Spectroscopy and dynamics of double proton transfer in formic acid dimer

Abstract

We present the isolated gas phase infrared spectra of formic acid dimer, (HCOOH)2, and its deuterated counterpart formic-d acid, (DCOOH)2, at room temperature. The formic acid dimer spectrum was obtained by spectral subtraction of a spectrum of formic acid vapor recorded at low pressure from that recorded at a higher pressure. The spectra of formic acid vapor contain features from both formic acid monomer and formic acid dimer, but at low and high pressures of formic acid, the equilibrium is pushed towards the monomer and dimer, respectively. A similar approach was used for the formic-d acid dimer. Building on the previous development of the Molecular Mechanics with Proton Transfer (MMPT) force field for simulating proton transfer reactions, molecular dynamics (MD) simulations were carried out to interpret the experimental spectra in the OH-stretching region. Within the framework of MMPT, a combination of symmetric single and double minimum potential energy surfaces (PESs) provides a good description of the double proton transfer PES. In a next step, potential morphing together with electronic structure calculations at the B3LYP and MP2 level of theory was used to align the computed and experimentally observed spectral features in the OH-stretching region. From this analysis, a barrier for double proton transfer between 5 and 7 kcal mol(-1) was derived, which compares with a CCSD(T)/aug-cc-pVTZ calculated barrier of 7.9 kcal mol(-1). Such a combination of experimental and computational techniques for estimating barriers for proton transfer in gas phase systems is generic and holds promise for further improved PESs and energetics of these important systems. Additional MD simulations at the semi-empirical DFTB level of theory agree quite well for the center band position but underestimate the width of the OH-stretching band.

Kinetic isotope effects and how to describe them

“…The reference data 110 were obtained at the MP2 level of theory and the fitting error over the entire grid is well below 1 kcal/mol (350 cm –1 ), but, of course, not of spectroscopic accuracy. As mentioned above, further adaptation can be achieved through morphing 98,100,114,115 . The additional MMPT-energy is written as $V_{\text{MMPT}}=true\{\begin{array}{cc}left{V}_0(R,\rho )+k·{\theta }^2,\hspace{0.17em}\text{for}\hspace{0.17em}\mathrm{a}\hspace{0.17em}\text{linear}\hspace{0.17em}\mathrm{H}-\text{bond}\hfill & \hfill \\ left{V}_0(R,\rho )+V_d(R,\rho ,d),\text{for}\hspace{0.17em}\mathrm{a}\hspace{0.17em}\text{non}-\text{linear}\hspace{0.17em}\mathrm{H}-\text{bond},\hfill \end{array}$where R is the distance of D and A, and r is the distance of D and H*.…”

Reactive molecular dynamics: From small molecules to proteins

“…Both processes can be cast within the framework of MS-ARMD 67,234 but alternative techniques have also been employed. Typical applications are the investigation of the change in the solvent structure around metal-complexes [235][236][237][238][239] or the spectroscopic signatures accompanying proton transfer, 68,69,[240][241][242][243][244][245] or the energetics of proton transfer in proteins. 246 A field by itself is the investigation of proton transfer in bulk water for which a myriad of FF-based techniques have been developed.…”

“…Furthermore, MS-ARMD can also be combined with MM with proton transfer (MMPT) 66 to follow proton transfer in the gas-and condensed phase. [67][68][69][70] It is instructive to briefly compare EVB and the two ARMD approaches. While all three methods start from an empirical force field perspective, the EVB constructs an n × n Hamiltonian with the force fields V nn for the pure states on the diagonal and mixing elements V mn (m 6 ¼ n) as off-diagonal elements.…”

Ultrafast dynamics induced by the interaction of molecules with electromagnetic fields: Several quantum, semiclassical, and classical approaches

“…MMPT with suitably morphed potential energy surfaces was also employed to analyze the gas-phase infrared spectra of formic acid dimer (FAD) 198 and of protonated oxalate 259 . For FAD, a combination of a symmetric double (SDM) and single minimum (SSM) surface yields a realistic description of the double proton transfer potential energy surface (see Fig.…”

“…15). 198 Conversely, for protonated oxalate, the two resonance forms of the molecule can be parametrized such that the change in bonding character of the CO-subunit (from single. to double-bonded) upon proton transfer is incorporated into the energy function 259 .…”