Molecular Electrostatic Potential Topography Research Articles

In silico exploration of O-H…X2+ (X = Cu, Ag, Hg) interaction, targeted adsorption zone, charge density iso-surface, O-H proton analysis and topographic parameters theory for calix[6]arene and calix[8]arene as model

In this work, the best adsorption targeted zones of the metal cations Ag2+, Cu2+ and Hg2+ at the surfaces or inside, outside the cavity of the calix[6] (CX[6]), calix[8]arene (CX[8]), NUBMOM (NUB-.) and LAYKUR (LAY-.) have been discussed. For X2+=Ag, Cu and Hg adsorbed onto the surface of CX[6,8], NUB-., and LAY-. and the morphologies of these complexes have been explained, the specific chosen surface structure has interfacial chemical properties to facilitate the stabilization of the cation in each targeted zone. The stability mechanisms have been investigated for the specific systems to understand the role of the cooperativity of the O…H (forming a donor-acceptor couple) bonding interactions for good selectivity to the cation in each host-guest in the acetonitrile solvent medium and the gas phase. For this purpose, all the host-guests chemical structures were investigated by the IR spectroscopy and O-H proton approach. The UV–visible absorption spectroscopy and the total DOS Orbital showed that all molecules possess a maximum absorption band in the range between 0.5 and 3.5 eV assigned to π–π* or n-π* transitions, the minimum band is characterized for the CX[6,8]-Cu2+, NUB-.Cu2+ and LAY-.Cu2+, while the highest band is specified for the complexation with the cation Ag2+. The Hirshfeld surface and the molecular electrostatic potential topography have demonstrated the selected targeted zone for the most stable configurations. The nature and the type of interaction formed between the chosen cation and the targeted area of the CX[6,8], NUB-. and LAY-. were typically studied by the Atom in Molecules (AIM) approach via non-covalent interaction (NCI) analyses. According to theoretical calculations, Cu2+, Ag2+ and Hg2+ cations were complexed with CX[6] and CX[8]-arenes in endo and exo-type form. In the endo complexes, it has been observed that Cu2+and Ag2+ cations enter the lower rim space where calixarene hydroxyl groups are located and form a complex in square planer geometry, as expected. This situation shows that the copper cation is planarly located in the calixarene core. These results show that the theoretical results are in good agreement with the experimental ones. In addition, our simulations point out the calix[6] arene and calix[8]arene were complexed with cations by “pinched” conformation, corresponding to best stable state.

Electrostatic Topographical Viewpoint of π-Conjugation and Aromaticity of Hydrocarbons.

A molecular electrostatic potential (MESP) topographical study has been conducted for a variety of conjugated hydrocarbons at B3LYP/6-311+G(d,p) level of theory to understand their π-conjugation features and aromaticity. The value of MESP minimum (Vm) is related to the localized and delocalized distribution of π-electron density. The Vm values are located interior to the rings in polycyclic benzenoid hydrocarbons (PBHs), whereas they lie outside the boundary of the rings in antiaromatic and in fused systems consisting of aromatic and antiaromatic moieties. The Vm points lie on top and bottom of the π-regions in linear polyenes and annulenes, while a degenerate distribution of CPs around the midpoint region of triple bonds is observed in alkynes. The eigenvalues λ1, λ2, and λ3 of the Hessian matrix at Vm(MESP minima) are used to define the aromatic character of the cyclic structures. The eigenvalues follow the trend λ1 ≫ λ2 > λ3 ≅ 0 in PBHs, λ1 > λ2 > λ3 ≅ 0 in linear polyenes, and λ1 > λ2 > λ3 ≠ 0 in antiaromatic systems. The difference in the aromatic character of PBHs from that of benzene is related to the deviations Δλ1, Δλ2, and Δλ3. The total deviation ∑i=13Δλi is found to be ≤ 0.011 au for all aromatic systems and lies between 0.011 and 0.035 au for all nonaromatic systems. For antiaromatic systems, its value is found to be above 0.035 au. Further, ∑i=13Δλi gives a direct interpretation of Clar's aromatic sextet structures for PBHs. In summary, MESP topography mapping is a powerful technique to quantify the localized and delocalized π-electron distribution in a variety of unsaturated hydrocarbon systems.

