Induction, Resonance and Secondary Electrostatic Interaction on

Article Cite This: J. Org. Chem. 2018, 83, 13446−13453

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Induction, Resonance, and Secondary Electrostatic Interaction on Hydrogen Bonding in the Association of Amides and Imides Xuhui Lin,† Xiaoyu Jiang,‡ Wei Wu,† and Yirong Mo*,§ †

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The State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, China ‡ College of Ecological Environment and Urban Construction, Fujian University of Technology, Fuzhou 350108, China § Department of Chemistry, Western Michigan University, Kalamazoo, Michigan 49008, United States S Supporting Information *

ABSTRACT: Both computations and experiments have confirmed that amides have stronger self-associations than imides. While this intriguing phenomenon is usually explained in the term of secondary electrostatic repulsion from the additional spectator carbonyl groups in imides, recently it was proposed that the π resonance effect from the spectator carbonyl which alters the balance between the acidity of the hydrogen-bond (Hbond) donor and the basicity of the H-bond acceptor is the major cause. In this work, we examined the roles of π resonance and the secondary electrostatic interaction in the formation of amide and imide dimers by deactivating the π conjugation from the spectator carbonyl and flipping the spectator carbonyl using the block-localized wave function method which is the simplest variant of valence bond theory. Energetic, geometrical, and spectral results show that three major forces, namely the σ induction effect (IE), π resonance effect (RE), and secondary electrostatic interaction (SEI), contribute to the different binding energies in the dimers of amides and imides. Whereas IE favors stronger binding among imides, both RE and SEI diminish the self-association of imides. Obviously, the negative force from RE and SEI exceeds the positive force from IE. Relatively, SEI plays a little bigger role than RE. electron system was small.28,29 In the study of the different selfassembled structures of guanine and xanthine, they also claimed that there is only a small stabilization by π conjugation, and the main force is the charge separation that goes with donor−acceptor orbital interactions in the σ-electron system.30 Alternatively, Beck and Mo studied the synergistic interplay between π delocalization and H-bonding interactions and found that the enhanced interactions largely come from the electrostatic attraction due to the π resonance.14 Recently, we studied a series of RAHB systems using the block-localized wave function (BLW) method which can quench the π conjugation and derive optimal electron-localized structures.31−33 Our results show that π resonance significantly shortens H-bonds, red-shifts DH stretching vibrational frequencies, and stabilizes molecules.34−36 In other words, π resonance does play a critical role in the enhancement of Hbonds. In RAHB systems, π resonance goes from the H-bond donor to the acceptor, leading to extra positive charge on the donor side and extra negative charge on the acceptor side and ultimately stronger H-bond. If π resonance goes reversely from the H-bond acceptor to the donor, the concerned RAHB may be weakend.37,38 Indeed, most recently, we demonstrated that by switching the direction of π resonance, an intramolecular Hbond could be weakened.39

1. INTRODUCTION Individually weak but collectively significant noncovalent interactions have been actively and broadly studied and explored along both theoretical and experimental directions. Among many types of noncovalent interactions, H-bonding (D−H···A) is obviously the most important and ubiquitous one in chemistry and biology. Many new forms of strong hydrogen bonds (H-bonds) whose strengths can reach up to 40 kcal/mol have been identified and recognized in the past two decades.1−6 Of these strong H-bonds, the resonanceassisted hydrogen bond (RAHB) proposed by Gilli and coworkers1,4−8 has received the most attention due to its popularity in chemical and biological systems. The RAHB highlights the interplay between the π-electron delocalization and H-bonds, and there have been enormous theoretical studies with attempts to not only understand the nature of RAHB but also utilize RAHB to modulate molecular interactions in engineered proteins, DNA, and self-assembling materials.9−22 Yet, there are controversies over the exact role of π resonance in RAHB. Góra et al. argued that π resonance makes little contribution to the stability of RAHB,23 while Alkorta et al. and Sanz et al. claimed that the stronger intramolecular RAHB in unsaturated compounds than in their saturated analogues results from the constraints imposed by the σ-skeleton framework rather than the π conjugation.11,24−27 Fonseca Cuerra et al. showed that in Watson− Crick DNA base pairs, the synergetic interplay between the π resonance and the donor−acceptor interactions in the σ© 2018 American Chemical Society

Received: August 30, 2018 Published: October 1, 2018 13446

DOI: 10.1021/acs.joc.8b02247 J. Org. Chem. 2018, 83, 13446−13453

Article

The Journal of Organic Chemistry

Figure 1. Secondary electrostatic interaction between OS (in red) and OHB (blue arrowed lines) enhanced by the π resonance in imides (shown in green).

