Pulsed nozzle Fourier transform microwave spectrometer: an ideal spectrometer to define hydrogen bond radius

Abstract

A pulsed nozzle Fourier transform microwave (PNFTMW) spectrometer has been recently fabricated at the Indian Institute of Science. The basic design is the same as that of Balle and Flygare. However, recent advances in microwave and computer technologies have helped in designing a state-of-the-art PNFTMW spectrometer. The range of the spectrometer is from 2 to 26.5 GHz. Strong signals from OCS and several of its low abundant isotopes as well as weakly bound complexes such as Ar-(H₂O), Ar-H₂S and Ar₃-H₂S have been obtained. For the OCS parent isotopomer, the observed line width is only 2.8 kHz, indicating the high resolution of the spectrometer. Small hyperfine splittings could also be observed from the ¹³C spin-rotation interaction (4.9 kHz) in OCS and the I=1 state of H₂O (10 kHz) in Ar-H₂O complex. This article gives the details of the newly fabricated PNFTMW spectrometer. By analysing the structural data available in the literature, most of it using PNFTMW spectrometer, we had recently defined a 'hydrogen bond radius' for all the hydrogen halides. This approach is extended to acetylene and water in this article, as models for the OH and CH groups involved in H bonding in biological systems. The H bond radius for H₂O is 0.78 ± 0.07 Å and that of C₂H₂ is 1.09 ± 0.05 Å.