Electronic Rearrangement in Molecular Plasmons: An Electron Density and Electrostatic Potential-Based Study.

Plasmonic modes in single-molecule systems have been previously identified by scaling two-electron interactions in calculating excitation energies. Analysis of transition dipole moments for states of polyacenes based on configuration interaction is another method for characterising molecular plasmons. The principal features in the electronic absorption spectra of polyacenes are a low-intensity, lower-in-energy peak and a high-intensity, higher-in-energy peak. From calculations using time-dependent density functional theory with the B3LYP/cc-pVTZ basis set, both these peaks are found to result from the same set of electronic transitions, that is, HOMO-n to LUMO and HOMO to LUMO+n, where n varies as the number of fused rings increases. In this work, the excited states of polyacenes, naphthalene through pentacene, are analysed using electron densities and molecular electrostatic potential (MESP) topography. Compared to other excited states the bright and dark plasmonic states involve the least electron rearrangement. Quantitatively, the MESP topography indicates that the variance in MESP values and the displacement in MESP minima positions, calculated with respect to the ground state, are lowest for plasmonic states. The excited-state electronic density profiles and electrostatic potential topographies suggest the least electron rearrangement for the plasmonic states. Conversely, high electron rearrangement characterises a single-particle excitation. The molecular plasmon can be called an excited state most similar to the ground state in terms of one-electron properties. This is found to be true for silver (Ag6 ) and sodium (Na8 ) linear chains as well.

Host-Guest Interactions Accompanying the Encapsulation of 1,4-Diazabicyclo[2.2.2]octane within endo-Functionalized Macrocycles.

The binding of novel endofunctionalized bis-urea/thiourea molecular receptors toward neutral 1,4-diazabicyclo[2.2.2]octane (DABCO) demonstrates stronger binding of the bis-thiourea macrocycles than for their urea analogues by employing M06-2X/6-31+G(d,p)-based density functional theory. The formation of such inclusion complexes is spontaneous, thermodynamically favorable, and facilitated via bifurcated N-H···N···H-N hydrogen bonding and C-H···π, dipole-dipole, and other noncovalent interactions, which are reflected in the frequency shift of their characteristic N-H vibrations in the calculated vibrational spectra of these complexes. The underlying noncovalent interactions are analyzed using the molecular electrostatic potential topography and quantum theory of atoms in molecules in conjunction with the noncovalent interactions reduced density gradient method. It has also been shown that the encapsulation of DABCO within the π-electron-rich cavity of such hosts brings about shielding of the guest protons confined within the host cavity whereas those facilitating hydrogen bonding engender the downfield signals in their calculated 1H NMR spectra.

Understanding Binding of Cyano-Adamantyl Derivatives to Pillar[6]arene Macrocycle from Density Functional Theory.

The ωB97X-based density functional theory has been employed to characterize molecular interactions between adamantane carbonitrile (ACN) or adamantane methyl carbonitrile (AMCN) and the ethylated pillar[6]arene (EtP[6]) molecular receptor. The inclusion complexes in 1:1 stoichiometry are stabilized through noncovalent interactions such as hydrogen bonding, C-H···π and dipole-dipole interactions. Gibbs free energies accompanying the encapsulation of ACN or AMCN within EtP[6] revealed that the formation of complex is spontaneous and thermodynamically favorable. Underlying interactions are unraveled through quantum theory of atoms in molecules and molecular electrostatic potential topography. Structural changes consequent to guest encapsulation have been rationalized through characteristic infrared and NMR spectra. The frequency downshifts for -C≡N stretching of the guest accompanying the complexation has been attributed to hydrogen bonding and C-H···π interactions. The methylene vibrations of ACN reveal the frequency shifts in opposite directions consequent to distinct binding features with EtP[6] host. The selective binding of AMCN further brings about a significant distortion of the host cavity. Calculated 1H NMR spectra of ACN and AMCN complexes show shielded signals for the adamantyl protons in consonance with experiment.

