Experimental Evidence of Synergistic Interactions in PyrroleâPhenol

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Experimental Evidence of Synergistic Interactions in PyrrolePhenol Complexes at Low Temperatures under Isolated Conditions Shubhra Sarkar, Nagarajan Ramanathan, and Kalyanasundaram Sundararajan J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b09076 • Publication Date (Web): 30 Oct 2018 Downloaded from http://pubs.acs.org on November 3, 2018

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The Journal of Physical Chemistry

Experimental Evidence of Synergistic Interactions in Pyrrole-Phenol Complexes at Low Temperatures under Isolated Conditions Shubhra Sarkar, N. Ramanathan, K. Sundararajan* Materials Chemistry & Metal Fuel Cycle Group, Homi Bhabha National Institute, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India.

Abstract The simultaneous possession of -electron clouds and acidic hydrogen atoms in pyrrole (C4H5N) and phenol (C6H5OH) framework, opens up the potentiality in exploring the synergistic interactions in their weakly bonded complexes. In this work, the synergistic hydrogen bonding in C4H5N-C6H5OH complexes is therefore investigated using FTIR spectroscopy under isolated conditions at low temperatures. Computations performed at DFT, DFT-GD3, M06 and MP2 level of theories employing aug-cc-pVDZ basis set, yielded three minima on the potential energy surface for the 1:1 complex of C4H5NC6H5OH. All the three optimized structures showed synergistic interactions, where both C6H5OH and C4H5N simultaneously act as a proton donor and acceptor at MP2/aug-ccpVDZ level of theory. In the global minimum complex A, the hydroxyl proton and the C-H group of C6H5OH interacts with the π-cloud of C4H5N. The first local minimum corresponds to complex B, where the N-H and π-electrons of C4H5N interact with π-electrons of C6H5OH. In complex C, the N-H and C-H groups of C4H5N interact with OH and π-cloud of C6H5OH, respectively. Complex A was the lowest energy structure at all levels of theory whereas the stabilization energies of complexes B and C varied depending upon the levels of theory used. Interestingly, the stabilization energies as predicted by DFT method are in accordance with Etter’s and Legon-Millen rules; however, a deviation in Legon-Millen rule was discerned with empirical (DFT-GD3, M06) and dispersion corrected (MP2) methods. On comparing the experimental vibrational wavenumber shifts in the N-H stretching and bending modes of C4H5N and O-H stretching mode of C6H5OH submolecules with the computed shifts, all the three complexes were identified in the N2 matrix. Natural Bond Orbital and Energy Decomposition analyses were performed to characterize the nature of the synergistic interaction in these complexes.

*Corresponding

author: email: [email protected] ACS Paragon Plus Environment

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1. Introduction Synergism among weak interactions is a subject of study as it strongly influences the total stabilization energy, which leads to enhanced stability. Pyrrole (C4H5N) and phenol (C6H5OH) can be taken as model systems to study these synergistic interactions. Furthermore, C4H5N and C6H5OH based compounds are well known for their role in biology, drug activities,1-6 and as a conducting polymer.7-12 Several reports are available on the self-aggregating nature of C4H5N and C6H5OH.13-16 C6H5OH can form dimers through O-H···O linkages,13 O-H of one C6H5OH moiety interacting with the oxygen of the other; whereas C4H5N can form dimers and multimers through N-H···π networks, the N-H of one C4H5N interacting with the π-cloud of the other.17-21 Several groups have carried out spectroscopic studies on C6H5OH22-28 and its interaction with water.29-41 Sodupe et al. have performed calculations at MP2 and B3LYP levels of theory on the ionization of C6H5OH-H2O and C6H5OH-NH3 hydrogen bonded complexes.30 They concluded that B3LYP method was the more accurate theory for the neutral as well as for the cationic complexes of C6H5OH with H2O and NH3. They have found that the geometrical relaxation leads to a non-proton transferred complex in C6H5OH-H2O and a proton-transferred complex in C6H5O-NH4+. Courty et al. have studied the gas phase photo-ionization behavior of C6H5OH with H2O, CH3OH and CH3OCH3 using semi-empirical model and by recording the ZEKE photoelectron spectra, one-color mass spectra and two-color fragmentation spectra and obtained the gas phase binding energy of the hydrogen bonded complexes.42 Iwasaki et al. have studied the hydrogen bonded clusters of C6H5OH with NH3, N(CH3)3, NH(C2H5)2, and N(C2H5)3 using IR-UV double resonance spectroscopy and ab initio calculations.43 They had found that the increase in the νO-H shifts of the complexes were consistent with the proton affinities of the amines. Mikami et al. using IR-UV double resonance spectroscopy and B3LYP/6-31G (d, p) level of theory studied the N-H···π interaction of C6H5OH with C2H2, C2H4 and ACS Paragon Plus Environment

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C6H6 in the neutral and cationic ground states.44 They observed that the frequency shifts in the O-H vibrational mode of C6H5OH did not depend on the source of the π-electrons and found that the shifts were lesser for the C6H5OH-C6H6 complex even though the benzene π-cloud has a high proton affinity. The reason for the lower shift was attributed to the formation of secondary C-H···π interaction between the C6H5OH-C6H6 complex. Pujarini et al. have revisited the O-H···π interaction by performing matrix isolation experiments through a series of fluorine substituted C6H5OH with C6H6.45 They discerned that the occurrence of red shift in the vibrational wavenumbers of the complexes depends upon the pKa values of the fluorophenols. Maity et al. have reported that the participation of C6H5OH in hydrogen bonding is entirely due to the acidic nature of O-H bond, which makes C6H5OH a proton donor. On the other hand, forming a hydrogen bond with a more acidic species, may turn C6H5OH into a proton acceptor, and in none of the cases, the aromatic π-cloud would participate in the bonding.46 Earlier, the complexes of C6H5OH with C2H2 were investigated using matrix isolation infrared spectroscopy, supported by computations performed at B3LYP/6311++G (d p) level of theory.47 Computations yielded three optimized structures for the C6H5OH-C2H2 complexes with O-H···π, C-H···O and C-H···π interactions and experimentally, the O-H···π and C-H···O C6H5OH-C2H2 complexes were observed in Ar matrix. Recently, we have reported the π-stacking complexes of C4H5N with N2 in Ar matrix.48 The π-stacking interaction was stabilized through the π-electron transfer from C4H5N ring to non-Lewis π-orbitals of N2 molecule, which was examined through quantum chemical calculations. Etter’s rule is generally used to rationalize and predict the hydrogen bonding interaction in the crystals49, which states that 1) All good proton donors and acceptors are used in hydrogen bonding 2) Six membered-ring intramolecular hydrogen bonds form in ACS Paragon Plus Environment

