Benzoic Acid Dimer Research Articles

Application of genetic algorithms in automated assignments of high-resolution spectra

This paper describes an automated assignment and fitting procedure for high-resolution rotationally resolved spectra. The method is based on the application of genetic algorithms (GA) and both frequency and intensity information of these spectra is used. The basic ideas behind the GA technique is introduced and the particular fitness function critical for the success of the GA evaluation is discussed. The meta-optimization of the internal GA parameters for an optimal exploration of the error landscape of the spectrum fitting is investigated. A number of typical examples are given in which the use of automated spectrum assignments with the GA method is of crucial importance in the analysis. Examples are given for fits of very dense spectra due to overlap of a number of rovibronic spectra in conformers and isotopomers and strongly congested spectra in dimer systems. It is also shown that since the GA performs an overall fit of the complete spectrum very good information on the orientation of the transition dipole moments is gathered. Contents PAGE 1. Introduction 354 2. The genetic algorithm 357 2.1. Introduction 357 2.2. The fitness function 358 2.2.1. Definition 358 2.2.2. Numerical evaluation of the fitness function 359 3. Optimization of the GA fits 360 3.1. Effect of the width (Δ w) of the weight function w(r) on the convergence of the GA 360 3.2. Meta-optimization of the internal GA parameters 361 3.3. Uncertainties in and correlations between the parameters from a GA fit 362 4. Methods 363 4.1. Experimental 363 4.2. The calculated spectra 364 4.2.1. The asymmetric rotor Hamiltonian 364 4.2.2. Internal rotation Hamiltonian 365 4.2.3. Intensities 366 4.3. The structure determination 367 4.4. The software used 368 5. Examples of the success of GA automatic assignments 368 5.1. GA fits of very dense rovibronic spectra 368 5.1.1. [7-D]phenol-N2 368 5.1.2. Benzonitrile-Ar 369 5.1.3. 4-methylphenol and its water complex 371 5.2. Simultaneous GA fits of a number of overlapping rovibronic spectra 373 5.2.1. 7-azaindole (Pyrrolo[2,3-b]pyridine) 374 5.2.2. Resorcinol (1,3-dihydroxybenzene) 375 5.2.3. Conformers and isotopomers of tryptamine 377 5.3. Strongly congested spectra in dimer systems 379 5.3.1. Benzoic acid dimer revisited 380 5.3.2. Benzonitrile dimer 385 5.3.3. Phenol dimer 387 5.4. Orientation of the transition dipole moment 389 5.5. GA fit of rotational contours 391 5.5.1. The FTIR spectrum of benzotriazole 391 5.5.2. The low-resolution LIF spectrum of azaindole-water 393 6. Summary 394 Acknowledgements 395 Appendix A 395 Appendix B 396 References 400 Supporting online material 403

4-(4-Pyridyl)benzoic Acid (PybenH) Dimer: An Efficient and Reasonable Design for a Long Linear Bidentate Building Block Employed in Metal–Organic Coordination Framework

Abstract Self-assembly of 4-(4-pyridyl)benzoic acid (pybenH), Ni2+, SCN− and an aromatic guest (naphthalene or dibenz[a,h]anthracene) gave two isostructural inclusion compounds, in which pybenH ligands form the corresponding dimers acting as bidentate building blocks with a length of ca. 20.86 Å.

Ground and Excited States of the Monomer and Dimer of Certain Carboxylic Acids

The ground-state properties of the monomer and the dimer of formic acid, acetic acid, and benzoic acid have been investigated using Hartree-Fock (HF) and density functional theory (DFT) methods using the 6-311++G(d,p) basis set. Some of the low-lying excited states have been studied using the time-dependent density functional theory (TDDFT) with LDA and B3LYP functionals and also employing complete-active-space-self-consistent-field (CASSCF) and multireference configuration interaction (MRCI) methodologies. DFT calculations predict the ground-state geometries in quantitative agreement with the available experimental results. The computed binding energies for the three carboxylic acid dimers are also in accord with the known thermodynamic data. The TDDFT predicted wavelengths corresponding to the lowest energy n-pi* transition in formic acid (214 nm) and acetic acid (214 nm) and the pi-pi* transition in benzoic acid (255 nm) are comparable to the experimentally observed absorption maxima. In addition, TDDFT calculations predict qualitatively correctly the blue shift (4-5 nm) in the excitation energy for the pi-pi* transition in going from the monomer to the dimer of formic acid and acetic acid and the red shift (approximately 19 nm) in pi-pi* transition in going from benzoic acid monomer to dimer. This also indicates that the electronic interaction arising from the hydrogen bonds between the monomers is marginal in all three carboxylic acids investigated.