2. COMPUTATIONAL METHODS

Amides and imides are both conjugated and planar functional groups and participate H-bonding with each other, which is one of the governing forces for protein folding40−42 and the structures of nucleic acids.43 The dimerization of amides and imides has been studied both experimentally44,45 and computationally46 as a better understanding can facilitate the rational design of self-recognition and self-assembling materials.47,48 Interestingly, Jeong et al.’s NMR measurements showed that amides (lactams) have stronger self-associations than imides, although the formers have lower acidity than the latter.45 Figure 1 shows the order of interaction energies, though all three dimers have similar primary H-bond interactions. This surprising finding was interpreted in the term of secondary electrostatic interaction (SEI, blue arrows in Figure 1) between the oxygen of the spectator carbonyl group (in red, denoted as OS) and the oxygen of the H-bond carbonyl (denoted as OHB) by Jorgensen and co-workers.49,50 Obviously, both oxygen atoms carry negative charges, and their electrostatic interaction is thus repulsive. There is one such destabilizing interaction in the heterodimer but two in the imide dimer. Based on an integrated experimental and computational study of a set of amides and imides, most recently Narváez et al. questioned the role of the SEI and proposed that the balance between the acidity of the H-bond donor (N−H group) and the basicity of the H-bond acceptor (CO group), which arises from the resonance effect of the spectator carbonyl, should be the primary cause for this phenomenon.51 In other words, it is the involved H-bonds that determine the binding order from amide dimer to imide dimer in Figure 1, and the repulsive SEI between OS and OHB is compensated by other intermolecular attractions.52 Due to the great potential of amides and imides in supramolecular chemistry and molecular recognition, it is essential to use alternative computational tools to examine the proposal by Narváez et al.51 and elucidate the nature of the stronger H-bonding in amide dimers than imide dimers. Although this unexpected phenomenon was initially observed in aqueous solutions, it also occurs in gas phase as computations showed. In this work, we used the BLW method31−33 to probe and compare the impacts of the π conjugation due to the addition of the spectator carbonyl groups on the H-bonding in a set of amides and imides. Apart from the dimers of amide (A1) and imide (I1) shown in Figure 1, we also studied the dimers of butyrolactam (A2) and succinimide (I2), where the spectator carbonyl group is a little far from its the interacting partner due to the geometry restraints.

2.1. Block-Localized Wave Function Method. Most popular computational approaches for bonding analyses either project out localized orbitals from delocalized canonical molecular orbitals (MOs)53 or build a molecular wave function from its fragmental canonical MOs,54 or partition molecular electron densities to atomic or fragmental parts55,56 based on certain criteria. These top-down approaches are essentially post-SCF methods as the concerned local orbitals are not re-optimized. Alternatively, valence bond (VB) theory adopts a bottom-up approach and builds wave functions for Lewis structures with local such as atomic orbitals and the final molecular wave function is expanded with a few Lewis (resonance) structures for a conjugated system.57−60 The resonance energy is computed with the energy difference between the final molecular wave function and the most stable resonance structure.61,62 Each resonance structure is defined with a Heitler−London−Slater−Pauling (HLSP) function as

ΨL = MLA(̂ φ1,2φ3,4 ··· φ2n − 1,2n)

(1)

where ML is the normalization constant, Â is the antisymmetrizer, and φ2i−1,2i is a bond function composed of nonorthogonal orbitals ϕ2i−1 and ϕ2i (or a lone pair if ϕ2i−1 = ϕ2i):