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preference to intermolecular hydrogen bonds 3) The best proton donors and acceptors remaining after intramolecular hydrogen bond formation form intermolecular hydrogen bonds to one another. Legon and Millen50 gave rules for the formation of X-H···B hydrogen bonds using the rotational spectra of several mixed dimers in the gas phase. These rules are 1) The axis of the HX molecule coincides with the supposed axis of a nonbonding electron pairs on B. 2) If B has no nonbonding electron pairs instead it has πbonding electron pairs, then the axis of HX molecules intersects the internuclear axis of atoms forming the π bond in B. 3) If B has both nonbonding and π-bonding electron pairs then the rule (1) is definite. The first and second rules describe the structure of the complex, while the third rule indicates that the lone pair of electrons is preferred over π-bonding electron pairs in the hydrogen bond formation. Both Etter’s and Legon-Millen predict a definite order in the hydrogen bonding based on these empirical rules. However, exceptions to these rules were noticed in the literature.46, 51-52 In this work, we present our experimental and computational results on the hydrogen bonded interaction between C4H5N and C6H5OH. The O-H of C6H5OH and the N-H of C4H5N can act as proton donor and the π-electron cloud in both the molecules can simultaneously act as proton acceptor.18, 48 Such heterogeneous sites, in these molecules, aid in the synergistic interaction where multiple hydrogen bonds lead to the stability of the complexes. Furthermore, the geometry of the complex depends on the relative hydrogen bonding abilities of the donor and acceptor molecules, which depends on the Etter’s and Legon-Millen rules. There are several reports on C4H5N π-electron cloud participating in hydrogen bonding,

15-18, 21

but there are a few studies involving the C6H5OH π-electron

cloud.46 DFT, DFT-GD3, M06 and MP2 level of theories employing aug-cc-pVDZ basis set were used to arrive at the possible geometries of the complexes and to compare with the experimental wavenumbers. Natural Bond Orbital (NBO) and Energy Decomposition ACS Paragon Plus Environment

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(ED) analyses were carried out to understand the synergistic interactions in these complexes. Additionally, Etter’s and Legon-Millen rules were tested on the C4H5NC6H5OH complexes to find out whether the hydrogen bonding is in accordance with these rules or not. 2. Experimental Methods and Computations Low temperature (~10 K) was achieved using a closed-cycle helium compressorcooled cryostat (RDK-408D2, Sumitomo Heavy Industries Ltd.). The cryostat is attached with the sample compartment containing a KBr window. The sample compartment and the mixing chamber were continuously pumped by a diffusion pump backed by a rotary pump to achieve the desired vacuum of 10-7 Torr. Ultra high pure N2 (Inox, 99.9995%) was used as the matrix gas. Pyrrole (C4H5N, Spectrochem, 99%) and phenol (C6H5OH, SigmaAldrich, >99%) were used by chilling them to 100 K and pumping off the volatile impurities by freeze-thaw-pump cycle method. Initially, C4H5N was equilibrated in the mixing chamber by maintaining at a suitable temperature. To this, a desired concentration of C6H5OH was added and then diluted with nitrogen gas to obtain manometric ratios of the sample mixture. The matrix to solute ratio (C4H5N/C6H5OH/N2) used in the experiments are 0.25:0.25:1000 to 1.0:0.5:1000. The gas mixture was allowed to deposit onto the cold KBr substrate through a dosing valve using a single jet effusive nozzle. Typically, 32 scans and a resolution 0.5 cm-1 were used to record the infrared spectra of the deposited sample with Bruker Vertex 70 infrared spectrophotometer using a Mercury Cadmium Telluride (MCT) detector. To aid the formation of the complex, the matrix was slowly annealed at 30 K for about 15 minutes and then cooled back to 10 K. The infrared spectrum of the annealed matrix is again recorded. All the spectra shown in this paper were recorded after annealing the matrix.

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Theoretical calculations were carried out using GAUSSIAN0953 package to arrive at the optimized geometries of the 1:1 C4H5N-C6H5OH complexes. B3LYP, B3LYP-GD3 (Grimme’s dispersion corrected54-56), M06 and MP2 levels of theories with aug-cc-pVDZ basis set were used to compute the optimized structures. Vibrational wavenumber calculation was performed on the optimized geometries, which gave all positive wavenumbers indicating that the geometries were indeed minima on the potential energy surface. Stabilization energies for the complexes, corrected separately, for both zero point energy (ZPE) and basis set superposition errors (BSSE) outlined by Boys and Bernardi.57 Natural bond orbital analysis (NBO) was performed for the optimized geometries using NBO 3.1 to obtain the delocalization energies and electron occupancies.58-59 Energy Decomposition Analysis (EDA) on the optimized geometries was done using GAMESS package.60-61 The structures of the complexes have been generated using Chemcraft software (version 1.8).62 3. Results and Discussion 3.1 Experimental Results Figure 1 illustrates the infrared spectra of the O-H and N-H stretching regions of C6H5OH and C4H5N in N2 matrix, annealed at 30 K. The 1 O-H stretching mode of bare C6H5OH in N2 matrix is observed at 3622.0 and 3626.8 cm-1, which agrees well with the reported data.47 The 1 N-H stretching mode of C4H5N appears as site-split features at 3513.8 and 3508.9 cm-1, and the peaks observed at 3503.4 and 3500.9 cm-1 are due to (C4H5N)2 where C4H5N acts as proton acceptor.63 When C4H5N and C6H5OH were codeposited and annealed, a new feature starts appearing at 3619.8 cm-1 in the O-H stretching region of C6H5OH and a broad intense peak is observed at 3508.4 cm-1 with a small shoulder at 3506.5 cm-1 in the N-H stretching region of C4H5N sub-molecule. All the new features increased in intensity as the concentration of C4H5N varied, clearly affirming the features are due to C4H5N-C6H5OH complex. ACS Paragon Plus Environment

