Absolute infrared band intensities and air broadening coefficient for spectroscopic measurements of formic acid in air

The absolute infrared intensities of the ν₂, ν₃ and ν₆ bands of formic acid have been evaluated in a 480 L White cell system using FTIR and ion chromatography techniques. The values obtained are, respectively; (4.2 ± 0.2) × 10⁻¹⁷ cm molec⁻¹ for the ν₆ band, (4.8 ± 0.2) × 10⁻¹⁷ cm molec⁻¹ for the ν₃ band and (0.57 ± 0.04) × 10⁻¹⁷ cm molec⁻¹ for the ν₂ band. The air broadening coefficient of transitions in the ν₆ band, has been measured using a tunable diode laser spectrometer, equal to (0.101 ± 0.005) cm⁻¹ atm⁻¹ (half width at half maximum). A computer search has been performed to find absorption lines of formic acid suitable for second derivative tunable diode laser measurement of this gas in ambient air.     

Compliant fields for molecular interactions: Water dimer and formic acid dimer

The redundancy-free internal valence compliance constants of open-chain water dimer and formic acid cyclic dimer have been determined by the combined use of the CNDO /Force method and the compliance constant formalism. The final compliant fields of these dimers have been refined with the help of experimental frequency data.     

Communication: The highest frequency hydrogen bond vibration and an experimental value for the dissociation energy of formic acid dimer

The highest frequency hydrogen bond fundamental of formic acid dimer, ν(24) (B(u)), is experimentally located at 264 cm(-1). FTIR spectra of this in-plane bending mode of (HCOOH)(2) and band centers of its symmetric D isotopologues (isotopomers) recorded in a supersonic slit jet expansion are presented. Comparison to earlier studies at room temperature reveals the large influence of thermal excitation on the band maximum. Together with three B(u) combination states involving hydrogen bond fundamentals and with recent progress for the Raman-active modes, this brings into reach an accurate statistical thermodynamics treatment of the dimerization process up to room temperature. We obtain D(0) = 59.5(5) kJ/mol as the best experimental estimate for the dimer dissociation energy at 0 K. Further improvements have to wait for a more consistent determination of the room temperature equilibrium constant.     

Ground-state and vibrationally assisted tunneling in the formic acid dimer

The previously developed instanton theory [G. V. Mil'nikov and H. Nakamura, J. Chem. Phys. 122, 124311 (2005)] is applied to the calculation of vibrationally assisted tunneling splitting of the deuterated formic acid dimer (DCOOH)2 with all the degrees of freedom taken into account. The ground-state tunnel splitting is determined by the density-functional theory combined with coupled cluster level of quantum chemistry to be 0.0038 cm(-1) which is comparable to the experimental value of 0.0029 cm(-1). Further, the tunnel splittings of fundamental excitations are estimated for frequencies below 300 cm(-1). In this energy range it is found that the excitation modes may either enhance or suppress tunneling as compared to the ground state. For the higher-frequency modes a rapid growth of the tunnel splitting is observed. At frequencies above 1000 cm(-1) the semiclassical solution becomes unstable and no reliable tunneling splittings can be obtained. This is in vast contrast to the adiabatic approximation to the instanton theory in which the tunnel splittings can be retrieved up to 3000 cm(-1). We discuss this disparity from the viewpoint of the multidimensional character of tunneling in hydrogen bonds and the adiabatic approximation is concluded to be inaccurate.     

Concerted hydrogen exchange tunneling in formic acid dimer

The effect of conformational relaxation on the quantum dynamics of the hydrogen exchange tunneling is studied in the D2h subspace of formic acid dimer. The fully coupled quantum dynamics in up to six dimensions are derived for potential energy hypersurfaces interpolated directly from hybrid density functional calculations with and without geometry relaxation. For a calculated electronic barrier height of 35.0 kJ/mol the vibrational ground state shows a tunneling splitting of 0.0013 cm(-1). The results support the vibrational assignment of Madeja and Havenith [J. Chem. Phys. 2002, 117, 7162-7168]. Fully coupled ro-vibrational calculations demonstrate the compatibility of experimentally observed inertia defects with in-plane hydrogen exchange tunneling dynamics in formic acid dimer.     