φ2i − 1,2i = Â {ϕ2i − 1ϕ2i[α(i)β(j) − β(i)α(j)]}

(2)

n

As such, eq 1 can be expanded to 2 Slater determinants, and the computational complexity with eq 2 limits the development and general applications of ab initio VB approaches. Enormous efforts have been put to introduce approximations and reduce the computational costs. It seems that the most efficient way is the use of nonorthogonal bond (i.e., ϕ2i−1 = ϕ2i in eq 2) or even fragmental orbitals.63−75 A notable success in this regard is the generalized VB (GVB) method by Goddard76 where only a few focused bonding orbitals (perfect-pairs) are localized and nonorthogonal and all remaining orbitals are delocalized and orthogonal. To combine the advantages of both MO and VB theories, we proposed the BLW method where a BLW is designated for the most stable resonance state in order to analyze the geometric, energetic, and spectral changes due to the electron delocalization in a molecule.31−33 It is assumed that all electrons and primitive basis functions (χμ) can be separated to k subgroups (blocks), and each orbital is block-localized and expanded in one block. If there are mi basis functions and ni electrons for block i, block-localized orbitals for this block can be expressed as ϕji =

mi

∑ Cjiμχμi

(3)

μ=1

and the subsequent BLW for a closed-shell is defined as ΨBLW = det|(φ11)2 (φ21)2 μ(φn1 /2)2 μ(φ1i)2 μ(φni /2)2 μ(φnk /2)2 | 1

i

k

= Â [Φ1Φ2μΦk ] (4) Orbitals in the same subspace are subject to the orthogonality constraint, but orbitals from different subspaces are nonorthogonal. Since eq 4 is self-consistently optimized by minimizing its energy, all 13447


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Figure 2. H-bond distances and the intermolecular interaction energies for various homodimers and heterodimers at the M06-2X/6-311+G(d,p) theoretical level.

Figure 3. EDD isosurface maps with the isovalue of 0.002 au in I1, A1-I1, and I1-I1. The orange/cyan colors indicate increasing/decreasing of the electron density. local orbitals are thus optimal. The BLW method is available at the density functional theory (DFT) level with the geometry optimization and frequency computation capabilities.33,77 2.2. Computational Details. DFT (M06-2X)78 computations with the basis set of 6-311+G(d,p) were performed throughout the work. All computations were performed with the GAMESS software,79 where the BLW code was ported to in our lab. The comparison of the geometrical parameters and vibrational frequencies computed with the standard M06-2X and the BLW-DFT methods reveals the impact of the π resonance on H-bonds in amides and imides. In all computations, geometries are maintained as planar for the sack of clarity. All binding energies are corrected for the basis set superposition errors with the counterpoise method proposed by Boys and Bernardi.80

Yet, optimizations show that the H-bond distances near the spectator carbonyl group are much shorter than amide−amide dimers (1.829 Å in A1-I1 compared to 1.883 Å in A1-A1, 1.800 Å in A2-I2 compared to 1.860 Å in A2-A2), while the other H-bond exhibits stretched bonding distances (1.953 Å in A1-I1 compared to 1.883 Å in A1-A1, 1.939 Å in A2-I2 compared to 1.860 Å for A2-A2). Obviously, there are some inherent factors other than the SEI that play roles in these dimer complexes. The first concern is the exact role of π conjugation. Here our focus is on the conjugation from the spectator carbonyl group to the rest parts of imide. To this end, the BLW method is applied in order to derive the strictly localized state of imide, where two π electrons are strictly localized on the spectator group (actually carbonyl group). The electron density changes due to the addition of the spectator carbonyl group can thus be visualized via electron density difference maps (EDD) which correspond to the differences between the regular DFT (delocalized) and BLW (localized) electron densities. Figure 3 shows the EDD maps for imide in its monomer and dimers of A1-I1 and I1-I1. On one hand, the EDD of I1 indicates that from amide to imide, the addition of the spectator carbonyl group moves the π electron density toward this newly added carbonyl group, though there is certain σ electron density movement in a reverse direction to offset the impact of π