Bonding and Reactivity Patterns from Electrostatic Landscapes of Molecules

The topographical analysis of molecular electron density (MED) and molecular electrostatic potential (MESP) offers insights into the bonding and reactivity patterns through the critical points (CPs) of these scalar fields. The MESP is found to be particularly useful for describing sites of electrophilic attack and weak intermolecular interactions. MESP is also shown to clearly distinguish between the lone pairs and π-delocalization. The concept of atoms in molecules (AIM) which has so far been primarily based on the gradients of MED, has recently been extended via the use of MESP. The portrayal of AIM through MESP clearly reveals the electron rich atoms in the molecule and also provides the details of the preferred direction of approach of an electrophile. This perspective briefly summarizes the prominent features of MESP topography and provides a future outlook.

Probing Molecular Interactions in Functionalized Asymmetric Quaternary Ammonium-Based Dicationic Ionic Liquids.

Electronic structure, binding energies, and spectral characteristics of functionalized asymmetric dicationic ionic liquids (DILs) composed of quaternary ammonium cations substituted with the ethoxyethyl and allyl/3-phenylpropyl/methoxyethoxyethyl/pentyl functionalities on two different nitrogen centers of the dication and the bis(trifluoromethanesulfonyl)imide (Tf2N-) anion were derived employing the dispersion-corrected density functional theory. DILs based on methoxyethoxyethyl-substituted cation reveal stronger binding toward the Tf2N- anion. The measured glass transition temperatures are found to be strongly dependent on the cation-anion binding facilitated through noncovalent interactions with dominant contributions from the electrostatics and hydrogen bonding. The manifestations of these interactions to vibrational spectra, in particular, to SO2 and CF3 stretchings in the complexes are presented. It has been demonstrated that the frequency down (red)-shift of the SO2 stretching in these DILs with varying substituent follows the order: methoxyethoxyethyl (35 cm-1) > allyl (23 cm-1) > pentyl (20 cm-1) > 3-phenylpropyl (5 cm-1), which is consistent with the strength of cation-anion binding. The CF3 stretching of the anion exhibits the frequency shift in the opposite direction with its hierarchy being reversed to that of SO2 stretchings; the largest upshift (blue shift) of 60 cm-1 was predicted for the DILs composed of 3-phenlpropyl substituted dications. The direction of such frequency shift has been rationalized through the difference molecular electron density maps in conjunction with the electron density at the bond critical point in the quantum theory of atoms in molecules. The underlying cation-anion binding has been analyzed through charge distribution analysis characterized in terms of molecular electrostatic potential topography. Furthermore, the observed decomposition temperatures of DILs are shown to correlate well with the maximum surface electrostatic potential parameters in quantum theory of atoms in molecules.

Cooperative Hydrogen Bonding, Molecular Electrostatic Potentials, and Spectral Characteristics of Partial Thia-Substituted Calix[4]arene Macrocycles.

Structure and spectral characteristics of the 2,14-dithiacalix[4]arene and its homooxa derivatives are obtained employing the dispersion-corrected ωB97X-D-based density functional theory. The conformational behavior of these receptors is governed by the nature and number of substituents at the bridging positions. The partial thia-substituted calix[4]arene scaffolds reveal the electron-rich regions reside near heteroatoms which emerge with deeper minima in molecular electrostatic potential topography. Underlying cooperativity of intramolecular hydrogen bonding manifests in characteristic OH vibrations of their infrared spectra. Moreover, the (1)H NMR reveals that hydrogen-bonded protons are deshielded, unlike those from tertiary butyl substituents. These inferences are in agreement with the experimental data.

