Searching of potential energy curves for the benzene dimer using dispersion-corrected density functional theory

The present work aims to establish the utility of dispersion-corrected density functional theory for potential energy curves of the benzene dimer, a problem that has received significant attention for a long time. The interaction energies of parallel-stacked, T-shaped and parallel-displaced benzene dimer configurations have been evaluated using both dispersion- and normal gradient-corrected Perdew-Burke-Ernzerhof functionals along with Dunning's augmented correlation-consistent polarized valence triple-zeta (aug-cc-pVTZ) basis functions and compared with explicit correlation methods. The potential energy curves for the parallel-stacked and parallel-displaced benzene dimers are in excellent agreement with highly accurate coupled cluster (CCSD(T)) results, while for the T-shaped benzene dimer the dispersion-corrected results show a distinct deviation, being closer in that case to the MP2 level of results. The overestimation of interaction energy in the T-shaped dimer may be attributed to the presence of a permanent dipole moment in this configuration and indicates a structural dependence of the dispersion-corrected density functional method.     

Hybrid density functional theory for pi-stacking interactions: application to benzenes, pyridines, and DNA bases

The suitability of a hybrid density functional to qualitatively reproduce geometric and energetic details of parallel pi-stacked aromatic complexes is presented. The hybrid functional includes an ad hoc mixture of half the exact (HF) exchange with half of the uniform electron gas exchange, plus Lee, Yang, and Parr's expression for correlation energy. This functional, in combination with polarized, diffuse basis sets, gives a binding energy for the parallel-displaced benzene dimer in good agreement with the best available high-level calculations reported in the literature, and qualitatively reproduces the local MP2 potential energy surface of the parallel-displaced benzene dimer. This method was further critically compared to high-level calculations recently reported in the literature for a range of pi-stacked complexes, including monosubstituted benzene-benzene dimers, along with DNA and RNA bases, and generally agrees with MP2 and/or CCSD(T) results to within +/-2 kJ mol(-1). We also show that the resulting BH&H binding energy is closely related to the electron density in the intermolecular region. The net result is that the BH&H functional, presumably due to fortuitous cancellation of errors, provides a pragmatic, computationally efficient quantum mechanical tool for the study of large pi-stacked systems such as DNA.     

Calculation of intermolecular interactions in the benzene dimer using coupled-cluster and local electron correlation methods

Potential energy curves for the parallel-displaced, T-shaped and sandwich structures of the benzene dimer are computed with density fitted local second-order M?ller-Plesset perturbation theory (DF-LMP2) as well as with the spin-component scaled (SCS) variant of DF-LMP2. While DF-LMP2 strongly overestimates the dispersion interaction, in common with canonical MP2, the DF-SCS-LMP2 interaction energies are in excellent agreement with the best available literature values along the entire potential energy curves. The DF-SCS-LMP2 dissociation energies for the three structures are also compared with new complete basis set estimates of the interaction energies obtained from accurate coupled cluster (CCSD(T)) and DF-SCS-MP2 calculations. Since LMP2 is essentially free of basis set superposition errors, counterpoise corrections are not required. As a result, DF-SCS-LMP2 is computationally inexpensive and represents an attractive method for the study of larger pi-stacked systems such as truncated sections of DNA.     

Structure and matrix isolation infrared spectrum of formyl fluoride dimer: blue-shift of the C-H stretching frequency

Infrared spectroscopy (IR) of formyl fluoride (HCOF) dimer is studied in low-temperature argon and krypton matrixes. New IR absorptions, ca. 17 cm(-1) blue shifted from the monomer C-H stretching fundamental, are assigned to the HCOF dimer. The MP2/6-311++G calculations were utilized to define structures and harmonic frequencies of various HCOF dimers. Among the four optimized structures, the dimer having two C-H...O hydrogen bonds possesses strongest intermolecular bonding. The calculated harmonic frequencies of this dimer structure are shifted from the monomer similarly as observed in the experiment. Thus, we suggest that the experimentally observed blue shifted C-H bands belong to the dimer with two C-H...O hydrogen bonds. This observation includes the HCOF dimer to the class of hydrogen bonded complexes showing blue shift in their vibrational energies.     