3. RESULTS AND DISCUSSION For the simplest amide (A1) and imide (I1) and the cyclic amide (A2) and imide (I2), the tendency of the interaction energy change for their dimers, as shown in Figure 2, is consistent with that in aqueous phase. Although the SEI concept49,50 can be qualitatively yet successfully used to explain the variation of the interaction energies by introducing the additional OS···OHB repulsion, it seemingly fails to elucidate the H-bond distances in the heterodimers. According to the SEI, we would expect a more stretched H-bond distance for the one in the vicinity of the spectator carbonyl group (in red) and a less stretched H-bond distance for the other one. 13448


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electrostatic energy based on the Mulliken charges on the Hbond donor and acceptor.34 Here we use a similar approach to estimate the SEI energy as qO qO Erep = 100 2 S HB R OS ··· OHB (5)

conjugation. The most remarkable is the depletion of electron density from the −NH group, leading to the enhancement of its H-bonding capability. This is evidenced by the shortening and enhancement of its H-bonding with amide (Figure 2). On the other hand, there is also electron density reduction on the OHB end which eventually also reduces its H-bonding capability as a H-bond acceptor. This is evidenced by the significant stretching of the H-bonding between OHB and the −NH group in the A1-I1 dimer (Figure 2). When two imides form a dimer, the π resonance from the spectator carbonyl groups may either strengthen or weaken the H-bonds in the I1-I1 dimer, taking the two H-bonds in A1-I1 as the extremes. The balance between the increased H-bond donor capability and the reduced H-bond acceptor capability eventually leads to a weaker H-bonding interaction compared with A1-A1. We particularly note that the OS···OHB repulsion in A1-I1 seems stronger than that in I1-I1 due to the reduced electron density (and negative charge) on OHB in I1 compared with A1 as well as the stretched OS···OHB distances. Until now, our conclusion seems consistent with Narváez et al.’s,51 i.e., the π resonance is the driving force for the weaker binding in the imide dimer than in the amide dimer. However, we need recognize that the above discussion is based on the DFT geometries. It is imperative to derive optimal geometries with the π conjugation “shut down”. Besides, although H-bonding is primarily dominated by the electrostatic attraction, there is a partial covalent nature which can be characterized by the electron transfer from the H-bond acceptor (usually a lone pair) to the H-bond donor (usually the σ antibond of D−H).3,18,81−86 For the dimers of amide and imide, when the carbonyl oxygen atom acts as a H-bond acceptor, it uses its electrons of σ symmetry to interact with σΝΗ*. Yet until now our focus has been on the π conjugation. Figure 3 clearly shows that the σ electron density movement is against the π electron density movement. As a consequence, the σ electron density on OS actually reduces, whereas the σ electron density on OHB increases. This complicates the picture we presented in the above. To further clarify the role of π conjugation from the spectator carbonyl oxygen OS, we performed geometry optimizations for the strictly localized states of I1-I1 and A1-I1 by confining the π electrons on the spectator carbonyl group(s) using the BLW method. Further, to evaluate the significance and role of the SEI, we flip the spectator carbonyl group(s) by 180° from its original position and obtain the flipped dimers F-I1-I1 and F-A1-I1, as shown in Figure 4. In the F-I1-I1 and F-A1-I1, the SEI is largely removed, while the BLW computations of I1-I1 and A1-I1 quench the π resonance effect due to the spectator carbonyl group(s). π resonance shifts electron density and consequently changes atomic charges. Previously, we demonstrated that there is a good correlation between the binding energy and the classical

Note that here Erep should be used relatively only and serve as an indicator as there is no scaling factor available to get the true electrostatic energy. Major computational results are compiled in Table 1. Table 1. H-Bond Distances (R, in Å), NH Stretching Vibrational Frequency (v, in cm−1), Repulsion Indictors (Erep) between OS and OHB, and the Interaction Energies (ΔEdim, in kcal/mol) for Various Homodimers and Heterodimers complex

method

R (H···O)a

v (NH)a

Erep

ΔEdim

A1-A1

DFT DFT BLW DFT BLW DFT BLW DFT BLW DFT DFT BLW DFT BLW

1.883 1.915 1.877 1.881 1.845 1.829/1.953 1.855/1.902 1.817/1.938 1.841/1.887 1.860 1.896 1.862 1.800/1.939 1.819/1.900

