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

Formic acid-methanol complexation vs. esterification: elusive pre-reactive species identified by vibrational spectroscopy

Vibrational spectra of the mixed dimer and the two mixed trimers of methanol and formic acid as well as some of their isotopologues are presented. Out of the eight expected OH stretching fundamentals of these three pre-reactive hydrogen-bonded complexes, the three modes mainly involving an acid OH group bound to the alcohol appear to be missing in the jet-cooled spectra despite a combination of infrared and Raman probing. A possibility of spectral overlap is discussed in the mixed dimer case, but largely discarded. The missing modes correspond to (fractional) concerted elongation of all engaged OH bonds, promoting synchronous degenerate proton transfer between the molecules. One other trimer mode is very tentatively attributed to a broad spectral feature, whereas all OH bonds contacting carbonyl groups can be unambiguously identified by four relatively narrow infrared absorptions. The spectral features are confirmed by vibrational perturbation theory and deviate in a subtle but systematic way from scaled harmonic predictions which were previously validated for the formic acid complex with a more acidic alcohol. Despite being exothermic and exergonic, ester formation can only be detected in the rarefied gas expansions after extended pre-mixture of the gases, which somewhat contrasts the recent microwave spectroscopic evidence of in situ ester formation and in particular the lack of pre-reactive complex signals.

Mid-infrared Laser Spectroscopy of Jet-Cooled Formic Acid Trimer: Mode-Dependent Line Broadening in the C-O Stretching Region

Building on recent progress in the vibrational spectroscopy of the formic acid trimer, we present the first high-resolution measurements of the jet-cooled laser absorption spectrum of (HCOOH)3. The spectra of the lowest- and highest-frequency C-O stretching fundamentals are analyzed whereas the third band is not observed, complicated by monomer and dimer absorptions at 1219 cm-1 (8.2 μm). Vibration-rotation parameters are obtained for the band at 1172.31512(68) cm-1 whereas the C-O stretch at 1246.33(5) cm-1 exhibits a significantly larger breadth, allowing only resolution of the coarse PQR structure. Vibrational predissociation can be ruled out, and intramolecular vibrational redistribution mechanisms are discussed, particularly coupling to the concerted proton exchange within the cyclic dimer subunit. Ultimately, the question remains open. The prospects of high-resolution measurements of other trimer bands or isotope substitution experiments, which might assist in revealing the mode-specificity of the underlying broadening mechanisms, are discussed.

On the vibrations of formic acid predicted from first principles

In this article, we review recent first principles, anharmonic studies on the molecular vibrations of gaseous formic acid in its monomer form. Transitions identified as fundamentals for both cis- and trans form reported in these studies are collected and supported by results from high-resolution experiments. Attention is given to the effect of coordinate coupling on the convergence of the computed vibrational states.

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

Raman and FTIR spectra of an acid–alcohol complex show complementary signatures from acidic and alcoholic OH stretching, proving its existence.

CC-stretched formic acid: isomerisation, dimerisation, and carboxylic acid complexation

The cis-trans-isomerism of the propiolic acid monomer (HC[triple bond, length as m-dash]C-COOH) is examined with linear Raman jet spectroscopy, yielding the first environment-free vibrational band centres of a higher-energy cis-rotamer beyond formic acid (HCOOH) in addition to all fundamentals and a large number of hot and combination/overtone bands of the trans-conformer. Two near-isoenergetic trans-fundamentals of different symmetry (CC[double bond, length as m-dash]O bend and OH torsion) prove to be a sensitive benchmarking target, as their energetic order is susceptible to the choice of electronic structure method, basis set size, and inclusion of vibrational anharmonicity. For the infrared- and Raman-active C[double bond, length as m-dash]O stretching fundamentals of the cyclic (C_2h) trans-propiolic acid dimer, resonance couplings are found that in part extend to the C_s-symmetric heterodimer of trans-propiolic and trans-formic acid. Exploratory vibrational perturbation theory (VPT2) calculations show that all perturbing states involve displacements of the OH moieties located on the doubly hydrogen bonded ring. The comparison of the infrared spectra of the propiolic acid dimer and its heterodimer with formic acid to that of several other carboxylic acid dimers from the literature reveals a notable similarity regarding a non-fundamental dimer band around 1800 cm^-1, which in most cases is so far unassigned. VPT2 calculations and a simple harmonic model suggest an assignment to a combination vibration of the symmetric and antisymmetric OH torsion.

Glycolic Acid as a Vibrational Anharmonicity Benchmark

Three energetically close pairs of vibrational states in glycolic acid are investigated by Raman spectroscopy in a supersonic jet to provide challenging benchmarks for vibrational and electronic structure theory and to solve some open issues in this prototypical hydroxy acid. The alcoholic OH stretching fundamental is located only 8 cm^-1 below the acidic OH stretch at 3586 cm^-1, much less shifted than predicted by previous anharmonic calculations and by experimental analogy to a fluorene derivative. This and further near-degeneracies in the CH and C=O stretching region are used to assess the predictive power of an exploratory set of quantum chemical calculations including anharmonic VPT2 corrections.

Shifting formic acid dimers into perspective: vibrational scrutiny in helium nanodroplets

A metastable dimer of formic acid has been prepared inside superfluid helium nanodroplets and examined using IR spectroscopy and quantum chemical calculations.

