Theoretical modeling of the formic acid dimer infrared spectrum: Shaping the O–H stretch band (original) (raw)
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Vibrational spectrum of the formic acid dimer in the OH stretch region. A model 3D study
Chemical Physics Letters, 2001
The vibrational spectrum of the formic acid dimer in the OH stretch region is calculated by solving a 3D vibrational Schr odinger equation and employing an electronic potential and dipole moment calculated at the MP2 level of theory. Two Raman-active bands of the symmetric OH stretch fundamental are located around 2600 cm À1 ; the gap between them ($300 cm À1 ) is caused by concerted proton tunneling. IR-active bands around 3100 cm À1 are assigned to the antisymmetric OH stretch fundamental, its ®rst overtone and two complex transitions. The calculated H =D isotope eect on the intensity of the antisymmetric OH fundamental is 1.92. Ó (J.M. Bowman). 0009-2614/01/$ -see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 -2 6 1 4 ( 0 1 ) 0 1 2 4 8 -9
Journal of Molecular Spectroscopy, 2008
The recent reaction surface Hamiltonian model for the double proton tunneling in formic acid dimer of Barnes et al. [G.L. Barnes, S.M. Squires, E.L. Sibert, J. Phys. Chem. B. 112 595.] has been applied to the calculation of the symmetric OH stretching Raman spectra. We interpret the full Raman spectra obtained through use of a simplified, single minimum spectrum. Extensive state mixing is found, leading to broad spectral features. Results compare well with the experimental measurements of Bertie et al. , J. Chem. Phys. 85 (9) (1986) 4779]. We also report improvements upon our previous approach and present ground state and fundamental frequencies as well as tunneling splittings obtained with our new method.
Theoretical interpretation of the infrared lineshapes of the H- and D-bonds in liquid formic acid
Chemical Physics, 2017
A full quantum theoretical approach has been used to study the ν O-H experimental IR line shapes of liquid formic acid. For this purpose, a previous model [M. E-A. Benmalti, A. Krallafa, N, Rekik, M. Belhakem, Spectrochimica Acta Part A 74 (2009) 58-66], which accounts for the proportion of cyclic dimers, has been successfully adapted. The present model thus incorporates the strong anharmonic coupling between the high frequency mode and the H-bond bridge, the Davydov coupling between the excited states of the two moieties, multiple Fermi resonances between the ν O-H (B u) mode and combinations of some bending modes, together with the quantum direct and indirect dampings. This model reproduces satisfactorily the main features of the experimental lineshapes of liquid hydrogenated and deuterated formic acid, by using a minimum set of independent parameters.
Local vibrational modes of the formic acid dimer – the strength of the double hydrogen bond
Molecular Physics, 2013
The 24 normal and 24 local vibrational modes of the formic acid dimer formed by two trans formic acid monomers to a ring (TT1) are analysed utilising preferentially experimental frequencies, but also CCSD(T)/CBS and ωB97X-D harmonic vibrational frequencies. The local hydrogen bond (HB) stretching frequencies are at 676 cm −1 and by this 482 and 412 cm −1 higher compared to the measured symmetric and asymmetric HB stretching frequencies at 264 and 194 cm −1. The adiabatic connection scheme between local and normal vibrational modes reveals that the lowering is due to the topology of dimer TT1, mass coupling, and avoided crossings involving the H•••OC bending modes. The HB local mode stretching force constant is related to the strength of the HB whereas the normal mode stretching force constant and frequency lead to an erroneous underestimation of the HB strength. The HB in TT1 is stabilised by electron delocalisation in the O=C-O units fostered by forming a ring via double HBs. This implies that the CO apart from the OH local stretching frequencies reflect the strength of the HB via their red or blue shifts relative to their corresponding values in trans formic acid.
Journal of Chemical Theory and Computation, 2010
Recent experimental studies of trans-formic acid (FA) in solid para-hydrogen (pH 2) highlighted the importance of vibrationally averaged dipole moments for the interpretation of the high-resolution infrared (IR) spectra, in particular for the CdO stretch (ν 3) mode. In this report, dipole moments for the ν 3 ground (v) 0) and excited (v) 1, 2, 3, and 4) anharmonic vibrational states in trans-FA are investigated using two different approaches: a single mode approximation, where the vibrational states are obtained from the solution of the one-dimensional Schrö dinger equation for the harmonic normal coordinate, and a limited vibrational configuration interaction (VCI) approximation. Density functional theory (B3LYP, BPW91) and correlated ab initio (MP2 and CCSD(T)) electronic methods were employed with a number of double-and triple-and correlation consistent basis sets. Both single mode and VCI approaches show comparable agreement with experimental data, which is more dependent on the level of theory used. In particular, the BPW91/cc-pVDZ level appears to perform remarkably well. Effects of solvation of FA in solid state Ar and pH 2 matrices were simulated at the BPW91/cc-pVDZ level using a conductor-like polarized continuum model (CPCM). The Ar and pH 2 solid-state matrices cause quite a substantial increase in the FA dipole moments. Compared to gas-phase calculations, the CPCM model for pH 2 better reproduces the experimental FA spectral shifts caused by interaction with traces of ortho-hydrogen (oH 2) species in solid pH 2. The validity of the single mode approach is tested against the multidimensional VCI results, suggesting that the isolated (noninteracting) mode approximation is valid up to the third vibrationally excited state (v) 3). Finally, the contribution of the ground anharmonic vibrational states of the remaining modes to the resulting ν 3 single mode dipole moments is examined and discussed.