3414 3433 3398 3419 3364 3284/3492 3329/3438 3303/3463 3338/3399 3390 3440 3394 3254/3494 3290/3443

− 0.81 0.77 − − 0.96 0.80 − − − 0.86 0.85 0.98 0.87

−15.22 −11.74 −13.62 −15.22 −18.03 −13.68 −14.40 −15.87 −16.69 −16.24 −12.78 −14.43 −14.76 −15.36

I1-I1 F-I1-I1 A1-I1 F-A1-I1 A2-A2 I2-I2 A2-I2

For heterodimers, the first data refer to the H-bond near the spectator carbonyl group, while the second data represent the other H-bond. a

For I1-I1, the repulsion indicators (Erep) in both the electron-delocalized (DFT) and electron-localized (BLW) states are very close (0.81 versus 0.77). Thus, the differences between the DFT and BLW data for I1-I1 in Table 1 can be largely ascribed to the resonance effect, while the SEI would be best reflected by the comparison between I1-I1 and F-I1-I1 with either the regular DFT or the BLW methods. Significantly, with the deactivation of the π resonance from the spectator carbonyl groups in I1-I1, the H-bonds are shortened to 1.877 Å, very close to but slightly shorter than the values in A1-A1. These bond distance changes are consistent with the variations of the stretching vibrational frequencies of the NH group. The present findings are quite surprising and unexpected for two reasons. First, the H-bond shortening with the quenching of π resonance indicates that π resonance is a destabilizing force in the self-association of imides.51 Second, the fact that the H-bonds in the localized I1-I1 are shorter than in A1-A1 is against the SEI concept as the repulsive force would stretch the H-bonds. At this point it is necessary to look at the flipped dimer F-I1-I1. It is interesting that the DFT optimization results in the H-bond distances of 1.881 Å, nearly the same as the values in A1-A1. But once the π conjugation from the spectator carbonyl groups is quenched, the H-bonds are notably enhanced and shortened to 1.845 Å, much shorter than the H-bonds in A1-A1. This discrepancy most likely comes from the often neglected σ induction effect of the

Figure 4. Flipped imide homodimer and heterodimer. 13449


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Figure 5. Contributions of the three factors including IE, RE, and SEI to the differences from the amide homodimer A1-A1 to the imide homodimer I1-I1. H-bond distances are in Å, and the energy values in red are in kcal/mol.

Figure 6. Contributions of the three factors including IE, RE, and SEI to the differences from the amide homodimer A1-A1 to the heterodimer A1I1.

role for the formation of imide dimer with reference to the amide dimer. Both the π resonance and secondary electrostatic interaction are responsible for the reduction of the binding energy in the imide dimer from the amide dimer. Among the three factors, the SEI, initially identified by Jorgensen and coworkers,49,50 is the strongest, but the resonance has a quite comparable impact on the reduced binding energy in the imide dimer. For the heterodimer A1-I1, we can perform a similar analysis with its flowchart shown in Figure 6. As the induction effect subsidizes exponentially with the distance, it only impacts the H-bond close to the spectator carbonyl group, with the other H-bond little changed. Figure 6 shows that the first H-bond shortens from 1.883 Å in A1-A1 to 1.841 Å in the localized FA1-I1. These data are very comparable to those in Figure 5. As a consequence, the stabilization due to the induction effect from the localized F-A1-I1 to A1-A1 amounts to 1.5 kcal/mol, about half of the value from the localized F-I1-I1 to A1-A1 (2.8 kcal/mol). Although the π resonance does shorten the first H-bond from 1.841 to1.817 Å together with the redshifting of the NH vibrational frequency, the other H-bond is stretched even more and thus weakened. Overall the resonance reduces the binding energy by 0.8 kcal/mol. The comparison of the geometries of the heterodimer A1-I1 and its flipped