Effect of Substituent Orientation on Selectivity between Polymerization and Cyclization: Polycondensation of (Alkylamino)benzoic Acid Dimer Phenyl Esters

AbstractSummary: The polymerization characteristics of (octylamino)benzoic acid dimer phenyl esters with para‐meta (2), meta‐para (3), and meta‐meta (4) orientation of the amino and carbonyl groups were investigated. While treatment of 2 or 3 with a base gave the cyclic dimer 6 as the main product, condensation of 4 with an initiator in the presence of a base afforded polymers with narrow molecular weight distributions.The condensation of 4 with an initiator in the presence of a base afforded polymers with narrow molecular weight distribution.imageThe condensation of 4 with an initiator in the presence of a base afforded polymers with narrow molecular weight distribution.

Vibrational coupling in carboxylic acid dimers

The vibrational level splitting in the ground electronic state of carboxylic acid dimers mediated by the doubly hydrogen-bonded networks are investigated using pure and mixed dimers of benzoic acid with formic acid as molecular prototypes. Within the 0-2000-cm(-1) range, the frequencies for the fundamental and combination vibrations of the two dimers are experimentally measured by using dispersed fluorescence spectroscopy in a supersonic jet expansion. Density-functional-theory calculations predict that most of the dimer vibrations are essentially in-phase and out-of-phase combinations of the monomer modes, and many of such combinations show significantly large splitting in vibrational frequencies. The infrared spectrum of the jet-cooled benzoic acid dimer, reported recently by Bakker et al. [J. Chem. Phys. 119, 11180 (2003)], has been used along with the dispersed fluorescence spectra to analyze the coupled g-u vibrational levels. Assignments of the dispersed fluorescence spectra of the mixed dimer are suggested by comparing the vibronic features with those in the homodimer spectrum and the predictions of density-functional-theory calculation. The fluorescence spectra measured by excitations of the low-lying single vibronic levels of the mixed dimer reveal that the hydrogen-bond vibrations are extensively mixed with the ring modes in the S1 surface.

Selective Cyclic Trimerization of 4-(Alkylamino)benzoic Acid Dimer Phenyl Esters

Abstract Treatment of 4-(alkylamino)benzoic acid dimer phenyl esters with a base, under conditions where monomeric 4-(alkylamino)benzoic acid phenyl esters polymerize to give well-defined aromatic polyamides, affords not polymers, but cyclic trimers in good yield.

Anharmonic midinfrared vibrational spectra of benzoic acid monomer and dimer

Anharmonic vibrational calculations for the benzoic acid monomer and dimer in the mid-IR regime (500-1800 cm(-1)) are reported. Harmonic frequencies and intensities are obtained at the DFT/B3LYP level of theory employing D95(d,p) and cc-pVTZ basis sets. Anharmonic corrections obtained from standard perturbation theory lead to redshifts of 1%-3%. In almost all cases, the resulting frequencies deviate by less than 1% from previous measurements [Bakker et al., J. Chem. Phys. 119, 11180 (2003)]. Calculated intensities are in qualitative agreement with the absorption experiment, with the cc-pVTZ values being superior to the D95(d,p) ones for a few modes of the dimer. The antisymmetric out-of-plane bending mode of the dimer, which is strongly blueshifted with respect to the monomer frequency, represents a remarkable exception: The harmonic frequencies obtained for the two basis sets differ notably from each other, and the anharmonically corrected frequencies deviate from the experimental value by 8% [D95(d,p)] or 3% (cc-pVTZ). Nonperturbative calculations in reduced dimensionality reveal that the relatively small total anharmonic shift (few tens of cm(-1)) comprises of partly much larger contributions (few hundreds of cm(-1)) which are mostly canceling each other. Many of the individual anharmonic couplings are beyond the validity of second-order perturbation theory based on cubic and semidiagonal quartic force constants only. This emphasizes the need for high-dimensional, nonperturbative anharmonic calculations at high quantum-chemical level when accurate frequencies of H-atom vibrations in double hydrogen bonds are sought for.

