Slow monomer vibrations in formic acid dimer: Stepping up the ladder with FTIR and Raman jet spectroscopy

Abstract

In an effort to extend the cold gas phase spectroscopic database of the cyclic formic acid dimer (FAD), we present and analyze the jet-cooled vibrational infrared and Raman spectrum of (HCOOH)2 in the monomer fingerprint region between 600 and 1500 cm−1. The present study bridges the gap between the intermolecular dimerization-induced and the carbonyl stretching fundamentals that have already been reexamined using jet-cooled or high-resolution spectroscopy. This completes the characterization of the jet-cooled vibrational (HCOOH)2 spectrum below the complex OH (CH) stretching fundamentals, and we report resonance-induced FAD combination/overtone transitions that will serve as a valuable reference for a theoretical modeling of its vibrational dynamics. As a by-product, several new formic acid trimer fundamentals are identified in the jet spectra and assigned with the help of second-order vibrational perturbation theory (VPT2). The polar formic acid dimer still eludes detection in a supersonic jet, but we are able to estimate an experimental upper-bound of the polar dimer-to-trimer-to-cyclic dimer intensity ratio to about 1:10:100 under typical expansion conditions. Using VPT2 with resonance treatment (VPT2+K), we reinvestigate the notorious ν22 resonance triad. Generally, we find that VPT2, which is, of course, inadequate for modeling the resonance-rich OH stretching spectrum of FAD, is performing very satisfactorily in predicting fundamental and two-quantum state term values for the slower modes below 1500 cm−1. As these modes are the building blocks for the ultrafast energy dissipation in the OH stretching region, the present work opens the door for its quantitative understanding.

Coupled proton vibrations between two weak acids: the hinge complex between formic acid and trifluoroethanol

“…For challenging vibrational assignments in the gas phase, it is helpful to probe the molecular complexes by both infrared and Raman spectroscopy, due to complementary intensities even in the absence of symmetry. We thus employ two linear but still sufficiently sensitive supersonic jet spectrometers 19 to record the OH infrared 26 and Raman 27 fingerprints of the mixed dimers and trimers formed by T and F.…”

“…As a substitute for pure coupled-cluster-quality quartic force fields, which are very expensive to compute, hybrid force fields with only MP2-level cubic and quartic force constants have proven themselves for modelling fundamental vibrational excitations of carboxylic acids and their clusters 14,19,[33][34][35] using second-order vibrational perturbation theory (VPT2). 36,37 In this work, we combine CCSD(T)-F12a/VDZ-F12 [38][39][40] harmonic force constants, computed with Molpro version 2018, [41][42][43] with a semi-diagonal MP2/aVTZ 44,45 quartic force field (f i,j,k,l with four distinct indices missing), computed with Gaussian 16 Rev.…”

“…50 In a recent vibrational benchmark study of the formic acid dimer (see ESI of ref. 19), it was noticed that Gaussian scales the cubic and quartic force constants if the substituted force field approach is employed since the new harmonic wavenumbers are utilised to convert the force constants from mass-weighted to dimensionless normal coordinates. 51 All VPT2 results shown here were therefore computed with the GUINEA (version 3.0) module of the CFOUR program package 52,53 (version 2.1) using the unscaled lower-level cubic and quartic force constants; results obtained with scaled force constants are discussed in Section 1 in the ESI.…”

“…16 The fact that our experimental methods are not size selective means that the corresponding homoaggregates of T and F must be suppressed or at least well characterised. For larger aggregates, this is achieved by high dilution, but the TT 17 and FF dimers 18,19 as well as the monomer signals of T 20 and F 21 have to be coped with. Favourably, the TF dimers are intermediate in hydrogen bond strength such that spectral overlap is minimised.…”

High-dimensional neural network potentials for accurate vibrational frequencies: the formic acid dimer benchmark

“…Since the harmonic frequencies are well described by the FAD-HDNNP, it is now appropriate to investigate the representation of mode couplings as a next step. A compact measure for the mode couplings near the equilibrium structure can be obtained from a comparison of the VPT2 frequencies computed using the PES with gas-phase experimental results 49,51,54,64,103 as well as direct ab initio results which are still feasible for FAD, although the computations are very demanding taking several weeks. The corresponding anharmonic fundamental transition frequencies are provided in Table 4.…”