Matrix Isolation Spectroscopy and Nuclear Spin Conversion of NH₃ and ND₃ in Solid Parahydrogen

Abstract

We present matrix isolation infrared absorption spectra of NH3 and ND3 trapped in solid parahydrogen (pH2) at temperatures around 1.8 K. We used the relatively slow nuclear spin conversion (NSC) of NH3 and ND3 in freshly deposited pH2 samples as a tool to assign the sparse vibration-inversion-rotation (VIR) spectra of NH3 in the regions of the ν2, ν4, 2ν4, ν1, and ν3 bands and ND3 in the regions of the ν2, ν4, ν1, and ν3 fundamentals. Partial assignments are also presented for various combination bands of NH3. Detailed analysis of the ν2 bands of NH3 and ND3 indicates that both isotopomers are nearly free rotors; that the vibrational energy is blue-shifted by 1-2%; and that the rotational constants and inversion tunneling splitting are 91-94% and 67-75%, respectively, of the gas-phase values. The line shapes of the VIR absorptions are narrow (0.2-0.4 cm(-1)) for upper states that cannot rotationally relax and broad (>1 cm(-1)) for upper states that can rotationally relax. We report and assign a number of NH3-induced infrared absorption features of the pH2 host near 4150 cm(-1), along with a cooperative transition that involves simultaneous vibrational excitation of a pH2 molecule and rotation-inversion excitation of NH3. The NSCs of NH3 and ND3 were found to follow first-order kinetics with rate constants at 1.8 K of k = 1.88(16) × 10(-3) s(-1) and k = 1.08(8) × 10(-3) s(-1), respectively. These measured rate constants are compared to previous measurements for NH3 in an Ar matrix and with the rate constants measured for other dopant molecules isolated in solid pH2.

Fingerprints of angulon instabilities in the spectra of matrix-isolated molecules

“…Our goal is to provide a microscopic description of the spectra shown using the angulon theory and thereby demonstrate that the broadening is due to an angulon instability, accompanied by a resonant transfer of 3 of angular (c) Schematics of relevant gas-phase molecular levels (black solid lines) and the corresponding spectroscopic transitions (blue arrows). The case of NH3 involves additional inversion doubling of the levels, which is not shown [67]. In the presence of helium, the R R1(1) transition (red line) takes place between the angulon states, 11(11, 0) and 22 (11,1 3 ) (dashed lines), which results in the line broadening encircled in (a) and (b).…”

“…(c) Schematics of relevant gas-phase molecular levels (black solid lines) and the corresponding spectroscopic transitions (blue arrows). The case of NH3 involves additional inversion doubling of the levels, which is not shown[67]. In the presence of helium, the R R1(1) transition (red line) takes place between the angulon states, 11(11, 0) and 22(11, 1 3 ) (dashed lines), which results in the line broadening encircled in (a) and (b).…”

The frequency-domain infrared spectrum of ammonia encodes changes in molecular dynamics caused by a DC electric field

“…infrared spectroscopy | electric field | orientation | inversion | tunneling A mmonia, a classic example of a symmetric-top molecule, rotates nearly freely inside a solid argon matrix. Only the lowest-energy rotational levels are significantly populated at cryogenic matrix temperature (1)(2)(3)(4)(5). Ammonia is fluxional with respect to inversion, with the ν 2 (umbrella mode) infrared fundamental band displaying features that encode the dynamics of inversion by tunneling through a barrier in a symmetric double-minimum potential (1).…”

Matrix Isolation Spectroscopy and Nuclear Spin Conversion of Propyne Suspended in Solid Parahydrogen

“…To study the NSC of propyne molecules isolated in solid pH 2 below lHe temperatures, room-temperature populations of excited rotational states of propyne must be deposited into the solid much faster than they can relax, as in NH 3 /pH 2 studies. 14 Thus, the RVD procedure was typically performed even more rapidly by setting Φ H 2 ≈ 400 mmol hr −1 to enhance observations of metastable propyne. Rapid-scan FTIR spectra recorded with modest time resolution (3−5 min depending on averaging) were collected during the deposition step while calibrating the dopant flowrate with Φ H 2 in order to enhance S/N and maximize contributions from freshly deposited rotating propyne molecules and, of course, immediately post deposition in order to monitor propyne NSC in as-deposited solid pH 2 samples near ∼1.7 K until equilibration.…”

“…The experimental apparatus for pH 2 matrix isolation spectroscopy in the Wyoming laboratory has been discussed previously, 14,29 so only details relevant to this study are presented below. Propyne-doped pH 2 solids are grown via the rapid-vapor-deposition (RVD) technique developed by Fajardo and Tam.…”