine, furfuyl amine,
ﬁirfural and cinnamaldehyde and some methoxy substituted benzenes- anisole, o
anisidine, p—anisidine and p-anisaldehyde are presented. A comparison of the local mode
parameters of the ring C-H and N-H oscillators in benzyl amine and furfuryl amine
indicates that the electron donation from the—CH2NHz group to the aromatic ring is
similar in both molecules. The hetero oxygen atom in furfuryl amine is thus not involved
in any interaction to the -CH2NH2 group attached to the ring. A direct inductive
interaction between the substituent aldehydic oxygen and the ring oxygen atom is
observed in furfural. In cinnarnaldehyde molecule the effect of lone pair effect is
analyzed. The presence of non-equivalent methyl C-H bonds in methoxy benzenes is
established and the possible factors inﬂuencing the local mode parameters of the ring C
H and methyl C-H bonds are discussed. The substituent effects are also analyzed in detail
for each molecule.
The third chapter discusses the overtone spectra of the aliphatic compounds- t
butyl amine and n-butyl amine. The effect of the amine group in these molecules is
discussed in detail. The analysis of overtone spectra of t-butyl amine reveals that the
methyl group in this molecule maintains Cgv local symmetry. The vibrational
Hlllliltonian of three-coupled oscillator system is used to predict the pure local mode
and local- local combination peaks. Calculated frequencies obtained by
diagonalizing the coupling matrices of the C3v Hamiltonian are consistent with
experimentally observed frequencies.
A number of applications of laser spectroscopy have been outlined in Chapter 4
and 5. The fourth chapter describes the experimental work done using a pulsed Nd: YAG
laser. Pulsed lasers are ideal for the study of the different physico-chemical events as the
delivery of photons to the system can be achieved in a well-deﬁned time interval, and
then the evolution of the subsequent physical and chemical events can be followed by
appropriate optical techniques. In our set up, the emitted photons are collected using a
monochromator —CCD detector assembly. Raman spectroscopy is a useful technique for
the identification of substances in any phase. The pulsed Raman spectra of 3
ﬂuorotoluene, anisole, t-butyl amine and p—anisidine are recorded and the assignments of
observed Raman shifts are done. The observed Raman spectra contain some new lines
corresponding to low lying states. The pulsed LIF spectrum of polymerized samples of
benzyl amine and n-butyl amine, and monomer sample of n-butyl amine are recorded and
analyzed.
The ﬁfth chapter deals with Tunable Diode Laser Absorption Spectroscopy
(TDLAS). It gives a brief review of TDLAS, its application and high-resolution
spectroscopic studies. The tunability of the laser wavelength is utilized to excite different
conformers of isobutanol molecule. The high-resolution ovenone spectrum of 0-H in the
second overtonc region in isobutanol is recorded using a linear and a long path length
high resolution set up and the conformations are studied.
The sumrna.ry and conclusion of the present studies is given after the last chapter.
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