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Figure 2 shows the ν1 N-H stretching mode of C4H5N, spanning the region 35223398 cm-1. The infrared absorption spectrum of C6H5OH and C4H5N alone in N2 matrix is shown in Figure 2a and 2b. The site split features observed at 3423.4, 3417.3 and 3415.2 cm-1 are due to (C4H5N)2 where C4H5N plays the role of a proton donor.63 The features observed at 3493.8, 3409.1, 3405.0 and 3402.2 cm-1 are due C4H5N-(H2O)2 complex in N2 matrix.64 The peaks observed at 3470.6/3465.8 and 3425.5/3417.1 cm-1 are due to (C6H5OH)2 and C6H5OH-H2O complexes, respectively.32 As the concentrations of C6H5OH were varied and annealed, new peaks were observed in the N-H stretching region of C4H5N submolecule at 3497.0, 3441.4, 3436.4, 3433.9, 3412.5 and 3406.7 cm-1. Figure 3 represents the ν16 N-H bending mode of C4H5N in N2 matrix. Figure 3a and 3b correspond to the C6H5OH and C4H5N alone spectrum, respectively. The features observed at 520.2/527.2 cm-1 are due to N-H bending mode of C4H5N. The peaks found at 543.6, 545.7, 550.6, 572.3 and 573.5 cm-1 are due (C4H5N)2.63 (C4H5N)3 peaks were observed at 585.8 and 590.1 cm-1.63 The feature at 527.9 cm-1 in C6H5OH alone spectrum in N2 matrix is due to δ(CC)+[ν(C-O)] mode of C6H5OH.65 During the co-deposition experiments, a new feature appears as a shoulder at 526.0 cm-1. Furthermore, the features at 545.7 and 550.6 cm-1 broaden and new peaks were observed at 546.2, 548.7 and 551.6 cm-1. The N-H bending feature due to the (C4H5N)2 appearing at 572.3/573.5 cm-1, merge to form a single broad peak at 573.5 cm-1 and a sharp peak observed at 587.9 cm-1 increases in intensity as the concentration of C6H5OH was varied. 3.2 Computational Results Computations were performed to arrive at the optimized geometries of the C4H5NC6H5OH complexes using different levels of theory such as B3LYP, B3LYP-GD3, MP2 and M06 with aug-cc-pVDZ basis set. Computations predicted three minima on the potential energy surface for the C4H5N-C6H5OH complexes and the structures of the complexes computed at MP2/aug-cc-pVDZ level of theory are shown in Figure 4a, 4b and ACS Paragon Plus Environment

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4c. For comparison, the structure of the complexes computed at B3LYP/aug-cc-pVDZ level of theory is shown in Fig.S1 of supporting information. The structural parameters of the complexes computed at B3LYP, B3LYP-GD3, M06 and MP2 level of theories with aug-cc-pVDZ basis set are listed in the Table.S1 of supporting information. The stabilization energies of the complexes along with their structure computed at different levels of theory using aug-cc-pVDZ basis set is illustrated in Table 1. Figure 5 shows the plot of BSSE corrected stabilization energies at different levels of theory using aug-cc-pVDZ basis set versus complexes A, B and C. It is clear from the plot that at all level of theories, the complex A (O-H···π) is lower in energy (global minimum) than the other two complexes B (N-H···π) and C (N-H···O) (local minima). Depending on the level of theory, the energies of the complexes, B and C fluctuate. At B3LYP/aug-cc-pVDZ level of theory, complex C is lower in energy than complex B, whereas the inclusion of Grimme’s dispersion correction at DFT level, the complexes becomes nearly isoenergetic. At M06 and MP2 level of theories, complex B is more stable than complex C, due to the incorporation of the hybrid functional and dispersion correction, respectively. Computations performed at B3LYP/aug-cc-pVDZ level of theory showed a decreasing order of stabilization energy; complex A < complex C < complex B, which obeys the first and third rule of Etter’s and Legon-Millen, respectively. The O-H of phenol and π-electron of pyrrole are better proton donors and acceptors, respectively and the non bonded pair of electrons form a stronger interaction (complex C) than the  electron (complex B). It should be mentioned at this level of theory, complex B and C are devoid of any secondary interactions. As the dispersion correction is incorporated either through empirical (DFT-GD3 and M06)

or by perturbation methods (MP2) to optimize the

geometry of the complexes, the stabilization energy ordering is complex A < complex B < complex C. Contrary to DFT level, apart from the primary interaction in complexes B and C, they are further stabilized by secondary interactions. Due to this secondary interaction ACS Paragon Plus Environment

