Slit jet infrared spectroscopy of hydrogen bonded N2HF isotopomers: Rotational Rydberg–Klein–Rees analysis and H/D dependent vibrational predissociation rates

Abstract

High resolution IR laser direct absorption spectra in a slit jet are presented and analyzed for nitrogen (15N14N–HF, 14N15N–HF, 15N15N–HF), and deuterium (14N14N–DF) substituted N2HF isotopomers. Both 14N15N–HF and 15N14N–HF isomers are observed, indicating a sufficiently deep minimum in the hydrogen bonding potential energy surface to quench internal rotation of the N2. The vibrationally averaged stretching potentials for each substituted species are recovered from rotational Rydberg–Klein–Rees (RKR) analysis. Features of the one-dimensional (1D) potential surface such as hydrogen bond length (RH-bond), harmonic force constant (kσ), and well depth (De) are then tested for isotopic invariance by direct comparison of the different isotopomers. Agreement among the various N substituted species for HF based complexes for either vHF=0 or 1 is excellent, and provides effective 1D potentials for the stretching coordinate between 3.39 and 3.75 Å. There is a 43 cm−1 (∼10%) strengthening of the hydrogen bond upon HF vibrational excitation, as quantitatively reflected in the experimental redshifts and the shape of the RKR potentials for vHF=0 and 1. The hydrogen bond is further strengthened by D/H isotopic substitution; this is a result of reduced vibrational averaging over DF vs HF bending motion, yielding a more linear, and hence stronger, hydrogen bond geometry. In contrast to the nearly apparatus-limited linewidths (Δνprediss∼7 MHz) observed for each of the N2HF isotopomers, the N2DF complexes yield significantly broadened lines with 73±9 MHz homogeneous linewidths due to vibrational predissociation. This tenfold increase in predissociation rates upon deuteration is in contrast to previous measurements in other HF/DF containing complexes, and indicates the importance of a near resonant vibrational channel to form N2(v=1)+DF(v=0). The energetic accessibility of this V→V channel would suggest an upper limit on the N2DF binding energy of D0≤547 cm−1, which is also consistent with upper limits on D0 from the rotational RKR analysis.