NH stretching vibrations of pyrrole clusters studied by infrared cavity ringdown spectroscopy

Abstract

The IR spectra for various sizes of pyrrole clusters were measured in the NH stretching vibration region by infrared cavity ringdown spectroscopy. The hydrogen-bonded structures and normal modes of the pyrrole clusters were analyzed by a density functional theory calculation of the B3LYP/6-311+G(d,p) level. Two types of pulsed nozzles, a slit and a large pinhole, were used to generate different cluster size distributions in a supersonic jet. A rotational contour analysis of the NH stretching vibration for the monomer revealed that the slit nozzle provides a warmer jet condition than the pinhole one. The IR spectra, measured under the warmer condition, showed the intense bands at 3444, 3392, and 3382 cm(-1), which were assigned to hydrogen-bonded NH stretching vibrations due to the dimer, the trimer, and the tetramer, respectively. On the other hand, the IR spectra measured under a lower temperature condition by a pinhole nozzle showed a broad absorption feature in addition to sharp bands. This broad absorption was reproduced by the sum of two Gaussians peaks at 3400 and 3372 cm(-1) with widths of 30 and 50 cm(-1) (FWHM), respectively. Compared with the spectra of the condensed phase, two bands at 3400 and 3372 cm(-1) were assigned to hydrogen-bonded NH stretching vibrations of larger clusters having liquid-like and solid-like structures, respectively.

Hydrogen-bonded structures of pyrrole-solvent clusters: Infrared cavity ringdown spectroscopy and quantum chemical calculations

“…1, the weak absorption intensities due to the NH stretching vibrations of the Py dimer and the trimer were observed at 3444 and 3392 cm −1 , respectively. 16 This result suggests that the Py-solvent binary clusters observed in the spectra are expected to consist of 2 or 3 molecules. Figure 1͑a͒ shows the IR spectrum obtained from the Py and H 2 O vapor mixtures.…”

“…16 Briefly, the ringdown cavity consisted of two highly reflective concave mirrors ͑R = 99.97% at 2.9 m, f = −3000 mm͒. The cavity length was 600 mm, and the cavity axis was set to 10 mm downstream from the pulsed nozzle.…”

“…In this study, we used a slit type of pulsed nozzle in order to prevent large clusters from being generated; a stainless-steel plate of 1 mm thick with a slit ͑100 m ϫ 10 mm͒ was directly mounted on the orifice flange of commercial electromagnetic pulsed valve with a circular orifice of 2 mm diameter. 16 The vacuum chamber was pumped by a 10 in. diffusion pump, and the typical background pressures without and during an operation of the pulsed nozzle were 3 ϫ 10 −6 and 5 ϫ 10 −5 Torr, respectively.…”

“…14,15 Recently, we reported on the IR spectra of various sizes of the Py clusters in the NH stretching vibration region by infrared cavity ringdown spectroscopy ͑IR-CRDS͒. 16 In this study, we successfully presented the hydrogen-bond formation between the NH groups and the electron cloud in the Py clusters. More recently, Dauster et al 17 reported on the NH fundamental and overtone stretching vibrations of the Py clusters by Fourier-transform infrared spectroscopy.…”

“…Among them, the bands at 3531, 3444, and 3392 cm −1 are the NH stretching vibrations of the Py monomer, dimer, and trimer, respectively. 16 Since the bands at 3689, 3576, and 3470 cm −1 are assigned to the OH stretching vibrations of ͑CH 3 OH͒ 1-3 , the remaining five bands can be assigned to the NH or OH stretching vibrations of the Py-CH 3 OH binary clusters. Among them, the bands at 3414 and 3507 cm −1 have already been assigned to the NH stretching vibration and combination bands of the Py-CH 3 OH 1:1 cluster, respectively.…”