A gradient-corrected density functional and MP2 study of phenol–ammonia and phenol–ammonia(+) hydrogen-bonded complexes (original) (raw)
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Journal of Physical Chemistry
Anharmonic vibrational frequency shifts of the phenol(+) O−H stretching mode upon complex formation with the open-shell ligand O 2 were computed at several DFT and MP2 levels of theory, with various basis sets, up to 6-311++G-(2df,2pd). It was found that all DFT levels of theory significantly outperform the MP2 method with this respect. The best agreement with the experimental frequency shift for the hydrogen-bonded minimum on the potential energy surfaces was obtained with the HCTH/407 functional (−93.7 cm −1 theoretical vs −86 cm −1 experimental), which is a significant improvement over other, more standard DFT functionals (such as, e.g., B3LYP, PBE1PBE), which predict too large downshifts (−139.9 and −147.7 cm −1 , respectively). Good agreement with the experiment was also obtained with the mPW1B95 functional proposed by Truhlar et al. (−109.2 cm −1 ). We have attributed this trend due to the corrected long-range behavior of the HCTH/407 and mPW1B95 functionals, despite the fact that they have been designed primarily for other purposes. MP2 method, even with the largest basis set used, manages to reproduce only less than 50% of the experimentally detected frequency downshift for the hydrogen-bonded dimer. This was attributed to the much more significant spin contamination of the reference HF wave function (compared to DFT Kohn−Sham wave functions), which was found to be strongly dependent on the O−H stretching vibrational coordinate. All DFT levels of theory outperform MP2 in the case of computed anharmonic OH stretching frequency shifts upon ionization of the neutral phenol molecule as well. Besides the hydrogen-bonded minimum, DFT levels of theory also predict existence of two other minima, corresponding to stacked arrangement of the phenol(+) and O 2 subunits. mPW1B95 and PBE1PBE functionals predict a very slight blue shift of the phenol(+) O−H stretching mode in the case of stacked dimer with the nearly perpendicular orientation of oxygen molecule with respect to the phenolic ring, which is entirely of electrostatic origin, in agreement with the experimental observations of an additional band in the IR photodissociation spectra of phenol(+)−O 2 dimer Dopfer, O. Chem. Phys. Lett. 2008, 457, 298]. The structural features of the minima on the studied PESs were discussed in details as well, on the basis of NBO and AIM analyses.
Physical Chemistry Chemical Physics, 2005
In order to study the effects of hydrogen bonding on the spectroscopic properties of (NH 3)(HF) and (NH 3)(DF) complexes, vibrational spectra (including fundamental, overtone and combination transitions) were calculated using the vibrational self consistent field (VSCF) method. This ab initio VSCF method accounts for both onedimensional anharmonicity and pair-wise mode-mode couplings for all vibrational modes of the molecule, using points on the potential energy surface (at the MP2/TZP level of theory in this study). An analysis of the coupling strength shows surprisingly important coupling effects from pair-wise interactions not expected to be major. This indicates the benefits of including all pair-wise mode-mode couplings for weakly bound systems. Hydrogen bonding induces B20% red shifts for the HF and DF stretch frequencies. The corrections due to anharmonicity for these modes are À6% and À5%, respectively. The anharmonic corrections for the intermolecular stretch of (NH 3)(HF) and (NH 3)(DF) are each about À5%. The NH 3 umbrella motion has virtually no anharmonic correction in the complex, whereas free ammonia experiences a À15% correction. Also, the closing motion as well as the opening motion is restricted. The 1 þ 1 combination transition of the proton stretching and intermolecular stretching modes has remarkably large intensity, larger even than the intensities for the first overtone of the proton stretching modes. The anharmonic frequency for the fundamental HF stretch, 3268 cm À1 , is in good agreement with the experimental gas phase result, 3215 cm À1. A comparison to solid rare-gas matrix data shows that the VSCF frequencies are a consistent improvement over the harmonic approximation. The experimental data also support the use of the MP2 level of theory for the associated electronic structure calculations.
Journal of Physical Chemistry
The performance of some recently proposed DFT functionals by Truhlar's group (mPW1B95, mPWLYP1W, PBELYP1W, and PBE1W [Dahlke, E. E.; Truhlar, D. G. J. Phys. Chem. B 2005, 109, 317. Zhao, Y.; Truhlar, D. G. J. Phys. Chem. A 2004, 108, 6908.]) was tested primarily with respect to computation of anharmonic vibrational frequency shifts upon hydrogen bond formation in small molecular/ionic dimers. Five hydrogenbonded systems with varying hydrogen bond strengths were considered: methanol-fluorobenzene, phenol-carbon monoxide in ground neutral (S 0 ) and cationic (D 0 ) electronic states, phenol-acetylene, and phenol-benzene(+). Anharmonic OH stretching frequency shifts were calculated from the computed vibrational potentials for free and hydrogen-bonded proton-donor molecules. To test the basis set convergence properties, all calculations were performed with 6-31++G(d,p) and 6-311++G(2df,2pd) basis sets. The mPW1B95 functional was found to perform remarkably better in comparison to more standard functionals (such as B3LYP, mPW1PW91, PBE1PBE) in the case of neutral dimers. In the case of cationic dimers, however, this is not always the case. With respect to prediction of anharmonic OH stretching frequency shifts upon ionization of free phenol, all DFT levels of theory outperform MP2. Some other aspects of the functional performances with respect to computation of interaction and dissociation energies were considered as well.