Tunneling dynamics of double proton transfer in formic acid and benzoic acid dimers

Direct dynamics calculations based on instanton techniques are reported of tunneling splittings due to double proton transfer in formic and benzoic acid dimers. The results are used to assign the observed splittings to levels for which the authors of the high-resolution spectra could not provide a definitive assignment. In both cases the splitting is shown to be due mainly to the zero-point level rather than to the vibrationally or electronically excited level whose spectrum was investigated. This leads to zero-point splittings of 375 MHz for (DCOOH)(2) and 1107 MHz for the benzoic acid dimer. Thus, contrary to earlier calculations, it is found that the splitting is considerably larger in the benzoic than in the formic acid dimer. The calculations are extended to solid benzoic acid where the asymmetry of the proton-transfer potential induced by the crystal can be overcome by suitable doping. This has allowed direct measurement of the interactions responsible for double proton transfer, which were found to be much larger than those in the isolated dimer. To account for this observation both static and dynamic effects of the crystal forces on the intradimer hydrogen bonds are included in the calculations. The same methodology, extended to higher temperatures, is used to calculate rate constants for HH, HD, and DD transfers in neat benzoic acid crystals. The results are in good agreement with reported experimental rate constants measured by NMR relaxometry and, if allowance is made for small structural changes induced by doping, with the transfer matrix elements observed in doped crystals. Hence the method used allows a unified description of tunneling splittings in the gas phase and in doped crystals as well as of transfer rates in neat crystals.

Conformational effects on vibronic spectra and excited state dynamics of 3-fluorobenzoic acid dimer

Two conformational isomers of 3-fluorobenzoic acid dimer (3-FBA(2)) have been identified in a supersonic jet expansion by use of laser-induced fluorescence excitation (FE), UV-UV hole-burning, and dispersed fluorescence (DF) spectroscopic methods. In the FE spectrum, the S(1) origins of the two isomeric species appear at a frequency gap of only 24 cm(-1), and the vibronic intensities of the redshifted dimer (dimer I) are about two times weaker than those of dimer II. However, ab initio quantum chemistry calculations at the MP2/6-31G(**) level of theory predict that all the isomeric species of 3-FBA(2) have almost the same binding energy (approximately 17 kcal/mol) in the ground state. Furthermore, unlike benzoic acid dimer, the present system shows intense activity for a low-frequency mode in both the FE and DF spectra. With the aid of DFT (B3LYP/6-311G(**)) predicted normal mode frequencies, we have assigned the mode to the in-plane gear (cogwheel) vibration of the cyclic hydrogen-bonded frame of the dimer. The Franck-Condon profiles for vibronic excitation of the mode indicate that the distortion of the cyclic hydrogen bond frame as a result of S(1)<--S(0) excitation is larger for dimer I than dimer II. Moreover, the fluorescence lifetime at the S(1) zero-point level of the former is also significantly smaller than the latter. Using the predictions of configuration interaction singles calculations, we have proposed that the spectral and dynamical differences between the two isomeric species observed in this study are manifestations of the different characteristics of their S(1) surfaces. By measuring FE, DF, and hole-burning spectra of a mixed dimer between 3-fluobenzoic acid and benzoic acid we have shown that the isomeric features in the homodimer spectra are due to two locally excited rotamers of the 3-fluorobenzoic acid moiety.