Modelling the vibrational behaviour of the cyclic carboxylic acid dimer. SQM force field of the formic acid dimer

The vibrational behaviour of the cyclic carboxylic acid dimer is modelled through the scaled quantum mechanical (SQM) force field of the cyclic formic acid dimer. The results indicate that the SQM force field technique is very well applicable to hydrogen bonded molecules. The frequency shifts observed on hydrogen bonding can be related to the shifts observed on lowering the temperature. This study also confirms that a clear distinction between cyclic carboxylic acid dimers and catamers can be made through the difference between infrared and Raman frequencies, and it is proved here that these conditions are also valid for weaker hydrogen bonded cyclic carboxylic acid dimers.     

Generalized approximation to the reaction path: the formic acid dimer case

A set of mass-weighted internal coordinates was derived and applied to the double proton transfer reaction in the formic acid dimer (FAD). The coordinate set was obtained starting from the Hirschfelder "mobile" by an optimization procedure consisting of a sequence of kinematic rotations. In FAD, the optimization procedure leads to three coordinates that do change significantly along the reaction path. These coordinates span the reaction space, whereas the remaining modes are treated in a harmonic approximation. The effect that the dimer dissociative motion has on the ground and excited vibrational states dynamics was explored. In the frequency region corresponding to the symmetric OH-stretch vibration four doublets have been identified with splittings of 2.76, 0.07, 0.60, and 4.03 cm(-1).     

Ground and asymmetric CO-stretch excited state tunneling splittings in the formic acid dimer

There has been some controversy concerning the assignment of measured tunneling splittings for the formic acid dimer in the vibrational ground state and the asymmetric CO-stretching excited state. The discussion is intimately related to the question whether the fundamental excitation of the CO-vibration promotes or hinders tunneling. Here we will address this issue on the basis of a five-dimensional reaction space Hamiltonian which includes three large amplitude coordinates as well as two harmonic modes whose linear superposition reproduces the asymmetric CO-vibrational mode. Within density functional theory using the B3LYP functional together with a 6-311++G(3df,3pd) basis set we obtain a ground state tunneling splitting which is about 2.4 larger than the one for the CO-stretching excited state.     

Absolute line intensities determination in the nu7 band of C2H4

Absolute intensities have been measured for 26 lines of C2H4 in the nu7 fundamental transition, using a tunable diode-laser spectrometer. These lines with 3< or = J"< or = 21, 2< or = Ka< or = 4, 2< or = Kc< or = 20 are located in the spectral range 920-980 cm(-1). The intensities have been measured by using two methods: the equivalent width method (EWM) and the line profile fit method (FPM). For the last one, three models have been tested: Voigt, Rautian and Galatry profiles.     

Vibrational action spectroscopy of the C-H and C-D stretches in partially deuterated formic acid dimer

Vibrational action spectroscopy of jet-cooled formic acid dimer measures the frequency of the C-H(D) stretching vibration and its coupling to nearby states. The action spectrum of (DCOOH)2 reveals a specific Fermi resonance between the C-D stretch and two antisymmetric combination states formed from the C-O stretch and DCO bend. A three-state deperturbation analysis shows that there is a relatively strong coupling between the fundamental vibration and each of the combination vibrations (mid R:13 cm(-1)mid R:) as well as between the combination states themselves (mid R:7 cm(-1)mid R:). This situation contrasts with that for the action spectrum of (HCOOD)2, where the C-H oscillator is isolated and not strongly coupled to other states.     

The determination of the second dissociation constant of sulfuric acid

The second dissociation constant of sulfuric acid is determined in 1M NaClO₄ at 25°C using an electrochemical cell without liquid junction consisting of a glass and a perchlorate electrode. By taking into account the association between the Na⁺ and SO ₄ ^2– ions an average value of 0.0184±0.0005 is found using three different methods. This corresponds with an apparent acidity constant KA ₂ ^* of 0.095±0.003     

Absolute infrared intensities of hydrocarbons

Dipole-moment derivatives, calculated by both the CNDO/2 method with different parameterizations and the INDO method, are compared to the experimental values determined from absolute infrared intensity measurements for the IR active modes of methane, ethane, ethylene and acetylene. A parameter refinement procedure is introduced in which the CNDO/2 molecular orbital parameters are adjusted through a damped least-squares treatment to give best agreement with the observed dipole-moment derivatives. It is found that the refinement does not substantially improve the agreement obtained with the original CNDO/2 parameterization. The INDO method gives somewhat poorer agreement than the CNDO/2 calculations. As an example of the applicability of the molecular orbital methods toward reproducing relative infrared intensities, the spectrum of cyclopropane in the gasphase is examined.