Ab initio calculations of structures and interaction energies of toluene dimers including CCSD(T) level electron correlation correction

The intermolecular interaction energy of the toluene dimer has been calculated with the ARS-F model (a model chemistry for the evaluation of intermolecular interaction energy between ARomatic Systems using Feller's method), which was formerly called as the AIMI model III. The CCSD(T) (coupled cluster calculations with single and double substitutions with noniterative triple excitations) interaction energy at the basis set limit has been estimated from the second-order Moller-Plesset perturbation interaction energy at the basis set limit obtained by Feller's method and the CCSD(T) correction term obtained using a medium-size basis set. The cross (C(2)) dimer has the largest (most negative) interaction energy (-4.08 kcal/mol). The antiparallel (C(2h)) and parallel (C(S)) dimers (-3.77 and -3.41 kcal/mol, respectively) are slightly less stable. The dispersion interaction is found to be the major source of attraction in the toluene dimer. The dispersion interaction mainly determines the relative stability of the stacked three dimers. The electrostatic interaction of the stacked three dimers is repulsive. Although the T-shaped and slipped-parallel benzene dimers are nearly isoenergetic, the stacked toluene dimers are substantially more stable than the T-shaped toluene dimer (-2.62 kcal/mol). The large dispersion interaction in the stacked toluene dimers is the cause of their enhanced stability.     

Geometries and stabilities of various configurations of benzene dimer: details of novel V-shaped structure revealed

Benzene dimer configurations namely T-shaped, parallel-displaced, sandwich, and V-shaped that were proposed by experimental studies are investigated using second- and fourth-order Møller–Plesset perturbation theory. The MP2 method with aug-cc-pVDZ and aug-cc-pVTZ basis sets unequivocally shows that the parallel-displaced configuration is considerably more stable than T-shaped structure. On the other hand, the MP4(SDTQ)/aug-cc-pVDZ level predicts that the T-shaped and parallel-displaced configurations are nearly isoenergetic, which is parallel to the previous results of estimated CCSD(T)/CBS level reported recently. The lowest energy T-shaped configuration is stabilized by 0.17 kcal/mol over the parallel-displaced configuration at the MP4(SDTQ)/aug-cc-pVDZ level. Although the structures of all the four different types of configurations are found to be stable at both MP2 and full MP4 methods, the V-shaped configuration is the least stable among them. The calculated interaction energy of ?2.3 kcal/mol for the lowest energy T-shaped structure at the MP4(SDTQ)/aug-cc-pVDZ level is in good agreement with the experimental value of ?2.4 ± 0.4 kcal/mol. We conclude that the MP4(SDTQ) with a reasonably good basis set can be used for systems involving π–π interactions to obtain qualitative and quantitative results.     

Intermolecular interactions of nitrobenzene-benzene complex and nitrobenzene dimer: significant stabilization of slipped-parallel orientation by dispersion interaction

The CCSD(T) level interaction energies of eight orientations of nitrobenzene-benzene complexes and nine orientations of nitrobenzene dimers at the basis set limit have been estimated. The calculated interaction energy of the most stable slipped-parallel (C(s)) nitrobenzene-benzene complex was -4.51 kcal/mol. That of the most stable slipped-parallel (antiparallel) (C(2h)) nitrobenzene dimer was -6.81 kcal/mol. The interaction energies of these complexes are significantly larger than that of the benzene dimer. The T-shaped complexes are substantially less stable. Although nitrobenzene has a polar nitro group, electrostatic interaction is always considerably weaker than the dispersion interaction. The dispersion interaction in these complexes is larger than that in the benzene dimer, which is the cause of the preference of the slipped-parallel orientation in these complexes.     

Accurate theoretical determination of the structure of aromatic complexes is complicated: the phenol dimer and phenol...methanol cases

The structure of the phenol dimer and phenol...methanol complexes was determined by gradient optimization using the Hartree-Fock (HF), MP2, DFT, and RI-DFT-D methods with various basis sets. Theoretical rotational constants were compared with experimental values and the following conclusions were made: (1) HF and DFT methods fail to predict cluster geometries; (2) MP2 with a medium basis set yields reliable cluster geometries but only because of a compensation for errors; (3) when the AO basis set is enlarged, the geometry becomes incorrect, and the theoretical geometry becomes reliable only when the higher correlation energy contributions (CCSD(T)) are included; and (4) the RI-DFT-D procedure covering the dispersion energy provides excellent geometries.     

Saturated hydrocarbon-benzene complexes: theoretical study of cooperative CH/pi interactions

High-level ab initio calculations at the CCSD(T)/aug-cc-pVTZ//MP2/aug(d,p)-6-311G(d,p) level were employed to investigate the cooperative CH/pi effects between the pi face of benzene and several modeled saturated hydrocarbons, propane, isobutane, cyclopropane, cyclobutane, cyclopentane, cyclohexane, cyclopentane, cyclooctane, and bicyclo[2.2.2]octane. In all cases, multiple C-H groups (2-4) are found to interact with the pi face of benzene, with one C-H group pointing close to the center of the benzene ring. The geometries of these complexes are governed predominantly by electrostatic interaction between the interacting systems. The calculated interaction energies (10-14 kJ mol(-1)) are 2-3 times larger than that of the prototypical methane-benzene complex. The trends of geometries, interaction energies, binding properties, as well as electron-density topological properties were analyzed. The calculated interaction energies correlate well with the polarizabilities of the hydrocarbons. AIM analysis confirms the hydrogen-bonded nature of the CH/pi interactions. Significant changes in proton chemical shift and stretching frequency (blue shift) are predicted for the ring C-H bond in these complexes.     