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

Formic acid dimer as the prototypical doubly hydrogen-bonded gas-phase species is discussed from the perspective of the three translational and the three rotational degrees of freedom which are lost when two formic acid molecules form a stable complex. The experimental characterisation of these strongly hindered translations and rotations is reviewed, as are attempts to describe the associated fundamental vibrations, their combinations, and their thermal shifts by different electronic structure calculations and vibrational models. A remarkable match is confirmed for the combination of a CCSD(T)-level harmonic treatment and an MP2-level anharmonic VPT2 correction. Qualitatively correct thermal shifts of the vibrational spectra can be obtained from classical molecular dynamics in CCSD(T)-quality force fields. A detailed analysis suggests that this agreement between experiment and composite theoretical treatment is not strongly affected by fortuitous error cancellation but fully converged variational treatments of the six pair or intermolecular modes and their overtones and combinations in this model system would be welcome.

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

An experimental benchmark data base on rotational constants, vibrational properties and energy differences for weakly and more strongly hydrogen-bonded complexes and their constituents from the spectroscopic literature is assembled. It is characterized in detail and finally contracted to a more compact, discriminatory set (ENCH-51, for Experimental Non-Covalent Harmonic with 51 entries). The meeting points between theory and experiment consist of equilibrium rotational constants and harmonic frequencies and energies, which are back-corrected from experimental observables and are very easily accessible by quantum chemical calculations. The relative performance of B3LYP-D3, PBE0-D3 and M06-2X density functional theory predictions with a quadruple-zeta basis set is used to illustrate systematic errors, error compensation and selective performance for structural, vibrational and energetical observables. The current focus is on perspectives and different benchmarking methodologies, rather than on a specific theoretical method or a specific class of compounds. Extension of the data base in chemical, observable and quantum chemical method space is encouraged.

Stretching of cis-formic acid: warm-up and cool-down as molecular work-out

A new technique to rotationally simplify and Raman-probe conformationally and vibrationally excited small molecules is applied to the cis-trans isomerism of formic acid. It quintuples the previously available gas phase vibrational data base on this excited form of a strongly anharmonic planar molecule despite its limited spectral resolution. The newly determined cis-formic acid fundamentals allow for a balanced vibrational benchmark on both rotamers of formic acid. Assuming the adequacy of vibrational perturbation theory, it reveals weaknesses of standard methods for these systems like B3LYP-D3(BJ)/aVQZ VPT2 or PBE0-D3(BJ)/aVQZ VPT2. The functionals ωB97-XD and M06-2X additionally suffer from severe integration grid size and symmetry dependencies. The vibrational benchmark suggests B2PLYP-D3(BJ)/aVQZ VPT2 and MP2/aVQZ VPT2 as partially competitive and in any case efficient alternatives to computationally demanding coupled cluster vibrational configuration interaction calculations. Whether this is due to fortuitous compensation between electronic structure and vibrational perturbation error remains to be explored.

Modulations of νO-H and νC═O Stretching Frequencies of Difluoroacetic Acid with Internal Rotation of CHF2 Rotor: A Combined Vapor Phase and Matrix Isolation Infrared Spectroscopy Study

Mid-infrared spectra of difluoroacetic acid (DFAA) have been measured by isolating the molecule in argon and nitrogen matrices at 8 K and also in the vapor phase at room temperature. In argon matrix, the O-H stretching fundamental (ν_O-H) of -COOH group appears as a doublet with band maxima at 3554 and 3558 cm^-1, and a similar doublet for C═O stretching fundamental appears at 1800 and 1810 cm^-1. In the vapor phase, the ν_O-H transition is featured with multiple peaks, and the observed band shape has been deconvoluted as superposition of two transitions both having A-type rotational band contours. We have attributed these transitions to the two internal rotational isomers corresponding to the two distinct minima along -CHF₂ torsional coordinate of the molecule. Natural bond orbital (NBO) analysis reveals that these torsional minima are the manifestations of different second order interactions involving bonding and antibonding orbitals corresponding to the rotor -CHF₂ and COOH groups of the molecule. By use of the theoretically predicted rotational constants of the rotamers, the band profile for ν_O-H has been simulated satisfactorily by means of the PGOPHER method, and this has allowed estimating accurately the energy difference between the two rotamers as 0.54 kcal/mol. The predicted energy barrier for interconversion between the rotamers is very small, ∼0.5 kcal/mol from rotamer II to rotamer I, which implies that the molecule could hop almost freely between the two rotameric forms at room temperature. As a result, the frequencies of the key stretching vibrational modes, like ν_O-H, ν_C═O, and ν_C-H, undergo modulation with internal rotation of the rotor -CHF₂ group. Such modulation of high frequency modes could be an efficient mechanism for acceleration of rotor-induced IVR (intramolecular vibrational redistribution) well documented in the literature. Furthermore, the spectra measured in matrix isolated environment show signatures for an energetically higher third rotamer, where -OH and -C═O groups are in anti orientation. It has also been shown that DFAA can easily form weak hydrogen bonded dimeric complexes with molecular nitrogen (N₂), which causes ν_O-H to undergo a red shift of ∼30 cm^-1 in argon matrix for all three DFAA monomers.

Dimers of acetic acid in helium nanodroplets

Two metastable dimers are created inside superfluid helium and studied using infrared spectroscopy to provide insight into condensed phase structures.

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

The NH stretch of pyrrole experiences downshifts when expanded with N₂ or embedded in pure/mixed N₂ matrices, no blueshift.