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Zusammenfassung Die Ableitungen des Dipol-Moments, die nach der CNDO/2-Methode mit verschiedenen Parametrisierungen sowie der INDO-Methode berechnet wurden, werden mit den experimentellen Ergebnissen aus Messungen der absoluten Infrarot-Intensitäten für die IR-aktiven Schwingungen von Methan, Äthan, Äthylen und Azetylen verglichen. Die CNDO/2-Parameter werden mit einer Methode der kleinsten Quadrate den beobachteten Dipol-Moment-Ableitungen angepaßt. Die Ergebnisse sind jedoch nicht wesentlich von denen der ursprünglichen CNDO/2-Methode verschieden. Die INDO-Ergebnisse sind nicht so gut wie die CNDO/2-Ergebnisse. Als Beispiel der Anwendbarkeit der MO-Methoden zur Berechnung von relativen IR-Intensitäten wird das Spektrum des Cyclopropans in der Gasphase untersucht.

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Résumé Les dérivées du moment dipolaire, calculées par la méthode CNDO/2 avec différentes paramétrisations et par la méthode INDO, sont comparées aux valeurs expérimentales déterminées à partir de mesures d'intensité absolue pour les modes actifs dans l'infra-rouge dans le méthane, l'éthane, l'éthylène et l'acétylène. Les paramètres sont ajustés de manière à donner le meilleur accord avec les dérivées du moment dipolaire. Cet ajustement n'améliore pas sensiblement l'accord obtenu avec la paramétrisation CNDO/2 originale. La méthode INDO donne des résultats moins bons que les calculs CNDO/2. Le spectre du cyclopropane en phase gazeuse est étudié comme exemple de l'applicabilité de la méthode des orbitales moléculaires au calcul des intensités relatives infra-rouge.

     

cis-trans formic acid dimer: experimental observation and improved stability against proton tunneling

We prepared the first cis-trans dimer of formic acid and measured its vibrational spectrum in a low-temperature Ar matrix. This preparation was done by selective vibrational excitation of the trans-trans noncyclic dimer. It was found that the stability of the cis-trans dimer against proton tunneling is strongly improved compared to the monomer, especially at elevated temperatures (>30 K). This surprising phenomenon was explained by differences in dynamical, energetic, and vibrational properties of the dimer and monomer. The obtained results show that the proton tunneling reactions can be strongly modified in the hydrogen-bonded solid network compared to the monomeric species.     

2D IR spectroscopy of the C-D stretching vibration of the deuterated formic acid dimer

We report transient grating and 2D IR spectra of the C-D stretching vibration of deuterated formic acid dimer. The C-D stretching transition is perturbed by an accidental Fermi resonance interaction that gives rise to a second transition. The transient grating results show that the population lifetime of these states, which are in rapid equilibrium, is 11 ps. 2D IR spectroscopy reveals the energies of the eigenstates in the regions of one quantum and two quanta of C-D stretching excitation. Using these eigenstate energies, we construct a simplified model for the zeroth-order states that we then use to simulate the 2D IR spectrum. The results of this simulation suggest that the model captures the essential features of the vibrational spectroscopy in the region of the C-D stretching transition and compares well with previous gas-phase spectroscopy of the C-D stretch of deuterated formic acid dimer.     

Double ionization and Coulomb explosion of the formic acid dimer by intense near-infrared femtosecond laser pulses

Ionization and fragmentation of formic acid dimers (HCOOH)(2) and (DCOOD)(2) by irradiation of femtosecond laser pulses (100 fs, 800 nm, ~1 × 10(14) W/cm(2)) were investigated by time-of-flight (TOF) mass spectrometry. In the TOF spectra, we observed fragment ions (HCOOH)H(+), (HCOOH)HCOO(+), and H(3)O(+), which were produced via the dissociative ionization of (HCOOH)(2). In addition, we found that the TOF signals of COO(+), HCOO(+), and HCOOH(+) have small but clear side peaks, indicating fragmentation with large kinetic energy release caused by Coulomb explosion. On the basis of the momentum matching among pairs of the side peaks, a Coulomb explosion pathway of the dimer dication, (HCOOH)(2)(2+) → HCOOH(+) + HCOOH(+), was identified with the total kinetic energy release of 3.6 eV. Quantum chemical calculations for energies of (HCOOH)(2)(2+) were also performed, and the kinetic energy release of the metastable dication was estimated to be 3.40 eV, showing good agreement with the observation. COO(+) and HCOO(+) signals with kinetic energies of 1.4 eV were tentatively assigned to be fragment ions through Coulomb explosion occurring after the elimination of a hydrogen atom or molecule from (HCOOH)(2)(2+). The present observation demonstrated that the formic acid dimer could be doubly ionized prior to hydrogen bond breaking by intense femtosecond laser fields.     