spectator carbonyl groups. As well recognized, the carbonyl group is a strong electron-withdrawing group, and its induction effect is expected to increase the adjacent H-bond donor (NH group) capability but has little impact on the H-bond acceptor (OHB) due to the long distance. This can be proved by the Mulliken charges on N and OHB by comparing the localized I1 (−0.348 and −0.393 using the BLW method) and A1 (−0.389 and −0.400 using the regular DFT method). Thus, there are three factors contributing to the different binding energies between A1-A1 and I1-I1, namely resonance effect (RE), secondary electrostatic interaction (SEI), and induction effect (IE). The comparison between the A1-A1 and the localized FI1-I1 determines the IE, the evolution from the localized F-I1I1 to the delocalized F-I1-I1 reflects the RE, whereas the SEI can be derived by the differences between the delocalized F-I1I1 and the delocalized I1-I1. Alternatively, while the quenching of the π conjugation from the spectator carbonyl groups in I1I1 measures the RE, the differences between the localized I1-I1 and A1-A1 results from the combined stabilizing IE and destabilizing SEI. Figure 5 shows the flowchart to estimate these three factors with the changes of binding energies listed in red. Figure 5 shows that, although there are deviations for the quantitative evaluation of the three factors with different pathways, it is clear that only the σ induction plays a stabilizing 13450


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The Journal of Organic Chemistry structure exhibits the SEI effect, which is again the strongest among the three factors discussed in this work. The above conclusion is applicable to the cyclic homodimers (A2-A2 and I2-I2) and heterodimer (A2-I2) as well, though in these cases the flipped structures are unavailable. The H-bonds in the localized I2-I2 (1.862 Å) are much shorter than those in the delocalized dimer (1.896 Å), but nearly identical to those in A2-A2 (1.860 Å). This confirms the negative role of π resonance in the self-association of I2 compared with A2. The repulsion energies in the localized and delocalized I2-I2 are similar, suggesting that there are little changes of the SEI with the resonance on or off. In fact, the comparable H-bond distances and vibrational frequencies in the localized I2-I2 and A2-A2 imply that the repulsive SEI is offset by the attractive induction effect. Similar to A1-I1, the H-bond in A2-I2 close to the spectator carbonyl group is significantly enhanced, while the other far away from the spectator carbonyl group is significantly weakened, measured by both the distances and vibrational frequencies.

Yirong Mo: 0000-0002-2994-7754 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the China Scholarship Council (for X.L.), the National Natural Science Foundation of China (no. 21733008 for W.W.), and Western Michigan University (for Y.M.).

