Sandhya Gopalakrishnan

The absorption spectroscopy of the lowest pseudorotational states of tetrahydrofuran

The rotational structure of the pseudorotational (PR) band n=0→n=2 has been observed in jet-cooled tetrahydrofuran in the 170–360 GHz frequency range. The observed transitions were analyzed together with the previously obtained microwave data of Meyer and co-workers [R. Meyer, J. C. Lopes, J. L. Alonso, S. Melandri, P. G. Favero, and W. Caminati, J. Chem. Phys. 111, 7871 (1999)]. The experimentally observed transitions provide direct spectroscopic evidence of the symmetry ordering of the lowest four observed PR states. Based on the symmetry properties of the pseudorotational states involved in this study, an analytical model of the potential energy surface (PES) along the pseudorotational path has been proposed that provides a consistent explanation of all the observed transition frequencies, including those from the early IR work. In addition, an analysis of the variation of the rotational constants of the molecule in different PR states has been performed using the proposed model. This analytical PES an...

Rotationally resolved B̃–X̃ electronic spectra of both conformers of the 1-propoxy radical

Five bands of the B–X laser induced fluorescence spectrum of jet-cooled 1-propoxy radical have been recorded with a spectral resolution of ≈200 MHz. The resolved rotational and fine structure of these bands has been assigned and analyzed providing rotational constants for both the X and B states as well as components of the electron spin-rotation tensor in the X state. By comparison of these constants with ones obtained from quantum chemistry calculations, two bands have been assigned to the gauche (G) conformer of 1-propoxy and 3 bands to the trans (T) conformer. The spectrum of each conformer abruptly terminates after the excitation of a single C–O stretch.

Theoretical prediction of spectroscopic constants of 1-alkoxy radicals

Abstract Spectroscopic constants for the conformers of the 1-alkoxy radicals were computed by ab initio and density functional methods. These parameters include ground and excited state rotational constants, harmonic frequencies, and A – X and B – X excitation energies. Elements of the electronic spin–molecular rotation tensor have also been predicted for the ground electronic state utilizing a simple transformation. The transformation employed the experimentally observed spin-rotation tensor of the ethoxy radical as a reference and the calculated rotational constants of the ethoxy radical and the investigated 1-alkoxy radicals. The predicted spectroscopic parameters are in good agreement with the experimental ones where available, and found to be useful parameters for assigning the rotationally resolved spectral bands to given conformers.

Dispersed fluorescence spectroscopy of primary and secondary alkoxy radicals

Dispersed fluorescence (DF) spectra of 1-propoxy, 1-butoxy, 2-propoxy, and 2-butoxy radicals have been observed under supersonic jet cooling conditions by pumping different vibronic bands of the B-X laser induced fluorescence excitation spectrum. The DF spectra were recorded for both conformers of 1-propoxy, three conformers of the possible five of 1-butoxy, the one possible conformer of 2-propoxy, and two conformers of the possible three of 2-butoxy. Analysis of the spectra yields the energy separations of the vibrationless levels of the ground X and low-lying A electronic state as well as their vibrational frequencies. In all cases, the vibrational structure of the DF spectra is dominated by a CO stretch progression yielding the nuCO stretching frequency for the X state and in most cases for the A state. In addition to the experimental work, quantum chemical calculations were carried out to aid the assignment of the vibrational levels of the X state and for some conformers the A state as well. Geometry optimizations of the different conformers of the isomers were performed and their energy differences in the ground states were determined. The results of the calculation of the energy separations of the close-lying X and A states of the different conformations are provided for comparison with the experimental observations.

Submillimeter wave vibration–rotation spectroscopy of Ar⋅CO and Ar⋅ND3

A new absorption spectrometer operating in the submillimeter wave region has been designed. The apparatus combines the previously reported fast scan submillimeter wave spectroscopic technique (FASSST) and a pulsed supersonic jet sample. It is specially designed for the rapid searching for unknown molecular transitions of weakly absorbing and/or low abundance species. Vibration–rotation transitions of the Ar⋅CO and Ar⋅ND3 van der Waals molecules have been observed using this apparatus. Transitions in Ar⋅CO involve the j=1→j=2 hindered rotor transition in the complex. Transitions in Ar⋅ND3 have been assigned as the rotational structure of the fundamental of the lowest frequency bending mode involving the two moieties. Transitions have been observed emanating from the levels correlating to the two lowest energy inversion components of ND3 yielding a determination of the inversion splitting in the complex.