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the geometry of the complexes are altered, which makes the complex B more stable than complex C. Clearly, a deviation from Legon-Millen third rule was noticed as the  interactions are stronger than the nonbonding interaction. It should be mentioned that at all levels of theories with aug-cc-pVDZ basis sets, complex A was the global minimum structure (with both primary O-H··· and secondary C-H··· interactions, which therefore manifests that the O-H of phenol is more acidic than pyrrole N-H and forms a stronger complex than complexes B and C (nonbonding and  interaction), obeying Etter ’ s first rule. Invariably at all levels of theory, the structures of the complexes A, B and C violate the Legon-Millen second rule. In complex A, there are synergistic (primary and secondary) interactions between the O-H and C-H of C6H5OH with π-cloud of C4H5N, respectively. The bond distances between the H13-C18, H13-C17, H10-C14 and H10-C15 are 2.339Å, 2.454Å, 2.671Å and 2.527Å. Furthermore, the bond angles O12-H13-C18,O12-H13-C17,C9-H10-C15 and C9-H10-C14 are 165.3º, 132.0º, 138.3º and 156.6º and the dihedral angles O12-H13-C18-H22, O12-H13-C17-H21, C6-C9-H10-C15, C6-C9-H10-C14 are 111.5º, 38.4º, 76º and 125.7º, which make the C6H5OH moiety tilt towards the C4H5N molecule. At B3LYP/aug-cc-pVDZ level of theory, the C6H5OH molecule is perpendicular to the C4H5N (as shown in Fig. S1a). In complex B, the bond angle N16-H23-C1 is 132.2º and the dihedral angles C17-N16-H23-C1,C15-N16-H23-C1 are -76º and 89.3º, which make the C4H5N molecule to be displaced parallel to the plane of the C6H5OH molecule, favoring a π···π* interaction between the aromatic π-electron clouds of these molecules. The bond distances between the H23-C1, H23-C2, C17-C7 and C17-C9 are 2.702Å, 2.831Å, 3.060Å and 3.289Å. At B3LYP/aug-cc-pVDZ level of theory the N-H group of C4H5N is perpendicular to the C6H5OH and has only N-H···π interaction (Fig. S1b). Clearly, the


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incorporation of dispersion correction alters the geometry of the complex and favors the synergistic interaction between the C4H5N and C6H5OH molecules. At MP2/aug-cc-pVDZ level of theory, complex C has both N-H···O and C-H···π interactions; the bond distance between the H23-O12 and H20 of C4H5N with all the carbons of C6H5OH is 2.167Å, and ~2.820Å, respectively.

The bond angle

N16-H23-O12 and C15-H20-C1 are 157.6º and 141.7º, and the dihedral angle C15-N16-H23-O12, C17-N16-H23-O12, C14-C15-H20-C1 and C14-C15-H20-C2 are -32.1º, 147.0º,134.4º and 162.5º, which orients the C4H5N molecule perpendicular to the plane of the C6H5OH molecule. At B3LYP level, the geometry of the molecule is completely different and there is only N-H···O interaction between C4H5N and C6H5OH molecule (Fig. S1c). At MP2/aug-cc-pVDZ level of theory, invariably all the complexes A, B and C demonstrate the synergistic ability of forming multiple hydrogen bonds (synergistic interactions), which increases the stability of the complexes. 3.3 Vibrational Assignments Table 2 shows the comparison of shift in the experimental wavenumbers of the complexes A, B and C with the computed values at MP2/aug-cc-pVDZ level of theory. 3.3.1 νO-H stretching region: As discussed earlier, the ν1 O-H stretching mode of bare C6H5OH was observed as a site split feature at 3622.0 and 3626.8 cm-1 in N2 matrix.32 As the concentration of C4H5N was varied, new peaks show up as a site-split feature at 3508.4/3506.5 cm-1, a red shift of 117.0 cm-1 from the C6H5OH monomer absorption, which compares well with the computed shift of 131.2 cm-1 for the O-H···π complex A. It should be mentioned that, the new feature observed at 3508.4/3506.5 cm-1 for the complex A, exactly falls on the site-split feature of (C4H5N)2 at 3508.9 cm-1, broadens and picks up in intensity as the C4H5N concentration was varied, clearly confirms the formation of O-H···π complex A at low temperatures. Computations predicted a red shift of 17.8 and ACS Paragon Plus Environment

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16.2 cm-1, for the N-H···π complex B and N-H···O complex C, respectively, in the ν1 O-H stretching mode of C6H5OH sub-molecule. Experimentally, a red shift of 2.2 cm-1 is observed in the ν1 O-H stretching mode of C6H5OH, which is assigned for the complex C. In complex A, where C6H5OH acts as a proton donor, the O-H vibrational shift was larger, whereas in complexes B and C, C6H5OH plays the role of proton acceptor and the corresponding shifts are smaller in the ν1 O-H stretching mode of C6H5OH submolecule. 3.3.2 νN-H stretching and bending region: For the O-H···π complex A, where C4H5N is a proton acceptor, the vibrational wavenumber shifts in the N-H stretching and bending mode in the C4H5N sub-molecule are smaller in comparison to those of the complexes B and C, where C4H5N is the proton donor. Experimentally, new features observed at 3497.0 and 546.2/548.7/551.6 cm-1 in the N-H stretching and bending modes of C4H5N, having a shift of -16.2 and +25.3 cm-1 compares well with the computed shift of -13.9 and +28.0 cm-1, respectively for the complex A. The site-split multiplets observed at 3441.4/3436.4/3433.9 cm-1 in the N-H stretching mode and at 573.5 cm-1 in the N-H bending mode, having a shift of -75.9 and +50.0 cm-1, respectively, matches with the computed shifts of -51.5 and +40.0 cm-1 for the complex B. For the N-H···O complex C, computations showed a shift of -56.7 and 64.8 cm-1, but the experimental shift are -103.6 and 64.4 cm-1 in the N-H stretching and bending modes of C4H5N sub-molecule, respectively. Table 3 gives the comparison of experimental vibrational shift of complexes A, B and C with the computed shift at different levels of theory with aug-cc-pVDZ basis set. It is clear from the table that the experimental shift for the complex A in the O-H stretching mode, matches well with the computed shift at B3LYP, M06 and MP2 levels; whereas, there is a deviation at the dispersion corrected B3LYP level. In the N-H bending and stretching modes, the computed shift at the MP2 level of theory corroborates well with the ACS Paragon Plus Environment