Characterization of structure and 1H NMR of methyl viologen encapsulated noria and substituted noria hosts from density functional theory

Density functional theory has been employed to characterize electronic structure, molecular electrostatic potential and 1H NMR of waterwheel-like macrocyclic host noria and its methyl derivative. Molecular electrostatic potential topography analyses reveal that substitution of methyl groups at reactive hydroxyls of noria engender deeper minima near the portal and inside the cavity of host. The complexes between noria or Me-noria hosts and methyl viologen (MV2+) exhibit distinct binding patterns. The structure of MV2+@noria shows penetration of MV2+ inside the central hole of the host whereas Me-noria complex possesses one of the aromatic rings of MV2+ resides near the host portal. Calculated interaction energy for complexation of Me-noria turns out to be 239kJmol−1, which is ∼20 times larger than that for noria host. Calculated 1H NMR chemical shifts predict deshielding of guest protons encapsulated within the cavity of host in the MV2+@noria complex compared to the isolated guest. The protons near the host portal led to up-field signals in the spectra of MV2+@Me-noria. Host-guest binding pattern and 1H NMR for complexation of noria and its methyl derivative are compared with those observed in well-known macrocycles such as β-cyclodextrin, cucurbit[8]uril, calix[5]arene and pillar[5]arene hosts.

Electronic structure, molecular electrostatic potential and spectral characteristics of pillar[6]arene hosts and their complexes with n-octyltriethylammonium ions

Electronic structure, charge distribution and (1)H NMR in pillar[6]arene (P6) conformers, their diisobutoxy derivatives and their host-guest complexes have been investigated by employing the density functional theory. It has been shown that a P6 conformer obtained by flipping of alternate hydroquinone units turns out to be of lowest energy, owing to the hydrogen bonded network at both rims of the host. As opposed to this, a conformer void of hydrogen bonding interactions has largely been destabilized. The O-HO interactions are analyzed using molecular electrostatic potential topography as a tool. Modification of a P6 host by substituting a diisobutoxy group at reactive phenols (DIBP6) renders rigid pillar-shape architecture to the host in which electron-rich regions are localized within the cavity and near portals. Complexation of n-octyltriethylammonium ions (n-OTEA) with P6 and DIBP6 reveals qualitatively different binding patterns. It has been shown that the conformer in which n-OTEA penetrates from the lower rim of the host and partially encapsulates within the P6 cavity turns out to be 1.4 kJ mol(-1) lower in energy than the complex showing complete guest encapsulation. Host-guest binding patterns, viz. encapsulation or portal interactions, can be distinguished from (1)H NMR chemical shifts. The shielding of ethyl and n-octyl chain protons in an n-OTEA⊂DIBP6 complex points to encapsulation of the guest which has been rationalized from natural bond orbital analyses. These inferences are in consonance with (1)H NMR experiments.

Quantitative Assessment of Substituent Effects on Cation−π Interactions Using Molecular Electrostatic Potential Topography

A molecular electrostatic potential (MESP) topography based approach has been proposed to quantify the substituent effects on cation-π interactions in complexes of mono-, di-, tri-, and hexasubstituted benzenes with Li(+), Na(+), K(+), and NH(4)(+). The MESP minimum (V(min)) on the π-region of C(6)H(5)X showed strong linear dependency to the cation-π interaction energy, E(M(+)). Further, cation-π distance correlated well with V(min)-π distance. The difference between V(min) of C(6)H(5)X and C(6)H(6) (ΔV(min)) is proposed as a good parameter to quantify the substituent effect on cation-π interaction. Compared to benzene, electron-donating groups stabilize the di-, tri-, and hexasubstituted cation-π complexes while electron-withdrawing groups destabilize them. In multiple substituted complexes, E(M(+)) is almost equal (∼95%) to the sum of the individual substituent contributions (E(M(+)) ≈ Σ(ΔE(M(+)))), suggesting that substituent effect on cation-π interactions is largely additive. The ΔV(min) of C(6)H(5)X systems and additivity feature have been used to make predictions on the interaction energies of 80 multiple substituted cation-π complexes with above 97% accuracy. The average mean absolute deviation of the V(min)-predicted interaction energy, E(M(+))(V) from the calculated E(M(+)) is -0.18 kcal/mol for Li(+), -0.09 kcal/mol for Na(+), -0.43 kcal/mol for K(+), and -0.67 kcal/mol for NH(4)(+), which emphasize the predictive power of V(min) as well as the additive feature of the substituent effect.