Journal of Chemical Theory and Computation, 2010
Recent experimental studies of trans-formic acid (FA) in solid para-hydrogen (pH 2 ) highlighted the importance of vibrationally averaged dipole moments for the interpretation of the high-resolution infrared (IR) spectra, in particular for the CdO stretch (ν 3 ) mode. In this report, dipole moments for the ν 3 ground (v ) 0) and excited (v ) 1, 2, 3, and 4) anharmonic vibrational states in trans-FA are investigated using two different approaches: a single mode approximation, where the vibrational states are obtained from the solution of the one-dimensional Schrö dinger equation for the harmonic normal coordinate, and a limited vibrational configuration interaction (VCI) approximation. Density functional theory (B3LYP, BPW91) and correlated ab initio (MP2 and CCSD(T)) electronic methods were employed with a number of double-and triple-and correlation consistent basis sets. Both single mode and VCI approaches show comparable agreement with experimental data, which is more dependent on the level of theory used. In particular, the BPW91/cc-pVDZ level appears to perform remarkably well. Effects of solvation of FA in solid state Ar and pH 2 matrices were simulated at the BPW91/cc-pVDZ level using a conductor-like polarized continuum model (CPCM). The Ar and pH 2 solid-state matrices cause quite a substantial increase in the FA dipole moments. Compared to gas-phase calculations, the CPCM model for pH 2 better reproduces the experimental FA spectral shifts caused by interaction with traces of ortho-hydrogen (oH 2 ) species in solid pH 2 . The validity of the single mode approach is tested against the multidimensional VCI results, suggesting that the isolated (noninteracting) mode approximation is valid up to the third vibrationally excited state (v ) 3). Finally, the contribution of the ground anharmonic vibrational states of the remaining modes to the resulting ν 3 single mode dipole moments is examined and discussed.
The Journal of Physical Chemistry A, 2012
We present a simulation of the OH stretching band in the gas-phase IR spectra of strongly hydrogen-bonded dimers of phosphinic acid and their deuterated analogs [(R 2 POOH(D), with R = CH 2 Cl, CH 3 ], which is based on a model for a centrosymmetric hydrogen-bonded dimer that treats the high-frequency OH stretches harmonically and the low-frequency intermonomer (i.e., O•••O) stretches anharmonically. This model takes into account the following effects: anharmonic coupling between the OH and O•••O stretching modes; Davydov coupling between the two hydrogen bonds in the dimer; promotion of symmetry-forbidden OH stretching transitions; Fermi resonances between the fundamental of the OH stretches and the overtones of the in-and out-of-plane bending modes involving the OH groups; direct relaxation of the OH stretches; and indirect relaxation of the OH stretches via the O•••O stretches. Using a set of physically sound parameters as input into this model, we have captured the main features in the experimental OH(D) bands of these dimers. The effects of key parameters on the spectra are also elucidated. By increasing the number and strength of the Fermi resonances and by promoting symmetry-forbidden OH stretching transitions in our simulations, we directly see the emergence of the ABC structure, which is a characteristic feature in the spectra of very strongly hydrogen-bonded dimers. However, in the case of the deuterated dimers, which do not exhibit the ABC structure, the Fermi resonances are found to be much weaker. The results of this model therefore shed light on the origin of the ABC structure in the IR spectra of strongly hydrogen-bonded dimers, which has been a subject of debate for decades.
Chemical Physics Letters, 2004
A direct dynamics calculation based on instanton techniques is reported of tunneling splittings in the formic acid dimer and its deuterated isotopomers for the zero-point level and the lowest vibrationally excited level of a nontotally symmetric CO-stretch vibration. A new method is introduced to calculate the effect of anharmonic couplings for skeletal modes that are not linearly coupled to proton tunneling modes. The calculated splittings are in good agreement with recent observations for (DCOOH) 2 provided the proposed tentative assignment is reversed such that the excited level has the smaller splitting.