The application of supersonic jet spectroscopy to the study of low-frequency, large-amplitude, anharmonic motions in the excited states of floppy molecules has been discussed. In particular, our work on inversion and torsional vibrations of 9, 10-dihydroanthracene and dihydrophenanthrene by laser-induced fluorescence excitation technique has been reviewed. The inversion of dihydroanthracene is nicely fitted with a quadratic Gaussian type potential function and the height of the barrier in the S₁ state is calculated to be 94 cm₋₁. The inter-ring torsional frequencies of dihydrophenanthrene appear as a prominent progression and Frank-Condon analysis shows that the S₁ state of the molecule is planar in contrast to a skew configuration in the ground state. A peculiar alternation of band gaps has been noticed.

Laser-induced fluorescence spectra of a 2:1 complex between 7-azaindole and water, known as `non-reactive dimer’ of the molecule, have been measured in a supersonic jet expansion. The dispersed fluorescence spectrum of the electronic origin band of the complex shows a very large number of low-frequency vibrational features corresponding to different intermolecular modes of the complex in the ground electronic state. Geometries of several possible isomeric structures of the complex and their vibrational frequencies at harmonic approximation were calculated by electronic structure theory method at MP2/6-31G^∗∗ level. An excellent agreement is observed between the measured and calculated intermolecular vibrational mode frequencies for the energetically most favoured structure of the complex, where the water molecule is inserted within one of the two N$\cdots$H-N hydrogen bonds of the 7AI dimer.

Mid infrared spectra of two O–H· · · π hydrogen-bonded binary complexes of acetic acid (AA) and trifluoroacetic acid (F₃AA) with benzene (Bz) have been measured by isolating the complexes in an argon matrix at ∼8 K. In a matrix isolation condition, the O–H stretching fundamentals (ν_O−H) of the carboxylic acid groups of the two molecules are observed to have almost the same value. However, the spectral red-shifts of ν_O−H bands of the two acids on complexation with Bz are largely different, 90 and 150 cm⁻¹ for AA and F₃AA, respectively. Thus, the O–H bond weakening of the two acids upon binding with Bz in a non-interacting environment follows the sequence of their ionic dissociation tendencies (pK_a) in aqueous media. Furthermore, ΔνO−H of the latter complex is the largest among the known π-hydrogen bonded binary complexes of prototypical O–H donors reported so far with respect to Bz as acceptor. It is also observed that the spectral shifts (Δν_O−H) of phenol-Bz and carboxylic acid-Bz complexes show similar dependence on the acidity factor (pK_a). Electronic structure theory has been used to suggest suitable geometries of the complexes that are consistent with the measured IR spectral changes. Calculation at MP2/6-311++G (d, p) level predicts a T-shaped geometry for both AA-Bz and F₃AA-Bz complexes, and the corresponding binding energies are 3.0 and 4.5 kcal/mol, respectively. Natural Bond Orbital (NBO) analysis has been performed to correlate the observed spectral behavior of the complexes with the electronic structure parameters.

The kinetic parameters of an important atmospheric reaction, CH₃O₂+NO₂+M→CH₃O₂NO₂+M, have been recorded by monitoring directly the changes in concentrations of methylperoxy radicals (CH₃O₂) in the gas phase employing a new mid-infrared quantum cascade laser (QCL)-based apparatus. CH₃O₂ radicals in our apparatus have been generated by pulsed UV laser (266 nm) photolysis of CH₃I in a gaseous mixture with oxygen. The absorption band corresponding to the mid-infrared O-O stretching fundamental of the peroxy radical, within a narrow spectral range, 1070–1120 cm^-1, has been recorded by tuning the wavelength of the QCL operated in CW mode. The kinetics of the aforementioned reaction of CH₃O₂ with NO₂ has been followed by analyzing the changes of the infrared (QCL) decay profile of CH₃O₂ at 9.1 μm (1098.9 cm^-1) maintaining a pseudo first order reaction condition. We noticed that the rate constant of the reaction at 298 K varies in the range of (1.21–3.08) ×10^-12 cm³ molecule^-1s^-1 for changing the total pressure in the range of 75–730 mbar. The absorption cross-section of CH₃O₂ at the probe wavelength (1098.9 cm^-1), has been estimated for the first time to be 8.3±0.4×10^-20 cm².