Molecular electrostatic potentials in Cucurbit[ n]uril ( n = 13–16) hosts

Charge distribution and 1H NMR chemical shifts in different Cucurbit[ n]uril {CB[ n] ( n = 13–16)} conformers, generated by flipping methylene groups adjacent to glycouril units, have been obtained by employing the density functional theory. The lowest energy conformer of CB[ n] has been endowed with symmetric architecture of methylene groups which engender uniform cavity and yield shallow minima near carbonyl oxygen’s on the host portals along the series. The effective cavity diameter and cavity height from the molecular electrostatic potential topography reveal that cavity diameter of these hosts varies from 16.1 Å for CB[13] to 20.9 Å for CB[16]; the cavity height of these hosts turn out to be nearly constant and equals ∼8.6 Å. CB[ n] family of hosts exhibit three different types of proton signals in the NMR spectra which include: methylene protons directing outside and towards the cavity of the host are denoted by H1 and H3 whereas the methine protons pointing outside the cavity are labeled as H2. Calculated 1H NMR chemical shifts in CB[ n] hosts follow the order: H3 > H2 > H1 in the gas phase and with water as solvent in consonant with the experimental spectra of lower CB[ n] homologues (J. Kim, I. Jung, S. Kim, E. Lee, J. Kang, S. Sakamoto, K. Yamaguchi, K. Kim, J. Am. Chem. Soc. 122 (2000) 540).

Appraisal of Through-Bond and Through-Space Substituent Effects via Molecular Electrostatic Potential Topography

Through-bond (TB) and through-space (TS) substituent effects in substituted alkyl, alkenyl, and alkynyl arenes are quantified separately using molecular electrostatic potential (MESP) topographical analysis. The deepest MESP point over the aromatic ring (V(min)) is considered as a probe for monitoring these effects for a variety of substituents. In the case of substituted alkyl chains, the TS effect (79.6%) clearly dominates the TB effect, whereas in the unsaturated analogues the TB effect (∼55%) overrides the TS effect.

Density Functional Investigations on the Charge Distribution, Vibrational Spectra, and NMR Chemical Shifts in Cucurbit[n]uril (n = 5−12) Hosts

Electronic structure, charge distribution, and vibrational frequencies of cucurbit[n]uril, CB[n] (n = 5-12), hosts have been derived using the density functional methods. CB[n] conformers with different orientations of methylene group connecting glycouril units have been investigated. The conformers that possess uniform CB[n] cavity turn out to be of lowest energy, and molecular electrostatic potential (MESP) herein engender shallow minima near ureido oxygens along the series. MESP topography has been utilized to estimate the cavity height and diameter; the ratio of which governs the shape (circular or elliptical) of the cavity. When this ratio is larger than unity (for CB[n] with n >or= 8), an elliptical host cavity is noticed. Calculated vibrational spectra reveal that carbonyl stretching frequency shift in successive CB[n] homologue decreases steadily from 1760 cm(-1) in CB[5] to 1742 cm(-1) in CB[12]. An increase in glycouril units along the CB[n] series influences significantly the intensity profile of C horizontal lineO and C-N stretching vibrations in the calculated infrared spectra. Furthermore, calculated (1)H chemical shifts predict that one of methylene protons directing outside the host cavity are deshielded, whereas the remaining proton near the carbonyl group exhibits downshifted signal in the NMR spectra.