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experimental shift whereas at the other levels of theory, there is a large deviation between the computed and experimental shift for the complex A. Interestingly, the synergistic O-H···π and C-H···π interactions were found to be present in the complex A, at all the levels of theory. In complex B, experimental shift showed a deviation from the computed shift calculated at B3LYP, B3LYP-GD3, M06 levels of theory with aug-cc-pVDZ basis set in the N-H and O-H stretching region of C4H5N and C6H5OH sub-molecules. The geometry optimized at the B3LYP/aug-cc-pVDZ level shows only an N-H···π interaction; whereas at MP2/aug-cc-pVDZ, due to the dispersion correction, and N-H···π and π···π* synergistic interactions, the C4H5N has a displaced parallel structure with the C6H5OH molecule and the experimental wavenumber compares well with the computed shift. For the complex C, the experimental and computed shifts are comparable in the O-H and N-H stretching regions at B3LYP, B3LYP-GD3, M06 levels of theory whereas at MP2 level of theory, there is a deviation. The experimental shift in the N-H bending mode for complex C matches reasonably well with the computed shift at other levels of theory whereas, there is a large difference in the N-H stretching wavenumbers at the MP2 level. Such discrepancies in the computed shift are probably due to the level of theory used to arrive at the optimized structures of the complexes. At B3LYP, B3LYP-GD3 and M06 levels of theory, the complex C has only N-H···O interaction. There are no synergistic interactions for which the red shift in the N-H stretching wavenumber of C4H5N submolecule was ca. 83-86 cm-1, agreeing well with the experimental red shift of 103.6 cm-1. At MP2 level of theory, apart from the primary N-H···O interaction, a secondary C-H···π interaction was formed between C4H5N and C6H5OH and these two interactions are acting opposite to each other. The incorporation of dispersion correction stabilizes the secondary C-H···π interaction, which probably weakens the N-H···O hydrogen bond distance and the corresponding red shift in N-H stretching mode of C4H5N submolecule ACS Paragon Plus Environment

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decreases in the complex C. This type of H-bond weakening due to secondary interaction has been reported by Mikami et al. in case of C6H5OH-C6H6 complexes.44 Hence, for the complex C, a non-synergistic structure is favorable in the matrix and it is stabilized by only N-H···O interaction as predicted by B3LYP/aug-cc-pVDZ level of theory (figure S1c). In a nutshell, complexes A (O-H···π, C-H···π) and B (N-H···π, π···π*) have synergistic interactions on the π-electron cloud of C4H5N and C6H5OH, respectively. MP2/aug-cc-pVDZ level of theory was able to predict the structures and the computed vibrational shifts agree reasonably well with the experimental wavenumbers. Although, MP2 level of theory predicts the structure of complex C to have interactions on oxygen (N-H···O) and π-electron cloud (C-H···π) of C6H5OH with C4H5N, the experimental shift confirms that the complex C may not possess a secondary interaction in the matrix. It is likely that the dispersion dominated complex C (as predicted by MP2 level of theory) is strongly destabilized due to the matrix surrounding and further computational work is essential to understand role of the matrix on to the complex C. It is obvious from our matrix isolation experiments that the structures optimized in the isolated gas phase in vacuum, need not be generated at cold isolated conditions as the low temperature matrix plays a profound role in altering the geometry of the complexes. 3.4 Nature of Interaction 3.4.1 NBO Analysis: Natural Bond Orbital Analysis (NBO) is a useful theoretical method to characterize the nature of interactions existing in the complexes. In the hydrogen bonded complexes, the charge transfer hyperconjugative interaction contribution is substantial, which induces a bond elongation with a concomitant red shift in the vibrational wavenumber of the proton donor.

In NBO analysis, the second order

perturbation energy (E2) gives the magnitude of delocalization interaction. Table 4 shows


13


the results of the NBO analysis of the complexes A, B and C computed at MP2/aug-ccpVDZ level of theory. For complex A, the hyperconjugative interaction was observed between the πelectron cloud of C4H5N [π(C17-C18)] to the antibonding orbital [σ*(O12-H13)] of C6H5OH. The E2 energy of this interaction is 6.33 kcal.mol-1. Furthermore, due to this interaction, there is a reduction in the occupancies of electrons in the donor π-orbital of C4H5N and a marginal increase in the occupancies of electrons in the acceptor σ*(O12H13) orbital of C6H5OH. The increase in electron density at the antibonding orbital elongates the O-H bond with an associated red shift in the vibrational wavenumber of C6H5OH sub-molecule in the O-H···π complex A. Furthermore, there is another hyperconjugative interaction between π(C14-C15) electron cloud of C4H5N with the antibonding orbital of σ*(C9-H10) of C6H5OH and the E2 energy is 2.82 kcal/mol. This interaction also exhibits a change in the electron occupancies of the donor and acceptor orbitals. Due to these hyperconjugative interactions between the C6H5OH with C4H5N molecules, the geometry of the C6H5OH molecule is tilted and favors the synergistic interaction and the experimental shift supports the formation of the complex in the matrix. For the complex B, the maximum delocalization E2 energy (2.55 kcal.mol-1) was obtained for a unique π···π* interaction between π(C2-C6) and π(C7-C9) of C6H5OH to the antibonding orbitals of π*(C14-C15) and π*(C17-C18) of C4H5N, respectively; similarly, there is an interaction between the π-electrons of C4H5N,π(C14-C15) and π(C17-C18) to the π*(C2-C6) and π*(C7-C9) antibonding orbital of C6H5OH, which makes the C4H5N parallel displaced to the plane of the C6H5OH molecule. The charge transfer hyperconjugative interaction between the π(C1-C3) of C6H5OH to the σ*(N16H23) of C4H5N is 1.22 kcal.mol-1, inducing a red shift in the N-H stretching vibrational wavenumber of C4H5N submolecule in the N-H···π complex B. In complex B, the