Approach to equilibrium after dilution of a monomer/dimer mixture. Measurement of the rate constant for dissociation of trityl dimer by stopped-flow methods

A general expression is derived for the approach to a new equilibrium when a monomer/dimer solution is diluted by a factor of n. The error introduced by a simple exponential treatment is analyzed. The response of a trityl monomer/dimer equilibrium to dilution by a factor of two in a stopped-flow apparatus has been used to measure the rate constant k₁₁. The results fall on the same Eyring plot with those obtained at lower temperatures, by another method, in the early 1930's by Ziegler and co-workers. © John Wiley & Sons, Inc.      

Laser isotope separation and the multiphoton dissociation of formic acid using a pulsed HF laser

The focussed beam from a single line [P₂ (5)] of a pulsed HF laser has been used to stimulate the decomposition of formic acid. The yield (Y is the number of product molecules per pulse / formic acid pressure) of the non-condensable (77 K) products, hydrogen and CO, has been studied as a function of laser radiant energy (from 25-115 mJ) and pressure (from 0.4-2.7 kPa). The intensity dependence of Y suggests that each dissociating formic acid requires the equivalent of at least 6 HF P₂(5) photons (260 kJ/mole). For pressures above about 0.6 kPa, Y_H2 = (?0.6 ± 1.7) × 10¹² + (2.4 ± 1.0) × 10¹² P and Y_CO = (?0.5 ± 6.1) × 10¹³ × (8.7 ± 3.7) × 10¹³ P. The linear dependerrce of yields indicates that a collisionally assisted decomposition process is important at these pressures. The efficiency of the conversion of photon energy to reaction products at a pressure of 2.7 kPa is ? 7% for CO and ? 0.2% for hydrogen. Selective excitation of HCOOH in equimolar mixtures of HCOOH/HCOOD, at a total pressure of 0.6 kPa, has provided a physically separated product, hydrogen gas, which is isotopically enriched in H versus D 25 fold as compared to the formic acid mixture. The degree of enrichment decreases as the total pressure of the mixture is increased. A possible mechanism accounting for isotope enrichment and the collisionally assisted dissociation is outlined.     

Double proton transfer and charge transfer transitions in hydrogen-bonded systems: formic acid dimer

The barriers for double proton transfer in the ground and lowest Π-Π^* and Π-Π^* excited states of the formic acid dimer have been calculated within a modified INDO scheme. Analysis of the nature of the excited electronic states, with emphasis on charge-transfer transitions, has been performed. The results indicate a lower barrier in the excited Π-Π^* states than in the ground state.     

Effects of external electric fields on double proton transfer kinetics in the formic acid dimer

Molecules can be exposed to strong local electric fields of the order of 10(8)-10(10) V m(-1) in the biological milieu. The effects of such fields on the rate constant (k) of a model reaction, the double-proton transfer reaction in the formic acid dimer (FAD), are investigated. The barrier heights and shapes are calculated in the absence and presence of several static homogenous external fields ranging from 5.14 × 10(8) to 5.14 × 10(9) V m(-1) using density functional theory (DFT/B3LYP) and second order M?ller-Plesset perturbation theory (MP2) in conjunction with the 6-311++G(d,p) Pople basis set. Conventional transition state theory (CTST) followed by Wigner tunneling correction is then applied to estimate the rate constants at 25 °C. It is found that electric fields parallel to the long axis of the dimer (the line joining the two carbon atoms) lower the uncorrected barrier height, and hence increase the raw k. These fields also flatten the potential energy surface near the transition state region and, hence, decrease the multiplicative tunneling correction factor. The net result of these two opposing effects is that fields increase k(corrected) by a factor of ca. 3-4 (DFT-MP2, respectively) compared to the field-free k. Field strengths of ～3 × 10(9) V m(-1) are found to be sufficient to double the tunneling-corrected double proton transfer rate constant at 25 °C. Field strengths of similar orders of magnitudes are encountered in the scanning tunneling microscope (STM), in the microenvironment of a DNA base-pair, in an enzyme active site, and in intense laser radiation fields. It is shown that the net (tunneling corrected) effect of the field on k can be closely fitted to an exponential relationship of the form k = aexp(bE), where a and b are constants and E the electric field strength.