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(1) Gilli, P.; Bertolasi, V.; Pretto, L.; Ferretti, V.; Gilli, G. Covalent versus Electrostatic Nature of the Strong Hydrogen Bond: Discrimination among Single, Double, and Asymmetric Single-Well Hydrogen Bonds by Variable-Temperature X-Ray Crystallographic Methods in β-Diketone Enol RAHB Systems. J. Am. Chem. Soc. 2004, 126, 3845−3855. (2) Custelcean, R.; Jackson, J. E. Dihydrogen Bonding: Structures, Energetics, and Dynamics. Chem. Rev. 2001, 101, 1963−1980. (3) Grabowski, S. J.; Sokalski, W. A.; Leszczynski, J. How Short Can the H···H Intermolecular Contact Be? New Findings that Reveal the Covalent Nature of Extremely Strong Interactions. J. Phys. Chem. A 2005, 109, 4331−4341. (4) Bertolasi, V.; Gilli, P.; Ferretti, V.; Gilli, G. Evidence for Resonance-Assisted Hydrogen Bonding. 2. Intercorrelation between Crystal Structure and Spectroscopic Parameters in Eight Intramolecularly Hydrogen Bonded 1,3-Diaryl-1,3-Propanedione Enols. J. Am. Chem. Soc. 1991, 113, 4917−4925. (5) Gilli, G.; Bellucci, F.; Ferretti, V.; Bertolasi, V. Evidence for Resonance-Assisted Hydrogen Bonding from Crystal-Structure Correlations on the Enol form of the β-Diketone Fragment. J. Am. Chem. Soc. 1989, 111, 1023−1028. (6) Gilli, P.; Bertolasi, V.; Ferretti, V.; Gilli, G. Evidence for Resonance-Assisted Hhydrogen Bonding. 4. Covalent Nature of the Strong Homonuclear Hhydrogen Bond. Study of the O-H···O System by Crystal Structure Correlation Methods. J. Am. Chem. Soc. 1994, 116, 909−915. (7) Gilli, P.; Bertolasi, V.; Ferretti, V.; Gilli, G. Evidence for Intramolecular N-H···O Resonance-Assisted Hydrogen Bonding in βEnaminones and Related Heterodienes. A Combined CrystalStructural, IR and NMR Spectroscopic, and Quantum-Mechanical Investigation. J. Am. Chem. Soc. 2000, 122, 10405−10417. (8) Gilli, P.; Bertolasi, V.; Pretto, L.; Lyčka, A.; Gilli, G. The Nature of Solid-State N-H···O/O-H···N Tautomeric Competition in Resonant Systems. Intramolecular Proton Transfer in Low-Barrier Hydrogen Bonds Formed by the ···O = C-C = N-NH···↔···HO-C = C-N = N··· Ketohydrazone-Azoenol System. A Variable-Temperature X-Ray Crystallographic and DFT Computational Study. J. Am. Chem. Soc. 2002, 124, 13554−13567. (9) Sobczyk, L.; Grabowski, S. J.; Krygowski, T. M. Interrelation between H-Bond and π-Electron Delocalization. Chem. Rev. 2005, 105, 3513−3560. (10) Lenain, P.; Mandado, M.; Mosquera, R. A.; Bultinck, P. Interplay between Hydrogen-Bond Formation and Multicenter πElectron Delocalization: Intramolecular Hydrogen Bonds. J. Phys. Chem. A 2008, 112, 10689−10696. (11) Alkorta, I.; Elguero, J.; Mó, O.; Yañez, M.; Del Bene, J. E. Are Resonance-Assisted Hydrogen Bonds ‘Resonance Assisted’? A Theoretical NMR Study. Chem. Phys. Lett. 2005, 411, 411−415. (12) Fuster, F.; Grabowski, S. J. Intramolecular Hydrogen Bonds: the QTAIM and ELF Characteristics. J. Phys. Chem. A 2011, 115, 10078−10086. (13) Trujillo, C.; Sánchez-Sanz, G.; Alkorta, I.; Elguero, J.; Mó, O.; Yáñez, M. Resonance Assisted Hydrogen Bonds in Open-Chain and Cyclic Structures of Malonaldehyde Enol: A Theoretical Study. J. Mol. Struct. 2013, 1048, 138−151.

4. CONCLUSIONS The experimental finding45 that amides (lactams) have stronger self-associations than imides has triggered many computational and further experimental studies. The overwhelming theory for this abnormal phenomenon is the secondary electrostatic interaction (SEI) proposed by Jorgensen and co-workers.49,50 Most recently, Narváez et al. conducted an integrated experimental and computational study of a set of amides and imides and identified the resonance effect as the key.51 In this work, we employed the BLW method which can uniquely deactivate the π resonance from the spectator carbonyl groups and derive the optimal geometries for the localized structures. In contrary with the works by Jorgensen et al. and Narváez et al., our analyses based on the energetic and geometrical changes indicate that the stronger self-association of imides compared with amides is driven by multiple factors of comparable magnitudes rather than by one dominant factor. More specifically, we identified three major factors behind the differences between amides and imides, namely the σ induction effect (IE), resonance effect (RE), and the SEI. While the IE would increase the H-bond donor (−NH group) capability but has little impact on the Hbond acceptor (−COHB group) capability for imides, the RE increases the H-bond donor capability but reduces the H-bond acceptor capability. Overall, the IE favors but the RE disfavors the self-associations of imides compared with amides. The SEI, as initially suggested,49,50 is also a negative force for the binding among imides and plays the biggest role in the three factors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.8b02247. Optimal geometries and energies at the M06-2X/6311+G(d,p) level (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wei Wu: 0000-0002-6139-5443 13451


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