14

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synergistic interaction between the two molecules changes the geometry and the experimental shift supports the formation in the matrix. In complex C, the delocalization interactions are between the non-bonding orbitals of oxygen of phenol to the N-H antibonding orbitals of C4H5N [n2(O12)→σ*(N16-H23) and n1(O12)→σ*(N16-H23)], the E2 energy of these interactions are 1.01 and 3.91 kcal.mol-1, respectively. In addition to this interaction, there is another charge transfer interaction between π(C2-C6), π(C1-C3) and π(C7-C9) of C6H5OH to the antibonding orbital of σ*(C15-H20) of C4H5N submolecule. The increase in the electron occupancy of the σ*(N16-H23) orbital, causes the N-H bond to elongate with a corresponding red shift of the N-H stretching wavenumber of C4H5N sub-molecule in the complex C. The difference in contribution of delocalization energy of the lone pairs of oxygen atom in C6H5OH is probably due to the different spatial orientations of the lone pair orbitals. The n1(O12)→σ*(N16-H23) is an axial overlap whereas, n2(O12)→σ*(N16-H23) is a lateral overlap; the former giving the highest interaction due to more facile interaction between the orbitals. The same trend has been observed in our previous work on the N-H···O interaction studies of C4H5N with H2O, CH3OH and CH3OCH3.64 3.4.2 Energy Decomposition Analysis: The energy decomposition (ED) analysis was performed in order to identify the dominant interactions in stabilizing the C4H5N-C6H5OH complexes A, B and C. Table 5 tabulates the results of the ED analysis for the C4H5NC6H5OH complexes performed using MP2/aug-cc-pVDZ level of theory. It is clear from the table that for all the complexes, the strongest attractive interaction is exchange followed by dispersion, electrostatic and polarization. For the complex A, the magnitudes of electrostatic (22%) and dispersion (23%) is nearly the same, whereas for the complexes B and C the dispersion contribution (28% and 29%) is larger than the electrostatic (17% and 19%). The above trend reveals that the larger the π…π* interactions, the dispersion contribution is predominant in stabilizing the complexes.66-67 Furthermore, the large ACS Paragon Plus Environment

15


exchange and repulsion terms show that there is a significant orbital overlap in the C4H5NC6H5OH complexes, which is supported by NBO analysis. Polarization energy contribution of the complexes A, B and C are 10%, 7% and 9% suggesting that C6H5OH and C4H5N orbitals undergo a very little change in their monomer geometry during the formation of complex. 4. Comparison of the O-H···π and N-H···O complexes of C4H5N with H2O, CH3OH and C6H5OH: Table 6 compares the various parameters of the N-H···O and O-H···π complexes of C4H5N with H2O, CH3OH and C6H5OH. All the computed values listed in the table were performed at MP2/aug-cc-pVDZ level of theory. From the table 6, it is obvious that H2O and CH3OH form stronger N-H···O complex than O-H···π complex, whereas the latter is more stable than former in C4H5NC6H5OH complex. The probable reason for observing the reverse trend in C4H5N-C6H5OH complex is due to the better proton donating ability of the phenolic O-H compared to H2O and CH3OH.It is noteworthy to mention that O-H···π C4H5N-C6H5OH complex is a global minimum whereas the O-H···π complexes of C4H5N-CH3OH and C4H5N-H2O were local minima on the potential energy surface. Among the O-H···π complexes, the stabilization energies and the computed red shifts of O-H stretching and bending modes follow the order: C4H5N-C6H5OH > C4H5N-CH3OH > C4H5N-H2O. Furthermore, the second order perturbation energy (E2) also shows a similar trend. Experimentally, features due to the O-H···π C4H5N-C6H5OH complex in the O-H stretching region of C6H5OH sub-molecule were observed in N2 matrix, whereas for the O-H···π complexes of C4H5N-CH3OH and C4H5N-H2O, experimental features could not be discerned. The hydrogen bonded distance for the O-H···π complexes is shorter for C4H5N-CH3OH followed by C4H5N-C6H5OH and C4H5N-H2O complexes but the computed red shift in the O-H stretching mode are larger for C4H5N-C6H5OH than the other two complexes. In C4H5N-C6H5OH complex, apart


16

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from the strong primary O-H···π interaction there also exist a secondary C-H···π interaction, which synergistically makes the O-H vibrational mode to red shift more than the other two complexes. In the O-H···π complexes of C4H5N-CH3OH andC4H5N-H2O, there were no synergistic interactions. Among the N-H···O complexes, C4H5N-H2O and C4H5N-CH3OH were the global minima and C4H5N-C6H5OH was the local minimum. The stabilization energies, N-H···O bond distances, E2 energies and the computed red shifts of the N-H bending and stretching modes of C4H5N follow the order: C4H5N-CH3OH > C4H5N-H2O > C4H5N-C6H5OH. The above trend is consistent with electron donating ability of the order CH3OH > H2O > C6H5OH. For CH3OH, due to the (+I) inductive effect of the CH3 group, the electron density is high on oxygen atom due to this the interaction is stronger and the corresponding red shift is maximum. In phenol, the availability of lone pair of electrons for bonding is less as it participates in resonance with the aromatic ring due to this the interaction and the red shift is the least. Furthermore, the magnitude of computed shift with the experiment in the N-H stretching mode shows a larger deviation for the C4H5NC6H5OH complex when compared to the other two complexes. In C4H5N-C6H5OH complex (complex-C), computations showed both N-H···O and C-H···π synergistic interactions, whereas in C4H5N-CH3OH and C4H5N-H2O complexes, there is only N-H···O interaction. It should be recalled from the table 3 that B3LYP and B3LYP-GD3 levels of theory with aug-cc-pVDZ basis set of C4H5N-C6H5OH complex showed a reasonable match of computed shift with experimental shift, since there is no synergistic interaction. It can be concluded that due to the secondary C-H···π interaction in C4H5N-C6H5OH complex, the N-H···O hydrogen bond distance gets weaker and the corresponding red-shift is smaller than the other two complexes.