Pictorial representation and validation of Clar's aromatic sextet theory using molecular electrostatic potentials

Topographical features of the molecular electrostatic potential (MESP) of a series of polycyclic aromatic benzenoid hydrocarbons have been analyzed at B3LYP/6-31+G(d,p) and MP2/6-31+G(d,p) levels of theory to study the relationship between π-electron distribution and Clar's aromatic sextet theory. For all the molecules, MESP isosurface plots provided an unambiguous visual representation of Clar's aromatic sextet theory. For molecules represented using a single dominant Clar structure, a complete agreement of Clar's theory was observed. For benzenoid structures showing more than one Clar structure, MESP isosurface lobes reflected the merger of possible Clar structures, and even in such cases, the most dominant configuration was clearly obtained in the MESP topography. The MESP at the ring critical point (Vrcp) of the π-electron cloud of every ring was found to be a good local measure of aromaticity. A hydrogenation scheme was introduced to calculate the aromatic stabilization energy (Earoma) of the peripheral rings. Earoma and Vrcp showed a good linear correlation. Furthermore, a linear correlation of Vrcp with the widely used geometric index of aromaticity, HOMA, was obtained. Thus, for a visual representation as well as rationalization of Clar's theory, MESP serves as an important descriptor of the π-regions, and Vrcp values give further insight into this theory by quantifying the local aromaticity of each ring in the polycyclic aromatic system. The theoretical results presented herein fully support the ideas of Clar's empirical sextet theory—a theory mainly based on experimental findings of the local reactivity of condensed aromatic systems.

Cavity diameter and height of cyclodextrins and cucurbit[n]urils from the molecular electrostatic potential topography

Understanding the interactions of cyclodextrins (CD) and cucurbit[n]uril (CB[n]) hosts with a variety of guest molecules following their encapsulation within the cavity of these macrocycles have become increasingly important in the recent years. The electronic charge distribution and the cavity dimension are some of the key factors those govern their interactions with cations or neutral guests. In the present work the molecular electrostatic potential (MESP) topography has been utilized to obtain the ‘effective’ cavity diameter and height of CB[n] (n = 6–8) homologues and 8 conformers each of α-, β- and γ-CD. It has been shown that the shape of CD cavity be it cone- or barrel-like stems from the hydrogen bonding patterns within primary hydroxyl groups. The width of CB[7] is comparable to the β-CD conformer that possesses either O6H–O5′ or intraglucose O6H–O5 interactions. The cavity diameters of α- and γ-CD are predicted to be respectively, 1.0 and 1.5 A larger than CB[6] and CB[8] hosts. MESP topography reveals that the cavities of CB[n] are less hydrophilic with largely hydrophilic portals as compared to CD hosts. Cremer–Pople puckering parameters were derived for all the CD conformers and CB[n]. It has been demonstrated that the clockwise and anticlockwise hydrogen bonding patterns in the lower as well as upper rims of different CD conformers are less distorted and exhibit a little deviation from the °C3 chair conformation of α-d-glucopyranose constituting monomeric unit of CD.

Molecular electrostatic potentials in aromatic substituted 4-hydroxyquino-2-lones: Glycine/NMDA receptor antagonists

Hydroxyquinolone derivatives have proven to be useful for inhibition at the glycine binding site of N-methyl-D-aspartate (NMDA) receptor. In this work the electronic structure, molecular electrostatic potential (MESP) and vibrational characteristics of a set of C(3) substituted 4-hydroxyquino-2-lone (HQ) derivatives, which act as Glycine/NMDA receptor antagonists, have been investigated using the density functional calculations. In the optimized structures a substituent at the C(3) site of HQ tends to adopt a helical structure. MESP investigations reveal that the ligands showing better inhibition activity should possess electron-rich regions extending over the substituent and carbonyl group of HQ. A correlation of inhibitory activity to the molecular electrostatic potential topography at the carbonyl oxygen as well as to the molecular electron density topography turns out to be a significant output of the investigation.

