The O–H⋯π hydrogen bonded phenol–benzene(+) radical cationic dimer: a gradient-corrected density functional study (original) (raw)
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Chemical Physics Letters, 2003
The global minimum on B3LYP, mPW1PW91 and PBE1PBE/6-31++G(d,p) potential energy surfaces (PESs) of the (phenol) þ 2 cationic radical dimer corresponds to O-H þ Á Á ÁO hydrogen-bonded structure, with an additional, although much weaker C-HÁ Á ÁO hydrogen bond, as revealed by AIM analysis. Excellent agreement with experimental data is obtained for the anharmonic vibrational frequency shift of the dangling O-H oscillator on the basis of one-dimensional DFT O-H stretching potentials. However, theoretical calculations suggest that the m(O-H þ Á Á ÁO) mode due to the hydrogen-bonded O-H oscillator should appear at significantly lower frequencies than it was first estimated on the basis of experimental dissociation spectroscopy combined with an ion trap technique data.
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.
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.
Chemical Physics, 2002
A gradient-corrected DFT and MP2 study of phenol-ammonia neutral and cationic dimers were carried out. Various combinations of exchange and correlation functionals were employed within the DFT approach: B3LYP, MPW1PW91, PBE1PBE, and the modified B3LYP(M) [J. Mol. Struct. 416 (1997) 1]. The BSSE-corrected PES of the neutral dimer was also explored at B3LYP, B3LYP(M) and MP2/6-31++G(2d,p) levels. A single minimum was located on the neutral complex PES corresponding to the hydrogen-bonded non-proton-transferred form, while the only minimum on cationic complex PES corresponds to a phenoxyl-ammonium proton-transferred form, regardless on the level of theory. Dissociation energies corrected for BSSE and including the relaxation energy terms are reported for both complexes. The calculated vertical IE values at all DFT levels of theory are in excellent agreement with the experimental data. While the anharmonic vibrational analysis based on all 1D DFT vibrational potentials overestimate the m(OH) mode frequency shift upon complexation, the value obtained from the 1D MP2 potential for the BSSEcorrected MP2/6-31++G(2d,p) PES is in excellent agreement with the experimental data (457 vs. 469 cm À1 ). Various contributions to the overall anharmonic m(OH) frequency shifts are considered, as well as the BSSE influence which was found to be very small. As revealed by the CFP 1D m(OH) DFT and MP2 vibrational potentials, the electrostatic interaction alone can not account neither for the substantial red shift of this mode upon hydrogen bonding, and especially for the substantial intensity enhancement.
The Journal of Organic Chemistry, 2010
Remote intramolecular hydrogen bonds (HBs) in phenols and benzylammonium cations influence the dissociation enthalpies of their O-Ha n dC-N bonds, respectively. The direction of these intramolecular HBs, para f meta or meta f para, determines the sign of the variation with respect to molecules lacking remote intramolecular HBs. For example, the O-H bond dissociation enthalpy of 3-methoxy-4-hydroxyphenol, 4, is about 2.5 kcal/mol lower than that of its isomer 3-hydroxy-4-methoxyphenol, 5, although group additivity rules would predict nearly identical values. In the case of 3-methoxy-4-hydroxybenzylammonium and 3-hydroxy-4-methoxybenzylammonium ions, the CBS-QB3 level calculated C-N eterolytic dissociation enthalpy is about 3.7 kcal/mol lower in the former ion. These effects are caused by the strong electron-withdrawing character of the-O • and-CH 2 þ groups in the phenoxyl radical and benzyl cation, respectively, which modulates the strength of the HB. An O-H group in the para position of ArO • or ArCH 2 þ becomes more acidic than in the parent molecules and hence forms stronger HBs with hydrogen bond acceptors (HBAs) in the meta position. Conversely, HBAs, such as OCH 3 ,i nt h e para position become weaker HBAs in phenoxyl radicals and benzyl cations than in the parent molecules. These product thermochemistries are reflected in the transition states for, and hence in the kinetics of, hydrogen atom abstraction from phenols by free radicals (dpph • and ROO •). For example, the 298 K rate constant for the 4 þ dpph • reaction is 22 times greater than that for the 5 þ dpph • reaction. Fragmentation of ring-substituted benzylammonium ions, generated by ESI-MS, to form the benzyl cations reflects similar remote intramolecular HB effects.