Chemical Physics, 2006
The p-hydrogen bonded minimum on the S 0 ground state potential energy hypersurface (PES) of the neutral phenol-acetylene dimer is located and analyzed in details. Three DFT approaches -B3LYP, mPW1PW91, and PBE1PBE/6-31++G(d,p) along with the MP2/6-31++G(d,p) level of theory were used for the presented analyses. Both the standard and the counterpoise-corrected PESs of the studied dimer were explored. Besides the weak hydrogen-bonding interaction of a p-type, also purely electrostatic interaction of a dipole-quadrupole type between the monomeric units is responsible for the stabilization of this global minimum on the studied PESs. The computed counterpoise-corrected interaction and dissociation energies at all levels of theory are in excellent agreement with the estimations based on experimental spectroscopic data. Anharmonic OH stretching frequencies for the free phenol and the p-hydrogen-bonded phenol-acetylene complex were computed on the basis of 1D DFT and MP2 OH stretching potentials. While all DFT levels significantly overestimate the experimentally measured m(OH) vibrational frequency shift, the corresponding MP2 value is in excellent agreement with the experimental data (70.2 vs. 68 cm À1 ). On the other hand, when the frequency shifts are computed within the harmonic approximation, all DFT levels are in fortuitous good agreement with the experiment, due to cancellation of errors. Passing to the counterpoise-corrected PESs has a little influence on the calculated harmonic and anharmonic vibrational frequency shifts of the phenol OH stretching mode. According to the charge-field perturbation analyses of the 1D OH stretching potentials, the purely electrostatic interaction of phenol with the proton-accepting acetylene molecule governs only a small portion of the overall OH vibrational frequency shift, while the role of electrostatics with this respect is more important at MP2 level of theory. However, all levels of theory lead to a conclusion that the dynamical changes of charge-transfer interaction in the course of OH stretching are the main factor governing the IR intensity enhancement of this vibrational transition upon p-hydrogen bonding, while the electrostatic interaction is of a second-order importance with respect to this. According to the second-order perturbation theory analysis of the Fock matrix (or its Kohn-Sham analog) within the NBO basis and the NBO deletion analyses, the charge-transfer between the monomeric units within the dimer is essentially one-directional (occurring from acetylene to phenol), and predominantly of a p ! r* type. The energetic effect of this interaction was estimated as well. The existence of another, much less stable minimum on the explored PESs, predicted on the basis of mainly electrostatic arguments, was also confirmed.
Journal of Physical Chemistry A, 2010
Vibrational predissociation spectra of the argon-tagged halide monohydrates, X -· H 2 O · Ar (X ) Cl, Br, or I), are recorded from ∼800 to 3800 cm -1 by monitoring the loss of the argon atom. We use this set of spectra to investigate how the spectral signatures of the hydrogen-bonding and large-amplitude hindered rotations of the water molecule are affected by incremental substitution of the hydrogen atoms by deuterium. All six vibrational modes of the X -· H 2 O complexes are assigned through fundamental transitions, overtones, or combination bands. To complement the experimental study, harmonic and reduced-dimensional calculations of the vibrational spectra are performed based on the MP2/aug-cc-pVTZ level of theory and basis set. Comparison of these results with those from the converged six-dimensional calculations of Rheinecker and Bowman [J. Chem. Phys. 2006, 125, 133206.] show good agreement, with differences smaller than 30 cm -1 . The simpler method has the advantage that it can be readily extended to the heavier halides and was found to accurately recover the wide range of behaviors displayed by this series, including the onset of tunneling between equivalent minima arising from the asymmetrical (single ionic hydrogen-bonded) equilibrium structures of the complexes.