17


5. Conclusion The complexes of C4H5N with C6H5OH were investigated using matrix isolation infrared spectroscopy. Quantum chemical calculations using DFT, DFT-GD3, M06 and MP2 levels of theory with aug-cc-pVDZ basis sets predicted three complexes A (O-H···π), B (N-H···O) and C (N-H···π).The uniqueness of this study is all the three complexes were trapped and discerned in N2 matrix. The complexes were identified from the shifts in the O-H stretching, N-H stretching and bending modes of C6H5OH and C4H5N, respectively. Computations predicted complex A to be the global minimum structure at all levels of theory having synergistic O-H···π and C-H···π interactions at the aforementioned level of theory. The energetics and geometries of complexes A, B and C varied with the levels of theory used to optimize these structures. At B3LYP/aug-cc-pVDZ level of theory the stabilization energies varied in the order complex A < complex C < complex B, which concur with the Etter’s and Legon-Millen first and third rule, respectively. As dispersion correction is applied through empirical or perturbation methods to optimize the geometry of the energy ordering is altered; complex A < complex B < complex C, showing a deviation from the above rules. At all levels of theory complex A is the global minimum having both O-H···π and C-H···π synergistic interactions but the structure violates LegonMillen second rule. Nevertheless, hydrogen bonding is highly directional; the present study on C4H5N-C6H5OH system is a definitive example where a strong deviation is noticed against generic Legon-Millen rules. The presence of synergistic interactions should thus be responsible for such a divergence. From the experimental shift in the N-H stretching mode of C4H5N submolecule, it can be concluded that the complexes A and B have synergistic interactions whereas in complex C, contrary to computational predictions, a structure devoid of secondary interaction is observed in N2 matrix. This study clearly points that synergistic interactions play an important role in altering the geometries of hydrogen bonded complexes with ACS Paragon Plus Environment

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enhanced stability. The low temperature matrix also has a control over the structure(s) with or without synergism as our studied system is one such classic example of which two structures trapped do have synergetic interactions and the third one not having this interaction. Supporting Information Structures of C4H5N-C6H5OH complexes and comparison of selected structural parameters of 1:1 C4H5N-C6H5OH complexes are given.

Acknowledgement Shubhra Sarkar acknowledges the grant of a research fellowship from IGCAR, DAE,

India.


19

C4H5N:C6H5OH:N2

3500.9

3506.5 3508.9

3503.4

3619.8

3508.4

     1.0:0.0:1000  

Absorbance

d

3622.0

c b

3626.8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 39

3513.8


a

3630

3620

3520

3512

3504

3496

-1

Wavenumber(cm ) Figure 1 Infrared spectra of the O-H and N-H stretching regions of C6H5OH and C4H5N in N2 matrix spanning the region 3630-3496 cm-1.


20

Page 21 of 39

C4H5N:C6H5OH:N2

b

3425.5 3465.8

3470.6

c

3417.1

3423.4

3500.8 3493.8

3503.4 3508.9

3513.8

3522

3415.2 3409.1 3405.0 3402.2

d

3417.3

3497.0

3441.4 3436.4 3433.9

3412.5 3406.7

       0.0:0.5:1000

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a

3460 -1 Wavenumber (cm )

3398

Figure 2 Infrared spectra of the N-H stretching region of C4H5N in N2 matrix covering the region 3522-3398 cm-1.


21


C4H5N:C6H5OH:N2

548.7

       0.0:0.5:1000

546.2

551.6

573.5

587.9

526.0

d

Absorbance

527.2 527.9

530.6

545.7

550.6

573.5 572.3

585.8

543.6

520.2

c

590.1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 39

592

554

b a

516 -1

Wavenumber (cm ) Figure 3 Infrared spectra of the N-H bending region of C4H5N in N2 matrix in the region of 592-516 cm-1.


22

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(a) 2.454Å

2.339Å 2.671Å

2.527Å

2.702Å

(b)

(c) 2.166Å

2.804Å

Figure 4 Computed structures of C4H5N-C6H5OH complexes optimized at MP2/aug-cc-pVDZ level of Paragon Plus theory (a) Complex A, (b) Complex B and ACS (c) Complex C Environment . 23


-1

-2

-1

Stabilization Energy (kcalmol )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 39

B3LYP/aug-cc-pVDZ B3LYP-GD3/aug-cc-pVDZ M06/aug-cc-pVDZ MP2/aug-cc-pVDZ

-3

-4

-5

-6

-7

-8 A

B Complexes

C

Figure 5 Plot of BSSE corrected stabilization energies versus complexes A, B and C at different levels of theory with aug-cc-PVDZ basis sets.


24

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Table 1 Raw/ZPE/BSSE corrected stabilization energies (kcal.mol-1) for the C4H5NC6H5OH complexes A, B and C computed at different level of theories with aug-ccpVDZ basis set. Level of theory B3LYP/aug-cc-pVDZ B3LYP-GD3/aug-cc-pVDZ M06/aug-cc-pVDZ MP2/aug-cc-pVDZ

A -4.04/-3.31/-3.40 -7.82/-6.90/-7.07 -6.58/-5.78/-5.78 -11.04/-9.99/-7.27

B -2.30/-1.93/-1.63 -6.48/-5.77/-5.62 -5.85/-4.72/-5.28 -11.34/-10.61/-6.71


25

C -3.75/-3.08/-3.31 -6.05/-5.26/-5.50 -4.85/-4.10/-4.36 -9.62/-8.63/-5.55


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Table 2 Comparison of computed with the experimental shift in N2 matrix, mode assignments for the C4H5N-C6H5OH complexes A, B and C. Computations were performed at MP2/aug-cc-pVDZ level of theory. Computeda v (cm-1) ν

∆νb

Experimental v (cm-1) Mode Assignment

N2 ν Pyrrole

∆νb ∆v

514.9(58)

-

520.2/527.2c

-

υ16 N-H bending

3672.4(76)

-

3519.3/3513.8/3510.9/3508.9c

-

υ1N-H stretching

-

O-H bending υ1O-H stretching

25.3c -16.2 168.0 -117.0c

υ16 N-H bending υ1N-H stretching O-H bending υ1O-H stretching

50.0 -75.9c -


64.4 -103.6c -2.2


Phenol 329.5(95) 3807.5(64)

-

542.9(68) 3658.5(72) 492.5(98) 3676.3(343)