A theoretical study on clusters of benzoic acid–water in benzene solutions

Abstract Structures and stability of benzoic acid dimer ((BA)2) and benzoic acid–water (BA–H2O) m:n complexes, with m and n = 1, 2, were studied in benzene solutions, using molecular dynamics (MD) simulations. It appeared that nearly all hydrogen bond (H-bond) complexes suggested from different partition experiments existed in MD simulations, with the probability depending on their size and temperature. The MD results revealed the probability of finding H-bonds between water molecules, as well as the non-self-association of water molecules in benzene solutions. Although the H-bonds in (BA)2 are quite strong in the gas phase and pure benzene, they can be opened by water molecules, forming microhydrates in benzene solutions. It was shown that, in order to provide insights into the structures and stability of the BA–H2O complexes in benzene solution, solvent molecules as well as dynamic and temperature effects have to be included in theoretical investigation.

Simulation of the Double Proton Transfer Dynamics in the Benzoic Acid Dimer

A numerical simulation of the rate constants of the double proton transfer in the benzoic acid dimer at partial and complete deuteration of hydrogen bonds in the dimer as well as at different hydrostatic pressures applied to the sample has been carried out on the basis of the model of a multistage process activated by equilibrium radiation. It has been shown that this model, as opposed to the known approach based on the synthesis of two processes — activation (Boltzmann–Arrhenius model) and molecular tunneling (Goldanskii model) — can describe both low- and high-temperature rate constants of reactions using a single set of parameters characterizing the process of interaction of the transforming molecule with the thermal field.

Isotope effects associated with tunneling and double proton transfer in the hydrogen bonds of benzoic acid.

The isotope effects associated with double proton transfer in the hydrogen bonds of benzoic acid (BA) dimers have been measured using field-cycling (1)H NMR relaxometry and quasielastic neutron scattering. By studying mixed isotope (hydrogen and deuterium) samples, the dynamics of three isotopologues, BA-HH, BA-HD, and BA-DD, have been investigated. Low temperature measurements provide accurate measurements of the incoherent tunneling rate, k(0). This parameter scales accurately with the mass number, m, according to the formula k(0)=(E/m)e(-Fm) providing conclusive evidence that the proton transfer process is a strongly correlated motion of two hydrons. Furthermore, we conclude that the tunneling pathway is the same for the three isotopologue species. Measurements at higher temperatures illuminate the through barrier processes that are mediated via intermediate or excited vibrational states. In parallel with the investigation of proton transfer dynamics, the theoretical and experimental aspects of studying spin-lattice relaxation in single crystals of mixed isotope samples are investigated in depth. Heteronuclear dipolar interactions between (1)H and (2)H isotopes contribute significantly to the overall proton spin-lattice relaxation and it is shown that these must be modeled correctly to obtain accurate values for the proton transfer rates. Since the sample used in the NMR measurements was a single crystal, full account of the orientation dependence of the spin-lattice relaxation with respect to the applied B field was incorporated into the data analysis.

Spectroscopic Consequences of Localized Electronic Excitation in Anthranilic Acid Dimer

The electronic and infrared spectroscopy of anthranilic acid (o-aminobenzoic acid) dimer has been studied in a supersonic jet. Fluorescence-dip infrared (FDIR) spectra have been obtained in both the ground and first excited electronic states. The ground-state FDIR spectrum shows a broad, highly shifted OH stretch absorption commensurate with a cyclic, doubly hydrogen bonded structure, as has been observed for other carboxylic acid dimers. Density functional theory calculations predict that the dimer retains the monomer propensity for the amino group to be adjacent to the carbonyl group of the carboxylic acid, producing a C2h ground-state geometry for the dimer. The presence of the amino group shuts off the double proton tunneling that is present in benzoic acid dimer. The excited-state FDIR spectrum shows NH stretch fundamentals that are the sum of the S0 and S1 FDIR spectra of anthranilic acid monomer, indicating that the electronic excitation is localized on one of the monomers in the excited electronic state. The ultraviolet spectrum of the dimer shows a strong Franck−Condon progression involving a 58 cm-1 vibration and many combination bands with this mode. Comparison with density functional theory calculations indicates that the 58 cm-1 mode involves the in-plane geared bend of the two monomer units, which has bu symmetry in the C2h ground state. While this non totally symmetric fundamental appears in the R2PI and IR−UV hole-burning spectra, the dispersed fluorescence spectra from the S1 origin, +58 cm-1 band, and +118 cm-1 band display intensity only in even members of the 58 cm-1 progression. This Franck−Condon activity is quantitatively fit by a model in which the excited-state vibrations are simple sums and differences of localized, shifted harmonic oscillator vibrational wave functions, producing unresolved ag/bu tunneling doublets associated with the large barrier that separates the two minima on the excited-state surface.