An experimental value for the B(1u) C-H stretch mode in benzene

We here present experimental infrared spectra on two (C(6)H(6))(C(6)D(6)) benzene dimer isomers in the gas phase. The spectra show that the two benzene molecules in the dimer are symmetrically inequivalent and have distinct IR signatures. One of the two molecules is in a site of low symmetry, which leads to the IR activation of fundamental modes that are IR forbidden by symmetry in the monomer. In the spectra, all four fundamental C-H stretch modes of benzene are observed. Modes in the dimer are shifted up to 3 cm(-1) to the red, compared to the modes that are known for the monomer. For the nu(13) B(1u) C-H stretch fundamental mode of benzene, a first experimental value of 3015(+2) (-5) cm(-1) is determined, in excellent agreement with anharmonic frequency calculations presented here.     

Estimates of the ab initio limit for pi-pi interactions: the benzene dimer

State-of-the-art electronic structure methods have been applied to the simplest prototype of aromatic pi-pi interactions, the benzene dimer. By comparison to results with a large aug-cc-pVTZ basis set, we demonstrate that more modest basis sets such as aug-cc-pVDZ are sufficient for geometry optimizations of intermolecular parameters at the second-order M?ller-Plesset perturbation theory (MP2) level. However, basis sets even larger than aug-cc-pVTZ are important for accurate binding energies. The complete basis set MP2 binding energies, estimated by explicitly correlated MP2-R12/A techniques, are significantly larger in magnitude than previous estimates. When corrected for higher-order correlation effects via coupled cluster with singles, doubles, and perturbative triples [CCSD(T)], the binding energies D(e) (D(0)) for the sandwich, T-shaped, and parallel-displaced configurations are found to be 1.8 (2.0), 2.7 (2.4), and 2.8 (2.7) kcal mol(-1), respectively.     

Direct infrared absorption spectroscopy of benzene dimer

The direct infrared (IR) absorption spectrum of benzene dimer formed in a free-jet expansion was recorded in the 3.3 μm region for the first time. This has led to the observation of the C-H stretching fundamental mode ν(13) (B(1u)), which is both IR and Raman forbidden in the monomer. Moreover, the IR forbidden and Raman allowed ν(7) (E(2g)) mode has been observed as well. These two modes were found to be red-shifted along with the IR allowed ν(20) (E(1u)) mode, as previously reported by Erlekam et al. [Erlekam; Frankowski; Meijer; Gert von Helden J. Chem. Phys.2006, 124, 171101], using ion-dip spectroscopy, contrary to the blue-shift predicted earlier by theoretical studies. The observation of the ν(13) band indicates that the symmetry is reduced in the dimer, confirming the T-shaped structure observed by Erlekam et al. Our experimental results have not provided any direct evidence for the presence of the parallel displaced geometry, the main objective of the present work, as predicted by theoretical calculations.     

High-electron-density C6H6 units: stable ten-pi-electron benzene complexes

The first stable benzene molecule with ten pi electrons is predicted. Stability is achieved through barium atoms acting as an electron-donating "matrix" to C6H6 in the inverted sandwich complex [Ba2(C6H6)]. The bis(barium)benzene complex has been computed at the density functional level of theory by using the hybrid functional mPW1PW91. Ab initio calculations were performed by using the coupled-cluster expansion, CCSD(T). Nucleus independent chemical shift (NICS) indices imply distinct aromatic character in the benzene ring of bis(barium)benzene. The D6h-symmetric structure with a 1A(1g) electronic ground state represents a thermochemically stable, aromatic benzene molecule with four excess pi electrons, stabilised by two barium ions. A possible molecular wire, built up from Ba end-capped thorium-benzene "sandwiches", is discussed.     

Resolution of identity density functional theory augmented with an empirical dispersion term (RI-DFT-D): a promising tool for studying isolated small peptides

Resolution of identity standard density functional theory augmented with a damped empirical dispersion term (RI-DFT-D) calculations have been carried out on a set of lowest energy minima of tryptophyl-glycine (Trp-Gly) and tryptophyl-glycyl-glycine (Trp-Gly-Gly) peptides. RI-DFT-D (TPSS/TZVP) results are in excellent agreement with benchmark data based on the CCSD(T) method. Experimental spectra could be assigned according to the calculated IR frequencies. Central processing unit (CPU) time requirements are only slightly higher than those needed for the DFT calculations. Consequently, RI-DFT-D theory seems to be a promising methodology for studying oligopeptides with accuracy comparable to ab initio quantum chemical calculations.     