The Journal of Physical Chemistry A, 2014
O−H stretching infrared fundamentals (ν OH) of phenol and a series of fluorophenol monomers and their 1:1 complexes with benzene have been measured under a matrix isolation condition (8 K). Spectral analysis reveals that ring fluorine substitutions have little effect on phenolic ν O−H as long as the molecules in the matrix are fully dispersed as monomers. The substitution effects are pronouncedly manifested only when the phenols are complexed with benzene, and the measured shift in phenolic ν OH from the monomer value varies from ∼78 cm −1 in phenol to ∼98 cm −1 in 3,4,5-trifluorophenol. The spectral shifts are found to display a linear correlation with the aqueous phase acid dissociation constants (pK a) of the phenols. The spectral changes predicted by electronic structure calculations at several levels of theory are found to be consistent with the observations. Such correlations are also found to exist with respect to different energetic, geometric, and other electronic structure parameters of the complexes. Atoms in Molecules (AIM) analysis shows a distinct bond critical point due to accumulation of electron density at the hydrogen-bonding site. The variation of electron densities both on the hydrogen bond as well the donor O−H group is in accordance with the experimentally observed ν O−H of the various fluorophenol−benzene complexes. Partitioning of binding energies into components following the Morokuma−Kitaura scheme shows that the π-hydrogen-bonded complexes are stabilized predominantly by dispersion interactions, although electrostatics, polarization, and charge-transfer terms have appreciable contribution to overall binding energies. NBO analysis reveals that hyperconjugative charge-transfers from the filled π-orbitals of the hydrogen bond acceptor (benzene) to the antibonding σ*(O− H) orbital of the donors (phenols) display correlations which are fully consistent with the observed variations of spectral shifts. The analysis also shows that the O−H bond dipole moments of all the phenolic species are nearly the same, implying that local electrostatics has only a little effect at the site of hydrogen bonding.
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–quad-rupole 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–acet-ylene complex were computed on the basis of 1D DFT and MP2 OH stretching p...
Journal of Physical Chemistry A, 1998
The assignment of the vibrational spectra of phenol has been reexamined on the basis of Raman and new IR measurements and theoretical analysis of the normal modes of vibrations in the electronic ground state. The infrared spectra of C 6 H 5 OH, C 6 D 5 OD, and C 6 D 5 OH have been studied in solution and vapor phases, as well as has the Raman spectra in solutions. New experimental data were obtained from infrared linear dichroism (IR-LD) studies of phenol aligned in uniaxially oriented nematic liquid crystal solution. The measured dichroic ratios and orientation factors indicate an effective C s symmetry of the molecule with coplanar orientation of OH bond with the benzene ring and supply unique information on the extent of symmetry lowering of benzene normal modes. The fundamental vibrational frequencies, force constants, and dipole derivatives have been calculated by ab initio quantum chemical methods applying the B3P86 density functional approximation with 6-311G** basis set. The force field optimized by means of a least-squares scaling procedure for phenold 0 (using six scale factors) was used to calculate the frequencies (with a mean deviation from the observed values less than 1%), normal modes, potential energy distributions, transition moment vectors, and IR intensities for phenol-d 0 , -d 1 , -d 5 , and -d 6 isotopomers. Compared to the deviations between the calculated and observed absorption intensities, a more satisfactory correlation was found between the calculated and experimentally determined Vibrational transition moment directions. The results indicate unanimously that the perturbation of the normal modes of benzene by the asymmetric hydroxyl substituent is so great that the previous practice of assigning the normal vibrations of phenol to those of benzene or even to C 2V symmetry species is not justified. †
Contributions, Section of Natural, Mathematical and Biotechnical Sciences, 2017
The quality of theoretical prediction of O-H stretching frequency shifts upon π-hydrogen bonding is analyzed for series of ten complexes between monosubstituted phenols and hexamethylbenzene. Computed O-H frequencies from density functional theory computations at B3LYP/6-311++G(2df,2p) were compared with literature spectroscopic data. The results reveal that the applied theoretical method predicts with an excellent accuracy the O-H frequency shifts [Δυ(OH)] upon π-hydrogen bond formation. Comparisons with analogous theoretical and experimental data for benzene complexes with substituted phenols reveal the magnitude of the methyl groups’ hyperconjugative effects on interaction energies and frequency shifts. The induced by phenol substituents variations in bonding energies and Δυ(OH) are ra-tionalized using theoretically evaluated and experimental parameters.
The Journal of Physical Chemistry A, 2001
The infrared spectra of phenol and phenol-OD are thoroughly reinvestigated, to resolve the contradictory assignment of some vibrations. The harmonic frequencies, integrated IR intensities, and potential energy distribution (PED) have been calculated by the B3LYP method with the 6-311++G(df,pd) basis set. The Fourier transform infrared (FT-IR) spectra of phenol and phenol-OD have been measured in carbon tetrachloride and cyclohexane solutions, in the frequency range 3700-400 cm -1 , and the experimental integrated infrared intensities are reported. On the basis of the results obtained, the detailed assignment of all the fundamental modes of Ph-OH and Ph-OD are presented. The study demonstrates that density functional B3LYP is clearly superior to the ab initio Hartree-Fock (HF) and second-order Möller-Plesset (MP2) methods in reliable prediction of the vibrational spectra of phenol. In particular, it is shown that scaling of the B3LYP-calculated frequencies of the CH and OH(OD) stretching vibrations by the scaling factor, derived by Baker et al. [J. Phys. Chem. A 1998, 102, 1412 gives excellent agreement between theoretical and experimental frequencies of these vibrations. Detailed theoretical investigations are performed for these troublesome normal modes in phenol and benzene, which show the largest deviations between the MP2-predicted frequencies and the experimental ones. It has been demonstrated that these modes have almost identical atomic displacements and potential energy distributions in both the molecules. The electron correlation effects and basis set dependences are examined, and the nature of these problematical vibrations in aromatic molecules is discussed.