28.0 -13.9 162.9 -131.2

554.9(104) 3620.9(85) 299.0(131) 3789.7(55)

40.0 -51.5 -30.6 -17.8

579.7(15) 3615.7(228) 365.2(106) 3791.3(60)

64.8 -56.7 35.6 -16.2

358.0 3622.0/3626.8c Complex A 546.2/548.7/551.6 3497.0 526.0 3508.4/3506.5 Complex B 573.5 3441.4/3436.4/3433.9 -d -d Complex C 587.9 3412.5/3406.7 -d 3619.8

aIntensity

in km.mol-1 is given in parenthesis. = monomer-complex. cAverage multiplet experimental wavenumbers of the monomers and C H N4 5 C6H5OHcomplexes A, B and C were taken to compute the experimental shift. dExperimental features were not observed. b


26

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Table 3 Comparison of the experimental with the computed shifts(cm-1) in the O-H stretchinga, N-H bendingb and N-H stretching modec of C4H5N-C6H5OH complexes, A, B and C at different levels of theory with aug-cc-pVDZ basis set. Complexes A B C

Experimental -117.0a 25.3b -16.2c -2.2a 50.0b -75.9c -2.2a 64.4b -103.6c

B3LYP -136.9 6.0 -6.3 -3.2 43.8 -33.0 -1.7 91.7 -85.8

B3LYP-GD3 -153.9 23.7 -7.6 -4.2 54.0 -20.3 3.2 91.0 -82.7


27

M06 -139.3 12.9 -10.9 -18.0 25.0 -20.0 -4.2 61.4 -55.4

MP2 -131.2 28.0 -13.9 -17.8 40.0 -51.5 -16.2 64.8 -56.7


Page 28 of 39

Table 4 Electron occupancies (a.u.) of various NBOs of C4H5N-C6H5OH complexes computed at MP2/aug-cc-pVDZ level of theory. The donor-acceptor delocalization interaction and second order delocalization energies (E2, kcal.mol-1) are also shown. Complexes

A

B

C

NBO

Occupancy

σ*(O12-H13) π(C17-C18) σ*(C9-H10) π(C14-C15) π(C1-C3) σ*(N16-H23) π(C2-C6) π(C7-C9) π*(C14-C15) π*(C17-C18) π(C14-C15) π(C17-C18) π*(C2-C6) π*(C7-C9)

0.01711(0.00554) 1.85196(1.86030) 0.01388(0.01034) 1.85420(1.86030) 1.70123(1.69101) 0.01797(0.01370) 1.67498(1.68104) 1.63539(1.66061) 0.31197(0.31218) 0.31526(0.31218) 1.86030(1.85953) 1.86030(1.85828) 0.34766(0.33769) 0.34996(0.36275)

n1(O12) n2(O12) σ*(N16-H23) π(C2-C6) π(C1-C3) σ*(C15-H20) π(C7-C9) π*(C14-C15)

1.98277(1.98413) 1.92157(1.92043) 0.02005(0.01370) 1.67351(1.68104) 1.68011(1.69101) 0.01145(0.00996) 1.66490(1.66065) 0.31247(0.31218)

Donor-acceptor delocalization interaction

Second order perturbation (E2) energy

π(C17-C18) →σ*(O12-H13)

6.33

π(C14-C15) →σ*(C9-H10)

2.82

π(C1-C3) →σ*(N16-H23) π(C2-C6) →σ*(N16-H23)

1.22 0.16

π(C2-C6) →π*(C14-C15) π(C7-C9) →π*(C17-C18) π(C14-C15) →π*(C2-C6) π(C14-C15) →π*(C7-C9) π(C17-C18) →π*(C7-C9) π(C17-C18) →π*(C2-C6)

0.59 0.95 0.49 0.38 0.08 0.06

n1(O12) →σ*(N16-H23) n2(O12) →σ*(N16-H23)

1.01 3.91

π(C2-C6) →σ*(C15-H20) π(C1-C3) →σ*(C15-H20) π(C7-C9) →σ*(C15-H20) π(C1-C3) → π*(C14-C15) π(C7-C9) → π*(C14-C15)


28

1.00 0.44 0.17 0.23 0.20

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Table 5 Energy Decomposition Analysis of the C4H5N-C6H5OH complexes computed at MP2/aug-cc-pVDZ level of theory. All the energies are in kcal.mol-1. Complex Electrostatic Exchange Repulsion Polarization Dispersion A -10.41 -21.23 35.73 -4.52 -10.97 B -9.25 -26.19 42.98 -4.14 -15.33 C -6.55 -14.71 24.62 -3.17 -9.97


29

Total -11.40 -11.94 -9.78


Page 30 of 39

Table 6 Comparison of various computed parameters with experimental shift of the N-H···O and O-H···π complexes of C4H5N with H2O, CH3OH and C6H5OH. N-H···O Complex

∆𝜐N-Hstr (cal/expt.)

∆𝜐N-Hbend (cal/expt.)

C4H5N-CH3OH C4H5N-H2O C4H5N-C6H5OH

-125.6/-163.6 -106.7/-128.2 -56.7/-103.6

87.8/73.5 89.4/73.6 64.8/64.4

O-H···π Complex

∆𝜐O-Hstr (cal/expt.)

∆𝜐O-Hbend (cal/expt.)

C4H5N-C6H5OH C4H5N-CH3OH C4H5N-H2O

-131.1/-117.0 -100.6/-a -68.6/-a

162.9/168.0 178.0/-a -4.2/-a

aExperimental

N-H∙∙∙O Stabilization bond Energy distance (kcalmol-1) (Å) 1.916 -6.00 1.959 -5.04 2.166 -5.55 O-H∙∙∙π Stabilization bond Energy distance (kcalmol-1) (Å) 2.396 -7.27 2.363 -4.87 2.405 -4.15

features were not observed.


30

E2 (kcalmol-1) 13.75 13.10 4.91

E2 (kcalmol-1) 6.33 4.73 3.71

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TOC Graphic

Legon-Millen Rules? Theory

A

B

C

Experiment


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