New results on methylalkali systems: Accurate vibrational study of methylsodium

This paper presents results (structural parameters and anharmonic vibrational spectra) obtained for the methylsodium system at different levels of theory from DFT and post-HF [CCSD(T)] calculations. These theoretical results are faced with the existing experimental data. An interpretation of the experimental reference is proposed from our calculations about the monomer and the dimer compounds.     

Blue shifts and unusual intensity changes in the infrared spectra of the enflurane···acetone complexes: spectroscopic and theoretical studies

Blue-shifting C-H···O hydrogen-bonded complexes between enflurane (CHFCl-CF(2)-O-CHF(2)) and deuterated acetone have been identified in CCl(4) solution by FT-IR spectroscopy. For the two ν(C-H) stretching vibrations of enflurane the observed blue shifts are +17 and +11 cm(-1). The corresponding two infrared ν(C-H) bands show the opposite changes of their intensity, one is decreasing, and the other is significantly increasing, upon formation of the hydrogen bonding. The structures, binding energies, and theoretical infrared spectra of the enflurane-acetone complexes were calculated by MP2 and B3LYP methods using the 6-311++G(d,p) basis set. The interaction energies were evaluated by the complete basis set limit (CBS) calculations at the HF, MP2, and CCSD(T) levels of theory. Although the MP2 method slightly overestimates the blue shifts, the MP2 predicted frequency difference and the relative IR intensities of two ν(C-H) stretching bands for the enflurane-acetone complexes show good agreement with experiment. Unfortunately, the B3LYP method predicts incorrect IR intensities of these hydrogen-bonded systems. The NBO analysis was performed to unravel the origin of the unusual intensity changes of two ν(C-H) stretching bands, in enflurane complexes.     

Highly Accurate CCSD(T) and DFT–SAPT Stabilization Energies of H‐Bonded and Stacked Structures of the Uracil Dimer

The CCSD(T) interaction energies for the H‐bonded and stacked structures of the uracil dimer are determined at the aug‐cc‐pVDZ and aug‐cc‐pVTZ levels. On the basis of these calculations we can construct the CCSD(T) interaction energies at the complete basis set (CBS) limit. The most accurate energies, based either on direct extrapolation of the CCSD(T) correlation energies obtained with the aug‐cc‐pVDZ and aug‐cc‐pVTZ basis sets or on the sum of extrapolated MP2 interaction energies (from aug‐cc‐pVTZ and aug‐cc‐pVQZ basis sets) and extrapolated ΔCCSD(T) correction terms [difference between CCSD(T) and MP2 interaction energies] differ only slightly, which demonstrates the reliability and robustness of both techniques. The latter values, which represent new standards for the H‐bonding and stacking structures of the uracil dimer, differ from the previously published data for the S22 set by a small amount. This suggests that interaction energies of the S22 set are generated with chemical accuracy. The most accurate CCSD(T)/CBS interaction energies are compared with interaction energies obtained from various computational procedures, namely the SCS–MP2 (SCS: spin‐component‐scaled), SCS(MI)–MP2 (MI: molecular interaction), MP3, dispersion‐augmented DFT (DFT–D), M06–2X, and DFT–SAPT (SAPT: symmetry‐adapted perturbation theory) methods. Among these techniques, the best results are obtained with the SCS(MI)–MP2 method. Remarkably good binding energies are also obtained with the DFT–SAPT method. Both DFT techniques tested yield similarly good interaction energies. The large magnitude of the stacking energy for the uracil dimer, compared to that of the benzene dimer, is explained by attractive electrostatic interactions present in the stacked uracil dimer. These interactions force both subsystems to approach each other and the dispersion energy benefits from a shorter intersystem separation.     

Infrared spectroscopic investigation of poly(2-methoxyethyl vinyl ether) during thermosensitive phase separation in water

Hydration changes of poly(2-methoxyethyl vinyl ether) (PMOVE) synthesized via living cationic polymerization have been investigated during a temperature-responsive phase separation in water by using infrared spectroscopy. An aqueous PMOVE solution has lower critical solution temperatures (LCSTs) of 66 degrees C in H2O and 65 degrees C in D2O at approximately 15 wt %. During phase separation, the C-H stretching (nu(C-H)) bands of PMOVE shift downward (red shift). In particular, the IR band assigned to the antisymmetric stretching vibration of the terminal methyl groups exhibits a remarkably large red shift by 16 cm-1. The band also exhibits a red shift with increasing polymer concentration at T < Tp. Density functional theory (DFT) calculations of the models of hydrated PMOVE indicate that the shift is due mainly to the breaking of hydrogen bonds (H-bonds) between the oxygen of the methoxy groups and water and partially to the breaking of the CH...O H-bond to them.