Hydrogen bond-induced vibronic mode mixing in benzoic acid dimer: a laser-induced fluorescence study.

Laser-induced dispersed fluorescence spectra of benzoic acid dimer in the cold environment of supersonic jet expansion have been reinvestigated with improved spectral resolution of measurements. The spectra are analyzed with the aid of the normal mode vibrations of the dimer calculated by the ab initio quantum chemistry method at the DFT/B3LYP/6-311+G(*) (*) level of theory. The analysis reveals that the low-frequency intermolecular hydrogen bond modes are mixed extensively with the carboxyl as well as aromatic ring vibrations upon electronic excitation. The mode mixing is manifested as the complete loss of mirror symmetry relation between the fluorescence excitation and dispersed fluorescence spectra of the S(1) origin, and appearance of large number of cross-sequence transitions when the DF spectra are measured by exciting the low-energy vibrations near the S(1) origin. The cross-sequence bands are found in all the cases to be the combinations of two nontotally symmetric fundamentals consisting of one of the intermolecular hydrogen bond modes and the other from the aromatic ring and carboxyl group vibrations. The implications of this mode mixing on the excited state dynamics of the dimer are discussed.

The ground-state tunneling splitting of various carboxylic acid dimers

Carboxylic acid dimers in gas phase reveal ground-state tunneling splittings due to a double proton transfer between the two subunits. In this study we apply a recently developed accurate semiclassical method to determine the ground-state tunneling splittings of eight different carboxylic acid derivative dimers (formic acid, benzoic acid, carbamic acid, fluoro formic acid, carbonic acid, glyoxylic acid, acrylic acid, and N,N-dimethyl carbamic acid) and their fully deuterated analogs. The calculated splittings range from 5.3e-4 to 0.13 cm(-1) (for the deuterated species from 2.8e-7 to 3.3e-4 cm(-1)), thus indicating a strong substituent dependence of the splitting, which varies by more than two orders of magnitude. One reason for differences in the splittings could be addressed to different barriers heights, which vary from 6.3 to 8.8 kcal/mol, due to different mesomeric stabilization of the various transition states. The calculated splittings were compared to available experimental data and good agreement was found. A correlation could be found between the tunneling splitting and the energy barrier of the double proton transfer, as the splitting increases with increased strength of the hydrogen bonds. From this correlation an empirical formula was derived, which allows the prediction of the ground-state tunneling splitting of carboxylic acid dimers at a very low cost and the tunneling splittings for parahalogen substituted benzoic acid dimers is predicted.

The infrared absorption spectrum of the gas phase neutral benzoic acid monomer and dimer

The IR absorption spectrum of the jet-cooled benzoic acid monomer and dimer have been recorded throughout the 500–1900 cm−1 range via ion dip spectroscopy. Both spectra show a wealth of vibrational modes and the monomer spectrum is remarkably different from that of the dimer. Density functional theory calculations show quantitative agreement with the experimental data. The C–O–H out-of-plane bending vibration in the dimer is poorly reproduced in the theoretical calculations and a more accurate description of the doubly hydrogen bonded structure is therefore still needed.

Theoretical Study of Multidimensional Proton Tunnelling in Benzoic Acid Dimer

Ab initio B3LYP/6-311++G** calculations have been carried out for the benzoic acid dimer for the stable and saddle point structures. The energy barrier for the proton tunneling amounts to 6.5 kcal/mol. The normal mode frequencies have been computed including modes coupled to the proton tunneling mode. Two-dimensional model potentials, formed from symmetric mode coupling potential and squeezed double well potential, have been fitted to the calculated energy barrier, geometries and frequencies, and used to analyze proton dynamics. The calculated proton tunneling energy splitting in the vibrationally ground states of the low-frequency modes is ~230 cm-1. The two-dimensional model PES predict monotonic increase of the tunneling splitting with the excitation of the planar modes. Depending of the sign of the coupling parameter out-of-plane modes can either suppress or promote the splittings.

