Hydrogen bonds (HBs) play a major role in defining the structure and properties of substances and pathways of chemical processes. Typically, O-H…N interactions are quite strong and it is rare to find examples of weak HBs involving O-H…N arrangements. Such weak interaction exists in HCOOH/N2 complexes, thereby providing a unique example. The clear identification of these complexes using conventional techniques such as FT-IR spectroscopy is sometimes problematic due to the overlapping of closely lying parent and complex infrared bands. Helium nanodroplet isolation spectroscopy utilizes helium nanodroplets as an ultra-cold and gentle matrix to trap molecular species of interest for subsequent spectroscopic study. An attractive feature of rovibrational spectroscopy in superfluid helium droplets is that solvent shifts of the vibrational origins are typically within 0.1% compared to the gas phase. This can be contrasted with spectroscopy in traditional matrix environments (e.g. Ar or Ne), in which the shifts are usually much larger. Infrared depletion spectra of HCOOH/N2 heterodimer embedded in helium nanodroplets were measured at 0.37 K and MP2/aug-cc-pVTZ calculations were done to interpret the experimental results. A set of observed features at 3552 to 3549 cm-1 and a feature at 2939.18 cm-1 are assigned to the O-H (ν1) and C-H (ν2) stretching vibrations of hydrogen bonded trans-HCOOH…N2(I), respectively. The shifts of the measured line positions of 20.01 and 5.24 cm-1 for the O-H and C-H stretching vibrations of the heterodimer (I) with respect to the monomer correlate well with the results from MP2 calculations. The red-shift in the O-H stretch vibration and an NCI analysis indicate the existence of a weak HB (15 ± 7 kJ mol-1) in trans-HCOOH…N2(I). According to the potential energy surface, the trans-HCOOH…N2(II) van der Waals complex is destabilized by 4 kJ mol-1 with respect to the complex (I) and the barrier for the (II) - (I) conversion is estimated to be ~0.3 kJ mol-1. This explains why only complex (I) is observed in helium droplets in contrast to the inert gas matrix where both complexes have been found.

Investigation of the helium nanodroplet isolated<i> trans</i>-HCOOH…N<sub>2</sub> complex by infrared laser spectroscopy

ACS Spring Meeting 2021

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Investigation of the helium nanodroplet isolated trans-HCOOH…N2 complex by infrared laser spectroscopy

Investigation of the helium nanodroplet isolated trans-HCOOH…N₂ complex by infrared laser spectroscopy