Electronic Structure, Molecular Electrostatic Potential, and NMR Chemical Shifts in Cucurbit[n]urils (n = 5−8), Ferrocene, and Their Complexes

Electronic structure and molecular electrostatic potential (MESP) in ferrocene (FC), cucurbit[n]urils (CB[n]) with n = 5-8, and their host-guest complexes are obtained within the framework of density functional theory. MESP topography that is employed to gauge the dimensions of the CB[n] cavity estimates that the cavity height increases from 7.25 to 7.70 A along CB[n] homologue series, whereas the diameter of the CB[8] (8.57 A) cavity is larger than twice that of CB[5] (3.91 A). MESP investigations reveal deeper minima near ureido oxygens in CB[5] along with large electron-rich regions at its portal. A lateral interaction of the guest FC with hydrophilic exterior of the CB[n] portal and its encapsulation within hydrophobic cavity of the host are analyzed. The present calculations suggest that CB[5] does not yield stable complexes in either case. FC interacts laterally with CB[6], and inclusion of the guest occurs, both parallel as well as perpendicular to the CB[n] axis, in the cavity of higher homologue. Self-consistent reaction field studies indicate that, in the presence of water as a solvent, encapsulation of FC in parallel fashion is favored within CB[7] and CB[8] cavities. NMR chemical shifts (delta(H)) of CB[n] protons remain practically unchanged with an increase in the cavity size; however, they are influenced significantly by water. The spectra thus obtained in aqueous solution agree with those observed experimentally. The delta(H) values in FC-CB[n] complexes indicate deshielding of FC protons directed toward portals, while those pointing toward nitrogens exhibit up-shifts in the spectra.

Electrostatic Potential Topography for Exploring Electronic Reorganizations in 1,3 Dipolar Cycloadditions

Topographical analysis of the molecular electrostatic potential (MESP) along a reaction path is employed for bringing out sequential electronic reorganizations for 1,3-dipolar cycloadditions of ethyne to fulminic acid as well as diazomethane. A simple and consistent set of rules for portraying electronic mechanisms of chemical reactions using the MESP topography is applied for this purpose. The MESP topography at each point on the concerted reaction path is associated with a classical electronic structure yielding a clear picture of the electronic reorganization along the reaction path.

Theoretical and experimental investigations on the structure and vibrational spectra of 4-{[(1 Z)-1-methyl-3-oxobut-1-enyl]amino}benzoic acid

Hartree–Fock and hybrid density functional methods have been used to determine the structure and energetics of 4-{[(1 Z)-1-methyl-3-oxobut-1-enyl]amino}benzoic acid ( 1) tautomers. The N–H⋯O (1.750 Å) intramolecular hydrogen bonded interactions are present in the keto tautomer and relatively strong O–H⋯N (1.616 Å) interactions prevail in the enol form. The two functional groups, viz. benzoic acid and 4-amino-pent-3-ene-2-one orient mutually with 154° in the keto tautomer in agreement with the X-ray data. Extended conjugation, which renders stability to the keto tautomer, has been manifested in the topography of molecular electrostatic potential. The deepest minimum in the electrostatic potential of the lowest energy tautomer emerges near carbonyl oxygen and is located near the carboxylic oxygen of benzoic acid for the enol form. X-ray crystal structure reveals the extended three-dimensional network comprising of C O⋯H intermolecular hydrogen bonds (1.82 Å). Quantum mechanically derived molecular electrostatic potential provide insights for the formation of supramolecular assemblies. The 1699 and 1240 cm −1 C O and C–N stretching vibrations in the observed infrared spectrum agree well with those predicted from the density functional calculations. The downshift of OH stretching to ∼2986 cm −1 in the observed infrared spectrum results from the C O⋯H intermolecular hydrogen bonded interactions.