Ne3−NH3 van der Waals Tetramer: Rotational Spectra and ab Initio Study of the Microsolvation of NH3 with Rare Gas Atoms

A broadband Fourier transform microwave spectrometer based on chirped pulse excitation

Designs for a broadband chirped pulse Fourier transform microwave (CP-FTMW) spectrometer are presented. The spectrometer is capable of measuring the 7-18 GHz region of a rotational spectrum in a single data acquisition. One design uses a 4.2 Gsampless arbitrary waveform generator (AWG) to produce a 1 mus duration chirped pulse with a linear frequency sweep of 1.375 GHz. This pulse is sent through a microwave circuit to multiply the bandwidth of the pulse by a factor of 8 and upconvert it to the 7.5-18.5 GHz range. The chirped pulse is amplified by a traveling wave tube amplifier and broadcast inside the spectrometer by using a double ridge standard gain horn antenna. The broadband molecular free induction decay (FID) is received by a second horn antenna, downconverted, and digitized by a 40 Gsampless (12 GHz hardware bandwidth) digital oscilloscope. The second design uses a simplified pulse generation and FID detection scheme, employing current state-of-the-art high-speed digital electronics. In this spectrometer, a chirped pulse with 12 GHz of bandwidth is directly generated by using a 20 Gsampless AWG and upconverted in a single step with an ultrabroadband mixer. The amplified molecular emission is directly detected by using a 50 Gsampless digital oscilloscope with 18 GHz bandwidth. In both designs, fast Fourier transform of the FID produces the frequency domain rotational spectrum in the 7-18 GHz range. The performance of the CP-FTMW spectrometer is compared to a Balle-Flygare-type cavity-FTMW spectrometer. The CP-FTMW spectrometer produces an equal sensitivity spectrum with a factor of 40 reduction in measurement time and a reduction in sample consumption by a factor of 20. The CP-FTMW spectrometer also displays good intensity accuracy for both sample number density and rotational transition moment. Strategies to reduce the CP-FTMW measurement time by another factor of 90 while simultaneously reducing the sample consumption by a factor of 30 are demonstrated.

Characterization of Weak NH−π Intermolecular Interactions of Ammonia with Various Substituted π-Systems

Among the several weak intermolecular interactions pervading chemistry and biology, the NH-pi interaction is one of the most widely known. Nevertheless its weak nature makes it one of the most poorly understood and characterized interactions. The present study details the results obtained on gas-phase complexes of ammonia with various substituted pi systems using both laser vibrational spectroscopy and ab initio calculations. The spectroscopic measurements carried out by applying one-color resonant two-photon ionization (R2PI) and IR-vibrational predissociation spectroscopy in the region of the NH stretches yield the first experimental NH stretching shifts of ammonia upon its interaction with various kinds of pi-systems. The experiments were complemented by ab initio calculations and energy decompositions, carried out at the second-order Møller-Plesset (MP2) level of theory. The observed complexes show characteristic vibrational spectra which are very similar to the calculated ones, thereby allowing an in-depth analysis of the interaction forces and energies. The interaction energy of the conformers responsible for the observed vibrational spectra has the maximum contribution from dispersion energies. This implies that polarizabilities of the pi-electron systems play a very important role in governing the nature and geometry of the NH-pi interaction. The larger polarizability of ammonia as compared to water and the tendency to maximize the dispersion energy implies that the characteristics of the NH-pi interactions are markedly different from that of the corresponding OH-pi interactions.