Dinitrogen as a Sensor for Metastable Carboxylic Acid Dimers and a Weak Hydrogen Bond Benchmarking Tool

Abstract

Molecular nitrogen as a weak hydrogen bond acceptor is added to formic and acetic acid and their monodeuterated isotopologues. FTIR spectroscopy of supersonic expansions in the O-H stretching region reveals the formation of the weakly bound N-carboxylic acid complexes. Their respective spectral downshifts from the monomer fundamental vibration are used to benchmark electronic structure calculations and vibrational perturbation theory. The small size of the investigated systems allows for a wide range of electronic structure levels to be explored. The O-H stretching vibration of an open dimer of acetic acid can be discriminated from the cyclic dimer vibrations by its higher susceptibility to coexpanded nitrogen.

Soft experimental constraints for soft interactions: a spectroscopic benchmark data set for weak and strong hydrogen bonds

“…We pick a relatively small subspace of compounds and observables, to illustrate some strategies and limitations of experimental benchmarking. Specifically, we focus on five well-studied strong hydrogen bond donors (hydrogen fluoride HF, [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] formic acid FA, [30][31][32][33][34][35][36][37] acetic acid AA, 30,32,[38][39][40] trifluoroethanol TE, [41][42][43] and hexafluoroisopropanol HP [44][45][46][47] ) and on some of their hydrogen-bonded dimers and trimers as well as complexes with very weak hydrogen bond acceptors (dinitrogen NN [48][49][50][51] and dioxygen OO [52][53][54]. We thus span the weakest and the strongest among the regular hydrogen bonds and initially include closely related systems to discover potential outliers in the experimental characterization.…”

Concerted Pair Motion Due to Double Hydrogen Bonding: The Formic Acid Dimer Case

“…It is important to build on the good VPT2 performance for the pair modes as test cases for alternative, presumably mostly variational methods 48,[92][93][94] , which reach further up on the energy scale. It may be too ambitious to aim at a quantitative description of the complex coupling pattern in the OH stretching region 36,41,[95][96][97] , but the more isolated couplings in the monomer fingerprint up to the carbonyl stretching region 81 are waiting to be understood in detail for FAD. Between these carbonyl stretching fundamentals and the OH stretching region, the PES starts to open up for other, metastable isomers 20,79,[98][99][100][101][102][103] , which need to be characterised.…”

“…Between these carbonyl stretching fundamentals and the OH stretching region, the PES starts to open up for other, metastable isomers 20,79,[98][99][100][101][102][103] , which need to be characterised. The enormous sensitivity of matrix isolation spectroscopy 104 can also be exploited towards this goal once the subtleties of spectral matrix shifts 23,24,96,105 are better Figure 7: Some diagonal-( x i,i ) and off-diagonal ( x i,j ) anharmonicity matrix elements for the six intermolecular modes, computed according to Eqs. (4) and (5).…”

Incremental NH stretching downshift through stepwise nitrogen complexation of pyrrole: a combined jet expansion and matrix isolation study

“…Anharmonic predictions for low frequency large amplitude motions are unphysical in all tested cases except the rigid P monomer, indicating limitations of the VPT2 approach when treating soft, nonrigid molecular systems such as nitrogen aggregates. 10 While localised high frequency modes such as NH stretching vibrations are typically less affected, errors are still introduced via coupling to the low frequency vibrations and could be of similar magnitude as the entire hydrogen bond induced shift.…”

“…5 Partial positive charges at the centre and negative charges at the molecule ends 6 lead to a more specific and directional guest-host interaction than for the similarly large argon host. 7 Stepwise nanocoating of hydrogenbonded organic compounds with matrix host molecules in supersonic expansions is a useful tool in understanding gas-to-matrix shifts [8][9][10] and may enable a more rigorous comparison of matrix isolation spectra to theoretical predictions.…”