Proton tunnelling in the hydrogen bonds of halogen-substituted derivatives of benzoic acid studied by NMR relaxometry: the case of large energy asymmetry

Abstract The concerted two proton transfer in the hydrogen bonds of para -halogen substituted derivatives of benzoic acid has been investigated using conventional NMR relaxometry combined with field-cycling measurements of the magnetic field dependence of the proton spin–lattice relaxation rate. Thus, the inverse correlation time describing the proton transfer process has been determined over a wide range of temperature. The energy difference between the two proton configurations was determined in all four compounds to be significantly larger than in the prototype model system of benzoic acid dimers. This energy difference exceeds the cut-off frequency of the acoustic phonon spectrum of the crystal as well as the frequency of two lowest modes promoting proton tunnelling. The low temperature limit of the tunnelling rate was found to be one order of magnitude higher than in benzoic acid. For the four compounds studied, this rate and its increase at higher temperature exhibit a different behaviour to benzoic acid which is attributed to the details of the level structure and energy gaps between states corresponding predominantly to one or the other proton configuration. A new adaptation of the phonon-assisted theory is proposed which applies to these cases of high energy asymmetry. To assist in the interpretation of the data, a powder neutron diffraction structure determination of 4-bromobenzoic acid is reported together with DFT calculations on the hydrogen bond structures of all members of the 4-halogen substituted derivatives.

Theoretical modeling of the OH stretch infrared spectrum of carboxylic acid dimers based on first-principles anharmonic couplings

Carboxylic acid dimers serve as prototypical systems for modeling the unusual spectral behavior of the hydride stretch fundamental. Large anharmonic effects associated with the pair of cooperatively strengthened OH⋯O=C hydrogen bonds produces complicated infrared spectra in which the OH stretch oscillator strength is spread over hundreds of wave numbers, resulting in a complicated band sub-structure. In this work cubic anharmonic constants are computed along internal coordinates associated with the intramolecular OH stretch, intermolecular stretch, and OH bend internal coordinates for the formic acid and benzoic acid dimers. These are then projected onto the normal coordinates to produce mixed states that are used in computing the OH stretch infrared spectrum. For the benzoic acid dimer the calculations accurately reproduce for three deuterated isotopomers the overall breadth and much of the vibrational sub-structure in the observed spectra. For the formic acid dimer, the spectrum is calculated using a model employing a subset of the cubic force constants as well as using the full cubic force field. The spectra calculated for the formic acid dimer are sparser and somewhat more sensitive to the exact positions of the anharmonically coupled states than that of the benzoic acid dimer. Again semiquantitative agreement with experiment is obtained.

Fluorescence-dip IR spectra of jet-cooled benzoic acid dimer in its ground and first excited singlet states.

The IR spectra of three isotopomers of the benzoic acid dimer have been recorded under jet-cooled conditions using the double resonance method of fluorescence-dip IR spectroscopy. In so doing, the spectra are assuredly due exclusively to dimers in the ground-state zero-point level at a rotational temperature of 3-5 K. Even under these conditions, the three isotopomers have remarkably broad spectra, extending from 2600 to almost 3150 cm-1. The spectra show extensive substructure consisting of some 15-20 transitions where only a single OH stretch fundamental should appear in the harmonic limit. The comparison of the undeuterated d0-d0 dimer with the ring-deuterated d5-d5 dimer tests the effect of mixing with the C-H stretches and overtones of the C-H bends. The mixed OH/OD ring-deuterated d6-d5 dimer shifts the frequency and changes the form of the OH stretch normal mode. The analogous OH stretch IR spectrum of the d0-d0 dimer out of the S1 excited-state zero-point level has also been recorded. In this case, much of the closely-spaced substructure is not apparent. What remains is a set of three bands separated from one another by about 180 cm-1. Preliminary results of model calculations of the anharmonic coupling, responsible for the broadening and substructure, are presented. These calculations indicate that it is OH stretch-OH bend coupling, rather than coupling with the intermolecular stretch, that is responsible for much of the observed structure and breadth.