The Journal of …, 2005
The results of harmonic and anharmonic frequency calculations on a guanine-cytosine complex with an enolic structure (a tautomeric form with cytosine in the enol form and with a hydrogen at the 7-position on guanine) are presented and compared to gas-phase IR-UV double resonance spectral data. Harmonic frequencies were obtained at the RI-MP2/cc-pVDZ, RI-MP2/TZVPP, and semiempirical PM3 levels of electronic structure theory. Anharmonic frequencies were obtained by the CC-VSCF method with improved PM3 potential surfaces; the improved PM3 potential surfaces are obtained from standard PM3 theory by coordinate scaling such that the improved PM3 harmonic frequencies are the same as those computed at the RI-MP2/cc-pVDZ level. Comparison of the data with experimental results indicates that the average absolute percentage deviation for the methods is 2.6% for harmonic RI-MP2/cc-pVDZ (3.0% with the inclusion of a 0.956 scaling factor that compensates for anharmonicity), 2.5% for harmonic RI-MP2/TZVPP (2.9% with a 0.956 anharmonicity factor included), and 2.3% for adapted PM3 CC-VSCF; the empirical scaling factor for the ab initio harmonic calculations improves the stretching frequencies but decreases the accuracy of the other mode frequencies. The agreement with experiment supports the adequacy of the improved PM3 potentials for describing the anharmonic force field of the G‚ ‚ ‚C base pair in the spectroscopically probed region. These results may be useful for the prediction of the pathways of vibrational energy flow upon excitation of this system. The anharmonic calculations indicate that anharmonicity along single mode coordinates can be significant for simple stretching modes. For several other cases, coupling between different vibrational modes provides the main contribution to anharmonicity. Examples of strongly anharmonically coupled modes are the symmetric stretch and group torsion of the hydrogen-bonded NH 2 group on guanine, the OH stretch and torsion of the enol group on cytosine, and the NH stretch and NH out-of-plane bend of the non-hydrogenbonded NH group on guanine.
Weak hydrogen bonds: insights from vibrational spectroscopic studies
International Reviews in Physical Chemistry, 2018
The review presents a critical analysis of the data obtained from vibrational spectroscopic studies on a narrow selection of weak hydrogen-bonded binary molecular complexes for measurements performed under isolated conditions, addressing the nature, properties, physical origins of the binding forces, and the role of such hydrogen bonds in dynamics of vibrational relaxations. In the recent history of studies of chemical bonding, hydrogen bond certainly occupies the centre stage. Although the bottom line of our knowledge for structure of hydrogen bonded systems is based on crystallographic data, it is well recognised that the constrained environment of a molecular crystal seriously perturbs the shallow interaction potentials of hydrogen bonds, and particularly their weaker variants. Binary complexes of different categories of molecular prototypes are the most convenient systems to look into the attributes and role played by the weak hydrogen bonds in promoting a chemically significant event. A variety of weak hydrogen bonded binary complexes, having mostly two types of binding motifs, CH••O and OH⋯π, have been considered for this review. The vital molecular parameter that has been primarily considered in the present analysis is the hydrogen bond induced spectral shift (Δν Χ-Η) of the stretching vibrational fundamental of the donor group (X-H), for measurements performed in inert gas matrixes and also in the gas phase. The changes in infrared spectral band shapes of ν X-H transitions have been considered to suggest the influence of the hydrogen bond in vibrational dynamics of the excited X-H stretching mode. Attempts are made to correlate the observed spectral shifts in homologous series of complexes for a particular binding motif with different energetic and electronic structure parameters, and those correlations have been used to get insights into the underlying molecular interactions and origin of vibrational spectral shifts. The other vital parameters of binary molecular complexes are the low-frequency intermolecular vibrations, which appear typically in terahertz range of the electromagnetic spectrum. A brief analysis of the available data for weak hydrogen bonded complexes, obtained by employing LIF spectroscopic method, is presented, and information obtained from complementary spectroscopic methods, like farinfrared absorption, are discussed. The spectral data presented are mostly from the published work of the authors.
A comprehensive analysis of hydrogen bond interactions based on local vibrational modes
International Journal of Quantum Chemistry, 2012
Local stretching modes for 69 different DH single bonds and 58 H• • • A H-bonds are calculated at the ωB97X-D/aug-cc-pVTZ level of theory to describe the changes in donor D and acceptor A upon forming the hydrogen-bonded complex. The intrinsic strength of the DH and AH interactions is determined utilizing the properties of a well-defined set of local, uncoupled vibrational modes. The local mode stretching force constant k a (HA) provides a unique measure of bond strength for both covalently and electrostatically bonded complexes. Generally applicable bond orders are derived, which can be related to the binding energies of the hydrogen bonded complexes. Although the red shifts in the DH stretching frequencies can be used to detect hydrogen bonding, they are not sufficient to assess the strength of hydrogen bonding. It is demonstrated that the calculated BSSE-corrected binding energies of hydrogen bonded complexes are related to the sum of bond order changes caused by hydrogen bonding.The covalent character of charge assisted hydrogen bonds is explained. Because local mode frequencies can also be derived from experimental normal mode frequencies, a new dimension in the study of hydrogen bonding is gained.